augmented reality essay

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Augmented Reality – The Past, The Present and The Future

Augmented reality has come a long way from a science-fiction concept to a science-based reality. Until recently the costs of augmented reality were so substantial that designers could only dream of working on design projects that involved it – today things have changed and augmented reality is even available on the mobile handset. That means design for augmented reality is now an option for all shapes and sizes of UX designers .

Augmented reality is a view of the real, physical world in which elements are enhanced by computer-generated input. These inputs may range from sound to video, to graphics to GPS overlays and more. The first conception of augmented reality occurred in a novel by Frank L Baum written in 1901 in which a set of electronic glasses mapped data onto people; it was called a “character marker”. Today, augmented reality is a real thing and not a science-fiction concept.

A Brief History of Augmented Reality (The Past)

Augmented reality was first achieved, to some extent, by a cinematographer called Morton Heilig in 1957. He invented the Sensorama which delivered visuals, sounds, vibration and smell to the viewer. Of course, it wasn’t computer controlled but it was the first example of an attempt at adding additional data to an experience.

Author/Copyright holder: Morton Heilig. Copyright terms and license: Public Domain.

Then in 1968, Ivan Sutherland the American computer scientist and early Internet influence, invented the head-mounted display as a kind of window into a virtual world. The technology used at the time made the invention impractical for mass use.

In 1975, Myron Krueger, an American computer artist developed the first “ virtual reality ” interface in the form of “Videoplace” which allowed its users to manipulate and interact with virtual objects and to do so in real-time.

Steve Mann, a computational photography researcher, gave the world wearable computing in 1980.

Of course back then these weren’t “virtual reality” or “augmented reality” because virtual reality was coined by Jaron Lainer in 1989 and Thomas P Caudell of Boeing coined the phrase “augmented reality” in 1990.

The first properly functioning AR system was probably the one developed at USAF Armstrong’s Research Lab by Louis Rosenberg in 1992. This was called Virtual Fixtures and was an incredibly complex robotic system which was designed to compensate for the lack of high-speed 3D graphics processing power in the early 90s. It enabled the overlay of sensory information on a workspace to improve human productivity

There were many other breakthroughs in augmented reality between here and today; the most notable of which include:

Bruce Thomas developing an outdoor mobile AR game called ARQuake in 2000

ARToolkit (a design tool) being made available in Adobe Flash in 2009

Google announcing its open beta of Google Glass (a project with mixed successes) in 2013

Microsoft announcing augmented reality support and their augmented reality headset HoloLens in 2015

The Current State of Play in Augmented Reality (The Present)

Augmented reality is achieved through a variety of technological innovations ; these can be implemented on their own or in conjunction with each other to create augmented reality. They include:

General hardware components – the processor, the display, the sensors and input devices. Typically a smartphone contains a processor, a display, accelerometers, GPS, camera, microphone etc. and contains all the hardware required to be an AR device.

Displays – while a monitor is perfectly capable of displaying AR data there are other systems such as optical projection systems, head-mounted displays, eyeglasses, contact lenses, the HUD (heads up display), virtual retinal displays, EyeTap (a device which changes the rays of light captured from the environment and substitutes them with computer generated ones),Spatial Augmented Reality (SAR – which uses ordinary projection techniques as a substitute for a display of any kind) and handheld displays.

Sensors and input devices include – GPS, gyroscopes, accelerometers, compasses, RFID, wireless sensors, touch recognition, speech recognition, eye tracking and peripherals.

Software – the majority of development for AR will be in developing further software to take advantage of the hardware capabilities. There is already an Augmented Reality Markup Language (ARML) which is being used to standardize XML grammar for virtual reality. There are several software development kits (SDK) which also offer simple environments for AR development.

There are apps available for or being researched for AR in nearly every industrial sector including:

Archaeology, Art, Architecture

Commerce, Office

Construction, Industrial Design

Education, Translation

Emergency Management, Disaster Recovery, Medical and Search and Rescue

Games, Sports, Entertainment, Tourism

Author/Copyright holder: Sonk54. Copyright terms and license: CC BY-SA 3.0

The Future of Augmented Reality

Jessica Lowry, a UX Designer, writing for the Next Web says that AR is the future of design and we tend to agree. Already mobile phones are such an integral part of our lives that they might as well be extensions of our bodies; as technology can be further integrated into our lives without being intrusive (a la Google Glass) – it is a certainty that augmented reality provides opportunities to enhance user experiences beyond measure.

This will almost certainly see major advances in the much-hyped but still little seen; Internet of Things . UX designers in the AR field will need to seriously consider the questions of how traditional experiences can be improved through AR – just making your cooker capable of using computer enhancements is not enough; it needs to healthier eating or better cooked food for users to care.

The future will belong to AR when it improves task efficiency or the quality of the output of an experience for the user. This is the key challenge of the 21st century UX profession.

Author/Copyright holder: Austin Berner. Copyright terms and license: Public Domain

The Takeaway

AR or augmented reality has gone from pipe dream to reality in just over a century. There are many AR applications in use or under development today, however – the concept will only take off universally when UX designers think about how they can integrate AR with daily life to improve productivity, efficiency or quality of experiences. There is an unlimited potential for AR, the big question is - how will it be unlocked?

References & Where to Learn More:

DID L. FRANK BAUM PREDICT AUGMENTED REALITY OR WARN US ABOUT ITS POWER? Some food for thought.

Ivan Sutherland’s research can be found here: http://90.146.8.18/en/archiv_files/19902/E1990b_123.pdf

Steve Mann’s research can be read here: "Eye Am a Camera: Surveillance and Sousveillance in the Glassage" . Techland.time.com

Rosenberg’s original research paper was published as: L. B. Rosenberg. The Use of Virtual Fixtures As Perceptual Overlays to Enhance Operator Performance in Remote Environments. Technical Report AL-TR-0089, USAF Armstrong Laboratory, Wright-Patterson AFB OH, 1992.

Find out more about ARQuake at Wiki.

Learn more about Google Glass at the New York Times.

Jessica Lowry’s article: Augmented reality is the future of design

Hero Image: Author/Copyright holder: Maurizio Pesce. Copyright terms and license: CC BY 2.0

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ORIGINAL RESEARCH article

The past, present, and future of virtual and augmented reality research: a network and cluster analysis of the literature.

• 1 Applied Technology for Neuro-Psychology Lab, Istituto Auxologico Italiano, Milan, Italy
• 2 Department of Psychology, Catholic University of the Sacred Heart, Milan, Italy
• 3 Instituto de Investigación e Innovación en Bioingeniería, Universitat Politècnica de València, Valencia, Spain

The recent appearance of low cost virtual reality (VR) technologies – like the Oculus Rift, the HTC Vive and the Sony PlayStation VR – and Mixed Reality Interfaces (MRITF) – like the Hololens – is attracting the attention of users and researchers suggesting it may be the next largest stepping stone in technological innovation. However, the history of VR technology is longer than it may seem: the concept of VR was formulated in the 1960s and the first commercial VR tools appeared in the late 1980s. For this reason, during the last 20 years, 100s of researchers explored the processes, effects, and applications of this technology producing 1000s of scientific papers. What is the outcome of this significant research work? This paper wants to provide an answer to this question by exploring, using advanced scientometric techniques, the existing research corpus in the field. We collected all the existent articles about VR in the Web of Science Core Collection scientific database, and the resultant dataset contained 21,667 records for VR and 9,944 for augmented reality (AR). The bibliographic record contained various fields, such as author, title, abstract, country, and all the references (needed for the citation analysis). The network and cluster analysis of the literature showed a composite panorama characterized by changes and evolutions over the time. Indeed, whether until 5 years ago, the main publication media on VR concerned both conference proceeding and journals, more recently journals constitute the main medium of communication. Similarly, if at first computer science was the leading research field, nowadays clinical areas have increased, as well as the number of countries involved in VR research. The present work discusses the evolution and changes over the time of the use of VR in the main areas of application with an emphasis on the future expected VR’s capacities, increases and challenges. We conclude considering the disruptive contribution that VR/AR/MRITF will be able to get in scientific fields, as well in human communication and interaction, as already happened with the advent of mobile phones by increasing the use and the development of scientific applications (e.g., in clinical areas) and by modifying the social communication and interaction among people.

Introduction

In the last 5 years, virtual reality (VR) and augmented reality (AR) have attracted the interest of investors and the general public, especially after Mark Zuckerberg bought Oculus for two billion dollars ( Luckerson, 2014 ; Castelvecchi, 2016 ). Currently, many other companies, such as Sony, Samsung, HTC, and Google are making huge investments in VR and AR ( Korolov, 2014 ; Ebert, 2015 ; Castelvecchi, 2016 ). However, if VR has been used in research for more than 25 years, and now there are 1000s of papers and many researchers in the field, comprising a strong, interdisciplinary community, AR has a more recent application history ( Burdea and Coiffet, 2003 ; Kim, 2005 ; Bohil et al., 2011 ; Cipresso and Serino, 2014 ; Wexelblat, 2014 ). The study of VR was initiated in the computer graphics field and has been extended to several disciplines ( Sutherland, 1965 , 1968 ; Mazuryk and Gervautz, 1996 ; Choi et al., 2015 ). Currently, videogames supported by VR tools are more popular than the past, and they represent valuables, work-related tools for neuroscientists, psychologists, biologists, and other researchers as well. Indeed, for example, one of the main research purposes lies from navigation studies that include complex experiments that could be done in a laboratory by using VR, whereas, without VR, the researchers would have to go directly into the field, possibly with limited use of intervention. The importance of navigation studies for the functional understanding of human memory in dementia has been a topic of significant interest for a long time, and, in 2014, the Nobel Prize in “Physiology or Medicine” was awarded to John M. O’Keefe, May-Britt Moser, and Edvard I. Moser for their discoveries of nerve cells in the brain that enable a sense of place and navigation. Journals and magazines have extended this knowledge by writing about “the brain GPS,” which gives a clear idea of the mechanism. A huge number of studies have been conducted in clinical settings by using VR ( Bohil et al., 2011 ; Serino et al., 2014 ), and Nobel Prize winner, Edvard I. Moser commented about the use of VR ( Minderer et al., 2016 ), highlighting its importance for research and clinical practice. Moreover, the availability of free tools for VR experimental and computational use has made it easy to access any field ( Riva et al., 2011 ; Cipresso, 2015 ; Brown and Green, 2016 ; Cipresso et al., 2016 ).

Augmented reality is a more recent technology than VR and shows an interdisciplinary application framework, in which, nowadays, education and learning seem to be the most field of research. Indeed, AR allows supporting learning, for example increasing-on content understanding and memory preservation, as well as on learning motivation. However, if VR benefits from clear and more definite fields of application and research areas, AR is still emerging in the scientific scenarios.

In this article, we present a systematic and computational analysis of the emerging interdisciplinary VR and AR fields in terms of various co-citation networks in order to explore the evolution of the intellectual structure of this knowledge domain over time.

Virtual Reality Concepts and Features

The concept of VR could be traced at the mid of 1960 when Ivan Sutherland in a pivotal manuscript attempted to describe VR as a window through which a user perceives the virtual world as if looked, felt, sounded real and in which the user could act realistically ( Sutherland, 1965 ).

Since that time and in accordance with the application area, several definitions have been formulated: for example, Fuchs and Bishop (1992) defined VR as “real-time interactive graphics with 3D models, combined with a display technology that gives the user the immersion in the model world and direct manipulation” ( Fuchs and Bishop, 1992 ); Gigante (1993) described VR as “The illusion of participation in a synthetic environment rather than external observation of such an environment. VR relies on a 3D, stereoscopic head-tracker displays, hand/body tracking and binaural sound. VR is an immersive, multi-sensory experience” ( Gigante, 1993 ); and “Virtual reality refers to immersive, interactive, multi-sensory, viewer-centered, 3D computer generated environments and the combination of technologies required building environments” ( Cruz-Neira, 1993 ).

As we can notice, these definitions, although different, highlight three common features of VR systems: immersion, perception to be present in an environment, and interaction with that environment ( Biocca, 1997 ; Lombard and Ditton, 1997 ; Loomis et al., 1999 ; Heeter, 2000 ; Biocca et al., 2001 ; Bailenson et al., 2006 ; Skalski and Tamborini, 2007 ; Andersen and Thorpe, 2009 ; Slater, 2009 ; Sundar et al., 2010 ). Specifically, immersion concerns the amount of senses stimulated, interactions, and the reality’s similarity of the stimuli used to simulate environments. This feature can depend on the properties of the technological system used to isolate user from reality ( Slater, 2009 ).

Higher or lower degrees of immersion can depend by three types of VR systems provided to the user:

• Non-immersive systems are the simplest and cheapest type of VR applications that use desktops to reproduce images of the world.

• Immersive systems provide a complete simulated experience due to the support of several sensory outputs devices such as head mounted displays (HMDs) for enhancing the stereoscopic view of the environment through the movement of the user’s head, as well as audio and haptic devices.

• Semi-immersive systems such as Fish Tank VR are between the two above. They provide a stereo image of a three dimensional (3D) scene viewed on a monitor using a perspective projection coupled to the head position of the observer ( Ware et al., 1993 ). Higher technological immersive systems have showed a closest experience to reality, giving to the user the illusion of technological non-mediation and feeling him or her of “being in” or present in the virtual environment ( Lombard and Ditton, 1997 ). Furthermore, higher immersive systems, than the other two systems, can give the possibility to add several sensory outputs allowing that the interaction and actions were perceived as real ( Loomis et al., 1999 ; Heeter, 2000 ; Biocca et al., 2001 ).

Finally, the user’s VR experience could be disclosed by measuring presence, realism, and reality’s levels. Presence is a complex psychological feeling of “being there” in VR that involves the sensation and perception of physical presence, as well as the possibility to interact and react as if the user was in the real world ( Heeter, 1992 ). Similarly, the realism’s level corresponds to the degree of expectation that the user has about of the stimuli and experience ( Baños et al., 2000 , 2009 ). If the presented stimuli are similar to reality, VR user’s expectation will be congruent with reality expectation, enhancing VR experience. In the same way, higher is the degree of reality in interaction with the virtual stimuli, higher would be the level of realism of the user’s behaviors ( Baños et al., 2000 , 2009 ).

From Virtual to Augmented Reality

Looking chronologically on VR and AR developments, we can trace the first 3D immersive simulator in 1962, when Morton Heilig created Sensorama, a simulated experience of a motorcycle running through Brooklyn characterized by several sensory impressions, such as audio, olfactory, and haptic stimuli, including also wind to provide a realist experience ( Heilig, 1962 ). In the same years, Ivan Sutherland developed The Ultimate Display that, more than sound, smell, and haptic feedback, included interactive graphics that Sensorama didn’t provide. Furthermore, Philco developed the first HMD that together with The Sword of Damocles of Sutherland was able to update the virtual images by tracking user’s head position and orientation ( Sutherland, 1965 ). In the 70s, the University of North Carolina realized GROPE, the first system of force-feedback and Myron Krueger created VIDEOPLACE an Artificial Reality in which the users’ body figures were captured by cameras and projected on a screen ( Krueger et al., 1985 ). In this way two or more users could interact in the 2D-virtual space. In 1982, the US’ Air Force created the first flight simulator [Visually Coupled Airbone System Simulator (VCASS)] in which the pilot through an HMD could control the pathway and the targets. Generally, the 80’s were the years in which the first commercial devices began to emerge: for example, in 1985 the VPL company commercialized the DataGlove, glove sensors’ equipped able to measure the flexion of fingers, orientation and position, and identify hand gestures. Another example is the Eyephone, created in 1988 by the VPL Company, an HMD system for completely immerging the user in a virtual world. At the end of 80’s, Fake Space Labs created a Binocular-Omni-Orientational Monitor (BOOM), a complex system composed by a stereoscopic-displaying device, providing a moving and broad virtual environment, and a mechanical arm tracking. Furthermore, BOOM offered a more stable image and giving more quickly responses to movements than the HMD devices. Thanks to BOOM and DataGlove, the NASA Ames Research Center developed the Virtual Wind Tunnel in order to research and manipulate airflow in a virtual airplane or space ship. In 1992, the Electronic Visualization Laboratory of the University of Illinois created the CAVE Automatic Virtual Environment, an immersive VR system composed by projectors directed on three or more walls of a room.

More recently, many videogames companies have improved the development and quality of VR devices, like Oculus Rift, or HTC Vive that provide a wider field of view and lower latency. In addition, the actual HMD’s devices can be now combined with other tracker system as eye-tracking systems (FOVE), and motion and orientation sensors (e.g., Razer Hydra, Oculus Touch, or HTC Vive).

Simultaneously, at the beginning of 90’, the Boing Corporation created the first prototype of AR system for showing to employees how set up a wiring tool ( Carmigniani et al., 2011 ). At the same time, Rosenberg and Feiner developed an AR fixture for maintenance assistance, showing that the operator performance enhanced by added virtual information on the fixture to repair ( Rosenberg, 1993 ). In 1993 Loomis and colleagues produced an AR GPS-based system for helping the blind in the assisted navigation through adding spatial audio information ( Loomis et al., 1998 ). Always in the 1993 Julie Martin developed “Dancing in Cyberspace,” an AR theater in which actors interacted with virtual object in real time ( Cathy, 2011 ). Few years later, Feiner et al. (1997) developed the first Mobile AR System (MARS) able to add virtual information about touristic buildings ( Feiner et al., 1997 ). Since then, several applications have been developed: in Thomas et al. (2000) , created ARQuake, a mobile AR video game; in 2008 was created Wikitude that through the mobile camera, internet, and GPS could add information about the user’s environments ( Perry, 2008 ). In 2009 others AR applications, like AR Toolkit and SiteLens have been developed in order to add virtual information to the physical user’s surroundings. In 2011, Total Immersion developed D’Fusion, and AR system for designing projects ( Maurugeon, 2011 ). Finally, in 2013 and 2015, Google developed Google Glass and Google HoloLens, and their usability have begun to test in several field of application.

Virtual Reality Technologies

Technologically, the devices used in the virtual environments play an important role in the creation of successful virtual experiences. According to the literature, can be distinguished input and output devices ( Burdea et al., 1996 ; Burdea and Coiffet, 2003 ). Input devices are the ones that allow the user to communicate with the virtual environment, which can range from a simple joystick or keyboard to a glove allowing capturing finger movements or a tracker able to capture postures. More in detail, keyboard, mouse, trackball, and joystick represent the desktop input devices easy to use, which allow the user to launch continuous and discrete commands or movements to the environment. Other input devices can be represented by tracking devices as bend-sensing gloves that capture hand movements, postures and gestures, or pinch gloves that detect the fingers movements, and trackers able to follow the user’s movements in the physical world and translate them in the virtual environment.

On the contrary, the output devices allow the user to see, hear, smell, or touch everything that happens in the virtual environment. As mentioned above, among the visual devices can be found a wide range of possibilities, from the simplest or least immersive (monitor of a computer) to the most immersive one such as VR glasses or helmets or HMD or CAVE systems.

Furthermore, auditory, speakers, as well as haptic output devices are able to stimulate body senses providing a more real virtual experience. For example, haptic devices can stimulate the touch feeling and force models in the user.

Virtual Reality Applications

Since its appearance, VR has been used in different fields, as for gaming ( Zyda, 2005 ; Meldrum et al., 2012 ), military training ( Alexander et al., 2017 ), architectural design ( Song et al., 2017 ), education ( Englund et al., 2017 ), learning and social skills training ( Schmidt et al., 2017 ), simulations of surgical procedures ( Gallagher et al., 2005 ), assistance to the elderly or psychological treatments are other fields in which VR is bursting strongly ( Freeman et al., 2017 ; Neri et al., 2017 ). A recent and extensive review of Slater and Sanchez-Vives (2016) reported the main VR application evidences, including weakness and advantages, in several research areas, such as science, education, training, physical training, as well as social phenomena, moral behaviors, and could be used in other fields, like travel, meetings, collaboration, industry, news, and entertainment. Furthermore, another review published this year by Freeman et al. (2017) focused on VR in mental health, showing the efficacy of VR in assessing and treating different psychological disorders as anxiety, schizophrenia, depression, and eating disorders.

There are many possibilities that allow the use of VR as a stimulus, replacing real stimuli, recreating experiences, which in the real world would be impossible, with a high realism. This is why VR is widely used in research on new ways of applying psychological treatment or training, for example, to problems arising from phobias (agoraphobia, phobia to fly, etc.) ( Botella et al., 2017 ). Or, simply, it is used like improvement of the traditional systems of motor rehabilitation ( Llorens et al., 2014 ; Borrego et al., 2016 ), developing games that ameliorate the tasks. More in detail, in psychological treatment, Virtual Reality Exposure Therapy (VRET) has showed its efficacy, allowing to patients to gradually face fear stimuli or stressed situations in a safe environment where the psychological and physiological reactions can be controlled by the therapist ( Botella et al., 2017 ).

Augmented Reality Concept

Milgram and Kishino (1994) , conceptualized the Virtual-Reality Continuum that takes into consideration four systems: real environment, augmented reality (AR), augmented virtuality, and virtual environment. AR can be defined a newer technological system in which virtual objects are added to the real world in real-time during the user’s experience. Per Azuma et al. (2001) an AR system should: (1) combine real and virtual objects in a real environment; (2) run interactively and in real-time; (3) register real and virtual objects with each other. Furthermore, even if the AR experiences could seem different from VRs, the quality of AR experience could be considered similarly. Indeed, like in VR, feeling of presence, level of realism, and the degree of reality represent the main features that can be considered the indicators of the quality of AR experiences. Higher the experience is perceived as realistic, and there is congruence between the user’s expectation and the interaction inside the AR environments, higher would be the perception of “being there” physically, and at cognitive and emotional level. The feeling of presence, both in AR and VR environments, is important in acting behaviors like the real ones ( Botella et al., 2005 ; Juan et al., 2005 ; Bretón-López et al., 2010 ; Wrzesien et al., 2013 ).

Augmented Reality Technologies

Technologically, the AR systems, however various, present three common components, such as a geospatial datum for the virtual object, like a visual marker, a surface to project virtual elements to the user, and an adequate processing power for graphics, animation, and merging of images, like a pc and a monitor ( Carmigniani et al., 2011 ). To run, an AR system must also include a camera able to track the user movement for merging the virtual objects, and a visual display, like glasses through that the user can see the virtual objects overlaying to the physical world. To date, two-display systems exist, a video see-through (VST) and an optical see-though (OST) AR systems ( Botella et al., 2005 ; Juan et al., 2005 , 2007 ). The first one, disclosures virtual objects to the user by capturing the real objects/scenes with a camera and overlaying virtual objects, projecting them on a video or a monitor, while the second one, merges the virtual object on a transparent surface, like glasses, through the user see the added elements. The main difference between the two systems is the latency: an OST system could require more time to display the virtual objects than a VST system, generating a time lag between user’s action and performance and the detection of them by the system.

Augmented Reality Applications

Although AR is a more recent technology than VR, it has been investigated and used in several research areas such as architecture ( Lin and Hsu, 2017 ), maintenance ( Schwald and De Laval, 2003 ), entertainment ( Ozbek et al., 2004 ), education ( Nincarean et al., 2013 ; Bacca et al., 2014 ; Akçayır and Akçayır, 2017 ), medicine ( De Buck et al., 2005 ), and psychological treatments ( Juan et al., 2005 ; Botella et al., 2005 , 2010 ; Bretón-López et al., 2010 ; Wrzesien et al., 2011a , b , 2013 ; see the review Chicchi Giglioli et al., 2015 ). More in detail, in education several AR applications have been developed in the last few years showing the positive effects of this technology in supporting learning, such as an increased-on content understanding and memory preservation, as well as on learning motivation ( Radu, 2012 , 2014 ). For example, Ibáñez et al. (2014) developed a AR application on electromagnetism concepts’ learning, in which students could use AR batteries, magnets, cables on real superficies, and the system gave a real-time feedback to students about the correctness of the performance, improving in this way the academic success and motivation ( Di Serio et al., 2013 ). Deeply, AR system allows the possibility to learn visualizing and acting on composite phenomena that traditionally students study theoretically, without the possibility to see and test in real world ( Chien et al., 2010 ; Chen et al., 2011 ).

As well in psychological health, the number of research about AR is increasing, showing its efficacy above all in the treatment of psychological disorder (see the reviews Baus and Bouchard, 2014 ; Chicchi Giglioli et al., 2015 ). For example, in the treatment of anxiety disorders, like phobias, AR exposure therapy (ARET) showed its efficacy in one-session treatment, maintaining the positive impact in a follow-up at 1 or 3 month after. As VRET, ARET provides a safety and an ecological environment where any kind of stimulus is possible, allowing to keep control over the situation experienced by the patients, gradually generating situations of fear or stress. Indeed, in situations of fear, like the phobias for small animals, AR applications allow, in accordance with the patient’s anxiety, to gradually expose patient to fear animals, adding new animals during the session or enlarging their or increasing the speed. The various studies showed that AR is able, at the beginning of the session, to activate patient’s anxiety, for reducing after 1 h of exposition. After the session, patients even more than to better manage animal’s fear and anxiety, ware able to approach, interact, and kill real feared animals.

Materials and Methods

Data collection.

The input data for the analyses were retrieved from the scientific database Web of Science Core Collection ( Falagas et al., 2008 ) and the search terms used were “Virtual Reality” and “Augmented Reality” regarding papers published during the whole timespan covered.

Web of science core collection is composed of: Citation Indexes, Science Citation Index Expanded (SCI-EXPANDED) –1970-present, Social Sciences Citation Index (SSCI) –1970-present, Arts and Humanities Citation Index (A&HCI) –1975-present, Conference Proceedings Citation Index- Science (CPCI-S) –1990-present, Conference Proceedings Citation Index- Social Science & Humanities (CPCI-SSH) –1990-present, Book Citation Index– Science (BKCI-S) –2009-present, Book Citation Index– Social Sciences & Humanities (BKCI-SSH) –2009-present, Emerging Sources Citation Index (ESCI) –2015-present, Chemical Indexes, Current Chemical Reactions (CCR-EXPANDED) –2009-present (Includes Institut National de la Propriete Industrielle structure data back to 1840), Index Chemicus (IC) –2009-present.

The resultant dataset contained a total of 21,667 records for VR and 9,944 records for AR. The bibliographic record contained various fields, such as author, title, abstract, and all of the references (needed for the citation analysis). The research tool to visualize the networks was Cite space v.4.0.R5 SE (32 bit) ( Chen, 2006 ) under Java Runtime v.8 update 91 (build 1.8.0_91-b15). Statistical analyses were conducted using Stata MP-Parallel Edition, Release 14.0, StataCorp LP. Additional information can be found in Supplementary Data Sheet 1 .

The betweenness centrality of a node in a network measures the extent to which the node is part of paths that connect an arbitrary pair of nodes in the network ( Freeman, 1977 ; Brandes, 2001 ; Chen, 2006 ).

Structural metrics include betweenness centrality, modularity, and silhouette. Temporal and hybrid metrics include citation burstness and novelty. All the algorithms are detailed ( Chen et al., 2010 ).

The analysis of the literature on VR shows a complex panorama. At first sight, according to the document-type statistics from the Web of Science (WoS), proceedings papers were used extensively as outcomes of research, comprising almost 48% of the total (10,392 proceedings), with a similar number of articles on the subject amounting to about 47% of the total of 10, 199 articles. However, if we consider only the last 5 years (7,755 articles representing about 36% of the total), the situation changes with about 57% for articles (4,445) and about 33% for proceedings (2,578). Thus, it is clear that VR field has changed in areas other than at the technological level.

About the subject category, nodes and edges are computed as co-occurring subject categories from the Web of Science “Category” field in all the articles.

According to the subject category statistics from the WoS, computer science is the leading category, followed by engineering, and, together, they account for 15,341 articles, which make up about 71% of the total production. However, if we consider just the last 5 years, these categories reach only about 55%, with a total of 4,284 articles (Table 1 and Figure 1 ).

TABLE 1. Category statistics from the WoS for the entire period and the last 5 years.

FIGURE 1. Category from the WoS: network for the last 5 years.

The evidence is very interesting since it highlights that VR is doing very well as new technology with huge interest in hardware and software components. However, with respect to the past, we are witnessing increasing numbers of applications, especially in the medical area. In particular, note its inclusion in the top 10 list of rehabilitation and clinical neurology categories (about 10% of the total production in the last 5 years). It also is interesting that neuroscience and neurology, considered together, have shown an increase from about 12% to about 18.6% over the last 5 years. However, historic areas, such as automation and control systems, imaging science and photographic technology, and robotics, which had accounted for about 14.5% of the total articles ever produced were not even in the top 10 for the last 5 years, with each one accounting for less than 4%.

About the countries, nodes and edges are computed as networks of co-authors countries. Multiple occurrency of a country in the same paper are counted once.

The countries that were very involved in VR research have published for about 47% of the total (10,200 articles altogether). Of the 10,200 articles, the United States, China, England, and Germany published 4921, 2384, 1497, and 1398, respectively. The situation remains the same if we look at the articles published over the last 5 years. However, VR contributions also came from all over the globe, with Japan, Canada, Italy, France, Spain, South Korea, and Netherlands taking positions of prominence, as shown in Figure 2 .

FIGURE 2. Country network (node dimension represents centrality).

Network analysis was conducted to calculate and to represent the centrality index ( Freeman, 1977 ; Brandes, 2001 ), i.e., the dimension of the node in Figure 2 . The top-ranked country, with a centrality index of 0.26, was the United States (2011), and England was second, with a centrality index of 0.25. The third, fourth, and fifth countries were Germany, Italy, and Australia, with centrality indices of 0.15, 0.15, and 0.14, respectively.

About the Institutions, nodes and edges are computed as networks of co-authors Institutions (Figure 3 ).

FIGURE 3. Network of institutions: the dimensions of the nodes represent centrality.

The top-level institutions in VR were in the United States, where three universities were ranked as the top three in the world for published articles; these universities were the University of Illinois (159), the University of South California (147), and the University of Washington (146). The United States also had the eighth-ranked university, which was Iowa State University (116). The second country in the ranking was Canada, with the University of Toronto, which was ranked fifth with 125 articles and McGill University, ranked 10 th with 103 articles.

Other countries in the top-ten list were Netherlands, with the Delft University of Technology ranked fourth with 129 articles; Italy, with IRCCS Istituto Auxologico Italiano, ranked sixth (with the same number of publication of the institution ranked fifth) with 125 published articles; England, which was ranked seventh with 125 articles from the University of London’s Imperial College of Science, Technology, and Medicine; and China with 104 publications, with the Chinese Academy of Science, ranked ninth. Italy’s Istituto Auxologico Italiano, which was ranked fifth, was the only non-university institution ranked in the top-10 list for VR research (Figure 3 ).

About the Journals, nodes, and edges are computed as journal co-citation networks among each journals in the corresponding field.

The top-ranked Journals for citations in VR are Presence: Teleoperators & Virtual Environments with 2689 citations and CyberPsychology & Behavior (Cyberpsychol BEHAV) with 1884 citations; however, looking at the last 5 years, the former had increased the citations, but the latter had a far more significant increase, from about 70% to about 90%, i.e., an increase from 1029 to 1147.

Following the top two journals, IEEE Computer Graphics and Applications ( IEEE Comput Graph) and Advanced Health Telematics and Telemedicine ( St HEAL T) were both left out of the top-10 list based on the last 5 years. The data for the last 5 years also resulted in the inclusion of Experimental Brain Research ( Exp BRAIN RES) (625 citations), Archives of Physical Medicine and Rehabilitation ( Arch PHYS MED REHAB) (622 citations), and Plos ONE (619 citations) in the top-10 list of three journals, which highlighted the categories of rehabilitation and clinical neurology and neuroscience and neurology. Journal co-citation analysis is reported in Figure 4 , which clearly shows four distinct clusters.

FIGURE 4. Co-citation network of journals: the dimensions of the nodes represent centrality. Full list of official abbreviations of WoS journals can be found here: https://images.webofknowledge.com/images/help/WOS/A_abrvjt.html .

Network analysis was conducted to calculate and to represent the centrality index, i.e., the dimensions of the nodes in Figure 4 . The top-ranked item by centrality was Cyberpsychol BEHAV, with a centrality index of 0.29. The second-ranked item was Arch PHYS MED REHAB, with a centrality index of 0.23. The third was Behaviour Research and Therapy (Behav RES THER), with a centrality index of 0.15. The fourth was BRAIN, with a centrality index of 0.14. The fifth was Exp BRAIN RES, with a centrality index of 0.11.

Who’s Who in VR Research

Authors are the heart and brain of research, and their roles in a field are to define the past, present, and future of disciplines and to make significant breakthroughs to make new ideas arise (Figure 5 ).

FIGURE 5. Network of authors’ numbers of publications: the dimensions of the nodes represent the centrality index, and the dimensions of the characters represent the author’s rank.

Virtual reality research is very young and changing with time, but the top-10 authors in this field have made fundamentally significant contributions as pioneers in VR and taking it beyond a mere technological development. The purpose of the following highlights is not to rank researchers; rather, the purpose is to identify the most active researchers in order to understand where the field is going and how they plan for it to get there.

The top-ranked author is Riva G, with 180 publications. The second-ranked author is Rizzo A, with 101 publications. The third is Darzi A, with 97 publications. The forth is Aggarwal R, with 94 publications. The six authors following these three are Slater M, Alcaniz M, Botella C, Wiederhold BK, Kim SI, and Gutierrez-Maldonado J with 90, 90, 85, 75, 59, and 54 publications, respectively (Figure 6 ).

FIGURE 6. Authors’ co-citation network: the dimensions of the nodes represent centrality index, and the dimensions of the characters represent the author’s rank. The 10 authors that appear on the top-10 list are considered to be the pioneers of VR research.

Considering the last 5 years, the situation remains similar, with three new entries in the top-10 list, i.e., Muhlberger A, Cipresso P, and Ahmed K ranked 7th, 8th, and 10th, respectively.

The authors’ publications number network shows the most active authors in VR research. Another relevant analysis for our focus on VR research is to identify the most cited authors in the field.

For this purpose, the authors’ co-citation analysis highlights the authors in term of their impact on the literature considering the entire time span of the field ( White and Griffith, 1981 ; González-Teruel et al., 2015 ; Bu et al., 2016 ). The idea is to focus on the dynamic nature of the community of authors who contribute to the research.

Normally, authors with higher numbers of citations tend to be the scholars who drive the fundamental research and who make the most meaningful impacts on the evolution and development of the field. In the following, we identified the most-cited pioneers in the field of VR Research.

The top-ranked author by citation count is Gallagher (2001), with 694 citations. Second is Seymour (2004), with 668 citations. Third is Slater (1999), with 649 citations. Fourth is Grantcharov (2003), with 563 citations. Fifth is Riva (1999), with 546 citations. Sixth is Aggarwal (2006), with 505 citations. Seventh is Satava (1994), with 477 citations. Eighth is Witmer (2002), with 454 citations. Ninth is Rothbaum (1996), with 448 citations. Tenth is Cruz-neira (1995), with 416 citations.

Citation Network and Cluster Analyses for VR

Another analysis that can be used is the analysis of document co-citation, which allows us to focus on the highly-cited documents that generally are also the most influential in the domain ( Small, 1973 ; González-Teruel et al., 2015 ; Orosz et al., 2016 ).

The top-ranked article by citation counts is Seymour (2002) in Cluster #0, with 317 citations. The second article is Grantcharov (2004) in Cluster #0, with 286 citations. The third is Holden (2005) in Cluster #2, with 179 citations. The 4th is Gallagher et al. (2005) in Cluster #0, with 171 citations. The 5th is Ahlberg (2007) in Cluster #0, with 142 citations. The 6th is Parsons (2008) in Cluster #4, with 136 citations. The 7th is Powers (2008) in Cluster #4, with 134 citations. The 8th is Aggarwal (2007) in Cluster #0, with 121 citations. The 9th is Reznick (2006) in Cluster #0, with 121 citations. The 10th is Munz (2004) in Cluster #0, with 117 citations.

The network of document co-citations is visually complex (Figure 7 ) because it includes 1000s of articles and the links among them. However, this analysis is very important because can be used to identify the possible conglomerate of knowledge in the area, and this is essential for a deep understanding of the area. Thus, for this purpose, a cluster analysis was conducted ( Chen et al., 2010 ; González-Teruel et al., 2015 ; Klavans and Boyack, 2015 ). Figure 8 shows the clusters, which are identified with the two algorithms in Table 2 .

FIGURE 7. Network of document co-citations: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank, and the numbers represent the strengths of the links. It is possible to identify four historical phases (colors: blue, green, yellow, and red) from the past VR research to the current research.

FIGURE 8. Document co-citation network by cluster: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank and the red writing reports the name of the cluster with a short description that was produced with the mutual information algorithm; the clusters are identified with colored polygons.

TABLE 2. Cluster ID and silhouettes as identified with two algorithms ( Chen et al., 2010 ).

The identified clusters highlight clear parts of the literature of VR research, making clear and visible the interdisciplinary nature of this field. However, the dynamics to identify the past, present, and future of VR research cannot be clear yet. We analysed the relationships between these clusters and the temporal dimensions of each article. The results are synthesized in Figure 9 . It is clear that cluster #0 (laparoscopic skill), cluster #2 (gaming and rehabilitation), cluster #4 (therapy), and cluster #14 (surgery) are the most popular areas of VR research. (See Figure 9 and Table 2 to identify the clusters.) From Figure 9 , it also is possible to identify the first phase of laparoscopic skill (cluster #6) and therapy (cluster #7). More generally, it is possible to identify four historical phases (colors: blue, green, yellow, and red) from the past VR research to the current research.

FIGURE 9. Network of document co-citation: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank and the red writing on the right hand side reports the number of the cluster, such as in Table 2 , with a short description that was extracted accordingly.

We were able to identify the top 486 references that had the most citations by using burst citations algorithm. Citation burst is an indicator of a most active area of research. Citation burst is a detection of a burst event, which can last for multiple years as well as a single year. A citation burst provides evidence that a particular publication is associated with a surge of citations. The burst detection was based on Kleinberg’s algorithm ( Kleinberg, 2002 , 2003 ). The top-ranked document by bursts is Seymour (2002) in Cluster #0, with bursts of 88.93. The second is Grantcharov (2004) in Cluster #0, with bursts of 51.40. The third is Saposnik (2010) in Cluster #2, with bursts of 40.84. The fourth is Rothbaum (1995) in Cluster #7, with bursts of 38.94. The fifth is Holden (2005) in Cluster #2, with bursts of 37.52. The sixth is Scott (2000) in Cluster #0, with bursts of 33.39. The seventh is Saposnik (2011) in Cluster #2, with bursts of 33.33. The eighth is Burdea et al. (1996) in Cluster #3, with bursts of 32.42. The ninth is Burdea and Coiffet (2003) in Cluster #22, with bursts of 31.30. The 10th is Taffinder (1998) in Cluster #6, with bursts of 30.96 (Table 3 ).

TABLE 3. Cluster ID and references of burst article.

Citation Network and Cluster Analyses for AR

Looking at Augmented Reality scenario, the top ranked item by citation counts is Azuma (1997) in Cluster #0, with citation counts of 231. The second one is Azuma et al. (2001) in Cluster #0, with citation counts of 220. The third is Van Krevelen (2010) in Cluster #5, with citation counts of 207. The 4th is Lowe (2004) in Cluster #1, with citation counts of 157. The 5th is Wu (2013) in Cluster #4, with citation counts of 144. The 6th is Dunleavy (2009) in Cluster #4, with citation counts of 122. The 7th is Zhou (2008) in Cluster #5, with citation counts of 118. The 8th is Bay (2008) in Cluster #1, with citation counts of 117. The 9th is Newcombe (2011) in Cluster #1, with citation counts of 109. The 10th is Carmigniani et al. (2011) in Cluster #5, with citation counts of 104.

The network of document co-citations is visually complex (Figure 10 ) because it includes 1000s of articles and the links among them. However, this analysis is very important because can be used to identify the possible conglomerate of knowledge in the area, and this is essential for a deep understanding of the area. Thus, for this purpose, a cluster analysis was conducted ( Chen et al., 2010 ; González-Teruel et al., 2015 ; Klavans and Boyack, 2015 ). Figure 11 shows the clusters, which are identified with the two algorithms in Table 3 .

FIGURE 10. Network of document co-citations: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank, and the numbers represent the strengths of the links. It is possible to identify four historical phases (colors: blue, green, yellow, and red) from the past AR research to the current research.

FIGURE 11. Document co-citation network by cluster: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank and the red writing reports the name of the cluster with a short description that was produced with the mutual information algorithm; the clusters are identified with colored polygons.

The identified clusters highlight clear parts of the literature of AR research, making clear and visible the interdisciplinary nature of this field. However, the dynamics to identify the past, present, and future of AR research cannot be clear yet. We analysed the relationships between these clusters and the temporal dimensions of each article. The results are synthesized in Figure 12 . It is clear that cluster #1 (tracking), cluster #4 (education), and cluster #5 (virtual city environment) are the current areas of AR research. (See Figure 12 and Table 3 to identify the clusters.) It is possible to identify four historical phases (colors: blue, green, yellow, and red) from the past AR research to the current research.

FIGURE 12. Network of document co-citation: the dimensions of the nodes represent centrality, the dimensions of the characters represent the rank of the article rank and the red writing on the right hand side reports the number of the cluster, such as in Table 2 , with a short description that was extracted accordingly.

We were able to identify the top 394 references that had the most citations by using burst citations algorithm. Citation burst is an indicator of a most active area of research. Citation burst is a detection of a burst event, which can last for multiple years as well as a single year. A citation burst provides evidence that a particular publication is associated with a surge of citations. The burst detection was based on Kleinberg’s algorithm ( Kleinberg, 2002 , 2003 ). The top ranked document by bursts is Azuma (1997) in Cluster #0, with bursts of 101.64. The second one is Azuma et al. (2001) in Cluster #0, with bursts of 84.23. The third is Lowe (2004) in Cluster #1, with bursts of 64.07. The 4th is Van Krevelen (2010) in Cluster #5, with bursts of 50.99. The 5th is Wu (2013) in Cluster #4, with bursts of 47.23. The 6th is Hartley (2000) in Cluster #0, with bursts of 37.71. The 7th is Dunleavy (2009) in Cluster #4, with bursts of 33.22. The 8th is Kato (1999) in Cluster #0, with bursts of 32.16. The 9th is Newcombe (2011) in Cluster #1, with bursts of 29.72. The 10th is Feiner (1993) in Cluster #8, with bursts of 29.46 (Table 4 ).

TABLE 4. Cluster ID and silhouettes as identified with two algorithms ( Chen et al., 2010 ).

Our findings have profound implications for two reasons. At first the present work highlighted the evolution and development of VR and AR research and provided a clear perspective based on solid data and computational analyses. Secondly our findings on VR made it profoundly clear that the clinical dimension is one of the most investigated ever and seems to increase in quantitative and qualitative aspects, but also include technological development and article in computer science, engineer, and allied sciences.

Figure 9 clarifies the past, present, and future of VR research. The outset of VR research brought a clearly-identifiable development in interfaces for children and medicine, routine use and behavioral-assessment, special effects, systems perspectives, and tutorials. This pioneering era evolved in the period that we can identify as the development era, because it was the period in which VR was used in experiments associated with new technological impulses. Not surprisingly, this was exactly concomitant with the new economy era in which significant investments were made in information technology, and it also was the era of the so-called ‘dot-com bubble’ in the late 1990s. The confluence of pioneering techniques into ergonomic studies within this development era was used to develop the first effective clinical systems for surgery, telemedicine, human spatial navigation, and the first phase of the development of therapy and laparoscopic skills. With the new millennium, VR research switched strongly toward what we can call the clinical-VR era, with its strong emphasis on rehabilitation, neurosurgery, and a new phase of therapy and laparoscopic skills. The number of applications and articles that have been published in the last 5 years are in line with the new technological development that we are experiencing at the hardware level, for example, with so many new, HMDs, and at the software level with an increasing number of independent programmers and VR communities.

Finally, Figure 12 identifies clusters of the literature of AR research, making clear and visible the interdisciplinary nature of this field. The dynamics to identify the past, present, and future of AR research cannot be clear yet, but analyzing the relationships between these clusters and the temporal dimensions of each article tracking, education, and virtual city environment are the current areas of AR research. AR is a new technology that is showing its efficacy in different research fields, and providing a novel way to gather behavioral data and support learning, training, and clinical treatments.

Looking at scientific literature conducted in the last few years, it might appear that most developments in VR and AR studies have focused on clinical aspects. However, the reality is more complex; thus, this perception should be clarified. Although researchers publish studies on the use of VR in clinical settings, each study depends on the technologies available. Industrial development in VR and AR changed a lot in the last 10 years. In the past, the development involved mainly hardware solutions while nowadays, the main efforts pertain to the software when developing virtual solutions. Hardware became a commodity that is often available at low cost. On the other hand, software needs to be customized each time, per each experiment, and this requires huge efforts in term of development. Researchers in AR and VR today need to be able to adapt software in their labs.

Virtual reality and AR developments in this new clinical era rely on computer science and vice versa. The future of VR and AR is becoming more technological than before, and each day, new solutions and products are coming to the market. Both from software and hardware perspectives, the future of AR and VR depends on huge innovations in all fields. The gap between the past and the future of AR and VR research is about the “realism” that was the key aspect in the past versus the “interaction” that is the key aspect now. First 30 years of VR and AR consisted of a continuous research on better resolution and improved perception. Now, researchers already achieved a great resolution and need to focus on making the VR as realistic as possible, which is not simple. In fact, a real experience implies a realistic interaction and not just great resolution. Interactions can be improved in infinite ways through new developments at hardware and software levels.

Interaction in AR and VR is going to be “embodied,” with implication for neuroscientists that are thinking about new solutions to be implemented into the current systems ( Blanke et al., 2015 ; Riva, 2018 ; Riva et al., 2018 ). For example, the use of hands with contactless device (i.e., without gloves) makes the interaction in virtual environments more natural. The Leap Motion device 1 allows one to use of hands in VR without the use of gloves or markers. This simple and low-cost device allows the VR users to interact with virtual objects and related environments in a naturalistic way. When technology is able to be transparent, users can experience increased sense of being in the virtual environments (the so-called sense of presence).

Other forms of interactions are possible and have been developing continuously. For example, tactile and haptic device able to provide a continuous feedback to the users, intensifying their experience also by adding components, such as the feeling of touch and the physical weight of virtual objects, by using force feedback. Another technology available at low cost that facilitates interaction is the motion tracking system, such as Microsoft Kinect, for example. Such technology allows one to track the users’ bodies, allowing them to interact with the virtual environments using body movements, gestures, and interactions. Most HMDs use an embedded system to track HMD position and rotation as well as controllers that are generally placed into the user’s hands. This tracking allows a great degree of interaction and improves the overall virtual experience.

A final emerging approach is the use of digital technologies to simulate not only the external world but also the internal bodily signals ( Azevedo et al., 2017 ; Riva et al., 2017 ): interoception, proprioception and vestibular input. For example, Riva et al. (2017) recently introduced the concept of “sonoception” ( www.sonoception.com ), a novel non-invasive technological paradigm based on wearable acoustic and vibrotactile transducers able to alter internal bodily signals. This approach allowed the development of an interoceptive stimulator that is both able to assess interoceptive time perception in clinical patients ( Di Lernia et al., 2018b ) and to enhance heart rate variability (the short-term vagally mediated component—rMSSD) through the modulation of the subjects’ parasympathetic system ( Di Lernia et al., 2018a ).

In this scenario, it is clear that the future of VR and AR research is not just in clinical applications, although the implications for the patients are huge. The continuous development of VR and AR technologies is the result of research in computer science, engineering, and allied sciences. The reasons for which from our analyses emerged a “clinical era” are threefold. First, all clinical research on VR and AR includes also technological developments, and new technological discoveries are being published in clinical or technological journals but with clinical samples as main subject. As noted in our research, main journals that publish numerous articles on technological developments tested with both healthy and patients include Presence: Teleoperators & Virtual Environments, Cyberpsychology & Behavior (Cyberpsychol BEHAV), and IEEE Computer Graphics and Applications (IEEE Comput Graph). It is clear that researchers in psychology, neuroscience, medicine, and behavioral sciences in general have been investigating whether the technological developments of VR and AR are effective for users, indicating that clinical behavioral research has been incorporating large parts of computer science and engineering. A second aspect to consider is the industrial development. In fact, once a new technology is envisioned and created it goes for a patent application. Once the patent is sent for registration the new technology may be made available for the market, and eventually for journal submission and publication. Moreover, most VR and AR research that that proposes the development of a technology moves directly from the presenting prototype to receiving the patent and introducing it to the market without publishing the findings in scientific paper. Hence, it is clear that if a new technology has been developed for industrial market or consumer, but not for clinical purpose, the research conducted to develop such technology may never be published in a scientific paper. Although our manuscript considered published researches, we have to acknowledge the existence of several researches that have not been published at all. The third reason for which our analyses highlighted a “clinical era” is that several articles on VR and AR have been considered within the Web of Knowledge database, that is our source of references. In this article, we referred to “research” as the one in the database considered. Of course, this is a limitation of our study, since there are several other databases that are of big value in the scientific community, such as IEEE Xplore Digital Library, ACM Digital Library, and many others. Generally, the most important articles in journals published in these databases are also included in the Web of Knowledge database; hence, we are convinced that our study considered the top-level publications in computer science or engineering. Accordingly, we believe that this limitation can be overcome by considering the large number of articles referenced in our research.

Considering all these aspects, it is clear that clinical applications, behavioral aspects, and technological developments in VR and AR research are parts of a more complex situation compared to the old platforms used before the huge diffusion of HMD and solutions. We think that this work might provide a clearer vision for stakeholders, providing evidence of the current research frontiers and the challenges that are expected in the future, highlighting all the connections and implications of the research in several fields, such as clinical, behavioral, industrial, entertainment, educational, and many others.

Author Contributions

PC and GR conceived the idea. PC made data extraction and the computational analyses and wrote the first draft of the article. IG revised the introduction adding important information for the article. PC, IG, MR, and GR revised the article and approved the last version of the article after important input to the article rationale.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The reviewer GC declared a shared affiliation, with no collaboration, with the authors PC and GR to the handling Editor at the time of the review.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fpsyg.2018.02086/full#supplementary-material

• ^ https://www.leapmotion.com/

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Keywords : virtual reality, augmented reality, quantitative psychology, measurement, psychometrics, scientometrics, computational psychometrics, mathematical psychology

Citation: Cipresso P, Giglioli IAC, Raya MA and Riva G (2018) The Past, Present, and Future of Virtual and Augmented Reality Research: A Network and Cluster Analysis of the Literature. Front. Psychol. 9:2086. doi: 10.3389/fpsyg.2018.02086

Received: 14 December 2017; Accepted: 10 October 2018; Published: 06 November 2018.

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*Correspondence: Pietro Cipresso, [email protected]

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• Published: 25 October 2021

Augmented reality and virtual reality displays: emerging technologies and future perspectives

• Jianghao Xiong 1 ,
• En-Lin Hsiang 1 ,
• Ziqian He 1 ,
• Tao Zhan   ORCID: orcid.org/0000-0001-5511-6666 1 &
• Shin-Tson Wu   ORCID: orcid.org/0000-0002-0943-0440 1  

Light: Science & Applications volume  10 , Article number:  216 ( 2021 ) Cite this article

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• Liquid crystals

With rapid advances in high-speed communication and computation, augmented reality (AR) and virtual reality (VR) are emerging as next-generation display platforms for deeper human-digital interactions. Nonetheless, to simultaneously match the exceptional performance of human vision and keep the near-eye display module compact and lightweight imposes unprecedented challenges on optical engineering. Fortunately, recent progress in holographic optical elements (HOEs) and lithography-enabled devices provide innovative ways to tackle these obstacles in AR and VR that are otherwise difficult with traditional optics. In this review, we begin with introducing the basic structures of AR and VR headsets, and then describing the operation principles of various HOEs and lithography-enabled devices. Their properties are analyzed in detail, including strong selectivity on wavelength and incident angle, and multiplexing ability of volume HOEs, polarization dependency and active switching of liquid crystal HOEs, device fabrication, and properties of micro-LEDs (light-emitting diodes), and large design freedoms of metasurfaces. Afterwards, we discuss how these devices help enhance the AR and VR performance, with detailed description and analysis of some state-of-the-art architectures. Finally, we cast a perspective on potential developments and research directions of these photonic devices for future AR and VR displays.

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Introduction.

Recent advances in high-speed communication and miniature mobile computing platforms have escalated a strong demand for deeper human-digital interactions beyond traditional flat panel displays. Augmented reality (AR) and virtual reality (VR) headsets 1 , 2 are emerging as next-generation interactive displays with the ability to provide vivid three-dimensional (3D) visual experiences. Their useful applications include education, healthcare, engineering, and gaming, just to name a few 3 , 4 , 5 . VR embraces a total immersive experience, while AR promotes the interaction between user, digital contents, and real world, therefore displaying virtual images while remaining see-through capability. In terms of display performance, AR and VR face several common challenges to satisfy demanding human vision requirements, including field of view (FoV), eyebox, angular resolution, dynamic range, and correct depth cue, etc. Another pressing demand, although not directly related to optical performance, is ergonomics. To provide a user-friendly wearing experience, AR and VR should be lightweight and ideally have a compact, glasses-like form factor. The above-mentioned requirements, nonetheless, often entail several tradeoff relations with one another, which makes the design of high-performance AR/VR glasses/headsets particularly challenging.

In the 1990s, AR/VR experienced the first boom, which quickly subsided due to the lack of eligible hardware and digital content 6 . Over the past decade, the concept of immersive displays was revisited and received a new round of excitement. Emerging technologies like holography and lithography have greatly reshaped the AR/VR display systems. In this article, we firstly review the basic requirements of AR/VR displays and their associated challenges. Then, we briefly describe the properties of two emerging technologies: holographic optical elements (HOEs) and lithography-based devices (Fig. 1 ). Next, we separately introduce VR and AR systems because of their different device structures and requirements. For the immersive VR system, the major challenges and how these emerging technologies help mitigate the problems will be discussed. For the see-through AR system, we firstly review the present status of light engines and introduce some architectures for the optical combiners. Performance summaries on microdisplay light engines and optical combiners will be provided, that serve as a comprehensive overview of the current AR display systems.

The left side illustrates HOEs and lithography-based devices. The right side shows the challenges in VR and architectures in AR, and how the emerging technologies can be applied

Key parameters of AR and VR displays

AR and VR displays face several common challenges to satisfy the demanding human vision requirements, such as FoV, eyebox, angular resolution, dynamic range, and correct depth cue, etc. These requirements often exhibit tradeoffs with one another. Before diving into detailed relations, it is beneficial to review the basic definitions of the above-mentioned display parameters.

Definition of parameters

Taking a VR system (Fig. 2a ) as an example. The light emitting from the display module is projected to a FoV, which can be translated to the size of the image perceived by the viewer. For reference, human vision’s horizontal FoV can be as large as 160° for monocular vision and 120° for overlapped binocular vision 6 . The intersection area of ray bundles forms the exit pupil, which is usually correlated with another parameter called eyebox. The eyebox defines the region within which the whole image FoV can be viewed without vignetting. It therefore generally manifests a 3D geometry 7 , whose volume is strongly dependent on the exit pupil size. A larger eyebox offers more tolerance to accommodate the user’s diversified interpupillary distance (IPD) and wiggling of headset when in use. Angular resolution is defined by dividing the total resolution of the display panel by FoV, which measures the sharpness of a perceived image. For reference, a human visual acuity of 20/20 amounts to 1 arcmin angular resolution, or 60 pixels per degree (PPD), which is considered as a common goal for AR and VR displays. Another important feature of a 3D display is depth cue. Depth cue can be induced by displaying two separate images to the left eye and the right eye, which forms the vergence cue. But the fixed depth of the displayed image often mismatches with the actual depth of the intended 3D image, which leads to incorrect accommodation cues. This mismatch causes the so-called vergence-accommodation conflict (VAC), which will be discussed in detail later. One important observation is that the VAC issue may be more serious in AR than VR, because the image in an AR display is directly superimposed onto the real-world with correct depth cues. The image contrast is dependent on the display panel and stray light. To achieve a high dynamic range, the display panel should exhibit high brightness, low dark level, and more than 10-bits of gray levels. Nowadays, the display brightness of a typical VR headset is about 150–200 cd/m 2 (or nits).

a Schematic of a VR display defining FoV, exit pupil, eyebox, angular resolution, and accommodation cue mismatch. b Sketch of an AR display illustrating ACR

Figure 2b depicts a generic structure of an AR display. The definition of above parameters remains the same. One major difference is the influence of ambient light on the image contrast. For a see-through AR display, ambient contrast ratio (ACR) 8 is commonly used to quantify the image contrast:

where L on ( L off ) represents the on (off)-state luminance (unit: nit), L am is the ambient luminance, and T is the see-through transmittance. In general, ambient light is measured in illuminance (lux). For the convenience of comparison, we convert illuminance to luminance by dividing a factor of π, assuming the emission profile is Lambertian. In a normal living room, the illuminance is about 100 lux (i.e., L am  ≈ 30 nits), while in a typical office lighting condition, L am  ≈ 150 nits. For outdoors, on an overcast day, L am  ≈ 300 nits, and L am  ≈ 3000 nits on a sunny day. For AR displays, a minimum ACR should be 3:1 for recognizable images, 5:1 for adequate readability, and ≥10:1 for outstanding readability. To make a simple estimate without considering all the optical losses, to achieve ACR = 10:1 in a sunny day (~3000 nits), the display needs to deliver a brightness of at least 30,000 nits. This imposes big challenges in finding a high brightness microdisplay and designing a low loss optical combiner.

Tradeoffs and potential solutions

Next, let us briefly review the tradeoff relations mentioned earlier. To begin with, a larger FoV leads to a lower angular resolution for a given display resolution. In theory, to overcome this tradeoff only requires a high-resolution-display source, along with high-quality optics to support the corresponding modulation transfer function (MTF). To attain 60 PPD across 100° FoV requires a 6K resolution for each eye. This may be realizable in VR headsets because a large display panel, say 2–3 inches, can still accommodate a high resolution with acceptable manufacture cost. However, for a glasses-like wearable AR display, the conflict between small display size and the high solution becomes obvious as further shrinking the pixel size of a microdisplay is challenging.

To circumvent this issue, the concept of the foveated display is proposed 9 , 10 , 11 , 12 , 13 . The idea is based on that the human eye only has high visual acuity in the central fovea region, which accounts for about 10° FoV. If the high-resolution image is only projected to fovea while the peripheral image remains low resolution, then a microdisplay with 2K resolution can satisfy the need. Regarding the implementation method of foveated display, a straightforward way is to optically combine two display sources 9 , 10 , 11 : one for foveal and one for peripheral FoV. This approach can be regarded as spatial multiplexing of displays. Alternatively, time-multiplexing can also be adopted, by temporally changing the optical path to produce different magnification factors for the corresponding FoV 12 . Finally, another approach without multiplexing is to use a specially designed lens with intended distortion to achieve non-uniform resolution density 13 . Aside from the implementation of foveation, another great challenge is to dynamically steer the foveated region as the viewer’s eye moves. This task is strongly related to pupil steering, which will be discussed in detail later.

A larger eyebox or FoV usually decreases the image brightness, which often lowers the ACR. This is exactly the case for a waveguide AR system with exit pupil expansion (EPE) while operating under a strong ambient light. To improve ACR, one approach is to dynamically adjust the transmittance with a tunable dimmer 14 , 15 . Another solution is to directly boost the image brightness with a high luminance microdisplay and an efficient combiner optics. Details of this topic will be discussed in the light engine section.

Another tradeoff of FoV and eyebox in geometric optical systems results from the conservation of etendue (or optical invariant). To increase the system etendue requires a larger optics, which in turn compromises the form factor. Finally, to address the VAC issue, the display system needs to generate a proper accommodation cue, which often requires the modulation of image depth or wavefront, neither of which can be easily achieved in a traditional geometric optical system. While remarkable progresses have been made to adopt freeform surfaces 16 , 17 , 18 , to further advance AR and VR systems requires additional novel optics with a higher degree of freedom in structure design and light modulation. Moreover, the employed optics should be thin and lightweight. To mitigate the above-mentioned challenges, diffractive optics is a strong contender. Unlike geometric optics relying on curved surfaces to refract or reflect light, diffractive optics only requires a thin layer of several micrometers to establish efficient light diffractions. Two major types of diffractive optics are HOEs based on wavefront recording and manually written devices like surface relief gratings (SRGs) based on lithography. While SRGs have large design freedoms of local grating geometry, a recent publication 19 indicates the combination of HOE and freeform optics can also offer a great potential for arbitrary wavefront generation. Furthermore, the advances in lithography have also enabled optical metasurfaces beyond diffractive and refractive optics, and miniature display panels like micro-LED (light-emitting diode). These devices hold the potential to boost the performance of current AR/VR displays, while keeping a lightweight and compact form factor.

Formation and properties of HOEs

HOE generally refers to a recorded hologram that reproduces the original light wavefront. The concept of holography is proposed by Dennis Gabor 20 , which refers to the process of recording a wavefront in a medium (hologram) and later reconstructing it with a reference beam. Early holography uses intensity-sensitive recording materials like silver halide emulsion, dichromated gelatin, and photopolymer 21 . Among them, photopolymer stands out due to its easy fabrication and ability to capture high-fidelity patterns 22 , 23 . It has therefore found extensive applications like holographic data storage 23 and display 24 , 25 . Photopolymer HOEs (PPHOEs) have a relatively small refractive index modulation and therefore exhibits a strong selectivity on the wavelength and incident angle. Another feature of PPHOE is that several holograms can be recorded into a photopolymer film by consecutive exposures. Later, liquid-crystal holographic optical elements (LCHOEs) based on photoalignment polarization holography have also been developed 25 , 26 . Due to the inherent anisotropic property of liquid crystals, LCHOEs are extremely sensitive to the polarization state of the input light. This feature, combined with the polarization modulation ability of liquid crystal devices, offers a new possibility for dynamic wavefront modulation in display systems.

The formation of PPHOE is illustrated in Fig. 3a . When exposed to an interfering field with high-and-low intensity fringes, monomers tend to move toward bright fringes due to the higher local monomer-consumption rate. As a result, the density and refractive index is slightly larger in bright regions. Note the index modulation δ n here is defined as the difference between the maximum and minimum refractive indices, which may be twice the value in other definitions 27 . The index modulation δ n is typically in the range of 0–0.06. To understand the optical properties of PPHOE, we simulate a transmissive grating and a reflective grating using rigorous coupled-wave analysis (RCWA) 28 , 29 and plot the results in Fig. 3b . Details of grating configuration can be found in Table S1 . Here, the reason for only simulating gratings is that for a general HOE, the local region can be treated as a grating. The observation of gratings can therefore offer a general insight of HOEs. For a transmissive grating, its angular bandwidth (efficiency > 80%) is around 5° ( λ  = 550 nm), while the spectral band is relatively broad, with bandwidth around 175 nm (7° incidence). For a reflective grating, its spectral band is narrow, with bandwidth around 10 nm. The angular bandwidth varies with the wavelength, ranging from 2° to 20°. The strong selectivity of PPHOE on wavelength and incident angle is directly related to its small δ n , which can be adjusted by controlling the exposure dosage.

a Schematic of the formation of PPHOE. Simulated efficiency plots for b1 transmissive and b2 reflective PPHOEs. c Working principle of multiplexed PPHOE. d Formation and molecular configurations of LCHOEs. Simulated efficiency plots for e1 transmissive and e2 reflective LCHOEs. f Illustration of polarization dependency of LCHOEs

A distinctive feature of PPHOE is the ability to multiplex several holograms into one film sample. If the exposure dosage of a recording process is controlled so that the monomers are not completely depleted in the first exposure, the remaining monomers can continue to form another hologram in the following recording process. Because the total amount of monomer is fixed, there is usually an efficiency tradeoff between multiplexed holograms. The final film sample would exhibit the wavefront modulation functions of multiple holograms (Fig. 3c ).

Liquid crystals have also been used to form HOEs. LCHOEs can generally be categorized into volume-recording type and surface-alignment type. Volume-recording type LCHOEs are either based on early polarization holography recordings with azo-polymer 30 , 31 , or holographic polymer-dispersed liquid crystals (HPDLCs) 32 , 33 formed by liquid-crystal-doped photopolymer. Surface-alignment type LCHOEs are based on photoalignment polarization holography (PAPH) 34 . The first step is to record the desired polarization pattern in a thin photoalignment layer, and the second step is to use it to align the bulk liquid crystal 25 , 35 . Due to the simple fabrication process, high efficiency, and low scattering from liquid crystal’s self-assembly nature, surface-alignment type LCHOEs based on PAPH have recently attracted increasing interest in applications like near-eye displays. Here, we shall focus on this type of surface-alignment LCHOE and refer to it as LCHOE thereafter for simplicity.

The formation of LCHOEs is illustrated in Fig. 3d . The information of the wavefront and the local diffraction pattern is recorded in a thin photoalignment layer. The volume liquid crystal deposited on the photoalignment layer, depending on whether it is nematic liquid crystal or cholesteric liquid crystal (CLC), forms a transmissive or a reflective LCHOE. In a transmissive LCHOE, the bulk nematic liquid crystal molecules generally follow the pattern of the bottom alignment layer. The smallest allowable pattern period is governed by the liquid crystal distortion-free energy model, which predicts the pattern period should generally be larger than sample thickness 36 , 37 . This results in a maximum diffraction angle under 20°. On the other hand, in a reflective LCHOE 38 , 39 , the bulk CLC molecules form a stable helical structure, which is tilted to match the k -vector of the bottom pattern. The structure exhibits a very low distorted free energy 40 , 41 and can accommodate a pattern period that is small enough to diffract light into the total internal reflection (TIR) of a glass substrate.

The diffraction property of LCHOEs is shown in Fig. 3e . The maximum refractive index modulation of LCHOE is equal to the liquid crystal birefringence (Δ n ), which may vary from 0.04 to 0.5, depending on the molecular conjugation 42 , 43 . The birefringence used in our simulation is Δ n  = 0.15. Compared to PPHOEs, the angular and spectral bandwidths are significantly larger for both transmissive and reflective LCHOEs. For a transmissive LCHOE, its angular bandwidth is around 20° ( λ  = 550 nm), while the spectral bandwidth is around 300 nm (7° incidence). For a reflective LCHOE, its spectral bandwidth is around 80 nm and angular bandwidth could vary from 15° to 50°, depending on the wavelength.

The anisotropic nature of liquid crystal leads to LCHOE’s unique polarization-dependent response to an incident light. As depicted in Fig. 3f , for a transmissive LCHOE the accumulated phase is opposite for the conjugated left-handed circular polarization (LCP) and right-handed circular polarization (RCP) states, leading to reversed diffraction directions. For a reflective LCHOE, the polarization dependency is similar to that of a normal CLC. For the circular polarization with the same handedness as the helical structure of CLC, the diffraction is strong. For the opposite circular polarization, the diffraction is negligible.

Another distinctive property of liquid crystal is its dynamic response to an external voltage. The LC reorientation can be controlled with a relatively low voltage (<10 V rms ) and the response time is on the order of milliseconds, depending mainly on the LC viscosity and layer thickness. Methods to dynamically control LCHOEs can be categorized as active addressing and passive addressing, which can be achieved by either directly switching the LCHOE or modulating the polarization state with an active waveplate. Detailed addressing methods will be described in the VAC section.

Lithography-enabled devices

Lithography technologies are used to create arbitrary patterns on wafers, which lays the foundation of the modern integrated circuit industry 44 . Photolithography is suitable for mass production while electron/ion beam lithography is usually used to create photomask for photolithography or to write structures with nanometer-scale feature size. Recent advances in lithography have enabled engineered structures like optical metasurfaces 45 , SRGs 46 , as well as micro-LED displays 47 . Metasurfaces exhibit a remarkable design freedom by varying the shape of meta-atoms, which can be utilized to achieve novel functions like achromatic focus 48 and beam steering 49 . Similarly, SRGs also offer a large design freedom by manipulating the geometry of local grating regions to realize desired optical properties. On the other hand, micro-LED exhibits several unique features, such as ultrahigh peak brightness, small aperture ratio, excellent stability, and nanosecond response time, etc. As a result, micro-LED is a promising candidate for AR and VR systems for achieving high ACR and high frame rate for suppressing motion image blurs. In the following section, we will briefly review the fabrication and properties of micro-LEDs and optical modulators like metasurfaces and SRGs.

Fabrication and properties of micro-LEDs

LEDs with a chip size larger than 300 μm have been widely used in solid-state lighting and public information displays. Recently, micro-LEDs with chip sizes <5 μm have been demonstrated 50 . The first micro-LED disc with a diameter of about 12 µm was demonstrated in 2000 51 . After that, a single color (blue or green) LED microdisplay was demonstrated in 2012 52 . The high peak brightness, fast response time, true dark state, and long lifetime of micro-LEDs are attractive for display applications. Therefore, many companies have since released their micro-LED prototypes or products, ranging from large-size TVs to small-size microdisplays for AR/VR applications 53 , 54 . Here, we focus on micro-LEDs for near-eye display applications. Regarding the fabrication of micro-LEDs, through the metal-organic chemical vapor deposition (MOCVD) method, the AlGaInP epitaxial layer is grown on GaAs substrate for red LEDs, and GaN epitaxial layers on sapphire substrate for green and blue LEDs. Next, a photolithography process is applied to define the mesa and deposit electrodes. To drive the LED array, the fabricated micro-LEDs are transferred to a CMOS (complementary metal oxide semiconductor) driver board. For a small size (<2 inches) microdisplay used in AR or VR, the precision of the pick-and-place transfer process is hard to meet the high-resolution-density (>1000 pixel per inch) requirement. Thus, the main approach to assemble LED chips with driving circuits is flip-chip bonding 50 , 55 , 56 , 57 , as Fig. 4a depicts. In flip-chip bonding, the mesa and electrode pads should be defined and deposited before the transfer process, while metal bonding balls should be preprocessed on the CMOS substrate. After that, thermal-compression method is used to bond the two wafers together. However, due to the thermal mismatch of LED chip and driving board, as the pixel size decreases, the misalignment between the LED chip and the metal bonding ball on the CMOS substrate becomes serious. In addition, the common n-GaN layer may cause optical crosstalk between pixels, which degrades the image quality. To overcome these issues, the LED epitaxial layer can be firstly metal-bonded with the silicon driver board, followed by the photolithography process to define the LED mesas and electrodes. Without the need for an alignment process, the pixel size can be reduced to <5 µm 50 .

a Illustration of flip-chip bonding technology. b Simulated IQE-LED size relations for red and blue LEDs based on ABC model. c Comparison of EQE of different LED sizes with and without KOH and ALD side wall treatment. d Angular emission profiles of LEDs with different sizes. Metasurfaces based on e resonance-tuning, f non-resonance tuning and g combination of both. h Replication master and i replicated SRG based on nanoimprint lithography. Reproduced from a ref. 55 with permission from AIP Publishing, b ref. 61 with permission from PNAS, c ref. 66 with permission from IOP Publishing, d ref. 67 with permission from AIP Publishing, e ref. 69 with permission from OSA Publishing f ref. 48 with permission from AAAS g ref. 70 with permission from AAAS and h , i ref. 85 with permission from OSA Publishing

In addition to manufacturing process, the electrical and optical characteristics of LED also depend on the chip size. Generally, due to Shockley-Read-Hall (SRH) non-radiative recombination on the sidewall of active area, a smaller LED chip size results in a lower internal quantum efficiency (IQE), so that the peak IQE driving point will move toward a higher current density due to increased ratio of sidewall surface to active volume 58 , 59 , 60 . In addition, compared to the GaN-based green and blue LEDs, the AlGaInP-based red LEDs with a larger surface recombination and carrier diffusion length suffer a more severe efficiency drop 61 , 62 . Figure 4b shows the simulated result of IQE drop in relation with the LED chip size of blue and red LEDs based on ABC model 63 . To alleviate the efficiency drop caused by sidewall defects, depositing passivation materials by atomic layer deposition (ALD) or plasma enhanced chemical vapor deposition (PECVD) is proven to be helpful for both GaN and AlGaInP based LEDs 64 , 65 . In addition, applying KOH (Potassium hydroxide) treatment after ALD can further reduce the EQE drop of micro-LEDs 66 (Fig. 4c ). Small-size LEDs also exhibit some advantages, such as higher light extraction efficiency (LEE). Compared to an 100-µm LED, the LEE of a 2-µm LED increases from 12.2 to 25.1% 67 . Moreover, the radiation pattern of micro-LED is more directional than that of a large-size LED (Fig. 4d ). This helps to improve the lens collection efficiency in AR/VR display systems.

Metasurfaces and SGs

Thanks to the advances in lithography technology, low-loss dielectric metasurfaces working in the visible band have recently emerged as a platform for wavefront shaping 45 , 48 , 68 . They consist of an array of subwavelength-spaced structures with individually engineered wavelength-dependent polarization/phase/ amplitude response. In general, the light modulation mechanisms can be classified into resonant tuning 69 (Fig. 4e ), non-resonant tuning 48 (Fig. 4f ), and combination of both 70 (Fig. 4g ). In comparison with non-resonant tuning (based on geometric phase and/or dynamic propagation phase), the resonant tuning (such as Fabry–Pérot resonance, Mie resonance, etc.) is usually associated with a narrower operating bandwidth and a smaller out-of-plane aspect ratio (height/width) of nanostructures. As a result, they are easier to fabricate but more sensitive to fabrication tolerances. For both types, materials with a higher refractive index and lower absorption loss are beneficial to reduce the aspect ratio of nanostructure and improve the device efficiency. To this end, titanium dioxide (TiO 2 ) and gallium nitride (GaN) are the major choices for operating in the entire visible band 68 , 71 . While small-sized metasurfaces (diameter <1 mm) are usually fabricated via electron-beam lithography or focused ion beam milling in the labs, the ability of mass production is the key to their practical adoption. The deep ultraviolet (UV) photolithography has proven its feasibility for reproducing centimeter-size metalenses with decent imaging performance, while it requires multiple steps of etching 72 . Interestingly, the recently developed UV nanoimprint lithography based on a high-index nanocomposite only takes a single step and can obtain an aspect ratio larger than 10, which shows great promise for high-volume production 73 .

The arbitrary wavefront shaping capability and the thinness of the metasurfaces have aroused strong research interests in the development of novel AR/VR prototypes with improved performance. Lee et al. employed nanoimprint lithography to fabricate a centimeter-size, geometric-phase metalens eyepiece for full-color AR displays 74 . Through tailoring its polarization conversion efficiency and stacking with a circular polarizer, the virtual image can be superimposed with the surrounding scene. The large numerical aperture (NA~0.5) of the metalens eyepiece enables a wide FoV (>76°) that conventional optics are difficult to obtain. However, the geometric phase metalens is intrinsically a diffractive lens that also suffers from strong chromatic aberrations. To overcome this issue, an achromatic lens can be designed via simultaneously engineering the group delay and the group delay dispersion 75 , 76 , which will be described in detail later. Other novel and/or improved near-eye display architectures include metasurface-based contact lens-type AR 77 , achromatic metalens array enabled integral-imaging light field displays 78 , wide FoV lightguide AR with polarization-dependent metagratings 79 , and off-axis projection-type AR with an aberration-corrected metasurface combiner 80 , 81 , 82 . Nevertheless, from the existing AR/VR prototypes, metasurfaces still face a strong tradeoff between numerical aperture (for metalenses), chromatic aberration, monochromatic aberration, efficiency, aperture size, and fabrication complexity.

On the other hand, SRGs are diffractive gratings that have been researched for decades as input/output couplers of waveguides 83 , 84 . Their surface is composed of corrugated microstructures, and different shapes including binary, blazed, slanted, and even analogue can be designed. The parameters of the corrugated microstructures are determined by the target diffraction order, operation spectral bandwidth, and angular bandwidth. Compared to metasurfaces, SRGs have a much larger feature size and thus can be fabricated via UV photolithography and subsequent etching. They are usually replicated by nanoimprint lithography with appropriate heating and surface treatment. According to a report published a decade ago, SRGs with a height of 300 nm and a slant angle of up to 50° can be faithfully replicated with high yield and reproducibility 85 (Fig. 4g, h ).

Challenges and solutions of VR displays

The fully immersive nature of VR headset leads to a relatively fixed configuration where the display panel is placed in front of the viewer’s eye and an imaging optics is placed in-between. Regarding the system performance, although inadequate angular resolution still exists in some current VR headsets, the improvement of display panel resolution with advanced fabrication process is expected to solve this issue progressively. Therefore, in the following discussion, we will mainly focus on two major challenges: form factor and 3D cue generation.

Form factor

Compact and lightweight near-eye displays are essential for a comfortable user experience and therefore highly desirable in VR headsets. Current mainstream VR headsets usually have a considerably larger volume than eyeglasses, and most of the volume is just empty. This is because a certain distance is required between the display panel and the viewing optics, which is usually close to the focal length of the lens system as illustrated in Fig. 5a . Conventional VR headsets employ a transmissive lens with ~4 cm focal length to offer a large FoV and eyebox. Fresnel lenses are thinner than conventional ones, but the distance required between the lens and the panel does not change significantly. In addition, the diffraction artifacts and stray light caused by the Fresnel grooves can degrade the image quality, or MTF. Although the resolution density, quantified as pixel per inch (PPI), of current VR headsets is still limited, eventually Fresnel lens will not be an ideal solution when a high PPI display is available. The strong chromatic aberration of Fresnel singlet should also be compensated if a high-quality imaging system is preferred.

a Schematic of a basic VR optical configuration. b Achromatic metalens used as VR eyepiece. c VR based on curved display and lenslet array. d Basic working principle of a VR display based on pancake optics. e VR with pancake optics and Fresnel lens array. f VR with pancake optics based on purely HOEs. Reprinted from b ref. 87 under the Creative Commons Attribution 4.0 License. Adapted from c ref. 88 with permission from IEEE, e ref. 91 and f ref. 92 under the Creative Commons Attribution 4.0 License

It is tempting to replace the refractive elements with a single thin diffractive lens like a transmissive LCHOE. However, the diffractive nature of such a lens will result in serious color aberrations. Interestingly, metalenses can fulfil this objective without color issues. To understand how metalenses achieve achromatic focus, let us first take a glance at the general lens phase profile \(\Phi (\omega ,r)\) expanded as a Taylor series 75 :

where \(\varphi _0(\omega )\) is the phase at the lens center, \(F\left( \omega \right)\) is the focal length as a function of frequency ω , r is the radial coordinate, and \(\omega _0\) is the central operation frequency. To realize achromatic focus, \(\partial F{{{\mathrm{/}}}}\partial \omega\) should be zero. With a designed focal length, the group delay \(\partial \Phi (\omega ,r){{{\mathrm{/}}}}\partial \omega\) and the group delay dispersion \(\partial ^2\Phi (\omega ,r){{{\mathrm{/}}}}\partial \omega ^2\) can be determined, and \(\varphi _0(\omega )\) is an auxiliary degree of freedom of the phase profile design. In the design of an achromatic metalens, the group delay is a function of the radial coordinate and monotonically increases with the metalens radius. Many designs have proven that the group delay has a limited variation range 75 , 76 , 78 , 86 . According to Shrestha et al. 86 , there is an inevitable tradeoff between the maximum radius of the metalens, NA, and operation bandwidth. Thus, the reported achromatic metalenses at visible usually have limited lens aperture (e.g., diameter < 250 μm) and NA (e.g., <0.2). Such a tradeoff is undesirable in VR displays, as the eyepiece favors a large clear aperture (inch size) and a reasonably high NA (>0.3) to maintain a wide FoV and a reasonable eye relief 74 .

To overcome this limitation, Li et al. 87 proposed a novel zone lens method. Unlike the traditional phase Fresnel lens where the zones are determined by the phase reset, the new approach divides the zones by the group delay reset. In this way, the lens aperture and NA can be much enlarged, and the group delay limit is bypassed. A notable side effect of this design is the phase discontinuity at zone boundaries that will contribute to higher-order focusing. Therefore, significant efforts have been conducted to find the optimal zone transition locations and to minimize the phase discontinuities. Using this method, they have demonstrated an impressive 2-mm-diameter metalens with NA = 0.7 and nearly diffraction-limited focusing for the designed wavelengths (488, 532, 658 nm) (Fig. 5b ). Such a metalens consists of 681 zones and works for the visible band ranging from 470 to 670 nm, though the focusing efficiency is in the order of 10%. This is a great starting point for the achromatic metalens to be employed as a compact, chromatic-aberration-free eyepiece in near-eye displays. Future challenges are how to further increase the aperture size, correct the off-axis aberrations, and improve the optical efficiency.

Besides replacing the refractive lens with an achromatic metalens, another way to reduce system focal length without decreasing NA is to use a lenslet array 88 . As depicted in Fig. 5c , both the lenslet array and display panel adopt a curved structure. With the latest flexible OLED panel, the display can be easily curved in one dimension. The system exhibits a large diagonal FoV of 180° with an eyebox of 19 by 12 mm. The geometry of each lenslet is optimized separately to achieve an overall performance with high image quality and reduced distortions.

Aside from trying to shorten the system focal length, another way to reduce total track is to fold optical path. Recently, polarization-based folded lenses, also known as pancake optics, are under active development for VR applications 89 , 90 . Figure 5d depicts the structure of an exemplary singlet pancake VR lens system. The pancake lenses can offer better imaging performance with a compact form factor since there are more degrees of freedom in the design and the actual light path is folded thrice. By using a reflective surface with a positive power, the field curvature of positive refractive lenses can be compensated. Also, the reflective surface has no chromatic aberrations and it contributes considerable optical power to the system. Therefore, the optical power of refractive lenses can be smaller, resulting in an even weaker chromatic aberration. Compared to Fresnel lenses, the pancake lenses have smooth surfaces and much fewer diffraction artifacts and stray light. However, such a pancake lens design is not perfect either, whose major shortcoming is low light efficiency. With two incidences of light on the half mirror, the maximum system efficiency is limited to 25% for a polarized input and 12.5% for an unpolarized input light. Moreover, due to the existence of multiple surfaces in the system, stray light caused by surface reflections and polarization leakage may lead to apparent ghost images. As a result, the catadioptric pancake VR headset usually manifests a darker imagery and lower contrast than the corresponding dioptric VR.

Interestingly, the lenslet and pancake optics can be combined to further reduce the system form. Bang et al. 91 demonstrated a compact VR system with a pancake optics and a Fresnel lenslet array. The pancake optics serves to fold the optical path between the display panel and the lenslet array (Fig. 5e ). Another Fresnel lens is used to collect the light from the lenslet array. The system has a decent horizontal FoV of 102° and an eyebox of 8 mm. However, a certain degree of image discontinuity and crosstalk are still present, which can be improved with further optimizations on the Fresnel lens and the lenslet array.

One step further, replacing all conventional optics in catadioptric VR headset with holographic optics can make the whole system even thinner. Maimone and Wang demonstrated such a lightweight, high-resolution, and ultra-compact VR optical system using purely HOEs 92 . This holographic VR optics was made possible by combining several innovative optical components, including a reflective PPHOE, a reflective LCHOE, and a PPHOE-based directional backlight with laser illumination, as shown in Fig. 5f . Since all the optical power is provided by the HOEs with negligible weight and volume, the total physical thickness can be reduced to <10 mm. Also, unlike conventional bulk optics, the optical power of a HOE is independent of its thickness, only subject to the recording process. Another advantage of using holographic optical devices is that they can be engineered to offer distinct phase profiles for different wavelengths and angles of incidence, adding extra degrees of freedom in optical designs for better imaging performance. Although only a single-color backlight has been demonstrated, such a PPHOE has the potential to achieve full-color laser backlight with multiplexing ability. The PPHOE and LCHOE in the pancake optics can also be optimized at different wavelengths for achieving high-quality full-color images.

Vergence-accommodation conflict

Conventional VR displays suffer from VAC, which is a common issue for stereoscopic 3D displays 93 . In current VR display modules, the distance between the display panel and the viewing optics is fixed, which means the VR imagery is displayed at a single depth. However, the image contents are generated by parallax rendering in three dimensions, offering distinct images for two eyes. This approach offers a proper stimulus to vergence but completely ignores the accommodation cue, which leads to the well-known VAC that can cause an uncomfortable user experience. Since the beginning of this century, numerous methods have been proposed to solve this critical issue. Methods to produce accommodation cue include multifocal/varifocal display 94 , holographic display 95 , and integral imaging display 96 . Alternatively, elimination of accommodation cue using a Maxwellian-view display 93 also helps to mitigate the VAC. However, holographic displays and Maxwellian-view displays generally require a totally different optical architecture than current VR systems. They are therefore more suitable for AR displays, which will be discussed later. Integral imaging, on the other hand, has an inherent tradeoff between view number and resolution. For current VR headsets pursuing high resolution to match human visual acuity, it may not be an appealing solution. Therefore, multifocal/varifocal displays that rely on depth modulation is a relatively practical and effective solution for VR headsets. Regarding the working mechanism, multifocal displays present multiple images with different depths to imitate the original 3D scene. Varifocal displays, in contrast, only show one image at each time frame. The image depth matches the viewer’s vergence depth. Nonetheless, the pre-knowledge of the viewer’s vergence depth requires an additional eye-tracking module. Despite different operation principles, a varifocal display can often be converted to a multifocal display as long as the varifocal module has enough modulation bandwidth to support multiple depths in a time frame.

To achieve depth modulation in a VR system, traditional liquid lens 97 , 98 with tunable focus suffers from the small aperture and large aberrations. Alvarez lens 99 is another tunable-focus solution but it requires mechanical adjustment, which adds to system volume and complexity. In comparison, transmissive LCHOEs with polarization dependency can achieve focus adjustment with electronic driving. Its ultra-thinness also satisfies the requirement of small form factors in VR headsets. The diffractive behavior of transmissive LCHOEs is often interpreted by the mechanism of Pancharatnam-Berry phase (also known as geometric phase) 100 . They are therefore often called Pancharatnam-Berry optical elements (PBOEs). The corresponding lens component is referred as Pancharatnam-Berry lens (PBL).

Two main approaches are used to switch the focus of a PBL, active addressing and passive addressing. In active addressing, the PBL itself (made of LC) can be switched by an applied voltage (Fig. 6a ). The optical power of the liquid crystal PBLs can be turned-on and -off by controlling the voltage. Stacking multiple active PBLs can produce 2 N depths, where N is the number of PBLs. The drawback of using active PBLs, however, is the limited spectral bandwidth since their diffraction efficiency is usually optimized at a single wavelength. In passive addressing, the depth modulation is achieved through changing the polarization state of input light by a switchable half-wave plate (HWP) (Fig. 6b ). The focal length can therefore be switched thanks to the polarization sensitivity of PBLs. Although this approach has a slightly more complicated structure, the overall performance can be better than the active one, because the PBLs made of liquid crystal polymer can be designed to manifest high efficiency within the entire visible spectrum 101 , 102 .

Working principles of a depth switching PBL module based on a active addressing and b passive addressing. c A four-depth multifocal display based on time multiplexing. d A two-depth multifocal display based on polarization multiplexing. Reproduced from c ref. 103 with permission from OSA Publishing and d ref. 104 with permission from OSA Publishing

With the PBL module, multifocal displays can be built using time-multiplexing technique. Zhan et al. 103 demonstrated a four-depth multifocal display using two actively switchable liquid crystal PBLs (Fig. 6c ). The display is synchronized with the PBL module, which lowers the frame rate by the number of depths. Alternatively, multifocal displays can also be achieved by polarization-multiplexing, as demonstrated by Tan et al. 104 . The basic principle is to adjust the polarization state of local pixels so the image content on two focal planes of a PBL can be arbitrarily controlled (Fig. 6d ). The advantage of polarization multiplexing is that it does not sacrifice the frame rate, but it can only support two planes because only two orthogonal polarization states are available. Still, it can be combined with time-multiplexing to reduce the frame rate sacrifice by half. Naturally, varifocal displays can also be built with a PBL module. A fast-response 64-depth varifocal module with six PBLs has been demonstrated 105 .

The compact structure of PBL module leads to a natural solution of integrating it with above-mentioned pancake optics. A compact VR headset with dynamic depth modulation to solve VAC is therefore possible in practice. Still, due to the inherent diffractive nature of PBL, the PBL module face the issue of chromatic dispersion of focal length. To compensate for different focal depths for RGB colors may require additional digital corrections in image-rendering.

Architectures of AR displays

Unlike VR displays with a relatively fixed optical configuration, there exist a vast number of architectures in AR displays. Therefore, instead of following the narrative of tackling different challenges, a more appropriate way to review AR displays is to separately introduce each architecture and discuss its associated engineering challenges. An AR display usually consists of a light engine and an optical combiner. The light engine serves as display image source, while the combiner delivers the displayed images to viewer’s eye and in the meantime transmits the environment light. Some performance parameters like frame rate and power consumption are mainly determined by the light engine. Parameters like FoV, eyebox and MTF are primarily dependent on the combiner optics. Moreover, attributes like image brightness, overall efficiency, and form factor are influenced by both light engine and combiner. In this section, we will firstly discuss the light engine, where the latest advances in micro-LED on chip are reviewed and compared with existing microdisplay systems. Then, we will introduce two main types of combiners: free-space combiner and waveguide combiner.

Light engine

The light engine determines several essential properties of the AR system like image brightness, power consumption, frame rate, and basic etendue. Several types of microdisplays have been used in AR, including micro-LED, micro-organic-light-emitting-diodes (micro-OLED), liquid-crystal-on-silicon (LCoS), digital micromirror device (DMD), and laser beam scanning (LBS) based on micro-electromechanical system (MEMS). We will firstly describe the working principles of these devices and then analyze their performance. For those who are more interested in final performance parameters than details, Table 1 provides a comprehensive summary.

Working principles

Micro-LED and micro-OLED are self-emissive display devices. They are usually more compact than LCoS and DMD because no illumination optics is required. The fundamentally different material systems of LED and OLED lead to different approaches to achieve full-color displays. Due to the “green gap” in LEDs, red LEDs are manufactured on a different semiconductor material from green and blue LEDs. Therefore, how to achieve full-color display in high-resolution density microdisplays is quite a challenge for micro-LEDs. Among several solutions under research are two main approaches. The first is to combine three separate red, green and blue (RGB) micro-LED microdisplay panels 106 . Three single-color micro-LED microdisplays are manufactured separately through flip-chip transfer technology. Then, the projected images from three microdisplay panels are integrated by a trichroic prism (Fig. 7a ).

a RGB micro-LED microdisplays combined by a trichroic prism. b QD-based micro-LED microdisplay. c Micro-OLED display with 4032 PPI. Working principles of d LCoS, e DMD, and f MEMS-LBS display modules. Reprinted from a ref. 106 with permission from IEEE, b ref. 108 with permission from Chinese Laser Press, c ref. 121 with permission from Jon Wiley and Sons, d ref. 124 with permission from Spring Nature, e ref. 126 with permission from Springer and f ref. 128 under the Creative Commons Attribution 4.0 License

Another solution is to assemble color-conversion materials like quantum dot (QD) on top of blue or ultraviolet (UV) micro-LEDs 107 , 108 , 109 (Fig. 7b ). The quantum dot color filter (QDCF) on top of the micro-LED array is mainly fabricated by inkjet printing or photolithography 110 , 111 . However, the display performance of color-conversion micro-LED displays is restricted by the low color-conversion efficiency, blue light leakage, and color crosstalk. Extensive efforts have been conducted to improve the QD-micro-LED performance. To boost QD conversion efficiency, structure designs like nanoring 112 and nanohole 113 , 114 have been proposed, which utilize the Förster resonance energy transfer mechanism to transfer excessive excitons in the LED active region to QD. To prevent blue light leakage, methods using color filters or reflectors like distributed Bragg reflector (DBR) 115 and CLC film 116 on top of QDCF are proposed. Compared to color filters that absorb blue light, DBR and CLC film help recycle the leaked blue light to further excite QDs. Other methods to achieve full-color micro-LED display like vertically stacked RGB micro-LED array 61 , 117 , 118 and monolithic wavelength tunable nanowire LED 119 are also under investigation.

Micro-OLED displays can be generally categorized into RGB OLED and white OLED (WOLED). RGB OLED displays have separate sub-pixel structures and optical cavities, which resonate at the desirable wavelength in RGB channels, respectively. To deposit organic materials onto the separated RGB sub-pixels, a fine metal mask (FMM) that defines the deposition area is required. However, high-resolution RGB OLED microdisplays still face challenges due to the shadow effect during the deposition process through FMM. In order to break the limitation, a silicon nitride film with small shadow has been proposed as a mask for high-resolution deposition above 2000 PPI (9.3 µm) 120 .

WOLED displays use color filters to generate color images. Without the process of depositing patterned organic materials, a high-resolution density up to 4000 PPI has been achieved 121 (Fig. 7c ). However, compared to RGB OLED, the color filters in WOLED absorb about 70% of the emitted light, which limits the maximum brightness of the microdisplay. To improve the efficiency and peak brightness of WOLED microdisplays, in 2019 Sony proposed to apply newly designed cathodes (InZnO) and microlens arrays on OLED microdisplays, which increased the peak brightness from 1600 nits to 5000 nits 120 . In addition, OLEDWORKs has proposed a multi-stacked OLED 122 with optimized microcavities whose emission spectra match the transmission bands of the color filters. The multi-stacked OLED shows a higher luminous efficiency (cd/A), but also requires a higher driving voltage. Recently, by using meta-mirrors as bottom reflective anodes, patterned microcavities with more than 10,000 PPI have been obtained 123 . The high-resolution meta-mirrors generate different reflection phases in the RGB sub-pixels to achieve desirable resonant wavelengths. The narrow emission spectra from the microcavity help to reduce the loss from color filters or even eliminate the need of color filters.

LCoS and DMD are light-modulating displays that generate images by controlling the reflection of each pixel. For LCoS, the light modulation is achieved by manipulating the polarization state of output light through independently controlling the liquid crystal reorientation in each pixel 124 , 125 (Fig. 7d ). Both phase-only and amplitude modulators have been employed. DMD is an amplitude modulation device. The modulation is achieved through controlling the tilt angle of bi-stable micromirrors 126 (Fig. 7e ). To generate an image, both LCoS and DMD rely on the light illumination systems, with LED or laser as light source. For LCoS, the generation of color image can be realized either by RGB color filters on LCoS (with white LEDs) or color-sequential addressing (with RGB LEDs or lasers). However, LCoS requires a linearly polarized light source. For an unpolarized LED light source, usually, a polarization recycling system 127 is implemented to improve the optical efficiency. For a single-panel DMD, the color image is mainly obtained through color-sequential addressing. In addition, DMD does not require a polarized light so that it generally exhibits a higher efficiency than LCoS if an unpolarized light source is employed.

MEMS-based LBS 128 , 129 utilizes micromirrors to directly scan RGB laser beams to form two-dimensional (2D) images (Fig. 7f ). Different gray levels are achieved by pulse width modulation (PWM) of the employed laser diodes. In practice, 2D scanning can be achieved either through a 2D scanning mirror or two 1D scanning mirrors with an additional focusing lens after the first mirror. The small size of MEMS mirror offers a very attractive form factor. At the same time, the output image has a large depth-of-focus (DoF), which is ideal for projection displays. One shortcoming, though, is that the small system etendue often hinders its applications in some traditional display systems.

Comparison of light engine performance

There are several important parameters for a light engine, including image resolution, brightness, frame rate, contrast ratio, and form factor. The resolution requirement (>2K) is similar for all types of light engines. The improvement of resolution is usually accomplished through the manufacturing process. Thus, here we shall focus on other three parameters.

Image brightness usually refers to the measured luminance of a light-emitting object. This measurement, however, may not be accurate for a light engine as the light from engine only forms an intermediate image, which is not directly viewed by the user. On the other hand, to solely focus on the brightness of a light engine could be misleading for a wearable display system like AR. Nowadays, data projectors with thousands of lumens are available. But the power consumption is too high for a battery-powered wearable AR display. Therefore, a more appropriate way to evaluate a light engine’s brightness is to use luminous efficacy (lm/W) measured by dividing the final output luminous flux (lm) by the input electric power (W). For a self-emissive device like micro-LED or micro-OLED, the luminous efficacy is directly determined by the device itself. However, for LCoS and DMD, the overall luminous efficacy should take into consideration the light source luminous efficacy, the efficiency of illumination optics, and the efficiency of the employed spatial light modulator (SLM). For a MEMS LBS engine, the efficiency of MEMS mirror can be considered as unity so that the luminous efficacy basically equals to that of the employed laser sources.

As mentioned earlier, each light engine has a different scheme for generating color images. Therefore, we separately list luminous efficacy of each scheme for a more inclusive comparison. For micro-LEDs, the situation is more complicated because the EQE depends on the chip size. Based on previous studies 130 , 131 , 132 , 133 , we separately calculate the luminous efficacy for RGB micro-LEDs with chip size ≈ 20 µm. For the scheme of direct combination of RGB micro-LEDs, the luminous efficacy is around 5 lm/W. For QD-conversion with blue micro-LEDs, the luminous efficacy is around 10 lm/W with the assumption of 100% color conversion efficiency, which has been demonstrated using structure engineering 114 . For micro-OLEDs, the calculated luminous efficacy is about 4–8 lm/W 120 , 122 . However, the lifetime and EQE of blue OLED materials depend on the driving current. To continuously display an image with brightness higher than 10,000 nits may dramatically shorten the device lifetime. The reason we compare the light engine at 10,000 nits is that it is highly desirable to obtain 1000 nits for the displayed image in order to keep ACR>3:1 with a typical AR combiner whose optical efficiency is lower than 10%.

For an LCoS engine using a white LED as light source, the typical optical efficiency of the whole engine is around 10% 127 , 134 . Then the engine luminous efficacy is estimated to be 12 lm/W with a 120 lm/W white LED source. For a color sequential LCoS using RGB LEDs, the absorption loss from color filters is eliminated, but the luminous efficacy of RGB LED source is also decreased to about 30 lm/W due to lower efficiency of red and green LEDs and higher driving current 135 . Therefore, the final luminous efficacy of the color sequential LCoS engine is also around 10 lm/W. If RGB linearly polarized lasers are employed instead of LEDs, then the LCoS engine efficiency can be quite high due to the high degree of collimation. The luminous efficacy of RGB laser source is around 40 lm/W 136 . Therefore, the laser-based LCoS engine is estimated to have a luminous efficacy of 32 lm/W, assuming the engine optical efficiency is 80%. For a DMD engine with RGB LEDs as light source, the optical efficiency is around 50% 137 , 138 , which leads to a luminous efficacy of 15 lm/W. By switching to laser light sources, the situation is similar to LCoS, with the luminous efficacy of about 32 lm/W. Finally, for MEMS-based LBS engine, there is basically no loss from the optics so that the final luminous efficacy is 40 lm/W. Detailed calculations of luminous efficacy can be found in Supplementary Information .

Another aspect of a light engine is the frame rate, which determines the volume of information it can deliver in a unit time. A high volume of information is vital for the construction of a 3D light field to solve the VAC issue. For micro-LEDs, the device response time is around several nanoseconds, which allows for visible light communication with bandwidth up to 1.5 Gbit/s 139 . For an OLED microdisplay, a fast OLED with ~200 MHz bandwidth has been demonstrated 140 . Therefore, the limitation of frame rate is on the driving circuits for both micro-LED and OLED. Another fact concerning driving circuit is the tradeoff between resolution and frame rate as a higher resolution panel means more scanning lines in each frame. So far, an OLED display with 480 Hz frame rate has been demonstrated 141 . For an LCoS, the frame rate is mainly limited by the LC response time. Depending on the LC material used, the response time is around 1 ms for nematic LC or 200 µs for ferroelectric LC (FLC) 125 . Nematic LC allows analog driving, which accommodates gray levels, typically with 8-bit depth. FLC is bistable so that PWM is used to generate gray levels. DMD is also a binary device. The frame rate can reach 30 kHz, which is mainly constrained by the response time of micromirrors. For MEMS-based LBS, the frame rate is limited by the scanning frequency of MEMS mirrors. A frame rate of 60 Hz with around 1 K resolution already requires a resonance frequency of around 50 kHz, with a Q-factor up to 145,000 128 . A higher frame rate or resolution requires a higher Q-factor and larger laser modulation bandwidth, which may be challenging.

Form factor is another crucial aspect for the light engines of near-eye displays. For self-emissive displays, both micro-OLEDs and QD-based micro-LEDs can achieve full color with a single panel. Thus, they are quite compact. A micro-LED display with separate RGB panels naturally have a larger form factor. In applications requiring direct-view full-color panel, the extra combining optics may also increase the volume. It needs to be pointed out, however, that the combing optics may not be necessary for some applications like waveguide displays, because the EPE process results in system’s insensitivity to the spatial positions of input RGB images. Therefore, the form factor of using three RGB micro-LED panels is medium. For LCoS and DMD with RGB LEDs as light source, the form factor would be larger due to the illumination optics. Still, if a lower luminous efficacy can be accepted, then a smaller form factor can be achieved by using a simpler optics 142 . If RGB lasers are used, the collimation optics can be eliminated, which greatly reduces the form factor 143 . For MEMS-LBS, the form factor can be extremely compact due to the tiny size of MEMS mirror and laser module.

Finally, contrast ratio (CR) also plays an important role affecting the observed images 8 . Micro-LEDs and micro-OLEDs are self-emissive so that their CR can be >10 6 :1. For a laser beam scanner, its CR can also achieve 10 6 :1 because the laser can be turned off completely at dark state. On the other hand, LCoS and DMD are reflective displays, and their CR is around 2000:1 to 5000:1 144 , 145 . It is worth pointing out that the CR of a display engine plays a significant role only in the dark ambient. As the ambient brightness increases, the ACR is mainly governed by the display’s peak brightness, as previously discussed.

The performance parameters of different light engines are summarized in Table 1 . Micro-LEDs and micro-OLEDs have similar levels of luminous efficacy. But micro-OLEDs still face the burn-in and lifetime issue when driving at a high current, which hinders its use for a high-brightness image source to some extent. Micro-LEDs are still under active development and the improvement on luminous efficacy from maturing fabrication process could be expected. Both devices have nanosecond response time and can potentially achieve a high frame rate with a well-designed integrated circuit. The frame rate of the driving circuit ultimately determines the motion picture response time 146 . Their self-emissive feature also leads to a small form factor and high contrast ratio. LCoS and DMD engines have similar performance of luminous efficacy, form factor, and contrast ratio. In terms of light modulation, DMD can provide a higher 1-bit frame rate, while LCoS can offer both phase and amplitude modulations. MEMS-based LBS exhibits the highest luminous efficacy so far. It also exhibits an excellent form factor and contrast ratio, but the presently demonstrated 60-Hz frame rate (limited by the MEMS mirrors) could cause image flickering.

Free-space combiners

The term ‘free-space’ generally refers to the case when light is freely propagating in space, as opposed to a waveguide that traps light into TIRs. Regarding the combiner, it can be a partial mirror, as commonly used in AR systems based on traditional geometric optics. Alternatively, the combiner can also be a reflective HOE. The strong chromatic dispersion of HOE necessitates the use of a laser source, which usually leads to a Maxwellian-type system.

Traditional geometric designs

Several systems based on geometric optics are illustrated in Fig. 8 . The simplest design uses a single freeform half-mirror 6 , 147 to directly collimate the displayed images to the viewer’s eye (Fig. 8a ). This design can achieve a large FoV (up to 90°) 147 , but the limited design freedom with a single freeform surface leads to image distortions, also called pupil swim 6 . The placement of half-mirror also results in a relatively bulky form factor. Another design using so-called birdbath optics 6 , 148 is shown in Fig. 8b . Compared to the single-combiner design, birdbath design has an extra optics on the display side, which provides space for aberration correction. The integration of beam splitter provides a folded optical path, which reduces the form factor to some extent. Another way to fold optical path is to use a TIR-prism. Cheng et al. 149 designed a freeform TIR-prism combiner (Fig. 8c ) offering a diagonal FoV of 54° and exit pupil diameter of 8 mm. All the surfaces are freeform, which offer an excellent image quality. To cancel the optical power for the transmitted environmental light, a compensator is added to the TIR prism. The whole system has a well-balanced performance between FoV, eyebox, and form factor. To release the space in front of viewer’s eye, relay optics can be used to form an intermediate image near the combiner 150 , 151 , as illustrated in Fig. 8d . Although the design offers more optical surfaces for aberration correction, the extra lenses also add to system weight and form factor.

a Single freeform surface as the combiner. b Birdbath optics with a beam splitter and a half mirror. c Freeform TIR prism with a compensator. d Relay optics with a half mirror. Adapted from c ref. 149 with permission from OSA Publishing and d ref. 151 with permission from OSA Publishing

Regarding the approaches to solve the VAC issue, the most straightforward way is to integrate a tunable lens into the optical path, like a liquid lens 152 or Alvarez lens 99 , to form a varifocal system. Alternatively, integral imaging 153 , 154 can also be used, by replacing the original display panel with the central depth plane of an integral imaging module. The integral imaging can also be combined with varifocal approach to overcome the tradeoff between resolution and depth of field (DoF) 155 , 156 , 157 . However, the inherent tradeoff between resolution and view number still exists in this case.

Overall, AR displays based on traditional geometric optics have a relatively simple design with a decent FoV (~60°) and eyebox (8 mm) 158 . They also exhibit a reasonable efficiency. To measure the efficiency of an AR combiner, an appropriate measure is to divide the output luminance (unit: nit) by the input luminous flux (unit: lm), which we note as combiner efficiency. For a fixed input luminous flux, the output luminance, or image brightness, is related to the FoV and exit pupil of the combiner system. If we assume no light waste of the combiner system, then the maximum combiner efficiency for a typical diagonal FoV of 60° and exit pupil (10 mm square) is around 17,000 nit/lm (Eq. S2 ). To estimate the combiner efficiency of geometric combiners, we assume 50% of half-mirror transmittance and the efficiency of other optics to be 50%. Then the final combiner efficiency is about 4200 nit/lm, which is a high value in comparison with waveguide combiners. Nonetheless, to further shrink the system size or improve system performance ultimately encounters the etendue conservation issue. In addition, AR systems with traditional geometric optics is hard to achieve a configuration resembling normal flat glasses because the half-mirror has to be tilted to some extent.

Maxwellian-type systems

The Maxwellian view, proposed by James Clerk Maxwell (1860), refers to imaging a point light source in the eye pupil 159 . If the light beam is modulated in the imaging process, a corresponding image can be formed on the retina (Fig. 9a ). Because the point source is much smaller than the eye pupil, the image is always-in-focus on the retina irrespective of the eye lens’ focus. For applications in AR display, the point source is usually a laser with narrow angular and spectral bandwidths. LED light sources can also build a Maxwellian system, by adding an angular filtering module 160 . Regarding the combiner, although in theory a half-mirror can also be used, HOEs are generally preferred because they offer the off-axis configuration that places combiner in a similar position like eyeglasses. In addition, HOEs have a lower reflection of environment light, which provides a more natural appearance of the user behind the display.

a Schematic of the working principle of Maxwellian displays. Maxwellian displays based on b SLM and laser diode light source and c MEMS-LBS with a steering mirror as additional modulation method. Generation of depth cues by d computational digital holography and e scanning of steering mirror to produce multiple views. Adapted from b, d ref. 143 and c, e ref. 167 under the Creative Commons Attribution 4.0 License

To modulate the light, a SLM like LCoS or DMD can be placed in the light path, as shown in Fig. 9b . Alternatively, LBS system can also be used (Fig. 9c ), where the intensity modulation occurs in the laser diode itself. Besides the operation in a normal Maxwellian-view, both implementations offer additional degrees of freedom for light modulation.

For a SLM-based system, there are several options to arrange the SLM pixels 143 , 161 . Maimone et al. 143 demonstrated a Maxwellian AR display with two modes to offer a large-DoF Maxwellian-view, or a holographic view (Fig. 9d ), which is often referred as computer-generated holography (CGH) 162 . To show an always-in-focus image with a large DoF, the image can be directly displayed on an amplitude SLM, or using amplitude encoding for a phase-only SLM 163 . Alternatively, if a 3D scene with correct depth cues is to be presented, then optimization algorithms for CGH can be used to generate a hologram for the SLM. The generated holographic image exhibits the natural focus-and-blur effect like a real 3D object (Fig. 9d ). To better understand this feature, we need to again exploit the concept of etendue. The laser light source can be considered to have a very small etendue due to its excellent collimation. Therefore, the system etendue is provided by the SLM. The micron-sized pixel-pitch of SLM offers a certain maximum diffraction angle, which, multiplied by the SLM size, equals system etendue. By varying the display content on SLM, the final exit pupil size can be changed accordingly. In the case of a large-DoF Maxwellian view, the exit pupil size is small, accompanied by a large FoV. For the holographic display mode, the reduced DoF requires a larger exit pupil with dimension close to the eye pupil. But the FoV is reduced accordingly due to etendue conservation. Another commonly concerned issue with CGH is the computation time. To achieve a real-time CGH rendering flow with an excellent image quality is quite a challenge. Fortunately, with recent advances in algorithm 164 and the introduction of convolutional neural network (CNN) 165 , 166 , this issue is gradually solved with an encouraging pace. Lately, Liang et al. 166 demonstrated a real-time CGH synthesis pipeline with a high image quality. The pipeline comprises an efficient CNN model to generate a complex hologram from a 3D scene and an improved encoding algorithm to convert the complex hologram to a phase-only one. An impressive frame rate of 60 Hz has been achieved on a desktop computing unit.

For LBS-based system, the additional modulation can be achieved by integrating a steering module, as demonstrated by Jang et al. 167 . The steering mirror can shift the focal point (viewpoint) within the eye pupil, therefore effectively expanding the system etendue. When the steering process is fast and the image content is updated simultaneously, correct 3D cues can be generated, as shown in Fig. 9e . However, there exists a tradeoff between the number of viewpoint and the final image frame rate, because the total frames are equally divided into each viewpoint. To boost the frame rate of MEMS-LBS systems by the number of views (e.g., 3 by 3) may be challenging.

Maxwellian-type systems offer several advantages. The system efficiency is usually very high because nearly all the light is delivered into viewer’s eye. The system FoV is determined by the f /# of combiner and a large FoV (~80° in horizontal) can be achieved 143 . The issue of VAC can be mitigated with an infinite-DoF image that deprives accommodation cue, or completely solved by generating a true-3D scene as discussed above. Despite these advantages, one major weakness of Maxwellian-type system is the tiny exit pupil, or eyebox. A small deviation of eye pupil location from the viewpoint results in the complete disappearance of the image. Therefore, to expand eyebox is considered as one of the most important challenges in Maxwellian-type systems.

Pupil duplication and steering

Methods to expand eyebox can be generally categorized into pupil duplication 168 , 169 , 170 , 171 , 172 and pupil steering 9 , 13 , 167 , 173 . Pupil duplication simply generates multiple viewpoints to cover a large area. In contrast, pupil steering dynamically shifts the viewpoint position, depending on the pupil location. Before reviewing detailed implementations of these two methods, it is worth discussing some of their general features. The multiple viewpoints in pupil duplication usually mean to equally divide the total light intensity. In each time frame, however, it is preferable that only one viewpoint enters the user’s eye pupil to avoid ghost image. This requirement, therefore, results in a reduced total light efficiency, while also conditioning the viewpoint separation to be larger than the pupil diameter. In addition, the separation should not be too large to avoid gap between viewpoints. Considering that human pupil diameter changes in response to environment illuminance, the design of viewpoint separation needs special attention. Pupil steering, on the other hand, only produces one viewpoint at each time frame. It is therefore more light-efficient and free from ghost images. But to determine the viewpoint position requires the information of eye pupil location, which demands a real-time eye-tracking module 9 . Another observation is that pupil steering can accommodate multiple viewpoints by its nature. Therefore, a pupil steering system can often be easily converted to a pupil duplication system by simultaneously generating available viewpoints.

To generate multiple viewpoints, one can focus on modulating the incident light or the combiner. Recall that viewpoint is the image of light source. To duplicate or shift light source can achieve pupil duplication or steering accordingly, as illustrated in Fig. 10a . Several schemes of light modulation are depicted in Fig. 10b–e . An array of light sources can be generated with multiple laser diodes (Fig. 10b ). To turn on all or one of the sources achieves pupil duplication or steering. A light source array can also be produced by projecting light on an array-type PPHOE 168 (Fig. 10c ). Apart from direct adjustment of light sources, modulating light on the path can also effectively steer/duplicate the light sources. Using a mechanical steering mirror, the beam can be deflected 167 (Fig. 10d ), which equals to shifting the light source position. Other devices like a grating or beam splitter can also serve as ray deflector/splitter 170 , 171 (Fig. 10e ).

a Schematic of duplicating (or shift) viewpoint by modulation of incident light. Light modulation by b multiple laser diodes, c HOE lens array, d steering mirror and e grating or beam splitters. f Pupil duplication with multiplexed PPHOE. g Pupil steering with LCHOE. Reproduced from c ref. 168 under the Creative Commons Attribution 4.0 License, e ref. 169 with permission from OSA Publishing, f ref. 171 with permission from OSA Publishing and g ref. 173 with permission from OSA Publishing

Nonetheless, one problem of the light source duplication/shifting methods for pupil duplication/steering is that the aberrations in peripheral viewpoints are often serious 168 , 173 . The HOE combiner is usually recorded at one incident angle. For other incident angles with large deviations, considerable aberrations will occur, especially in the scenario of off-axis configuration. To solve this problem, the modulation can be focused on the combiner instead. While the mechanical shifting of combiner 9 can achieve continuous pupil steering, its integration into AR display with a small factor remains a challenge. Alternatively, the versatile functions of HOE offer possible solutions for combiner modulation. Kim and Park 169 demonstrated a pupil duplication system with multiplexed PPHOE (Fig. 10f ). Wavefronts of several viewpoints can be recorded into one PPHOE sample. Three viewpoints with a separation of 3 mm were achieved. However, a slight degree of ghost image and gap can be observed in the viewpoint transition. For a PPHOE to achieve pupil steering, the multiplexed PPHOE needs to record different focal points with different incident angles. If each hologram has no angular crosstalk, then with an additional device to change the light incident angle, the viewpoint can be steered. Alternatively, Xiong et al. 173 demonstrated a pupil steering system with LCHOEs in a simpler configuration (Fig. 10g ). The polarization-sensitive nature of LCHOE enables the controlling of which LCHOE to function with a polarization converter (PC). When the PC is off, the incident RCP light is focused by the right-handed LCHOE. When the PC is turned on, the RCP light is firstly converted to LCP light and passes through the right-handed LCHOE. Then it is focused by the left-handed LCHOE into another viewpoint. To add more viewpoints requires stacking more pairs of PC and LCHOE, which can be achieved in a compact manner with thin glass substrates. In addition, to realize pupil duplication only requires the stacking of multiple low-efficiency LCHOEs. For both PPHOEs and LCHOEs, because the hologram for each viewpoint is recorded independently, the aberrations can be eliminated.

Regarding the system performance, in theory the FoV is not limited and can reach a large value, such as 80° in horizontal direction 143 . The definition of eyebox is different from traditional imaging systems. For a single viewpoint, it has the same size as the eye pupil diameter. But due to the viewpoint steering/duplication capability, the total system eyebox can be expanded accordingly. The combiner efficiency for pupil steering systems can reach 47,000 nit/lm for a FoV of 80° by 80° and pupil diameter of 4 mm (Eq. S2 ). At such a high brightness level, eye safety could be a concern 174 . For a pupil duplication system, the combiner efficiency is decreased by the number of viewpoints. With a 4-by-4 viewpoint array, it can still reach 3000 nit/lm. Despite the potential gain of pupil duplication/steering, when considering the rotation of eyeball, the situation becomes much more complicated 175 . A perfect pupil steering system requires a 5D steering, which proposes a challenge for practical implementation.

Pin-light systems

Recently, another type of display in close relation with Maxwellian view called pin-light display 148 , 176 has been proposed. The general working principle of pin-light display is illustrated in Fig. 11a . Each pin-light source is a Maxwellian view with a large DoF. When the eye pupil is no longer placed near the source point as in Maxwellian view, each image source can only form an elemental view with a small FoV on retina. However, if the image source array is arranged in a proper form, the elemental views can be integrated together to form a large FoV. According to the specific optical architectures, pin-light display can take different forms of implementation. In the initial feasibility demonstration, Maimone et al. 176 used a side-lit waveguide plate as the point light source (Fig. 11b ). The light inside the waveguide plate is extracted by the etched divots, forming a pin-light source array. A transmissive SLM (LCD) is placed behind the waveguide plate to modulate the light intensity and form the image. The display has an impressive FoV of 110° thanks to the large scattering angle range. However, the direct placement of LCD before the eye brings issues of insufficient resolution density and diffraction of background light.

a Schematic drawing of the working principle of pin-light display. b Pin-light display utilizing a pin-light source and a transmissive SLM. c An example of pin-mirror display with a birdbath optics. d SWD system with LBS image source and off-axis lens array. Reprinted from b ref. 176 under the Creative Commons Attribution 4.0 License and d ref. 180 with permission from OSA Publishing

To avoid these issues, architectures using pin-mirrors 177 , 178 , 179 are proposed. In these systems, the final combiner is an array of tiny mirrors 178 , 179 or gratings 177 , in contrast to their counterparts using large-area combiners. An exemplary system with birdbath design is depicted in Fig. 11c . In this case, the pin-mirrors replace the original beam-splitter in the birdbath and can thus shrink the system volume, while at the same time providing large DoF pin-light images. Nonetheless, such a system may still face the etendue conservation issue. Meanwhile, the size of pin-mirror cannot be too small in order to prevent degradation of resolution density due to diffraction. Therefore, its influence on the see-through background should also be considered in the system design.

To overcome the etendue conservation and improve see-through quality, Xiong et al. 180 proposed another type of pin-light system exploiting the etendue expansion property of waveguide, which is also referred as scanning waveguide display (SWD). As illustrated in Fig. 11d , the system uses an LBS as the image source. The collimated scanned laser rays are trapped in the waveguide and encounter an array of off-axis lenses. Upon each encounter, the lens out-couples the laser rays and forms a pin-light source. SWD has the merits of good see-through quality and large etendue. A large FoV of 100° was demonstrated with the help of an ultra-low f /# lens array based on LCHOE. However, some issues like insufficient image resolution density and image non-uniformity remain to be overcome. To further improve the system may require optimization of Gaussian beam profile and additional EPE module 180 .

Overall, pin-light systems inherit the large DoF from Maxwellian view. With adequate number of pin-light sources, the FoV and eyebox can be expanded accordingly. Nonetheless, despite different forms of implementation, a common issue of pin-light system is the image uniformity. The overlapped region of elemental views has a higher light intensity than the non-overlapped region, which becomes even more complicated considering the dynamic change of pupil size. In theory, the displayed image can be pre-processed to compensate for the optical non-uniformity. But that would require knowledge of precise pupil location (and possibly size) and therefore an accurate eye-tracking module 176 . Regarding the system performance, pin-mirror systems modified from other free-space systems generally shares similar FoV and eyebox with original systems. The combiner efficiency may be lower due to the small size of pin-mirrors. SWD, on the other hand, shares the large FoV and DoF with Maxwellian view, and large eyebox with waveguide combiners. The combiner efficiency may also be lower due to the EPE process.

Waveguide combiner

Besides free-space combiners, another common architecture in AR displays is waveguide combiner. The term ‘waveguide’ indicates the light is trapped in a substrate by the TIR process. One distinctive feature of a waveguide combiner is the EPE process that effectively enlarges the system etendue. In the EPE process, a portion of the trapped light is repeatedly coupled out of the waveguide in each TIR. The effective eyebox is therefore enlarged. According to the features of couplers, we divide the waveguide combiners into two types: diffractive and achromatic, as described in the followings.

Diffractive waveguides

As the name implies, diffractive-type waveguides use diffractive elements as couplers. The in-coupler is usually a diffractive grating and the out-coupler in most cases is also a grating with the same period as the in-coupler, but it can also be an off-axis lens with a small curvature to generate image with finite depth. Three major diffractive couplers have been developed: SRGs, photopolymer gratings (PPGs), and liquid crystal gratings (grating-type LCHOE; also known as polarization volume gratings (PVGs)). Some general protocols for coupler design are that the in-coupler should have a relatively high efficiency and the out-coupler should have a uniform light output. A uniform light output usually requires a low-efficiency coupler, with extra degrees of freedom for local modulation of coupling efficiency. Both in-coupler and out-coupler should have an adequate angular bandwidth to accommodate a reasonable FoV. In addition, the out-coupler should also be optimized to avoid undesired diffractions, including the outward diffraction of TIR light and diffraction of environment light into user’s eyes, which are referred as light leakage and rainbow. Suppression of these unwanted diffractions should also be considered in the optimization process of waveguide design, along with performance parameters like efficiency and uniformity.

The basic working principles of diffractive waveguide-based AR systems are illustrated in Fig. 12 . For the SRG-based waveguides 6 , 8 (Fig. 12a ), the in-coupler can be a transmissive-type or a reflective-type 181 , 182 . The grating geometry can be optimized for coupling efficiency with a large degree of freedom 183 . For the out-coupler, a reflective SRG with a large slant angle to suppress the transmission orders is preferred 184 . In addition, a uniform light output usually requires a gradient efficiency distribution in order to compensate for the decreased light intensity in the out-coupling process. This can be achieved by varying the local grating configurations like height and duty cycle 6 . For the PPG-based waveguides 185 (Fig. 12b ), the small angular bandwidth of a high-efficiency transmissive PPG prohibits its use as in-coupler. Therefore, both in-coupler and out-coupler are usually reflective types. The gradient efficiency can be achieved by space-variant exposure to control the local index modulation 186 or local Bragg slant angle variation through freeform exposure 19 . Due to the relatively small angular bandwidth of PPG, to achieve a decent FoV usually requires stacking two 187 or three 188 PPGs together for a single color. The PVG-based waveguides 189 (Fig. 12c ) also prefer reflective PVGs as in-couplers because the transmissive PVGs are much more difficult to fabricate due to the LC alignment issue. In addition, the angular bandwidth of transmissive PVGs in Bragg regime is also not large enough to support a decent FoV 29 . For the out-coupler, the angular bandwidth of a single reflective PVG can usually support a reasonable FoV. To obtain a uniform light output, a polarization management layer 190 consisting of a LC layer with spatially variant orientations can be utilized. It offers an additional degree of freedom to control the polarization state of the TIR light. The diffraction efficiency can therefore be locally controlled due to the strong polarization sensitivity of PVG.

Schematics of waveguide combiners based on a SRGs, b PPGs and c PVGs. Reprinted from a ref. 85 with permission from OSA Publishing, b ref. 185 with permission from John Wiley and Sons and c ref. 189 with permission from OSA Publishing

The above discussion describes the basic working principle of 1D EPE. Nonetheless, for the 1D EPE to produce a large eyebox, the exit pupil in the unexpanded direction of the original image should be large. This proposes design challenges in light engines. Therefore, a 2D EPE is favored for practical applications. To extend EPE in two dimensions, two consecutive 1D EPEs can be used 191 , as depicted in Fig. 13a . The first 1D EPE occurs in the turning grating, where the light is duplicated in y direction and then turned into x direction. Then the light rays encounter the out-coupler and are expanded in x direction. To better understand the 2D EPE process, the k -vector diagram (Fig. 13b ) can be used. For the light propagating in air with wavenumber k 0 , its possible k -values in x and y directions ( k x and k y ) fall within the circle with radius k 0 . When the light is trapped into TIR, k x and k y are outside the circle with radius k 0 and inside the circle with radius nk 0 , where n is the refractive index of the substrate. k x and k y stay unchanged in the TIR process and are only changed in each diffraction process. The central red box in Fig. 13b indicates the possible k values within the system FoV. After the in-coupler, the k values are added by the grating k -vector, shifting the k values into TIR region. The turning grating then applies another k -vector and shifts the k values to near x -axis. Finally, the k values are shifted by the out-coupler and return to the free propagation region in air. One observation is that the size of red box is mostly limited by the width of TIR band. To accommodate a larger FoV, the outer boundary of TIR band needs to be expanded, which amounts to increasing waveguide refractive index. Another important fact is that when k x and k y are near the outer boundary, the uniformity of output light becomes worse. This is because the light propagation angle is near 90° in the waveguide. The spatial distance between two consecutive TIRs becomes so large that the out-coupled beams are spatially separated to an unacceptable degree. The range of possible k values for practical applications is therefore further shrunk due to this fact.

a Schematic of 2D EPE based on two consecutive 1D EPEs. Gray/black arrows indicate light in air/TIR. Black dots denote TIRs. b k-diagram of the two-1D-EPE scheme. c Schematic of 2D EPE with a 2D hexagonal grating d k-diagram of the 2D-grating scheme

Aside from two consecutive 1D EPEs, the 2D EPE can also be directly implemented with a 2D grating 192 . An example using a hexagonal grating is depicted in Fig. 13c . The hexagonal grating can provide k -vectors in six directions. In the k -diagram (Fig. 13d ), after the in-coupling, the k values are distributed into six regions due to multiple diffractions. The out-coupling occurs simultaneously with pupil expansion. Besides a concise out-coupler configuration, the 2D EPE scheme offers more degrees of design freedom than two 1D EPEs because the local grating parameters can be adjusted in a 2D manner. The higher design freedom has the potential to reach a better output light uniformity, but at the cost of a higher computation demand for optimization. Furthermore, the unslanted grating geometry usually leads to a large light leakage and possibly low efficiency. Adding slant to the geometry helps alleviate the issue, but the associated fabrication may be more challenging.

Finally, we discuss the generation of full-color images. One important issue to clarify is that although diffractive gratings are used here, the final image generally has no color dispersion even if we use a broadband light source like LED. This can be easily understood in the 1D EPE scheme. The in-coupler and out-coupler have opposite k -vectors, which cancels the color dispersion for each other. In the 2D EPE schemes, the k -vectors always form a closed loop from in-coupled light to out-coupled light, thus, the color dispersion also vanishes likewise. The issue of using a single waveguide for full-color images actually exists in the consideration of FoV and light uniformity. The breakup of propagation angles for different colors results in varied out-coupling situations for each color. To be more specific, if the red and the blue channels use the same in-coupler, the propagating angle for the red light is larger than that of the blue light. The red light in peripheral FoV is therefore easier to face the mentioned large-angle non-uniformity issue. To acquire a decent FoV and light uniformity, usually two or three layers of waveguides with different grating pitches are adopted.

Regarding the system performance, the eyebox is generally large enough (~10 mm) to accommodate different user’s IPD and alignment shift during operation. A parameter of significant concern for a waveguide combiner is its FoV. From the k -vector analysis, we can conclude the theoretical upper limit is determined by the waveguide refractive index. But the light/color uniformity also influences the effective FoV, over which the degradation of image quality becomes unacceptable. Current diffractive waveguide combiners generally achieve a FoV of about 50°. To further increase FoV, a straightforward method is to use a higher refractive index waveguide. Another is to tile FoV through direct stacking of multiple waveguides or using polarization-sensitive couplers 79 , 193 . As to the optical efficiency, a typical value for the diffractive waveguide combiner is around 50–200 nit/lm 6 , 189 . In addition, waveguide combiners adopting grating out-couplers generate an image with fixed depth at infinity. This leads to the VAC issue. To tackle VAC in waveguide architectures, the most practical way is to generate multiple depths and use the varifocal or multifocal driving scheme, similar to those mentioned in the VR systems. But to add more depths usually means to stack multiple layers of waveguides together 194 . Considering the additional waveguide layers for RGB colors, the final waveguide thickness would undoubtedly increase.

Other parameters special to waveguide includes light leakage, see-through ghost, and rainbow. Light leakage refers to out-coupled light that goes outwards to the environment, as depicted in Fig. 14a . Aside from decreased efficiency, the leakage also brings drawback of unnatural “bright-eye” appearance of the user and privacy issue. Optimization of the grating structure like geometry of SRG may reduce the leakage. See-through ghost is formed by consecutive in-coupling and out-couplings caused by the out-coupler grating, as sketched in Fig. 14b , After the process, a real object with finite depth may produce a ghost image with shift in both FoV and depth. Generally, an out-coupler with higher efficiency suffers more see-through ghost. Rainbow is caused by the diffraction of environment light into user’s eye, as sketched in Fig. 14c . The color dispersion in this case will occur because there is no cancellation of k -vector. Using the k -diagram, we can obtain a deeper insight into the formation of rainbow. Here, we take the EPE structure in Fig. 13a as an example. As depicted in Fig. 14d , after diffractions by the turning grating and the out-coupler grating, the k values are distributed in two circles that shift from the origin by the grating k -vectors. Some diffracted light can enter the see-through FoV and form rainbow. To reduce rainbow, a straightforward way is to use a higher index substrate. With a higher refractive index, the outer boundary of k diagram is expanded, which can accommodate larger grating k -vectors. The enlarged k -vectors would therefore “push” these two circles outwards, leading to a decreased overlapping region with the see-through FoV. Alternatively, an optimized grating structure would also help reduce the rainbow effect by suppressing the unwanted diffraction.

Sketches of formations of a light leakage, b see-through ghost and c rainbow. d Analysis of rainbow formation with k-diagram

Achromatic waveguide

Achromatic waveguide combiners use achromatic elements as couplers. It has the advantage of realizing full-color image with a single waveguide. A typical example of achromatic element is a mirror. The waveguide with partial mirrors as out-coupler is often referred as geometric waveguide 6 , 195 , as depicted in Fig. 15a . The in-coupler in this case is usually a prism to avoid unnecessary color dispersion if using diffractive elements otherwise. The mirrors couple out TIR light consecutively to produce a large eyebox, similarly in a diffractive waveguide. Thanks to the excellent optical property of mirrors, the geometric waveguide usually exhibits a superior image regarding MTF and color uniformity to its diffractive counterparts. Still, the spatially discontinuous configuration of mirrors also results in gaps in eyebox, which may be alleviated by using a dual-layer structure 196 . Wang et al. designed a geometric waveguide display with five partial mirrors (Fig. 15b ). It exhibits a remarkable FoV of 50° by 30° (Fig. 15c ) and an exit pupil of 4 mm with a 1D EPE. To achieve 2D EPE, similar architectures in Fig. 13a can be used by integrating a turning mirror array as the first 1D EPE module 197 . Unfortunately, the k -vector diagrams in Fig. 13b, d cannot be used here because the k values in x-y plane no longer conserve in the in-coupling and out-coupling processes. But some general conclusions remain valid, like a higher refractive index leading to a larger FoV and gradient out-coupling efficiency improving light uniformity.

a Schematic of the system configuration. b Geometric waveguide with five partial mirrors. c Image photos demonstrating system FoV. Adapted from b , c ref. 195 with permission from OSA Publishing

The fabrication process of geometric waveguide involves coating mirrors on cut-apart pieces and integrating them back together, which may result in a high cost, especially for the 2D EPE architecture. Another way to implement an achromatic coupler is to use multiplexed PPHOE 198 , 199 to mimic the behavior of a tilted mirror (Fig. 16a ). To understand the working principle, we can use the diagram in Fig. 16b . The law of reflection states the angle of reflection equals to the angle of incidence. If we translate this behavior to k -vector language, it means the mirror can apply any length of k -vector along its surface normal direction. The k -vector length of the reflected light is always equal to that of the incident light. This puts a condition that the k -vector triangle is isosceles. With a simple geometric deduction, it can be easily observed this leads to the law of reflection. The behavior of a general grating, however, is very different. For simplicity we only consider the main diffraction order. The grating can only apply a k -vector with fixed k x due to the basic diffraction law. For the light with a different incident angle, it needs to apply different k z to produce a diffracted light with equal k -vector length as the incident light. For a grating with a broad angular bandwidth like SRG, the range of k z is wide, forming a lengthy vertical line in Fig. 16b . For a PPG with a narrow angular bandwidth, the line is short and resembles a dot. If multiple of these tiny dots are distributed along the oblique line corresponding to a mirror, then the final multiplexed PPGs can imitate the behavior of a tilted mirror. Such a PPHOE is sometimes referred as a skew-mirror 198 . In theory, to better imitate the mirror, a lot of multiplexed PPGs is preferred, while each PPG has a small index modulation δn . But this proposes a bigger challenge in device fabrication. Recently, Utsugi et al. demonstrated an impressive skew-mirror waveguide based on 54 multiplexed PPGs (Fig. 16c, d ). The display exhibits an effective FoV of 35° by 36°. In the peripheral FoV, there still exists some non-uniformity (Fig. 16e ) due to the out-coupling gap, which is an inherent feature of the flat-type out-couplers.

a System configuration. b Diagram demonstrating how multiplexed PPGs resemble the behavior of a mirror. Photos showing c the system and d image. e Picture demonstrating effective system FoV. Adapted from c – e ref. 199 with permission from ITE

Finally, it is worth mentioning that metasurfaces are also promising to deliver achromatic gratings 200 , 201 for waveguide couplers ascribed to their versatile wavefront shaping capability. The mechanism of the achromatic gratings is similar to that of the achromatic lenses as previously discussed. However, the current development of achromatic metagratings is still in its infancy. Much effort is needed to improve the optical efficiency for in-coupling, control the higher diffraction orders for eliminating ghost images, and enable a large size design for EPE.

Generally, achromatic waveguide combiners exhibit a comparable FoV and eyebox with diffractive combiners, but with a higher efficiency. For a partial-mirror combiner, its combiner efficiency is around 650 nit/lm 197 (2D EPE). For a skew-mirror combiner, although the efficiency of multiplexed PPHOE is relatively low (~1.5%) 199 , the final combiner efficiency of the 1D EPE system is still high (>3000 nit/lm) due to multiple out-couplings.

Table 2 summarizes the performance of different AR combiners. When combing the luminous efficacy in Table 1 and the combiner efficiency in Table 2 , we can have a comprehensive estimate of the total luminance efficiency (nit/W) for different types of systems. Generally, Maxwellian-type combiners with pupil steering have the highest luminance efficiency when partnered with laser-based light engines like laser-backlit LCoS/DMD or MEM-LBS. Geometric optical combiners have well-balanced image performances, but to further shrink the system size remains a challenge. Diffractive waveguides have a relatively low combiner efficiency, which can be remedied by an efficient light engine like MEMS-LBS. Further development of coupler and EPE scheme would also improve the system efficiency and FoV. Achromatic waveguides have a decent combiner efficiency. The single-layer design also enables a smaller form factor. With advances in fabrication process, it may become a strong contender to presently widely used diffractive waveguides.

Conclusions and perspectives

VR and AR are endowed with a high expectation to revolutionize the way we interact with digital world. Accompanied with the expectation are the engineering challenges to squeeze a high-performance display system into a tightly packed module for daily wearing. Although the etendue conservation constitutes a great obstacle on the path, remarkable progresses with innovative optics and photonics continue to take place. Ultra-thin optical elements like PPHOEs and LCHOEs provide alternative solutions to traditional optics. Their unique features of multiplexing capability and polarization dependency further expand the possibility of novel wavefront modulations. At the same time, nanoscale-engineered metasurfaces/SRGs provide large design freedoms to achieve novel functions beyond conventional geometric optical devices. Newly emerged micro-LEDs open an opportunity for compact microdisplays with high peak brightness and good stability. Further advances on device engineering and manufacturing process are expected to boost the performance of metasurfaces/SRGs and micro-LEDs for AR and VR applications.

Data availability

All data needed to evaluate the conclusions in the paper are present in the paper. Additional data related to this paper may be requested from the authors.

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Xiong, J., Hsiang, EL., He, Z. et al. Augmented reality and virtual reality displays: emerging technologies and future perspectives. Light Sci Appl 10 , 216 (2021). https://doi.org/10.1038/s41377-021-00658-8

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What Is Significant in Modern Augmented Reality: A Systematic Analysis of Existing Reviews

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Augmented reality (AR) is a field of technology that has evolved drastically during the last decades, due to its vast range of applications in everyday life. The aim of this paper is to provide researchers with an overview of what has been surveyed since 2010 in terms of AR application areas as well as in terms of its technical aspects, and to discuss the extent to which both application areas and technical aspects have been covered, as well as to examine whether one can extract useful evidence of what aspects have not been covered adequately and whether it is possible to define common taxonomy criteria for performing AR reviews in the future. To this end, a search with inclusion and exclusion criteria has been performed in the Scopus database, producing a representative set of 47 reviews, covering the years from 2010 onwards. A proper taxonomy of the results is introduced, and the findings reveal, among others, the lack of AR application reviews covering all suggested criteria.

1. Introduction

Augmented Reality (AR) is a technological field that has already fueled diverse application areas for a few decades. A typical definition of AR can be found in [ 1 ]: “Augmented reality is a system that enhances the real world by superimposing computer-generated information on top of it”. If we would like to search for the first appearance of the term, we should go back to 1992 and the paper by Caudell et al. [ 2 ]. In this specific publication, the authors present a head mounted display (HMD) that is used to aid the aircraft manufacturing process.

Ever since that publication, there has been a thriving increase of works in several application areas where AR could be utilized. The first known paper that surveys the state-of-the-art in AR applications is [ 3 ] by Azuma. The author already recognized, at this early stage, six different application classes: medical, manufacturing and repair, annotation and visualization, robot path planning, entertainment, and military aircraft. The works referenced span the years range of 1989–1997, and the author describes the main characteristics of these systems, as well as the problems that the up-to-date systems faced in terms of registration and sensing.

In the latest years, there has been an apparent rise in published material concerning AR as one can see in Figure 1 , where the Scopus database was searched for journal papers containing the words “augmented” and “reality” in their title, without any further screening.

Scopus articles referring to augmented reality per year (1993–2021).

As the years advanced, there were even more application fields that found support in AR, such as education, tourism, retail, and geoscience. In each of these fields, many new findings were published. Researchers had to find their way into exploring the current state-of-the-art in each different area, depending on the specific AR application they were aiming at. Thus, reviews of specific AR applications areas, as well as reviews of specific technical aspects of AR, started to emerge. In Figure 2 we can see the annual trend of journal articles found in Scopus database, containing the words “augmented”, “reality”, and “review” or “survey” in their title, without any subsequent screening.

Scopus review articles referring to augmented reality per year (1997–2021).

The increased interest both for AR itself, as well as for surveying its application areas and technical aspects, rendered it crucial, in our mind, to perform a review of what areas have attracted more interest by the researchers, measured by the number of reviews in each area and the extent to which the application characteristics have been analyzed. As a result, the areas which deserve to be further considered by researchers for AR application development will be identified, as well as the weaknesses that existing AR systems reveal in practice, in order to suggest future research directions. In order to do so, a sort of “umbrella” review will be performed, in the sense that existing reviews in all AR application areas will be covered. The only similar attempt made before was that by Nesenbergs et al. [ 4 ], but it only focused on the specific application area of remote higher education also covering Virtual Reality (VR). Another point that should be stressed is that no attention was paid to the specific query that has been executed at each instance and each database, presuming that the authors performing the survey have a broader knowledge of the scientific area and, thus, perform an acceptable search.

The research questions that are going to be answered based on the results are the following:

• RQ1: Are all application areas adequately covered by current AR reviews?
• RQ2: Are all technical aspects of proposed AR systems covered by AR reviews?
• RQ3: Is it possible to establish common taxonomy criteria for surveying any AR applications area?
• RQ4: Is it possible to recognize which technical aspects of AR are considered more significant depending on the AR application area?

The layout of the rest of the paper is as follows. In Section 2 a brief retrospect of publications conducting AR reviews, but not focusing on a particular application area, is presented. In Section 3 , the adopted search and screening process is described in detail, in order to end up with the set of review papers that are going to be analyzed. In Section 4 , the selected taxonomy criteria are presented, together with a brief description of them. In Section 5 , our findings are presented, and a discussion on the posed research questions follows. Lastly, in Section 6 , conclusions are drawn about the performed study and future directions are proposed.

2. Related Work

After the original review paper by Azuma [ 3 ], an incremental review was presented by Azuma et al. [ 5 ] four years later, where all the up-to-date developments in the field were considered. These included trends of that time in displays, tracking sensors, calibration, user interfaces, interaction, rendering, as well as on mobile, collaborative, and commercial applications, together with a discussion on existing problems and limitations. In 2011, Carmigniani et al. [ 6 , 7 ] went through all related computer vision methods, devices, interfaces, and AR systems. They also made a special mention to mobile systems and subsequently presented applications in advertising, entertainment, education, and medicine. In 2012, Berryman [ 8 ] presented a brief overview of AR history, concept, and uses, specifically mentioning marketing, entertainment, sightseeing, industry, fashion, and medicine. Mekni et al. [ 9 ] presented medical, military, manufacturing, visualization, entertainment and games, robotics, education, marketing, navigation and path planning, tourism, geospatial, urban planning, and civil engineering applications of AR. In 2017, Chatzopoulos et al. [ 10 ] focused on applications of Mobile Augmented Reality (MAR) such as tourism and navigation, entertainment and advertisement, training and education, geometry modeling and scene construction, assembly and maintenance, information assistant management, and big data driven MAR, and provided an overview of related user interfaces and user experience, system components, tracking and registration, network connectivity, data management, system performance, and sustainability and challenging problems. Cipresso et al. [ 11 ] later stressed the differences between VR and AR, and, for both technologies, identified document clusters corresponding to areas of research, as well as networks of document co-citations. Chen et al. [ 12 ] presented a brief overview of AR display, registration and interaction technologies, AR SDKs, application areas, and the AR cloud. In [ 13 ], Merino et al. discussed both technology-centric and human-centric evaluations present in Mixed Reality (MR) and AR literature. More recently, Arena et al. [ 14 ] discriminated between games, medical applications, and others briefly described the hardware, software, and design limits of AR systems, and made a reference to AR in Industry 4.0.

Apart from reviews regarding AR as a whole, several papers have reviewed developments of specific technical aspects. As far as input device technology is concerned, some good examples are papers reviewing skin-integrated vibrohaptic devices [ 15 , 16 ], wearable sensors and integrated functional devices [ 17 ], thermo-haptic materials and devices [ 18 , 19 ], elastomeric haptic devices [ 20 ], and active materials [ 21 ].

In terms of display technology, a significant effort has been made to report technological advances. Yin et al. [ 22 ], Zhan et al. [ 23 ], and Xiong et al. [ 24 ] present general reviews in this field. On the other hand, in [ 25 ], Huang et al. focus on liquid-crystal-on-silicon technology, while Xiong et al. refer to holographic optical elements [ 26 ] and planar liquid crystal polarization optics [ 27 ]. El Jamiy et al. [ 28 ] surveyed existing works on depth perception for HMDs.

Zhou et al. [ 29 ] are concerned about three different technical aspects of AR systems, namely, tracking, interaction, and display, while Bostanci et al. [ 30 ] focus solely on user tracking, and Goh et al. [ 31 ] exclusively on interaction for MAR. User experience is considered by Arifin et al. [ 32 ], and, specifically for MAR, by Irshad et al. [ 33 ].

Kurkovsky et al. [ 34 ] focus on the particularities of handheld AR in terms of navigation and tracking, content management, and usability. In [ 35 ], Qiao et al. discuss the convergence of the Web and AR, as a successor to MAR. Si-Mohammed et al. [ 36 ] present the state-of-the-art in fusing Brain–Computer interfaces with AR. Norouzi et al. [ 37 ] refer to the fusion of AR with IVA (Intelligent Virtual Agents) and the IoT (Internet of Things). From a different aspect, Lampropoulos et al. [ 38 ] discuss the merging of AR with deep learning, semantic web, and knowledge graphs.

Last but not least, a special mention should be made to surveys on collaborative AR [ 39 , 40 , 41 , 42 , 43 , 44 ], which has enabled the enhancement of either co-located or remote, synchronous or asynchronous, shared workspace or shared experience applications, among others.

3. Searching and Screening Process

In order to end up with a decent collection of survey papers in the field of AR applications, the Scopus database was chosen for searching. This specific database includes a large number of exclusively peer-reviewed journals (40,079 as of 4 April 2022) covering all fields of science over a large period of time (1970–). The search was made using the expression:

This means that:

• The title has to contain the words: “augmented”, “reality”, and either “review” or “survey”. 467 papers were originally returned as hits.
• The search is limited to a year range of 2010–2022 so that relatively recent results are also taken into account by the review paper (20 papers removed).
• Results were limited once again by demanding the document type to be designated as “article” or “review”. Sometimes, the item is characterized as a research article by the publisher, although it clearly contains a review (153 papers removed).
• The results are, subsequently, limited to source type of “journal”: review papers are traditionally lengthy and do not normally fit to a conference (2 papers removed).

Other commercial databases, such as Web of Science, were not considered due to access limitations through the author’s institution. Free ones such as Google Scholar were also not utilized due to their automatic mechanism of item inclusion [ 45 ]. Thus, it was found to be proper to limit the search to Scopus alone, although there may be a chance that the results are biased due to the expert inclusion criteria. The PRISMA flowchart [ 46 ] was adopted and is as shown in Figure 3 .

Flow diagram of the article selection process (initially: 467, finally 47).

The remaining 292 papers, resulting after the above-referenced exclusion process, were all sought for downloading via the Scopus Document Download Manager, and only 96 of them could eventually be retrieved. The subsequent manual screening process was based on the following criteria:

• Qualitative taxonomy per article: The review paper should contain a taxonomy of AR application works in tabular format, based on several criteria. A researcher needs to be able to pinpoint all necessary technical aspects of an existing AR application field in order to be able to easily identify strengths and weaknesses of existing methods and propose ones with real novelty. Based on this, 37 articles were excluded for not presenting such a taxonomy.
• Application oriented reviews: Only reviews focusing on a specific AR application field are sought, and not ones focusing on specific technical aspects of applications. 5 articles were excluded by this criterion.
• No framework/protocol: No existing frameworks or protocols for performing reviews are to be assessed. This way, 2 articles were excluded.
• English language: Only articles written in English were considered, as it is the most common and accessible language to the great majority of researchers. Three (3) articles were excluded because they were written in Spanish.
• No umbrella reviews: The present work can be considered an “umbrella” review, in the sense that it covers other review works. However, it probably is the first one to conduct such an analysis. In fact, only a single such article was found, and it was only covering a broader application area, not all possible AR application areas, as in the present case.
• Sufficient number of reviewed AR works: Since, sometimes, reviews cover also other technological areas such as VR together with AR, it is possible that some reviews do not cover a sufficient number of AR application works, and their taxonomy is thus not tailored to AR applications. This way, one review paper was excluded, which only referenced a single AR application.

After this careful screening, the remaining 47 articles underwent the subsequent analysis.

4. Criteria Selection and Definitions

The 47 papers that were eventually selected for reviewing contained several taxonomy criteria. After a careful study, it was concluded that there are several criteria appearing in different works that could be useful for reference by future research works. It should also be noted that the criteria names vary from one article to another. Another point is that no criteria that were not deemed as useful for drawing conclusions were taken into account, such as publication name, authors’ country of origin, or participants’ ages. In the following, each of the selected criteria is presented and defined, using a standard name each time.

4.1. Hardware

The specific criterion basically refers to the display type employed. Most commonly encountered types are [ 47 ]:

• Head-worn: The (either video or optical) see-through display is very close to the user’s eyes, since it is attached to their head (e.g., HMD). Probably the most expensive technology, but the hands are left free for interaction with the surrounding environment.
• Hand-held: The users hold the see-through display in their hands. This technology acts like a magnifying lens and its cost is definitely lower than that of head-worn displays, but at least one hand is occupied.
• Spatial: The display is positioned at a specific location and it is either video, optical, or projective. Usually, spatial displays are intended for applications with minimum interaction (e.g., HUD in military aircrafts).

4.2. Field of Interest

This criterion refers to the use case of the application and its description varies, depending on the specific area. For example, in [ 48 ], which is concerned about orthopedic surgery, possible fields of interest are placement, osteotomies, tumor surgery, trauma, and training/education (called “categories” in the specific paper). In the same fashion, in [ 49 ], concerning cultural heritage studies, the possible fields are museums, cultural heritage sites, and art galleries.

4.3. Method

This refers to the kind of approach followed by authors to conduct their research, such as pilot study, qualitative study, quantitative study, prototype description (such as in [ 50 ] concerned about healthcare), or simulation-based game, collaborative learning simulator, or inquiry-based simulator (such as in [ 51 ] in the context of education).

Otherwise, the objective or purpose of the work under concern is another area-specific criterion. For example, in healthcare-oriented papers the aim could be preoperative, intraoperative, or training (as in [ 52 ], concerning orthopedic surgery). In education-oriented reviews the purpose can be, for example, the preliminary evaluation of a procedure training application, or the comparison of a didactic aid based on AR with images and video [ 53 ].

4.5. Main Outcomes

The main results concluding the specific research. Another area-specific descriptive criterion shows, for example, that the patient’s balance and walking speed improved or that their muscle strength increased significantly [ 54 ], or that using AR can enhance mobile and remote learning, or that it can enhance students’ focus and attentiveness [ 55 ].

4.6. Sample

The number of participants that took part in the experiments. This may refer, for example, to patients as far as healthcare applications are concerned, or students when talking about education applications.

4.7. Software

This could refer either to a SDK for developing AR applications (ARToolKit, ARCore, Vuforia Engine, ARKit, Augment, etc.), a game engine (e.g., Unity), a 3D graphics API (e.g., OpenGL) or, simply, the language in which the application was developed from scratch (e.g., C++, Java, Python). The software implements all interaction, visualization, and registration tasks.

4.8. Tracking

This refers to the way the presentation of the AR content is triggered. A common categorization of tracking techniques is [ 56 ]:

• Magnetic tracking: A device that bases its function in magnetic field properties is used to calculate the position of a receiver with respect to a transmitter.
• - Infrared tracking: Infrared light sources (LEDs) are, usually, attached to the target of interest and a proper camera in the environment receives the emitted light (configuration can be inverted with respect the LED and camera positions).
• * Fiducial or marker-based tracking: A static planar image (marker) attached to specific targets is required to activate the augmented content. Examples are QR codes, logos, and product packaging. In this case, the virtual content is anchored to the marker (it is displayed in a specific location with regard to the marker).
• * Natural feature or markerless tracking: Instead of resorting to a marker, these techniques scan the whole environment for naturally existing features that are unique and might trigger the superposition of the virtual items. It is preferable that the natural image contains enough edges and corners to make it easier to be recognized.
• * Model based tracking: A 3D object model instead of a planar marker is used to trigger the augmentation.
• - 3D structure tracking: This category is based on creating range images, usually by means of a pair of an infrared projector and an infrared sensor (e.g., Kinect). Such a device can perform full 3D reconstruction of a scene.
• Inertial tracking: Sensors such as gyroscopes and accelerometers are employed in order to measure all three angles of rotation of the object being tracked, as well as its change of position.
• GPS or Location-based tracking: This trigger type is simply based on current geographic coordinates, rendering such techniques suitable for wide area applications, such as those demanding directional guidance.
• Hybrid tracking: As one expects, this category fuses input from different kinds of sensors in order to improve tracking performance and overcome problems of specific sensor types.

4.9. Limitations

This criterion concerns the weaknesses of the proposed work. For example, in an image-guided therapy system [ 57 ], a drawback would be having to attach a marker to the bone. In an educational context [ 58 ], a limitation would be the low graphics processing power in mobile devices.

4.10. Modalities

Modalities refer to the different input or output sensory channels for human–computer interaction [ 59 ]:

• - Vision: For example, when the camera tracks an AR marker.
• - Tactility: Clicking a mouse button is an example.
• - Audition: e.g., a voice command.
• - Kinesthetics: For example, sensing the position or movement of hands.
• - Vision: For example, presentation of 3D graphics on a screen.
• - Audition: e.g., sound effects accompanying a visual augmented item.

5. Results and Discussion

5.1. ar application areas coverage.

Since the gathered papers concerned reviews of specific AR application areas, the first thing that comes into question is how these articles are distributed with respect to application field. The pie chart in Figure 4 reveals the fact that the lion’s share belongs to the areas of healthcare and education, scoring 41% (19 papers) and 36% (17 papers), respectively. This indeed comes as no surprise, since these two are the main branches of the social sector. Next comes the field of industrial applications with 9% (4 papers), an area that has attracted a lot of attention since the very beginning of AR. The rest of the areas that have been encountered after the aforementioned searching and screening process are robotics, engineering, interior design, tourism, entertainment, and chemistry, all having the same minimum percentage of 2% (1 paper). Lastly, a single paper by Parekh et al. [ 60 ] addressing three different areas, namely healthcare, entertainment, and retail, has been categorized under the label “various”, thus producing the remaining 2% of the pie chart. Since this paper does not focus on a single field, it is going to be considered in the subsequent analysis of all three application fields. It should be noted here that the absence of other well known fields of AR applications, such as military, collaborative, or geospatial, in this distribution, does not suggest that no reviews in the specific fields exist, but that they did not qualify for being further analyzed.

Distribution of review articles over application fields.

5.2. Healthcare

Starting with healthcare, which regards 41% of total articles, or 19 out of 47 (plus the one mentioned above), it should be first stressed that, under the term “healthcare”, several subcategories are considered, such as surgery, nursing, dentistry, rehabilitation, physical therapy, emergency medicine, and so on. In Table 1 , a taxonomy of the 19+1 healthcare reviews is presented, based on criteria appearing in any of them.

Criteria for healthcare-oriented AR review articles.

It is evident from Table 1 that there is not a single review article employing all ten taxonomy criteria. The one being closer to performing a complete taxonomy is the one by Viglialoro et al. [ 61 ], with 8 out of 10, omitting only software and limitations in their tabular taxonomy. This finding suggests there is room for a thorough review in healthcare applications of AR that would provide a full taxonomy in tabular format for assisting future researchers in their work.

In Figure 5 we can see which criteria are most and which are least used by researchers in reviews of AR in healthcare.

Distribution of AR healthcare reviews over criteria (min: 2–max: 16).

We can see that researchers in this area tend to pay more attention to the field of interest (targeted application), the method employed to perform the research, and the hardware (mostly display type) in use. On the contrary, software, limitations, tracking, and modalities seem to attract reviewers’ attention less.

5.3. Education

Second in order of interest to researchers comes the field of education in all its forms, including professional training and learning. Seventeen out of forty-seven surveys, a significant 36%, refer to this application area. In Table 2 , a taxonomy of the 17 education-oriented reviews is presented, based on criteria appearing in any of them.

Criteria for education-oriented AR review articles.

Again, there is no article to score 10 out of 10 in presenting a clear tabular taxonomy based on all selected criteria. The one closest to achieving it is the review paper by Ajit et al. [ 51 ], with a score of 9 out of 10, leaving the sample criterion behind. According to the presented taxonomy requirements, a complete review paper should address these criterion as well, since it is significant for measuring the objectivity of the study.

In Figure 6 we can see which criteria are most and which are least used by researchers in reviews of AR in education.

Distribution of AR education reviews over criteria (min: 3–max: 13).

Two of the three top criteria found in healthcare oriented reviews are also among the first three here: hardware and field of interest. However, in the third position we find the method and the main outcomes, which are descriptive fields capturing a great percent of the essence of any work. Modalities, limitations, software, and tracking are still at the lower end of reviewers’ preferences.

5.4. Other Areas

Although, in reality, all other application areas could prove equally important for assessing AR usage in them, as it was done with healthcare and education, the findings after the inclusion and exclusion process described above were too few to perform a quantitative analysis per criterion. For this reason, it was decided to consider all of them together in the current subsection. For convenience, a column was added, showing the specific application area to which each paper belongs, as one can see in Table 3 .

Criteria for AR review articles oriented towards other areas.

Although, in this case, there is increased diversity in the nature of applications, since they come from different areas, the ones achieving the highest score in providing the selected taxonomy criteria are the publications by Li et al. [ 87 ] in the field of engineering, and the one by Ho et al. [ 88 ] in the field of industry, both with a 7 out of 10. In Figure 7 we can see which criteria are most and which are least used by researchers in reviews of AR in education.

Distribution of AR education reviews over criteria (min: 2–max: 8).

Again, it is obvious that the field of interest and the hardware are the two of the three top trends in criteria selection by previous authors, together with tracking, which shares third place with method. To no surprise, the tracking method is referenced by reviews in the fields of industry, engineering, and robotics, which have a tendency to provide more technical details. In the last four places, we find the main outcomes, aim, limitations, and modalities.

5.5. Answers to Research Questions

After all the above-described analysis, we are ready to answer the research questions posed in Section 1 :

RQ1: Are all application areas adequately covered by current AR reviews? : The answer is obviously negative. Even in the most widely covered application areas, such as healthcare and education, there is no single review paper that covers all taxonomy criteria that were considered to be significant for including. At this point, it should be noted once again that examined review papers were expected to compare existing works with criteria being presented in a tabular format, so that future researchers can follow the state-of-the-art easily. Some of the selected criteria may be descriptive by nature (e.g., method, aim, main outcomes, etc.) but an experienced researcher can capture their essence in a few words. Some of the papers not selected after screening may contain useful comparison information in a totally textual format, making it difficult for the reader to come up with a conclusion about the pros and cons of the referenced works. Eventually, there are certain areas apart from all the ones encountered in the selected papers which deserve more attention. In fact, there are works proposing reviews in the field of e-commerce [ 96 ], architecture [ 97 ], construction [ 98 ], or human resource management (HRM) [ 99 ] that do not follow a systematic taxonomy such as the one proposed in the present paper.

RQ2: Are all technical aspects of proposed AR systems covered by AR reviews? : Although the focus of each researcher performing a review may be on different technical details of AR technology, it is apparent, from the present study, that certain aspects are considered less significant by different authors, depending on their scientific background. For example, the study by Liang et al. [ 90 ] concerning tourism does not at all refer to hardware, software, tracking, and modalities, which comprise the technical body of each application, but the authors come from a School of Hospitality, Food, and Tourism Management, and there is no coauthor from, e.g., a Computer Science Department. As another example, Longo et al. [ 52 ] present a review paper in healthcare applications of AR without any reference to all the above four criteria, but they come from a Department of Orthopedic and Trauma Surgery and a Research Unit Nursing Science.

RQ3: Is it possible to establish common taxonomy criteria for surveying any AR applications area? : As one can conclude from the results presented in the previous subsections, it is possible to apply all ten selected criteria to create a taxonomy of applications in any scientific area employing AR technology, since hardware, software, tracking, and modalities are technical details specific to AR, and all the rest (field of interest, method, aim, main outcomes, sample, and limitations) are general criteria that could be practically used in any case study, irrespective of technology (all six are descriptive, except for sample). These ten criteria comprise a set that enables reviewers to perform a taxonomy that will assist fellow scientists in proposing novelties in the field of AR.

RQ4: Is it possible to recognize which technical aspects of AR are considered more significant depending on the AR application area? : In the present paper, it is claimed that there is a degree of correlation between the popularity of a technical aspect of AR as a criterion in existing AR application area reviews and the extent to which this technical aspect is important to authors for the particular application. Indeed, it is evident from the results presented in the previous subsections that the display component is the most popular as a taxonomy criterion for authors conducting a survey in healthcare and education applications, in which the optical stimulus is the most important for the outcome of the AR use, whether the end-user is a patient, a healthcare specialist, or a student. Other aspects that drew authors’ attention to a lesser extent are software, tracking, and modalities. One reason for this may be that, in certain areas, such as healthcare and education, standard (not custom) software (e.g., ARToolKit or Vuforia/Unity), or a standard tracking method (e.g., marker-based tracking), or standard modalities (e.g., vision both for input and output) is usually employed and, thus, the authors may sometimes consider the specific criteria as redundant for taxonomy. Another reason could be that the reviewers come from a scientific area different than Computer Science and, thus, are not interested in exploring the technical aspects of the method, rather than its social implications.

6. Conclusions and Future Work

In the present umbrella review, it was first attempted to select a representative set of published AR application reviews, after careful screening based on specific eligibility conditions. Afterwards, a set of ten taxonomy criteria was presented that can be used in future reviews in the area. The findings were subsequently presented with respect to the selected criteria, and a further analysis of them followed in the form of answers to four specific research questions. It was thus deduced that no AR application area is fully covered until today according to the proposed taxonomy, and that the technical aspects of employed AR technology are sometimes neglected by the authors. Additionally, it was proved that the ten taxonomy criteria that were selected can be utilized in any AR application area and provide a complete picture of the state-of-the-art. Lastly, it was found out that the hardware components are deemed a more significant technical aspect by reviewers compared to software, tracking, and modalities.

It is the strong belief of the author that the present work will help future review authors in the area to conduct their surveys in a way that will assist researchers to identify the strengths and weaknesses of previous methods and introduce technical novelties.

Regarding future work, it would be a good idea to perform umbrella reviews per AR application area in order to identify potential lack of proper technical coverage in area reviews. Another point would be to propose a more fine-grained taxonomy that would reveal which previously employed AR systems perform better in practice, and thus decide which research directions would be more fruitful.

Abbreviations

The following abbreviations are used in this manuscript:

Funding Statement

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Data availability statement, conflicts of interest.

The author declares no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

THE UNIQUE NATURE OF ICT

BYLINE: Keith Brawner, PhD, Program Manager, ICT; Senior Scientist/Engineer, US Army DEVCOM Soldier Center 

Dr. Keith Brawner is the Program Manager of the Institute for Creative Technologies University Affiliated Research Center (ICT UARC) for the Undersecretary of Defense for Research and Engineering , as managed by the US Army DEVCOM Soldier Center , where he is also the lead for AI initiatives. 

In 2011, I was working for the Army Research Laboratory and received a presentation from ICT on the new realistic graphics being developed within military training technologies. That was when the UARC (in affiliation with USC) first captured my imagination. It was (as now!) at the bleeding edge of technology, and had recently won its first Academy Award. It struck me as a one-of-a-kind place within the DoD. 

As a UARC for the Undersecretary of Defense for Research and Engineering (USD(R&E)), ICT must have unique Core Competencies which separate it from other UARCs, and it does. But it also maintains distinctiveness in all aspects. This makes it a joy to work with – and to utilize for national defense. ICT’s unique core competencies in the areas of realism, simulation, the behaviors within simulations, and the application of simulation technology into education, training, and operations make it world-class. 

In the area of realism ICT has won Academy Awards, which is certainly unique among the DoD, but it has also developed the HDR Shop used in many games and simulations. The visualization of these realistic technologies birthed the virtual and augmented reality industries directly through spin-out activities. At the individual level of simulation, ICT’s Virtual Human Toolkit has been utilized in dozens of military training tools, deployed in dozens of STEM-outreach applications, and has more commercial items than I can discuss. In fact, an industry is starting to rise around these technologies.

At the collective level of simulation, ICTs technologies have instructed key decision-makers on how to manage country-level peace-keeping. ICTs human behaviors within simulations are currently training many individuals on interpersonal communications, such as within the SHARP schoolhouses, and also doing research into how to better model OPFOR actions for training both large and small combat operations. 

Insofar as education, training, and operations – ICT has been unique in its works with the Army University on teaching AI, with the training injects into the Synthetic Training Environment program, and the operational deployment of counter-IED training semi-trucks training into Iraq and Afghanistan which trained tens of thousands of Warfighters. Within the research community, it is somewhat unique to deploy things downrange.

ICT is also unique in its business model as a DoD UARC partnered with a private university. The business model of the ICT for the DoD is to produce the research and results within its areas, and provide deep technological reach-back capability across all major scientific disciplines. In the coming years, ICT’s status as a unit within the Viterbi School of Engineering, alongside its sister institute, the Information Sciences Institute, will leverage USC talent in areas such as microelectronics and quantum computing to enhance DoD training and operations. 

ICT freely gives its technology back to the Government, but also maintains a robust commercial technology transition office, collaborating on numerous technologies with private industry for non-defense applications. The robustness and seamlessness of this utilization is unparalleled. 

ICT is unique in its service associated with the military, and can serve as trusted and unbiased advisors into Government. The DoD has utilized this directly in their development of the One World Terrain program, to name just one project deserving of the Army’s top civilian honor – the Distinguished Civilian Service Award. The ability of the UARC to provide technical expertise side-by-side with Government employees is unique to UARCs in their policy guidance allowing “close to inherently Governmental Functions.” 

By being headquartered in Los Angeles, ICT draws on the entertainment capital of the world, where one in eight jobs is related to the creative industries, and many of its residents are triple hyphenates, holding several concurrent roles within the industry. Many of ICT’s staff and freelance contributors are creatives, contributing professional-quality scripts and scenarios to Government projects, delivering exquisitely shot materials, featuring top notch acting talent. ICT’s collaborations with the entertainment industry have brought a new level of realism and

engagement to military training simulations. Imagine the excitement as a soldier dons a headset and steps into a fully immersive virtual battlefield, complete with Hollywood-caliber sound design, special effects, and even actors playing key roles. 

ICT is also unique in its contracting mechanisms and flexibility. The ICT UARC, for its areas of specialty, has the type of contracting only dreamt about within acquisition circles. The USD(R&E) authority allows my office to have ~1 week to the start of execution on critical projects, virtually unlimited ceiling, templated requirements, and guaranteed deliverables, Government rights, and functionally no fee. Simply put, we pay for work and we get it because of ICTs unique contractual relationship as a trusted DoD research center. 

The ICT itself is a hotbed of creativity and cutting-edge research, where interdisciplinary teams of military scientists, technologists, and artists come together to explore the vast potential of technology in military training and operations. This unique partnership has produced groundbreaking advancements in virtual and augmented reality, artificial intelligence, and machine learning, all with the goal of enhancing military readiness and capability. 

In running a research center on behalf of the Army on behalf of the DoD, the uniqueness of the ICT work, business, military service, creative enterprises, contracting, academic relationships, entertainment industry connections, and, most importantly, the assistance it has provided to the nation, are unparalleled within its charter. Each day I am proud and honored to serve as its Program Manager.

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Home — Essay Samples — Information Science and Technology — Augmented Reality — Augmented Reality Books: Transforming Reading and Learning

Augmented Reality Books: Transforming Reading and Learning

• Categories: Augmented Reality Impact of Technology Technology in Education

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Published: Oct 25, 2023

Words: 801 | Pages: 2 | 5 min read

Table of contents

Introduction, the use of augmented reality in children's books, the use of augmented reality in educational books, the use of augmented reality in fiction and non-fiction books.

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Introduction to Virtual and Augmented Reality

• First Online: 12 January 2022

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• Ralf Doerner 5 ,
• Wolfgang Broll ,
• Bernhard Jung ,
• Paul Grimm ,
• Martin Göbel &
• Rolf Kruse  

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What is Virtual Reality (VR)? What is Augmented Reality (AR)? What is the purpose of VR/AR? What are the basic concepts? What are the hard- and software components of VR/AR systems? How has VR/AR developed historically? The first chapter examines these questions and provides an introduction to this textbook. This chapter is fundamental for the whole book. All subsequent chapters build on it and do not depend directly on one another. Therefore, these chapters can be worked through selectively and in a sequence that suits the individual interests and needs of the readers. Corresponding tips on how this book can be used efficiently by different target groups (students, teachers, users, technology enthusiasts) are provided at the end of the chapter, as well as a summary, questions for reviewing what has been learned, recommendations for further reading, and the references used in the chapter.

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Augmented Reality: A Comprehensive Review

Augmentierte und Virtuelle Realität

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Bernhard Jung

Recommended Reading

Angel E, Shreiner D (2015) Interactive computer graphics: a top-down approach with WebGL . Pearson Education, Harlow – Textbook covering the basics of computer graphics, e.g., discussing the generation of images with the computer. It also introduces OpenGL and WebGL, a programming library for computer graphics, and discusses the possibilities of using graphics processors (GPUs) in the form of so-called shaders.

Rabin S (2009) Introduction to game development , 2nd edition. Charles River Media, Boston – a standard work on computer games. Due to the manifold points of contact between VR and computer games, literature from the field of computer games is also relevant.

Original scientific literature can be found in specialist journals and conference proceedings which can be researched and accessed in digital libraries (e.g., dl.acm.org , ieeexplore.org , link.springer.com ) or via search engines (e.g. scholar.google.com ). In the field of VR the IEEE VR Conference ( ieeevr.org ) takes place annually. Moreover, there is the Eurographics Symposium on Virtual Environments (EGVE) as well as the VR Conferences of euroVR, which are partly jointly organized as Joint Virtual Reality Conference (JVRC). With the focus on AR, ISMAR, the IEEE Symposium for Mixed and Augmented Reality, is held annually. In addition, there are special events that focus on aspects of user interfaces of VR and AR, such as the ACM VRST conference or the 3DUI, the IEEE Symposium for 3D User Interfaces. There are also further events dealing with special applications of VR, for instance in the industrial sector (e.g., VRCAI – ACM International Conference on Virtual Reality Continuum and Its Applications in Industry). Some scientific journals also focus on VR and AR, e.g., Presence – Teleoperators and Virtual Environments by MIT Press, Virtual Reality by Springer Verlag or the Journal of Virtual Reality and Broadcasting (jVRb) as an open access e-journal.

In addition to conference proceedings and professional journals that deal primarily with VR and AR, literature is also recommended that deals with essential aspects of VR and AR, such as Computer Graphics (e.g., ACM SIGGRAPH and the ACM Transactions on Graphics), Computer Vision (e.g., IEEE ICCV) or Human–Machine Interaction (e.g. ACM SIGCHI).

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Doerner, R., Broll, W., Jung, B., Grimm, P., Göbel, M., Kruse, R. (2022). Introduction to Virtual and Augmented Reality. In: Doerner, R., Broll, W., Grimm, P., Jung, B. (eds) Virtual and Augmented Reality (VR/AR). Springer, Cham. https://doi.org/10.1007/978-3-030-79062-2_1

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The metaverse reached the peak of hype and has been left for dead. But make no mistake, the technologies that will power this 3D internet are firing on all cylinders. This guide covers the history of the metaverse and explains the latest developments and where it’s headed.

Augmented reality (ar).

• Alexander S. Gillis, Technical Writer and Editor

What is augmented reality (AR)?

Augmented reality (AR) is the integration of digital information with the user's environment in real time. Unlike virtual reality ( VR ), which creates a totally artificial environment, AR users experience a real-world environment with generated perceptual information overlaid on top of it.

Augmented reality has a variety of uses, from assisting in the decision-making process to entertainment. AR is used to either visually change natural environments in some way or to provide additional information to users. The primary benefit of AR is that it manages to blend digital and three-dimensional ( 3D ) components with an individual's perception of the real world.

AR delivers visual elements, sound and other sensory information to the user through a device like a smartphone, glasses or a headset. This information is overlaid onto the device to create an interwoven and immersive experience where digital information alters the user's perception of the physical world. The overlaid information can be added to an environment or mask part of the natural environment.

Boeing Computer Services, Research and Technology employee Thomas Caudell coined the term augmented reality in 1990 to describe how the head-mounted displays that electricians use when assembling complicated wiring harnesses worked. One of the first commercial applications of augmented reality technology was the yellow first-down marker that began appearing in televised football games in 1998.

This article is part of

What is the metaverse? An explanation and in-depth guide

• Which also includes:
• Is the metaverse dead? Here's what happened and what's next
• Top metaverse platforms to know about in 2024
• 4 virtual reality ethics issues that need to be addressed

Today, smartphone games, mixed-reality headsets and heads-up displays (HUDs) in car windshields are the most well-known consumer AR products. But AR technology is also being used in many industries, including healthcare, public safety, gas and oil, tourism and marketing.

How does augmented reality work?

Augmented reality is deliverable in a variety of formats, including within smartphones, glasses and headsets. AR contact lenses are also in development. The technology requires hardware components, such as a processor, sensors, a display and input devices. Mobile devices, like smartphones and tablets, already have this hardware onboard, making AR more accessible to the everyday user. Mobile devices typically contain sensors, including cameras, accelerometers, Global Positioning System (GPS) instruments and solid-state compasses. For AR applications on smartphones, for example, GPS is used to pinpoint the user's location, and its compass is used to detect device orientation.

Sophisticated AR programs, such as those used by the military for training, might also include machine vision , object recognition and gesture recognition. AR can be computationally intensive, so if a device lacks processing power, data processing can be offloaded to a different machine.

Augmented reality apps work using either marker-based or markerless methods. Marker-based AR applications are written in special 3D programs that let developers tie animation or contextual digital information into the computer program to an augmented reality marker in the real world. When a computing device's AR app or browser plugin receives digital information from a known marker, it begins to execute the marker's code and layer the correct image or images.

Markerless AR is more complex. The AR device doesn't focus on a specific point, so the device must recognize items as they appear in view. This type of AR requires a recognition algorithm that detects nearby objects and determines what they are. Then, using the onboard sensors, the device can overlay images within the user's environment.

Differences between AR and VR

VR is a virtual environment created with software and presented to users in such a way that their brain suspends belief long enough to accept a virtual world as a real environment. Virtual reality is primarily experienced through a headset with sight and sound.

The biggest difference between AR and VR is that augmented reality uses the existing real-world environment and puts virtual information on top of it, whereas VR completely immerses users in a virtually rendered environment.

The devices used to accomplish this are also different. VR uses VR headsets that fit over the user's head and present them with simulated audiovisual information . AR devices are less restrictive and typically include devices like phones, glasses, projections and HUDs.

In VR, people are placed inside a 3D environment in which they can move around and interact with the generated environment. AR, however, keeps users grounded in the real-world environment, overlaying virtual data as a visual layer within the environment. So, for example, while VR places a user in a simulated environment, VR could overlay a web browser in front of the user in their living room. For spatial computing headsets, like Apple Vision Pro or Meta Quest 3, where the device is blocking the user's natural vision, a technique called passthrough is used. Here, the headset mirrors what the device's front-facing cameras see on the headset's display.

Although it can be interchanged with AR, the term mixed reality refers to a virtual display over a real-world environment with which users can interact. For example, Apple Vision Pro can project a virtual keyboard that the wearer can use to type. The key difference between mixed reality and AR is the user's ability to interact with the digital display.

Top AR use cases

AR can be used in the following ways, among others:

• Retail. Consumers can use a store's online app to see how products, such as furniture, will look in their own homes before buying.
• Entertainment and gaming. AR can be used to overlay a video game in the real world or enable users to animate their faces in different and creative ways on social media.
• Navigation. A user can overlay a route to their destination over a live view of a road. AR used for navigation can also display information about local businesses in the user's immediate surroundings.
• Tools and measurement. Mobile devices can use AR to measure different 3D points in the user's environment.
• Art and architecture. AR can help artists visualize or work on a project.
• Military. Data can be displayed on a vehicle's windshield, indicating destination directions, distances, weather and road conditions.
• Archaeology. AR aids archaeological research by helping archeologists reconstruct sites. 3D models help museum visitors and future archeologists experience an excavation site as if they were there.

Examples of AR

Examples of AR include the following:

• Target app. The Target retail app feature See it in Your Space lets users take a photo of a space in their home and digitally view an object, such as a picture on the wall or a chair, to see how it will look there.
• Apple Measure app. The Measure app on Apple iOS acts like a tape measure by letting users select two or more points in their environment and measure the distance between them.
• Snapchat. Snapchat can overlay a filter or mask over the user's video or picture.
• Pokemon Go. Pokemon Go is a popular mobile AR game that uses the player's GPS sensors to detect where Pokemon creatures appear in the user's surrounding environment for them to catch.
• Google Glass. Google Glass was Google's first commercial attempt at a glasses-based AR system. This small wearable computer, which was discontinued in 2023, let users work hands-free. Companies such as DHL and DB Schenker used Google Glass and third-party software to help frontline workers in global supply chain logistics and customized shipping.
• U.S. Army Tactical Augmented Reality (TAR). The U.S. Army uses AR in an eyepiece called TAR. TAR mounts onto the soldier's helmet and aids in locating another soldier's position.
• Apple Vision Pro. Apple Vision Pro is a spatial computing device that offers AR, VR and mixed-reality features. It live-maps a user's environment, and it offers passthrough and the ability to pin projections like web browsing windows to specific places in the user's environment. Users can control the device using gestures.
• Meta Quest 3. Meta Quest 3 is a mixed-reality headset that offers many similar features as Apple Vision Pro, including passthrough and productivity features. Users control this headset through gestures or controllers.

Future of AR technology

AR technology is growing steadily as the popularity and familiarization of apps and games like Pokemon Go or retail store AR apps increase.

Apple continues to develop and update its open source mobile augmented reality development tool set, ARKit. Companies, including Target and Ikea, use ARKit in their flagship AR shopping apps for iPhone and iPad. ARKit 6, for example, enables the rendering of AR in high dynamic range 4K and improves image and video capture. It also provides a Depth API, which uses per-pixel depth information to help a device's camera understand the size and shape of an object. It includes scene geometry that creates a topological map of a space along with other features.

ARCore, Google's platform for building AR experiences on Andriod and iOS, continues to evolve and improve. For example, ARCore uses a geospatial API that sources data from Google Earth 3D models and Street View image data from Google Maps. Similar to ARKit's Depth API, ARCore has improved its Depth API, optimizing it for longer-range depth sensing.

Improved AR, VR and mixed-reality headsets are also being released. For example, Meta improved its Quest 2 headset with Meta Quest 3, which was released in October 2023. This new headset is slimmer, lighter and more ergonomic than Quest 2.

In February 2024, Apple released Apple Vision Pro, bringing more competition to the AR and VR headset market. Vision Pro is targeted at early adopters and developers at a much higher price point than Quest 3. Meta Platforms is pursuing a wider audience at a $499 price point, while Apple is pricing Vision Pro at about $3,499. It's expected that Apple will produce a non-Pro variant of its headset at a more affordable price in the future. Developers of Apple Vision Pro will have to work with the visionOS software development kit. However, they can still use familiar Apple tools, such as ARKit, SwiftUI or RealityKit to build apps.

Other potential future advancements for AR include the following:

• More powerful and lighter devices.
• The use of artificial intelligence for face and room scanning, object detection and labeling, as well as for text recognition.
• The expansion of 5G networks that could make it easier to support cloud-based AR experiences by providing AR applications with higher data speeds and lower latency.

AR, VR and mixed-reality technologies are being used in various industries. Learn how each of these technologies differ .

Continue Reading About augmented reality (AR)

• Apple's spatial computing could make offices less relevant
• Augmented reality emerges in customer experience tech
• Workplace benefits of immersive learning with AR and VR
• Apple Vision Pro may energize adoption of VR/AR learning
• Spatial computing redraws the world of work

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A Framework for Drivers and Barriers to Augmented Reality Usage in Online Retailing: A Behavioural Reasoning Theory Perspective

36 Pages Posted: 6 Jul 2024

Diksha Agarwal

Indian Institute of Management Lucknow

Rajeev Kumra

T A Pai Management Institute

Harish Kumar

Great Lakes Institute of Management, Gurgaon; Management Development Institute

Despite prolific attention, only a handful of retailers have begun using AR for their consumers. The study aims to systematically identify and assess the factors influencing consumers' decisions to embrace or resist AR technology in retailing. Additionally, it explores the role of the human value - 'openness to change' in shaping these usage decisions. To fulfil the research objective, behavioural reasoning theory was employed as a new theoretical background. Data collected from 328 participants using a scenario-based survey technique reveal factors motivating (e.g., perceived local presence, ownership control) and hindering AR's usage intention (e.g., value barrier, risk barrier, response time lag) in a coherent theoretical framework, many of which are new to AR marketing literature. Further, we found a significant influence of consumer openness to change in the decision-making process. These insights offer retailers and AR developers strategic value in crafting customer-centric online shopping experiences, thereby enriching the understanding of AR and metaverse usage dynamics. The novelty of the study lies in developing a theoretical framework integrating and testing the reasons for and against AR usage intention in a single framework while establishing the role of human value of 'openness to change' as a significant antecedent to their reasoning processes.

Keywords: - Augmented Reality, Consumer Behaviour, Behavioural Reasoning Theory, Online Retailing, Metaverse

Suggested Citation: Suggested Citation

Diksha Agarwal (Contact Author)

Indian institute of management lucknow ( email ).

Prabandh Nagar Off. Sitapur Road, Lucknow 226013 Uttar Pradesh India

T A Pai Management Institute ( email )

PB No 9 80 Badagabettu Manipal, 576104 India

Great Lakes Institute of Management, Gurgaon ( email )

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Management Development Institute ( email )

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Augmented reality: what it is, how it works, examples

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Table of content

Virtual reality and augmented reality, what distinguishes them?

Augmented reality, what it is: some examples, augmented reality, what is a smart glass, augmented reality enters the operating room, sport and augmented reality, what is the hawk-eye, augmented reality museum, what it is, holograms, between augmented reality and virtual reality, augmented reality: the sectors in which it can be used.

Augmented reality a.k.a AU gives the interactive experience to the users of the real-world environment where the real world’ objects reside and enhanced by the perceptual information which is generated by computers. It is also the integration and strengthening of the possibilities that are offered to us by our five senses with high-tech devices.

The practical examples are Smart glasses like Google Glass or gesture control devices like Myo Armband, a hi-tech bracelet that allows you to remotely control PCs, tablets, smartphones, drones and robots are a case of ""integrated"", enhanced reality. In short, when we ask ourselves what augmented reality is we are doing nothing but discussing our daily interactions - human and between us and machines - enriched thanks to increasingly sophisticated electronic devices.

The augmented reality should not be confused with virtual reality.   The latter creates an artificial environment, built on the computer, and makes it credible by using technologies that give the feeling to those who use them to find themselves immersed in that scenario. Think for example of flight simulators or visors for 3D games and movies.

The augmented reality starts from what is around us, it already exists, but it is modified with the addition of animations and digital contents that allow having a deeper knowledge of the environment that surrounds us.

If virtual reality is therefore an artificial reality, augmented reality can be defined as an ""enriched"" reality.

The augmented reality is likely in the next few years to change our habits, our way of seeing the world. A silent revolution that is producing interesting, visible results - it is appropriate to say this - already in different sectors: from medicine to the military, from sport to marketing, from tourism to entertainment. But not only: augmented reality it also helps us in carrying out normal daily activities such as driving a car. There are ""enhanced"" satellite navigators that use a smartphone's camera, pointed at the road, to provide us with real-time information on traffic, the presence of speed cameras and places of interest. The road signs that appear directly on the windshield of the car, without distracting the driver are nothing more than augmented reality.

Here's what happens on this off-road vehicle. Not only additional indications like those on the speed and inclination of the vehicle while facing a slope but also the bonnet of the car that appears transparent to us, allowing us to see the road at all times, even that portion that is normally “obscured” by the front of our car.

The latest generation of smartphones is just one example of devices that support augmented reality. In truth, the accessory that is most often cited when we talk about augmented reality is the ""smart"" glasses, better known as smart glasses. Like the now-famous Google Glass,   real wearable computers on which you can read emails, SMS of our smartphone, directions to reach a destination or with which we can take a picture, shoot a video or surf the Internet. The Google Glass can be controlled only with the voice, as indeed other competing for smart glasses: from the Microsoft Hololens lenses (we'll talk better later) to the Smart Eyeglass from Sony via Epson's smart glasses, to name a few.

Smart glasses have already been tested in the operating room and in the future, they could be widely used in the field of robotic surgery, which is already a form of augmented reality.

The advantages deriving from their use are different: from the recording of the intervention for educational purposes to the monitoring of the vital parameters of the patient, up to the possibility of requesting a consultation in real-time from a colleague surgeon who is on the other side of the world.

The surgical application is for Myo Armband, the smart bracelet that allows you to control digital devices of various kinds using only your arms and hands. In this way, during an operation, it will be possible for example to consult the medical record or other useful information on the operation to be performed without leaving the operating table.

The augmented reality plays an important role - at times even decisive for the final result - even in sports. For some years now, in tennis and recently in football too, the Hawk-eye has been introduced, from the English word ""falcon's eye"", a system that allows verifying and eventually correcting an arbitrator's decision instantly. How does it work? Through several cameras positioned in different corners of the system, the trajectory of the ball is reconstructed to see if it has ended up outside the tennis court or if - in the case of football - it has crossed the goal line. However, the Hawk-eye is not foolproof and has a margin of error of some millimeters.

The augmented reality assists you even when you are on vacation. If you are in a city of art and the itinerary chosen by your guide does not satisfy you, know that there are museums that you can visit with augmented reality viewers or through dedicated software for smartphones and tablets. In this way, by pointing the device towards the work you are interested in, you can access a series of detailed information, usually of a historical and stylistic nature, which is normally reserved only for professionals.

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What we have just seen are just a few examples that explain how augmented reality is changing the way we live. As augmented reality enters our lives it becomes a simple reality. It will also be so with holograms. It's hard to say if it's more augmented or virtual. A hologram is a three-dimensional reconstruction in the space of images of objects or living beings.

The holograms can be used for educational, entertainment or   medical purposes . Imagine a surgeon who visualizes the patient's organs before surgery thanks to a hologram. An intervention that maybe will be carried out, thanks to robotic surgery.

In the field there are several projects on which multinationals like Samsung or Google are working. Holograms are halfway between virtual reality and augmented reality. They are a mixed reality because they are integrated into our world. They are and move in our dimension.

Augmented reality can be used in many sectors to improve work processes. For example it can be useful for technical assistance and remote maintenance, for training, for design, in marketing, to improve safety at work, for physical rehabilitation. There are many applications of augmented reality, just use your imagination. For example, the Aprilia motorcycle company will use it to improve bike preparation.

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The Sunday Papers

Sunday’s are for being mildly optimistic about the future for a few short hours. Before something bad happens, let’s read this week’s best writing about games (and game related things!)

For The Baffler, Noah Caldwell-Gervais wrote about games that employ repetition, both cynically and transformatively.

This is just the era we live in, our own stagnant age in the Lands Between. With Disney and its subsidiaries sucking all of the air out of the room to repackage the same concept over and over, Hollywood has reached the stale conclusion that the same story can be told repetitively. The embrace of AI across multiple mediums just intensifies this dilution of what feels meaningful. The worst games have always felt like an uncritical amalgamation of what’s familiar and proven, and AI is a tool designed to manufacture nothing but the obvious and average. Books with no point and nothing to say because they have no lived experience behind them; music that sounds familiar but says nothing because the AI that made it has never experienced emotion; animation that is detailed but stilted and lifeless because all the AI knows how to do is copy the humans who have gotten it right. It is a crushing tidal wave of cheap slop, a response to the hunger for more content that makes content both infinite and empty, starving even as it feeds. The incentive to do something new, or take a risk, or ever definitively say “This experience is over now” is vanishingly small against the profits that come from cyclically remonetizing what is already familiar.

Ed Zitron wrote about shareholder supremacy, nihilistic capitalism, and the legacy of Jack Welch. It’s a massive read, but well worth it if, like me, you lack the words and historical context to articulate why these things make you angry. I’m still going to refuse to elaborate on my “men in suits bad” stance, though, because it annoys people.

The unique problem that Sundar Pichai and the rest of the rot barons currently face is that there aren’t any hyper-growth markets left, and they’ve been desperately adapting to that reality since 2015. So many promises — augmented reality, home robotics, autonomous cars, and even then, artificial intelligence writ large — never quite materialized into viable business units, making big tech that little bit more desperate. I have repeatedly and substantively proven that both Meta and Google made their products worse in pursuit of growth, and they’ve done so by following a roadmap drawn by Jack Welch, a sociopathic scumbag that realized that he could turn General Electric into a shambling monstrosity of a company that could shapeshift into whatever the street needed. And I believe that this same financial nihilism is what empowers people like Mira Murati and Sam Altman, but also millions more middle managers and absentee CEOs like the kinds I’ve been writing about for the last three years. Our economy is run by people that have never built anything, running companies that they contort to make a number go up for shareholders they rarely meet — people like David Zaslav, the CEO of Warner Brothers Discovery who intentionally chose to not release Coyote Vs. Acme, a fully-produced and ready-to-debut movie featuring Warner Brothers’ core brands, choosing instead to save money on its tax bill…. And where do you think David Zaslav gets his fucking management philosophy from? Huh? Can you guess? Can you guess who it might be?

For The Guardian, Julian Benson simulated a year in the UK under each party in Democracy 4.

At the start of each party’s term in parliament, I mirror the manifesto costings in tax cuts and spending increases as closely as possible. For the Conservative party, that means cutting national insurance contributions and civil service headcount across the state sector; for Labour, I implement tax policies that target the same people affected by the closure of non-dom loopholes and VAT on private schools; and for the Liberal Democrats, a major tax on banks, corporations, technology companies and the aviation industry coupled with massive investment in state health and welfare services. All the parties rely on £5bn or more raised from cracking down on tax avoidance. While that seems like a policy that would hit onlynthe very wealthy, in reality, according to the IFS , it would hit taxpayers at every level. I recreated this with a tax that hit the Everybody demographic, and I can tell you, they weren’t thrilled about it

Second Wind’s Sebastian ‘Frost’ Ruiz’s Cold Takes are always great, and this one was pleasingly inside-baseball. Here’s the last two minutes or so of Kerrang! TV. I agree with maybe a third of this review of Shadow Of The Erdtree , but god is it cathartic. Music this week is One More Day Won’t Hurt by Soft Play. Have a great weekend!

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Computer Science > Robotics

Title: proact: an augmented reality testbed for intelligent prosthetic arms.

Abstract: Upper-limb amputees face tremendous difficulty in operating dexterous powered prostheses. Previous work has shown that aspects of prosthetic hand, wrist, or elbow control can be improved through "intelligent" control, by combining movement-based or gaze-based intent estimation with low-level robotic autonomy. However, no such solutions exist for whole-arm control. Moreover, hardware platforms for advanced prosthetic control are expensive, and existing simulation platforms are not well-designed for integration with robotics software frameworks. We present the Prosthetic Arm Control Testbed (ProACT), a platform for evaluating intelligent control methods for prosthetic arms in an immersive (Augmented Reality) simulation setting. Using ProACT with non-amputee participants, we compare performance in a Box-and-Blocks Task using a virtual myoelectric prosthetic arm, with and without intent estimation. Our results show that methods using intent estimation improve both user satisfaction and the degree of success in the task. To the best of our knowledge, this constitutes the first study of semi-autonomous control for complex whole-arm prostheses, the first study including sequential task modeling in the context of wearable prosthetic arms, and the first testbed of its kind. Towards the goal of supporting future research in intelligent prosthetics, the system is built upon on existing open-source frameworks for robotics.

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