Virtual Reality

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Introduction

Virtual Reality or VR is a computer-simulated artificial multisensory 3D environment that can mimic the properties and imagery of the physical world, be completely based in fantasy, or a mix of both. The system uses position-tracking and responds to the user’s inputs. In VR, the senses are temporarily fooled into believing that the artificial environment is real. The goal of a true VR experience is to create presence - the feeling of physically being somewhere else, of being in another reality [1].

Virtual Reality is an interactive and immersive medium that can be used to create unique experiences that are unattainable elsewhere. VR has the power to transform games, films and other forms of media. Some enthusiasts call VR the "ultimate input/output device" or the "last medium" because any subsequent medium can be created within VR, using only software [1] [2].

While Augmented Reality enhances the real world with digital content, Virtual Reality completely replaces the real world with a virtual one, creating a brand new digital environment for the users to explore [1] [2].

Main characteristics

Interactive - The user’s input controls the system and guides the behavior of the VR experience, while also modifying the virtual environment. This type of interaction engages the user, connecting him to the application in a more natural way since the environment responds directly to the stimuli [1].

Immersive - An immersive experience has to provide a sense of presence as well as a sense of engagement. Immersion can be divided into three different aspects:

1. According to Bierbaum (2000), “For a VR application to be immersive, it must be perceptually immersive by providing ‘the presentation of sensory cues that convey perceptually to users that they’re surrounded by the computer-generated environment.’” Therefore, the VR must provide the user with an all-encompassing sensory input [1].

2. The second aspect of immersion is the sense of presence. This implies that the VR experience must give the user the sense they are “in” the virtual world [1].

3. The final aspect is engagement. It is the degree “to which the user has a sense they are deeply involved in the environment.” [1]

Multisensory - providing a virtual experience that uses multiple human sensory systems increases the level of immersion. While current VR systems cannot provide a full range of stimuli to all human senses, it is expected that in the future this problem will be solved and the VR experience will be completely or almost indistinguishable from reality. The more senses are involved in the VR experience, the higher the degree of engagement and, consequently, this results in a greater sense of presence [1].

Synthetic - The environment is artificial, created by a computer in real-time [1].

Hardware Technologies

Head-mounted Display

VR is currently created by head-mounted displays (HMDs) such as the Oculus Rift. HMDs utilize stereoscopic displays and specialized lenses along with motion tracking hardware to give the illusion that the user is physically inside the virtual world.

To create the illusion of depth, a display is placed very close to the users' eyes, covering their entire field of view. Two images that are very similar but have different perspectives are channeled into each eye to create parallax, the visual phenomenon where our brains perceive depth based on the difference in the apparent position of objects.

Specialized lenses are placed between the display and our eyes. The lenses allow our eyes to focus on the images on the display, even though the display is only a few inches in front of our faces. Without lenses, our entire VR world would become blurry because human eyes have trouble focusing on things that are very close.[3]

The headset tracks the movement of your head and changes the images shown on the display based on it. This process creates the sensation that users are located within the virtual environment. Users of these devices are not only able to experience the computer-simulated environments but also interact with them. Various input methods, from the traditional game controllers and keyboards to the futuristic hand gestures and voice commands, are available or under development.

Motion Tracking

HMD tracks the movement of your head and updates the rendered scene based on its orientation and location. This process is similar to how we look around in real life. There are 2 types of tracking: rotational tracking and positional.

Rotational tracking tracks the 3 rotational movements: pitch, yaw, and roll. It is performed by IMUs such as accelerometers, gyroscopes and magnetometers.

Positional tracking tracks the 3 translational movements: forward/back, up/down and left/right. Positional tracking is usually more difficult than rotational tracking and is accomplished through different Types and Systems.

Motion tracking is not only used to track your head in HMDs but also used to track your hands and rest of your body through various input devices.

Input Devices

Input Devices allow the users to influence and manipulate the virtual realm they are in. These devices include traditional input methods such as gamepad, mouse and keyboard and novel devices that track the position and orientation of your hands, fingers, feet and other body parts.

Platforms

Oculus Rift (Platform)

SteamVR

PlayStation VR

OpenVR

Daydream

OSVR

WebVR

Windows 10 VR

Additional Information

VR Headset Demo Locations

Devices

See also: Virtual Reality Devices


Popular VR Headsets:

DevicesRequiresDisplayResolutionRefresh RateField of ViewTrackingRotational TrackingPositional TrackingUpdate RateLatencyInputConnectivity
ANTVRPC
Xbox One
PlayStation 4
Smartphone
Aspherical lens1920 x 1080100° diagonal3DOFGyroscope
Magnetometer
Accelerometer
Transformable ControllerU2B (USB-2.4GHz and Bluetooth) module
Acer Windows Mixed Reality HeadsetPCDual LCD display2880 x 1440 (1440 x 1440 per eye)95 Degree Horizontal6DOFGyroscope
Accelerometer
Magnetometer
Inside-out trackingMotion to Photon: Less than 10msKeyboard/Mouse
Xbox One GamePad
Motion Controllers
HDMI
USB
Apple Vision ProDual Micro OLED4k per eye120 degreesHands
Eyes
Face
Legs?
Daydream ViewDaydream Ready Phones5 / 5.5 inch AMOLED (Pixel / Pixel XL)1920 x 1080 / 2560 x 1440 (Pixel / Pixel XL)60 Hz90° (nominal)3DOFAccelerator
Gyrometer
Proximity
NoneRotational: 1000 Hz
High accuracy
Daydream Controller
FOVEPC5.8 inch low persistence OLED2560 x 144060 - 90 Hz100°Eye TrackingIMUTheir own systemEye Tracking: 120 FPSUSB 3.0
Display port
3.5mm headphone port
Figment VRDepends on the smartphoneDepends on the smartphone3DOFIMUs in SmartphoneNone
Google CardboardSmartphoneDepends on the smartphoneDepends on the smartphone3DOFIMUs in SmartphoneNone
HP Reverb G2Dual LCD 2.89’’ diagonal with Pulse Backlight technology2160 x 2160 pixels per eye90Hz~114 degrees
Fresnel-Aspherical
6DOF
Inside/out
HP Reverb G2 ControllersUSB 3.0 type C
BlueTooth
HTC VivePCDual Panel2160 x 1200 (1080 x 1200 per eye)90 Hz110° (diagonal)6DOFGyroscope
Accelerometer
Laser Position Sensor
Base StationsRotational: 1000Hz
Positional: 60Hz
7ms (no load)
15ms (medium load)
Motion to Photon
Controllers in both hands2 HDMI ports
2 USB ports
1 headphone jack
HTC Vive Developer EditionsPCDual Panel2160 x 1200
1080 x 1200 per eye
90 Hz110° (diagonal)6DOFGyroscope
Accelerometer
Laser Position Sensor
Base Stations????Controllers in both hands2 HDMI ports
2 USB ports
1 headphone jack
HTC Vive ProPCDual Panel2880 x 1600 (1440 x 1600 per eye)110° (diagonal)6DOFGyroscope
Accelerometer
Laser Position Sensor
Base StationsRotational: TBC
Positional: TBC
Motion to Photon
TBC
Controllers in both handsDisplayPort 1.2
USB-C 3.0 port
Bluetooth (Version TBD) (TBD: 1 headphone jack???)
Impression PiSmartphoneDepends on smartphoneDepends on the smartphone6 DOFIMU BoardIMU Board
Infared Cameras?
IR Projector?
LG 360 VRLG G51.88" IPS LCD x 2 EA
920 x 720 per Eye
639 ppi Real RGB
960x720 pixels at 693ppi (per eye)60HzMotion to Photon: over 50ms
Lenovo Windows Holographic HMDPC1440x1440-pixel OLED displayInside-out TrackingTwo CamerasInput DevicesHDMI and USB Connectors
Meta Quest 2IPS LCD1832 x 1920 per eye72Hz
90 Hz
90° (estimated)6DOFGyroscope
Accelerometer
Oculus InsightRotational: 1000Hz
Positional: 60Hz
Motion to Photon: less than 5msRedesigned Oculus TouchOculus Link
USB-C
OSVR HDK1PC5.5 inch LCD (1.0)
5.5 inch low-persistence OLED (1.1 - 1.3)
1920 x 1080
960 x 1080 per eye
60 Hz (1.0 - 1.2)
120 Hz (1.3)
100° (nominal)
90° (H and V)
3DOF
6DOF
Gyroscope
Accelerometer
Magnetometer
IR-LED faceplate and External Infrared CameraPositional: 100 Hz
Rotational: 400Hz
??Leap Motion VR
Gamepads
Mouse and Keyboard
1 external and 2 internal USB 3.0 ports
OSVR HDK2PC5.5 inch OLED2160 x 1200 (1080 x 1200 per eye)110 degrees3DOF
6DOF
Gyroscope
Accelerometer
Magnetometer
???Rotational: 400Hz
Positional: 100 Hz
?????1 external and 2 internal USB 3.0 ports
Oculus QuestOLED1440×1600 per eye72Hz96°×94° (estimated)(ref 5)6DOFGyroscope
Accelerometer
Oculus InsightOculus Touch Controllers
Controller-free Hand Tracking
Oculus Link
USB-C
Oculus RiftPCDual OLED Panels2160 x 1200 (1080 x 1200 per eye)90 Hz110° (diagonal)6DOFGyroscope
Accelerometer
Magnetometer
Oculus SensorRotational: 1000Hz
Positional: 60Hz
Motion to Photon: less than 5msXbox One controller
Oculus Touch
Oculus Remote
HDMI
USB
Oculus Rift DK1PC7 inch LCD1280 x 800
640 x 800 per eye
110° (Nominal)3DOFGyroscope
Accelerometer
Magnetometer
NoneRotational: 1000 HzTracking: 2ms
End-to-end: 50-60ms
Oculus Rift DK2PC5.7 inch OLED (PenTile)1920 x 1080
960 x 1080 per eye
75 Hz
72 Hz
100° (nominal)6DOFGyroscope
Accelerometer
Magnetometer
Separate Camera
Near Infrared CMOS Sensor
Rotational: 1000 Hz
Positional: 60 Hz
~30 msUSB
HDMI
Oculus Rift SLCD1280×1440 per eye80Hz6DOFOculus InsightOculus TouchHeadset cable required
Oculus Santa CruzInside-out trackingInput Devices1 HDMI
2 USB 3.0 Ports
Pico G23K LCD
Blue ray reduction
2880 x 160090Hz
615 ppi
101 degrees3DOF1 3DOF ControllerN/A
Pico Neo 24k
Single LCD panel
1
920 × 2
160 per eye
75Hz101 degrees
Fresnel
6DOF
Inside/out
Electromagnetic
2 Pico Neo ControllersN/A
Pico Neo 3 Pro4k
3664 x 1920 LCD
5.5"
PPI 773
72/90Hz98 degrees
Fresnel
6DOF
Inside-out tracking
2
Updated Pico Neo Controllers
Displayport
Pico Neo CVDual 1.5K AMOLED1500 x 1500 pixels/eye90Hz102 degrees6 Degrees of Freedom (6DoF)
Pimax 8KCustom low-latency LCD screen8K (4K for each eye)Monocular 75Hz / (Up to 90Hz although struggling to better 80Hz) (Both eyes 150Hz / 180Hz through Brainwarp)200-degree FOVGesture Tracking (optional)Yes18ms (Low MTP Latency)
PlayStation VRPlayStation 45.7 inch OLED1920 x 1080
960 x 1080 per eye
90 Hz100°6DOFAccelerometer
Gyroscope
PlayStation Camera?Less than 18msDualShock 4 Wireless Controller
PlayStation Move Motion Controller
PlayStation VR Aim
HDMI
USB
Project Alloy
Samsung Gear VR (2015/2016)All Samsung Smartphones 2015 and newer5.7 / 5.1 inch Super AMOLED (RGBG PenTile)2560 x 1440
1280 x 1440 per eye
60 Hz96° (nominal)3DOFAccelerator
Gyrometer
Proximity
NoneRotational: 1000 Hz
High accuracy
Motion to Photon: Less than 6 msTouchpad
Back Button
Volume Key
Focus adjustment wheel
MicroUSB to the smartphone
Samsung Gear VR Innovator EditionGalaxy Note 4
Galaxy S6
Galaxy S6 Edge
5.7 / 5.1 inch Super AMOLED (RGBG PenTile)2560 x 1440
1280 x 1440 per eye
60 Hz Low-persistence96° (nominal)3DOFAccelerator
Gyroscope
Magnetometer
NoneRotational: 1000 Hz
High accuracy
Motion to Photon: Less than 20 msTouch Pad
Back Button
Volume Key
MicroUSB to the smartphone
StarVRPCDual 5.5" LCD Quad HD Displays5120 x 1440 (2560 x 1440 per eye)210° (horizontal)
130° (vertical)
6DOFGyroscope
Accelerometer
Magnetometer
External optical sensor with fiducial markers
Sulon QPCOLED2560x1440 pixelsDual noise-suppressing embedded microphones
Gamepads
Controllers compatible with Windows 10
Mouse and Keyboard
Bluetooth 4.0
Wi-Fi 802.11ac
Valve Index1440x1600 RGB LCDs80/90/120/144Hz~120-130 degreesValve Index ControllersUSB 3.0
DisplayPort 1.2
Wearality SkySmartphoneDepends on smartphoneDepends on smartphone150°3DOFIMUs in SmartphoneNone
  • To make changes to the table, please edit the the infobox of the corresponding device. See Template:Device Infobox for reference.

Apps

VR Apps

Developer Resources

Game Engines

Unity

Unreal Engine

WebVR

Use Cases

See also: Virtual Reality Use Cases
Use Case Examples
3D modeling / design Medium, Tilt Brush, Gravity Sketch
Aerial Photography Skywand
Anatomy VR Human Anatomy, YouVR
Animal testing Janelia Research Campus
Arcades and Theme Parks THE VOID, Rise of the Demon, Nomadic VR, Anvio VR, Dreamscape Immersive
Architecture IrisVR, Arch Virtual, SmartVizX
Astronomy dSky Celestrion
Automotive design WorldViz
Big data visualisation Masters of Pie, Envelop VR, Virtualitics
Cinema Jaunt VR
Circus Two Bit Circus
Cognitive training Cerevrum
Computer-aided design VRED
Construction IDEAbuilder
Courtroom Institute of Forensic Medicine at the University of Zurich
Desktop Virtual Desktop, BigScreen, Multiscreens, Mure VR, Envelop VR
Ecommerce Trillenium
Education Unimersiv, Lecture VR, IndyLab VR, SuperChem VR (Chemistry Lab)
Exercise VirZoom
Eye examination EyeNetra
Finance QuantVR
Flying drones FLYBi, Ghost 2.0
Gaming Many
Industrial training EON Reality, VR Mobile Crane Simulator
Interior design Planner VR
Journalism Emblematicgroup
Language learning House of Languages
Manufacturing Virtalis
Marketing 10x Army
Mental health CleVR, USC Institute for Creative Technologies
Meditation Zen Zone, Guided Meditation VR
Pain relief DeepStream VR, KindVR
Performance Theatre VR
Public speaking SpeechCenter
Psychedelics
Physical rehabilitation VRecover
Recruitment Wade & Wendy
Simulation
Sports spectating NextVR, Livelike
Sports training STRIVR Labs
Social networking AltspaceVR, High Fidelity, Oculus Social
Surgery training Surgical Theater, Medical Realities
Surgical imaging Block Coronary Artery Surgery
Surveillance
Travel
Vision treatment Vivid Vision

Virtual Reality History timeline

Figure 1. Stereoscopic images (Image: www.vrs.org.uk)
Figure 2. Link Trainer (Image: www.vrs.org.uk)
Figure 3. Sensorama (Image: www.vrs.org.uk)
Figure 4. Virtual Environment Reality workstation technology (Image: www.sciencefocus.com)
Figure 5. VR Arcade Machines (Image: www.vrs.org.uk)

Virtual reality has a long history of development. While the main advancements happened after the introduction of electronics and computer technology, there are precursors to the ideas and implementation of VR that date as far back as the 1800s. For example, focusing solely on VR as a means of creating the illusion of being someplace else, then the earliest attempts at virtual reality could be considered the panoramic murals (or 360-degree murals). These would fill the viewer’s field of vision with the intention of making them feel a sense of presence at a certain historical event or scene [4] [5].

What follows is a timeline of the main historical dates and events in the development of VR.

1838 - Stereoscopic viewers and photos

Charles Wheatstone demonstrated that the brain processes different two-dimensional images for each eye into a single three dimensional object (Figure 1). The stereoscope was invented in the same year and used twin mirrors to project a single image. When viewing two side by side stereoscopic images through a stereoscope, it gave the sense of depth and immersion [4] [5] [6].

In 1839, William Gruber also patented the View-Master stereoscope which was used for “virtual tourism” and still is produced today. The design principles of the stereoscope can still be found in the Google Cardboard and low-budget VR headsets for smartphones [4] [6].

It could be argued that since the creation of stereoscopic images, people have been interested in making images more three dimensional to enrich its experience [6].

1929 - Link Trainer

Edward Link creates the first commercial flight simulator - the Link Trainer (Figure 2). It was entirely electromechanical, “controlled by motors that linked to the rudder and steering column to modify the pitch and roll.” It had a small motor-driven device that simulated turbulence and other disturbances. These flight simulators were used by over 500,000 pilots during World War II for initial training and improving skills [4] [6].

1936 - Pygmalion’s Spectacles

Science fiction writer Stanley G. Weinbaum wrote a short story - Pygmalion’s Spectacles - that had the idea of a pair of goggles that allowed the user to experience a different world through holographic recordings, smell, taste, and touch. This concept can be easily equated to the VR devices that are currently available or under development [4] [6] [7].

1956 - The Sensorama

Cinematographer Morton Heilig develops the Sensorama, which was patented only in 1962 and might be considered the first true VR system. It was an arcade-style cabinet that stimulated all the senses. It had a stereoscopic 3D display, stereo speakers, vibrating seat, fans, and a scent producer. It was intended to fully immerse the person in a film. Heilig created six short films for his invention titled Motorcycle, Belly Dancer, Dune Buggy, Helicopter, A date with Sabina and I’m a coca cola bottle! Heilig intended the Sensorama to be one in a line of products for the “cinema of the future”. Unable to secure financial backing, his vision never became reality [4] [5] [7] [8] [2].

1960 - First VR Head-Mounted Display

After the Sensorama, Morton Heilig invented the first example of a virtual reality headset - the Telesphere Mask. It only worked with non-interactive films and didn’t have motion tracking. Nevertheless, the headset provided stereoscopic 3D and wide vision with stereo sound [4] [5].

1961 - First motion tracking HMD

The true precursor of the HMDs available today was developed by two Philco Corporation engineers, Comeau and Bryan. It was called Headsight and it incorporated a video screen for each eye and a magnetic motion tracking system. This system was linked to a closed circuit camera. The device wasn’t developed for virtual reality applications. Instead, its goal was to allow immersive remote viewing of dangerous situations by the military. The head movements of the used would be replicated by a remote camera, allowing him to look around the environment. While the Headsight was a step in the evolution of the virtual reality headset, it lacked the integration of a computer and image generation [4].

1965 - The Ultimate Display

Ivan Sutherland developed the concept of the “Ultimate Display”. This device could simulate the natural world so realistically that a user could not tell the difference between actual reality and virtual reality. The concept comprised of a virtual world viewed through an HMD and had augmented 3D sound and tactile feedback; computer hardware that created the virtual environment and maintained it in real time; and interactivity between users and objects from the VR world in a realistic way. Sutherland suggested that the device would serve as a “windows into a virtual world”, and his idea would become a core blueprint for the concepts that encompass current VR [4] [5] [2].

1968 - Sword of Damocles

Ivan Sutherland and Bob Sproull created the Sword of Damocles, an HMD that was held by a mechanical arm mounted on a ceiling. The device was connected to a computer and displayed simple wireframe graphics to the user. The arm tracked the user’s head movements but was difficult to use. The contraption was also too heavy and bulky for comfortable use [4] [7] [2].

1969 - Artificial Reality

Myron Kruegere developed a series of experiences called “Artificial Reality”. He developed computer-generated environments that responded to the people in it. He created several projects such as Glowflow, Metaplay, and Psychic Space leading to the development of the Videoplace technology. This enabled communication between people at a distance in a responsive computer-generated environment [4].

1975 - Videoplace

Myron Kruegere created the Videoplace, which was the first interactive VR platform. The virtual reality surrounded the user and responded to movements and actions without the use of goggles or gloves. The Videoplace was a mix of several other artificial reality systems that he had developed [2] [9].

1982 - Sayre gloves

The Sayre glove was the first wired glove. It was invented by Daniel J. Sandin and Thomas Defanti from an idea by Richard Sayre. Both scientists were from the Electronic Visualization Laboratory at the University of Illinois, Chicago. The glove used light emitters and photocells in the fingers. When flexed, the quantity of light reaching the photocell changed, translating the finger movements into electrical signals [7].

1985 - NASA project

The Virtual Environment Workstation Project at NASA’s Ames Research Center in Mountain View, California, was founded with the purpose of producing a VR system that allowed astronauts to control robots outside a space station (Figure 4). The HMD that was developed had super-wide optics (almost an 180-degree field of view) [7].

1987 - The “Virtual Reality” name is coined

Before this date, even though there had been developments in VR, there wasn’t a term to describe the field. In 1987, Jaron Lanier (founder of the Visual Programming Lab, VPL) finally coined the term “virtual reality”. Lanier, through his company, developed a range of VR gear like the Dataglove and the EyePhone headset. The company also made the first surgical simulator, the first vehicle prototyping simulator, and the first architecture simulators [4] [5] [7].

1991 - Virtuality Group

By this time, VR devices started to be available to the public (although owning cutting-edge VR was still out of reach). The Virtuality Group launched several arcade games and machines in which players would use a set of VR goggles (Figure 5). The machines had immersive stereoscopic 3D visuals, handheld joysticks, and some unit were networked together for multiplayer gaming. There were some discussions about bringing Virtuality to Atari’s Jaguar console, but the idea was abandoned [4] [7].

1993 - Sega’s virtual reality headset

At the Consumer Electronics Show in 1993, Sega announced a virtual reality headset for the Sega Genesis console. The prototype had head tracking, stereo sound and LCD screens in the visor. The company intended to have a general release of the product but technical difficulties stopped that from happening and the headset would remain in the prototype phase [4] [7].

1995 - Nintendo Virtual Boy

The Virtual Boy was a 3D gaming console, marketed as the first portable console that could display 3D graphics. It was released in Japan and North America, and it was a commercial failure for the Japanese company. Some of the reasons for the failure were the lack of color in graphics (only red and black), lack of software support, and difficulty in using the console in a comfortable position. Production of the console was halted in 1996 [4] [7].

Virtual reality in the 21st century

After 1997, the public interest in VR saw a decrease. Nevertheless, the first fifteen years of the 21st century had several advancements in the field of virtual reality. Computer technology, including small and powerful mobile technologies, increased in power while prices were getting more accessible [4] [7]. The interest in VR regained momentum after Palmer Luckey created the first prototype of the Oculus Rift, in 2011, and launched a kickstarter campaign for its development in 2012. The campaign was successful, raising $2.5 million. In March 2014, Facebook bought the company Oculus VR for $2 billion dollars. After this, virtual reality blew up, with multiple companies investing in the development of their own VR systems. The rise of smartphones with high-density displays and 3D capabilities has also enabled the development of lightweight and practical VR devices [4] [8] [9].

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Bierbaum, A.D. (2000). VR Juggler: A Virtual Platform for Virtual Reality Application Development. Masters of Science Thesis, Iowa State University, Iowa
  2. 2.0 2.1 2.2 2.3 2.4 2.5 Mazuryk, T. and Gervautz, M. (1996). Virtual Reality - History, applications, technology and Future (Technical Report). Retrieved from https://www.cg.tuwien.ac.at/research/publications/1996/mazuryk-1996-VRH/TR-186-2-96-06Paper.pdf
  3. http://doc-ok.org/?p=1360
  4. 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 4.12 4.13 4.14 4.15 4.16 Virtual Reality Society. History of Virtual Reality. Retrieved from https://www.vrs.org.uk/virtual-reality/history.html
  5. 5.0 5.1 5.2 5.3 5.4 5.5 The Franklin Institute. History of Virtual Reality. Retrieved from https://www.fi.edu/virtual-reality/history-of-virtual-reality
  6. 6.0 6.1 6.2 6.3 6.4 Gemsense. Virtual Reality: History, projections and developments. Retrieved from http://gemsense.cool/virtual-reality-developments/
  7. 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 Evenden, I. (2016). The history of virtual reality. Retrieved from http://www.sciencefocus.com/article/history-of-virtual-reality
  8. 8.0 8.1 Robertson, A. and Zelenko, M. Voices from a virtual past. Retrieved from https://www.theverge.com/a/virtual-reality/oral_history
  9. 9.0 9.1 Freefly VR. Time travel through virtual reality. Retrieved from https://freeflyvr.com/time-travel-through-virtual-reality/