Vergence-accommodation conflict

People playing 3D video games have often reported eye strain afterward, or that the 3D effect is disorienting. This is because of VAC.{{Cite web |title=Is it normal the 3DS effect hurt my eyes? - Nintendo 3DS |url=https://gamefaqs.gamespot.com/boards/997614-nintendo-3ds/72524629 |access-date=2022-09-26 |website=gamefaqs.gamespot.com}} There is not a strong consensus on the extent of visual damage, if any, that may occur due to overexposure to VAC. Even though this is the case, users of classic stereoscopic devices report being unable to look at the 3D screen for a long period of time.{{Cite web |title=Is Your Nintendo 3DS Bad for Your Health? |url=https://www.pcmag.com/archive/is-your-nintendo-3ds-bad-for-your-health-262584 |access-date=2022-09-26 |website=PCMAG |language=en}}

Vergence-accommodation conflict can be quantified; typically, by comparing the optical power required to focus on objects at the vergence distance with the optical power required to focus on objects at the accommodation distance.{{Cite journal |last1=Shibata |first1=Takashi |last2=Kim |first2=Joohwan |last3=Hoffman |first3=David M. |last4=Banks |first4=Martin S. |date=2011-07-05 |title=The zone of comfort: Predicting visual discomfort with stereo displays |url=https://jov.arvojournals.org/article.aspx?articleid=2121032 |journal=Journal of Vision |language=en |volume=11 |issue=8 |pages=11 |doi=10.1167/11.8.11 |pmid=21778252 |pmc=3369815 |issn=1534-7362}} In this context, optical power is equal to the reciprocal of distance, with units of Diopter (m⁻¹). Hence the difference between the reciprocal of the vergence distance and the reciprocal of the accommodation distance characterizes the extent of VAC.

In the example of a virtual reality head-mounted display, the accommodation distance corresponds to the distance of the virtual image plane. Often the optics is designed to place a virtual screen somewhere between 2 meters and infinity. That is, for a virtual display at a 2-meter distance, the target accommodation distance expressed in diopters would be 0.5 D. In contrast, the vergence distance in a stereoscopic display can change freely based on the location of target content. For example, a virtual object by means of binocular disparity can be placed at a 30 cm distance, corresponding to 3.33 Diopters. In such a case, the magnitude of the VAC for a person with normal vision would be 3.33-0.5=2.83 diopters.

The vergence-accommodation conflict is caused due to factors in human physiology like the accommodation reflex. VAC occurs when the human brain receives mismatching cues between vergence and accommodation.{{Cite web |date=2022-09-22 |title=Resolving the Vergence-Accommodation Conflict in Head-Mounted Displays |url=https://3dvar.com/Kramida2016Resolving.pdf |access-date=2022-09-22 |archive-url=https://web.archive.org/web/20220922032435/https://3dvar.com/Kramida2016Resolving.pdf |archive-date=2022-09-22 |url-status=live}}{{Cite journal |last1=Zhou |first1=Yao |last2=Zhang |first2=Jufan |last3=Fang |first3=Fengzhou |date=2021-12-01 |title=Vergence-accommodation conflict in optical see-through display: review and prospect |journal=Results in Optics |language=en |volume=5 |pages=100160 |doi=10.1016/j.rio.2021.100160 |s2cid=241361232 |issn=2666-9501|doi-access=free |bibcode=2021ResOp...500160Z }}{{Cite journal |last=Konrad |first=Robert |date=2015-11-06 |title=What is the vergence-accommodation conflict and how do we fix it? |url=https://doi.org/10.1145/2810048 |journal=XRDS: Crossroads, the ACM Magazine for Students |volume=22 |issue=1 |pages=52–55 |doi=10.1145/2810048 |s2cid=9330080 |issn=1528-4972|url-access=subscription }}{{Cite web |date=2014-11-06 |title=3D technologies and eyesight: use not recommended for children under the age of six, use in moderation for those under the age of 13 |url=https://www.anses.fr/en/content/3d-technologies-and-eyesight-use-not-recommended-children-under-age-six-use-moderation-0 |access-date=2022-09-22 |website=Anses |language=en}} It often causes headaches and visual fatigue.{{Cite journal |last1=Hoffman |first1=David M. |last2=Girshick |first2=Ahna R. |last3=Akeley |first3=Kurt |last4=Banks |first4=Martin S. |date=2008-03-01 |title=Vergence–accommodation conflicts hinder visual performance and cause visual fatigue |url=https://jov.arvojournals.org/article.aspx?articleid=2122611 |journal=Journal of Vision |language=en |volume=8 |issue=3 |pages=33.1–30 |doi=10.1167/8.3.33 |issn=1534-7362 |pmc=2879326 |pmid=18484839}} The vergence-accommodation conflict is one of the main causes of virtual reality sickness.{{Cite journal |last1=Lawson |first1=Ben D. |last2=Stanney |first2=Kay M. |date=2021 |title=Editorial: Cybersickness in Virtual Reality and Augmented Reality |journal=Frontiers in Virtual Reality |volume=2 |doi=10.3389/frvir.2021.759682 |issn=2673-4192|doi-access=free }}

Most people can tolerate some extent of VAC, without noticeable onset of adverse effects. While it depends on a particular person and viewing distance, VAC of around up to 0.4 Diopters is within comfort limits of most people.

The vergence-accommodation conflict can have permanent effects on eyesight. Children under the age of six are recommended to avoid 3D displays that cause VAC. Meta Half Dome prototypes addressed the problem with variable focus lenses that matched focal depth to vergence stereoscopic depth.{{Cite web |last=VR |first=Oculus |title=Introducing the Team Behind Half Dome—Facebook Reality Labs' Varifocal Prototype {{!}} Meta Quest Blog |url=https://www.oculus.com/blog/introducing-the-team-behind-half-dome-facebook-reality-labs-varifocal-prototype/ |access-date=2023-06-13 |website=www.oculus.com |language=en}} The first prototype used bulky mechanical actuators to refocus the lens. The third prototype used a stack of 6 liquid crystal lens layers where each layer could be turned on and off by applying a voltage, and this creates 64 discrete focal planes.{{Cite web |date=2020-07-28 |title=Facebook's Display Research Lead: Varifocal Half-Dome 3 'Almost Ready For Prime Time' |url=https://www.uploadvr.com/half-dome-3-prime-time/ |access-date=2023-06-13 |website=UploadVR |language=en}} There are currently no production products using this technology.

All first-generation VR and augmented reality (AR) headsets are fixed-focus devices that can cause VAC. Popular examples of these devices include the Oculus Quest 2, HTC Vive, Valve Index and the Microsoft HoloLens. VAC can be experienced by bringing a virtual object very close to one's eyes in the headset and trying to focus on it.{{Citation |title=Vergence-Accommodation Conflict: Facebook Research Explains Why Varifocal Matters For Future VR |url=https://www.youtube.com/watch?v=YWA4gVibKJE |language=en |access-date=2022-09-22|publisher=VR Trailers & Clips|date =18 July 2020| via=YouTube|format=Video}}

Not all 3D displays cause the vergence-accommodation conflict. New types of displays are being developed that do not cause VAC, such as holographic displays and light field displays.

VAC can also be experienced when using other technologies, including:

• when viewing a stereogram through a stereoscope{{citation needed|date=September 2022}}
• when viewing an autostereogram using an eye defocusing technique{{citation needed|date=September 2022}}
• when viewing 3D cinema and 3D TV{{Cite web |last=Digital |first=Brandon Costa, Director of |date=2011-10-25 |title=Vergence-Accommodation Conflict: Why Bad 3D Literally Makes You Sick |url=https://www.sportsvideo.org/2011/10/25/why-bad-3d-literally-makes-you-sick/ |access-date=2022-09-22 |website=Sports Video Group |language=en}} (not as pronounced as other kinds of media like VR)
• autostereoscopic displays, such as the one found in the Nintendo 3DS

VR and AR hardware companies often ask software developers not to show virtual content too close to the user in the devices.{{Cite web |title=Comfort - Mixed Reality |url=https://learn.microsoft.com/en-us/windows/mixed-reality/design/comfort |access-date=2022-09-22 |website=learn.microsoft.com |date=19 October 2021 |language=en-us}}{{Cite web |date=2019-08-26 |title=Magic Leap Developer - Game Design: Best Practices |url=https://ml1-developer.magicleap.com/en-us/learn/guides/best-practices-in-game-design |url-status=live |archive-url=https://web.archive.org/web/20220922071934/https://ml1-developer.magicleap.com/en-us/learn/guides/best-practices-in-game-design |archive-date=2022-09-22 |access-date=2022-09-22 |website=Magic Leap Developer}} However, this is only a software mitigation and often times the effect can still be noticed.

In the same way that VR users can become acclimated to motion in VR (through practice and exposure to overcome motion sickness), they can likewise train their pupils to maintain distance focus while slowly bringing a virtual object closer to their face.

The solution to the vergence-accommodation conflict is to avoid looking at anything which causes the phenomenon. In VR and AR, new types of displays have been developed since the 2010s that can minimize or eliminate VAC to non-issue levels. These displays include varifocal, multifocal,{{Cite journal |last1=Zhan |first1=Tao |last2=Xiong |first2=Jianghao |last3=Zou |first3=Junyu |last4=Wu |first4=Shin-Tson |date=2020-03-30 |title=Multifocal displays: review and prospect |journal=PhotoniX |volume=1 |issue=1 |pages=10 |doi=10.1186/s43074-020-00010-0 |s2cid=214754949 |issn=2662-1991|doi-access=free }} holographic, [https://www.youtube.com/watch?v=LowxBE6iKQk pin-mirror] and [https://www.youtube.com/watch?v=p5trOS3NaHw light field displays].{{Citation |title=Douglas Lanman (NVidia) - Light Field Displays at AWE2014 | date=2 June 2014 |url=https://www.youtube.com/watch?v=8hLzESOf8SE |language=en |access-date=2022-09-26}}

Varifocal displays are a concept explored mainly in VR display solutions. The basic principle relies on dynamically adjusting focal distance of displays based on the gaze direction. The technique requires an eye-tracking solution and means of modulating focal distance of a screen. Modulation of a focal distance can be, for example, [https://www.youtube.com/watch?v=JdM5SnSnuyQ&t=4s physical actuation of the screen] in relation to a fixed eyepiece optics, alternatively it can be utilization of varifocal{{Cite journal |doi=10.1364/OE.27.015627 |title=Design and demonstration of a vari-focal optical see-through head-mounted display using freeform Alvarez lenses |year=2019 |last1=Wilson |first1=Austin |last2=Hua |first2=Hong |journal=Optics Express |volume=27 |issue=11 |pages=15627–15637 |pmid=31163757 |bibcode=2019OExpr..2715627W |s2cid=174815078 |hdl=10150/633572 |hdl-access=free }}{{Cite book |last1=Stevens |first1=R. E. |last2=Rhodes |first2=D. P. |last3=Hasnain |first3=A. |last4=Laffont |first4=P.-Y. |title=Digital Optics for Immersive Displays |chapter=Varifocal technologies providing prescription and VAC mitigation in HMDS using Alvarez lenses |editor-first1=Bernard C |editor-first2=Hagen |editor-first3=Wolfgang |editor-last1=Kress |editor-last2=Stolle |editor-last3=Osten |date=2018-05-21 |chapter-url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/10676/106760J/Varifocal-technologies-providing-prescription-and-VAC-mitigation-in-HMDs-using/10.1117/12.2318397.full |publisher=SPIE |volume=10676 |pages=142–158 |doi=10.1117/12.2318397|bibcode=2018SPIE10676E..0JS |isbn=9781510618787 |s2cid=173178593 }} lens element(s). While varifocal approach mitigates or entirely solves VAC, it cannot convey realistic monocular focus cues. To try to add realism, these techniques rely on [https://www.cl.cam.ac.uk/~rkm38/pdfs/march2022_impact_of_focus_cues.pdf image processing techniques to simulate focus cues].    

Multifocal displays are another way of overcoming VAC. The principle of operation relies on availability of multiple image focal planes (screens), which from the perspective of a viewer are available simultaneously at all times. This gives the ability to accommodate eyes within the available range of focal distances and perceive realistic monocular focus (image blur) cues similarly to natural viewing conditions. Essentially multifocal displays discretize the depth dimension and split or slice the 3D content according to the available configuration of depth planes to minimize VAC. The topic of multifocal displays has been generously researched for at least several [http://bankslab.berkeley.edu/publications/Files/stereo_display_prototype04.pdf decades],{{Cite journal |last1=MacKenzie |first1=Kevin J. |last2=Hoffman |first2=David M. |last3=Watt |first3=Simon J. |date=2010-07-01 |title=Accommodation to multiple-focal-plane displays: Implications for improving stereoscopic displays and for accommodation control |url=https://jov.arvojournals.org/article.aspx?articleid=2191682 |journal=Journal of Vision |language=en |volume=10 |issue=8 |pages=22 |doi=10.1167/10.8.22 |pmid=20884597 |issn=1534-7362|doi-access=free }}{{Cite journal |doi=10.1364/AO.39.003209 |title=Multifocal planes head-mounted displays |year=2000 |last1=Rolland |first1=Jannick P. |last2=Krueger |first2=Myron W. |last3=Goon |first3=Alexei |journal=Applied Optics |volume=39 |issue=19 |pages=3209–3215 |pmid=18349886 |bibcode=2000ApOpt..39.3209R |s2cid=16434842 }} nevertheless, there is only a limited offering of commercially available [https://lightspace3d.com/ multifocal] near-eye displays.

Light field displays are one of the best ways to solve the vergence-accommodation conflict. They share features with integral imaging displays.

[http://www.creal.com CREAL], a near-eye display manufacturer for AR headsets/glasses, developed a light field display technology that projects the light rays just like they exist in the real world. This way, the virtual content has a real depth, and each eye can change focus naturally between the virtual objects, from up close to infinity.

SeeReal Technologies, a manufacturer of displays for 3D-enabled mobile devices, claim that their displays can generate visuals that do not have fixed accommodation.{{Cite book|chapter-url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/6803/68030M/Large-holographic-displays-as-an-alternative-to-stereoscopic-displays/10.1117/12.766163.full|chapter=Large holographic displays as an alternative to stereoscopic displays|first1=R.|last1=Häussler|first2=A.|last2=Schwerdtner|first3=N.|last3=Leister|editor-first1=Andrew J |editor-first2=Nicolas S |editor-first3=John O |editor-last1=Woods |editor-last2=Holliman |editor-last3=Merritt |title=Stereoscopic Displays and Applications XIX |date=29 February 2008|publisher=SPIE|volume=6803|pages=219–227|via=www.spiedigitallibrary.org|doi=10.1117/12.766163|s2cid=62176480 }} The company developed the display used in the Takee 1 smartphone.{{Cite web |last=Kano3D |date=2020-12-29 |title=Takee 1: The first Holographic Smartphone gets a new Update after 6 Years! |url=https://www.tridimensional.info/2020/12/takee-1-the-first-holographic-smartphone-gets-a-new-update-after-6-years/ |access-date=2022-09-28 |website=Tridimensional.info |language=en-US}} However, SeeReal's solution requires eye tracking, which can limit the 3D capabilities of the displays such as the field of view of the 3D effect.{{citation needed|date=October 2022}}

• Accommodative convergence
• Binocular vision
• Depth perception
• Health effects of 3D
• Holographic display
• Multiscopy
• Polarized 3D system
• Stereopsis

{{reflist}}

• {{youTube|fNzuzssl4eU|Why 3D sucks - the vergence accommodation conflict}}
• {{youTube|8hLzESOf8SE|Douglas Lanman (NVidia) - Light Field Displays at AWE2014}}

{{Display technology}}

{{Stereoscopy}}

Category:Stereoscopy

Category:Binocular rivalry

Category:Visual disturbances and blindness