The vergence-accommodation conflict as it can occur in virtual reality
Vergence-accommodation conflict (VAC), also known as accommodation-vergence conflict, is a visual phenomenon that occurs when the brain receives mismatching cues between vergence and accommodation of the eye. This commonly occurs in virtual reality devices, augmented reality devices, 3D movies, and other types of stereoscopic displays and autostereoscopic displays. The effect can be unpleasant and cause eye strain.
Two main ocular responses can be distinguished: vergence of eyes, and accommodation. Both of these mechanisms are crucial in stereoscopic vision. Vergence or independent inward/outward rotation of eyes is engaged to fixate on objects and perceive them as single. Incorrect vergence response can cause double vision. Accommodation is the eye’s focusing mechanism and it is engaged to produce a sharp image on a retina. Both of these mechanisms are neurally linked forming the accommodation-convergence reflex[1] of eyes. One can distinguish vergence distance—a distance of a point towards which both eyes are converging, and an accommodation distance—a distance of a region in space towards which the focus or refractive power of the crystalline lens has been adjusted to produce a sharp image on the retina.
In normal conditions the human visual system expects vergence and accommodation distances to match. When viewing most artificial 3D images or displays, vergence and accommodation distances for the most part are mismatched. The human visual system has not evolved to view these types of artificial 3D images comfortably, so VAC can be a very unpleasant sensation for the viewer.[2][3][4]
VAC is often encountered when viewing stereograms, 3D movies, or virtual reality (VR). It can cause visual fatigue and headaches after a short period of time; It is one of the main contributors to virtual reality sickness. The phenomenon can make it impossible to focus on objects close to the eye in VR, limiting the development of VR software.[5]
VAC is very difficult to overcome when designing new types of 3D displays.[5]
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People playing 3D video games have often reported eye strain afterward, or that the 3D effect is disorienting. This is because of VAC.[6] 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.[7]
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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.[8] 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.
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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.[9][10][11][12] It often causes headaches and visual fatigue.[13] The vergence-accommodation conflict is one of the main causes of virtual reality sickness.[14]
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 [8] 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.[12] Meta Half Dome prototypes addressed the problem with variable focus lenses that matched focal depth to vergence stereoscopic depth.[15] 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.[16] There are currently no production products using this technology.