Adding stereoscopic information to displays (i.e., presenting slightly different images to the two eyes) yields a compelling 3D sensation, and this has proven popular and useful for medical imaging, cinema, television, and many other applications. However, despite the clear advantages of stereoscopic displays, there is a well-known problem.
In natural viewing, images arrive at the eyes with varying binocular disparity, so as viewers look from one point in the visual scene to another, they must adjust their eyes' vergence. The distance at which the lines of sight intersect is the vergence distance. Failure to converge at that distance results in double images. The viewer also adjusts the focal power of the lens in each eye (i.e., accommodates) appropriately for the fixated part of the scene. The distance to which the eye must be focused is the accommodative distance. Failure to accommodate to that distance results in blurred images. Vergence and accommodation responses are coupled in the brain: specifically, changes in vergence drive changes in accommodation and changes in accommodation drive changes in vergence. Such coupling is advantageous in natural viewing because vergence and accommodative distances are nearly always identical.
In stereoscopic displays, images have varying binocular disparity thereby stimulating changes in vergence as happens in natural viewing. But the accommodative distance remains fixed at the display distance, so the natural correlation between vergence and accommodative distance is disrupted, leading to the so-called vergence–accommodation conflict. The conflict causes several problems. First, differing disparity and focus information cause perceptual distortions.1,2 Second, viewers experience difficulties in simultaneously fusing and focusing a stimulus.2, 3 Finally, attempting to adjust vergence and accommodation separately causes visual discomfort and fatigue in viewers.2, 4,5
In our work, we considered the problem of discomfort, which is probably the most serious of these problems. The optometric literature provides guidelines on the types of vergence–accommodation conflicts that can be managed without discomfort. Those guidelines, which were derived from patients' reports when fitted with new spectacles, are summarized by ‘Percival's zone of comfort.’ There are, however, some important differences between the conflicts that occur with optical correction and with viewing stereoscopic displays, so Percival's zone may not be useful for understanding discomfort associated with stereoscopic viewing in entertainment, communication, and medicine. To find out, we measured viewer discomfort associated with stereoscopic viewing when the vergence–accommodation conflict took on different values. We quantified user discomfort using two questionnaires at the end of the experimental sessions.
We used a custom volumetric display that allows us to manipulate vergence and accommodation cues independently.6 Our results (see Figure 1) show that Percival's zone is a reasonable approximation of the conflicts that can be managed without discomfort for viewers of stereoscopic displays.5
Figure 1. The comfort zone plotted in (A) diopters and (B) meters. Viewing distance corresponds to the accommodative stimulus. The black diagonal line represents natural viewing where vergence and accommodative distances are the same. The red and blue lines represent estimates from our data of the far and near boundaries of the comfort zone, respectively. The dashed horizontal lines represent typical viewing distances for mobile devices, desktop displays, television, and cinema. Notice that longer viewing distances yield a larger range of comfortable vergence distances when expressed in meters.
We also found that an individual's phoria (an optometric concept concerning how accommodation drives vergence) is predictive of the conflicts that cause discomfort and the ones that do not. Phoria falls into three categories. One is orthophoria, in which the person's vergence and accommodation naturally go to the same distance. A second is esophoria, where the person tends to converge closer than he/she accommodates and therefore has to combat that tendency in natural viewing. The third category is exophoria, in which the person tends to converge farther than he/she accommodates and therefore has to work against that tendency in natural viewing. We found, perhaps unsurprisingly, that people with esophoria experience more discomfort when the stereo content is behind the display screen (near the red lines in Figure 1) and that people with exophoria experience more discomfort when the stereo content is in front of the screen (near the blue lines in Figure 1). Others have found that discomfort from viewing stereo 3D display starts to diminish in middle age.7
One can use our data to construct guidelines for comfortable viewing of stereoscopic media. Those guidelines would differ according to display size, viewing distance, and of course the viewer's phoria andage. Having shown that vergence–accommodation conflict in stereo 3D viewing causes viewer discomfort and fatigue, we are now developing technology that minimizes the conflict and, therefore, should mitigate discomfort. We are also examining other causes of viewer discomfort, such as the effect of tilting the head to the side while viewing stereoscopic imagery.
University of California, Berkeley
Martin Banks is professor of optometry and vision science and affiliate professor of psychology and bioengineering.
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