Abstract
In the real world, the sense of sight is dominant for humans, and having a normal vision is essential to perform well in many common tasks. There, ophthalmology has several tools to assess and correct a person’s vision. In VR, when wearing an HMD, even a user with normal vision is challenged by additional hurdles that affect the virtual environment’s perceptual acuity, negatively impacting their performance in the application task. Display resolution, but also soiled lenses and bad vergence adjustment are examples of possible issues. To better understand and tackle this problem, we provide a study on assessing visual acuity in a VR setup. We conducted an experimental evaluation with users and found out, among other results, that visual acuity in VR is significantly and considerably lower than in real environments. Besides, we found several correlations of the measured acuity and task performance with difficulty adjusting the HMD and use of prescription glasses.
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Notes
- 1.
Reference removed for blind review.
- 2.
1 Paris foot is equivalent to 324.8393 mm. 1 Paris foot = 1.06575 feet.
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Acknowledgements
This work was funded by the Brazilian funding agencies Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) - Finance Code 001, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) project 311251/2020-0, and FAPERGS PqG 17/2551-0001192-9.
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Appendices
Appendix 1 – Background on Visual Acuity in Ophthalmology
Visual acuity refers to the clarity of vision and the ability to distinguish details in objects. Anatomically, it is the ability of the eye to focus the image on the retina [30]. It is also the capability of the eye to distinguish small details appearing on the visual field at a specified distance [23]. Acuity can also be split into two types: static, when the object is perceived stationary; dynamic, when the observer, the object or both are in motion [27].
Standard objects used to assess acuity are often called optotypes. The most common set of optotypes used to measure static VA are the Snellen chart and the Landolt C, also known as a Landolt ring. Both were created more than 100 years ago. There are also other more recent optotypes in use today [8, 26]. We present them and their different uses for visual acuity assessment in the next subsections.
1.1 Snellen-Type Optotype
Herman Snellen, in 1862 [29] created a Table 1 composed by letters of different sizes (optotypes - Table 1) representing a visual angle of 5 min of arc (5’) at a distance of 5 m. The letters are composed by elements of \(\frac{1}{5}\) of this measure.
In his study [29], Snellen specifies the dimensions of the characters and the spaces that separate them. The visual acuity (V) is the maximum distance at which the optotype is recognized (d) divided by the distance at which it should be to form an angle of 5 arc-minutes (D) [29] as in Eq. 1:
If d and D are equal and the optotype is visible at 20 Paris feetFootnote 2, then \(V=\frac{20}{20}=1\) is defined as a normal visual acuity.
In the Snellen proposal, the minimum resolution angle is 1 arc-minute, as seen in Fig. 3. To determine the size of an optotype in the Snellen chart, the formula of Eq. 2 is used:
where H is the height of the optotype (in mm), D is the presentation distance (in meters), V is the visual acuity (in tenths) and the constant 14.6 represents the tangent of 5 multiplied by 10,000 to compensate for the use of millimeters and tenths in the other components.
In a Snellen chart, some letters are more easily readable than others and each row has a different number of letters. This causes the phenomena of non-proportional grouping and spacing between letters and rows, making reliability and reproducibility of using a Snellen chart low. Nevertheless, it is widely used and universally accepted.
1.2 Pelli-Robson Contrast Sensitivity
Besides the high-contrast VA measurement (black optotypes on a white background) provided by Snellen charts, other contrast levels can also be used to obtain a second measure of acuity. The principle is to use gray optotypes on the same white background, showing successively lighter and lighter grays.
Contrast is defined as the relative difference of luminance between a target and the background. The whole human visual system (HVS) is involved in object detection, meaning that while the eyes capture and convert light into electric signals, the brain processes and makes the decisions about the visual perception of objects [1, 32]. Contrast is used to determine what is detectable by the HVS. The objects are visible if they have a contrast greater than the contrast sensitivity (CS) [16, 32], which is defined as the minimum contrast necessary to detect a grid in some specified spatial frequency
CS was first measured in 1889 [28], but its value was recognized only after Bodis-Wolner work in 1972 [4].
Pelli et al. [25, 26] first proposed a chart with variable contrast letters sized at half a degree that can measure the CS of an individual with spatial frequencies between 3 and 5 c/deg. That is the best interval to determine whether an individual has a loss of sensitivity in the spatial frequency. Later, they came up with a new chart with single sized letters that change in contrast at each row to obtain information about the contrast sensitivity of any individual. Hence, they created a model that allows to choose the best parameters to accurately maximize the measurements provided by the test [25, 26].
The most widely used chart presents a set of Sloan font letters with size of \(0.5^{\circ }\) at a distance of 3 m, although it can be used at shorter distances to assess individuals with subnormal vision. The chart is read from left to right, from top to bottom. Each row contains two groups of three letters. The letters within each group have the same contrast, while each successive group has lower contrast than the previous one. As seen in Fig. 4, there is a total of 48 optotypes on a white background, divided in 16 groups. The first group is black (contrast is \(100\%\)), and each subsequent group has a contrast reduction factor of 0.707 (0.15 log units). Thus, the contrast of the last group is \(0.56\%\) (2.25 log units below 100% [35].
The Pelli-Robson chart is considered a suitable technique to asses the visual function [19].
1.3 Glare and Disability Glare
Glare is a light phenomenon that causes difficulty, and may even disable, viewing of an object due to very bright light of artificial or natural origin. The light scatters in opacified regions of the eye capsule, causing ofuscating bright regions to appear in the field of view. Cataract is the most associated condition with glare testing. While most vision quality analyses are performed in a fixed viewing position and direction, glare depends on the viewing position and direction within a space [3], in such a way that specific central and peripheral glare tests are used. Lacava [18] concluded that the glare test associated with the contrast sensitivity test shows that the visual acuity provided by the Snellen Table does not correspond to everyday vision. Although the measurement of visual acuity using contrast sensitivity is not unanimous, it is considered more informative than the measurement of visual acuity using onkly the Snellen chart [35].
Hoskins [13] states that glare testing and contrast sensitivity play a role in quantifying or describing visual impairment in some patients.
1.4 Luminance, Contrast, Resolution and Field of View
In modern optics, the ability of the eye to resolve a line pair is one of many ways to determines the human eye-plus-brain acuity. This acuity is measured in the fovea zone as \(1/a=1.7\), where a is the number of arc minutes of field of view necessary to discriminate the two lines. This is roughly \(0.59'\) (arc minutes), or \(171.62\,{\upmu }\)rad microradian. As two pixels are necessary to see the two lines, it is said that the resolution of the human eye in good light conditions is about \(1'\) arc minute (\(290.89\,{\upmu }\)rad microradians. Outside the fovea zone, the resolution of the eye decreases considerably [29], so a moderate variation in contrast or illumination will reflect very little on the person’s visual acuity. Visual perception is rather influenced by the difference in intensity between the object and the background (contrast), the spatial frequency (inverse of the line thickness in regular optotypes) and the area of the object. As for the field of view, there is no consensus and it varies among people, but it is accepted that it is somewhere above \(180^\circ \) horizontally, limited at \(220^\circ \). The binocuar vision is, in turn, limited to the central \(120^\circ \) of the total field of view (FoV).
When referring to displays, the term resolution is often used to mean either display pixel pitch or pixel count, which may be confusing. When referring to HMDs, resolution more accurately refers to cycles (or lines) per unit angle that can be resolved [34], as seen above for the human vision. Typical VR optics have a focal length of about 40 mm [9], which amplifies the pixel size. So, HMDs use a larger amount of smaller pixels when compared to screens to try and increase both the perceived angular resolution and the FoV. An estimation is that to provide 60 pixels per degree (1 pixel per arc minute) or Snellen acuity of 20/20 for a FoV of \(150^\circ \), an HMD would require \(9600\times 9000\) pixels per eye [34].
Besides resolution, the luminance and contrast provided by a display are other items that could impact acuity. Luminance is the amount of visible light emitted per unit projected area of the display. It is relative to the amount of light emitted by the display system being expressed in candelas per squared meter (\(cd/m^2\)) [15]. Contrast, on the other hand, is the ratio between the highest and lowest luminance provided by the display.
Luminance is sometimes confused with brightness. In the real world, it can reach much higher values than in display systems, such as \(1.6\times 10^9\) cd/m\(^2\) for the sun at noon versus 50–300 cd/m\(^2\) at a maximum resolution on a computer monitor [15].
The technology used in today’s HMDs construction is based on two approaches [2]. The first, similar to the display of smartphones, televisions and computer monitors, is based on liquid crystals (LCD - Liquid-Crystal Displays), while the other is based on OLED (Organic Light-Emitting Diode). These technologies allow for different ranges in terms of luminance, color, contrast, refresh rate, etc., which combined with optical lenses and design decisions compose the final experience and acuity of these displays. In Table 2 we present a comparison of the technical specifications of some popular HMDs.
Appendix 2 – Virtual Charts and Optotypes Used
Appendix 3 – Raw Data from Questionnaires
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da Fontoura, V.S., Maciel, A. (2021). Characterizing Visual Acuity in the Use of Head Mounted Displays. In: Magnenat-Thalmann, N., et al. Advances in Computer Graphics. CGI 2021. Lecture Notes in Computer Science(), vol 13002. Springer, Cham. https://doi.org/10.1007/978-3-030-89029-2_44
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