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Amy Yuan, Michael Ehrlich; A model to predict Snellen visual acuity in patients with a common presenting complaint. Invest. Ophthalmol. Vis. Sci. 2016;57(12):211.
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© ARVO (1962-2015); The Authors (2016-present)
Patients commonly present to general ophthalmology clinic with a chief complaint of decreased visual acuity, specifically manifested as being unable to see the “horizontal scrolling bar at the bottom of the television screen”. In this study, we aimed to approximate how this complaint translated into a Snellen visual acuity.
Snellen acuity x/y is measured with optotypes constructed so that numerator x denotes the testing distance, and denominator y denotes the distance at which the letter subtends 5 arcmins. In the patient who is unable to see his television’s scrolling bar, we need only know the size of the TV, the distance that the patient sits from his TV, and the height of the bar. Suppose the television has diagonal size d with aspect ratio a:b, and the patient is sitting distance x away, watching a channel with a scrolling bar whose height is c% of the television’s height. The absolute height of the scrolling bar is determined, from which we compute y by simple trigonometric relationships (example in Figure 1), to arrive at Snellen acuity x/y.
We generated a mapping from TV size and viewing distance to Snellen acuity (Figure 2), and produced a table of visual acuity associated with a range of common TV sizes and preferred viewing distances for clinical use. These results make assumptions on television aspect ratio, size of scrolling bar, position of scrolling bar, and visual axis relative to screen. Based on this model, expected visual acuity corresponding to this chief complaint ranged from 20/100 – 20/130. The model does not account for the effects of contrast and motion of the text, and likely underestimates true visual acuity.
We created a model to estimate Snellen visual acuity for a common complaint of decreased vision. We also discuss factors that may cause deviation of measured from predicted results.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
Figure 1 shows a sample calculation of visual acuity for a patient unable to read the scrolling bar on a 27 inch TV while sitting 5 feet away. With our model, this patient would have predicted acuity of 20/160.
Figure 2 shows the relationship between Snellen acuity and distance from the TV as a multiple of the TV diagonal. Studies on TV viewing distance for medium sized screens suggest a preferred distance of 2.5-3.5x the TV diagonal, which corresponds to a visual acuity of 20/100 – 20/130 (inset).
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