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William H. Seiple, Edward Pei, Vernon Odom, Bruce Rosenthal; The Role of Lighthing and Contrast in Low Vision. Invest. Ophthalmol. Vis. Sci. 2011;52(14):1889.
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© ARVO (1962-2015); The Authors (2016-present)
Lighting and contrast have long been the interventions of choice for patients with low vision. Is common wisdom that increasing the lighting level increases contrast and improves reading performance? We questioned this wisdom based upon known physical properties and psychophysical contrast and luminance gain mechanisms.
We measured reading speeds for printed sentences (one sentence per page) under four levels of attenuation, ranging from 0 log units to 1.8 log units. For each light level, five sentence were read at a given font size (ranging in size from an equivalent of 20/20 to an equivalent of 20/160). We also measured reading speeds to sentences of varying contrast levels from 3.4% to 94% under the four luminance conditions. Two font sizes were used in the contrast experiments (equivalent to 20/40 and 20/100).
Five normally sighted subjects and five patients with low vision due to macular disease were tested. Reading speeds from the five sentences at each condition were averaged and plotted as a function of size or contrast. Reading speed as a function of size was best fit by a saturating function. Reading speed increased with increasing size, but reading speed saturated near a size of 20/40 for the normally sighted subjects. Although reading speeds decreased as a function of luminance level for the smallest text size (20/20), the point of saturation did not change with luminance. That is, reading speed for text sizes at and larger than 20/40 did not increase with increasing luminance. Reading speed as a function of contrast also had saturating function. Similar to the acuity data, luminance only had an influence on reading speed only at the lower contrast levels. At contrasts above 20%, luminance had no influence on reading speed for either font size. Patients showed similar saturating function, with the horizontal position of their data shift corresponding to their acuity and/or contrast threshold elevation.
Increases in reading speed with increasing luminance were only observed for text near a subject’s acuity level. For text of ≥ 0.6 dB above a subject’s acuity threshold, increasing light level did not change reading speed. Therefore, magnification aids would be predicted to have a greater influence on reading speed than lighting alone. For contrast, increasing luminance does not change the physical contrast of the text. However, we found no deficits in performance at supra-threshold contrast levels over the range of luminances used in our study.
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