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Johnell O. Brooks, Richard A. Tyrrell, Joanne M. Wood, Benjamin R. Stephens, Efty P. Stavrou; Comparing estimated and actual visual acuity at high and low luminance. Journal of Vision 2005;5(8):173. doi: 10.1167/5.8.173.
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© ARVO (1962-2015); The Authors (2016-present)
How well do we know our own visual limitations? To evaluate whether observers appreciate the extent to which low luminance and low contrast affect their own visual acuity, we compared observers' estimates of their own visual acuity with their actual visual acuity. Estimates of acuity were measured using newly refined methodologies. Twenty observers (M = 69 years) were given systematic training in reporting their size estimates using both a magnitude estimation (verbal) technique and a manual matching technique using calipers (manual). The stimuli used during training were Es ranging in size from the observers' threshold to logMAR 1.4 (21.9 cm high) viewed at high luminance (85 cd/m2) at 6 m. Following training, observers dark adapted and then made acuity estimates while wearing goggles containing ND filters (ND 4, 3, 2, 1, or 0) while sitting 6 m from a blank white wall. At each luminance level, they indicated the height of stimuli that they imagined they would be “just able to see as an E” on the wall. Because there was no reliable difference between the verbal and manual acuity estimates for either high or low contrast stimuli, these measures were averaged and then compared with measures of actual acuity. At the higher luminances, the observers' mean high contrast acuity estimates were quite accurate. At lower luminances, however, observers dramatically overestimated their high contrast acuity. At the lowest luminance, the mean estimated high contrast acuity (20/55) was 0.75 log units better than the actual mean acuity (20/300). This trend to overestimate acuity at low luminances was even stronger for low contrast stimuli, where the mean estimated acuity (20/90) was 0.95 log unit better than the actual mean acuity (20/800). We conclude that observers fail to appreciate the extent to which their own acuity is degraded as luminance decreases. This may help explain why drivers are comfortable overdriving their headlights at night.
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