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J.C. Rabin, S. Aljarudi, Y. Liu, T. Nkadi, S. Tsai; A Re–Examination of the Impact of Quantal Fluctuations on Visual Performance . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3612.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The importance of night vision for the military, law enforcement, and driver safety, coupled with advances in wavefront sensing and refractive surgery, have revitalized interest in factors limiting vision at low luminance. It is classically assummed that contrast threshold is relatively constant over much of the luminance range (Weber’s Law; WL), while stimulus variability (quantal fluctuations; QF) limits detection at low light levels. Our purpose is to re–examine the luminance range over which QF limit visual performance and how this relates to target detail. Methods: The stimulus was log visual acuity (VA) and contrast sensitivity (CS) letter charts back–illuminated in a fluorescent light box in a dark room. Neutral filters before the letter charts were used to assess VA and CS at luminances ranging from 0.1 to 120 cd/m2 in 0.4 log steps. CS was tested at 4m, 2m and 1m (20/50, 20/100 and 20/200 letter size). In control experiments CS was assessed with sine wave gratings across the same luminance range and with artificial pupils to minimize optical factors. Five visually normal trained observers participated. Results: In contrast to WL which predicts that CS is constant with luminance, both VA and CS (20/50; 12 cycles/deg) varied systematically across photopic and mesopic luminance ranges (0.1 to 50 cd/m2; r2=0.98, p<0.001). CS varied linearly with the square root of luminance (slope=0.5, log–log scale), a result predicted by QF (Barlow, 1958). Testing with optimized pupil size indicated that the decrease in performance was not attributable to optical factors. As letter size was increased (20/100 and 20/200), the slope of log CS vs. log luminance became non–monotonic (slope=0.5 at low luminance; slope=0 at high luminance). High spatial frequency (SF) sine wave grating CS (12 cycles/deg) also varied with the square root of luminance for small (0.5 deg) and large (5 deg) fields, indicating that this relation is independent of field size for high SFs. Low SF CS (2 cycles/deg) remained constant with changes in luminance. Conclusions: High SF CS varies with the square root of luminance across the mesopic and photopic range, a finding best explained by the quantal nature of light. Thus stimulus factors impose a major limitation on target detection at decreased luminance. Because higher order aberrations are most detrimetnal when pupil size is large (e.g., at low luminance), QF may limit the degree of vision improvement achieved by correcting aberrations with wavefront procedures and adaptive optics. Barlow HB (1958) J Physiol 141:337–350.
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