The purpose of this study was to evaluate the effects of luminance on
visual acuity (VA) and small letter contrast sensitivity (SLCS). Compu
ter-generated letter charts were used to measure VA and SLCS [6/7.5 (2
0/25) Snellen equivalent] as a function of stimulus luminance. Letter
size (VA) and contrast (SLCS) were varied in equal logarithmic steps,
making the task and scoring procedure comparable for the two types of
measurement. Both VA and SLCS decreased with decreasing luminance, but
the effect was far greater in the contrast domain. Reducing luminance
from 116 cd/m2 to 0.23 cd/m2 produced a 3 x reduction in VA, but a 17
x reduction in SLCS. The greater sensitivity of SLCS to luminance end
ured even after correction for greater measurement variability. SLCS i
s a sensitive approach for detecting resolution loss undisclosed by st
andard measures of VA. It may be useful for monitoring visual loss fro
m light attenuation in early cataracts, and for detecting subtle resol
ution loss from neural or pathologic factors in ocular and neuro-ophth
almologic disease.