The effect of surround luminance on measurements of contrast sensitivity

We investigated how the luminance of an immediate surround to a visual targ et affects the measurement of contrast sensitivity. We examined four young, healthy, subjects under typical lighting conditions for each test but vari ed the surround luminance systematically by controlling a backlit surround. The effect of varying the surround on contrast sensitivity measurements ma de with a Pelli-Robson chart was minimal. The only statistically significan t difference was between measurements made with a 900 cd.m(-2) surround com pared to those made with 5.6, 9, or 30 cd.m(-2) surrounds when tested at a viewing distance of 4 m. The higher luminance surround produced higher leve ls of contrast sensitivity. The effect of the surround luminance was greate r when measuring contrast sensitivity to sinewave gratings using a method o f constant stimuli. Optimum performance was found using a surround luminanc e of 9 cd.m(-2) (20% of the mean target luminance). Statistically, a wide r ange of surround luminances (3-30 cd.m(-2), or 7-67% mean target luminance) gave similar levels of visual performance, but higher luminance surrounds led to reduced contrast sensitivity. Possible reasons for the difference in the effect of the surround depending upon the test are discussed. Ophthalm ic practitioners using contrast sensitivity measurements with letter charts probably need not make special efforts to illuminate the test surround app ropriately. However, practitioners using sinewave gratings, especially comp uter generated sinewave gratings, to measure contrast sensitivity, may have their measurements affected by inappropriately lit task surrounds. (C) 199 9 The College of Optometrists. Published by Elsevier Science Ltd. All right s reserved.