Effects of luminance and exposure time on contrast sensitivity in spatial noise

Using gratings with and without spatial noise we measured r.m.s. contrast sensitivity as a function of (i) retinal illuminance, (ii) amount of light added onto the screen, and (iii) exposure time. Our experiments showed that contrast sensitivity in external spatial noise was independent of decreasing retinal illuminance and exposure time as long as contrast sensitivity was lower with noise than without. Thereafter the decrease of contrast sensitivity was identical with and without noise. Although contrast sensitivity without external spatial noise was independent of the amount of added light, contrast sensitivity in external spatial noise was found to increase with the amount of added light until it reached the sensitivity measured without noise. Thereafter contrast sensitivities with and without noise were identical. Our results (i) provide experimental evidence for the validity of the generally accepted hypothesis that in the human contrast detection mechanism signal-to-noise ratio is constant at threshold and (ii) indicate that external spatial noise is the principal source of noise when it reduces contrast sensitivity.