Mesopic lightness, brightness, and brightness contrast

Abstract

At mesopic mean luminances a fixed physical contrast produces less brightness contrast than at photopic mean luminances.1 This suggests that lightnesses of surfaces might also be altered at low luminances. I measured lightness, brightness, and brightness contrast in cathode-ray tube (CRT) simulations of achromatic paper patchworks. The illuminance of the standard pattern was fixed, producing 0.3, 3, or 30 cd/m2. The illuminance on the test pattern was varied from trial to trial in a lightness-constancy paradigm. Contrast efficiency declined at low mean luminances, i.e., constant brightness contrast required more luminance contrast at mesopic luminances than at photopic luminances. However, mean luminance had only a minor effect on surface lightnesses. Minor failures of lightness constancy occurred at the lowest mean luminances, but they were proportionately smaller than the loss of brightness contrast in the same pattern. These results have implications for imaging applications. Luminances in pictures frequently range downward from about 100 cd/m2, the maximum luminance of common television sets, CRT monitors, and photographs under room lights. Often, some scene content falls in the photopic range and some in the mesopic range. The results indicate that brightness contrast may decrease in low luminance regions without major changes of apparent surface color.