Luminance gradients and lightness constancy.

The ability of the visual system to perceive surface color correctly despite changes in the level of illumination is referred to as lightness constancy. It has been shown that strong cues of changes in the level of illumination are provided by smooth luminance changes. A smooth luminance gradient has been placed at the border dividing the lower from the higher luminances side of a classical-lightness-constancy-type display, presented on a CRT-monitor (Bruno, 1994). Observers performed a matching task in a within-subject design: 3 luminance ratios (30:1-10:1-2:1) x 4 conditions, i.e., gradient throughout the whole display dividing border (“congruent” condition), gradient throughout the inner background dividing border only (first “incongruent” condition), gradient throughout the outer background dividing border only (second “incongruent” condition), and no gradient (control condition). Compared to the control condition, congruent condition holds better lightness constancy, while the two incongruent conditions show only little failures of constancy, independently from the specific tested ratio. For the congruent condition there is the maximum increase in lightness constancy, but, since the loss of constancy in the incongruent conditions is modest, it seems that the visual system considers smooth luminance gradients as important sources of information on illumination changes even if edge cues are partially incongruent.