Gelb lighting, Type II constancy, and failure of Type I temporal constancy

Lightness constancy and simultaneous lightness contrast (SLC) illustrate, in different ways, the effect of context on color appearance. Gilchrist (1988) defined constancy as Type I constancy and SLC as Type II constancy. Agostini & Bruno (1996) showed that under Gelb lighting (an illumination border is made to coincide with the outer border of the display and the rest of the visual field is left in near darkness), the size of SLC effect approximately doubles. Successively, Agostini & Galmonte (1999) found that in Type II constancy displays Gelb lighting affects both the lightness of the gray patch and that of its background. The aim of the present research is to test whether under Gelb lighting the size of lightness induction effect on a Type II constancy display is affected by temporal changes of the absolute amount of illumination. There were c2 conditions; in the first one the overall background was black, whereas in the second was white. Their luminances were kept constant during the experiment. The experimental display and a Munsell scale (white background) were suspended in mid air and tested under Gelb lighting at 13 different levels of luminance. For each level observers performed a match for the 2 reflectances (black and middle gray) of the experimental display. Even though the coplanar luminance ratio between the tested surfaces is always constant, a temporal increase of the absolute amount of the level of illumination produces an increase of the lightening effect in both conditions and on both surfaces. In the overall black background condition the differences between the mean lightness matches for the 2 surfaces decrease as the absolute luminance on the experimental display increases. In the overall white background condition the lightening is lower. The differences between the mean lightness matches for the gray and the black surfaces are approximately constant and close to the difference between their objective Munsell values. By varying the intensity of Gelb lighting: 1. in general, the coplanar ratio principle seems to fail; and 2. in both conditions, an increase of the absolute amount of the level of illumination produces an increase of the lightening effect on both the tested surfaces