Journal of Experimental Psychology: Human Perception and Performance 1983, Vol. 9, No. 6, 936-944 Copyright 1983 by the American Psychological Association, Inc. Lightness Constancy Through a Veiling Luminance Alan L. Gilchrist Institute for Cognitive Studies, Rutgers—The State University Alan Jacobsen State University of New York at Stony Brook Observers were asked to select samples from a Munsell chart to match the lightness of seven identified surfaces in an outdoor scene they were shown. A separate group that was given the same task but viewed the same scene covered with a veiling luminance equal in intensity to the highest luminance in the scene selected almost the same matches. The same lightness constancy results were obtained using an abstract laboratory display to rule out memory color. These results challenge ratio and contrast theories because a veiling luminance, by adding a constant luminance to every poing in the image, dramatically alters luminance ratios, tightness constancy was not obtained, however, when these three-dimensional real-world-type displays were replaced by a flat, Mondrian-type display consisting of surface grays from white to black, whether or not colored regions were present in the display; lightness matches were consistent with ratio predictions both with and without the veil. It has long been understood that luminance (the absolute amount of light reflected from a surface) does not correlate with lightness (the perceived reflectance, or whiteness/black- ness, of a surface). In fact, this understanding lies at the heart of the central and longstanding problem of lightness constancy. When the general level of illumination in the environ- ment is increased, for example, the luminance of surfaces also increases, yet the perceived lightness of each surface remains remarkably constant. This problem gains sharpness when seen in relation to the underlying, fundamental assumption that the basic response of the vi- sual system is to the luminance at each point in the visual field. For more than a hundred years theoretical attempts have been made to explain how veridical lightness percepts could be derived from an input consisting of lu- minance values. In 1948 Wallach, in a significant departure, proposed that lightness was based not on lu- minance but on luminance ratios of adjacent parts of the visual field. This ratio idea has since acquired great popularity, for which we can list at least four reasons, (a) Wallach's ex- periments gave such clear ratio results, (b) The ratio idea seemed to solve Hering's paradox: how to get two values, reflectance and illu- Requests for reprints should be sent to Alan Gilchrist, Institute for Cognitive Studies, Rutgers—The State Uni- versity, Newark, New Jersey 07102. mination, from one value, luminance, without recourse to the vagaries of "taking illumination into account." (c) The idea has "ecological validity." Wallach noted that when illumina- tion changes, ratios remain constant, (d) The ratio idea seemed to mesh beautifully with a known physiological mechanism, namely, lat- eral inhibition between receptor cells. In the meantime, further evidence for ratios has been accumulating. A large part of this evidence indicates that color perception (both chromatic and achromatic) derives from in- formation extracted from edges, or gradients, in the retinal image and that the information is physiologically encoded in terms of the ratio of the two intensities that form the edge. Whittle and Challands (1969) showed that two edges (or rather the regions they bound) appear identical if the ratio of change at the two edges is the same despite different absolute levels. Walraven (1976) extended this finding to chromatic color. Yarbus (1967) reported many analogous findings using stabilized image techniques. D. Hood (Note 1) recorded neural impulses from single cells in the optic nerve of the frog that respond directly to ratios. Despite all of this, there are reasons to sug- gest that the ratio idea is still not relative enough. We have discovered a new constancy, one that is commonplace in the everyday world, one in which lightness remains constant in the face of great changes in both luminances and luminance ratios. 936