The effect of object shape and pose on perceived lightness

Purpose: The shape and pose of an object with respect to a directional light source affect the luminance of the light reflected from the object to an observer. If perceived lightness provides a stable representation of object surface reflectance (i.e. if there is lightness constancy), the visual system must compensate for the effects of shape and pose changes. Little is known about how well and over what range of scenes this happens. The experiments reported here were designed to help us learn more. Methods: Observers viewed grayscale objects placed in an experimental chamber. The illumination in the chamber was provided by a single incandescent bulb, located near the top of the chamber and not directly visible to observers. On each trial of the experiment observers indicated which of two surfaces, located on two distinct objects, appeared to have a higher lightness. In judging lightness, observers were explicitly instructed to consider surface reflectance rather than the luminance of the reflected light. Across trials the reflectance of the test surface remained fixed, while the reflectance of the match surface was varied using a staircase procedure. We determined the point of subjective lightness equality, which we refer to as the match reflectance. Match surface reflectance was changed during an intertrial interval by substitution of identically shaped but differently painted match objects. The observer's view was occluded by a shutter duringthe intertrial interval. Results: When the test and match surfaces were both planar, match reflectances varied greatly as a function of match surface pose. Match reflectances also varied as a function of match object shape (planar, cube, or sphere), although the most striking effect here was a large increase in individual differences when the match object was a cube. Conclusion: The data indicate that there can be large failures of lightness constancy with respect to changes in object shape and pose.