Perceived angle of incidence and achromatic surface color

Theory and evidence proposing illumination, and therefore necessarily angle of incidence of light, as a perceptual cue for lightness judgments are described. In two experiments, angle of incidence was varied by having 46 Ss view an upright trapezoid that monocularly appeared flat and binocularly, upright. In the first experiment, numbers of cues to the direction of a fixed source above the trapezoid were varied and combined. In a second experiment, the background for an illumination gradient was varied. Angle of incidence and cues to the location of the source did predict the trends in lightness judgments but not the magnitude of the effects. An alternative explanation to cue theory is proposed.