Human observers compensate for secondary illumination originating in nearby chromatic surfaces in making achromatic settings

Purpose: In three-dimensional scenes, the light arriving at a matte surface patch can come directly from a light source (primary illumination) but can also arrive after being absorbed and reradiated by one or more surfaces (secondary illumination). The light 'transferred' from one planar matte surface to another depends on the distance between them, the size of each patch, and on their relative orientation. For some orientations, no direct light transfer is possible. Bloj, Kersten & Hurlbert (1999) established that changes in apparent relative orientation of surface patches can affect perceived surface color. In this study, we investigated whether humans take into account the precise orientation of an adjacent surface patch when setting a test patch to achromatic. Methods: The stimuli were stereo image pairs of three-dimensional scenes composed of a small number of specular and matte objects. The scenes were rendered with achromatic diffuse and punctate light sources. Each scene contained a saturated orange matte cube. Attached to one side of the cube was a dark gray matte rectangular patch which could appear at any of seven different rotations about the vertical axis different ranging from 50 degrees to -50 degrees. The test patch was a small square located within this rectangle. In each trial a randomly chosen scene was displayed in a Wheatstone stereoscope. Observers adjusted the test patch to be achromatic. Over the course of the experiment, the adjustment task was repeated 10 times for each orientation. The order of presentation of conditions was randomized. Three observers participated in the study. Results: The orientation of the test patch relative to the side of the saturated cube affected observers' achromatic settings. As the test patch rotated further from the saturated surface, the contribution of the light reflected from the cube decreases approximately with the cosine of the rotation, which for these conditions is correct.