Lightness and brightness over spatial illumination gradients.

We extended our studies of lightness and brightness in complex scenes to cathode-ray-tube simulations of an array of 35 gray reflective patches under spatially varying illuminants. There were three illuminance profiles, an abrupt step, a linear gradient, and a simulation of side illumination, with nine steepnesses of each. In half the sessions observers adjusted a test patch at one end of the illumination gradient in order to match the lightness of a standard patch at the other end of the gradient. In the remaining sessions they matched the brightness of the test patch to that of the standard. For all three illuminance profiles the lightnesses of the patches matched when they had approximately the same simulated reflectance; i.e., there was excellent lightness constancy even though the illuminance gradients were clearly visible.