Koffka's invariance theorem, highest luminance anchoring, and the area rule apply to both lightness and perceived illumination

Helmholtz, Hering, and Katz all suggested that perceived illumination within a field is determined by average luminance. Zdravkovic et al. (2011) reported evidence that illumination is tied to highest luminance. In a series of experiments, we tested perceived illumination using a new technique. Looking into two square windows in the far wall of a vision tunnel, observers could see a patch of the far wall in each of two chambers. Each patch contained two shades of grey. They turned a knob to adjust the illumination level in one chamber to match that of the other, in effect creating the appearance of a single room with two windows. Lightness matches were taken using a Munsell chart. The stimuli placed in the chambers varied in luminance range, spatial frequency, and relative area. Illumination was matched for highest luminance, not average. Spatial frequency made no difference. Significant area effects were also found, both for perceived lightness and for perceived illumination, that is, the larger the area of the darker region in an aperture, the lighter and more dimly-illuminated it appeared, but only when the darker region had more than half of the area in an aperture, consistent with the area rule of anchoring theory. The area effects were complementary for lightness and perceived illumination, consistent with Koffka’s invariance theorem. These results suggest that lightness and perceived illumination are complementary and both are anchored by the highest luminance, allowing an obvious expansion of anchoring theory to cover perceived illumination.