Experiments in perceived illumination

Classic theorists, like Helmholtz, Hering, and Katz, suggested that perceived illumination within a field is determined by average luminance. Zdravković et al (2011 Journal of Experimental Psychology: Human Perception and Performance) reported evidence that illumination is tied to highest luminance. 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. 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. Area effects were also found. If an aperture contained a large dark grey region and a small light grey region, illumination level appeared lower and surface lightness appeared higher than if the aperture contained equal light and dark regions. These results, which are consistent with Koffka's invariance theorem and with the role of highest luminance in anchoring theory, provide a foundation for integrating the perception of illumination with perception of surface lightness.