Lightness Perception and Lightness Illusions
Edward H. Adelson
In The New Cognitive Neurosciences, 2nd ed., M. Gazzaniga, ed. Cambridge, MA: MIT Press, pp. 339-351, (2000).
Abstract: A gray surface in sunlight may have much higher luminance than it has in the shade, but it still looks gray. To achieve the task of "lightness constancy," the visual system must discount the illumination and other viewing conditions and estimate the reflectance. Many different physical situations, such as shadows, filters, or haze, can be combined to form a single, simple mapping from luminance to reflectance. The net effect of the viewing conditions, including additive and multiplicative effects, may be termed an "atmosphere." An "atmospheric transfer function" maps reflectance into luminance. To correctly estimate lightness, a visual system must determine a "lightness transfer function" that performs the inverse. Human lightness computation is imperfect, but performs well in most natural situations. Lightness illusions can reveal the inner workings of the estimation process, which may involve low-level, mid-level, and high-level mechanisms. Mid-level mechanisms, involving contours, junctions, and grouping, appear to be critical in explaining many lightness phenomena.