Anchoring of lightness values by relative luminance and relative area

Surface lightness is widely thought to depend on the relative luminance coming from neighboring surfaces. But relative luminance can produce only relative lightness values. Specific lightness values can be derived only with an anchoring rule that specifies how relative luminance values in the retinal image are mapped onto the lightness scale. We explored the anchoring rules governing very simple images consisting of two adjacent surfaces that fill the entire visual field. These were painted onto the interior of a large hemisphere that surrounded the observer's head. Lighter and darker radial sectors of the same two shades of gray were painted onto nine such hemispheres, but with different relative areas. The region of highest luminance was always seen as white. The lightness of the darker sector depended on relative area, appearing lighter as the darker sector became larger, but this effect was stronger when the darker sector was larger than the lighter, a pattern of results shown to be consistent with over a dozen prior studies of relative area and lightness.