Recent progress in the study of how we perceive black, gray, and white surfaces has been based heavily on the insight that the retinal image is encoded in terms of relative, rather than absolute, light intensities, or luminances. This research will investigate two serious problems for models based on relative luminance, known as ratio models. In one study, reports by observers in a completely homogeneous visual field that grows either brighter or darker for an extended period of time, at a rate too slow for detecting the change, will indicate whether the visual system senses absolute luminance in the absence of visible spatial and temporal change. The homogeneous visual field will eliminate spatial changes in luminance and the gradual change of luminance over time will eliminate the detection of temporal change. The hypothesis that only relative luminances are encoded suggests the counter-intuitive result that under these conditions the human observers will not be able to discriminate a very bright field from a very dark field. Deduction of visual processes underlying surface color perception requires a specification of whether relative or absolute luminances, or both, are available to the visual system. In the second study, observers will view various displays through a glass panel that reflects a homogeneous sheet of light, known as a veiling luminance; their experience will be much like looking through a fog. Although contrasts, or relative luminances, are dramatically reduced under these conditions, observers are able to identify gray shades correctly if the display behind the veil is sufficiently complex. Systematic variation of the display will enable the determination of factors that allow correct identification. These results should carry strong implications regarding how relative luminances are processed so as to produce visual experience of white and black surfaces.
