Perceiving the intensity of light

The relationship between luminance (i.e., the photometric intensity of light) and its perception (i.e., sensations of lightness or brightness) has long been a puzzle. In addition to the mystery of why these perceptual qualities do not scale with luminance in any simple way, “illusions ” such as simultaneous brightness contrast, Mach bands, Craik–O’Brien–Cornsweet edge effects, and the Chubb–Sperling–Solomon illusion have all generated much interest but no generally accepted explanation. The authors review evidence that the full range of this perceptual phenomenology can be rationalized in terms of an empirical theory of vision. The implication of these observations is that perceptions of lightness and brightness are generated according to the probability distributions of the possible sources of luminance values in stimuli that are inevitably ambiguous. A fundamental problem in vision (and perception generally) was recognized at the beginning of the 18th century by George Berkeley (1709/1975), who pointed out that the sources underlying visual stimuli are unknowable in any direct sense. The light that falls on the eye from any region of a scene conflates the contributions of reflec-tance, illumination, and transmittance (as well as a host of subsidiary factors that affect these parameters). As a result, the physical prove