The Effect of Cues to Illumination on Apparent Lightness

Abstract

The observation that the apparent lightness, or albedo, of an object does not appear to change despite variations in the amount of incident illumination is defined as lightness constancy. Helmholtz (1962) proposes that lightness constancy occurs because, in addition to the data from the amount of light stimulating the retina, the perceptual system utilizes information about the amount of light falling on the target. Through the process of 'unconscious inference' these two sources of information are combined to produce an appropriate perception of lightness, corresponding to the physical reflectance of the target. Woodworth (1938) suggests that the term 'cues to illumination' be used to designate data about the light incident on the object, in order to avoid the more mentalistic terms in the Helmholtzian hypothesis. A number of experiments have attempted to show that cues for illumination may affect the perception of the brightness of a surface. The typical paradigm involves presenting a target both in the presence and in the absence of cues to the nature of incident light or shadow. Some typical experimental manipulations of this nature include those of Gelb (1929) and Kardos (1934). Gelb (1929) introduces a white piece of paper to indicate the presence of a spotlight on a disc, while no paper exists in the control condition. Kardos (1934) uses the presence of a penumbra to indicate that a shadow is falling on the target, while no penumbra is present for the control group. Similarly, MacLeod (1932) uses a shadow on the background as a cue, and Beck (1971) uses a shadow from a card in front of the target. Hochberg and Beck (1954) use shadows from nearby objects in an enriched environment as cues to the direction of illumination.