The Perception of Luminosity on Different Backgrounds and in Different Illuminations

Abstract

Observers were presented with target surfaces of varying luminance and asked to report whether they appeared luminous or opaque. In one experiment the targets were presented against three backgrounds, white, gray, and black. In another experiment the targets were presented within Mondrian patterns that were either brightly or dimly illuminated. The results indicate that, across a variety of conditions, a target begins to appear luminous when its luminance is about 1.7 times that of a surface that would appear white in the same illumination, whether or not a white surface is available in the visual field for comparison. Defined in this way the luminosity threshold exhibits the two main kinds of constancy characteristic of surface grays, constancy with respect to changes in the illumination level and constancy with respect to changes in the reflectance of the immediate background. This finding, while challenging a range of potential rules, places the problem of defining the conditions that produce luminosity squarely within the problem of lightness perception for opaque surfaces.

The maintenance of apparent luminance of an object.

“…Across all experiments, the relative contrasts of the stimuli (stimulus luminance-background luminance) varied from 0.745 to 0.800. Depending on the contrast level, targets could appear self-luminous, (contrasts levels above 1.7), white (contrasts from 1.1 to 1.7), gray (contrasts from 0.5 to 0.9), or black (contrasts below 0.5), for the relatively simple stimulus configurations in these experiments (Bonato & Gilchrist, 1994;Heggelund, 1974). The contrast levels were chosen to give the appearance of gray surfaces that were darker than the background and to avoid the percepts of luminous white and black objects.…”

“…or a change in the reflectance of the background. A study by Bonato and Gilchrist (1994) suggested, however, that observers interpret the absolute change of luminance across the background as a change of illumination. Using a stimulus configuration similar to this experiment, a single central target either on a large background surround or on a ganzfeld surround, they found that the background tended to maintain an appearance of white despite large changes in the absolute luminance of the background and in the relative contrast of the background with the target.…”

“…As suggested by Gilchrist and Bonato (1995), objects have absolute values of lightness, such as black, gray, and white, which must be determined beyond values of relative lightness by a rule assigning or anchoring a particular stimulus to an absolute lightness value. For relatively simple displays of single targets against a large uniform surround, the absolute lightness of the target appears to be determined by the assignment of white to the background (see also Bonato & Gilchrist, 1994).…”

Measuring surface achromatic color: Toward a common measure for increments and decrements

“…Unmatchable surfaces have sometimes been reported when a simple disk-ring arrangement has been compared with a more complex stimulus (Bruno, 1992;Bruno, Bernardis, & Schirillo, 1997). Most important, a number of investigators have noted that within the same stimulus space, some surfaces can appear definitely too bright, almost self-emitting, for matching on a decremental scale to be possible (Bonato & Gilchrist, 1994;Heinemann, 1955;MacLeod, 1947). This peculiar mode of appearance has been called fluorescence (Evans, 1959(Evans, , 1974 or, more recently, superwhite (Gilchrist et al, 1999).…”

Effects of luminance oscillations on simulated lightness discriminations

“…The relationship of targets with their background can affect both their appearance (Bonato & Gilchrist, 1994;Heggelund, 1974) and detectability (Boynton, Ikeda, & Stiles, 1964;Ehrenstein & Spillmann, 1983;Patel & Jones, 1968;Short, 1966). Consequently, two sets of observations were gathered, one with the simulated reflectances (and therefore the luminances) of both targets greater than that of the background and the other with the targets' simulated reflectances (and luminances) lower than that of the background.…”