Foveal simultaneous contrast as a function of inducing-field area.

Abstract

The apparent brightness of a 33-foot test field square was studied as a function of the area and luminance of an inducing-field square, the area varying from zero to double that of the test field. A binocular matching technique was used in 2 experiments. These differed in the method of holding constant the spatial separation between the test and inducing fields while the inducing-field area was varied. Expt. I held constant the distance between the near borders of the two fields. Expt. II held constant the distance between their centers. It is pointed out that neither method of spatial-separation control is justified and that it is theoretically more correct to assume that along with in-ducing-field area variation spatial separation is also being varied. The results for both experiments show that the test-field apparent brightness decreases as the area of the inducing field increases, but only for the inducing-field luminances equal to or greater than the test-field luminance. In Expt. II, the test-field apparent brightness is relatively less affected by the smaller inducing-field areas than in Expt. I, although the effect is the same for the largest area. An equation, expressing the relation between test-field apparent brightness and inducing-field luminance and area, is developed. This equation is based upon the assumption that spatial separation is being varied along with inducing-field area. A satisfactory fit is obtained for the data of both experiments.