Light Adaptation and the Brightness of Brief Foveal Stimuli*

Abstract

Brightness scales for differing conditions of adaptation and contrast have been established by the combined procedures of ratio production and equal-brightness judgments under adapting conditions which differ systematically for the two eyes. Under stimulus conditions in which light adaptation alone is systematically varied, the equivalence of brightness ratios at differing levels of adaptation is demonstrated for a wide range of adapting luminances. Preliminary findings suggest that this equivalence also holds for chromatic adapting and test stimuli. Changes in the brightness of brief foveal stimuli due to light adaptation may be describable within the framework of a general power law, if all test-stimulus luminances are expressed as luminance above psychological zero brightness (that is, as luminance above threshold for each specific condition).Changes in brightness due to light adaptation may be measured either by the use of a preadapting stimulus or by presentation of test stimuli on a prevailing surround. When adaptation is defined in terms of the production of a criterion luminance threshold, the adaptive effects of the two procedures may be shown to be equivalent.