Original and historical data regarding luminance additivity are considered in terms of the photometric systems of absolute foveal threshold, direct brightness matching, incremental threshold and flicker photometry. Evidence is provided in support of an empirically-based vector model which describes both bichromatic luminance additivity and colour-matching data at or near threshold, and new experiments are described which confirm that red-plus-green mixtures are grossly less than additive at threshold and high intensities. Also reported is the result that green-plus-violet mixtures are less than additive at threshold but superadditive (the whole is greater than the sum of the parts) at high levels. The facts of luminance additivity among normal and color blind observers are discussed within the framework of a physiologically based trichromatic opponent-colors theory which suggests that absolute threshold, direct brightness matching and incremental threshold judgments are mediated by the combined outputs of an opponent-type chromatic system and a nonopponent achromatic system, whereas flicker judgments are mediated only by the nonopponent system. © 1969.
