Abstract

The hue of a short-wavelength foveal increment on a concentric 5500° white background was measured as a function of background luminance. The test stimulus was 1 s in duration and 2° in diameter. The background was 7° in diameter and was presented continuously. For each background luminance tested, the luminance of the test spot was set 0.6 log unit above increment threshold, and a quantitative hue-naming procedure was used to determine its relative redness (or greenness) and blueness. At scotopic and low photopic background levels, the test spot appeared mainly blue, with a 10–40% red component. For the next 2–3 log units of the photopic range, the hue balance shifted sharply toward red. In some cases the relative red component reached 80%. At still higher photopic levels, however, this trend reversed dramatically. For most subjects, redness disappeared entirely, leaving a mixture of blue and green. The violet-to-cyan transition coincided with a pronounced steepening of the threshold vs background intensity (tvi) function, presumably caused by selective saturation of short-wavelength cone responses.¹ Test spots that appeared mainly blue had clearly visible borders, whereas spots that appeared mainly red tended to appear spatially diffuse. These results reinforce previous evidence² that short-wavelength cone activity produces a reddish purple hue and middle-wavelength cone activity produces a bluish cyan.

© 1985 Optical Society of America