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
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