Abstract

Drum¹ measured threshold-versus-adapting-intensity (TVI) functions for chromatic increments on achromatic backgrounds. The TVI slopes became greater than unity at background levels above 4 log trolands (log td) for stimulus wavelengths across the visible spectrum. Because only one background level above 4 log td was tested, however, the detailed shapes of the curves and the full extent of the steepening could not be resolved. We now report more detailed TVI measurements at four test wavelengths (430, 490, 575, and 660 nm) and background illuminances at 0.2 log-unit intervals from 4.0 log td to 5.6 log td. Foveal, 2° diameter test stimuli were presented for 1 s on a concentric, perceptually white (5128 K color temperature), 7° diameter, steady background. Thresholds were obtained by the method of adjustment, after which the test stimulus illuminances were increased 0.6 log-unit and the subject estimated the percentages of red, yellow, green, blue, and white. Average TVI slopes for two subjects were 2.75 for 430 nm, 1.55 for 490 nm, 1.15 for 575 nm, and 1.4 for 660 nm, which is in qualitative agreement with published estimates of the ratio of chromatic to achromatic sensitivity. Also, the percentage of white apparent in the suprathreshold increments generally increased with increasing background illuminance despite substantial increases in excitation purity. These findings imply that steady, intense achromatic backgrounds can produce response saturation in color-oppo-nent mechanisms.

© 1990 Optical Society of America