This study investigated the effect of exposure duration on the luminances required to reach absolute, detection, chromatic, and correct-hue thresholds. Dominant wavelengths 642, 584, 521, and 468 nm were investigated by the method of constant stimuli, with 64.5- and 2.5-min-diam stimulus sizes, for a series of nine exposure durations ranging from 5 to 1413 msec. The results show that, for the small stimulus at brief durations, the luminance–exposure-time relationship approaches Bloch’s law for the absolute-, detection-, and chromatic-threshold functions. For the corresponding correct-hue function, and for all thresholds, brief durations, large stimulus, the slope of the function relating luminance to exposure duration is less than that predicted by Bloch’s law. The slopes relating luminance to exposure duration for the long exposures are greater for the small than for the large stimuli. There is a tendency for the two longer wavelengths to be seen at lower luminances than the two shorter wavelengths, for all thresholds and exposures. The extent of the differences among wavelengths changes as a function of the kind of threshold; the greatest difference occurs for the correct-hue threshold. Photochromatic intervals α and β are smallest for the 642-nm stimulus, over the range of durations. At this wavelength, the four thresholds can be described by a single function relating luminance and exposure duration. The largest photochromatic intervals α and β are found for the 521-nm stimulus. The results are discussed in terms of the relative action time among hues and the threshold-tritanopia hypothesis.
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