interval, photochromatic
Range of low luminances between the absolute threshold of light perception and the threshold of hue. The length of this interval varies with wavelength, being nearly nil around 650 nm (Fig. I15). Syn. achromatic interval. See Purkinje shift; duplicity theory.
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Fig. I15 Photochromatic interval, PHI (P, curve of the threshold of hue; S, curve of the absolute threshold of light)
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