Hue discrimination is poor when measured with fine gratings in which alternate bars differ in wavelength but not in luminance, or with small test objects that differ in wavelength from their background. Hue discrimination improves if a small luminance contrast is added. This effect is strongest for small test objects, and decreases as the size of the test object increases, or as the spatial frequency of the grating decreases, so that luminance contrast has little effect on hue discrimination measured with a 2° bipartite field. When measured with equiluminous square-wave gratings, an observer’s sensitivity to small wavelength differences increases monotonically as spatial frequency is decreased. When a luminance contrast is included, maximum sensitivity occurs when the grating has a spatial frequency of about 3 cycles/deg, and decreases at higher and lower spatial frequencies, so that its shape resembles the luminance modulation transfer function of the visual system. Hue-discrimination performance is better when measurements are made with the 3-cycles/deg grating with luminance contrast than with a conventional bipartite field.
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