Abstract
The Abney effect in colour-appearance systems (Munsell, NCS) means that the lines for identical apparent hue (at constant lightness) do not coincide with the straight lines for constant dominant wavelength. The curvature of constant-hue lines in chromaticity diagrams reflects the fact that cone signals are nonlinear functions of the rate of photon absorption. The most widely used nonlinear intensity-response function in vision is the Naka - Rushton function, which in an intermediate range can be approximated by a square-root function. Our purpose has been to study the Abney effect in the Munsell and NCS colour atlases in order to develop a mathematical-physiological description on this basis, designing the redness - greenness and yellowness - blueness perceptual variables and the perceptual hue function in each colour system. The description is applicable to both biological and machine-vision systems.
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