The JPC79 colour–difference formula has represented a substantial improvement over earlier formulae and is being applied successfully in industrial shade passing. Modifications are described which overcome certain problems and, to some extent, simplify the formula. With available experimental data, the modified version performed even better than the original formula. Perceptibility data are fitted better by increasing the JPC79 lightness weighting by a factor of two. The new formula, designated CMC (: c), has the best overall performance of any formula so far published.
The paper describes an adaptometer which has been built to investigate local adaptation in the extra-foveal retina (Troxler's Effect). The instrument provides extra-foveal stimuli of subtense up to 12°using Illuminant A, and observations involve the continuous adjustment of the luminance of a foveal comparison field to maintain equality of subjective brightness. The time course of local adaptation can be automatically recorded, and has been measured for test stimuli at a fixed level of initial subjective brightness with variation of area and eccentricity. Subsidiary experiments have been performed with a grating field, with pre-adaptation and with reversed contrast. Troxler's Effect as an inductive process of neutralization of boundaries of contrast explains the results better than the hypothesis that local adaptation is an ' area ' effect.
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