Determinations at the same brightness by six normal observers have been made of the color-temperature difference corresponding to a chromaticity difference just doubtfully perceptible over the range of 1800 to 11,000°K. It has been found that this temperature difference corresponds closely to a constant difference in the spectral centroid of light (Priest’s empirical relation); it is also closely proportional to the square of the color temperature (Davis’ representation of Priest’s empirical relation); furthermore it corresponds with good approximation to a constant difference in the “red” trilinear coordinate (Judd’s empirical relation). These experimental results have therefore approximately checked three empirical relations previously derived from less complete data. Priest’s spectral-centroid relation and Davis’ representation of it both agree so closely with the experimental data that the discrepancies are not known to be real, but Judd’s empirical relation yields discrepancies about twice as large as the experimental uncertainty. The verification of Priest’s spectral-centroid relation confirms the tentative conclusion by Davis and Gibson that between 2000 and 3000°K a given color-temperature difference causes for the normal observer nearly the same size chromaticity difference regardless of whether the radiators be viewed directly or through a blue filter not more highly selective than a “daylight filter.” This result has a bearing on the phenomenon of “color constancy”; that is, it helps to explain why the colors of objects are approximately constant for illuminants differing as widely as incandescent-lamplight and natural daylight.
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