Saturation and the Foveal Achromatic Interval*

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Abstract
Abney’s curves for chromatic threshold are similar to curves for colorimetric purity as presented by Priest and Brickwedde. Determinations of chromatic threshold Ec and achromatic threshold Ea in two of our own subjects show that the curve for the chromatic threshold is similar to that of Abney and also similar to the curve for colorimetric purity. The curve for the achromatic threshold is below the curve for the chromatic threshold and reproduces the luminosity curve for the fovea. Six other subjects showed chromatic threshold curves similar to those of Abney. Defined as logEc/Ea the curve for achromatic interval has a maximum near 570 nm and, in general, falls to greatly reduced values at 420 in the blue and 700 in the red. This curve also gives chromatic thresholds in photopic photometric units based on achromatic thresholds (i.e., foveal luminosity). In the blue and red regions of the spectrum, the achromatic threshold has a value that approaches the chromatic. This result, not obtained by Purdy in the blue, is discussed. It is pointed out that colorimetric-purity thresholds and chromatic thresholds seem both to be reciprocal measures of saturation. Chromatic thresholds may have an advantage in not involving a physical mixture of color with white.

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