Saturation constancy in surface colors

The level of reflectance of pigmented surfaces observed in daylight affects saturation growth in different ways, depending on the wavelength of the samples. Numerical and matching judgments collected in previous experiments were replotted in families of monochromatic (constant hue) saturation power functions for blue, green, yellow, and red. For each hue the set of functions intersected at a point at which the colorimetric purity and saturation were invariant with reflectance. The points of intersection were: blues, 0.081 colorimetric purity (Pc) and 2.7 cromes; greens, 0.257 Pc and 4.6 cromes; yellows, 0.694 Pc and 8.3 cromes; and reds, 0.144 Pc and 3.6 cromes. A straight line fitted to these four intersecting points makes it possible to interpolate other intersecting points for other hues. Two additional experiments were designed to compare a function of saturation with a function of lightness for a set of different hues (heterochromatic functions). Observers judged saturation in one experiment and lightness in the other. Lightness grows linearly with reflectance, and saturation increases as the 0.5 power of Pc, suggesting a different perceptual discrimination for the two dimensions. The relation between changes in quantity and percentage of color perceived was tested in an additional experiment. Observers assigned the following saturation values to the samples with Pc close to the converging points: 13% of color for blues, 20% for reds, 25% for greens, and 40% for yellows.