A hypothesis is proposed to predict the perceived lightness of chromatic object colors, which is well known as the Helmholtz‐Kohlrausch effect or the lightness‐reflectance ratio effect (L/Y effect). The hypothesis is, “When two chromatic object colors with different hues have the same values for each of three attributes (whiteness, blackness, and chromaticness), the two colors have the same perceived lightness, including a chromatic‐component contribution.” The hypothesis is tested by two kinds of experiments: perceivedlightness matching using the NCS Color Atlas, and chromatic tiles determined by Wyszecki. Both results strongly confirm the hypothesis. Further, contour lines of constant L/Y ratios are estimated, and are very similar to those reported previously.
The Natural Color System (NCS) is analyzed by using a nonlinear color‐appearance model. Perceptual uniformity was examined for each of the NCS attributes of blackness (s), chromaticness (Cr), and hue. Some nonuniformities were found in chromaticness and hue spacing, which were probably the result of the assessing and scaling method used in developing the system. In addition, a colorimetric interpretation is given to a principal plane in the NCS consisting of the colors with (s,Cr) = (50,0) and (0,100). The colors with (0,100) have different values of Y for different hues. Based on the analysis, a method is developed to predict the NCS attributes from the values x, y, Y of an object color by using the nonlinear color‐appearance model. The present analysis clarifies the importance of the NCS system in studying the color appearance of object colors.
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