Chromaticity and characterization of whiteness for surface colors

Whiteness is an important colorimetric characteristic for surface colors. The CIE whiteness formula, the most widely used formula, only characterizes the whiteness of a surface color under CIE standard D65 and requires a sample to be within a small chromaticity region. In this study, 20 observers evaluated the whiteness appearance of 88 samples under four light settings at different CCT levels (i.e., 3000, 4000, 5000, and 6500 K). The 88 samples were carefully selected and the spectral power distributions of the light settings were carefully designed using a spectrally tunable LED device, so that the chromaticities of the samples under each light settings uniformly covered a wide range along the yellow/blue direction in a color space, which had never been realized before. The results, together with the two recent studies, allowed the derivation of ellipsoids for classifying the whiteness appearance for surface colors. For the samples within the derived ellipsoids, though the Uchida whiteness formula with CAT02 (W_Uchida,CAT02) had a higher correlation to the perceived whiteness than the CIE whiteness formula with CAT02 (W_CIE,CAT02), samples that were perceived as white and had a high chroma with a hue angle of blue due to the high violet/ultraviolet radiation in the illumination may had a negative W_Uchida,CAT02 value. A comprehensive whiteness formula that can accurately characterize the whiteness appearance for surface colors under an arbitrary light source by considering different conditions is still necessary and the work is undergoing.