Chromatic luminance, colorimetric purity, and optimal aperture‐color stimuli

Abstract

Chromatic luminance (i.e., luminance of a monochromatic color) is the source of all luminance, since achromatic luminance arises only from mixing colors and their chromatic luminances. The ratio of chromatic luminance to total luminance (i.e., chromatic plus achromatic luminance) is known as colorimetric purity, and its measurement has long been problematic for nonspectral hues. Colorimetric purity (p c ) is a luminance metric in contrast to excitation purity, which is a chromaticity-diagram metric approximating saturation. The CIE definition of p c contains a fallacy. CIE defines maximum (1.0) p c for spectral stimuli as monochromatic (i.e., optimal) stimuli, and as the line between spectrum ends for nonspectrals. However, this line has \0.003 lm/W according to CIE colorimetric data and is therefore effectively invisible. It only represents the limit of theoretically attainable colors, and is of no practical use in color reproduction or color appearance. Required is a locus giving optimal rather than invisible nonspectral stimuli. The problem is partly semantic. CIE wisely adopted the term colorimetric purity, rather than the original spectral luminance purity, to permit an equivalent metric for spectrals and nonspectrals, but the parameter of equivalence was never clear. Since 1 p c denotes optimal aperture-color stimuli for spectrals, arguably 1 p c should denote optimal stimuli consistently for all stimuli. The problem reduces to calculating optimal aperture-color stimuli (''optimal'' in energy efficiency in color-matching) for nonspectrals, shown to comprise 442 þ 613 nm in all CIE illuminants. This remedy merely requires redefinition of 1 p c for nonspectrals as the line 442-613 nm, and gives meaningful p c values over the hue cycle allowing new research of chromatic luminance relations with color appearance.

Chroma, chromatic luminance, and luminous reflectance. Part I: Basic research and illustration of relations

“…Optimal color stimuli for nonspectral colors are compounds admixed from 442 þ 613 nm. 1 Optimal Although colorimetric purity (p c ) has been reported to resemble inverse chromaticness, [10][11][12][13] the similarity can only be broad. Chromaticness cannot be the close inverse of colorimetric purity, since the latter is a constant maximum (1.0 p c ) for all wavelengths whereas perceived chromaticness is relative to (varies with) wavelength or hue angle; e.g., chroma for value 5 is high in the blue stimuli and low in yellow.…”

“…1 using the line 442-613 nm as optimum purity (1.0 p c ) for nonspectral colors. The line represents optimal aperture-color stimuli in all CIE illuminants, 1 and is similar to the locus used empirically in television and photographic color reproduction. 3,19 This improves on the current CIE convention using the effectively invisible purple line (between spectrum extremes) as max p c for nonspectral colors, which is not used by color reproduction media 19 or by (object color) color order systems such as Munsell and NCS.…”

“…1). The main use of p c is to indicate the proportion of chromatic luminance in the total luminance of a color.…”

“…1,3 Excitation purity is a spatial metric dependent on the color space and is a valuable indicator of chromaticness. In contrast, colorimetric purity is a luminance metric, independent of the color space, and is such a poor indicator of chroma that it is broadly the inverse of chroma.…”

“…Historically, such research has been limited to the spectrum (about 3/4 of the hue cycle according to Munsell and NCS hue circles). A previous paper 1 redefined colorimetric purity (the proportion of chromatic luminance in the total luminance of a sample; see equations below) for nonspectrals and for very short-and long wavelengths, allowing research of chromatic luminance relations with color appearance over the complete hue cycle.…”