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, 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 I pc denotes optimal aperture-color stimuli for spectrals, arguably I pc 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 + 613 nut in all CIE illuminants. This remedy merely requires redefinition of I p, for nonspectrals as the line 442-613 nm, and gives meaningful pc values over the hue cycle allowing new research of chromatic luminance relations with color appearance. (c) 2007 Wiley Periodicals, Inc.