The negative values in the representation of color by R-G-B-values is unpleasant. Thus the Commission Internationale de l'Éclairage (CIE) defined in 1931 another base in terms of (virtual) primaries , (the luminous-efficiency function) and , which allows to match all visible colors as linear combinations with positive coefficients only (the so called CHROMATICITY VALUES ), i.e. any visible color can be expressed as , see
Normalization to gives new coordinates , (and ), which are independent on luminous energy . The visible chromatic values in this coordinate system form a horseshoe shaped region, with the spectrally pure colors on the curved boundary. Warning: brown is orange-red at very low luminance (hence is not shown in this diagram). Standard white light (approximative sunlight) is located at point near .
The DOMINANT WAVELENGTH of some color is given by the intersection of the ray from to the color with the curved boundary formed by the pure colors.
Some colors (purples and magentas) are non-spectral, i.e. have no dominant wavelength (since the intersection of the rays hit the boundary in the flat part). But they have have a COMPLEMENTARY DOMINANT WAVELENGTH, lying on the opposite side.
COLOR COMPLEMENTARY to some color are opposite to on the line through . E.g. we have the following complementary pairs: red-cyan, green-magenta, and blue-yellow.
EXCITATION PURITY is a ratio of the distances from the color and the dominant wavelength to .
The CIE chromaticity diagram can also be used to visualize the COLOR GAMUTS (i.e. the ranges of producible colors) for various output devices:
The chromaticity values for standard NTSC RGB phosphor are:
A disadvantage of the CIE 1931 standard is that equal distances in the coordinates are not perceived as being equal. This was corrected by 1976 CIE LUV standard.
Andreas Kriegl 2003-07-23