CIE 1931 Color Space - Tristimulus Values

Tristimulus Values

The human eye has photoreceptors (called cone cells) for medium- and high-brightness color vision, with sensitivity peaks in short (S, 420–440 nm), middle (M, 530–540 nm), and long (L, 560–580 nm) wavelengths (there are also low-brightness, monochromatic "night-vision" receptors, called rod cells, which have peak sensitivity around 490–495 nm). Thus, in principle, three parameters describe a color sensation. These tristimulus values of a color can be conceptualized as the amounts of three primary colors in a tri-chromatic additive color model.

Associating tristimulus values with colors is the job of a color space. CIE XYZ, one of many such spaces, is a commonly used standard, and serves as the basis from which many other color spaces are defined. A color-matching function associates specific tristimulus values with specific colors.

Consider two light sources made up of different mixtures of various wavelengths. Such light sources may appear to be the same color; this effect is called metamerism. Such light sources have the same apparent color to an observer when they produce the same tristimulus values, no matter what are the spectral power distributions of the sources.

It is not possible to stimulate one type of cone cell only, because the sensitivity curves of the three cone cells overlap. In tri-chromatic additive color spaces with color triangles defined by discrete (i.e., non-overlapping) primary colors (e.g. RGB color spaces), this yields negative RGB values for some colors that correspond to certain tristimulus values. To avoid these negative RGB values, and to have one component that describes the perceived brightness, "imaginary" primary colors and corresponding color-matching functions have been formulated. The resulting tristimulus values are defined by the CIE 1931 color space, in which they are denoted X, Y, and Z.