The CIE theory is a model, or system, used to measure colors in relation to the wavelengths they contain. Based on the three cones in the eye, the theory defines three supersaturated primaries that can be combined to form all colors.
The CIE color theory is based on the tristimulus theory of color perception. Based on the different wavelengths seen by the three color receptors, or cones, in the human eye, the tristimulus theory attempts to plot the responses of the eye into a three-dimensional model, which is then projected onto one plane and translated into a two-dimensional graphic.
The CIE theory can be used to specify ranges of colors, or gamuts, that can be produced by a particular light source. A computer's gamut, for example, is fairly small and must therefore only be used to view specific wavelengths of colors.
The CIE color theory led to a model called Lab. Within this model, colors are described using three components: luminance or lightness, and two chromatic components (one ranges from green to red and the other from blue to yellow).