CIE 1931 color model
The CIE 1931 color model describes a color by three tristimulus values X, Y and Z. The color space is a three-dimensional plot of the tristimulus values X, Y and Z, but often the plot is expressed in terms of chromaticity coordinates and a chromaticity diagram. The chromaticity coordinates are derived as fractional components of the tristimulus values.
The experimental conditions for CIE 1931 color model are based on a 2 degree standard observer.
The CIE 1931 color model was enhanced to the CIE 1964 color model by a 10 degree standard observer in 1964.