Chromaticity coordinates in CIE-XYZ system.

 For tristimulus value X, Y, Z the chromaticity coordinates are given by

x=X/X+Y+Z

y=Y /X+Y+Z

z=Z/X+Y+Z

 Thus, a color is specified by two chromaticity coordinates (x, y) and the Y where Y is the luminance and (x, y) can be thought of as color of the stimulus devoid of brightness.

A plot of chromaticity coordinates for the physical colors forms a “chromaticity diagram”. Two such diagrams are shown in Fig (2.13.) for chromaticities and   (x, y).

Figure 2.13(a) The (r₀,g₀) chromaticity diagram for the Standard Observer.

Figure 2.13(b) 1931 CIE-xy chromaticity diagram

These chromaticity diagrams also show the chromaticity coordinates of each spectral color. The pure spectral colors are plotted on the elongated horse-shoe shaped curve called “spectral-locus”.The straight line joining the two extremes of the spectral locus is called the “line of purples”.