Monochromatic sources (AX ^ 0) are located on the perimeter of the chromaticity diagram. However, as the spectral linewidth of a light source gets broader, the color location in the chromaticity diagram moves towards the center of the chromaticity diagram. If the spectral width of a light source becomes comparable to the entire visible range, the light source is white and thus located near the center of the chromaticity diagram.

The dominant wavelength of a test light source is defined as the wavelength (i. e. monochromatic color) located on the perimeter of the chromaticity diagram that appears to be closest to the color of the test light source. The dominant wavelength is determined by drawing a straight line from the equal-energy point to the (x, y) chromaticity coordinate of the test light source, and by extending the straight line to the perimeter of the chromaticity diagram. The intersection point is the dominant wavelength of the light source. The procedure is schematically shown in Fig. 17.9.

The color purity or color saturation of a light source is the distance in the chromaticity diagram between the (x, y) color-coordinate point of the test source and the coordinate of the equal-energy point divided by the distance between the equal-energy point and the dominant - wavelength point. The color purity is thus given by

(17.14)

where a and b are shown in Fig. 17.9 and (x, y), (xee, yee), and (xd, yd) represent the chromaticity coordinates of the light source under test, of the equal-energy reference illuminant, and of the dominant-wavelength point, respectively. Thus the color purity is the relative distance of a light source under test from the center of the chromaticity diagram. Generally, the color purity is 100% for monochromatic light sources (AX ^ 0) located on the perimeter of the chromaticity

Note that the dominant wavelength and color purity are an alternative way to uniquely characterize the location of an emitter on the chromaticity diagram. Dominant wavelength and color purity are quite intuitive quantities (more so than the numerical (x, y) chromaticity coordinates) and they are therefore frequently preferred.