Polar Coordinate Representation of color:

 Another way of representing the chromaticity of a color is by polar coordinates, with some preselected reference normalizing white, e.g., CIE illuminant C, as the origin. The two polar coordinates are Dominant Wavelength (or complementary wavelength) and Excitation Purity. Dominant wavelength of a color is the wavelength of the spectral color that, when additively mixed in suitable proportions with the specified reference white, yields a match with the color being considered. This is shown as point D for the color  of  Fig 2.15. For colors in the lower right-hand portion of the chromaticity diagram the straight line connecting the color under consideration with the reference white may have to be projected backwards in order to intersect the spectral locus. In such a case, this complementary wavelength is specified instead of the dominant wavelength. This complementary wavelength of a test color is the wavelength of the spectral color that when mixed in suitable proportions with the test color yields a match with the specified reference white. Fig (2.15) shows the complementary wavelength (point ) for color .

Every color has either a complementary wavelength or a dominant wavelength. Some colors have both.

The radial coordinate of this polar color specification after normalization is called the excitation parity. Excitation parity is the ratio of two length (CS 1 /CD) on the chromaticity diagram. The first length is the distance between the origin (reference white) and the color  being considered; the second length CD is the distance along the same direction from the origin to the spectrum locus D (or the line of purples). Obviously, 100 percent excitation purity usually corresponds to a spectral color.

Qualitatively, the dominant wavelength of a color indicates what part of the spectrum has to be mixed with the reference neutral color to match the given color, and the excitation purity indicates how “far” the color lies from the reference neutral color. Dominant wavelength and excitation purity are related to the commom specifications of perceived color, namely hue and saturation. Thus, a dominant wavelength of 600nm (orange) and purity 50 percent represent a color of orange hue of about half the saturation of spectral orange. In fact, such a color is perceived as more pinkish than this, but not enough to vitiate the usefulness of the designation, Colors of constant dominant wavelength are perceived to have nearly constant lines.