Dominant Wavelength

In color science, the dominant wavelength and complementary wavelength are ways of describing non-spectral (polychromatic) light mixtures in terms of the spectral (monochromatic) light that evokes an identical perception of hue.

On the CIE color coordinate space, a straight line drawn between the point for a given color and the point for the color of the illuminant can be extrapolated out so that it intersects the perimeter of the space in two points. The point of intersection nearer to the color in question reveals the dominant wavelength of the color as the wavelength of the pure spectral color at that intersection point. The point of intersection on the opposite side of the color space gives the complementary wavelength, which when added to the color in question in the right proportion will yield the color of the illuminant (since the illuminant point necessarily sits between these points on a straight line in CIE space, according to the definition just given).

In situations where no particular illuminant is specified, it is common to discuss dominant wavelength in terms of some standard (usually white) illuminant, such as flat spectrum white light. For the purposes of this geometrical discussion, an analogy may be observed between the horseshoe shaped CIE 1931 color space and a circular slice of HSV color space, where the CIE flat spectrum white point at (1/3,1/3) is analogous to the HSV white point at (0,0). This comparison clarifies the derivation of the ideas of hue and complementary color common in uses of the HSV space.