In color science, the dominant wavelength is a way of describing non-spectral
(polychromatic) light mixtures in terms of the spectral (monochromatic)
light that would evoke an identical perception of hue.
On the CIE color 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).
For many power distributions of natural light, the set of spectra mapping
to the same color perception also includes a stimulus that is just a narrow
band at a single frequency, i.e. a pure spectral light (usually with some
flat spectrum white light added to desaturate). The wavelength of this
pure spectral light that will evoke the same color perception as the given
complicated light mixture is the dominant wavelength of that mixture.
The dominant wavelength ld is determined
Let P be a color of a sample.
The straight line joining the white point W
and P, meets the visual spectrum
at the dominant wavelength ld.