The excitation purity of any color possessing a dominant
wavelength is an exactly defined ratio of the distances in the chromaticity
diagram indicating how far the given color is displaced towards the spectrum
color from the achromatic color.
The excitation purity of EP
is determined by the following distance ratio:
xP, xW and xld are the chromaticity
coordinates at the color P,
the white point W and the dominant
color ld respectively,
and the white point is given by xW = yW = zW = 1/3
For a color without a dominant wavelength as Q,
the excitation purity EPc is determined
from the complementary wavelength.
is calculated using the complementary wavelength lc.