Editing Colorfulness (section)

== Excitation purity ==

[[Image:Excitation Purity.svg|300px|thumb|right|'''Excitation purity''' is the relative distance from the white point. Contours of constant purity can be found by shrinking the spectral locus about the white point. The points along the line segment have the same hue, with p<sub>e</sub> increasing from 0 to 1 between the white point and position on the [[Spectral color|spectral locus]] (position of the color on the horseshoe shape in the diagram) or (as at the saturated end of the line shown in the diagram) position on the [[line of purples]].]]

The '''excitation purity''' (purity for short) of a stimulus is the difference from the illuminant's [[white point]] to the furthest point on the chromaticity diagram with the same [[dominant wavelength]]; using the [[CIE 1931 color space]]:<ref>{{cite book|isbn=0-240-51417-3|url=https://archive.org/details/focalencyclopedi00lesl|url-access=registration|quote=excitation purity.|title=The Focal Encyclopedia of Photography|publisher=[[Focal Press]]|first=Leslie D.|last=Stroebel|author2=Zakia, Richard D.|page=[https://archive.org/details/focalencyclopedi00lesl/page/121 121]|year=1993|edition=3E}}</ref>
<math display=block>p_e = \sqrt{\frac{\left(x - x_n\right)^2 + \left(y - y_n\right)^2}{\left(x_I - x_n\right)^2 + \left(y_I - y_n\right)^2}}</math>
where <math>\left(x_n, y_n\right)</math> is the chromaticity of the white point and <math>\left(x_I, y_I\right)</math> is the point on the perimeter whose line segment to the white point contains the chromaticity of the stimulus. Different color spaces, such as CIELAB or CIELUV may be used, and will yield different results.