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Relative luminance Y {\displaystyle Y} follows the photometric definition of luminance L {\displaystyle L} including spectral weighting for human vision, but while luminance L {\displaystyle L} is a measure of light in units such as c d / m 2 {\displaystyle cd/m^{2}} , Relative luminance Y {\displaystyle Y} values are normalized as 0.0 to 1.0 , with 1.0 being a theoretical perfect reflector of 100% reference white. Like the photometric definition, it is related to the luminous flux density in a particular direction, which is radiant flux density weighted by the luminous efficiency function y of the CIE Standard Observer.
The use of relative values is useful in color or appearance models that describe perception relative to the eye's adaptation state and a reference white. For example, in prepress for print media, the absolute luminance of light reflecting off the print depends on the specific illumination, but a color appearance model using relative luminance can predict the appearance by referencing the given light source.