Relative luminance ${\displaystyle Y}$ follows the photometric definition of luminance ${\displaystyle L}$ including spectral weighting for human vision, but while luminance ${\displaystyle L}$ is a measure of light in units such as ${\displaystyle cd/m^{2}}$, Relative luminance ${\displaystyle Y}$ values are normalized as 0.0 to 1.0 (or 1 to 100), with 1.0 (or 100) 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(λ) 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.