Relative luminance in the context of Chromaticity

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 ${\displaystyle {\overline {y}}(\lambda )}$ 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.