Relative luminance
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 (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 ) {\displaystyle {\overline {y}}(\lambda )} of the CIE Standard Observer.