Relative luminance () follows the photometric defintion of Luminance including spectral weighting for human vision, but while luminance is a measure of light in units such as , relative luminance values are normalized as 0.0 to 1.0 (or 1 to 100, with 1.0 being a theoretical perfect reflector of 100% reference white.
The use of relative values is useful in Colour Appearance Models that describe perception relative to the eye’s adaptiation state and a reference white.
For CIE colour spaces XYZ and xyY, the letter refers to relative luminance.