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Relative Luminance
Relative luminance (\(Y\)) follows the photometric defintion of Luminance \(L\) including spectral weighting for human vision, but while luminance \(L\) is a measure of light in units such as \(cd/m^2\), relative luminance \(Y\) 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 \(Y\) refers to relative luminance.