Relative luminance follows the photometric definition of luminance, but with the values normalized to 1 or 100 for a reference white^[1]. Like the photometric definition, it is related to the luminous flux density in a particular direction, which is radiant flux density weighted by the luminosity function of the CIE Standard Observer. Luminance (also called luminosity) is a photometric measure of the density of luminous intensity in a given direction. ... The luminosity function is a standard function established by the Commission Internationale de lÉclairage to account for the variable sensitivity of the human eye to radiation at different wavelengths. ...

The use of relative values is useful in systems where absolute reproduction is impractical. For example, in prepress for print media, the absolute luminance of light reflecting off the print depends on the illumination and therefore absolute reproduction cannot be assured.

Relative luminance in colorimetric spaces

For colorspaces such as XYZ, xyY, etc. the letter Y refers to relative luminance. No computation is required to find relative luminance when it is explicit in a color representation in such spaces. A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components (e. ... In the study of the perception of color, one of the first mathematically defined color spaces was the CIE XYZ color space (also known as CIE 1931 color space), created by the International Commission on Illumination (CIE) in 1931. ...

For RGB colorspaces that use the ITU-R BT.709 primaries (or sRGB, which defines the same primaries), relative luminance can be calculated from linear RGB components:^[2] The RGB color model utilizes the additive model in which red, green, and blue light are combined in various ways to create other colors. ... The title given to this article is incorrect due to technical limitations. ...

Y = 0.2126 R + 0.7152 G + 0.0722 B

For other sets of primary chromaticities (defined by their x and y chromaticity coordinates), different linear coefficients are needed to get relative luminance. In general, the coefficients are all positive, the green coefficient is largest and blue smallest, and the three form the middle row of the RGB-to-XYZ color transformation matrix.

For nonlinear gamma-compressed R'G'B' spaces as typically used for computer images, a linearization of the R'G'B' components to RGB is needed before the linear combination.^[3]

For L*a*b* space, the L* component is simply a nonlinear version of the relative luminance Y, so Y is easy to compute as a function of L*. CIE L*a*b* (CIELAB) is the most complete color model used conventionally to describe all the colors visible to the human eye. ...

Note that (relative) luminance should not be confused with luma, the weighted sum of the nonlinear gamma compressed R'G'B' components. For colorspaces that use luma, such as Y'UV or Y'CbCr (where Y' represents luma), computation of relative luminance can still be done. The R'G'B' components can be transformed into linear light components by undoing the gamma compression; these linear primary components can then be used to calculate luminance. Gamma correction is the name of an internal adjustment made in the rendering of images through photography, television, and computer imaging. ...

References

1. ^ Poynton, Charles. Digital Video and HDTV: Algorithms and Interfaces. ISBN 1-55860-792-7
2. ^ Michael Stokes, Matthew Anderson, Srinivasan Chandrasekar, and Ricardo Motta, "A Standard Default Color Space for the Internet - sRGB", online see matrix at end of Part 2.
3. ^ Maureen C. Stone, A Field Guide to Digital Color, A K Peters, Ltd, 2003. online

See also

• CIE 1931 color space
• Chromaticity