Converting R′G′B′ sources (such as the output of a three-CCD camera) into luma and chroma allows for chroma subsampling: because human vision has finer spatial sensitivity to luminance ("black and white") differences than chromatic differences, video systems can store and transmit chromatic information at lower resolution, optimizing perceived detail at a particular bandwidth.
[3] Luma is the weighted sum of gamma-compressed R′G′B′ components of a color video—the prime symbols ′ denote gamma compression.
The word was proposed to prevent confusion between luma as implemented in video engineering and relative luminance as used in color science (i.e. as defined by CIE).
The formula used to calculate relative luminance uses coefficients based on the CIE color matching functions and the relevant standard chromaticities of red, green, and blue (e.g., the original NTSC primaries, SMPTE C, or Rec.
709 (and sRGB) primaries, the linear combination, based on pure colorimetric considerations and the definition of relative luminance is: The formula used to calculate luma in the Rec.
709 spec arbitrarily also uses these same coefficients, but with gamma-compressed components: where the prime symbol ′ denotes gamma compression.
In consumer equipment, the matrix required to perform this conversion may be omitted (to reduce cost), resulting in inaccurate color.
This lowered bandwidth, coupled with high frequency chroma components, can cause visible errors in luminance.