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 luminance as used in color science (i.e. as defined by CIE). Luminance is formed as a weighted sum of linear RGB components, not gamma-compressed ones1. SMPTE EG 28 recommends the symbol Y' to denote luma and the symbol Y to denote luminance.2
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