Luminance (video)

In video, luma represents the brightness in an image (the "black-and-white" or achromatic portion of the image). Luma is typically paired with chrominance. Luma represents the achromatic image, while the chroma components represent the color information. 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.

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). Relative luminance is formed as a weighted sum of linear RGB components, not gamma-compressed ones. Even so, luma is erroneously called luminance.SMPTE EG 28 recommends the symbol Y′ to denote luma and the symbol Y to denote relative luminance.

While luma is more often encountered, relative luminance is sometimes used in video engineering when referring to the brightness of a monitor. 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). For the Rec. 709 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.

For digital formats following CCIR 601 (i.e. most digital standard definition formats), luma is calculated with the formula Y′ = 0.299 R′ + 0.587 G′ + 0.114 B′. Formats following ITU-R Recommendation BT. 709 use the formula Y′ = 0.2126 R′ + 0.7152 G′ + 0.0722 B′. Modern HDTV systems use the 709 coefficients, while transitional 1035i HDTV formats may use the SMPTE 240M coefficients (Y′ = 0.212 R′ + 0.701 G′ + 0.087 B′). These coefficients correspond to the SMPTE RP 145 primaries (also known as "SMPTE C") in use at the time the standard was created.

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