Courtesy Wikipedia

As applied to analog television signals, two different words are used, luminance and luma, meaning two different things. Luma is a new word proposed by the NTSC in 1953 to prevent confusion between the Y' component of a color signal and the traditional meaning of luminance. While luminance is the weighted sum of the linear RGB components of a color video signal, proportional to intensity, luma is the weighted sum of the non linear R'G'B' components after gamma correction has been applied, and thus is not the same as either intensity or luminance.

An incorrect luma setting can cause visible flicker in a television display.

For standard-definition television systems using older phosphor characteristics modeling for the RGB primaries, (non-linear) luma can be calculated from gamma-adjusted RGB using the CCIR 601 formula Y' = 0.299 R' + 0.587 G' + 0.114 B'. Modern HDTV systems use a different definition of RGB phosphor characteristics and the ITU-R BT.709 formula Y' = 0.2126 R' + 0.7152 G' + 0.0722 B'. In both formulas, scaling and offsets used on both sides of the equation are assumed to be the same (e.g., ranging from 16 for black to 235 for white when used for digital video such as CCIR 601). Otherwise, some adjustments to the formula are needed to account for different scaling and offsets.