Luminance is a measure of physical light intensity modified by the spectrum sensitivity
measured in human vision. In other words, red, green, and blue contribute to the
perception of brightness, but not equally. Since two of the cone cell types in the eye
favor light in the green portion of the spectrum, the luminance channel is weighted to
mostly represent the green color channel. This makes the luminance channel appear to
have the equivalent brightness that we see in a color image.
color model derives luminance from the Commission Internationale de
l’Eclairage (CIE) XYZ color system, in which the Y component represents luminance
(X and Z represent color components).
In video systems, a gamma adjustment is applied to the RGB color channels to make
efficient use of the bandwidth available for carrying and recording signals. The
gamma-adjusted channels are called R´G´B´, and the derived black-and-white channel
is called Y´, or luma. For more information about gamma, see “
Measuring and Setting Video Levels