What is luma?
Video systems approximate the lightness response of vision by computing a luma component Y’ as a weighted sum of nonlinear R’G’B’ primary components: Each RGB signal is subject to a square root function, comparable to the 0.4-power function defined by L*. Luma is often carelessly referred to as luminance. For more information, consult the companion document Frequently Asked Questions about Color.
It is useful in a video system to convey a component representative of luminance and two other components representative of colour. It is important to convey the component representative of luminance in such a way that noise (or quantization) introduced in transmission, processing and storage has a perceptually similar effect across the entire tone scale from black to white. The ideal way to accomplish these goals would be to form a luminance signal by matrixing RGB, then subjecting luminance to a nonlinear transfer function similar to the L* function. There are practical reasons in video to perform these operations in the opposite order. First a nonlinear transfer function – gamma correction – is applied to each of the linear R, G and B. Then a weighted sum of the nonlinear components is computed to form a signal representative of luminance. The resulting component is related to brightness but is not CIE luminance. Many video engineers call it luma and give it the symbol Y’. It is oft
