The luminance signal is formed by combining a proportion of 30% red, 59% green and 11% blue from the colour signal.
This combined output becomes the luminance (brightness/ monochrome) signal. It is written as Y. In TV usually derived from the RGB signals originating from a camera or telecine, by a matrix or summation.
Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press, London, Boston