8 - Constant luminance

Abstract

This chapter provides detailed information on the way luminance can be formed as a weighted sum of linear (red, green, blue) RGB values that are proportional to optical power. It also explains why and how all video systems depart from the principle. Transmitting relative luminance preferably after the imposition of a nonlinear transfer function is called the Principle of Constant Luminance. Video systems depart from this principle and implement an engineering approximation. A weighted sum of linear RGB is not computed. Instead, a nonlinear transfer function is applied to each linear RGB component, then a weighted sum of the nonlinear gamma corrected R'G'B' components forms what is called “luma”. (Many video engineers carelessly call this luminance). The chapter also introduces the way in which the luma and color difference signals are formed and sub sampled. In a true constant luminance system, no matter how the color difference signals are handled, all of the relative luminance is carried by the luminance channel. In the rearranged system, most of the relative luminance is conveyed through the “Y” channel; however, some relative luminance can be thought of as “leaking” into the color difference components. If the color difference components were not sub sampled, this would present no problem.