Luminance (Y) is the luminance of a video display. It is a measure of the impression of brightness. Since the human eye is much better at resolving differences in brightness than nuances in color, the luminance determines the impression of sharpness of the video image.
In color television systems, the luminance signal is composed of the addition of color components of the primary colorsred (R), green (G) and blue (B). These color components are due to the eye's sensitivity, which is much less sensitive in the blue and red range than in the green range. This spectral brightness response is described by the V-lambda curve and has the following condition for the Standard Definition Television( SDTV) luminance signal: Y = 0.299(R) + 0.587(G) + 0.114(B). If the luminance signal is composed of these color components, then a brightness-correct black-and-white reproduction of color images is guaranteed.
For HDTV, V-lambda applies: Y = 0.21(R) + 0.71 (G) + 0.08(B). According to this, dark colors are reproduced darker for HDTV.