Luminance and lightness
Perceptual coding is essential to maximize the performance of an image coding system. In commercial imaging, we rarely use pixel values proportional to luminance; instead, we use pixel values that approximate lightness. This chapter introduces luminance and lightness.
Relative luminance, denoted Y, is what I call alinear-light quantity; it is directly proportional to physical radiance weighted by the spectral sensitivity of human vision. Luminance involves light having wavelengths in the range of about 400 nm to 700 nm. (Luminance can also be computed as a properly weighted sum of linear-light red, green, and blue tristimulus components according to the principles and standards of the CIE.)
In Colour science for video ...