What is lightness?
Human vision has a nonlinear perceptual response to brightness: a source having a luminance only 18% of a reference luminance appears about half as bright. The perceptual response to luminance is called Lightness and is defined by the CIE [1] as a modified cube root of luminance: Yn is the luminance of the white reference. If you normalize luminance to reference white then you need not compute the quotient. The CIE definition applies a linear segment with a slope of 903.3 near black, for (Y/Yn) < 0.008856. The linear segment is unimportant for practical purposes but if you don't use it, make sure that you limit L* at zero. L* has a range of 0 to 100, and a "delta L-star" of unity is taken to be roughly the threshold of visibility. Stated differently, lightness perception is roughly logarithmic. You can detect an intensity difference between two patches when the ratio of their intensities differs by more than about one percent. Video systems approximate the lightness response of vision
