The light that falls onto the retina is processed by photoreceptors, better known as rods and cones. The rods respond to light of relatively low luminance levels, while
the less sensitive cones are more responsive to higher luminance levels.
There are three types of cones that together produce the sensation of color. Each cone has a different response curve
The cones are characterized by the peak value of the response curves: S, M and L, which respectively stand for short, middle and long wavelengths.
L, M and S cone responses to light
two colors will appear the same to us if they produce the same cone responses.
the number of S-cones on the retina is lower than the M- and L- cone numbers.
luminance is weighted by the human cone response curves. Luminance is often normalized according to a specified reference white. Lightness is the perceptual response to luminance.