So you can see that luminance takes into account two major factors: 1. the energy of the chromatic light and 2. the spectral sensitivity of the observer.
The luminance channel is also referred to as the achromatic channel or the spectrally non-opponent channel. The strongest input to this channel is the sum of the long wavelength sensitive (L) and middle wavelength sensitive (M) cone receptors. The observer spectral sensitivity is primarily a function of the L and M cone excitation.
There is some evidence that the short wavelength sensitive (S) cone receptors have an input to the luminance channel but that it is a subtractive input. This result is obtained under very specific experimental conditions and is a very small effect.
A major pathway of the luminance channel is via the magnocellular pathways. This is the pathway that goes through the magnocellular layers of the lateral geniculate nucleus (LGN). However, there is a school of thought that believes that a L+M signal also goes through the parvocellular pathway.
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