Although the concept of "brightness" is familiar to most people, its measurement is not. There are instruments which measure brightness or more correctly luminance. The term luminance is defined by the CIE.

Simply put this equation say that luminance is equal to a constant times the sum of the spectral radiance times the photopic spectral sensitivity.

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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