- The human eye with normal vision has three kinds of cone cells that sense light, having peaks of spectral sensitivity in short (“S”, 420 nm – 440 nm), middle (“M”, 530 nm – 540 nm), and long (“L”, 560 nm – 580 nm) wavelengths.
- These cone cells underlie human colour perception in conditions of medium and high brightness; in very dim light colour vision diminishes, and the low-brightness, monochromatic “night vision” receptors, denominated “rod cells”, become effective.
- Thus, three parameters corresponding to levels of stimulus of the three kinds of cone cells, in principle describe any human colour sensation. Weighting a total light power spectrum by the individual spectral sensitivities of the three kinds of cone cells renders three effective values of stimulus; these three values compose a tristimulus specification of the objective colour of the light spectrum.
- The three parameters denoted “S”, “M”, and “L”, are indicated using a 3-dimensional space denominated the “LMS colour space”, which is one of many colour spaces devised to quantify human colour vision.
- Stimuli that account for colour perception: can be specified by a set of tristimulus values, defined as the “amounts of the 3 reference colour stimuli, in a given trichromatic system, required to match the colour of the stimulus considered”.
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