Because the curves overlap, some tristimulus values do not occur for any incoming light combination. The response curve as a function of wavelength varies for each type of cone. These amounts of stimulation are sometimes called tristimulus values. For each location in the visual field, the three types of cones yield three signals based on the extent to which each is stimulated. Each cone type adheres to the principle of univariance, which is that each cone’s output is determined by the amount of light that falls on it over all wavelengths. Light, no matter how complex its composition of wavelengths, is reduced to three color components by the eye. It has been estimated that humans can distinguish roughly 10 million different colours. The set of all possible tristimulus values determines the human color space.
For example, it is not possible to stimulate only the mid-wavelength (so-called “green”) cones the other cones will inevitably be stimulated to some degree at the same time.
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