Although we perceive a seemingly endless variety of hues, the first stage of human colour vision is relatively simple, since individual photoreceptors are blind to colour and generate a univariant signal that depends solely on the number of photons that each absorbs. The existence of three different daytime cone photoreceptor types with distinct spectral sensitivities means that human colour vision is a three-variable, trichromatic system, such that colour matches can be defined by just three numbers. We are able to perceive colour by comparing the outputs of the three cone types: long-wavelength, middlewavelength and short-wavelength sensitive cones. Yet, despite the simplicity of the underlying cone signals, the colours that we perceive depend on many other factors, including individual differences, chromatic adaptation, the influence of surround colours, the influence of preceding colours, colour constancy and cognitive factors.
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