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A theory proposed to explain colour and brightness perception and constancies. It postulates that the colour of an object is not determined by the spectral composition of the light stimulus coming from an object, but is determined by information obtained from a comparison of three lightnesses generated by the light absorption of the three types of cone cell. The name of the theory 'retinex' reflects the fact that the perception of the image results from the retina and the cortex. See pigment visual.