The human visual system plays an integral part in the theory of color. Within the human eye are two element which are responsible for the perception of light. These are the rods and cones. The rods contain the elements that are sensitive to light intensities. They are used almost exclusivly at night for humans night vision. The cones provide humans with vision during the daylight and are believed to separated into three types, where each type is more sensitive to a particular wavelength. An estimate of the spectral sensitivities is shown in
The perception of color by the human visual system is based on the tristimulus theory. The theory states that color vision results from the action of three cone receptor mechanisms with different spectral sensitivies. When light of a particular wavelength is presented to the eye, these mechanisms are stimulated to different degrees, and the ratio of activity in the three mechanisms results in the perception of a color. Each color is therefore, coded in the nervous system by its own ratio of activity in the three receptor mechanism.