the human eye

Cones consist of three cell types, each "tuned" to a distinct wavelength response maximum centered at either 430, 535, or 590 nanometers. The basis for the individual maxima is the utilization of three different photopigments, each with a characteristic visible light absorption spectrum. The photopigments alter their conformation when a photon is detected, enabling them to react with transducin to initiate a cascade of visual events. Transducin is a protein that resides in the retina and is able to effectively convert light energy into an electrical signal. The population of cone cells is much smaller than rod cells, with each eye containing between 5 and 7 million of these color receptors. True color vision is induced by the stimulation of cone cells. The relative intensity and wavelength distribution of light impacting on each of the three cone receptor types determines the color that is imaged (as a mosaic), in a manner comparable to an additive RGB video monitor or CCD color camera.

When all three types of cone cell are stimulated equally, the light is perceived as being achromatic or white. For example, noon sunlight appears as white light to humans, because it contains approximately equal amounts of red, green, and blue light. An excellent demonstration of the color spectrum from sunlight is the interception of the light by a glass prism, which refracts (or bends) different wavelengths to varying degrees, spreading out the light into its component colors. Human color perception is dependent upon the interaction of all receptor cells with light, and this combination results in nearly trichromic stimulation. There are shifts in color sensitivity with variations in light levels, so that blue colors look relatively brighter in dim light and red colors look brighter in bright light. This effect can be observed by pointing a flashlight onto a color print, which will result in the reds suddenly appearing much brighter and more saturated.

When only one or two types of cone cells are stimulated, the range of perceived colors is limited. For example, if a narrow band of green light (540 to 550 nanometers) is used to stimulate all of the cone cells, only the ones containing green photoreceptors will respond to produce a sensation of seeing the color green. Human visual perception of primary subtractive colors, such as yellow, can arise in one of two ways. If the red and green cone cells are simultaneously stimulated with monochromatic yellow light having a wavelength of 580 nanometers, the cone cell receptors each respond almost equally because their absorption spectral overlap is approximately the same in this region of the visible light spectrum. The same color sensation can be achieved by stimulating the red and green cone cells individually with a mixture of distinct red and green wavelengths selected from regions of the receptor absorption spectra that do not have significant overlap. The result, in both cases, is simultaneous stimulation of red and green cone cells to produce a sensation of yellow color, even though the end result is achieved by two different mechanisms. The ability to perceive other colors requires the stimulation of one, two, or all three types of cone cells, to various degrees, with the appropriate wavelength palette.

Although the human visual system features three types of cones cells with their respective color pigments plus light-receptive rod cells for scotopic vision, it is the human brain that compensates for variations of light wavelengths and light sources in its perception of color. Metamers are pairs of different light spectra perceived as the same color by the human brain. Interestingly, colors that are interpreted as the same or similar by a human are sometimes readily distinguishable by other animals, most notably birds.

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