Color in the eye

The ability of the human eye to distinguish colors is based upon the varying sensitivity of different cells in the retina to light of different wavelengths. Humans being trichromatic, the retina contains three types of color receptor cells, or cones. One type, relatively distinct from the other two, is most responsive to light that we perceive as violet, with wavelengths around 420 nm; cones of this type are sometimes called short-wavelength cones, S cones, or blue cones. The other two types are closely related genetically and chemically. One of them, sometimes called long-wavelength cones, L cones, or red cones, is most sensitive to light we perceive as greenish yellow, with wavelengths around 564 nm; the other type, known as middle-wavelength cones, M cones, or green cones is most sensitive to light perceived as green, with wavelengths around 534 nm. Light, no matter how complex its composition of wavelengths, is reduced to three color components by the eye. For each location in the visual field, the three types of cones yield three signals based on the extent to which each is stimulated. These amounts of stimulation are sometimes called tristimulus values. The response curve as a function of wavelength for each type of cone is illustrated above. Because the curves overlap, some tristimulus values do not occur for any incoming light combination. 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. The set of all possible tristimulus values determines the human color space. It has been estimated that humans can distingui

h roughly 10 million different colors.[6] The other type of light-sensitive cell in the eye, the rod, has a different response curve. In normal situations, when light is bright enough to strongly stimulate the cones, rods play virtually no role in vision at all.[7] On the other hand, in dim light, the cones are understimulated leaving only the signal from the rods, resulting in a colorless response. (Furthermore, the rods are barely sensitive to light in the "red" range.) In certain conditions of intermediate illumination, the rod response and a weak cone response can together result in color discriminations not accounted for by cone responses alone. These effects, combined, are summarized also in the Kruithof curve, that describes the change of color perception and pleasingness of light as function of temperature and intensity. The Kruithof curve relates color temperatures that are perceived as pleasing to the illuminance (lighting level of an environment). It is named after the Dutch physicist Arie Andries Kruithof.[2] Lighting conditions lying inside the bounded region of the plot were empirically assessed as being pleasing, whereas conditions outside the region were displeasing.[3] For example, daylight at a color temperature of 6500 K and an illuminance of 104 to 105 lux results in natural color rendition, but the same color temperature would appear bluish under low luminance. At typical office illuminance levels (400 lux), pleasing color temperatures are between 3000 and 6000 K, whereas at typical home illuminance levels (75 lux), pleasing color temperatures are between 2400 and 2700 K, which can be achieved with incandescent lights.