Tetrachromacy is the ability to perceive four basic color types instead of the three colors, red, green, and blue, which are perceived by normal human eyes. Certain birds and even some other animals have this ability, letting them see into the ultraviolet range, inaccessible to regular human vision. This is enabled by these creatures having four types of cone cells, one for each of the basic colors they can perceive. Humans, with three basic color types, have only three kinds of cone cells in their eyes. This results in a four-dimensional mixing space of these perceived colors, as opposed to a mere three-dimensional mixing space for human vision.
Some species which have tetrachromatic eyes include:
Goldfish and Zebrafish, which are fish that have it, as well as….
Reindeer, which have it and use it, and….
Zebra Finches and Columbidae, which are birds that have it.
These last ones are interesting because not only can they see an extra part of the spectrum, the near ultraviolet spectrum, but they also use this to attract and find mates, as the color their feathers appear in ultraviolet light is related to preferences for mating found in these birds.
Finally, some humans may also have tetrachromatic vision, with four distinct cone cell types, resulting in a sharper distinction between colors that normal, three cone cell people. However, this is not quite true tetrachromacy because even if there are four separate types of cones in the eye, the visual sensory organs might not deal with the fourth one as a separate channel, but just lump it together with one of the other three, and it is not known if this would happen or not. As for seeing in ultraviolet, this is usually impossible in humans, because the cornea and lens block those wavelengths from entering at all. Still, some people don’t have a lens, and they can sometimes see ultraviolet light, appearing to them as a bright blue-like color. Also, one more interesting fact is that at low light levels, humans have been seen to exhibit a bit of tetrachromacy by using their rod cells, normally used for light intensity only, as a fourth color receptor unit. That is also an instance of something like a kind of tetrachromatic vision in human beings.