Monochromacy - Types

Types

There are two known types of monochromacy. "Animals with monochromatic vision may be either rod monochromats or cone monochromats. These monochromats contain photoreceptors which have a single spectral sensitivity curve." There is also a third, theoretical type that has never been identified.

• Rod monochromacy is the condition of having only rods in the retina. A rod monochromat is truly unable to see any color and can see only shades of grey.
• Cone monochromacy is the condition of having both rods and cones, but only a single kind of cone. A cone monochromat can have good pattern vision at normal daylight levels, but will not be able to distinguish hues. In humans, who have three different types of cones, there are three differing forms of cone monochromacy. There are three types named according to the single functioning cone class:

1. Blue cone monochromacy, also known as S-cone monochromacy
2. Green cone monochromacy, also known as M-cone monochromacy
3. Red cone monochromacy, also known as L-cone monochromacy

• Cone monochromacy, type II, if its existence were established, would be the case in which the retina contains no rods, and only a single type of cone. Such an animal would be unable to see at all at lower levels of illumination, and of course would be unable to distinguish hues. In practice it is hard to produce an example of such a retina, at least as the normal condition for a species.