Organisms with monochromacy are called monochromats. Monochromats are truly color blind and can see only shades of black, gray and white. The perceptual effect of any arbitrarily chosen light from its visible spectrum can be matched by any pure spectral light.
The normal explanation of monochromacy is that the organism's retina contains only a single kind of light receptor cell, or at least that only one kind is active at any particular level of illumination. In vertebrates, which typically have two kinds of receptors, called rods and cones, active at low and higher levels of illumination respectively, there are two main kinds of monochromacy:
In principle there could also be a second kind of cone monochromacy, 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, but it would have good pattern vision at normal daylight levels (though it would not be able 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; there are animals (for example, many birds) with very cone-rich retinas, but they all tend to have multiple types of cones. Some individuals do possess diseases or injuries that lead to nyctalopia, or night blindness, where rod cells stop responding properly to light.
In cone monochromats, at low light intensities the rods and cones may be active simultaneously, allowing some degree of color discrimination. However it is unlikely that this will be functionally significant since the neural apparatus for hue discrimination would presumably not be present in an animal that was monochromatic most of the time.
It used to be confidently claimed that most mammals other than humans and our fellow primates were monochromats. In the last half-century, however, evidence of at least dichromatic color vision in a number of mammalian orders has accumulated. Two of the orders of sea mammals, the pinnipeds (which includes the seal, sea lion, and walrus) and cetaceans (which includes dolphins and whales) clearly are cone monochromats, since the short-wavelength sensitive cone system is genetically disabled in these animals. The same is true of the owl monkeys, genus Aotus.
Both rod and cone monochromacy occur as very rare forms of color blindness in humans. Rod monochromacy, or maskun, is the more common of the two. The majority of people described as color blind, however, are either dichromats or anomalous trichromats.
Monochromacy has been subdivided into typical (rod monochromacy) and atypical forms.
Clinically, some monochromats have normal visual acuity and others have poor visual acuity.