Monochromacy
Monochromacy (mono meaning one and chromo color) is, among animals, the condition of possessing only one type of cone cell for color vision, which results in a monochromatic-like (in most cases black and white) vision.
Organisms with monochromacy are called monochromats. The perceptual effect of any arbitrarily chosen light from the visible spectrum can be matched by any pure spectral light.
Many species, such as all marine mammals, the owl monkey, and the Australian sea lion (pictured at right) are monochromats under normal conditions. However, in other mammal species, including humans, individuals having such vision are considered carriers of full color blindness disease. In humans that disease is called achromatopsia.
Causes
There are two kinds of visual receptors in humans:
Rods, which are extremely abundant in numbers (about 120 million) in the periphery of the human retina. Rods only respond to faint levels of light and are very light sensitive, therefore, completely useless in daylight because bright light bleaches them.