In order to see color, humans and animals must have at least two different kinds of color-sensitive cells. These cells, called cones, send messages about color to our brains. If the brain can understand the messages then it will see color. Cones work in bright light and help you see color during the day. Cones also help you pick out details like the leaves on a tree or the fine print in a newspaper. When it’s dark your cones turn off. That’s why it’s hard to see color at night. You have about six million cones on your retina. There are three different kinds, each sensitive to a different color: red, green and blue. Your brain mixes the messages from these cones to see all the colors of the rainbow.
Some people can not see some or all colors, this condition is called color blindness. Color blindness occurs when some of the cones in the eye’s retina are not working properly or may be missing. This sometimes happens due to an injury or disease, but most color blindness is inherited and is present from birth. The most common form of color blindness is red/green. A person who has red/green color blindness either cannot see red and green at all or can see only a limited number of shades. Very rarely a person will have yellow/blue color blindness and even more rarely a person can be absolutely color blind, where they perceive the world only in shades of gray. For humans, the most popular test for color blindness was developed in 1916 by Dr. Shinobu Ishihara.
In 1969 a scientist named Anita Rosengren studied color vision in dogs and found that they do see in color. However dogs can’t see as many colors as you do. Its eyes have only two different kinds of cones and your eyes have three. Dogs can tell the difference between blue and yellow, but can’t tell red from green. The same is true for cats, squirrels and many other animals.
While some animals can not see as many colors as we do, some can see more. While you only have three different kinds of cones, butterflies have four and pigeons have five. Scientists study why animals see more colors than we do. For example, in 1910 Karl Von Frisch discovered that the bee can see ultraviolet (UV) light. UV is a color that is invisible to our eyes. Bees use this UV vision to see special patterns on flower petals. These patterns help a bee to see where the nectar is.
The mantis shrimp has amazing color vision. A mantis shrimp has at least 12 different kinds of color-sensitive cells. The mantis shrimp probably sees more colors than any other animal. A vision scientist named Dr. Thomas Cronin remarked, “People like me study animal vision because we are inherently interested in the problems and we see a puzzle out there that we’d like to solve. Why do animals see ultraviolet light for example? We don’t see it, what’s the advantage? The thing that caught me most by surprise in my career was discovering that mantis shrimps have so many different kinds of color receptors. It was completely unexpected. ”