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Colour Vision :

The human eye has receptors that are sensitive to three primary colors, red, green and blue. The brain is able to blend these three primary colors so that the eye is able to discriminate very slight differences. A person with normal color vision can see approximately 8,000 colors in nearly 8 million different shades and tints.

The retina is made up of 10 layers of different kinds of cells. These cells are connected to the brain by approximately 1 million tiny nerve fibers. When stimulated by light, these nerve fibers transmit electrical impulses from the eye to the brain, where the signals are interpreted to give vision. The retina is the focus of our "color receptors".

The very back layer of cells in the retina is called the photoreceptors. There are two types of these cells; rod and cones. Rod function well in dimly lit situations and can perceive only black, white and shades of gray. Rods are located in the outer parts of the retina, away from central vision. Cones are the second type of receptor and they are located primarily in the central part of the retina. This type of receptor functions to provide daytime vision and the important central detail vision, such used for reading small print. There are three types of cones; red, green and blue cones. These three types of cones combine to provide for the wide range in color vision. There are only about 1/3 as many cones as rods.

Color vision testing can be used to identify color defects in your vision. There are many types of color vision tests, from the general screening methods that test your gross perception of color, to other more sensitive tests, which are much more time consuming. The most common type of color vision loss is inherited and occurs from birth. But several diseases are also known to cause color vision losses later in life.


To test for colour vision, Ishihara Charts are used as shown here. The patients are asked to read the numbers written in these charts.
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