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Colour Vision

The human eye sees light by stimulating receptors at the back of the eye ( the retina). These photoreceptors are known as rods and cones. The rods are used for night vision but cannot be used to distinguish colour. Cones are useful during the day and for colour perception.

The cones, each contain a light sensitive pigment specific to a particular range of wavelengths. If the genes which make up the light sensitive pigment are wrong, then the cones will be sensitive to different wavelengths of light hence colour vision deficiency.

The common misconception is that people with colour vision deficiency can only see black and white. It is extremely rare to be totally color blind (monochromasy - complete absence of any color sensation).

People with normal colour vision will have fully functional red, green and blue cones. A person who has colour vision deficiency will have one or more cones not functioning properly. When only one or more of the three cones are not quite right and their peak sensitivity is shifted the person has a mild colour vision deficiency called anomalous trichromasy. This includes protanomaly and deuteranomaly. A more severe color deficiency is present when one or more of the cones light sensitive pigments is really wrong. This is called dichromasy and includes protanopia and deuteranopia.

What causes colour vision deficiency?

Some eye diseases and conditions may cause colour vision deficiency in adulthood. Children born with colour vision deficiency mostly inherit the condition.

The commonest forms of congenital defective color vision, the red-green deficiencies, are due to simple recessive hereditary abnormalities in the "sex-linked X chromosomes". Thus means that men are mainly affected because women have two X chromosomes and men have only one X and a Y chromosome. If a man's one X chromosome is color defective he will be color deficient, where as , a woman must inherit two color defective X chromosomes to be color deficient. For a woman to be color deficient, her father must be colorblind and her mother colorblind or be a carrier of the defective gene.

Different types of colour blindness

Protanomaly: ( one out of 100 males)

Any redness seen in a color by a normal observer is seen less saturated and bright by the protanomalous viewer. Red, orange, yellow, and yellow-green appear greener and paler, the extent varies according to severity.

When driving in bright sunlight or in rainy or foggy weather, protanomalous individuals can easily mistake a blinking red traffic light from a blinking yellow or amber one.

Deuteranomaly: ( five out of 100 males)

Red, orange, yellow, green region of the spectrum appear somewhat shifted towards red. However unlike protanomalous individuals, brightness of colour is not reduced.

People with deuteranomaly can easily do tasks which require normal colour vision. They may not be aware of the colour vision deficiency until tested in a routine eye examination.

Protanopia: ( one out of 100 males)

Both protanopia and deuteranopia are usually reffered to as red-green colour blindness. They are both dichromats which means they only have one out of the three types of cones functioning.

For the protanope, the brightness of red, orange, and yellow is much reduced compared to normal. Reds may be confused with black or dark gray. As a result, red traffic lights may appear to be extinguished, and pink flowers may appear just blue to the protanope. They muse brightness as a cue to distinguish reds from yellows and greens.

Deuteranopia: ( one out of 100 males)

Deuteranope suffers the same hue discrimination problems as the protanope, but without the abnormal dimming.

Tritanope: ( one out of 10′000 persons)

Tritanope confuse blue with green and yellow with violet. They do not confuse blue and yellow. Alcohol, cataract and head injury may cause blue-yellow colour deficiency in adulthood.

Normal Protanope Deuteranope