How Colour Vision Works

The human eye sees light by stimulating receptors (rods & cones) at the back of the eye (the retina). Rods are used for night vision, where Cones are useful during the day and for colour perception.  Each cone contains 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 causing a colour vision deficiency.

The Different Types of Colour Vision Problems

The common misconception is that people with colour vision deficiency can only see black and white. It is extremely rare to be totally colour blind. People with normal colour vision will have 3 fully functional cones whereas 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 that person is said to have a mild colour vision deficiency – this includes protanomaly and deuteranomaly. A more severe colour deficiency is present when one or more of the cones light sensitive pigments is effectively missing – this includes protanopia and deuteranopia.

What Causes Colour Vision Deficiencies

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

The most common forms of congenital defective colour vision, the red-green deficiencies, are due to simple recessive hereditary abnormalities in the “sex-linked X chromosomes”. This 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 colour defective he will be colour deficient, whereas, a woman must inherit two colour defective X chromosomes to be colour deficient. For a woman to be colour deficient, her father and mother must be colour blind or be a carrier of the defective gene.

The Different Types of Colour Vision Problems

Protanomaly: (1 out of 100 males)

Any redness seen in a colour 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: (5 out of 100 males)

Red, orange, yellow and 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: (1 out of 100 males)

Both protanopia and deuteranopia are usually referred 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 blue to the protanope. They use brightness as a cue to distinguish reds from yellows and greens.

Deuteranopia: (1 out of 100 males)

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

Tritanope: (1 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.

How You Screen For Colour Vision Problems

A common test you might come across during a standard eye test is the ISHIHARA test.  This test doesn’t pick up subtleties and will fail almost everyone regardless of how mild or severe their colour vision problem is.

The Extra Colour Tests You Can Do

How Colour Vision Problems Can Impact Your Child

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