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Colour Blindness

Did you know the human eye is capable of distinguishing up to 10 million colours?  Colour discrimination is critical to how we interact with our environment and adds beauty to our world. It is also a common language. For example, we can ask a friend “Would you hand me the red container on the self?” because we share a mutual perception for the colour red.

Colour vision perception is a complex process beginning with a structure in the eye called the retina. Within the retina are receptor cells we call rods and cones. The cone receptors correspond primarily to vision in our central visual field and are best at detecting detail and colour. There are 3 different cones types, each responding best to a certain range of light’s wavelengths. The brain processes the response of each of these cones to “triangulate” the perceived colour. The visible colour spectrum extends from violet to red.

Some people have altered colour vision perception often referred to as colour blindness or colour deficiency. There are many forms and causes of altered colour vision but I will describe two of the more common inherited causes of the so called “red-green color blindness”: dichromacy and anomalous trichromacy. Dichromacy is the more severe colour deficiency caused by the absence or malfunction of one of the 3 types of cones. Anomalous trichromacy is more common and occurs when one of the 3 cones does not function properly.

In colour blindness, the colours that cannot be perceived depend on which cone is abnormal or absent. Problems with the longer wavelength or red sensitive cone results in either protanopia (a dichromacy) or protanomaly (an anomalous trichromacy). Red light may appear gray or duller. Red and greens may be difficult to distinguish.

If the medium wavelength or green sensitive cones are affected it is called deuteranopia (a dichromacy) or deuteranomaly (an anomalous trichromacy). Green appears duller or gray and differences in hues of red, orange, yellow and green are challenging.

These colour deficiencies are called “sex-linked” because many of the genes for colour vision are located on the X chromosome. Because of this males are about 8 times more likely to inherit a red-green deficiency. The most common red-green deficiency is deuteranomaly.

Most people with colour blindness adapt well to their environment. It is important to recognize altered colour perception in children so that teaching methods can be adjusted and leeway provided for responses to colour based problems. A few occupations requiring accurate colour discrimination may be difficult or impossible to achieve. Some coloured filters, software, apps and other devices can provide some limited assistance in overcoming reduce colour discrimination.

You optometrist can test for colour vision deficiency, starting at about age five. There are some interesting online tests with links listed below. Interpret these results with caution because proper room lighting and monitor calibration with external hardware are crucial to the accuracy of the test results.

Colour IQ Test

PPI Colour Vision Test

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