– Ed

What is colour blindness?

A general consensus is, if you suffer from colour blindness you cannot see any colours at all apart from black, white, and shades of gray. Wrong! This form of complete colour blindness is a very rare subtype of colour vision deficiency. A much more common type is red-green colour blindness. Generally speaking, colour deficiency would be more appropriate than colour blindness. Colour Blindness begins when you have only a very light change of colour perception and it ends with complete colour blindness combined under the umbrella term monochromacy.

People with colour blindness aren’t aware of differences among colour that are obvious to the rest of us. People who don’t have the more severe types of colour blindness may not even be aware of their condition unless they’re tested in a clinic or laboratory.

Inherited colour blindness is caused by abnormal photopigments. These colour-detecting molecules are located in cone-shaped cells within the retina, called cone cells. In humans, several genes are needed for the body to make photopigments, and defects in these genes can lead to colour blindness.

There are three main kinds of colour blindness, based on photopigment defects in the three different kinds of cones that respond to blue, green, and red light. Red-green colour blindness is the most common, followed by blue-yellow colour blindness. A complete absence of colour vision —total colour blindness – is rare.

Sometimes colour blindness can be caused by physical or chemical damage to the eye, the optic nerve, or parts of the brain that process colour information. Colour vision can also decline with age, most often because of cataract – a clouding and yellowing of the eye’s lens.

Who gets colour blindness?

As many as 8 percent of men and 0.5 percent of women with Northern European ancestry have the common form of red-green colour blindness. Men are much more likely to be colour blind than women because the genes responsible for the most common, inherited colour blindness are on the X chromosome. Males only have one X chromosome, while females have two X chromosomes. In females, a functional gene on only one of the X chromosomes is enough to compensate for the loss on the other. This kind of inheritance pattern is called X-linked, and primarily affects males. Inherited colour blindness can be present at birth, begin in childhood, or not appear until the adult years.

Your optometrist can help you decide which lens design will best suit  your needs.

How do we see colour?

Normal human retina

What colour is a strawberry? Most of us would say red, but do we all see the same red? Colour vision depends on our eyes and brain working together to perceive different properties of light.

We see the natural and artificial light that illuminates our world as white, although it is actually a mixture of colours that, perceived on their own, would span the visual spectrum from deep blue to deep red. You can see this when rain separates sunlight into a rainbow or a glass prism separates white light into a multi-colour band. The colour of light is determined by its wavelength. Longer wavelength corresponds to red light and shorter wavelength corresponds to blue light.

Strawberries and other objects reflect some wavelengths of light and absorb others. The reflected light we perceive as colour. So, a strawberry is red because its surface is only reflecting the long wavelengths we see as red and absorbing the others. An object appears white when it reflects all wavelengths and black when it absorbs all wavelengths.

What are the different types of colour blindness?

The most common types of colour blindness are inherited. They are the result of defects in the genes that contain the instructions for making the photopigments found in cones. Some defects alter the photopigment’s sensitivity to colour, for example, it might be slightly more sensitive to deeper red and less sensitive to green. Other defects can result in the total loss of a photopigment. Depending on the type of defect and the cone that is affected problems can arise with red, green, or blue colour vision.

Red-Green colour blindness

The most common types of hereditary colour blindness are due to the loss or limited function of red cone (known as protan) or green cone (deutran) photopigments. This kind of colour blindness is commonly referred to as red-green colour blindness.

  • Protanomaly: In males with protanomaly, the red cone photopigment is abnormal. Red, orange, and yellow appear greener and colours are not as bright. This condition is mild and doesn’t usually interfere with daily living. Protanomaly is an X-linked disorder estimated to affect 1 percent of males.
  • Protanopia: In males with protanopia, there are no working red cone cells. Red appears as black. Certain shades of orange, yellow, and green all appear as yellow. Protanopia is an X-linked disorder that is estimated to affect 1 percent of males.
  • Deuteranomaly: In males with deuteranomaly, the green cone photopigment is abnormal. Yellow and green appear redder and it is difficult to tell violet from blue. This condition is mild and doesn’t interfere with daily living. Deuteranomaly is the most common form of colour blindness and is an X-linked disorder affecting 5 percent of males.
  • Deuteranopia: In males with deuteranopia, there are no working green cone cells. They tend to see reds as brownish-yellow and greens as beige. Deuteranopia is an X-linked disorder that affects about 1 percent of males.

Blue-Yellow colour blindness

Blue-yellow colour blindness is rarer than red-green colour blindness. Blue-cone (tritan) photopigments are either missing or have limited function.

  • Tritanomaly: People with tritanomaly have functionally limited blue cone cells. Blue appears greener and it can be difficult to tell yellow and red from pink. Tritanomaly is extremely rare. It is an autosomal dominant disorder affecting males and females equally.
  • Tritanopia: People with tritanopia, also known as blue-yellow colour blindness, lack blue cone cells. Blue appears green and yellow appears violet or light grey. Tritanopia is an extremely rare autosomal recessive disorder affecting males and females equally.

Complete colour blindness

People with complete colour blindness (monochromacy) don’t experience colour at all and the clearness of their vision (visual acuity) may also be affected. There are two types of monochromacy:

  • Cone monochromacy: This rare form of colour blindness results from a failure of two of the three cone cell photopigments to work. There is red cone monochromacy, green cone monochromacy, and blue cone monochromacy. People with cone monochromacy have trouble distinguishing colours because the brain needs to compare the signals from different types of cones in order to see colour.
    When only one type of cone works, this comparison isn’t possible. People with blue cone monochromacy, may also have reduced visual acuity, near-sightedness, and uncontrollable eye movements, a condition known as nystagmus. Cone monochromacy is an autosomal recessive disorder.
  • Rod monochromacy or achromatopsia: This type of monochromacy is rare and is the most severe form of color blindness. It is present at birth. None of the cone cells have functional photopigments. Lacking all cone vision, people with rod monochromacy see the world in black, white, and gray. And since rods respond to dim light, people with rod monochromacy tend to be photophobic – very uncomfortable in bright environments. They also experience nystagmus. Rod monochromacy is an autosomal recessive disorder.

How is colour blindness diagnosed?

Ishihara Colour Tests

Eye Care Professionals use a variety of tests to diagnose colour blindness. These tests can quickly diagnose specific types of colour blindness.

The Ishihara Colour Test is the most common test for red-green colour blindness. The test consists of a series of coloured circles, called Ishihara plates, each of which contains a collection of dots in different colours and sizes. Within the circle are dots that form a shape clearly visible to those with normal colour vision, but invisible or difficult to see for those with red-green colour blindness. There are several other tests to diagnose colour vision defects.

Are there treatments for colour blindness?

There is no cure for colour blindness. However, people with red-green colour blindness may be able to use a special set of lenses to help them perceive colours more accurately. These lenses can only be used outdoors under bright lighting conditions. Visual aids have also been developed to help people cope with colour blindness. There are iPhone and iPad apps, for example, that help people with colour blindness discriminate among colours. Some of these apps allow users to snap a photo and tap it anywhere on the image to see the colour of that area. More sophisticated apps allow users to find out both colour and shades of colour. These kinds of apps can be helpful in selecting ripe fruits such as bananas, or finding complementary colours when picking out clothing.

How does colour blindness affect daily life?

Colour blindness can make it difficult to read colour-coded information such as bar graphs and pie charts. This can be particularly troubling for children who aren’t yet diagnosed with colour blindness, since educational materials are often colour-coded. Children with red-green colour blindness may also have difficulty reading a green chalkboard when yellow chalk is used. Art classes, which require selecting appropriate colours of paint or crayons, may be challenging.

Colour blindness can go undetected for some time since children will often try to hide their disorder. It’s important to have children tested, particularly boys, if there is a family history of colour blindness. Many school systems offer vision screening tests that include colour blindness testing. Once a child is diagnosed, he or she can learn to ask for help with tasks that require colour recognition.
Simple everyday tasks like cooking meat to the desired colour or selecting ripe produce can be a challenge for adults. Children might find food without bright colour as less appetising. Traffic lights pose challenges, since they have to be read by the position of the light. Since most lights are vertical, with green on bottom and red on top, if a light is positioned horizontally, a colour blind person has to do a quick mental rotation to read it. Reading maps or buying clothes that match colours can also be difficult. However, these are relatively minor inconveniences and most people with colour blindness learn to adapt. Lastly, it is important to be aware of a colour defect before launching into studies for a particular career such as electrical engineering. It would be unfortunate to discover at a late stage that another career path would have been more apropriate.

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