Ask An Optom: what is “colour blindness” and how is it diagnosed?

How many Australians have colour vision deficiency?

According to the Australian Institute of Health and Welfare (AIHW), more than half a million Australians have colour blindness, with men 16 times more likely to be colour vision deficient than women.

To further understand colour vision deficiencies and how colour vision testing works to diagnose the condition, we spoke with Dr Amanda Douglass, an optometrist and Senior Lecturer in Optometry and Vision Science at Deakin University.

What are the primary types of colour vision deficiencies and how do they differ in terms of the colours affected?

Colour is perceived by our brain based on signals generated by three different photoreceptors at the back of the eye called cones. We have one cone type that is maximally sensitive to red, one to green, and one to blue. The brain compares the responses from these different cones through channels (red vs green, blue vs yellow and a brightness channel) and generates a perception of colour. Colour vision deficiencies occur because of changes anywhere in the colour pathway from the eye to the brain and are classified depending on which part of the pathway is affected.

Colour vision deficiencies can be either congenital (deficiencies we are born with) or acquired (deficiencies we are not born with, but which we acquire for some reason at some point in our life).

Congenital colour vision deficiencies are caused by genetic changes to the cones, most commonly either the red or green cones. Deficiencies vary from person to person in terms of the number of colours but also in the spectrum of colours the person can see. If a person has extreme difficulties with the red/green pathway then they see the world in a spectrum of blue and yellow, although more commonly people can still see the affected colour but are just less sensitive to it.

The pictures below simulate the limited colours that people with the most common types of colour deficiency can see.

Acquired colour vision deficiencies can occur due to changes anywhere in the colour pathway from the eye to the brain due to eye conditions, systemic diseases, some medications and intracranial injuries.

What are the common misconceptions about colour vision deficiencies you often encounter?

The term colour blindness is a bit misleading; there are only a tiny proportion of people in the world who are truly colour blind and see only shades of grey. This occurs when there is only one receptor able to detect the light and the brain is therefore unable to make comparisons between different receptors to determine colour (i.e. they only have the brightness channel remaining). Most people who are called colour blind are really colour deficient and see the world on a more limited spectrum of colours, as either the red vs green or yellow vs blue channel is altered or not working. For people with red-green colour blindness this may result in the confusion of not only reds and greens but also other groups of colours.

It's also important to understand that people have differing severity of colour deficiencies. Some people are completely missing a channel and will have a profound deficiency, others will have three channels but one will not be working very well so they will have difficulty differentiating some colours but may be able say to tell the difference between bright red and bright green lights.

How do Ishihara test plates work to assess colour vision?

Colour vision deficiencies reduce the number of colours that an individual can see. Ishihara plates have pictures made up of coloured dots with varying levels of brightness. The colours are chosen so that people with a red/ green deficiency (this is the only type that is tested by the Ishihara plate) see a bunch of dots of similar colours. Those with normal colour vision can see numbers formed by clustering of dots of a certain colour. In the pictures above the pink pencil is easily distinguished from the green pencil by someone with normal colour vision, yet the colours are difficult to distinguish for someone with a colour deficiency. The Ishihara plates are great for screening for congenital defects which are red/green but they tell us nothing about the severity of the colour deficiency, or other categories of deficiency.

What advancements have been made in the field of colour vision testing beyond the traditional Ishihara plates?

For me, seeing the advancement of colour vision testing into computerised testing has been exciting. We have moved from tests developed around the time of the second World War to computerised tests such as the Konan CCT-HD (developed by the American Airforce and commercialised by Konan) and the Colour Assessment and Diagnosis (CAD) developed by City University. Both tests allow colour vision to be tested relatively quickly and provide both a good indication of the type of defect as well as its severity.

What are some of the challenges that individuals with colour blindness may face in daily life?

Depending on the severity, colour vision deficiency can have a range of impacts. Clashes in clothing is the one that often first comes to mind, however with a bit of help from someone with normal colour vision outfits can be arranged. Daily challenges tend to occur more with cooking, such as telling if fruit is ripe or rotten or if meat is cooked. Colour deficient parents can also have difficulty determining if their child is sunburnt. Picking up a red football or cricket ball against the green grass is also a common difficulty depending on the exact shade. Traffic lights can be difficult if a person is not aware of their deficiency. This is a particular issue in acquired defects where someone has learnt to drive based on the colour signals rather than the position of the traffic lights.

How might colour deficiencies impact career choices and job opportunities for those affected?

Most occupations do not have a colour vision standard and being colour deficient may not affect a person’s job prospects at all. There are a small number of occupations which have colour vision standards, specifically where confusing colours could pose significant safety risks, such as commercial pilots, train drivers or maritime workers.