Red-green colour deficiency, red-green colour blindness and total colour blindness

Life is filled with a vast array of colours – but not everyone perceives colours to the same degree: colour blindness and red-green colour deficiency are widespread colour visual impairments. And often those affected don't even notice. However, there is a test for colour blindness and red-green colour deficiency that provides results quickly. But what kinds of colour blindness and colour deficiencies are there? And how do you diagnose them?

Not all forms of colour blindness are the same. Based on the cause and symptoms, a distinction is made between colour deficiencies, partial colour blindness and total colour blindness. A person can be born with colour perception deficiencies or develop them over the course of their life. For example: many difficulties identifying colours are the result of an eye disease like macular degeneration. Certain medications taken over a long period of time or illnesses affecting the optic nerve can also result in problems seeing colours. This includes optic atrophy, i.e. the death of photoreceptor cells in the optic nerve. This has different causes, including an inflammation of the optic nerve, increased pressure in the brain or alcohol poisoning. Clouding of the lens of the eye as we age and changes in the brain can also limit our ability to perceive colours.

People suffering from hereditary colour perception difficulties often only notice after many years of living with this condition. It might take the form of a chance conversation with someone who perceives colours normally (‘It looks more blue to me...’), or when performing a task that requires a precise categorisation of colours before the person with the colour deficiency realises they don't perceive the world the same way as other people. Many professions require perfect colour vision and do not accept colour blind people or those with colour perception deficiencies. This includes policemen, painters, lacquerers, those working at CAD workstations, dentists, electricians and chemical lab assistants. Perfect colour vision is also a must in many artistic/design professions and the fashion industry. For this reason, many professions require job candidates to take a colour blindness test, such as in the transportation industry. Future pilots also must also prove that they are not colour blind, as do those applying for a motorboat licence.

The retina of the human eye consists of two types of sensory cells: rods and cones. Rods primarily help us see bright-dark contrasts, while cones are responsible for colour vision. People with normal vision have three different types of cones, each of which is responsible for a certain colour range: L cones for red, S cones for blue and M cones for green. L, S and M refer to the area of the colour spectrum covered by the particular cones: L stands for ‘long’ wavelengths, S for ‘short’ wavelengths and M for ‘medium’ wavelengths. The wavelength of the light entering the eye stimulates the colour pigments in the cones, thereby triggering different colour sensations in the brain. If a type of cone does not work properly or fails to work at all, then this limits the person's ability to perceive colours, causing a colour impairment or colour blindness. Cones are also only active at a certain level of brightness. When it's dark, only the rods responsible for brightness-darkness contrasts are at work. That is why everything matches in the dark.

People with a colour deficiency only perceive certain colours and not others because one part of their receptors – the cones – do not work properly. There are different kinds of colour deficiencies. The most common is the red-green colour deficiency, which people often (incorrectly) refer to as red-green colour blindness or just colour blindness. It affects 9% of men but only 1% of women. There are two types of red-green colour deficiencies: a difficulty perceiving green (deuteranomaly) and a difficulty perceiving red (protanomaly). Someone suffering from deuteranomaly has difficulty perceiving green because the necessary sensory cells – the cones for the colour green – are defective. Green colours appear flatter or less vibrant in comparison to people with normal colour vision, and often a person does not notice until confronted with a situation where they fail to notice a difference between different hues of green. Depending on the severity, those affected find it difficult to distinguish between red and green and often between blue and purple as well as pink and grey – especially in poor light. Symptoms of red colour deficiency are similar: since the red cones do not work correctly, these people often have difficulty perceiving the colour red correctly and distinguishing it from green. There is no treatment for either form of red-green deficiency.

If someone suffers from anomalous trichromacy, a colour perception deficiency, then all the necessary cones are there. However, their sensitivity is extremely limited, which is why colours appear less intense and those affected often confuse them. For example: a limited ability to perceive red can mean that someone with anomalous trichromacy only notices red traffic lights much later than other drivers.

If a person suffers from partial colour blindness, a portion of the sensory cells required to perceive colours are either non-existent or don't work. Dichromacy refers to someone who only has two types of functional cones, monochromacy to a person who only has one. Those affected can perceive some colours, just not the entire spectrum. With green colour blindness (deuteranopia), for example, there are no functional cones for the colour green; people with tritanopia, a blue colour deficiency, are missing cones for the colour blue. A person with protanopia lacks functional red cones. The result: they can only perceive a very limited colour spectrum. Those affected cannot distinguish at all between red and green. They either do not perceive traffic signals or the red brake lights of the vehicles ahead on the road – or only do so once it's too late. There is no treatment for partial colour blindness.

Total colour blindness (achromatopsia or achromasy) usually refers to hereditary colour blindness where the person only perceives grey scales instead of colours. The rate of achromatopsia is identical in men and women. Those affected have significantly reduced visual acuity and are extremely sensitive to light (photophobia). None of the three types of cones used for colour perception work with people who are completely colour blind. They can only see by using the rods in their eyes, i.e. the sensory cells that are responsible for vision when it is dark. For this reason, people with achromasy perceive around 500 different bright-dark gradients. Complete colour blindness is usually hereditary, but it can also result from cerebral achromatopsia following a stroke, a traumatic brain injury or other forms of brain damage.

The following colour vision tests are used to determine whether or not someone is colour blind or has a colour deficiency.

Ishihara test (pseudoisochromatic plates)
Ishihara colour plates are used to diagnose a red-green colour or a blue-yellow colour deficiency (tritanomaly). Each circular plate is covered with coloured dots that form a particular number depending on the person's ability to see colours. For example: when shown the same plate, people with normal vision see 74, whereas those with a red-green colour deficiency see 21 instead, making it possible to reliably diagnose a person's particular colour deficiency.

Nagel anomaloscope
The anomaloscope is used to diagnose red-green colour blindness or colour deficiency. The patient is asked to mix red and green light to achieve a certain shade of yellow (sodium yellow). This makes it possible to make a precise diagnosis for the particular type of colour deficiency. For example: people with a green colour deficiency tend to add too much green.

Farnsworth test
The Farnsworth test makes it possible to diagnose red-green and blue-yellow colour deficiencies. Patients are asked to sort tiles with different hues. Depending on the type of impairment, the person being tested tends to produce a typical pattern so that conclusions can be drawn about the particular type of colour deficiency.