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

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

Not all forms of color blindness are the same. Based on the cause and symptoms, a distinction is made between color deficiencies, partial color blindness and total color blindness. A person can be born with color perception deficiencies or develop them over the course of their life. For example: many difficulties identifying colors 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 colors. 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 as we age and changes in the brain can also limit our ability to perceive colors.People suffering from hereditary color 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 colors normally ("It looks bluer to me...”), or when performing a task that requires a precise categorization of colors before the person with the color deficiency realizes they don't perceive the world the same way as other people. Many professions require perfect color vision and do not accept color blind people or those with color perception deficiencies. This includes policemen, painters, lacquerers, those working at CAD workstations, dentists, electricians and chemical lab assistants. Perfect color 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 color blindness test, such as in the transportation industry. Future pilots also must also prove that they are not color blind, as do those applying for a motorboat license.

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 color vision. People with normal vision have three different types of cones, each of which is responsible for a certain color range: L cones for red, S cones for blue and M cones for green. L, S and M refer to the area of the color 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 color pigments in the cones, thereby triggering different color 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 colors, causing a color impairment or color 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 color deficiency only perceive certain colors and not others because one part of their receptors – the cones – do not work properly. There are different kinds of color deficiencies. The most common is the red-green color deficiency, which people often (incorrectly) refer to as red-green color blindness or just color blindness. It affects 9% of men, but only 1% of women. There are two types of red-green color deficiency: 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 color green – are defective. Green colors appear flatter or less vibrant in comparison to people with normal color 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 gray – especially in poor light. Symptoms of red color deficiency are similar: since the red cones do not work correctly, these people often have difficulty perceiving the color 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 color perception deficiency, then all the necessary cones are there. However, their sensitivity is extremely limited, which is why colors 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 color blindness, a portion of the sensory cells required to perceive colors 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 colors, just not the entire spectrum. With green color blindness (deuteranopia), for example, there are no functional cones for the color green; people with tritanopia, a blue color deficiency, are missing cones for the color blue. A person with protanopia lacks functional red cones. The result: they can only perceive a very limited color 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 of them on the road – or only do so once it's too late. There is no treatment for partial color blindness.

Total color blindness (achromatopsia or achromasy) usually refers to hereditary color blindness where the person only perceives gray scales instead of colors. 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 color perception work with people who are completely color 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 color 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 color vision tests are used to determine whether or not someone is color blind or has a color deficiency.

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

Nagel anomaloscope The anomaloscope is used to diagnose red-green color blindness or color 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 precisely diagnose the particular type of color deficiency. For example: people with a green color deficiency tend to add too much green.

Farnsworth test The Farnsworth test makes it possible to diagnose red-green and blue-yellow color 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 color deficiency.