Colour Blindness: Types, Causes and How Testing Works
Protan, deutan and tritan deficiencies explained, how common each is, why it affects men more often, and what online screening can and cannot tell you.
Colour blindness is a misleading name. Most people with a colour vision deficiency can see colour; they see some colours as more alike than other people do. The most common form, difficulty telling reds from greens, affects about one man in twelve of Northern European descent. This article explains the types, why it is so much more common in men, how testing works, and what an online test can and cannot do.
How normal colour vision works
The retina holds three types of cone cell, each most sensitive to a range of wavelengths: long (L, roughly red), medium (M, roughly green) and short (S, roughly blue). The brain compares the signals from the three types to build the sensation of colour. A colour vision deficiency occurs when one type of cone is missing, or when its sensitivity is shifted so that it overlaps too much with another.
The types
| Type | What is affected | What it looks like |
|---|---|---|
| Protanomaly / protanopia | L cones shifted or missing | Reds look darker and less saturated; red can be confused with dark green, brown or black |
| Deuteranomaly / deuteranopia | M cones shifted or missing | Greens and reds, and some browns, oranges and pinks, are confused |
| Tritanomaly / tritanopia | S cones shifted or missing | Blues and greens, and yellows and pinks or violets, are confused (rare) |
| Achromatopsia | Cone function absent or severely reduced | Little or no colour perception; often with poor acuity and light sensitivity (very rare) |
The suffix "-anomaly" means the cone is present but shifted (a milder deficiency), and "-opia" means it is missing (more severe). Deuteranomaly is the most common deficiency overall.
How common is it?
Among people of Northern European descent, roughly 8% of men and 0.5% of women have a red-green deficiency (Birch, 2012). Rates are somewhat lower in many other populations. Tritan deficiencies are far rarer, with estimates of the order of 1 in several thousand people, and complete colour blindness is rarer still.
Why men are affected far more often
The genes for the L and M cone pigments sit on the X chromosome. Men have one X chromosome, so a single faulty copy produces a deficiency. Women have two, so they usually need faulty copies on both, although a woman who carries one faulty copy can pass it on to her sons. Tritan deficiency is carried on an autosome (chromosome 7) and affects men and women equally.
Acquired colour vision changes
Colour vision can also change later in life. Causes include eye diseases such as glaucoma, macular degeneration and diabetic retinopathy; neurological conditions; certain medications and chemical exposures; and the natural yellowing of the lens with age, which makes blues harder to discriminate. A sudden change in colour vision is a reason to see an eye professional.
How colour vision is tested
- Pseudo-isochromatic plates such as the Ishihara test (devised by Shinobu Ishihara in 1917): numbers or shapes hidden in a pattern of dots. Fast and good for detecting red-green deficiencies; less good for grading severity.
- Arrangement tests such as the Farnsworth D-15 and the Farnsworth-Munsell 100 Hue test: you order coloured caps by hue. They reveal the type and rough severity of a deficiency.
- The anomaloscope: you match a yellow light to a mix of red and green light. It is the gold standard for diagnosing red-green deficiency and is used by specialists.
Our colour vision test is an arrangement test in the style of the Farnsworth-Munsell approach: you drag chips into a smooth progression of hue.
What an online test can and cannot tell you
A screen is not a calibrated instrument. The colours you see depend on the display, brightness, colour temperature and room lighting, so the same test can give different results on different devices. Turn off night mode and blue-light filters, use a bright screen and avoid glare. An online test can suggest a possible deficiency, and it can show roughly which part of the hue circle gives you trouble, but it cannot diagnose. If colour matters for your work or studies (aviation, electrical trades, some design and medical roles, and some armed services have colour vision requirements), get a clinical test.
Living with it
- Do not rely on colour alone. Designers should add labels, patterns or shapes alongside colour, and choose colour-safe palettes such as blue and orange.
- Use tools. Phone apps and operating-system settings can recolour or label colours; browser extensions can simulate or adjust colours.
- Special lenses. Coloured glasses marketed to people with colour blindness can enhance colour contrast for some people under some conditions, but they do not restore normal colour vision. Studies show modest effects.
- It is not a disease. Most people with a deficiency adapt well and manage everyday life, though tasks like judging ripeness, reading colour-coded maps or charts, or checking wiring can be harder.
Sources
- Birch, J. (2012). Worldwide prevalence of red-green color deficiency. Journal of the Optical Society of America A, 29(3), 313–320.
- Deeb, S. S. (2005). The molecular basis of variation in human color vision. Clinical Genetics, 67(5), 369–377.
- Farnsworth, D. (1943). The Farnsworth-Munsell 100-Hue and Dichotomous Tests for Color Vision. Journal of the Optical Society of America, 33(10), 568–578.
- Gómez-Robledo, L., et al. (2018). Do EnChroma glasses improve color vision for colorblind subjects? Optics Express, 26(22), 28693–28703.