How color vision works, briefly
Most people have three kinds of cone cell in the retina. L cones respond most to long wavelengths (reds), M cones to medium (greens) and S cones to short (blues). The brain compares their signals to work out color. Color blindness, more precisely called color vision deficiency, happens when one cone type is missing or responds at the wrong wavelengths.
That gives three families, named after the Greek for first, second and third:
| Family | Cone affected | Missing cone | Shifted cone |
|---|---|---|---|
| Protan | L (red) | Protanopia | Protanomaly |
| Deutan | M (green) | Deuteranopia | Deuteranomaly |
| Tritan | S (blue) | Tritanopia | Tritanomaly |
"-opia" means the cone type is absent, so the person sees with two cone types (dichromacy). "-anomaly" means all three are present but one is shifted, so colors are weaker rather than gone. A seventh condition, achromatopsia, means no cones work at all and the world is seen in shades of gray.
Red-green: by far the most common
Protan and deutan types together are what people usually mean by color blindness. The genes for the L and M cones sit on the X chromosome. Men have one X, so a single altered copy is enough; women need two. That is why commonly cited estimates for people of Northern European ancestry are around 8% of men but only about 0.5% of women. Rates are lower in many other populations.
Deuteranomaly alone accounts for more than half of all cases. It ranges from barely noticeable to almost as strong as deuteranopia, and many people with a mild form only find out when they fail a screening test.
What changes in practice:
- Protan types see reds as darker. A red traffic light can look almost black, and red text on a dark background may disappear. In the scene above, protanopia turns the red light into a dim olive (#62571A).
- Deutan types don't darken reds as much, but red, orange, yellow and green collapse toward one yellow-brown axis. In our chart, the default red and green bars are about ΔE 70 apart for typical vision and under 5 apart with deuteranopia: practically the same color.
Both groups mix up pairs that seem obvious to others, such as orange and lime, or red and olive. The cards above list the three pairs from a test set that move closest together for each type, with the color difference before and after.
Blue-yellow: rare and different
Tritan types involve the S cones, whose gene is on chromosome 7, not the X, so men and women are affected equally. Inherited tritan deficiency is rare; estimates vary widely, from about 1 in 10,000 to far fewer. It can also be acquired: aging of the lens, some eye diseases and certain medications can reduce blue sensitivity. With tritanopia, blue and green become hard to separate, and so do pink and yellow.
Achromatopsia
Complete achromatopsia affects around 1 in 30,000 people. Because the cones handle sharp daytime vision, people with achromatopsia usually also have reduced visual acuity, strong sensitivity to bright light and involuntary eye movements. The simulation shows only the color part: every color becomes its brightness. Sky blue and lime green, for example, end up almost exactly the same gray.
What the simulation can and can't show
The scene uses the model published by Machado, Oliveira and Fernandes in 2009, which is widely used for screen simulations. The anomalous types are shown at a moderate severity. Real experience varies by person, severity, screen and lighting, and no one with a deficiency sees the world as a "broken" version of typical vision; their brain has adapted to it all their life. Use the simulations to understand which colors are risky, not to know exactly what someone sees.
Testing and diagnosis
The color blind test on this site uses Ishihara-style dot plates to screen for red-green and blue-yellow differences, and the hue test checks how finely you can order similar colors. Both are screening tools only. A diagnosis needs an eye care professional, who may use printed plates under standard lighting, arrangement tests or an anomaloscope. There is currently no cure for inherited color vision deficiency. Tinted "color blind glasses" can increase contrast between some colors for some people, but they don't restore normal color vision.
Designing for every type
About one in twelve men will have trouble with red-versus-green signals, so:
- never use color as the only signal; add labels, icons or patterns,
- check charts and maps with the color blindness simulator, which runs your own image or palette through the same model,
- make text meet contrast targets with the contrast checker, since contrast also helps people with low color discrimination.