What is color blindness, really?

Short answer

What we call "color blindness" is, in most cases, not an inability to see color at all, but difficulty telling certain pairs of colors apart — most often red and green. The cause is that one of the three cone types is missing or has a shifted sensitivity. Because these genes are carried on the X chromosome, it's far more common in men; complete color blindness, in which the world appears entirely gray, is very rare.

One cone missing or shifted

As we saw in BI-09, color vision rests on comparing the responses of the three cone types. If one of these types is entirely absent, or if its sensitivity curve has drifted too close to its neighbor's, the comparison weakens and some color differences disappear. That's why scientists often prefer the term color vision deficiency over "color blindness": the person isn't so much blind as lacking certain distinctions.

The types of deficiency are named for which cone is affected. Problems in the long-wavelength (L) cone are called protan, in the medium-wavelength (M) cone deutan, and in the short-wavelength (S) cone tritan. Each comes in two degrees: if the cone type is entirely absent, the person sees with two cone types — that is, they're dichromatic; if the cone is present but its sensitivity has shifted, the person sees with three cone types but some distinctions weaken, which is called anomalous trichromacy.

Protan and deutan deficiencies together are known as "red–green color blindness" and make up the overwhelming majority of cases. For these people, some of the differences among red, orange, yellow, and green are erased; a ripe and an unripe tomato, or a red and a green indicator light, can look alike. In people with protan deficiency, red lights also look darker than they are, because the cone that best captures long wavelengths is missing.

One of the first people to describe this condition scientifically was the English chemist John Dalton, known for atomic theory. In 1794 he described the oddities in his own color perception in a paper, and proposed that they came from the fluid inside his eye being colored. That explanation turned out to be wrong, but the curiosity didn't end there: DNA taken in the 1990s from Dalton's preserved eye tissue showed that he was a deuteranope, lacking the pigment gene for the medium-wavelength cone.

Why is it more common in men?

The cause of red–green deficiencies lies in genes, and the pigment genes for the L and M cones sit on the X chromosome. Women have two X chromosomes, men one X and one Y. If a man carries a variant in these genes on his single X chromosome, he has no second copy to compensate, and color vision deficiency results. For a woman to experience the deficiency, she generally needs a variant on both X chromosomes; women carrying a single copy usually see normally but can pass the trait on to their children.

The result is a large difference between the two sexes. Large-scale screenings in populations of European descent find red–green deficiency in roughly eight percent of men, but in only about four in a thousand women. The rates vary between populations too: in men of Chinese and Japanese descent, they have been reported at between 4 and 6.5 percent.

Eight percent isn't a small number: in a class of thirty, half of them boys, on average one boy will have this condition, and he's often unaware of it. Tritan deficiencies, by contrast, are linked not to the X chromosome but to a different chromosome; that's why they appear at similar rates in women and men, and are far rarer.

The Ishihara test

Color vision deficiency is often noticed during a school screening, a driver's license exam, or a job application. The best-known screening tool is the set of plates the Japanese ophthalmologist Shinobu Ishihara published in 1917. Each plate is a circle made of dots in different sizes and colors; some of the dots form a number. The dots forming the number and the dots in the background are chosen from colors that someone with normal color vision easily tells apart, but that someone with a red–green deficiency confuses.

The test is fast and cheap, but it has limits. It mainly catches red–green deficiencies and doesn't detect tritan deficiencies well. The plates' colors are sensitive to print quality and lighting; copies shown on a screen can give misleading results on an uncalibrated display. To determine the type and degree of deficiency precisely, clinics use more detailed instruments such as the anomaloscope.

Complete color blindness is rare

There are also people who truly see the world without color, only as light and dark, but their numbers are very small. The inherited form of this is achromatopsia: the cones largely don't function, and rods carry vision instead. People with achromatopsia don't just fail to see color; daylight is uncomfortably bright for them and making out detail is hard, because the cones responsible for daytime vision aren't working.

The loss of color perception after brain damage is a separate matter; color disappears even though the eye itself is intact. The story of this phenomenon is told in PS-08.

Example — the charging light and the fruit stand

Many devices' charging indicators glow red while charging and green when full. For someone with a red–green deficiency, these two lights can look nearly identical; instead of the light's color, they have to look at the device's screen or whether the light is blinking. Telling ripe and unripe tomatoes apart at a fruit stand, or red and green apples, can be similarly hard; the person relies on other cues like texture, brightness, and shape. Flushing in the face, sunburn, or redness in the skin also fall right into this vanished distinction and are harder for these people to notice — and this holds true no matter the skin tone.

These examples also carry a design lesson: if information is conveyed by color alone, a considerable part of the population loses it. How this problem is solved in interfaces and in graphics is the subject of UX-07 and VG-07.

Common misconception

Summary

1. Color vision deficiency is usually the absence or shift of one cone type; the protan, deutan, and tritan types are named for the cone affected.
2. Because the L and M cone genes sit on the X chromosome, red–green deficiency occurs in roughly 8 percent of men of European descent and roughly 4 in every thousand women.
3. Ishihara plates screen quickly for red–green deficiency but have limits; inherited achromatopsia, in which the world appears colorless, affects roughly one in 30,000 people.

Test yourself

1. (Easy) What are the two types of red–green color vision deficiency?
2. (Easy) Who published the Ishihara test, and in what year?
3. (Medium) Why is color blindness more common in men?
4. (Medium) What is the difference between dichromacy (two-cone vision) and anomalous trichromacy?
5. (Hard) Why can a version of the Ishihara test shown on a screen give misleading results, and which type of deficiency does the test fail to catch well?
6. (Hard) What does the fact that tritan deficiencies occur at similar rates in both sexes tell us about their inheritance?

Answers

1. Protan and deutan; they affect the long-wavelength and medium-wavelength cones respectively ("One cone missing or shifted").
2. The Japanese ophthalmologist Shinobu Ishihara, in 1917 ("The Ishihara test").
3. The pigment genes for the L and M cones sit on the X chromosome. Because men have only a single X chromosome, they have no second copy to compensate for a variant in these genes; in women, generally both copies need to be affected. That's why the rate in men of European descent is roughly 8 percent, and in women around 4 in every thousand ("Why is it more common in men?").
4. In dichromacy, one cone type is entirely absent and the person sees with two cones; in anomalous trichromacy, all three cone types are present, but one has a shifted sensitivity and some distinctions weaken ("One cone missing or shifted").
5. The plates' colors are sensitive to printing and lighting; an uncalibrated screen can alter these colors. The test mainly catches red–green deficiencies and doesn't detect tritan deficiencies well ("The Ishihara test").
6. Tritan deficiencies are linked not to the X chromosome but to a different chromosome; because they aren't tied to a sex chromosome, they occur at similar frequency in both sexes ("Why is it more common in men?").

Sources

• Birch, J., "Worldwide prevalence of red-green color deficiency," Journal of the Optical Society of America A 29(3): 313–320, 2012.
• Hunt, D. M., Dulai, K. S., Bowmaker, J. K., and Mollon, J. D., "The Chemistry of John Dalton's Color Blindness," Science 267, 1995.
• NCBI GeneReviews, "Achromatopsia."
• OMIM, #190900 "Tritanopia."

Next step

UX-07 unpacks this lesson's design consequence: it covers interfaces, error fields, and status indicators that pair color with shape, icons, text, or pattern, testing them with a color vision deficiency simulation. VG-07 carries the same question into graphics: it shows, with before/after examples, how to avoid the red–green pairing, use direct labeling, and rely on patterns.