Seeing colour
Colour blindness and its kinds
A difference in the cone cells that makes some colours hard to tell apart. Usually red and green, and far more common in men.
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Colour blindness, more accurately called colour vision deficiency, means that some colours which most people see as different look alike. Very few affected people see in black and white. The usual form makes reds, greens, browns and oranges hard to separate. It is inherited, lifelong and common: about one man in twelve and one woman in two hundred among people of European descent.
The red-green kinds
Most cases involve the long or medium-wavelength cones. If one type is missing, the person has two working cone types and is called a dichromat. Protanopia is the loss of the long type and deuteranopia the loss of the medium type. More often the cone is present but shifted, so that its response sits too close to its neighbour's. These milder forms are protanomaly and deuteranomaly, and deuteranomaly is the most common of all. The genes for both cone types lie on the X chromosome. Men have only one X, which is why they are affected far more often.
The rarer kinds
Tritan deficiency affects the short-wavelength cones and confuses blue with green and yellow with violet. It is rare and affects men and women equally, because its gene is not on the X chromosome. Complete colour blindness, called achromatopsia, is rarer still. People with it have no working cones, see in greys, and find bright light painful. It is unusually common on the Pacific atoll of Pingelap in Micronesia, where much of the population descends from a few survivors of a typhoon in the 1770s. Colour vision can also be damaged later in life by eye disease and by some medicines.
Testing and living with it
The best-known test was published in 1917 by the Japanese eye doctor Shinobu Ishihara. Each plate shows a number made of coloured dots on a ground of other dots, which a person with a red-green deficiency cannot pick out. There is no cure. Tinted glasses can increase the contrast between some colours but do not restore normal vision. Most people manage well, though traffic lights, ripe fruit, charts and electrical wiring can be awkward. Many countries, India among them, set colour vision standards for jobs such as train driver and pilot.
Sources and further reading
- Color blindness on Wikipedia
- National Eye Institute: Color Blindness
- National Eye Institute: Types of Color Vision Deficiency
- NHS: Colour vision deficiency
- MedlinePlus Genetics: Achromatopsia
Original text by the Rang desk, written with AI assistance and checked against the sources above. Meanings of colour vary from place to place and person to person. If we have something wrong, tell us.