What is color blindness?
Most people see color with three kinds of cells. Color blindness is what happens when one kind is missing or works differently. It is common, almost always inherited, and almost never “seeing in black and white”.
Three kinds of cones
The eye has three kinds of color cells, called cones. Each is most sensitive to a different part of the spectrum: long wavelengths (L, often called the “red” cone), medium (M, “green”) and short (S, “blue”). The brain makes color by comparing them.
A fourth kind of cell, the rods, sees in dim light. Rods see no color at all.
Each curve is scaled to its own peak. Sources: Stockman & Sharpe (2000) for the cones, CIE (1951) for the rods.
A cone can be missing, or shifted
Missing: strong
One kind of cone does not work at all, so two color signals remain instead of three (dichromacy). Some pairs of colors become impossible to tell apart.
Shifted: mild
All three cones work, but one responds to slightly the wrong colors (anomalous trichromacy). The same pairs become hard to tell apart, not impossible. This is the most common form.
Which cone, what it is called, how common
| Kind | Affects | Strong and mild forms | How common | Often confused |
|---|---|---|---|---|
| Protan the “red” type | the L (“red”) cone | protanopia / protanomaly | about 1 in 100 Japanese men | red with dark brown; blue-green with gray and some reds. Reds also look darker. |
| Deutan the “green” type | the M (“green”) cone | deuteranopia / deuteranomaly | the most common: about 1 in 25 Japanese men | red, orange and light brown; blue-green with gray and purple. |
| Tritan the “blue-yellow” type | the S (“blue”) cone | tritanopia / tritanomaly | rare: between 1 in 13,000 and 1 in 65,000 people | blue with green, and yellow with violet. |
| No color vision monochromacy | no working cones: rods only | complete achromatopsia | very rare: about 1 in 30,000 people | every color, judged by brightness alone. Bright light is also painful, and sharpness is low. |
Protan and deutan together are called red-green color blindness. About 8% of men and 0.4% of women of European descent have it, and between 4% and 6.5% of Chinese and Japanese men.
The same colors, through other eyes
Switch between the views and watch which colors become the same. For a deutan, red, orange, lime and brown drift together, and so do blue and purple.
- red
- orange
- yellow
- lime
- green
- teal
- blue
- purple
- pink
- brown
- gray
Showing: Normal
It rides on the X chromosome
The genes for the “red” and “green” cones sit on the X chromosome. Men have one X, so a single changed copy is enough. Women have two, so a changed copy is usually covered by the other one.
In Japan, about 1 woman in 10 carries the change without being color blind, and about 1 in 500 is color blind herself. A color blind man usually has a carrier mother.
Each child gets one of the mother's two X chromosomes, at random. Half the sons are color blind; half the daughters are carriers.
A small glossary
- Normal vision
- all three kinds of cones working (trichromacy).
- Red-green color blindness
- protan and deutan together. It is what the Ishihara test screens for.
- Strong / mild
- strong: one kind of cone is missing (dichromacy). Mild: it is there but shifted (anomalous trichromacy). Many people fall in between, and are called moderate.
- Rods only
- rods are the cells used for seeing in the dark. They see no color, and some colors, like orange and red, look much darker to them than to cones.
- Confusion colors
- colors that look the same to a given type of color blindness. The plates are built from them.