Human beings usually have three types of color-sensitive cones, which respond strongest to the colors blue, green and red respectively. In some cases, one of these these receptor types is only weakly developed or not present at all leading to a limited discrimination of certain color ranges. In almost all cases of color-anomalous vision either green or red sensitive cones are affected; as the response curve of red and green receptor cones are very similar, the resulting color vision deficiency is very similar for red and green cone anomalies, leading to very similar color perceptions in both cases.
The Prevalence of Color-Deficient Vision
7-10% of men have red-green color-anomalous vision or color deficient vision, a decreased ability to perceive differences between some of the colors that others can distinguish. Only ca. 0.4% of women suffer from color-anomalous vision.
With 6.3% of men, by far the most common form of “color deficient vision” is anomalous trichromacy, i.e. color vision with three types of color receptors present, but with limited sensitivity in either the red or the green sensitive cones. These are green-weakness (deuteranomaly) and red-weakness (protanomaly).
Another 2.4% of men suffer from dichromatic color vision, i.e. the absence of either the red or the green sensitive cones. These are red-deficient (protanopic) and green-deficient (deuteranopic) vision.
“Color-Blindness” – An Unfortunate Term
The term “color blindness” is very unfortunate as it implies inability to see colors, whereas for the afflicted persons the world looks just as it should be – albeit with more or less limited discrimination of some color ranges. In fact, persons with a mild to moderate red- or green-weakness will often not be aware of their deficient color vision. Red-green discrimination is still possible – albeit to a lesser degree – and since no comparison to “normal color vision” is possible, nothing short of an optometry test or very specialized color discrimination tasks will make the deficiency explicit or conscious.
Why Color-Deficient Vision is more Complicated than that
If we only responded to the presence or absence of single spectral colors – the colors in a rainbow – then color-deficient vision would simply be a matter of lowered ability to discriminate between different frequencies. But our color vision takes a combination of dozens, hundreds of spectral colors and assigns it a (more or less arbitrary) position in our 3-dimensional color space, based on the most predominant color range. What this means is that we don’t say:
“This signal is composed of frequencies 440, 520 and 690 with intensities 3, 3 and 10 respectively”, but rather we say it’s “light red”.
But of course, there are frequencies that we are not aware of just outside our visual spectrum, so there could be a strong signal in the infrared or ultraviolet range with intensity 50, making the overall signal much more ultraviolet than red.
In Some Cases Color-Deficient Viewers will see more
The case where a person with color-deficient vision sees “less” than persons with “normal vision” is quite well known and has been showcased with countless Ishihara diagrams.
But, persons with color-deficient vision can actually see details that “normal-visioned” persons can’t. Such Reverse-Ishihara diagrams are much less known and put the joke on the “normal” people.
Being able to discriminate between green and orange, for example, can lead to a sort of camouflage of information that is quite available to “red-green blind” subjects.
Therefore, our first impulse to discount “color blindness” as a disability is not quite correct. Persons with color-deficient vision don’t necessarily see less, they simply see the world differently, being far more able to differentiate red/green colors based on luminance than subjects with “normal vision”.