How color vision works
- To see anything at all we need some tiny little helpers insideour eyeballs, the so called photorecptors. There are two different types ofthem: rods and cones. Both of them are sitting on the retina and passinformation of light on to our brain. There are about 120million rods which are very sensitive to light but not to color.
- The cones are the photoreceptors which areresponsible for our color vision. They are only about 6 to 7million of them, gathering together very closely in the center of the retina,the so called fovea centralis.
- And here comes the clue: Each of those cones is carrying one out of three differentphotopigments and therefore reacts differently on colored light sources. Foreach of this three types there exists a specific color absorption curve withpeaks at different points in the color spectrum.
- S-cones: sensitive to short wavelength light with a peak at ca. 420nm (blue)
M-cones: sensitive to medium wavelength light, peak at ca. 530nm (green)
L-cones: sensitive to long wavelength light, peak at ca. 560nm (red) - Mixing together the information of those three different typesof cones makes up our color vision. This is also the reason that only threemain colors are needed if we want to mix together all visible colors, becausewe only have three sources of information for mixing our whole color spectrum.
Types of color vision deficiency
- Based on this knowledge about our visual system we easily canput together the list of different forms of color blindness. All of them have adirect relation to the available photoreceptors in your eye and are accordinglycategorized.
- Monochromatism: Either no cones available or just one type of them.
Dichromatism: Only two different cone types, the third one is missing completely.
Anomalous trichromatism: All three types but with shifted peaks of sensitivity for one of them. This results in a smaller color spectrum. - Dichromats and anomalous trichromats exist again in threedifferent types according to the missing cone or in the latter case of itsmalfunctioning.
- Tritanopia/Tritanomaly: Missing/malfunctioning S-cone (blue).
Deuteranopia/Deuteranomaly: Missing/malfunctioning M-cone (green).
Protanopia/Protanomaly: Missing/malfunctioning L-cone (red). - For a better understanding you can also call them blue-, green-,or red-weakness respectively -blindness. Unfortunately this terms didn’t really madetheir way and are not used very often.
- You could ask know: “What about red-green color blindnessor blue-yellow color vision deficiency? These are the ones I know but I can’tfind them in your lists above.”
- That’s right. The problem with this well known terms is, thatthey are not telling the truth! Many people think that if you suffer from blue-yellowcolor blindness this are the only colors you can’t distinguish. But that’swrong. Color blindness doesn’t relate to justtwo color hues you can’t distinguish, it is the whole color spectrum which isaffected.More on this a little later in this article and in thenext chapter of Color Blind Essentials where we will have a closer look at red-green colorblindness.
- But to solve the puzzle: blue-yellow color blindness relates totritan defects and red-green color blindness to all types of protan or deutandefects.
| TYPE | DENOMINATION | PREVALENCE | |
| MEN | WOMEN | ||
| Monochromacy | Achromatopsia | 0.00003% | |
| Dichromacy | Protanopia | 1.01% | 0.02% |
| Deuteranopia | 1.27% | 0.01% | |
| Tritanopia | 0.0001% | ||
|
Anomalous Trichromacy | Protanomaly | 1.08% | 0.03% |
| Deuteranomaly | 4.63% | 0.36% | |
| Tritanomaly | 0.0002% |
- The above list shows you the prevalence rates of each type ofcolor vision deficiency. The ratios between the most frequently occurring typesfor men can simply be remembered as: 1 protanope to 1 protanomalous trichromatto 1 deuteranope to 5 deteranomalous trichromats.