Red-green blindness is a disease also called Daltonism after its discoverer, John Dalton. It is relatively often wrongly referred to as color blindness. In doing so, those affected can actually recognize blue and yellow tones without any problems; only the distinction between red and green tones does not succeed for them.
Even more frequent than red-green blindness, whose affected persons cannot distinguish the two colors at all, is red-green weakness. Persons affected by it can distinguish the colors only when they are particularly strong. For they see the color tones significantly duller and more colorless than people with normal vision.
To understand why there is such a red-green weakness, one must first understand how color perception works. When light falls on the eye, there are various sensors there, so-called photoreceptors, that perceive this. You can imagine them like little men who incessantly send messages to the brain. You can imagine the messages in terms of content something like this:
There, where I am, there is currently light.
There, where I am, there is currently no light.
There, where I am, there is currently blue light.
There, where I am, there is currently no red light.
Each photoreceptor can always capture only a very small part of the visual field, the part from which light can fall on exactly this receptor. In this part of the visual field it can also only determine whether a light is there right now or not.
There are two types of receptors: rods and cones. Rods determine whether light is currently present at a place or not. That is especially important for seeing at dusk.
Cones, namely, need more light in order to determine the presence of light at all. After all, they do not simply determine whether there is currently any light. They determine whether there is currently light of a certain color. However, each cone can recognize only one color. The more cones lying close together, for example, determine that green light is currently arriving at them, the more strongly an object appears green to us.
There are three types of cones: they are responsible for green, red, and blue. All other color tones in the visual field result from mixtures. If, for example, cones lying next to each other determine that an object is both green and blue, one sees the object as turquoise.
In people with red-green visual weakness, a large part of the cones responsible for red and green does not function. This has the consequence that they hardly see green at this place and the object appears only blue to them. The reason for this is genetic: the cones have a certain structure which normally, when green light falls on them, changes and thereby automatically sends a message like the ones mentioned above to the brain: “There, where I am, there is currently green light.” In people with red-green weakness, the structure of many cones is different; they therefore do not react correctly and often also falsely, when such light is present, still send: “There, where I am, there is currently no green light.”
The same problem can also occur with red cones. Strictly speaking, the people who have a red-green weakness have either a red weakness or a green weakness, but the consequences of both are very similar. A blue weakness, in which the blue cones are affected, also exists, but it is far rarer.
Most affected persons with red-green weakness are men. Anyone who wants to determine whether he himself might be affected can simply look at a test on the Internet:
https://www.brillen-sehhilfen.de/sehtest/farbsehtest.php
Make an appointment online right away.