How do colour blindness correcting glasses work ?
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Colorblindness happens due to a lack or deficiency in one or more color-sensing cone cells in your retina. [This graph](https://upload.wikimedia.org/wikipedia/commons/thumb/1/1e/Cones_SMJ2_E.svg/1200px-Cones_SMJ2_E.svg.png) shows the sensitivity of each of the three kinds of cones in your eyes. To identify one frequency, your brain combines the data from the cells to see how much of each was activated. So, orange light – with a wavelength of, say, 600nm – would activate *some* green cells and a lot of red cells. Much deeper red light of 650nm would activate few, if any, green cells, and a lot of red cells.
Consider, then, the color *brown* which is actually [dull orange light with bright colors around it](https://www.youtube.com/watch?v=wh4aWZRtTwU). Since it’s orange, it will activate some red cells and some green cells. Since there’s less of it than what we perceive as *orange*, it will activate fewer of both red and green cells, but the ratio between them will stay the same.
Now, consider someone with fewer than normal red cone cells in their eyes. They still have *some*, they can still actually see red light, but with fewer red cone cells it’s much harder to differentiate between slightly different shades of red. For this person, red light *always* activates fewer red cells than normal because there are fewer red cells to be activated. If they look at something that is green, it activates a lot of green a very little red – which is what people with normal vision also see. Consider if that person looks at something that is brown, though. Brown *normally* activates fewer red cells *and* fewer green cells. But this person can’t see the difference between there being no red because it isn’t there and no red because they just can’t see it well. With brown, there is also very little green, either. So is it brown, or just dark green? This person can’t see the difference.
They can see green just fine. If you put something red against something white, they can probably tell you that it’s red. They can probably do the same for something that is dark green. But as soon as you put something dark green next to something red or brown, they can’t see the difference.
Colorblindness correcting glasses are custom-ish made (or at least, the better, more expensive kind are) for the individual to correct the *ratios* between the cones that are activated. There’s no way to *increase* the red cone activation, but you *can* decrease the green so that while everything is darker and fewer overall cones are activated, they are at least activated in the same *relative* amount as they would be for someone with normal vision. For our example person with red-brown colorblindness, they would be able to distinguish the browns from the dark greens and dark reds.
However, this only works for people with a *deficiency* in one type of cone cell. They can’t help someone who lacks that cell entirely. Again, they can’t *increase* the activation, just bring everything down to match. If one of them is zero, or even just very low, you can’t match it. I’m also not sure how well it works with someone that has a deficiency in more than one type of cell. I’m sure it *could* work but I imagine there would be diminishing returns.
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