Why do Protan & Deutan look so similar?


Say hello to this pretty lady. You know what makes her prettier? Being color blind. Let’s use her to simulate some colorblindness real quick.

As you probably know, since, well, you clicked on the thumbnail, there are three main types of partial colorblindness: protan, deutan and tritan. These are the forms that affect the red, green and blue cones, respectively…

Then whhhyyyy do protan and deutan basically look the same?

I’ve seen this question so many times, and it’s a great question, because intuitively… you’d kinda expect the outcomes to be like… similarly different. So today on chromaphobe… why are tritans so weird?

Lets start with a color wheel and plot the primary colors – the cone colors – red, green and blue… Whether we get rid of red, green OR blue, the wheel kinda has this symmetry to it; so it would make sense that no two types of color blindness should be more similar to each other than the third. The problem with this, is thinking of color vision in RGB space. It makes sense, most of you have red, green and blue cones… but you don’t THINK of color in this way… you don’t look at this color and think “ my cones are firing at 40% red, 40% blue and 30% green.” If I ask you what color this is, the colorblind will say “nah, I’m not playing this game again.” But the color normal of you will probably say something along the lines of “dark purple with some gray mixed in” …or “royal heath” if you’re some color snob.

Essentially, we like to think of color in terms of color, brightness and colorfulness, or more precisely, hue, saturation and luminance. A normal human retina generates signals in RGB, it has to, that’s how your cones work, but along the way to your brain, it gets transformed. By the time it gets to the deep recesses of your conscious mind, that color is now recognized in an HSL color model.

Now before you think about it too hard, no color blindness also doesn’t really make sense in an HSL color model either. Instead, we actually have to look at the middleman of this RGB to HSL transformation… the OPPONENT PROCESS CHANNELS. I’ve talked about these before and I’ll talk about them again, because they are probably the single most important concept for understanding color blindness. Just like RGB and HSL, there are also three parameters, or channels, called red-green, blue-yellow and luminance channels and you can calculate them pretty easily from the raw cone signals:

  • The luminance channel is just the sum of all of your cone signals. It’s just the brightness component.
  • The red-green channel is simply the difference between the red and green cone signals, and
  • The blue-yellow channel is simply the difference between the blue and yellow cone signals.

Dalton: what’s this nonsense? Yellow cone? There’s no such th…

Shutup Dalton, you’re ruining the suspense… What Dalton was trying to say, is there’s no such thing as yellow cones… we’ll get back to that.

Protan and deutan color blindness are two types of red-green color blindness. So called NOT because they are affected by the red and green cones, which they are, but because they affect the red-green opponent channel. Clearly, if you lose either of those cones, the red-green channel is pretty much dead because your nervous system can’t COMPARE red and green signals if it doesn’t HAVE both red and green signals.

This would explain why protan and deutan are identical… which… wasn’t the question, and isn’t true. They aren’t identical, they’re similar… so… then… we can ask… what makes them different? Well, its all about those yellow cones… which don’t exist… but there IS a yellow cone signal. Think of it like a virtual cone, that gives a signal that is just the average of your red cone and green cone signals, because red and green… make yellow.

This means if you are missing your red cone, your virtual yellow cone signal just shifts greener, and if you’re missing your green cone, your virtual yellow cone signal just shifts redder. Your red cone and green cone are actually very close together on the spectrum though, so the effect of protanopia or deuteranopia on your yellow signal and therefore your blue-yellow opponent channel is slight, which explains the slight difference between how protan and deutan simulations look!

Also notice that the distance between the blue and yellow signals is higher for deutans than for color normals… which means despite losing their entire red-green opponent channel, the “enhanced” deutan blue-yellow channel can actually discern some very specific ranges of colors better than color normals. Like… shades of tan or khaki, look, we gotta take what we can get, right?

This virtual yellow cone also explains why there are two types of red-green colorblindness, protan and deutan, but only one type of blue-yellow color blindness: tritan.There would actually be a second type of blue-yellow colorblindness and it would be called tetartanopia. Where tritans are missing their blue cone signal, tetartans would be missing their yellow cone signal. But… using what we just learned, you can probably figure out that to be completely missing your yellow signal, you have to be missing both red AND green cones, which not only wrecks your blue-yellow signal, but also your red-green signal. Such a person doesn’t have tetartanopia… they have full on blue cone monochromacy and see in black and white.

Another factor that may make the protan and deutan simulations look a bit different-er, is the minor effect on the luminance channel. The visible range of colors usually spans 400-700nm, but protans, who are missing their red cones, only perceive the range of 400-670nm. A small change, but that means reds will look dimmer, or even black to protans compared to both color normals and deutans.

The kicker is, this brightness adjustment is ignored in a lot of color blindness simulations, including the one I’ve been using in this video, so if I want the protan simulation to look right, I have to pre-darken those reds… and NOW, protan and deutan look a bit more distinct, don’t they?

In the end, protan and deutan… they are simply different sides of the same coin… and tritan… well, it’s a different coin entirely.

This is chromaphobe.





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