Why are we sensitive to red green and blue?

01 August 2013



We're told that the rods and cones in ones retina are sensitive to red, green and blue. Why not red, yellow and blue the three primary colours? It seems more logical to me.


Dave - Okay, I've built some experiments on this recently and basically, this is due to what you think the three primary colours are, aren't necessarily actually the three primary colours. There's two different kinds of primary colours; one is light, so there's red, green and blue which can trigger the three different kinds of cones in your eye and you can fake pretty much any colour by using red, green and blue. That's what the TV does. The other kind of primary colours are when you want to go painting. Painting works in almost the opposite way around, you're starting off with white light reflecting off your white piece of paper and then you add things to it to absorb colours. So, the three primary colours of paint are yellow which absorbs blue light, sort of purple which absorbs green light, and turquoise which absorbs red light, and then you can add those together in different quantities and take away whichever mix of colours you like, and end up with fooling your eye to present any colour you like. Red, yellow and blue aren't really any kind of primary colours. They're just what primary school teachers tell you and they're just confusing. Chris - There you go, Robin.

Robin - Confusing all others, yes. Thank you very much then.

Chris - I mean, the bottom line here is that in the back of your eye, you have these cones and they have dye pigment molecules in them that are sensitive to lights of different colours. They respond different amounts to different colours and the eyes, basically comparing what amount of activation there is of these different dyes in proportion to each other. It knows that if you have a certain amount of the red one and a certain amount of the green one, this must be a colour somewhere in between the two.

Robin - Yes, and they reflect in some sort of negative way, as you say, the turquoise reflects in red and...

Dave - Absorbing red, yeah.

Ginny - So, the other way you can think of it is from an evolutionary point of view and actually, primates like us, we're quite unusual in being able to see the spectrum of colours that we can see. So, the red, green distinction is thought to be relatively recent and that was thought to be because it was really important for us to be able to tell when fruits were ripe. So, it really became important for us to be able to see a ripe red strawberry in a green bush. So, we evolved this ability that a lot of other animals don't have. So, we now use vision a lot more than things like smell which is much more important for other animals. That might be because of this same reason we evolve this ability to tell red and green apart so well, and then a lot of monkeys and things use, you know, red bottoms and things like that to signal sex rather than smell.

Chris - And besides from traffic light, only red doesn't mean stop.

Ginny - Exactly.

Chris - Because Ian, some animals and insects see in domains like ultraviolet. Bees can see UV, can't they?

Ian Burgess - Indeed, yes. Actually, the spectrum that we see is really quite tiny and if you look at what insects can see, much of it is shifted as you say towards the UV. But it is not only much broader, it's much more spectacular when you look at it in an insect's eye view.


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