Colour is Weird (Part 1)

The basics of colour are a little more complicated than the basics of sound. But the weirdness of colour is a lot stranger than the weirdness of sound. The basic idea of colour is the familiar “rainbow” spectrum, containing all the “spectral colours” like ROYGBIV. Each of these colours is an electromagnetic wave vibrating at a different frequency (just like different sound pitches have different frequencies). For example, Red vibrates at 400 trillion Hz. Other colours, which are not found in the rainbow, can be made from a mixture of spectral colours. For example, you get magenta by combining spectral red and blue (perhaps by hanging two crystal elephants in your sunny window):

The usual explanation for perceiving colours is that your eye is undoing this mixing and breaking colours back down into components of Red, Green and Blue. This is done by the “cone” cells of the retina – there are three types of cone in the eye, with one being sensitive to Red, one sensitive to Green, and one to Blue. When you look at magenta the Red and Blue cones are activated, but the Green cones are not. Your brain interprets this a magenta. Simple enough idea. But completely false. Colour is weirder than that.

One of the best explanations of what is going on when you see magenta on your screen was created by Jamie Wong. I cannot improve on it. So go read it here http://jamie-wong.com/post/color/. Don’t be intimidated by the diagram at the top. Skip it and go to the text below. And then come back! There is much more weirdness after that.

Did you go to Jamie’s page? That’s the introductory weirdness. Now you know that perceived colour is the LMS cone responses, each happening at all frequencies, which is a non-linear transformation of the colour distribution of the light coming from the source object (how much is present of each frequency). One weird thing Jamie mentions in passing is that there is no pure colour that activates just one of the cone types. Even pure spectral Red doesn’t just activate the L cones – it activates all three (e.g., lots of L activation, but only a very tiny bit of S).

Perceiving colour is not a question of which cones get activated (which is what most people believe). It is the ratio of all three cone activations that matters. And since different objects, with different camera responsiveness, and different computer display gamut, and different gamma setting, and different people’s retinas, can all ultimately result in the same LMS activations, then two objects can appear to be exactly the same colour, but for very different reasons.

That’s only a little weird. Here’s something weirder that Jamie didn’t go into. Notice that the LMS response curves each stretch out with left and right tails. In principle they are infinitely wide. This means, for example, that the S cones are actually a tiny bit sensitive to UV light (since the S response curve has a small tail stretching out past the visible violet). Humans can therefore actually see ultraviolet light. Just not very well. But it is possible to create conditions to enhance this ability.

Imagine if you wore some kind of welder’s goggles that had a filter to block out visible light. Wearing them, everything would look quite black. Your eyes would slowly adjust, with dilating pupils.  And eventually you might begin to faintly make out the UV lighting around you). You would need goggles that block visible light, but allow UV to get through. If you have access to theatre lighting gels, this can be approximated with three layers of Congo Blue filter plus one layer of Primary Red. If you put those into the frame of some welder’s goggle, go outside on a very sunny day (since the sun puts out lots of UV light) and wait for your eyes to adapt. You will begin to see the nearest UV colours with your own eyes. Pretty weird. But be very careful. This is a risky activity to actually try. The darkness of the goggles will cause your pupils to dilate a lot. And you will expose those wide open eyes to the sun’s UV. You risk getting a painful sunburn on your retinas (sometimes called “snowblindness”). In fact, don’t do this at all. Forget the whole idea. Just know that I tried it once for ten minutes and saw the world in UV with my own eyes (and I didn’t get a sunburn). It was pretty weird. But weirder still to come...