Colour-blindness

I’m one of the 8% of all men who are colour-blind, most of us with an inability to distinguish red from green. This isn’t a major problem but it is a nuisance when designers don’t take it into account, using indicators which change colour from red to green (e.g. battery chargers), or PowerPoint presentations with green text on a red background. The other 92% of you must find it hard to imagine what the world looks like to us. To me, grass looks bright orange, blotting paper is grey, and the colour red just doesn’t leap to my attention the way it probably does to you.

Colour-blindness is a physical problem with the eye, as is short-sightedness, so it’s not a matter of just not having "learned" the colours – we actually cannot perceive the difference between some of them. Subtle differences in colour perception may be quite widespread – people with "normal" colour vision sometimes start by testing me on the colour of this, the colour of that … and before long are arguing with each other over a particular colour! There is even a suggestion that some women have super colour vision as a result of inheriting two different X chromosomes.

To imagine what it’s like to be colour-blind, try turning down the "colour" setting on your television or monitor until there’s almost no colour on the screen. To begin with, everything looks very black and white, but in time your brain adapts and the colours appear to come back, or at least you don’t notice they’re missing. But now you don’t have very good sensitivity between them, so you might start to confuse a greenish red with an orangeish red, that kind of thing. The world still looks just as "colourful" as normal, but you have lost some of your colour discrimination.

Through my eyes

Human eyes have 3 types of colour sensors in them – red, green and blue. For this reason, all the kit we use to capture and reproduce colour (film, digital camera sensors, colour monitors, print media) only needs those 3 colours to trick us into thinking that an image is in "full-colour"

I am a "protanope" which means that there’s something wrong with just the red receptors in my retinas. I’m not sure if it’s that they don’t work very well, or they’re missing, or they’re just tuned to the wrong colour. To give you an idea what this is like, below left is a picture I took of the sunset in Sri Lanka. On the right is the same picture with all the red removed. The two pictures look identical to me.

To perhaps make this easier for you to understand, here’s a colourful picture of a toy:

The red, green and blue components look like this:

If we just remove the red and keep the green and blue (as I did with the Sri Lanka picture above) you’ll just see a picture without any red in it. But does this really represent what I perceive? Probably not because since I have never seen red, perceptually I am probably "stretching out" the colours I can see so that they fill my perceived colour space. So that’s what I’ve done below: I’ve thrown away the red, then shifted the green to be yellow (yellow is between red and green in the spectrum):

then I’ve added back the blue to give this:

It may appear at a glance to be a truly "colourful" picture, but actually it’s missing 1/3 of its colour information.

If some people had 4 types of colour sensor in their eyes, instead of the usual 3, then they’d be able to tell the difference between colours that we couldn’t, and everyone would seem colour-blind to them. [Update: Some recent research implies that this may indeed be the case – some women inherit a defective gene on one of their X chromosomes, and a normal gene on the other, giving them 4 colours]

Tests

If you think you might be colour-blind, your optician can give you a simple test. The old Ishihara test is most common – you are shown a series of pictures, each of which consists of a bunch of randomly-sized dots forming a pattern (e.g. the digit "7") a background of other randomly-sized dots. The random sizing of the dots prevents you from using shape as a recognition mechanism, forcing you to use only colour. If you can’t distinguish e.g. a symbol made of red dots on a background of green dots, you may be a protanope, just like me.

Don’t worry, it’s no big deal unless you want to fly a fighter plane, so just enjoy seeing the world a bit differently from everyone else.

(The true colours of the printing inks used to print the test shown above are different from those red/green/blue colours used by computers, so the above picture isn’t spectrally correct, and you can’t take this test online).

Cures

There are no real cures yet for colour-blindness. It’s understood quite well genetically now, so eventually we’ll probably edit it out of the gene pool but for now we are stuck with it.

Meanwhile there are contact lenses which filter the image that your left eye sees differently from that of your right, to try to recreate the missing information. For example if you can’t sense red then the lenses might filter out the red from just one eye. Everything will look normal if you look at a green or blue object – both eyes are seeing the same thing. But if you look at a  red object, for example a rose, instead of looking "dark" as it did before, it glows in a most amazing way, the result of your brain trying to cope with the fact that it’s getting different information from each eye. This is a neat idea and you can try it for yourself if you can find a piece of red filter (hold it in front of one eye for around 5 minutes until you adapt to the light difference). However, a friend who tried the contact lenses said it gave him a headache, probably because it interferes with the eye’s focussing mechanism which makes use of the image convergence between your two eyes (if they’re seeing different colours, it’s harder for the brain to know whether this is because your eyes are not pointing in quite the same direction, and/or because the lens in your eye acts like a prism and so different colours focus at slightly different distances. 

Even if in the real world you can’t be sure what colour something is, at least you can when you’re using a computer. A big thank you to the Microsoftie who wrote the software that makes the name of the colour pop up when you’re picking colours from the standard Office 8×6 colour palette. And outside of this, please do use the excellent WhatColor shareware application to tell you what other colours are on the screen, or buy the excellent new eyePilot software. You can also use an online tool at Visicheck which alter the colours to improve discrimination according to your particular kind of colour-blindness.

An excellent review of colour-blindness which goes much deeper than this article, with several examples, has now been published here. It includes an online tool to which you can submit any picture and see how it would look to someone with different types of colour-blindness.

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