good to be colour blind

Colour blindness is mainly a male affliction. Something like 8% of all men in the world are colour blind though, as I have mentioned before, this doesn’t mean that they cannot see colour but, rather, means that their colour discrimination is not as good as that of so-called normal observers (the rest of us, in common vernacular). See my earlier post. So we normally think of colour blindness as being something undesirable, something that ideally we would like to be able to cure.

Interesting then that new research at Anglia Ruskin University has suggested that colour blindness may even be an advantage. The study was led by Dr Andrew Smith and showed that colour-blind monkeys (tamarins, to be exact) were better than their ‘normal’ counter-parts at catching camouflaged insects (such as crickets). I guess what this means is that the camouflage is designed (I guess I should say, has evolved) to be effective when viewed by normal tamarins. So the colour-blind tamarins may be better off in some sense.

Dr Smith is also quoted as saying that there is some evidence that, in humans, dichromats (who have two classes of cone rather than three) may see better in dim light than trichromats. For further information see http://www.businessweekly.co.uk/academia-a-research/13403-colour-blind-monkeys-have-advantage-in-catching-camouflaged-prey.

What is a colour space?

In my job I probably use the phrase “colour space” every day and have done for the last 20 years. So imagine my surprise when I was talking with a colleague recently and after a few minutes he said “Can I stop you for a second there Steve – when you say colour space, what exactly do you mean?”.

A colour space is like a map. A map of New York, for example, shows the location of various landmarks with reference to the xy coordinates (the position in horizontal x and vertical y units on the map). A colour space or colour map does the same thing with colours. Perhaps the simplest colour space is the spectrum, see below:
 

As we look from right to left on the spectrum the wavelengths changes from around 700nm on the far left to about 400nm on the far right. So this map shows colour with reference to wavelength. Although it is a commonly used colour space it is limited because it only really describes how hue changes with wavelength. Hue is only one of three ways in which colour can change or vary.

The most well-known really useful colour space then is the CIE chromaticity diagram – see below.

The CIE chromaticity diagram shows colours arranged on a 2-D plane. We can easily refer to any colour by how far from the left it is (the x coordinate) and how far from the bottom it is (the y coordinate). This space only shows two of the dimensions of colour; the hues are arranged in a somewhat circular way and the colourfulness increases as we move outwards from the white point (a position near to the centre of the diagram). However, we can also consider the third component of colour (brightness) if we imagine a dimension coming out of the page towards you (http://colourware.wordpress.com/2009/07/18/cie-system-of-colorimetry/). The CIE defines several different colour spaces; the CIELAB colour space, for example, is another 3-D space that defines a colour by its L*, a* and b* values.

It is useful to think of an image-display device as also having a colour space. Consider the display on which you are probably reading this blog. The display shows colour by changing the amount of the red, green and blue light emitted at each point on the screen. The diagram below is a representation of what the RGB colour space of your display device may look like.

 

In the RGB cube, black is in the bottom left. As the RGB values increase colours are created and white results from each of the RGB primaries at full strength. So the RGB colour space defines the relationship between RGB values and colour. However, here’s the really interesting thing: The colour space for different display devices is very different. Even if we take a single device – such as the one that you are reading this blog on – then as we change settings (the brightness, the contrast, the gamma, the colour temperature, etc.) then the colour space changes. That is, the relationship between RGB and colour changes as you change those settings. This is a huge problem. Imagine if there were many maps of New York and each showed the position of, say, the Empire State Building to be in a different position. How confusing would that be? Well, that’s the problem with colour-display technology. If we didn’t do anything about this problem then every time we looked at a colour image on a different display device the colours could change markedly. This is why we need colour management. Colour management can make compensations to the RGB values that are sent to each display device so that the colours always appear the same (well, nearly the same). To make this compensation the colour management software (which is embedded in your Windows or Apple operating system) needs to know about the colour space of each device connected to the computer. Each device needs to have a profile that describes the relationship of its own colour space with respect to some standard colour space. 

How good is colour management? Well, that depends upon many factors. Most printers, cameras, scanners, and screens (LCD, CRT, etc.) come with a driver that includes a crude colour profile. This ensures that there is a basic level of colour management and for a great majority of users this is more than adequate. However, if you want better performance then you need to think about making some measurements that will allow a more accurate colour profile to be built. In a recent blog I described a new device that you can buy to enable you to do this – http://colourware.wordpress.com/2009/07/29/colormunki-colour-management/. There are many such devices on the market. I highly recommend Andrew Rodney’s book Color Management for Photographers which is both clear and accurate (though the edition I have works on Adobe’s CS2 package whereas the latest package is CS4).

However, no matter how hard you try, colour management is never likely to be perfect. This is because different devices have different colour gamuts; a printer is likely to be able to display some colours that your display physically cannot and vice versa.

WebExhibits colour site

I just came across a really interesting site – see http://www.webexhibits.org/

It includes some great interactive colour-related ideas. It looks like a fantastic teaching resource.

Indigo – a colour of the rainbow?

From time to time I come across web pages and groups of people who get irrate about indigo being in the rainbow. There is even a facebook group called “Get Indigo out of the rainbow”. It was Newton who suggested that the rainbow contains seven colours: red, orange, yellow, green, blue, indigo and violet. It has been suggested that, at the time, Newton was trying make some anology with the musical scale and the octave (with its seven intervals) and hence was keen to identify seven colours in the rainbow or visible spectrum. Many modern commentators claim that only six distinct colours can be observed in the rainbow.

Interestingly, the facebook group referred to above would like to eject indigo from the spectrum on the basis that it is not a primary or secondary colour but rather a tertiary colour. The group shows the following colour wheel:

In this so-called painters’ wheel the primary colours are red, yellow and blue and the secondary colours are orange, green and violet. It is argued that since six of the colours in the rainbow are primary or secondary colours in the colour wheel and indigo is not, then indigo has no right to be there. This is wrong on so many levels it is hard to know where to start.

The first thing I would have to say is that this argument seems to ignore the difference between additive and subtractive mixing. Additive mixing – http://colourware.wordpress.com/2009/07/13/additive-colour-mixing/ - describes how light is mixed and the additive primaries are red, green and blue. The additive secondaries are cyan, magenta and yellow. Orange is not in sight – and yet surely if we are to make an argument for inclusion in the spectrum based on primaries (and/or secondaries) then it is the additive system that we should be using since the spectrum is emitted light.  

The optimal subtractive system primaries are cyan, magenta and yellow (with the secondaries being red, green and blue) though the artists’ colour wheel (which is like the painters’ wheel above) has red, blue and yellow as the primaries. 

In my opinion there is nothing special about the colours that we see in the spectrum. Indeed, orange is clearly a mixture of red and yellow and does not seem to me to be a particularly pure colour. I just do not think that arguments to exclude indigo from the spectrum based upon colour wheels or primary colours is valid. That said, I have already mentioned that many people believe that indigo cannot be seen in the spectrum as a separate colour; but this is a phenomenological observation not dogma. I am one of those who believe that indigo and violet cannot be distinguished in the spectrum and therefore I agree with the aims of the facebook group even if I do not agree with their arguments.

The really interesting question is why we see six (or even seven) distinct colour bands in the spectrum when the wavelengths of the spectrum vary smoothly and continuously? I have postulated some possible reasons for this in an earlier post – http://colourware.wordpress.com/2009/07/20/colour-names-affect-consumer-buying/ - but it is far from a complete and convincing explanation. It may explain why we see distinct colours in the rainbow, but why six and why those six in particular. Comments on this would be very very welcome.

Colour 101

I am really looking forward to some interesting topics such as

Is black a colour?

and

Does colour exist?

But, before I get into these tough topics I would like to present some basic and rudimentary notions about colour and what it is. Look in any textbook on colour and you’re almost certain to find a picture of the electromagnetic spectrum looking something like this:

spectrum

 

It was Newton, of course, who famously studied the relationship between wavelength and colour. Light is a form of energy called electromagnetic radiation. Light can be characterised by its wavelength and our visual systems are sensitive to wavelengths in the approximate range 400-700nm (we’ll deal with the exact wavelength range later). So we call radiation in this range the visible spectrum or, more simply, light. In my diagram above the short wavelengths are on the right and the longer wavelengths on the left. So we might simplistically think that, for example, light at 400nm is blue or violet and that light at 700nm is red. It’s nowhere near as simple as this but it would do no harm to think that way for the present.

The spectrum above raises two interesting questions straight away however. The first is, why – since the wavelength of light varies continuously from about 400nm to about 700nm – do we see these specific and discrete colours? When I was at school I learned the mnenomic Richard Of York Gave Battle in Vain to remember the order of the colours in the spectrum. But why don’t we see a continuous range of colours – or, to be technically more precise – hues? The answer is something called categorical perception. However, just as interesting is my second question. Why do the two ends of the spectrum look rather similar. OK, red and violet are not the same. But certainly, red is closer to violet perceptually than it is, to say, green. And yet in wavelength terms red is closer to green! I’ll be returning to this issue of circularity of hue in a later post. However, if you would like to explore either of these phenomena yourself then I would encourage you to spend time looking at a rainbow. When sunlight strikes droplets of water in the air (this often happens on a sunny day after a rainstorm) the wavelengths separate (a process called refraction) and we see the visible spectrum. Newton achieved this by passing sunlight through a glass prism but the effect is the same, and equally enjoyable.

Interestingly, although Newton observed 7 colours when he separated white light with his glass prism, most scientists today agree that it is really only possible to discern 6 colours and that indigo cannot be distinguished from violet in the visible spectrum. Again, don’t take my word for it. Go out and look a rainbow now!!! The following relationships between colour and wavelength are often quoted:

Red —- 635-700nm

Orange —- 590-635nm

Yellow —- 560-590nm

Green —- 490-560nm

Blue —- 450-490nm

Violet —- 400-450nm

However, be very careful. Newton famously wrote that “to speak properly, the rays are not coloured”. Now, I wonder what he could have meant by that?