Tag Archives: wavelength

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:
 myspectrum

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.

chromdiagram

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.

 

rgb

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).

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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.

cure for colour blindness

One of the reasons I enjoy travelling by train is that it gives me an opportunity to read a newspaper from front to back (something I very much enjoy but rarely have time to do). Yesterday I was travelling to Bristol where I was delivering a lecture at the IMPACT6 Printmaking conference on colour management and took the train from Leeds to Bristol during which I was able to read The Times. I couldn’t fail to notice the story about a potential cure for colour blindness – http://www.timesonline.co.uk/tol/news/science/medicine/article6837392.ece

Congenital red-green colour blindness occurs when either the L- or M-cone class is either missing (making the sufferer a dicromat) or shifted in terms of peak wavelength of sensitivity (resulting in anomalous trichromacy) – see http://colourware.wordpress.com/2009/07/04/colour-blindness-news/.

Scientists working at the universities of Seattle and Florida have restored normal colour vision to two colour-blind monkeys by injecting a virus with a modified gene (called L opsin) that is known to be responsible for red-green colour blindness. The success of this work is remarkable in that it suggests that the brain is able to rewire itself to take advantage of the new receptors. 24 weeks after the injection the monkeys were able to correctly distinguish patterns of grey, green and red dots that they had previously been unable to distinguish.

colour_385x185_615124a

Jay Neitz, professor ophthalmology at the University of Washington, is now looking to start work that could lead to a similar treatment for humans.

The work has just been published in Nature – http://www.nature.com/news/2009/090916/full/news.2009.921.html 

The rays are not coloured

So when Newton wrote that the rays are not coloured, what exactly did he mean?

Well, he meant that even though we may say loosely that light at 400nm is blue and light at 700nm is red this implies that the blueness and the redness are properties of light.  Although there are philosophical arguments that would support colour as a property of light (and we’ll get on to those arguments in a later post) for now I would like to put forward my view (which is, I believe, consistent with Newton’s) that colour is not the property of light.

The evidence that supports my view is that light at 700nm may look red to most people most of the time, it doesn’t look red to all of the people most of the time or even to most of the people all of the time. For a very striking example please consider the image below:

illusion

In this example, you will see some blue spirals and some green spirals. But physically the blue and green are the same. In terms of wavelengths, exactly the same wavelengths (in exactly the same proportions) are being reflected from the areas that you perceive as being green and the those you perceive as being blue. If you think in terms of digital (RGB) terms, the RGB values of the green areas and the blue areas are the same – both are about R = 9, G = 20, B = 160. We know now that the colour that you perceive for a wavelength of light or a group of wavelengths depends upon the colours that are close by. This is often expressed as contrast or assimilation. When contrast occurs colours become less like the colours that they are next to an image; when assimilation occurs colours become more like the colours that they are next to. Contrast and assimilation effects result in you seeing two colours, a blue and a green, when physically only one colour exists.

Straight away some of you can see that I am falling into loose language straight away because I am using colour in two different ways. On the one hand I am saying the two colours are physically the same and on the other hand I am saying that the two colours are perceptually different (blue and green). Which is it? It all depends upon how you define colour. My stance is that I define colour as a perceptual phenomonon – it’s something we see and experience. Others may argue that the two colours are really the same and that it is a mere illusion that they look different – I, on the other hand, would argue that the two colours are different. It’s not an illusion – you see a blue and a green, don’t you?

This is what Newton was referring to when he said that “to speak properly, the rays are not coloured” – I believe that Newton was aware of this problem with language – that colour can be used to represent several things. But when we speak properly we realise that the rays are not coloured.