Fundamentals of colour science
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What is colour?

An intrinsic property of substances (Aristotle)

Aristotle believed that light is something transmitted from an object to the eye. The colour of the object is then an intrinsic property, like its weight or taste.

Something substances do to light

Aristotle believed that the rainbow's colours were not like normal colours: they are caused by reflection of sunlight from raindrops in distant clouds. He reasoned that each droplet acts like a tiny mirror, and that such mirrors can change white light into coloured light. This gave rise to the idea that colour arises when objects somehow alter light.

Goethe's theory of colour builds on this idea. He regarded light as some kind of fundamental and homogeneous thing - a kind of universal radiance - that becomes coloured when disturbed and modified by differing degrees of darkness. So in Goethe's model there are two polar extremes: light and dark, with all the colours arrayed between them. This is similar to the ancient Greek view of colour.

But rather confusingly, he somehow equated light with yellow (there was a common view that sunlight was yellow) and dark with blue, and suggested that red could be made by mixing yellow and blue. Goethe's influence on artists, e.g. Turner, Kandinsky, theosophy & Mondrian. [Images: Turner's Deluge (1843), Kandinsky, Mondrian]

Light itself (Newton)

Misconception that Newton in the 17th C showed that 'white' sunlight could be split by a prism into the visible spectrum. In fact, this was long known - even to Aristotle. But the idea was that the prism was somehow modifying the light. Newton showed that it was in fact revealing the components of white light. He showed that by using a lens to refocus the spectrum, one could reconstitute white light. And he showed that the individual colours were irreducible.

So Newton turned colour theory away from substances and towards light.

What is light?

Electromagnetic spectrum (Maxwell). Newton divided the spectrum into seven fundamental colours - arbitrarily. He recognized that the extreme ends of this spectrum faded into similar colours - red and violet - and he united them in the colour wheel. Later colour wheels were simpler and more symmetrical: six colours. The modern colour version of the colour wheel is the CIE diagram (Commission Internationale de l'Eclairage), which is less elegant but more scientifically informative.

Additive and subtractive mixing

To many artists, Newton's theory of colour was completely unsatisfactory. In particular, it said strange things about colour mixing: that all colours mix to white. And light mixes in other strange ways: red and green make yellow, for instance. This is one of the principal objections of Goethe to the Newtonian theory.

Painters knew that almost any colour could be made by mixing just a few 'primaries'. Since the 17th C, these were generally regarded as red, yellow and blue, with white and black for lightening and darkening. But in the mid-19th C, Maxwell showed that he could make light of any colour by mixing three different primaries: red-orange, blue-violet and green. Mixing light is not like mixing pigments (additive and subtractive).

Complementary colours

Artists and designers had known for a long time that certain colours go well together - they seem to enhance one another. In the 19th C, the French chemist Michel-Eugene Chevreul formalized these relationships in his theory of 'simultaneous contrasts'. He was the director of the Gobelins dye works in Paris, where he was set the task of finding out why the dyes seemed dull. He realised that there was nothing wrong with the dyes themselves, but that threads of complementary colours were being woven too close together. When that happens, our eye can't resolve them and they merge into a kind of greyness. But if the coloured patches are bigger, the colours look brighter. And at the point just before they start to merge, they seem to shimmer. This effect was sought after by the Impressionists, who often placed patches of complementary colours together to enhance their brilliance. The shimmering effect was what George Seurat and the other Neo-Impressionists were looking for in their pointillism technique.

Colour and perception

One thing we do have to thank Goethe for is the notion that colour is as much about perception as about light. It's not just about what reaches our eye, but how we perceive it. Red and green light remain just that when they are mixed: they don't turn into light of a 'yellow' wavelength. It's our visual system that turns the effect of red and green light into a sensation of yellow. Maxwell agreed with this: he said 'The science of colour must· be regarded as essentially a mental science.'

Just a brief word about how colour vision works. The English scientist Thomas Young suggested at the start of the 19th century that, rather than having cells sensitive to every different wavelength of visible light, we had just three types, sensitive to the three primary colours, and the sensation of colour came from the mixture of responses from each cell type. The German physicist and physiologist Hermann von Helmholtz later showed that this was essentially correct. We have three types of colour-sensitive cells in our retinas (cones). It's usually said that these are sensitive to red, green and blue light; actually, their strongest sensitivities are to violet, yellow and green. And there's nothing fundamental about the fact that there are three types - colour vision is possible with only two types of cell, and in fact most New World monkeys have only two. Some animals - some fish and birds - have four types of cone cell, making their colour discrimination more acute than ours.

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