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Colour Correction - part 1 of 1 2 3 4 5

by Mike McNamee Published 01/09/2006

We first wrote our eight-part series on colour correction back in October 2002. Say it quickly, but that is 48 months, back to a time when Professional Imagemaker had 32 pages and Photoshop was a mere baby of seven versions. Two things have changed in the intervening period - the SWPP/BPPA membership has grown (by about 300%, so there are more people who missed the first series) and the technology has moved on (so the technical content needs amending). One thing has not changed - human perception. This has remained essentially the same for millions of years and remains fascinating to the world at large - Google for "colour perception" and you return 25,000,000 hits!

One of the most potent features of digital imaging is the ability to colour-correct before output. This strength comes from two quarters, the ability to measure within the image and the ability to correct using numerical values. Both add great precision to the procedure, so much in fact that you can colour correct with impaired colour vision, providing you know the target numbers of various parts of the image. The downside is that too many of us spend too long tinkering with files before printing, when the job used to be done by a laboratory technician who did it all day and was as quick as lightning. In recent research, the participants were asked to adjust target colours using RGB sliders until they matched reference colours (which included the 24 of the Macbeth Chart, plus six others). They averaged 50 to 60 seconds per colour. This puts a mark in the sand over how long it might take to colour correct a file. It also explains why you have to automate the process if you are not to spend most of your week in front of the monitor - a one-click colour balance in a RAW file handler suddenly looks very attractive indeed. Of all the technology that has advanced since we last wrote on the topic it is the quality of RAW file handling software. In Photoshop all the major adjustment tools have changed very little, the implementation of 16-bit working across the application being the greatest.


Visual Perception humans and others Most of us take colour for granted, although it has not always been so. Primitive cultures tended not to bother about colour and had a limited vocabulary of colours (that is they did not assign names to specific colours). Colour is in any case a conceptual thing that only really exists in the mind of the viewer. It ceases to exist in a darkened room. Animals see in colour quite differently to humans; dogs for example are limited in their colour discrimination. Our New World cousins have different colour to us Old World types. Forty million years ago primates developed an additional skill that enabled them to differentiate green and red - pretty handy if you are up a tree trying to decide which fruit is ripe and which is not. Colour vision works a bit like your scanner. In the scanner, the detector looks for red, green and blue light and mixes them together to define the colour of the pixel. The eye works in a similar way. "Cones", which are situated close to the central axis of the eye, have pigments that are (each) one of three colours. They are usually thought of as red, green and blue although they are, in reality, more accurately described as yellow-green, greenyellow and blue. This does not inconvenience us as the brain does clever tricks and makes red look like red, etc. The cones are the source of our colour vision and it was a third cone type that formed 40 million years ago to give us our enhanced 24-bit vision. Dogs have 16-bit vision (2 by 8-bits of colour!) - this is true of most non-primate mammals. Fish and some poor Aussie nocturnal creatures have poor old 8-bit vision and see in monochrome. Monochrome vision in humans is very rare although about 10% of the population of an island in Micronesia are afflicted. Most birds and (rarely) some humans have additional cones and are termed Tetrachromatic or Pentachromatic - some can sense ultraviolet light as well.

While the cones are the source of our enhanced colour vision they can also be the downfall of some people. If your genetics are mixed up you could end up with one of your cone sets missing or perhaps with cones that are too similar to each other in their colour detection. The genetic basis of colour vision is what makes men more vulnerable to colour vision defects, 16 times in fact (0.43% of women, 8.14% of men to be precise). Girls take X chromosomes from each parent while boys take one of the mother's Xs and one of the father's Ys. As the X chromosome carries the pigment-making signals for the cones, the boys are disadvantaged. At the same time, a similar gene mutation is responsible for some women becoming tetrachromats.


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1st Published 01/09/2006
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