Colour Correction

Perception & Measurement

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If you are colour-blind or colour defective there are certain occupations that you are barred from. You won’t be much use in a photographic laboratory, an electronics company or as a police officer. The first is obvious, the second is because you would mix up all those wires and the third is because the judge will want a better description of the getaway car than “it was a reddy, greeny-grey colour”.

Assuming you are not a colour defective, i.e. something biologically wrong such as a missing pigment, how do you measure up when judging colour? This is an important question because experience tells us that some people seem to be better at judging colour than others. We spoke to one of the UK’s leading experts in this subject, Jennifer Birch of the City University. In her book on the subject she says, “…most people with normal colour vision can acquire superior colourmatching skills if they are sufficiently motivated.” Certainly in the tests conducted on my Photoshop students over the years, only two have failed the Farnsworth D15 colour discrimination test, the main difference between people is the speed at which they do the test, not whether they get the correct answers.

This is encouraging for those who worry that they have poorer colour vision than their peers, but there is no doubt that some people seem to have better judgement than others, the trick is to learn from our peers who are judged good at it. Colour discrimination goes off with time away from the job; you need to break yourself back in after a long holiday or time off work! The problem is two-fold for the colour printer – deciding which way the colour is out and then how to make it right! The advice that filters through seems to be consistent; train yourself up, learn from a more experienced colleague and, lastly, don’t give up. It might also pay to visit the site at http://www.geocities. com/heartland/8833/coloreye.html and test your colour vision. If you find that you are colour-blind take up monochrome photography or try harder!

Here are some general pointers:

1. The eye is quite discriminating over colour – watch that you don’t end up chasing an impossible standard.

2. The trained eye is even more discriminating, better than many instruments.

3. Practice improves performance.

4. Learning alongside a skilled practitioner is useful.

5. Viewing conditions are important and include:
- sufficient illumination
- correct light colour
- correct surrounding colours (don’t have jazzy wallpaper!)
- no glare, especially on your monitor
6. Differentiating cyans is the hardest task.

7. The size of the block of colour exaggerates its colour. A small piece of off-colour grey might look OK in a colour print, that same bias on a 16x12 monochrome print will look way out.

8. The colours surrounding the colour under examination exert a big influence. We had a classic example on the front cover of the issue before last. Look at the SWPP/BPPA logo shown as normal and how we had to distort it to make it “correct” on the cover. At its correct Pantone value, the BPPA was almost invisible against the model’s arm. This is where colour by numbers really breaks down, we arrived at the adjusted colour of the logo purely by eye, there is a 21 delta E difference between the colours (we’ll explain delta E later!).

We can use the list above to guide our behaviour when making colour judgement. In general terms you should rely on your female partner, wife, sister, etc to assist you. As well as two brains being better than one, if you call them in late in the process they will not have been sitting there accumulating eye strain and colour bias, a process known as chromatic adaptation. Also you should take a break from your labours before making a final decision on the colour balance of your image, your first judgement is most likely to be correct and this includes your first impression after a break. Also you should take your time and while this goes against the advice in the last sentence to some extent it is a fact that only experienced colourists are able to make fast judgements.

Another trick you can employ to assist is to highlight the numerical adjustment values in a Photoshop dialogue box (by double clicking them to make them show blue). Now, when you hold the shift key down and click on the up or down arrows, the values jump by 10 units, producing a rapid shift in colour. If you move without the shift key the values jump one digit and you might have adapted to the shift by the time you get 10 clicks in. By jumping in big increments you can do a Goldilock’s Porridge adjustment (i.e. – this one is too big; this one is too small; this one is just right!).

A final word before we get into the nuts and bolts of adjusting colour. Do not waste your time chasing perfection in your greys from older, six-colour ink jet printers – you will never get there if you are very discriminating. The latest Ultrachrome K3 ink sets will satisfy 99% of even fanatical photographers and 100% of normal punters. Bespoke profiling in general will get the job done with most printer and ink combinations although metamerism may still rear its head with some ink sets.

Measuring Colour

This series of articles is planned to be a reasonably complete explanation of the topic and we must therefore set the scene with some care. We have so far explained how colour is perceived, now we need to explain how it is measured. We have to understand the relationship between measuring and correcting colour so we know where to put our effort in.

Providing you have either a colorimeter or a spectrophotometer, measuring colour is simple. You simply place the image on a flat surface, plonk the instrument onto the relevant bit of the image and press the measure button. The instrument then sends the colour values to your computer or to its own little screen. It is from here that things get complicated! There are literally dozens of ways of expressing the colour values. This is a bit like currency. A jar of coffee may cost £3.45, $5.1 or €5.48. The coffee in your shopping trolley does not change but the numbers it costs vary according to how you are going to pay for it. Colour is the same. The colour in the image stays the same but the numbers (currency) you use to describe its value vary according to how you measure it.

BELOW: The X-Rite Digital Swatchbook is one of the most popular measuring instruments. It measures single colours but can also average a series of readings, indispensible if you have to match to the colour of a garment or even a brick!

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Photo Quote: The virtue of the camera is not the power it has to transform the photographer into an artist, but the impulse it gives him to keep on looking. - Brooks Anderson,