A quick explanation of separations

Whenever you convert an image file from RGB to CMYK you will be deciding on a number of criteria which determine how it will print. Two of the critical criteria are often referred to as TAC and GCR. TAC stands for Total Area Coverage and defines the amount of ink that can be used to create any printed dot before that amount of ink becomes a problem to the printer.

GCR stands for Grey Component Replacement and in essence relates to the amount of black used within any colour to recreate that colour on the press.

Colours in an image are made up of cyan, magenta, yellow and possibly black. The image could be a metallic silver car, or a vignetted background to a cosmetics advert - or any number of other possible subjects. The subject could actually be reproduced with little or no black, regardless of its colour. If GCR is applied the combination of cyan magenta and yellow within a particular colour (whose cumulative effect would be grey) will be removed and replaced with black.

The decision as to how to treat the conversion from RGB to CMYK for a neutral part of an image (ie how much of the neutral colour to create using the black channel only and how much to recreate using the other three process colours) is defined by the GCR level. If one uses 100 per cent GCR it means as much of the black (for which read 'grey' in GCR) component of that specific colour as possible will be recreated using the black channel (K, of key). If there is zero per cent GCR, all of the colour will be recreated using the other three process colours (cyan, magenta and yellow). Bear in mind that if you had 100 per cent, 100 per cent magenta and a 100 per cent yellow it would, to all intents and purposes, appear black. So why does it matter which level of GCR you select? Well, there is a trade-off between the amount of GCR used, the stability of the colour reproduction on the press and the suitability of the content matter. The more GCR one uses, the less likely it is to slightly change when printed.

Any particular profile describing a CMYK colour space will have only the black generation parameters (GCR and UCR) which were defined when it was created. Certain types of content are better handled with little or no GCR, while others do not react well to GCR at all. This means that there will never be a profile which will be appropriate for all subject matters, even if it describes the target colour space accurately.

Consequently, whilst we are recommending a particular conversion profile (ISOWwebcoated.icc) we are only recommending it if you are uncertain as to what else to use. We feel that it gives a reasonably compromise solution to the problem. However, you may well decide to use an alternative profile for a specific job, or indeed for all of your work.