On Dec 20, 2009, at 10:20 AM, Krzysztof Kosiński wrote:
Is CMYK rendered exactly the same by every printer? Are there such things as ICC profiles for printers? Why don't the print companies use those profiles with the customers' color-managed RGB documents?
No, CMYK is rendered differently by each printer. In fact, two of the same model printer by the exact same company will render those colors differently. (Not "can", but "will". The only question is the amount that those colors will differ)
And, yes, ICC profiles are very common for printers. Or to be more precise, profiles for a specific printer with a specific type of paper and a specific type of ink are common.
For some workflows, explicit profiles are not always required to be included. For example, sRGB is a very specific colorspace with specific details, and ICC profiles exist for it. But since it is a common fallback, it is built in to many systems. LittleCMS, for example, has a call to return the sRGB profile... so one does not require an explicit .icc file for it. The W3C has declared its standards (incuding SVG) to use sRGB, so for those one does not need to explicitly attach an external ICC file.
There are several different "standardized" CMYK profiles out there that are comparable to sRGB. So one can use one of those profiles, "Sep Euro UCR340" or "U.S. Web Coated (SWOP) v2" perhaps, and then the print shop will do a conversion from that to the specific offset printer they have on the second floor when they run their job. There will be some data loss in that conversion, but if the target profile is close enough to the actual one used for the final print, then it will minimize issues.
For home users, on the other hand, one should have a specific ICC file per printer to be used. Test samples should be printed and then measured with a colorimeter to get that specific printer's results with that specific paper and that specific ink set. Then when proofs are printed locally the user can be sure it will be close to the final result. Given how low cost these have become, anyone who does any art jobs for at least some pay really has no excuse for not doing so.
And as to the question of using those profiles for RGB documents... sometimes one needs to work in CMYK. That used to be required, but nowadays wide-gamut RGB for photos and such will give better results. For things like overprint, sharpness in text, etc. an artist might need to go to explicit CMYK, but otherwise it can be compared to a programmer trying to optimize code by manually unrolling loops, etc. Compilers became smarter than programmers back in the early nineties, but many people kept the habits that now hurt instead of helped.
Oh, and a final caveat is that colors can and do change over time. Monitors will change their behavior over time (LCD's becoming dimmer, shifting hue, etc.), printers can change over time as things wear or even as the next ink cartridges are installed. So for a person's local system (printer, monitor, scanner) ICC profile files should be created periodically (perhaps monthly).