Color calibration

Wednesday 10 May 2006This is 17 years old. Be careful.

For work (which I’ll be saying much more about very shortly), I’ve been having to consider color calibration with a printer. By printer I don’t mean a thing the size of a bread box that sits on my desk and squirts ink onto paper. I mean a person who runs a printing shop using room-sized machines like an HP Indigo 5000.

I send him PDF files, and sometimes they look fabulous, and sometimes they look like someone dipped them in a yellow wash. I know a bit about printing technology, for example, I was Digital’s PostScript expert for a while. I can even throw around terms like tristimulus values and color gamut without completely faking it.

But he and I don’t see eye-to-eye on this problem. My PDF files are RGB, mostly because it was the simplest thing to do. He wants CMYK, but my understanding is that the mapping from RGB to CMYK is a very fluid thing, depending a great deal on the actual printing technology and printing machine used. How could I convert the colors into CMYK better than the HP Indigo’s RIP could itself?

Does anyone have any insight or good pointers for me? I took a look at the International Color Consortium site (you know, the ICC in ICC profiles), but man, that’s one thick site.

How does one make a PDF file that can be confirmed correct once it is printed?


This might be helpful:

You could also post to Python's image-sig mailing list; some regulars know a lot about stuff like this.
The authoring program (Photoshop, Illiustrator etc.) can generate a CMYK file from the RGB. The look up table that it uses to do the conversion usually comes from a device driver specific to the target device. I would think that the printer would be able to provide you with that driver. It's true that the device, if given an RGB, does it's own look up translation (it has to since the actual inks are CMYK), but in theory the driver has settings and can be fine tuned. What is your authoring program?
I suspect the major source of trouble isn't the RGB to CMYK conversion itself. The key concept you're looking for is color profiles.

They're kind of analogous to charset encodings. To properly display a string you have to have the character data *and* its encoding. To properly display/print an image you need the pixel data *and* its working color profile. (To complicate things, we also have variance across devices -- this is where calibration and device profiles come in. But device profiles don't need to travel with the image as long as proper calibration is being done by both parties.)

In short, if you want to see the closest possible thing to what's going to come out of the press, you need to 1) Make sure you're working with software that understands color profiles, 2) calibrate your monitor, and 3) agree with your printer on a working color profile ("Adobe RGB 1998" is widely used).

I worked in the magazine business for many years. I always consoled myself with the fact that since exact mapping between transmissive color (monitor) and reflective color (print) is impossible, the fact that we get as close as we do is pretty amazing and something to be thankful for.
Thanks for the useful links, and the words of encouragement.

Fred (if that is your real name!): we are generating our own PDFs with PDFlib, effectively, it's like spitting out straight PostScript. The good thing about this is that I have tight control over what's in the PDF. The bad thing is that I have to do everything "manually" in the code.
Fred is my real name, just ask Lucy and Ethel...

Paul is right about the color profiles. You might put your PDF file into Illustrator or Photoshop to take advantage of the color calibration therein.

I suspect you are using an LCD monitor, so calibration is going to be blunt at best. That is one thing that CRTs still do better....
You might also want to speak to the guys at ReportLab, who probably know more about PDF and Python than most people. I also know they have been fairly closely in touch with the H-P group that markets the Indigo, though I don't know whether that led to any collaboration that might help you.

You can find all kinds of infomation and nitty gritty details on imposition in the Scribus wiki available via

Basic color calibration of your system, then color profile choice
are critical. IIRC yellow washout may be the out of gaumut color for when you've chosen colors that can't be represented in CMYK space.


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