Re: Funny problem: FM/PDF color setting

Subject: Re: Funny problem: FM/PDF color setting
From: Dick Margulis <margulis -at- fiam -dot- net>
To: "TECHWR-L" <techwr-l -at- lists -dot- raycomm -dot- com>
Date: Tue, 22 Oct 2002 07:09:46 -0400




debbywu -at- benq -dot- com wrote:


Due to the company's VI guideline, I used the PANTONE colours for my
coloured texts and rectangles in FM, and when the doc was turned into PDF,
the colours went wrong. Not too wrong though, from reddish purple to
bluish purple, but it was still annoying.
In the end I decided to print out both FM and PDF on our laser colour
printer for a look. Now guess what happened? Yuppy, both doc showed the
same correct colour.


Debby,

I'm going to try to explain what's happening, but I have to make a couple of guesses about what you are doing.

The Pantone Matching System provides printed color chips for matching to printed inks. These are reflected colors. The colors on your monitor are not produced by printing ink on paper but by exciting phosphors that radiate red, green, and blue (RGB) light. These are transmitted colors. The color spaces (called color gamuts) of reflected and transmitted colors are different.

Further, Pantone offers more than one set of colors. Spot colors are inks mixed to the color you want to print. Process colors are colors produced by overprinting various percentages of cyan, magenta, yellow, and black (CMYK) inks. The color gamut of spot colors is much larger than the color gamut of process colors. Your laser colour printer uses process colors (because it has CMYK toners), not spot colors.

Pantone also licenses their color system to software producers to provide simulated color chips that enable you to pick colors on your monitor to _represent_ (but not match) colors you intend to print. Because the color gamuts do not match, the colors themselves match only approximately. Further, unless you have color calibrated your monitor, the similarity is really not very good.

You haven't said how you picked your Pantone colors. I don't know if you picked a swatch on screen or looked at a PMS spot color book and held a chip up to the screen or looked at a PMS process color book and held a swatch up to the screen. In the latter two cases, you're lucky you even came close. But I'm going to assume you picked a simulated Pantone color on your monitor and wonder why the same color specs did not translate into a matching color in the document.

Because the reflected and process printed color gamuts are not the same, image and publishing software has to apply some sort of algorithm to translate between RGB colors and CMYK colors. Some programs (geared toward display on a monitor) use one algorithm and some programs (aimed at printing on paper) use another. The result, in your case, is that the monitor displays differ between FM and Acrobat, but the printed documents match. If you were to fiddle with the colors so that the monitor displays matched, then the printed output would not match. The reality is that you can't have it both ways. Your PDF has the correct CMYK colors specified, but Acrobat is not converting them to RGB for display the way you expect. It can't be helped.

Dick





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