Chuck Gardner's Photography Class
Part Six - Preparing Digital Files for Offset Printing
by Chuck Gardner
My day job for the past 20 years or so has been running an offset printing operation. I prefer that photographers and designers supply all images as close as possible to the state that they came out of the camera or scanner. If the file has been cropped or edited in PhotoShop I prefer getting a copy in the native, unflattened, and unsharpened PhotoShop format so color in layers can be adjusted separately if necessary. The exception is re sampling, for reasons explained below.

It makes little sense for a photographer to darken and correct out a magenta color cast apparent on his monitor, only to have the file arrive at the printer and have it too dark and green on its monitors which are calibrated to simulate ink on paper. There will always be a difference between the color of the monitor of the photographer and the printer but the monitors and proofing devices at any good printer these days are carefully calibrated to simulate the appearance of the entire reproduction chain, which typically includes outputting the file onto hard dot patterns on separate C, Y, M, K (black) films, exposing plates using the film, and finally printing the dot pattern with a set of pigmented inks (which can vary significantly from brand to brand) on paper which can vary greatly in brightness, color, and finish (i.e., gloss, dull, matte) and absorption / dot gain).

It is possible for a printer to create color compensation profiles for each paper/ink combination used. We just did this at our shop with the assistance of Heidelberg Philippines. A standard test pattern is scanned, imageset, plated, and printed then measured with a photospectrometer. By comparing the readings on the press sheet with the values on the original, a table is built by the software which compensates for the gamut of the ink set and the brightness of the paper. For example, on a very white gloss coated stock a light ivory tone might reproduce with a 5% yellow and 2% magenta and yellow dots, however if the same tone is printed on a dull cream stock no dots at all are needed because the paper is already the desired color. Printing the 5% yellow and 2% magenta and yellow dots on the cream stock will make the highlights look dull and muddy.

Once the profiles for the various paper/ink combinations are determined they can be used on a high-end scanner or PhotoShop to change the scanning or display settings for hue, contrast, and brightness to simulate the color gamut and contrast of the ink / paper to the extent possible. Reproducing the same file on the two difference stocks would require two different output profiles, one for each paper.

If you view a digital camera file on a monitor in PhotoShop calibrated to an ink/paper ICC profile, colors like bright purples which printing inks can't reproduce will automatically be dulled-down on the monitor image to the shade of purple that can be printed. The overall brightness and contrast of the screen will be lower, simulating the range of reflectances of the press sheet. What you see on the screen can be, if the calibration is done well, very close to how the file will look when printed.

Similar calibration can by done with high-end proofers. For example, each time the color ribbon is changed in an Imation Rainbow dye sublimation printer the starting points for the calibration must be downloaded from the Imation web site using the lot number of the ribbon for reference. A detailed test subject is then printed and used to fine tune the calibration. The same test target can be imageset and printed on press, with the Rainbow calibration adjusted as needed to match the printed results.

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