Inkset Matching in Photoshop

by Battle Vaughan

Pre-press professionals who use Adobe Photoshop sometimes find that the picture on their computer screen looks different from the one that ends up in the newspaper. Often the problem can be traced to an important detail: Photoshop needs the user to provide monitor, ink and separation preferences.

To make CMYK conversions from an RGB image, Photoshop uses the information stored in its printing-inks setup and separation setup tables. It then uses the information in the monitor setup table to show, on the screen, a representation of those colors. If any of this setup information is incorrect, then the colors are too.

Photoshop's ink-setup screen

Users often mistakenly select the generic SNAP or Ad-Litho inksets provided in Photoshop. Research by Knight-Ridder's technology department at several newspapers has shown some deficiencies in these canned ink setups. In fact, different sites using the same ink setups had widely varying results.

Why? Because their paper stock, press conditions, ink vendors, etc., were different. The lesson: To get accurate color, you must match your ink-setup table to the particular inkset on your press. Fortunately, this isn't hard to do if you enlist the help of your ink vendor and a simple set of test patches.

Knight-Ridder's approach is to create a CMYK press-test form in Photoshop, print it with the presses set at their target ink densities (K1.05, M.90, C.90, Y.85, plus or minus .05), and then read the resulting patches with a spectrophotometer. If you don't happen to have one lying around, your ink provider will probably read the form for you.

Some sites have saved paper by running the test on bundle wrappers, and others have run the tests in editorial space. However it's printed, the test form needs to have the same patches as the Photoshop ink table--100 percent patches of C, M, Y, C+M, C+Y, M+Y, CMY and K. You should read W from the paper, which must be your regular production stock.

The latest version of NAA's Digital Color Test Form (available for download at http://www.naa.org) has the proper patches, or you can easily make your own in Photoshop. If you do, be sure to create the file as CMYK and not RGB, and that the patches are large enough for the instrument that will read them. The SNAP press-test form usually lacks the CMY patch and may have print areas too small for some spectrophotometers to read.

As the spectrophotometer reads the form, it programs the resulting CIE L*a*b* numbers into Photoshop as a custom ink table. Photoshop checks the monitor-setup file, neutralizes the monitor with its Knoll Gamma tool and gives the separation-setup table the information it needs--maximum ink percent, maximum black and amount of UCR or GCR. You can enter gray balance and dot gain into the ink table as well.

I have carefully avoided the word "calibration" in this discussion--what we're doing here is basic color matching. True color calibration will come with the arrival of ColorSync 2.0 from Apple, Linotype-Hell and others. Their color-management systems will provide accurate measurements and compensate for variables in each device in the system.

In setting up these systems, we will use some of the measurement skills we developed in setting up the Photoshop ink table, so get on good terms with your spectrophotometer. While high-end units cost upward of $10,000, less sophisticated ones are around $2,000. ColorTron by Light Source and the Digital Swatchbook by X-Rite are available now. We will use them to produce the device profiles at the heart of color management.

These systems have two parts. The first is the color profile, which defines the difference between a scanner, printer or other device and a reference color space. The second is software that compensates for the color characteristics of each device. ColorSync 2.0 will use a non-proprietary device profile format, agreed on by members of the International Color Consortium. We'll be measuring device profiles on monitors, scanners, proofers and presses, and all the steps in the production process will "understand" how the others work with color. More than a hundred manufacturers are reportedly developing ICC-compliant software and hardware, from self-calibrating monitors to on-press spectrophotometers.

Until we reach this level, though, simple inkset matching can have a profound effect on your Photoshop color separations.

Battle Vaughan is a picture editor at The Miami Herald and a member of the Knight-Ridder Newspapers Talent Bank in the pre-press and photo-lab areas. E-mail, wizone@herald.infi.net; phone, (305) 376-3749.