Proofing By the Numbers

Dunwoody College of Technology proves that matching a color proof to the press isn't purely academic.

LAST FALL, DUNWOODY College of Technology instructor Pete Rivard and his students set out on a mission. Armed with a new color halftone proofer, spectrophotometers, and color management (CM) software Rivard set out to prove just how close a proof can match a press—in this case a narrow-web flexo press, running UV inks on pressure-sensitive label and paper stocks.

Rivard's experiment is meaningful for a number of reasons. First, he showed that yes, it is possible to define a set of conditions under which a proof can match a press. Secondly, he found that the proofer could predict the press each time the conditions are repeated. And third, he discovered that the process can be understood and used by 19-year-olds with no experience in the trade.

The project was a collaboration between Dunwoody, Latran Technologies, and the Flexographic Trade Association (FTA). Rivard was challenged with unofficially certifying the Latran Prediction digital halftone proofer for accuracy under F.I.R.S.T. specifications.

The Dunwoody team began by choosing the stocks and inks they would test. They set their press to the "sweet spot" for gray balance, solid ink densities, and hue saturation running an IT.8 control target and adhering to F.I.R.S.T. tolerances and guidelines. Using X-Rite and Gretag automated spectrophotometers, Rivard read the press sheets into Heidelberg CM software. The data from the readings was used to create an ICC profile—a computer file that describes the 3-dimensional color map of a given color output.

The press ICC profile was then applied to the control image as it was RIPed to the Prediction proofer. Theoretically, the RIP would remap the RGB data of the source file to CMYK equivalents within the color space described by the profile—effectively constraining the capabilities of the proofer to more closely resemble those of the press.

The differences between the press results and the profiled proof results are measured in Delta E increments—which is the straight-line distance in LAB color space between two colors. (Humans with average sight can perceive variations of one Delta E.) After making some adjustments to the color selection and sequencing of the Latran Prediction's image transfer sheets, Rivard achieved a remarkably close approximation between the proof and the press—even for films and foil substrates. "We were successful far beyond what I thought we'd get to," he says. "We could hold the press sheet and proof side-by-side and not be able to tell which was which."

Rivard says it's key for companies that want to get their proofing process under control to leave it in the hands of the prepress and press operators who know the day-to-day realities of their equipment's performance. Either that, or to a 19-year-old with no preconceived ideas about what isn't possible.

(Dunwoody College of Technology [www.dunwoody.edu] in Minneapolis offers curricula and consulting on the printing sciences.)

For a product roundup of digital halftone/continuous tone proofing systems, see the charts below.

Charts 1 | 2 | 3