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Jun 1, 2007 12:00 AM
You know the common statement when the proofs don't match the press: “It must be a ‘bad’ film, plate or proof!” I've been the person who had to visit the printer, conduct a press check and determine what was wrong. This helped me understand the entire printing process, from scanning to proofing, platemaking and printing. Through process control measurements, we can define where the problem lies.
The control of process color printing is very complex and has many variables. My intent is to convey the information necessary to produce quality color printing through measurement, calibration and process control. We will look at how to measure color reproduction through each department, from scanning through print.
Let's begin with the discussion of color. It's certainly subjective! Different people “see” color differently. Some people are more sensitive in grays, some in reds, and some in blues. Usually, people who work around color printing pay close attention to neutrals. There is a good reason for this: When images are in balance, they are neutral throughout the tonal scale. If they are out of balance, they are quantified as “casted” toward magenta, cyan or yellow. If you look at a Lab CMC color tolerance chart, the elliptical shapes represent the measurable Delta differences in lightness, hue and saturation. The smallest ones are in the neutral gray region.
When viewing color, it is important to do so in an area that simulates 5,000 Kelvin. It is very important to have the correct lighting condition for viewing color proofs and originals. I can remember working all evening to color-correct an advertiser's four-color proof. When the new proof was made, it looked great! The following morning, the agency called and said the flesh tones were “way too red.” I realized the client was viewing this proof under the morning sunlight, which is “warmer” than our viewing conditions. He agreed the color was OK after viewing it under 5,000K controlled lighting.
It is imperative that we standardize the printing process, and to do that, we need a target, such as SWOP — these guidelines help us define our process control and establish acceptable tolerances.
In scanning, for example, how can we covert the original — print or digital — to the printing process without knowledge of the color reproduction system, proof and/or press?
When we proof the image, do we have something to measure? Typically, the proof only contains the color image. The biggest problem in the industry today is that proofs are made without any process control element. Proofs come from everywhere — laser printers, dye sublimation printers and inkjet technology. Some of these systems can be profiled to match your print condition; others cannot. How do you know? I recommend placing a PIA/GATF Proof Comparator on every proof and measuring any changes from proof to proof in Lab color space or with a densitometer.
In platemaking, we control the size of the printing dot. Prior to computer-to-plate technology, we never actually measured the size of the dot on the printing plate; we measured the film output and controlled the dot size through calibration. Now, we have the tools to measure and calibrate the plate.
The press is still the largest variable. Ink, water, paper, blankets, pressures, temperature and humidity all impact print quality. Newsprint has its own special circumstances, including printing without room for a color bar or any measurement device.
In the rest of this series, we will investigate each of these areas in detail: scanning, proofing, platemaking and press work. We'll discuss the use of gray component replacement (GCR), color management and digital capture in the manufacturing process.
Contact me with any comments or questions, and I'll e-mail you a PDF of my “Process Control Reference Guide.”
Dan Remaley has 30 years' experience in color lithography. He is senior technical consultant of process control for PIA/GATF. Contact him at (412) 259-1814 or firstname.lastname@example.org.
This is part 1 in a five-part online series on process control for accurate color reproduction.