Color proofing: Hitting Color, part 3

Mar 22, 2007 9:31 AM, By Dan Remaley

The next step in the production cycle, after scanning, is proofing. There are a variety of reasons for a proof: Copy editors need it for type corrections and legal restrictions; page layout people need it for color breaks, formatting and positioning issues; scanner operators need it for color evaluation and color correction.

I divide proofing into these three categories:

• Soft proofing.
• Copier/laser.
• Contract.

Contract soft proofing

Soft proofing, until most recently, was only considered “representative” of the final product. The copier/laser proof is a hard copy, but limited in color accuracy. The contract proof is designed to be as close to the final product as possible. However, things are changing. A couple years ago, at a PIA/GATF Tech Alert Conference in Pittsburgh, two “Contract Soft Proofing” units were demonstrated. Nearly half of the surveyed viewers of these devices said they produced an acceptable match to the printed samples.

A soft, (non-contract) proof can be made by viewing a PDF file on a color monitor. The intent is to satisfy content, positioning, layout, etc. If we need a proof without contract quality to mark up or share, then a copier/laser proof will work. Since this study, contract proofing has grown in leaps and bounds. In many workflows, this soft proof is considered a “contract proof.”

The contract proof is the ultimate example of what the printed piece should look like. In the past, we used the same CMYK colors as the printing press. In the very early years, we used a system of dry toners applied by hand, called Cromalin. This Dupont product could match nearly any press ink color by mixing different values of these toners. The next generation of proofing was the 3M color key material; again, these precoated colorants attempted to match the printing ink colors. These CMYK colored materials were mounted on a paper stock by taping the material near the top of each sheet, leaving each one loose to lift up for single-color viewing. The next generation of proofing allowed these colorants to be mounted on a plastic base or actual stock. Matchprint and Waterproof are examples of this technology.

I’ll limit the examples to 3M’s Matchprint system. The film output imagesetters are linear in nature (i.e. a 50 percent file size = 50 percent on film). Although some shops modified these numbers for particular print conditions, the normal setup was linear. These color sheets are laminated to a base material and exposed with an ultraviolet exposure unit, developed, and the unexposed material washes away. The exposed area hardens and remains on the carrier. This process is repeated for all colors. With the use of a PIA/GATF Proof Comparator, we can measure the exposure and values of the proof, including SID (solid ink density), dot gain (tone value increase), print contrast, gray balance, etc.

CTP changes things

With the invention of CTP, film is no longer in the workflow. The new proofing systems are digital by design.

We need to understand how film-based proofs work. Let’s go back to scanning for a moment. We scanned for a TVI of around 20 percent in the midtone. This means we removed “weight” in the image equal to 20 percent in the midtones, and a correctly made proof will represent this 20 percent increase visually. Now here’s the interesting part: We know the film has a 50 percent screen value—the proof viewed with a magnification loupe looks like 50 percent—but if we measure it with a densitometer, it will measure 70 percent (total dot area, or a gain of 20 percent).

When we measure color proofs, it’s the total gain we’re interested in, or the combination of optical and mechanical gain. Color proofs are designed to have a lot of optical gain because the colorants never absorb into the paper like a press sheet’s ink will. On the printing press, there’s little optical gain but more physical gain. It’s the total gain we are interested in measuring.

Many people don’t understand the “optical” component to this dot gain measurement. These color proofs are designed so that the some of the light is diffused and not reflected back to the viewer. Therefore, we perceive it to be “darker.” This amount of reflection is measured by the densitometer and recognized as “dot area” or “dot gain.” At press, there is also some optical gain, but there is more mechanical gain from the ink absorbing into the paper. It’s the total gain we are interested in and measure on the color bars.

This is why it’s so important to place a measurement patch on the proof to determine the characteristics of the proof. If the proof measures only 15 percent gain and the scan represented (-20 percent) the proof will appear “light”; otherwise, a proof with 35 percent gain and a scan with (-20 percent) will appear “darker.” These are just contrast differences. What about color differences? If the proof gain is higher in one color, only the color will be out of gray balance. This is why color proofing is so critical. All of the decisions about color are based on this proof, scanning corrections, customer corrections and presswork. For dot-based proofs (i.e., Kodak Approval) or film-based systems, the colorants, densities and dot gains must match the print condition, inks and paper.

In the new digital world, everything has changed. digital color proofing systems with color management capabilities can be made to match any print condition. These digital machines must be able to use ICC profiles to make these conversion from one device to another, and not all color devices have this ability. The use of ICC profiles with digital proofers, with as many as six colors or more and a wider color gamut than the press, can be adjusted to match the press. A digital proof is measured in Lab color space with a Spectrophotometer. These patches are printed on a “good” press sheet and measured in Lab. The same patches are printed with the proofer and adjusted to match the printed press sheet.

What exactly is a “good” press sheet?

My definition is a press sheet that has the correct density, dot gain and gray balance.

The viewing conditions of these off-press proofs is very critical. View them under controlled lighting at 5,000 Kelvin, in a designated area for viewing color in prepress and at press. PIA/GATF makes a control strip that can be attached to the proof to determine if the color temperature, 5,000K is correct. Without the correct color viewing condition, images will be too warm (+magenta) or too blue (+cyan), showing tungsten or fluorescent, respectively.

Monitor-based soft proofing has some limitations, but these are being overcome with automatic calibration and tighter specifications.

The proof values are very important as we move to platemaking and presswork. Being a negative process, dot gain is a natural phenomenon and it increases at every step. Visit SWOP.org for a complete list of proofing systems that are “SWOP Certified,” meaning the digital and soft proofing systems were approved to be a visual match to a SWOP printed sheet.

In part 4 of this series, we’ll tackle the platemaking area, and conventional and digital printing.

Dan Remaley has 30 years of experience in color lithography. He is senior technical consultant of process control for PIA/GATF. Contact him at (412) 259-1814 or dremaley@piagatf.org.

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