We made it. Final destination, the pressroom. I hope we didn’t lose anyone.

The pressroom is the ultimate customer of scanning and prepress. The measurement and control of these variables is paramount for process control in the pressroom. In theory, we are at the last device. In reality, it’s the first.

We must first know the print conditions of the press before scanning, proofing and platemaking can be accurate. Fortunately, we have some guidelines, like specifications for non-heatset advertising printing, or SNAP.

In our journey, we have dealt with film- and computer-generated plates. We’ll believe that everything has been accurate in prepress. With good plates, we are now ready to print.

Our machine-specific press settings should be checked and measured. With new blankets we should run a GATF/SNAP test form. This form should be evaluated and measured against the SNAP numbers for density, dot gain, gray balance, print contrast, etc.

Any mechanical problems will show on the test form’s diagnostic elements and should be corrected immediately. Hopefully, the press prints within specification. If not, you may need a GATF consultant to help with this evaluation.

Ink density

Solid ink density is the measurement of a solid printed patch on the paper, including the paper density. It generally relates to ink film thickness; however, an ink with a higher pigment load will measure with the same density even if its film thickness isn’t as great. This helps control dot gain and meet density requirements.

Dot gain measures the increase in tone value from the original file to the printed sheet. An imaged piece of film will measure a dot gain of 50 percent. After plating it becomes 54 percent to 58 percent. When printed, it becomes 82 percent, which includes all these steps.

Have you heard that computer-to-plate prints sharper? It does, but why? The main reason is that we make the plates linear (similar to the imagesetter) but CTP plates don’t experience the same 4 percent to 8 percent bump in dot gain seen in film-generated plates.

Be aware that you may need to add weight to the CTP system to match images that printed fine in the past.

Too much gain?

On the other hand, if you printed with too much gain in the past, CTP will help your print condition. Beware that CTP can mask problems present in the pressroom.

All of the following specifications are from SNAP and are available at www.naa.org.

The density values include paper and are “Status T” response.

At press we need a good densitometer, one that reads density, dot gain, gray balance and print contrast. These measurements are used to establish the printing condition and its relationship to SNAP or the proof. In all prepress areas I have described the importance of gray balance control. Now we need to measure it at press.

The first method is to use the densitometer to measure the density of the midtone three-color build (40C/30M/30Y). Set the densitometer to read all filters and measure the density. The overall density should be around .65 (+/- .05). Now go through the color channels and record the density of each color, C-M-Y. These measurements are the corresponding red-green-blue filter readings.

When the RGB numbers are the same, you have a neutral color. If the numbers are higher than desired, but equal, we have a darker gray. If the numbers are lower than desired, but equal, we have a lighter gray. If any of the numbers are greater than .03 the color will be casted in that direction.

Measuring patches

The second method is to measure patches of 50 percent as dot gain. The procedure is to set the densitometer to measure dot area or dot gain. The procedure is: zero out the paper, then measure the solid patch, then the 50 percent tint. The result will be dot gain, a value of increase from the 50 percent patch (i.e., 32 percent).

If we measure as dot area the value would read 82 percent. This value is the same for all colors. When these values are correct, gray balance will be achieved and appear gray.

One other measurement is print contrast. This measurement is the difference between the solid ink density and a 75 percent tint. The contrast reference is a measurement of print quality. When the percentage drops, the 75 percent patch begins to fill in and plug, showing poor print quality.

All of this applies if the scanning and proofing are controlled. If not, the pressman makes adjustments to the solid ink density to match a proof or sample. The increase or decrease in density reflects a change in dot gain — more ink, more gain, less ink, less gain.

By printing to a standard, we can define the print condition and prepress can adjust for a stable and controlled process. SNAP has the following value and tolerance:

Cyan .90 32 percent gain 13 percent print contrast

Magenta .90 32 percent gain 12 percent print contrast

Yellow .85 32 percent gain 15 percent print contrast

Black 1.05 32 percent gain 16 percent print contrast

Density Tolerance +/- .05 Dot gain tolerance +/- 3 percent print contrast +/- .5 percent

Now that we realize the tolerances required and can measure the standard deviation of the press, we can help the pressroom by using gray component replacement in scanning. The use of GCR will allow for a wider deviation of the press without changing the color at press.

The concept is to remove the graying component of the Y-M-C values and replace with black. This process reduces the size of the three-color dots and increases the value of the black dots. The reduced size of Y-M-C dots allows the press to increase or decrease dot gain without changing the color on press. The only concern is the black printer: If printed too heavy, the color will be darker and muddy (see Newspapers & Technology, February 2003).

Today’s hot topic is color management. The idea is to measure each device and profile its color space. By attaching this profile to the file, the next device can interpret the desired color. In order to do this correctly we need to print an IT.8 target on press at the correct density, dot gain, print contrast and gray balance.

This data, through software, can be used in proofing and scanning to match the established print condition. It absolutely cannot happen without process control. Without defined measurement procedures and controlled tolerances, color management will fail.

If you have any questions or comments, please call. Thank you for the opportunity to share this series of articles with you. I hope we have helped your process control methods.

For those who are interested, Dan Remaley will be teaching Process Control Boot Camp at GATF in Pittsburgh on May 13-14.

Dan Remaley is a process control technician for the Graphic Arts Technical Foundation in Pittsburgh. He has 30 years of experience and has been with GATF for five years. He welcomes comments, questions and private inquiries and can be reached at 412.741.6860, extension 450 or via e-mail at dremaley@gatf.org.