All of the process control elements we have discussed so far have been in color, color scanning and color proofing. We will now enter the black-and-white world of film output and platemaking. No color here, or is there? The majority of newspapers still use film but a growing number have made the transition to direct-to-plate. We’ll cover both systems.

First, let’s review how metal plates evolved from film. Second, we’ll address computer-to-plate issues.

Film generated from scans and/or digital files should have a maximum density of 3.5 or better (D-max).

Calibration critical

The scan or files should be calibrated so that all dot percentages from 0-100 percent are represented faithfully. This process is called linearization and it means that when a 30 percent dot of cyan and 30 percent magenta are in the digital file for a purple color, the value on the film is 30 percent as well.

Let’s say, however, that the film value is 40 percent rather than the requested 30 percent upon output. Then we need to calibrate the filmsetter. This calibration procedure usually only requires the adjustment of the light-on-film number.

If we increase the LOF number the dots become smaller. If we lower the LOF number the dots become larger. This is because we are exposing the black area, or background, of the film.

In some cases it may be necessary to adjust this calibration curve to match a specific print condition, but it’s a good idea to start from a linear position.

If we have high gain numbers at press and would like to print to a standard like Specifications for Non-Heatset Advertising Printing, or SNAP, we may need to reduce the film dot sizes to print correctly at press. Remember, any changes made to the film output will be reflected in the film-based proof.

Exposure next step

Now that we have an accurate film output we expose it to a negative-working metal plate. The exposure is made with a high-intensity 6- or 8-kilowatt bulb. This high ultraviolet content bulb has a short lifespan, generally 1,000 “on-time” hours. On-time hours represent the entire time the platemaker is on, not the total exposure time.

This high UV content is what makes the exposed emulsion stick to the aluminum plate. As the bulb ages, the UV content drops and the emulsion, or dots, drop off the plate.

Some plate frames have integrators that measure the amount of UV light exposed to the plate but don’t measure the quality or content of the UV spectrum. If you’re in a two-shift environment, change your bulb every 1,000 hours. That’s every three months (a lot of printers wait until it burns out).

Each plate exposed should be measured with a Plate Control Target from the Graphic Arts Technical Foundation or a UGRA scale. These targets have a continuous-tone gray wedge, microlines, and dot percent scales in 10-percent increments.

Place a scale in the plate bend area (non-printing area) of the plate and expose with the film negatives, or as a separate exposure. We need to measure visually (microlines) or with a plate reader the percentage values of these patches.

Grayscale info

First we evaluate the grayscale information. The manufacturer generally recommends a solid step 4 or 5. Each grayscale step is a .15-density difference. Increasing the exposure moves the step scale higher. A finer measurement is of the microlines. These are a series of clear and black lines of equal spacing per segment. The segments vary from a small distance of 2 microns to a large distance of 20 microns. The correct exposure for most plates is 6 to 8 microns.

When I travel to printing plants the No. 1 error in their process is overexposed plates. When the plates are overexposed the value of dot size changes — not very much in the 10 percent or 90 percent area, but very much in the 50 percent area. On a correctly exposed plate with a 6- to 8-micron exposure, a 50 percent original dot will become 54 percent on the plate. Many shops I visit have an 18- to 20-micron exposure and their 50 percent becomes 60 percent on the plate.

Ever wonder why your images are “flat” and the color is “muddy” at press? Check your plate exposure. I prefer to use a plate reader for evaluating these plates, but the visual check of the microlines, grayscale, and highlight/shadow patches may be sufficient.

CTP alters process

Our process control strategy began in scanning, proofing, film output and now platemaking. This is all controlled for a consistent product delivered to press. For those of you that have made the move to CTP, the process changes a little. With CTP there isn’t any film produced and the file goes directly to platemaking.

Most platesetters are installed linear. This means that a 50 percent value in the original file will remain 50 percent on the plate. In the film-based world the plate would gain about 4 percent, making the plate value 54 percent.

Here is the great debate: Linear or not? In my opinion, if you are trying to match SNAP or some other film-based standard, you’ll need to add this 4 percent bump to the midtones. Remember, it’s the total gain at press that were interested in, so if your gain is excessive then a linear plate may be just fine. However, adjusting plates becomes addictive and soon you may be color-correcting each plate for a bad press condition — not a very good idea, because you are simply masking the print problem.

How to measure

Whatever your choice, each plate should have a GATF Digital Plate Control Target in the plate bend area. Our target has two complete tonal scales to show if any changes or curves were applied to the plate.

How do we measure these plates? There are several plate-reading devices for measuring the actual dot area of the plate. Reflection densitometers can be used but a defined N-factor must be changed in the instrument for accurate readings. However, the densitometer can be use to check consistency from plate to plate.

By now, we realize it’s all about the size of the dot and the color of the dot. My next article will cover the pressroom process control. We have visited every department and discussed the tools and methods necessary for process control and quality reproduction. Each of these departments has an impact on what, and how, the press will print. We must think of this in terms of a process.

On to the pressroom…

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