Film output & platemaking: Hitting Color, part 4

Mar 29, 2007 9:21 AM, By Dan Remaley

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

Calibration is key

Let’s review how metal plates evolved from film, then we’ll address computer-to-plate (CTP) issues. The film generated from scans or digital files should have a maximum density of 3.5 or better (D-max). They should be calibrated so that all dot percentages from zero to 100 percent are represented faithfully. This process is called linearization: It means that when a 30 percent cyan dot and 30 percent magenta are in the digital file for a “purple” color, the value on the film is 30 percent, as well. If, when the film is output, the film value is 40 percent instead of the requested 30 percent, we need to “calibrate” the filmsetter.

This calibration procedure usually requires only the adjustment of the LOF (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 might 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 SNAP for newsprint, we might 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.

Optimal exposure

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-KW bulb. This high ultraviolet content bulb has a short lifespan, generally 1,000 “on time” hours, which 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. Change your bulb every 1,000 hours. In a two-shift environment, that’s every three months.

UGRA plate scale (film)

PIA/GATF’s Plate Control Target for digital plates

Each plate exposed should be measured with a PIA/GATF Plate Control Target or a UGRA scale. These targets have a continuous-tone gray wedge, micro lines and dot percent scales in 10 percent increments. Place a scale in the plate bend area (nonprinting 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.

First, we evaluate the grayscale information; the manufacturer generally recommends a “solid step” 4 or 5. Each gray scale step is a 0.15 density difference. Increasing the exposure moves the step scale higher. A finer measurement is of the microlines, a series of clear and black lines of equal spacing per segment. The segments vary from a small distance of two microns to a large distance of 20 microns. The correct exposure for most plates is six to eight microns.

When I travel to printing plants, this is the No. 1 error in their process: 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. Correct plate exposure, six to eight microns, will result in a 50 percent original dot becoming 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 on 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.

If you have made the move to CTP, the process changes a little. Most platesetters are installed linear. This means a 50 percent value in the original file will remain 50 percent at plate. In the film world, the plate would gain about 4 percent, making the plate value 54 percent. Here is the great debate: linear or nonlinear. In my opinion, if you are trying to match SWOP or some other film-based standard, you will need to add this 4 percent “bump” to the midtones. Remember, it’s the total gain at press we are interested in, so if your gain is excessive, then a linear plate might be just fine. However, adjusting plates becomes addictive. Soon, you might be color correcting each plate for a “bad” press condition, which is not a very good idea because you’re simply “masking” the print problem. Whatever your choice, each plate should have a PIA/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.

Concentrate on curves

Each plate (C, M, Y and K) will need a separate and distinct curve for each color. It would be impossible to have one plate curve for all colors, yet this is what I see in many printers around the country. Black is the first printing unit and hits all the other blankets. The yellow doesn’t and prints with less gain than black. Thus, a different curve is needed.

The plate curves are created from measuring a printed tone scale with linear plates, or with known plate values, on each press unit, then measuring against the proper printed values for SWOP or GRACoL. We then add or subtract from the plate to achive the correct dot gain (TVI), and gray balance.

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. The densitometer can be use to check consistency from plate to plate, however.

By now, you realize it’s all about the size of the dot and the color of the dot. My next article will cover pressroom process control.

We’re visiting every department and discussing 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 ...

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.