Hitting Halftone Targets Through Linearization, Part 2
Learn how to apply the principles of linearization at each stage of production
Last month, I introduced the concept of linearization and explained why it is an indispensable function for any screen-printing shop that wants to print halftones with accurate and repeatable results. We learned that the objective of linearization is to accurately translate and carry the tone and color values of original art through each step of the reproduction process. By assessing each step to determine where and how these values shift, we can reliably adjust the process to compensate and arrive at final prints that match the original. All of us who've printed process-color work have agonized and struggled to produce high-quality, consistent, and repeatable halftones. But if we make a concerted effort to apply the principles of linearization, the majority of our struggles will quietly disappear.
Here we conclude our discussion of linearization by introducing a step-by-step approach for applying its principles and removing the inconsistencies that plague our halftone prints. Throughout this discussion, we need to keep in mind that for linearization to lead to the same correct results each time a job is printed, it must be applied in a measured and repeatable manner. We begin by considering imagesetting, the first step in the process where color and tone information may drift from the values in the original.
All imagesetters must be characterized and calibrated in order to deliver predictable results. The tolerances necessary for producing good halftone work are beyond the range of human visual perception. This being the case, we will assess our imagesetter with a transmission densitometer, using it to complete two very important operations. The first is to make sure that we have sufficient Dmax or maximum density in imaged areas of our film, while the second is to verify our Dmin, or background density in clear areas.
The density refers to how much light is passed through or held back by the clear and imaged areas of the positive. We would like to have the most transparent base possible. In reality, most 4-mil (0.004-in.) polyester will have a Dmin of 0.04. Our Dmax should be 3.0 at the absolute minimum. This means that 99.9% of the light is being held back. Traditional graphic-arts practices usually call for Dmax to be as high as 4.0, representing that 99.99% of the light is being held back. With a good hybrid developer for photographic imagesetting, it is not uncommon to have Dmax in excess of 5.0 (99.999%).
Why is a high Dmax so important? The Dmax assures that the dot we produce will be opaque to the UV light used to expose our screens, which raises an important concern. Our exposure equipment is generally targeted to UV wavelengths in the 365-420 nm range of the electromagnetic spectrum. This being the case, we need to make sure that the photocell in our densitometer is correctly filtered to measure this range of wavelengths. Densitometer makers have different filter sets available to accommodate various UV ranges. Additionally, note that some thermal films appear as a transparent dark brown color to the naked eye, but are opaque to UV in the screenmaking stage. So don't let your eyes fool you; measure with the proper filtration on your instrument to obtain an accurate reading of Dmax.
The first step at the imagesetter is to run an internal exposure test. This produces a series of solid patches across the film surface, along with a minimum, median, and maximum dot for each. These dots are usually 2-4%, 50%, and 96-98%, plus the solid (100%). Assuming that we are using a photographic imagesetter, we will select a test laser-power exposure value. The imagesetter will now make a series of exposures, at differing laser intensity, across the film. The target exposure value is usually in the middle of the exposure run.
Before we can determine the dot area at different tonal values, we must take two readings with the densitometer. The first will zero the Dmin for the clear area of the film. Place the exposed and developed positive under the sensor in a clear background area. Take your reading using the density function, and zero the densitometer.
Now, read a solid black area. This is to establish the positive's Dmax. Select the step that is closest to 4.0 (or the value your imagesetter manufacturer recommends.) When you have found this step, measure the min, median, and max dot areas. They should be reasonably close to the expected target values (within