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Give it Your Best Dot

(October 2007) posted on Wed Oct 03, 2007

Are you printing the optimum halftone line count for your screens? Find out how to test the highest resolution that your screens and screenmaking procedures can deliver.


By Tom Trimingham

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Checking your films can be a really quick process and doesn’t need to be overly scientific. All you need is a loupe and, if possible, a light table. The things that you are looking for are the edge quality and the density of your dots on the film.

The edge quality of your films is simply how clean and sharp the edges of your dots and straight lines appear under magnification. Poor edge quality will cause a wide range of problems, but most noticeably will contribute to excessive dot gain and tonal compression. The effect of tonal compression is that the upper values in the 70-90% will all flow together and the 0-40% will appear really light (Figure 1). This happens because a dot that is not round or has a larger surface area tends to expand faster than a dot that is in a round shape. A round dot is better at holding shape because the surface tension of the ink—the force that keeps water droplets as round, dome-shaped drops on a flat surface—helps to maintain its shape. Ink flows more easily away from the center of a dot that takes an irregular shape. Remember that from computer to film to screen is already two reproductions and that images never get any better as they go throughmultiple copies. The goal is to keep them somewhat close to the original.

When the edge quality checks out, then it’s time to look at the film density. The Dmax, or density, of the film is a measurement of the amount of light that can penetrate the black areas of the positive. Some shops acquire a densitometer to read and record the opacity of their positives to make sure they’re achieving a high-quality barrier for proper screen exposure. A visual test on a light table will suffice for most of the printers out there. You can do this by taking your films to a light table, overlapping two films, and assessing the top film’s opacity. Can you read the bottom film through the top one’s blackest areas? Consider it a red flag if you can.


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