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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

click an image below to view slideshow

I’ve encountered quite a lot of resistance to this concept from experienced printers due to the added labor of putting an extra coat on some of the screens for underbases, but they can’t stand the moiré, either. So I contend that the extra time spent at the front end is well worth the time saved in trying to battle moiré on press.

One significant note to add at this point is that many CTS systems seem to produce far fewer frequency issues than film because they can get such an absolute seal directly on the screen’s surface. I still believe that the same ratio of dots to thread count should be observed using these systems so that the floating dots in the higher percentages will have enough threads to hold their shapes properly.

 

Testing the dots

The films are black and the screens are prepped, so it’s time to test a variety of halftone resolutions to determine which will cause the fewest headaches. First you will need an appropriate test file. This file is best executed in Adobe Photoshop, where you can create a grayscale document at 1200 dpi that you can then paste in varying resolution gradients of different line counts (Figure 3). Another option could be to create a CorelDRAW file and then import separate bitmaps of the different frequencies.

The idea with this test file is to create steps of different resolutions that will clearly show a preference when printed on a variety of screen meshes. The final prints will illustrate whether a specific resolution works best on a given mesh and how high the films and screen process can go with resolution before tonal compression and excessive dot gain make it less appealing.

I typically don’t mess with the angle of the dots for this test. I have tried all of the angles over the years, and through trial and error I now always print everything through 22.5° with no recurring issues. Expose and print your test film on all appropriate mesh counts and then evaluate which line count is best for the meshes.


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