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Ensuring Uniform Vacuum Draw in Screen Exposure

(June 2002) posted on Mon Jul 01, 2002

Discover the consequences of poor vacuum drawdown, and learn how to test the completeness and consistency of vacuum on your exposure unit.

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By Mark A. Coudray

To view the rings, put your face next to the vacuum glass, about 2-3 in. away from the glass. Adjust your vision so that you are viewing the area where the inside surface of the glass comes in contact with the film positive. Your viewing angle should be about 15°; relative to the glass. The best conditions for viewing Newton's Rings are under subdued indirect lighting, such as you might find in a typical screen room. If you do not easily see the rings, you may have a low vacuum or localized vacuum situation.


You may not have a vacuum frame that allows you to view the exposure side of the glass. This would be the case if you are using a fully enclosed exposure unit. With such units, the easiest way to make sure that you have good vacuum and screen/film contact is to create a test exposure with a positive that you know is in perfect contact with the emulsion.


The test image should be a halftone of 55 or 65 line/in. The halftone should contain 1% to 10% steps, in 1% increments. The idea is to determine what the finest dot is that you can expose. Your test exposure will be done on 305-, 355-, or 390-thread/in. mesh. If you do not use fine mesh counts, change the line count and use a coarser mesh (e.g., 35 line/in. on a 230 thread/in. mesh).


To assure perfect contact, we will lightly spray the positive with adhesive and apply it to the screen (I suggest using 3M's Scotch 77 Spray Cement). Hold the can 18 in. above the positive and lightly mist the surface. By holding the can high above, you allow the aerosol adhesive to slightly dry before it hits the emulsion surface of the positive. When you are done, the positive should be lightly tacky. You do not want to permanently glue it to the screen--this is a temporary situation. Make and spray adhesive on a few sample test patterns, then apply them on old screens to get an idea of how tacky you want the positive to be.



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