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| Posted on 1/11/04 at 01:09 AM | |
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Three things I find useful in determining exposure time are the technical data sheets that come with your emulsion (or you can probably print out the TDSs from your manufacturers website), an exposure calculator (that is a film positive that has various resolution targets, as well as filters that will simulate several exposure times from one exposure.), and a plain old calculator, which helps me process the data from the exposure calculator and the TDS. Even with those aids there can be a lot of trial and error until you find the perfect exposure since some variables - such as the effect of dyed fabric - can be hard to quantify. Just off the top of my head, I recall that my emulsion's TDS says to multiply the base exposure by a factor of 1.1 to 2 or 3 for dyed mesh. And that is just one variable. Try to minimize your variables by setting standards in your shop and log your results so you can refer to them later. And remember, if you change anything (mesh count, color of mesh, coating method, even film positives can make a difference; clear films have almost no effect on exposure time, while vellum should increase your exposure by a factor of about 2!) your ideal exposure time will change. Also, you should do a test screen a couple of times a year and/or after changing exposure lights, either using an exposure calculator, or a step wedge test, since the amount of actinic light (that's what hardens the emulsion) a light source puts out will decrease with age. It is said that a light will lose it's ability to expose a screen in a reasonable amount of time long before the light would ever burn out. Speaking of 'burn out', I think I need to get some sleep... One final thought: While Technical Data Sheets and Exposure Calculators are good starting points, remember that they are just that: Guides. You need to refer to them, interpret them, and apply them to your situation. That is all. ____________________ Roger L. Moore. Shameless Self-promotion Screenprinting and Design. | |
 
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