Need Exposure Time/Calculator Help

[kmel]

Here's what I'm working with:

Homemade exposure unit with 10 40 watt unfiltered blacklight tubes 4" from glass
125 mesh screens (white)
CP Tex emulsion
Matte Mylar positives (same thing as Duralar I think)
Exposure calculator on transparent film

The spec sheet for the emulsion gives estimated times for cool white fluorescents and metal halide lights but not UV blacklights. Any idea where to start with exposure time? 3-4 min worked with other, crappier emulsions in the past.

And should I layer a blank sheet of my matte mylar over the exposure calculator when I run the tests?

[paul204]

When you say "exposure calculator," what do you mean?

A true exposure calculator is a calibrated step wedge test that will tell you how much UV energy is being shot at your screen. These come with instructions and are very easy to use. Order a 21-step from Stouffer Graphic Arts if you don't have one.

What most people think an exposure calculator is is a positive printed with various halftone screens, rules and text sizes. This isn't a calculator, but an output calibrator, so once you know your burn time from using the calibrated step wedge test you can see how much a 55 lpi screen or 6pt Helvetica 55 gets eaten up and how much you have to transfer when doing output.

Does that make sense? Buy a step wedge test, follow instructions. Afterwards, do controlled tests with text and halftones to see how much you have to compensate in output for undercutting during exposure.

[vrooooom]

Speaking of putting mylar over your calculator, you should use your calculator with whatever you plan to output your separations on. It doesn't make sense to figure out the best burn time for transparencies and then try to do everything with oiled up bond paper. Even mylars will block some transmission, so put it on a mylar if you plan to use those.

Like paul said, if your calculator doesn't have filters, you're going to have to result to doing a step wedge test or other manual test to burn a number of exposure lengths on one screen.

If you have a calculator with filters, you generally start at twice the length of a recommended burn. If you haven't changed emulsion types (from fast, presensetized emulsion to diazo dual cures, etc.) then double your previous number may be a good starting point.

[kmel]

I have the exposure calculator from Murakami, it does have an overlay.

[paul204]

That murakami calculator isn't really telling you if your emulsion has cross-linked, it's just telling you if you are reproducing your image accurately, and in my opinion, it's pretty useless. The 5% and 95% values you're supposed to base the results of your tests on will be different when you output them yourself. I again would recommend buying a step-wedge grayscale test and creating an output calibrator similar to murakami's, but print it yourself.

[Andymac]

I don't know murikami's calculator. most of those UV tube units expose pretty fast.
print out something with fine detail on your material. Coat your screen properly. What is CP tex? is that like 'I have a car, it's a 4 wheel model' when someone asks what kind of car you have....
Tape the positive in place so it can't move.
Step expose with a block out sheet (rubylith or black paper) half minute intervals about 10 steps. 30 sec - 5 minute range
Wash it out.
what worked?
Putting mylar under a calculator is not giving you a proper reading, as your own print will be just on the one layer of mylar, not with a second thickness of film.
You will need to play with your exposure times, no matter what results you get. Better to be overexposed and worry about the detail later.

Do your testing BEFORE you take on your first job with a deadline....just sayin'

[kmel]

CP Tex is from Chromaline, sorry!

I ended up doing the Murakami not-exposure-calculator against the emulsion with the mylar layered on top of that (before I read the last two comments). It got me in the ballpark but I would have been way better off with a step wedge.

Anymac, how did you know I was on a deadline?

[jimmyfredman]

a long time...

[Andymac]

Anymac, how did you know I was on a deadline?

I have a crystal squeegee, it shows me the past, the present, and the future. You're going to have pinholes and blurred prints.....and then drink beer.