I have just purchased a Lumitron Workhorse lightbox, i have no idea how long to expose my screens for anymore as previously i was using 4 x 400w lamps...
I have done many step tests with them, but that involved physically moving a sheet of paper every 30 seconds to reveal the next row of halftone designs.
Basically, the only way i can think to do a step test is to switch my light box on for 30 seconds at a time and move the sheet of paper like with my original halogen set up.
This seems very impractical, especially since my acetate faces down and my screen sits over it, so i'd have to lift my screen, in fact thinking about it, it's impossible... nevermind the fact that every time i open it the vacuum loses it's 'suck'.
SO, can anyone recommend a good way of judging exposure time on a vacuum light box?
I am UK based, i can't find ANY distributors of the Stouffer 21 step test which is so easily attainable in the states.
Try asking your emulsion rep/supplier nicely for a real-ass exposure calculator. Failing that, buy one. It makes this all painless and I don't think that leaving the vacuum off is such a hot idea. Stouffers are ok, what you really want is something like this, Exposure Calculator - MacDermid Autotype | MacDermid Autotype MacDermid claims to have distributors in the uk. I would also see about whatever brand of emulsion you are currently using and see if they make a calculator.
Taping the acetate was my idea, but it seems ridiculous to have to set my timer to 30 seconds constantly and move it.
And i can't turn the vacuum off, the power button instantly switches the vacuum on alongside the timer which controls the lights.
I'll have a look at that MacDermid calculator, see if i can find a supplier over here.
Kiwo (My emulsion brand) do have an expo check but it seems to be a print off and use one.
Considering the fact that you should only have to do a step test once or twice I don't think it's so ridiculous to have to turn it on an off every 30 seconds. Just do it and get it over with. Once you find the right time you're good to go. You've probably wasted more time looking for another solution than you would have if you'd just taped the film on and done the step test.
I think I paid about $7-$14 for an exposure calculator. I throw it in the corner of the first screen that I do for every project and compensate for variables such as age of the lights, age of the emulsion or, if doing solar exposure, time of year and position of sun.
After doing step tests and eye-balling what I thought was the best exposure, I found out with the use of the calculator that I was underexposing by at least a 1/2 step. Not such a huge deal given the latitude of the emulsion but it's nice to get it dialed in.