How to do a step test on a vacuum lightbox
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.