I have 2 of these, I think 19" from the glass and burn a screen in 6 minutes.
These are not "black lights", they are lights with a black housing. At 20w 4 of them would take a long time to expose. Use a standard 500w halogen work light with the glass removed. Around 16" seams to work well. Then do a step wedge test to find your optimum exposure time. Good luck.Low Voltage 20-Watt Black Flood Light-8301-9602-01 at The Home Depot
I want to build a 20x24 exposure box and I was curious if 4 of these would work?
mine is built essentially the same way, except i have mine connected the other way... on the long side...When I put mine together with 2 - 500W halogens, I put them side by side the long way. It's hard to explain why, but I figured if I put them the other way the light would "overlap" more and cause inconsistent screen exposures. Sure, there is light that "overlaps" in this, but I don't think it's as much or as intense. Keep in mind, this was just my "theory".