Is there an formula to determine the distance necessary for a metal halide lamp to expose a given vacuum frame size? I am new to the metal halide world. Does the intensity of a metal halide lamp differ from the inside to the outside, meaning that screens would expose faster in the center? If so, how is this compensated for? I watched a video before about how to make a homemade point light source exposure unit, and they recommended a 21" distance. However, the vacuum frame I have now is much larger than the one he built in that video, so I am sure that the distance will be greater. The frame that I have is 67" x 89". I just want to know what considerations need to be taken when setting up my frame and lamp. Thanks!
sqrt of x squared + y squared is c squared which gives you your diagonal distance for your screen. In your case it's sqrt(67^2+89^2) which is 111.4". If your unit uses the given "standard" of 1.5 times the diagonal length of the screen or screen area that would be 167" or 14 feet which is nuts. I'd say go for 85% ofthat diagonal distance and also use the diagonal distance of the inner dimension of the largest frame you intend on exposing. You can look at spec of the diffusion pattern for that light to get a better estimate, but this should get you on the right track.
I recently built a unit that is 1.5 times the diagonal length of the exposure unit and I'm modifying that immediately
Thanks a lot guys for all of the help. I have a 3000 watt lamp and I'm starting to think that it may be too small for my frame. I'm not going to be using the entire size of, just enough to make frames for all over printing, so i'd say approximately 36" x 48", maybe a little bigger.
At 36x48 you have a diagonal distance of 60", but that's not the inside diameter of the frame and you're not exposing an image that goes from the absolute inside to the absolute inside of the frame either, so lets say about 34 x 46 which is 58" (not much difference, but in any case), take 85% of that and you have 50" which is not bad at all.
That 3000 watt bulb isn't too small for your frame, it just needs to be placed at the appropriate distance. The best best though, is to borrow a UV spectrum meter to determine if you're getting any hot spots and you can modify your distance appropriately. You can always raise or lower the bulb esp. considering you're building the exposure unit.
Distance of Lamp to Exposure Glass 
