DIY Exposure Unit Question.

[JackThrasher]

I am building an exposure unit and I am planing to put a 400w Metal Halide bulb in it. Ant recommendations as to the distance I will need from the screen? And would I be able to burn halftones with that type of light source?

[Frog]

I am building an exposure unit and I am planing to put a 400w Metal Halide bulb in it. Ant recommendations as to the distance I will need from the screen? And would I be able to burn halftones with that type of light source?

For distance, assuming that you have no specially designed reflector, the rule of thumb is 1.5 times the diagonal of your desired exposure area.

As for halftones, they are much more a product of how closely you can press the film to the emulsion.
Vacuums are the single biggest improvement in holding resolution, and reducing undercutting.

Surprisingly though, for many non-demanding jobs, many folks do pretty well with units designed with foam and draw-down latches.

[JackThrasher]

So should I build it with a diagonal reflective surface from the light source to the glass? Or should that be black, with a reflective ballast? The NuArc 2000w exposure unit I use at my day job has it set up that way... Would that be a good BluePrint to follow design wise?

[Northland]

I am building an exposure unit and I am planing to put a 400w Metal Halide bulb in it. Ant recommendations as to the distance I will need from the screen? And would I be able to burn halftones with that type of light source?

I converted a fluorescent unit (15 lamps) to metal halide a couple years ago.... I chose to use three 400w lamps, because I wanted to get plenty of curing punch. Some have suggested the multiple lights sources  is a huge no-no, but I prefer to deal with the chance of undercutting and avoid underexposed screens. I found 400w fixtures (with lamps) for $25/ea, so it's a pretty cheap retro.

If I stumble across a 1000w MH fixture, I'd back up and redo the thing.... but for now, I'm enjoying better screens than my old beamed blacklight fluorescent provided.

Photos:
http://www.flickr.com/photos/northlandsp/sets/72157625703927040/show/

[screenxpress]

Man, you always have some of the neatest DIYs and your presentations/explanations are always top-notch!!

[Sbrem]

Metal Halide will give you the proper color temperature (with the proper bulb for your emulsions) but one is sufficient for halftone (the best halftones come from using a "point source" light). My theory is that more than one source undercuts your dots; however, some of the testing I've heard about here regarding the new LED light sources use an array, but the fast exposure times seems to take care of the undercutting issues. But I digress, use one lamp, and if you can get higher wattage lamps, all the better. If you enclose it, make sure it gets plenty of air...

Steve

[JackThrasher]

I had to put my project on hold for the time being... Gonna get back to it next summer though!! I really appreciate all the fantastic info!!! Cheers to all and happy printing

[Denis Kolar]

About 4.5 years ago I was going to build my own exposure unit too.
Same set-up, 400W MH light, build a box, vacuum, blanket and all that jazz

Luckily, a day before I went to Home Depot to buy supplies I checked eBay one more time.
Now, I use Nuarc 40-1K with a 1000W Mercury light and I love my decision to buy it instead of building my own.
Paid around $400 for it and it still kicks ass. Burning screens in 75-90 seconds, depending on the mesh. Using Chromaline SBQ emulsion.

Do not do many screens a month, so that time is OK, no need for a 10-20 seconds exposure.

Check eBay, Craigslist and Digitsmith before building one yourself.

[Homer]

I got  Douthitt unit here you can snag from me for 250.00...

[mimosatexas]

Why can't you live closer to me Homer...I always want what you're sellin'

[blue moon]

Metal Halide will give you the proper color temperature (with the proper bulb for your emulsions) but one is sufficient for halftone (the best halftones come from using a "point source" light). My theory is that more than one source undercuts your dots; however, some of the testing I've heard about here regarding the new LED light sources use an array, but the fast exposure times seems to take care of the undercutting issues. But I digress, use one lamp, and if you can get higher wattage lamps, all the better. If you enclose it, make sure it gets plenty of air...

Steve

the reason LEDs can be used in an array is because they are a directional light source. Light emitting from it is not scattered, but is limited to a cone shaped area. So the light three rows over is not contributing to the light pattern over here.

FL tube on the other hand bounce the light all over the place and thus produce the undercutting.

pierre 

[JackThrasher]

I got  Douthitt unit here you can snag from me for 250.00...

What's your time frame on selling it?

[Homer]

I got  Douthitt unit here you can snag from me for 250.00...

What's your time frame on selling it?

pfft...yesterday....damn thing is taking up space....