"Correct" Exposure Distance Between BLB bulbs and Negative/Paper

[Keith Pitman]

I'm preparing to construct an exposure unit to make ziatype and platinum/palladium prints. As I've got this drawn out right now, the distance between the bottom of the BLB bulbs and the surface of the negative/paper sandwich when exposing would be 2 1/2 to 3 inches. Is this too close? What are the consequences, if any, of having the lights too close for exposure? Finally, if this is too close, what is the recommended distance range?

 

[Trevor Crone]

A ballpark figure is around 6". Too close and you could get uneven illumination and over heating of the negative.

 

[Monophoto]

The concern is that if the lights are too far away, printing time will be longer. If the lights are too close, the light falling on the printing frame could be uneven.

I read a recommendation in one of these threads that the spacing between the bulb and the surface of the printing frame be about the same as the spacing between bulbs. I used compact spiral BLBs in my UV box spaced on 6" centers, and with the bottom of the bulb about 6" from the printing frame.

 

[juan]

I use the squiggly BLB bulbs - six to cover an 8x10, and mine are about two inches above the paper.
juan

 

[Kerik]

3" should be fine. That's how my box is set up and it's pushing 10-years of steady use. Are you using UV CFLs or UV tubes? My box has 12 4-foot UV tubes about 1.5 inches on center. That said, a few more inches won't have a huge effect on your printing time. The inverse square law for brightness of illumination is based on a point light source and does not apply to a large diffuse source as is the case for this type of exposure unit.

 

[Vaughn]

The Edwards Engineered Products Exposure Box Kit I put together has the bulbs less than 1/4" apart and 3" of space between the bottom and the bulbs (so taking in the thickness of the contact printing frame, the neg-to-bulb distance is closer to 2". Works fine.)

If you can find them, BL bulbs are much cheaper than BLB bulbs. The BL bulbs might even be a little faster since they do not have the visible light filter built-in. (might be able to utilize a little of the blue end of the spectrum.)

Vaughn

 

[Keith Pitman]

Thanks for the responses. I'm going to try the box with bulbs at the distance I built it. If I have problems, I'll modify it later.

 

[Trevor Crone]

Thanks for the responses. I'm going to try the box with bulbs at the distance I built it. If I have problems, I'll modify it later.

Good luck. Please feed back on how you get on.

Trevor.