My DIY UV exposure unit - finishing step?

[yeknom02]

Not sure whether I should post this here or in DIY, but here goes:

I recently built a UV exposure unit out of a series of fluorescent light fixtures (see attached photo). The thing is, I now need to suspend this over any alternative process contact printing I'd be doing (Cyanotypes and Carbon prints). However, I'm unsure what height to set the lights. There's got to be a "sweet spot" region, because if I set the lights too low, I would be too close to the bulbs and get banding across the image, whereas I don't want to set them too high, because of increased exposure times thanks to light intensity falloff. (Also, it would be a chore to move). Does anyone have any suggestions as to how to determine the height at which I should set these bulbs?

The bulbs are F15T8 unfiltered blacklight bulbs, if that makes a difference.

77098

[E. von Hoegh]

Use an adjustable suspension, and trial and error.

[Brian Ellis]

I built a unit like that years ago. I don't think the height is all that critical, it's not like there's only one sweet spot and if you miss it you're going to get "banding" or hour-long exposures. I just put the lights far enough up that the contact printing frame could easily slide under them plus maybe 6 or 7 inches between the lights and the glass in the frame. The tubes spread the light out pretty well so I don't think "banding" will be a real concern unless they're almost touching the frame. My exposure times for gum and van dyke brown were in the five minute range, adding another minute or so by perhaps placing the lights a little higher than the theoretical perfect height didn't bother me.

I suppose you could build an adjustable suspension as E. suggests but for me, with my relative lack of handyman skills, that would have been more trouble than it was worth.

[BrianShaw]

I wouldn't build a UV unit without adjutability, but is there any merit to thinking about integrating a turntable into such a design -- like microwave ovens do sometimes (or once did) to avoid hot spots?

[Colin Graham]

I worried about distance too when I was making mine. Initially I had 5" of clearance but then added a vacuum frame drawer that reduced the distance to 3". No adverse effects. The bulbs in mine are closer together, though- 2" on center, F20T12/BL bulbs, which leaves only about a 3/4" gap between bulbs. I do use a cooling fan, heat from the bulbs might impact certain processes at close distances. Here are some pictures of mine if you're interested- http://colinflanarygraham.com/darkshop/?p=337

If you want to be sure though, maybe expose a sensitized sheet with no negative to a light gray value to test evenness of illumination.

[E. von Hoegh]

I built a unit like that years ago. I don't think the height is all that critical, it's not like there's only one sweet spot and if you miss it you're going to get "banding" or hour-long exposures. I just put the lights far enough up that the contact printing frame could easily slide under them plus maybe 6 or 7 inches between the lights and the glass in the frame. The tubes spread the light out pretty well so I don't think "banding" will be a real concern unless they're almost touching the frame. My exposure times for gum and van dyke brown were in the five minute range, adding another minute or so by perhaps placing the lights a little higher than the theoretical perfect height didn't bother me.

I suppose you could build an adjustable suspension as E. suggests but for me, with my relative lack of handyman skills, that would have been more trouble than it was worth.

It looks like it's built on a sheet of plywood. All that's needed is a bridle at each end, hang it by chains from sturdy screw hooks in the ceiling. Adjust by hooking different lengths of chain.

[Jim Noel]

Not sure whether I should post this here or in DIY, but here goes:

I recently built a UV exposure unit out of a series of fluorescent light fixtures (see attached photo). The thing is, I now need to suspend this over any alternative process contact printing I'd be doing (Cyanotypes and Carbon prints). However, I'm unsure what height to set the lights. There's got to be a "sweet spot" region, because if I set the lights too low, I would be too close to the bulbs and get banding across the image, whereas I don't want to set them too high, because of increased exposure times thanks to light intensity falloff. (Also, it would be a chore to move). Does anyone have any suggestions as to how to determine the height at which I should set these bulbs?

The bulbs are F15T8 unfiltered blacklight bulbs, if that makes a difference.

77098

An informal test was done by Judy Seigel a few years ago in which she put black tape on every other tube, laid the sensitized paper directly on the tubes, and had no banding. My tubes are 1/2" above the glass of my printing frame and I have never seen any banding.

[yeknom02]

Well, my original intention was to put something like a table frame together around the plywood's perimeter, then have this board rest bulb side down on it for exposing whatever's beneath.

Due to using the cheapest fixtures I could get my hands on, the bulbs are 3.25 to 3.5 inches (82 to 89 mm) apart.

[Keith Fleming]

Wouldn't it be easier to build a wooden frame around three sides of the unit with slots into which you slide an inverted contact printing frame? Strips of wood about 3/8-inch square, fastened on the inner part of the sides would support the printing frame. It would be fairly simple to use multiple strips that would permit the printing frame to be at various distances from the bulbs. My own unit, which I bought used, is sheet metal, the lights shine up, and an inverted printing frame slides into the top. It has the advantage of the heavy light units being on the bottom of the unit, and that allowed the manufacturer to design the "box" with a lighter construction.

Keith

[Gary Beasley]

I would have the distance no less than the spacing between the bulbs as a starting point.

[Colin Graham]

Here's an example of a box made with simple extendable legs- http://unblinkingeye.com/Articles/UVBox/uvbox.html

BTW, removing the fixtures from the metal housings saves a lot of weight, allows the bulbs to be placed closer together, plus opens up more options for available fixtures. I found a great deal on electronic ballast 4' shop light fixtures but had no room for such a big box. There were no decent fixtures available in the 2' range. Shelling the 4' units allowed them to be easily reconfigured for 2' bulbs.