>
>
> On Sun, 21 Jul 1996, John Rudiak wrote:
> > My tubes are spaced one-eighth of an inch apart, and I have my glass one
> > inch away from the tubes.
>
> John, I'm intrigued by that "one-eighth inch apart," which I assume means
> you have separate ballasts. My engineering skills don't reach that high
> so I settled for butting ready-made strips together, which makes the
> bulbs about 1 inch apart. But don't you have trouble changing bulbs? I
> thought you had to have room to get the fingers in there.
>
> However, what I'm really wondering is about the one-inch distance to
> paper stage, which raises 2 questions:
>
> 1. Have you found a good way to test for hot spots? The best I could come
> up with was a weak cyanotype coat (cut with water) on paper the full size
> of the paper stage with a brief exposure,say 1/3 "normal," to see if
> there were differences in the blue density. (When you give full exposure
> tendency is for the emulsion to max-out & hotspots disappear.)
>
> 2. Aside from speed and so forth, is the 1-inch distance maximum for
> sharpness? Last year in these pages Mike Ware cited some square of the
> sugar cube law that I didn't follow, but his hypothesis (at least at the
> time, at least as I understood it), was that a 3-inch distance from the
> light bed to the paper stage would give the sharpest image.
>
> An optics mavens on the line?
>
> Cheers,
>
> Judy
>
I built my exposing unit from parts obtained by disassembling shop lights
which I bought at WalMart for $8 each. I threw away the sheet metal and
used the ballasts and wires and T12 connectors, which I screwed onto
plywood. 1/8 inch apart was about as close as I could get them, as I
figured the closer together they were, the more light would result per
square inch. I found (I suppose because of the geometry of the round
tubes and their proximity to each other) that I get extremely even light,
nop matter how close I put the paper to the tubes. I even turned the
exposing unit upside down (so the tubes faced up) and place a contact
printing frame on the tubes themselves (frame upside down) and got an
evenly exposed print. Also it seems that the exposure is the same
whether the print is right next to the tubes or a couple inches away.
This didn't make a lot of sense to me until I read Richard Sullivans
recent post about the inverse square law relating mostly to a point source.
John