1. evenness
Even exposure should be easy to obtain with diffused lightsource as the UV
tubes. People have claimed that putting the contact frame in almost direct
contact with the tubes does not even show visible banding (as long as the
tubes are close enough). With that I assume that the diffusion is really
great, so if the distance between tubes are not very close, one can simply
place the contact frame further away (say 5-7 inches away) which leads to the
issue of whether exposure will be very long....
2. exposure
UV tubes are quite diffused and cannot be considered a point source, so
inverse square law of exposure does not apply (unless the distance between
the tubes and the print is really far away). The question is if the fall off
is not proportional to inverse square, is it better or worse (in terms of
exposure time). From the data shown, it looks like the fall off is better
(for example, going from 6" to 24" of exposure distance (4 times), the change
in exposure only goes from 6s to 15s. There is only a factor of 2.5 times
instead of 16 times if inverse square law applies.
Exposure time for gum prints is about 1-5 mins (of course this depends on the
number to tubes, densities of negatives, and other factors). Cyanotype might
take longer.
3. wattage of the tubes
The wattage itself does not tell you much about exposure. For example, a 5-ft
40W tubes might expose longer than a 2-ft 20 W tubes. This is mathematically
simple, but I have not thought about it before, so it is new and interesting.
I think this is a useful information for anyone thinking about building an
exposure unit himself because I myself had some misconception before.
4. painting on the wall and/or back of the exposure unit
White paint should be used instead of aluminium foil. Aluminium is
absorptive, might cause unevenness, and might be dangerous in electrical
environment because it is metal.
Hope this helps. If there is any wrong information, please also point it out
to the list. Thanks!
Dave