erobkin@uwc.edu
Date: 04/26/00-09:27:31 AM Z
Mr. Edwards comments about the distance effect with light sources applies in
the strict form he stated it only to point sources without reflectors. For
a line source such as a flourescent bulb the fall off is inverse linear for
distances that are small relative to the long dimension of the bulb. For
plane sources, and a flat arrary of flourescent bulbs is a good
approximation, the illumination is independent of distance for distances
that are small relative to the smallest dimension of the array. Reflectors
behind the bulbs act to increase the useful distances. How small "small"
would need to be would have to be tested but for the light boxes that have
been discussed on alt photo I'd expect to have to get well beyond the
working spaces that have been mentioned before any inverse relationships
kicked in enough to matter. All real sources look like point sources if
you get far enough away.
So for a bunch of 24 inch bulbs lined up next to each other in an exposure
unit the light levels should be substantially the same at 2 inches or at 4
inches. You would have to get greater separations before you would begin to
see edge fall off.
Easy to test with long sensitive strips so you can see the light fall off
towards the ends.
ER
One thing that makes it difficult to "ferret" out good information from
these
discussion groups is the nature of light itself. If you have the correct
wavelength, and that a big assumption, generally exposure times vary
(almost)
directly with the intensity of the light. However, light intensity varies
with the inverse square of the distance from the source. And, distance is
the one BIG thing most people leave out of their discussions If a light
source is 2" from your negative and you get a 10 minute exposure, move the
light 4" from you negative and you will get a 40 minutes exposure, or four
times the time. You doubled the distance and reduced the light by 1/4th.
Then, add the variables of a 1000 watt halide at 14" and a 500 watt halogen
at 6," and 20 watt fluorescent tube at 2" to the equation, all with
different
wavelengths, and it really becomes confusing.
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