From: Sarah Van Keuren (svk@steuber.com)
Date: 04/27/00-07:35:50 PM Z
Since Jon Edwards who makes the Edwards exposure unit is not on the List, I
am facilitating his communication on the topic. He replied to segments of
emails from the List that I forwarded to him. Sarah Van Keuren
<< 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 fluorescent 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 array of fluorescent 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 >>
Regarding exposure time vs. distance with U.V. lights - I recently
halved my printing distance - from 12 inches to 6 inches, expecting the
inverse square law to shorten my exposusre times. This is with 8 48
inch BL tubes. Printing times are exactly the same!
Thanks,
John Richardson
Jon Edwards replies:
Yes, John I would expect no difference either at that distance due to the
nature of the BL tubes, try 2" and see what you get. The correct distance
between the tubes should be 1/4-1/2". See the discussion below.
ER has a good point and John Richardson is correct in his finding too. I
will attempt to explain why, for what I know of the Pt/Pd process, and maybe
it will apply to your other alt. processes as well
ER's statements about a bank of fluorescent bulbs with a reflector is
correct
when we directly measure the intensity of the light versus distance. Here
is
an example. About 4 years ago a Prof. at Lee College in Waco, Texas
(everyone now know where Wacko is located) had been using one of my 18X20 UV
light sources making 7X14 Pt/Pd prints with a contact print frame. With the
print frame his negative was located 2" from the tubes. He obtained a used
vacuum frame and installed the light source over the vacuum frame on a neat
little railroad track system. Now his distance to the negative was 5" and
he
lost all his print times. His edges fell off and he overexposed the center,
and was not able to make a print of equal quality, as he had with the print
frame. Same everything, only the distance changed. Even at 5" the bank of
tubes still covered the negative completely as it had before. The light
black
light intensity measurements were as follows:
1" 940 lux
2" 900
3" 845
4" 785
5" 690
8" 380
>From the numbers, yes, there is a linear relationship of distance to the
measured intensity or light levels as ER states, and consequently there
should not be that much difference in exposure times between 900 lux and 690
lux. But, there was a big difference in the quality of the results. So
with
the diffuse light of fluorescent tubes the exposures times are much more
sensitive to the distance, than the linear relationship of the intensity of
the light. Small changes in intensity seem to magnify exposure times and
degrade quality.
-Jeffrey D. Mathias wrote:
There is much more than optimum or fastest printing speed. The image
quality is very important. I have not seen any information (data and
observations) related to the quality of a print and spectrum of exposure
light for any photo process. One option of finding could be that some
speed could be traded for some particular quality. The resulting
optimum light source for a particular image and personal taste may
differ, perhaps similar to a painter choosing a particular pallet.
-- Jeffrey D. Mathias http://home.att.net/~jeffrey.d.mathiasI agree with Jeffrey that there is too much emphasis on speed and not quality. The important thing is to test your light source for edge fall off and corner to corner consistency in both directions.
Sarah Van Keuren wrote to Jon: Another person wrote before your letter came that 365 nm and below is good for gum exposures and that over 385 does nothing. This is very different from your belief that the best range for gum is 400-500 nm. I go with your opinion from my experience but I question the long exposures you think people have for gum on your units. I've written to Ernestine Ruben to see if I remember correctly that her gum exposures average about 6 minutes in your unit.
Jon replied: I really don't know what is the best range for gum since I do not have any experience with the process. My somewhat dubious conclusion about the 400-500 nm range was based solely on the writer's conclusion about getting better results with the halogen over the BL's and the best results using metal halide. Its like comparing apples to oranges to piglets. I don't think my print times are good either, because most of the people I talk with are just starting out and have not mastered the art and science yet. So the information they feed me is probably not valid, since they are in the learning process. Sometimes I think print times are like sex, nobody really wants to discuss it much. Print times are part of the artist's secrets to making an exceptional print, and often they are not willing to tell the truth or give all the information. Or, they boast how fast their light source is (when they had a thin negative). Also, I find a lot of printers are not willing to share how much burning and dodging they did on a print either. So, I am not a reliable source when it comes to gum. I will send out some inquiries to my gum printer friends and see what info I come back with.
There has always been a lot of discussions about print times and light sources. I think I need to figure out how to write a grant to do a definitive study of the various processes, correct wavelength of light required, and best light source configuration, for the major alt. processes. It would be an interesting and complex project just to design the experiments.
Keep me posted, the best to all
Jon Edwards http://www.EEPJON.com
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