From: Sandy King (sanking@clemson.edu)
Date: 12/11/01-04:26:53 PM Z
Robert,
The inverse square law only applies to point source lights. I
knowfrom experience that it does not apply to banks of fluorescent
tubes when used at typical exposing distances of 2-6".
I can not give you the math or optical laws for this, however, there
was a thread on this subject on the list some time ago, perhaps a
year or more? At this point I don't know how to search the archives
and hope that perhaps someone else can direct you to the thread.
As for your current problem, I assume you are making small prints
with the two tubes? As for ballast, yes, it does make some difference
in that electronic ballast will give you about 10% more lumens per
watt than magnetic ballast. But in practice this amounts to less than
a stop of exposure so I don't think your ballast is the problem.
As for the rest I most defer to the gum experts on the list.
Sandy King
>Sandy (et al.),
>
> I'm very curious about your mention that the inverse square law does not
>apply to a bank of fluorescent tubes. Might you explain a bit more, or even
>give the math (or a reference to it)?
> Also, I am exposing gum dichromate emulsion to a bank (so far, just two)
>of F40BL bulbs (is there another that might be better?), and wonder whether
>the type of balast I use makes any difference. As it is, the bulbs come on,
>but I'm having some problems with the gum not hardening. It is probably not
>the exposure, since I get a very readable (if faint) dichromate image, but
>at this point I wonder about everything.
>
> Any info from anyone would be appreciated,
>
> Robert Schaller
>
>
>on 12/11/01 10:30 AM, Sandy King at sanking@clemson.edu wrote:
>
>> Jeff Foster wrote:
>>
>>
>>> so last night I went down to my shop and hooked up a ground wire to
>>> all three ballast (via the screw holding them to the wood top) and
>>> then to the ground on the ac cord. bingo - no delayed start and no
>>> flicker! very cool solutions. I looked closely at the wiring
>>> schematic on the ballast label and it does show a little ground
>>> symbol leading from the case...hmmm.
>>
>>
>> Congratulations. Wiring schematics can be useful when all else fails!!
>>
>>>
>>> thanks to all who supplied insight - now on to printing ( banding
>>> test - even with the inverse square law for light fall off....)
>>
>>
>> Interestingly, the inverse square law only applies to point source
>> lights, not to a bank of fluorescent tubes being used very close to
>> the exposing plane, essentially functioning as a diffuse light
>> system. In other words, contrary to expectations, doubling the
>> distance of the tubes from 2" to 4" from the exposing plane will
>> *not* result in a 2X increase in exposure time. It will in fact be
>> far less, perhaps barely enough to even notice.
>>
>> Sandy King
>>
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