From: Robert W. Schramm (schrammrus@hotmail.com)
Date: 09/15/00-07:46:17 PM Z
Sandy,
Of course you are right. Actually the closest thing to a point
source is the sun and it even is not a point. However, since I
got into a discussion of this once with Dick Sullivan as regards
the inverse square law and did some fancy math involving calculus
to get some answers, I can tell you that by the time you get to be
10 light source diameters away you have, in effect a "point source"
and the inverse square law works. At closer distances it goes as
1/distance. My four bulbs are about 10" apart and I can get up
to 12 feet away. I used four lamps because I wanted to get even
illumination at a large distance. A light meter indicates that
at least over a distance of about six ft. by six ft. illumination
is pretty constant.
As a sidebar. If you used just one lamp with a clear bulb, you could
get a point source effect at much smaller distances...say two feet
... since the size of the arc is less than an inch even though the
glass envelope is much larger. You would have to use the clear glass
bulbs though (some are frosted).
Point sources are like black body radiators, they are both theoretical
concepts but we can approximate them.
Well thats enough physics lecture for now.
Best wishes,
Bob
>From: Sandy King <sanking@hubcap.clemson.edu>
>Reply-To: alt-photo-process-l@sask.usask.ca
>To: alt-photo-process-l@sask.usask.ca
>Subject: Re: Point source UV
>Date: Tue, 12 Sep 2000 22:11:34 -0400
>
>Bob,
>
>Just wanted to point out the obvious, of which you may already be
>aware. If you are printing with a bank of four mercury vapor yard
>lights, even at 10 feet from the negative, you don't have a point
>source UV. You have a diffuse light system!!
>
>Sandy King
>
>
>
>
>
>
>>Sarah et all,
>>
>>Around here you can buy a mercury vapor "yard light" for about
>>$35. It has a large clear bulb and produces a goodly amount of
>>UV. Not hard to wire up and the bulb will last a long time.
>>I've got a bank of four of them attached to the ceiling of my garage
>>about 10 feet from the floor. The exposure times are rather long at
>>this distence but I can make really big prints.
>>
>>Bob Schramm
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