From: Sandy King (sanking@hubcap.clemson.edu)
Date: 09/15/00-09:45:51 PM Z
Bob,
My interest in whether this multiple light unit should be considered
a point source or diffuse light source lies not in whether or not the
inverse square law works but in how much sharpness is lost when using
reversed negatives. It is clear, for example, that the use of diffuse
light sources such as BL tubes will result in considerable loss of
sharpness with reversed negatives. Even when using the sun as the
light sources there will be some loss of sharpness with reversed
negatives, except in cases where the sun is at a right angle to the
printing frame.
I would be interested to know if you have done any experiments with
your bank of four mercury vapor lights at 10' from the printing frame
to test sharpness with reversed negatives?
And by the way, you mention that your exposures with this unit are on
the long side. How long is long?
Sandy King
>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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>>
>
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