From: Sandy King (sanking@clemson.edu)
Date: 12/11/01-10:21:21 PM Z
Anyone with a UV bank of fluorescent tubes and a step wedge can
easily verify that the inverse square law does not apply when
exposing with such a unit. You can easily test the premise.
Using whatever process you are familiar make one exposure of the step
wedge with the tubes at at 2" from the exposing plane. Assume that
the exposure is 20 seconds. Develop and process the step wedge and
note the first step that gives you maximum density. Now make another
another exposure with the tubes at 4" from the exposing plane. Apply
a correction for time assuming that the inverse square law applies
and give 4X the first exposure, or 80 seconds. Develop and process
the second step wedge. For the inverse square law to apply both step
wedges should record the first maximum density on the same step of
the wedge, after exposure and processing. They will not. The step
wedge that received the 80 second exposure will be very much
overexposed when compared to the one that received 20 seconds.
There is no magic here. The inverse square law only applies to light
rays diverging from a common point, as with a point source light. In
a UV bank of tubes the total amount of radiation that falls on the
exposing plane is a combination of rays: point source, near parallel,
and rays intermediary between the two. The inverse square law only
applies to those rays which emanate from a common point, not to the
near parallel and intermediary rays.
Sandy King
> >light has no "choice" but to obey the laws of physics, that
>>includes the inverse square law. Light is not magic, it can and must be
>>described in a rational way.
>
>I am puzzled by the explanation why the square inversion law should not
>apply for the bank of lights, and I'm too not convinced that this is
>true, but on the other hand light IS magic. Funny things happen to it
>(although on a different scale). If you are curious and have a moment to
>spare please read a very short book by Richard Feynman: "QED - The
>Strange Theory of Light and Matter".
>Have fun.
>Ben
--
This archive was generated by hypermail 2b30 : 01/02/02-04:47:33 PM Z CST