From: Michael Mutmansky (psu4ever@ix.netcom.com)
Date: 04/27/00-02:53:40 PM Z
Actually, the inverse square law is specifically associated wit POINT SOURCES,
and other light sources that can be approximated as point sources. In the
lighting industry, the general rule of thumb is that the source must be 5X the
distance of the largest dimension of the light source. That equates to 20 feet
away for a 4 foot lamp...
All this does is give you the ability to APPROXIMATE non-point sources (because
there is no true point source) to a simple formula.
That said, the illuminance level under a large bank of lamps closely spaced
together follows the Parallel Plane (or Infinite Plane) Theory. This theory
states that, given that the distance between the source and the receiving
surface are small relative to the dimentions of the surface, the illumance
level WILL NOT CHANGE WITH DISTANCE.
Therefore, if you have a light source that is 4' by 20" (like mine) as long as
the distance between the source and the contact printer remains small compared
to the 20" dimension, there will be a neglegable change in illuminance. The
place you will notice a change is at the edges, where the Parallel Plane theory
does not apply. But, to keep good uniformity on the print, you have to have a
light source big enough to allow you to only use the middle part, so this
should not be of great consequence.
I had noticed that the Arentz book has the Inverse Square Law misapplied, and I
thought that I might email Dick to correct the text.
---Michael
Ed Stander wrote:
> Sandy:
> The Inverse Square law DOES hold for fluorescent lights. It would be
> rather sad if the laws of physics only held true for incandescents. The
> trick here lies in defining the distance correctly. The light received
> from a bank of lights does not come only from the center of the bulb. It
> bounces around a lot and effectively comes from everywhere the light
> bounces from. So what distance do you measure? As long as you are within
> the enclosure of the box, a move away from the lights may simply bring new
> rays into play. Once you move far enough away, however, the inverse square
> law works fine. If you wish to test this, simply make white paper blinders
> for each bulb. Wrap the paper over the top of each bulb, and have it open
> towards the print. This will limit the amount of scatter. Do this and I
> think you'll find tat Newton's law works just fine.
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