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# Re: Lighting, from another angle

```> From: JOlson8590 at aol_com
> Date: Thu, 31 Oct 1996 02:28:46 -0500
>
> Thus far, I have seen no mention of the Law of Inverse Squares as it
> affects lighting.

Because, for most of our applications, it does not affect our lighting.

> Very briefly, the intensity of light (any light)
> shining on a surface varies inversely as the square of the distance
> from the light to the surface. This is true, regardless of the
> efficiency of the reflector, etc.

This is only true for a POINT source of light without a reflector. MH
bulbs are close to a point source. Of course, two or three MH bulbs
over a tank with a reflector complicate this somewhat.

This is NOT true for fluorescent bulbs which are a linear source.  For
most of the length of the bulb, the intensity is proportional to the
inverse of the distance from the surface (1/r).  This has been
verified by actual measurements.

Light from near the ends of the tubes diminishes faster with distance
since the ends become more like a point source.

Also note that a water filled, non-planted aquarium will act much
like a "wave guide" (the sides will reflect much of the light back
into the water), so there is not much loss in intensity from the
surface to the gravel.  Once you add plants and ornaments, this effect
is disrupted.  Thus a tall, well planted tank needs more intensity at
the surface than a shorter tank if short foreground plants are to be
well lighted.

> For a simple quickie, lets put the fluorescent tube 2 inches from the
> water surface, and then raise it to 12 inches above the water surface.
> 2 x 2 is 4, 12 x 12 is 144.  (squaring the distances) 4 / 144 is 1 /
> 36. The intensity of the light at the surface is 36 TIMES AS BRIGHT
> when the light is 2 inches from the surface as it is when the same
> light is 12 inches from the surface.

That would be true for a point source of light, NOT a FL light.

> On the tanks here at the Aquarium Center (Des Moines, Iowa) the
> difference in light intensity at the BOTTOM of the tank (the gravel)
> is (rounded) two and a half times as bright at the 2 inch from the
> surface position as it is at 12 inches from the surface.

If the surface is 36 times as bright, why is the bottom suddenly only
2.5 times as bright?  You didn't actually measure this, did you?

George in Cold and Wet Colorado
```