# Re: [APD] How much light actually gets down here?

```The loss of lightfal over a given size of area decreases as a square of the distance from the point or origination --simply put, if you draw lines fromthe pint source to the perimeter of the area, then lower the area to twice the distance, the line will describe an area that increases as a square of the original distance (for example, iirc, an 8" diameter comprises about half the area of a 10" diameter) . This rule of thumb is often cited and comes from genralizations about incandescent lighting.

The square of distance rule yields an easy calculation with a single point source of light. However, with a line source, you have an infinite number of point sources (all along the length and around the tube), so the calculation becomes more complex. For any one of these, the square of ditance rule holds, but as the area is lowered, more point sources shed light on the area (an ovesimplification but that's generally the way to picture it). Taking all that into account, the lightfall roughly halves wtih distance, ignoring side reflections, and a few other complications.

With an MH, one could rely on the square of distance rule since the light emits from a very small source, not quite a point, but close enough for gardening purposes.

The reflector won't make a terrific difference -- it does mean that lgiht light come from more point sources (from a wider area) but not tremendously so.

sh

----- Original Message ----
From: Vaughn Hopkins <hoppycalif at yahoo_com>
To: aquatic plants digest <aquatic-plants at actwin_com>
Sent: Thursday, May 3, 2007 11:06:26 AM
Subject: Re: [APD] How much light actually gets down here?

. . . the bulb sitting above the tank with no reflector loses light
proportional to the square of the distance the light travels . . , but
with a good reflector it loses less than that, and with a perfect
reflector it loses very little.
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