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Re: Light intensity

Wright Huntley points out:

>In free space, radiation falls off in intensity as the inverse square of the
>distance from a limited source (point).

And I add:

In practical terms, you can figure how much light you lose with distance:

* fluorescent tubes are linear sources and light falls off roughly inversely 
proportional to distance - twice as far, 1/2 the light.

* metal halide bulbs are close to points sources and light falls off roughly 
proportional to the square of the distance - twice as far, 1/4 the light. 

I measures these effects with my trusty Lux meter. 

This assumes that the relector is NOT perfect and is NOT directing all the light 
towards the target. A really good reflector will greatly corrupt the simple 
relations described above. 

And, as Wright further points out, all bets are off once the light gets in the 
water. I also measured this in a 29 gallon tanks the results were bizarre. My 
website has some details under lighting. 

George Booth in Ft. Collins, Colorado (booth at frii_com)