Re: Measuring light in tank

[George Booth <booth at lvld_agilent.com>]

> Date: Sun, 9 Jul 2000 21:29:30 EDT
> From: Zchryk at aol_com
>
>     I have an uncommon light set-up (diy aluminum canopy with raised 
> flourescent fixtures, n.o. bulbs on ice cap electronic ballast), and I want 
> to quantify the light reaching the plants.
>     Using a photography light meter in a baggie, ...
>
>     Have visited George Booths' site where he describes readings with a 
> lux-meter but don't know how that compares.  
>     Can anyone help me evaluate these readings?

Unfortunately, photography light meters have a very coarse resolution. Each 
change in f-stop is 1/2 or 2x the light level.  So, your readings "w/filter" may 
truly indicate an intensity of around 2400 to 4800 lux. Those values are 
reasonable for FL bulbs but the error range is quite large, eh?.  You will find 
the light meter does not have the resolution to compare different bulbs in your 
setup (+/- 20% difference) or to compare your intensity with readings from a 
luxmeter. 

The readings "w/o filter" seem *way* too high. For example, we get readings from 
our 175w MH bulbs of 100,000 lux (new) to 75,000 lux (6 months old) a few inches 
below the buld and 20,000 to 10,000 lux 14" from the bulb. A reading of 38,000 
lux mid-water with n.o. bulbs is bogus. 


George Booth in Ft. Collins, Colorado (booth at frii_com)
  http://www.frii.com/~booth/AquaticConcepts

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