Re: Lighting and Lux

[Wright Huntley <jwwiii at pacbell_net>]

> ------------------------------
> 
> Date: Wed, 12 Jun 2002 15:45:39 EDT
> From: Billinet at aol_com
> Subject: Re: Lighting and Lux
> 
> Hello,


Hi Bill,


> 
> Lux, or illuminance per square meter, varies dramatically with the distance 
> from the light source, inversely by the square of the distance, as a matter 
> of fact.  It can almost disappear at the bottom of a deep tank.  


This statement is far from true, but has become an unfortunate part of 
aquarium mythology.

The inverse-square law applies only to point sources in an environment 
without any "guiding." Neither is at all true of most aquarium situations.

> That's why 
> 20 watts can grow glosso beautifully in a six inch deep tank, and not at all 
> in a 15 inch tank, both with the same surface area.


Sorry, but this, too, appears quite contrary to my actual experience. 
[Glosso is a bad example, as it is so strongly influenced by other 
factors more than absolute light level. Current, hardness, nutrients and 
temp. would have to be matched to really do it experimentally.]


> 
> It would be most useful to have a table that would show the lux that is 
> produced by various bulb and reflector combinations, at distances of from, 
> say, two inches through 20 inches from the light source.  I think that would
> just about eliminate most of the lighting questions that we see and eliminate 
> a lot of questions about "how many watts per gallon?" and "what is the best 
> light?"


It would require a thousand such tables, and still wouldn't eliminate 
most of the questions, I fear.

The W/G rule-of-thumb is based on the approximation that standard tanks 
have a standard shape (sort of), and that, once light is in the water, 
the dominant source of loss with distance tends to be 
absorption/reflection by the plants and scatter from contaminants and 
substrate. The light is inside a light pipe, and attenuation due to 
absorption is fairly modest at normal tank depths.

Watts/G are used rather than lumens, because they tend to correlate a 
little better with PAR (Photosynthetically Active Radiation) than does 
lumens.


> 
> Has anyone seen such a table?  Would it be hard for a knowledgeable person to 
> construct one?  I'd think it could be done either mathematically or 
> experimentally, the latter with a waterproof lux meter and a flexible neck.


Mathematical construction would require accurate statement of tube shape 
  and emission distribution, plus reflector properties. Both vary 
wildly, in practice, so rough rules work easiest.

George Booth has reported some results, of the latter sort, on his site, 
I think. Unfortunately, plants don't use much of the energy measured by 
any true Lux meter, and they do need a lot that the meter does not read 
at all. The results would thus need a lot of interpretation.

Wright


-- 
Wright Huntley -- 290 521-0557 -- 731 Loletta Ave, Modesto CA 95351

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