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[APD] Re: Luxed out

Message: 2
Date: Mon, 13 Oct 2003 14:27:42 +0100
From: "Beek, Graham" <Graham_Beek at ds-s.com>
Subject: [APD] Luxed out
To: "APD (E-mail)" <aquatic-plants at actwin_com>

Hi group,

I've become a little confused over lux and it's meaning.

Join the crowd. Even our usually-accurate Scott got this one messed up. :-) Lux is a standard photometric unit that isn't just a "European thing." As I recall it's the metric equivalent of the foot-Lambert, and describes how well a surface is illuminated. It is a lousy choice for lamp rating, BTW. Lumens is better, IMHO.

Looking for a new T8 tube, I was reading the blurb on Hagens range which include LifeGlo, Sunglo, Powerglo, Floraglo etc and found that with increasing tube length the lux increased. I thought lux was a measurement of light (lumens) per metre, so surely the length of the tube would be irrelevant? Am I missing something?

It is, but that's per *square* meter. Doesn't mean much for plants, and often is very misleading. Converting from lumens of apparent brightness to a surface measurement requires definition of the measurement apparatus. Yes, longer tubes will generally be able to deposit higher lux.

Also, Hagen describe their PowerGlo tubes as 'High Intensity' and Sunglo as a 'standard tube that replicates natural sunlight', however the Sunglo tube has a higher lux measurement than the PowerGlo (150 or so compared with 110). This confused me even more.....

In case it matters the PowerGlo is 18000K, the Sunglo is 5000ish (can't
remember the exact number).

Those are color descriptions that have nothing whatsoever to do with power or brightness.

Any clarification would be greatly appreciated!


Let's start basic. Products intended for the human eye, like lamps, are measured with photometric units that are based on a "standard observer" as defined by the CIE, years ago. How the meat looks in the butcher case, and how the dept. store window looks provided the impetus for such definitions.

Since the human eye sees green with about 10X the sensitivity of blue or red, and plants see and use red and blue at several times what they usually collect in the green (they reflect much of that away).

Radiometric units, like Watts/cm2 and Einsteins are better for plant discussions, but for how the tank *looks* to you, the photometric units are better. Conversion between them is difficult and rarely accurate, as all photometric units are based on emperical curves obtained by sampling a lot of observers. Simple, accurate algebraic conversions are out of the question.

Bottom line is to take photometric values as just how the light will look to you, but use better tools for getting the plants fed enough photons. I often guess a wider spectrum by picking the *lowest* lumens/Watt lamp in a given family. :-) Most always, it is true. Lower color temps often have richer red, while higher are sometimes red deficient while providing much more blue. The two chlorophyls (A & B) require both.


Wright Huntley -- 760 872-3995 -- Rt. 001 Box K36, Bishop CA 93514

           Mencken's maxim—every election is a sort
             of advanced auction of stolen goods.

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