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Re: watts per gal

> ------------------------------
> Date: Sat, 8 Jun 2002 22:25:30 -0400
> From: "Gordon L. Mc Lellan" <gordon2 at dimension-x_net>
> Subject: watts per gal
> questions about the whole "watts per gal" idea in tank lighting.
> I'm not an electrical engineer, a mathematician, or a botanist, so what I'm
> thinking is most likely wrong, or something like that  [:)]

I'm an electrical engineer, specializing in optics. That has never kept 
me from being wrong, AFAIK. <VBG>

> Numerous websites have formulas to calculate how many "watts per gallon" of
> light you need to grow low-mid-high light plants...

That advice is free, as is mine. Worth every penny, too. :-)

> The questions I have are:
> Isn't watts a rather unreliable measure of light output?  I mean, a 75 watt
> standard incandescent puts off as much light as a standard 20 watt cool
> white
> fluorescent, or a 13 watt compact fluorescent twist-bulb.  Which wattage do
> the formula's speak of... I assume they mean watts fluorescent (since that
> appears
> to be the most popular)?

The sites you have read should have been specific about that. The "rule 
of thumb" we have used here means wide-spectrum fluorescent at normal 
output, typically T-12 tubes o/e. It is close for metal halide, but no 
where near useful for halide or other incandescent lights.

You need a lot more if they are "cool white" or some other spectrally 
limited phosphor. You need much less if they are T-8, T-5 (power 
compact) and have high-efficiency reflectors.

> Are there any formula that use something more related to actual light
> output, like lumens or candlepower?  

PAR might be a better rating for plants (Photosynthetically Active 
Radiation) but it is never given for regular tubes. Lumens and Lux were 
developed for describing how a decorated store window looks to the 
"standard observer" (presumably human) and are biased against those 
parts of the spectrum most useful for plants (red and blue). We have 
found that tubes with a wide spectrum that plants need ("daylight," 
tri-phosphor and "sunshine") often have lower Lumens per Watt than the 
greenish tubes that cater more to the human eye response. The human eye 
is 10X as sensitive to green as it is to red or blue. Lumens, 
Foot-Lamberts and lux all overvalue green rather strongly, but most 
plants reflect much of it away.

> Or formula that are calibrated to the
> wattage to light output ratio for standard fluorescent bulbs (can't afford
> the power compacts yet).

Most folks mean standard fluorescents when they are talking Watts/G but 
too-often fail to specify that. I tend to multiply the actual Watts by 
1.5 or 2 when dealing with AH Supply "Brite Kits" for example. "Cool 
White" gets divided by 2 or 3. [Still works, but you usually need a 
*lot* more of it for the same plant effect.]


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

                     It ain't what you know
                     that gets you in trouble;
                     it's what you know for sure
                     that ain't so. -- Mark Twain