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Re: lumen ratings, watts/gallon

Dr. Dave wrote:

> Yes, tank dimensions, hood reflectivity, natural light and a host of
> other factors will influence the effectiveness of your 'Watts per gallon'.

Here's one of those complicating factors...

Erik Olsen pointed out in a later letter in this same digest that lux and
lumen are measures adjusted to match the perception of the human eye (I
think they're called "photometric" measures) - and they're strongly
weighted to green light.  PAR measures the total intensity of light over a
range of wavelengths (I think its called a "radiometric" measure) from red
light through violet light.

Significance?  I noticed that the lumen output claimed on some 40 watt
Sylvania Grolux lights were very low compared to the output claimed for
other 40 watt flourescents.  I can't remember exactly what the values
were, so I won't repeat them, but I think the Grolux output was less than
1/3 the output of a GE daylight tube.

My plants, of course don't respond as if that is true, and it only takes a
quick look at the spectrum to figure out why. Grolux tubes provide very
little green light, so their lumen (human perception) output is low.  They
provide a lot of red and blue light, so their PAR (plant-perceived light
intensity) is actually fairly high.

The other side of this coin is that a tube that produces a lot of green
light (triphosphor bulbs, for instance) will rate a high lumen output, and
will actually appear brighter to our eyes than other tubes, but they may
not produce as much PAR as Grolux, or other lights that are missing the
big green peak in their spectrum and look fairly dim to us.

This sort of complication aside, it isn't too hard to see the reasoning
behind a watts-per-gallon guideline.

The light intensity at the water surface is the total amount of light
(lumens) reaching the surface divided by the area of the surface. The
amount of light available to plants below the surface declines with
distance away from the surface.  If you wanted a qualitative measure of
average light intensity in a tank, then you would divide the light
intensity (lumens per area) at the surface, by the depth of the tank. That
works out to lumens per volume of tank - lumens per gallon, for instance.

Most flourescent lights work within a fairly small range of lumens/watt.
So if you can describe a light intensity in lumens per gallon, then you
can also describe it in watts per gallon.

Works for me, anyway.