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Re: lighting small tanks, watts per gallon rule? (long)

> Date: Sun, 14 Mar 1999 20:53:45 -0500
> From: "wayne jones" <waj at MNSi_Net>
> One of the things that really bugs me is spectral charts for individual lamps.
> They never have any meaningful units so you cannot use them to compare one lamp
> against another. If manufacturers would simply publish charts plotting watts or
> lumens or einsteins/m2 versus frequency one could easily determine what lamp
> produces the most PAR. Then we could have a formula of PAR/gallon. I can't even
> get meaningful information on lamps that are specifically designed for aquarium
> use.

C'mon guys. One more time. 

Power spectrum is (relative) Watts vs inverse frequency (or wavelength). All the
lamp outfits use that standard format.

It *can't* be expressed in "lumens" vs frequency, for lumens -- by definition --
have a particular spectrum built in. That spectrum is the viewing sensitivity of
the "standard observer." [A human eye thing.] It's 10 times higher in green than
in either blue or red. Plants usually reflect away green, absorb red strongly,
and blue somewhat less. Lumens *inverts* what we usually want for optimum plant

PAR is only a bit better, for it assumes a wider spectrum, but rarely does it
agree very well with the action spectrum of *your* particular plants, anyway.
Again, no such thing as PAR vs frequency. Same reason.

Einsteins count photons, and photons have different energy at different
wavelenths, so a "spectrum" of Einsteins vs frequency would have the frequency
squared and look pretty strange if not totally useless.

Bob Dixon had commented: 

> >But with the high variablity in lumens per watt these days, that still doesn't
> >really tell us anything.  

Wrong. It gives us an excellent *indirect* clue. The good tubes that have lower
lumens/Watt are usually putting more energy in the ends of the spectrum where
plants are more growth active, and less in the more visible green region. Tubes
with high lumens/Watt will make the tank *look* brighter, but "feed" the plants

[One glaring exception is some of the poorly-designed fish-shop tubes. They have
lower lumens per Watt because they are just poor designs with unstable
phosphors. They often even lose substantial brightness as they age, unlike all
good tubes that retain up to 90% until they will not even fire any more.]

Just mix and match to get the combo of plant growth and appearance that you
like. I have one 55G with two GE "P&A" tubes and a "cool white" for a total of
120W. The plants grow quickly (for no CO2) and the tank looks nice. If I turn
off the cw, it dies visually. It "looks" like I turned off way more than half
the light!

> >Yes, PAR would be eeven more effective still, but that is hard
> >to detirmine based on the available info for the different types of bulbs.

Read the spectra as *relative* Watts vs inverse frequency (or wavelength) The
peak is normalized to one, and all other values are just a percentage of that.
[Yes, it *is* frustrating that they assume you know that and never, ever label
the vertical axis.]

For pleasing your eye, look for lots of energy (area under the curve) in the
green-yellow region (500-600nm) and for your plants look for plenty in the deep
red (700nm) and near blue (450nm) regions. 

CRI can tell how "white" the light looks, but is useless when mixing tubes. Two
with CRI lower than 80 can have a combined CRI of nearly 100.

Color Temperature is also an "appearance" scale, and is fairly worthless. I lean
toward the 3000K range for plant performance and the 5000-6000K range for
appearance, if that's all I have to go by. Anything much higher than 6500K may
tend to look too harsh and glaringly blue. It may also lack the red that plants

In summary, you still need about 2W/G for good viewing and good growth. If you
go higher, you will probably be CO2 limited. Mix tubes to balance viewing
pleasure and plant growth. Deep, narrow tanks may need less light than that, due
to light-pipe effect. [The rule of thumb assumes the "normal" tank dimensions.]

It's not rocket science, so don't *try* to get too numerically accurate. You'll
just get frustrated. Watch your plants to see what works best for you.


Wright Huntley, Fremont CA, USA, 510 494-8679  huntley1 at home dot com

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