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Re: Watts, Lux, Lumens

Visit http://www.thekrib.com/Plants/Tech/intensorama.html for my summary
on the subject of watts, lux, lumens and PAR. Erik has done a nice job
of hyper linking several items from this article to other excellent Krib
entries by George Booth and others. The whole subject has been "done to
death" and the Krib information is fairly complete with one exception
(PAR ratings for popular lamps).

Mark Stahlke <mstahlke at denver_infi.net> wrote:
>    I've been looking into lighting upgrades for my 37 gal. planted tank.
> I understand that my Eclipse 3 hood provides inadequate watts per unit area
> and limits my choice of bulbs to 24 inch T8s. I want to make an enlightened
> decision so I started investigating different bulbs.
>    As I studied different flourescent bulbs, It occurred to me that not all
> watts
> are created equal. I was looking at Hagen's line of bulbs (www.hagenpets.com)
> and I noticed that various bulbs of the same wattage have vastly different lux
> and
> lumen ratings. Not to mention spectral characteristics.
>    Now I'm confused. I know what watts are but I don't understand lux and
> lumens.
> I looked up those terms in my dictionary but it wasn't very illuminating.
>    As my poor addled brain struggled to assimilate this flood of information
> several
> questions came up.
>    What spectral characteristics are best for aquatic plants?

Plants absorb light wavelength from 400 nM to 700 nM (nano-meters). This
is termed Photosynthetically Active Radiation (PAR) and this is measured
in microeinsteins per second per square meter. (umol/m2/s)

It turns out that plants don't really care that much where the spectra
are as long as they fall within the range 400 - 700 nM. See

Gro & Sho and Gro-Lux lamps are designed to radiate light in the spectra
that plants absorb best however they have very low lumen ratings. NOTE:
This does not mean they are inefficient at converting electrical power
into useful energy for plants. If you choose lights with very high lumen
efficiencies, you are not necessarily getting lights that are producing
the highest PAR/watt.

Metal halide lights typically have high lumen/watt ratings (~80) and
produce a fairly wide spectrum of light. T-8s with good electronic
ballasts rate about 90 lumens/watt but the spectrum is narrower and
tuned to human visual response. Although I'm not aware of any PAR data
for T-8s or MH, I suspect that MH do have a higher PAR/watt rating.

The problem with MH lighting is that you start off with bulbs at about
175 watts and jump to 250 watts and then 400 watts. This is perfect for
lighting a large room full of leafy green plants but tends to be
overkill for typical aquarium sizes. The T-8 lamps are a better
investment because the light can be used more effectively. They are
ideal when you want to hit the optimum of about 700 lux or 150 PAR which
is the "optimum" for Crypts, Echinodorus and similar low light plants.
(see http://www.thekrib.com/Plants/Tech/lighting.html#10 ) 

It turns out that PAR is correlated more to wattage than it is to lumen
output (assuming that we're using efficient ballasts and fresh
fluorescent bulbs) so this probably falls somewhere around 1.5 watts/gal
on an 18" tall tank. The interesting thing is that the slow growth
plants DON'T reward you with more growth when you give them more light
than they require to saturate photosynthesis!

In our typical aquariums, you would not be able to saturate
photosynthesis because you are starving the plants for one or more
nutrients in order to stave off algae. You can get tremendous gains in
growth rates by paying close attention to providing enough carbon
dioxide, potassium, magnesium and calcium dissolved in your water and by
carefully and selectively increasing the fertility of your substrate
(such as clay fertilizer balls). Large changes in any single growth
regulating factor are most likely to benefit the fastest growing plants
in your aquarium, the algae!

>    Would a formula based on lux or lumens per unit area be more meaningful
> than
> watts per unit area? Watts are after all a measure of electrical power not
> light.

Actually, until we get data published on the PAR ratings of various
fluorescent bulbs, the watt ratings may be more representative of PAR
than the lumen ratings. Light intensity is really energy intensity and
energy is measured in watts x hours (or kwH). Speaking of PAR
measurements, if anybody in the Cincinatti Ohio area has T-8 lamps or MH
lamps, please contact me or Pete Mohan as we still haven't been able to
make any PAR measurements and Pete has access to the equipment to do the


1) use electronic ballasts for fluorescent lighting
2) use energy efficient T-8s such as GE SPX-50 if you can but don't
waste time; get what's available and cheap.
3) use mylar, aluminum or white paint inside your hood to maximize
4) for large aquariums, use 175 watt MH pendant lamps with parabolic
5) for incredibly huge aquariums use the monster MH lamps
6) if you live in the tropics, use filtered sunlight (its FREE! and you
probably DON'T need the extra heat in your home)
> Can anyone shed some light on this mystery?

[Chestnut alert] :-O

For that old pun we should deduct 3 lux from your score but I'm afraid
I've been guilty the same pun myself!

Steve Pushak                              Vancouver, BC, CANADA 

Visit "Steve's Aquatic Page"      http://home.infinet.net/teban/
 for LOTS of pics, tips and links for aquatic gardening!!!