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Re: lumen per liter

Rod Hanks asked in apd696:

"The old formula of watts of flourescent light (2 to 4 watts per
gallon) is breaking down, since newer bulb types have different
watts-to-light ratios.  Is there a rule of thumb based on lumens? "

dc wrote:

Yes there is, although not so well known in the US. In the book "The Optimum
Aquarium" by Kaspar Horst/Horst Kipper (founders of Dupla) recommended a
range of 30~50 lumen/liter depend on low or high light plant.

I reply:

That seems pretty much in line with what I think. If you have very good
reflectors 200 lumens per gallon will grow anything I have tried but then
again I havn't tried everything. The problem with a lumens per liter (or
gallon) rule is that you have to know some hard to get information about
your system in order to apply the rule. For one thing lamp lumens vary
widely depending on the type of spectrum it is putting out even though the
plant usable light is equal. So the rule must specify the type of spectrum
being used for comparison. For example, the mean lumens of an F40 Chroma50
lamp is 1amp is 1870 lumens. The mean lumens of an F40 841 lamp is 3050. Big
difference in lumens but I don't think there is a big difference in plant
usable light. The 200 lumens per gallon is a value your system would produce
if you were using 841 lamps so there is no point in even looking at the
rated lumens of the lamp you are actually using.

Another problem is that the ballasts used to operate fluorescent lamps also
vary widely in how brightly they will operate a lamp. This can be anywhere
from 60% of the rated lamp lumens to 120% of the rated lamp lumens. Ballasts
in N.A. are rated with a ballast factor which specifies at what percentage
of the lamps rated output the ballast will drive the lamp. Unfortunately,
that factor is never marked on the ballast itself.

Another variable is the fixture efficiency of a given system. Large diameter
low output lamps have to be spaced so close together that they block light
and make it impossible to design any kind of effective specular reflectors.
Small diameter high output lamps can be spaced far enough apart that
effective reflectors can be employed. I don't know what the difference is
between an average setup and the best setups in this regard but I think you
can use a conservative estimate of a 20% improvement in how much light makes
it into the tank with small diameter lamps and specular reflectors. Please
note that specular reflectors will do nothing to improve how much light
makes it into the tank with T12 lamps.

Lastly it is difficult to compare MH lamps with fluorescent lamps on a lumen
per lumen basis. There aren't any MH lamps with spectrums that closely match
an 841 fluorescent spectrum. There are a few very high CRI MH lamps that
closely match the spectrum of some very high CRI fluorescent lamps. Those
lamps just like their fluorescent counterparts have lower lumens per watt
outputs even though the plant usable output appears to be the same.

You could have a rule that used the lumen output of a wide spectrum lamp as
a reference to evaluate various sytems that would be more or less accurate
for both MH and fluorescent lamps. It would be something like 120 lumens per
gallon provided that the fixture was quite efficient at getting light into
the tank. For example, 6 Chroma50s would provide 150 lumens per gallon in a
75 gallon tank. Since those lamps are large diameter due to lamp spacing
difficulties the fixture could only be considered to be of average
efficiency but at least something like white paint could be used which
wouldn't make it as bad as it could be. If you have a decent ballast with a
.88 B.F. the total lamp output will be 132 lumens per gallon. Given that the
fixture efficiency is maybe 10% below optimum the setup will still give you
almost 120 lumens per gallon. I think most people report that 6 Chroma50s
will grow pretty much anything in a 75 gallon tank. So, the 120 lumens per
gallon of wide spectrum lighting should be enough and the same rule should
apply to 950 or daylight deluxe lighting or the wide spectrum MH lamps that
I was talking about as they all have similar spectrums. Interestingly
enough, those particular lamps would be considered to be very efficient
under this kind of rule. The 100 watt version is about 6000 initial lumens.
They don't maintain the lumens as well as some fluorescents but I guess they
would still have a mean lumen rating of 5000. Two of them over a 75 gallon
tank would make 130 wide spectrum lumens per gallon. They even come in a PAR
type configuration which have excellent built in reflectors.

As you can see it is not so easy to apply a lumens per gallon rule and if
you apply it incorrectly you are going to get a prediction at least as bad
as the watts per gallon rule. Even with the best information you still have
to make assumptions and guess about fixture efficiency and some of the
values for lamp lumens or ballast factor if they are not available. In the
end it is probably better to simply ask if a certain type of lighting over a
particular size tank will grow the plants that you want. No rule beats the
experience of the people on this list.


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