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Is this enough light?
I have seen a lot of posts lately trying to address the issue of how much
light you need for a 120 gallon or about the efficiency of different lamps.
I think is very misleading to talk about individual system components rather
than a lighting system as a whole. So I tried to find the worst fluorescent
sytem and the best system and compare them so that people can get an idea of
how variable fluorescent systems can be. My comparison is using comparable
spectrums so that the actual lumen rating of each lamp is not really an
issue. One of the worst ones I found is a trigger start ballast with (2) 2'
NO F20 T12 CW lamp. This system as near as I can figure is rated at about 41
lumens per watt and produces a total of 1470 lumens with a nominal wattage
of 40 watts. One of the best systems uses a Sylvania HL electronic ballast
with (1) 4' NO F32 T8 841 lamp. This system produces 91 lumens per watt for
a total of 3540 lumens. The nominal wattage is 32 watts. So not even taking
into account differences due to lamp spectrum, lamp spacing, reflector
quality and glass cover/no glass cover the electronic system is 2.2 times
more efficient than the trigger start system and produces 2.4 times more
light. When you look at the nominal watts per gallon rating though, the
trigger start system is supposed to be actually better. On top of all this
is the fact that if the electronic sytem had a very high quality reflector,
no glasss cover and a lamp with a spectrum well suited for growing plants, I
think that it would be easily be three times as effective as a the trigger
start system with a glass cover, no reflector and a poor spectrum lamp.
This is why it is very difficult to say if a particular system is sufficient
for growing plants without more information about that system. The most
important piece of information is the ballast factor and this is never
published by companies that sell lighting systems in the aquarium trade.
Without it you cannot calculate the system lumens and you really cannot get
a good idea of which systems are suitable for which tank. Other things like
the reflectivity of the reflector or the input wattage of the ballast would
also be very nice. For that matter why not just publish the system lumens
for a given fixture using 4100K lamps and then it would be possible to make
some intelligent comparisons.
I think that this tank requires about 20,000 system lumens combined with a
high quality reflector but without more information who can say for sure?
That said it is quite likely that four 55 watt lamps or six T8 lamps or six
T12 lamps or possibly even two 96 watt PCs can effectively light a 120
gallon tank as they are basically be all about the same efficiencies and
the watts per gallon rule more or less does apply.