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This topic is like the black hole of
aquatic plant keeping so I should know
by now not to get involved but I cannot
help myself so I apologize in advance.

What PAR gives us is the lamps ability
to turn electricity into potentially
useful photons. Lamps with high PAR/watt
values are potentially (but not
necessarily)  very useful.  It really
has nothing to do with determining which
is the better lamp spectrum. Nobody
knows which spectrum is optimum for
growing aquatic plants and even if that
could be defined and even if the lamp
could be built and even if that spectrum
were the best for every aquatic plant
and not different for every one, I doubt
if anybody would use it. The lamp would
probably not look that great and people
would probably decide to mix them with
cool whites so their tanks would look
In fact, from anecdotal evidence we can
see that if a lot of photons are pumped
into a  planted tank and all other
nutrients are supplied in sufficiency a
lot of plant growth occurs. We also see
different people using lamps with vastly
different spectra who are all still
getting outstanding results. If one
person can use a broad spectrum lamp,
another uses Triphosphors and someone
else is using CWs mixed with grow lamps
how important can spectrums really be.
It is on this basis that I believe that
the color of the photon is not nearly as
important as the  number of photons and
that I think that therefore PAR
combined with how the lamps looks in
your tank is the next best thing to
knowing the (unattainable?) optimum
spectrum for your plants and how well a
given lamp matches that spectrum.

That is not to say that lamps do not
exist with spectrums that are indeed
generally more efficient at producing
desirable plant growth  but it is very
difficult to prove for sure one way or
the other. Anecdotal evidence seems to
point to, for example, Chroma 50s but is
it's spectra so much better that it is
worth putting up with it's inferior lamp
life and poor system efficiency? Their
large diameters and relatively low
intensities make it practically
impossible to increase lighting
intensity beyond certain levels and make
it impossible to design a good
reflector. Try adding one moreT12 lamp
to 4 existing lamps in a 12" wide space.
After you pack all the lamps closer
together will you really get very much
more light? Better yet, try and figure
out how to build a good reflector for
the four lamps that is able to direct
the light downward so that the light can
penetrate the water and not reflect off
the water. This stuff is a snap with T5
PCs which means that in the end you are
going to get way more photons/watt
entering the water. The particular
wavelengths of these photons should also
be sufficiently correct to sustain good
plant growth. In my mind, the spectrum
of just about any 800 series phosphor
looks good and is sufficient for good
plant growth so why not take advantage
of the greater efficiency of T8 and T5
lamps and the better ballasts and
reflectors that usually come with them.