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I am in agreement with Bob. I've thought for some time that all the
concern about color spectrum in your bulbs is overrated as long as the
intensity is adequate. It is possible to determine what wavelengths of
light are being absorbed by isolating a plant's photosynthetic pigments and
using a spectrophotometer--in fact our intro biology classes do such an
experiment--but this is not practical for the hobbiest, and probably not
You can indeed determine PAR by using a proper light meter (not a Lux
meter), but these are very expensive (the cheapest one I've seen is
$475.00). The unit for PAR is micromoles/m2/sec--this is a measure of the
number of photons striking a given area per unit time. My understanding is
that most aquatic plants are saturated at around 200 micromol/m2/sec (this
is from a conversation--I have not verified this number w/ the literature,
so anyone who knows better feel free to correct me). For comparison,
midday sun output on a clear day is ca. 2000 micromol/m2/sec, but much of
this is reflected from the water's surface (the amount of reflection
depends on the angle of incidence) and more light is lost with depth.
I've been told that you could probably build a PAR meter for around
$25--the trick is that it must be calibrated against an existing PAR
meter--but I am not enough of a techie to know how to do it. I've seen
some very clever designs posted to this list--any suggestions from you folks?
BTW, I once asked our plant physiology tech if I could borrow his Lux
meter--he laughed at me.