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RE: Lumens and Spectrum

Katy wrote:

>I'm sure this topic has been beat to death, but I need some reassurance...
>There's too many opinions floating around on the Internet.
>1. Is comparing lumens the best way to judge lamp output for AQUATIC plants? Is
>lumens the most imporant out of other factors like CRI and Kelvin?
>2. Do AQUATIC plants make use of all available light including the green at low
>light levels like 2-4 wpg ?
>I know other factors influence these answers. I went to the krib, but there's
>almost too much info there... Thanks!

I'll chime in on this one since i wrestled with these issues at one time and
 i think i understand them. i'm sure you've actually read a lot of this stuff
before but i'll try to make it straightfoward. if i'm wrong someone please 
correct me.
 its a little long.

Lumens is not necessarily the most important thing to look at when buying bulbs
for your plant tank.  lumen output ratings are basically a measure of how bright
a bulb will look to the human eye.  the human eye's perception of light 
(which is generally very shoddy to begin with - which can be a 
useful adaptation, but that's another story) is heavily weighted towards the 
band of the spectrum.now, plants are adapted to make use of a fairly wide range 
spectral outputs, but generally can absorb green the least. (although this 
isn't important, 
its the ability of plant pigments to absorb a photon of a given wavelength that
matters, rather than the energy of that photon.  in this respect all photons 
are created 
equal once they have been absorbed)that is of course why most plants look  
basically green. 
 they reflect mostly green light rather than absorbing it. this is why you'll 
find that 
plant bulbs like grolux have a low lumen output rating and look rather dim.
they only look dim to us.  to the plants they're nice and bright with less 
 visible blues and reds.so certainly you can use just about whatever bulb you 
 and the plants will get some useful light, but your standard fluorescents tend 
 put out a lot more greens than other colors so you might have to make your 
tank look 
extremely bright using these bulbs to get the equivalent photosynthetic 
activity you'd
 get out of fewer grolux bulbs.  this is why a lot of people will mix and match 
and chroma 50's or standard cool whites. i've actually found you can get a very 
effect out of a few warm whites (2700K) and an actinic bulb thrown in for 
(actinics don't have a color temp per se since they're more or less 
light sources and don't mimic black body radiators).
this should also be pretty good for plants since its mostly reds and blues.
as far as CRI and color temperature go, that can get a little complicated
and may not be terribly useful.  the thing about color temperature vs CRI is
that color temperature will tell you basically what color the bulb looks 
(2700 is warm yellowy orange, 5000 is pinky white, 6500 is cold blue/green 
you see in movies that use daylight balanced film to show you how awful 
fluorecent light looks),
 but it won't tell you much about what actual wavelengths of light
are given off.  you can reach the same color temp by mixing and matching 
spectral outputs.  its just like painting.  the thing is, if you achieve a 
looking color temp using a little blue, a little green, a little red, or 
something like that, 
instead of a broad continuous spectral output
like you get from the sun your CRI may be low.  this is because certain
colors of light just aren't being given off by your bulb(yellow maybe, or 
violet, whatever),
 so they can't be reflected by the object you're looking at
to be seen by you.  so the objects color looks a little wierd.  When looking at 
all these
factors it can be very hard to chose a single good bulb because much of the 
needed information
often isn't available or in a readily useful form. trial and error is often 
that's not a helpful statement, so i'll finish by saying you can surely get 
good growth 
and decent viewing out of a mixture of cheap bulbs, but overall efficiency of 
electricity used
to produce light leading to photosynthesis will probably not be terribly high.

good luck

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