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Re: [APD] lighting temperature question

The color temperature rating is an approximation of the
overall spectral output of the bulb. It's based on the
notion of what the color of the light would be that was
emitted by a block body heated to a given temperature --
the hotter the body is heated, the higher up the spectrum
the light that is emitted, starting at infrared, runing
through the visbile spectrum, and beyond.

With light bulbs, especially multiphosphor fluorescent
bulbs that do not have output that is continuous across the
spectrum, the colortemp is not quite as meaningfull and one
might hope. 

I a bulb has just one big spike in it's out put in the red
range, it would have a relatively low color temp rating. If
it has one peak up on the high energy blue end of the
spectrum, it would have a relatively high rating. If it has
two peaks, one at the red end and one at the high end, the
rating would be in the middle -- sort of an average. A bulb
with two peaks close together can have the same rating as a
bulb with two peaks far apart. A bulb with most of its
light down at one end of th spectrum but with a very
narrow, very high peak way at the other end will have a
higher rating than one without that high peak -- but the
observable color will be pretty much the same.

And bulbs from diff vendors but with the same ratings won't
necessarily look the same color. the most like sunlight
I've seen are some URI aquasuns rated at 10000K while most
bulbs with that high of a rating definitely look bluish.

And some bulbs with lower ratings, around 2000-4000 don't
look reddish as one might expect, but quite green.

The cheaper phosphors that can be used in fluorescent lamps
make a narrow band of greenish sort of white light. Adding
more expensive phosphors allows more output in other parts
of the spectrum. Compared to plants, humans are much more
sensitive to the green region of the spectrum while plants
are more sensitive to the red and blue. So a brighter
looking bulb might not seem brighter to your plants -- it
depends on whether the extra brightness comes from the
green region of the spectrum or not. And the converse is
true too -- what looks dimmer to you might seem stronger to
the plants.

But realize we're talking about shades of diff of *white*
light. The point being that the diffs don't make much diff
and you don't need to split hairs growing plants. Any high
quality multiphosphor fluorescent will do. 

Basically, your plants don't care what color temp bulb you
get, but the higher ones, within a given manufacturer, will
tend to have more blue and the lower ones more yellow or
red. So get what pleases your eye.

Very accomplished aquatic gardeners have used bulbs all
over the spectrum -- literally.


>  On 5/2/05, Ben Hong <bkhong at rogers_com> wrote: 
> > 
> > At least that's what I think it means... What is the
> difference between 
> > the
> > different 'k' measurements for light output? i.e.
> 2700k, 5000k,
> > 10,000k..etc.
> > 
> > For a 10 gallon planted tank, how much difference will
> this make?

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