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Re: comment on lumens (V3 #s 461 & 464)

"K. A. Bryant" wrote:
> Thu, 20 Aug 1998 14:42:32 -0700 (MST), Roger Miller wrote:
> <snip>
> >This difference is because the lumen measure is weighted toward green
> >light - the light to which our eyes are most sensitive.  Light of that
> >color is near the minimum in the photosynthetic action spectrum.  That
> >makes lumens nearly irrelevant to whether or not a light is good for
> >growing plants.
> Are lumen measurements any more helpful if you are comparing the light
> output of two lamps with the same color temperature?

Not much, if any. Generally, the *lower* the lumens, the more light is
going into the useful action spectrum of many plants (all other factors
being equal).

> Can anyone supply a reference on how lumens are measured?  I'd like to
> understand how it is that "the lumen measure is weighted toward green light"
> and what this weighting amounts to.

Lumens are based on measurements done back in the 30s by the
International Committee on Illumination (ICI or CIE in French) where a
large number of people were tested to determine a "standard observer."
Lumens are a *psychophysical* measurement, describing how "bright" the
light looks to the "standard observer."

Your eye is at least ten times as sensitive to green as to blue-violet
or deep red. Hence, a good bright-looking lamp will be quite rich in
green. Your visual acuity is higher in green, so reading under
"cool-white" flourescents avoids the 1/8 diopter of chromatic aberation
you have in blue or red. Everything is just brighter and clearer.

Color Temperature is another psychophysical measure, describing how a
source "looks" like a blackbody of a given temperature. It has nothing
whatsoever to do with the *actual* blackbody temperature! [The standard
reference is _Science of Color_ by Optical Committee on Colorimetry,
BTW. About $50 at Amazon.com]

Both measures are great for picking the lamps to illuminate your store's
window display. That's *all* they were designed to do. Using them to
pick plant lights is misleading in the extreme, at times, and fairly
worthless otherwise. They do tell you quite a bit about how your tank
will "look," but not very much about how well your plants will grow.

Exceptions abound. My *Riccia fluitans* flourishes under cool whites.
Its action spectrum is different, apparently, from many other plants. I
suspect several more of Amano's favorites may be in this category.


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