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Re: watts vs. lumens

> Date: Mon, 15 Mar 1999 20:16:38 +0100
> From: "Ole Larsen" <olet at larsen_dk>
> Subject: watts vs. lumens

> Sorry, I donīt know the expression PAR

Photosynthetically Active Radiation. Sounds really nice, but has only a little
more meaning than lumens, IMHO. It is defined in terms of the recipient, and not
the source of radiation. Unfortunately, the defined "plant" it isn't our usual
recipients, aquatic plants and people. ;-)

> Lumens is a measure of what comes out as light ( candela/steradian) and thus
> better, but far from good enough

This statement is quite misleading, and lies at the heart of a lot of the
misinformation we encounter here on the APD. Lumen is a psychophysical term that
describes the way *humans* perceive optical radiation. Candela is a member of
that same photometric system. They are defined in terms of the human observer,
and not the source radiation. They tell you next to nothing about what actually
"comes out."

Here in APD we define light in a much broader sense, usually. Lumens *are* great
for predicting how bright the light will "look." Too many lumens per Watt always
cheats the plants out of energy *they* need, for the manufacturers of "cool
white" tubes have gotten really very good at not wasting red and blue light we
don't see well, anyway.

Red and blue are about 20% of the output of a very high lumens/Watt tube.
Unfortunately they are about 80% of the photosynthetically active spectrum of
our plants. That's why plants *can* grow well under "cool white" lights. You
just may need several times as many Watts of it.


Wright Huntley, Fremont CA, USA, 510 494-8679  huntley1 at home dot com

One big difference between a Libertarian and a Demopublican is the 
Libertarian knows it's not a waste to vote against a Republocrat.