mg queried thusly:

I was in WalMart today getting some replacement bulbs - I use 4 2'
flourescent bulbs for one set of plants - when I noticed lumen output
varied widely between bulbs. Aquarium/Plant bulbs had 750, Home and
Bath bulbs had 1250. And for the same space, I could get 3 or 4 times
as many lumens with compact flourescents - the circular halo-looking
things. All at 20 W apiece.

More lumens means more light, right? Is spectrum a factor, or for the
same price why wouldn't I just go for the highest lumen count for the
space? What am I missing?

Intensity is certainly one factor, and it becomes important with
plants that demand a lot of light.

However, another factor is photosynthetically active radiation
(PAR), which is light that plants can absorb and use. A good web page on
the whole subject is he

http://www.aquabotanic.com/lightcompare.htm

There's an interesting table about 3/5 of the way down on PUR
efficiency ratio in uE/s/watt that should be kept in mind when comparing
"Gro-Lux" to regular fluorescent tubes. They really aren't worth the extra
expense.

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-AJHicks
Chandler, AZ