Growing under lights

Tim Eck
Sun, 23 Feb 2014 13:25:27 PST
Sodium lights draw a very large amount of electricty, require a special
transformer, and are expensive to buy and run. They heat the room they
illuminate much more than do fluorescent lights of any design; while perhaps
advantageous for some plants, this is not desirable for winter-growing
plants. Sodium bulbs also diminish in light output over time and must be
replaced regularly.

I was going to let this go, but most of this paragraph is just plain false.
Sodium vapor lights put out far more usable light per watt than fluorescents
and less heat per watt than fluorescents (as a corollary), although they do
burn hotter than fluorescents.  Needless to say, they heat the room less
than fluorescents for the same amount of useable light.
Fluorescent tubes also diminish in light output over time as does every
lighting fixture I know of.
Fluorescents also require a special transformer that goes by the same name
(ballast) as those for sodium vapor lights.  Both can be driven by
"electronic" current limiting devices instead of ballasts, saving some
wasted heat.  The reason they use ballasts is they are plasmas and will
exhibit thermal runaway (as will mercury vapors, metal halides and LEDs) in
the absence of current limiting devices .  Tungsten and tungsten halogen
both have negative feedback (increased resistance) with heat, requiring no
ballast,  and both are black body resistance emitters rather than emitting
in special wavelengths.  Tungsten halogen is more efficient than tungsten
because it can burn much hotter since a halogen gas mixture scrubs the
emitted tungsten off the bulb surface and re-deposits it on the hottest part
of the filament, building up the thin parts of the filament and extending
its lifetime. As this is a finely engineered phenomenon, they should never
be used with a dimmer (as is true with the ballast type lamps).
I was going to replenish my fluorescents when I saw 70 watt sodium vapors at
Lowes for $42 each and there was no way I could match that economy with good
fluorescent grow lights.  

More information about the pbs mailing list