Metal Halide/Halogen bulbs
Thanks for all of the comments.
I wonder if we are not actually in agreement? I used the term "Metal Halide"
instead of Halogen, or Tungsten Halogen, _knowing they are different things
entirely._ MV bulbs are Mercury Vapor Arc bulbs, Low Pressure Sodium are
Sodium Vapor Arc Bulbs, which is the reason for the super narrow spectrum.
Two Bright Yellow lines, quite close together on the Spectrum, from the
excited state of Sodium. Of course, the ABSORPTION spectrum is just the
opposite, namely the same two lines are ABSORBED by the unexcited vapor.
That is why astronomers really don't mind Low Pressure Sodium - it is really
easy to filter out when doing those 2-hour time exposures they are wont to
do. And yes, the ballasts are VERY different. Also, the Arc type bulbs take
a while to start up, the Filament types go on instantaneously. That is a
pretty good clue as to what is going on in the reflector.
The term, "Metal Halide" has been used quite commonly on the List, and I
presumed (Fatal presumption! :-( ) that the term was generally accepted. I
didn't want to make things more confusing, which, it turns out, was
_guaranteed_ to make things more confusing. Sorry. I will perform
appropriate penance. :-(
Thus far all of the "metal halide" bulbs I have personally seen installed
over aquariums, both fresh and salt water, were actually "Tungsten Halogen"
bulbs. In fact, I just got out the box for one of the beasts, and it is
CALLED "Metal Halide," which of course, it is NOT. My desk lamp by the
computer came in a box labeled "Metal Halide" bulb, and I can promise you
that it really is a "Tungsten Halogen" bulb. Flashlight bulbs are also
commonly labeled as if they were true Metal Halide bulbs, which they are not.
"Krypton" is a common label, which means the rare gas Krypton is used to
"preserve" the tungsten filament.
Could some of you take a look inside those parabolic reflectors and see what
kind of bulbs are actually installed? I have a really BIG curiosity bump
about this. Are there some true ARC bulbs out there for growing plants?
Brand names and such would sure help. One cannot rely on advertisements to
be accurate. A wide disparity between the facts as known by the Engineering
Section and the Ad from the Advertising Department is not at all uncommon!