Electric Lamps (a brief history since Edison)
Here's a very brief rundown on lamp types for the curious:
Electric lamps can be divided into incandescent (conventional and
quartz-halogen) and discharge. Incandescent lamps (of either variety) put
most of their energy out in the infrared, and so are of little interest
here (think of them as tiny expensive electric space heaters that happen to
Discharge lamps include fluorescent, mercury, metal-halide, low-pressure
sodium, and high-pressure sodium. They all produce light by passing a
current through a gas, exciting the odd electron here and there. The
electrons emit light as the return to their ground state, and this directly
or indirectly is the source of the illumination.
Fluorescent lamps use low-pressure mercury, which emits primarily up in the
ultraviolet, and (except for blacklight and germicidal lamps) fluorescents
are coated internally with phosphors that glow under ultraviolet. The
visible light they produce is almost all from these phosphors. Some types
of fluorescents make excellent light sources for planted tanks.
Mercury lamps increase the pressure, which shifts more of the direct output
into blue and green emission lines (these can be supplemented with
phosphors to produce better color rendering); these are the blue-green
lights you see illuminating parking lots and the like. These are useless
as grow-lamps; neither you nor your plants' chloroplasts would be thrilled.
Metal-halide lamps introduce metal halides into the mercury lamp; this adds
all sorts of nice emission lines to the spectrum, and greatly improves
color rendition. These, too, make fine light sources for plants (and
people looking at plants).
Low-pressure sodium lamps are not seen much, at least in the United States.
They are the most efficient light source, because they put almost all
their energy into one yellow-orange emission line (and the human eye is
pretty sensitive there). Unfortunately, you can't see colors under this
illumination; everything looks like shades of grey illuminated by a fairly
ghastly yellow light. I have no idea off-hand what the average chloroplast
would think, but you certainly wouldn't like looking at plants under this
light, so the point is moot.
High-pressure sodium lights are very widely used in the United States for
street-lighting. If it is bright, kind of yellow with purply overtones,
and you can pretty much distinguish color under it, it is high-pressure
sodium. GE, at least, has some "white" sodium lamps that actually render
colors fairly well (but nowhere near as well as MH or fluorescent), so
perhaps someday these will make reasonable light sources for planted tanks
(but I don't think "someday" is today).
Right now, compact (biaxial) fluorescents on electronic ballasts can be
both more efficient and better at rendering colors than metal-halide lamps
(in the wattages used to light tanks, at least; I haven't bothered to try
to calculate what you'd need in fluorescent lamps to equal MH lamps over
300 watts). Metal-halide lamps are more compact than fluorescents, though.
As several people have pointed out, you can buy all of this much more
inexpensively from a lighting distributor than from a pet store.
Incidentally, if you decide to go with MH lamps, I would suggest choosing
lamps designed for the particular burning position of your fixtures; these
have very significant efficiency and lifespan advantages over "universal"
lamps. You can get horizontal, vertical base-up, and even vertical
base-down lamps. Be sure, too, that your fixtures are enclosed, so that
arc-tube or envelope ruptures will neither shower your tank with hot
fragments, nor expose you and your tank to short-wave UV (some of the MH
lamp failure modes are a bit dramatic, but a good fixture will be designed
to contain the disaster).
If there's interest, I can supply a more detailed summary of the different
lamp types (fluorescents, in particular, come in a zillion or two different
configurations). Perhaps, though, this has already been done (I didn't see
it in the Krib, but then I'm notorious for missing the obvious).
Incidentally, if you are reading this on a CRT, you might like to consider
the impact ambient light has on that activity, too. Discharge lamps on a
conventional ballast are horrible illumination (they flicker at almost the
same rate as your CRT, most likely), but also extremely common (especially
if you are reading this at work, you naughty person you). On the other
hand, compact biaxial fluorescents on an electronic ballast are pretty
good; they don't flicker much at all, and what fluctuation they have
probably won't interact noticeably with your CRT's flicker. You can even
find them (complete with fancy phosphor and electronic ballast) at
supermarkets sometimes (GE sells some designed to replace incandescent
lamps; they are slightly larger than some of the incandescents they
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