Here's a very brief rundown on lamp types for the curious:
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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 light up).
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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.
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Fluorescent lamps use low-pressure mercury, which emits primarily up in the ultraviolet, and (except for blacklight and germicidal lamps) fluorescent's are coated internally with phosphors that glow under ultraviolet. The visible light they produce is almost all from these phosphors. Some types of fluorescent's make excellent light sources for aquariums.
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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.
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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.
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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.
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High-pressure sodium lights are very widely used in the United States for street-lighting. If it is bright, kind of yellow with purple 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 tanks (but I don't think "someday" is today).
WHEN WIRING VHO BULBS
Use only the 3 piece German end caps rated for VHO's. The slip on type caps are notorious for slipping off and on due to warming and cooling of the bulb and are not rated for the heat of VHO's. When installing lighting, it is best to use what they call "stand-offs" that are attached to the bottom of the end cap and these are screwed directly to your canopy. Ends caps can't slip off when these are installed properly. If you're using clips to hold your bulbs, have someone put pressure against the end of the bulb so that you tighten caps correctly. Old style end caps have 8 positions for adjusting the position of the light bulb. 2 of the positions are such that the pins align with the slot used to install the bulb. This is a poor contact position for the pins and a definite design flaw. Also remember that Ice Cap and Workhorse are different types of ballasts. Ice Cap uses specific circuitry and wiring patterns. Both pins on the end of the bulb require contact to function. Workhorse uses a common for all bulbs and individual power leads. Only 1 pin on each end of the bulb is needed to light the bulb.