Now we're getting serious

HID or High Intensity Discharge are the big bright lamps you see in grocery stores, street lighting and industrial lighting. They can be very large and draw a lot of power. Indeed 2000 watt and 6000 watt lamps exist, however small ones, down to 70 watts are available.

Tradeoffs

These lamps produce a lot of light output quite efficiently, however they can be quite expensive to install initially and may require a fan for cooling in the housing/reflector as they can produce phenomenal amounts of heat. These lamps are used by aquarists who need lots of light, such as marine reef tanks, of large freshwater plant tanks.

HID lamps requite a ballast, and almost every bulb requires it's own type of ballast. The ballasts are expensive and bulky and are not something you trot on down to the corner hardware store to pick up, although larger hardware stores may have some; they are usually reasonably priced. You'll have to go to a lighting supplier for most of them however.

HID lamps are built like halogen bulbs. A small capsule contains the vapor that an arc is sent through. This capsule is in turn encased in the much larger outer bulb body. There is quite a bit of UV generated by the inner capsule that is filtered by the outer capsule. All these bulbs carry warnings not to operate them if the outer capsule is broken.

Types

There are three basic types of HID lamps: mercury vapor, sodium vapor and metal halide.

Mercury vapor

When you see a bright light illuminating some industrial building and it has a decided bluish cast - that's mercury vapor. Mercury vapor lamps have an output spectra that is almost entirely blue-white, with very little red. Worse, the spectra is not continuous, there are spectral peaks at certain wavelengths. These lamps, although not useless - there is no doubt very good results can be obtained with them - are equivalent to cool white fluorescents. Yes they work, but why bother going to this expense and trouble when other bulbs will yield much greater success?

One interesting variation on this theme is the self ballasted bulb. These bulbs (around 250 watts) require no ballast, they just screw into a standard medium base (ie. incandescent) fixture and voila, light. The downside is these bulbs are not as efficient as regular mercury vapor lamps because they use the resistive properties of the large filaments as a ballast, and worse of all these bulbs are very expensive, around $100 plus or minus $30. Of course with mercury vapor lamps having a 10,000 hour lifespan the high cost of the bulb must be considered in view of the lack of expense for a ballast.

Sodium vapor lamps

These lamps come in two varieties, high pressure sodium and low pressure sodium, although this is rather a moot point, as the light they output is monochromatic (pure) yellow, and is all but useless in terms of aquaria. It's rather a shame, as they are a full ten times more efficient then incandescent bulbs, in fact these are the most efficient bulbs made, and have a 24,000+ hour lifespan. These are one of the cheapest HID bulbs to purchase, and can be found in most hardware stores for around $80 for bulb and ballast. Spare bulbs are around $30. Recent advances in high pressure sodium bulbs such as the Philips "Sun Agro" have improved output spectra, and are quite popular for terrestrial plants, although they haven't as yet gained great acceptance with aquatic gardeners.

Metal Halide

Like sodium vapor, these lamps come in two versions, regular and color corrected (HQI) versions. The HQI versions have a uniform, sunlight like output spectra, whereas the standard halide bulb has a lot of yellow, some blue and not much red. Unlike sodium vapor, these lamps are very useful to the aquarist needing a lot of light. They can be found nominally in 250, 400, and 1000 watt sizes, from most manufacturers, but Osram also makes a 70 watt and a 150 watt size. The 70 watt bulb is only 2 x 3 inches, although is unfortunately a 3000K color temperature bulb. You have to go to a 250 watt bulb to get 5400K color temperature.

These bulbs range in life from 6000 to 10,000 hours. Bulbs are around $50, ballasts are around $100.