Tungsten halogen lamps are basically incandescent lamps that operate at higher pressure and temperature than standard incandescent lamps, producing a whiter light and longer life. The "tungsten" part of the name comes from the filament material - tungsten. The halogen part refers to the type of chemical additive that bestows the lamp with its special properties.
Another way halogen is an improvement over standard incandescent is in light output. Incandescent lamps have a tendency as they burn to slowly deposit their allotment of tungsten onto the glass bulb, turning the bulb black. This black coating blocks some of the light from coming out of the bulb. Eventually, the filament gets thinner, breaks, and the lamp fails.
The halogen gas in the lamp significantly reduces the deposition of evaporated tungsten onto the glass bulb. No bulb blackening = more light!
The most energy efficient halogen lamp is one with an infrared reflective (IR) coating. The coating redirects the infrared energy back onto the filament, while letting the visible energy pass through the coating. The redirected (or recycled) infrared energy improves the output to approximately 82% infrared heat and 18% visible light - instead of the 90-10 heat to light ratio of a standard lamp.
The Halogen Cycle
1Tungsten atoms evaporate from filament.
2Tungsten atoms combine with halogen atoms.
3The resulting compound returns to the filament, redepositing tungsten.
4Halogen atoms are released to recombine with other tungsten atoms.
There are two general families of halogen lamps: line voltage and low voltage. Line voltage simply means the voltage available at the wall outlet in your home - 120 volts in North America. Europe and much of the rest of the world uses 220 volts. Low voltage usually means 12 volts (and that's the same all over the world).
Here are some examples of line voltage halogen lamps:
Line Voltage halogen lamps tend to be mostly PAR lamps, but can also be tubular-shaped.
Low voltage halogen lamps have shorter, fatter filaments compared to line voltage lamps. Due to this, the lamps can be smaller in size and the light more precisely aimed.