3. Tungsten halogen lamps

Halogen lamps are also a type of incandescent lamp.  They are more expensive to buy but last up to 10,000 hours.  They can be either 240V lamps, which are usually tubular and are often used in up lighters and outdoor floodlights or low voltage lamps (typically 12 volt). They are generally bi-pin capsule lamps or MR16 (50mm) dichroic lamps, used in down lighting. 

Some halogen lamps require special light fittings.  Low voltage halogen lamps are slightly more efficient than normal lamps of the same wattage but they use a transformer that can consume from 10% to 30% of lamp energy, reducing the efficiency gain.

Halogen lamps were developed as a result of the drive to improve the efficacy and reduce the size of incandescent lamps. This resulted in a worsening of the blackening of the inner surface of the lamp envelope as a result of evaporation of tungsten from the hot filament. 

In the 1950's it was found that the addition of a small amount of a halogen (a group of chemical elements with certain properties, one of which has a marked tendency to chemically combine with metals) could prevent the blackening.  The five halogen elements are iodine, bromine, chlorine, fluorine and astatine.  Lamps with this addition became known as tungsten halogen lamps.

These lamps are sometimes referred to as quartz halogen lamps, as the envelope is almost always made not from glass, but from quartz, which can better withstand the high temperatures at which these lamps operate (although some lower wattage lamps that do not generate too much heat, use 'hard glass'.  This is a cheaper material which has thermal properties between soda glass and quartz).