Halogen lamp
The small size of halogen lamps permits their use in compact optical systems for projectors and illumination.
[1] Standard and halogen incandescent bulbs are much less efficient than LED and compact fluorescent lamps, and therefore have been or are being phased out in many places.
The presence of the halogen, however, sets up a reversible chemical reaction cycle with this evaporated tungsten.
At moderate temperatures the halogen reacts with the evaporating tungsten, the halide formed being moved around in the inert gas filling.
At some point, however, it will reach higher temperature regions within the bulb where it then dissociates, releasing tungsten back onto the filament and freeing the halogen to repeat the process.
However, the overall bulb envelope temperature must be significantly higher than in conventional incandescent lamps for this reaction to succeed: it is only at temperatures of above 250 °C (482 °F)[11] on the inside of the glass envelope that the halogen vapor can combine with the tungsten and return it to the filament rather than the tungsten becoming deposited on the glass.
For a fixed power and life, the luminous efficacy of all incandescent lamps is greatest at a particular design voltage.
Halogen lamps made for 12 to 24 volt operation have good light outputs, and the very compact filaments are particularly beneficial for optical control (see picture).
The ranges of multifaceted reflector "MR" lamps of 20–50 watts were originally conceived for the projection of 8 mm film, but are now widely used for display lighting and in the home.
Undoped quartz halogen lamps are used in some scientific, medical, and dental instruments as a UV-B source.
Their small size helps to concentrate the heat on a smaller envelope surface, closer to the filament than a non-halogen incandescent.
Because of the very high temperatures, halogen lamps can pose fire and burn hazards.
[27] Halogen Torchère floor lamps have been banned in some places, such as dormitories, because of the large numbers of fires they have caused.
[28] Halogen bulbs operate at high temperatures and the tall height of the lamps can bring them near flammable materials, such as curtains.
[29] Some safety codes require halogen bulbs to be protected by a grid or grille, especially for high-power (1–2 kW) bulbs used in theatre, or by the glass and metal housing of the fixture, to prevent ignition of draperies or flammable objects in contact with the lamp.
[citation needed] Any surface contamination, notably the oil from human fingertips, can damage the quartz envelope when it is heated.
Contaminants, because they absorb more light and heat than the glass does, will create a hot spot on the bulb surface when the lamp is turned on.
[Note 1] The designation MR means "Multifaceted Reflector", with the number following this still corresponding to eighths of an inch in diameter of the overall bulb.
Low wattage halogen lamp arrays are widely used by monitor lizard keepers.
Halogen lamps' thick glass lenses are safe to use inside high humidity reptile enclosures.
Banks of powerful tubular halogen lamps were used to simulate the heat of re-entry of space vehicles.
Round spotlights with built-in multifaceted reflector lamps are widely used in residential and commercial lighting.
Tungsten halogen lamps are used in the majority of theatrical and studio (film and television) fixtures, including Ellipsoidal reflector spotlights, Source Four, and Fresnels.
