Editing Halogen lamp (section)

==Halogen cycle==
In ordinary incandescent lamps, evaporated tungsten mostly deposits onto the inner surface of the bulb, causing the bulb to blacken and the filament to grow increasingly weak until it eventually breaks. The presence of the halogen, however, sets up a reversible chemical reaction cycle with this evaporated tungsten. The halogen cycle keeps the bulb clean and causes the light output to remain almost constant throughout the bulb's life. At moderate temperatures the halogen reacts with the evaporating tungsten, the [[Tungsten(V) bromide|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 [[Dissociation (chemistry)|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 {{convert|250|C|F}}<ref name=":0">{{cite web |url=https://ntrs.nasa.gov/search.jsp?R=19910010012 |title=NASA Technical Reports Server (NTRS) - Vibration and thermal vacuum qualification test results for a low-voltage tungsten-halogen light |newspaper=Https |date=1 February 1991 |author=Sexton, J. Andrew|access-date= 19 January 2019}}</ref> 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.<ref name="Wolke2009">{{cite book|author=Robert Wolke|title=What Einstein Told His Barber: More Scientific Answers to Everyday Questions|url=https://books.google.com/books?id=AVf4AAAAQBAJ&pg=PA52|date=29 July 2009|publisher=Random House|isbn=978-0-307-56847-2|page=52}}</ref> A 300 watt tubular halogen bulb operated at full power quickly reaches a temperature of about {{convert|540|C|F}}, while a 500 watt regular incandescent bulb operates at only {{convert|180|C|F}} and a 75 watt regular incandescent at only {{convert|130|C|F}}.<ref>{{cite web|title=Torchiere Halogen Lamps and Plastic Shades - Policies and Procedures|url=https://www.colorado.edu/firelifesafety/sites/default/files/attached-files/halogen_lamps.pdf|archive-url=https://web.archive.org/web/20121021153315/http://www.colorado.edu/firelifesafety/sites/default/files/attached-files/halogen_lamps.pdf|url-status=dead|archive-date=21 October 2012|publisher=University of Colorado Boulder|author=Fire and Life-Safety Group}}</ref>

The bulb must be made of [[Quartz glass|fused silica (quartz)]] or a high-melting-point glass (such as [[Aluminosilicate#Aluminosilicate glasses|aluminosilicate glass]]). Since quartz is very strong, the gas pressure can be higher,<ref>Some lamps have as much as 15 times atmospheric pressure when cold, and some lamps increase pressure five-fold at [[operating temperature]]. Kane and Sell 2001, page 76–77</ref> which reduces the rate of evaporation of the filament, permitting it to run a higher temperature (and so [[luminous efficacy]]) for the same average life. The tungsten released in hotter regions does not generally redeposit where it came from, so the hotter parts of the filament eventually thin out and fail.

Quartz iodine lamps, using elemental iodine, were the first commercial halogen lamps launched by GE in 1959.<ref>Zubler and Mosby Illuminating Engineering 1959 54.734</ref><ref>{{cite web |url=http://home.frognet.net/~ejcov/newhalogen.html |title=The Tungsten-Halogen Lamp |first=Edward J. |last=Covington |access-date=2016-03-04 |url-status=dead |archive-url=https://web.archive.org/web/20160305025647/http://home.frognet.net/~ejcov/newhalogen.html |archive-date=5 March 2016 |df=dmy-all }}</ref> Quite soon, bromine was found to have advantages, but was not used in elemental form. Certain hydrocarbon bromine compounds gave good results.<ref name="autogenerated1" /><ref>T'Jampens and van der Weijer Philips Technical Review 1966 27.173</ref> Regeneration of the filament is also possible with fluorine, but its chemical reactivity is so great that other parts of the lamp are attacked.<ref name="autogenerated1">Burgin and Edwards Lighting Research and Technology 1970 2.2. 95–108</ref><ref>Schroder Philips Technical Review 1965 26.116</ref> The halogen is normally mixed with a [[noble gas]], often [[krypton]] or [[xenon]].<ref name=ullmann>{{cite book |author1=Häussinger, Peter |author2=Glatthaar, Reinhard |author3=Rhode, Wilhelm |author4=Kick, Helmut |author5=Benkmann, Christian |author6=Weber, Josef |author7=Wunschel, Hans-Jörg |author8=Stenke, Viktor |author9=Leicht, Edith |author10=Stenger, Hermann |chapter=Noble gases |title=Ullmann's Encyclopedia of Industrial Chemistry |publisher=Wiley |year=2002 |doi=10.1002/14356007.a17_485|isbn=3527306730 }}</ref> The first lamps used only tungsten for filament supports, but some designs use [[molybdenum]] – an example being the molybdenum shield in the H4 twin filament [[headlight]] for the European Asymmetric Passing Beam.

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. More recently, wider beam versions have become available designed for direct use on supply voltages of 120 or {{nowrap|230 V}}.