A halogen lamp, also known as a tungsten halogen, quartz-halogen or quartz iodine lamp, is an incandescent lamp consisting of a tungsten filament sealed into a compact transparent envelope that is filled with a mixture of an inert gas and a small amount of a halogen such as iodine or bromine. The combination of the halogen gas and the tungsten filament produces a halogen cycle chemical reaction which redeposits evaporated tungsten to the filament, increasing its life and maintaining the clarity of the envelope. For this to happen, a halogen lamp must be operated at a higher envelope temperature (250° C; 482° F) than a standard vacuum incandescent lamp of similar power and operating life; this also produces light with higher luminous efficacy and color temperature. The small size of halogen lamps permits their use in compact optical systems for projectors and illumination. The small glass envelope may be enclosed in a much larger outer glass bulb for a bigger package; the outer jacket will be at a much lower and safer temperature, and it also protects the hot bulb from harmful contamination and makes the bulb mechanically more similar to a conventional lamp that it might replace.
Standard and halogen incandescent bulbs are much less efficient than LED and compact fluorescent lamps, and have been banned in many jurisdictions because of this.
Tungsten halogen lamps are used in the majority of theatrical and studio (film and television) fixtures, including Ellipsoidal reflector spotlights, Source Four, and Fresnels. PAR Cans are also predominately tungsten halogen.
Projection lamps are used in motion-picture and slide projectors for homes and small office or school use. The compact size of the halogen lamp permits a reasonable size for portable projectors, although heat-absorbing filters must be placed between the lamp and the film to prevent melting. Halogen lamps are sometimes used for inspection lights and microscope stage illuminators. Halogen lamps were used for early flat-screen LCD backlighting, but other types of lamps are now used.