The tungsten-halogen lamp is similar to the conventional non-halogen incandescent lamp in that it employs a tungsten filament in a gas-filled, light-transmitting envelope and emits the same type of light. The major differences are that a trace of halogen vapor (e.g., iodine or bromine) is added to the inert fill gas, the gas pressure and bulb temperature are much higher than in non-halogen lamps, and the bulb is made of fused quartz (silica, SiO₂), high-silica glass or aluminosilicate “hard” glass to withstand the high operating pressures and temperatures. The first of these lamps employed fused quartz bulbs and iodine vapor and were thus called “quartz-iodine” lamps. Since other high-temperature bulb materials and other halogens may be employed, the more generic term “tungsten-halogen” is now used.

Tungsten-halogen lamps operate in a “halogen regenerative cycle” which maintains constant light output and color temperature throughout the life of the lamp. The halogen cycle permits the use of more compact bulbs than those of conventional tungsten-filament lamps of equal ratings, and also permits either increasing lamp life to approximately twice that of conventional tungsten filament lamps having comparable wattage and color temperature or increasing color temperatures and lumen outputs to values significantly above those of conventional tungsten filament lamps having comparable life and wattage.
 
The Halogen Cycle
In conventional gas-filled tungsten-filament lamps, tungsten molecules evaporate from the hot filament, are carried by convection currents of the inert fill gas to the relatively cool inner surface of the bulb, and are deposited to form a thin film which gradually increases in thickness during the life of the lamp. These phenomena cause depreciation of lumen output and efficacy in two ways. First, deposition of the evaporated tungsten on the bulb wall builds up a film of increasing opacity which absorbs increasing portions of the light produced by the filament and thus reduces the total light output. Second, evaporation of tungsten from the filament reduces the filament wire diameter, increasing the resistance and thus (at constant voltage) decreasing the amperes, wattage, lumens, lumens per watt, and color temperature.

In tungsten-halogen lamps, the effects described above are reduced or retarded by the regenerative action of the halogen cycle, which operates by virtue of the temperature gradient between the filament and the bulb. As a general concept:

a. The filament, fill gas, and bulb are initially at some low temperature (e.g., ambient, for a cold start).

b. When power is applied, the filament rapidly rises to its operating temperature (2800K to 3400K depending on application), heating the fill gas and the bulb. The bulb wall rises to an operating temperature of 400°C to 1000°C, and the fill gas rises to temperatures ranging from that at the filament to that at the bulb wall. This temperature gradient causes convection currents in the fill gas.

c. As the bulb wall rises above temperatures in the range 200°C to 250°C (depending on nature and amount of halogen vapor), the halogen cycle begins to operate. Tungsten molecules evaporated from the filament combine with the halogen vapor to form a tungsten halide (e.g., tungsten iodide or tungsten bromide). The halide does not condense on the hot wall of the bulb but is circulated by convection back to the region of the filament.

d. At the filament where the temperature exceeds 2500°C, the tungsten halide dissociates, the tungsten is deposited on the filament, and

e. The free halogen vapor is recirculated to continue the regenerative cycle. This cycle thus keeps the bulb wall clean by preventing deposition of tungsten and results in much higher lumen maintenance over the life of the lamp than that obtained for conventional tungsten-filament lamps.

Physical Characteristics

Typical Construction Features
Bulb shapes and sizes. To maintain the high temperatures and pressures required for operation of the halogen cycle, tungsten-halogen lamp bulbs are significantly smaller and have generally thicker walls than the bulbs of non-halogen incandescent lamps of comparable wattage. The bulb shapes are usually tubular (T) and sometimes globular (G). The bulbs are generally designated by letter(s) for the shape and a number giving the maximum diameter in eighths of an inch, as for other lamps. Note that this designation system does not include the bulb length, nor does it tell whether the bulb is single-ended or double-ended.

Bulb materials must be capable of withstanding high operating temperatures (generally between 400°C and 1000°C) and pressures. Fused quartz (silica, SiO₂), with a melting point of 1650°C, is usually used because it can easily operate at wall temperatures which may be as high as 1100°C. For wall operating temperatures up to about 600°C, high-silica glass (96% silica glass, e.g., Vycor® ) is sometimes used, especially for short-life photographic lamps. Aluminosilicate “hard” glass is used in low-voltage tungsten-halogen lamps rated at 80W or less, with wall operating temperatures less than about 400°C.

Atmospheres of tungsten-halogen lamps comprise an inert gas with about 0.1% to 1.0% of a halogen vapor added. The inert gas may be xenon, krypton, argon, or nitrogen, or a mixture (e.g., krypton/argon or argon/nitrogen) having the highest atomic weight consistent with cost as well as arc-resistance suitable to the lamp design and the operating voltage. The halogen vapor may be pure iodine (I₂) or a compound of iodine (e.g., CH₃I) or of bromine (e.g., HBr, CH₃Br, or CH₂Br₂). Iodine is still used in long-life lamps for general illumination but bromine is now used in most tungsten-halogen lamps, especially those for photographic and reprographic applications. The minimum bulb wall temperature for operation of the halogen-cycle is about 200°C for bromine which is significantly lower than the 250°C for iodine. Bromine is also colorless while iodine has a very slight absorption in the yellow-green.

Unlike conventional tungsten-filament lamps which operate with an internal gas pressure of about one atmosphere, most tungsten-halogen lamps operate with an internal gas pressure of several atmospheres to reduce the rate of tungsten evaporation. The combined effects of higher pressure and the halogen cycle give halogen lamps much longer life than that of comparable non-halogen incandescent lamps operating at the same filament temperatures. Conversely, when the two types are designed for equal life, halogen lamps will operate at higher filament temperatures with consequently greater luminance and efficacy.