Halogen Lamp - Halogen Cycle

Halogen Cycle

In ordinary incandescent lamps, evaporated tungsten mostly deposits on the bulb. The halogen sets up a reversible chemical reaction cycle with the tungsten evaporated from the filament. The halogen cycle keeps the bulb clean and the light output remains almost constant throughout life. At moderate temperatures the halogen reacts with the evaporating tungsten, the halide formed being moved around in the inert gas filling. At some time it will reach higher temperature regions, where it dissociates, releasing tungsten and freeing the halogen to repeat the process. The overall bulb envelope temperature must be higher than in conventional incandescent lamps for the reaction to work.

The bulb must be made of fused silica (quartz) or a high-melting-point glass (such as aluminosilicate glass). Since quartz is very strong, the gas pressure can be higher, 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. Regeneration of the filament is also possible with fluorine, but its chemical activity is so great that other parts of the lamp are attacked.

It is possible to see a live demonstration of the Tungsten-Halogen cycle in this accelerated video. The lamp is equipped with an open tube that permits the halogen gas to be withdrawn and re-introduced as desired. When switched on, the filament is operating in a vacuum. After a few seconds the bulb is observed to blacken; this is caused by tungsten atoms that evaporate from the filament and condense on the bulb wall. Once completely blackened, the halogen gas is re-introduced back into the bulb. It quickly begins to react with the tungsten that has been deposited on the relatively cold bulb wall, and transports it back to the hot filament. The result is that the wall is returned to its original clarity. In this experiment the concentration of halogen gas used is higher than normal so as to achieve the rapid clean-up. In a standard lamp, the speed of the halogen regenerative cycle is much slower, but it operates continuously to prevent the bulb from blackening and thus maintaining a constant light output during lamp life.

Quartz iodine lamps, using elemental iodine, were the first commercial halogen lamps launched by GE in 1959. Quite soon, bromine was found to have advantages, but was not used in elemental form. Certain hydrocarbon bromine compounds gave good results. 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.

High temperature filaments emit some energy in the UV region. Small amounts of other elements can be mixed into the quartz, so that the doped quartz (or selective optical coating) blocks harmful UV radiation. Hard glass blocks UV and has been used extensively for the bulbs of car headlights. Alternatively, the halogen lamp can be mounted inside an outer bulb, similar to an ordinary incandescent lamp, which also reduces the risks from the high bulb temperature. Undoped quartz halogen lamps are used in some scientific, medical and dental instruments as a UV-B source.

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 range of MR-16 (50 mm diameter) reflector lamps of 20 W to 50 W 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 are available designed for direct use on supply voltages of 120 or 230 V.