In shortwave infrared lamps, what is the TUNGSTEN HALOGEN CYCLE?
Tungsten halogen cycle lamps (sometimes called emitters) have tungsten filaments.
Are halogen gas filled, and require high temperature quartz glass tubes. These lamps fall into the shortwave segment of the light spectrum
Tungsten halogen cycle lamps exhibit unique traits over standard Incandescent bulbs.
Directly summed up; glass remains clear (doesn’t darken) and filament life is doubled.
This means that the lumen output over the life of the lamp remains high (brighter) so that the Lumens Per Watt (efficiency) remains high and the color of light remains and stays whiter.
Lamps lasting longer is an obvious benefit.
Halogen is the main family element name on the periodic table that includes iodine, bromine, chlorine, fluorine and astatine. These days, bromine is the most commonly used halogen element in Tungsten halogen cycle lamps.
The Tungsten halogen cycle is as follows…
Tungsten metal filament is heated by electricity to point of incandescence (emitting photons).A few tungsten particles evaporate (boil off) from the filament and are carried away by convection current where they combine with bromide vapor forming tungsten bromide. If the quartz wall is above 250 degrees centigrade bromide particles will not adhere to it. They continue to circulate in the hot gas envelope by convection current and when they come back close to the hot filament the particles reduce back to tungsten metal which is randomly re-deposited back onto the filament thereby releasing the bromide vapor, and the whole process then repeats itself.
If every tungsten particles happened by chance to land back exactly where it came off, the filament could last forever, but they do not, and eventually some parts of the filament get thinner and eventually burn out,
For the Tungsten halogen cycle to work, the lamp has to be on long enough to heat up completely and keep the quartz at over 250 degree centigrade. To make this happen, manufactures design their lamps positioning high temperature quartz glass as close to the filament as possible (but far enough away so that the quartz doesn’t melt) and designing the entire gas envelope so that the coolest area’s are at least 250 degrees centigrade. Any area inside the quartz envelope which is lower than 250 degrees will attract tungsten bromide particles and the tungsten molecule will revert and cling tightly to the cool areas on the quartz and cool internal supports thereby causing the Tungsten halogen cycle to fail.
Obviously short cycling the lamp or using a dimmer (running the lamp at lower than design voltage) will keep the lamp from fully heating up causing the cycle to fail.