Tungsten Halogen Lamp – Working Principle, Spectrum & Construction
A Tungsten Halogen lamp also known as Halogen Lamp is an incandescent light source. It consists of a tungsten filament, enclosed in an environment of an inert gas and a small amount of a halogen (bromine or iodine). The combination of the tungsten filament and the halogen results in a chemical reaction called the halogen cycle, which increases the lifetime of the filament.
Due to high temperature tungsten filament gets evaporated during working & also due to conventional flow of gas inside the bulb, the evaporated tungsten is transported away from the filament. The wall of the bulb is relatively cool. Hence the evaporated tungsten then is adhered to the inner bulb wall. This is not the case when halogen like iodine is used in the bulb container.
The temperature of the filament of the halogen lamp is maintained at about 3300K. Hence here also tungsten will be evaporated from lamp filament. Due to conventional flow of gas inside the bulb, the evaporated tungsten atoms are transported away from the filament to relatively lower temperature zone where they combine with the iodine vapor and form the tungsten iodide. The temperature required for combination of tungsten and iodine is 2000K.
Then the same convectional flow of gas inside the bulb carries the tungsten iodide to the wall of relatively lower temperature. But the bulb is so designed that the temperature of the glass wall remains between 500K and 1500K and at that temperature tungsten iodide does not adhere to the bulb wall. It goes back to towards the filament due to same convectional flow of gas inside the bulb. Again, at the close vicinity of the filament where temperature is more than 2800K, the tungsten iodide gets broken into the tungsten and iodine vapor. Because this is the required temperature for breaking tungsten iodide into tungsten and iodine atoms is >2800K.
Then these tungsten atoms further proceed and get re-deposited on the filament to compensate previously vaporized tungsten. After that they again get evaporated due to high filament temperature and become free to acquire iodine to form iodide. This cycle repeats again and again. Hence the filament does not get evaporated permanently so temperature of the filament can be maintain at very high level compared to normal incandescent lamp which makes it more efficient i.e. more lumen/watt rating. As there is no permanent evaporation of filament, the lifespan of the Tungsten Halogen Lamps gets much longer with clarity of illumination.
The spectral output of halogen lamps is continuous, and is similar to that of a blackbody radiator. A major portion (up to 85%) of the emitted light lies in the infrared and near infrared regions; the rest (15-20%) lies in the visible region, and less than 1% of the light falls in the ultraviolet region.
Quartz is widely used to make halogen bulb glass. Quartz is transparent silica and pure silicon dioxide. It is very stronger and it withstands higher temperature as compared to the borosilicate or alumina silicate glass. Quartz bulb can be soft material above 1900K. Again around the filament 2800K must be maintained to get continuous halogen cycle. So the distance between the filament and quartz bulb wall must be maintained in such a way that the quartz bulb wall gets temperature below 1900K. The bulb wall should be stronger and smaller in volume such that the lamp can be operated at the inner pressure of several atmospheres. Again higher pressure inside the bulb reduces the rate of evaporation of the tungsten filament. A certain amount of nitrogen and argon are mixed in addition to the halogen gas inside the bulb to maintain this higher gas pressure inside. Thus the lamp can be operated at the higher temperature and with higher luminous efficacy for long time. Most of the lamps in present days are with bromine instead of iodine.