what is Halogen cycle

Halogen cycle The main characteristics of an incandescent lamp, namely its luminous efficacy and service life, are determined to a large extent by the filament temperature. The higher the filament temperature, the higher the luminous efficacy but the shorter the life of the lamp. This reduction in the life of the lamp is a consequence of the rate at which tungsten evaporates from the filament. This rate increases rapidly with increasing temperature. Not only does it blacken the inside of the bulb, it causes the filament ultimately to burn out. This blackening of the bulb can be effectively countered by adding halogens to the filler gas to prevent evaporated tungsten from depositing on the bulb wall by a "halogen cycle process". The tungsten that evaporates from the filament during normal operation migrates towards the bulb wall by diffusion or thermal convection In the temperature region <1400>1400 K) where they dissociate back to tungsten and halogen, leaving the halogen to repeat the cycle. Some of the tungsten is transported back to the filament, but not to its original location. The "normal halogen cycle" therefore merely prevents the bulb from blackening, it does not extend the life of the lamp. The lamp comes to the end of its life when the filament melts at one of its hot spots.