What is the benefit of Halogen lamps over incandescent lamps?
As a result of grain boundary etching and the development of porosity due to tungsten evaporation (near the end of life, a cross-section of the filament resembles Swiss cheese) within the filament, the resistance increases during life so the current decreases at constant volts. However, the current decrease is not substantial. Light output over life will depend upon both operating wattage and any bulb blackening caused by tungsten deposits on the bulb wall. The change in electrical resistance in halogen lamps is minimal because the tungsten halogen cycle replaces material in the filament and the higher gas pressure inside the bulb suppresses the evaporation of tungsten. This combined with the bulb cleaning cycle results in halogen lamps maintaining over 90% lumen output and wattage over life.
