Tungsten-halogen lamps and regenerative mechanisms

The presence of iodine, bromine or chlorine in a tungsten-halogen lamp provides only an envelope cleaning mechanism; longer filament life and/or higher luminous-efficacy entirely result from the smaller size and physical changes that are permitted in the absence of envelope blackening. Since the introduction of the first practical lamp, patented in 1958, much progress has been made in the technology of tungsten-halogen lamps leading to the development of a wide range of sources. In recent times some attention has been given to the tungsten-fluorine cycle, which provides a self-healing filament mechanism. Experimental lamps employing this principle show up to a 45% increase in life.