Step 1: Tungsten atoms evaporate from the hot filament and move toward the cooler
wall of the bulb.
Step 2: Tungsten, oxygen and halogen atoms combine at the bulb-wall to form tungsten oxyhalide molecules.
Step 3: The bulb-wall temperature keeps the tungsten oxyhalide molecules in a vapor. The molecules
move toward the hot filament where the higher temperature breaks them apart.
Step 4: Tungsten atoms are re-deposited on the cooler regions of the filamentnot in the
exact places from which they evaporated. Breaks usually occur near the connections between the tungsten filament
and its molybdenum lead-in wires where the temperature drops sharply.
For more information about tungsten halogen lamps see Webnote