162. If you have halogen headlights on your car, then you employ the iodide process every time you drive at night. The white hot tungsten filament reacts with iodine vapor in the bulb, becoming tungsten tetaiodide, which plates pure tungsten back on the filament. The tight, closed-loop iodine cycle keeps your tungsten filament from evaporating at the very high temperature. That, in a nutshell, is the crystal bar process.