Headlights with “H4” two-filament bulb

Fog lamps with halogen technology were available from 1966, but in 1971, Bosch introduced halogen “H4”-lamps with combined high-beam and low-beam bulbs. As with the forerunner double-filament lamps, light was produced using a glowing tungsten wire. However, in halogen lights the glass bulb of the light bulb was filled with a halogen (iodine or bromine). This allowed the filament to reach a temperature close to the melting point of the tungsten wire, thus producing greater and constant light efficiency and increasing the service life of the light.