Halogen lamp
A halogen lamp is a special type of incandescent lamp. The light is produced by an electrically heated tungsten filament. The flow of current heats the filament to such a high temperature that it emits visible light. Unlike a conventional incandescent lamp, the gas filling also contains a small amount of a halogen, such as bromine or iodine. This enables the so-called halogen cycle.
The halogen cycle
During operation, a small amount of tungsten continuously evaporates from the filament due to the high temperature. In a conventional incandescent lamp, this tungsten can be deposited on the inside of the lamp bulb, leading to increasing blackening.
In a halogen lamp, under suitable conditions, the evaporated tungsten reacts with the halogen. The resulting compounds return to the hot region near the filament, where they decompose. This causes tungsten to be deposited once more, and the halogen becomes available for the cycle again.
This cyclic process reduces the blackening of the lamp bulb and allows the filament to operate at higher temperatures. As a result, halogen lamps can achieve higher luminous efficacy and a higher colour temperature than traditional incandescent lamps.
However, the process does not completely restore the filament. Tungsten is not necessarily deposited exactly at the points where it previously evaporated. Consequently, even a halogen filament becomes thinner during operation and may eventually burn through. Nevertheless, a halogen lamp generally has a significantly longer lifespan than a conventional incandescent bulb.
Halogen lamps in vehicle headlamps
Halogen bulbs have been used for decades as light sources in headlights. Different standardised types, such as H1, H4 or H7, differ in terms of, amongst other things, their base, electrical design and the number or arrangement of their filaments.
In an H4 bulb, for example, two filaments for dipped and high beam are housed within a single bulb. An H7 bulb, on the other hand, has a single filament and is used for a single lighting function.
In older, appropriately approved headlamp systems, markings such as HC, HR or HCR indicate halogen light sources for dipped or high beam, respectively. Further information on these markings can be found under incidcations on lights.
Halogen and LED
Halogen bulbs produce light through the high temperature of a filament. LEDs, on the other hand, are based on electroluminescence in a semiconductor. The two technologies therefore differ fundamentally in terms of how they work.
In particular, LEDs enable higher luminous efficacy, compact light sources and other possibilities in terms of design and light distribution. Nevertheless, halogen lamps are still found in numerous vehicles and applications.