Halogen cycle: long service life, no bulb blackening

The same as classic incandescent lamps, halogen lamps are radiators of temperature. However, by contrast, halogen lamps do not show the typical bulb blackening that incandescent lamps do. The reason for this is the halogen cycle, due to which the bulb remains clear throughout the entire service life of the bulb. Therefore, the halogen lamp always emits a constant light flux, achieving a longer service life than the classic incandescent lamp. To do this, traces of halogens are required in the filling gas of the halogen lamp.

Definition: temperature radiator

Temperature radiators such as standard incandescent lamps and halogen incandescent lamps create light through current passing through a tungsten wire. This tungsten wire can be heated up to temperatures of over 3000° Celsius. In so doing, up to ten percent of the energy applied is converted into visible light.

Black tungsten particles flow from the incandescent coil to the cooler bulb edge.

Bulb blackening of standard incandescent lamp

When the lamp is on, tungsten molecules evaporate from the glowing tungsten wire. These are then deposited on the cooler inner wall of the bulb. The result is the typical bulb blackening on standard incandescent lamps of increasing age. Part of the generated light is retained by the bulb blackening. Thus the amount of light emitted by an incandescent lamp falls over time. Classic incandescent lamps thus have a large bulb in order to keep the light loss as low as possible. This means that the tungsten particles can be spread over a larger area and the quantity of deposited tungsten molecules stays low on each surface unit.

Halogen in filling gas prevents the tungsten atoms from being deposited on the glass of the bulb.

Halogen supplement prevents bulb blackening

In contrast to a standard incandescent lamp, in a halogen lamp, the glass always stays clear. This is ensured by tiny quantities of a halogen in their filling gas, such as iodine or bromine. The halogens may not be able to prevent the tungsten atoms from evaporating from the hot coil, but ensure that no tungsten atoms are deposited on the inside of the bulb. The next image shows just how this works.

Halogens catch the tungsten particles and bring them back to the incandescent coil.

Halogen and tungsten combine to form tungsten halogenide

Before the evaporating tungsten particles can reach the inner side of the bulb, the tungsten and halogen molecules combine to form tungsten halogenides. These gaseous tungsten halogenides do not form a coating on the bulb, but due to the thermal convection, move freely in the bulb until they reach the incandescent coil again.

Tungsten particles stay on the coil and the halogens flow out again.

Continuous halogen cycle

Due to the high temperature, the tungsten halogenides split back into the halogen and tungsten upon reaching the coil. The tungsten particles are not redeposited on the hot coil again but on the cooler parts of the coil, such as the "coil leg". Then the halogens are available again for the halogen cycle. This means that the tungsten atoms have no opportunity to be deposited on the inside of the glass bulb, turning it black. And so even the smallest halogen bulb will always remain clear. The result is that the unavoidable reduction in light flux, as is seen on standard incandescent bulbs, is completely avoided throughout the service life.


The halogen cycle means that halogen lamps can be considerably smaller in size than standard incandescent lamps. This allows more expensive materials to be used for bulbs and the filling gas, improving the lamp quality. The result is a considerable increase in light quality and service life. All the halogen lamps stand out through their constant light flux and a longer service life.