Gas-discharge lamp
A gas-discharge lamp is an electric light source in which light is produced by an electric discharge in a gas or mixture of gases. Unlike a halogen lamp, the light is not produced by a heated filament.
An electrical voltage is applied between two electrodes. If this voltage is sufficiently high, the gas in the discharge chamber becomes ionised and thus electrically conductive. A gas discharge occurs, during which excited atoms, molecules or ions emit electromagnetic radiation. Depending on the gas mixture used and the design of the lamp, this results in either directly visible light or radiation that is converted into visible light through further processes.
Different types of gas discharge lamps
Gas discharge lamps form a large group of different light sources. These include, for example, neon tubes, fluorescent lamps, as well as mercury vapour, sodium vapour and metal halide lamps.
Xenon lamps or xenon gas-discharge lamps are particularly well known for use in vehicle lighting. They were primarily used in headlights and differ significantly from halogen lamps both in their mode of operation and in their technical requirements.
Operation of gas-discharge lamps
A gas discharge cannot be operated directly and continuously from a voltage source in the same way as a conventional ohmic load. Gas-discharge lamps therefore require a suitable electrical circuit for ignition and for limiting or regulating the lamp current.
Depending on the lamp type, high ignition voltages may be required for this. After ignition, the electrical properties of the gas discharge change, which is why a ballast controls or limits the lamp’s subsequent operation.
In the case of xenon headlights, this task is performed by an electronic ballast.
Properties
Compared with conventional incandescent lamps, gas-discharge lamps can achieve higher luminous efficacy and a longer service life. However, their properties vary greatly depending on the type of lamp. These include, amongst others, luminous flux, colour temperature, colour rendering, start-up behaviour and the required control electronics.
In vehicle lighting, xenon gas-discharge lamps have now been replaced by LEDs in many applications. However, gas-discharge technology is still found in existing vehicles and other technical lighting applications.