Gas discharge lamps and metal-halide lamps
The terms "gas discharge lamp" and "metal-halide lamp" have a basic meaning. In discharge lamps, light is produced by passing electricity through an ionised gas or metal vapour contained in a pressurised, sealed vessel. This results in the release of energy in the form of directly visible light or ultraviolet light. In the case of ultraviolet light, this is converted into directly visible light by a luminescent substance applied to the inside of the illuminant. This functional principle gives gas discharge lamps and metal-halide lamps their names.
Discharge lamps with ballast unit
Discharge lamps usually consist of a discharge vessel, often tubular, made of glass, quartz glass or aluminium oxide ceramic. Inside the vessel, an electrical field builds up between two electrodes, where the gas discharge finally occurs. However, discharge lamps require current limiting, as they would otherwise be damaged by unregulated energy consumption. This function is usually performed by an electronic ballast unit that is integrated in the lamp. The discharge vessel usually contains a gas, gas mixture or vapours of metals. These can be noble gases such as xenon, krypton or neon, plasma-forming substances such as the vapours of sodium and mercury, or mixtures of halogens and metals, as well as other gases. The substances vary depending on the model.
Discharge lamps generate radiation by means of an electric arc
Discharge lamps produce radiation by means of an electric arc between two electrodes. This radiation either emerges directly from the lamp as visible light (e.g., in metal-halide lamps) or is converted from UV radiation to visible light by a phosphor layer on the bulb wall (e.g., in fluorescent lamps). The operating pressure in the discharge tube is either low (low-pressure discharge lamps) or high (high-pressure discharge lamps). High-pressure discharge lamps are particularly energy-saving: small and compact in design, they deliver extremely large amounts of light and have a long service life. Metal-halide lamps and sodium vapour lamps are particularly popular – they can be double-ended, tubular or ellipsoidal, depending on the space.