High-pressure sodium discharge lamps

The review article deals with the high-pressure sodium (HPS) discharge lamp, characterised by a sodium vapour pressure above 5 kPa and a power density of about 100 Wcm¿3. In its short history ¿ the sodium-resistant translucent alumina used as the discharge tube material was invented only in the late 1950s ¿ continuous research and development has led to a good understanding of the discharge physics, and to an improvement of the lamp properties. The various physical aspects, like plasma temperature, emitted spectrum and energy balance, are discussed. Technical aspects like discharge-tube design, tube material and seal, and the ignition and stabilisation of the lamp are also covered. The HPS lamp is a compact light source emitting golden-white light with luminous efficacies of up to 140 l mW¿1. It has already found widespread applications, mainly for outdoor lighting. Further developments in the field of design diversification, especially lamps with lower wattage and improved light quality, will make this light source also suitable for indoor applications.