Low-pressure sodium discharge lamps

Abstract

The review article deals with the low-pressure sodium lamp, which, since its very introduction, has been the most efficient light source available. The principle of this lamp is that light is generated as the result of an electric discharge with sodium as emitting material and an inert gas as a ‘buffer gas’. The spectrum emitted by this lamp consists of the yellow sodium resonance lines of 589.0 and 589.6 nm wavelength. Before it was first introduced commercially in 1932 several difficulties had to be overcome. A glass for the discharge tube that was resistant to the highly reactive sodium had to be developed, the composition of the rare-gas filling had to ensure easy ignition of the lamp and the heat insulation had to ensure the wall temperature reached 260°C so as to give the correct sodium vapour pressure. One of the developments of the lamp that took place before the 1960s was that the detachable vacuum jacket, meant for heat insulation, was replaced by a sealed-on evacuated tube. The glass of the discharge tube has also undergone many improvements, resulting in a longer life for the lamp. The introduction of an indium-oxide coating as an infra-red reflecting layer has increased the luminous efficacy of the lamp up to 180 lm/W. The electrodes of the lamp also have an influence on the performance of the lamp. Other important phenomena, such as the imprisonment of resonance radiation, sodium migration and sodium depletion, greatly influence the life of the lamp and its luminous efficacy. Several experimental and theoretical studies give further insight into these discharge processes. Continuing research and development effort will probably lead to an increased luminous efficacy of the low-pressure sodium lamp in the future and the introduction of lower wattage lamps. Recently, hybrid ballast has been introduced with lower ballast losses than the conventional leakage-reactance transformer; also better insulation of the bend of the U-shaped tube has been applied in order to reduce the increase in lamp voltage that occurs during its life. The low-pressure sodium lamp finds application in street lighting, safety and security lighting.