The low-pressure sodium lamp

Summary form only given. For many years before the introduction of the sodium vapour lamp, scientists had been aware of the remarkably high luminous efficacy of the sodium discharge. However, many technical problems had to be overcome before these lamps could be marketed. The first commercial low pressure sodium lamps were introduced in the early 1930's and to this day they remain the most efficient light sources available. The high efficacy is due partly to the fact that these lamps emit nearly monochromatic yellow light, which is very close to the peak sensitivity of the human eye. Sodium lamps have come a long way since their introduction, and efficacies are now approaching 200 lumens per watt. Despite increasing competition from other types of discharge lamp, low pressure sodium lamps of the 'SOX' type find widespread use in road and security lighting, particularly in Great Britain and many other parts of Europe. This paper will review the operation and development of the low pressure sodium lamp, and show what makes it different from the many other types of discharge lamp available.