Extending the life of low-power high-pressure sodium lamps

Abstract

As new low power ratings of high pressure sodium lamps were designed and developed it was found that lamp lives were unexpectedly short compared with higher power ratings. The principal feature of failed lamps was extensive blackening of the arc tube with emitter material from the electrodes. Improvements to electrode construction and emitter processing offered some improvements but lamp lives were still not satisfactory. A detailed study was made of the processes occurring in the region of the electrode when the lamps were started. This revealed that sodium amalgam in electrical contact with electrode assemblies was causing the lamps to act temporarily as rectifiers, for time periods of up to two minutes, after the lamps were switched on. A new type of end plug was designed to prevent electrical contact between the sodium amalgam and the electrode assembly. Optimised 70W high-pressure sodium lamps made with this new end plug have been found to live six times longer than lamps containing the usual end construction.