Degradation analysis in switched low pressure sodium lamps

Abstract

This paper presents the results of a laboratory investigation of the degradation of the parameters of low pressure SOX 35 watt lamps under switched conditions, and an analysis of these results to give reliability figures for these lamps. In order to define lamp failure, three parameters were chosen and failure was represented by the following changes from the 200 hrs. burning level: a drop of 33 per cent in light output; a rise of 40 per cent in power consumption; and a drop of 55 per cent in luminous efficacy. A statistical sample of the lamps was cycled in the laboratory at different switching rates and parameter degradation was measured at regular intervals. The changes in the lamp parameters with time and with differing switching rates are presented in this paper. A statistical analysis of the lamp failure data was then performed using the Weibull distribution whose parameters were estimated using maximum likelihood. In this manner measures of stress due to switching and switching rate and also characteristic lives for the lamps for the different failure criteria were estimated and are presented here. The results of the analysis suggest that if light output were the only factor of concern, long characteristic lives of over 13, 000 hours could be expected. However, it is proposed that the most important criterion in measuring lives and reliabilities of the 35 W SOX lamps tested should be the decrease of luminous efficacy yielding characteristic lives of the order of 9, 000 hours.