ABSTRACT
INTRODUCTION It is well known that starting contributes to the loss of light output of metal halide lamps. Tests
show the lumen maintenance of cycled lamps to be 20-30% lower than that of continuously operated lamps at rated life (Figure 1). Starting related loss of light output is generally attributed to tungsten sputtering during the glow and glow-to-arc transition phases of the starting cycle where high voltages and low pressures prevail near the cathode fall [1]. During normal operation, mercury pressure minimises the diffusion of sputtered and evaporated tungsten atoms, and reduces the light degradation in continuously operated lamps. Based on such results and assumptions of faster glow to arc transition at high fill pressures, it has been suggested that the lumen maintenance of metal halide lamps could be dramatically improved by a substantial increase in buffer gas pressure. However, tests have traditionally been conducted on probe start lamps at relatively low fill pressures of 30 to 60 ton as lamps with fill pressures higher than 60 torr do not start reliably on conventional ballasts.
