Why does the lumen maintenance of sodium–scandium metal halide lamps improve by VHF operation?

Abstract

Lifetime experiments show that sodium scandium metal halide lamps perform better on VHF (Very High Frequency) drivers than on LF (Low Frequency) CWA ballasts. The question why, will be addressed with focus on arc tube aspects. It is argued that at high frequency operation sodium loss is less, and that the absence of thermal fluctuations in the electrode tip causes less damage and cracking to this part of the electrode. Sudden lm/W drops, observed with CWA-operated lamps, most probably occur when the arc attaches on such a corroded and cracked surface. Thorium is effective as an emitter both in the CWA and the VHF operation mode, despite the absence of cataphoretic transport to the cathode in the VHF case. IntroductionThe sodium scandium metal halide lamps discussed here are designed to be operated on conventional magnetic Constant Wattage Autotransformer (CWA) systems . Tests reported in literature as well as own investigations have shown that these lamps perform very well on electronic Very High Frequency (VHF) systems [1,2]. Especially the lumen per watt (LPW) maintenance on VHF is better than on the conventional CWA system. Despite of this advantage, in the majority of lighting systems for these lamps still the conventional gear is used because of their long lifetime and also the higher price of electronic drivers. In the lumen per watt maintenance of metal halide lamps several effects play a role like spectral changes and wall blackening due to the deposition of tungsten. Lamouri et al. [1] report that 150 W lamps burned on magnetic ballasts revealed more blackening of the arc tube than on electronic drivers. A test with fast cycling of the lamps (2 minutes ON and 28 minutes OFF) showed that the lumen depreciation is not related to starting but must occur during normal operation [1]. Furthermore, chemical analysis of the lamp filling after 9000 hrs of operation showed no significant difference between lamps operated on magnetic ballasts versus electronic drivers [1]. This excludes spectral differences from being the major cause of difference in light output. The authors conclude that the current crest factor and the operating frequency are not responsible for the improved lumen maintenance on electronic drivers. They suggest that the cathode peak temperature is the dominant factor. With respect to differences in voltage rise the reported results are not unequivocal. A test with horizontal burning 350 W lamps resulted in less voltage rise, and so less sodium loss, for lamps operated on VHF [2]. For 150 W lamps burning in vertical orientation for 9000 hrs the voltage rise was similar regardless of the ballast type [1]. This paper will address the question why these lamps perform better on VHF and will focus on the lamp side of the system, on physical, chemical and mechanical effects of the very high frequency operation mode.