Investigation of Tungsten Erosion in Fluorescent Lamps during Cold Start by Laser-induced Fluorescence

摘要

Investigations of fluorescent lamps (FL) are often focused on electrodes because the lifetime of lamps is limited by the electrode durability. In genera!, a commercial electrode system consists of a tungsten coil coated with a work function reducing emitter mix of alkali oxides, such as BaO, SrO and CaO. The spot temperatures, during steady state, typically are in the range of 1200 - 1400 K. Until now investigations of electrodes have been directed mainly toward preheated ignition [1-4]. Driven by the desire of saving costs, electronic control gear (ECG) without preheating circuits are becoming more common. A FL driven with such a 'cold start ECG' starts in a glow mode. In this mode, which last typically for tens of ms, the discharge current is relatively low (few mA), but the lamp voltage may exceed 500 V because of the very high cathode fall. This causes high energy ion bombardment of the electrode which heats the electrode, and induces a transition from glow to arc mode. In this mode the electrode emits thennionically and the cathode fall drops to the 12 - 15 V range. Unfortunately, the high energy ion bombardment during the glow mode leads also to intense sputtering of electrode material, including tungsten as well as emitter. Thus, cold started FL often suffer from early failures due to coil fracture. Havcrlag ct al. [5] have shown that coil breakage is caused by tungsten sputtering at one of the emitter-free ends mainly during the glow-to-arc transition. Our investigations on both linear and compact fluorescent lamps by fast emission spectroscopy and high-speed video observation support that behaviour [6].