Starting Processes and the Influence of the Radioactivity of Thorium on the Statistical Lag Time in Metal Halide Lamps
High pressure metal halide discharge lamps are widely used as high efficiency lighting sources in a wide variety of applications from high wattage (175 to 1500 W) outdoor and stadium lighting to the more recent lower wattage (75 to 150 W) lamps used in retail store and display lighting1,2. Both low and high wattage lamps usually contain thorium; either in the form of ThI4 as an additive to the chemical fill in the lamp, or as ThO2 in the pressed, sintered and drawn tungsten wire used as the electrode material in the lamp. The presence of thorium in the discharge improves the colour rendering properties of the light from the lamp due to the many emission lines in the radiative spectrum of atomic thorium3. The presence of a few monolayers of elemental thorium on the surface of the tungsten electrodes also aids in the starting of the lamp and lowers the operating temperature of the thermionically emitting electrode by lowering the work function of W from φ = 4.6 eV to φ ≃ 2.7 eV4. The stages involved in the starting of these lamps are complex and are briefly outlined in the next section.
KeywordsMetal Halide Tungsten Electrode Metal Halide Lamp Daughter Nuclide Daughter Radionuclide
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