2001 Volume 85 Issue 11 Pages 888-894
The translucence phenomenon of arc tubes for inductively coupled electrodeless metal halide lamps was evaluated and analyzed, methods for decreasing the translucence were investigated.
The surface of the fused silica was found to be etched in the hot region, and the fused silica was deposited on the cold-region surface. The change in translucence occurred as a result of this SiO2 transport phenomenon.
Effects of the lamp parameters were evaluated. The translucence phenomenon is promoted under the condition that the dose material contained rare-earth metal halides.
Another study showed that the partial pressures of free metal atoms around the wall of the arc tube (the pressures were controlled by adjusting the filled Xe pressure and added SnI2amount) strikingly affect the translucence level.
The effects of the temperature of the arc tube and the effects of the electric field created by the induction coil were found to be not so great as the effects of the lamp fills and the partial pressures of the free atoms.
OH radicals in the fused silica, which may promote the translucence change of the fused silica, were found to invade the outer and inner surfaces of the arc tube wall due to the use of an H2-O2 burner in the bulb-forming process.
Removal of these OH radicals with hydrofluoric-acid surface-etching treatment strikingly decreased the translucence change level.
Therefore, removing the OH radicals from the inner surface of the arc tube is an important and basic condition for decreasing the translucence level of arc tubes.
In arc tubes without an outer bulb, OH radicals that originate in H2O vapor in air invade the arc tubes. Calculation of distributions of OH radicals showed that after 20, 000 hours of operation of arc tubes with lmm thickness, OH radicals reachthe inner wall and the arc tube becomes translucent.
This calculated result agreed well with the results of life-time testing.