Fabry-Perot measurements of barium temperature in fluorescent lamps

Abstract

Abstract.A scanning Fabry-Perot interferometer (FPI) is used to determine the temperature of barium atoms that are liberated from the electrodes of fluorescent lamps during their steady-state operation. Barium, a constituent of the work function lowering emitter material that is placed on the tungsten coil that forms the electrode, is liberated primarily by evaporation from the hot (~1300°K) thermionic electrode. However, there may be situations or modes of operation in which barium is, in addition, sputtered, a condition which may lead to increased end-darkening, shortened life, and increased mercury consumption in the lamp. Using the FPI diagnostic, the occurrence of sputtering is inferred when barium temperatures are much greater than the electrode temperature. The FPI diagnostic senses resonance radiation ( 553 nm λ = ) emitted by barium atoms excited in the low pressure discharge environment, and infers temperature from the Doppler broadened linewidth. The diagnostic has proven to be successful in a number of situations. Measurements have been made on rare gas discharges and on Hg-argon discharges for different discharge currents, gas pressures, and auxiliary coil currents. Measurements are phase resolved for a.c. driven discharges.