The Chemical Composition of Plasmas Generated in High Pressure Discharge Lamps Containing Tin and Sodium Halides

Abstract

In order to model high pressure discharge lamps which contain metal halides, the chemical composition in the plasma region is required. We have developed a procedure for calculating the composition of a plasma in local thermodynamic equilibrium, between 1500 and 6000 K, including the effect of radial diffusion. Mixtures of tin halides and sodium halides, when added to high pressure mercury discharges, produce highly efficient white light sources. We have determined the saturated vapor phase composition of some tin halide/sodium halide mixtures using the results from Knudsen effusion/ mass spectrometric, vapor pressure, and transpiration studies. We report herein a detailed study of the SnC12 + NaC1 (50:50) system and a preliminary investigation of the SnC12 + NaI (50:50) system together with the plasma compositions for the corresponding lamps. These results show that radial diffusion has a highly significant effect on the plasma composition.Our understanding of high pressure sodium lamps has been deepened through the development of a computer model (1). The model is based on solving the energy balance equation to give the radial temperature distribution of an arc. We assume that the electrical energy dissipated in an elemental volume equals the sum of the net heat and radiation losses. The variation of the chemical composition of the plasma with temperature stands at the heart of the model, and is used in the calculations of thermal and electrical conductivities and of radiation losses.A knowledge of the temperature profile enables us to predict the electrical and spectral characteristics of the arc. We have used our model to investigate the variation of efficacy for a high pressure sodium lamp with arc tube bore and current (2), and with arc tube temperature and electrical power dissipation (3). The model has thus contributed directly to the development and improvement of high pressure sodium lamps.Metal halide lamps are able to combine good color appearance and good color rendition with compact size, giving rise to highly efficient white light sources. A large variety of metal halide lamps are available on the market, these are generally confined to powers above 150W. Such lamps are employed in many applications: industrial and commercial interior lighting; exterior floodlighting; theater and stage lighting; set lighting; image projection systems; photoprinting; water sterilization; plant growth; street lighting. The present trend is towards lower power lamps, say below 150W, for use in display lighting. The development of successful low wattage metal halide lamps has proved to be a considerable challenge to the lamp manufacturing industry. To date, only a few low power metal halide lamps are available.The addition of tin halide/sodium halide mixtures to high pressure mercury discharges has been shown to produce lamps which combine high efficacy with good color rendition (4). The inclusion of metal halide systems in high pressure lamps gives rise to both atomic and molecular radiation, the latter being broad-ba...