Demixing in a metal halide lamp, results from modeling

Metal Halide (MH) lamps are high pressure discharge devices, containing a complex chemical mixture, to emit light on a broad spectrum while maintaining good efficacies. Lamps of this type were first exhibited by General Electric at the 1964 World Fair in New York [1]. They typically consist of an inner burner the size of a cigarette filter, surrounded by a larger protective outer wall. The inner burner is made from polycrystalline alumina or quartz and is filled with noble gasses, mercury (about 10 mg) and salt additives (a few milligram). Under operating conditions the inner burner has a gas pressure of several tens of atmospheres, most of which is mercury vapor. The salt additives, such as sodium, scandium and dysprosium idide, are present in very low concentrations in the gas phase. Yet it is these minority species that provide most of the light output. A well known [2, 3, 4] phenomena in MH lamps is that, when operated vertically, the metal halides in the lamp tend to demix; the concentration of metal halides in the gas phase is much greater at the bottom of the lamp. This can be observed directly from the light output, a demixed lamp shows a blue white mercury discharge at the top of the lamp and a much brighter and whiter discharge at the bottom of the lamp [5]. Demixing, or axial segregation as it is also called, has a negative impact on the lamp’s efficacy.