In an inductively coupled electrodeless metal halide lamp dosed with NaI and ScI3, dosing a small amount of SnI2 into its arc tube suppresses the free I2 gas generation, which extends the life of the lamp. The free I2 gas is thought to be left over as a result of Na and Sc loss reaction with the quartz. It is assumed the Na loss is caused by its migration into the quartz and that the Sc loss is caused by the reaction between the quartz and Sc clusters on the quartz. The Na loss rate depends on the partial pressure of the Na and the Sc loss rate depends on the nucleation rate of the Sc clusters, which can be derived from the nucleation theory of clusters. Thermodynamical calculations showed that the Na and Sc partial pressures become 1/100-1/10000, and the nucleation rate of the Sc clusters becomes negligible with SnI2 dosing. The rate of the free I2 gas generation was calculated, assuming it is proportional to the Na pressure and the Sc cluster nucleation rate. The proportional coefficients were determined from the results of lifetime tests and furnace-heating tests. The calculations showed that the SnI2 dose sharply reduces the rate of free I2 gas generation, making the lamp-life longer.
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