An investigation of the effect of impurities on the striking voltage of metal halide lamps

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A glow discharge is excited in metal halide lamps, which contain mercury-thallium iodide, mercury-sodium iodide and mercury-sodium-thallium-indium iodide, in order to determine the impurity concentration spectroscopically. A series of band spectra of mercury hydride and a line spectrum of iodine are observed, and the spectral intensities at 4017 Å (mercury hydride band) and at 2062 Å (iodine line) are measured for this purpose. In mercury-thallium iodide lamps, it was found that hydrogenous impurity present in the arc tube decreases with operating time with a time constant of 60〜150 minutes, which is about the same as the diffusion time of hydrogen through the fused silica tube at the operating temperature. In mercury-sodium iodide lamps, the impurity concentration decreases at first with the same rate as that of the mercury-thallium iodide lamps, and then ceases to decrease gradually after 2 hours operation. Mercury-sodium-thallium-indium iodide lamps show nearly the same behavior as the mercury-sodium iodide lamps. A cross-check is made between the striking voltage and these spectral intensities, and it was found that the impurity concentrations more than 2×10^<-3> Torr affect the striking voltage of the lamps.
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