Arc lamps with efficiencies in the range from 70 to 110 lm/W, with good color rendition, have been made by a combination of metallic iodides and mercury in a medium-pressure vapor arc. Sodium iodide and thallium iodide with mercury were found to give the highest efficiency although other compounds such as indium iodide also provide improved color rendition. Careful control of the vapor pressure of the components, attained by control of lamp temperature and correct proportion of the iodides, is necessary in order to attain good color balance and efficiency. In case of arc tubes similar in dimensions to the 400-W mercury lamp, the new lamps operate with similar volt-ampere characteristics.

Data on the luminous efficiency, spectral characteristics, chemical reactions, and arc performance are shown and discussed. Several effects such as reduction of excitation temperature by the iodides appear to be responsible for increased efficiency.

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