References(3) The conventional high pressure sodium lamps have the yellowish light source color, and find limited applications. We have conducted the study in an effort to improve this defect. In general, the color characteristics of a light source are represented in terms of a color temperature and a color rendering index. Our study, however, revealed the fact that even when the color temperature and the color rendering index are the same, the light source color greatly varies depending upon the specifications of an arc tube and, especially, depending upon the composition ratios of sodium amalgam contained in the arc tube, and that the ability for discriminating the color difference changes, too. We have succeeded in obtaining a lamp of a type which is adapted to a ballast for a 400-watt mercury lamp, with a wattage of 350 W, a luminous efficacy of 105 lm/W, a color temperature of 2150 K, a color rendering index of 65, and a color discrimination index of 29, with the whitish light source color.
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