| 摘要 |
PURPOSE:To optimize the temperature of the coldest area located at the rear part of the electrode and minimize the initial color variation so as to enhance efficiency by making one end periphery of the first thermally insulating film almost coincide with the end surface of the electrode and restricting the region where the first thermal insulating film is applied. CONSTITUTION:One end periphery of a first thermal insulating film almost coincides with the end surface of the electrode. The distance (a) between the end surface of the electrode on the central axis of the light-emitting tube and the inner end of the press seal section and the distance (b) between one end periphery of the first thermal insulating film and its other end periphery are adjusted according to the formula 0.6<=b/1<=0.9 in order to restrict at least the region where the first thermal insulating film is applied. Since the electrode- encircling area of the light-emitting tube located at the rear part of the electrode is free from the first thermal insulating film, the temperature of the coldest area located at the rear part of the electrode is optimized and the vapor pressure of the metal halide is prevented from being excessively high and is optimized.
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