| 摘要 |
The present invention provides a high pressure discharge lamp which is provided with a discharge tube containing a pair of electrodes therein and being filled with mercury, an inert gas and a halogen gas, the amount of the mercury filled being 0.12 to 0.35 mg/mm<SUP>3</SUP>, the halogen gas being at least one gas selected from the group consisting of Cl, Br and I and being present in an amount of 10<SUP>-7 </SUP>to 10<SUP>-2 </SUP>mumol/mm<SUP>3</SUP>, and the electrodes being mainly composed of tungsten, and the tungsten as a material of the electrodes contains not more than 12 ppm of potassium oxide (K<SUB>2</SUB>O). In such a high pressure discharge lamp, blackening of the discharge tube due to the potassium oxide (K<SUB>2</SUB>O) contained in the tungsten, and decrease in illumination maintenance can be prevented, thus providing a high pressure discharge lamp with a long lifetime, a lighting optical apparatus using the high pressure discharge lamp as a light source, and an image display system using the lighting optical apparatus. In another aspect, the amount of -OH in the glass of the discharge tube is less than 3 ppm. In a further aspect, the discharge lamp has a restriking voltage of less than 20 V.
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