| 摘要 |
Provided is a metal halide lamp that can (i) prevent flicker in an illuminated surface of an arc tube, which is caused by violent movement of an electric arc when the lamp is lit while being tilted, (ii) suppress an early reduction in a luminous flux maintenance factor, and (iii) suppress formation of cracks in an enclosure of the arc tube. The lamp comprises (i) an arc tube 3 that (a) is arranged inside an outer tube 2, (b) has at least one selected from the group consisting of Ce and Pr enclosed therein as a light emitting material, and (c) is made of translucent ceramic, and (ii) a sleeve 4 arranged outside the arc tube 3, so as to surround a part of a discharge space 13 in the arc tube 3 extending between a pair of electrodes 12. The relationship 0.7<L/D<3 is satisfied in the lamp, with L denoting the inter-electrode distance [mm] and D denoting a largest inner diameter [mm] of a part of the arc tube 3 surrounding the part of the discharge space 13. The relationship R/r≦̸−0.0019P+2.625 (R/r>1) is also satisfied in the lamp, with R denoting an average inner diameter [mm] of a part of the sleeve 4 surrounding the part of the discharge space 13, r denoting an average outer diameter [mm] of the part of arc tube 3, and P denoting a power rating [W] of the lamp.
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