Metal vapor discharge lamps of high luminance, high efficiency and long life, such as a metal halide lamp (hereinafter, simply referred to as “lamp”), have been widely used in many places for the above features.
A conventional lighting fixture that uses the above lamp as a light source includes, in addition to the lamp, a reflector that has a concave reflecting surface that reflects light emitted from the lamp in a desired direction. The reflector's light output opening is covered with, for example, a front glass plate (what is called a closed type lighting fixture). Note that the reason for covering the light output opening is to prevent broken pieces of the lamp from scattering outside of the lighting fixture when the lamp (discharge tube) is broken for some reasons.
Recently, there has been a demand for so-called an open type lighting fixture that is a conventional lighting fixture whose light output opening is not covered with a front glass. To meet such a demand, a lamp with the following structure is suggested. For example, the lamp is composed of a discharge tube, an inner tube and an outer tube, which is so-called a triple-tube structure. The inner tube houses the discharge tube, and the outer tube houses the inner tube. If the inner tube is damaged because of the breakage of the discharge tube, broken pieces of the inner tube remain within the outer tube. (e.g. Patent Document 1).
Note that each of the inner and outer tubes of the lamp has a tubular shape whose one end is open and another end is closed. The discharge tube is housed in the inner tube of which the other end is pinch-sealed. In addition, the outer circumference of the other end of the inner tube is bonded to the inner circumference of the other end of the outer tube with use of an adhesive (e.g. cement).    Patent Document 1: Japan Unexamined Patent Publication No. H11-96973
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