ARC TUBE, DISCHARGE LAMP, AND ILLUMINATION DEVICE

摘要

Provided are an arc tube and a lamp capable of maintaining a high lamp efficiency even if lamp power fluctuates. The arc tube (3) is formed of a hermetically sealed glass tube (19), at both ends (13, 15) of which electrodes (17, 18) are arranged. The glass tube (19) has a swelling portion (37) formed in a central region corresponding to a substantial center of the inter-electrode region sandwiched by filament coils (25, 26) of the two electrodes (17, 18). A coldest point part is present in an end region other than the inter-electrode region when being lighted with a rated lamp power and is present in the swelling portion (37) when being lighted with a predetermined lamp power lower than the rated lamp power. The lamp has the arc tube (3) and a cap electrically connected to the electrodes (17, 18) in the arc tube (3).