(57) According to one embodiment, a discharge lamp (1) includes a light-emitting section
(10), a pair of sealing sections (20, 20), and a pair of electrodes (30, 30). One
ends of the pair of electrodes (30, 30) are disposed to be opposed to each other in
a discharge space (13). An inner circumferential surface of the light-emitting section
(10) includes a pair of taper surfaces (10a, 10a) expanding in an inner diameter from
respective both end portions (12, 12) toward a center portion of the light-emitting
section (10) in the direction in which the pair of electrodes extends and a cylindrical
side circumferential surface (10b) contiguous to the pair of taper surfaces (10a,
10a) and extending along the direction in which the pair of electrodes (30, 30) extends.
Length IL (mm) of the side circumferential surface (10b) in the direction in which
the pair of electrodes (30, 30) extends, an inner diameter ID (mm) in the side circumferential
surface (10b) of the light-emitting section (10), and maximum thickness t (mm) of
the light-emitting section (10) satisfy the following Expression 1:
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