Patent ID: 8004195

Claim:
A discharge bulb comprising: an arc tube main body comprising: a discharge arc chamber disposed in a center portion of the arc tube main body in a longitudinal direction, in which two discharge electrodes are disposed to oppose to each other and a luminous material is sealed together with a starting rare gas; a tube portion disposed at each end portion of the arc tube main body, each of the tube portions being in communication with the discharge arc chamber and holding an inserted respective one of the discharge electrodes, wherein a wall for forming the discharge arc chamber has a taper portion whose diameter is reduced gradually from a cylinder portion of the arc tube main body in a center area in the longitudinal direction to the tube portion of the arc tube main body, the taper portion being connected to a pore of the tube portion, and an inner diameter Di of the cylinder portion is about 1.0 mm≦Di≦about 2.5 mm, and a projection length Le of the discharge electrode into the discharge arc chamber is about 1.5 mm≦Le≦about 2.5 mm.