Patent ID: 7741779

Claim:
A metal vapor discharge lamp including an arc tube, an inner tube housing the arc tube, and an outer tube housing the inner tube, the arc tube having a pair of electrodes therein that oppose each other substantially in line, wherein the following relational expression is satisfied: 2× A+B≧ 1.06; A≧0.3, where A represents, in millimeters, a shortest distance between the arc tube and the inner tube along a line in a radial direction of the inner tube, and B represents, in millimeters, a distance between the inner tube and the outer tube along the line, and the following relational expressions are further satisfied: α≦5; and β≦2.5 where α represents, in millimeters, a distance between the arc tube and the inner tube in a cross section 2 in which the distance between the arc tube and the inner tube is shortest, the cross section 2 being taken perpendicularly to an imaginary line that connects the pair of the electrodes, the cross section 2 being present between the pair of the electrodes, α being defined by the following relational expression: α=(Inside Diameter of Inner Tube in Cross Section 2−Outside Diameter of Arc Tube in Cross Section 2) 2, and β represents, in millimeters, a distance between the inner tube and the outer tube in a cross section 1 in which the distance between the inner tube in a cross section 1 in which the distance between the inner tube and the outer tube and the outer tube is shortest, the cross section 1 being taken perpendicularly to the imaginary line that connects the pair of the electrodes, the cross section 1 being present between the pair of the electrodes, β being defined by the following relational expression: β=(Inside Diameter of Outer Tube in Cross Section 1−Outside Diameter of Inner Tube in Cross Section 1)/2.