PURPOSE: To improve light utilization factor by providing an emitting window on a part of the curved surface of an outer tube shaped in an ellipsoid of revolution and a reflecting film on the other curved surface and placing a luminous tube at a focus on the emitting window side of the ellipsoid of revolution.
CONSTITUTION: A part of light flux which is emitted from a luminous tube 2 placed at a position equivalent to a focus on an emitting window 6 side of an outer tube 3 shaped in an ellipsoid of revolution directly goes out of the emitting window 6. On the other hand, the remainder of the light flux is reflected by the coating portion 5 of the outer tube 3, passed near another focus, reflected again by the coating portion 5 and returned near to the focus on the emitting window 6 side. While repeating such processes, almost all light flux except for few loss transmitted and absorbed into the coating portion 5 goes out of the emitting window 6 on an equal amount basis as light flux directly emitted from the luminous tube 2. As a result, almost all light flux emitted from the luminous tube 2 can go out of the emitting window 6, resulting in better light utilization factor.