LIGHT SOURCE DEVICE

PURPOSE:To eliminate hindrance to a light beam which irradiates a rectangular surface, by providing a photodetecting element in a space part formed of primary regularly reflected light from a rotationally symmetric reflecting plate and light projected on a part other than the surface irradiated with direct projection light from a lamp. CONSTITUTION:The rotationally symmetric reflecting plate 2 is used for uniform illuminance on the rectangular irradiated surface 11, and the space part is formed by the straight projection light projected from the lamp 1 upon the part other than the irradiated surface 11 or the primary regularly reflected light from the reflecting plate 2. The photodetecting element 7 is disposed in the slanting- line part 16 of the space part, so the straight projection light from a light source 1 is photodetected without shielding the irradiated surface 11 from the irradiation light beam, obtaining light with enough intensity. Further, even when an element which has relatively low photoelectric conversion efficiency is used as the photodetecting element 7, an enough output is obtained, which is advantageous in terms of cost and improves noise resistance performance.