ANGLE DETECTING DEVICE

PURPOSE:To shape a laser beam emitted from a laser light source to an accurate circle so that the angle of the beam can be accurately measured by reflecting the beam with a circular mirror having a diameter smaller than that of the beam. CONSTITUTION:An optical system member 2 composed of a lens, etc., transforms a laser beam 10 emitted from an optical collimator 1 (laser light source) into parallel rays. A half mirror 3 transmits the beam 10 and, at the same time, reflects a beam 11 form a laser circular mirror 4. The mirror 4 has a diameter smaller than that of the beam 10 and is mounted on a plate member 5 treated for preventing reflection. A four-quadrant angle detector 6 detects the angle of the beam 10 based on the light quantity received by each channel in four quadrants by making the beam 11 incident to a sensor light receiving sections. Since the beam 10 is reflected by using the mirror 4 having a smaller diameter, the beam 10 can be shaped to an accurate circle. In addition, the occurrence of stray light can be prevented, because the outer peripheral section of the beam 11 which is not reflected by the mirror 4 is absorbed by the plate member 5 treated for preventing reflection.