| 摘要 |
PURPOSE:To constitute a small optical system by providing a detecting prism having a single reflecting surface set so as to become approximately a critical angle with respect to the optical axis of a reflected light beam. CONSTITUTION:The reflecting surface 11 is set to a critical angle or slightly smaller than said angle with respect to an incident rays of light (parallel flux) in the focused state. When a disk 6 is displaced in directions (a) and (b) to deviate from the focused state, the light and dark states against a paper surface including a central rays of light are reverse each other with a vertical surface as a boundary, because reflecting intensity on the reflecting surface 11 is sharply changed due to a slight change in the incident angle. On the other hand, rays of light are reflected uniformly in the focused state, and therefore such light and dark never appears. A photodetector 12 detects the luminous energy distribution of the reflected rays of light from the reflecting surface 11 and is constructed with two light receiving areas 12A and 12B available from splitting an area with a central rays of light (optical axis) as a boundary. |
