OPTICAL FOCUS POSITION DETECTOR

摘要

PURPOSE:To facilitate the arrangement of a masking element and a photodetector and to obtain the desired sensitivity of detection, by putting a single-direction lens into a focus detecting optical path and setting an element that masks a part of the light between two focuses of the detected light. CONSTITUTION:The light 23 radiated from a desired focus position is refracted by a lens 2 to be converged at a point P1. The light passed through a single- direction lens 19 is converged at a point P2 at a section parallel to the working axis 19' of the lens 19 including the center axis of the light. A mask 20 is inserted to the position Q where the section of light becomes circular and then into the center of the light so that the tip 20' of the mask 20 forms 45 deg. to the axis 19'. The half of the light which is not masked by the mask 20 is made incident to a detector pair 21. The focus is detected from the difference of output signals between detectors 21a and 21b. In such a way, the arrangement is facilitated for both the masking element and the detector with the desired sensitivity of detection.