JPS5760306A - Method and device for detecting focal point - Google Patents

Abstract

PURPOSE:To reduce the size of the optical system and, at the same time, to always detect accurate focusing condition, by setting the surface of reflection of the detection prism so that it will become smaller than the critical angle against the incident ray when the ray is in focus. CONSTITUTION:Light from a light source 21 is transformed into a parallel luminous flux at a lens of collimater 22, and the flux is focused on a disk 26 through a polarized prism 23, an 1/4 wavelength plate 24 and an objective lens 25. The light reflected by the disk 26 is reflected by the polarized prism 23. The reflected luminous flux is made incident to a detection prism 27 and a luminous flux reflected by a surface of reflection 27a of the prism 27 is received by a photodetector 28. This surface of reflection 27a is set so that it will become smaller than the critical angle against at least a part of the incident ray when the ray is in focus. Since, when this arrangement is made, only a detection prism having a constant inclined angle is required, the optical adjustment can be made easily. Moreover, because focusing error signals which are the output of the photodetector 28, excellently change with the focusing location as the center, the stabiity near the focusing location is improved and, at the same time, high sensitive detections can be made at locations far from the focusing point.