| 摘要 |
PURPOSE:To reduce the thickness of a focus detecting device by setting a focus detecting means at a position opposite to a light source centering on a prism and leading the light returned from an object to the focus detecting means through a face different from an incident face of the prism. CONSTITUTION:A luminous flux 2 received from a light source 1 is partly reflected by a 1st face 3a of an approximately cubic prism 3 and condensed on an information recording carrier 5 via an objective lens 4. While a reflected luminous flux 9 is made incident on the prism 3 through the face 3a after passing through the lens 4 again and then reflected internally by a 2nd face 3b. This internally reflected light is radiated through a 3rd face 3c and detected by a photodetector 6 and a focused state is detected from the change of the spot shape of the light detected by the photodetector 6. The source 1 and the photodetector 6 are set opposite to each other centering on the prism 3 and have no interference to each other. Thus, no distance is required in the optical axis direction of the lens 4 and therefore the thickness of a focus detecting device is reduced. |
