| 摘要 |
PURPOSE:To miniaturize light pick-up optical system and to detect a focus without being influenced by an environmental temperature change by unifying and providing a second lambda/4 plate and a convex lens mirror equipped with a reflecting part at a poralizing beam splitter, and providing a knife edge and a photodetecting element at the reflecting side of the convex mirror. CONSTITUTION:At a light path after it is S-poralized and reflected by a reflecting film 14a of a poralizing beam splitter 14, the second lambda/4 plate 17 is fitted in bringing it into close contact with the poralizing beam splitter 14, and a convex lens mirror 18 is provided in the front direction of the light path. The light through the second lambda/4 plate 17 is directed to the convex lens mirror 18, reflected by a reflecting part 18a, comes to be a converged light flux and directed to the second lambda/4 plate 17 side again. By passing through the second lambda/4 plate 17 two times, the light comes to be a P poralization again, is made incident on a poralizing beam splitter 1, and thereafter, transmits a reflecting film 14a, proceeds and is made incident on a two-divided photodetecting element 19 for detecting a focus at an opposite side while a part is enclosed by a knife edge 20.
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