| 摘要 |
A focus error detection device for use with a system for optically recording and reading out information, and which can produce a symmetrical focus error signal with respect to the amount of displacement of the reflecting surface of the recording medium from an in-focus position. A second cylindrical lens whose axial direction is perpendicular to the axial direction of a first cylindrical lens is provided, and lateral magnification between the position of the receiving surface of a photo detector and a conjugate plane, which is located adjacent to the in-focus position of the reflecting surface, is made uniform in each direction. By this feature, the output signal of the photo detector varies in a symmetric manner in both directions from the in-focus position of the reflection surface.
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