| 摘要 |
Autofocus apparatus for use in an optical system utilizing a recording medium (20) formed from a birefringent medium (21) and a radiation-sensitive medium (22). The autofocus apparatus automatically focuses radiation through the birefringent medium and substantially at the interface (24) between the birefringent medium and the radiation-sensitive medium and includes a source of linearly polarized radiation (100, 110, 120, 130), a quarter-wave plate (150) to convert the linearly polarized radiation to circularly polarized radiation, and an actuator (165) controlled lens (160) for focusing the radiation onto the recording medium, whereby radiation reflected from the first surface of the birefringent medium and the interface between the birefringent medium and the radiation-sensitive medium passes back through the lens and quarter-wave plate and is deflected by a beamsplitter (140) to a polarization detector (180). The polarization detector isolates polarization components which arise from reflection at the interface and the isolated polarization components impinge upon a detector (190). The detector provides a focus-error signal which is applied, in turn, to the actuator (165) to maintain focus.
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