| 摘要 |
In an optical scanning device suited for scanning two types of record carriers having a different thickness of their transparent layers and wherein an astigmatic element is used in combination with a quadrant detector for generating a focus error signal (Sf), this signal, which is generated when one type of record carrier is scanned, can considerably be improved by arranging an aspheric surface profile in only the central part of the beam coming from said one type of record carrier.
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