| 摘要 |
An optical compensator for use in an optical scanning device for scanning optical record carriers, there being at least two different information layer depths within two different ones of the carriers, the optical record carriers including a first optical record carrier (3'), a second optical record carrier and a third optical record carrier, the scanning device including a radiation source system (7) for producing first, second and third radiation beams for scanning the first, second and third record carriers, respectively, the first, second and third radiation beams having different predetermined wavelengths. The optical compensator has a non-periodic phase structure through which each of the first, second and third radiation beams are arranged to pass, the non-periodic phase structure including a plurality of stepped annular zones separated by steps, the zones forming a non-periodic radial pattern, the stepped annular zones introducing first, second and third different wavefront modifications into at least part of the first, second and third radiation beams, respectively. The optical compensator includes radial height variations in the plurality of stepped annular zones, the radial height variations being arranged such that non-zero contributions are provided to each of the first, second and third wavefront modifications by the optical compensator in each of the stepped annular zones.
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