| 摘要 |
An optical scanning device (1) for scanning optical record carriers (2) having cover layers of different thicknesses. The scanning device has a first mode for scanning a first optical record carrier, a second mode for scanning a second optical record carrier and a third mode for scanning third optical record carrier. The device comprises an objective lens system (10) and a switchable optical element (8) having a first discrete state and a different, second discrete state. The element comprises: a fluid system including a first fluid (18) and a different, second fluid (19); a wavefront modifier (17); and a fluid system switch for acting on the fluid system to switch between the first and second discrete states of the element. When the element is in the first discrete state, the wavefront modifier (17) is substantially covered by the first fluid (18), and when the element is in the second discrete state, the wavefront modifier (17) is substantially covered by the second fluid (19). The wavefront modifier includes a non-periodic phase structure including a plurality of stepped annular zones separated by steps, the zones forming a non-periodic radial pattern, and is characterized in that the wavefront modifier further includes an aspherical surface. The scanning device is arranged to operate using the element (8) in the second state to generate spherical aberration when in the second mode, the aspherical surface of the element (8) in the second state compensating for more than half of the spherical aberration due to the objective lens system (10) and second record carrier in combination.
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