Patent attributes
An optical scanning device for scanning a multi-layer optical record carrier when positioned in a scanning location in the device, the device being adapted for scanning a first information layer at a first information layer depth within the record carrier and a second information layer at a second information layer depth within the record carrier. The device comprises: a radiation source for generating a radiation beam; an objective lens, located in an optical path between the radiation source and the scanning location, for converging a radiation beam to a spot on an information layer; and-an optical switching arrangement switchable between a first state, in which the device is arranged to scan a said first information layer, and a second state, in which the device is arranged to scan a said second information layer. The optical switching arrangement comprises a compensator arranged to generate a different amount of spherical aberration in a radiation beam when in said first state and when in said second state. The compensator is further arranged to generate a different amount of vergence in a radiation beam when in said first state and when in said second state. The different amounts of spherical aberration and vergence are selected such that a free working distance between said objective lens and said optical record carrier remains substantially constant when switching between said first and second states.
