OPTICAL SCANNING DEVICE

摘要

An optical scanning device for scanning an information layer of a first optical record carrier having a first cover layer thickness and an information layer of a second optical record carrier having a second, different cover layer thickness. The device includes an objective lens system for converging a radiation beam on the information layers. The objective lens system includes a first lens element and a second lens element spaced apart along an optical axis. The objective lens system further includes a switchable optical element comprising a first fluid, and a chamber positioned between said first lens and said second lens. The objective lens system is switchable between a first configuration in which the first fluid occupies an optically active portion of the chamber such that the objective lens system has a first focal length for scanning the information layer of the first optical record carrier, and a second configuration in which the first fluid does not occupy the optically active portion of the chamber such that the objective lens system has a second, different focal length for scanning the information layer of the second optical record carrier. The objective lens system is arranged to satisfy the condition: Focal2-Focall > 0.9 (T2-T1)/N, where Tl is the first cover layer thickness, T2 is the second cover layer thickness, T2 > Tl, N is the refractive index of the second cover layer, Focal 1 is the first focal length, and Focal2 is the second focal length.