Abstract: An optical element comprises a fluid chamber and a device for providing a magnetic field over at least a portion of the fluid chamber The fluid chamber has side portions and end portions, and contains a first fluid and a second fluid. The fluids are non-miscible and the second fluid is capable of being influenced by a magnetic field. The end portions of the fluid chamber are connected together only by the side portions.

Abstract: A switchable diaphragm (9) and a device for beam deflection (10) are placed in the servo branch of an optical drive, in a path of light beams of different diffraction orders. This permits redirection of light beam orders towards a detection means (8) on an individual basis and selection of light orders at a detection means (8) depending on requirements. The number of detectors in the device detection means (8) can thus be reduced, thereby also reducing additional components associated with those detectors and saving on cost and complexity. With such device functionality, new methods for measuring beam landing and spherical aberration are developed.