Apparatus for reading an optically recorded record carrier provided with periodic surface undulations and for deriving a focussing control signal from such surface undulations

摘要

An apparatus for reading a record carrier on which information is stored in an optically readable structure of trackwise arranged areas and intermediate areas where at least one of the surfaces of the record carrier body has periodic surface undulations along the track direction. A focussing error signal is derived by reflecting a focussed light beam on the record surface and by measuring the amplitude and phase of the portion of the reflected beam that corresponds to the surface undulations.