Opto-electronic focussing error detection arrangement

An improved opto-electronic focussing error detection arrangement for detecting, in an optical imaging system, a deviation between a radiation reflecting surface and a focussing plane of an objective system, in particular for use in an apparatus for reading a record carrier with an optical radiation-reflecting information structure and an apparatus for optically writing information in a record carrier, which focussing error detection arrangement comprises a beam-splitting element disposed in the path of a radiation beam which is reflected by said surface and, disposed behind the beam-splitting element, a radiation-sensitive detection system which comprises two detectors each associated with one of the subbeams formed by the beam-splitting element, the outputs of the detectors being connected to the inputs of an electronic circuit in which a focussing-error signal is derived from the detector signals, wherein the detectors are disposed in a plane in which the reflected beam is focussed if the said deviation is zero, that each detector takes the form of a grating and comprises at least three radiation-sensitive strips, that said electronic circuit moreover derives a positional-error signal from the detector signals, and that each detector is connected to a separate selection device which effectively divides the detector into two detector sections, the bounding line of the detector sections being adjustable depending on the positional error signal which is applied to a control input of the selection circuit.