Abstract

Apparatus for scanning the information surface of an optical disc record carrier with a radiation beam. Errors in beam focus on such surface are detected by providing two optical gratings in succession in the path of the radiation reflected from the record carrier. The gratings have a constant grating period, which may be equal or in a fixed ratio, the directions of the grating strips of the two gratings intersecting each other at a relatively small angle, thereby forming a Moire interference pattern in the radiation emergent from the two gratings which rotates due to changes in beam focus. This changes the relative amounts of radiation received by an assembly of radiation detecting elements adjacent the second grating, producing a focus error control signal which can be employed for correcting beam focus.