| 摘要 |
An apparatus for scanning an information surface (2) of an optical record carrier (1) is described. An error in the focussing of the scanning beam (b) on the information surface (2) is detected by means of two gratings (R1, R2) which are arranged one behind the other in the path of the beam (b) from the record carrier, which have grating periods (P) which are adapted to each other and whose grating strips intersect each other at a small angle ( theta ), use being made of a composite radiation-sensitive detection system (12). |
