An optical device for scanning an optical record carrier (20) has a radiation source (25) that can emit two radiation beams of different wavelength along different optical paths (28, 29). The emitted radiation is focused on the record carrier and reflected back to a detection system (40). A beam combiner 43 is arranged in the optical path between the radiation source and the detection system for combining the two radiation beams on one optical path, thereby allowing the use of a single detection system for both radiation beams. The beam combiner includes a grating that passes one of the beams undiffracted and diffracts the other beam mainly in the first order. The profile of the grating lines show alternating slanting grooves and slanting lands.