Focus control for a medium scanning system

Abstract

An optical disc device and a method are for scanning a medium via a beam of radiation while focusing the beam, in particular for scribing a visible label on a record carrier that has a label side provided with a radiation sensitive layer for creating the visible label. The device has a head for providing the beam and a detector for generating a detector signal (CA) from radiation reflected from the medium. A focus system is provided for generating a focus control signal for focusing the beam of radiation to a spot on the medium. A focus excitation signal (505) is added to the focus control signal and a focus correction signal is generated based on detecting a center of gravity in the detector signal, the weight of the detector signal being determined in dependance of the focus excitation signal.

Dependencies are inferred from the claim text.