A device is for scribing a visible label on the medium includes a head for providing a beam and generating a detector signal in dependence of radiation reflected from the medium, and a focus unit or generating a focus control signal for focusing the beam. A label control unit controls the scribing in dependence of label data. The focus unit detects a focus error by including a focus excitation signal in the focus control signal and detecting a deviation in the detector signal in dependence of the focus excitation signal. The label control unit is for controlling the focus unit in dependence on the label data to detect the focus error during said scribing and without interrupting said scribing.