The invention refers to a method for calibrating an optical arrangement (5) prior to its intended use in respect to a global coordinate system (xyz_global), the optical arrangement (5) comprising a carrier unit (11) made of a rigid material, an optical acquiring unit (12) and a light emitting unit (13) both releasably connected to the carrier unit (11). In order to facilitate the calibration and in order to make calibration independent from the intended use of the optical arrangement (5), it is suggested that an optical acquiring unit (12) or a light emitting unit (13) is calibrated in respect to a reference coordinate system (xyz_ref) offline and independently from the optical arrangement (5). Then values of a conversion matrix (52) for converting the calibration data (51) acquired in respect to the reference coordinate system (xyz_ref) into corresponding calibration data (50') in respect to the global coordinate system (xyz_global) are determined. This can be achieved by performing once a calibration of the entire optical arrangement (5) in respect to the global coordinate system (xyz_global). During the intended use of the optical arrangement (5), the calibration data (51) acquired for the unit (12, 13) and/or the respective values of the conversion matrix (52) are considered when generating control signals for the unit (12, 13) and/or when processing sensor signals received from the unit (12, 13).