A calibration method and machine for detecting a location mark (12) printed on plate elements (10) within a processing machine (1) having an introducer (20) for positioning the plate elements (10) into a plurality of gripping members (31) of a conveyor (30) which conveys them in rhythmical motion into successive stations (3, 4, 5). The introducer (20) includes a fastening device to the plate elements. The introducer (20) is driven by a control unit (40) that also drives at least one illuminating device and at least one sensor (7). A plate element (10) is selected from a batch of them. The fastening device fixes the plate element (10) to the introducer (20). The introducer (20) is driven to perform a succession of back-and-forth movements with the plate element (10). At the time of each back-and-forth movement, the illumination device subjects the location mark (12) printed on the plate element (10) to a different illumination, and the sensor (7) performs a corresponding measurement, so as to obtain a succession of measurements. The calibrated illumination parameters that will be used during the processing of the whole batch of plate elements (10) are determined according to the succession of measurements obtained.