A method for the calibration of an optical sensor system in a sensor housing, having at least one sensor and at least one light source, a control device for controlling the sensor system, a calibration medium and a drive which moves the calibration medium under the control of the control device into an optical path of the sensor system in a calibration mode for the calibration of the sensor system, and moves the calibration medium out of the optical path of the sensor system in an operating mode for the testing of measuring objects, in particular banknotes, wherein the calibration medium has more than one reference (6.1 to 6.8), the drive moves the calibration medium under the control of the control device into the optical path of the sensor system in such a way that one of the references (6.1 to 6.8) covers a detection area of the sensor system, and the sensor system measures the reference (6.1 to 6.8) located in the detection area, the drive moves the calibration medium under the control of the control device at least one time by a predetermined stretch (b) or a multiple of the predetermined stretch (b), so that another one of the references (6.1 to 6.8) covers the detection area and the sensor system measures the other reference (6.1 to 6.8) located in the detection area, and the predetermined stretch (b) corresponds to an extent of the calibration mediums' individual references (6.1 to 6.8) in whose direction the drive moves the calibration medium, characterized by a first calibration step in which a further calibration medium independent of the sensor system, having standardized references, is introduced into a focus area of the sensor system located outside of the sensor housing and covers the entire detection area of the sensor system where the respective measuring object to be tested in each case is located during the operating mode, wherein the light source of the sensor system excites the further calibration medium with an excitation signal and signals successively generated by the standardized references of the further calibration medium due to the excitation signal are detected by at least one sensor of the sensor system and measuring signals are generated; and wherein an adjustment of the sensor system is effected due to the measuring signals, and a second calibration step, which is performed immediately after the first calibration step, wherein the calibration medium, having non-standardized references (6.1 to 6.8), is introduced into the optical path of the sensor system at a place which is shifted into the sensor housing by a distance (d) in relation to the focus area of the sensor system and covers the entire detection area of the sensor system, wherein the light source of the sensor system excites the calibration medium with the excitation signal and signals successively generated by the references (6.1 to 6.8) of the calibration medium are detected by the sensor and measuring signals are generated, and wherein the measuring signals of the second calibration step are stored, and further characterized in that during the calibration mode, the calibration medium is again introduced into the optical path of the sensor system at the place which is shifted into the sensor housing by the distance (d) in relation to the focus area of the sensor system and covers the entire detection area of the sensor system, wherein the light source of the sensor system excites the calibration medium with the excitation signal and signals successively generated by the references (6.1 to 6.8) of the calibration medium are detected by the sensor and measuring signals are generated, and wherein the measuring signals of the calibration mode are compared with the stored measuring signals of the second calibration step; and an adjustment of the sensor system is effected, if one or several of the measuring signals of the calibration mode deviate from the stored measuring signals of the second calibration step.