The invention relates to a method for laser marking and to a device which comprises: a) a control device for detecting graphics information which describes a graphic with different brightness values; b) the control device is configured to generate a position control signal and a laser control signal depending on the graphics information; c) a laser, the laser light output of which can be controlled by the laser control signal; and d) a beam guiding lens for guiding the laser light which is emitted by the laser onto the security document; and e) at least one position control device which can be actuated by the position control signal for positioning the laser light in a temporally adapted manner to the laser light output or laser pulse energy which is fixed by the laser control signal relative to the security document, with the result that a precisely positioned marking of the security document with the graphic which has the different brightness values can be carried out, wherein the device (1) comprises an absolutely measuring head and a decoupling unit for decoupling a defined proportion of the laser light from a beam path for the laser marking, wherein the measuring head outputs a calibration signal which represents the absolutely measured light output/pulse energy, wherein the control unit is configured to perform at least one calibration step during operation of the device, in which at least one calibration step the laser control signal is generated for a brightness value and is kept constant during the calibration step, the power output or energy which is available for the marking is concluded from the measured power output or energy of the decoupled defined proportion of the laser light and is compared with the setpoint laser light output/setpoint laser pulse energy, and the calibration is performed via a change in the assignment function (18) if a difference above a predefined tolerance threshold is determined during the comparison.