We describe a device for calibrating an irradiation system of an apparatus for producing a three-dimensional workpiece, the irradiation system comprising an irradiation unit for selectively irradiating an irradiation beam onto an irradiation plane, wherein the device comprises: a control unit configured to control the irradiation system to irradiate the irradiation beam onto the irradiation plane, and an optical detection unit coupled to the control unit, wherein the optical detection unit comprises an optical detector and an objective lens for optically detecting a portion of the irradiation plane, wherein the optical detection unit is configured to detect a position of a spot of the irradiation beam on the irradiation plane, wherein the objective lens is adapted to be arranged, with respect to an irradiation beam path of the irradiation beam, between the optical detector and an irradiation beam scanner of the irradiation system, wherein the optical detection unit is configured to detect the position of the spot of the irradiation beam in multiple focal planes based on a focal length of the optical detection unit being adjustable, wherein the optical detection unit is configured to output a signal to the control unit in response to the optical detection unit detecting the position of the spot of the irradiation beam on the irradiation plane, and wherein the control unit is configured to control the irradiation system based on the signal output from the optical detection unit to the control unit.