An additive manufacturing apparatus (400; 410) arranged for calibrating a laser scanner, the additive manufacturing apparatus comprising a building area upon which a 3D object is built, the additive manufacturing apparatus comprising: a calibration plate comprising reference markings, the calibration plate being positioned in the building area, or the calibration plate being positioned outside of the building area closer to the laser scanner; the laser scanner, the laser scanner being configured to send a laser beam to a predefined location on the calibration plate which forms a laser marking on the calibration plate; an image acquisition assembly comprising an image acquisition device, the image acquisition device configured to photograph at least a portion of the calibration plate associated with the predefined location on the calibration plate; and a computer control system comprising one or more computers having a memory and a processor, the computer control system configured to: cause the laser scanner to direct the laser beam to a plurality of predefined locations on the calibration plate thereby creating one or more laser markings; receive an image of at least a portion of the calibration plate including at least one of the one or more laser markings; determine laser marking coordinates for each predefined location based on the position of the formed laser marking relative to the reference markings on the calibration plate; and determine scanner corrections based on the determined laser marking coordinates. a calibration plate comprising reference markings, the calibration plate being positioned in the building area, or the calibration plate being positioned outside of the building area closer to the laser scanner; the calibration plate being positioned in the building area, or the calibration plate being positioned outside of the building area closer to the laser scanner; the laser scanner, the laser scanner being configured to send a laser beam to a predefined location on the calibration plate which forms a laser marking on the calibration plate; an image acquisition assembly comprising an image acquisition device, the image acquisition device configured to photograph at least a portion of the calibration plate associated with the predefined location on the calibration plate; and a computer control system comprising one or more computers having a memory and a processor, the computer control system configured to: cause the laser scanner to direct the laser beam to a plurality of predefined locations on the calibration plate thereby creating one or more laser markings; receive an image of at least a portion of the calibration plate including at least one of the one or more laser markings; determine laser marking coordinates for each predefined location based on the position of the formed laser marking relative to the reference markings on the calibration plate; and determine scanner corrections based on the determined laser marking coordinates. cause the laser scanner to direct the laser beam to a plurality of predefined locations on the calibration plate thereby creating one or more laser markings; receive an image of at least a portion of the calibration plate including at least one of the one or more laser markings; determine laser marking coordinates for each predefined location based on the position of the formed laser marking relative to the reference markings on the calibration plate; and determine scanner corrections based on the determined laser marking coordinates.