Apparatus for additively manufacturing three-dimensional objects by means of successive layerwise selective irradiation and consolidation of layers of a build material which can be consolidated using at least one energy beam, wherein an irradiation device is adapted to generate and guide the energy beam to at least one position of a build plane, wherein a determination unit is adapted to determine at least one parameter of radiation propagating in a process chamber of the apparatus, wherein a calibration element is arrangeable or arranged in the process chamber, wherein the calibration element comprises at least one calibration section that is adapted to emit measurement radiation upon irradiation with the or an energy beam and in that the determination unit is adapted to detect the measurement radiation, wherein a control unit is adapted to calibrate the irradiation device.