Method for calibrating at least one apparatus (1) 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 by means of an energy beam that can be generated via an irradiation element of an irradiation device (7) of the apparatus (1), wherein a determination unit (2) is provided for determining at least one parameter of radiation (3) inside a process chamber (5), wherein a calibration beam source (4) is arranged or generated inside the process chamber (5) of the apparatus (1), in particular in a build plane (9) or a region above the build plane (9), wherein at least one parameter, in particular the intensity, of radiation (3) emitted by the calibration beam source (4) is determined via the determination unit (2).