Real-time measurement of the composition and spectroscopic characteristics of the radiation field inside the ATLAS detector and cavern will be carried out with MediPix2 devices. In view of this application, a calibration of the low energy threshold was performed by illuminating the devices with low energy photons, using 241Am sources and x-rays. A procedure was thereafter established for the calibration of all MediPix2 devices to be used in ATLAS. Using x-ray tubes with filters at the Czech Metrology Institute, photons of energies between 8 keV and 250 keV were obtained and the detection efficiency of the MediPix devices to these radiations was determined. An exponential decrease of the photon intensity as a function of detector depth was observed, illuminating the detector with the beam almost parallel to the surface. The properties of the decrease of intensity were used to determine the exact incident angle of the beam.