The ELSA facility located at CEA DAM DIF consists of a 30 MeV, 15 ps rms, 1 nC compact linac. X-rays are produced either in the MeV energy range through interaction between the electron beam and a solid Ta conversion target (Bremsstrahlung radiation) or in the 10-80 keV energy domain through interactions between electrons with a Nd:YAG laser (inverse Compton Scattering radiation). ELSA is dedicated to X-ray test of detectors and thus requires a good knowledge of the X-ray beam. We present here a spatial ans spectral characterization of a new high energy flat panel detector devoted to the X-ray beam parameter measurements. This characterization was achieved with X-ray portable generators and radioactive sources. A methodology for using this flat panel in dosimetry measurements is also proposed and scheduled for experimental validation on ELSA in 2025.