For two years now, CEA INES is involved in the development of insulated metal substrates (IMS) for CPV receivers. In an effort for establishing the reliability of such a new design compared to state-of-the-art direct bonded copper (DBC) design, accelerated ageing test have been carried out. During these tests, several characterization tools were used including current voltage measurements, X-ray tomography and electroluminescence. A new method for the characterization of thermal inhomogeneities has been developed, the so-called Enhanced Electroluminescence (EEL) which is described in this paper.