[en] Both synchrotron monochromatic X-ray topography, in the reflection mode, and synchrotron white-beam X-ray topography, in the transmission mode, were used for the investigation of defects in α-HgI₂ crystals. The combination of these two techniques appeared to be very fruitful for the characterization of these defects either in relation to the growth conditions or with the deformation introduced during the processing of lamellae for the detector fabrication. These capabilities were particularly enhanced by using the ESRF which is a 3rd generation synchrotron source. (orig.)