An X-ray detector includes a directly converting semiconductor layer for converting an incident radiation into electrical signals with a band gap energy characteristic of the semiconductor layer, and at least one light source for coupling light into the semiconductor layer, wherein the generated light, for the simulation of incident X-ray quanta, has an energy above the band gap energy of the semiconductor layer. One embodiment includes at least one evaluation unit for calculating an evaluation signal from the electrical signals generated when the light is coupled into the semiconductor layer, and at least one calibration unit for calibrating at least one pulse discriminator on the basis of the evaluation signal. This provides the prerequisites for a rapidly repeatable calibration of the X-ray detector taking into account of the present polarization state without using X-ray radiation. Another embodiment additionally relates to a calibration method for such an X-ray detector.