1. A method for calibration of a transformation of at least two X-ray attenuation values determined using different X-ray spectra for a material to a value for the density and a value for the atomic number of the material, the method comprising: recording a first distribution of first X-ray attenuation values obtained from a calibration phantom using a first X-ray spectrum, and a second distribution of second X-ray attenuation values obtained from the calibration phantom using a second X-ray spectrum, wherein the calibration phantom includes at least three calibration samples arranged physically separately from one another and including at least one of different densities and atomic numbers; calculating a density function which associates a value for a density of the material with a combination of a first recorded X-ray attenuation value for a material with a second recorded X-ray attenuation value for the material; and calculating an atomic number function, associating a value for an atomic number of the material with a combination of the first recorded X-ray attenuation value for the material with the second recorded X-ray attenuation value for the material; and determining a value for the density and for the atomic number of the calibration sample with the aid of the density function and the atomic number function from the first and second X-ray attenuation values recorded for a calibration sample; determining a discrepancy between the determined values and the actual density and atomic number of the calibration sample; and using the discrepancy as the basis to produce a mapping rule which changes the values determined by the density function and the atomic number function to the actual values.