Asymmetry measurements, which are especially sensitive on small amplitudes, provide an additional handle to the nuclear structure apart from cross section data. The use of linearly polarised photons as a tool in nuclear physics demands a reliable description of its production process and predictions of polarisation with small systematic uncertainties. The modelling of the bremsstrahlung spectra so far[1,2] seems not sufficient to describe different radiators and collimation angles with full consideration of all experimental deficiencies over a large energy range. Therefore several improvements were applied which allow to describe the absolute spectra as well as the relative ones to a high accuracy.