Method for controlling a flux distribution of evaporated source material in a system for thermal evaporation with electromagnetic radiation, wherein the system comprises an electromagnetic radiation source for providing an electromagnetic radiation, a vacuum chamber containing a reaction atmosphere and a detector for measuring electromagnetic radiation, wherein a source material and a target material to be coated are arranged in the vacuum chamber and the radiation source is arranged such that its electromagnetic radiation impinges at an angle, preferably at an angle of 45°, on a source surface of the source material for a thermal evaporation and/or sublimation of the source material below the plasma threshold, and wherein the detector for measuring electromagnetic radiation is arranged such that electromagnetic radiation reflected on the source surface reaches the detector, the method comprising the following steps: a) defining a desired distribution of a flux of source material evaporated from the source surface and an impinging distribution of electromagnetic radiation required for the desired distribution, b) determining an expected distribution of electromagnetic radiation reflected on the source surface based on the desired distribution and the impinging distribution of step a), c) providing electromagnetic radiation with the required impinging distribution defined in step a) by the electromagnetic radiation source, d) measuring electromagnetic radiation reflected on the source surface by the detector, e) determining a measured distribution of electromagnetic radiation reflected on the source surface based on the measurement data of step d), f) determining differences between the expected distribution determined in step b) and the measured distribution determined in step e), g) redetermining the required impinging distribution of electromagnetic radiation provided by the electromagnetic radiation source to minimize the differences determined in step f), and h) providing electromagnetic radiation with the required impinging distribution redetermined in step g) by the electromagnetic radiation source. a) defining a desired distribution of a flux of source material evaporated from the source surface and an impinging distribution of electromagnetic radiation required for the desired distribution, b) determining an expected distribution of electromagnetic radiation reflected on the source surface based on the desired distribution and the impinging distribution of step a), c) providing electromagnetic radiation with the required impinging distribution defined in step a) by the electromagnetic radiation source, d) measuring electromagnetic radiation reflected on the source surface by the detector, e) determining a measured distribution of electromagnetic radiation reflected on the source surface based on the measurement data of step d), f) determining differences between the expected distribution determined in step b) and the measured distribution determined in step e), g) redetermining the required impinging distribution of electromagnetic radiation provided by the electromagnetic radiation source to minimize the differences determined in step f), and h) providing electromagnetic radiation with the required impinging distribution redetermined in step g) by the electromagnetic radiation source.