(EN) A method of measuring the transfer function of an X-ray optical component (12) over wide range of X-ray energies, which includes steps of : using the X-ray optical component (12) between an X-ray source (10) and a scattering target (16) to obtain a first spectrum; obtaining a second scatter spectrum from the same or a similar target without the X-ray optical component (12) between the X-ray source (10) and the scattering target (16); and calculating the transfer function by the ratio of the first scatter spectrum to the second scatter spectrum. The method is used to improve the accuracy of X-ray quantitative methods in an apparatus where an X-ray optical component (12) is used between the X-ray source (10) and the specimen (16) to be investigated by utilizing the method described above.