This method essentially consists of analyzing x-ray filtration data in terms of Laplace transforms and obtaining the spectral energy distribution in terms of a sum of the corresponding inverse Laplace transforms. Some of the functions which are especially suited for this application are given and a graphical method of analyzing filtration data in terms of these functions is described. Examples of the practical application of this method to several low energy x-ray machines are included with a discussion of the advantages, disadvantages, and probable uncertainties which are involved.