A system and method are presented for determining an optimized set of estimation coefficients for use in the gray scale conversion of measured intensity ratios derived from radiographic images of an object. Radiographic images for known materials may be obtained using different source energy levels. Measured training ratios for each known material may be determined intensity values derived from pairs of radiographic images. Standard attenuation data for each known material may also provided. A genetic algorithm may be used to obtain an optimized set of estimation coefficients for the known materials using initial candidate sets of estimation coefficients, the measured training ratios, and the standard attenuation data. The optimized set of estimation coefficients may be used in the gray scale conversion of measured ratios derived from radiographic images of unknown materials for the determination of the atomic number of the unknown materials.