Abstract: A method of determining an effective atomic number, Zeff, of an object may include obtaining a plurality of radiographic images of the object. Each radiographic image can be obtained using a different independent X-ray energy level. An intensity value for each pixel in a region of interest in each radiographic image can be determined. A plurality of measured ratios, R, can be formed using attenuation coefficients from a pair of different radiographic images. At least one adjusted measured ratio, Rm, can be calculated based on the plurality of measured ratios, R, and at least one corresponding estimation coefficient, ?. Zeff values can be assigned based on a comparison of the at least one adjusted measured ratio, Rm to a material attenuation database.