Abstract: Device and method are provided for separating images according to material species in nondestructive x-ray or .gamma.-ray imaging. Using the x-ray (or .gamma.-ray) attenuation coefficient of materials as functions of x-ray energy, simultaneous linear equations are set up and solved for obtaining the image for a specific material when multiple x-ray (or .gamma.-ray) energies are used. The device disclosed in this invention features real-time, material-specific image separation. Applications include x-ray security inspection and industrial x-ray or .gamma.-ray noninvasive structural inspection.