[en] The gage determines the weight per unit area and/or the thickness of a sheet of material, passed by the gage, by accounting for local thickness variations and material composition. For this purpose transmission as well as backscattering of an X- or γ-ray directed towards the sheet is picked up by means of separate detectors. The electric signals coming from the detectors are fed into a computer, calculating at first the transmission and backscattering intensities for the material with the aid of special mathematical formulas. For the calculation of a corrected signal corresponding to the absorption coefficient of the material these backscattering intensities are combined with nominal values of the composition and calibration data. This signal, together with the signal for the transmission intensity, is used for the determination of the mass per unit area and the thickness. (DG)