Abstract: An apparatus and method for gauging the thickness of a moving sheet material, typically a metal, which has a variable alloy composition over its length. This is accomplished by measuring the spectral energy of a radiation beam which has not been absorbed by the material and passed perpendicularly through the material by utilizing a length absorption filter radiation detector. The length absorption filter spectrally filters the radiation into separate wavelengths and measures the intensity of these wavelengths. Electrical outputs representing these measured values is output to a processor which then determines the thickness of the material based on the transmission ratios measured for each measured energy band and the absorption coefficients of the elements in the material. This result is then used to control the rolling mill to provide real-time on-line feedback.