Abstract
All X-ray computerised tomography systems that are available or proposed base their reconstructions on measurements that integrate over energy. X-ray tubes produce a broad spectrum of photon energies and a great deal of information can be derived by measuring changes in the transmitted spectrum. It is shown that for any material, complete energy spectral information may be summarised by a few constants which are independent of energy. A technique is presented which uses simple, low-resolution, energy spectrum measurements and conventional computerised tomography techniques to calculate these constants at every point within a cross-section of an object.