A method of deriving simultaneous dual energy scans on a conventional EMI CT5005 scanner is described. The only change that has to be made to the scanner involves the detector collimator where alternate slits are covered by a thin metal foil. The detector readings from the covered and uncovered slits are separated by software before picture processing to give two pictures from one scan--one at the usual X-ray energy and the other at a higher X-ray energy due to the beam hardening produced by the foil. A procedure is then outlined for converting these into pictures showing the effective electron density and the effective atomic number for the whole scan field.