The non destructive testing of industrial pieces is evaluated on the basis of numerical radiographies.The context of the study is the online control of the fuel rods production. A direct model of a numerical radiography formation is proposed and detailed for an acquisition system consisting of a CCD video connected to a converter screen by an optical system. As this approach does not allow the determination of the measured matter thickness from the X-ray photograph, an approximate model based on realistic approximations of the industrial non destructive testing, has been developed. For the specific cases it is possible to inverse the model and to reach the quantitative information present in the x-ray photograph, in other words, the map of the X-rays measured matter thickness. It becomes then possible to access to the quantitative parameters of the possible defects present in the measured specimen, such as the surface and the bulk. To reach the 3 D information on the defects a 3 D reconstruction method, from 3 X-rays photographs, is proposed.The inverse problem is solved by the non convex energy minimization. (A.L.B.) 109 refs.