in the sense that it only allows measurement at a given moment device 3 will be for example a fixed lens or optical fiber connecting an image element of screen 2 to the detector 4 in the case of an optical fiber means not shown will be provided for moving the screen 2 in the two directions x and y so as to sequentially position each image element in front of the inlet face of the optical fiber a layer of optical fibers may also be provided or any light guide in the form of a linear bar in front of the screen 2 and able to be transformed into a concentrated guide in front of the input of detector 4 in this case a single movement of the plate is required along y if the bar covers a row of image elements along x thus after being exposed to the x rays passing through the body 5 the screen 2 stores a resultant image of the image formation of the body under the control of the operator the stored image may be visualized at will for that excitation potentials are applied to electrodes 23 and 24 by means of generators 6 and 7 and sequential switches 16 17 successively connecting the generators 6 and 7 to the different electrodes furthermore detector 4 detects the light beam emitted by each intersection point at each excitation order and delivers an electric signal at an output s to the processing circuit sensor 4 may also be an assembly of detectors 40 41 aligned as is shown in.