|International Symposium on Computerized Tomography for ...|
Though advances have been made in digital information processing, most of radiographs are still performed with conventional films, especially due to limitations in the spatial resolution of the digital radiographic systems. As each point of a radiograph contains a projection of what the radiation has encountered in passing from the source through the material, the internal structures encountered are thus superimposed on each other in the acquired image, which complicates interpretation. So testers are facing the delicate task of establishing a complete diagnosis including the detection, the localization, the characterization and the accurate positioning of incoming faults within the thickness of the part being tested. Since 1998 EDF has been involved in the evaluation of the performance of RADView LIBERTY film digitizer and of software tools developed by its R&D Division, such as :
RADView workstations and software provide a simple, easy-to-use PC-based solution for managing, displaying and analyzing radiographic images. The system includes a high-resolution monitor, image printer, flatbed document scanner, workstations, optical disk storage device and image digitizer (5). The overall quality of the digitization protocol has been evaluated. Firstly, by interpretation of the images obtained, testers can assess whether the digital radiograms can be compared to conventional ones.
Secondly, to evaluate the image quality of the processing system, classical parameters as spatial resolution, contrast resolution, dynamic range, local and global distortion have been estimated. Specific guidelines for EDF film digitization procedures have been written, based on the digitization of a standard film (6). To determine the detail perceptibility of the imaging system, i.e. performance of digitization, an object with preliminary known geometry and with different levels of object contrast has been used (steel specimen composed of various electro-eroded slots). The obtained perceptibility depends on both the width and the depth of the slots. Conventional exposures of such an object, under controlled conditions, have been compared to digitized images in order to decide whether the expected information is in fact visible on the image. Herewith the information capability of the system has been determined (fig. 1).
So single image digitization is not sufficient to detect very thin defects. That is why EDF uses « digital double loading », which means :
The obtained image has increased signal to noise ratio and a very good quality that helps for interpretation.