This paper presents the procedure to determine the size of the focal spot of an X-ray tube in order to evaluate the quality of radiographic images of welded joints. This paper collects important data of studies related to this project, also it takes into account fundamental aspects of international standards related to Non-Destructive Testing and how to determine the focal spot size of an industrial radiography tube. The “Pinhole Imaging” method allows to determine the real size of the focal spot of X-ray tubes on a radiographic film. The method development is disclosed, data collection and statistical processing are described. An experimental statistical analysis was applied to 70 radiographic tests to measure the focal spot of each one. With the results of focal spot sizes obtained it was possible to calculate the geometric unsharpness in welded specimens, later to know the value of the relative error between the reference value of the theoretical focal spot and the experimental focal spot.
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