Radiography in Modern Industry - Kodak

Figure 93: Double-exposure (parallax) method for localizing defects.If the flaw is sufficiently prominent, both exposures may be made on the same film. (Oneexposure "fogs" the other, thus interfering somewhat with the visibility of detail.) The distance ofthe flaw above the film plane is given by the equation:whered = distance of the flaw above the film plane,a = tube shift,b = change in position of flaw image, andt = focus-film distance.If the flaw is not sufficiently prominent to be observed easily when both exposures are made onthe same film, two separate radiographs are necessary. The shadows of the marker (M 2 ) aresuperimposed and the shift of the image of the flaw is measured. The equation given in thepreceding paragraph is then applied to determine the distance of the flaw from the film.Often it is sufficient to know to which of the two surfaces of the part the flaw is nearer. In suchcases, the shifts of the images of the flaw and the marker (M 1 ) are measured. If the shift of theimage of the flaw is less than one-half that of the marker (M 1 ), the flaw is nearer the film plane; ifgreater, it is nearer the plane of the marker (M 1 ).The above methods of calculation assume that the image of the bottom marker (M 2 ) remainsessentially stationary with respect to the film. This may not always be true--for example, if thecassette or film holder is not in contact with the bottom surface of the specimen or in a situationwhere large tube shifts are used.In such cases, a graphical solution is convenient. It can be shown that if the markers are placedfairly close to the flaw, the image shifts are proportional to the distances from the film plane.Thus, a straight line graph can be drawn of image shift against distance from the bottom (filmside) of the specimen.Radiography in Modern Industry 152