T.O. 33B-1-16-456.6.2.4.2If the discontinuity is sufficiently prominent, both exposures may be made on the same film. The distance of thediscontinuity above the film in either case is given by the following expression:dbta b=+Where:d = distance of discontinuity above film planea = distance of the source shiftb = change in position of discontinuity image on radiographst = source film distance6.6.2.4.3In some cases, it is sufficient to know which of the two surfaces of the object is nearest to the discontinuity. In thiscase, the shift of the discontinuity and marker images are measured. If the shift of the discontinuity image is less thanone-half the shift of the markers that are on the source side of the object, then the discontinuity is nearer to the film. Ifthe shift is greater than one-half, the discontinuity is nearer the markers on the source side of the object.6.6.2.5 ThicknessMeasurement.Sometimes it is impossible to determine the thickness of an object using conventional mechanical measurementtechniques. In these instances, a special radiographic technique for the measurement of material thickness may beemployed. Although the mathematical development of a relationship between film density and the thickness of anabsorber is too complex for practical use, an empirical method of thickness measurement has proven useful. Byimaging the object of interest and a step wedge of the same material on a single film, it is possible to obtain a goodestimate of the thickness of the material section. It is imperative that the composition and structure of the step wedgebe the same as that of the material being measured if any accuracy is to be achieved. Thickness is determined bymeasuring the resultant film density and finding the step on the wedge that is nearest to that density. For best results,the section of interest and the step wedge should be placed as close to one another as possible to avoid variations in theuniformity of the radiation beam. This technique may also be employed to measure void dimensions (parallel to thebeam direction).6.6.2.6 Stereo (Three Dimensional Techniques).Objects viewed with a normal pair of eyes appear in their true perspective and correct spatial relationship because of aproperty of the eyes, which is called stereoscopic vision. That is, each eye receives a slightly different view and the twoimages are combined by interpretation to give the impression of three dimensions. A single radiographic image doesnot possess perspective. Therefore it does not give the impression of depth. However, some estimate of depth can bejudged from detail observed by an experienced radiographer. The mechanics of stereoradiography are relatively simple.Two radiographs are made from two positions of the X-ray tube. These positions can be thought of as the "left eye" andthe "right eye." As a matter of fact, the two positions represent the distance between the eyes. A so-called stereoscopeis used to view the images (See Figure 6-19). Each eye sees only one image but the brain blends these two images intoone.
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