On a radiograph, a three-dimensional object is presented in a two-dimensional plane (the film). The appearance of both the object and its defects depends on the orientation of radiation relative to the object.
As shown in figure 1-12, the image of a gas cavity in a casting may be circular or elongated depending on beam orientation.
In general, the beam of radiation should be at right angles to the film and a specimen should whenever possible be laid flat on the film cassette. Special angle shots are, however, sometimes useful.
Figure 2-12 (A) shows a situation whereby detection of lack-of-side wall fusion in a V-weld is not performed optimally. Angled radiation (B) is more likely to show up this type of weld defect.