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What is image distortion?
In this article:
- Image Distortion Occurs When 3D Objects Are Projected onto 2D Film: Radiographic distortion arises because a three-dimensional object is represented on a flat surface, altering the appearance of both the object and its internal defects.
- Beam Orientation Significantly Affects Image Accuracy: The shape and visibility of features—such as gas cavities—can vary depending on the angle at which the radiation beam intersects the object, leading to circular or elongated images.
- Optimal Imaging Requires Perpendicular Beam Alignment: To minimize distortion, the radiation beam should ideally be perpendicular to the film, and the object should lie flat on the film cassette whenever possible.
- Angled Shots Can Be Useful for Specific Defect Detection: In some cases, such as identifying lack-of-sidewall fusion in welds, angled radiation improves flaw visibility, even if it introduces some distortion.
- Understanding Distortion Is Key to Accurate Interpretation: Recognizing how geometry and beam direction influence radiographic images helps inspectors differentiate between real defects and projection artifacts.
What is image distortion?
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.
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