It refers to the non-proportional increase in the dimensions of a radiographed object relative to the actual dimensions of that object. It happens when there is a change in the angle of the incidence x-ray beam or when the receptor is not parallel to the object (Figure 10). 2,4,5,7,8
Figure 10. Distortion of the shape of the rectangle due to a change in the angle of incidence of an x-ray beam.