a. The eye is limited in resolution.
b. Discontinuities are detectable only in the direction of primary radiation.
c. Variations in material density are not considered.
d. Definition or sharpness of transition between densities is not considered.
e. Actual defects are usually irregular in shape while penetrameters have a definite size and shape.
Definition or Detail.
Definition or detail in radiography is the sharpness of the image outline reproduced on the film. The size of the focal
spot, the physical condition of exposure and the film resolution determine the definition. If a screen is used, then the
screen resolution will also affect the definition. In addition to the focal spot size, the object-to-film distance is an
important factor in the sharpness of shadow picture (see Figure 6-41). The resolution of the film is a function of grain
size. Refer to Section III for detail discussion of film and related items.
Figure 6-41. Pinhole Picture of Focal Spot.
Since the radiograph is a shadow picture, the geometric interrelationship between the elements of the radiographic
system is important. Ideal X-ray focal spots and radioisotope sources should be pinpoints. With such sources, we
would obtain sharp images under all conditions. All our radiation sources have finite size since X-ray tube focal spots
must be large enough to withstand the energy dissipated as heat to prevent melting and target destruction. The
radioactive activity of an isotope is proportional to the source strength in curies, so the smaller the size, the lower the
To better understand geometrical relationship, refer to Figure 6-42 which illustrates various conditions true to X-ray
and light shadow formations. Diagram A in Figure 6-42 shows that the size of the shadow is to the size of the object as