The application of the geometric principles of shadow formation to radiography leads to five general rules. Although these rules are stated in terms of radiography with x-rays, they also apply to gamma-ray radiography.
1. The focal spot should be as small as other considerations will allow, for there is a definite relation between the size of the focal spot of the x-ray tube and the definition in the radiograph. A large-focus tube, although capable of withstanding large loads, does not permit the delineation of as much detail as a small-focus tube.
Long source-film distances will aid in showing detail when a large-focus tube is employed, but it is advantageous to use the smallest focal spot permissible for the exposures required.
Long source-film distances will aid in showing detail when a large-focus tube is employed, but it is advantageous to use the smallest focal spot permissible for the exposures required.
B and H in the Figure 13 show the effect of focal spot size on image quality. As the focal spot size is increased from 1.5 mm (B) to 4.0 mm (H), the definition of the radiograph starts to degrade. This is especially evident at the edges of the chambers, which are no longer sharp.
2. The distance between the anode and the material examined should always be as great as is practical. Comparatively long-source distances should be used in the radiography of thick materials to minimize the fact that structures farthest from the film are less sharply recorded than those nearer to it. At long distances, radiographic definition is improved and the image is more nearly the actual size of the object.
3. A to D in the Figure 13 show the effects of source-film distance on image quality. As the source-film distance is decreased from 68 inches (A) to 12 inches (D) the image becomes more distorted until at 12 inches it is no longer a true representation of the casting. This is particularly evident at the edges of the casing where the distortion is greatest.
4. The film should be as close as possible to the object being radiographed. In practice, the film--in its cassette or exposure holder--is placed in contact with the object.
In B and E of Figure 13, the effects of object-film distance are evident. As the object-film distance is increased from zero (B) to 4 inches (E), the image becomes larger and the definition begins to degrade. Again, this is especially evident at the edges of the chambers that are no longer sharp.
5. The central ray should be as nearly perpendicular to the film as possible to preserve spatial relations.
As far as the shape of the specimen will allow, the plane of maximum interest should be parallel to the plane of the film.
Finally, in F and G of the Figure 13, the effects of object-film-source orientation are shown. When compared to B, image F is extremely distorted because although the film is perpendicular to the central ray, the casting is at a 45° angle to the film and spatial relationships are lost. As the film is rotated to be parallel with the casting (G), the spatial relationships are maintained and the distortion is lessened.
Figure 13: These graphics illustrate the effects on image quality when the geometric exposure factors are changed. 
CALCULATION OF GEOMETRIC UNSHARPNESS
The width of the "fuzzy" boundary of the shadows in B, C, and D in the above figure is known as the geometric unsharpness (Ug). Since the geometric unsharpness can strongly affect the appearance of the radiographic image, it is frequently necessary to determine its magnitude.
From the laws of similar triangles, it can be seen (In Figure 14) that:
where Ug is the geometric unsharpness, F is the size of the radiation source, Do is the source- object distance, and t is the object-film distance. Since the maximum unsharpness involved in any radiographic procedure is usually the significant quantity, the object-film distance (t) is usually taken as the distance from the source side of the specimen to the film.
Figure 14: Geometric construction for determining geometric unsharpness (Ug).
Do and t must be measured in the same units; inches are customary, but any other unit of length-- say, centimetres--would also be satisfactory. So long as Do and t are in the same units, the formula above will always give the geometric unsharpness Ug in whatever units were used to measure the dimensions of the source. The projected size of the focal spots of x-ray tubes are usually stated in millimetres, and Ug will also be in millimetres. If the source size is stated in inches, Ug will be in inches.
For rapid reference, graphs of the type shown in the figure below can be prepared by the use of the equation above. These graphs relate source-film distance, object-film distance and geometric unsharpness. Note that the lines of Figure 15 are all straight. Therefore, for each source-object distance, it is only necessary to calculate the value of U for a single specimen thickness, and then draw a straight line through the point so determined and the origin. It should be emphasized, however, that a separate graph of the type shown in Figure 15 must be prepared for each size of source.
Figure 15: Graph relating geometric unsharpness (Ug) to specimen thickness and source- object distance, for a 5-millimetre source size.
