What is determining geometric unsharpness in radiographs?

What is determining geometric unsharpness in radiographs?

Three factors govern the discernibility of defects in a radiograph:

1. Geometrical effects:

  • Size of the source
  • Source-to-object distance
  • Defect-to-film distance

2. Film properties (governing image quality):

  • Graininess
  • Contrast
  • Fog
  • Inherent unsharpness

3. Quality of radiation applied.

What is determining geometric unsharpness in radiographs?

Geometric unsharpness

X-ray tubes and radioactive sources always produce radiographs with a certain amount of blurring – the “geometric unsharpness”, Ug in fig. 1-11, because of the finite dimensions of the focal spot or source size.

The magnitude of this unsharpness, Ug , is given in the following equation:

Ug Formula

The maximum value of Ug related to a defect situated at a maximum distance from the film (and for which a = t) can be calculated from the formula:

Ug max Formula

In this situation the unsharp images of each of the two edges of the defect may overlap, as shown in example C. The result is that image C not only becomes unsharp, but also suffers a reduction in contrast compared to image A, made with a point source and image B made with a relatively small source.

Inherent unsharpness

Not only the silver halide crystals directly exposed to X-radiation are formed into grains of silver, but also (albeit to a lesser degree) the surrounding volume of emulsion. This cross-sectional area represents the “inherent unsharpness” or “film unsharpness” Uf .

So, even in the absence of geometric unsharpness, if the radiation energy is high enough, film unsharpness can occur: the so called “inherent unsharpness”. If a steel test plate with a sharp thickness transition is radiographed with high energy X-rays, there will be a gradual transition of film density across the image of the “step” from A to B.

Without inherent unsharpness, the film would show an absolutely sharp transition between the two densities, as shown in figure 3a-11. In practice, the density change across the image is as shown in figures 3b, 3c and 3d-11.

The width of this transitional area (Uf), expressed in mm, is a measure of film unsharpness. 

Table 1-11 and figure 4-11 show experimentally determined values of inherent unsharpness for film exposed at various radiation energy levels. These values are based on the use of filters and thin lead intensifying screens; thicker screens produce slightly higher values. If no lead screens are used, Uf is 1.5 to 2 times smaller. Uf is influenced mainly by radiation intensity and the type of intensifying screens used; the type of film is hardly of any consequence.

The distance between film and intensifying screen is of great importance for the value of Uf .

Good contact between film and intensifying screen is imperative and can be achieved by vacuum-packing of film and screens together.

Table 1-11. and Fig. 4-11.

From the above information it can be deduced that Uf increases at higher radiation energies. 

Total unsharpness

Total film unsharpness Ut is determined by the combination of Ug and Uf . The two values cannot be just added up to arrive at a figure for Ut .

In practice, the following formula produces the best approximation for film unsharpness Ut :

Ut Formula
Ut Formula

Broadly, if one value of unsharpness (Ug or Uf ) is more than twice the value of the other, the total unsharpness is equal to the largest single value; if both values of unsharpness are equal, total unsharpness is about 2 = 1.4 times the single value.

If necessary, Ug can be reduced by increasing the focus-to-film distance. This can only be done to a limited extent because, due to the inverse square law, exposure times would become extremely long. As a compromise an optimum focus-to-film distance F is chosen whereby Ug = Uf .

Fig. 1-11 Geometric usharpness

Consequently, Ug can be reduced to any required value by increasing the source-to-film distance. However, in view of the inverse square law this distance cannot be increased without limitation, as extremely long exposure-times would result. The formula also indicates that geometric unsharpness assumes more and more importance as the distance between defect and film increases.

A special case arises, however, when one uses a micro focus X-ray tube with a focal spot size in the range 10-50 µm. With such a small focus size, the image can be deliberately magnified by using a short source-to-specimen distance, and a large specimen-to-film distance, and still retain an acceptably small value of Ug. The advantage of this technique, called the “projective magnification method”, is that the graininess always present in a photographic image is less of a disturbing factor in the discernibility of very small defects.

Figure 2-11 shows the effect of geometric unsharpness on the image of a defect smaller than the focus size.

Fig. 2-11. Geometric unsharpness effect
Fig. 3-11. Inherent (film) unsharpness