How do I calculate the source-to-film distance?
In this article:
- Source-to-Film Distance (SFD) Directly Influences Image Sharpness: The SFD affects geometric unsharpness (Ug), which must be minimized to achieve high-quality radiographic images with maximum detail and flaw detectability
- SFD Can Be Calculated Using a Standard Formula: To match geometric unsharpness (Ug) with film unsharpness (Uf), the required SFD (F) can be calculated using the formula:
F=d⋅fUgF=Ugd⋅f
where dd is the object-to-film distance and ff is the focal spot size - Radiation Energy and Material Thickness Guide SFD Selection: The chosen radiation energy (kV or isotope) and the thickness of the object influence both the required SFD and the achievable image quality
- Graphical Tools and Standards Support SFD Planning: Standards like EN 1435 provide nomograms and charts to determine minimum SFD values for different quality levels and inspection categories
- Balancing SFD with Exposure Time Is Crucial: Increasing SFD improves sharpness but reduces radiation intensity due to the inverse square law, requiring longer exposure times and careful planning
How do I calculate the source-to-film distance?
Preceding paragraphs of this chapter described the effects of geometric unsharpness and the possibility to influence this by adjusting the source-to-film distance. This section will expand on this.
To obtain a radiograph which is as sharp as possible, so as to show maximum detail, the total unsharpness should be kept to a minimum. The radiation energy level selected for making the radiograph, see chapter 9, can serve as a lead. It is, after all, determined by the thickness of the material to be radiographed, but is at the same time also responsible for film unsharpness Uf , which can be extracted from table 1-11 and or figure 4-11.
It is no use to try and keep geometric unsharpness Ug far below the value of Uf , as in that case Uf determines the total unsharpness anyhow. If the aim is to make geometric unsharpness Ug equal to the value of Uf , the source-to-film distance (F) required can be calculated from the following formula :
Instead of calculating F, various code-based procedures and guidelines provide graphs from which minimum distance (Fmin) can be determined.
Figure 5-11 shows a nomogram on the basis of EN 1435, from which the minimum focus distance for two quality levels (category A and B) can be extracted.
How do I check the development process and film archiving properties?