In radiography, we want the image of a part to show edges and small details clearly. If an edge is sharp, it changes from light to dark over a very short distance. If an edge is unsharp, it changes gradually across a wider band. That blurred band is called the penumbra.
A useful way to picture it is a shadow on a wall. A tiny torch makes a fairly sharp shadow. A wide lamp makes a softer shadow because light comes from different parts of the lamp. Industrial X-ray tubes and gamma sources are not perfect mathematical points, so their shadows can also have soft edges.
Key point: geometric unsharpness is not caused by the material being inspected. It is caused by the geometry of the exposure setup.
f = effective focal spot size | a = source to front of subject | b = front of subject to film (gap + thickness)
The formula says the blur gets larger when the focal spot size f gets larger, or when the object is farther from the film or detector. The blur gets smaller when the source is farther away from the object.
At Year 10 maths level, read Ug = f × b / a as: multiply the source size by the object-to-film distance, then divide by the source-to-object distance.
| Symbol | Plain meaning | If it gets bigger... | How to improve sharpness |
|---|---|---|---|
| f | Size of the source or focal spot | Blur increases | Use a smaller focal spot or source where practical |
| b | Distance from the front of the subject to the film or detector | Blur increases | Keep the part close to the detector |
| a | Distance from the source to the front of the subject | Blur decreases | Move the source farther away, if exposure conditions allow |
Because rays come from a wider area. Each part of the focal spot makes a slightly shifted shadow, and the shifted shadows overlap into a soft edge.
Ug increases. The shadow has more distance to spread before it reaches the film or detector.
Source far from object, if other factors are unchanged. Greater source-to-object distance makes the rays more parallel and reduces geometric blur.
Radiographic images can also lose detail from motion, poor contact, detector limits, scatter, screen effects, or processing. This calculator focuses only on geometry.
Moving the source farther away improves sharpness, but it also reduces intensity at the detector. Exposure time or technique settings may need to change.