What is geometric unsharpness in radiography and how is it calculated?
Effect of source size (F = the maximum dimension of the radiating source):
A radiation source is not point-like and always has an area of several millimeters, and therefore cannot produce a sharp image. Smaller dimensions of the radiation source produce sharper images on the film.
Geometric unsharpness cannot be completely eliminated because there are no point-like light sources.
Distance between object and film (d = object-to-film distance)
Another way to reduce geometric unsharpness is to bring the film as close as possible to the surface of the test object.
In other words, keep the distance between the surface of the test object on the source side and the film as small as possible.
Distance from source of radiation to object (D = source-to-object distance)
Another way to reduce geometric unsharpness is to increase the distance between the radiation source and the surface of the test object. The greater the distance from the radiation source, the smaller the geometric unsharpness.
Geometric unsharpness calculator
How to calculate geometric unsharpness in radiography? Geometric unsharpness is calculated using the formula:
Ug = F * d / D
Now calculate the geometric unsharpness in radiographic testing with our calculator.
Geometric unsharpness limitations according to ASME Code V Article 2
Example of geometric unsharpness limitations:
The following limits are specified in ASME Code V Article 2 – T-274.
Material Thickness, mm [in.]
Ug-max, mm [in.]
< 50 [< 2]
0.51 [0.020]
50 through 75 [2 through 3]
0.76 [0.030]
> 75 through 100 [>3 through 4]
1.02 [0.040]
> 100 [>4]
1.78 [0.070]
Geometric unsharpness limitations according to ISO 17636-2
The upper limit of geometric unsharpness is specified by the minimum distance [fmin] in accordance with ISO 17636-2.
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