Geometric unsharpness refers to the loss of definition that is the result of geometric factors of the radiographic equipment and setup. It occurs because the radiation does not originate from a single point but rather over an area.
The three factors controlling unsharpness are source size, source to object distance, and object to film distance. The source size is obtained by referencing manufacturers specifications for a given X-ray or gamma ray source. As the source size decreases, the geometric unsharpness also decreases. For a given size source, the unsharpness can also be decreased by increasing the source to object distance, but this comes with a reduction in radiation intensity.
The object to film distance is usually kept as small as possible to help minimize unsharpness. However, there are situations where geometric enlargement occurs, such as when the object distance from the film is increased, which will reduce the definition.
Codes and standards used in industrial radiography require that geometric unsharpness be limited. In general, the allowable amount is 1/100 of the material thickness up to a maximum of 0.040 inch. These values refer to the degree of penumbra shadow in a radiographic image. Since the penumbra is not nearly as well defined, it is difficult to measure it in a radiograph. Therefore it is typically calculated.
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The three factors controlling unsharpness are source size, source to object distance, and object to film distance. The source size is obtained by referencing manufacturers specifications for a given X-ray or gamma ray source. As the source size decreases, the geometric unsharpness also decreases. For a given size source, the unsharpness can also be decreased by increasing the source to object distance, but this comes with a reduction in radiation intensity.
The object to film distance is usually kept as small as possible to help minimize unsharpness. However, there are situations where geometric enlargement occurs, such as when the object distance from the film is increased, which will reduce the definition.
Codes and standards used in industrial radiography require that geometric unsharpness be limited. In general, the allowable amount is 1/100 of the material thickness up to a maximum of 0.040 inch. These values refer to the degree of penumbra shadow in a radiographic image. Since the penumbra is not nearly as well defined, it is difficult to measure it in a radiograph. Therefore it is typically calculated.
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Download Ug Calculator V1.1
