Source-to-Film Distance and Object-to-Film Distance 

SFDSource-to-Film Distance – is the distance between the radiation source and the film in radiographic testing, as measured in the direction of the beam.

OFD Object-to-Film Distance – is the distance between the radiation side of the test object and the film surface, measured along the central axis of the radiation beam. In BS EN 1435: 1997, ‘Non-destructive testing of welds – Radiographic testing of welded joints’, which was recently superseded by the new ISO EN BS standard (see below), this was denoted by b.

The other distance used in radiographic testing is the source-to-object distance, which in the superseded BS EN 1435: 1997 was denoted by f. It can be calculated from SFD–OFD. The SFD and OFD are two of the three factors (the third being source size) that determine the geometric unsharpness of the image. The geometric unsharpness refers to the loss in definition on the film, which is due to the geometry of the testing set-up.

For more information on radiography see:

The Capabilities and Limitations of NDT – Part 3. Radiographic Methods, by Dr R Halmshaw. Available from www.bindt.org/shopbindt/books

BS EN ISO 17636-1: 2013, ‘Non-destructive testing of welds. Radiographic testing. X- and gamma ray techniques with film’.

What the hec?! articles are not intended to be the definitive account on the topic or acronym in question. Readers’ comments and contributions are welcomed. Email: ndtnews@bindt.org