The success of radiographic interpretation is dependent upon the quality of the
film presented. If the film does not meet the minimum applicable standards for
quality then it should be rejected and reshot. The manufacturer’s interpreter
may, for economic reasons, not be inclined to reject radiographs which do not
meet the minimum quality standards. Therefore any third party viewing the
radiographs should be extremely careful to correctly assess the quality of the
radiographs prior to endorsing the relevant report, otherwise they will be open
to criticism should the film become the subject of any subsequent legal inquiry.
When assessing a film for quality a number of items must be considered. These
are discussed below.
All radiographs must be permanently and uniquely marked with sufficient
information so as to permit their identification with the component radiographed
at a later stage. It is often useful to include such items as the date of test and
heat treatment or repair status of the component in the identification.
Radiographic identification could appear on the radiograph as a radiographic
image but there is usually no reason why it should not be added by any other
suitable means. A written procedure should be in force describing the standard
method to be used for identifying radiographs.
Location markers on a radiograph serve two functions: they permit the
radiograph to be identified with the area of the component radiographed and
they serve to prove that the component has been fully covered by the
technique used. Refer to the sections above on radiographic techniques for
details. Wherever possible location markers should permanently identify the
radiograph with the area radiographed. Items such as pressure vessels are
usually hard stamped with a permanent radiographic datum. A written
procedure should be in force which describes the standard method used for the
placement of location markers.
It is important that the film density is within the specified range since a film
having low film density will also have inferior film contrast. EN ISO 17636-1
requires a minimum film density of 2.0 for Class A radiography and a minimum
of 2.3 for Class B. ASME V article 2 requires a minimum of 1.8 for X-ray and 2
for gamma ray techniques. In most cases (including EN ISO 17636-1 and ASME
V article 2) the minimum figures for film density apply to the area of interest
(the diagnostic area) on the radiograph. In weld radiography, for example, film
density should generally be measured on the weld area between the location
markers (which identify the ends of the diagnostic film length).
Density is usually assessed by using a measuring device known as a
densitometer. Anyone accepting radiographs which do not meet the applicable
density requirements is open to criticism at a later stage should litigation follow
a component failure.
ASME V article 2 requires that the film density within the area of interest must
not vary by more than minus 15 or plus 30% from the value measured through
the body of the IQI. If necessary additional IQIs can be used in order to satisfy
this requirement for exceptional areas.
Occasionally an upper limit is specified for film density. ASME V article 2, for
example, specifies an upper limit of 4.
Radiographic sensitivity is not directly related to the minimum detectable defect
size. However, a radiograph that meets the applicable code requirement for
radiographic sensitivity is much more likely to provide good defect sensitivity
than a radiograph which fails to meet the code requirements.
The sensitivity of a radiograph depends upon the parameters chosen to produce
that radiograph (see the section on the production of a radiograph). If any of
the relevant parameters are altered the sensitivity will be affected. It is
therefore essential to use IQIs to prove that adequate radiographic quality has
been attained. Except in the case of the panoramic technique, which has been
described above, at least one IQI should generally appear on each radiograph.
Anyone viewing radiographs should be careful to check the radiographic
sensitivity meets the requirements of the applicable code. Anyone who fails to
do is open to criticism should litigation follow a component failure.
Artefacts and other unwanted images.
In film radiography an artefact can be defined as any image resulting from a
cause that is not directly associated with the object that has been radiographed.
Artefacts can be produced by mechanical or chemical damage to the film and by
damaged or dirty intensifying screens. Sometimes radiographic images may be
formed by things such as debris on the inside of a pipe. These images, while
they are strictly speaking not artefacts, can also interfere with the proper
interpretation of the radiograph.
When radiographs are produced on a commercial basis it is not possible for
every film to be free from artefacts. An artefact only becomes significant when
it cannot be identified as such or when it hinders the interpretation of the film.
These two factors are rather subjective but if any doubt exists then the
interpreter should call for a repeat radiograph. A list of possible artefacts is
given in the next section.
Interpretation of radiographic images.
Three types of image may appear on a radiograph, due to:
2 Surface irregularities in the component.
3 Internal discontinuities in the component.
Every image within the diagnostic area of a radiograph must be identified as
one of these three. It is not permissible to reject a component simply because
an image appearing within the diagnostic area cannot be interpreted or because
artefacts which are not within the diagnostic area. The following sections
attempt to give a description of various types of image which may be seen. The
ability to successfully identify all flaws and artefacts on a radiographic images is
a skill which can only be perfected with time and experience.