Interpretation of Radiographs
Introduction
Interpretation of radiographs is a skill only gained through long experience.
The interpretation of a radiograph should not be confused with the acceptance
or rejection of a component. The radiograph must first be interpreted and any
defects observed assessed against the applicable standard. A weld or casting
must be accepted on its merits or rejected for its faults and should neither be
accepted nor rejected due to difficulties encountered in the interpretation of
radiographs. Any radiograph not meeting code requirements with regard to
radiographic quality must be rejected.
In circumstances where there is doubt as to the nature of a radiographic image
it is often necessary to visually inspect the component or to cross check the
radiographic results using another NDT method.
Viewing conditions
The success or failure of radiographic interpretation is highly dependent upon
the film viewing conditions. The eye is very sensitive to small variations in film
density once it has developed night vision. Anyone carrying out radiographic
interpretation should therefore not begin to view radiographs until night vision
has developed. Since this cannot be achieved in a brightly lit room it is
important that the films are viewed in low ambient light. Night vision takes
several minutes to develop and so the films should not be viewed immediately
upon entering the viewing room. Five minutes is the recommended period that
should elapse before critical interpretations are made. It is also important that
film is properly masked on the viewer so that the light falling on the eye comes
from the radiograph only. If the film is not adequately masked the eye will be
blinded by the bright light coming from around the film.
Radiographs are easily damaged, therefore the viewing room must be clean and
dry and the radiographs must be handled with care. The viewer should be
mounted on a table or bench large enough to allow the films to be spread out
without the danger of them falling to the floor. A well shielded reading lamp will
allow reports to be read or notes to be made, without unduly increasing the
overall ambient lighting.
The radiographs should be viewed at a normal reading distance (normally less
than 400mm). A low power magnifier (2 or 3X) may occasionally be helpful, but
it should not be necessary for routine examination. In accordance with
governing standards requirements the visual acuity of the radiographic
interpreter must be J1 (corrected or uncorrected) in at least one eye.
Additionally a perception of grey shades should be verified.
The viewing of radiographs is often undertaken in the dark room where the film
was processed. This is satisfactory provided that the viewing bench or table is
clean and well away from the processing tanks. Under normal circumstances
films should NEVER be viewed whilst wet. There are two reasons for this:
1 The film emulsion is swollen with water and the images are not as clear as
when the film is dry.
2 Emulsion is very delicate and any attempt to mask the film will result in
scratches or marks on the film, effectively ruining it.
National standards generally require that the illuminance of a radiographic film
viewer be sufficient to produce a transmitted light intensity of at least 30 and
preferably 300 candela per square metre (cd/m2). This means that a viewer
suitable for viewing radiographic film with a density of 3.0 must have an
illuminance of at least 30,000 cd/m2 with as much as 300,000 cd/m2 being
desirable.
Reporting
The initial interpretation of a radiograph should always be undertaken by the
manufacturer or designated representative. Other interested parties should be
presented with a report which includes an interpretation of each film. They
check this and agree or disagree with it. The radiographic report should contain
the following as a minimum:
– Identification of the item radiographed.
– Date of manufacture.
– Date of radiography.
– Exposure details including the type of equipment used and the tube voltage
for X-ray and the type of isotope for gamma ray techniques.
– Type of film used.
– Type and thickness of the intensifying screens used.
– Geometric details, particularly the FFD or SFD and the effective focus or
source dimension.
– Details of the component being radiographed, including the type of material
and method of manufacture, the thickness, the heat treatment condition
and the repair status.
– Method of film processing.
– Film density achieved.
– Radiographic sensitivity achieved.
– Technician’s name, signature and date.
– Interpreter’s name, signature and date.
– An interpretation of each film and a statement of the component’s
acceptability or not.
– Code or standard applicable to the radiographic technique.
– Acceptance code or standard.
– Reference to a written procedure or technique sheet.
