(developed film exposed to x-ray or gamma radiation) are generally
viewed on a light-box. However, it is becoming increasingly common
to digitize radiographs and view them on a high resolution monitor.
Proper viewing conditions are very important when interpreting
a radiograph. The viewing conditions can enhance or degrade the
subtle details of radiographs.
Before beginning the evaluation of a radiograph,
the viewing equipment and area should be considered. The area
should be clean and free of distracting materials. Magnifying
aids, masking aids, and film markers should be close at hand. Thin cotton gloves should be available and worn to prevent fingerprints
on the radiograph. Ambient light levels should be low. Ambient
light levels of less than 2 fc are often recommended, but subdued
lighting (rather than total darkness) is preferable in the viewing
room. The brightness of the surroundings should be about the same
as the area of interest in the radiograph. Room illumination must
be arranged so that there are no reflections from the surface
of the film under examination.
Film viewers should be clean and in good working
condition. There are four groups of film viewers. These include
strip viewers, area viewers, spot viewers, and a combination of
spot and area viewers. Film viewers should provide a source of
defused, adjustable, and relativity cool light as heat from viewers
can cause distortion of the radiograph. A film having a measured
density of 2.0 will allow only 1% of the incident light
to pass. A film containing a density of 4.0 will allow only 0.01% of the incident light to pass. With such low levels of
light passing through the radiograph, the delivery of a good light
source is important.
The radiographic process should be performed in
accordance with a written procedure or code, or as required by
contractual documents. The required documents should be available
in the viewing area and referenced as necessary when evaluating
components. Radiographic film quality and acceptability, as required
by the procedure, should first be determined. It should be verified
that the radiograph was produced to the correct density on the
required film type, and that it contains the correct identification
information. It should also be verified that the proper image
quality indicator was used and that the required sensitivity level
was met. Next, the radiograph should be checked to ensure that
it does not contain processing and handling artifacts that could
mask discontinuities or other details of interest. The technician
should develop a standard process for evaluating the radiographs
so that details are not overlooked.
Once a radiograph passes these initial checks, it
is ready for interpretation. Radiographic film interpretation
is an acquired skill combining visual acuity with knowledge of
materials, manufacturing processes, and their associated discontinuities.
If the component is inspected while in service, an understanding
of applied loads and history of the component is helpful. A process
for viewing radiographs (e.g. left to right, top to bottom, etc.) is
helpful and will prevent overlooking an area on the radiograph.
This process is often developed over time and individualized.
One part of the interpretation process, sometimes overlooked,
is rest. The mind as well as the eyes need to occasionally rest
when interpreting radiographs.
When viewing a particular region of interest, techniques
such as using a small light source and moving the radiograph over
the small light source, or changing the intensity of the light
source will help the radiographer identify relevant indications.
Magnifying tools should also be used when appropriate to help
identify and evaluate indications. Viewing the actual component
being inspected is very often helpful in developing an understanding
of the details seen in a radiograph.
Interpretation of radiographs is an acquired skill
that is perfected over time. By using the proper equipment and
developing consistent evaluation processes, the interpreter will
increase his or her probability of detecting defects.