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Radiographic
Film
 X-ray
films for general radiography consist of an emulsion-gelatin
containing radiation sensitive silver halide crystals, such as silver bromide or silver chloride, and a flexible, transparent, blue-tinted base. The emulsion is
different from those used in other types of photography films
to account for the distinct characteristics of gamma rays and
x-rays, but X-ray films are sensitive to light. Usually, the emulsion
is coated on both sides of the base in layers about 0.0005 inch
thick. Putting emulsion on both sides of the base doubles the
amount of radiation-sensitive silver halide, and thus increases
the film speed. The emulsion layers are thin enough so developing,
fixing, and drying can be accomplished in a reasonable time. A
few of the films used for radiography only have emulsion on one
side which produces the greatest detail in the image.
When x-rays, gamma rays, or light strike the grains of the sensitive
silver halide in the emulsion, some of the Br- ions are liberated and captured by the Ag+ ions. This change is of such a small nature that
it cannot be detected by ordinary physical methods and is called a "latent (hidden) image." However, the exposed grains are now more sensitive to the reduction process when exposed to a chemical solution (developer),
and the reaction results in the formation of black, metallic silver.
It is this silver, suspended in the gelatin on both sides of the
base, that creates an image. See the page on film processing for additional information.
 Film Selection
The
selection of a film when radiographing any particular component
depends on a number of different factors. Listed below are some
of the factors that must be considered when selecting a film and
developing a radiographic technique.
- Composition, shape, and size of the part being examined
and, in some cases, its weight and location.
- Type of radiation used, whether x-rays from an x-ray generator
or gamma rays from a radioactive source.
- Kilovoltages available with the x-ray equipment or the
intensity of the gamma radiation.
- Relative importance of high radiographic detail or quick
and economical results.
Selecting the proper film and developing the optimal radiographic
technique usually involves arriving at a balance between a number
of opposing factors. For example, if high resolution and contrast
sensitivity is of overall importance, a slower and finer
grained film should be used in place of a faster film.
 Film Packaging
Radiographic
film can be purchased in a number of different packaging options.
The most basic form is as individual sheets in a box. In preparation
for use, each sheet must be loaded into a cassette or film holder
in the darkroom to protect it from exposure to light. The sheets
are available in a variety of sizes and can be purchased with
or without interleaving paper. Interleaved packages have a layer
of paper that separates each piece of film. The interleaving paper
is removed before the film is loaded into the film holder. Many
users find the interleaving paper useful in separating the sheets
of film and offer some protection against scratches and dirt during
handling.
Industrial x-ray films are also available in a form in which
each sheet is enclosed in a light-tight envelope. The film can
be exposed from either side without removing it from the protective
packaging. A rip strip makes it easy to remove the film in the
darkroom for processing. This form of packaging has the advantage
of eliminating the process of loading the film holders in the
darkroom. The film is completely protected from finger marks and
dirt until the time the film is removed from the envelope for
processing.
Packaged film is also available in rolls, which allows the radiographer
to cut the film to any length. The ends of the packaging are sealed
with electrical tape in the darkroom. In applications such as
the radiography of circumferential welds and the examination of
long joints on an aircraft fuselage, long lengths of film offer
great economic advantage. The film is wrapped around the outside
of a structure and the radiation source is positioned on axis
inside, allowing for examination of a large area with a single
exposure.
Envelope packaged film can be purchased with the film sandwiched
between two lead oxide screens. The screens function to reduce
scatter radiation at energy levels below 150keV and as intensification
screens above 150 keV.
Film Handling
X-ray film should always be handled carefully to avoid physical
strains, such as pressure, creasing, buckling, friction, etc.
Whenever films are loaded in semi-flexible holders and external
clamping devices are used, care should be taken to be sure pressure
is uniform. If a film holder bears against a few high spots, such
as on an un-ground weld, the pressure may be great enough to produce
desensitized areas in the radiograph. This precaution is particularly
important when using envelope-packed films.
Marks resulting from contact with fingers that are moist or contaminated
with processing chemicals, as well as crimp marks, are avoided
if large films are always grasped by the edges and allowed to
hang free. A supply of clean towels should be kept close at hand
as an incentive to dry the hands often and well. Use of envelope-packed
films avoids many of these problems until the envelope is opened
for processing.
Another important precaution is to avoid drawing film rapidly
from cartons, exposure holders, or cassettes. Such care will help
to eliminate circular or treelike black markings in the radiograph
that sometimes result due to static electric discharges.
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