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c. If the density of the thinnest section is too high, the range of thicknesses is too great for satisfactory
imaging. One solution is to raise the kilovoltage substantially, as this reduces contrast one of thin
areas. A better solution is to load the cassette with two films of different speed and expose them
simultaneously (see paragraph 6.6.2.2.). In the latter case, care is needed to insure that an acceptable
image density is obtained for all areas of interest.
6.6.2.2
Multiple Film Techniques.
Multiple film techniques permit the inspection of multi-thickness components with a single radiographic exposure.
Since the major expense associated with radiographic inspection can be attributed to setup, it is desirable to image a
multi-sectional component in a single exposure, rather than set up an exposure for each cross section of the component.
Multiple film techniques permit this objective to be achieved. For example, a cassette may be loaded with both Class 1
and Class 3 film to provide wide latitude. The faster Class 3 film provides a readable density film for the thicker
sections of the component inspected, while the slower Class 1 film may provide the appropriate film density for the
thinner sections of the component. Thus, in a single radiographic exposure, two images may be generated which cover
the required latitude for the inspection of a multi-thickness component. With very complex parts, it is even possible to
use three films and cover a very wide latitude. Several parameters must be considered in the choice of a multiple film
technique. In addition to the exposure parameters that are always of concern in radiographic inspection, the
radiographer must be concerned with the choice of film to be used and the combination of these films with various
screens. Lead screens have a definite effect on the quality of a radiographic image. First, lead screens are very dense,
and they preferentially absorb the lower energy scattered radiation. This reduces the fog on the final image and
provides a higher contrast, higher quality image. Secondly, at energies above 125 kVp, lead screens provide a definite
intensification. This intensification is due to the efficient conversion of X-ray photons into electrons in lead foils.
These electrons in turn expose the X-ray film and thus provide intensification in the final image. These properties of
lead screens may be useful in developing a multiple film technique. The film combination selected is based on the
range of thicknesses that must be covered in a single exposure. The simplest multiple film techniques employ two
different films, such as a Class 1 and a Class 2 film, to provide a range of densities for the inspection of an object. The
exposure is then determined that provides the best combination of contrast and sensitivity in the two films. In double-
film radiography it should be recognized that the film nearest the X-ray source acts as an absorber and the underneath
film receives less exposure than the source side film. This absorption effect is of considerable magnitude at low
kilovoltages and decreases with increased radiation energies. The choice of film positions will affect the range of
materials that can be visualized in a single exposure. Once a technique with a particular film combination has been
established, care must be used to assure that the same film is always placed in the same position in the film holder. If
the positions of the films are inadvertently switched, the resulting densities in the final images will be different than
expected. Two films of the same class may be loaded in a film holder and exposed at the same time to effectively
reduce exposure time by one-half. The two films are then interpreted by superimposition in front of the illuminator.
6.6.2.3
Multiple Film Techniques with Lead Screens.
The above combinations of films can be used for radiography of multi-thickness materials, but the optimum in image
quality may not be attained. Therefore, the use of lead screens may be introduced to help regulate the relative speeds of
the films used. As an example, assume that the combination of a Class 1 and a Class 2 film could not provide the
required latitude for a given component. Lead screens could be used to increase the latitude of the total exposure. Most
lead screens consist of thin lead foils backed on one side by cardboard, rubber, or vinyl. With this configuration, lead
screens have a filtration effect on the films that are beneath them and intensification effect on those films that face the
foil-coated side. If a combination of Class 1 and Class 2 film is used and insufficient latitude is provided, the latitude
may be increased by placing the faster Class 2 film on top with a backed 0.005-inch lead screen between the two films
with the lead screen in contact with the Class 2 film. This increases the latitude through two effects. First, the lead foil
intensifies the top Class 2 film and second, the lead acts as a filter, slowing the response of the Class 1 film. Thus, over
all the latitude of the exposure is increased. If on the other hand, the latitude was excessive with the two films and no
screens; the opposite effect can be achieved by placing the slower film on top and the faster film on the bottom, with
lead screens in between facing the slower film. This combination speeds up the slower film by intensification and
slows down the faster film by filtration. Thus, the total latitude is reduced. When only a slight increase in latitude is
required, two sheets of the same film type may be employed and lead screens may be used as described above to achieve
a relative speed difference between the two films. There is no end to the combinations that may be employed in
multiple film radiography. Using the principles that are outlined above, any capable radiographer should be able to
