It is necessary to identify exposed film so as to be able to identify its subject. Methods are:
a. Lead lettering.
b. Lead tape inscribed with ballpoint (low energies only).
c. Metal die stamp to impress the film (after processing).
d. Stick-on labels (after processing).
e. Thin lead sheet impressed with metal die stamps.
Cassettes and Film Holders.
Films should not be left in cassettes or film holders more than 24 hours, due to
interaction between films and lead screens.
Cassettes and film holders are used to protect the film from light exposure while the film is being transported and while
it is being exposed. These film holders are of various designs made to hold the various film sizes.
The term cassette is usually applied to rigid film holders. Cassettes have a bakelite or magnesium front to allow
transmission of the X-rays; the back contains a lead foil lining to absorb the back-scattered radiation. Cassettes are
normally used with calcium tungstate or lead screens. They provide uniform compression on the film and screens to
assure good physical contact between the film and screen for ultimate in image sharpness. Cassettes are comparatively
heavy and somewhat difficult to handle.
Cardboard holders are used extensively in industrial radiography. These are simply a heavy Kraft paper envelope
between hinged cardboard covers. The back has a lead foil lining to absorb back-scattered radiation. Always place the
holder with the "tube side" marking toward the radiation source. If the holder position is reversed, the radiation is
filtered by the lead foil backing and will result in images of lower density. The cardboard holders are economical and
durable. Lead screens can be inserted into the envelope with the film for making lead screen radiographs. Intimate
film-screen contact is normally accomplished by placing the object to be inspected on the cardboard holder.
Flexible Film Holders.
Flexible film holders are used where it is necessary to contour the film to accomplish good film-to-test-object contact.
However, sharp bends should be avoided. These holders are made of a lightproof flexible material. Lead screens with
a rubber or vinyl backing are available to permit contouring and flexible positioning of the film for exposure.
Vacuum cassettes are especially useful when used in conjunction with lead or fluorescent screens. The air is pumped
out of these cassettes, insuring intimate film-screen contact. They are very flexible, allowing the film to be positioned
in a confined space.
Application of Cassettes and Film Holders.
Plain film holders are generally used to radiograph thin sections of materials at low kilovoltage, 150 kVp or lower. The
sensitivity is reduced when using plain holders on thick sections due to backscatter. Using film holders with a lead
sheet backing will reduce the backscatter. At kilovoltages lower than 30 kVp, standard film holders cannot be used