is that a film makes more visible small discontinuities in the specimen when the image is dense. Image densities of 2.0
or more are usually recommended or required for high sensitivity to discontinuities in critical areas of parts. This will
be discussed further in a later section. Film contrast should be distinguished carefully from subject contrast (a flat
sheet specimen will give negligible contrast with any film). Subject contrast is affected by X-ray kilovoltage or gamma-
In summary, the overall image contrast with any given specimen will depend upon:
a. Kilovoltage of X-ray beam or characteristics of gamma radiation.
b. Type of screens used.
c. Image density.
d. Processing conditions.
e. Film contrast.
The film characteristic that is the reverse of contrast is film latitude. The higher the contrast, the smaller the latitude;
the lower the contrast, the greater the latitude. Latitude is, therefore, the range of radiation intensities that a film is
capable of recording. Latitude is also the term used to indicate the range of material thicknesses that can be visualized
in the final image. Often in the radiography of castings or circular rods, where it is necessary to visualize a large range
of thicknesses, wide latitude is desirable.
Storage of Unexposed Film.
Film emulsions are sensitive to heat, humidity, and certain chemical fumes. Because of this harmful effect, unexposed
film should be stored in a cool (below 75F), dry (relative humidity below 60 percent) area remote from stray
background radiation. It is possible to refrigerate films for additional protection, but if this is done, the film shall be
brought to ambient temperatures before use. Film emulsions are also pressure sensitive and film packages shall be
stored on edge rather than flat. X-ray film is sensitive to the cosmic radiation that exists everywhere. This radiation
will cause fogging. Fog is the darkening of the radiograph by scattered radiation, exposure to light, or pre-exposure to
radiation. It can also be caused by over-development or aging. It should be noted that fog brings no information to the
film and merely creates a high background that reduces contrast and image visibility. The very high-speed films, being
more sensitive to exposure, are more susceptible to fogging than the slower emulsions.
Expiration Dates of Radiographic Film.
An expiration date is marked on the film boxes at the time of manufacture. To prevent exceeding the expiration date,
film should be ordered in quantities such that long-term storage is not necessary. The inventory of film should be
rotated in such a manner as to use the older film first. Film that exceeds its shelf life date shall not be put in salvage.
Its usability will be verified by first processing an unexposed sheet to determine clearing and fog level. Then, if the
clearing and fog level are satisfactory, make a radiograph of a step-wedge and penetrameters to determine the
sensitivity and contrast of the film in question. If these limits are acceptable, continue using the film and extend its
shelf life by six months, utilizing AF Form 2032. At the end of the extended period re-verify the film using the
aforementioned procedure. If the film does not meet acceptable quality levels, use the film for training, for clearing the
automatic film processor, for detection of foreign objects, or should the quantity warrant ship it to the Air Education
and Training Command (AETC) at NAS Pensacola for use at the NDI Technical Training School. For Navy and
Marine Corps send film to NAVAL AIR Technical Training Center (NATTC) ATTN: NDI School, 230 Chevalier
Field AVE, Pensacola, FL 32508. If after a one-year period past its original shelf life the film is not used, it will be
considered unacceptable for crack detection and will be utilized for the alternate purposes mentioned. X-ray films
present no greater fire hazard in storage in the X-ray laboratory and filing room than an equal quantity of paper
records. There is no necessity for expensive vaults, equipped with elaborate fire protection devices. The storage area
must be kept reasonable clean.