T.O. 33B-1-1
6-35
e. Close the holder cover taking care that the lead screen on the cover enters the holder pocket without
binding. Some holders may not have a lead screen on the cover. In this case when the use of the screen
is desired, place it in the holder pocket face downward.
f. Lower the holder cover and fasten the locking device. If the holder has a spring back, turn the latch to
lock the holder.
6.4.2.10
Prepackaged Film.
X-ray film suppliers offer X-ray film in prepackaged, flexible envelopes or in rolls. The prepackaged film eliminates
the film loading operation in the darkroom. They are available with double films loaded in the same package, and with
a lead compound screen absorber incorporated in the package. They are convenient to use and are preferred for many
industrial applications. Prepackaged film is especially convenient for field inspection.
6.4.3
Screens.
In industrial radiographic applications, screens are often used in direct contact with the X-ray film. The term screens
refers to a layer of a material whose purpose is to increase the imaging radiation impinging on the film and/or decrease
the scatter radiation reaching the film. There are two types of intensifying screens, fluorescent and lead.
6.4.3.1
Fluorescent Screens.
6.4.3.1.1
Principles.
The fluorescent screens are made of calcium tungstate crystals or other crystals of chemical salts that fluoresce (emit
visible light) when bombarded by X-rays. These materials have the ability to convert X radiation into electromagnetic
photons in the visible light of ultraviolet spectrums. The light emanating from the fluorescent screen exposes the film
emulsion and supplements the original X radiation. Fluorescent screens can reduce the quantity of radiation necessary
to produce a given film density by a factor of 8 to 100. However, they also cause image noise and unsharpness because
of the spreading effect of the visible light emitted from the screen. Fluorescent screens produce radiographic images of
inferior quality. For this reason, their use is usually limited to those situations where exposure speed is more important
than image quality, or where the radiation quantity available is inadequate to perform the task. Whenever there is a
need to perform a radiographic inspection using a combination of screens and film, they SHALL be of the same plane
dimensions and in close contact with each other during exposure.
6.4.3.1.2
Care.
Fluorescent light from intensifying screens obeys all the laws of visible light and cannot pass through opaque bodies, as
do X-rays. To avoid extraneous shadows caused by absorption of the fluorescent light by foreign matter during
exposure, do not allow dust and dirt particles to collect between film and screen surfaces. Stains upon the screens must
also be avoided. Make every effort to keep the screens clean. Calcium tungstate screens may be stored in the
processing room but away from chemicals and other sources of contamination.
6.4.3.2
Lead Screens.
6.4.3.2.1
Description and Use.
Lead (Pb) foil and lead oxide screens are used extensively in industrial radiographic operations. When used properly
they improve the image contrast and final radiographic sensitivity. The thin lead oxide screens used in light-tight,
prepackaged film envelopes are very thin, usually less than 0.001 inch. Their primary function is to intensify the
image.
Lead screens consist of a sheet of lead alloy (94 percent lead, 6 percent antimony) mounted on a cardboard or plastic
base. They are generally used in pairs, one on each side of the film. Typically, the front screen is 0.005 inch thick and
the rear screen is 0.010 inch thick. Below 100 kVp, the attenuation of the primary beam through the front screen will
increase exposure time, and it is not until the kVp exceeds 160 that any significant reduction in exposure results.
Sample results are shown in Table 6-11. However, the quality of the image is improved at all kVp settings.
