T.O. 33B-1-1
6-49
economy resulting from the use of smaller size film. The image can be enlarged for viewing by a magnifying system or
by projection. In general, this system permits radiographic sensitivity of about four to five percent. The
photoradiography accessory is available as an assembly and usually consists of a light-tight hood, a fluorescent screen
or image intensifier assembly and the camera. Various type cameras are available, some, of which employ sheet film
and others using 70-mm roll film.
6.6.3.7
Polaroid Radiograph.
If a convenient, permanent image is needed and the time required for conventional film radiography is prohibitive; an
alternate may be radiography with other film. One of these is Polaroid radiography. Just as Polaroid photography
facilitates very rapid development of photographic images, there are available Polaroid X-ray films which provide the
same advantages. These require the special Polaroid film holders and a film processor if the larger sizes are used. In
some cases the typical Polaroid 4- by 5-inch adapter can be used. Polaroid radiographic films are used just as regular
films are used in conventional film radiography. They have their own characteristic curves and an appropriate
exposure technique should be used. However, after the exposure has been made, rather than process the films by
conventional techniques, they are dry developed as a Polaroid photograph is, and results are available after about one
minute. Presently available Polaroid films provide for either viewing by reflected or transmitted light. Polaroid
radiographs provide nearly instant interpretation and also provide a permanent image. However, Polaroid radiographs
are low in contrast and detail resolution compared to conventional film. Polaroid radiographs can be made to establish
the geometrical alignment of the X-ray beam with the part before a typical film radiograph is exposed. This technique
is useful in those cases where critical alignment is required.
6.6.3.8
Photothermographic Film.
The photothermographic process uses a special "dry silver" film which is heat processed, eliminating the need for
chemical processing. The film is sensitive to visible green light. Therefore, to produce the image, rare earth phosphor
intensifying screens are placed in intimate contact with the film. When struck by X-rays, the screens fluoresce,
forming an image on the film. Since the film itself is insensitive to X-rays, care must be taken to assure that the coated
side of the film is in direct contact with the coated side of the screen during the exposure. Since this film is dependent
upon the screens for forming the latent image, only screens approved by the film's manufacturer SHALL be used. To
aid in maintaining the necessary contact, vacuum cassettes SHALL be used for holding the film and screens, unless an
approved procedure states otherwise. Photothermographic film less sensitivity than Class 4 films; therefore, it is not
suitable for most critical applications and SHALL NOT be used for critical crack detection. Photothermographic film
is processed by exposing the film to heat in a special thermal processor. The heat causes the latent image in the silver
halide grains to form in the reducible silver salts. This process is very fast; typically requiring is to 20 seconds to
process a 14- by 17-inch film. During this process the radiograph is also stabilized, requiring no additional processing.
The image produced should remain stable for years under normal storage conditions. However, exposure of the film to
bright light for several days could cause some discoloration of the white background.
6.6.4
Computed Tomography (CT).
Computed Tomography (CT) is a radiation inspection method that can provide quantitative density and geometric
images of thin cross sections of an inspection object. The method, adapted for nondestructive testing after extensive use
in medical radiology, employs a computer to reconstruct an image of a cross-sectional plane through the object. CT
inspection of a tree, for example, would look very much like the surface of a tree stump, showing the varying density of
the winter and summer wood rings and an accurate representation of the tree growth rings. CT information is derived
from a large number of observations of radiation intensity over many different viewing angles. Using CT, one can, in
effect, slice open the test object, examine its internal features, perform dimensional inspections and identify any
material or structural anomalies that may exist. As compared to conventional radiography, a major advantage of CT
inspection is that internal structures are not hidden or shadowed by other structures that may be in the beam path. In
addition, CT inspection can provide quantitative information about density variations and spatial locations within the
inspected material. An obvious disadvantage is that currently used CT image reconstruction methods require full
access to the inspected part; a full 180 degrees of data must be collected by the scanner. In addition, the inspection
object must be small enough to fit in the CT handling and scanning system. Systems large enough to handle missiles
up to 9 feet in diameter are in use. Additional information is available from reference 12.
