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linear attenuation coefficient (m) can be changed by changing radiation energy. This in turn will change the ratio IT/I0
or the percent radiation transmitted through a part of thickness, x. In industrial radiographic applications, the
difference in thickness (often due to discontinuities) is the actual parameter from which interpretation is made.
Therefore, the greater the change in the radiation transmitted due to a particular change in material thickness, the more
obvious is the thickness change revealed in the final image. This radiation difference due to material thickness change
is called the material contrast. The material contrast is a function of the absorption characteristics of the part being
inspected and the radiation energy level. When measurements have been made and a numerical value has been
established, it is called the material contrast factor.
6.5.5.2
Percent Radiation Transmission.
When monochromatic radiation is used, the percentage of radiation transmission can be calculated from the formal
laws of attenuation. Since this condition seldom exists in actual practice, the percent of radiation transmitted must be
empirically measured. When the proper recorder is used, the actual measurements will include the scattered radiation
as well as the transmitted primary beam, both of which can be expected to expose a film or interact with any other
recorder in a typical industrial radiographic set-up.
SECTION VI
SPECIAL RADIOGRAPHIC TECHNIQUES
6.6
SPECIAL RADIOGRAPHIC TECHNIQUES.
6.6.1
Introduction.
The previous sections of this chapter have been primarily concerned with conventional film radiography. While film
radiography offers a very versatile tool for the detection and identification of material discontinuities, there are a variety
of special techniques that may be employed to extend the capabilities of conventional radiography. Special techniques
may be placed into two broad categories. The first relates to radiographic techniques with a specific objective of
extending the capabilities of the inspection method in general. This first category would include such techniques as
multi-thickness, multiple film, triangulation, thickness measurement and stereo (three-dimensional techniques). The
second category relates to special imaging methods, such as radioscopy with techniques such as image intensifiers, or
X-ray vidicon, photoradiography, Polaroid radiography, photothermographic film and radiographic paper. Special
radiographic methods that are not included in authorized inspection manuals SHALL NOT be used without written
approval of the appropriate depot engineering activity.
6.6.2
Special Purpose Techniques.
6.6.2.1
Multi Thickness Techniques.
Most real life situations involve the radiography of parts of widely varying thicknesses and sometimes of two or more
materials. If it is possible to concentrate on one area with a nearly constant thickness, optimization of image density is
straightforward. Often, however, it is necessary to obtain an acceptable exposure for two or more difference thicknesses
on the same image. Small thickness variations, for example, of 0.8 to 0.6 inches, can lead to large variation in density,
from 1.2 to 1.7 respectively. The aim is to insure that all areas of interest have densities that are not so low so as to lose
film contrast and not so high that they cannot be evaluated. An acceptable range of densities is 1.0 to 3.5. The
procedure recommended during technique development is to identify the thickest area of interest, and then from
exposure charts and trial-and-error, determine the exposure and kilovoltage that gives a density of 1.0. A trial shot will
then show the density of the image of the thinnest area of interest. There are three possible courses of action:
a. If the density of the image of the thinnest section is approximately 3.5 and the image can be
satisfactorily interpreted, the technique is optimized.
b. If this density is too low, the exposure should be increased to raise the average density of thick and thin
areas.
