T.O. 33B-1-1
6-55
geometric unsharpness that can be tolerated sets the lower limit for the adjustable parameter. Further demands on
sharpness are paid for in intensity. The unsharpness is inversely proportional to the source-object distance, whereas the
intensity is inversely proportional to the square of this distance. Thus, the trade-off of intensity for sharpness is not an
equitable one. Nonetheless, this uneven exchange is necessary in many cases because it is very important to achieve
good geometric definition. The basic principles of shadow formation must be given primary consideration to insure
satisfactory sharpness and low distortion of radiographic images. Distortion cannot be entirely eliminated since some
of the test object will be further away from the film than others and radiation from any source cannot be made ideally
parallel; images will always be less than perfect. In summary then, five general rules can be stated which promote
quality assurance from geometric considerations.
a. The X-rays should proceed from as small a local spot as other considerations will allow.
b. The distance between the source and the object should be as great as practical.
c. The film should be as close as possible to the object being radiographed.
d. The central beam should be as nearly perpendicular to the film as possible.
e. As far as the shape will allow, planes within the specimen plane of interest should be parallel with the
film.
6.7.2.6
Film Placement.
After the film and film holder have been chosen, the film position in relation to the part must be considered. In
production radiography of small parts, this is a simple matter of laying the parts on the film holder. With complex
structures, film positioning is not usually as simple. A few rules can be of assistance in such inspection situations.
a. Always position the film as close to the area of interest as possible.
b. Attempt to locate the film so that the plane of the area of interest and the film are perpendicular to the
radiation beam. This is to prevent distortion in the final image.
6.7.2.6.1
In positioning the film, care should be used to prevent sharp bends in the film or applying pressures to the film holder
that can produce pressure marks or crimp marks (artifacts) on the final image. In radiography of curve surfaces, the
source and film should be positioned, if possible, to take the best advantage of the inverse square law and to prevent as
much distortion as possible. Flexible film holders should be used in order to place the film as near as possible to the
surface of the test object. It may be noted in Figure 6-25 that the distance from source to the entire surface of the film is
nearly constant and the thickness of the test object is also a constant to the path of radiation. This preferred positioning
is not always possible, but it should be used when practical.
