subject contrast resulting from correct kilovoltage and alignment of the beam with the plane of the likely flaw; a sharp
image due to good geometry and control of secondary radiation; and optimum density to give good film contrast.
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. The following rules can be of assistance in such inspection
Always position the film as close to the area of interest as possible.
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.
Positioning. 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.
Curved Surfaces. In radiography of curved 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.
Source-to-Film Distance. The sharpest image would be formed by having a Source-to-Film Distance (SFD) so
great that the rays would be parallel at the film plane. However, since radiation intensify or quantity is diminished in
relationship to the inverse square of the distance, the radiation quantity available to expose the film would be very small,
and exposure times would become impractical. Therefore, in the production of the radiographic image, economics and
practicability must be considered. It is recommended that the longest practical SFD be used for critical exposures to
improve image sharpness.
Degree of Sensitivity. There is a need to be able to quantatively define how sensitive a radiographic image is.
The devices which achieve this aim are known as penetrameters. Another description is Image Quality Indicators (IQI).
By whatever name, they provide an indication of what the film reader can be expected to see 5-2 in the actual part being
inspected. A wide range of penetrameters is specified for use by various industries.
Use only the penetrameter, if any, specified by the inspection technique. Damage to the Image
may otherwise result.
Thickness Measurement. Sometimes it is impossible to determine the thickness of an object using conventional
mechanical measurement technique. In these instances, a special radiographic technique for the measurement of
material thickness may be employed. Although the mathematical development of a relationship between film density
and the thickness of an absorber is too complex for practical use, an empirical method of thickness measurement has
proven useful. By imaging the object of interest and a step wedge of the same material on a single film, it is possible to
obtain a good estimate of the thickness of the material section. It is imperative that the composition and structure of the
step wedge be the same as that of the material being measured if any accuracy is to be achieved. Thickness is
determined by measuring the resultant film density and finding the step on the wedge which is nearest to that density.
For best result, the section of interest and the step wedge should be placed as close to one another as possible to avoid
variation in the uniformity of the radiation output. This technique may also be employed to measure the dimensions of
Best Access to Object. Best access to object being inspected should be made prior to radiographic inspection.
5-7. Special Techniques. Conventional film radiography has its own capabilities and limitations. Special radiographic
techniques are used in some situations to provide a more rapid means of imaging. The following paragraphs describe
some of these techniques.
Fluoroscopy. Fluoroscopy is based on the ability of X-rays and gamma rays to produce fluorescence in some
objects. Specially formulated fluorescent screens are used. These fluoresce (emit visible light) in proportion to the
amount or radiation striking them. Thus, an instantaneous visible image is produced and the results may be instantly
Portable X-Ray Units. X-ray vidicon, photoradiography, and the polaroid radiograph are methods which use
portable X-ray units to provide a more rapid means of Imaging.