Exposure of Film.
In their action on photographic film, X-rays differ from ordinary light. Examination of microscopic sections through
the sensitive layer of exposed films has shown that X-rays, unlike light, produce an equal distribution of grains of
reduced silver throughout the whole thickness of the layer whereas light produces an effect mainly on the surface of the
emulsion. Consequently, a greater blackening of the emulsion can be produced by increasing the thickness of the
emulsion and by coating both sides of the base of X-ray film. This darkening effect may then be used to obtain a
photographic record, or radiograph, which is produced by the passage of X-rays or gamma rays through an object and
onto a film. Thus a radiograph is a shadow picture of an object and its interior; dark regions on the film represent the
more penetrable regions of the part and lighter areas on the film represent the more dense areas of the part. Film may
be coupled with various screens to improve the image and reduce problems associated with scattered radiation.
Radiography satisfies the three primary requirements of any nondestructive inspection:
a. There is an energy form that can be usefully produced in a controlled manner.
b. This energy form is capable of interacting with material in a manner that causes a change in the energy
form, but not in the material.
c. After such interaction the energy form may be detected and may be interpreted to define what material
condition produced the observed result.
Where To Use Radiography.
There are some basic guidelines that may be used to determine situations to which radiography is applicable.
a. The defect, which is of interest, must cause a detectable change in apparent thickness, density or
composition of the test material.
b. The material should be reasonably homogeneous, so that an indication of a defect may be recognized.
c. The configuration of the part to be tested, or the area that surrounds it, must be such that access to both
sides is available.
d. The defect to be detected must be properly oriented in the path of the radiation beam.
Radiography is not a cure-all and should be used only when the above conditions are satisfied. Multiple film
techniques and other special methods, which will be covered in Section V, make radiography a versatile tool for
a. Radiography is a useful nondestructive inspection method for detecting internal discontinuities in many
b. Radiography may be applied to the inspection of castings, welds, and assembled components. Various
metals, both ferrous and nonferrous, as well as non-metallics, such as ceramics and plastics, can
successfully be inspected.