Table 6-12. Approximate Radiation Energies Compatible with Various Absorbers.
It should be noted that, as the radiation energy increases, the differences between absorbing materials become less
pronounced than at lower energies. Due to the photoelectric absorption, the atomic number of an absorber has a large
effect upon radiation absorption at energies of 100 kV or less. At high energies in the 1 MeV range, the material
density becomes the major controlling factor in determining radiation absorption. A 10 percent change in radiation
energy has a very definite effect at low energies. In the MeV energy ranges, this same percent change in energy can
hardly be detected in transmission characteristics.
An alteration in the filament current (ma) produces a direct change in the quantity of radiation emitted but has no effect
upon the radiation energy. Further more, filament current (ma) and time are usually interchangeable. That is, the
product of milliamperage and time is constant for the same photographic effect. This is known as the reciprocity law
and is valid for X-ray and gamma exposures, with or without lead screens, over the range of radiation intensities and
exposure times used in industrial radiography with one exception. This exception is the use of fluorescent screens;
their use is discussed in the section entitled films, film holders and screens. For very low or high intensities, the
reciprocity law fails because of changes in the efficiency of the response of the film emulsion to unit radiation. If high
production radiography were required, then a source with a high radiation output would be economical. Usually, the
high-output equipment requires a source with a comparatively large focal spot. Therefore, rate of radiation output is
often directly related to focal spot size, and the unsharpness due to geometry can become detrimental to image quality.
The geometrical setup used to produce a radiographic image is an important factor that contributes to final image
quality. Geometrical relationships affect the image sharpness and help control image distortion.
The central ray of the X-ray source should be aligned perpendicular to the part being radiographed, and the film should
be located in the same plane as the part. This positioning projects the image of the part upon the film in the true shape
of the object. Any deviation from these relative positions of source, object and film will produce images with some