Understanding Neutron Radiography Reading V-Kodak Part 2 of 3

Limitations Of Exposure Charts
Although exposure charts are useful industrial radiographic tools, they must
be used with some caution. They will, in most cases, be adequate for routine
practice, but they will not always show the precise exposure required to
radiograph a given thickness to a particular density. Several factors have a
direct influence on the accuracy with which exposures can be predicted.
Exposure charts are ordinarily prepared by radiographing a stepped wedge.
Since the proportion of scattered radiation depends on the thickness of
material and, therefore, on the distribution of the material in a given specimen,
there is no assurance that the scattered radiation under different parts will
correspond to the amount under the same thickness of the wedge. In fact, it is
unreasonable to expect exact correspondence between scattering conditions
under two objects the thicknesses of which are the same but in which the
distribution of material is quite different. The more closely the distribution of
metal in the wedge resembles that in the specimen the more accurately the
exposure chart will serve its purpose. For example, a narrow wedge would
approximate the scattering conditions for specimens containing narrow bars.
Charlie Chong/ Fion Zhang
Radiography in Modern Industry. Rochester, NY: Eastman Kodak Co. 1980