Understanding Neutron Radiography Reading V-Kodak Part 2 of 3

■ Penetrameters and Visibility of Discontinuities
It should be remembered that even if a certain hole in a penetrameter is
visible on the radiograph, a cavity of the same diameter and thickness may
not be visible. The penetrameter holes, having sharp boundaries, result in an
abrupt, though small, change in metal thickness whereas a natural cavity
having more or less rounded sides causes a gradual change. Therefore, the
image of the penetrameter hole is sharper and more easily seen in the
radiograph than is the image of the cavity. Similarly, a fine crack may be of
considerable extent, but if the x-rays or gamma rays pass from source to film
along the thickness of the crack, its image on the film may not be visible
because of the very gradual transition in photographic density. Thus, a
penetrameter is used to indicate the quality of the radiographic technique and
not to measure the size of cavity that can be shown. In the case of a wire
image quality indicator of the DIN type, the visibility of a wire of a certain
diameter does not assure that a discontinuity of the same cross section will
be visible. The human eye perceives much more readily a long boundary than
it does a short one, even if the density difference and the sharpness of the
image are the same.
Charlie Chong/ Fion Zhang
Radiography in Modern Industry. Rochester, NY: Eastman Kodak Co. 1980