Understanding Neutron Radiography Reading V-Kodak Part 2 of 3

As Figure 47 shows, the slope (or steepness) of the characteristic curves is
continuously changing throughout the length of the curves. The effects of this
change of slope on detail visibility are more completely explained in "The
Characteristic Curve". It will suffice at this point to give a qualitative outline of
these effects. For example, two slightly different thicknesses in the object
radiographed transmit slightly different exposures to the film. These two
exposures have a certain small log E interval between them, that is, have a
certain ratio. The difference in the densities corresponding to the two
exposures depends on just where on the characteristic curve they fall, and
the steeper the slope of the curve, the greater is this density difference. For
example, the curve of Film Z (See Figure 47), is steepest in its middle portion.
This means that a certain log E interval in the middle of the curve
corresponds to a greater density difference than the same log E interval at
either end of the curve. In other words, the film contrast is greatest where the
slope of the characteristic curve is greatest. For Film Z, as has been pointed
out, the region of greatest slope is in the central part of the curve. For Films X
and Y, however, the slope-- and hence the film contrast continuously
increases throughout the useful density range. The curves of most industrial
x-ray films are similar to those of Films X and Y.
Charlie Chong/ Fion Zhang
Radiography in Modern Industry. Rochester, NY: Eastman Kodak Co. 1980