Understanding Neutron Radiography

The slope of a curve at a particular point is expressed as the slope of a
straight line drawn tangentially to the curve at that point. When applied to the
characteristic curve of a photographic or radiographic material, the slope of
such a straight line is called the gradient of the film material at the particular
density.
In Fig. 7.2, the tangents to the curve have been drawn at two points, and the
corresponding gradients (ratio a.a'/b.b') have been evaluated. Note, that the
gradient is less than 1.0 in the toe and much greater than 1.0 in the central
portion of the characteristic curve. Now consider two slightly different
thicknesses in a subject, and assume that the thinner section transmits 20%
more radiation than the thicker. The difference in logarithm of relative
exposure (log E) is 0.08 and is independent of the exposure time. If this
subject is radiographed with an exposure that puts the developed densities
on the toe of the chacteristic curve where the gradient is 0.5, the Xray
intensity difference of 20% is represented by a density difference of 0.04 (see
Fig. 7.3), corresponding to a difference in light transmission of 10%.
Charlie Chong/ Fion Zhang
Practical.NR Chapter 7