Understanding Neutron Radiography Reading V-Kodak Part 2 of 3

■ Overlay Methods
An example of a transparent overlay is shown in the Figure 50. The numbers
on the horizontal line are exposure values. They can be taken, for example to
be milliampere-minutes, milliampereseconds, curie-minutes, curie-hours, or
an exposure factor. Further, all numbers on the line can be multiplied by the
same value, without affecting the use of the device. For instance, multiplying
by 10 makes the scale go from 10 to 10,000 (rather than from 1 to 1,000) of
whatever exposure unit is convenient. Note that the overlay must be made to
fit the characteristic curves with which it is to be used, since it is essential for
the horizontal scales of both characteristic curves and overlay to agree. The
use of the overlay will be demonstrated by solving again some of the same
problems used as illustrations in the foregoing section. Note that the vertical
lines on the overlay must be parallel to the vertical lines on the graph paper of
the characteristic curve, and the horizontal line must be parallel to the
horizontal lines on the graph paper.
Charlie Chong/ Fion Zhang
Radiography in Modern Industry. Rochester, NY: Eastman Kodak Co. 1980