FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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SENSITOMETRIC QUALITY CONTROL (Continued)
The sensitometric values may be detennined directly from a stepped wedge or
from calculations derived from an H&D curve. The indicated value for speed
is measured by reading the density of the step on a wedge closest to D 2.00,
but above D 2.00. This is adeguate for quality control work because it
states that for a given film emulsion and a given exposure we wish to achieve
a certain density. The actual density level is unimportant as long as it is
above D 2.00. The step chosen is called the speed point.
Indicated contrast is measured by selecting two steps, one to either side of
the speed point, on the stepped wedge. The resulting densities are substracted
to produce the difference or contrast value. This is also called the
delta D (AD) meaning difference of density or contrast.
Calculated speed is derived frcm the H&D curve. The coordinate is D 2.00
plus B+F. At this point on the H&D curve a vertical line is dropped to the
abscissa where the relative mAs is read. An alternative is to use Du Pont
Graph Paper which has a built-in Bit Speed Scale which relates to relative or
arithmetic speed. Step 11 or Bit Speed of 16 is assigned a relative value of
100.
Calculated contrast may be calculated one of two ways:
o Gross or Work which has coordinates of 1.00 and 2.00
o ANSI, with coordinates of 1.50 plus B+F and 3.50 plus B+F
In both cases a line is drawn between the coordinate points to form the
average gradient slope line. There are three common ways to measure the
slope or rate of inclination:
o Tangent of the angle
o Parallel rule method
o Rise divided by run (Y/X)
Obviously, indicated values are faster and they can be just as accurate for
quality control work. The values derived should be tabulated for a month and
then an average found. This forms the aim point or index or standard which
is placed on a chart. Upper and lower control limits are established at +/-
10%.
WHEN TO TEST
Test before a work film shows a problem because then it is often too late and
the work film, plus all the other films in processing, must be repeated which
is very expensive. Test prior to the processing of work film such as at the
beginning of each shift. Test during a shift to insure the process is
holding consistent; that the process is not changing. Record the data on the
trend chart. Testing is cheaper than repeats and downtime.