magnitude of the COV will be dependant on the ratio of the primary axis dimensions of
the ellipse. Figure 6-11 provides a graph of COV values plotted against the ratio of the
radii of an ellipse. These COV values represent an ideal case for a perfectly defined
boundary. This plot can be used to assess the quality of a defect boundary generated for
an elliptical defect. Consider the defect boundary that is shown in Figure 6-9 (Defect IB
in Specimen A-i). The approximate ratio of the principal axis dimensions for this defect
is 0.35. This corresponds to an inherent COV of 0.35 on the graph in Figure 6-11. The
computed COV for this defect was 0.33. This example illustrates that the absolute
magnitude of COV should not be used to assess the quality of a computed defect
boundary. Instead of the absolute magnitude, the difference between the computed COV
and the inherent COV for the defect under consideration should be used.
24.2
24 2
S 5 10 15 2o 215 30 3
A B
Figure 6-10. Reduced accuracy in area computations due to a weak signal. A) Thermal
image for weak signal. B) Gradient intensity.
The second source of reduced accuracy in area computations, non-uniform heating,
is illustrated in Figure 6-12. The thermal image highlights the temperature gradient that
develops across the box which was drawn around the defect. This thermal gradient is
also apparent in the gradient image provided in Figure 6-12B. As a result, the defect
boundary that was generated using the gradient area method is skewed to one side. The
COV computed for the radius values was 0.25.