For Defect A75 and Defect IB on Specimen A-i, this procedure results in a better
estimate for the defect area than the boundary trace method. The estimated area for
Defect A75 was 2.6 cm2 and the estimated area for Defect IB was 6.4 cm2 (see Figure 6-
9B). Unfortunately, the performance of the gradient area method is not always
guaranteed. Results from the current study indicate several factors that can reduce
accuracy in area computations using the gradient area method:
* Weak signal for the defect (low SBR value)
* High temperature gradient due to non-uniform heating
* Insufficient pixel resolution for small defects
An example of a defect which generated a weak signal is provided in Figure 6-10.
This thermal image was collected for Defect A75 in Specimen A-3. The corresponding
gradient image (shown in Figure 6-10B) does not provide a well-defined defect
boundary. Oddly enough, the estimated area for this defect was 3.3 cm2 which is within
14% of the actual value (2.8 cm2). Based on observation of the gradient image, however,
this estimate does not appear to be meaningful.
The coefficient of variation (COV) of the radius values generated by the gradient
area method is a useful quantity for assessing the quality of the defect boundary. COV is
defined as the standard deviation of a series of numbers divided by the mean value.
When the gradient area method was applied using this center point, a total of 152 radius
values were obtained. The apparent center point of the defect in Figure 6-10B is labeled
"o". For the case shown in Figure 6-10B, the COV for these values was 0.54. The COV
for the defect shown in Figure 6-9A, a well-defined defect, was 0.10.
In an ideal case, the COV for a circular defect should be zero. For an elliptical
defect, however, there will always be variation in the computed radius values. The