A = ATdef
1.5
B = Tper max -Tper avg
0 1 2
10 10 10
Time (sec)
Figure 6-7. Signal for undetected defect
The only remaining parameter that was extracted for each area was ATper. ATper is
defined as the average temperature rise experienced by the defect area boundary due to
heating. This value was obtained by subtracting Tavgper @ t = 240 sec from Tavgper @ t
0. Table 6-2 provides a summary of all the parameters that were extracted from each
ATdef VS. time plot.
Table 6-3. Parameters extracted from ATdefVS. time plot for each defect
Parameter Description
ATmax Maximum defect signal strength
Average temperature rise experienced by the perimeter of the
defining area due to heating (Tper avg @ t = 0 Tper vg @ t =
ATper 240)
tb Time at which ATdef becomes dominated by the defect of interest
tmax Time at which ATmax occurs
tl/2 Time required for ATmax to decay by a factor of two
Area computations
To determine the size of a defect from a thermal image, it is possible to draw a line
around the boundary of the defect and count the number of pixels inside the boundary.
This method will be referred to as the boundary trace method. The number of pixels can
be converted to an area by applying a length factor that is obtained by identifying two
points in a thermal image where the true separation distance is known (typically the two