## 3 Data Processing and Image Elaboration Procedures
### Reference Absolute Contrast  
Even when undamaged zone definition is straightforward, considerable variations of results are observed with different location choice of reference zone and with different dimension of the identified damaged area. The source distribution image (SDI) was developed for selecting free-defects portions of specimen that receives the same thermal flow of the corresponding inspected spot [24]. In Figures 6a and 7a, examples of thermal map analysis at the beginning of the cooling phase allows to estimate the heat distribution quality, as well as the presence of heat pre-accumulation in real component and in the artificial plate. Although heat distribution appears not to be uniform due to wide component dimensions, we observed the cooling phase to produce more uniform temperature maps over time and local temperature gradients remaining only in the defected regions. The preliminary algorithm considers the selection of the reference intact spots, corresponding to the previously determined defective regions as indicated in Figure 8a. On the bases of equal excitation energy, absolute contrast evaluation is enhanced with reduced errors. After choosing a thermal frame where the defects are more distinguishable, the algorithm provides an interactive analysis procedure for selected defective and intact zones section. This procedure is automatically iterated many times for all defects; Figure 7b shows an example on Plate I for processing a 25 mm diameter defect (diff1), a 10 mm diameter defect (diff2) and reference intact zone (int1). In the same way, on the central zone of the multi-stringer component, Figure 6b highlights the left (diff1) and right (diff2) damaged areas beside the central cut and the reference intact area (int1) as an example for calculation, since the component has been previously loaded under compression and failure is accumulated around the artificial discontinuity. From temperatures acquired in the selected spots, the MATLAB routine elaborates the absolute contrasts, as calculated in the selected zones. The code provides diagrams of \(C_{A}\) during time, for a more reliable comparison between different damaged zones. Figures 8-11 show examples of typical  
Figure 6: (**a**) Map of isotherm at the beginning of the cooling phase for real component and (**b**) raw thermal image for the central damaged zone of real component using heating time of 7 s.  
The following Tables 6 and 7 report the detected maximum values of absolute contrast (\(C_{A\_MAX}\)), as evaluated for two example defects in different computation zones, displayed in Figure 7b. The first table shows results for rectangular shape computation zone for defective zones of 20 \(\times\) 30 pixel and of 10 \(\times\) 15 pixel for same Defect 1 and zones of 8 \(\times\) 14 pixel and of 5 \(\times\) 10 pixel for same Defect 2. The general result is a significant contrast variation for the same defect as function of selected computational zones extension. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{2}{c}{\(C_{A\_MAX}\) of Defect 1 (\({}^{\circ}\)C)} & \multicolumn{2}{c}{\(C_{A\_MAX}\) of Defect 2 (\({}^{\circ}\)C)} \\ \hline \multicolumn{2}{c}{**Intact Zones (px\({}^{2}\))**} & \multicolumn{1}{c}{**Area 20 \(\times\) 30**} & \multicolumn{1}{c}{**Area 10 \(\times\) 15**} & \multicolumn{1}{c}{**Area 8 \(\times\) 14**} & \multicolumn{1}{c}{**Area 5 \(\times\) 10**} \\ \hline
**14 \(\times\) 12** & 4.02 & 4.72 & 0.9 & 0.99 \\
**21 \(\times\) 30** & 4.87 & 5.57 & 1.74 & 1.83 \\
**14 \(\times\) 20** & 3.73 & 4.43 & 0.62 & 0.72 \\
**19 \(\times\) 20** & 3.41 & 4.11 & 0.37 & 0.46 \\
**13 \(\times\) 20** & 3.84 & 4.55 & 0.75 & 0.84 \\ \hline
**Mean Value** & 3.97 & 4.68 & 0.87 & 2.00 \\
**Standard Deviation** & 0.55 & 0.55 & 0.52 & 0.55 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Maximum absolute contrast value for rectangular computation shape zone.