## 2 Materials and Methods
### Data Processing
#### 2.3.2 Line Scan Thermography (LST)  
In the case of LST, data are acquired in a dynamic configuration, i.e., the camera and the heating source (IR heater) move in tandem above the surface of the samples at a constant scanning speed. This allows reconstruction of a pseudo-static sequence following the procedure described in previous works [21; 22]. It is also possible to process the dynamic sequences by PPT as described in [9; 23].  
The original thermographic data are in a three-dimensional (3D) sequence with frames represented in spatial coordinates \(x\), \(y\) through time \(t\). The complete sequence usually consists of several hundred frames. For the sake of clarity, Figure 4a shows only 4 representative thermograms arbitrarily selected at different times. It can be seen for instance that early in the sequence, i.e., at \(t_{1}\), the first plate (S44) can be completely seen, while the second plate (S48) is seen partially. As the scanning continues, plates appear and disappear from the camera field-of-view (FOV) as illustrated at different times (\(t_{2}\), \(t_{3}\), and \(t_{4}\)). The acquisition ends after all plates are scanned and there is no longer any sample in the camera FOV.  
The dynamic sequence is then recomposed in such a way that the original \(y\) dimension and time \(t\) are transposed as the observation time \(t^{\prime}\) and coordinate \(y^{\prime}\), respectively, whilst the x coordinate remains unchanged. The reconstructed pseudo-static matrix is shown in Figure 4b and consists of several images (equal to the spatial resolution in the scanning direction, the \(y\) coordinate in this example) distributed along the observation time \(t^{\prime}\). In the process, the positions of the reconstructed \(y^{\prime}\) lines are adjusted by interpolation to compensate for the lack of synchronization between the scanning speed and the camera frame rate.  
The observation time, \(t^{\prime}\), which is the time during which every line of pixels along the \(y\) coordinate (in the direction of the scan) is registered by the camera after thermal  
Figure 4: LST processing: (**a**) dynamic sequence, line scan at 25 mm/s; (**b**) pseudo-static reconstructed sequence and ROI sequence showing defect 1 of sample S45; and (**c**) thermal profiles of defect 3 and the sound area around it.  
stimulation, is calculated from the length \(Y\) (in mm) of the FOV in the direction of the scan and the scanning speed \(v_{y}\):  
\[t^{\prime}=\frac{Y}{v_{y}}\begin{bmatrix}\text{mm}\\ \hline\text{\framebox{mm}}\end{bmatrix} \tag{1}\]  
The length of the FOV in the scanning direction \(Y\), is calculated by knowing the distance between the camera and the sample surface and the lens focal length.  
For instance, for a lens with a focal length = 13.1 mm, with the camera located 267 mm from the sample and a scanning speed of \(v_{y}\) = 25 mm/s: \(Y\) = 166.3 mm and \(t^{\prime}\) = 6.64 s.  
An ROI surrounding defect 1 of specimen S45 can be identified in Figure 4b (white square in the thermogram at \(t^{\prime}_{a}\)) and then cropped from the reconstructed sequence showing the thermal evolution of the ROI through the observation time \(t^{\prime}\) as also seen in Figure 4b. The thermal profiles of defect vs. sound areas are shown in Figure 4c.  
After reconstruction, the LST pseudo-static matrix can be processed using advanced signal processing techniques such as TSR, PCT, and PPT, as is conducted with PT [22]. Examples of signal processing of LST (and PT) data are presented in Section 3.