## 4 Results  
### CFRP\({}_{\text{FBH}}\) Coupon  
The results of the different post-processing techniques on the CFRP\({}_{\text{FBH}}\) coupon sample are gathered in Figure 5. The true sizes and locations of the FBHs are marked on the figures. Firstly, Figure 5a,b presents DTT's maxigramT and timegram. In the maxigramT (Figure 5a), only the three shallowest FBHs (0.85 mm, 1.64 mm, and 2.47 mm depth) were detected. Additionally, there were clear effects of the fiber orientations, which distorted the background uniformity. On the other hand, the timegram (Figure 5b) also hinted at the presence of the two deepest FBHs. The timegram was noisy since its values for sound pixels represent random fluctuations in the thermal contrast. The noisiness of the timegram can be significantly reduced by performing a threshold based on the maximum value of maxigramT[22], however, this also removes the signature of low-contrast defects. This additional thresholding was therefore not considered in this article. Furthermore, the effect of lateral heat diffusion can be observed for the shallower defects, where the values in the timegram increase with the distance away from the FBH's center. Consequently, the defects did not have a sharp outline in the timegram, which would complicate accurate defect sizing. The result of TSA is displayed in Figure 5c. Several integration limits were carefully checked, and an integration range of 50 s was found to provide a good detectability of the defects. This is a sensible integration range since it ranges over the timegram values of all FBHs (i.e., it encapsulates  
Figure 5: Results of processing techniques for the CFRP\({}_{\text{FBH}}\) coupon: (**a,b**) DTT maxigramT\(\Delta T_{max}\) and timegram \(t_{\Delta T,max}\); (**c**) TSA (0–50 s); (**d,e**) FDT maxigramF\(\Delta\varphi_{max}\) and frequencygram \(f_{\Delta\varphi,max}\); and (**f**) ASBI damage index map. the times of maximum thermal contrast, see also Figure 4b). In the TSA output, the three shallowest FBHs (0.85 mm, 1.64 mm, and 2.47 mm depth) were clearly detected, while the presence of the fourth FBH (3.68 mm depth) was hinted. The background was significantly more uniform and less noisy than in DTT's \(\mathrm{m}\mathrm{a}\mathrm{i}\mathrm{r}\mathrm{g}\mathrm{m}\mathrm{T}\) and \(\mathrm{m}\mathrm{i}\mathrm{r}\mathrm{g}\mathrm{m}\mathrm{T}\). Of course, due to the integration procedure over a relatively long temporal range, the effects of lateral heat diffusion blur the edges of the shallow defects with respect to DTT's \(\mathrm{m}\mathrm{a}\mathrm{i}\mathrm{r}\mathrm{g}\mathrm{m}\mathrm{T}\). In order to obtain better defect sizing, it would be beneficial to make the integration procedure adaptive in a pixel-wise manner. In that way, shallow defects will be associated with a short integration time in order to limit lateral diffusion effects. Deep defects, on the other hand, will have long integration times in order to increase their contrast. However, such an adaptive integration procedure was out of scope in this manuscript. The resulting images from FDT are presented in Figure 5d,e. The \(\mathrm{m}\mathrm{a}\mathrm{i}\mathrm{r}\mathrm{g}\mathrm{m}\mathrm{T}\) (see Figure 5d) provides a good indication of the (note the logarithmic color scale). While DTT's \(\mathrm{m}\mathrm{a}\mathrm{i}\mathrm{r}\mathrm{g}\mathrm{m}\mathrm{T}\) and FDT's \(\mathrm{m}\mathrm{a}\mathrm{i}\mathrm{r}\mathrm{g}\mathrm{m}\mathrm{T}\) mainly serve for defect detection, DTT's timegram and FDT's frequencygram can be used for quantitative depth inversion. The main limitation for the timegram lies in its limited detectability of deep defects (FBHs detected clearly up to 2.47 mm depth, see Figure 5b), however, quite uniform values were obtained over a defect's signature. On the other hand, the frequencygram could detect significantly deeper defects (detected up to 4.51 mm, see Figure 5e), but the limited frequency resolution makes it practically very difficult to perform depth inversion for deep defects (deep defects had the same frequency values).