# Depth Retrieval Procedures in Pulsed Thermography: Remarks in Time and Frequency Domain Analyses
## 3 Results and Discussion
### Depth Retrieval with Blind Frequency
However, the interesting point observed by this comparison is the fact that less noise content can be observed in the latter case, which is important when the depth of a shallow defect has to be estimated [36]. Nonetheless, a possible approach for eliminating further the produced noise can be based on repeated measurements, normalising the phase contrast plots. By studying the effects from the detected defects of Series B and as the depth of the detected features was increased, the phase contrast was affected by the detected feature's size. More specifically, as can be seen in Figure 10, larger detected insertions in Series B produced greater phase contrasts, regardless the selection of the sound area. However, the size of the insertions had an impact only to the produced contrast as all defects at this depth are visible in approximately the same frequency range. The noise reduction observed in the previous inspection through the selection of the whole surface as a sound area can be also seen from the respective plots of Series B defects, as the Figure 10b is much freer of noise with respect to Figure 10b. The above presented results for Series B insertions were further confirmed from the plot results of the deeper defects of this panel, illustrated in Figure 11. As can be seen from the correspondent plots the greater is the defect depth the shorter is the frequency range in which the defects are obvious. For instance, the insertions of Series C have a frequency range of visibility from the minimum available frequency (0.149 Hz) to approximately 1 Hz, while for the deepest defects (Series E) the corresponding frequency range is between 0.149 Hz to approximately 0.5 Hz. However, shorter detection frequency ranges perform clearer determinations of the blind frequency, thus less noise content is presented in these cases. The plot results presented in Figure 11 further indicate the impact of feature's lateral dimensions on the produced contrast, as smaller inserts, regardless of their location, produced weaker phase contrast with respect to the larger ones. Figure 11: _Cont_. Figure 10: Phase contrast profiles of the Series B insertions (0.4 mm) after: (**a**) selecting the left top corner; and (**b**) the whole surface as a sound area. Figure 9: Phase contrast profiles of the Series A insertions (0.2 mm) after: (**a**) selecting the left top corner; and (**b**) the whole surface as a sound area. From the phase contrast measurements and the determination of the blind frequency, the depths of all the detected defects into the CFRP panel were quantitatively estimated. Table 1 summarises the experimental values of the blind frequencies and the calculated depths as the product of the diffusion length and the correlation factor \(C=1.82\). In general, frequency domain analysis and phase data manipulation provided an efficient estimation of the inserts depth, regardless their location and size. Additionally, the similarity of results in terms of error production for inserts having the same depth indicates that the potential non-uniform heat deposition on the inspected surface has a negligible impact on the quantitative information retrieval. Moreover, as regards the quantitative information retrieval and their correlation with the investigated depth, it can be generally seen that defects nearer to the surface produced a larger divergence from the actual depth contrary to the deeper ones where the error production was lower. The above remark was somehow expected as it is attributed to the increase noise levels produced as higher is the frequency range of detection, incommoding the accurate detection of the blind frequency. The comparison of the two different procedures used for the depth measurements with the phase, referring to the two different sound areas selection, indicates that the selection of the entire surface as a sound area either provided similar results with these produced from the random area selection or it was reducing the error production as in the cases of the Series A and C defects.