## 5 Conclusions  
In this article, the defect detectability of both time- and frequency-domain analysis techniques for flash thermography was evaluated. Both processing techniques that evaluate a single time (or frequency) bin, i.e., DTT and FDT, and techniques that perform an integration over a range of time (or frequency) bins, i.e., TSA and ASBI, are of interest. Three CFRP samples, with different defect types (FBH, impact damage, and cluster of production defects), sizes, and depths were inspected.  
The time-domain results illustrate the advantage of standardizing the dataset before applying amplitude-based processing algorithms in order to suppress effects of non-uniform excitation/emissivity. The results further indicate that the time-domain integration (i.e., TSA) outperformed the time-domain single-bin technique (i.e., DTT) when a defect exhibited only a limited amount of thermal contrast (e.g., deep FBHs and complex cluster). However, when concerned with shallow defects in close proximity (e.g., BVID), the integrated technique tended to merge the individual defect locations into one global defected zone. This merging was a result of the integration procedure, which was affected by lateral heat diffusion, and reduced the defect detectability and sharpness of a defect. While DTT's timegram delivered a better defect detectability than the maxigramT, it typically suffered from a noisy background and was more affected by lateral heat diffusion. DTT's maxigramT provides a sharp outline of the detected defects, while the defect contour obtained through TSA was not sharp due to effects of lateral heat diffusion.  
The single-bin evaluation approach in the frequency domain (i.e., FDT) provided improved defect detectability (i.e., higher contrast-to-noise and defect-to-background ratios) and a higher inspectable defect depth than its time-domain counterpart. However, since FDT generally refers to low indicative frequencies, its maxigramF and frequencygram were both strongly affected by lateral heat diffusion. Moreover, the frequency-domain results also show a higher sensitivity to the non-uniform heating. The integrated frequency-domain technique (i.e., ASBI) provided good defect detectability and could highlight deeper defects than TSA.  
Conceptualization, G.P., S.H. and M.K.; Funding acquisition, S.H. and M.K.; Methodology, G.P., S.H. and M.K.; Resources, W.V.P. and M.K.; Software, G.P. and S.H.; Supervision, S.H. and M.K.; Validation, G.P., S.H., J.S. and M.K.; Writing--original draft, G.P.; Writing--review and editing, S.H., J.S., W.V.P. and M.K. All authors have read and agreed to the published version of the manuscript.  
The authors acknowledge the SBO project DETECT-IV (Grant no. 160455), which fits in the SIM research program MacroModelMat (M3) coordinated by Siemens (Siemens Digital Industries Software, Belgium) and funded by SIM (Strategic Initiative Materials in Flanders) and VLAIO (Flemish government agency Flanders Innovation and Entrepreneurship). The authors also acknowledge Fonds voor Wetenschappelijk Onderzoek-Vlaanderen (FWO-Vlaanderen) through grants 1S11520N and 1148018N. The authors further thank SABCA Limburg for providing the aircraft panel.  
The authors declare no conflict of interest.