# Depth Retrieval Procedures in Pulsed Thermography: Remarks in Time and Frequency Domain Analyses
## 5 Conclusions  
In the present study, a CFRP specimen with internal simulated delaminations of various sizes and depth locations was investigated by means of PT for the retrieval of quantitative depth information. Hidden features located in a depth range from 0.2 mm to 1 mm and varying in size from 3 mm to 15 mm were quantitatively characterised using two different analysis procedures through which the impact of different analysis parameters was also assessed. The produced results indicated that the two different analyses provided efficient depth estimations for the inserted features, while the comparison of temporal and frequency domain quantification in terms of error production showed that frequency domain analysis produced measured depths closer to their nominal values. Moreover, the quantification in the frequency domain was further improved by considering the entire surface of the panel as the sound area because the defective zones in the present case occupy a small region with respect to the entire surface. Contrarily, when working with peak slope time and especially when an uneven heating application is presented, depth quantification may be performed selecting different sound areas for depths determinations, ensuring that both the defective and the sound area have received the same amount of heat energy. As regards the selection of the experimental and analysis parameters, time resolution had a great impact on the determination of the characteristic times used for depth estimations and this parameter produced large errors when defects near the surface have to be quantitatively determined. On the other hand, as frequency is inversely proportional to time, the temporal resolution had an impact on the frequency resolution as well. Contrary to temporal data analysis, the low frequency resolution impacted the results of the deeper defects, as the frequency visibility range was lower for these features.  
Figure 15: Correlation results after: (**a**) selecting a sound area from the background; and (**b**) selecting the entire surface as a sound area.  
For the continuation of this research, and because these quantification methods require the value of thermal diffusivity \(\alpha\) to be available, it is highly recommended to also assess the produced depth accuracy when this value is known. In such a manner, the impact degree of this parameter could be more accurately estimated. Moreover, based on this preliminary study, the application of quantitative infrared thermography for depth prediction in different materials and/or for the characterisation of different defects would provide a clearer view in the assessment of these depth prediction methods. In this direction, a comparative study incorporating different methodologies for depth prediction, such as the Logarithmic Peak Second-Derivative Time method [41], techniques based on the development of numerical modelling [42], or the recently developed Pulse Compression Thermography [43; 44; 45], which has shown to provide results with enhanced SNR, can define more representatively the strengths and weaknesses of each quantitative technique.  
Authors would like to thank Prof. [PERSON] and Dr. [PERSON] from Computer Vision and Systems Laboratory of Universite Laval, for the providing of the raw pulsed thermographic data and more importantly for their continuous support and willingness to provide any information throughout the conduction of this research.  
[PERSON] wrote the paper and designed and implemented the experimental case study presented above. [PERSON] supervised the present research study.  
The authors declare no conflict of interest.