# Depth Retrieval Procedures in Pulsed Thermography: Remarks in Time and Frequency Domain Analyses
###### Abstract  
In the present study, a Carbon Fibre Reinforced Polymer (CFRP) sample of trapezoid shape, consisting of internal artificial delaminations of various sizes and depth locations, is investigated by means of optical pulsed thermography for the retrieval of quantitative depth information. The main objectives of this work are to evaluate the produced depth estimation accuracy from two contrast-based depth inversion procedures as well as to correlate the acquired results with characteristics such as the location and size of the detected features and with analysis parameters such as the selection of the sound area. Quantitative analysis is performed in both the temporal and frequency domains, utilising, respectively, the informative parameters of thermal contrast peak  
Nowadays, PT is recognised as a testing procedure that can provide inspectors not only with the ability to detect defects, but also the means to quantify these defects for continuous monitoring and assessment [7]. This is a result of the advances in thermographic imaging technology that have orientated the analysis to the independent spatial and/or temporal treatment of each pixel of the thermal images' sequence, along with the development and use of advanced signal processing techniques [8; 9]. Signal processing techniques have been developed for the interpretation of infrared thermographic data, targeting the defect detectability enhancement and noise reduction on the final imaging output, while the importance of these processing tools is also devoted to the preparation of data for the continuation to the phase of quantitative characterisation [10; 11]. Regardless of the domain where the thermal data are processed (the most common are temporal domain processing and frequency domain processing), different techniques have been proposed to quantitatively characterise detected defects. These techniques are based either on analytical methods [12] or inverse methods that are implementing numerical calculations and experimental measurements [13]. In the former case, experimental data can somehow produce results in a straightforward manner, as these are used to solve analytical formulas. On the other hand, by involving inverse techniques, quantitative characterisation requires the establishment of a numerical model taking into account many parameters. Usually, only one parameter is of interest (e.g., depth) which can be estimated by correlating the numerical and the experimental results. Quantitative characterisation by means of pulsed, and more generally speaking of active thermal imaging, includes mainly depth information retrieval [14; 15; 16; 17; 18] and the estimation of defect's lateral dimension [19; 20; 21], while several studies have also investigated the determination of the thermal properties either of the material being inspected [22; 23] or of the hidden features [24]. Based on the above mentioned, the present study focused to the application of optical Pulsed Thermography for the quantitative characterisation of induced artificial defects in a CFRP component, aiming to highlight the strengths and limitations of this technique in similar quantitative inspections. The quantitative depth estimation procedures applied in this study evaluated the performance of two contrast-based methods, one in the time and one in the frequency domain, respectively, and in particular through the study of the temperature-contrast slope temporal variations and the monitoring of the phase-contrast variations as a frequency function. These analytical methods for depth prediction require the need for a sound area determination and thus a comparison was performed regarding the influence degree of the sound area selection in depth prediction accuracy working with either raw temperature or phase data.