# Quantification of the Effective Detectable Period for Concrete Voids of CLP by Lock-In Thermography
## 5 Results and Discussion
### SNR Comparison of Simulated Thermography  
A high SNR in thermography means that the contrast between the defective part and the healthy part is high, and it can be seen that the detection ability is high and flaw detection is easy. Identifying a specific shape or object in an image or video can generally be achieved by contrasting the surroundings and the signal. When this contrast is expressed in decibels through SNR calculation, arithmetically, \(\pm\)6 dB represents twice the signal strength between the surroundings and the object, and it is difficult to recognize a contrast close to 0 dB in human vision. The higher the SNR, the more advantageous the object recognition. As a quantitative evaluation method for the length or width of defects detected in various nondestructive testing techniques such as ultrasonic testing (UT), the 6 dB or 12 dB drop method is usefully used [27]. To use this technique in TT, it is considered that a contrast of at least 6 dB should be displayed.  
Figure 14a-c shows the temperature, amplitude, and phase SNR for each case as a graph based on the selected comparative analysis time, and Figure 15a-c shows each thermal image of Case 3 as an example. As a result, a high contrast of more than 35 dB was shown in all cases, and it is judged that the detection of concrete voids on the back of the CLP will be smooth with the LIT inspection technique. In addition, in terms of SNR, the condition of 0.02 Hz was relatively higher than that of 0.01 Hz, and the effect of the difference in the void was large, but the effect of the difference in depth was very small.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Case**} & \multicolumn{3}{c}{**EDP**} \\ \cline{2-4}  & **Temperature** & **Amplitude** & **Phase SNR (Step)** \\ \cline{2-4}  & **SNR (Frame)** & **SNR (Step)** & **Phase SNR (Step)** \\ \hline \multirow{2}{*}{Case 1} & \multirow{2}{*}{27–151} & 1–15, 30–65, 82–111 & 8–18, 56–69, 106–115 \\ \hline \multirow{2}{*}{Case 2} & \multirow{2}{*}{23–151} & 1–12, 36–59, 89–107 & 20–28, 58–68, 102–104, 107–113 \\ \hline \multirow{2}{*}{Case 3} & \multirow{2}{*}{12–151} & 1–14, 28–62, 82–111 & 3–5, 8–18, 57–69, 106–115 \\ \hline \multirow{2}{*}{Case 4} & \multirow{2}{*}{16–151} & 1–17, 29–64, 81–113 & 4–6, 14–17, 20–28, 52–55, 58–69, 102–104, 107–115 \\ \cline{1-1} \cline{2-4}  & & 81–113 & 52–55, 58–69, 102–104, 107–115 \\ \hline \multirow{2}{*}{Analysis time} & \multirow{2}{*}{74 (frame)} & \multirow{2}{*}{58 (step)} & \multirow{2}{*}{58 (step)} \\ \hline \hline \end{tabular}
\end{table}
Table 4: EDP and comparative analysis time.  
Figure 14: _Cont._  
Figure 14: Comparison of LIT simulation results by case: (**a**) temperature SNR (#74 frame); (**b**) amplitude SNR (#58 step); and (**c**) phase SNR (#58 step).  
Figure 15: _Cont._  
The detection ability of the active thermography test can generally be expected to be advantageous as the heat intensity is high and the time is long. However, if the performance of the flaw detector (temperature resolution, etc.) is good and the heat flux can be delivered evenly, it is estimated that good detection ability can be secured with less heat energy and test time, as shown in this simulation result.