# Condition Monitoring of Wind Turbine Blades Using Active and Passive Thermography
## 5 Results and Discussion
### Image Processing and Quantitative Evaluation
#### 5.2.2 Transform-Based Techniques: Step Phase and Amplitude Thermography  
Pulsed thermograms were converted to a frequency domain using the FFT and the phase images were analyzed. Since the surface was excited uniformly during step heating, both phase and amplitude images of the transformed step heating thermograms were evaluated. Figure 9a,b shows a comparison of the raw pulsed thermograms and the normalized phase contrast obtained using the FFT. PPT significantly increased the contrast so that all defects, except D5, were detected. The same normalization method as the one used for the MF method was applied to the results obtained using the transformed-based techniques. The SNR values for the phase images depicted in Figure 9 are presented in Table 8, which shows that larger defects that were closer to the surface had higher SNRs. The first column of defects is an exception to this observation, mainly due to non-uniform heating of the surface. \begin{table}
\begin{tabular}{c c c c c} \hline  & **1** & **2** & **3** & **4** & **5** \\ \hline
**A** & 44.3 (139\%) & 41.6 (129\%) & 34.9 (95\%) & 28.5 (90\%) & 15.7 (416\%) \\
**B** & 35.6 (95\%) & 37 (105\%) & 34.5 (95\%) & 38.8 (201\%) & 22.7 (280\%) \\
**C** & 28.2 (58\%) & 30.2 (72\%) & 30.3 (77\%) & 27.6 (151\%) & 19.4 \\
**D** & 13.5 (10\%) & 18.4 (53\%) & 16 (85\%) & 11.3 & 6.6 \\ \hline \end{tabular}
\end{table} 
Table 6: SNR (dB) for the \(t\)-statistic on step heating thermal data. \begin{table}
\begin{tabular}{c c c c c c} \hline  & **1** & **2** & **3** & **4** & **5** \\ \hline
**A** & 40.2 (117\%) & 37.6 (107\%) & 32.7 (83\%) & 27 (87\%) & 13.8 (354\%) \\
**B** & 33 (82\%) & 33.8 (88\%) & 32.2 (82\%) & 28.3 (120\%) & 17.8 (198\%) \\
**C** & 27.2 (52\%) & 28.9 (65\%) & 28.6 (68\%) & 26.1 (137\%) & 18.5 \\
**D** & 13.5 (10\%) & 17.2 (44\%) & 16.5 (92\%) & 11.6 & 2.7 \\ \hline \end{tabular}
\end{table} 
Table 7: SNR (dB) for the \(F\)-statistic on step heating thermal data. Figure 9: (**a**) Raw thermogram and (**b**) phase image (acquisition time = 53.2 s) obtained from the thermal image sequence recorded during cooling after flashing the surface. \begin{table}
\begin{tabular}{c c c c c} \hline  & **1** & **2** & **3** & **4** & **5** \\ \hline
**A** & 33.2 (79\%) & 31.9 (76\%) & 29.1 (63\%) & 24.3 (68\%) & 11.6 (282\%) \\
**B** & 30.6 (68\%) & 29.9 (66\%) & 28.5 (60\%) & 25.4 (97\%) & 15.3 (185\%) \\
**C** & 26.4 (48\%) & 27.1 (54\%) & 26.5 (55\%) & 24 (118\%Figure 10a,b shows the phase and amplitude images at the minimum frequency where the FFT was applied to the step heating thermograms captured during cooling after heating for 75 s.  
The diameter-to-depth ratio of the minimum detectable defect in each of phase images of PPT and SPAT was 2, while the amplitude of SPAT provided a better value of 1.33. It can be concluded from these results that the application of the FFT transform to step heating thermography was more effective than it was for flash thermography. This was especially true for the amplitude data, where clearer results with higher contrasts were achieved. Phase images revealed that defects with a better contrast compared to the amplitude images, in some cases. This demonstrated that a reliable inspection could be achieved by evaluating both results. Figure 11 shows the results of an FFT transform applied to thermograms captured during heating for 75 s. The phase image captured all defects--assumed to be indicated by the signals that were more than three times the background level--demonstrating that phase images extracted from the heating data provided more visibility compared to the data obtained from cooling. The amplitude images obtained during cooling were less noisy and contained more detail, which allowed the shape of defects to be determined. Figure 12a presents a further analysis of the normalized amplitude contrast distribution of the thermograms captured during cooling after 75 s of heating. The curves in this figure are the amplitude variation along the lines shown in Figure 5a.