“…It also can be seen from Table 6 that the introduction of polarization thermal imaging can significantly improve the detection accuracy, whose error rate compared with infrared thermography is down as much as 21.8%, 17.6%, 31.4%, and 22.2% in the hollowing defect areas of the circle, rectangle, triangle, and square under the same experimental treatments. (2) In order to verify the superiority of the improved Canny algorithm proposed in this paper in extracting the contours of building hollowing defects using polarization thermal images, the performances of the Roberts, Sobel, Prewitt, Log [48], Canny, and the output contour size with pixel area method [49] algorithms were compared with each other. The six detection algorithms lead to different errors between the processing results and the actual sizes, and the output edge detection results of hollowing defects processed using these algorithms and the morphological processing results of corresponding images are shown in Figures 11 and 12 After the integrated comparison of the edge detection results of polarization thermal images of hollowing using several methods, it can be found that, as shown in Figure 11a,c,d, the true edge points of hollowing defects extracted using the Roberts, Prewitt, and Log algorithms are missing to varying degrees, with the most severe edge feature loss of hollowing defects in the images processed using the Prewitt algorithm.…”