# Analysis of Fine Crack Images Using Image Processing Technique and High-Resolution Camera
## 4 Measurement Results and Analysis
### Measured Values of Line Widths According to Working Distance  
4.2.1 Measured Values of Line Widths according to Differences in the Resolution of Imaging Equipment  
As written above, the _GSD_ of the pixel value measured by each variable through image processing was calculated according to the working distance to measure the width value of the virtual image. Figure 10 shows the width values of individual virtual images with the same focal length of 50 mm applied, measured by working distance using individual units of imaging equipment. Here, the ground truth represents the width value of the actual virtual image, and the shaded part of the red box is the percentage within the limit of error, which is less than 10% (PWL-10). In addition, the working position where the size of one measured pixel has the same value as the width of the virtual image was expressed as the measurement limit distance of the imaging equipment. As a result of measurement of the width of CC50-0.1 in Figure 10a, there were 56 working positions within the measurement limit distance of 850 mm, and the number of working positions that had values within PWL-10 was 22, accounting for 39.29%. However, in the case of SC50-0.1, the number of working positions that had values within PWL-10 was 38, accounting for 38.77% of the 98 working positions within the measurement limit distance. With regard to CC50-0.3, the number of working positions within the measurement limit distance of 2450 mm was 216, and the number of working positions that had values within PWL-10 was 105, which is 48.6% in ratio. However, for SC50-0.3, the number of working positions that had values within PWL-10 within the measurement limit distance of 4010 mm was 178, accounting for 49.2% of the entire 362 working positions within the measurement limit distance. Here, all working positions were within the working distance of 1 m except for those at 930 and 940 mm, which had values within PWL-10, and the ratio was 96.7%. Within the working distance of 1.5 m, there were 99 working positions, and the ratio of those working positions that had values within PWL-10 was 89.1%, as can be seen in Figure 10b. The results of the measurement of the width of a 0.5 mm virtual image are shown in Figure 10c. In the case of CC50-0.5, the ratio of working positions within 1 m that had values within PWL-10 was 95%, and it was seen that errors increased as the working distance increased. For the accumulated shooting section, 376 working positions were identified within the measurement limit distance of 4050 mm, and the number of working positions that had values within PWL-10 was 188, with a ratio of 50.26%. In the case of SC50-0.5, it was found that the values measured at all working positions in the entire section within a working distance of 1.5 m corresponded to PWL-10. The number of working positions that had values within PWL-10 within a working distance of 2 m was 144 out of 161 locations, with a ratio of 89.4%. It was confirmed that the precision showed a tendency to decrease after that, since the shooting section grew as the working distance increased. Within the measurement limit distance of 6650 mm, 321 out of 624 working positions showed values within PWL-10, with an accuracy of 51.3%. Table 5 shows the resulting measurement values of each image with different resolutions for the same focal length. The measurement accuracy increased \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Classification**} & \multicolumn{3}{c}{**Equivalent Resolution**} \\ \cline{2-4}  & \multicolumn{3}{c}{**Number of Pixels (Each)**} \\ \cline{2-4}  & **SC50** & **SC100** & **SC135** \\ \hline
0.1 mm & 2(4) & 5(7) & 8(8) \\
0.3 mm & 7(11) & 16(21) & 23(25) \\
0.5 mm & 12(19) & 26(36) & 38(41) \\ \hline \hline \end{tabular} () The number of measurement pixels according to each focal length at min. working distance.  
\end{table} 
Table 4: The number of pixels measured by focal length changes at a working distance of 600 mm.