# 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
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Classification**} & \multicolumn{4}{c}{**PWL-10**} & \multicolumn{2}{c}{**Unit: \%**} \\ \cline{2-7}  & \multicolumn{2}{c}{**SC50**} & \multicolumn{2}{c}{**CC50**} & \\ \cline{2-7}  & **0.1 mm** & **0.3 mm** & **0.5 mm** & **0.1 mm** & **0.3 mm** & **0.5 mm** \\ \hline \multirow{8}{*}{Working distance (m)} & 0.85 & 52.17 & 100 & 100 & 39.29 & 87.71 & 100 \\ \cline{2-7}  & 1.00 & 39.34 (38.77) * & 96.72 & 100 & 78.87 & 95.77 \\ \cline{1-1} \cline{2-7}  & 1.50 & 89.18 & 100 & 72.07 & 88.28 \\ \cline{1-1} \cline{2-7}  & 2.00 & 72.04 & 89.44 & 46.11 (48.61) * & 72.51 \\ \cline{1-1} \cline{2-7}  & 2.50 & 65.40 & 89.57 & & 65.40 \\ \cline{1-1} \cline{2-7}  & 3.00 & 52.87 & 72.41 & & 52.87 \\ \cline{1-1} \cline{2-7}  & 4.00 & 49.03 (49.17) * & 70.63 (51.44) * & 47.92 (50.26) * \\ \hline \hline \end{tabular} (*) Value of PWL-10 at the measurement limit distance.  
\end{table} 
Table 5: Value of PWL-10 within working distance by resolution difference. In the results of measurement of a 0.5 mm wide virtual image, in the case of SC100-0.5, 663 out of 1274 working positions within the measurement limit distance of 13,300 mm had values within PWL-10, representing a ratio of 52.41%, as seen in Figure 13. When checked by the shooting section, it was found that measurement precision was secured over the entire section, from the minimum working distance to 3 m. As for SC135-0.5, all working positions within 4 m had values within PWL-10, and the precision decreased afterward so that 98.20% of all working positions within 5 m had values within PWL-10. The resulting value was 89.49% in the shooting section within 5.5 m, but the precision increased slightly after that so that 91.26% of all working positions in the shooting section within 6.5 m had values within PWL-10. The reason for the slight increase after decreasing is that the number of working positions that had values within PWL-10 remained the same, while the working distance continued to increase. In the shooting section within 7 m, the value was 86.66%, and as the working distance increased, 900 out of 1727 sections within the measurement limit distance of 17,960 mm had values within PWL-10, representing a ratio of 52.70%. The overall precision seems to have increased slightly compared to SC100-0.5, and the reason is judged to be the fact that not only the maximum shooting section but also the number of working positions that had values within PWL-10 increased. Figure 11: Results of 0.1 mm width measurement. Figure 12: Results of 0.3 mm width measurement. The levels of measurement accuracy by shooting section according to changes in focal length are summarized in Figure 14 and Table 6. The resulting values of SC135-0.1 and SC50-0.5, SC100-0.3 and SC50-0.5, and SC135-0.3 and SC100-0.5 show similar trends. It was found that in the case of SC135-0.5, at least 95% measurement accuracy was secured up to 5 m. For widths smaller than 0.3 mm, which is the crack measurement limit, but not smaller than 0.1 mm, which is the purpose of this study, SC50-0.3, SC100-0.3, and SC135-0.3 secured at least 90% measurement accuracy within working distances of 1 m, 3 m, and 4 m. However, in the case of 0.1 mm width, at least 85% and 95% measurement accuracy levels were secured within a 1 m working distance at focal lengths of 100 mm and 135 mm, respectively. However, as mentioned above, it is judged that when measuring 0.1 mm cracks, if a focal length of 50 mm is applied, the cracks can hardly be measured except for certain working positions. If microcracks not wider than 0.1 mm are measured by applying imaging equipment with the same resolution as the one applied in this experiment and a focal length of 50 mm, it is judged that the measurement should be carried out at distances shorter than the minimum working distance from the crack surface. In addition, to secure at least 90% measurement accuracy within the working distance of 1 m, 135 mm should be applied as the minimum focal length.