# 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
slightly, but this seems to be a result of the increase in the measurement limit distance. For instance, as can be seen in Table 5, in the case of CC50-0.1, the ratio of working positions that had values within PWL-10 was 39.29% within the limit distance of 850 mm, but in the case of SC50-0.1, the ratio was the same when the same limit distance was applied. In conclusion, it was demonstrated that the SC equipment had 2.76 times higher resolution corresponding to the sensor area and 1.65 times larger horizontal and vertical dimensions than the CC equipment, but no significant difference in measurement accuracy was shown between the two equipment units within their respective limit distances. Figure 10: Width measurement results of Canon and Sony cameras. However, relative comparisons by limit distance and shooting section of the CC equipment showed that the SC equipment had somewhat higher precision, and it was confirmed that the difference in resolution increased working distance as well as accuracy. In addition, it was judged that the size measurement of a specific object is based on the numbers of horizontal and vertical pixels, not the pixel value in the space corresponding to the sensor. At the current level of imaging equipment, when measuring a 0.1 mm crack by applying a focal length of 50 mm, a minimum working distance from the crack surface is required, and it is judged that crack measurement is somewhat difficult without increasing the resolution and focal length. 2.2 Measured Values of Line Widths According to Increases in Focal Length Using the Same Imaging Equipment  
Earlier, the resulting measurement values of the widths of virtual images according to differences in resolution at the same focal length of 50 mm were checked. In this chapter, the widths of virtual images were measured while increasing the focal length at the same resolution, and the resulting values were analyzed. Figure 11 shows the resulting measurement values of a 0.1 mm wide virtual image. With regard to SC100-0.1, the number of working positions that had values within PWL-10 was 96 out of 218 total working positions within the measurement limit distance of 2740 mm, representing a ratio of 46.49%. As for SC135-0.1, the ratio of working positions that had values within PWL-10 was 95.55% within a working distance of 1 m, and the ratio decreased as the working distance increased to show values lower than 82.10% at a working distance of 1.5 m. The ratio showed a tendency to decrease rapidly after that. Within the measurement limit distance of 3700 mm, 136 out of 301 working positions had values within PWL-10, representing a ratio of 47.94%. In Figure 12, with regard to SC100-0.3, 379 out of 746 working positions within the measurement limit distance of 8020 mm had values within PWL-10, representing a ratio of 51.45%. In this case, when checked by the shooting section, the measurement accuracy at which all working positions in the entire section, ranging from the minimum working distance to 1.5 m, had values within PWL-10, as shown. However, in the case of SC135-0.3, 100% measurement accuracy was shown over the entire shooting section within 2 m, and the measurement accuracy gradually decreased afterward as the distance increased so that 97.95% measurement accuracy was secured within 3 m. The measurement accuracy was 91.01% at the 4 m working position and then showed a tendency to decrease from the working position, so the number of working positions that had values within PWL-10 became 517 out of 1014 working positions within the measurement limit distance of 10,830 mm, representing a ratio of 51.94%.