# Analysis of Fine Crack Images Using Image Processing Technique and High-Resolution Camera
## 6 Conclusions
There was no significant difference in measurement accuracy within the limit distance of the imaging equipment. However, relative comparisons by shooting section showed that the SC equipment had somewhat higher precision than the CC equipment. * Increasing the focal length enables more precise measurement by increasing the measurement limit distance and pixel measurement values, as well as reducing the _GSD_ value of imaging equipment. When measurement accuracy was checked by section while increasing the working distance from the minimum working distance, it was found that within a 1 m working distance, with regard to SC50-0.1 and SC100-0.1, 39.34 and 85.18% of working positions, respectively, had values within PWL-10, and as for SC135-0.1, 95.55% of working positions had values within PWL-10. In the case of SC50-0.3, the ratio was 96.72% within 1 m of the working distance, and for SC100-0.3 and SC135-0.3, all working positions within working distances of 1.5 m and 2 m, respectively, had values within PWL-10. In the case of SC50-0.5, all working positions within a working distance of 1.5 m had values within PWL-10, and as for SC100-0.5 and SC135-0.5, all working positions within working distances of 3 m and 4 m, respectively, had values within PWL-10. Based on the results of analysis of PWL-10 in this study, cracks were measured using a unit of 61-megapixel imaging equipment, and according to the results, in the case of 0.3 mm wide or wider cracks, a working distance of 1 m was secured when the focal length was 50 mm, and working distances of 3 and 4 m were secured when the focal lengths were 100 and 135 mm, respectively. However, with regard to cracks not wider than 0.1 mm, focal lengths of 100 and 135 mm showed measurability within 1 m, but a focal length of 50 mm was judged to hardly enable measurement, except for certain working positions. * Many elements affect width measurement. However, based on the results of this study, width measurement is highly dependent on the measured pixel values. In order to accurately measure width, the measured pixel values must be in decimal units, but that is theoretically impossible. Field measurement tests were conducted to verify measurement parameters identified by the results of the indoor experiment. Anapproximate curve was predicted based on the pixel value according to the working distance, and an estimated pixel value was derived using power law distribution. As the working distance increased, the crack measurement value (Wm) did not show a clear trend in error change and fluctuated with ground truth. However, the crack width (Wa) calculated by the estimated pixel indicated an error of up to 5% and showed great agreement with the ground truth. From the evaluation, it was confirmed that the number of pixels corresponding to the working distance had a great influence on the measurement accuracy of crack width when using image processing. Once the working distance is accurately defined, it is judged that the number of pixels measured at each shooting location can be identified through the approximation method. * If microcracks not wider than 0.1 mm are measured by applying imaging equipment with the same resolution as applied in this experiment and a focal length of 50 mm, the measurement should be carried out at distances shorter than the minimum working distance from the crack surface. To secure at least 90% measurement accuracy for widths not larger than 0.1 mm at working distances not longer than 1 m, a focal length of 135 mm should be applied. In addition, the resolution value required when a focal length of 50 mm is applied was estimated using the _GSD_ value calculated by applying a focal length of 135 mm, and according to the results, it is thought that the resolution range of the imaging equipment should be at least about 400 million pixels. Conceptualization, H.J. and D.C.; methodology, H.J., B.J., M.H. and D.C.; software, H.J. and M.H. ; validation, H.J. and D.C.; formal analysis, H.J. and D.C.; investigation, H.J. and M.H. ; resources, D.C.; data curation, B.J.