# Analysis of Fine Crack Images Using Image Processing Technique and High-Resolution Camera
## 6 Conclusions  
In this paper, indoor measurement experiments were conducted to measure cracks in concrete structures and to present guidelines for image acquisition and processing using imaging equipment. Two units of DSLR imaging equipment, which is applied in various fields, were selected, and lenses with focal lengths of 50, 100, and 135 mm were applied. Linear virtual images with widths of 0.1, 0.3, and 0.5 mm were applied to the indoor measurement experiments, and video images were obtained at 10 mm intervals from the minimum working distance to the measurement limit distance to measure the line width values of the virtual images through image processing. The crack measurement limits, as well as optimal working distances of the selected imaging equipment and optical equipment, were derived to analyze measurement accuracy according to the difference in resolution and focal lengths. Field measurement tests were conducted to verify measurement parameters identified by the results of the indoor experiment. The conclusions drawn from this study are as follows. * The pixel values were measured using the CC (Canon camera) and SC (Sony camera) equipment units with performances of 22.1 and 61 million pixels, respectively, and applying the same focal lengths. 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, respectively. As a result of applying the same  
Figure 19: Measurement results of concrete crack. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{\ \begin{tabular}{c} **WD** \\ **(mm)** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} _GSD_ \\ **(mm/pixel)** \\ \end{tabular}  } & \multicolumn{3}{c}{**Measured**} & \multicolumn{3}{c}{**Approx. **} & \multicolumn{3}{c}{**Real**} \\ \cline{3-8}  & & **Pm** & **Wm** & **Em** & **Pa** & **Wa** & **Ea** & **Pr** & **Ground** \\  & **(ea)** & **(mm)** & **(\%)** & **(ea)** & **(mm)**  
focal length, it was found that the measured number of pixels and the measurement limit distance increased because the resolution and size of the virtual image increased. However, despite an increase in resolution by 1.65 times, the measured pixel values of the SC50 were estimated to be 1.33, 1.10, and 1.19 times higher, respectively, than the CC50 at the minimum working distance. In addition, when the virtual image size increased by three times from 0.1 to 0.3 mm, the pixel value of CC50 at the minimum working distance increased by 3.3 times, and when it increased by five times from 0.1 mm to 0.5 mm, the pixel value increased by 5.5 times. As for SC50, the pixel value increased by 2.75 times and 4.75 times, respectively. * As for increasing the focal length at the same resolution, the value showed a similar tendency with the results of increasing the resolution. However, the pixel value of SC100 was 1.85 times larger at average than that of SC50 at the minimum working distance. When the size of the virtual image increased by three times and by five times, the pixel value of SC100 was estimated at the same rate and by 5.13 times. As a result of experiments, the pixel value was more affected by the increase in focal length than the increase in resolution. However, even if the resolution, focal length, and size of the object increased, the pixel values did not increase at the same rate. It is considered that the difference between the pixel value and the rate could occur because of the minimum working distance, in accordance with the physical features of an optical lens. * According to the results, in the case of CC50-0.1 and SC50-0.1 mm, 39.29% and 38.77% of the working positions, respectively, had values within PWL-10 within the respective measurement limit distances. With regard to the 0.3 mm wide virtual image, 48.61 and 49.17%, respectively, of the working positions had values within PWL-10. For the 0.5 mm virtual image, 50.26 and 51.44%, respectively, of the working positions had values within PWL-10 within the limit distance.