## 3 Photoelectric Scene Design and Verification
### Concrete Equivalent Exposure Scene Design and Validation
#### 3.2.3 Concrete Image Photoelectric Behavior  
In the 21 equivalent exposure scenes, images from the exposure time control scene and the corresponding illuminance control scene are theoretically identical, yet slight difference exists between equivalent exposure scenes due to the difficulty in illumination distribution uniformity on the specimen surface by LED illuminance control. Histograms of each equivalent exposure scene of two control style groups are shown in Figure 4,  
Figure 3: Arrangement of the experimental devices.  
Figure 2: Concrete specimen image area.  
and it can be seen that the concrete image histograms nearly follow the law of the normal distribution curve, and there is a slight difference in the histograms in each equivalent scene. Figure 4 indicates that the peak values and mean values of the image histograms showed no significant difference in exposure time-controlled scenes and illumination-controlled scenes at each equivalent exposure level. Therefore, the histograms for both control groups exhibit remarkable similarity, which serves as additional validation of the equivalent exposure scenario despite the control manner.  
Moreover, to further understand concrete image photoelectric behavior with exposure changes, the relationships between the histogram mean value (HMV) and standard deviation (HSD) changes with exposure are plotted in Figure 5. From Figure 5a, it can be seen that in the low-exposure interval of 0-154 k\(\cdot\)s, the HMV is in linear proportion to equivalent exposure, while in a higher exposure interval, greater than 154 k\(\cdot\)s, the HMV remains at the maximum constant value of 65,535, which indicates extensive overexposure in image capture. In Figure 5b, the HSD also changes linearly with equivalent exposure in the low-exposure interval of 0-106 k\(\cdot\)s, but in a higher exposure interval, the HSD also decreases nearly quadratically with equivalent exposure. According to imaging experience, the HMV and HSD differences in the histograms were caused by the difficulty in uniform illumination control in each scene, which was achieved by the variation in light intensity and location shown in Figure 3.  
By comparing the mean values and standard deviations of concrete image capture from the 21 equivalent exposure scenes, it can be seen that images from the same exposure scene are nearly identical, and equivalent exposure can be considered a core control index for scene characterization.  
Figure 4: Histogram of concrete images under different exposure scenes.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Device Name** & **Model** & **Parameter and Specification** \\ \hline \multirow{4}{*}{Camera} & \multirow{4}{*}{QHY600} & Shutter 40 us\(\cdot\)3600 s \\  & & Resolution 9600 \(\times\) 3194 pixels \\ \cline{3-3}  & & Image format TIF \\ \cline{3-3}  & & Bit depth 8/10/12/16 \\ \hline \multirow{2}{*}{Lens} & \multirow{2}{*}{Nikon AF Zoom-Nikkor} & Aperture F2.8–22 \\  & & Focal length 80–200 mm \\ \hline \multirow{2}{*}{Illuminometer} & \multirow{2}{*}{TES-1339R} & Measurement range 0.1–999,900 lx \\  & & Measurement speed five times per second \\ \cline{3-3}  & & Resolution 0.01 lx \\ \hline \hline \end{tabular}
\end{table}
Table 3: Experimental devices and parameters.