## 3 Design, Simulation and Implementation for Scattering Measurement System
### Modeling and Implementation for Surface Scattering Measurement System  
According to the above design parameters, the surface scattering measurement system had been designed and modeled. The design diagram of the high-precision surface scattering measurement system is shown in Figure 8. The system is mainly composed of the aiming-and-positioning system and the measuring system. The main function of the aiming-and-positioning system is to facilitate the real-time observation for the region to be measured, so as to improve the flexibility and accuracy of the measuring system. The main function of the scattering-rate measuring system is to collect the scattered light efficiently, so as to achieve the high-precision scattering-rate measurement for any region on the coating surface. Before using the high-precision integrating-sphere scattering-rate system for measurement, the aiming-and-positioning system should be firstly used to aim at the measuring region. When aiming, one can observe the spot position on the sample plate through the image collected by CCD camera, and then adjust the laser incident position according to the image to make the spot coincide with the region interested. Before the reflectivity measurements, the laser incident angle and the horizontal and vertical offset positons are adjusted by using the three-dimensional displacement adjustment platform and collimator to ensure that the incident laser beam vertically passes through the integrating sphere from the center of the incident beam opening to the center of the sample opening. Once the laser incidence angle is adjusted, the position of the laser spot relative to the sample opening remains unchanged, and then the relative position of the sample plate is adjusted by using the three-dimensional displacement platform to select the region to be measured. After the region to be measured has been selected, the laser at 1064 nm should be turned on to record the luminous flux received by the infrared detector. According to the calibration relationship between the simulated luminous flux and detector photocurrent, the scattering rate for the specific region on the sample surface to the corresponding laser wavelength can be calculated. The photograph of high-precision three-hole-integrating-sphere coating-scattering-rate measurement system is shown in Figure 9. The specific technical parameters of the system are shown in Table 1. The flowchart of high-precision scattering-rate measuring system is shown in Figure 10.  
Figure 8: Diagram of high-precision surface-scattering measurement system model. Figure 10: Flowchart of scattering-rate measurement. Figure 9: Photograph of high-precision three-hole-integrating-sphere coating-scattering-rate measurement system. (1) laser, (2) optical filter, (3) beam splitter, (4) CCD camera, (5) three-hole integrating sphere, (6) liquid-nitrogen injection port, (7) LED light switch, (8) system frame and three-dimensional adjustment platform of sample, (9) laser power meter, (10) incident beam opening and (11) measuring opening. As can be seen in Figure 10, it is necessary to measure the background noise to reduce the measuring uncertainty of the system before employing the system for measurement. Then, the laser-aiming-and-positioning system was used to complete the laser aiming for the measured region. Finally, the reflectivity of the selected region could be measured. The specific steps can be summarized as follows. The first step is to install the spherical-missing plate and to record the background current \(I_{0}\) received by the infrared detector without turning on the laser. The second step is to remove the spherical-missing plate and to install the standard reference sample plate, which is made of gold-plating and sand-blasting process. In this paper, the surface reflectance of reference sample is consistent with the internal reflectance in the integrating sphere, both of which are 96%.