## 3 Design, Simulation and Implementation for Scattering Measurement System  
### Design and Simulation for Aiming-and-Positioning System  
The scattering-rate measurement system based on three-hole integrating sphere was mainly composed of the aiming-and-positioning system, the scattered-light collection and the data-processing system. The laser wavelength was 1064 nm in this paper. The laser wavelength at 1064 nm is invisible to the naked eye. Therefore, in order to observe and adjust the position of the incident spot on the sample plate, it is necessary to design the corresponding visible-light aiming-and-positioning system.  
The green laser at 532 nm was used as the light source of the aiming-and-positioning system. The beam splitter was placed between the laser and the integrating sphere, and a charge coupled device (CCD) camera was mounted on the side of the beam splitter to observe the position of the laser spot. Due to the high brightness of the laser and the closed space in the integrating sphere, only a green spot could be observed in the CCD camera when the laser was incident into the integrating sphere. In other words, the relative position of the measuring spot at the wavelength of 1064 nm on the sample plate would not be observed. Therefore, it is necessary to install a micro light emitting diode (LED) light inside the integrating sphere to improve the contrast for the convenience of observation. In this paper, blue light was used to excite yellow fluorescent material in TracePro to simulate the white light produced by LED.  
Figure 3 shows the composition diagram of the aiming-and-positioning system and the simulation results obtained by TracePro. It could be seen that the laser passed through the beam splitter and reached the sample surface to form a spot. A small part of the light reflected from the sample surface was reflected by the beam splitter into the CCD camera for imaging, so that the relative position of the spot on the sample plate could be observed. Then, the position of the sample would be adjusted according to the CCD image to make the laser irradiate the coating surface region.  
The above aiming-and-positioning system model was imported into TracePro, the images received by CCD camera were observed and recorded. The comparison results without and with LED lighting are shown in Figure 4. It was obvious from Figure 4a that the laser spot could only be observed in the integrating sphere without LED illumination, and the relative position of the spot on the sample plate could not be observed. In this  
Figure 3: Schematic diagram of the aiming-and-positioning system and the TracePro simulation result: (**a**) composition of aiming-and-positioning system, and (**b**) TracePro simulation result.  
case, the relative position between the laser spot and the sample could not be adjusted. After adding LED lighting source, as shown in Figure 4b, the CCD camera could clearly distinguish the relative position of the incident light spot on the sample. In this way, one can adjust the three-dimensional position of the sample plate according to the observed spot position, so as to achieve the accurate measurement of the region interested.