## 2 Principle
### Construction of Binocular Measurement System for Projection of Ultraviolet Light Source  
Based on the above-motioned background, a UV light source and a physical grating are selected as fringe projection equipment, and a binocular measurement system based on a UV light source projection is built, as shown in Figure 4.  
The chosen light source wavelength is 365 nm and the power is 3W, shown in Figure 4. The experimental device is shown in Figure 4. An IDS industrial camera with the resolution of 1920 \(\times\) 1200 pixels was used, and the field of view of the projection device is about 100 \(\times\) 100 mm\({}^{2}\). The system components are shown in Figure 4.  
The aperture of the camera was adjusted to its maximum value and the system parameters, such as the exposure time of the camera, have also been fixed. The deformed fringe images of the formed metal parts under the illumination of the UV light source were obtained, as shown in Figure 5. Figure 5 shows the center line distribution of the Fourier spectrum of a fringe pattern. After filtering out the fundamental frequency component, and calculating the corresponding wrapped phase after spatial phase unwrapping, the unwrapped phase distribution shown in Figure 5 can be used to reconstruct 3D morphology at the corresponding sampling time. The measurement system is built with a filter with a central wavelength of 365 nm, a bandwidth of 20 nm, and a cutoff band transmittance of 10\({}^{-4}\), which is placed in the front of the binocular camera to suppress the interference light outside the band. Figure 6 shows the deformed fringe image when one single such filter is added in front of the camera, in which a large area is overexposed due to the strong laser intensity of the additive manufacturing system. After the superposition and combination of two filters, the deformed fringe image of the processing process is obtained again as shown in Figure 6. It can be seen that the superposition of two filters can filter out the influence of visible light, and the subsequent 3D shape reconstruction of the processing workpiece can be carried out. Figure 4: Binocular measurement system based on UV light source projection: (**a**) test of UV light source projection on the industry site; (**b**) system components of 3D morphology detection. Figure 5: Test results of metal plate surface: (**a**) deformed fringe with UV light source, (**b**) distribution along the center line of Fourier spectrum of (**a**), (**c**) unwrapping phase of (**b**). In the actual additive manufacturing process, there are often splashes of metal powder. Figure 7a shows a frame of a deformed fringe image in the additive manufacturing process. Considering the detection speed requirement in additive manufacturing, PMP, which has a higher reconstruction accuracy for complex objects, could not be chosen as the phase analysis method because a large number of acquired images (at least three frames) is required to complete a single measurement. In this work, FTP was chosen as the means of calculating and acquiring phase information. Additionally, the Fourier fringe analysis method also can filter out the high-frequency noise from the interference of dust on the actual measurement. By using Fourier fringe analysis, the corresponding wrapped and unwrapped phase is shown in Figure 7b,c, respectively. Figure 7d\(-\)f show the section line of the Fourier spectrum and phase information, respectively. The usage of the selected light source, filter and Fourier fringe analysis algorithm can calculate the unwrapped phase of each recorded fringe pattern by two cameras, and provides a basis for the matching of homonymous points in the binocular system. The phase unwrapping starting point can be marked on the grating to guide the acquisition of the same spatial point in the measurement field of view as the same starting point during phase unwrapping processes of two-phase distribution captured by cameras.