## 3 Experiment and Discussion
### Measurement Results of Additive Manufacturing Process of Metal Parts  
The additive manufacturing environment for the metal plate is shown in Figure 11. As there will be a large amount of metal powder splashing during the printing process of the DED system, to avoid the metal powder from adhering to the surface of the camera filter and the lens of the projection device, as well as to ensure the quality of the projection fringe and the recorded deformed fringe image, three blowpipes (numbered Outlet 1-3 in Figure 11) are placed on the upper end of the camera filter and the lens of the projection system, respectively. Air is supplied to cool the light source during the experiment, as shown in Outlet 4 in Figure 11. The whole measuring system is fixed on a tripod. The working distance was adjusted to ensure that the printing area is located at the projection focal plane, and the baseline distance and respective focal length between two cameras were adjusted in order to guarantee that the printing area can be clearly imaged by two cameras, respectively. The protective gas blown by the print head can effectively ensure the cleanliness of the measured object surface without dust. The additive manufacturing process of metal parts were measured. Figure 12a shows the deformed fringe image of the metal plate after the first layer is printed. The additive manufacturing is carried out in the dotted area of the metal plate, and a total of 15 layers are printed. Figure 12c shows the deformed fringe image of the metal plate after printing, and Figure 12b shows the fringe change of the corresponding area before and after additive manufacturing. The deformed fringe image of the printing area after the printing of each layer is shown in Figure 12d. All of the obtained deformed fringe images have been used for 3D reconstruction according to the aforementioned manner, and the height change after the printing of each layer can be identified. Figure 13 shows the reconstructed height distribution of 3D print metal at 15 times shown in Figure 12d. The above number of each subfigure is the corresponding printing layer. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Ball-Bar Gauge Standard Parts** & **Ground Truth Value** & \
\begin{tabular}{c} **Measurement Result** \\ **Measured Value** \\ \end{tabular} &
\begin{tabular}{c} **Error** \\ \end{tabular} \\ \hline Ball A diameter & 50.7991 & 50.7507 & 0.0484 \\ \hline Ball B diameter & 50.7970 & 50.8436 & 0.0466 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Accuracy evaluation results of dumbbell gauge standard parts (unit: mm). Figure 10: Measurement results of the ball-bar gauge standard part, (**a**) deformation fringe image measured on ball A; (**b**) height distribution of ball A; (**c**) standard deviation distribution of ball A; (**d**) deformation fringe image measured for ball B; (**e**) height distribution of ball B; (**f**) standard deviation distribution of ball B.  
Figure 11: Working environment for the additive manufacturing of metal plates. Figure 12: Deformation fringe pattern taken during additive manufacturing of metal parts (taken by the left camera). (**a**) deformation fringe of the first printed layer; (**b**) comparison of fringe changes of metal parts after printing the 1 st and 15 th layers; (**c**) deformed fringe image of 15 th printed layers; (**d**) deformed fringe images of different layers; (**e**) Fourier spectrum of the dashed region of the (**a**); (**f**) wrapped phase of (**e**); (**g**) unwrapping phase of (**f**). It can be seen from Figure 13 that when the number of printing layers is greater than 7, the height distribution of metal parts cannot be fully obtained with part of the region data missing. This is because the height changes greatly when the printing layer increases, resulting in a local shadow of images and mutual occlusion of binocular cameras, and finally leading to the loss of phase and height data. Figure 14a shows the height change 15 groups along the 500 th row and the 250-500 th column.