## 5 Multi-Channel Scanning System  
Thus far, the time-multiplexing technique has been used to increase the viewing zone and screen size of holographic displays. Although the viewing zone angle has been sufficiently increased to 40-360\({}^{\circ}\), the screen size has been limited to 2-4 inches. For a further increase in screen size, the space-multiplexing technique has been introduced into the viewing zone scanning system [27].  
Figure 10 illustrates the multi-channel holographic display system that uses both time-multiplexing and space-multiplexing techniques. A multi-channel system consists of multiple projection systems using the MEMS SLMs and a planar scanner. The projection system shown in Figure 2 is modified to enable us to tile all screens of the multiple projection systems seamlessly. The planar scanner is a screen that supports large images and can redirect the images in a horizontal direction. The planar scanner consists of an off-axis Fresnel lens and a vertical diffuser. The rotation of the off-axis Fresnel lens rotates the viewing zone around the rotation axis. The vertical diffuser diffuses light vertically so that the rotation of the viewing zone is transformed into the horizontal movement of the viewing zone. Thus, the planar scanner can scan the viewing zone in the horizontal direction. The multi-channel system provides a scalable screen size by alternating the number of projectors.  
The proposed multi-channel technique was verified using a two-channel holographic display shown in Figure 11. The screens of the projection systems #1 and #2 were horizontally tiled on the planer scanner to increase the screen size to 7.4 inches (156 \(\times\) 104 mm\({}^{2}\)). The focal length of the Fresnel lens was \(f\) = 800 mm. The scan angle of the planer scanner was 43.0\({}^{\circ}\) which was equal to the viewing zone angle of the two-channel system. The 3D images were produced with a frame rate of 60 Hz. As the viewing zone was scanned twice during one rotation of the planar scanner, the rotation speed of the planar scanner was set to 1800 rpm.  
Figure 9: Color 3D images produced by RGB scanning holographic display: (**a**) three objects, (**b**) earth, and (**c**) cube.  
The photographs of the 3D images produced by the two-channel system are shown in Figure 12. From the experimental results, both the large screen size and the wide viewing zone angle were obtained.  
The methodology for the enlargement of the screen size described in this section makes use of the space-multiplexing technique. The use of the time-multiplexing technique to increase the screen size is described in the following section. Many research works have made use of the space-multiplexing technique to increase the screen size of the holographic displays [10; 11; 12; 28]. The combination of the 2D scanning system and two DMDs was used to increase the screen size to 10 \(\times\) 20 mm\({}^{2}\)[28]. The space-multiplexing technique has also been used for the holographic displays using liquid-crystal (LC) SLMs to increase the screen size because of the low frame rate of the LC SLMs. Holographic display systems using 9 and 16 LC SLMs were developed to provide a screen size of 2.5 and 3.3 inches, respectively [11; 12].  
Figure 11: Two-channel holographic display system.  
Figure 10: Multi-channel holographic display using multiple MEMS SLMs.