## 2 Viewing Zone Scanning System  
In conventional holographic display techniques, the SLM diffracts light to generate the viewing zone (Figure 1a). Therefore, the SLM pixel pitch should be reduced to enlarge the viewing zone. When the pixel pitch is reduced, the resolution should be increased to enlarge the screen size. Thus, a small pixel pitch is needed only for the viewing zone enlargement. Regarding viewing zone enlargement, the scanning system outperforms the light diffraction system (Figure 1b). A combination of a 1D light modulation using the acousto-optic modulator and a 2D light scanning using the polygon mirror and the galvano scanner was proposed [23]. In this article, the combination of 2D light modulation using MEMS SLM and 1D or circular light scanning proposed in our research group [13; 19; 20; 21] is reviewed. The latter combination requires a slower scanning speed than the former. Figure 2 shows the holographic display system comprising the MEMS SLM and the 1D scanning system [19]. A magnifying imaging system enlarges the screen size of the MEMS SLM to increase the screen size of the display system. Thus, the pixel pitch increases, and the viewing zone is reduced. The reduced viewing zone is scanned by the 1D scanner to enlarge the viewing zone. Hence, both the viewing zone and screen size are enlarged. The single-sideband (SSB) filter is used to eliminate the conjugate image and the zero-order diffraction light, which are not required for hologram reconstruction [24]. The width of the reduced viewing zone should exceed the pupil diameter of the human eye to ensure wavefront reconstruction. Figure 3 shows the experimental system based on the viewing zone scanning system. A digital micromirror device (DMD) was used for the MEMS SLM. The resolution was 1024 \(\times\) 768, the pixel pitch was 13.68 \(\upmu\)m, and the frame rate was 13.333 kHz. A galvano mirror with a diameter of 50 mm (2.0 inches) was used as the horizontal scanner. A laser diode with a wavelength of 635 nm was used as the light source. The magnification of the imaging system was determined to be 2.86 such that the image was the same size as the galvano mirror (2.0 inches), which is the same size as the screen of the holographic display. The imaging system had an enlarged pixel pitch of 39.1 \(\upmu\)m. The distance between the galvano mirror and the viewing zone was determined such that the width of the viewing  
Figure 1: Generation of viewing zone: (**a**) conventional technique using a sub-micron pixel pitch SLM and (**b**) combination of MEMS SLM and spatial scanning. zone was larger than the average pupil diameter of the human eye (5 mm). This distance was set to 600 mm and the width of the viewing zone was then 9.74 mm. The scan angle of the galvano mirror was 40.0\({}^{\circ}\). The viewing zone angle was enlarged to 40.0\({}^{\circ}\), considering that the viewing zone and scan angles are equal. The scanning frequency was 60 Hz, and 222 reduced viewing zones were generated during a single scan. A substantial overlap existed between the reduced viewing zones. The delay and the hysteresis characteristics of the rotation of the galvano mirror were measured in advance, and the hologram calculation was performed considering the measured rotation angles of the galvano mirror. Figure 4 shows the experimentally obtained reconstructed images. The 3D images were captured from different horizontal directions. The viewing zone was enlarged effectively. Figure 5 shows the reconstructed images when the focus of the camera was adjusted at different depth positions. The focused pattern looked sharp. Thus, the generated 3D images could be focused on by the eyes. The holographic display system using a 2D scanning system was proposed in Refs. [14; 15] which used a resonance scanner for the high-speed horizontal scan and a galvano mirror for the low-speed vertical scan.