## 3 Circular Scanning System  
When the scanning scheme is changed from a linear scan to a circular scan, a 360\({}^{\circ}\) viewing zone can be realized [20]. The 3D images are generated on the table screen, which can be observed from 360\({}^{\circ}\) directions. As the MEMS SLM can operate at a sufficiently high frame rate, reduced viewing zones are generated without gaps on a circle. Hence, multiple viewers can observe 3D images without using the viewer tracking technique.  
Figure 5(a) shows a schematic diagram of the table screen 360\({}^{\circ}\) holographic display. A magnifying imaging system and a rotating screen are combined. The hologram patterns displayed on the MEMS SLM are projected onto the rotating screen. As the rotating screen is an off-axis lens, the viewing zone is generated outside the rotation axis (Figure 5(b)). When the screen lens rotates, the viewing zone rotates circularly around the rotation axis. The rotating lens becomes a table screen.  
In the experimental system of the circular scanning display, a DMD with a frame rate of 22.727 kHz was used. A laser diode with a wavelength of 635 nm was used as the light source. The reflective rotating screen was used to permit the DMD projection system to be positioned above the rotating screen. The rotating screen consisted  
Figure 4: Reconstructed images captured from different horizontal positions produced by the viewing zone scanning system: (**a**) plane, (**b**) castle, and (**c**) ship.  
Figure 5: Reconstructed images captured by focusing on various depth positions: (**a**) “3D” at +100 mm, (**b**) “TUAT” at +30 mm, and (**c**) a circle at \(-\)100 mm.  
lens and an aluminum-coated vertical diffuser. A Fresnel lens was used as the rotating screen because a lightweight lens with a large diameter was needed. The focal length of the Fresnel lens was 700 mm. The light passed through the Fresnel lens twice, and thus, the effective focal length was 350 mm. The off-axis lens shift, which was equal to the distance between the lens center and the screen center, was 172 mm. The screen diameter was 100 mm (3.94 inches), and the image size was 80.0 \(\times\) 60.0 mm\({}^{2}\). A lenticular lens with a fine lens pitch was used as the vertical diffuser. As 800 reduced viewing zones were generated during a single circular scan, the frame rate of the 3D image generation was 28.4 Hz. The width of the reduced viewing zone was 5.81 mm.  
Figure 7 shows the generated 360\({}^{\circ}\) images captured from the four directions around the table screen. The ghost images were observed in addition to the 360\({}^{\circ}\) images; these ghost images were caused by light reflecting on the surface of the rotating screen, not the conjugate images. The generated 3D images had a smooth motion parallax, and multiple viewers could observe the 3D images simultaneously.  
Figure 6: Circular scanning holographic display using MEMS SLM: (**a**) system configuration and (**b**) circular scanning using an off-axis lens.  
Figure 7: 360\({}^{\circ}\) holographic images generated by circular scanning holographic display; photographs were captured from four different directions: (**a**) symbols and (**b**) two planes.  
The 360\({}^{\circ}\) holographic display using a circular scanning system was also proposed in Ref. [16]; in this system, four DMDs, two parabolic mirrors, and a rotating mirror were used to provide a screen diameter of 3.2 in. This system has the capacity to generate floating 3D images, although the diameter of the parabolic mirrors should be several times larger than that of the 3D images. The circular scanning system illustrated in Figure 6 required a rotating Fresnel lens with a diameter roughly equal to that of 3D images.