# Full-Parallax Multiview Generation with High-Speed Wide-Angle Dual-Axis Scanning Optics
## 4 High-Speed Wide-Angle Dual-Axis Scanning Optics  
Figure 10 shows the optical structure of the full-parallax multiview display implemented in this study. The view is generated by three DMDs respectively assigned to red, green, and blue (RGB) colors and subsequently passes through 4-\(f\) optics with a stop. The optical stop defines the field of view at the individual SVZ, and the size of the stop is directly related to the width of the SVZ. The image is projected onto the first scanning mirror of the dual-axis scanner. This scanner comprises two scanning mirrors along with high-NA scanning optics. Scanning optics play an important role in the effective placement of the two rotational axes of scanners on a common plane. The scanning optics has a basic 4-\(f\) structure. The front and rear focal planes have rotational axes for each scanner and are conjugate to each other. After the dual-axis scanner, the image tilted at a predetermined angle is magnified by 4-\(f\) optics and appears in the center of the ball lens. Generally, a full-parallax multiview display using temporal multiplexing requires a high-speed operating spatial light modulator (SLM) and a dual-axis optical scanner. A dual-axis scanner is required to determine the characteristics of the system such as viewing angle, view volume, and frame rate. Although two types of dual-axis scanners, micro-electromechanical system (MEMS) and OXCT, are widely used, these have limited applicability in full-parallax multiview displays owing to intrinsic problems. A MEMS scanner typically has only one scanning mirror, which is tilted via micro actuators around two axes. Therefore, the available beam size is small and the scanning angle is insufficient. An OXCT scanner contains two scanning mirrors connected in a cross configuration, and the gap between the two scanning mirrors causes pincushion error. Therefore, a dual-axis optical scanner with a large beam size and a wide scanning angle that is not susceptible to undesirable pincushion error is required. We used a resonance scanner and a galvanometer to implement the high-speed dual-axis scanner. The resonance scanner exhibits a very high resonant frequency; however, it is difficult to finely adjust the frequency. By contrast, a galvanometer is relatively slow, but it is easy to electrically control the scanning phase. The phase of the galvanometer, that rotates around the x-axis, is tuned to be synchronized with the resonance scanner, which rotates the view around the y-axis. Arranging the scanners in this manner enabled the implementation of a raster scan. To avoid pincushion errors, the two rotation axes must be placed on a common plane, and the 4-\(f\) structure is preferable. This is because  
Figure 9: WDFs of the views at the fourth and fifth eye positions. (**a**) WDF at the fourth eye position; (**b**) WDF at the fifth eye position. if two scanners are connected with an imaging lens, the beam reflected from the second scanning mirror diverges due to the diverging spherical phase, and the aperture of the optics following it will have to be increased. We designed wide-angle scanning optics using commercial lenses inspired by a hypergon lens, which is symmetrical with respect to an optical stop and has the advantages of wide angles and low distortion. We used the optical design software, optics studio (Zemax), to design optical structure for scanning optics. Figure 11 shows two high-NA scanning optics based on the 4-\(f\) structure. Figure 11a shows scanning optics with two lens groups symmetrical to each other. These optics have a sufficiently large NA; however, there is insufficient space to place the scanning mirror, which poses a practical challenge. Figure 11b depicts the addition of a concave lens between lens groups 1 and 2 to increase the working distance. This ensures that there is enough space to place the scanning mirror. Additionally, geometric aberration is reduced in the off-axis region after the insertion of the concave lens.