Enlargements of Viewing Zone and Screen Size of Holographic Displays Using MEMS SLM Combined with Scanning Systems

: The problems of conventional holographic display techniques, which are the requirements of a sub-micron pixel pitch and ultra-high resolution for spatial light modulators (SLMs) to enlarge the viewing zone and screen size, can be addressed using microelectromechanical systems (MEMS) SLMs combined with spatial scanning systems. Various scanning systems have been efﬁciently combined with high-speed image generation of MEMS SLMs based on the time-multiplexing technique. The horizontal scanning system enlarged the viewing zone and screen size, the circular scanning system provided 360 ◦ three-dimensional (3D) images, and the RGB scanning system generated color 3D images. The screen size can be increased scalably using a multichannel system based on the space-multiplexing technique. The use of a short laser pulse illumination system eliminates the mechanical scanning system and greatly simpliﬁes the display system. The measurements of the accommodation responses of human eyes showed that 3D images generated by the screen scanning holographic display have a possibility to solve the visual fatigue issue caused by the vergence–accommodation conﬂict, which prevents the long-time usage of conventional 3D displays.


Introduction
Due to the production of three-dimensional (3D) images that satisfy all physiological factors of human depth perception [1,2], holographic displays are ideal 3D displays devoid of visual fatigue caused by the vergence-accommodation conflict [3].However, conventional techniques to construct holographic displays require that spatial light modulators (SLMs) exhibit a sub-micron pixel pitch and an ultra-high resolution to provide a wide viewing zone and a large screen size.This article reviews techniques to enlarge the viewing zone and screen size of holographic displays using microelectromechanical systems (MEMS) SLMs combined with spatial scanning systems.
For the conventional holographic display techniques, when hologram patterns are displayed using the SLM with a resolution of N × M and a pixel pitch of p, the screen size (i.e., the 3D image size) is given by Np × Mp, and the light diffraction determines the viewing zone angle given by 2 sin −1 (λ/2p), where λ is the wavelength of light.For instance, to obtain a viewing zone angle of 30 • and a screen size of 40 inches, the resolution should be 886,600 × 498,000, and the pixel pitch should be 0.97 µm when λ = 0.5 µm.SLMs with such a high resolution and a small pixel pitch are difficult to develop.Although many efforts have been made to develop SLMs with ultra-high resolution and a sub-micron pixel pitch [4][5][6][7][8][9], substantial time is required to develop SLMs that can provide an effective practical viewing zone angle and screen size.
Several techniques that do not depend on pixel pitch reduction and an increase in SLM resolution have been developed for enlarging the viewing zone angle and screen size.One method is the spatial-multiplexing technique that employs multiple SLMs [10][11][12].This technique requires optical systems that combine many SLMs.Another method is the timemultiplexing technique, which employs a high-speed SLM combined with one-dimensional (1D) [13], two-dimensional (16) [14,15], and circular scanning systems [16][17][18].
Among several types of high-speed SLMs, MEMS SLM has the highest frame rate with moderate resolution.This article reviews the holographic displays developed by our research group, which are based on the time-multiplexing technique using MEMS SLMs combined with various scanning schemes, such as the screen [13], viewing zone [19], circular [20], and RGB scanning [21].The measurements of the accommodation responses of the eyes to the scanning holographic display system [22] are also explained.

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.
Among several types of high-speed SLMs, MEMS SLM has the highest fra with moderate resolution.This article reviews the holographic displays developed research group, which are based on the time-multiplexing technique using MEM combined with various scanning schemes, such as the screen [13], viewing zo circular [20], and RGB scanning [21].The measurements of the accommodation re of the eyes to the scanning holographic display system [22] are also explained.

Viewing Zone Scanning System
In conventional holographic display techniques, the SLM diffracts light to g the viewing zone (Figure 1a).Therefore, the SLM pixel pitch should be reduced to the viewing zone.When the pixel pitch is reduced, the resolution should be incre enlarge the screen size.Thus, a small pixel pitch is needed only for the viewin enlargement.Regarding viewing zone enlargement, the scanning system outperfo light diffraction system (Figure 1b).A combination of a 1D light modulation u acousto-optic modulator and a 2D light scanning using the polygon mirror galvano scanner was proposed [23].In this article, the combination of 2D light mod using MEMS SLM and 1D or circular light scanning proposed in our research [13,[19][20][21] is reviewed.The latter combination requires a slower scanning speed t former.Figure 2 shows the holographic display system comprising the MEMS SLM 1D scanning system [19].A magnifying imaging system enlarges the screen siz MEMS SLM to increase the screen size of the display system.Thus, the pix increases, and the viewing zone is reduced.The reduced viewing zone is scanned 1D scanner to enlarge the viewing zone.Hence, both the viewing zone and screen enlarged.The single-sideband (SSB) filter is used to eliminate the conjugate image zero-order diffraction light, which are not required for hologram reconstruction [ width of the reduced viewing zone should exceed the pupil diameter of the huma ensure wavefront reconstruction.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 × 768, the pixel pitch was 13.68 µm, 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 µm.The distance between the galvano mirror and the viewing zone was determined such that the width of the viewing 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 • .The viewing zone angle was enlarged to 40.0 • , 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 3 shows the experimental system based on the viewing zone scanning A digital micromirror device (DMD) was used for the MEMS SLM.The resolut 1024 × 768, the pixel pitch was 13.68 μm, and the frame rate was 13.333 kHz.A mirror with a diameter of 50 mm (2.0 inches) was used as the horizontal scanner.diode with a wavelength of 635 nm was used as the light source.The magnificatio imaging system was determined to be 2.86 such that the image was the same siz galvano mirror (2.0 inches), which is the same size as the screen of the holographic The imaging system had an enlarged pixel pitch of 39.1 μm.The distance betw galvano mirror and the viewing zone was determined such that the width of the zone was larger than the average pupil diameter of the human eye (5 mm).This d was set to 600 mm and the width of the viewing zone was then 9.74 mm.The sca of the galvano mirror was 40.0°.The viewing zone angle was enlarged t considering that the viewing zone and scan angles are equal.The scanning freque 60 Hz, and 222 reduced viewing zones were generated during a single scan.A sub overlap existed between the reduced viewing zones.The delay and the hy characteristics of the rotation of the galvano mirror were measured in advance, hologram calculation was performed considering the measured rotation angle galvano mirror.Figure 3 shows the experimental system based on the viewing zone scanning A digital micromirror device (DMD) was used for the MEMS SLM.The resolu 1024 × 768, the pixel pitch was 13.68 μm, and the frame rate was 13.333 kHz.A mirror with a diameter of 50 mm (2.0 inches) was used as the horizontal scanner diode with a wavelength of 635 nm was used as the light source.The magnificatio imaging system was determined to be 2.86 such that the image was the same siz galvano mirror (2.0 inches), which is the same size as the screen of the holographic The imaging system had an enlarged pixel pitch of 39.1 μm.The distance betw galvano mirror and the viewing zone was determined such that the width of the zone was larger than the average pupil diameter of the human eye (5 mm).This was set to 600 mm and the width of the viewing zone was then 9.74 mm.The sc of the galvano mirror was 40.0°.The viewing zone angle was enlarged t considering that the viewing zone and scan angles are equal.The scanning freque 60 Hz, and 222 reduced viewing zones were generated during a single scan.A su overlap existed between the reduced viewing zones.The delay and the h characteristics of the rotation of the galvano mirror were measured in advance, hologram calculation was performed considering the measured rotation angle galvano mirror.Figure 4 shows the experimentally obtained reconstructed images.The 3D were captured from different horizontal directions.The viewing zone was e effectively.Figure 5 shows the reconstructed images when the focus of the cam adjusted at different depth positions.The focused pattern looked sharp.T generated 3D images could be focused on by the eyes.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.This system demonstrated horizontal and vertical viewing zone angles of 48 • and 3.2 • , respectively, and had a screen size of 10 × 10 mm 2 , which was obtained by fully utilizing the high frame rate of the DMD.The horizontal scanning system shown in Figure 3 used a vertical diffuser to increase the vertical viewing zone [19].In this manner, the vertical viewing zone angle was increased to >50 • , although the vertical parallax was lost.The holographic display system using a 2D scanning system was proposed in Refs.14 and 15 which used a resonance scanner for the high-speed horizontal scan and a galvano mirror for the low-speed vertical scan.This system demonstrated horizontal and vertical viewing zone angles of 48° and 3.2°, respectively, and had a screen size of 10 × 10 mm2, which was obtained by fully utilizing the high frame rate of the DMD.The horizontal scanning system shown in Figure 3 used a vertical diffuser to increase the vertical viewing zone [19].In this manner, the vertical viewing zone angle was increased to >50°, although the vertical parallax was lost.

Circular Scanning System
When the scanning scheme is changed from a linear scan to a circular scan, a 360° viewing zone can be realized [20].The 3D images are generated on the table screen, which can be observed from 360° 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 6a shows a schematic diagram of the table screen 360° holographic display.A magnifying imaging system and a rotating screen are combined.The hologram patterns   The holographic display system using a 2D scanning system was proposed in Refs.14 and 15 which used a resonance scanner for the high-speed horizontal scan and a galvano mirror for the low-speed vertical scan.This system demonstrated horizontal and vertical viewing zone angles of 48° and 3.2°, respectively, and had a screen size of 10 × 10 mm2, which was obtained by fully utilizing the high frame rate of the DMD.The horizontal scanning system shown in Figure 3 used a vertical diffuser to increase the vertical viewing zone [19].In this manner, the vertical viewing zone angle was increased to >50°, although the vertical parallax was lost.

Circular Scanning System
When the scanning scheme is changed from a linear scan to a circular scan, a 360° viewing zone can be realized [20].The 3D images are generated on the table screen, which can be observed from 360° 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 6a shows a schematic diagram of the table screen 360° 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 6b).When the screen lens rotates, the viewing zone rotates circularly around the rotation axis.The rotating lens becomes a table screen.

Circular Scanning System
When the scanning scheme is changed from a linear scan to a circular scan, a 360 • viewing zone can be realized [20].The 3D images are generated on the table screen, which can be observed from 360 • 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 6a shows a schematic diagram of the table screen 360 • 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 6b).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 of an off-axis Frensel 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 Frensel 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 × 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.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 of an off-axis Frensel 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 Frensel 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 × 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° images captured from the four directions around the table screen.The ghost images were observed in addition to the 360° 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 7 shows the generated 360 • images captured from the four directions around the table screen.The ghost images were observed in addition to the 360 • 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.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 of an off-axis Frensel 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 Frensel 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 × 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° images captured from the four directions around the table screen.The ghost images were observed in addition to the 360° 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.The 360 • 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.

RGB Scanning System
By virtue of the high-speed image generation of the MEMS SLM, the time-multiplexing technique can be used not only for viewing zone enlargement but also for color image generation.The RGB lasers are combined with a viewing zone scanning holographic display to generate color 3D images [21].
The viewing zone scanning system shown in Figure 2 is modified to be able to generate 3D color images.Holograms for R, G, and B colors are displayed sequentially by the MEMS SLM, and the MEMS SLM is illuminated by the R, G, and B lasers with the synchronization of the hologram pattern generation by the MEMS SLM.Then, as shown in Figure 8, the reduced viewing zones for the three colors are scanned simultaneously on the viewing zone.For the original viewing zone scanning system described in Section 2, the width of the reduced viewing zone depends on the wavelength of light.For the RGB scanning system, a rectangular aperture is used for the SSB filter, and its width is determined for blue light having the shortest wavelength among the three colors.Since the images of the SSB filter become the reduced viewing zones, the widths of the reduced viewing zones and the amount of overlap among the reduced viewing zones become identical for the three colors.The directions of the three laser lights illuminating the MEMS SLM should be appropriately adjusted considering the grating equation.In this case, due to the color of 3D images changing in the vertical direction, the vertical diffuser must be placed close to the galvano mirror, although the vertical diffuser was optional for the monochromatic display systems.The details of this system are described in Ref. [18].
floating 3D images, although the diameter of the parabolic mirrors sho larger than that of the 3D images.The circular scanning system illu required a rotating Fresnel lens with a diameter roughly equal to that

RGB Scanning System
By virtue of the high-speed image generation of the MEM multiplexing technique can be used not only for viewing zone enlar color image generation.The RGB lasers are combined with a view holographic display to generate color 3D images [21].
The viewing zone scanning system shown in Figure 2 is mod generate 3D color images.Holograms for R, G, and B colors are displa the MEMS SLM, and the MEMS SLM is illuminated by the R, G, an synchronization of the hologram pattern generation by the MEMS SL in Figure 8, the reduced viewing zones for the three colors are scanned the viewing zone.For the original viewing zone scanning system des the width of the reduced viewing zone depends on the wavelength o scanning system, a rectangular aperture is used for the SSB filte determined for blue light having the shortest wavelength among the the images of the SSB filter become the reduced viewing zones, the w viewing zones and the amount of overlap among the reduced view identical for the three colors.The directions of the three laser ligh MEMS SLM should be appropriately adjusted considering the grati case, due to the color of 3D images changing in the vertical direction, must be placed close to the galvano mirror, although the vertical diffu the monochromatic display systems.The details of this system are de Figure 9 shows the produced color 3D images.The wavelength lasers were 488, 515, and 635 nm, respectively.The color profile con the three laser wavelengths was performed before the hologram calcu colors in order to improve the color representation.The color 3D imag produced.As the DMD can generate only binary images, the error modified for the hologram pattern generation was developed [25].change in the vertical direction was mitigated by the vertical diffu Figure 9 shows the produced color 3D images.The wavelength of the B, G, and R lasers were 488, 515, and 635 nm, respectively.The color profile conversion considering the three laser wavelengths was performed before the hologram calculation for the three colors in order to improve the color representation.The color 3D images were successfully produced.As the DMD can generate only binary images, the error diffusion technique modified for the hologram pattern generation was developed [25].Although the color change in the vertical direction was mitigated by the vertical diffuser, the 3D images became redder when they were viewed from the left side and more cyan when they were viewed from the right side.
FOR PEER REVIEW 7 of 14 became redder when they were viewed from the left side and more cyan when they were viewed from the right side.The generation of 3D color image using MEMS SLMs has been proposed by numerous research groups [15,21,26].There are two techniques that can be used to generate 3D color images: the time-multiplexing technique using a single MEMS SLM and the space-multiplexing technique using three MEMS SLMs.The majority of works in the literature make use of the time-multiplexing technique because of the high frame rate of the DMDs.

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°, 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 × 104 mm 2 ).The focal length of the Fresnel lens was f = 800 mm.The scan angle of the planer scanner was 43.0° which was equal to the viewing zone angle of the two-channel system.The generation of 3D color image using MEMS SLMs has been proposed by numerous research groups [15,21,26].There are two techniques that can be used to generate 3D color images: the time-multiplexing technique using a single MEMS SLM and the spacemultiplexing technique using three MEMS SLMs.The majority of works in the literature make use of the time-multiplexing technique because of the high frame rate of the DMDs.

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 • , 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 × 104 mm 2 ).The focal length of the Fresnel lens was f = 800 mm.The scan angle of the planer scanner was 43.0 • 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.The photographs of the 3D images produced by the two-channel syst in Figure 12.From the experimental results, both the large screen size viewing zone angle were obtained.The photographs of the 3D images produced by the two-channe in Figure 12.From the experimental results, both the large screen viewing zone angle were obtained.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 × 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].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 wid viewing zone angle were obtained.

Scanning System Using Pulse-Modulated MEMS SLM
For viewing zone scanning holographic displays, the viewing zone angle equals the scan angle of the scanning system, and the screen size equals the optical window size of the scanning system.Therefore, a large scan angle and a large optical window are required for the scanning system.Among various scanning systems, this requirement can be satisfied by mechanical scanners, such as the galvano scanner.However, the use of the mechanical system reduces system reliability and stability.The use of the pulse-modulation of laser light to expose the MEMS SLM has been proposed to eliminate the mechanical scanning systems from the holographic display systems employing the MEMS SLM [29].
Figure 13 illustrates the holographic display using the pulse-modulated MEMS SLM.Short-pulse laser light exposes the MEMS SLM.The MEMS SLM consists of two-dimensionally aligned MEMS mirrors.The MEMS mirrors have binary states, i.e., an on state and an off state, and quickly rotate to alter their states from on to off, and vice versa.All MEMS mirrors alter their states simultaneously to display 2D images.The proposed technique uses a rotating state of the MEMS mirrors, which is not conventionally utilized to generate images.The pulse width of the laser is made shorter than the rotating time of the MEMS mirrors.Hologram patterns are sequentially displayed by the MEMS SLM.When the short-pulse laser light exposes the MEMS mirrors in the rotating state, the direction of light reflected by the MEMS mirrors is determined by the timing of the laser pulse exposure.Therefore, 2D images displayed by the MEMS SLM can be scanned by appropriately controlling the timing of the short-pulse generation.Thus, the pulse-modulation of the MEMS SLM enables the scanning of the viewing zone.When the rotation angles of the MEMS mirrors for the on and off states are respectively represented by +α and −α, the possible scan angle is from +2α to −2α.Therefore, the use of pulse-modulated MEMS SLM can enlarge the viewing zone angle to 4α.
appropriately controlling the timing of the short-pulse generation modulation of the MEMS SLM enables the scanning of the viewi rotation angles of the MEMS mirrors for the on and off states are respe by +α and −α, the possible scan angle is from +2α to −2α.Therefor modulated MEMS SLM can enlarge the viewing zone angle to 4α.The DMD was also used to verify the technique employing th MEMS SLM.The MEMS mirrors of the DMD had a rotating angle o possible scan angle was from +24° to −24°.Thus, the viewing zone ang The DMD was also used to verify the technique employing the pulse-modulated MEMS SLM.The MEMS mirrors of the DMD had a rotating angle of α = 12 • .Thus, the possible scan angle was from +24 • to −24 • .Thus, the viewing zone angle could be enlarged to 48 • .The rotation time of the MEMS mirrors was 3.83 µs.The pulse width of the laser light was set to 20 ns.So, the number of scan points was 192.The frame rate of the DMD was 22,727 Hz.Thus, the frame rate of displaying 3D images was 59.2 Hz.An FPGA was used to control the pulse signal generation.Figure 14 shows the photograph of the experimental system.The elimination of the mechanical scanning system greatly simplified the display system.
Sci. 2022, 12, x FOR PEER REVIEW to 48°.The rotation time of the MEMS mirrors was 3.83 μs.The light was set to 20 ns.So, the number of scan points was 192.The was 22,727 Hz.Thus, the frame rate of displaying 3D images was used to control the pulse signal generation.Figure 14 shows experimental system.The elimination of the mechanical sc simplified the display system.The pulse modulation of the DMD was originally proposed for the Lidar syst This technique was also used in the construction of the light field display [31].A light scanning using the pulse modulation of the MEMS SLM significantly simpl display systems, the brightness of 3D images was low because the rotation tim MEMS mirrors was short.The development of a specialized MEMS SLM for u displays consisting of MEMS mirrors that have a longer rotation time is importan

Screen Scanning System
Screen scanning holographic displays using the MEMS SLM [13,32] w proposed before the proposal of viewing zone scanning holographic displays to the screen size and the viewing zone.The grayscale and color image ge techniques were also developed for screen scanning display systems [33,34].Th scanning systems scan the light beam on the display screen to enlarge the screen s the light beam should have a large diverging angle to obtain a large viewin Therefore, screen scanning systems require a large lens to project the screen of the SLM onto the display screen with a large diverging angle.The requirement for size imaging lens was the problem of the screen scanning systems.However, the zone scanning systems do not require a large-size lens because they perform The pulse modulation of the DMD was originally proposed for the Lidar system [30].This technique was also used in the construction of the light field display [31].Although light scanning using the pulse modulation of the MEMS SLM significantly simplified the display systems, the brightness of 3D images was low because the rotation time of the MEMS mirrors was short.The development of a specialized MEMS SLM for use in 3D displays consisting of MEMS mirrors that have a longer rotation time is important.

Screen Scanning System
Screen scanning holographic displays using the MEMS SLM [13,32] were also proposed before the proposal of viewing zone scanning holographic displays to enlarge the screen size and the viewing zone.The grayscale and color image generation techniques were also developed for screen scanning display systems [33,34].The screen scanning systems scan the light beam on the display screen to enlarge the screen size, and the light beam should have a large diverging angle to obtain a large viewing zone.Therefore, screen scanning systems require a large lens to project the screen of the MEMS SLM onto the display screen with a large diverging angle.The requirement for a large-size imaging lens was the problem of the screen scanning systems.However, the viewing zone scanning systems do not require a large-size lens because they perform image magnification like commercial video projectors.
Conventional 3D displays have the issue of visual fatigues caused by the vergenceaccommodation conflict [3].The accommodation, which is the focusing function of the eyes, does not work correctly for 3D images generated by conventional 3D displays.To confirm that the accommodation works correctly for the 3D images generated by the holographic displays, the accommodation responses to the holographic images generated by the screen scanning system were measured [22].Figure 16 shows the results of the accommodation measurements.Observably, the accommodation responses to the holographic images were similar to those to real objects.

R PEER REVIEW
12 of 14 by the screen scanning system were measured [22].Figure 16 shows the results of the accommodation measurements.Observably, the accommodation responses to the holographic images were similar to those to real objects.

Screen Comparisons of Scanning Schemes
The components used to build the experimental systems based on the various scanning schemes described in this article are shown in Table 1, and the parameters describing the performance of the experimental systems are shown in Table 2.The scanning systems increased the viewing zone angle to almost 40°, with the exception of the screen scanning system.The conventional holographic display techniques require a pixel pitch of 0.93 μm for the SLMs to obtain the viewing zone angle of 40° for λ = 635 nm.
The viewing zone scanning system had a larger viewing zone angle and a smaller screen size than the screen scanning system, although the optical system of the former system was much simpler than that of the latter.The frame rate of the 3D image display for the circular scanning system was reduced to about half that of the other scanning systems because more scan points were required to provide a 360-degree viewing zone.In spite of using a single DMD, the display performances for the RGB scanning system remained identical to those for the monochrome (viewing zone scanning) system because the framerate of the DMD was sufficiently high.The multi-channel scanning system

Screen Comparisons of Scanning Schemes
The components used to build the experimental systems based on the various scanning schemes described in this article are shown in Table 1, and the parameters describing the performance of the experimental systems are shown in Table 2.The scanning systems increased the viewing zone angle to almost 40 • , with the exception of the screen scanning system.The conventional holographic display techniques require a pixel pitch of 0.93 µm for the SLMs to obtain the viewing zone angle of 40 • for λ = 635 nm.
The viewing zone scanning system had a larger viewing zone angle and a smaller screen size than the screen scanning system, although the optical system of the former system was much simpler than that of the latter.The frame rate of the 3D image display for the circular scanning system was reduced to about half that of the other scanning systems because more scan points were required to provide a 360-degree viewing zone.In spite of using a single DMD, the display performances for the RGB scanning system remained identical to those for the monochrome (viewing zone scanning) system because the framerate of the DMD was sufficiently high.The multi-channel scanning system

Figure 1 .
Figure 1.Generation of viewing zone: (a) conventional technique using a sub-micron pi SLM and (b) combination of MEMS SLM and spatial scanning.

Figure 1 .
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.

Figure 3 .
Figure 3. Experimental system for the viewing zone scanning holographic display.

Figure 3 .
Figure 3. Experimental system for the viewing zone scanning holographic display.

Figure 4 .
Figure 4. Reconstructed images captured from different horizontal positions produced by the viewing zone scanning system: (a) plane, (b) castle, and (c) ship.

Figure 4 .
Figure 4. Reconstructed images captured from different horizontal positions produced by the viewing zone scanning system: (a) plane, (b) castle, and (c) ship.

Figure 4 .
Figure 4. Reconstructed images captured from different horizontal positions produced by the viewing zone scanning system: (a) plane, (b) castle, and (c) ship.

Figure 6 .
Figure 6.Circular scanning holographic display using MEMS SLM: (a) system configuration and (b) circular scanning using an off-axis lens.

Figure 6 .
Figure 6.Circular scanning holographic display using MEMS SLM: (a) system configuration and (b) circular scanning using an off-axis lens.

14 Figure 6 .
Figure 6.Circular scanning holographic display using MEMS SLM: (a) system configuration and (b) circular scanning using an off-axis lens.

Figure 7 .
Figure 7. 360 • holographic images generated by circular scanning holographic display; photographs were captured from four different directions: (a) symbols and (b) two planes.

Figure 8 .
Figure 8. RGB viewing-zone scanning scheme to produce color 3D images.

Figure 8 .
Figure 8. RGB viewing-zone scanning scheme to produce color 3D images.

Figure 14 .
Figure 14.Experimental system of holographic display using pulse-mod

Figure 14 .
Figure 14.Experimental system of holographic display using pulse-modulated MEMS SLM.

Figure 15
Figure15shows the reconstructed images.The effective scan angle was from −18 • to +24 • , because undesirable distributions appeared in the minus-end scan region.The viewing zone angle was enlarged to 42 • .The undesirable light distributions were generated by the small vibration of the MEMS mirrors in the off state when the other MEMS mirrors changed from the off state to the on state.