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Viewing-zone scanning holographic displays can enlarge both the screen size and the viewing zone. However, limitations exist in the screen size enlargement process even if the viewing zone is effectively enlarged. This study proposes a multi-channel viewing-zone scanning holographic display comprising multiple projection systems and a planar scanner to enable the scalable enlargement of the screen size. Each projection system produces an enlarged image of the screen of a MEMS spatial light modulator. The multiple enlarged images produced by the multiple projection systems are seamlessly tiled on the planar scanner. This screen size enlargement process reduces the viewing zones of the projection systems, which are horizontally scanned by the planar scanner comprising a rotating off-axis lens and a vertical diffuser to enlarge the viewing zone. A screen size of 7.4 in. and a viewing-zone angle of 43.0° are demonstrated.
© 2016 Optical Society of America
1. Introduction
Eyes naturally respond to three-dimensional (3D) images generated via holography because this process reconstructs the wavefronts of light emitted from objects. All physiological factors of human 3D perception [1], including vergence, binocular parallax, motion parallax, and accommodation, work properly for 3D images produced via holography. Thus, holographic 3D images do not create the visual fatigue associated with the vergence–accommodation conflict [2], which prevents the widespread use of conventional 3D displays. However, the electronic implementation of holography has a drawback in terms of limited screen size and viewing zone. We previously proposed a viewing-zone scanning holographic display [3] that uses two-dimensional (2D) light modulation by a microelectromechanical systems (MEMS) spatial light modulator (SLM) and one-dimensional (1D) spatial scanning by a mechanical scanner to enlarge both the screen size and the viewing zone. However, the amount of screen size enlargement attainable by this system is limited. In this study, a multi-channel viewing-zone scanning holographic display to enable the scalable enlargement of the screen size is proposed.
Based on light diffraction properties, an SLM with a pixel pitch of p can generate 3D images with a viewing zone angle of 2 sin−1(λ / 2p), where λ is the light wavelength. For an SLM with a resolution of N × M, the screen size is given by Np × Mp. Therefore, the pixel pitch needs to be reduced to increase the viewing zone angle and the resolution needs to be increased to increase the screen size. For example, to obtain a screen size of 40 in. and a viewing zone angle of 30°, the pixel pitch should be 0.97 μm and the resolution should be approximately 886,000 × 498,000 when λ = 0.5 μm. As ultra-high resolution SLMs are required to attain practical holographic displays, much research has been conducted on the development of ultra-high resolution SLMs [4, 5].
As there is a physical limitation to the degree to which the resolution of an SLM can be increased, spatial multiplexing techniques have been developed to increase the screen size and the viewing zone angle by using multiple SLMs [6–9]. In [6], five SLMs with resolutions of 3,200 × 960 were aligned in the horizontal direction. In [7], twelve SLMs with resolutions of 1,024 × 768 were arranged along a curved horizontal line. In [8] and [9], display systems were respectively demonstrated employing 3 × 3 and 4 × 4 arrays of SLMs with resolutions of 3,840 × 2,160 and pixel pitches of 4.8 μm. The latter system provided a screen size of 3.3 in. and a viewing zone angle of 5.6°.
Time-multiplexing techniques have also been developed to increase the screen size and the viewing zone angle. The combination of 1D light modulation by an acousto-optic modulator (AOM) and 2D spatial scanning using a galvano scanner and a polygon mirror has been proposed [10]; in this system, narrow-width horizontal hologram patterns generated by the AOM are scanned in a raster-scanning manner. The hologram display system comprising 36 AOMs was shown to provide a screen size of 6.6 in. and a viewing zone angle of 30°. Recently, development of a system using anisotropic leaky-mode modulators has been reported [11]. The combination of 2D light modulation by a MEMS-SLM and 1D spatial scanning by a galvano mirror was proposed by our group [3, 12–16]. Our MEMS-SLM systems generate hologram patterns at a high frame rate and require only horizontal scanning with a slower speed than needed by the AOM systems. Two types of scanning system have been proposed for the MEMS-SLM systems: screen scanning systems [12–15] and viewing-zone scanning systems [3, 16]. The former type is suitable for obtaining a larger screen size; a color display system with a screen size of 6.2 in. and a viewing zone angle of 10.2° has been demonstrated [15]. The viewing-zone scanning type is suitable for obtaining a larger viewing zone angle, and a display system with a screen size of 2.0 in. and a viewing zone angle of 40.0° has been demonstrated [3]. A 360-degree display system has also been demonstrated based on the viewing-zone scanning system [16]. A holographic display that uses eye-tracking and beam steering systems has been proposed [17, 18]. In this holographic display system, the beam steering system is used instead of the beam scanning system; as this system does not require an SLM to operate at a high frame rate, a liquid crystal display panel with a large screen can be used instead.
Holographic display systems using both time- and spatial-multiplexing techniques have been developed [19, 20]. In [19], a temporal and spatial multiplexing technique was proposed, wherein a high-speed SLM is used to practically increase the resolution, with the screen size increased by tiling multiple screens. Four sets of ferroelectric liquid-crystal (FLC) SLMs with resolutions of 1,280 × 1,024 and frame rates of 1 kHz were used to construct a holographic display with a screen size of 5.5 in. and a pixel pitch of 6.6 μm (as the light wavelength of this system was not mentioned, we can calculate the viewing zone angle to be 4.3° providing that λ = 0.5 μm). In [20], 3 × 8 sets of FLC-SLMs with resolutions of 1,280 × 1,024 were aligned two-dimensionally and six-step horizontal scanning was performed by a galvano scanner in order to obtain a screen size of 10 in. and a viewing zone angle of 2.2°.
In this study, the temporal and spatial multiplexing technique is applied to a viewing-zone scanning holographic display in order to further increase the screen size, i.e., a multi-channel viewing-zone scanning holographic display is proposed in order to simultaneously provide a large screen size and a large viewing zone angle.
2. Viewing-zone scanning system
In this section, we briefly explain a previously proposed viewing-zone scanning system [3].
Figure 1 shows a schematic of the viewing-zone scanning system. It consists of a MEMS-SLM, a magnifying imaging system, and a horizontal scanner. The MEMS-SLM generates hologram patterns at a high frame rate. The magnifying imaging system consists of two lenses and has two functions: to image the screen of the MEMS-SLM onto the mirror of the horizontal scanner with an increased magnification, and to converge light at one point after the image formation. Because the scanning mirror is the screen of this display system, the screen size is increased. However, the pixel pitch is also increased, which in turn reduces the viewing zone and localizes it at the light convergence point. To enlarge the viewing zone, the reduced zone is scanned horizontally by the horizontal scanner. In this way, both the screen size and the viewing zone are enlarged. In order to practically attain wavefront reconstruction, the width of the reduced viewing zone should be larger than the pupil diameter of the eye. When a vertical diffuser is placed in the vicinity of the scanning mirror, the viewing area is increased in the vertical direction.
The experimental system in [3] was constructed using a digital micromirror device (DMD) as the MEMS-SLM and a galvano mirror as the horizontal scanner. The system performance was limited by the galvano mirror rather than the DMD. When the scanning frequency was 60 Hz, the maximum diameter of available scanning mirrors was about 2.0 in., although the scanning angle was increased to about 40°. Therefore, the constructed system had a screen size of 2.0 in. and a viewing zone angle of 40°. Although a large viewing zone angle was achieved, the amount of enlargement of the screen size was limited.
3. Multi-channel viewing-zone scanning system
The previous system comprised a single projection system, which consists of an MEMS-SLM and a magnifying imaging system, and a horizontal scanner. In this study, a multi-channel system comprising multiple projection systems and a planar scanner is proposed for the further enlargement of the screen size. Figure 2 illustrates the proposed multi-channel viewing-zone scanning holographic display. The projection systems are modified so that the screens of the multiple projection systems are tiled seamlessly. The planar scanner is a flat-type horizontal scanner that supports large-size images. The screen size can be scalably increased by increasing the number of projector systems.
Figure 3 depicts the modified projection systems. As shown in Fig. 3(a), it also employs two lenses: an imaging lens and a screen lens. The imaging lens produces an enlarged image of the MEMS-SLM screen on the screen lens, and the screen lens converts the incoming light into a plane wave when the MEMS-SLM displays null hologram patterns. Thus, the two lenses constitute a telecentric imaging system. A single-sideband (SSB) filter is placed on the focal plane of the imaging lens in order to eliminate zero-order diffraction light and the conjugate image component [21, 22]. By using rectangular screen lenses, the multiple projection systems can be tiled seamlessly, as shown in Fig. 2. Because the tiled screen lenses also emit plane waves, light from the tiled screen lenses converges to one point, as shown in Fig. 3(b), when a common screen lens is attached to the tiled screen lenses. Thus, a common reduced viewing zone is generated for all projection systems. The width of the common reduced viewing zone is given by λf / p, where the pixel pitch of the enlarged images is denoted by p and the focal length of the common screen lens is denoted by f.
Fig. 3 Multi-channel projection system: (a) modified projection system, and (b) tiling of multiple projection systems.
Figure 4 shows the planar scanner which can support large images. It comprises a rotating off-axis lens and a vertical diffuser. The common screen lens shown in Fig. 3(b) is used as the rotating lens; however, as its lens axis is shifted away from the rotation axis, the reduced viewing zone is produced outside the rotation axis. When the off-axis lens rotates, the viewing zone also rotates around the rotation axis. The vertical diffuser enlarges the viewing zone in the vertical direction. When the vertical light diffusion is sufficiently large, the viewing zone moves horizontally when the off-axis lens rotates, as shown in Fig. 4; in this way, the rotary movement is transformed into horizontal movement. Owing to the thin and light features, a Fresnel lens can be used as the large-diameter rotating lens. Finally, a planar scanner with a large optical window is constructed. When the distance between the lens axis and the rotation axis is denoted by d, the scan width is given by 2d and the width of the enlarged viewing zone is given by 2d + λf / p. The viewing zone angle is given by 2 tan−1 (d / f + λ / 2p).
When the frame rate of the MEMS-SLM is denoted by fSLM and the scanning frequency of the planar scanner is denoted by fscan, the number of reduced viewing zones generated during a single scan is given by N = fSLM/fscan. The interval of the reduced viewing-zone generation becomes largest when the reduced zones are generated around the rotation axis, which is given by d sin(π / N). The interval should be smaller than the width of the reduced viewing zones; therefore, the maximum width of the enlarged viewing zone is given by (λf / p)[2 / sin(π / N) + 1]. The smallest interval of the reduced viewing-zone generation is given by d (1− cos(π / N)), which occurs when the reduced viewing zones are generated at the left and right ends of the enlarged viewing zone.
4. Experiments
A two-channel display system was constructed to demonstrate the effectiveness of the proposed multi-channel system.
A DMD (DiscoveryTM 4100 [Texas Instruments, Inc.]) was used as the MEMS-SLM. The resolution was 1,024 × 768, the pixel pitch was p = 13.68 μm, and the maximum frame rate was fSLM = 22.727 kHz.
First, the magnification of the projection systems was determined so that the width of the reduced viewing zone became the average pupil diameter of 5 mm. The distance between the screen and the light convergence point was set to 800 mm and the wavelength of the laser diode used to illuminate the DMD was 635 nm; consequently, the magnified pixel pitch was 102 μm and the magnification was 7.43. The DMD was placed in the portrait direction and the screen size was enlarged to 78.0 × 104 mm2. A rectangular Fresnel lens whose size was identical to the enlarged screen size was combined with the planar scanner as described below. Figure 5 shows one of the two projection systems.
Fig. 5 Constructed projection system (screen lens is combined with planar scanner so it does not appear in this photo).
The screens of the two projection systems were tiled in the horizontal direction, making the screen size of the two-channel system 156 × 104 mm2 (7.4 in.)
The planar scanner consists of an off-axis Fresnel lens and a vertical diffuser. The frame rate of the holographic image generation was set to fscan = 60 Hz. During each rotation of the off-axis Fresnel lens, the viewing zone is scanned twice, i.e., from left to right and vice versa. Thus, the rotation speed of the Fresnel lens becomes 1,800 rpm. The number of reduced viewing zones generated during a single scan is N = 378, resulting in a viewing zone that can be enlarged to 1,208 mm in width, in which case the distance between the lens axis and the rotation axis becomes d = 602 mm, which would be impractical when using commercially available Fresnel lenses. The focal length of the Fresnel lens should be f = 800 mm because it is equal to the distance between the screen and the light convergence point. We chose a Fresnel lens with a diameter of 850 mm, which was one of the largest Fresnel lenses commercially available. With this lens, the distance between the lens axis and the rotation axis became d = 312.5 mm and the width of the viewing zone was enlarged to 630 mm at a distance of 800 mm from the screen. The effective diameter of the off-axis Fresnel lens was 200 mm and its thickness was 2.0 mm. The viewing zone angle was enlarged to 43.0°.
Figure 6 shows a photograph of the constructed planar scanner. The off-axis Fresnel lens was rotated by the servo motor. A lenticular lens was used as the vertical diffuser, with the direction of the cylindrical lenses comprising the lenticular lens aligned vertically. The lens pitch of the lenticular lens was 0.07 mm. The lenticular lens was attached to the exit side of the planar scanner, and the two screen lenses corresponding to the two projector systems were attached to the incident side of the planar scanner.
A photograph of the constructed two-channel experimental system is shown in Fig. 7. One projector was placed behind the planar scanner and the other was placed in the perpendicular direction, with light coming from it bent by a mirror into the planar scanner. To block the light from the light emitting diodes on the driver boards, which would otherwise be observed with the reconstructed images, the driver boards were covered with black papers.
Grid images were projected by the two projector systems in order to evaluate the tiling of the two screens. Figure 8 shows the image captured at the center position in the enlarged viewing zone without rotating the off-axis Fresnel lens. The image distortion was small and the two images were tiled precisely. The measured width of the enlarged viewing zone was 632 mm.
The hologram patterns were calculated by the point-based method, which represents a 3D object by an aggregate of object points. The half-zone plates having only horizontal parallax corresponding to the objects points were generated and summed to obtain an object wave [21, 22]. Because the DMDs can display only binary patterns, the hologram patterns were binarized by an error diffusion algorithm [23]. The binarized hologram patterns were divided into left and right hologram patterns and displayed through channels #1 and #2, respectively.
Reconstructed images generated by the experimental system are shown in Fig. 9. In the reconstructed images of panels (a), (b), (c), and (d), the number of object points was 710, 3,391, 29,298, and 72,164, respectively. The corresponding calculation times for generating 378 hologram patterns displayed during a single scan were 6.9, 7.8, 17.9, and 43.5 s. The calculation was performed on a personal computer with an Intel Core i3 CPU (M380, 2.53 GHz). The images were captured from five different horizontal positions in the enlarged viewing zone. The reconstructed images could be observed in a wide area and had very smooth motion parallax; however, a vertical black line was observed between the screens of the two projection systems.
Fig. 9 Reconstructed images captured from five different horizontal positions in enlarged viewing zone: (a) apple (see Visualization 1), (b) head (see Visualization 2), (c) T-REX (see Visualization 3), and (b) castle (see Visualization 4).
5. Discussion
As shown in Fig. 9, a vertical black line was observed in the reconstructed images. This might have been caused by the side surfaces of the two Fresnel lenses at which the lenses were conjected. We tried to eliminate this line by adhering the two Fresnel lenses using a photo-curing resin; however, the line could not be eliminated completely. A non-uniform light intensity distribution was observed in each screen lens. This was caused by the light intensity distribution of the illumination laser light on the DMD’s screen. The non-uniformity could be reduced by increasing the diameter of the Gaussian beam illuminating the DMDs; it could also be corrected by calculating the hologram patterns to generate reconstructed images with intensity distributions modulated inversely proportional to the illumination light intensity distributions.
The first reconstructed images that we produced contained two identical images that were shifted in the vertical direction. This effect was caused by a gap between the rotating off-axis Fresnel lens and the vertical diffuser. Rotation of the Fresnel lens generates viewpoints on a circle, and the viewpoints on the upper and lower halves of the circle are converted to viewpoints scanned from left to right and vice versa by the vertical diffuser. The vertical diffuser acts as a practical screen because rays are diffused vertically on it. When there is a gap between the vertical diffuser and the off-axis Fresnel lens, the vertical position of the practical screen changes when the off-axis Fresnel lens rotates. The vertical shift of the practical screen is given by (g / l) d sin θ, where the gap is denoted by g and the rotation angle of the screen is denoted by θ. In the experimental system, the gap was g = 10.0 mm, so the maximum vertical shift was ± 3.91 mm; therefore, vertically separated double images were observed. In order to avoid the double image generation, hologram patterns were calculated by shifting the 3D images in the opposite direction to the vertical shift. The reconstructed images shown in Fig. 9 were produced using this image correction method, and Fig. 10 shows the effects of the correction of the double image generation.
Fig. 10 Effect of the correction of the double image generation: (a) without the correction, and (b) with the correction.
The reconstructed images could be observed at any position in the enlarged viewing zone. Because the intervals of the reduced viewing zones were smaller than their widths, the reduced viewing-zones overlapped. The width of each reduced viewing zone was 5.00 mm, with a largest interval of 2.60 mm at the center of the enlarged viewing zone and a smallest interval of 0.0108 mm at both ends. The reconstructed images appeared brighter when they were observed from the positions nearer the ends of the enlarged viewing zone.
The screen size could be increased by increasing the number of projection systems. The practical limitation to this process is the optical window size of the planar scanner, which is determined by the size of the Fresnel lens and the torque of the servo motor. As described in Sec. 3, it is difficult to increase the size of the Fresnel lens as long as commercial Fresnel lenses are used. From the torque calculations, the diameter of the Fresnel lens could be increased to about 300 mm when the thickness is 2.0 mm.
6. Conclusion
This study proposed a multi-channel viewing-zone scanning holographic display to enable the scalable enlargement of a screen size. The display consists of multiple projection systems and a planar scanner. A two-channel system was constructed, with each channel using a DMD and a magnifying imaging system to project an enlarged image of the DMD’s screen. The two enlarged images corresponding to the two channels were tiled seamlessly on a planar screen comprising a rotating off-axis Fresnel lens and a vertical diffuser. Reconstructed images with a screen size of 7.4 in. and a viewing zone angle of 43.0° were successfully produced.
Funding
Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number 15H03987.
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