Horizontally scanning holography using a spatial light modulator based on microelectromechanical system, which we previously proposed for enlarging both the screen size and the viewing zone, utilized a screen scanning system with elementary holograms being scanned horizontally on the screen. In this study, to enlarge the screen size and the viewing zone, we propose a viewing-zone scanning system with enlarged hologram screen and horizontally scanned reduced viewing zone. The reduced viewing zone is localized using converging light emitted from the screen, and the entire screen can be viewed from the localized viewing zone. An experimental system was constructed, and we demonstrated the generation of reconstructed images with a screen size of 2.0 in, a viewing zone width of 437 mm at a distance of 600 mm from the screen, and a frame rate of 60 Hz.
© 2014 Optical Society of America
Holography  is an ideal three-dimensional (3D) display technique because it can generate sharp 3D images for the eye to focus on; this reduces the accommodation–vergence conflict, which causes visual fatigue . The accommodation–vergene conflict is one of the problems that prevent the widespread use of conventional 3D displays. The electronic implementation of holography requires ultra-high resolution spatial light modulators (SLMs), and current SLMs can provide only a limited screen size and viewing zone. The viewing zone is a region in an observation space of holographic displays where an entire display screen can be viewed. To address this problem, we previously proposed horizontally scanning holography using an SLM based on microelectromechanical systems (MEMS) technology . In this study, we propose a new display scheme for horizontally scanning holography, in which a reduced viewing zone is scanned spatially to provide a larger screen size and a wider viewing zone.
Horizontally scanning holography fully utilizes the high-speed operation of an SLM based on MEMS (MEMS SLM). Figure 1 shows a schematic of the previously proposed horizontally scanning holography system. The MEMS SLM generates hologram patterns at a high frame rate. The anamorphic imaging system de-magnifies hologram patterns in the horizontal direction, and magnifies them in the vertical direction. The vertically stretched hologram patterns, which are elementary holograms, are scanned horizontally on the screen by the horizontal scanner to increase the screen size. Because the pixel pitch of the elementary holograms decreases in the horizontal direction, the horizontal viewing zone angle increases. A hologram display system having a screen size of 3.5 in. and a viewing zone angle of 15° has been demonstrated using the MEMS SLM with a frame rate of 13.333 kHz . Techniques to improve the grayscale representation of the reconstructed images have been developed because the MEMS SLM can generate only binary patterns [4, 5]. The responses of eye accommodation, which is an eye focusing function, to the reconstructed images were measured to ensure that eyes could focus on the reconstructed images . Recently, the generation of color reconstructed images with a screen size of 6.2 in. and viewing zone angles of 14.7° (R), 11.8° (G), and 11.2° (B) was achieved using the MEMS SLM with a frame rate of 22.727 kHz .
Several techniques have also been proposed to increase the screen size and the viewing zone angle of the holographic displays. A technique that combined an acousto-optic modulator and a two-dimensional (2D) mechanical scanner was proposed by a group at MIT [8, 9]. This technique uses one-dimensional (1D) light modulation and 2D scanning, whereas our technique uses 2D light modulation and 1D scanning. Recently, the technique proposed by the MIT group was improved to reduce the requirement of high-speed horizontal scanning . A technique that combines an eye tracking system and a viewing-zone steering system has also been proposed [11, 12]. When the screen size is enlarged, the viewing zone is reduced. The reduced viewing zone is adjusted to the position of the eye using the viewing-zone steering system. Our research group also proposed the resolution redistribution technique; the frameless display modules based on this technique were developed to allow further enlargement of the screen size by tiling multiple modules . Straight-forward techniques that increase the resolution of the holographic displays by use of multiple SLMs have also been proposed [14–17].
To enlarge the viewing zone, several research groups have proposed scanning techniques. Kim  proposed the combination of an eye-tracking system and a rotating mirror. The Fourier transform holography configuration was used to generate reconstructed images around the rotating mirror. They demonstrated hologram generation using the phase modulation by a liquid-crystal on a silicon (LCOS) type SLM. Matoba  also proposed the combination of phase modulation using an LCOS-type SLM and a rotating mirror. The Fresnel holography configuration was used, and they demonstrated hologram reconstruction into three horizontal directions. For the present study, the Fresnel holography imaging system has been modified to generate a localized viewing zone. A large number of localized viewing zones are generated using the high-speed operation of the MEMS SLM, which are scanned horizontally to provide a wide viewing zone.
In this study, the viewing-zone scanning type horizontally scanning holography using MEMS SLM is proposed; (1) the screen size is increased, (2) the viewing zone is reduced and localized, and (3) the reduced and localized viewing zone is scanned horizontally to increase the viewing zone. Experimental verification of the proposed technique is shown, and comparisons with the previous screen scanning system are provided.
2. Proposed system
The main concept of the proposed viewing-zone scanning system is illustrated in Fig. 2(a). The screen of the MEMS SLM is magnified to increase the hologram screen size. Because the pixel pitch increases, the viewing zone is reduced. The reduced viewing zone is scanned horizontally to enlarge the viewing zone. For comparison, the conventional screen scanning system is illustrated in Fig. 2(b). In this case, the pixel pitch is reduced to increase the viewing zone. The reduced MEMS SLM images, i.e., elementary holograms, are scanned horizontally on the screen to increase the hologram screen size.
Figure 3 presents a schematic of the proposed viewing-zone scanning type horizontally scanning holography system. The system consists of a MEMS SLM, a magnifying imaging system, and a horizontal scanner.
As shown in Fig. 4, the magnifying imaging system consists of two lenses that provide two functions. One function is to enlarge the hologram screen size. To this end, the screen of the MEMS SLM is imaged onto the mirror of the horizontal scanner with an enlarged magnification. The other function is to localize the viewing zone. Light is converged to a single point after being reflected by the mirror. Because of the enlarged magnification, the pixel pitch increases so that the viewing zone is reduced. Because the viewing zone is produced at the point where light converges, the reduced viewing zone is localized at this point as shown in Fig. 4. In this reduced and localized viewing zone, an entire reconstructed image can be observed.
The reduced viewing zones are scanned horizontally by the horizontal scanner to enlarge the viewing zone. By generating hologram patterns at a high frame rate by the MEMS SLM and synchronizing the generated patterns with the horizontal scanner, both the screen size and the viewing zone are increased.
The conjugate image and the zero-order diffraction light must be removed from the hologram reconstruction. In the proposed system, these are removed by a single-sideband (SSB) filter [20, 21] placed on the Fourier plane of the imaging system, i.e., on the focal plane of Lens1 shown in Figs. 3 and 4.
The magnification of the imaging system is denoted by M, and the length between the scanning mirror and the light convergence point is denoted by l. The resolution and the pixel pitch of the MEMS SLM are denoted by X × Y and p, respectively. The scanning angle of the horizontal scanner is denoted by ± ϕ. The wavelength of light is denoted by λ. As such, the hologram screen size is enlarged to (MXp) × (MYp). The size of the reduced viewing zone is given by (λl/(Mp)) × (λl/(2Mp)). The viewing zone is enlarged to (2ltan ϕ) × (λl/(2Mp)). The interval of generating the reduced viewing zones must be equal to or smaller than the width of the reduced and localized viewing zone, i.e., λl/(Mp).
The effect of the viewing zone localization is described as follows. Figure 5(a) illustrates the viewing zone formation proposed in this study. The diffraction angle α is given by α = λ/Mp. When an eye is placed in the localized viewing zone, the entire hologram display screen can be viewed. The wavefront from the entire screen produces a retinal image. Figure 5(b) illustrates the conventional Fresnel holography reconstruction, where the eye is able to see a part of the display screen. In conventional Fresnel holography reconstruction, the width of the observable area is d + lα, where d denotes the pupil diameter. A partial wavefront emitted from the display screen contributes to the retinal image formation. When a scanning mirror is placed on the screen, different segments of the screen can be viewed so that the retinal image is produced from plural partial wavefronts. Because these wavefronts are generated at different times, the retinal image is the sum of the intensity distributions for the partial wavefronts. Therefore, the retinal image is different from that generated by a single wavefront so that the reconstructed images are degraded. Figure 5(c) illustrates the conventional Fourier transform holography configuration. Although the reconstructed image size and the viewing zone angle are different from those of Fresnel holography, again, the eye can only see a part of the screen. Here, the diffraction angle β is given by β = w/f, where f is the focal length of the Fourier transform lens.
For the proposed viewing-zone scanning system, the scan angle of the horizontal scanner determines the size of the viewing zone, and the mirror size of the horizontal scanner determines the screen size. In contrast, for the previous screen scanning system, the scan angle determined the screen size, and the mirror size determined the size of the viewing zone.
An experimental system was constructed to verify the effectiveness of the proposed viewing-zone scanning type horizontally scanning holography system.
A digital micromirror device (DMD) was used as the MEMS SLM. The DMD used was a DiscoveryTM 3000 (Texas Instruments, Inc.) with a resolution of 1024 × 768, pixel pitch of 13.68 μm, and a screen size of 0.689 in. The frame rate was 13.333 kHz. The DMD was illuminated by collimated laser light emitted from a laser diode at a wavelength of 635 nm.
A galvano mirror was used as the horizontal scanner. The galvano mirror used was the Micro MaxTM series 671 (Cambridge Technology, Inc.) with a diameter of 50 mm and a scanning angle of ± 20.0°. The image update signal from the DMD was used to generate the drive voltage for the galvano mirror. The number of hologram patterns displayed during a single scan was set to 222, thereby providing a scanning frequency of 60.1 Hz. The holograms were displayed using both clockwise and counterclockwise rotating directions, resulting in a mirror vibration frequency of 30.0 Hz.
The magnification of the magnifying imaging system was M = 2.86. The distance from the scanning mirror to the point where light converges was l = 600 mm. The screen size was enlarged to 40.0 mm × 30.0 mm (2.0 in), and the pixel pitch was increased to 39.1 μm. The viewing zone was reduced in size to 9.74 mm × 4.87 mm. The width of the enlarged viewing zone was 437 mm. The interval of generating the reduced viewing zones was 1.98 mm.
Figure 6 provides a photograph of the constructed experimental system used to conduct viewing-zone scanning type horizontally scanning holography.
The reconstructed images generated by the experimental system that were captured at different horizontal positions in the enlarged viewing zone are shown in Fig. 7. Because the width of the enlarged viewing zone was larger than 400 mm, the reconstructed images could be easily observed with both eyes and provided sufficient motion parallax. As shown in Fig. 7, the observed images changed greatly depending on the observation position. The reconstructed images had smooth motion parallax because the enlarged viewing zone consisted of dense viewing zones. Figure 8 shows the reconstructed images when a vertical diffuser was placed just in front of the scanning mirror toward the viewing zone. A lenticular lens was used as the vertical diffuser. The lenticular lens consists of vertically aligned cylindrical lenses. In the vertical direction, light is diverged by the cylindrical lenses. In the horizontal direction, light is not diverged because the cylindrical lenses have no lens power. The vertical pitch of the cylindrical lenses was 0.10 mm, and the vertical diverging angle was 52°. Because the height of the original viewing zone was less than 5 mm, the use of the vertical diffuser greatly increased the freedom of the vertical viewing position. However, use of a vertical diffuser eliminates the vertical parallax so that the reconstructed images had only horizontal parallax.
Figure 9 presents the photographs of reconstructed images captured for three patterns that were generated at different depth positions: “3D” and “TUAT” were respectively generated at distances of + 100 mm and + 30 mm in front of the screen, and a circle was generated at a distance of −100 mm behind the screen. In each photograph, the focused pattern is observed to be sharp whereas the other two patterns are blurred.
The reconstructed images provided by the viewing-zone scanning system developed in the present study have full-parallax, although those provided by the screen scanning system have only horizontal parallax. However, the image change in the vertical direction was small because the height of the viewing zone was approximately 5 mm, and viewers needed to adjust their eyes to the viewing zone. The vertical diffuser allowed viewers to easily see the reconstructed images. Compared to the reconstructed images shown in Fig. 7, the reconstructed images shown in Fig. 8 appear to be less sharp, which is caused by the blurring effect of the vertical diffuser in the vertical direction.
When the reconstructed images were observed near the left or right ends of the enlarged viewing zone, flicker was observable. The time intervals between the forward and backward scans were smaller or larger than 1/60 s near the ends so that the larger time intervals caused flicker. Around the center of the enlarged viewing zone, flicker was not observable because the time intervals were approximately 1/60 s. The vibration frequency of the galvano mirror determines the frame rate. For the galvano mirror, there is a trade-off between the maximum vibration frequency and the mirror size. Therefore, the screen size should be reduced to increase the frame rate. In the previous screen scanning system, flicker was not observable because, as shown in Fig. 2(b), light was always provided to the entire viewing zone.
We have experimentally verified the effectiveness of the proposed technique. Here, we compare the display parameters of the presently developed viewing-zone scanning system with those of the previously developed screen scanning system . Table 1 lists the display parameters of both systems. In both systems, the frame rate of the DMD was 13.333 kHz. From Table 1, we can see that the viewing-zone scanning system has a larger viewing zone, whereas the screen scanning system has a larger display screen.
In the experimental system, reduced viewing zones with a width of 9.74 mm were generated at an interval of 1.98 mm. If the eye’s pupil diameter is assumed to be 5 mm (the average pupil diameter), two or three reduced viewing zones simultaneously enter the pupils. The wavefronts are different for different viewing zones because the wavefronts are calculated by referring to the positions of the viewing zones. However, the reconstructed images were not blurred. The 3D objects reconstructed from the different wavefronts were displayed at identical positions because the wavefronts were calculated referring to both the positions of the viewing zones and the positions of the 3D objects. Because the same wavefront is scanned horizontally for one scan interval (1.98 mm), the reconstructed images are blurred horizontally. This horizontal blur becomes smaller when the reconstructed images are displayed nearer to the screen. The horizontal blur was not obvious in the experimental results because the reconstructed images were displayed near the screen. There is substantial overlap among the reduced viewing zones. For the screen scanning system, a technique to improve the grayscale representation of the reconstructed images was proposed, which utilizes the overlap among the elementary holograms and the intensity modulation of laser illumination . The grayscale representation of the viewing-zone scanning system could also be improved by utilizing the overlap among the reduced viewing zones and the illumination laser modulation.
We have proposed the viewing-zone scanning system for horizontally scanning holography using a MEMS SLM. The proposed system increases both the screen size and the viewing zone. An experimental system was constructed using a DMD having a frame rate of 13,333 Hz. The screen size was enlarged to 2.0 in. and the width of the viewing zone was enlarged to 437 mm at a distance of 600 mm from the screen. The frame rate was 60 Hz. The generation of reconstructed images was successfully demonstrated.
This work was supported by Strategic Information and Communications R&D Promotion Programme (SCOPE) of the Ministry of Internal Affairs and Communications Government of Japan (MIC), Japan.
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