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Abstract
We propose off-axis virtual-image display and camera systems, which integrate a vertically-standing holographic off-axis mirror, blur-compensation optical systems, and digital imaging devices. In the system, the holographic mirror is used for an off-axis reflector, which realizes an upright and thin screen for virtual-image formation. By combining it with a display unit, an off-axis virtual-image display is realized, where the virtual image can be seen behind the upright holographic mirror. Simultaneously, by combining it with a camera unit, an off-axis camera is implemented, which realizes frontal shooting of objects by a camera placed at an off-axis position. Since both the off-axis display and the camera can be implemented by a single holographic mirror, it can be applied to a two-way visual-telecommunication system with a thin screen, which implements eye contact and the observer--image distance. A problem with the proposed system is image blur, which is caused by the chromatic dispersion of the holographic mirror. To solve this, we designed optical blur-compensation systems using a diffractive optical element and a diffuser or a lens. Experimental results verify the concept of the proposed systems with clarifying the effect of designed blur-compensation methods.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
A holographic optical element (HOE) is capable of implementing various flexible optical functions on a thin, flat, and transparent film based on wavefront recording and reconstruction. Many applications of HOEs exploit their flexibility in performing optical functions and their see-through characteristics. HOEs have been applied to head-up displays (HUDs) [1], head-mount displays (HMDs) [1–6], bidirectional displays [7], see-through diffusive screens [8], projection-type three-dimensional (3D) displays [9–12], 3D user interfaces [13], wearable eye-gaze detection systems [14], solar-power generation systems [15], vibration and temperature measurements [16,17], and 3D telepresence systems [18].
The HOE can implement off-axis mirror, which is referred as holographic mirror. One interesting application of the holographic mirror is realization of off-axis virtual-image display. The virtual-image display is the display that forms a virtual image behind the screen. Here the observers do not perceive the frame of the display, which reduces material presence of the display for observers. This non-existent sense of the display device realizes novel user experiences. For example, the user experience based on the realistic fusion of cyber-physical image information is realized. The simplest realization of the virtual-image display is the use of a tilted half mirror as shown in Fig. 1(a). Here a tilted half mirror reflects light from the display toward an observer, which forms a virtual image. This configuration is as same as the one in the optical illusion called Pepper’s ghost [19], if a real object is placed near the virtual image. In this realization, the optical system is quite simple; however, the screen becomes thick due to the tilted placement. On the other hand, the use of the holographic mirror instead of the half mirror can reduce the thickness due to the upright placement like Fig. 1(b). Similar configuration has been applied to, e.g., compact HUDs and HMDs [1–6]. As a substitute of a physical display, a combination of a diffuser and a projector can also be used as illustrated in Fig. 1(c). This realizes the magnification and optical modulation of the image to be displayed. With this setup, the viewing angle of the virtual image is simply determined by the diffusion angle of the diffuser.
Fig. 1 Schematic of the (a) virtual-image display by a slanted half mirror, (b)–(c) off-axis virtual-image displays by a upright holographic mirror with a display or a projector, and (d) off-axis camera by the holographic mirror. In off-axis virtual-image displays, the image can be seen behind the holographic mirror. In off-axis camera, the object is captured from the front by a camera at the off-axis position.
Another application of the holographic mirror is the off-axis camera as shown in Fig. 1(d). By combining the holographic mirror and a camera, the person facing the holographic mirror is captured from the front by a camera at an off-axis position. Here the image capture is performed as if a virtualized camera was existing behind the holographic mirror. The merit of the virtual camera based on the off-axis image capture is the nonexistence perception of the imaging device by the subject to be captured [14,20].
As indicated in Figs. 1(c)–1(d), the off-axis virtual-image display and the off-axis camera can be implemented with the same optical system [20]. Thanks to this unique feature, the holographic mirror can be applied to the bidirectional imaging screen, where the screen displays the virtual image to the observer and also captures the image of the observer from the front. Such display is used for, e.g., the interactive display, which enables scene- and observer-responsive image presentation. As the another usage, the visual-telecommunication system where the users can faces each other with eye contact and comfortable distance is also possible.
A major issue on the use of a holographic mirror with natural broadband light is the generation of spatial blurring in the virtual image. Since the virtual image is formed by diffraction from a hologram, the light is dispersed, which blurs the image. To deal with this, the simplest approach is to limit the spectrum of the transmitted light [14]. However, there is a tradeoff between the brightness and sharpness of an image. Signal processing, such as deconvolution filtering or image recovery/pre-optimization, is another approach to this problem [21–23]. However, this cannot work when the blur is not invertible due to the degradation of the optical transfer function. Optical dispersion compensation is an alternative approach. Until today, several dispersion compensation techniques for reflection-type imaging holograms have been invented [1]. The basic idea is the insertion of the disperser like a prism [1] or a grating [24] to cancel out the dispersion of the hologram. The use of two identical holograms has also been proposed for the compensation [25, 26]. In case of our off-axis virtual-image display and camera based on a holographic mirror, such compensation technique is effective in a certain amount; however, the not-negligible size of blur remains in the image as discussed in Sec. 2.
In this paper, we propose the novel off-axis virtual-image display and camera systems, which integrates a holographic off-axis mirror, optical dispersion compensation systems, and digital imaging devices. To generate sharp virtual images by the holographic-mirror-based system, two types of optical-modulation system using a diffractive optical element (DOE) and a lens or a diffuser are designed. Sec. 2 describes the operating principle of the proposed systems. Sec. 3 presents the fabrication process of the holographic mirror. Sec. 4 shows experimental verification of the proposed off-axis virtual-image display. Sec. 5 presents the experimental results of the proposed off-axis camera. The concluding section summarizes this paper.
2. Dispersion compensation for off-axis virtual-image display and camera
2.1. Model of blur through a holographic mirror
The virtual image formed by a holographic mirror is blurred by chromatic dispersion. Figure 2 shows a schematic illustrating the generation of the blur. When light is incident on a hologram, the diffracted light is dispersed. The dispersion causes spatial blur of the formed virtual image. Figure 2 explains the magnitude of the blur by using a point source and its virtual image. The range of dispersion angles ΔθHM by a holographic mirror is given by
where λo is the central wavelength of the propagating light and θHM is the diffraction angle of the holographic mirror. ΔλHM is the spectral width diffracted by the holographic mirror, which is determined by the wavelength selectivity of the hologram. By the dispersion, the virtual image formed by the holographic mirror is blurred along the dispersion direction. The size of the blur b′vimg is where zvimg is the depth of a virtual image from a holographic mirror. Note that Eq. (2) makes use of the paraxial approximation. According to Eq. (2), the size of the blur b′vimg linearly increases with the depth of the virtual image, wavelength selectivity of the hologram, and the diffraction angle.Fig. 2 Schematic of the generation of spatial blur in a virtual image caused by the chromatic dispersion of a holographic mirror.
Here we denote the angle of viewing direction to the holographic mirror as θview as shown in Fig. 2. When the virtual-image plane is parallel to the holographic mirror, the blur size on the virtual-image bvimg is the geometrical projection of b′vimg to the image plane as follows,
When the observer sees the image perpendicularly to the holographic mirror, θview = 0 and bvimg = b′vimg.2.2. Proposed systems with optical blur compensation
2.2.1. Blur-compensated off-axis virtual-image display
To form sharp virtual image with holographic mirror, compensation of chromatic dispersion and resulting blur is necessary. In the history of the research on holographic displays, there are lots of studies on dispersion compensation. The original idea was developed for realizing white-light illumination with the transmission hologram. Here the basic idea is implementing pre-compensation by dispersive optical elements to cancel out the holographic dispersion [24,27,28]. The idea has also been applied to holographic lenses [29,30] and reflection holograms [26,31]. In case with a holographic mirror, the dispersion compensation method for reflection HOE that uses an additional identical HOE can be used [25]. Figure 3(a) shows the schematic of the method. In the method, the dispersion from the holographic screen is cancelled by the pre-dispersion of another identical holographic mirror. However, in principle, the image is blurred with the distance of two holographic mirrors. From now, we simply consider the dispersion compensation when θview = 0, which is referred as single view angle dispersion compensation in [31]. The size of suppressed blur of the virtual image bvimg(conv.) based on the conventional method is modeled as follows:
where z[HM1−HM2] is the distance of two holographic mirrors along the diffracted light, and ΔθHM is the range of dispersion angle of the holographic mirrors. Comparing with Eq. (2), the size of blur with the system of Eq. (4) does not depend on the depth of virtual image from the holographic mirror because angular dispersion is compensated by an additional holographic mirror. However, a certain amount of spatial dispersion still remains as in Fig. 3(a). It is ignorable when the optical system is compact like HMDs or uses only the narrow-band illumination like a laser; however, it is an issue to be solved when the system is large and works with the broadband light source, which is required for, e.g., life-size bright visual-telecommunication systems. For example, when z[HM1−HM2] = 1.0 m, ΔλHM = 10 nm, λo = 532 nm, and θHM = 45°, the size of blur in a virtual image bvimg(conv.) becomes 1.9 cm. Furthermore, the use of two holographic mirrors in a system significantly reduces photon efficiency of the system. Considering practical applications, the dispersion compensation should be achieved without adding HOEs.Fig. 3 Schematic diagram of holographic-mirror-based off-axis virtual-image displays with (a) conventional and (b) proposed blur-compensation methods.
We therefore propose an alternative optical design for further blur compensation in the off-axis virtual-image display. Figure 3(b) illustrates the proposed system. The system comprises a projector, a DOE, a diffuser, and a holographic mirror. The focusing distance of the projector is set to a diffuser plane. The image on a diffuser is pre-dispersed by a DOE. In principle, the DOE can be replaced with other dispersive optical elements like a prism or an HOE. The diffuser emits the diffused image toward a holographic mirror. The pre-dispersed image on a diffuser compensates the dispersion by a holographic mirror. When the magnitude of pre-dispersion mapped on a diffused image is carefully designed, the spatial blur of a virtual image is compensated.
Here we denote ΔθDOE as the range of dispersion by a DOE, and Δθ′DOE as that by the light rays that incidents onto a holographic mirror after modulations as shown in Fig. 3(b). The required condition for the blur compensation is,
Δθ′DOE is determined by the optical design of the compensation optics and ΔθDOE, and ΔθHM is determined by Eq. (1). To satisfy Eq. (5), the design condition of the proposed off-axis virtual-image display system in Fig. 3(b) is as follows: where zDOE is the distance between the DOE and the diffuser, and zHM is the distance between the holographic mirror and the diffuser, and θDOE is a diffraction angle of a DOE, respectively. When Eq. (6) is satisfied, the lateral blur in a virtual image is compensated. As a result, the proposed system can display a sharp virtual image through an upright holographic mirror. Note that the compensation is strictly valid only with the center region of the image when the observer sees the holographic mirror perpendicularly. The peripheral region of the image is blurred based on the gap between bvimg and b′vimg.2.2.2. Blur-compensated off-axis camera
The concept of virtualization of imaging device is also applicable to the camera system, where a camera can be placed at an off-axis position from the observer and a holographic mirror like Fig. 1(d). Then, the compensation of blur caused by the holographic mirror is also necessary. In past studies, Zhou et al. use a bandpass filter to suppress the blur [14]; however, the image is still blurred within the transmissive spectrum of the filter. Lv et al. use deconvolution filtering after capturing [21]; however, the effectiveness is limited by the optical transfer function of the optical system. We therefore propose the blur-compensation system based on optical dispersion compensation.
In Sec. 2.2.1, we proposed a blur-compensated optical system based on a diffuser and a DOE. The system is effective for the application to the virtual-image display; however, it is not applicable to the virtual camera system in practice. In practical image capturing of natural scenes, the power of illumination, the distance of subject, and the diffusion property of subject cannot be controlled unlike projector-based display systems. In such cases, the brightness of the mapped image on a diffuser tends to be insufficient for image capturing. Therefore, we design an alternative brighter optical design for blur compensation than that based on a diffuser.
Figure 4 illustrates the design of the proposed off-axis camera with blur compensation. The system comprises a holographic mirror, a convex lens, a DOE, and a camera. By the convex lens, the object (an observer in Fig. 4) is imaged between the lens and a DOE, and the holographic mirror is imaged on the DOE. As a result, the dispersion of the holographic mirror ΔθHM is converged on the DOE at Δθ′HM. The condition for dispersion and blur compensation is as follows:
Δθ′HM is determined by the optical system and ΔθHM, and ΔθDOE is determined by Eq. (1). The design condition of the proposed off-axis camera in Fig. 4 satisfying Eq. (7) is as follows: where zHM is the distance between the holographic mirror and the lens, and zDOE is the distance between the DOE and the lens, respectively. When the optical system satisfies the condition of Eq. (8), the dispersion and blur in imaging is compensated at the image plane of the camera. Then the camera captures a sharp image of the object through the DOE, in which the captured image is the real image formed between the lens and the DOE as indicated in Fig. 4. The system works as if the virtual camera (located at the left end in Fig. 4) was shooting the observer from the space behind the upright holographic mirror. Thanks to the lens-based compensation optics, the virtual camera can capture blur-compensated images through the holographic mirror. Note that the proposed off-axis camera is also based on the single view angle dispersion compensation as well as the off-axis virtual-image display described in the previous section.Thanks to the reduced-size real-image formation and forward light transmission by a lens, the brightness of a lens-based blur-compensated imaging system is much higher than that of a diffuser-based system presented in Sec. 2.2.1. Contrary, the viewing angle of the camera is limited because the formed real image is not diffused after holographic diffraction. This limitation is a problem for the display application; however, this is not a problem for the virtual camera because the position of the camera can be fixed. This is why we adopt the lens-based blur-compensation system for the virtual camera, and the diffuser-based system for the virtual-image display.
The compensation optics for the proposed off-axis virtual-image display and camera can be integrated into a single optical system by, e.g., inserting a half mirror near the single holographic mirror [20]. The off-axis virtual-image display and camera can be applied to the life-size visual telecommunication system, where the observer can have eye contact with the person displayed in the virtual image. Since the holographic screen displays the virtual image, the observer recognizes the displayed virtualized person as if he/her was in the air behind the screen, which can implement a larger image–observer distance than conventional physical displays. Furthermore, the holographic mirror is transparent film and works with vertically standing arrangement so that the screen can blend into artificial straight objects. For example, the holographic mirror as a screen can be attached on a window, doors, and walls. The application of the proposed off-axis virtual-image display and camera to the visual telecommunication system will be presented in a future work.
3. Experiment: fabrication of holographic mirror based on tiling exposure
A holographic mirror is a reflection-type volume hologram, which is fabricated by the holographic recording on a photosensitive material. The material is exposed by the interference fringes from two coherent beams to record the hologram. To fabricate a large holographic mirror to serve as such a screen, the simplest approach is to expand the beams with collimation. However, this method has three problems: (1) the requirement of linear upscaling of the collimating optics, (2) the non-homogeneity of the diffraction efficiency over the entire area of the mirror, and (3) the trade-off between the size of a beam and its intensity per unit area. To solve these problems, we adopted the approach of tiling small holographic mirrors realized by lateral scans of exposures. In this paper, we define the small hologram to be tiled as elemental hologram. The exposure method based on scanning and tiling has been termed as holographic printing in past studies, and it has been studied mainly for the purpose of exposing holographic stereograms [32–36]. Holographic printing has also been applied for the creation of computer-generated volume holograms [37,38] and HOEs [10,39]. Herein, we apply holographic printing to produce a large holographic mirror without using large collimation optics. The resulting tiled holographic mirror functions as a single large holographic mirror with a spatially uniform diffraction efficiency.
The holographic printer employed in our experiments is very simple as shown in Fig. 5. In this printer, a diode-pumped, solid-state (DPSS) continuous-wave (CW) laser operating at 532 nm (Samba 100mW by Cobolt) was used as a coherent light source. An acousto-optic modulator (AOM) (R23080-1-LTD by Gooch & Housego) was placed in front of the laser as an electrical shutter. A half-wave plate (HWP) and a polarization beam splitter (PBS) cube were arranged to generate reference and signal beams with varying the intensity ratio. By controlling the HWP, we implement the 1:1 beam-intensity ratio on a photopolymer. Each beam was transmitted through polarization-maintaining single-mode optical fibers (pmSMFs) (KineFLEX by QIOPTIQ) via alignment devices, and the beams were delivered to a position close to a photosensitive material. We used photopolymer (Bayfol HX200 by Covestro) with 16 μm thickness as the photosensitive material in our experiments. The signal and reference beams were interfered on the photopolymer to record an elemental holographic mirror. The photopolymer was attached to a 2 mm thick glass plate. The glass plate, mounted on a two-axis motorized stage, was mechanically scanned for tiling the elemental holographic mirrors. The motorized stage, the laser, and the AOM were controlled from a single computer. In the experiment, the angle between the two beams was fixed at 135°. After exposure, the photopolymer was bleached by illuminating ultraviolet light. The irradiance of the interfering light beams used to expose the elemental holographic mirror was 7.4 × 102 mW/cm2 as a total of signal and reference beams. The duration of each exposure was 10 ms, and the diameter of each beam was 1.1 mm on the photopolymer. The total count of tiled elemental holograms was 175 × 175. The spatial interval covered in each scan was 0.8 mm, and the time interval between scans was 2 s. The total time for the exposure was around 20 h. The total size of the recorded holographic mirror was 14 cm × 14 cm.
3.1. Evaluation
After fabrication by holographic printing, we evaluated the holographic mirror. Figure 6 shows the fabricated holographic mirror with diffracted light. The diffracted light forms a virtual image of a planer paper with a star symbol illuminated by a white light-emitting diode (LED). Herein, the holographic mirror is mounted on a glass plate. Figure 6(a) focuses on the surface of the holographic mirror, whereas the Fig. 6(b) focuses on the surface of the virtual image. As a reference, a name card was placed on a virtual-image plane. The close-up in Fig. 6(a) shows a dot-like structure, which is caused by random fluctuations of the diffraction efficiency among the elemental holographic mirrors. When focusing on the virtual image, the pixel-like structure of the holographic mirror is blurred out. Note that this image was recorded using a camera with a lens having a 100 mm focal length and an f-number of 2.8, where its depth of field is about the same as human eyes.
Fig. 6 The fabricated holographic mirror with diffracted light forming a virtual image of a planer paper. The paper was illuminated by an external light source, and a star symbol was printed on a paper. Pictures focus on (a) the surface of the holographic mirror and (b) the virtual-image plane. A name card was placed on a virtual-image plane as a reference.
Figure 7(a) shows the setup employed to measure the diffraction efficiency of the holographic mirror based on ISO 17901-1. The spectrum Iλ of the collimated white-LED illumination (HL01 by Pi PHOTONICS) incident to the hologram and the spectrum Tλ of the transmitted light were measured using a fiber spectrometer (USB2000 by Ocean Optics). From the measured spectral transmittance, we evaluated the diffraction efficiency η of the holographic mirror as follows:
Figure 7(b) shows the ηλ in Eq. (9) with the fabricated holographic mirror. Note that the data of Fig. 7(b) are the average value over the four corners and the center of the hologram. The full width at half maximum (FWHM) of the diffracted wavelength was 8.9 nm, where the peak wavelength was 526.4 nm. Based on Eq. (9), the diffraction efficiency η of the fabricated holographic mirror was found to be 72.4 %.Fig. 7 (a) Setup for and (b) result of a spectral-transmittance measurement of the holographic mirror for evaluating diffraction efficiency.
4. Experiment: verification of blur-compensated off-axis virtual-image display
We experimentally verified the concept of the proposed off-axis virtual-image display integrating optical blur-compensation system illustrated in Fig. 3(b). The picture of experimental setup is shown in Fig. 8. We used the holographic mirror fabricated in Sec. 3. We used a digital projector (EH-TW5200 by EPSON) with an internal metal halide lamp, together with a DOE (VIS Holographic Grating by Edmund optics) having 1200 grooves per millimeter (GPM) in 50 mm square. As a diffuser described in Fig. 3(b), we used an A4 screen with a reflective geometry. To record the experimental results, we placed a camera in front of the holographic mirror, which mimics the view of an observer. In this research, we assume the application or the proposed systems with vertically arranged optical system like Fig. 3(b). To emulate this on the optical table, we rotated the projected images 90°. The experimental parameters of the system are summarized in Table 1. The parameters for blur compensation, such as zDOE and θDOE, were initially designed using Eq. 6 and were then experimentally adjusted to cancel the experimental errors in the system parameters, e.g., in θHM.
Table 1. Experimental parameters for the blur-compensated off-axis virtual-image display.
Figure 9 shows the images projected on the diffuser with and without a blur-compensation-purpose DOE. The images without the DOE were captured by replacing the DOE with a normal mirror. Note that the direction of light-propagation via the DOE (diffraction) and the mirror (reflection) is different, thus we placed them with the different surface angles. To confirm the dispersive blur, resolution, and appearance of a natural image in our system, we projected three target images: a pattern of crossing lines, the 1951 USAF resolution test chart, and a portrait of one of the authors. Without compensation, the images on the diffuser were the same as the original target images. In contrast, with a compensation system, the projected images were dispersed horizontally.
Fig. 9 Images projected on the diffuser in an off-axis virtual-image display system without and with a blur-compensation-purpose DOE.
We observed the virtual images of the projected images on the diffuser through the holographic mirror with and without the DOE. Figure 10 shows the experimental results captured by a camera. As discussed in Sec. 2.1, the images were blurred vertically, i.e., along the dispersion direction without compensation optics. In the virtual images of the 1951 USAF resolution test chart, the horizontal resolution was severely blurred by the dispersion without the blur-compensation-purpose DOE; however, it was successfully compensated by the proposed method with the DOE. According to the visual assessment, the vertical spatial resolution of the virtual image was improved from less than 0.12 cycle/mm to 0.42 cycle/mm, whereas the horizontal resolution remained at 0.49 cycle/mm. The experimental amount of the blur matches that of the model expressed by Eq. (2), which can also be confirmed by the virtual image of the vertical line in the target with the pattern of crossing lines. We confirmed the improvement in the vertical resolution of the virtual image also with a natural image (the author’s portrait), which indicates the possibility of applying the proposed blur-compensated off-axis virtual-image display system to the visual telecommunication system.
Fig. 10 Experimentally observed virtual images in an off-axis virtual-image display without and with a blur-compensation-purpose DOE. The scale bar indicates 1 cm at the image plane.
Figure 11 visualizes the implemented depth of the virtual image from the holographic mirror. To verify that the displayed image appears behind the holographic mirror, the pictures were taken with changing the viewing position and focus of the observation. The plane of the virtual image was located at a black curtain behind the transparent holographic mirror. The motion parallax and defocus can be seen, which presents the distance between the virtual-image plane and the surface of a holographic mirror. Visualization 1 is a video that shows the virtual image and optical system with changing the viewing position. The visualization demonstrates the dynamically-changing virtual-image display using movie projection by a digital projector.
Fig. 11 Virtual images by the proposed method with changing observing viewpoints and focusing distances, which express the distance between the virtual image and the holographic mirror (see also Visualization 1).
5. Experiment: verification of blur-compensated off-axis camera
We also experimentally verified the proposed off-axis camera with optical blur-compensation system illustrated in Fig. 4. The photograph of the experimental setup is shown in Fig. 12. We used the same holographic mirror used in the previous section. The object was placed in front of the holographic mirror in the observer’s space. On the axis of the diffracted light by the hologram, we placed a convex lens whose diameter was 100 mm and a DOE (VIS Holographic Grating by Edmund optics) having 1200 grooves per millimeter (GPM) in 50 mm square. A color charged-coupled-device (CCD) camera (Flea3 by FLIR) was placed near the DOE. The experimental parameters are listed in Table 2. Like the experiment in the previous section, the positions and parameters of the lens and the DOE were initially determined from Eq. (8) and the focusing distance of the lens f, and we modified them experimentally to compensate for experimental errors in the system parameters. As supplementary information, we note that zobj was 1130 mm, and the diameter of the lens was 100 mm.
Table 2. Experimental parameters for the blur-compensated off-axis camera.
Figure 13 shows the experimentally captured images of the 1951 USAF resolution test chart and a Japanese doll. The top and bottom rows correspond to the captured image without and with the blur-compensation-purpose DOE, respectively. Like the previous section, the experiment without the DOE was performed by replacing the DOE with a normal mirror, which were placed with different surface angles. As shown in the images of the resolution chart appearing in the left column of Fig. 13, the dispersion-based blur along the vertical axis was effectively compensated by the proposed method. From the captured images of the resolution chart, the vertical spatial resolution was experimentally improved from less than 0.80 cycle/mm to 1.46 cycle/mm. In the experimental result, horizontal resolution also increased from 2.31 cycle/mm to 3.54 cycle/mm. In principle, the horizontal resolution is not improved by dispersion compensation. However, in practice, vertical bars in the resolution chart was distorted by a lens, which added dispersive image components also on vertical bars. Although the vertical resolution was made closer to the horizontal resolution than non-compensation setup, a gap between both resolutions remained. In addition, the image after blur compensation seems to be still blurred. We consider that these effects are caused by the aberration by the inserted lens such as astigmatism and spherical aberration. The use of a well-designed compound lens system will help to solve these problems. The circular image region is due to vignetting by the lens. With the DOE, the image was enlarged vertically by the effect of off-axis projection by the DOE. This can easily be compensated by signal processing, if necessary. The right column in Fig. 13 presents the imaging results with a Japanese doll used as a 3D diffusing object. Its face and texture were successfully resolved by the proposed method. This result confirms the possibility of applying this method to the visual telecommunication system.
Fig. 13 Experimentally captured images by the off-axis camera without and with a blur-compensation-purpose DOE. The scale bar indicates 5 mm at the image plane.
6. Conclusion
In this paper, we proposed off-axis virtual-image display and camera systems integrating optical blur-compensation systems. The systems are composed of a upright holographic mirror, optical blur-compensation systems with a diffuser or a lens, and digital imaging devices. A key technology is a off-axis reflective diffraction by the holographic mirror, which enables upright arrangement of the mirror. It realizes a thin screen for the virtual-image display and off-axis camera. In this paper, we presented the concept of proposed systems and experimental verification of them. In our proposed virtual-image display system, a projector and a diffuser are placed under the holographic mirror for displaying a virtual image, and a DOE is also inserted between them for blur compensation. In our proposed virtual camera, a camera is placed under the holographic mirror to capture the frontal image, and a lens and a DOE are also inserted to compensate the blur. The experimental results successfully verify the compensation of blur in the proposed holographic-mirror-based systems based on the quantitative assessment using a resolution chart and the visual assessment using a natural object including a face. In future work, we plan to apply the proposed optical systems to visual telecommunication systems, where the observer can have eye contact and keep a distance to the virtual image that appears behind the screen.
Our current holographic mirror was fabricated using only a green laser. The reflection-type volume hologram has the wavelength selectivity, and thus it can be colorized using multi-wavelength lasers during the exposure. In a future project, we will fabricate a full-color holographic mirror to demonstrate its capability for full-color off-axis virtual-image display and camera. In addition, we will also address the enlargement of the image size. In our setup for an off-axis camera, the scaling up of the lens is required for enlarging the image; however, the implementation of such a large convex lens is difficult in practice. To deal with this issue, the use of a flat lens such as a Fresnel or holographic lens is one of solutions. Then the blur correction in peripheral visual field might be required. Furthermore, we will investigate the way to compensate the blur even when the viewing angle is not perpendicular to the holographic mirror. As in this paper, the blur of peripheral field of the virtual image is not a big problem when the size of the virtual image is not so large and the viewpoint is limited to perpendicular space to the holographic mirror; however, this problem will need to be solved to increase the image size and the viewing space. Eventually, we will integrate the off-axis virtual-image displays and camera into a single optical system with a single holographic-mirror screen, and demonstrate the life-size two-way visual telecommunication system.
Acknowledgments
The authors would like to thank Covestro Deutschland AG for providing the photopolymer holographic recording material.
References
1. R. J. Withrington, “Optical display systems utilizing holographic lenses,” US Patent, 3940204 A (1975).
2. R. A. Farrar, “Helmet-mounted holographic aiming sight,” US Patent, 3633988 A (1970).
3. I. Kasai, Y. Tanijiri, T. Endo, and H. Ueda, “A practical see-through head mounted display using a holographic optical element,” Opt. Rev. 8, 241–244 (2001). [CrossRef]
4. H. Mukawa, K. Akutsu, I. Matsumura, S. Nakano, T. Yoshida, M. Kuwahara, and K. Aiki, “A full-color eyewear display using planar waveguides with reflection volume holograms,” J. Soc. Inf. Display 17, 185–193 (2009). [CrossRef]
5. H.-J. Yeom, H.-J. Kim, S.-B. Kim, H. Zhang, B. Li, Y.-M. Ji, S.-H. Kim, and J.-H. Park, “3D holographic head mounted display using holographic optical elements with astigmatism aberration compensation,” Opt. Express 23, 32025–32034 (2015). [CrossRef] [PubMed]
6. J. A. Piao, G. Li, M. L. Piao, and N. Kim, “Full color holographic optical element fabrication for waveguide-type head mounted display using photopolymer,” J. Opt. Soc. Korea 17, 242–248 (2013). [CrossRef]
7. M. K. Hedili, M. O. Freeman, and H. Urey, “Transmission characteristics of a bidirectional transparent screen based on reflective microlenses,” Opt. Express 21, 24636–24646 (2013). [CrossRef] [PubMed]
8. M.-L. Piao, K.-C. Kwon, H.-J. Kang, K.-Y. Lee, and N. Kim, “Full-color holographic diffuser using time-scheduled iterative exposure,” Appl. Opt. 54, 5252–5259 (2015). [CrossRef] [PubMed]
9. G. Jang, C.-K. Lee, J. Jeong, G. Li, S. Lee, J. Yeom, K. Hong, and B. Lee, “Recent progress in see-through three-dimensional displays using holographic optical elements,” Appl. Opt. 55, A71–A85 (2016). [CrossRef] [PubMed]
10. K. Wakunami, P.-Y. Hsieh, R. Oi, T. Senoh, H. Sasaki, Y. Ichihashi, M. Okui, Y.-P. Huang, and K. Yamamoto, “Projection-type see-through holographic three-dimensional display,” Nat. Commun. 7, 12954 (2016). [CrossRef] [PubMed]
11. M. Yamaguchi, “Light-field and holographic three-dimensional displays,” J. Opt. Soc. Am. A 33, 2348–2364 (2016). [CrossRef]
12. T. Nakamura and M. Yamaguchi, “Rapid calibration of a projection-type holographic light-field display using hierarchically upconverted binary sinusoidal patterns,” Appl. Opt. 56, 9520–9525 (2017). [CrossRef] [PubMed]
13. M. Yamaguchi and R. Higashida, “3D touchable holographic light-field display,” Appl. Opt. 55, A178–A183 (2016). [CrossRef] [PubMed]
14. M. Zhou, O. Matoba, Y. Kitagawa, Y. Takizawa, T. Matsumoto, H. Ueda, A. Mizuno, and N. Kosaka, “Fabrication of an integrated holographic imaging element for a three-dimensional eye-gaze detection system,” Appl. Opt. 49, 3780–3785 (2010). [CrossRef] [PubMed]
15. T. Kasezawa, H. Horimai, H. Tabuchi, and T. Shimura, “Holographic window for solar power generation,” Opt. Rev. 23, 997–1003 (2016). [CrossRef]
16. V. Bavigadda, R. Jallapuram, E. Mihaylova, and V. Toal, “Electronic speckle-pattern interferometer using holographic optical elements for vibration measurements,” Opt. Lett. 35, 3273–3275 (2010). [CrossRef] [PubMed]
17. M. Kumar and C. Shakher, “Measurement of temperature and temperature distribution in gaseous flames by digital speckle pattern shearing interferometry using holographic optical element,” Opt. Laser. Eng. 73, 33–39 (2015). [CrossRef]
18. P. Blanche, A. Bablumian, R. Voorakaranam, C. Christenson, W. Lin, T. Gu, D. Flores, P. Wang, W.-Y. Hsieh, M. Kathaperumal, B. Rachwal, O. Siddiqui, J. Thomas, R. A. Norwood, M. Yamamoto, and N. Peyghambarian, “Holographic three-dimensional telepresence using large-area photorefractive polymer,” Nature 468, 80–83 (2010). [CrossRef] [PubMed]
19. D. Ochi, A. Kamera, K. Takahashi, M. Makiguchi, and K. Takeuchi, “VR technologies for rich sports experience,” in SIGGRAPH (ACM, 2016), pp. 21.
20. T. Nakamura, S. Kimura, K. Takahashi, Y. Aburakawa, S. Takahashi, S. Igarashi, and M. Yamaguchi, “HOE-based screen for virtual-image projection and scene capture,” in International Display Workshops (ITE, 2017), 3D4-4.
21. Y. Lv, X. Zhang, D. Zhang, L. Zhang, Y. Luo, and J. Luo, “Reduction of blurring in broadband volume holographic imaging using a deconvolution method,” Biomed. Opt. Express 7, 7516–7524 (2016). [CrossRef]
22. M. Grosse, G. Wetztein, A. Grundhöfer, and O. Bimber, “Coded aperture projection,” ACM Trans. Graph. 29, 1–12 (2010). [CrossRef]
23. S. Lee, C. Jang, S. Moon, J. Cho, and B. Lee, “Additive light field displays: realization of augmented reality with holographic optical elements,” ACM Trans. Graph. 35, 60 (2016). [CrossRef]
24. D. J. De Bitetto, “White-light viewing of surface holograms by simple dispersion compensation,” Appl. Phys. Lett. 9, 417–418 (1966). [CrossRef]
25. N. F. Hartman, “Heads-up display with holographic dispersion correcting,” US Patent, 4613200 A (1986).
26. T. Kubota, “Image sharpening of Lippmann hologram by compensation of wavelength dispersion,” Proc. SPIE 1051, 12–17 (1989). [CrossRef]
27. C. B. Burckhardt, “Display of holograms in white light,” Bell Syst. Tech. J. 45, 1841–1844 (1966). [CrossRef]
28. P. G. Boj, M. Pardo, and J. A. Quintana, “Display of ordinary transmission holograms with a white light source,” Appl. Opt. 25, 4146–4149 (1986). [CrossRef] [PubMed]
29. J. N. Latta, “Analysis of multiple hologram optical elements with low dispersion and low aberrations,” Appl. Opt. 11, 1686–1696 (1972). [CrossRef] [PubMed]
30. H. Madjii-Zolbanine and C. Froehly, “Holographic correction of both chromatic and spherical aberrations of single glass lenses,” Appl. Opt. 18, 2385–2393 (1979). [CrossRef]
31. A. Klein, “Dispersion compensation for reflection holography,” Master Thesis, Massachusetts Institute of Technology, (1996).
32. M. Yamaguchi, N. Ohyama, and T. Honda, “Holographic three-dimensional printer: new method,” Appl. Opt. 31, 217–222 (1992). [CrossRef] [PubMed]
33. M. Klug, M. Holzbach, and A. Ferdman, “Method and apparatus for recording 1-step full-color full-parallax holographic stereograms,” US Patent, US6330088B1 (1998).
34. D. Brotherton-Ratcliffe, F. M. Vergnes, A. Rodin, and M. Grichine, “Holographic Printer,” US Patent, US7800803B2 (1999).
35. K. Hong, S. Park, J. Yeom, J. Kim, N. Chen, K. Pyun, C. Choi, S. Kim, J. An, H.-S. Lee, U. Chung, and B. Lee, “Resolution enhancement of holographic printer using a hogel overlapping method,” Opt. Express 21, 14047–14055 (2013). [CrossRef] [PubMed]
36. J. Jeong, J. Yeom, C. Jang, C.-K. Lee, K. Hong, and B. Lee, “Viewing characteristics improved integral imaging system using holographic printing technique,” in Imaging and Applied Optics (OSA, 2015), paper JT5A.1. [CrossRef]
37. T. Yamaguchi, O. Miyamoto, and H. Yoshikawa, “Volume hologram printer to record the wavefront of three-dimensional objects,” Opt. Eng. 51, 075802 (2012). [CrossRef]
38. W. Nishi and K. Matsushima, “A wavefront printer using phase-only spatial light modulator for producing computer-generated volume holograms,” Proc. SPIE 9006, 90061F (2014). [CrossRef]
39. K. Wakunami, R. Oi, T. Senoh, H. Sasaki, Y. Ichihashi, and K. Yamamoto, “Wavefront printing technique with overlapping approach toward high definition holographic image reconstruction,” Proc. SPIE 9867, 98670J (2016).
