Introduction. Sharp, vivid, and large three-dimensional (3D) color images can be reconstructed using the traditional optical holography [1]. Recently, full-parallax high-definition computer-generated holograms (FPHD-CGHs) with tens of billions of pixels can also reconstruct amazing, deep 3D scenes in monochrome [2,3]. The latest pixel scale has reached 0.18 to 0.36 trillion pixels [4,5]. A technique using red–green–blue (RGB) color filters was proposed to realize the full-color reconstruction of FPHD-CGHs [6]. However, this technique requires an RGB laser emitting monochromatic light at three wavelengths to reconstruct high-quality color 3D images because of the broadband spectral property of RGB color filters [7].

To use conventional white LEDs as the illumination instead, a technique using computer-generated volume holograms (CGVHs) is an attractive alternative for reconstructing full-color 3D images in CGH. Because the fringe of a volume hologram cannot be directly printed by ordinary printing devices, wavefront printers have been proposed for printing CGVHs [8–10], and full-color CGVHs printed using wavefront printers at RGB wavelengths have also been reported [11–13]. Unfortunately, these full-color 3D images are blurred and unclear. This is most likely because the interference fringe was printed by tiling very small CGVH segments.

A technique that stacks three reflection CGHs whose fringe patterns are printed on dichroic mirror films has been proposed for full-color CGH reconstruction [14]. However, sophisticated film and pattern formation technologies are required to fabricate reflection CGHs with dichroic mirror films. To avoid this problem, we proposed a technique for stacked CGVH [15]. In this technique, three reflection CGVHs for RGB colors are stacked for full-color reconstruction (Fig. 1). The stacked CGVH can reconstruct high-quality full-color 3D images. Unlike a holographic stereogram printed by a holographic printer [16], the stacked CGVH produces not only binocular disparity but also a proper accommodation of the eye. Because the 3D image does not cause any sensory conflict, we can reconstruct very deep 3D scenes with the stacked CGVH. However, the size of the stacked CGVH is limited by the power of the laser used. In this paper, we propose a novel technique to increase the size of the stacked CGVHs without sacrificing their quality.

 

Fig. 1. Principle of full-color reconstruction in stacked CGVHs.

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Principle. A stacked CGVH is composed of three CGVHs fabricated by contact-copy of the original amplitude CGHs, whose fringes are printed using laser lithography as a pattern of chromium (Cr) thin films on a glass substrate [15]. We attach a photopolymer to the glass substrate and irradiate the original CGH with transfer laser light through the photopolymer (Fig. 2(a)). The Cr thin film has a high reflectance, and hence the 3D image reconstructed by the original printed CGH is transferred to the photopolymer as a reflection volume hologram. Because of the wavelength selectivity of the volume hologram, the CGVH illuminated with white light reconstructs almost monochromatic 3D images at the wavelength of the transfer light. Consequently, the three stacked CGVHs in Fig. 1 can be used to obtain full-color 3D images.

 

Fig. 2. (a) Conventional contact-copy using a Cr reflection fringe, where the transfer light agrees with the reference wave used to calculate the fringe pattern of the original CGH. (b) Problem with tiling the contact-copy of amplitude CGHs.

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In this technique, however, the maximum size of the CGVH is determined by the power of the laser used as the light source of the transfer light, because the transfer light must irradiate the entire area of the original CGH where the photopolymer is attached. If the intensity of the transfer light is lower than a threshold, the 3D image of the original CGH cannot be transferred to the photopolymer. Specifically, when using a 100-mW laser, the maximum area that can be transferred is approximately 5 × 5 cm².

To increase the size beyond the limit imposed by the laser power, we introduce the tiling contact-copy (TCC) technique (Fig. 3). In this technique, rectangular collimated transfer light, formed by a collimator lens and aperture, irradiates part of the original CGH through the photopolymer. The whole area is transferred by moving the original CGH and photopolymer using an automatic stage. Because laser irradiation is performed while stopping the stage operation in each rectangular area, the contact-copy is not affected by stage vibrations.

 

Fig. 3. Principle of tiling contact-copy (TCC).

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Two problems typically arise in practice when fabricating a large CGVH using TCC. First, in the original contact-copy technique (Fig. 2(a)), the transfer light is the same as the reference wave used to calculate the fringe pattern of the original CGH [15]. This ensures that the transfer light exactly reconstructs the 3D image recorded on the original CGH. Note that the reference wave of the original CGH is commonly a spherical wave. However, the transfer light of the TCC must be collimated light that is incident perpendicularly to the original CGH, as shown in Fig. 3. Additionally, when irradiating a tile through the aperture in the TCC, as shown in Fig. 2(b), part of the light diffracted by the original CGH spills out into the area of the neighboring tile in the photopolymer layer. As a result, the whole diffracted light is not recorded in the CGVH; i.e., there are regions of the fringe at the edge of the tile where the reconstructed image of the original CGH is incompletely transferred to the CGVH. This results in visible boundaries between the tiles of the resultant CGVH.

To resolve these problems, we use a phase-only reflection CGH instead of an amplitude CGH for the original CGH in the TCC (Fig. 4). The phase-only fringe, formed as a transparent surface relief of the photoresist, is printed using laser lithography. A reflection layer of a Cr thin film is formed between the surface relief and glass substrate. The transfer light irradiates the surface relief through the photopolymer, and the 3D image of the original CGH is reconstructed by the transfer light that passes through the transparent surface relief twice by the reflection of the Cr layer. In this technique, the photopolymer can be closely attached to the original printed CGH such that there is no large gap between them. Therefore, visible boundaries are not caused in the resultant CGVH, unlike when an amplitude CGH is used for the original CGH. Additionally, phase-only CGHs have the advantage of a high diffraction efficiency.

 

Fig. 4. Contact-copy of a phase-only reflection CGH made of photoresist and Cr thin film.

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Fabrication and optical reconstruction. An actual stacked CGVH was fabricated using TCC. The 3D scene recorded in the original CGHs and the parameters are shown in Fig. 5 and Table 1, respectively. Each object wave field $O(x,y)$ of the three original CGHs for the primary colors, approximately 34 × 10⁹ pixels in size, was calculated in full parallax using the polygon-based method [17]. The silhouette method was used to process occlusions [18]. The phase-only fringe pattern of the object wave fields are calculated by

 

Fig. 5. Three-dimensional scene of the fabricated stacked CGVH. Red arrows and figures indicate the depth of the points.

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Table 1. Parameters of the Original CGHs and 3D Scene

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Figure 6 shows the TCC setup. The beams of three lasers are combined into a coaxial beam using dichroic mirrors and then converted to rectangular collimated transfer light using a spatial filter, a lens, and apertures. Note that the aperture is duplicated to avoid the strong effects of the complicated reflection light from the original CGH. Only one laser is turned on; the other lasers are simply switched off for CGVH fabrication. The TCC parameters are summarized in Table 2. The overall size of each CGVH is the same as that of the original CGH, i.e., 104.8 mm × 104.8 mm.

 

Fig. 6. Setup for TCC. M, mirror; DM, dichroic mirror; S, shutter; SF, spatial filter; CL, collimator lens.

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Table 2. Summary of the TCC Parameters

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Figure 7(a) shows photographs of the optical reconstruction of the three monochrome CGVHs transferred using TCC. Here, a white pigtail LED is used for the illumination light source, and the camera is focused on the object images. Figure 7(b) shows a photograph capturing the surface of the same green CGVH. We cannot detect the boundaries in these photographs. In contrast, there are visible boundaries in Fig. 7(c), which captures the surface of another green CGVH fabricated by transferring an amplitude CGH.

 

Fig. 7. Photographs of the optical reconstruction of the monochrome CGVHs. (a) Photos of individual CGVHs focused on the object image. (b) Photo of the green CGVH focused on the surface of the CGVH. (c) Photo of another green CGVH transferred from the amplitude CGH and focused on the surface of the CGVH.

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A full-color reconstruction obtained by stacking three CGVHs is shown in Fig. 8, where a pigtail white LED is again used for illumination. A brilliant 3D image reconstructed by the stacked CGVH is shown in Figs. 8(a)–8(c). In addition, Fig. 8(d) reveals that we cannot detect the boundaries of the tiles.

 

Fig. 8. Photographs of the optical reconstruction of the stacked CGVH (see Visualization 1). Photos taken from the (a) center, and (b) left and right viewpoints. Photos focused on the (c) background image and (d) surface of the stacked CGVH.

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Discussion. We calculated the phase-only fringe of the original CGH using a spherical reference wave to avoid problems with conjugate images and non-diffraction light. These problems should not occur in phase modulation CGHs theoretically. However, in practice, conjugate images and non-diffraction light are caused in the phase-only CGH because the phase modulation with the surface relief is not perfect. Therefore, we need an off-axis reference wave to generate the fringe pattern of the original printed CGH.

Here, the reconstructed light of the original CGH is transferred to the photopolymer using a collimated transfer light that is approximately a plane wave. Hence, the illumination light of the original CGH is different from the reference wave used to calculate the fringe pattern. Nevertheless, when lighting the stacked CGVH with the same spherical wave as the reference wave, the designed 3D image of the original CGHs is reconstructed normally. It is difficult to explain the reason for this based on volume hologram theories. However, we can explain the reason if the CGVH can be treated as a thin amplitude hologram.

When the object is far from the hologram, the amplitude distribution of the object wave is approximately constant because it spreads over the whole of the original CGH. In this case, because the denominator of Eq. (1) is constant, the phase-only fringe can be represented as

In summary, we successfully used TCC to create a 10-cm square quality full-color stacked CGVH whose three original CGHs are composed of approximately 34 billion pixels. The viewing angle is estimated to be 35.5° in blue color in both horizontal and vertical directions. Moreover, in this technique, the size of the stacked CGVH is no longer dependent on the laser power.