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Abstract
Two-dimensional (2D)/three-dimensional (3D) convertible or mixed display is one of the most important factors for the fast penetration of 3D display into the display market. In this paper, we propose a 2D/3D mixed frontal projection system that mainly contains a liquid crystal micro-lens array (LCMLA) and a quarter-wave retarding film with pinholes (QWRF-P). The LCMLA exhibits the focusing effect or no optical effect depending on the polarization direction of the incident lights. The forward incident lights pass through the LCMLA without any bending. After passing through the QWRF-P twice, half of the backward lights change the polarization direction with 90°, and the other half remains. Using our designed system, different display modes, including 2D display, 3D display, and 2D/3D mixed display, can be realized. The unique feature of the proposed 2D/3D mixed frontal projection system is that it can switch the display modes by simply changing the image sources without the need of any active optical devices. Moreover, the proposed system is compact, simple and space-efficient, which is suitable for the application in glassless 3D cinema and home 3D theatre.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, integral imaging has been given considerable attention, because it can reconstruct the light field of the three-dimensional (3D) scene and provide full parallaxes, quasi-continuous viewing points and various physiological depth cues [1–6]. Projection-type integral imaging is the easiest way to provide large-sized display and can be widely used in exhibitions, movies, advertisements, and so on [7–9]. The front projection-type integral imaging, also called reflection-type integral imaging, were studied by many researchers, since it can largely increase the spacing efficiency. Several front projection-type integral imaging systems, such as using micro-concave mirror array [10] and micro-convex mirror array [11] can achieve good 3D display effect. The manufacturing cost of the micro-concave or micro-convex mirror array is high. The combination of a large aperture lens and a lens array also can implement the frontal projection-type integral imaging [12]. The recently proposed single frontal projection system by using a liquid crystal lens array and a quarter-wave retarding film has compact system architecture [13].
Two dimensional (2D) / 3D convertible or mixed feature is one of the most important factors for the fast penetration of 3D display into the display market. Former researches have performed the researches of 2D/3D convertible display by using liquid crystal active barrier [14–15], liquid crystal active lens [16–19], the convertible back light units [20–21], and lens-array holographic optical elements [22–23]. Most of them adopt the active devices as the core component for implementing 2D/3D conversion which makes it difficult in the large-sized application. A method for implementing 2D/3D convertible feature in the projection-type integral imaging by using concave half mirror array doesn’t need any active devices for the 2D/3D switching [24]. But two projectors are needed, one is rear projector for 2D displays, and the other is front projector for 3D displays, which increases the system complexity and wastes the projection room.
For the viewers who are already accustomed to the high resolution of current 2D displays, 3D experience with severely reduced image resolution is not pleased although 3D displays can provide depth information that is absent in 2D displays. In a 3D image, the depth of background often closes to the display panel, which almost equals to zero, but it still suffers from the reduced image resolution. Hence, 2D/3D mixed or hybrid display was needed in which a full-resolution 2D background displayed on 2D display mode can compensate the resolution of background, meanwhile, a 3D foreground image displayed on 3D display mode can maintain the depth information of foreground. Chou. et al. proposed a 2D/3D hybrid integral imaging display by using a liquid crystal micro-lens array (LCMLA) [25], in which the LCMLA has the focusing effect or no optical effect determined by the polarization state of the incident light. However, an active device (twisted nematic cell) is required to quickly switch the 2D and 3D display modes, and it cannot be implemented in a frontal projection system. In our former work, we proposed a 2D/3D mixed display by adding a switchable diffuser element and a front projector in front of an integral imaging display device [26], in which the front projector is used for 2D display and the integral imaging display device reconstructs 3D images. However, the system is complex in structure and high cost.
To address the above problems, this study presents a 2D/3D mixed frontal projection system based on integral imaging. The main innovations of our approach are summarized as follows: the proposed system can realize 2D display, 3D display and 2D/3D mixed display; it is simple and compact; and any active device is not needed for the switching between different display modes.
2. Structure and principle of the 2D/3D mixed frontal projection system
Figure 1 shows the structure of the proposed 2D/3D mixed frontal projection system. It mainly consists of a projector, a polarizer, a polarization-dependent LCMLA, a quarter-wave retarding film with pinholes (QWRF-P), and a polarization-preserving screen (PPS). The projector projects 2D, 3D, or 2D/3D mixed image source according to the display mode of the system. The polarization-dependent LCMLA contains a concave-planar isotropic polymer layer, a planar-convex optical positive liquid crystal layer, an alignment layer and a glass substrate. The liquid crystal layer has optically birefringence property with the ordinary refractive index (no) and the extraordinary refractive index (ne), and the liquid crystal molecule is oriented along the x-axis. The refractive index (np) of the isotropic polymer layer matches with the ordinary refractive index of the liquid crystal material. As a result, the polarization-dependent LCMLA exhibits focusing effect to the x-polarized light and no optical effect to the y-polarized light. The QWRF-P is actually a quarter-wave retarding film with pinholes arranged evenly on it, and it contains quarter-wave retarding units and pinhole units. The quarter-wave retarding units and pinhole units alternately arrange with the uniform unit size. The QWRF-P can be fabricated by punching pinholes on a quarter-wave retarding film. The PPS locating at the imaging plane of the projector, functions as a receiving screen to receive the projected 2D, 3D, or 2D/3D mixed image source, and reflect the light with its polarization state preserved.
Figure 2 shows the propagation path of the projected light in the proposed system. The forward light projected from the projector goes through the polarizer and turns to y-polarized light shown as red arrows. Because the polarization state of the forward light is paralleled to the fast axis of the positive liquid crystal layer, and the ordinary refractive index of the liquid crystal layer no matches with the refractive index of the isotropic polymer layer np, the forward light passes through the polarization-dependent LCMLA directly without any bending. When arriving at the QWRF-P, the forward y-polarized light is divided into two parts. One half meets the quarter-wave retarding units (with its fast axis 45° oriented with respect to the x-axis) on the QWRF-P, thus becomes left-circularly polarized lights shown as light blue arrows. The PPS preserves the polarization state and reverses the propagation direction of the light. After reflected back by the PPS, the left-circularly polarized light is converted into right-circularly polarized light shown as dark blue arrows due to half wave loss. Then, the backward right-circularly polarized light passes through the quarter-wave retarding unit (with its fast axis 135° oriented) and converts into x-polarized light shown as green arrows. Because the incident polarization state of the x-polarized light is paralleled to the slow axis of the positive liquid crystal layer, and the extraordinary refractive index of the liquid crystal layer ne is higher than the refractive index of the isotropic polymer layer np, the x-polarized light is periodically refracted and focused at the focal plane to achieve 3D display. While, the other half of the forward y-polarized light meets the pinhole units on the QWRF-P and passes through without change of polarization state. Then, the y-polarized light is reflected back by the PPS, and passes through the pinhole units of the QWRF-P again with its polarization state maintaining. As a result, the backward y-polarized light directly passes through the polarization-dependent LCMLA to achieve 2D display. Therefore, 2D display, 3D display or 2D/3D mixed display can be easily achieved by simply changing the projected image sources of the projector.
Fig. 2. Light propagation path in the proposed system, (a) forward light path and (b) backward light path.
Since the quarter-wave retarding units and the pinhole units on the QWRF-P are staggered arranged evenly, and only half of the projected light can be effectively used to present 2D, 3D, or 2D/3D mixed images, the image to be projected should be sampled with the sampling interval being equal to the unit size of the QWRF-P. As a result, the resolution of the proposed system is reduced by half, and the uniformity of the projected image is determined by the unit size of the QWRF-P. Inspired by the methods proposed in [27–28], a movable QWRF-P could be used to improve the image resolution. By fast moving the QWRF-P along the horizontal or vertical axis back and forth with its moving step being equal to the unit size of the QWRF-P, and switching the projected images synchronously, 2D, 3D or 2D/3D mixed images with full resolution can be obtained due to the visual retention effect.
To effectively improve the uniformity of the projected image, a high-density QWRF-P is needed. Thus, we also proposed to superimpose two QWRF-Ps to enhance the density of QWRF-P. Two QWRF-Ps with the same unit sizes p are superimposed with a shifting of half unit in both horizontal and vertical directions. So, we get four kinds of phase retarding situations, as shown in Fig. 3.
For situation i), half-wave retarding is obtained, so that, the polarization state of the light isn’t changed after passing through the two QWRF-Ps twice. For situations ii) and iii), quarter-wave retarding is obtained, so that, the polarization state of the light changes to the orthogonal state after passing through twice. For situation iv), there is no phase retardance, and the polarization state of the light doesn’t change similar to situation i). As a result, the two superimposed QWRF-Ps with the unit size of p is equivalent to one QWRF-P with the unit size of p/2. Therefore, the uniformity of the projected image is doubled in both horizontal and vertical directions.
3. Experimental results
In the experiment, we developed an experimental prototype based on the proposed 2D/3D mixed frontal projection approach, as shown in Fig. 4. The detailed parameters of the prototype are listed in Table 1. The projector has the resolution of 1280 × 800, the projection distance is 110 mm, and the projected size is 4 inch. Thus, the pixel pitch of the projected image is 62.5 µm. The polarization-dependent LCMLA has the lens pitch of 1 mm, focal length of 9.9 mm and size of 50 mm × 50 mm, as shown in Fig. 5(a). Figures 5(b) and 5(c) show good focusing effect of the polarization-dependent LCMLA to the x-polarized light and no optical effect to the y-polarized light, respectively. The QWRF-P was fabricated by punching pinholes on a quarter-wave retarding film, and it contains 50 × 50 pinholes with the pinholes size of 1 mm, as shown in Fig. 5(d). The central operating wavelength of the quarter-wave retarding unit is 550 nm, and the retardation errors are 2.9%, 1.1% and 7.0% at 638 nm, 520 nm and 450 nm, respectively, as shown in Fig. 5(e). Thus, the quarter-wave retarding unit has generally good phase retardation in the entire visible wavelength range of the projector.
Fig. 5. (a) Polarization-dependent LCMLA, (b) focusing and (c) no optical effects of the polarization-dependent LCMLA, (d) QWRF-P and (e) retardation errors curve of the QWRF-P.
Table 1. Specifications of the developed 2D/3D mixed frontal projection prototype.
Figure 6(a) shows the 3D scene. The foreground includes a rocket and a Jupiter which have distinct 3D depths, and the background includes sky and mountains whose depth are zero. We generated the light field image of the 3D foreground and the 2D/3D mixed image source, as shown in Figs. 6(b) and (c), respectively.
Figure 7 shows the 2D display and 3D display results of the frontal projection prototype. Although the experimental results coarsely verified the proposed approach, the uniformity of the displayed 2D and 3D images are poor. Because the punching accuracy of the drilling machine is low and the unit size of the QWRF-P is big. To improve the image uniformity, we superimposed two QWRF-Ps with half-unit shift as we proposed in Fig. 3. The modified 2D display and 3D display results are shown in Fig. 8. Compared to Fig. 7, the prototype with two QWRF-Ps significantly improved the image clarity and uniformity. The profiles and edge details of both 2D and 3D images are clearer.
Fig. 7. (a) 2D display, (b) left and (c) right views of 3D display of the frontal projection prototype using one QWRF-P.
Fig. 8. (a) 2D display, (b) left and (c) right views of 3D display of the frontal projection prototype using two QWRF-Ps.
Figure 9 shows the 2D/3D mixed display results of the frontal projection prototype by using two QWRF-Ps. The top, bottom, center, left and right views confirm the successful reconstruction of the 3D foreground. The relative movements between the Jupiter and the rocket in both horizontal and vertical directions are obvious. As shown in the red rectangles, the rocket moves up and the Jupiter moves down when the viewing points changes from top to bottom. The gap between the rocket and Jupiter is getting farther when the viewing points changes from left to right, as shown in the yellow rectangles. The boosters of the rocket show obvious relative movement in horizontal direction.
4. Conclusions
In this paper, a 2D/3D mixed frontal projection system was proposed. The polarization-dependent LCMLA has focusing and transparency effects according to the polarized state of the light. The forward light projected by the projector goes through the system without any bending which makes the frontal projection is possible. Half of the backward light was modulated to display 3D images, while another half wasn’t modulated to display 2D images. The developed experimental prototype verified the effectiveness of proposed approach, and realized 2D, 3D, and 2D/3D mixed display. The switching between different display modes needs no active device, and the system is compact, simple and space-efficient. The image uniformity was also improved by superimposing two QWRF-Ps with half-unit shifting.
QWRF-P and polarization-dependent LCMLA are the main factors which degrades the image quality. To further improve the qualities of 2D, 3D, and 2D/3D mixed images, the following solutions could be helpful. Firstly, a drilling machine with higher precision can fabricate QWRF-Ps with higher density and smaller unit size, so that the image uniformity will be greatly improved. Secondly, a polarization-dependent LCMLA with better focusing and transparent effects could improve the image definition. Thirdly, a polarization-dependent LCMLA with small focal length could enlarge the 3D viewing angle, so that viewers can experience better motion parallax. Fourthly, by adding a polarization switcher and combing the time-division multiplexing technology, the quarter-wave retarding unit and pinhole unit both can work for 3D image or 2D image on different states, thus the resolutions of the 2D, 3D, 2D/3D mixed image could be doubled. The proposed system could be a potential candidate for the practical application in the future glasses-free 3D cinema and home 3D theatre.
Funding
National Natural Science Foundation of China (61775151, 61975138).
Disclosures
The authors declare no conflicts of interest.
References
1. M. Martínez-Corral and B. Javidi, “Fundamentals of 3D imaging and displays: a tutorial on integral imaging, light field, and plenoptic systems,” Adv. Opt. Photonics 10(3), 512–566 (2018). [CrossRef]
2. S. J. Xing, X. Z. Sang, X. Yu, D. Chen, P. Bo, and G. Xin, “High-efficient computer-generated integral imaging based on the backward raytracing technique and optical reconstruction,” Opt. Express 25(1), 330–338 (2017). [CrossRef]
3. M. Guo, Y. Si, Y. Lyu, S. Wang, and F. Jin, “Elemental image array generation based on discrete viewpoint pickup and window interception in integral imaging,” Appl. Opt. 54(4), 876–884 (2015). [CrossRef]
4. H. Hua and B. Javidi, “A 3D integral imaging optical see-through head-mounted display,” Opt. Express 22(11), 13484–13491 (2014). [CrossRef]
5. J. Wei, S. Wang, Y. Zhao, and M. Piao, “Synthetic aperture integral imaging using edge depth maps of unstructured monocular video,” Opt. Express 26(26), 34894–34908 (2018). [CrossRef]
6. J. L. Zhang, X. R. Wang, Y. J. Chen, Q. P. Zhang, S. Yu, Y. Yuan, and B. T. Guo, “Feasibility study for pseudoscopic problem in integral imaging using negative refractive index materials,” Opt. Express 22(17), 20757–20769 (2014). [CrossRef]
7. H. Liao, M. Iwahara, N. Hata, and T. Dohi, “High-quality integral videography using a multiprojector,” Opt. Express 12(6), 1067–1076 (2004). [CrossRef]
8. J. S. Jang and B. Javidi, “Depth and lateral size control of three-dimensional images in projection integral imaging,” Opt. Express 12(16), 3778–3790 (2004). [CrossRef]
9. L. Zhang, Y. Yang, X. Zhao, Z. Fang, and X. Yuan, “Enhancement of depth-of-field in a direct projection-type integral imaging system by a negative lens array,” Opt. Express 20(23), 26021–26026 (2012). [CrossRef]
10. Y. Jeong, S. Jung, J. H. Park, and B. Lee, “Reflection-type integral imaging scheme for displaying three dimensional images,” Opt. Lett. 27(9), 704–706 (2002). [CrossRef]
11. J. S. Jang and B. Javidi, “Three-dimensional projection integral imaging using micro-convex-mirror arrays,” Opt. Express 12(6), 1077–1083 (2004). [CrossRef]
12. M. Okui, J. Arai, Y. Nojiri, and F. Okano, “Optical screen for direct projection of integral imaging,” Appl. Opt. 45(36), 9132–9139 (2006). [CrossRef]
13. Z. Wang, M. Xu, G. Lv, Q. Feng, A. Wand, and H. Ming, “Single frontal projection autostereoscopic three-dimensional display using a liquid crystal lens array,” Opt. Express 28(2), 1621–1630 (2020). [CrossRef]
14. S. U. Kim, J. Kim, J. H. Suh, J. H. Na, and S. D. Lee, “Concept of active parallax barrier on polarizing interlayer for near-viewing autostereoscopic displays,” Opt. Express 24(22), 25010–25018 (2016). [CrossRef]
15. Y. A. Zhang, T. Jin, L. C. He, Z. H. Chu, T. L. Guo, X. T. Zhou, and Z. X. Lin, “Controllable liquid crystal gratings for an adaptive 2D/3D auto-stereoscopic display,” Opt. Commun. 384, 16–24 (2017). [CrossRef]
16. B. Lee and J. H. Park, “Overview of 3D/2D switchable liquid crystal display technologies,” Proc. SPIE 7618, 761806 (2010). [CrossRef]
17. L. Shi, A. K. Srivastava, A. M. W. Tam, V. G. Chigrinov, and H. S. Kwok, “2D–3D switchable display based on a passive polymeric lenticular lens array and electrically suppressed ferroelectric liquid crystal,” Opt. Lett. 42(17), 3435–3438 (2017). [CrossRef]
18. M. Park, H. Park, K. Joo, T. Lee, and H. Kim, “Polarization-dependent liquid crystalline polymeric lens array with aberration-improved aspherical curvature for low 3D crosstalk in 2D/3D switchable mobile multi-view display,” Opt. Express 26(16), 20281–20297 (2018). [CrossRef]
19. C. Zhang, X. Tang, C. Chen, G. Huang, H. Cui, B. Si, T. C. Chung, and T. S. Jen, “Design of GRIN liquid crystal lens for autostereoscopic 2D/3D switchable display,” Solid State Phenom. 181-182, 301–304 (2011). [CrossRef]
20. J. H. Jung, Y. Kim, Y. Kim, J. Kim, K. Hong, and B. Lee, “Integral imaging system using an electroluminescent film backlight for three-dimensional-two-dimensional convertibility and a curved structure,” Appl. Opt. 48(5), 998–1007 (2009). [CrossRef]
21. S. W. Cho, J. H. Park, Y. Kim, H. Choi, J. Kim, and B. Lee, “Convertible two-dimensional-three-dimensional display using an LED array based on modified integral imaging,” Opt. Lett. 31(19), 2852–2854 (2006). [CrossRef]
22. H. L. Zhang, H. Deng, J. J. Li, M. Y. He, D. H. Li, and Q. H. Wang, “Integral imaging-based 2D/3D convertible display system by using holographic optical element and polymer dispersed liquid crystal,” Opt. Lett. 44(2), 387–390 (2019). [CrossRef]
23. K. Hong, J. Yeom, C. Jang, G. Li, J. Hong, and B. Lee, “Two-dimensional and three-dimensional transparent screens based on lens-array holographic optical elements,” Opt. Express 22(12), 14363–14374 (2014). [CrossRef]
24. J. Hong, Y. Kim, S. Park, J. Hong, S. Min, S. Lee, and B. Lee, “3D/2D convertible projection-type integral imaging using concave half mirror array,” Opt. Express 18(20), 20628–20637 (2010). [CrossRef]
25. P. Y. Chou, J. Y. Wu, S. H. Huang, C. P. Wang, Z. Qin, C. T. Huang, P. Y. Hsieh, H. H. Lee, T. H. Lin, and Y. P. Huang, “Hybrid light field head-mounted display using time-multiplexed liquid crystal lens array for resolution enhancement,” Opt. Express 27(2), 1164–1177 (2019). [CrossRef]
26. H. Ren, Y. Xing, H. L. Zhang, Q. Li, L. Wang, H. Deng, and Q. H. Wang, “2D/3D mixed display based on integral imaging and a switchable diffuser element,” Appl. Opt. 58(34), G276–G281 (2019). [CrossRef]
27. J. Y. Zou, T. Zhan, J. H. Xiong, and S. T. Wu, “Increasing the pixel density for VR displays with a polarization grating,” J. Soc. Inf. Disp. 28(4), 315–323 (2020). [CrossRef]
28. J. Y. Wu, P. Y. Chou, K. Peng, Y. P. Huang, H. Lo, C. Chang, and F. Chuang, “Resolution enhanced light field near eye display using e-shifting method with birefringent plate,” J. Soc. Inf. Disp. 26(5), 269–279 (2018). [CrossRef]
