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Optical See-through 2D/3D Compatible Display Using Variable-Focus Lens and Multiplexed Holographic Optical Elements

Optical See-through 2D/3D Compatible Display Using Variable-Focus Lens and Multiplexed... hv photonics Article Optical See-through 2D/3D Compatible Display Using Variable-Focus Lens and Multiplexed Holographic Optical Elements 1 1 , 2 1 1 1 Qinglin Ji , Huan Deng *, Hanle Zhang , Wenhao Jiang , Feiyan Zhong and Fengbin Rao College of Electronics and Information Engineering, Sichuan University, Chengdu 610065, China; jiqinglin@stu.scu.edu.cn (Q.J.); jiang7ogremy@163.com (W.J.); zhongfeiyan@stu.scu.edu.cn (F.Z.); raofengbin@stu.scu.edu.cn (F.R.) School of Instrumentation Science and Optoelectronics Engineering, Beihang University, Beijing 100191, China; hanlezhang@buaa.edu.cn * Correspondence: huandeng@scu.edu.cn Abstract: An optical see-through two-dimensional (2D)/three-dimensional (3D) compatible display using variable-focus lens and multiplexed holographic optical elements (MHOE) is presented. It mainly consists of a MHOE, a variable-focus lens and a projection display device. The customized MHOE, by using the angular multiplexing technology of volumetric holographic grating, records the scattering wavefront and spherical wavefront array required for 2D/3D compatible display. In particular, we proposed a feasible method to switch the 2D and 3D display modes by using a variable- focus lens in the reconstruction process. The proposed system solves the problem of bulky volume, and makes the MHOE more efficient to use. Based on the requirements of 2D and 3D displays, we calculated the liquid pumping volume of the variable-focus lens under two kinds of diopters. Keywords: optical see-through 2D/3D; variable-focus lens; holographic optical elements Citation: Ji, Q.; Deng, H.; Zhang, H.; Jiang, W.; Zhong, F.; Rao, F. Optical 1. Introduction See-Through 2D/3D Compatible Display Using Variable-Focus Lens Augmented reality (AR) is a technology which allows computer generated virtual and Multiplexed Holographic Optical imagery to exactly overlay physical objects [1,2]. AR display mainly includes optical Elements. Photonics 2021, 8, 297. see-through type [3], video see-through type [4] and reflection type [5,6] display devices. https://doi.org/10.3390/ At present, the optical see-through display device has become the mainstream technical photonics8080297 solution, because it allows the user to directly watch the real environment light through an image combiner, and improves the user ’s perception of the real world. As a kind of Received: 28 June 2021 true three-dimensional (3D) display technology, integral imaging has a compact structure Accepted: 23 July 2021 and can provide various physiological depth cues [7,8]. It is conductive for the natural Published: 27 July 2021 integration of virtual 3D images and real scenes, and is suitable to be integrated into AR display devices [9,10]. However, due to optical modulation of the lens array, integral Publisher’s Note: MDPI stays neutral imaging is not compatible with two-dimensional (2D) display. Although 3D display with regard to jurisdictional claims in can provide depth information that is absent in 2D display, the current 3D resolution published maps and institutional affil- is severely degraded. The 2D display has the advantage of high resolution and large iations. viewing angle, but it can’t replace 3D display. Therefore, some researchers have proposed 2D/3D compatible displays, such as switching backlight units by using polarized pinhole array [11,12] and polymer dispersed liquid crystal (PDLC) [13], or using dynamic parallax grating technology [14], but these technologies cannot be applied to optical see-through Copyright: © 2021 by the authors. AR display devices. Licensee MDPI, Basel, Switzerland. The technology that is key to realizing optical see-through AR display is the image This article is an open access article combiner that overlaps the virtual images and the real scenes together. The image combiner distributed under the terms and can be further divided into two categories: reflection-type devices [15,16] and diffraction- conditions of the Creative Commons type devices [17,18]. Compared with reflection-type devices, the diffraction-type devices Attribution (CC BY) license (https:// have many advantages, such as unique angle selectivity and wavelength selectivity and creativecommons.org/licenses/by/ flexibility of design, as well as high diffraction efficiency and transparency. It can replace 4.0/). Photonics 2021, 8, 297. https://doi.org/10.3390/photonics8080297 https://www.mdpi.com/journal/photonics Photonics 2021, 8, x FOR PEER REVIEW 2 of 9 biner can be further divided into two categories: reflection-type devices [15,16] and dif- fraction-type devices [17,18]. Compared with reflection-type devices, the diffraction-type Photonics 2021, 8, 297 2 of 9 devices have many advantages, such as unique angle selectivity and wavelength selec- tivity and flexibility of design, as well as high diffraction efficiency and transparency. It can replace one or several optical elements in the system and is an ideal image combiner one or several optical elements in the system and is an ideal image combiner [19]. Yeom [19]. Yeom et al. proposed a 2D/3D convertible projection system using see-through in- et al. proposed a 2D/3D convertible projection system using see-through integral imaging tegral imaging based on HOE. The use of prism reduced the number of projectors and based on HOE. The use of prism reduced the number of projectors and enabled a compact enabled a compact form factor, but the utilization of the HOE was only 50%, that is, one form factor, but the utilization of the HOE was only 50%, that is, one half of the HOE was half of the HOE was used for 2D display and the other half for 3D display [20]. Chou. et used for 2D display and the other half for 3D display [20]. Chou. et al. proposed a 2D/3D al. proposed a 2D/3D hybrid integral imaging display based on a liquid crystal mi- hybrid integral imaging display based on a liquid crystal micro-lens array (LCMLA), in cro-lens array (LCMLA), in which the LCMLA has the focusing effect or no optical effect which the LCMLA has the focusing effect or no optical effect determined by the polarization determined by the polarization state of the incident light. Therefore, the system added state of the incident light. Therefore, the system added devices such as twisted nematic cell devices such as twisted nematic cell and polarizer to control the polarization state, and and polarizer to control the polarization state, and increased the redundancy [21]. In our increased the redundancy [21]. In our previous work, we proposed attaching a PDLC previous work, we proposed attaching a PDLC film to the lens array HOE to constitute a film to the lens array HOE to constitute a 2D/3D projection screen. The lens array HOE 2D/3D projection screen. The lens array HOE and PDLC film were used to realize integral and PDLC film were used to realize integral imaging 3D display and 2D display, respec- imaging 3D display and 2D display, respectively. However, the PDLC film degraded the tively. However, the PDLC film degraded the transparency of the system, and the opti- transparency of the system, and the optical see-through properties could not be achieved cal see-through properties could not be achieved in 2D mode [22]. in 2D mode [22]. In this paper, we propose an optical see-through 2D/3D compatible display by using In this paper, we propose an optical see-through 2D/3D compatible display by using variable-focus lens and multiplexed holographic optical elements (MHOE). Based on variable-focus lens and multiplexed holographic optical elements (MHOE). Based on angular multiplexing technology, the MHOE records the scattering wavefront of a dif- angular multiplexing technology, the MHOE records the scattering wavefront of a diffuser fuser for 2D display and the spherical wavefront array of micro-lens array (MLA) for 3D for 2D display and the spherical wavefront array of micro-lens array (MLA) for 3D display. display. The incident angles of the projected angles vary with the changes in the varia- The incident angles of the projected angles vary with the changes in the variable-focus ble-focus lens to meet the two Bragg diffraction conditions of 2D and 3D displays. lens to meet the two Bragg diffraction conditions of 2D and 3D displays. Therefore, our Therefore, our approach does not require polarization devices and reduces the systematic approach does not require polarization devices and reduces the systematic complexity, complexity, enhances the effective utilization of MHOE and maintains optical enhances the effective utilization of MHOE and maintains optical see-through properties see-through properties for both 2D and 3D displays, all of which are highly beneficial for for both 2D and 3D displays, all of which are highly beneficial for AR applications. AR applications. 2. Structure and Principle 2. Structure and Principle Figure 1 shows the structure of our proposed optical see-through 2D/3D compatible Figure 1 shows the structure of our proposed optical see-through 2D/3D compatible display system. It is mainly composed of a MHOE, a variable-focus lens and a projector. display system. It is mainly composed of a MHOE, a variable-focus lens and a projector. Essentially, the MHOE is a volumetric holographic grating. The scattering wavefront of a Essentially, the MHOE is a volumetric holographic grating. The scattering wavefront of a diffuser and the spherical wavefront array of MLA are recorded on the single volumetric diffuser and the spherical wavefront array of MLA are recorded on the single volumetric holographic grating by two-step recording with angular multiplexing technology. The hologr variable-focus aphic grat lens ing b hasy t two wo-st optep r icalestates cording of wit diopter h anj guland ar mu j lt , and iplexing it modulates technology. the light The 1 2 variable-focus lens has two optical states of diopter φ1 and φ2, and it modulates the light transmitted by the projector respectively to meet the two Bragg diffraction conditions of tra MHOE. nsmitted by the projector respectively to meet the two Bragg diffraction conditions of MHOE. (a) (b) Figure 1. Schematic diagrams of system structure: (a) 3D image reconstruction when the probe beam satisfies the Bragg diffraction condition 1; (b) 2D image display when the probe beam satisfies the Bragg diffraction condition 2. In Figure 1a, the probe beam from the projector contains the amplitude information of elemental image array, and illuminates the MHOE after passing through the variable-focus Photonics 2021, 8, x FOR PEER REVIEW 3 of 9 Figure 1. Schematic diagrams of system structure: (a) 3D image reconstruction when the probe beam satisfies the Bragg diffraction condition 1; (b) 2D image display when the probe beam satisfies the Bragg diffraction condition 2. In Figure 1a, the probe beam from the projector contains the amplitude information of elemental image array, and illuminates the MHOE after passing through the varia- ble-focus lens with the diopter of φ1. Then, the MHOE’s Bragg diffraction condition 1 is satisfied, and the spherical wavefront array is diffracted out. The elemental image aligns with the element of MHOE to reconstruct the 3D image based on the theory of integral Photonics 2021, 8, 297 3 of 9 imaging. In Figure 1b, the probe beam from the projector contains the 2D image, and il- luminates the MHOE after passing through the variable-focus lens with the diopter of φ2. At this time, the MHOE’s Bragg diffraction condition 2 is satisfied, and the scattering lens with the diopter of j . Then, the MHOE’s Bragg diffraction condition 1 is satisfied, wavefront is diffracted out to achieve 2D display. The light from the real scene whose and the spherical wavefront array is diffracted out. The elemental image aligns with the wavelengths and incident angles don’t match the Bragg diffraction conditions, directly element of MHOE to reconstruct the 3D image based on the theory of integral imaging. passes through the MHOE. Hence, the system shows good optical see-through proper- In Figure 1b, the probe beam from the projector contains the 2D image, and illuminates ties. the MHOE after passing through the variable-focus lens with the diopter of j . At this In the reconstruction process, the variable-focus lens is the key component that plays time, the MHOE’s Bragg diffraction condition 2 is satisfied, and the scattering wavefront is a role in the angle selectivity of MHOE. It can accurately change the incident angles of the diffracted out to achieve 2D display. The light from the real scene whose wavelengths and reconstructed beam under different diopter conditions in order to meet the Bragg incident angles don’t match the Bragg diffraction conditions, directly passes through the matching angles. In the 3D mode, the divergent angle of projected beam is 2U1, and it MHOE. Hence, the system shows good optical see-through properties. doesn’t change after passing through the variable-focus lens with the diopter of φ1 = 0D. In the reconstruction process, the variable-focus lens is the key component that plays The conical beam still maintains the divergent angle of 2U1 when it arrives at the MHOE, a role in the angle selectivity of MHOE. It can accurately change the incident angles of the the incident angles of the two boundary beams are θl1 and θr1, respectively, as shown in reconstructed beam under different diopter conditions in order to meet the Bragg matching Figure 2a. The θl1 and θr1 can be expressed as angles. In the 3D mode, the divergent angle of projected beam is 2U , and it doesn’t change after passing through the variable-focus lens with the diopter of j = 0D. The conical beam θ =° 90 − β+U , 1 (1) l11 still maintains the divergent angle of 2U when it arrives at the MHOE, the incident angles of the two boundary beams are q and q , respectively, as shown in Figure 2a. The q and l 1 r 1 l 1 θ =° 90 − β−U , (2) r11 q can be expressed as q = 90 b + U , (1) l1 where β is the complement angle between the MHOE surface and the optical axis of q = 90 b U , (2) r1 1 projector. On the 2D mode, the diopter of variable-focus lens is φ2. The divergent angle of projected beam turns into 2U2 after passing through the variable-focus lens. The incident where b is the complement angle between the MHOE surface and the optical axis of angles of the two boundary beams onto the MHOE are θl2 and θr2, respectively, as shown projector. On the 2D mode, the diopter of variable-focus lens is j . The divergent angle of in Figure 2b. The U2, θl2 and θr2 can be described as projected beam turns into 2U after passing through the variable-focus lens. The incident angles of the two boundary beams onto the MHOE are q and q , respectively, as shown l 2 2 in Figure 2b. The U , q and q can be describedUy − asϕ 2 l 2 r 2 U = (3) n' U yj U = , (3) θ =° 90 − β+U , (4) l22 q = 90 b + U , (4) l2 2 q θ ==° 9090−b β− UU , , (5) (5) r2 2 r22 where y is the height of the projected beam formed onto the back surface of the variable- where y is th0 e height of the projected beam formed onto the back surface of the varia- focus lens, n is the refractive index of the liquid in the variable-focus lens. ble-focus lens, n’ is the refractive index of the liquid in the variable-focus lens. (a) (b) Figure 2. The projected beam matching with the Bragg matching angles under different diopters of the variable-focus lens: (a) 3D mode; (b) 2D mode. Compared with the conventional optical elements, the HOEs possess useful properties originating from their volumetric holographic grating structures, namely their selectivity and multiplex ability. The selectivity means HOEs have optical see-through properties, and it is easy to utilize the multiplex ability to superpose different functions of optical elements in the single volumetric holographic grating. Figure 3 illustrates the design and two-step recording process of MHOE used in this paper. Figure 3a shows the first recording process Photonics 2021, 8, x FOR PEER REVIEW 4 of 9 Figure 2. The projected beam matching with the Bragg matching angles under different diopters of the variable-focus lens: (a) 3D mode; (b) 2D mode. Compared with the conventional optical elements, the HOEs possess useful proper- ties originating from their volumetric holographic grating structures, namely their selec- tivity and multiplex ability. The selectivity means HOEs have optical see-through prop- Photonics 2021, 8, 297 4 of 9 erties, and it is easy to utilize the multiplex ability to superpose different functions of optical elements in the single volumetric holographic grating. Figure 3 illustrates the de- sign and two-step recording process of MHOE used in this paper. Figure 3a shows the first recording process that records the spherical wavefront array generated by the MLA. that records the spherical wavefront array generated by the MLA. The reference beam The reference beam illuminates the holographic plate with the divergent angle of 2U1. illuminates the holographic plate with the divergent angle of 2U . The indent angles of the The indent angles of the two boundary beams are θl1 and θr1. The signal beam illuminates two boundary beams are q and q . The signal beam illuminates the MLA vertically and l 1 r 1 the MLA vertically and generates spherical wavefront array. The spherical wavefront generates spherical wavefront array. The spherical wavefront array is projected onto the array other is projected onto the other side of th side of the holographic plate, and interfer e holo es with graphic plate, the reference and inter beam in fere the s with the region of reference beam in the region of L1. As a result, the spherical wavefront array is recorded. L . As a result, the spherical wavefront array is recorded. Figure 3b presents the second Fig recor ure ding 3b pre process sents the seco that recor nd recordin ds the scattering g procwavefr ess thatont records the sca of a diffuser.tteri The ng wa reference vefront of beam a d illuminates iffuser. Th the e reference be holographicam illumin plate witha the tes th diver e ho gent logr angle aphic p of l2 ate U w . The ith the dive indent angles rgent ang of the le two boundary beams are q and q . Meanwhile, the signal beam illuminates the diffuser of 2U2. The indent angles of the two boundary beams are θl2 and θr2. Meanwhile, the 2 r 2 vertically and generates scattering wavefront. The scattering wavefront is projected onto signal beam illuminates the diffuser vertically and generates scattering wavefront. The the other side of the holographic plate, and interferes with the reference beam in the region scattering wavefront is projected onto the other side of the holographic plate, and inter- of L . feres with the reference beam in the region of L2. (a) (b) Figure 3. Two-step recording process of MHOE: (a) First recording with the MLA; (b) second recording with the diffuser. Figure 3. Two-step recording process of MHOE: (a) First recording with the MLA; (b) second recording with the diffuser. 3. Experiments and Results 3. Experiments and Results 3.1. Fabrication of Variable-Focus Liquid Lens and MHOE 3.1. Fabrication of Variable-Focus Liquid Lens and MHOE In experiments, we fabricated a variable-focus liquid lens. The operating mechanism In experiments, we fabricated a variable-focus liquid lens. The operating mechanism of the liquid lens is that its focal length is controlled by pumping liquid in and out of the of the liquid lens is that its focal length is controlled by pumping liquid in and out of the lens chamber, which, in turn, changes the curvature of the liquid profile. Therefore, the lens chamber, which, in turn, changes the curvature of the liquid profile. Therefore, the volume of pumping liquid is controlled to adjust the diopter of the liquid lens to satisfy volume of pumping liquid is controlled to adjust the diopter of the liquid lens to satisfy two two Bragg diffraction conditions. The radius of curvature (R) and volume change ΔV Bragg diffraction conditions. The radius of curvature (R) and volume change DV follow follow the relationship [23]: the relationship [23]: ! ! r r 22 2   2 2 2 π dd d p d d d 22 2 DΔ= VVR = 222 R − − 2RR RR− 2R2R ++ RR− , , (6) (6) 3 4 4 4 34 4 4   where d is the diameter of the lens aperture. The variable-focus liquid lens is considered as where d is the diameter of the lens aperture. The variable-focus liquid lens is considered a thin lens, and its effective diopter (j) can be calculated: as a thin lens, and its effective diopter (φ) can be calculated: (n 1) j = , (7) where the refractive index of the liquid in the experiment is n = 1.33. Figure 4a shows the changes in the radius of curvature R and diopter j with the variation in volume change DV. In the practical experiment, the j was 14.29D and the R was 23 mm, which we chose while DV was 8.6 mL. Figure 4b,c show the lens effects with the lens diopter of 0D and 14.29D, respectively. Photonics 2021, 8, x FOR PEER REVIEW 5 of 9 n'1 − () , (7) ϕ = where the refractive index of the liquid in the experiment is n’ = 1.33. Figure 4a shows the changes in the radius of curvature R and diopter φ with the variation in volume change Photonics 2021, 8, 297 ΔV. In the practical experiment, the φ was 14.29D and the R was 23mm, which we chose 5 of 9 while ΔV was 8.6mL. Figure 4b,4c show the lens effects with the lens diopter of 0D and 14.29D, respectively. (a) (b) (c) Figure 4. Variable-focus liquid lens: (a) Curves of the radius of curvature and the diopter of liquid lens with the variation Figure 4. Variable-focus liquid lens: (a) Curves of the radius of curvature and the diopter of liquid lens with the variation in in the volume change; (b) imaging effect at 0D; (c) Imaging effect at 14.29D. the volume change; (b) imaging effect at 0D; (c) Imaging effect at 14.29D. Figure 5a,b show the experimental setups for the two-step recording of MHOE. The Figure 5a,b show the experimental setups for the two-step recording of MHOE. The fabricated variable-focus liquid lens was placed in the optical path of the reference beam fabricated variable-focus liquid lens was placed in the optical path of the reference beam to to change the incident angles of the reference beam to match the Bragg diffraction condi- change the incident angles of the reference beam to match the Bragg diffraction conditions. tions. The detailed parameters are shown in Table 1. The pitch and focal length of the The detailed parameters are shown in Table 1. The pitch and focal length of the recorded recorded MLA are 1 mm and 3.3 mm, respectively, and the scattering angle of the dif- MLA are 1 mm and 3.3 mm, respectively, and the scattering angle of the diffuser is 20 . fuser is 20°. (a) (b) Figure 5. Experimental setups for the two-step recording of MHOE: (a) Recording of MLA, (b) re- Figure 5. Experimental setups for the two-step recording of MHOE: (a) Recording of MLA, (b) record- cording of a diffuser. ES, electronic shutter; SF, spatial filter; CL, collimating lens; BS, beam splitter; ing of a diffuser. ES, electronic shutter; SF, spatial filter; CL, collimating lens; BS, beam splitter; M1, M1, M2, mirror; HP, holographic plate. M2, mirror; HP, holographic plate. Photonics 2021, 8, x FOR PEER REVIEW 6 of 9 Photonics 2021, 8, 297 6 of 9 Table 1. Parameters of the experiment for recording process. Components Parameters Values Table 1. Parameters of the experiment for recording process. θl1 57.2° Bragg matching angles 1 Components Parameters Values θr1 32.8° Bragg diffraction conditions q 57.2 θl2 45° l 1 Bragg matching angles 1 Bragg matching angles 2 q 32.8 θr2 45° Bragg diffraction conditions q 45 l 2 Bragg matching angles 2 Width L1 23 mm q 45 Recording region Width L2 18 mm Width L 23 mm Recording region Pitch 1 mm MLA Width L 18 mm Focal length 3.3 mm Pitch 1 mm Diffuser Scattering angle 20° MLA Focal length 3.3 mm Thickness 15 ± 1 µm Diffuser Scattering angle 20 Sensitive wavelength 532 nm Thickness 15  1 m Photopolymer plate Sensitivity 10 mJ/ cm² Sensitive wavelength 532 nm Averaged refractive index 1.47 Photopolymer plate Sensitivity 10 mJ/cm Refractive index modulation >0.02 Averaged refractive index 1.47 Refractive index modulation >0.02 In order to obtain high diffraction efficiency, we actually used two holographic plates and recorded each optical element independently. The MHOE was obtained by In order to obtain high diffraction efficiency, we actually used two holographic plates using a frame (GCM-1301M in Daheng Optics) to clip the two holographic plates to- and recorded each optical element independently. The MHOE was obtained by using gether. The fabricated MHOE was tested with reference beams under the corresponding a frame (GCM-1301M in Daheng Optics) to clip the two holographic plates together. Bragg diffraction conditions. Figure 6a,b show the diffraction effects of the MLA and The fabricated MHOE was tested with reference beams under the corresponding Bragg diffuser, respectively. The real-world scene behind the MHOE is clearly observed with- diffraction conditions. Figure 6a,b show the diffraction effects of the MLA and diffuser, out visual obstruction, which verifies the see-through properties of MHOE, as shown in respectively. The real-world scene behind the MHOE is clearly observed without visual Figure 6c. Figure 6d shows the relation between normalized diffraction efficiency and obstruction, which verifies the see-through properties of MHOE, as shown in Figure 6c. angle deviation. The diffraction efficiency starts to decline sharply at about ±5°. The an- Figure 6d shows the relation between normalized diffraction efficiency and angle deviation. gular difference between the two Bragg matching angles is 12.2°, which is large enough The diffraction efficiency starts to decline sharply at about 5 . The angular difference to avoid the crosstalk between the two display modes. between the two Bragg matching angles is 12.2 , which is large enough to avoid the crosstalk between the two display modes. (a) (b) (c) (d) Figure 6. Performance testing of MHOE: (a) Diffraction effect of the MLA; (b) diffraction effect of the diffuser; (c) optical Figure 6. Performance testing of MHOE: (a) Diffraction effect of the MLA; (b) diffraction effect of the diffuser; (c) optical see-through properties of MHOE; (d) relationship between normalized diffraction efficiency and angle deviation. see-through properties of MHOE; (d) relationship between normalized diffraction efficiency and angle deviation. 3.2. Experimental Results 3.2. Experimental Results We developed the experimental system based on the proposed scheme as shown in We developed the experimental system based on the proposed scheme as shown in Figure 7a. The projector has the resolution of 1280 × 800. The projected image for the 3D Figure 7a. The projector has the resolution of 1280  800. The projected image for the 3D mode is provided as shown in Figure 7b. The character “3” and letter “D” are in front of mode is provided as shown in Figure 7b. The character “3” and letter “D” are in front of the MHOE with the depth of 35 mm and behind the MHOE with the depth of −35 mm, the MHOE with the depth of 35 mm and behind the MHOE with the depth of 35 mm, respectively. In addition, a 2D text “Sichuan University” is prepared for 2D display as shown in Figure 7c. Photonics 2021, 8, x FOR PEER REVIEW 7 of 9 Photonics 2021, 8, x FOR PEER REVIEW 7 of 9 Photonics 2021, 8, 297 7 of 9 respectively. In addition, a 2D text “Sichuan University” is prepared for 2D display as respectively. In addition, a 2D text “Sichuan University” is prepared for 2D display as shown in Figure 7c. shown in Figure 7c. (a) (b) (c) (a) (b) (c) Figure 7. (a) Experimental system; (b) elemental image array; (c) 2D image. Figure 7. (a) Experimental system; (b) elemental image array; (c) 2D image. Figure 7. (a) Experimental system; (b) elemental image array; (c) 2D image. On the 3D display mode, the projected beam meets the Bragg diffraction condition 1. On the 3D display mode, the projected beam meets the Bragg diffraction condition 1. On the 3D display mode, the projected beam meets the Bragg diffraction condition 1. The 3D image is reconstructed, and four different perspectives from top left, top right, The 3D image is reconstructed, and four different perspectives from top left, top right, The 3D image is reconstructed, and four different perspectives from top left, top right, bottom left and bottom right viewing points are shown in Figure 8a. Obvious horizontal bottom left and bottom right viewing points are shown in Figure 8a. Obvious horizontal bottom left and bottom right viewing points are shown in Figure 8a. Obvious horizontal and and vertical vertical parallaxes parallaxes c can an b be e c clearly learly seen. seen. Add Additionally itionally, , t the he ch character aracter “3” “3” is is b bigger igger t than han and vertical parallaxes can be clearly seen. Additionally, the character “3” is bigger than let letter ter “D “D”, ”, w which hich dem demonstrates onstrates t that hat “ “3” 3” is recon is reconstr struct ucted ed in in front front of of tthe he MHOE w MHOE while hile “D “D” ” letter “D”, which demonstrates that “3” is reconstructed in front of the MHOE while “D” i is s behi behind nd the MHOE. On the MHOE. On the the 2 2D D di display splay mo mode, de, the the pr projected beam sa ojected beam satisfies tisfies the Bra the Bragg gg is behind the MHOE. On the 2D display mode, the projected beam satisfies the Bragg dif diffraction fraction co condition ndition2, 2and , and F Figur igu e re 8b 8b shows shows t the displayed he displayed 2D image. 2D imThe age.r The eal object real ob “dice” ject diffraction condition 2, and Figure 8b shows the displayed 2D image. The real object behind the MHOE is always visible on both 2D and 3D display modes, which verifies the “dice” behind the MHOE is always visible on both 2D and 3D display modes, which “dice” behind the MHOE is always visible on both 2D and 3D display modes, which good optical see-through properties of the system. verifies the good optical see-through properties of the system. verifies the good optical see-through properties of the system. (a) (b) (a) (b) Figure 8. Experimental results of 2D/3D compatible display with optical see-through properties: (a) Figure 8. Experimental results of 2D/3D compatible display with optical see-through properties: (a) Figure 8. Experimental results of 2D/3D compatible display with optical see-through properties: 3D images display captured from four different viewing directions; (b) 2D image display. 3D images display captured from four different viewing directions; (b) 2D image display. (a) 3D images display captured from four different viewing directions; (b) 2D image display. 4. Conclusions 4. Conclusions 4. Conclusions In this paper, an optical see-through 2D/3D compatible display system is proposed In this paper, an optical see-through 2D/3D compatible display system is proposed by In this paper, an optical see-through 2D/3D compatible display system is proposed by using a variable-focus lens and a MHOE. The scattering wavefront and spherical using a variable-focus lens and a MHOE. The scattering wavefront and spherical wavefront by using a variable-focus lens and a MHOE. The scattering wavefront and spherical wavefront array are recorded in the MHOE using angular multiplexing technology to array are recorded in the MHOE using angular multiplexing technology to achieve 2D/3D wavefront array are recorded in the MHOE using angular multiplexing technology to achieve 2D/3D display, respectively. The variable-focus lens is used to adjust the incident display, respectively. The variable-focus lens is used to adjust the incident angles of the achieve 2D/3D display, respectively. The variable-focus lens is used to adjust the incident angles of the projected beam. By analyzing the relationship between the liquid change projected beam. By analyzing the relationship between the liquid change and the radius of angles of the projected beam. By analyzing the relationship between the liquid change and the radius of curvature of the variable-focus lens, the optimal diopter of the varia- curvature of the variable-focus lens, the optimal diopter of the variable-focus lens is picked and the radius of curvature of the variable-focus lens, the optimal diopter of the varia- ble-focus lens is picked up to match the Bragg diffraction conditions. The incident angles up to match the Bragg diffraction conditions. The incident angles of the projected beam ble-focus lens is picked up to match the Bragg diffraction conditions. The incident angles of the projected beam are calculated and measured. When the MLA and diffuser were are calculated and measured. When the MLA and diffuser were recorded on holographic of the projected beam are calculated and measured. When the MLA and diffuser were plates recorded based on on hologr angular aphic multiplexing plates based technology on angular , the mult variable-focus iplexing technology, lens is placed the var in the ia- recorded on holographic plates based on angular multiplexing technology, the varia- ble-focus lens is placed in the path of the reference beam to adjust the incident angles of path of the reference beam to adjust the incident angles of the reference beam through ble-focus lens is placed in the path of the reference beam to adjust the incident angles of the reference the variation in beam throug the liquid pumping h the variatio volume. n in the li During quid pu reconstr mpiuction, ng volu the me. Du incident ring recon- angles the reference beam through the variation in the liquid pumping volume. During recon- struction, the of the projected incid beam ent ang satisfied les of the pro the Bragg jected bea matching m sa angles tisfied the Bra under the gg ma same tchi amount ng anglof es struction, the incident angles of the projected beam satisfied the Bragg matching angles liquid pumping volume, and the spherical wavefront array of the MLA and the scattering under the same amount of liquid pumping volume, and the spherical wavefront array of under the same amount of liquid pumping volume, and the spherical wavefront array of wavefront of diffuser are reconstructed. As a result, 3D images with proper horizontal and the MLA and the scattering wavefront of diffuser are reconstructed. As a result, 3D im- the MLA and the scattering wavefront of diffuser are reconstructed. As a result, 3D im- vertical parallaxes are reconstructed on 3D display mode and 2D image is displayed on ages with proper horizontal and vertical parallaxes are reconstructed on 3D display ages with proper horizontal and vertical parallaxes are reconstructed on 3D display 2D display mode, and both modes show good optical see-through properties. The images; mode and 2D image is displayed on 2D display mode, and both modes show good opti- mode and 2D image is displayed on 2D display mode, and both modes show good opti- quality would be further improved if a variable-focus liquid lens with multi-chamber is used. The proposed system can be a good candidate in AR application. Photonics 2021, 8, 297 8 of 9 Author Contributions: Conceptualization, Q.J.; methodology, H.D., H.Z. and W.J.; software, F.Z.; validation, Q.J. and F.R.; data curation and writing-original draft preparation, Q.J.; writing—review and editing, H.D. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant numbers 61972147 and 61775151, and the China Postdoctoral Science Foundation, grant number 2021M690287. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request. Conflicts of Interest: The authors declare no conflict of interest. References 1. Zhou, F.; Duh, H.B.L.; Billinghurst, M. Trends in augmented reality tracking, interaction and display: A review of ten years of ISMAR. In Proceedings of the 7th IEEE/ACM International Symposium on Mixed and Augmented Reality, Washington, DC, USA, 15–18 September 2018; pp. 193–202. 2. Lee, Y.H.; Zhan, T.; Wu, S.T. Prospects and challenges in augmented reality displays. Virtual Real. Intell. 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Jang, C.; Lee, C.K.; Jeong, J.; Li, G.; Lee, S.; Yeom, J.; Hong, K.; Lee, B. Recent progress in see-through three-dimensional displays using holographic optical elements. Appl. Opt. 2016, 55, A71–A85. [CrossRef] [PubMed] 20. Yeom, J.; Jeong, J.; Jang, C.; Li, G.; Hong, K.; Lee, B. Three-dimensional/two-dimensional convertible projection screen using see-through integral imaging based on holographic optical element. Appl. Opt. 2015, 54, 8856–8862. [CrossRef] 21. Chou, P.Y.; Wu, J.Y.; Huang, S.H.; Wang, C.P.; Qin, Z.; Huang, C.T.; Hsieh, P.Y.; Lee, H.H.; Lin, T.H.; Huang, Y.P. Hybrid light field head-mounted display using time-multiplexed liquid crystal lens array for resolution enhancement. Opt. Express 2019, 27, 1164–1177. [CrossRef] Photonics 2021, 8, 297 9 of 9 22. Zhang, H.L.; Deng, H.; Li, J.J.; He, M.Y.; Li, D.H.; Wang, Q.H. Integral imaging-based 2D/3D convertible display system by using holographic optical element and polymer dispersed liquid crystal. Opt. Lett. 2019, 44, 387–390. [CrossRef] [PubMed] 23. Ren, H.; Fox, D.; Andrew Anderson, P.; Wu, B.; Wu, S.T. Tunable-focus liquid lens controlled using a servo motor. Opt. Express 2006, 14, 8031–8136. [CrossRef] [PubMed] http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Photonics Multidisciplinary Digital Publishing Institute

Optical See-through 2D/3D Compatible Display Using Variable-Focus Lens and Multiplexed Holographic Optical Elements

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hv photonics Article Optical See-through 2D/3D Compatible Display Using Variable-Focus Lens and Multiplexed Holographic Optical Elements 1 1 , 2 1 1 1 Qinglin Ji , Huan Deng *, Hanle Zhang , Wenhao Jiang , Feiyan Zhong and Fengbin Rao College of Electronics and Information Engineering, Sichuan University, Chengdu 610065, China; jiqinglin@stu.scu.edu.cn (Q.J.); jiang7ogremy@163.com (W.J.); zhongfeiyan@stu.scu.edu.cn (F.Z.); raofengbin@stu.scu.edu.cn (F.R.) School of Instrumentation Science and Optoelectronics Engineering, Beihang University, Beijing 100191, China; hanlezhang@buaa.edu.cn * Correspondence: huandeng@scu.edu.cn Abstract: An optical see-through two-dimensional (2D)/three-dimensional (3D) compatible display using variable-focus lens and multiplexed holographic optical elements (MHOE) is presented. It mainly consists of a MHOE, a variable-focus lens and a projection display device. The customized MHOE, by using the angular multiplexing technology of volumetric holographic grating, records the scattering wavefront and spherical wavefront array required for 2D/3D compatible display. In particular, we proposed a feasible method to switch the 2D and 3D display modes by using a variable- focus lens in the reconstruction process. The proposed system solves the problem of bulky volume, and makes the MHOE more efficient to use. Based on the requirements of 2D and 3D displays, we calculated the liquid pumping volume of the variable-focus lens under two kinds of diopters. Keywords: optical see-through 2D/3D; variable-focus lens; holographic optical elements Citation: Ji, Q.; Deng, H.; Zhang, H.; Jiang, W.; Zhong, F.; Rao, F. Optical 1. Introduction See-Through 2D/3D Compatible Display Using Variable-Focus Lens Augmented reality (AR) is a technology which allows computer generated virtual and Multiplexed Holographic Optical imagery to exactly overlay physical objects [1,2]. AR display mainly includes optical Elements. Photonics 2021, 8, 297. see-through type [3], video see-through type [4] and reflection type [5,6] display devices. https://doi.org/10.3390/ At present, the optical see-through display device has become the mainstream technical photonics8080297 solution, because it allows the user to directly watch the real environment light through an image combiner, and improves the user ’s perception of the real world. As a kind of Received: 28 June 2021 true three-dimensional (3D) display technology, integral imaging has a compact structure Accepted: 23 July 2021 and can provide various physiological depth cues [7,8]. It is conductive for the natural Published: 27 July 2021 integration of virtual 3D images and real scenes, and is suitable to be integrated into AR display devices [9,10]. However, due to optical modulation of the lens array, integral Publisher’s Note: MDPI stays neutral imaging is not compatible with two-dimensional (2D) display. Although 3D display with regard to jurisdictional claims in can provide depth information that is absent in 2D display, the current 3D resolution published maps and institutional affil- is severely degraded. The 2D display has the advantage of high resolution and large iations. viewing angle, but it can’t replace 3D display. Therefore, some researchers have proposed 2D/3D compatible displays, such as switching backlight units by using polarized pinhole array [11,12] and polymer dispersed liquid crystal (PDLC) [13], or using dynamic parallax grating technology [14], but these technologies cannot be applied to optical see-through Copyright: © 2021 by the authors. AR display devices. Licensee MDPI, Basel, Switzerland. The technology that is key to realizing optical see-through AR display is the image This article is an open access article combiner that overlaps the virtual images and the real scenes together. The image combiner distributed under the terms and can be further divided into two categories: reflection-type devices [15,16] and diffraction- conditions of the Creative Commons type devices [17,18]. Compared with reflection-type devices, the diffraction-type devices Attribution (CC BY) license (https:// have many advantages, such as unique angle selectivity and wavelength selectivity and creativecommons.org/licenses/by/ flexibility of design, as well as high diffraction efficiency and transparency. It can replace 4.0/). Photonics 2021, 8, 297. https://doi.org/10.3390/photonics8080297 https://www.mdpi.com/journal/photonics Photonics 2021, 8, x FOR PEER REVIEW 2 of 9 biner can be further divided into two categories: reflection-type devices [15,16] and dif- fraction-type devices [17,18]. Compared with reflection-type devices, the diffraction-type Photonics 2021, 8, 297 2 of 9 devices have many advantages, such as unique angle selectivity and wavelength selec- tivity and flexibility of design, as well as high diffraction efficiency and transparency. It can replace one or several optical elements in the system and is an ideal image combiner one or several optical elements in the system and is an ideal image combiner [19]. Yeom [19]. Yeom et al. proposed a 2D/3D convertible projection system using see-through in- et al. proposed a 2D/3D convertible projection system using see-through integral imaging tegral imaging based on HOE. The use of prism reduced the number of projectors and based on HOE. The use of prism reduced the number of projectors and enabled a compact enabled a compact form factor, but the utilization of the HOE was only 50%, that is, one form factor, but the utilization of the HOE was only 50%, that is, one half of the HOE was half of the HOE was used for 2D display and the other half for 3D display [20]. Chou. et used for 2D display and the other half for 3D display [20]. Chou. et al. proposed a 2D/3D al. proposed a 2D/3D hybrid integral imaging display based on a liquid crystal mi- hybrid integral imaging display based on a liquid crystal micro-lens array (LCMLA), in cro-lens array (LCMLA), in which the LCMLA has the focusing effect or no optical effect which the LCMLA has the focusing effect or no optical effect determined by the polarization determined by the polarization state of the incident light. Therefore, the system added state of the incident light. Therefore, the system added devices such as twisted nematic cell devices such as twisted nematic cell and polarizer to control the polarization state, and and polarizer to control the polarization state, and increased the redundancy [21]. In our increased the redundancy [21]. In our previous work, we proposed attaching a PDLC previous work, we proposed attaching a PDLC film to the lens array HOE to constitute a film to the lens array HOE to constitute a 2D/3D projection screen. The lens array HOE 2D/3D projection screen. The lens array HOE and PDLC film were used to realize integral and PDLC film were used to realize integral imaging 3D display and 2D display, respec- imaging 3D display and 2D display, respectively. However, the PDLC film degraded the tively. However, the PDLC film degraded the transparency of the system, and the opti- transparency of the system, and the optical see-through properties could not be achieved cal see-through properties could not be achieved in 2D mode [22]. in 2D mode [22]. In this paper, we propose an optical see-through 2D/3D compatible display by using In this paper, we propose an optical see-through 2D/3D compatible display by using variable-focus lens and multiplexed holographic optical elements (MHOE). Based on variable-focus lens and multiplexed holographic optical elements (MHOE). Based on angular multiplexing technology, the MHOE records the scattering wavefront of a dif- angular multiplexing technology, the MHOE records the scattering wavefront of a diffuser fuser for 2D display and the spherical wavefront array of micro-lens array (MLA) for 3D for 2D display and the spherical wavefront array of micro-lens array (MLA) for 3D display. display. The incident angles of the projected angles vary with the changes in the varia- The incident angles of the projected angles vary with the changes in the variable-focus ble-focus lens to meet the two Bragg diffraction conditions of 2D and 3D displays. lens to meet the two Bragg diffraction conditions of 2D and 3D displays. Therefore, our Therefore, our approach does not require polarization devices and reduces the systematic approach does not require polarization devices and reduces the systematic complexity, complexity, enhances the effective utilization of MHOE and maintains optical enhances the effective utilization of MHOE and maintains optical see-through properties see-through properties for both 2D and 3D displays, all of which are highly beneficial for for both 2D and 3D displays, all of which are highly beneficial for AR applications. AR applications. 2. Structure and Principle 2. Structure and Principle Figure 1 shows the structure of our proposed optical see-through 2D/3D compatible Figure 1 shows the structure of our proposed optical see-through 2D/3D compatible display system. It is mainly composed of a MHOE, a variable-focus lens and a projector. display system. It is mainly composed of a MHOE, a variable-focus lens and a projector. Essentially, the MHOE is a volumetric holographic grating. The scattering wavefront of a Essentially, the MHOE is a volumetric holographic grating. The scattering wavefront of a diffuser and the spherical wavefront array of MLA are recorded on the single volumetric diffuser and the spherical wavefront array of MLA are recorded on the single volumetric holographic grating by two-step recording with angular multiplexing technology. The hologr variable-focus aphic grat lens ing b hasy t two wo-st optep r icalestates cording of wit diopter h anj guland ar mu j lt , and iplexing it modulates technology. the light The 1 2 variable-focus lens has two optical states of diopter φ1 and φ2, and it modulates the light transmitted by the projector respectively to meet the two Bragg diffraction conditions of tra MHOE. nsmitted by the projector respectively to meet the two Bragg diffraction conditions of MHOE. (a) (b) Figure 1. Schematic diagrams of system structure: (a) 3D image reconstruction when the probe beam satisfies the Bragg diffraction condition 1; (b) 2D image display when the probe beam satisfies the Bragg diffraction condition 2. In Figure 1a, the probe beam from the projector contains the amplitude information of elemental image array, and illuminates the MHOE after passing through the variable-focus Photonics 2021, 8, x FOR PEER REVIEW 3 of 9 Figure 1. Schematic diagrams of system structure: (a) 3D image reconstruction when the probe beam satisfies the Bragg diffraction condition 1; (b) 2D image display when the probe beam satisfies the Bragg diffraction condition 2. In Figure 1a, the probe beam from the projector contains the amplitude information of elemental image array, and illuminates the MHOE after passing through the varia- ble-focus lens with the diopter of φ1. Then, the MHOE’s Bragg diffraction condition 1 is satisfied, and the spherical wavefront array is diffracted out. The elemental image aligns with the element of MHOE to reconstruct the 3D image based on the theory of integral Photonics 2021, 8, 297 3 of 9 imaging. In Figure 1b, the probe beam from the projector contains the 2D image, and il- luminates the MHOE after passing through the variable-focus lens with the diopter of φ2. At this time, the MHOE’s Bragg diffraction condition 2 is satisfied, and the scattering lens with the diopter of j . Then, the MHOE’s Bragg diffraction condition 1 is satisfied, wavefront is diffracted out to achieve 2D display. The light from the real scene whose and the spherical wavefront array is diffracted out. The elemental image aligns with the wavelengths and incident angles don’t match the Bragg diffraction conditions, directly element of MHOE to reconstruct the 3D image based on the theory of integral imaging. passes through the MHOE. Hence, the system shows good optical see-through proper- In Figure 1b, the probe beam from the projector contains the 2D image, and illuminates ties. the MHOE after passing through the variable-focus lens with the diopter of j . At this In the reconstruction process, the variable-focus lens is the key component that plays time, the MHOE’s Bragg diffraction condition 2 is satisfied, and the scattering wavefront is a role in the angle selectivity of MHOE. It can accurately change the incident angles of the diffracted out to achieve 2D display. The light from the real scene whose wavelengths and reconstructed beam under different diopter conditions in order to meet the Bragg incident angles don’t match the Bragg diffraction conditions, directly passes through the matching angles. In the 3D mode, the divergent angle of projected beam is 2U1, and it MHOE. Hence, the system shows good optical see-through properties. doesn’t change after passing through the variable-focus lens with the diopter of φ1 = 0D. In the reconstruction process, the variable-focus lens is the key component that plays The conical beam still maintains the divergent angle of 2U1 when it arrives at the MHOE, a role in the angle selectivity of MHOE. It can accurately change the incident angles of the the incident angles of the two boundary beams are θl1 and θr1, respectively, as shown in reconstructed beam under different diopter conditions in order to meet the Bragg matching Figure 2a. The θl1 and θr1 can be expressed as angles. In the 3D mode, the divergent angle of projected beam is 2U , and it doesn’t change after passing through the variable-focus lens with the diopter of j = 0D. The conical beam θ =° 90 − β+U , 1 (1) l11 still maintains the divergent angle of 2U when it arrives at the MHOE, the incident angles of the two boundary beams are q and q , respectively, as shown in Figure 2a. The q and l 1 r 1 l 1 θ =° 90 − β−U , (2) r11 q can be expressed as q = 90 b + U , (1) l1 where β is the complement angle between the MHOE surface and the optical axis of q = 90 b U , (2) r1 1 projector. On the 2D mode, the diopter of variable-focus lens is φ2. The divergent angle of projected beam turns into 2U2 after passing through the variable-focus lens. The incident where b is the complement angle between the MHOE surface and the optical axis of angles of the two boundary beams onto the MHOE are θl2 and θr2, respectively, as shown projector. On the 2D mode, the diopter of variable-focus lens is j . The divergent angle of in Figure 2b. The U2, θl2 and θr2 can be described as projected beam turns into 2U after passing through the variable-focus lens. The incident angles of the two boundary beams onto the MHOE are q and q , respectively, as shown l 2 2 in Figure 2b. The U , q and q can be describedUy − asϕ 2 l 2 r 2 U = (3) n' U yj U = , (3) θ =° 90 − β+U , (4) l22 q = 90 b + U , (4) l2 2 q θ ==° 9090−b β− UU , , (5) (5) r2 2 r22 where y is the height of the projected beam formed onto the back surface of the variable- where y is th0 e height of the projected beam formed onto the back surface of the varia- focus lens, n is the refractive index of the liquid in the variable-focus lens. ble-focus lens, n’ is the refractive index of the liquid in the variable-focus lens. (a) (b) Figure 2. The projected beam matching with the Bragg matching angles under different diopters of the variable-focus lens: (a) 3D mode; (b) 2D mode. Compared with the conventional optical elements, the HOEs possess useful properties originating from their volumetric holographic grating structures, namely their selectivity and multiplex ability. The selectivity means HOEs have optical see-through properties, and it is easy to utilize the multiplex ability to superpose different functions of optical elements in the single volumetric holographic grating. Figure 3 illustrates the design and two-step recording process of MHOE used in this paper. Figure 3a shows the first recording process Photonics 2021, 8, x FOR PEER REVIEW 4 of 9 Figure 2. The projected beam matching with the Bragg matching angles under different diopters of the variable-focus lens: (a) 3D mode; (b) 2D mode. Compared with the conventional optical elements, the HOEs possess useful proper- ties originating from their volumetric holographic grating structures, namely their selec- tivity and multiplex ability. The selectivity means HOEs have optical see-through prop- Photonics 2021, 8, 297 4 of 9 erties, and it is easy to utilize the multiplex ability to superpose different functions of optical elements in the single volumetric holographic grating. Figure 3 illustrates the de- sign and two-step recording process of MHOE used in this paper. Figure 3a shows the first recording process that records the spherical wavefront array generated by the MLA. that records the spherical wavefront array generated by the MLA. The reference beam The reference beam illuminates the holographic plate with the divergent angle of 2U1. illuminates the holographic plate with the divergent angle of 2U . The indent angles of the The indent angles of the two boundary beams are θl1 and θr1. The signal beam illuminates two boundary beams are q and q . The signal beam illuminates the MLA vertically and l 1 r 1 the MLA vertically and generates spherical wavefront array. The spherical wavefront generates spherical wavefront array. The spherical wavefront array is projected onto the array other is projected onto the other side of th side of the holographic plate, and interfer e holo es with graphic plate, the reference and inter beam in fere the s with the region of reference beam in the region of L1. As a result, the spherical wavefront array is recorded. L . As a result, the spherical wavefront array is recorded. Figure 3b presents the second Fig recor ure ding 3b pre process sents the seco that recor nd recordin ds the scattering g procwavefr ess thatont records the sca of a diffuser.tteri The ng wa reference vefront of beam a d illuminates iffuser. Th the e reference be holographicam illumin plate witha the tes th diver e ho gent logr angle aphic p of l2 ate U w . The ith the dive indent angles rgent ang of the le two boundary beams are q and q . Meanwhile, the signal beam illuminates the diffuser of 2U2. The indent angles of the two boundary beams are θl2 and θr2. Meanwhile, the 2 r 2 vertically and generates scattering wavefront. The scattering wavefront is projected onto signal beam illuminates the diffuser vertically and generates scattering wavefront. The the other side of the holographic plate, and interferes with the reference beam in the region scattering wavefront is projected onto the other side of the holographic plate, and inter- of L . feres with the reference beam in the region of L2. (a) (b) Figure 3. Two-step recording process of MHOE: (a) First recording with the MLA; (b) second recording with the diffuser. Figure 3. Two-step recording process of MHOE: (a) First recording with the MLA; (b) second recording with the diffuser. 3. Experiments and Results 3. Experiments and Results 3.1. Fabrication of Variable-Focus Liquid Lens and MHOE 3.1. Fabrication of Variable-Focus Liquid Lens and MHOE In experiments, we fabricated a variable-focus liquid lens. The operating mechanism In experiments, we fabricated a variable-focus liquid lens. The operating mechanism of the liquid lens is that its focal length is controlled by pumping liquid in and out of the of the liquid lens is that its focal length is controlled by pumping liquid in and out of the lens chamber, which, in turn, changes the curvature of the liquid profile. Therefore, the lens chamber, which, in turn, changes the curvature of the liquid profile. Therefore, the volume of pumping liquid is controlled to adjust the diopter of the liquid lens to satisfy volume of pumping liquid is controlled to adjust the diopter of the liquid lens to satisfy two two Bragg diffraction conditions. The radius of curvature (R) and volume change ΔV Bragg diffraction conditions. The radius of curvature (R) and volume change DV follow follow the relationship [23]: the relationship [23]: ! ! r r 22 2   2 2 2 π dd d p d d d 22 2 DΔ= VVR = 222 R − − 2RR RR− 2R2R ++ RR− , , (6) (6) 3 4 4 4 34 4 4   where d is the diameter of the lens aperture. The variable-focus liquid lens is considered as where d is the diameter of the lens aperture. The variable-focus liquid lens is considered a thin lens, and its effective diopter (j) can be calculated: as a thin lens, and its effective diopter (φ) can be calculated: (n 1) j = , (7) where the refractive index of the liquid in the experiment is n = 1.33. Figure 4a shows the changes in the radius of curvature R and diopter j with the variation in volume change DV. In the practical experiment, the j was 14.29D and the R was 23 mm, which we chose while DV was 8.6 mL. Figure 4b,c show the lens effects with the lens diopter of 0D and 14.29D, respectively. Photonics 2021, 8, x FOR PEER REVIEW 5 of 9 n'1 − () , (7) ϕ = where the refractive index of the liquid in the experiment is n’ = 1.33. Figure 4a shows the changes in the radius of curvature R and diopter φ with the variation in volume change Photonics 2021, 8, 297 ΔV. In the practical experiment, the φ was 14.29D and the R was 23mm, which we chose 5 of 9 while ΔV was 8.6mL. Figure 4b,4c show the lens effects with the lens diopter of 0D and 14.29D, respectively. (a) (b) (c) Figure 4. Variable-focus liquid lens: (a) Curves of the radius of curvature and the diopter of liquid lens with the variation Figure 4. Variable-focus liquid lens: (a) Curves of the radius of curvature and the diopter of liquid lens with the variation in in the volume change; (b) imaging effect at 0D; (c) Imaging effect at 14.29D. the volume change; (b) imaging effect at 0D; (c) Imaging effect at 14.29D. Figure 5a,b show the experimental setups for the two-step recording of MHOE. The Figure 5a,b show the experimental setups for the two-step recording of MHOE. The fabricated variable-focus liquid lens was placed in the optical path of the reference beam fabricated variable-focus liquid lens was placed in the optical path of the reference beam to to change the incident angles of the reference beam to match the Bragg diffraction condi- change the incident angles of the reference beam to match the Bragg diffraction conditions. tions. The detailed parameters are shown in Table 1. The pitch and focal length of the The detailed parameters are shown in Table 1. The pitch and focal length of the recorded recorded MLA are 1 mm and 3.3 mm, respectively, and the scattering angle of the dif- MLA are 1 mm and 3.3 mm, respectively, and the scattering angle of the diffuser is 20 . fuser is 20°. (a) (b) Figure 5. Experimental setups for the two-step recording of MHOE: (a) Recording of MLA, (b) re- Figure 5. Experimental setups for the two-step recording of MHOE: (a) Recording of MLA, (b) record- cording of a diffuser. ES, electronic shutter; SF, spatial filter; CL, collimating lens; BS, beam splitter; ing of a diffuser. ES, electronic shutter; SF, spatial filter; CL, collimating lens; BS, beam splitter; M1, M1, M2, mirror; HP, holographic plate. M2, mirror; HP, holographic plate. Photonics 2021, 8, x FOR PEER REVIEW 6 of 9 Photonics 2021, 8, 297 6 of 9 Table 1. Parameters of the experiment for recording process. Components Parameters Values Table 1. Parameters of the experiment for recording process. θl1 57.2° Bragg matching angles 1 Components Parameters Values θr1 32.8° Bragg diffraction conditions q 57.2 θl2 45° l 1 Bragg matching angles 1 Bragg matching angles 2 q 32.8 θr2 45° Bragg diffraction conditions q 45 l 2 Bragg matching angles 2 Width L1 23 mm q 45 Recording region Width L2 18 mm Width L 23 mm Recording region Pitch 1 mm MLA Width L 18 mm Focal length 3.3 mm Pitch 1 mm Diffuser Scattering angle 20° MLA Focal length 3.3 mm Thickness 15 ± 1 µm Diffuser Scattering angle 20 Sensitive wavelength 532 nm Thickness 15  1 m Photopolymer plate Sensitivity 10 mJ/ cm² Sensitive wavelength 532 nm Averaged refractive index 1.47 Photopolymer plate Sensitivity 10 mJ/cm Refractive index modulation >0.02 Averaged refractive index 1.47 Refractive index modulation >0.02 In order to obtain high diffraction efficiency, we actually used two holographic plates and recorded each optical element independently. The MHOE was obtained by In order to obtain high diffraction efficiency, we actually used two holographic plates using a frame (GCM-1301M in Daheng Optics) to clip the two holographic plates to- and recorded each optical element independently. The MHOE was obtained by using gether. The fabricated MHOE was tested with reference beams under the corresponding a frame (GCM-1301M in Daheng Optics) to clip the two holographic plates together. Bragg diffraction conditions. Figure 6a,b show the diffraction effects of the MLA and The fabricated MHOE was tested with reference beams under the corresponding Bragg diffuser, respectively. The real-world scene behind the MHOE is clearly observed with- diffraction conditions. Figure 6a,b show the diffraction effects of the MLA and diffuser, out visual obstruction, which verifies the see-through properties of MHOE, as shown in respectively. The real-world scene behind the MHOE is clearly observed without visual Figure 6c. Figure 6d shows the relation between normalized diffraction efficiency and obstruction, which verifies the see-through properties of MHOE, as shown in Figure 6c. angle deviation. The diffraction efficiency starts to decline sharply at about ±5°. The an- Figure 6d shows the relation between normalized diffraction efficiency and angle deviation. gular difference between the two Bragg matching angles is 12.2°, which is large enough The diffraction efficiency starts to decline sharply at about 5 . The angular difference to avoid the crosstalk between the two display modes. between the two Bragg matching angles is 12.2 , which is large enough to avoid the crosstalk between the two display modes. (a) (b) (c) (d) Figure 6. Performance testing of MHOE: (a) Diffraction effect of the MLA; (b) diffraction effect of the diffuser; (c) optical Figure 6. Performance testing of MHOE: (a) Diffraction effect of the MLA; (b) diffraction effect of the diffuser; (c) optical see-through properties of MHOE; (d) relationship between normalized diffraction efficiency and angle deviation. see-through properties of MHOE; (d) relationship between normalized diffraction efficiency and angle deviation. 3.2. Experimental Results 3.2. Experimental Results We developed the experimental system based on the proposed scheme as shown in We developed the experimental system based on the proposed scheme as shown in Figure 7a. The projector has the resolution of 1280 × 800. The projected image for the 3D Figure 7a. The projector has the resolution of 1280  800. The projected image for the 3D mode is provided as shown in Figure 7b. The character “3” and letter “D” are in front of mode is provided as shown in Figure 7b. The character “3” and letter “D” are in front of the MHOE with the depth of 35 mm and behind the MHOE with the depth of −35 mm, the MHOE with the depth of 35 mm and behind the MHOE with the depth of 35 mm, respectively. In addition, a 2D text “Sichuan University” is prepared for 2D display as shown in Figure 7c. Photonics 2021, 8, x FOR PEER REVIEW 7 of 9 Photonics 2021, 8, x FOR PEER REVIEW 7 of 9 Photonics 2021, 8, 297 7 of 9 respectively. In addition, a 2D text “Sichuan University” is prepared for 2D display as respectively. In addition, a 2D text “Sichuan University” is prepared for 2D display as shown in Figure 7c. shown in Figure 7c. (a) (b) (c) (a) (b) (c) Figure 7. (a) Experimental system; (b) elemental image array; (c) 2D image. Figure 7. (a) Experimental system; (b) elemental image array; (c) 2D image. Figure 7. (a) Experimental system; (b) elemental image array; (c) 2D image. On the 3D display mode, the projected beam meets the Bragg diffraction condition 1. On the 3D display mode, the projected beam meets the Bragg diffraction condition 1. On the 3D display mode, the projected beam meets the Bragg diffraction condition 1. The 3D image is reconstructed, and four different perspectives from top left, top right, The 3D image is reconstructed, and four different perspectives from top left, top right, The 3D image is reconstructed, and four different perspectives from top left, top right, bottom left and bottom right viewing points are shown in Figure 8a. Obvious horizontal bottom left and bottom right viewing points are shown in Figure 8a. Obvious horizontal bottom left and bottom right viewing points are shown in Figure 8a. Obvious horizontal and and vertical vertical parallaxes parallaxes c can an b be e c clearly learly seen. seen. Add Additionally itionally, , t the he ch character aracter “3” “3” is is b bigger igger t than han and vertical parallaxes can be clearly seen. Additionally, the character “3” is bigger than let letter ter “D “D”, ”, w which hich dem demonstrates onstrates t that hat “ “3” 3” is recon is reconstr struct ucted ed in in front front of of tthe he MHOE w MHOE while hile “D “D” ” letter “D”, which demonstrates that “3” is reconstructed in front of the MHOE while “D” i is s behi behind nd the MHOE. On the MHOE. On the the 2 2D D di display splay mo mode, de, the the pr projected beam sa ojected beam satisfies tisfies the Bra the Bragg gg is behind the MHOE. On the 2D display mode, the projected beam satisfies the Bragg dif diffraction fraction co condition ndition2, 2and , and F Figur igu e re 8b 8b shows shows t the displayed he displayed 2D image. 2D imThe age.r The eal object real ob “dice” ject diffraction condition 2, and Figure 8b shows the displayed 2D image. The real object behind the MHOE is always visible on both 2D and 3D display modes, which verifies the “dice” behind the MHOE is always visible on both 2D and 3D display modes, which “dice” behind the MHOE is always visible on both 2D and 3D display modes, which good optical see-through properties of the system. verifies the good optical see-through properties of the system. verifies the good optical see-through properties of the system. (a) (b) (a) (b) Figure 8. Experimental results of 2D/3D compatible display with optical see-through properties: (a) Figure 8. Experimental results of 2D/3D compatible display with optical see-through properties: (a) Figure 8. Experimental results of 2D/3D compatible display with optical see-through properties: 3D images display captured from four different viewing directions; (b) 2D image display. 3D images display captured from four different viewing directions; (b) 2D image display. (a) 3D images display captured from four different viewing directions; (b) 2D image display. 4. Conclusions 4. Conclusions 4. Conclusions In this paper, an optical see-through 2D/3D compatible display system is proposed In this paper, an optical see-through 2D/3D compatible display system is proposed by In this paper, an optical see-through 2D/3D compatible display system is proposed by using a variable-focus lens and a MHOE. The scattering wavefront and spherical using a variable-focus lens and a MHOE. The scattering wavefront and spherical wavefront by using a variable-focus lens and a MHOE. The scattering wavefront and spherical wavefront array are recorded in the MHOE using angular multiplexing technology to array are recorded in the MHOE using angular multiplexing technology to achieve 2D/3D wavefront array are recorded in the MHOE using angular multiplexing technology to achieve 2D/3D display, respectively. The variable-focus lens is used to adjust the incident display, respectively. The variable-focus lens is used to adjust the incident angles of the achieve 2D/3D display, respectively. The variable-focus lens is used to adjust the incident angles of the projected beam. By analyzing the relationship between the liquid change projected beam. By analyzing the relationship between the liquid change and the radius of angles of the projected beam. By analyzing the relationship between the liquid change and the radius of curvature of the variable-focus lens, the optimal diopter of the varia- curvature of the variable-focus lens, the optimal diopter of the variable-focus lens is picked and the radius of curvature of the variable-focus lens, the optimal diopter of the varia- ble-focus lens is picked up to match the Bragg diffraction conditions. The incident angles up to match the Bragg diffraction conditions. The incident angles of the projected beam ble-focus lens is picked up to match the Bragg diffraction conditions. The incident angles of the projected beam are calculated and measured. When the MLA and diffuser were are calculated and measured. When the MLA and diffuser were recorded on holographic of the projected beam are calculated and measured. When the MLA and diffuser were plates recorded based on on hologr angular aphic multiplexing plates based technology on angular , the mult variable-focus iplexing technology, lens is placed the var in the ia- recorded on holographic plates based on angular multiplexing technology, the varia- ble-focus lens is placed in the path of the reference beam to adjust the incident angles of path of the reference beam to adjust the incident angles of the reference beam through ble-focus lens is placed in the path of the reference beam to adjust the incident angles of the reference the variation in beam throug the liquid pumping h the variatio volume. n in the li During quid pu reconstr mpiuction, ng volu the me. Du incident ring recon- angles the reference beam through the variation in the liquid pumping volume. During recon- struction, the of the projected incid beam ent ang satisfied les of the pro the Bragg jected bea matching m sa angles tisfied the Bra under the gg ma same tchi amount ng anglof es struction, the incident angles of the projected beam satisfied the Bragg matching angles liquid pumping volume, and the spherical wavefront array of the MLA and the scattering under the same amount of liquid pumping volume, and the spherical wavefront array of under the same amount of liquid pumping volume, and the spherical wavefront array of wavefront of diffuser are reconstructed. As a result, 3D images with proper horizontal and the MLA and the scattering wavefront of diffuser are reconstructed. As a result, 3D im- the MLA and the scattering wavefront of diffuser are reconstructed. As a result, 3D im- vertical parallaxes are reconstructed on 3D display mode and 2D image is displayed on ages with proper horizontal and vertical parallaxes are reconstructed on 3D display ages with proper horizontal and vertical parallaxes are reconstructed on 3D display 2D display mode, and both modes show good optical see-through properties. The images; mode and 2D image is displayed on 2D display mode, and both modes show good opti- mode and 2D image is displayed on 2D display mode, and both modes show good opti- quality would be further improved if a variable-focus liquid lens with multi-chamber is used. The proposed system can be a good candidate in AR application. Photonics 2021, 8, 297 8 of 9 Author Contributions: Conceptualization, Q.J.; methodology, H.D., H.Z. and W.J.; software, F.Z.; validation, Q.J. and F.R.; data curation and writing-original draft preparation, Q.J.; writing—review and editing, H.D. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Foundation of China, grant numbers 61972147 and 61775151, and the China Postdoctoral Science Foundation, grant number 2021M690287. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the findings of this study are available from the corresponding author upon reasonable request. Conflicts of Interest: The authors declare no conflict of interest. References 1. Zhou, F.; Duh, H.B.L.; Billinghurst, M. Trends in augmented reality tracking, interaction and display: A review of ten years of ISMAR. In Proceedings of the 7th IEEE/ACM International Symposium on Mixed and Augmented Reality, Washington, DC, USA, 15–18 September 2018; pp. 193–202. 2. Lee, Y.H.; Zhan, T.; Wu, S.T. Prospects and challenges in augmented reality displays. Virtual Real. Intell. 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Journal

PhotonicsMultidisciplinary Digital Publishing Institute

Published: Jul 27, 2021

Keywords: optical see-through 2D/3D; variable-focus lens; holographic optical elements

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