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3D compatible integral imaging display system using lens-array holographic optical element and polymer dispersed liquid crystal
Optics Communications 456 (2020) 124615
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
See-through 2D/3D compatible integral imaging display system using lens-array holographic optical element and polymer dispersed liquid crystal Han-Le Zhang a,b , Huan Deng b , Hui Ren b , Min-Yang He b , Da-Hai Li b , Qiong-Hua Wang a ,∗ a b
School of Instrumentation and Optoelectronic Engineering, Beihang University, Beijing 100191, China School of Electronics and Information Engineering, Sichuan University, Chengdu 610065, China
ABSTRACT A see-through 2D/3D compatible integral imaging display system including two projectors and a projection screen is proposed. The projection screen consists of a lens-array holographic optical element (LAHOE) and polymer dispersed liquid crystal (PDCL) film. The projection screen can operate at two different modes: the see-through and scattering display mode. With voltage, the projection screen works in the see-through display mode, 3D image for Bragg matched image can be presented, and 2D image can also be presented. Without voltage, the projection screen works in the scattering display mode, the 2D and 3D images can be presented. The prototype of the 2D/3D compatible integral imaging display system is developed. Experimental results confirm the feasibility of the proposed system.
1. Introduction Three-dimensional (3D) display is an attractive topic in the field of display technology, which includes holographic display [1–4], integral imaging display [5–8], volumetric display [9,10], etc. Integral imaging firstly proposed in 1908 by G. Lippmann [11] is considered to be one of the most promising 3D displays. It has the advantages of continuous viewpoint, full parallax, no need of special wearable equipment [12– 14]. It will open up a new market for display industry. However, integral imaging has some issues that need to be solved, such as 2D/3D compatible display, see-through 3D display, etc. Augmented reality has developed rapidly in recent years, which presents virtual images overlaid on real word scenes. 3D display can provide virtual images that are naturally merged into the real scene, which include physiological depth cues. The development of optical see-through 3D display is the trend of the next generation of augmented reality. Recently, lens-array holographic optical element (LAHOE) recorded on the photopolymer material was proposed for the optical see-through 3D display [15–18]. The LAHOE is transparent to the ambient light which makes it suitable for see-through display. In order to realize the rapid commercialization of 3D display, it is necessary to realize the see-through 2D/3D compatible integral imaging display, and it also can be used in automobile navigation, commercial exhibitions and so on. For example, when a see-through 2D/3D compatible integral imaging display is used in an automobile, it can represent 2D real-time vehicle condition information, such as speed, fuel volume, time, etc., and also provide 3D navigation information for the driver. When a see-through 2D/3D compatible integral imaging display is used for commercial exhibitions, it can represent 3D merchandise detail display, and also provide 2D information description for the consumers.
In order to realize the 2D/3D compatible display for the conventional integral imaging, many teams did a lot of research. National Chiao Tung University released a 2D/3D hybrid integral imaging display system by using fast switchable hexagonal liquid crystal lens array [5]; University of Science and Technology of China developed a convertible 2D/3D display using an edge-lit light guide plate based on integral imaging [19]; Pukyung National University released a 3D/2D convertible integral imaging display system using an active mask [20]; Sun Yat-sen University developed a 2D/3D switchable directional-backlight autostereoscopic display using PDLC films [21]; University of Calabria released a 2D/3D switchable display through PDLC reverse mode parallax barrier [22]. However, these 2D/3D compatible display methods or systems cannot be applied to see-through display. In order to realize the 2D/3D compatible integral imaging and see-through display simultaneously, Seoul National University developed a 2D/3D convertible projection screen see-through integral imaging based on holographic optical element (HOE) [17], and released a two-dimensional and three-dimensional transparent screens based on lens-array holographic optical elements [18]. In [17] and [18], the HOE needs to be recorded many times, and the diffraction efficiency of each recording is different, which makes the fabrication of HOE more difficult. In addition, there are still many problems to be solved in commercial display applications in terms of system complexity and display size. In our former research work, we reported an integral imagingbased 2D/3D convertible display system by using HOE and PDLC [23], but 2D and 3D contents cannot be displayed simultaneously. In this paper, we proposed a see-through 2D/3D compatible integral imaging display system using projection screen and two projectors. The projection screen consists of a LAHOE film and a PDLC film. The
∗ Corresponding author. E-mail addresses: [email protected] (D.-H. Li), [email protected] (Q.-H. Wang).
https://doi.org/10.1016/j.optcom.2019.124615 Received 31 May 2019; Received in revised form 19 August 2019; Accepted 22 September 2019 Available online 24 September 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
H.-L. Zhang, H. Deng, H. Ren et al.
Optics Communications 456 (2020) 124615
LAHOE film and it does not affect the Bragg condition required for the LAHOE film to reconstruct 3D images. Projector II projects 2D images onto the screen, and the 2D images are presented. The ambiguity of visible light of the PDLC film is 92%. So, it is only equivalent to a ground glass screen, and the 2D image can be presented. 2.1. Fabrication and reconstruction principle of the LAHOE film The fabrication principle of the LAHOE film is shown in Fig. 2(a). The LAHOE film is fabricated by the photopolymer material using the method of volume hologram, and the LAHOE film has angular and wavelength selectivity. In the fabrication process, the green collimated beam is incident into the micro-lens array with the angle a = 45◦ to form the spherical wavefront array. The size of the micro-lens array is 150 mm × 150 mm, the pitch of the elemental lens is 1 mm, and the focal length is 3.3 mm. The spherical wavefront array interferes with the spherical wave which incidents into the photopolymer material with a pair of angles of 𝜃1 and 𝜃2 , then the interference fringes are recorded on the photopolymer material. The spherical wave is generated by the green laser point source. The spherical wavefront array is opposite to the spherical wave incident into the photopolymer material. The reconstruction principle of the LAHOE film is shown in Fig. 2(b). Compared with the conventional HOE, the spherical waves are used as reference beam in our fabrication process. The LAHOE film is fabricated by the volume hologram principle which has angular and wavelength selectivity. When the Bragg condition is satisfied, the LAHOE film can reconstruct the diffracted beam, i.e., the projection beam in the reconstruction process is same as the spherical wave in the fabrication process, the LAHOE film can reconstruct the spherical wavefront array of the micro-lens array. When the incident angle of the projection beam deviates from 𝜃1 and 𝜃2 , the Bragg condition cannot be satisfied, and the diffracted beam of the LAHOE is disappeared or distorted. When the deviation angle of the projection beam is within a certain range, the spherical wavefront array reconstructed by the LAHOE film is distorted. When the deviation angle of the projection beam is sufficiently large, the LAHOE cannot reconstruct the spherical wavefront array. Then the LAHOE film cannot be used as an imaging optics. This characteristic of the LAHOE film makes it possible to realize 2D and 3D compatible display of integral imaging in different directions.
Fig. 1. Schematic of the 2D/3D compatible integral imaging display system.
projection screen can operate at two different modes: the see-through and scattering display mode. With voltage, the projection screen works in the see-through display mode, the 3D image for the Bragg-matched image can be presented, and the 2D image also can be presented. Without voltage, the projection screen works in the scattering display mode, the 2D and 3D images can be presented. The fabrication and reconstruction principle of the LAHOE film are analyzed, the see-through and scattering display mode of the projection screen are proposed, and the imaging characteristics of the projection screen are studied. The experimental results show that the 2D/3D compatible integral imaging display and see-through display can be realized in the seethrough display mode, and the 2D/3D compatible integral imaging display also can be realized in the scattering display mode. The issues of the system complexity and limited display size are solved. Our system is compact. It can directly realize 2D/3D compatible integral imaging display without any additional collimation optics, and the display size is no longer limited. 2. Principle The schematic of the 2D/3D compatible integral imaging display system composed of a projection screen, projectors I and II are shown in Fig. 1. The projection screen includes an LAHOE film and a PDLC film, and the PDLC film is tightly behind the LAHOE film. The projection screen can switch between two different modes: the see-through and scattering display modes. With voltage, the projection screen works in the see-through display mode, projector I projects 3D images onto the projection screen, and the incident angles of the projection beam are the same as that of the reference beam in the fabrication process of the LAHOE film. The Bragg condition is satisfied, and the 3D images are reconstructed. It is equivalent to put a thin transparent glass on the back of the LAHOE film, and it does not affect the Bragg condition which is required for the LAHOE film to reconstruct 3D images. Projector II projects 2D images onto the projection screen, and then the 2D images are presented. The visible light transmittance of the LAHOE film and the PDLC film in the transparent mode are 80% and 84%, respectively, thus the visible light transmittance of the projection screen is about 67.2%. The averaged refractive index of the LAHOE film is the same as that of the PDLC film, which is 1.47. It is equivalent to a glass with low transmittance. So, the 2D image can be presented. Without voltage, the projection screen works in the scattering display mode, the display process of the 3D images is the same as that in the see-through display mode. Since the PDLC can scatter the ambient light, it is equivalent to put a thin ground glass on the back of the
2.2. Principle of the projection screen The working principle of the PDLC film in the transparent mode and the projection screen in the see-through display mode are shown in Fig. 3. The PDLC film is composed of liquid crystal (LC) molecules which are encapsulated by polymers [24]. With voltage, the PDLC film operates at transparent mode and the projection screen works in the see-through display mode. The LC molecules of the PDLC film from an irregular arrangement to an orderly arrangement. Thence the refractive index of the LC molecules is equal to that of the polymers, and the light can pass through the PDLC film, as shown in Fig. 3(a). The LAHOE film has angular and wavelength selectivity, so the projection screen also has angular and wavelength selectivity. Projector I project 3D images into the screen with a pair of angles of 𝜃1 and 𝜃2 . The Bragg condition is satisfied, and the 3D images are reconstructed, as shown in Fig. 3(b). It is equivalent to put a thin transparent glass on the back of the LAHOE film, it does not affect the Bragg condition required for the LAHOE film to reconstruct 3D images. Projector II projects 2D images into the screen with a pair of angles of 𝜃3 and 𝜃4 , and the 2D images are presented, as shown in Fig. 3(b). The visible light transmittance of the projection screen is about 67.2%. It is equivalent to a glass with low transmittance. When the projection screen works in the see-through display mode, which can be applied to augmented reality, head-up display, etc. Without voltage, the PDLC film operates at scattering mode and the projection screen works in the scattering display mode, as shown 2
H.-L. Zhang, H. Deng, H. Ren et al.
Optics Communications 456 (2020) 124615
Fig. 2. (a) Fabrication and (b) reconstruction principle of the LAHOE film.
Fig. 3. (a) Working principle of the PDLC film in the transparent mode, (b) the projection screen works in the see-through display mode.
Fig. 4. (a) Working principle of PDLC film in the scattering mode, (b) the projection screen works in the scattering display mode.
in Fig. 4. The LC molecules of the PDLC film changes from ordered arrangement to irregular arrangement. The refractive index of the LC molecules and the refractive index of the polymers become different,
and the light cannot pass through the PDLC film, as shown in Fig. 4(a). In the scattering mode, the projection screen also has angular and wavelength selectivity. The reconstruction principle of 3D and 2D 3
H.-L. Zhang, H. Deng, H. Ren et al.
Optics Communications 456 (2020) 124615 Table 1 Experiment parameters for the fabrication of the LAHOE and PDLC.
images are the same as in the see-through display mode. Projector I projects the 3D images into the screen with a pair of angles of 𝜃1 and 𝜃2 , and 3D image is reconstructed. It is equivalent to fitting a ground glass behind the LAHOE film. Projector II projects 2D images into the screen with a pair of angles of 𝜃3 and 𝜃4 , and the 2D images are presented. It is only equivalent to a ground glass screen, as shown in Fig. 4(b). When the projection screen works in the scattering display mode, it can be used to present 2D and 3D images, etc.
Components
Parameters
Values
LAHOE film
Resolution Thickness Averaged refractive index Sensitive wavelength Sensitivity Visible light transmittance
12 000 line/mm 15 μm 1.47 532 nm 10 mJ/cm2 80%
PDLC film
Thickness Response time Visible light transmittance Ambiguity of visible light Averaged refractive index
650 μm 200 ms 84% 92% 1.47
2.3. Viewing characteristics of the 2D/3D compatible display system The viewing characteristics of the 2D/3D compatible integral imaging display system include the viewing angle, the depth of the 3D image, the resolution of the 2D and 3D images. As shown in Fig. 4(b), according to the geometric optics, the viewing angle of the 3D image 𝜃 can be expressed: 𝜃 = arctan
2𝑑 tan 𝛼 2𝑑 tan 𝛼 − arctan , 2𝑑 − 𝑝 tan 𝛼 2𝑑 + 𝑝 tan 𝛼
LAHOE film can reconstruct the spherical wavefront array only when the incident angle of the reference beam satisfies the Bragg condition. When the incident angle of the projection beam deviates from 𝜃1 and 𝜃2 , the Bragg condition cannot be satisfied, and the diffraction beam of the LAHOE film is disappeared or distorted. The maximum deviation angle which cannot be reproduced the diffracted beam is measured. The measurement formula of the diffraction efficiency can be expressed [25]:
(1)
where a is tilt angle of the spherical wavefront array, p is the pitch of the elemental lens in the LAHOE film, d is the distance between the LAHOE film and the focal plan of the LAHOE film, L is the view distance of the 3D image. When the projection screen works in the seethrough display mode, it is equivalent to a glass with low transmittance. When the projection screen works in the scattering display mode, it is equivalent to a ground glass screen. So, the viewing angle of the 2D image is the same as the conventional projection screen. The image on the projection screen is a second projection image after homography transformation, so there is no lateral distortion. Suppose the projected image size of the projector is M, the resolution of the 3D image can be expressed: 𝑅𝐼 =
𝑀 , 𝑝
𝜂=
𝐼𝐷 , 𝐼𝑇 + 𝐼𝐷
(3)
where 𝐼D and 𝐼T represent the intensity of the diffracted and transmitted beam, respectively. During the measurement, the incident angle of the reference beam is controlled by a rotation table, and the intensity of the diffracted beam of the LAHOE film is obtained by an optical power meter. The diffraction efficiency of the LAHOE film is measured, as shown in Fig. 6(b). When the deviation angle of the projection beam is between −10◦ and 10◦ , the diffraction beam is almost disappeared. Therefore, the deviation angle of the reference beam is about 20◦ , as shown in Fig. 6(b).
(2)
and the resolution of the 2D image is equal to the resolution of projector II.
3.2. Experiment on the see-through 2D/3D compatible display system
3. Experiments
The experimental setup diagram of the see-through 2D/3D compatible integral imaging display system is shown in Fig. 7. In the see-through display mode, the projection beam presented by projector I is incident into the projection screen with a pair of angles 𝜃1 = 59◦ , and 𝜃2 = 75◦ . The projection beam contains information about the elemental image array (EIA) which satisfies the Bragg condition, and the 3D image is reconstructed. The projection beam presented by projector II is incident into the projection screen with a pair of angles 𝜃3 = 75◦ , and 𝜃4 = 91◦ . The projection beam contains information about the 2D image, and the 2D image is presented. In order to prevent crosstalk between the reconstructed 3D and 2D images, the incident angles of projectors I and II are strictly controlled. The projection screen has angular and wavelength selectivity, when the deviation angle of the projection beam is sufficiently large, the projection screen cannot reconstruct the spherical wavefront array. It is assumed that the deviation angle of the reference beam is 𝛺, which cannot be reconstructed the spherical wavefront array by the projection screen. The interval between the incident angles of projector I and II must be greater than 𝛺, so that the reconstructed 3D and 2D images cannot produce crosstalk. The corresponding formula can be expressed:
3.1. Fabrication of the LAHOE film The schematic of fabricating the LAHOE film is shown in Fig. 5. The green-sensitive photopolymer materials are used to fabricate the LAHOE film. The thickness, resolution, sensitivity and average refractive index of the photopolymer materials are 15 μm, 12 000 line/mm, 10 mJ/cm2 and 1.47, respectively. The green laser is divided into two beams in the horizontal and vertical directions by a beam splitter (BS). The laser beam in the horizontal direction is expanded by a spatial filter (SF) and a collimating lens (CL), then the laser beam is incident into the micro-lens array with the angle a = 45◦ to form a spherical wavefront array. The size of the micro-lens array is 150 mm × 150 mm, the pitch of the elemental lens is 1 mm, and the focal length is 3.3 mm. Another laser beam in the vertical direction passes through the divergent lens (DL) to from spherical wave. When the spherical wave is incident into the holographic material, the incident angle is controlled at 𝜃1 = 59◦ , and 𝜃2 = 75◦ . The spherical wavefront array and the spherical wave are incident into the holographic material from opposite directions. They interfere with each other and the interference pattern is recorded on the holographic material. After the post-processing, the fabrication of the LAHOE film is done. The detailed parameters of the experiment are listed in Table 1. The electronic shutter (ES) is used for controlling the exposure of the holographic materials. An LAHOE film with a size of 80 mm × 80 mm is fabricated. When the original spherical wave is used for reproducing the spherical wavefront array of the micro-lens array, the Bragg condition of the LAHOE film is satisfied, and the wavefront array of the micro-lens array is reproduced, which is shown in Fig. 6(a). The
𝛺 < 𝜋 − 𝜃2 − 𝜃3 ,
(4)
𝛺 < 𝜋 − 𝜃1 − 𝜃4 ,
(5)
where 𝜃1 and 𝜃2 are the incident angles of projector I, 𝜃3 and 𝜃4 are the incident angles of projector II, as shown in Fig. 7. According to Eqs. (4) and (5), the interval between the incident angles of projector I and II are greater than 𝛺. So, there is no crosstalk between the reconstructed 3D and 2D images. 4
H.-L. Zhang, H. Deng, H. Ren et al.
Optics Communications 456 (2020) 124615
Fig. 5. Schematic of fabricating the LAHOE film.
Fig. 6. (a) Reconstructed spherical wavefront array of the LAHOE film, and (b) diffraction efficiency versus the angle deviation of the projection beam.
In the scattering mode, the projection beam presented by projector I is incident into the projection screen with a pair of angles 𝜃1 = 59◦ , and 𝜃2 = 75◦ . The projection screen is equivalent to a ground glass behind the LAHOE film, which does not affect the Bragg condition of the LAHOE film to reconstruct 3D image. The projection beam presented by projector II is incident into the projection screen with a pair of angles 𝜃3 = 75◦ , and 𝜃4 = 91◦ . It is only equivalent to a ground glass screen, and the 2D image is presented. According to the angular selectivity of the projection screen, there is no crosstalk between the reconstructed 3D and 2D images. A projector (Philips PPX4935) is used to project the 3D and 2D contents. The distance between the projection lens of the projectors and the projection screen is 31 cm. Projectors I and II are 12 cm apart from each other in vertical height. Projector I is 18 cm away from the optical table, and placed parallelly to the optical table. Projector II is 30 cm away from the optical table, and placed at an angle of 20◦ downward. The height of the projection screen is 19.5 cm from the optical table. The distance between the projection screen and the car model is 1.5 cm. The resolution of the projector is 1280 × 720. The 3D content is used for projector I, which is included of 75 × 75 elemental images. It is generated by a computer and consisted of the characters ‘‘3’’ and ‘‘D’’, ‘‘3’’ is located at −10 mm, and ‘‘D’’ is located at +10 mm, which is shown in Fig. 8(a). The 2D content is used for projector II, which is generated by a computer and consisted of the words ‘‘Sichuan’’ and ‘‘University’’, and it is shown in Fig. 8(b). The parameters of the 2D/3D compatible integral imaging display prototype are shown in Table 2. When the projection screen works in the see-through display mode, it can be applied to augmented reality, head-up display, etc. Projection
Fig. 7. Experimental setup diagram of the see-through 2D/3D compatible integral imaging display system.
beam I presented by projector I which is incident into the projection screen, and the 3D image is reconstructed. The projection beam II presented by projector II which is incident into the projection screen, and the 2D image is presented. The 2D and 3D images can be presented. The left, right, top and bottom viewpoints are obtained. The horizontal and vertical parallaxes are viewed in Fig. 9(a). The obvious horizontal and vertical parallaxes can be present in Visualization 1. The presented 2D 5
H.-L. Zhang, H. Deng, H. Ren et al.
Optics Communications 456 (2020) 124615
Fig. 8. (a) 3D content used for projection beam of projector I, (b) 2D content used for projection beam of projector II.
Fig. 9. (a) Presented 2D and 3D images in the see-through display mode of the projection screen (see Visualization 1), (b) presented 2D and 3D images in the scattering display mode of the projection screen (see Visualization 2).
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H.-L. Zhang, H. Deng, H. Ren et al.
Optics Communications 456 (2020) 124615
Table 2 Parameters of the prototype system. Components
Parameters
Values
Projectors I and II
Model Resolution
Philips PPX4935 1280 × 720
Projection screen in the see-through display mode
Visible light transmittance 2D image viewing angle 2D image resolution 3D image viewing angle 3D image resolution
67.2% ≥120◦ 1280 × 720 8.7◦ 75 × 75
Projection screen in the scattering display mode
Ambiguity of visible light 2D image viewing angle 2D image resolution 3D image viewing angle 3D image resolution
92% ≥120◦ 1280 × 720 8.7◦ 75 × 75
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and 3D images can be combined with the ‘‘Car’’, which can realize the function of virtual and real fusion. When the projection screen works in scattering display mode, it also can be used to present 2D and 3D images. Projection beams I and II are incident into the projection screen. The 2D and 3D images can be presented. The left, right, top and bottom viewpoints are obtained. The horizontal and vertical parallaxes are viewed in Fig. 9(b). The horizontal and vertical parallaxes can be clearly observed in Visualization 2. 4. Conclusion In this paper, we propose a see-through 2D/3D compatible integral imaging display system using an LAHOE film and a PDLC film. The projection screen can directly realize 2D/3D compatible display without any additional collimation optics, and the display size is no longer limited. When the projection screen works in the see-through display mode, it can be used to present 2D and 3D images, which can realize the see-through 2D/3D compatible integral imaging display and can be applied to augmented reality, head-up display, etc. When the projection screen works in the scattering display mode, it also can be used to present 2D and 3D images, which can also realize the 2D/3D compatible integral imaging display. The prototype of the system is developed. Experimental results confirm the feasibility of the proposed system. Funding information National Key Research and Development Program of China under Grant No. 2017YFB1002900 and National Natural Science Foundation of China (NSFC) (61535007, 61775151). References [1] Y. Zhao, L.C. Cao, H. Zhang, D.Z. Kong, G.F. Jin, Accurate calculation of computer-generated holograms using angular-spectrum layer-oriented method, Opt. Express. 23 (20) (2015) 25440–25449. [2] Q. Gao, J. Liu, J. Han, X. Li, Monocular 3d see-through head-mounted display via complex amplitude modulation, Opt. Express 24 (15) (2016) 17372.
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
See-through 2D/3D compatible integral imaging display system using lens-array holographic optical element and polymer dispersed liquid crystal Han-Le Zhang a,b , Huan Deng b , Hui Ren b , Min-Yang He b , Da-Hai Li b , Qiong-Hua Wang a ,∗ a b
School of Instrumentation and Optoelectronic Engineering, Beihang University, Beijing 100191, China School of Electronics and Information Engineering, Sichuan University, Chengdu 610065, China
ABSTRACT A see-through 2D/3D compatible integral imaging display system including two projectors and a projection screen is proposed. The projection screen consists of a lens-array holographic optical element (LAHOE) and polymer dispersed liquid crystal (PDCL) film. The projection screen can operate at two different modes: the see-through and scattering display mode. With voltage, the projection screen works in the see-through display mode, 3D image for Bragg matched image can be presented, and 2D image can also be presented. Without voltage, the projection screen works in the scattering display mode, the 2D and 3D images can be presented. The prototype of the 2D/3D compatible integral imaging display system is developed. Experimental results confirm the feasibility of the proposed system.
1. Introduction Three-dimensional (3D) display is an attractive topic in the field of display technology, which includes holographic display [1–4], integral imaging display [5–8], volumetric display [9,10], etc. Integral imaging firstly proposed in 1908 by G. Lippmann [11] is considered to be one of the most promising 3D displays. It has the advantages of continuous viewpoint, full parallax, no need of special wearable equipment [12– 14]. It will open up a new market for display industry. However, integral imaging has some issues that need to be solved, such as 2D/3D compatible display, see-through 3D display, etc. Augmented reality has developed rapidly in recent years, which presents virtual images overlaid on real word scenes. 3D display can provide virtual images that are naturally merged into the real scene, which include physiological depth cues. The development of optical see-through 3D display is the trend of the next generation of augmented reality. Recently, lens-array holographic optical element (LAHOE) recorded on the photopolymer material was proposed for the optical see-through 3D display [15–18]. The LAHOE is transparent to the ambient light which makes it suitable for see-through display. In order to realize the rapid commercialization of 3D display, it is necessary to realize the see-through 2D/3D compatible integral imaging display, and it also can be used in automobile navigation, commercial exhibitions and so on. For example, when a see-through 2D/3D compatible integral imaging display is used in an automobile, it can represent 2D real-time vehicle condition information, such as speed, fuel volume, time, etc., and also provide 3D navigation information for the driver. When a see-through 2D/3D compatible integral imaging display is used for commercial exhibitions, it can represent 3D merchandise detail display, and also provide 2D information description for the consumers.
In order to realize the 2D/3D compatible display for the conventional integral imaging, many teams did a lot of research. National Chiao Tung University released a 2D/3D hybrid integral imaging display system by using fast switchable hexagonal liquid crystal lens array [5]; University of Science and Technology of China developed a convertible 2D/3D display using an edge-lit light guide plate based on integral imaging [19]; Pukyung National University released a 3D/2D convertible integral imaging display system using an active mask [20]; Sun Yat-sen University developed a 2D/3D switchable directional-backlight autostereoscopic display using PDLC films [21]; University of Calabria released a 2D/3D switchable display through PDLC reverse mode parallax barrier [22]. However, these 2D/3D compatible display methods or systems cannot be applied to see-through display. In order to realize the 2D/3D compatible integral imaging and see-through display simultaneously, Seoul National University developed a 2D/3D convertible projection screen see-through integral imaging based on holographic optical element (HOE) [17], and released a two-dimensional and three-dimensional transparent screens based on lens-array holographic optical elements [18]. In [17] and [18], the HOE needs to be recorded many times, and the diffraction efficiency of each recording is different, which makes the fabrication of HOE more difficult. In addition, there are still many problems to be solved in commercial display applications in terms of system complexity and display size. In our former research work, we reported an integral imagingbased 2D/3D convertible display system by using HOE and PDLC [23], but 2D and 3D contents cannot be displayed simultaneously. In this paper, we proposed a see-through 2D/3D compatible integral imaging display system using projection screen and two projectors. The projection screen consists of a LAHOE film and a PDLC film. The
∗ Corresponding author. E-mail addresses: [email protected] (D.-H. Li), [email protected] (Q.-H. Wang).
https://doi.org/10.1016/j.optcom.2019.124615 Received 31 May 2019; Received in revised form 19 August 2019; Accepted 22 September 2019 Available online 24 September 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
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Optics Communications 456 (2020) 124615
LAHOE film and it does not affect the Bragg condition required for the LAHOE film to reconstruct 3D images. Projector II projects 2D images onto the screen, and the 2D images are presented. The ambiguity of visible light of the PDLC film is 92%. So, it is only equivalent to a ground glass screen, and the 2D image can be presented. 2.1. Fabrication and reconstruction principle of the LAHOE film The fabrication principle of the LAHOE film is shown in Fig. 2(a). The LAHOE film is fabricated by the photopolymer material using the method of volume hologram, and the LAHOE film has angular and wavelength selectivity. In the fabrication process, the green collimated beam is incident into the micro-lens array with the angle a = 45◦ to form the spherical wavefront array. The size of the micro-lens array is 150 mm × 150 mm, the pitch of the elemental lens is 1 mm, and the focal length is 3.3 mm. The spherical wavefront array interferes with the spherical wave which incidents into the photopolymer material with a pair of angles of 𝜃1 and 𝜃2 , then the interference fringes are recorded on the photopolymer material. The spherical wave is generated by the green laser point source. The spherical wavefront array is opposite to the spherical wave incident into the photopolymer material. The reconstruction principle of the LAHOE film is shown in Fig. 2(b). Compared with the conventional HOE, the spherical waves are used as reference beam in our fabrication process. The LAHOE film is fabricated by the volume hologram principle which has angular and wavelength selectivity. When the Bragg condition is satisfied, the LAHOE film can reconstruct the diffracted beam, i.e., the projection beam in the reconstruction process is same as the spherical wave in the fabrication process, the LAHOE film can reconstruct the spherical wavefront array of the micro-lens array. When the incident angle of the projection beam deviates from 𝜃1 and 𝜃2 , the Bragg condition cannot be satisfied, and the diffracted beam of the LAHOE is disappeared or distorted. When the deviation angle of the projection beam is within a certain range, the spherical wavefront array reconstructed by the LAHOE film is distorted. When the deviation angle of the projection beam is sufficiently large, the LAHOE cannot reconstruct the spherical wavefront array. Then the LAHOE film cannot be used as an imaging optics. This characteristic of the LAHOE film makes it possible to realize 2D and 3D compatible display of integral imaging in different directions.
Fig. 1. Schematic of the 2D/3D compatible integral imaging display system.
projection screen can operate at two different modes: the see-through and scattering display mode. With voltage, the projection screen works in the see-through display mode, the 3D image for the Bragg-matched image can be presented, and the 2D image also can be presented. Without voltage, the projection screen works in the scattering display mode, the 2D and 3D images can be presented. The fabrication and reconstruction principle of the LAHOE film are analyzed, the see-through and scattering display mode of the projection screen are proposed, and the imaging characteristics of the projection screen are studied. The experimental results show that the 2D/3D compatible integral imaging display and see-through display can be realized in the seethrough display mode, and the 2D/3D compatible integral imaging display also can be realized in the scattering display mode. The issues of the system complexity and limited display size are solved. Our system is compact. It can directly realize 2D/3D compatible integral imaging display without any additional collimation optics, and the display size is no longer limited. 2. Principle The schematic of the 2D/3D compatible integral imaging display system composed of a projection screen, projectors I and II are shown in Fig. 1. The projection screen includes an LAHOE film and a PDLC film, and the PDLC film is tightly behind the LAHOE film. The projection screen can switch between two different modes: the see-through and scattering display modes. With voltage, the projection screen works in the see-through display mode, projector I projects 3D images onto the projection screen, and the incident angles of the projection beam are the same as that of the reference beam in the fabrication process of the LAHOE film. The Bragg condition is satisfied, and the 3D images are reconstructed. It is equivalent to put a thin transparent glass on the back of the LAHOE film, and it does not affect the Bragg condition which is required for the LAHOE film to reconstruct 3D images. Projector II projects 2D images onto the projection screen, and then the 2D images are presented. The visible light transmittance of the LAHOE film and the PDLC film in the transparent mode are 80% and 84%, respectively, thus the visible light transmittance of the projection screen is about 67.2%. The averaged refractive index of the LAHOE film is the same as that of the PDLC film, which is 1.47. It is equivalent to a glass with low transmittance. So, the 2D image can be presented. Without voltage, the projection screen works in the scattering display mode, the display process of the 3D images is the same as that in the see-through display mode. Since the PDLC can scatter the ambient light, it is equivalent to put a thin ground glass on the back of the
2.2. Principle of the projection screen The working principle of the PDLC film in the transparent mode and the projection screen in the see-through display mode are shown in Fig. 3. The PDLC film is composed of liquid crystal (LC) molecules which are encapsulated by polymers [24]. With voltage, the PDLC film operates at transparent mode and the projection screen works in the see-through display mode. The LC molecules of the PDLC film from an irregular arrangement to an orderly arrangement. Thence the refractive index of the LC molecules is equal to that of the polymers, and the light can pass through the PDLC film, as shown in Fig. 3(a). The LAHOE film has angular and wavelength selectivity, so the projection screen also has angular and wavelength selectivity. Projector I project 3D images into the screen with a pair of angles of 𝜃1 and 𝜃2 . The Bragg condition is satisfied, and the 3D images are reconstructed, as shown in Fig. 3(b). It is equivalent to put a thin transparent glass on the back of the LAHOE film, it does not affect the Bragg condition required for the LAHOE film to reconstruct 3D images. Projector II projects 2D images into the screen with a pair of angles of 𝜃3 and 𝜃4 , and the 2D images are presented, as shown in Fig. 3(b). The visible light transmittance of the projection screen is about 67.2%. It is equivalent to a glass with low transmittance. When the projection screen works in the see-through display mode, which can be applied to augmented reality, head-up display, etc. Without voltage, the PDLC film operates at scattering mode and the projection screen works in the scattering display mode, as shown 2
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Optics Communications 456 (2020) 124615
Fig. 2. (a) Fabrication and (b) reconstruction principle of the LAHOE film.
Fig. 3. (a) Working principle of the PDLC film in the transparent mode, (b) the projection screen works in the see-through display mode.
Fig. 4. (a) Working principle of PDLC film in the scattering mode, (b) the projection screen works in the scattering display mode.
in Fig. 4. The LC molecules of the PDLC film changes from ordered arrangement to irregular arrangement. The refractive index of the LC molecules and the refractive index of the polymers become different,
and the light cannot pass through the PDLC film, as shown in Fig. 4(a). In the scattering mode, the projection screen also has angular and wavelength selectivity. The reconstruction principle of 3D and 2D 3
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Optics Communications 456 (2020) 124615 Table 1 Experiment parameters for the fabrication of the LAHOE and PDLC.
images are the same as in the see-through display mode. Projector I projects the 3D images into the screen with a pair of angles of 𝜃1 and 𝜃2 , and 3D image is reconstructed. It is equivalent to fitting a ground glass behind the LAHOE film. Projector II projects 2D images into the screen with a pair of angles of 𝜃3 and 𝜃4 , and the 2D images are presented. It is only equivalent to a ground glass screen, as shown in Fig. 4(b). When the projection screen works in the scattering display mode, it can be used to present 2D and 3D images, etc.
Components
Parameters
Values
LAHOE film
Resolution Thickness Averaged refractive index Sensitive wavelength Sensitivity Visible light transmittance
12 000 line/mm 15 μm 1.47 532 nm 10 mJ/cm2 80%
PDLC film
Thickness Response time Visible light transmittance Ambiguity of visible light Averaged refractive index
650 μm 200 ms 84% 92% 1.47
2.3. Viewing characteristics of the 2D/3D compatible display system The viewing characteristics of the 2D/3D compatible integral imaging display system include the viewing angle, the depth of the 3D image, the resolution of the 2D and 3D images. As shown in Fig. 4(b), according to the geometric optics, the viewing angle of the 3D image 𝜃 can be expressed: 𝜃 = arctan
2𝑑 tan 𝛼 2𝑑 tan 𝛼 − arctan , 2𝑑 − 𝑝 tan 𝛼 2𝑑 + 𝑝 tan 𝛼
LAHOE film can reconstruct the spherical wavefront array only when the incident angle of the reference beam satisfies the Bragg condition. When the incident angle of the projection beam deviates from 𝜃1 and 𝜃2 , the Bragg condition cannot be satisfied, and the diffraction beam of the LAHOE film is disappeared or distorted. The maximum deviation angle which cannot be reproduced the diffracted beam is measured. The measurement formula of the diffraction efficiency can be expressed [25]:
(1)
where a is tilt angle of the spherical wavefront array, p is the pitch of the elemental lens in the LAHOE film, d is the distance between the LAHOE film and the focal plan of the LAHOE film, L is the view distance of the 3D image. When the projection screen works in the seethrough display mode, it is equivalent to a glass with low transmittance. When the projection screen works in the scattering display mode, it is equivalent to a ground glass screen. So, the viewing angle of the 2D image is the same as the conventional projection screen. The image on the projection screen is a second projection image after homography transformation, so there is no lateral distortion. Suppose the projected image size of the projector is M, the resolution of the 3D image can be expressed: 𝑅𝐼 =
𝑀 , 𝑝
𝜂=
𝐼𝐷 , 𝐼𝑇 + 𝐼𝐷
(3)
where 𝐼D and 𝐼T represent the intensity of the diffracted and transmitted beam, respectively. During the measurement, the incident angle of the reference beam is controlled by a rotation table, and the intensity of the diffracted beam of the LAHOE film is obtained by an optical power meter. The diffraction efficiency of the LAHOE film is measured, as shown in Fig. 6(b). When the deviation angle of the projection beam is between −10◦ and 10◦ , the diffraction beam is almost disappeared. Therefore, the deviation angle of the reference beam is about 20◦ , as shown in Fig. 6(b).
(2)
and the resolution of the 2D image is equal to the resolution of projector II.
3.2. Experiment on the see-through 2D/3D compatible display system
3. Experiments
The experimental setup diagram of the see-through 2D/3D compatible integral imaging display system is shown in Fig. 7. In the see-through display mode, the projection beam presented by projector I is incident into the projection screen with a pair of angles 𝜃1 = 59◦ , and 𝜃2 = 75◦ . The projection beam contains information about the elemental image array (EIA) which satisfies the Bragg condition, and the 3D image is reconstructed. The projection beam presented by projector II is incident into the projection screen with a pair of angles 𝜃3 = 75◦ , and 𝜃4 = 91◦ . The projection beam contains information about the 2D image, and the 2D image is presented. In order to prevent crosstalk between the reconstructed 3D and 2D images, the incident angles of projectors I and II are strictly controlled. The projection screen has angular and wavelength selectivity, when the deviation angle of the projection beam is sufficiently large, the projection screen cannot reconstruct the spherical wavefront array. It is assumed that the deviation angle of the reference beam is 𝛺, which cannot be reconstructed the spherical wavefront array by the projection screen. The interval between the incident angles of projector I and II must be greater than 𝛺, so that the reconstructed 3D and 2D images cannot produce crosstalk. The corresponding formula can be expressed:
3.1. Fabrication of the LAHOE film The schematic of fabricating the LAHOE film is shown in Fig. 5. The green-sensitive photopolymer materials are used to fabricate the LAHOE film. The thickness, resolution, sensitivity and average refractive index of the photopolymer materials are 15 μm, 12 000 line/mm, 10 mJ/cm2 and 1.47, respectively. The green laser is divided into two beams in the horizontal and vertical directions by a beam splitter (BS). The laser beam in the horizontal direction is expanded by a spatial filter (SF) and a collimating lens (CL), then the laser beam is incident into the micro-lens array with the angle a = 45◦ to form a spherical wavefront array. The size of the micro-lens array is 150 mm × 150 mm, the pitch of the elemental lens is 1 mm, and the focal length is 3.3 mm. Another laser beam in the vertical direction passes through the divergent lens (DL) to from spherical wave. When the spherical wave is incident into the holographic material, the incident angle is controlled at 𝜃1 = 59◦ , and 𝜃2 = 75◦ . The spherical wavefront array and the spherical wave are incident into the holographic material from opposite directions. They interfere with each other and the interference pattern is recorded on the holographic material. After the post-processing, the fabrication of the LAHOE film is done. The detailed parameters of the experiment are listed in Table 1. The electronic shutter (ES) is used for controlling the exposure of the holographic materials. An LAHOE film with a size of 80 mm × 80 mm is fabricated. When the original spherical wave is used for reproducing the spherical wavefront array of the micro-lens array, the Bragg condition of the LAHOE film is satisfied, and the wavefront array of the micro-lens array is reproduced, which is shown in Fig. 6(a). The
𝛺 < 𝜋 − 𝜃2 − 𝜃3 ,
(4)
𝛺 < 𝜋 − 𝜃1 − 𝜃4 ,
(5)
where 𝜃1 and 𝜃2 are the incident angles of projector I, 𝜃3 and 𝜃4 are the incident angles of projector II, as shown in Fig. 7. According to Eqs. (4) and (5), the interval between the incident angles of projector I and II are greater than 𝛺. So, there is no crosstalk between the reconstructed 3D and 2D images. 4
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Optics Communications 456 (2020) 124615
Fig. 5. Schematic of fabricating the LAHOE film.
Fig. 6. (a) Reconstructed spherical wavefront array of the LAHOE film, and (b) diffraction efficiency versus the angle deviation of the projection beam.
In the scattering mode, the projection beam presented by projector I is incident into the projection screen with a pair of angles 𝜃1 = 59◦ , and 𝜃2 = 75◦ . The projection screen is equivalent to a ground glass behind the LAHOE film, which does not affect the Bragg condition of the LAHOE film to reconstruct 3D image. The projection beam presented by projector II is incident into the projection screen with a pair of angles 𝜃3 = 75◦ , and 𝜃4 = 91◦ . It is only equivalent to a ground glass screen, and the 2D image is presented. According to the angular selectivity of the projection screen, there is no crosstalk between the reconstructed 3D and 2D images. A projector (Philips PPX4935) is used to project the 3D and 2D contents. The distance between the projection lens of the projectors and the projection screen is 31 cm. Projectors I and II are 12 cm apart from each other in vertical height. Projector I is 18 cm away from the optical table, and placed parallelly to the optical table. Projector II is 30 cm away from the optical table, and placed at an angle of 20◦ downward. The height of the projection screen is 19.5 cm from the optical table. The distance between the projection screen and the car model is 1.5 cm. The resolution of the projector is 1280 × 720. The 3D content is used for projector I, which is included of 75 × 75 elemental images. It is generated by a computer and consisted of the characters ‘‘3’’ and ‘‘D’’, ‘‘3’’ is located at −10 mm, and ‘‘D’’ is located at +10 mm, which is shown in Fig. 8(a). The 2D content is used for projector II, which is generated by a computer and consisted of the words ‘‘Sichuan’’ and ‘‘University’’, and it is shown in Fig. 8(b). The parameters of the 2D/3D compatible integral imaging display prototype are shown in Table 2. When the projection screen works in the see-through display mode, it can be applied to augmented reality, head-up display, etc. Projection
Fig. 7. Experimental setup diagram of the see-through 2D/3D compatible integral imaging display system.
beam I presented by projector I which is incident into the projection screen, and the 3D image is reconstructed. The projection beam II presented by projector II which is incident into the projection screen, and the 2D image is presented. The 2D and 3D images can be presented. The left, right, top and bottom viewpoints are obtained. The horizontal and vertical parallaxes are viewed in Fig. 9(a). The obvious horizontal and vertical parallaxes can be present in Visualization 1. The presented 2D 5
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Optics Communications 456 (2020) 124615
Fig. 8. (a) 3D content used for projection beam of projector I, (b) 2D content used for projection beam of projector II.
Fig. 9. (a) Presented 2D and 3D images in the see-through display mode of the projection screen (see Visualization 1), (b) presented 2D and 3D images in the scattering display mode of the projection screen (see Visualization 2).
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Optics Communications 456 (2020) 124615
Table 2 Parameters of the prototype system. Components
Parameters
Values
Projectors I and II
Model Resolution
Philips PPX4935 1280 × 720
Projection screen in the see-through display mode
Visible light transmittance 2D image viewing angle 2D image resolution 3D image viewing angle 3D image resolution
67.2% ≥120◦ 1280 × 720 8.7◦ 75 × 75
Projection screen in the scattering display mode
Ambiguity of visible light 2D image viewing angle 2D image resolution 3D image viewing angle 3D image resolution
92% ≥120◦ 1280 × 720 8.7◦ 75 × 75
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and 3D images can be combined with the ‘‘Car’’, which can realize the function of virtual and real fusion. When the projection screen works in scattering display mode, it also can be used to present 2D and 3D images. Projection beams I and II are incident into the projection screen. The 2D and 3D images can be presented. The left, right, top and bottom viewpoints are obtained. The horizontal and vertical parallaxes are viewed in Fig. 9(b). The horizontal and vertical parallaxes can be clearly observed in Visualization 2. 4. Conclusion In this paper, we propose a see-through 2D/3D compatible integral imaging display system using an LAHOE film and a PDLC film. The projection screen can directly realize 2D/3D compatible display without any additional collimation optics, and the display size is no longer limited. When the projection screen works in the see-through display mode, it can be used to present 2D and 3D images, which can realize the see-through 2D/3D compatible integral imaging display and can be applied to augmented reality, head-up display, etc. When the projection screen works in the scattering display mode, it also can be used to present 2D and 3D images, which can also realize the 2D/3D compatible integral imaging display. The prototype of the system is developed. Experimental results confirm the feasibility of the proposed system. Funding information National Key Research and Development Program of China under Grant No. 2017YFB1002900 and National Natural Science Foundation of China (NSFC) (61535007, 61775151). References [1] Y. Zhao, L.C. Cao, H. Zhang, D.Z. Kong, G.F. Jin, Accurate calculation of computer-generated holograms using angular-spectrum layer-oriented method, Opt. Express. 23 (20) (2015) 25440–25449. [2] Q. Gao, J. Liu, J. Han, X. Li, Monocular 3d see-through head-mounted display via complex amplitude modulation, Opt. Express 24 (15) (2016) 17372.
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