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Two-dimensional (2D) and three-dimensional (3D) transparent screens can be created using lens-array holographic optical elements (HOEs). Lens-array HOEs can be used to perform 2D and 3D imaging for Bragg matched images while maintaining the transparent properties of the images in the background scenes. 2D or 3D imaging on the proposed screen is determined by the relative size of an elemental-lens on the lens-array to a pixel on the projected image. The 2D and 3D displays on the lens-array HOEs are implemented by the diffusion of light on each elemental-lens and by taking advantage of reflection-type integral imaging, respectively. We constructed an HOE recording setup and recorded two lens-array HOEs having different optical specifications, permitting them to function as 2D and 3D transparent screens. Experiments regarding 2D and 3D imaging on the proposed transparent screens are carried out and the viewing characteristics in both cases are discussed. The experimental results show that the proposed screens are capable of providing 2D and 3D images properly while satisfying the see-through properties.
© 2014 Optical Society of America
1. Introduction
Transparent display technology has developed rapidly in recent years, due to its potential for use in a wide variety of applications in virtual reality and more sophisticated screens. Since this technique provides virtual images that are overlaid on a real-world scene, it would be perfectly suitable for virtual reality [1–3]. A head mounted display for a wearable device, a head-up display for navigating information, and a wall display on a regular glass window are examples of its applications. There are several ways to implement a transparent display. A transparent flat panel display is one candidate for a transparent display – for example, a transparent liquid crystal display and a transparent organic light emitting diode display [4, 5]. A transparent display based on a flat panel display, however, has limitations in terms of scaling to large-sized displays. Another method, which permits a large-sized transparent display to be implemented at a relatively low cost, is the use of a transparent screen that can diffuse projected images without losing their transparency. Hedili et al. used a microlens array [6, 7] and Hsu et al. adopted the principle of nanoparticle scattering [8] to implement a transparent screen. However, these methods had disadvantages such as the complicated fabrication procedure and wavelength selective problems, respectively.
Several research groups have recently reported on the development of transparent screens which are able to display three-dimensional (3D) virtual images, advancing one step further from displaying two-dimensional (2D) images. These studies adopted autostereoscopic 3D display techniques, such as a super multi-view display [9] and integral imaging methods [10–13], for the transparent screen. However, the total systems for these methods were unavoidably bulky because they contain additional optical structures of conventional autostereoscopic 3D displays in their systems. One promising approach to implementing a 3D screen is the use of a holographic screen as a vertical diffuser for multi-projection 3D displays [14–16]. By using the vertical diffusing characteristic of the holographic screen, these methods are capable of providing high resolution horizontal parallax only 3D images with multiple image projectors. However, the holographic screen is not appropriate for use as a transparent 3D screen for optical see-through virtual reality systems, because it cannot satisfy the see-through characteristics and is comprised of a bulky large sized system due to its semi-transparent characteristics and requirements of the multiple image projectors, respectively.
In our previous work, a 3D transparent screen was implemented by using a lens-array holographic optical element (HOE), which can be used to provide sufficient full-color integral imaging with satisfying the transparent property [13]. Since the lens-array HOE is made by a hologram recording method that is different from the typical, complicated implementation methods used for other 3D transparent screens, it would be expected to be easy to implement and also to have the advantage of being amenable to mass production, because of its simple optical recording. Furthermore, full parallax autostereoscopic 3D images can be provided by using a single image projector.
In this report, we propose a method for creating 2D and 3D (2D/3D) transparent screens based on lens-array HOEs. In the proposed method, the lens-array HOE can be used to create a 2D or 3D transparent screen simply by varying the ratio of the pitch of an elemental-lens on the lens-array HOE to that of a pixel on the projected image. The principles of this 2D/3D imaging using a conventional lens-array are described first. To provide transparent properties for 2D/3D imaging on the lens-array, lens-array HOEs were adopted for the proposed method instead of the conventional lens-arrays. The lens-array HOEs for implementing frontal projection-type 2D/3D transparent screens are made by a reflection-type hologram recording on a photopolymer [17]. Subsequently, the recording scheme and parameters of the reflection-type hologram are presented, and the 2D/3D imaging conditions of the recorded lens-array HOEs are presented in terms of optical parameters. Lastly, the feasibility of the proposed method was verified by display experiments using two lens-array HOEs with different optical parameters. The viewing characteristics of the proposed 2D/3D transparent screens were analyzed and were also experimentally verified.
2. Proposed method for implementing 2D/3D transparent screens
2.1 Principles of 2D/ 3D imaging on conventional lens-arrays with image projectors
The lens-array, which consisted of multiple lenses in a 2D array, has a variety of applications. It can be used as a diffuser for applications for a 2D screen, for achieving uniform illumination, to control of light, and so on [18–21]. When a lens-array is used for integral imaging, it functions as an optical element for autostereoscopic 3D imaging [22–24]. The roles of a lens-array as 2D/3D screens are the focus of this paper. The function of a lens-array as a 2D or 3D screen is determined by the relative size of an elemental-lens on the lens-array compared to the size of a projected pixel. Principles of 2D/3D imaging on lens-arrays are illustrated in Figs. 1(a) and 1(b), respectively, when the images are projected from image projectors. In the figures, images are assumed to be projected in parallel to the optic-axis of the lens-array for the sake of simplicity, and the projected pixels, which have different colors and intensity information, are denoted as p1, p2, and p3.
Fig. 1 Principles of 2D/3D imaging on a conventional lens-array: (a) 2D imaging when the lens size is equal to or smaller than the projected pixel size, and (b) 3D imaging when the lens size is at least twice as large as the projected pixel size.
When the size of the elemental-lens is smaller than or equal to the size of the projected pixels, light from each pixel will diverge with a diffusing angle equivalent to the numerical aperture of the elemental-lens. In this case, the information on each pixel is maintained within the diffusing angle as the conventional diffuser, and 2D images are diffused in the observer’s direction. Figure 1(a) shows 2D imaging on a lens-array when both the projected pixel and elemental-lens are the same size. On the other hand, 3D imaging can be achieved by a lens-array when the size of the elemental-lens is at least twice larger than the size of the projected pixel. When this condition is satisfied, every pixel projected on a one elemental-lens is crossed at the same focal point of the elemental-lens, and is then projected separately toward its refracted direction. This situation satisfies the principles of projection-type integral imaging, and the observer is able to observe full-parallax autostereoscopic 3D images within a viewing angle that is equivalent to the numerical aperture of the elemental-lens [22–24]. Figure 1(b) illustrates an example of 3D imaging on a lens-array when the size of the elemental-lens is three times larger than the sizes of the projected pixels.
2.2. Recording of lens-array HOEs for reflection-type 2D/3D transparent screens
Although a lens-array can be used for 2D/3D screens, as discussed above, the transparent screen which satisfies see-through properties cannot be implemented using a conventional lens-array. However, instead of a conventional lens-array, the lens-array HOE is capable of providing see-through properties and 2D/3D imaging, simultaneously [11]. In this section, the principles of hologram recording for fabricating a lens-array HOE are discussed in detail. We have used a photopolymer in our studies for a holographic material. It is possible for both frontal and rear projection-type screens to be implemented using lens-array HOEs. However, we employed frontal projection-type screens for the sake of the efficiency of space and the advantage of wavelength multiplexing for full-color imaging on the photopolymer during the hologram recording procedure [25].
The most distinct advantage of using HOEs based on the volume hologram for imaging applications is its capabilities of multiplexing techniques. Since multiple holograms can be recorded in the same area of the holographic material, multi-color recording and spatially overlapping multiple optical elements are possible when wavelength multiplexing and angular multiplexing are used, respectively. Since the photopolymer has a limited dynamic range, however, we give a higher priority to wavelength multiplexing for full-color imaging than the other multiplexing techniques. Hence, we have realized the production of 2D and 3D transparent screens separately on different types of lens-array HOEs, which will be discussed in this section.
The proposed 2D/3D transparent screens developed on the basis of the lens-array HOEs are implemented by a reflective hologram recording for implementing frontal projection-type screens. Figure 2(a) shows a schematic diagram of a reflective hologram recording scheme for generating a lens-array HOE. An expanded laser beam is divided into a reference and a signal path by a beam splitter, and the two beam paths are arranged by two mirrors so as to be incident on a holographic material from two opposite directions for recording the reflective hologram. The reference beam is projected with an incidence angle θ, while the signal beam is incident normal to the photopolymer. The photopolymer film facing the conventional lens-array, which is for the reference optical element, is located at the intersecting position of the reference and signal beam paths.
Fig. 2 (a) Schematic diagram for recording a lens-array HOE and (b) optical parameters of the recorded lens-array HOE.
During the recording of a hologram, the optical properties of the conventional lens-array are duplicated on the recorded lens-array HOE as a grating of interference patterns between the reference and signal waves. The viewing characteristics of the lens-array HOEs as 2D/3D transparent screens are closely related to the optical properties of the lens-array HOEs, and they can be evaluated by the optical parameters depicted in Fig. 2(b). The lens-array HOE reconstructs the duplicated waves of the conventional lens-array with lens size plens and focal length f when a displaying beam corresponding to the reference wave in the hologram recording is projected from the image projector on the lens-array HOE. The size of the pixel on the projected image from the image projector is denoted by pproj. The dimension of the projected pixel size is changed on the lens-array HOE because of the oblique projection angle (θ). The size of the projected pixel in the vertical direction is maintained as pproj. In horizontal direction, however, the pixel size is scaled to pproj/cosθ due to the oblique incidence on the horizontal plane. We denote the scaled pixel size in the horizontal direction as p'proj.
The dimensional features of the proposed transparent screens based on the lens-array HOEs are determined by the relation between plens and p'proj as Eqs. (1) and (2), because the 2D/3D imaging on the lens-array is related to the relative sizes of the elemental-lens and the projected pixel as discussed in the previous section.
The optical parameters, plens and f, are to be decided by the specifications of the reference lens-array, and pproj can be adjusted by the image projector. Therefore, the 2D and 3D features of the proposed transparent screens can be implemented by choosing the appropriate reference lens-arrays and image projectors according to the conditions given in Eqs. (1) and (2).
The transparent property of the proposed 2D and 3D screens is achieved by the principles of a volume hologram. The lens-array HOE functions as a 2D or 3D screen only for Bragg matched incident light with a wavelength and incidence angle that are identical to those of the reference beam used in recording the hologram [26]. For other light from the real world scene, except for the Bragg matched light, the lens-array HOE has no effect of any optical functions because the light simply passes through it as if it were a transparent film.
2.3 Viewing characteristics of the proposed 2D/ 3D transparent screens
The viewing characteristic of the proposed 2D transparent screen can be represented by the diffusion angle. When an image is projected on the lens-array HOE satisfying Eq. (1), the lens-array HOE diffuses the projected image within the diffusion angle (Ω) which can be evaluated by the optical parameters in Fig. 2(b) as
In the case of integral imaging, 3D images can be displayed within the limited viewing angle without any distortions. The viewing angle is one of the important viewing characteristics in integral imaging [27], and generally depends on the specifications of the elemental-lens. In the case of integral imaging displayed by the 3D transparent screen proposed herein, the viewing angle can be expressed by the optical parameters of the lens-array HOE. The mathematical expression of the viewing angle (Ω) for the 3D transparent screen is equal to the diffusion angle of the 2D transparent screen, as presented in Eq. (3).
The lateral resolutions of the diffuser and the integral imaging are inversely proportional to the sizes of the grain and the elemental-lens. Since both the grain and elemental-lens sizes are considered to be equal to the elemental-lens size (plens) on the lens-array HOE, the lateral resolution (Rl) in both the 2D and 3D transparent screens can be represented as Rl = 1/plens.
Another important viewing characteristic of the 3D transparent screen is the angular resolution of the displayed 3D image [28]. The angular resolution in the integral imaging is defined by the number of perspectives per unit angle inside the viewing angle. Since each pixel behind the elemental-lens generates different perspectives, the number of perspectives in the case of integral imaging is considered to be equal to that of pixels for the single elemental-lens. The angular resolution in the vertical direction (Rav) of the 3D transparent screen can be simply defined as
On the other hand, the angular resolution in the horizontal direction (Rah) is smaller than the angular resolution in the vertical direction due to the scaled projected pixel along the horizontal direction, and can be obtained as
3. Experiments
3.1 Experiments on recording the lens-array HOEs for the proposed 2D/3D transparent screens
An experimental setup for recording the lens-array HOEs was constructed in a manner that would permit 2D/3D transparent screens to be created by the proposed method. Figure 3 shows a photograph of the experimental arrangements for recording reflection-type lens-array HOEs as described in Fig. 2(a). Red (671 nm), green (532 nm), and blue (473 nm) lasers are used as light sources to record full-color holograms. The three lasers are combined into a single beam path and expanded by a spatial filter and a collimating lens. An electric shutter and circular variable neutral density filters are used to control the energy exposed on the photopolymer at the location where the lens-array HOEs will be recorded. A 2 inch cubic beam splitter and two 3 inch mirrors are used to arrange the reference and signal beam paths. The signal beam is incident normal to the photopolymer while the reference beam is illuminated with an incidence angle (θ) of 45þ.
Fig. 3 Photograph of an experimental setup for recording the lens-array HOEs for transparent screens.
We recorded two lens-array HOEs using two different commercially available reference lens-arrays, as described in Refs [29, 30]. The specifications of the reference lens-arrays for the 2D/3D transparent screens are listed in Table 1.The size of the lens in the reference lens-array for 3D transparent screen is relatively larger, 1 mm, with a focal length of 3.3 mm, compared to that for the 2D transparent screen which has a lens size of 0.125 mm and a focal length of 0.4 mm. The diffusion and viewing angles of the recorded lens-array HOEs calculated by Eq. (3) are 17.8þ and 17.2þ, respectively. The size of the recorded lens is also related to the lateral resolutions of the 2D/3D transparent screens as discussed in the previous section. The lateral resolutions for the 2D and 3D transparent screens are 8 pixels/mm and 1 pixels/mm, respectively. In this experimental setup, both 2D and 3D transparent screens are recorded in the shape of a circle with a diameter of 45 mm.
Table 1. Reference lens-arrays used to record the lens-array HOEs for 2D/3D transparent screens
Diffraction efficiencies for red, green, and blue colors on the recorded lens-array HOEs are directly related to the color representation in the display procedure. In order to display appropriate colors on the proposed transparent screens, the diffraction efficiencies for the three colors must be carefully adjusted by controlling exposure energies during the recording of both lens-array HOEs for the 2D/3D transparent screens. Exposure conditions and measured diffraction efficiencies for both 2D and 3D transparent screens are listed in Table 2. A spectrometer (Ocean Optics, USB4000-VIS-NIR) with a white light source (Ocean Optics, HL-2000-FHSA) was used for measuring diffraction efficiencies of both lens-array HOEs. As shown in Table 2, both lens-array HOEs provided similar values of diffraction efficiencies for red, green, and blue colors: 19.7%, 20.3%, and 24.3% on the 2D transparent screen, and 19.1%, 23.2%, and 22.1% on the 3D transparent screen, respectively. Photographs of the recorded lens-array HOEs for 2D/3D transparent screens under white light are shown in Figs. 4(a) and 4(b), respectively.
Table 2. Exposed energies and diffraction efficiencies for red, green, and blue color HOEs in implementing 2D/3D transparent screens
Since the reference lens-array used for recording the 2D transparent screen has a low fill factor with a 180 μm lens pitch and a 125 μm lens size, it simultaneously partially performs the function of a diffusor and an undesirable function of a mirror. This ambiguous problem, however, can be simply solved by using a reference lens-array with a high fill factor in the recording procedure. The diffusion angle of the lens-array HOE for the 2D transparent screen was evaluated using the measurement setup shown in Fig. 5(a). A collimated laser beam with a beam waist of 5 mm and a wavelength of 532 nm was incident on the HOE with incidence angle θ, which was identical to that in the recording setup. The diameter of the area of diffused light superimposed on purely reflected light was 41 mm, and it was measured at a distance of 120 mm from the HOE, as seen in Fig. 5(b). From these measured values, the diffusion angle for the 2D transparent screen is 17.1þ which approximates to the calculated value in Table 1. The viewing angle for the 3D transparent screen is discussed in the next section.
Fig. 5 Measurement of the diffusion angle Ω on the lens-array HOE for the 2D transparent screen: (a) schematic diagram of optical paths of light, and (b) photograph of the measurement setup.
3.2 Experiments for displaying images on the proposed 2D/3D transparent screens
In order to demonstrate the proposed transparent screens, we carried out display experiments by using the lens-array HOEs and an image projector. Since the lens-array HOEs are recorded by means of the collimated reference beam in the hologram recording procedure, it is also necessary for the image projector for the display experiments to provide collimated images for the proposed transparent screens to avoid Bragg mismatch causing image blurring and color separation [23]. Figure 6 shows a photograph of the experimental setup for displaying see-through 2D or 3D images using the lens-array HOEs as the proposed transparent screens. Relay optics and a telecentric lens were located in front of the image projector to generate the collimated image projection. The lens-array HOE as a proposed transparent screen is located on the path of the image projection with an incident angle of 45þ, which is identical to that of the reference beam path in the hologram recording setup. A background object ‘cube’ is located behind the transparent screen to confirm the transparent property of our proposed method. The sizes of the projected pixels (pproj) are adjusted by relay optics and the telecentric lens to be 128 μm and 25.6 μm for the 2D/3D transparent screens, respectively. These sizes of projected pixels are slightly larger and about 40 times smaller than the size of elemental-lenses on lens-array HOEs that are used for the 2D and 3D transparent screens, respectively, which satisfy the 2D/3D imaging conditions discussed in section 2.2. The angular resolutions in the horizontal and vertical directions of the 3D transparent screen calculated by Eqs. (4) and (5) are 1.6 perspectives/degree and 2.3 perspectives/degree, respectively.
Fig. 6 Photograph of the experimental arrangements used for displaying 2D or 3D images on the lens-array HOE as a transparent screen in the presence of a background object ‘cube’.
When the 2D images shown in Fig. 7(a) are projected from the image projector, the see-through 2D images are captured on the proposed 2D transparent screen, as seen in Fig. 7(b) with the background object appearing behind the transparent screen. Since the proposed 3D transparent screen provides 3D images based on the principles of the projection-type integral imaging, the elemental images must be projected [22–24]. The elemental images used in the display experiment for the 3D transparent screen were generated by computer graphics. They include 3D information related to the three characters ‘O’, ‘S’, and ‘A’ of red, green, and blue colors with a depth discrepancy of 20 mm, 0 mm, and −20 mm, respectively. From the elemental images shown in Fig. 8(a), the 3D images were produced around the 3D transparent screen with the background scene of the real object ‘cube’, as shown in Fig. 8(b), captured from five different viewing directions. The top/right and bottom/left perspective images are captured 8þ and −8þ apart from the viewing position of the center perspective image, respectively. From these captured positions, the viewing angle for the 3D transparent screen is 16þ.
Fig. 7 Images for the 2D transparent HOE screen: (a) 2D images projected to the screen, and (b) see-through 2D images displayed on the screen with a real object ‘cube’ for a background.
Fig. 8 Images of the 3D transparent HOE screen: (a) elemental images projected to the screen, and (b) perspective see-through 3D images around the screen captured from five different viewing directions with a background object ‘cube’.
The feasibility of the proposed 2D/3D transparent screens is clearly verified from the results of the display experiments. In the case of the 2D transparent screen, the projected 2D images are properly diffused on the 2D transparent screen with their colors maintained. The 3D imaging on the proposed 3D transparent screen is also verified by the appropriate disparities among the three virtual characters from five different viewing directions. In both cases, the background object can clearly be observed through the transparent screens, which confirms the transparent property of the proposed 2D/3D screens.
Regarding further possibilities for the proposed method, we note that it is also possible to simultaneously display both 2D and 3D virtual images on the proposed lens-array HOE. In the recording procedure, two lens-arrays which satisfy the 2D/3D imaging conditions presented in Eqs. (1) and (2) respectively, can be recorded on the same area of the photopolymer by an angular multiplexing technique using two reference beams which have different incident angles. Then two image projectors, which satisfy both the 2D/3D imaging conditions and incident angles, can display both 2D and 3D contents at the same time in the angular multiplexed lens-arrays HOE.
4. Conclusion
We propose 2D and 3D transparent screens, via the use of lens-array HOEs. The basic principles and relationship between the sizes of the projected pixel and the elemental-lens for 2D and 3D imaging on a conventional lens-array are described. The lens-array HOEs for 2D/3D imaging are fabricated on a photopolymer to permit see-through properties, thin thickness, and low cost, which are unobtainable using conventional lens-arrays. In order to examine the optical features of the proposed method, viewing characteristics, such as diffusion or viewing angle, lateral resolution, and angular resolution, for 2D/3D imaging on the proposed 2D/3D transparent screens were evaluated by the optical parameters of the lens-array HOEs. We formulated an HOE recording setup, and prepared two lens-array HOEs with different specifications for 2D/3D transparent screens. The elemental-lens and projected pixel of the recorded lens-array HOE for a 2D transparent screen are almost the same sizes. On the other hand, the lens-array HOE for a 3D transparent screen has an elemental-lens that is 40 times larger than the projected pixel size. Experimental results for displaying 2D/3D images on the proposed 2D/3D transparent screens verify the validity of 2D/3D imaging by the lens-array HOEs, in which the see-through property was satisfactory. In the present work, we only used wavelength multiplexing for the full-color imaging on the proposed 2D or 3D transparent screen by the lens-array HOE due to the limited dynamic range of the photopolymer. If a photopolymer with a higher dynamic range were available, however, both 2D and 3D transparent screens could be recorded together on the same area of the photopolymer by using an angular multiplexing technique. It would then be possible to simultaneously display full-color 2D and 3D images on a single transparent screen by two image projectors which have different projection angles. We expect that the proposed see-through 2D and 3D imaging methods on the lens-array HOE with the possibility of simultaneous 2D and 3D imaging have the potential for use in a wide variety of applications in virtual reality systems.
Acknowledgment
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2007-0054847). The authors acknowledge the support by Bayer Material Science AG for providing the photopolymer Bayfol® HX film used for recording the full-color HOE.
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