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We proposed a concept of an active parallax barrier using a liquid crystal-on-polarizing interlayer (LPI) for near-viewing autostereoscopic displays. In contrast to a conventional two-panel configuration where two independent panels are stacked together for displaying and parallaxing purposes, a monolithic one-panel architecture was demonstrated with the help of the LPI. The LPI was constructed using a polarizer sheet, one side of which provided the support for the active parallax barrier and the other served as the substrate for the image panel. For the active parallax barrier, an array of periodically patterned indium-tin-oxide electrodes was first prepared on the LPI and bi-level structures were subsequently fabricated for the cell gap and the liquid crystal alignment. Our monolithic one-panel architecture allows the near-viewing distance property which is essential for mobile applications.
© 2016 Optical Society of America
1. Introduction
Stereoscopic displays [1–4] provide vivid images with the depth perception generated by the spatial-multiplexed [3, 5, 6] or time-multiplexed [4, 7, 8] binocular disparity using a specific eyewear. In recent years, a variety of three-dimensional (3D) displays based on lenticular lens arrays [9–13], parallax barriers [14–16], integral imaging [17–21], and holography [22–25] have been developed for realizing 3D images without a viewing aid. Except for specific areas including entertainment, simulations, and training, two-dimensional/3D (2D/3D) convertible displays are still required for usual 2D as well as 3D contents. For such convertible displays, liquid crystals (LCs) have been widely used for constructing active lenticular lens arrays or active parallax barriers [11, 26–30]. Especially, the active parallax barrier is relatively simple to be implemented and easily applicable for usual 2D LC displays (LCDs). For mobile applications, 2D/3D convertible displays should allow the near-viewing distance. In the parallax case, the viewing distance is proportional to the pixel dimension as well as the separation between the image panel and the barrier plate (pixel-barrier separation) [31]. Moreover, the reduction of the pixel-barrier separation is critical for the achievement of high resolution in 3D mobile environment. Another point is that for a stacked configuration of an image panel and a barrier panel, the viewing distance always exceeds the value governed by the pixel-barrier separation so that it is barely possible to realize the near-viewing image.
In this work, we demonstrated a new type of an autostereoscopic display with an active parallax barrier based on a LC-on-polarizing interlayer (LPI) for near-viewing. The key concept comes from the reduction of the pixel-barrier separation by implementing the LPI between the imaging cell and the switching cell instead of simply stacking two independent cells in series. The LPI was constructed using a plastic polarizer sheet where an array of periodically patterned electrodes was first prepared on one side and a switching electrode on the other side using indium-tin-oxide (ITO). Bi-level structures of photo-curable monomers were then produced on the two sides for the LC alignment and the cell gap. The use of such LPI was found to reduce the viewing distance below 300 mm, which is desirable for mobile environments. This approach would be directly applicable for high resolution 3D mobile displays with the pixel dimension of about 40 μm even below in the world-leading mobile products.
2. Basic principle of the LC-on-polarizing interlayer
The schematic diagrams showing a conventional stacked configuration and our monolithic architecture with the LPI for the 2D/3D convertible LCD are depicted in Fig. 1. In the conventional configuration as shown in Fig. 1(a), the viewing distance d is defined to be d = g(i + e)/i where g, i, and e denote the pixel-barrier separation, the pixel pitch, and the interpupillary distance, respectively. Since i is predefined in pixel design and e is a constant, the distance g between the two panels is the only adjustable parameter for the viewing distance of d. The minimum value of g is then the total thickness of two inner substrates and a polarizer between them. Taking typical values of i = 100 μm, e = 65 mm, and g = 1.5 mm, the viewing distance is approximately 1 m. This is not suitable for mobile applications.
Fig. 1 (a) Schematic diagram showing the conventional structure of 2D/3D convertible LCD and its operating principle. Here, d, g, i, and e denote the viewing distance, the pixel-barrier separation, the pixel pitch, and the interpupillary distance, respectively. Purple and orange dotted lines represent the light rays from the left and right pixels, respectively. (b) Proposed 2D/3D convertible LCD using LPI for the near-viewing distance. (c) Illustration of bi-level structures on both sides of the polarizer sheet.
In contrast, our monolithic one-panel approach as shown in Fig. 2(b) eliminates the two inner substrates so that it enables to reduce the viewing distance due to the decrease of g. Figure 1(c) shows the LPI with bi-level structures which provides one surface in the imaging cell and the other in the switching cell for the LC alignment and the cell gap. Note that the microgroove patterns in the bi-level structures are capable of aligning the LC molecules along the microgroove direction [32–34]. The periodic cross patterns serve as spacers to maintain the uniform cell gap. An array of periodically patterned ITO electrodes on one side (upper surface) of the LPI is used for producing active parallax barriers in the switching cell and the electrode on the other side (lower surface) is for the imaging cell.
Fig. 2 Fabrication of the 2D/3D convertible autostereoscopic LCD with LPI: (a) Preparation of patterned ITO electrodes on a polarizer sheet. Here, PIL denotes the polarization direction of the polarizer sheet. (b) Construction of the bi-level structures and the cell assembly. Here, Rt and Rb denote the rubbing direction on the top and bottom substrates, respectively. (c) The top view of the bi-level structure on the polarizer sheet observed using the SEM. (d) The side view of the LPI with the bi-level structures.
3. Experimental
In light of the idea described above, the 2D/3D convertible autostereoscopic LCD for near-viewing applications was fabricated as shown in Fig. 2.
An array of periodically patterned ITO electrodes was first prepared on the upper side of the polarizer sheet by transfer printing of fluorous-polymer (EGC-1700, 3M NovecTM) for a sacrificial layer with the help of an elastomeric stamp of poly(dimethylsiloxane) (PDMS, GE silicones) [35]. The pitch and the width of the patterned ITO were denoted as λ and w, respectively. They should be adjusted according to the number of views of the original image. i.e. w is given by w = λ(N-1)/N in the N-view mode. EGC-1700 was transfer-printed on only the top side of the polarizer sheet and ITO was then sputtered onto both sides of the polarizer sheet at room temperature. The fluorous-polymer patterns were lifted off in a fluorous solvent (HFE-7100, 3M Novec) as shown in Fig. 2(a). Note that the ITO electrode was prepared on the whole surface of the bottom side of the polarizer sheet while patterned ITO electrodes were on the top side of it. Figure 2(b) shows the fabrication of the bi-level structures and the assembly of the LPI between the top and bottom substrates. The ultraviolet (UV) ozone treatment of the polarizer sheet was carried out for 10 minutes to increase the surface wettability. A photo-curable monomer (NOA65, Norland Products Inc.) was then imprinted on the polarizer sheet using a mold of PDMS and irradiated with UV light at 50 mW/cm2 for 5 minutes. The bi-level structures were composed of microgrooves for the LC alignment and cross-shaped columnar spacers for the cell gap. Along both the x-axis and y-axis, the cross-shaped columnar spacers were located apart 1.2 mm from each other and provided the good uniformity in the cell gap. Note that the denser distribution of the spacers yields the less transmission. The top and bottom ITO substrates were spin-coated with polyimide (AL22620, JSR) at the rate of 3000 rpm for 30 seconds, and annealed at 180 °C for 1 hour. The polyimide layers were rubbed to promote the homogeneous alignment of the LC molecules. The rubbing directions on the top and bottom substrates (denoted by Rt and Rb) were parallel to the microgroove direction on the LPI. For permanently stacking the top and bottom substrates on the cross-shaped spacers in one panel architecture, a thin layer (about 1 μm thick) of a photo-curable material with low viscosity (NOA74, Norland Products Inc.) was used and exposed to UV light at 50 mW/cm2 for 5 minutes. Finally, a nematic LC (ZLI-2293, Δn = 0.13, Δε = 10, Merck) was injected by capillary action to the imaging cell as well as the switching cell.
Figure 2(c) shows the images of the LPI observed using a scanning electron microscope (SEM). Cleary, the cross-shaped spacer and microgrooves were well constructed. The thickness of the bi-level structure was measured to be 21 μm from Fig. 2(d). The total thickness of the LPI from the bi-level structure on the top surface to that on the bottom surface of the polarizer sheet was estimated to be 210 μm. The cell gaps of both the imaging and the switching cells were identical to be 4 μm.
4. Electro-optical properties
We first examine the electro-optical properties of our monolithic device in one-panel architecture. The experimental geometry was shown in Fig. 3(a). Here, Φ is defined as the angle with respect to the x axis. The polarization directions of the LPI, the bottom polarizer, and the top polarizer are denoted by PIL, P1, and P2, respectively. As shown in Fig. 3(a), the directions of PIL, P1, and P2 were Φ = 90°, 0°, and 0°, respectively. Figure 3(b) shows the microscopic textures of our device under the applied voltage of V1 = 0, 7, 14, and 20 V, respectively, in the imaging cell. Here, V2 was fixed at 0 V. At V1 = 0 V, the highly uniform alignment of LC was achieved due to the microgrooves in the bi-level structure. Small light leakage, observed in the vicinity of the spacers at a high value of V1, was attributed to the LC deformations around the spacers. Note that the light leakage can be reduced using smaller spacers that may be produced either simply using a mold with fine physical features or other high resolution lithographic process. Figure 3(c) shows the microscopic textures under several different voltages of V2 = 0, 5, 10, and 15 V, respectively, in the switching cell. In this case, V1 was fixed at 0 V. As V2 increases, only the LC molecules on the patterned ITO electrode were reoriented along the vertical direction so that parallax barriers were formed. In the intermediate range of V2, the lens effect occurred in the bright region due to the index gradient of the LC.
Fig. 3 (a) The geometry for the measurements of the electro-optical properties of the imaging cell and the switching cell. (b) Microscopic textures under the applied voltage of V1 = 0 V, 7 V, 14 V, and 20 V, respectively, for fixed V2 = 0 V. (c) Microscopic textures under the applied voltage of V2 = 0 V, 5 V, 10 V, and 15 V, respectively, for fixed V1 = 0 V. Scale bars are 400 μm.
The color difference between Figs. 3(b) and 3(c) is attributed to the vertical variance of the focal plane in addition the mismatch of the cell gap between two cells. It should be noted that the difference of the operating voltage between the imaging cell and the switching cell was due to the thickness difference between the bi-level structures. The thickness of the bi-level structure can be reduced using a photo-curable material with low viscosity. For the thinner bi-level structure, the lower operating voltage was needed. The rising and falling times estimated from the transmittance curve (from 10% to 90% and vice versa) were 39 ms and 6 ms, respectively.
5. Multiscopic performance at the near-viewing distance
Let us describe the multiscopic performance of our device within the framework of the 3D representation of 2D original images.
The pitch (λ) and the width (w) of the ITO electrode were given as λ = 2ie/(i + e) and w = ie/(i + e). If e is much larger than i, λ and w become approximately 2i and i, respectively. For the two-view mode, λ and w were set to be 400 μm and 200 μm, respectively. In this case, two different original images (2IM2 consisting of two alternating slices of the 2D image and 2IM4 having extra black slices in addition to the two slices of the 2D image) were prepared to estimate the cross-talk between the image seen in the left eye and that in the right eye. In 2IM2, two letters ‘S’ and ‘M’ (each 2 cm wide) were divided into slices of 200 μm wide. The two different types of the slices were placed in an alternating manner. In 2IM4, extra black slices (B; each 100 μm wide) were inserted between the two slices of ‘S’ and ‘M’ (each 100 μm wide). The total width of a set of four slices (S-B-M-B) was then 400 μm. Here, let us define two angle parameters as shown in Fig. 4(a); one is the angle (θc) between a camera and the normal axis of the screen from the center of the cell and the other is the angle periodicity (θp) for the repetition of the captured image by the camera. For the case of the viewing distance which is much larger than the screen size, the angle periodicity (θp) can be written as
where n is an integer representing the n-th pixel and Δκn denotes the magnitude of the horizontal variance between the corresponding pixel and the parallax barrier. The offset in θc for the image capture can be varied with Δκn. It means that the point of view is laterally shifted according to Δκn.Fig. 4 (a) The schematic diagram of the experiment for measuring the multiscopic imaging capability of the proposed cell. Here, θc denotes the angle between a camera and the normal axis of the screen from the center of the cell and θp denotes the angle periodicity for the repetition of the captured image by the camera. (b) 2IM2 and the captured images of ‘S’ and ‘M’. (c) 2IM4 and the captured images of ‘S’ and ‘M’. Scale bars are 1 cm.
For the viewing distance of about 300 mm, a thin optical spacer of 0.7 mm thick was introduced in our case since the viewing distance of our device itself was about 60 mm, indicating that our device was indeed capable of reducing the viewing distance for high-resolution pixels of a few tens of micrometers. The angular range of the entire multiscopic image was −45° ≤ θc ≤ + 45° and Nθp ≤ 90°. For all the experiments we performed, the active parallax barriers in the switching cell were operated at V1 = 30 V. As shown in Figs. 4(b), for the 2IM2 case, severe crosstalk occurred, meaning that both of ‘S’ and ‘M’ were appeared at θc = ± 5°. In contrast, for the 2IM4 case, the crosstalk was drastically diminished since the black slices, placed between two slices of ‘S’ and ‘M’, significantly eliminated the interference between the two images.
From the captured images of ‘M’ in the two cases (2IM2 and 2IM4), we simply estimate the amount of the crosstalk quantitatively. Note that the viewer-crosstalk (C) at a specific point (x,y) is given as follows [36].
where IS denotes the intensity of the intended signal and IL is the intensity of the leaked light from the wrong viewing zone. In our experiment, the point in the captured image was defined as (x,y). The intensity at each point I(x,y) was obtained from the captured image with the help of an image analysis software available in public (ImageJ, NIH, USA). The ideal non-crosstalk case corresponds to the parameter C = 0. Considering that the intensity at the point where only the image of ‘M’ appeared is the intended signal IS, the intensity IL can be obtained by IL = I - IS. The analysis was carried out at 15,000 points in the captured image. Figures 5(a) and 5(b) show the histograms of the counts for the crosstalk as a function of the parameter C in the 2IM2 and 2IM4 cases. Clearly, for the 2IM2 case, the counts were substantially distributed around 0.4 (away from 0) whereas for the 2IM4 case, they were largely concentrated near 0. This means that the black slices, separating two slices of ‘S’ and ‘M’, plays a significant role in the reduction of the crosstalk. However, the addition of the black slices reduces the pixel resolution. The reduction of the relative dimension of the black slice to the image slice is one way of improving the image quality while reducing the viewer crosstalk.We also evaluated the multiscopic performance of our device using two different original images (3IM6 consisting of three alternating ‘S’, ‘N’, and ‘U’ slices with black slices in the three-view mode and 4IM8 consisting of four alternating ‘M’, ‘I’, ‘P’, and ‘D’ slices with black slices in the four-view mode) as shown in Figs. 6(a) and 6(b), respectively. In both cases, all slices were 100 μm wide. In this experiment, λ and w were adjusted in accordance with the opening ratio for each mode. In the three-view mode with the opening ratio of 33%, λ and w were 600 μm and 400 μm, respectively. In the four-view mode with the opening ratio of 25%, λ and w were 800 μm and 600 μm, respectively. Figures 6(c) and 6(d) shows the captured images at different values of θc at the distance of 300 mm from the center of our device. In the 3IM6 case, three letters of ‘S’, ‘N’, and ‘U’ were captured at θc = 9°, 0°, and −9°, respectively. In 4IM8 case, four letters of ‘M’, ‘I’, ‘P’, and ‘D’ were captured at θc = 31°, 21°, 10°, and 0°, respectively. It is clear that that the experimental results were in good agreement with the expected value of θp = 10 ± 2° from Eq. (1) above.
Fig. 6 Three-view and four view multiscopic performance: (a) 3IM6 in the three-view mode, (b) 4IM8 in the four-view mode. All slices are 100 μm wide. (c) Captured images of ‘S’, ‘N’, and ‘U’ in the three-view mode. (d) Captured images of ‘M’, ‘I’, ‘P’, and ‘D’ in the four-view mode. Scale bars are 1 cm.
6. Conclusion
We have demonstrated a new concept of 2D/3D convertible autostereoscopic LCD based on the LPI for near-viewing applications. The viewing distance of our device was decreased with decreasing the pixel-barrier separation by the introduction of the LPI between the image cell and the switching cell. This approach provides more flexible design in the viewing distance in 3D displays, particularly for mobile applications. The concept of the LPI and similar interlayers will be useful for more sophisticated autostereoscopic display systems.
Acknowledgments
This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. This work was in part supported by Samsung Display Company and the Brain Korea 21 Plus Project in 2016.
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