A wide-viewing-angle 3D/2D convertible display system with a thin structure is proposed that is able to display three-dimensional and two-dimensional images. With the use of a transparent display device in front of a conventional integral imaging system, it is possible to display planar images using the conventional system as a backlight source. By experiments, the proposed method is proven and compared with the conventional one.
©2005 Optical Society of America
Integral imaging (InIm), which is also called integral photography, is one of the most attractive methods for displaying three-dimensional (3D) images without the need of any special glasses . InIm is able to provide full-color moving pictures and continuous viewpoints within the viewing angle, prompting a variety of research in this field recently. Through such studies, the early problems of InIm have been overcome significantly [2–12]. In spite of the progress with 3D, including InIm, however, it will still be some time before the commercialization of 3D display systems can be realized. The reason is that the mainstream of the two-dimensional (2D) display has been changed into high-definition (HD) flat panel display (FPD) recently. With the advent of HD broadcasting, demands for HD display devices have increased enormously and the performance of 2D display devices has improved, whereas prices have been significantly decreased. Even the latest 3D technologies have a lower performance than the conventional HD display device. In addition, there is minimal 3D content, most of which is intended for 3D theater and not home application. Therefore, the demand for 3D/2D convertible display is increased as a buffer between 3D and 2D technologies. An important final goal of 3D display research is to replace the 2D televisions with 3D television systems. For 3D display systems to infiltrate into current television markets, it is essential for the systems to be convertible with 2D display systems.
Recently, our group proposed a 3D/2D convertible InIm display system using a collimated backlight source . However, the system is bulky because a large lens is used to collimate the backlight. For most FPD devices, the thickness of the display system is of major concern, and the previously proposed system has a handicap in that respect. In this paper, a 3D/2D convertible display system based on InIm and an extra liquid crystal display (LCD) is proposed. The proposed system needs only a slightly larger thickness than most 2D FPDs. The proposed method is proven by some preliminary experiments.
First it is necessary to review the basic principles of InIm and LCD, on which the principle of the proposed method has its basis. Figure 1 shows a basic principle of the InIm. The InIm consists of a display device and a lens array. The display device displays small images that contain the 3D information of the original object. Those images are called elemental images. The lens array is composed of many elemental lenses of the same size and focal length. Elemental images are integrated through the lens array to form a 3D image at the central depth plane (CDP), as shown in Figs. 1(a) and 1(b). The location of the CDP is determined by the following equation, which is called lens’s law:
where a means the distance between the display device and the lens array, b is the location of the CDP measured from the lens array, and f is the focal length of the elemental lens. From Eq. (1), if a is larger than f, then b has a positive value, and the 3D image is formed in front of the lens array. This is called real InIm and is shown in Fig. 1(a). On the contrary, if a is smaller than f, then b has a negative value, and it means that the 3D image is located behind the lens array. This structure is called virtual InIm and is shown in Fig. 1(b) [14, 15].
The basic principle of LCD is very simple and well known. The LCD is basically a spatial light modulator (SLM), which is able to transmit or block the light through itself. In most LCDs, a backlight unit (BLU) is adapted to supply a white light to the LCD panel. Each red/green/blue (R/G/B) pixel in the LCD panel controls the transmittance of the induced light and displays an image. In other words, the LCD panel is transparent when it is set to display a white screen. Therefore, if we place a white-screen-mode LCD panel (with BLU removed) in front of any display device, it is possible to observe the images on that display device through the LCD panel. This principle plays an important role in the 3D display mode of the proposed method.
Combining these principles of 3D and 2D display methods, it is possible to construct a 3D/2D convertible display system. The basic structure of the proposed system is shown in Fig. 2. The proposed system has three main devices: display device 1 (DD1), display device 2 (DD2), and a lens array. The DD1 is a normal FPD such as an LCD, a plasma display panel, or even a cathode ray tube. The DD2 is an essential device that plays the most important role in 3D/2D conversion. The DD2 has to be transparent in 3D display mode and has to display 2D images in 2D display mode. With the basic principles of LCD, one can easily see that an LCD panel is ideal for DD2 because it can be used as an SLM. The lens array can be located between DD1 and DD2. It is attached to DD2 in our experimental setup.
Compared with the previously proposed 3D/2D convertible InIm , the proposed system has a much thinner structure. In Fig. 2 the gap a is the distance between the lens array and DD1, as used in Eq. (1). Therefore, this gap a has a value larger than f, the focal length of the lens array, if the 3D image is to be formed in front of the lens array (real InIm). If the 3D image is to be formed behind the lens array, then a has to be smaller than f (virtual InIm). For both cases, a needs to be slightly larger or smaller than f to form a 3D image with good quality.
The principle of the 2D display mode is shown in Fig. 3. In this mode, the DD2 is a main device and it displays 2D images. The DD1 and the lens array play the role of a BLU. This is made possible when the DD1 is set to display a white screen. If a white screen is displayed on DD1, the white light is emitted from it and goes to the DD2 (LCD panel) through the lens array. The R/G/B pixels of DD2 modulate the incident light, and finally 2D images can be displayed. A 3D/2D convertible system needs to provide 2D images of comparable or better quality than do conventional 2D display devices. Since the DD2 is an LCD panel, the proposed system is able to satisfy the requirement in the 2D display mode. The lens array does not play any role in 2D display mode because the DD2, the main display device, is located in front of it. In addition, the DD1 and the lens array can be supposed as an active BLU. Most LCD display systems have simple BLUs that generate white light. However, in our case the DD1 and the lens array, which are used as a BLU, are also a display system and so it is possible to control the luminance of each pixel. Therefore, it is possible for the proposed system to display higher-quality 2D images by adjusting the active BLU (the DD1 and the lens array). We will perform further research on this improved 2D display mode in the future. In this paper, we deal only with the basic 2D display mode without BLU control.
In the 3D display mode, the basic principle is the same as that of the conventional InIm. In this case, the DD1 is the main display device. The DD1 displays elemental images of the 3D image, and the lens array reconstructs them as a 3D image around the CDP. The DD2 is set to be transparent so that the reconstructed 3D images can be observed through it. From the principles of the LCD, it becomes obvious that the DD2 can be transparent by displaying a white screen (white screen state). The principle of the basic 3D display mode is shown in Fig. 4.
With the proposed method, the proposed system can display a 3D image. However, the viewing angle of InIm is not sufficient in the conventional method. Various methods have been proposed to widen the viewing angle of InIm. Among them, the elemental-lens-switching method is an efficient and helpful one [16, 17]. In this paper, we adapt this technique to the basic 3D display mode in order to construct a wide-viewing-angle display system. In Ref. 17, the authors used an orthogonally polarized mask and a polarization switching device. In the method proposed in this paper, however, we do not need to add a physical mask because the DD2 itself can display masks and even move them. Therefore, the generation and switching of the mask are possible electrically without any additional device in the proposed method. The principles of the improved wide-viewing-angle 3D display mode are shown in Figs. 5(a) and 5(b). In Fig. 5(a), the odd columns of elemental lenses in the lens array are blocked by the image of odd column masks in DD2. The elemental images on the DD1 are reformed by the same method as in the previously proposed method [16, 17]. Since the odd half of the lens array is blocked and the elemental images are modified, only half of the 3D image is reconstructed while its viewing angle is widened by two times. In Fig. 5(b), the even columns of the elemental lenses are blocked, and the elemental images are also changed. The other half of the 3D image is then reconstructed with a doubled viewing angle. By switching between these two phases with enough speed to induce the afterimage effect, it is possible to construct a wide-viewing-angle 3D display system using the proposed method.
However, it is not sufficient to apply just the switching mask method. The elemental images also need to have a widened field of view. The horizontal field of view of the elemental images can be widened through image resizing  or the computer-generated integral imaging (CGII) [4, 19]. In this method, the CGII is used to generate the elemental images.
3. Experimental result
Several preliminary experiments have been performed to prove the proposed principles. Two identical 17-inch LCD monitors are used for DD1 and DD2. For DD2, the LCD monitor was disassembled and only the LCD panel from it was used. In combining two display devices, the direction of the polarizer of DD2 was adjusted to maximize the total luminance of the system. The lens array is composed of 13 by 13 elemental lenses. The pitch of each elemental lens is 10 mm and the focal length of it is 22 mm. The location of CDP is set to be 100 mm from the lens array, and the gap a was adjusted to be 28 mm. As mentioned in the Section 2, the lens array is attached to DD2. The sum of thicknesses of the DD2 and the lens array is nearly 5mm. Therefore, the total additional thickness is 33 mm for a 17-inch display system. Since the previously proposed 1.8-inch 3D/2D convertible InIm system needs a thickness of larger than 100 mm , the system proposed in this paper is the more efficient one. Figure 6(a) shows a front view of the experimental setup. The DD2 (LCD panel) is located in front of the DD1 (LCD monitor), and the lens array is attached behind the DD2. The side view of the experimental setup is shown in Fig. 6(b). The thickness of DD1 is 60 mm whereas the gap a is 28 mm. It is also shown that both the DD2 and the lens array are very thin and their thicknesses are almost negligible. Therefore, the additional thickness is just nearly half of DD1. The polymer-dispersed liquid crystal (PDLC) can be used to increase the uniformity of DD2. Since the DD1 and the lens array cannot emit the isotropic white light, there may be some degradation of image quality in 2D display mode. The PDLC is an active diffuser that can be transferred from a transparent glass to a diffuser when the induced voltage is controlled . Therefore, if we insert the PDLC behind the DD2 and use it as a diffuser plate in 2D display mode, the uniformity of the 2D display mode can be increased. This method will be researched further, and only basic experiments are performed in this paper.
The experimental result of the 2D display mode is shown in Figs. 7(a) and 7(b). In Fig. 7(a), some part of a text document is displayed and captured by a charge-coupled device (CCD) camera. We magnified the original captured image to make it easy to find distortion, and the text contents consist of a small number of pixels so that any distortion on it can be detected more easily. As shown in Fig. 7(a), the texts are displayed without any distortion even though it is magnified. In Fig. 7(b), an image of a rose is displayed well. From Figs. 7(a) and 7(b), therefore, it is easily recognized that the proposed system can display a 2D image without distortion.
In the 3D display mode, two images of banana and cherry are used. The location of the banana and cherry is set to 110 mm and 90 mm from the lens array, respectively. The elemental-lens-switching method is also adopted in real time to widen the viewing angle. The experimental results in 3D display mode are shown in Figs. 8(a), 8(b), and 8(c). Each figure shows a 3D image captured at a different viewpoint, from left 10 degrees to right 10 degrees. From the figures, it is recognized that the relative positions of the banana and cherry are changed as a result of the change of the viewpoints. For comparison with the basic 3D display mode, the same 3D images without the elemental-lens-switching method are reconstructed and shown in Figs. 9(a), 9(b), and 9(c). As shown in Figs. 9(a) and 9(c), the reconstructed images are severely distorted because the locations of the viewpoints exceed the viewing angle of the basic 3D display mode. Comparing these two results, it is proven that the improved 3D display mode provides a wide-viewing-angle 3D image by InIm.
In this paper, a novel method is proposed to construct a wide-viewing-angle 3D/2D convertible display system based on the principles of InIm and LCD. By changing the role of the DD1 and the DD2, it is possible to switch between 3D display mode and 2D display mode electrically. In 2D display mode, the proposed system is suitable for HD format because the LCD panel is the main display device. The elemental-lens-switching InIm technique is also adopted to widen the viewing angle in the 3D display mode. The proposed method is proven and compared with the conventional InIm by experimental results. The proposed method can be helpful in realizing a 3D/2D convertible display system as a stepping stone between 2D and 3D technologies.
This work was supported by the Next Generation Information Display R&D Center, one of the 21st Century Frontier R&D Programs funded by the Ministry of Commerce, Industry and Energy of Korea.
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