A three-dimension(3D)/two-dimension(2D) convertible display system is proposed. The proposed system realizes a thin structure by adopting a pinhole array to generate point light sources for 3D display mode from a backlight source. The optical efficiency of 2D display mode is also enhanced using polarization control. By experiments, the proposed method was proven and compared with a previous one.
©2006 Optical Society of America
The three-dimensional (3D) display is regarded as the final form of display technique because it can show various aspects of the original objects as if they really existed ideally. Nowadays, with the progresses in display fields, the 3D display attracts much attention and various 3D display methods have been researched. Among them, the integral imaging (InIm), which is also called integral photography, is one of the most advanced methods to display full parallax 3D images without any special glasses . Recently, the use of active devices makes the InIm support real-time full color moving pictures , and many early problems of the InIm have been overcome by continuing improvements [3–12]. However, the 3D display itself does not have enough demands to be commercialized because the basis for the massive 3D display market is not prepared yet. The mainstream of current display market is a high-definition (HD) two-dimensional (2D) flat panel display (FPD). As a result, the 3D-2D convertible display is regarded as a stepping stone from the 2D display to the 3D display and various researches have been performed to realize it.
Recently, our group has proposed a few 3D-2D convertible InIm systems [6, 13–14]. One of them uses a point light source array and a collimated back light source . This system can change the 3D and 2D display modes without any mechanical movement and supports a high quality of 2D images. The previously proposed system, however, has a critical problem that it needs a large lens and huge space which is more than ten centimeters. The method proposed in this paper solves this difficulty by adopting a pinhole array on a polarizer (PAP). Fabricating a pinhole array on a polarizer of liquid crystal display (LCD) panel makes it possible to enhance the optical efficiency in the 2D display mode nearly tens of times compared with the use of pinholes on a mask, while the thickness of the proposed system is only a little bit larger than most 2D displays. The proposed method is proven by some preliminary experiments.
For the 3D-2D convertible display, it is needed to guarantee the 2D image quality since the main function of the 3D-2D convertible display system is a 2D display. The previously proposed method switched between 3D and 2D display modes by generating and eliminating the point light sources using a polymer-dispersed liquid crystal (PDLC). The basic principles of the previous method are shown in Fig. 1.
In the previous method, the point light sources are generated from the lens array and parallel light. The PDLC transmits or diffuses the parallel light to switch the system between the 3D and 2D display modes. The most important factor is the point light source. Both the collimating lens and the lens array are needed to display 3D images through the point light source because the lens array cannot form the point light sources from most diffusive backlight units (BLUs). On the other hand, these devices are the main ringleaders of large thickness of the previous method and need to be replaced with advanced techniques. The key point to reduce the size of the previous system is to make the point light sources using diffused light sources which mean the normal BLUs, not a special one.
The simplest device to generate the point light source from diffusing light is a pinhole array. The pinhole array is a device to transmit the light only through the opened small apertures whereas the other areas block the light. Therefore, the point light sources can be formed from the normal BLUs through the pinhole array as shown in Fig. 2.
As shown in Fig. 2, the point light sources are generated in the locations of the pinholes even if the backlight is a diffusing one. The use of normal BLU brings many improvements to the previous method. Above all, the reduction of the system size is the most important one. If the pinhole array is used, the BLU can be attached closely as the 2D LCD systems. The only required space is the gap between the pinhole array and the display panel for which several millimeters is enough. Therefore, a compact 3D-2D convertible system can be realized with the pinhole array. However, the pinhole array method still needs to be improved since it has low optical efficiency. Only small bit of light from the BLU can transmit the pinhole array through the small aperture. This is an inherent problem of a pinhole array. The more important problem is that the pinhole array is not needed in the 2D mode but it cannot be removed electrically. Since the main use of the 3D-2D convertible system is for 2D display, the performance of the 2D display is more important than that of the 3D display. As a result, the pinhole array in the 2D mode is an obstacle for high quality 2D images and needs to be removed electrically for higher optical efficiency.
To resolve the problem, our method uses a PAP. The structure and principles of a PAP are shown in Fig. 3. The polarizer has a structure of a number of small apertures on it. When the induced light has an orthogonal polarization with the PAP, the light can transmit only through opened apertures and the polarizer becomes a pinhole array as shown in Fig. 3(a). If the polarization of induced light rotates by π/2, the light can transmit through all areas of the PAP and the pinholes vanish. Therefore, the PAP becomes a transparent film as shown in Fig. 3(b) and the optical efficiency can be enhanced significantly. In this structure the aperture ratio of the PAP determines the differences of the optical efficiency in the 3D modes. The aperture ratio will be increased if the size or the number of the pinholes is increased. However, since the pinhole size limits the number of available pinholes and also affects the size of voxel and viewpoints of the 3D image, it is needed to optimize the trade-off between the size and the number of the pinholes.
The structure of the proposed method is shown in Fig. 4. In the proposed structure, the pinhole array is replaced with the PAP. A polarization switcher is added between the BLU and the PAP instead of the PDLC. The polarization switcher determines the polarization of the backlight and switches the PAP between the 3D and the 2D modes. Therefore, the 3D-2D conversion is possible electrically. On the aspects of a 3D mode, the proposed method has advantages compared with the previous collimated light source method. The previous method uses a lens array to form a point light source in the 3D mode. Since each lens in the lens array cannot ideally integrate the light into one focal point, there is a leakage of light which reduces the quality of 3D image. On the other hand, the PAP forms almost ideal point light sources, and the quality of the 3D image can be improved.
3D mode (orthogonal polarization) A more detailed principle of the 3D mode is shown in Fig. 5. In Fig. 5, the elemental images are shown with pixel-structures. Each pixel of the display panel displays the corresponding information to integrate a voxel of the reconstructed 3D image. The real 3D image in Fig. 5 is composed of 5 voxels which are integrated from several elemental pixels on display panel. As a result, the reconstructed 3D image is divided into separated facets  as shown in Fig. 5. The number of the voxels increases, the resolution of the 3D image also increases. Since the number of voxels is the same as that of the pinholes, the resolution of the 3D mode is determined by the number of pinholes in the PAP.
3. Experimental results
Several preliminary experiments have been performed to prove the proposed principles. A spatial light modulator (SLM) with pixel pitch of 0.036mm and resolution of XGA is used as the display device. The PAP is fabricated by using a drilling machine and is composed of 70 by 70 apertures which have size of 0.1mm each. Therefore, the resolution of the 3D image is 70 by 70 and there are nearly 3 pixels in the voxel of the 3D image. The space between apertures is set to be 0.5mm and therefore the aperture ratio of the PAP in the 3D mode is about 3.2% and the size of the elemental image is 0.5mm (14 by 14 pixels per elemental image). The BLU is a commercial incoherent flat white light source which has luminance of 4000cd/m2 and is mostly used for mobile LCDs. The BLU uses a cold cathode fluorescent lamp as a white light source and various optical diffusing elements are added to increase the uniformity of light distribution. No other special technique is used for the BLU in the experiments to prove that the proposed method is suitable for normal BLUs. The side view of the experimental setup is shown in Fig. 6. The SLM is located in front of the BLU and the PAP is located between them. Although the PAP is covered by the SLM and not clearly shown, it is located right behind the SLM as shown in Fig. 6 and the system has a thin size compared with the previous method. The experimental setup has a thickness of several centimeters because it was set on optical table and the holders of each device need enough space to be established. If the system is integrated into one module, however, the thickness is expected to be decreased to nearly one centimeter.
In the 3D mode, the images of two letters ‘S’ and ‘U’ are formed at 30mm front of and behind the point light source array, respectively. In other words, the ‘S’ is a real image and the ‘U’ is a virtual one. The elemental images for them are generated by using computergenerated integral imaging. Therefore, it is possible to integrate both images orthoscopic. However, it is also needed to consider the pickup of the real 3D object in acquiring of the elemental images and the pseudoscopic-to-orthoscopic conversion technique is needed for that case [16, 17]. In the experiments, a polarizer is added between the BLU and the PAP as a polarization switcher to control the polarization of the backlight. The experimental results in 3D mode are shown in Fig. 7, which shows pictures of the 3D images that are captured at different positions. As shown in Fig. 7, the relative positions of the front image and the rear image are changed with the viewpoints. Therefore, the two images are proved to be 3D images.
In the 2D mode, two pictures of a landscape and a flower are displayed and captured by the CCD camera. The 2D images are displayed with full resolution (1024 by 748) of the SLM and therefore have good image qualities. The experimental results in 2D mode are shown in Fig. 8.
The conversion between the 3D and 2D modes is captured and shown as a movie in Fig. 9. In the 3D mode, the 3D images are observed at various positions and it is recognized that the two character images move with the viewing position. With the change of the polarization of the BLU, the system is converted to the 2D mode and the 3D image becomes a 2D elemental image array. In the 2D mode, we displayed several images which are contained in the Windows XP.
The experimental results to prove the principles of PAP are shown in Fig. 10. Figure 10(a) shows the PAP in the 3D mode. There are numbers of small apertures on the PAP. Figure 10(b) shows the PAP in the 2D mode and light transmits through all areas. In the experimental setup, the full white luminance with the SLM is 6.5cd/m2 in the 3D mode and is 200cd/m2 in the 2D mode. Therefore, the optical efficiency of the 3D mode is 3.25% of the 2D mode and nearly same as the aperture ratio of the PAP. These results prove that the optical efficiency is improved significantly for 2D mode-the main function of the system, in the proposed method. Although the optical efficiency is not sufficiently high for commercial displays, the experimental setup is not a complete module but an arrangement of optical components. Therefore, the optical efficiency can be improved by optimization and modularization of the setup. The efficiency of the 3D mode can also be increased by increasing the number of pinholes on the PAP.
In this paper, a novel method is proposed to construct a compact 3D-2D convertible display system using a PAP. The PAP is switched between the pinhole array and the transparent film electrically by controlling the polarization of the induced light. The proposed system also provides a high optical efficiency and a high image quality for the 2D mode-the main function of the 3D-2D convertible display. The proposed method is proven by experimental results and can be helpful in realizing a compact 3D-2D convertible display system for mobile applications.
Authors would like to acknowledge the financial support for this research, resulting from the award of a Samsung Electronics Co., Ltd.
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