Multi-projection 3D displays using multiplexing techniques in autostereoscopic displays

Projection displays have the capability of manipulating their images in size and resolution as well as deflecting the direction of projected images. These characteristics are of great benefit for the implementation of 3D displays, which require dense directional images (or view images) for achieving natural perception of 3D images. Adopting a multiple projection system can greatly increase the image quality and performance of 3D displays.

Multi-projection 3D displays generate directional images by projection optics.^1,2 Basically, a directional image is created by a single projection optic system. It can show 3D images with high spatial resolution, and the angular resolution becomes the number of projection optics, which is identical to the number of directional images. Another method is using multiple projection-optic systems for creating each directional image. This method is advantageous for achieving higher angular resolution, because the spatial resolution of a display device is converted to angular resolution. Also, a hybrid method which converts only some portion of spatial resolution to angular resolution is possible using special optical configurations. However, if the number of projection optics is similar to the number of horizontal pixels, the difference between these methods will be negligible. Consequently, adopting more multiple projection optics is advantageous in all kinds of the multi-projection systems, but the physical dimension of the projection optics could be a huddle in expansion of the systems and would narrow the applications of the multi-projection 3D displays. We introduce several methods to overcome the limitations.

Polarization duplicated multi-projection
Uniaxial crystal which has birefringence can be used for duplicating view images.³ Some projection displays project polarized images such as projectors using laser light sources or liquid crystal spatial light modulator. Generally, all the primary colors do not have identical polarization state in those projectors. If the uniaxial crystal is integrated with the polarization optics, the polarization state can be used to control the optical path of the projection system. Consequently, this system can simply double the number of projection optics because single projection optics system is separated by two projection optics by the polarization state. Moreover, by adjusting the birefringence property, projection optics can be more densely positioned overcoming the physical dimension of projection optics. With this method, the complexity of the system can be greatly reduced, which also lowers the operation load and stability of the system.

Fig. 1. Polarization duplicated multi-projection system: (top left) projection images are doubled after passing the birefringent optics, (top right) experimental system, and (bottom center) ten view images are observed through five projectors.

Compressive multi-projection
Another method of increasing the image quality is adopting compressive light field method.⁴ By placing a liquid crystal display (LCD) which controls the transmittance of the light rays propagating the space, the set of directional view images can be regarded as a multiplication of images from projection optics and the transmittance of the LCD. Image data for the LCD and projection optics can be calculated by optimizing the directional images with given viewing position, specifications of displays, and optical geometry of the system.

Because the position of directional images can be freely chosen, compressive multi-projection system can provide full parallax with larger number of directional images compared to the conventional multi-projection systems.

Fig. 2. Compressive multi-projection system: (top left) projected images are attenuated at the LCD panel and create view images, (top right) experimental system, and (bottom center) experimental results.

Light-guided multi-projection
One of the problems of multi-projection systems is that they require enough projection distance to show an image. For this reason, most applications of the multi-projection system are limited for large-scale 3D systems. Because adopting waveguide can fold the projection distance multiple times, the waveguide multi-projection method can greatly reduce the projection distance less than one-fifth of the original projection distance conserving the advantage of the multi-projection system such as high quality of 3D images.^5,6 This method can widen the applications of the high quality multi-projection system which requires smaller form factor.



Fig. 3. Light-guided multi-projection system: (top left) projected images are expanded while propagating inside the light-guide, (top right) experimental system, and (bottom center) experimental results of ten view images.

See-through multi-projection
Most of the multi-projection systems employ anisotropic diffuser to increase the vertical viewing zone and visibility of the reconstructed 3D images. As it is a kind of a diffuser, the screen is mostly opaque. However, if the diffuser is made by thin metal layer, it can be partially transparent. This new optical device is called transparent anisotropic diffuser (TAD).⁷ If TAD is adopted in the multi-projection system, see-though multi-projection display can be implemented which can be used for augmented reality applications. As the viewing quality of the system can be adapted to the application in multi-projection systems, TAD can be applied in various configurations such as AR screens or head-up displays for automobile or aircrafts.



Fig. 4. See-through multi-projection system: (top left) conceptual diagram, (top right) experimental system, and (bottom center) experimental results demonstrating see-through property and parallax views.

In summary, new multi-projection 3D systems are introduced which use various multiplexing methods and new optical devices These systems demonstrated the advantage of multi-projection system such as high spatial resolution images, natural expression of 3D images as well as showing flexibility of the system. In addition to the spatial multiplexing method employing multiple projectors, additional multiplexing methods such as polarization duplication or compressive multi-projection methods can further enhance the image quality of the systems. It is also promising that adopting new optical systems such as wedge projection or transparent anisotropic diffuser can greatly widen the applications of the multi-projection systems without compromising the advantages of the multi-projection system.

This research was supported by "The Cross-Ministry Giga KOREA Project" of The Ministry of Science, ICT and Future Planning, Korea [GK15D0200, Development of Super Multi- View (SMV) Display Providing Real-Time Interaction].

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Byoungho Lee received his PhD from the Department of Electrical Engineering and Computer Science, University of California, Berkeley, California, USA in 1993. He has been in the faculty of the School of Electrical and Computer Engineering, Seoul National University since September 1994, where he is now serving as the head. He is a Fellow of SPIE, OSA, and IEEE, and a member of the Korean Academy of Science and Technology. Currently, he is on the editorial board of Advances in Optics and Photonics, Light: Science and Applications, and Applied Physics B. He serves as the editor-in-chief of the Journal of Information Display. He served as a vice-president of the Korean Information Display Society and is serving as a vice-president of the Optical Society of Korea.

Soon-gi Park is currently working as a research scientist in Korea Institute of Science and Technology. He received his BS and MS from the Department of Information Display at Kyung Hee University in 2009 and 2011, respectively. In 2015, he received his PhD in electrical engineering and computer science from Seoul National University. From October 2015 to December 2016, he was a postdoctoral researcher in the Department of Electrical and Electronical Engineering at Tokyo University of Agriculture and Technology, Tokyo, Japan. His research focuses on 3D display systems including various autostereoscopic displays, multi-projection displays, and optical systems for augmented reality including see-through displays.

Keehoon Hong received his BS in electrical and electronic engineering from Yonsei University, Seoul, Korea, in 2008 and his PhD in electrical engineering and computer science from Seoul National University, Seoul, Korea, in 2014. He is currently working as a Senior Researcher in Electronics and Telecommunications Research Institute, Daejeon, Korea. His research interests include autostereoscopic display, digital holographic display, and holographic optical elements.

Jisoo Hong received his BS and MS in electrical engineering from Seoul National University, Korea in 2002 and 2004, respectively. From 2004 to 2008 he was a senior research engineer with LG Electronics, Korea. In August 2012, he received his PhD from his alma mater. In 2013, he was a postdoctoral researcher in the Department of Physics at University of South Florida, Tampa, Florida. Presently, he is a senior researcher with KETI (Korea Electronics Technology Institute). He is interested in 3D display techniques ranging from light field to holographic representation and incoherent digital holography for biomedical imaging and 3D photography.