Three-dimensional dancing bear shows the way to practical holographic display
Three-dimensional holographic displays can provide truly realistic images because they provide all the depth cues required by the human visual system and eliminate visual discomfort. However, the development of a practical holographic display system is limited by the availability of a spatial light modulator (SLM) with a large pixel count and closely spaced pixel element (small pixel pitch). Such a SLM is critical to achieve a large displayed image and a wide viewing angle.1 Over the past five years, we have developed three holographic display systems to combat this problem, each with an increasing degree of sophistication, resolution, and color reproduction quality.
In our first system, we focused on producing a basic monochrome 3D hologram. We used a single digital micromirror device (DMD) with 2 megapixels (MPixel) at a 1920×1080 pixel resolution and 10.8μm pixel pitch for rendering the computer-generated holograms. To illuminate these holograms, we used an expanded, collimated laser beam generated by a 655nm wavelength red laser diode.2, 3 We were able to produce a 3in monochrome 50 Mpixel display at a 25Hz refresh rate by using a two-axis scanning mirror device to optically tile 24 reconstructed sub-holograms computed from a pre-divided 3D object.3
In our second system, we wanted to reproduce full-color images on our holographic displays. For this system, we used five FLCoS (ferroelectric liquid crystal on silicon) SLMs, each with 1.3 MPixel at 13.62μm pixel pitch, and red/green/blue (RGB) lasers at 655, 532, and 473nm, respectively. We physically tiled five FLCoSs with the aid of a beam splitter/combiner to form a seamless image (see Figure 1). In total, we were able to display full-color 3D objects with a 3in diagonal from 6.5 MPixel holograms at a 60Hz refresh rate.4 A new space division multiplexing (SDM) technique was developed to achieve full-color reconstruction of 3D objects. The SDM technique randomly assigned different small areas on the hologram plane to different colors to form masked RGB holograms rendered to the five FLCoSs. Three masks were fabricated to allow RGB laser beams to pass through to illuminate the masked RGB holograms. Only the small hologram areas assigned to each color were illuminated by their corresponding light. With this system, we can dynamically reconstruct full-color 3D objects from network transmitted hologram data at a rate of 9.44Gbps.4 A reconstructed full-color full-parallax animated 3D dancing bear with a 3in diagonal is shown in a short video that is available online.5
In our latest system, we were able to produce ultra-high resolution (189–378 MPixel) full-color 3D holograms on 5–10in diagonal screens at a 60Hz refresh rate by using 24 FLCoSs and two sets of RGB lasers at 640, 532, and 450nm. To display holograms with such large pixel counts, we combined the physical tiling and optical scan tiling techniques described before (see Figure 2). The 24 FLCoSs were physically tiled into a SLM array (3×8) to achieve a pixel count of 31.5 MPixel with the aid of a large beam splitter/combiner. Each column consisted of eight FLCoSs that were seamlessly tiled along the vertical direction. A gap between two adjacent columns was reserved to be filled by optical scan tiling with a one-axis galvanometric scanning mirror along the horizontal direction to further increase the pixel count to 189–378 MPixel with 6–12 scan steps.
The sequence and timing for the optical scan tiling were specially designed to form a whole hologram frame within the eye integration time limit of about 50ms. Demagnification optics (0.5×) were implemented to enhance the viewing angle. We used a time division multiplexing technique that allowed us to time-sequentially render RGB holograms to FLCoSs at high speeds and to reconstruct the RGB components of the object with the synchronized illumination of RGB lasers. Full-color and full-parallax 3D holographic video was transmitted at 45Gbps via six 10Gbps network channels from our hologram loading platform to the hologram launching platform. System control based on a field-programmable gate array was implemented to synchronize the FLCoSs, RGB lasers, and scanning mirror. Figure 3 shows a snapshot of the reconstructed 3D holographic video with a 5in diagonal. Using the SDM technique for color mixing, we were ultimately able to achieve a 10in diagonal full-color display of 3D objects.
We have also developed a new split look-up table algorithm and implemented it on a graphics processing unit (GPU)-based computation platform to achieve fast hologram generation.6 Full-parallax computer-generated holograms with 189 MPixel can be computed within 40s for up to 16 million 3D object points. To reduce laser speckles, multiple holograms for the same object with different sets of random phases imposed on the object points were computed and reconstructed within the eye integration time limit of about 50ms.1, 4 For example, the speckle noise was effectively reduced by displaying 24 holograms at 1440Hz for each object frame.5 More technical details of our work will be presented in an upcoming Photonics West invited paper.7
There are some bottlenecks that hinder the practical realization of holographic display systems. New SLM device technology with submicron pixel pitch is the key to increasing the viewing angle to 90°, while tens of terapixels are needed to achieve display sizes of several tens of inches.8 Besides the increase in pixel count of a single SLM, physical tiling and optical scan tiling techniques provide an effective way to increase the total pixel count of displayed holograms. Compact RGB laser modules with an illumination area large enough to cover the physically tiled SLMs is also one of the key components required for the development of compact 3D holographic display systems. Other obstacles such as data transmission bandwidth, storage capacity, and hologram computation speed also need to be overcome on the way to the commercialization of 3D holographic display technology. Our future work will include addressing some of these technology bottlenecks.
Xuewu Xu is a scientist at DSI. His current research interests include holographic displays, spatial light modulators, and display systems. He is a member of the Society for Information Display, as well as the Program Committee of Digital Holography and 3D Imaging (DH 2013).