## 3 Design Concept and Methodologies  
In the following are our design goals of tabletop holographic display implementation. What we had in mind from the start is that the system should be designed to evolve eventually to a commercially viable system. 1. The size of the hologram image should be reasonably big enough so that viewers can perceive the 3-dimentional volume and some details of the image. 2. The hologram image should be seen by multiple viewers at the same time, and freely from different positions and perspectives so that each viewer can easily perceive the whole shape of rendered objects and all viewers can share the whole scene information in order to discuss about, or collaboratively manipulate, on it. 3. The image should be rendered in such a way that binocular disparity and motion parallax is perceived to a satisfactory level. In other words, the image should deliver the perception of real objects being there, showing all around perspective views of the object with full parallax. 4. No physical apparatus of the display should occupy the image space, so that the hologram image is shown in free air, enabling manipulation or interaction with the image as if we do with real objects. 5. High quality color hologram image with dynamic content, i.e., the hologram video should be presented. Also note thatvergence-accommodation conflict-free property should basically be supported since the system is real holographic display. Performance specification of the display system to be realized is drawn from these requirements, which is summarized in Table 2. \begin{table}
\begin{tabular}{c c c} \hline
**Requirement** & \(\rightarrow\) & **Specification** \\ \hline
1. Size of the hologram image & \(\rightarrow\) & 7.63 cm or bigger \\ \hline
2. 360 degree viewable freely by multiple users & \(\rightarrow\) & Horizontal viewing angle 360 degrees \\ \hline
3. Volumetric image with full & \(\rightarrow\) & Vertical viewing angle 20 degrees \\ parallax & & \\ \hline
4. Image should not occupy & \(\rightarrow\) & Floating image in the center of the display with 45 degrees of downward \\ physical space & \(\rightarrow\) & viewing direction \\ \hline
5. Color hologram video & \(\rightarrow\) & R/G/B full color video rendering \\ \hline \end{tabular}
\end{table} 
Table 2: Performance specification of the system. Figure 1: Current status of state-of-the-art holographic displays. A drawing of the system design goal and schematic diagram of the main system specifications are shown in Figure 2.  
It is quite challenging to realize these requirements into working display system with currently available commercial SLM devices, since most of them have a physical size of modulation area less than one inch and diffraction angle less than 2 degrees. Figure 3 shows this issue in a schematic diagram. We need to enlarge virtual modulation device to bigger than three inches and also enlarge the field of view to cover 360 degrees. Viewing angle of 360 degrees can only be achieved by either temporally scanning or spatially tiling a small viewing angle of native SLM device. The former method needs temporal multiplexing with fast SLM and the latter one needs spatial tiling with multiple SLMs. The very first thing to do in the system design is to decide on the basic architecture with the selection of appropriate SLM device. We decided to adopt the temporal multiplexing scheme and fast SLM, based on the comparison of expected system complexity and maximum data throughput of SLMs. Usually the performance of an SLM device is denoted by SBP, which is cited most often to compare different SLM devices. SBP simply represents the total pixel numbers of an SLM device, while total data throughput of a device comes from the combination of pixel count and frame rate. Total data throughput actually determines whole system performance in terms of hologram image rendering, thus is more appropriate to judge the performance of an SLM.