## 2 Prior Art of Holographic Display Implementation  
Since Prof. [PERSON] of MIT Spatial Media Group introduced the first prototype system of electronic holographic display [2], called Mark-1, there have been several different approaches with different SLM device [3; 4; 5; 6; 7; 8; 9; 10; 11]. The basic challenge is overcoming the Space Bandwidth Product (SBP) limitation of the SLM device used, by using spatiotemporal multiplexing techniques for wider viewing angle and bigger image size. Table 1 shows the most prominent systems reported so far with their characteristics and specifications. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Systems** & **SLM** & **Architecture** & **Characteristics** & **References** \\ \hline MIT Holo-video & & & P: HPO, & \\ display & AOM & Multichannel & C: MC (1992), FC & [2] \\ (Mark-I, II, III, IV) & & mechanical scanning & (2012) & [3] \\ \hline \multirow{3}{*}{SeeReal VISIO 20} & \multirow{3}{*}{LCD (pixel pitch: \(\sim\)100 um)} & \multirow{3}{*}{Viewing window tracking} & P: HPO, & \\  & & & ISD: 20 inch & \\ \cline{1-1}  & & \(\sim\)100 um) & & \\ \cline{1-1}  & & & VA: NA (eye & \\ \cline{1-1}  & & & tracking) & \\ \hline SNU Curved & & & P:FP & \\ \cline{1-1} holographic & \multirow{3}{*}{LCD display} & \multirow{3}{*}{LCD multiple LCDs} & P:ID: 0.5 inch & \\ \cline{1-1}  & & & VA: 22.8\({}^{\circ}\) & \\ \hline \multirow{3}{*}{NICT Full color display} & \multirow{3}{*}{4K LCoS} & \multirow{3}{*}{4 \(\times\) 4 spatial tiling} & \multirow{3}{*}{ISD: 85 mm} & [6] \\ \cline{1-1}  & & & VA: 5.6\({}^{\circ}\), & [7] \\ \cline{1-1}  & & & FR: 20 fps & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Electronic holographic display systems. They used almost all different types of SLMs, including acousto-optic modulator (AOM), liquid crystal display (LCD), digital micromirror device (DMD), liquid crystal on silicon (LCoS), and rewritable photorefractive photopolymer, which reflects that none of them has satisfactory capability to build a high quality holographic display system; for an extensive review and analysis of these systems and similar approaches, please refer to [12]. All of these holographic display systems are of the 'front-viewing' type implementations. Holographic 3-dimensional images are seen in front or behind the display depending on the content to be rendered. Since the viewing angle is small, it can hardly give any volumetric feeling or perception different from conventional multiview displays, while image size and quality is much inferior to them. So, even though the theory tells strength of the holographic display, practical implementations do not yet realize desired superiority of the technology. This situation is depicted in Figure 1. Recently, there have been several other approaches for different type of holographic displays trying to render more volumetric holographic images, either floating in the air [13] or seen from 360 degrees around [14; 15]. In reference [13], only one viewer can see the image, and in reference [15], one can see only side views, while the display in reference [14] can show top side views, thus neither one is fulfilling the capability of rendering true volumetric image. On the other hand, there have also been several tabletop display systems demonstrated their quite impressive capability of volumetric 3-dimensional image rendering in the free air, which easily enables some manipulations and interactions with the content [16; 17; 18]. These displays shows the usefulness of the tabletop displays, but crucial weakness remains regarding human factor issue, since it cannot support visual accommodation cue of the human 3D vision. This can only be achieved by holographic technology, though super-multiview or integral imaging has also been claimed to have a very similar property [19; 20; 21]. In summary, the tabletop holographic display system with rendering capability of more realistic volumetric 3D images is yet to be developed, which is far better than typical 'front-viewing' type ones.