Viewing window position control on holographic projection system by electrically focused tunable lens

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Space bandwidth product (SBP) is one of the most significant limitation for displaying the digital holographic display. Due to the SBP problem, the size and viewing angle of displayed holograms cannot be enlarged simultaneously. To overcome the SBP problem, holographic projection system has been researched. It uses a field lens to converge diffracted light from a spatial light modulator (SLM) into a viewing window, where the observer can see whole hologram image displayed on the SLM. However, it has a problem that the viewing distance between the display and observer cannot be controlled and fixed on the viewing window plane. We propose a method to control the position of viewing window formation in the holographic projection system by using an electrically focus tunable lens. We added the focus tunable lens in the holographic projection system, and the position of the viewing window can be controlled by its lens power variation. The principle of controlling viewing window in the proposed system is described, and the relationship among the optical power of focus tunable lens, location and size of the viewing window is analyzed. A computer generated hologram encoding based on Fresnel diffraction theory is developed to generate hologram contents for the proposed system with consideration of varying optical power of the focus tunable lens. Test-bed is built to verify the feasibility of the proposed method, and the experimental results confirm that the proposed method can effectively control the viewing window position of the holographic projection system.