Augmented reality head-mounted displays (AR-HMDs) based on waveguides (WGs) are compact in size and comfortable to wear. However, conventional WG-type AR-HMD systems commonly project virtual contents onto single fixed focal plane, often leading to a vergence-accommodation conflict (VAC). This conflict can result in motion sickness and visual fatigue for the user. To mitigate the VAC, display’s depth of field (DOF) should be expanded. For this, we present a multi-focal lens holographic optical element (HOE)-based WG-type display. The multi-focal lens HOE consists of several spatial focal areas with focal lengths of -30 cm, -60 cm, and -100 cm, respectively. The configuration of HOE can be extended the DOF in AR display, as each area on HOE possesses a different refractive power relative to its recorded focal length. The proposed HOE is fabricated using a photopolymer and interferogram recording technique with photomask patterns. Experimental results demonstrate that the DOF is extended from 30 cm to 100 cm, as indicated by the fact that the virtual contents are spatially focused on different depth positions. In conclusion, we believe that the proposed optical system offers significant benefits in terms of reducing VAC while maintaining a compact form factor.
Augmented Reality (AR) has been attracted considerable attention according to the demand for non-face-to-face services. The principle of AR is overlapping a virtual image in the real world. To display a virtual image at a proper position, depth of field is a significant factor. In this paper, we propose a multi-variable focal lens system that can dynamically tune a depth of field. By using a multifocal lens that has several different focal lengths, an image has depth information corresponding to each focal length. A focus tunable lens controls a focused area and magnification to display the appropriate position and size. The proposed system has a huge advantage in form factor and fever issues owing to its simple architecture. In order to verify the feasibility of the system for AR, numerical simulations are performed. The system divides a 2D image into focused and defocused areas. Focused and defocused areas show feasibility that can be tuned by the multifocal lens and focus tunable lens. The results show the depth range from 0.3 m to 2 m (3.3D to 0.5D), which is determined by the design of the system.
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