Study On Large Field Of View Optical System Of Virtual Reality Head-mounted Display

Virtual reality is the technology that makes people interact with the digital environments which is produced by computer. It has widely used in aerospace, military training, simulation, virtual design, health care, education, entertainment and other fields. Virtual reality technology includes not only immersive virtual reality which is separated the true world absolutely, but also augmented reality which is overlapped on the true world. Head-mounted display(HMD) is the key equipment in virtual reality technology, which bears the simulation of visual information. Immersion is an important feature of virtual reality, which is achieved by large field of view(FOV) HMD, and the larger the FOV, the stronger the sense of immersion. However, for a fixed display, the resolution will be decreased by the field of view increasing at the same time. With the development of display manufacturing technology, the number of pixels and pixel density are rising speedily. The new displays provide more choice for the HMD and also put forward higher requirements for the design of the optical system of the HMD. Partial binocular overlap is an effective way to expanding binocular field of view and to ensure the resolution constant at the same time, provided that the excellent design of the optical system with large FOV. This article conducted in-depth research on the large field of view of virtual reality head-mounted display optical system. The key contents include the following work:(1) The calculation of the size and outline and the aberration characteristics of visual optical system were analyzed theoretically. The design difficulties of coaxial and off-axis optical system which brought by increasing FOV were proposed. These analyses provide a theoretical basis for the design of HMD optical system. Based on the characteristics of the human eyes, the design requirements and the main indicators of HMD were analyzed.(2) The relationship of the binocular FOV and monocular FOV and binocular overlapped FOV of divergent overlap was derived firstly. The aberration correction limitations of the double ellipsoid HMD which could achieve large FOV and low distortion were analyzed. Then the double free-form mirrors large FOV HMD optical system was proposed. Compared with the double ellipsoid structure, the new structure makes the system MTF increased by 200%, and simplifies the relay lens group and the subsequent lens group, and avoids the "W" or "M" shape of the aspheric lens surface. The monocular FOV is 106.3°(H)×80°(V). The exit pupil diameter is 8mm. The maximum relative distortion is 6.97% and the monocular pixel resolution is 1920×1200. Through divergent overlap of optical system tilt outside and tilt outside 8°of each optical system, the binocular FOV is 122.3°(H)×80°(V) and the horizontal binocular overlapped FOV is 90.3°.(3) The aberration characteristics of coaxial wide-angle eyepiece were analyzed firstly. Then a new large FOV coaxial optical system which was used in immersive virtual reality was proposed. The new optical system uses three pieces of glass lens and a plastic lens, and includes five aspheric surfaces. The diagonal FOV of the new optical system is 114°. The relative distortion of the whole FOV is less than 9.6%. The monocular pixel resolution is 1920×1080. The exit pupil diameter is 10 mm and the monocular FOV is105.6°(H)×73.08°(V). Through divergent overlap of optical system tilt outside and tilt outside 18°of each optical system, the binocular FOV is 141.6°(H)×73.08°(V) and the horizontal binocular overlapped FOV is 69.6°.(4) The machining tolerances and assembly tolerances of the new large FOV coaxial optical system were analyzed and the aspheric surface accuracy tolerances were verified by numerical analysis.Then the glass aspheric surfaces were grinded by ultra-precision CNC grinding optical machining centers and polished through computer controlled optical surfacing technology. The plastic aspheric lenses were processed through ultra-precision single point diamond turning technology. The aspheric surfaces accuracy of plastic aspheric lenses was tested by high-precision contacting contour measuring machine. The mechanical structures of the large FOV coaxial optical system were processed into thin-walled form by machining center. The IPD of the binocular structure is adjustable within 55~71mm by left-handed and right-handed screw thread. The image was corrected according the characteristics of partial binocular overlap. For the binocular scence inconsistencies when the HMD is titled, the image translation correction model was proposed.