| Near-eye display,also known as head-mounted display or wearable display,is a technology for creating computer-generated digital images in the normal view of users by a specially designed optical system.The device based on near-eye display technology is also one of the essential solutions to realize the application of virtual reality or augmented reality.It can make users feel immersed virtual reality experience or enhanced reality experience.Therefore,it has great potential in application of military,education,medical,entertainment,advertising and other fields.The device based on near-eye display technology is regarded as the next generation of mobile intelligent terminals.Although the research on near-eye display technology has lasted for decades,there is not existed a mature technology which can make the head-mounted device light enough,meeting the concept of "human-oriented".The ideal near-eye display scheme should have the characteristics of compact size,light weight,large angle of view,high resolution and providing true three-dimensional display effect without fatigue.In recent years,the development of near-eye display technology is becoming rapid with more researchers paying attention to this area.It is believed that these features can be well integrated into a cleverly designed optical display system in the near future.In this dissertation,we focus on reducing the volume of the near-eye display system,improving image sharpness,enhancing the visual angle of display,and increasing depth cues based on the in-depth investigation and analysis of the current status and problems of the near-eye display technology.The techniques of pure reflection optical system,digital light field,computational holography and virtual retinal projection are studied and discussed in detail,and the corresponding near-eye display optical systems are built using these techniques.The main research contents and innovative work of this paper are as follows:(1)Wide view angle of near-eye display system based on off-axis three-mirror structureOff-axis pure reflection structure is an important method to realize near-eye display system.At present,this kind of structure has the characteristics of chromatic aberration free,high light efficiency and compact structure.But the structure has the disadvantage of narrow view angle.To overcome this shortcoming,a new optical system design method designing off-axis three-mirror near-eye display system is proposed in this dissertation.Different from the traditional method,the proposed method in this dissertation used the ideal reflector as the initial surface,eliminating the optimization interference caused by the aberration of the actual surface.The method focused on the optimization of the space position of the lens.This method can be used in the design of pure reflection system with large view angle.Based on this method,a pure reflective near-eye display system with three free-form reflectors was designed to satisfy the physiological structure of human head.The system had a 40-degree diagonal view angle.At 60 cycles/mm spatial frequency,the maximum distortion of the system was less than 5%,and the value of modulation transfer function(MTF)was greater than 0.1.(2)Real time and high pixel efficiency near-eye display system based on light filed technologyAt present,the digital light field technology is used to achieve multi-depth in near-eye display.However,there are some problems in light field display,such as slow calculation speed and low resolution.In this dissertation,the generation and display process of light field near-eye display system were analyzed,and a high-pixel utilization real-time near-eye display method based on light field technology was proposed.According to the position of the human pupil,the method can produce the corresponding near-eye light field image with high speed by projecting rays backward parallelly.The acceleration of ray tracing algorithm was realized by combining GPU hardware.The real-time rendering and 3D scene display were realized by building a corresponding near-eye light field display system.This method allowed the eye to observe more object pixels,and it can achieve high pixel utilization.The system can make a single eye receive 640*360 resolution,and it can render and display a three-dimensional scene with nearly 60 frames per second.The efficiency of light field generation was 7 times higher than that of ordinary rasterization method.(3)Large view angle and compact near-eye display system based on computational holography technologyComputational holography(CGH)can really reconstruct the three-dimensional wavefront of an object,and its application to near-eye display can achieve the effect of continuous depth effect.However,the current CGH near-eye display system has many problems,such as long system length,small view angle and high computational cost.In order to solve these problems,a compact large-view fast near-eye display method based on CGH is proposed in this dissertation.This method used divergent light as illumination source and Fourier optical transformation system to realize a compact CGH near-eye display system.The system had a diagonal view angle of more than 60 degrees and an optical length of 40 mm.In addition,in the process of generating computed holograms,the characteristics of human view window were taken into account,which greatly reduced the data redundancy in the traditional generation algorithm.Using only 1.5%of the computing resources,the reconstructed effect can be close to that of the traditional CGH.(4)Ultra-lightweight near-eye display system based on virtual retina displayAt present,in order to solve the problem of convergence-accommodation conflict,most near-eye display schemes had the problem of too heavy and complex head redundant devices.Therefore,an ultra-light near-eye display method based on virtual retinal display is proposed in this dissertation.Virtual retinal display can solve the conflict of convergence and accommodation without consuming computational resources and sacrificing resolution,which can greatly simplify the complexity of the display system.In addition,this dissertation separated the optical projection device from the electronic control device,transferring the display-independent parts to the handheld device terminal,further reduced the size of the display system.The system had a diagonal view angle of more than 60 degrees,a weight of less than 30 grams.The near-eye display system was capable of providing depth cues.In this dissertation,four kinds of near-eye display optical systems were evaluated and compared comprehensively.The advantages and disadvantages of each solution were summarized.According to the research results,it is predicted that the near-eye display system using volume holographic optical elements to realize virtual retina display is the most feasible scheme in the future. |
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