基于双全息光学元件的3D视网膜投影显示器

3D Retina Projection Display Using Doublet Holographic Optical Elements

为克服非相干光源照明条件下基于全息光学元件的视网膜投影显示系统存在的色散、出瞳小、无法获得单眼调焦深度信息等局限,提出了一种基于双全息光学元件的3D视网膜投影显示器。用全息光学元件取代玻璃透镜,用发光二极管取代激光。用反射型体全息光栅和反射型体全息透镜构成的光学结构对系统的色散进行补偿,获得了清晰的图像,系统分辨率达到11.6 lp/mm。结合双全息光学元件结构,利用时分复用技术和角度复用技术实现了具有密集视点的3D视网膜投影显示。该结构可有效扩展系统的出瞳,解决视网膜投影显示系统出瞳小的问题。同时在0.45~2.0 m的深度范围内,实现具有单眼调焦深度信息的真3D显示,显示范围覆盖人眼的辐辏调焦响应敏感范围,可有效缓解辐辏调焦矛盾引起的眩晕和视觉疲劳问题。该系统结构紧凑、价格便宜且避免了散斑噪音和安全隐患,具有良好的应用前景。

Near-Eye Displays(NEDs)are considered as a next-generation display platform.They have shown great application potential in communication,education,healthcare,and so on.As an important NED technique,retinal projection display has attracted much attention for its advantages of large depth of field,wide viewing angle,and simple structure.Traditional retinal projection display can alleviate the Vergence-Accommodation Conflict(VAC)problem to a certain extent.But,this kind of system has problems such as bulky,limited exit pupil area,and inability to evoke monocular accommodation information.For this reason,a variety of retinal projection display technologies based on Holographic Optical Elements(HOE)have been proposed.However,the HOE is an optical element based on diffraction,such systems usually need to use a laser as the light source.Lasers are expensive,have speckle problems,and have potential safety hazards to the eyes.To address these issues,a 3D retinal projection display based on doublet HOEs has been proposed in this paper.The performance of traditional retinal projection display system is improved by using a cheap and safe LED light source and HOEs.A retinal projection display system based on doublet HOEs is made.The display experiments prove the effectiveness of the system.The doublet HOEs are used to compensate for the chromatic dispersion of HOE,which reduces the image blur,and increases the display sharpness.The doublet HOEs compose of a reflective holographic grating and a reflective holographic lens.The reflective holographic grating is made by the interference exposure of two parallel beams.The reflective holographic lens is made by the interference exposure of a parallel beam and a divergent spherical wave.The aperture of the holographic grating and holographic lens is both 30 mm×45 mm.The focal length of the holographic lens is 90 mm.The diffraction efficiencies of the holographic grating and lens are 61%and 58%respectively.The display effects of single HOE and doublet HOEs system are tested by using USAF1951 resolution plate as display target.The results show that the horizontal resolution of the system increases from 2.6 lp/mm to 11.6 lp/mm,and the vertical resolution increases from 2.06 lp/mm to 11.6 lp/mm.These indicate that the doublet HOEs can effectively compensate for the dispersion caused by the diffraction of HOE illuminated by a broadband light source.The doublet HOEs structure can be used to expand the application of HOE in the near eye display system illuminated by incoherent light sources.Combined with the doublet HOEs,LED array,and high performance digital micro-mirror device,a 3D retinal projection display system with full parallax and dense viewpoints is realized by using time division multiplexing and angular multiplexing technology.The LED array is used to generate a viewpoint array corresponding to different viewing angle.By injecting corresponding parallax images for each viewpoint in time sequence,the proposed experimental system realizes the true 3D display with monocular focusing depth cues within the depth range of 0.45 m to 2 m.The display range can cover the sensitive range of human eye with VAC which is from 0.5 m to 2 m.It can alleviate the visual fatigue caused by the VAC problem effectively.The pupil area of the system is expanded from one viewpoint to 7 mm×7 mm,solved the limitation of small exit pupil of retinal projection system.The imaging experiments of 3D scene with three different depth planes are designed to quantitatively characterize the focusing and defocusing effect of images by evaluating the image contrast in different planes.The results further prove the 3D imaging effects of the proposed retinal projection display system.The proposed system uses HOEs to replace the bulky glass lenses,which makes the system structure simple and compact.It also uses the incoherent LED array to replace the laser as the lighting source,which avoids speckle noise and potential safety hazards,and has good application prospects.