Design of a bright polarized head-mounted projection display

In optical see-through head-mounted displays, it has been a common challenge that the displayed image lacks brightness and contrast compared with the direct view of a real-world scene. Consequently, such displays are usually used in dimmed lighting conditions, which limits the feasibility of applying such information displays outdoors or in scenarios where well-lit environments, such as in operation rooms, are required. The lack of image brightness is aggravated in the design of a see-through head-mounted projection display (HMPD). For instance, the overall flux transfer efficiency of existing HMPD designs is less than 10%. The design of a polarized head-mounted projection display (p-HMPD) is presented. The images of a p-HMPD system can potentially be three times brighter than those in existing HMPD designs. It is further demonstrated that the p-HMPD design is able to dramatically improve image brightness, contrast, and color vividness with experimental results. Finally, the design of a compact optical system and helmet prototype is described.     

Design of a polarized head-mounted projection display using ferroelectric liquid-crystal-on-silicon microdisplays

It has been a common problem in optical see-through head-mounted displays that the displayed image lacks brightness and contrast compared with the direct view of a real-world scene. This problem is aggravated in head-mounted projection displays in which multiple beam splitting and low retroreflectance of a typical retroreflective projection screen yield low luminous transfer efficiency. To address this problem, we recently proposed a polarized head-mounted projection display (p-HMPD) design where the polarization states of the light are deliberately manipulated to maximize the luminous transfer efficiency. We report the design of a compact p-HMPD prototype system using a pair of high-resolution ferroelectric liquid-crystal-on-silicon (FLCOS) microdisplays. In addition to higher resolution, the FLCOS displays have much higher optical efficiency than a transmissive-type liquid crystal display (LCD) and help to further improve the overall light efficiency and image quality. We detail the design of a compact illumination unit for the FLCOS microdisplay, also commonly referred to as the light engine, and a projection lens, both of which are key parts of the p-HMPD system. The performances of the light engine and projection lens are analyzed in detail. Finally, we present the design of a compact p-HMPD prototype using the custom-designed light engine and projection optics.     

Design of an ultralight and compact projection lens

Driven by the need for lightweight head-mounted displays, we present the design of an ultralight and compact projection lens for a head-mounted projective display (HMPD). An HMPD consists of a pair of miniature projection lenses, beam splitters, and miniature displays mounted on the helmet and retroreflective sheeting materials placed strategically in the environment. The HMPD has been proposed recently as an alternative modality for three-dimensional visualization. After demonstrating the concept, building a first-generation custom-designed prototype, and investigating perception issues and application potentials, we designed an ultralight and compact projective lens with a diffractive optical element (DOE), plastic components, and aspheric surfaces for the next-generation prototype. The key contribution here lies in the conception, optimization, and assessment of the projection optics. Thus a brief review of the HMPD technology and related research is followed by a detail discussion of the conception and optimization of the ultralight and high-performance projection optics. The design of the DOE will be particularly described in detail. Finally, the diffraction efficiency of the DOE will be evaluated, and the overall performance of the optics will be assessed in both object space for the optical designer and visual space for possible end-users of the technology.     

Engineering of head-mounted projective displays

Head-mounted projective displays (HMPD's) are a novel type of head-mounted display. A HMPD consists of a miniature projection lens mounted upon the user's head and retroreflective sheeting material placed strategically in the environment. First, the imaging concept of a HMPD is reviewed and its potential advantages and disadvantages are discussed. The design and a bench prototype implementation are then presented. Finally, the effects of retroreflective materials on the imaging properties and the optical properties of HMPD's are comprehensively investigated.     

单片式LCOS微型投影仪照明系统设计

照明系统作为投影仪的重要组成部分,对于整个系统的亮度和均匀度起着决定性作用.本文提出了一种特殊的复合抛物面聚光器(Compound Parabolic Concentrator,CPC)作为回复反射器微型投影仪照明系统,目的在于利用CPC特殊的几何结构对UHP(Ultra High Performance)光源发出的部分光线进行收集循环利用,以提高整个系统的光能利用率.通过在Tracpro软件中建立整个单片式LCOS微型投影仪照明系统进行光线模拟,结果表明:使用特殊CPC回复反射器以后,投影到屏幕上的能量提高约为单纯利用椭球形反光碗收集光线时的两倍.     

Efficient polarization converter for projection displays

In the waveguiding limit, a twisted nematic liquid crystal cell behaves as an achromatic polarization rotator. We propose and demonstrate the application of such a polarization rotator to convert unpolarized light into linearly polarized light with almost 100% efficiency. This polarization converter has a 2:1 aspect ratio, which is close to the 16:9 ratio for modern televisions. It can be used therefore in a projection display with polarization-dependent light valves such as a liquid crystal light valve. Both transmittive and reflective light valves can be used. The temperature dependence of the achromatic polarization rotator is also studied.     

Ultraslim fixed pattern projectors with inherent homogenization of illumination

For a given illumination source brightness, the transmitted flux of common single-aperture projection optics scales with all three system dimensions, thus preventing the realization of slim devices along with a high lumen output. In this article we introduce a multichannel approach, called "array projector," which breaks this constraint, thus enabling the realization of ultraslim but high flux systems with inherent homogenization for still image content. The concept is based on regular two-dimensional arrangements of absorbing object structures and projective microlenses superposing their individual images on the screen. After deriving first-order scaling laws for the multichannel projector in contrast to common single-aperture optics, specification of system parameters is shown considering aberrations of a single-channel and collective effects of the array. The technological realization of a sample system is shown and characterized in terms of modulation transfer, homogeneity, depth of focus and flux.     

High-definition imaging system based on spatial light modulators with light-scattering mode

We have developed a prototype high-definition imaging system using polymer-dispersed liquid-crystal (PDLC) light valves, which can modulate unpolarized light with high spatial resolution and exhibit a high optical efficiency, based on the light-scattering effect. We fabricated high-definition light valves with a fine polymer-matrix structure in a PDLC film by controlling the curing conditions used during the photopolymerization-induced phase separation and formation process. This device has excellent characteristics, such as a high resolution, with 50 lp/mm for a limiting resolution and greater than 20 lp/mm at the 50% modulation transfer function point, and a reflectivity of greater than 60%. An optically addressable full-color projection display was designed, consisting of three PDLC light valves, a schlieren optical system based on shift-decentralization optics with a xenon lamp illumination and input-image sources with 1.5 million pixels, including electrical image compensation of the gamma characteristics. We succeeded in displaying pictures on a 110-inch screen with a resolution of 810 TV lines and a luminous flux of 1900-2100 American National Standards Institute lumens.     

Magneto-optic displays

Both the monochromatic and white light performance of a magneto-optic display, using bismuth substituted iron garnets are discussed. It is shown that for monochromatic light, efficiencies up to 28% and contrast ratios up to 180:1 can be achieved. For a practical device, using a white light source, efficiencies up to 8% with contrast ratios in excess of 20:1 can be achieved. It is also shown that a projection display, with a screen illuminance of 0.08 lumens/cm²(∼ 280FL) can be made, using a x20 projection lens and a quartz halogen lamp.     

数字影像输出的关键技术:光引擎系统——数字成像技术进展(六)

随着微电子、光学技术等的长足进展、使屏幕显示技术已经广泛的应用到很多的领域，在大屏幕投影显示领域，光引擎技术是获得大亮度，高影像清晰度和真实的色彩还原性图像输出的关键技术。     

快速响应光阀的制备及在真三维立体显示中的应用

固态体积式真三维立体显示系统主要由数据传输,图像处理模块,高速彩色投影光路模块,显示模块(显示体)以及外围控制电路组成。显示模块由多层液晶光阀组成,光线透过显示体后能量衰减严重,造成显示画面黯淡,模糊,此外,由于光阀响应时间慢,系统刷新频率低,屏幕存在严重的闪烁问题。本文通过在液晶中添加少量聚合物和手性物添加剂制备出快速响应的聚合物稳定胆甾相液晶光阀,其开态透过率达到88%,响应时间小于1ms。将其应用于真三维立体显示系统,显示画面清晰稳定,无闪烁。     

双目光学透视式头盔显示器的实验研究

采用分立元件研制双目光学透视式头盔显示器,对双目透视式头盔显示中的关键问题进行实验研究.用偏振分光棱镜作为组合镜,通过理论分析,发现图像源和外界环境的亮度随偏振片方位角呈余弦变化规律,并通过调整偏振片使头盔显示器的图像与外界环境达到最佳融合状态.调查研究了头盔显示器双目重叠范围的改变对视觉效果的影响,数据结果显示,双目重叠范围为单目视场的33～50%时效果最佳.最后根据视差原理编程制作立体图像对,分别显示于头盔显示器的左右眼,实现立体显示.     

Spatial light modulators for high-brightness projection displays

We fabricated polymer-dispersed liquid-crystal light valves (PDLCLV's) consisting of a 30-mum-thick hydrogenated amorphous-silicon film and a 10-mum-thick polymer-dispersed liquid-crystal (PDLC) film composed of nematic liquid-crystal (LC) microdroplets surrounded by polymer. The device can modulate high-power reading light, because the PDLC becomes transparent or opalescent independent of the polarization state of the reading light when either sufficient or no writing light is incident on the PDLCLV. This device has a limiting resolution of 50 lp/mm (lp indicates line pairs), a reading light efficiency of 60%, a ratio of intensity of light incident on the PDLC layer to intensity of light radiated from the layer, and an extinction ratio of 130:1. The optically addressed video projection system with three PDLCLV's, LC panels of 1048 x 480 pixels as input image sources, a 1-kW Xe lamp, and a schlieren optical system projected television (TV) pictures of 600 and 450 TV lines in the horizontal and the vertical directions on a screen with a diagonal length of 100 in. The total output flux of this system was 1500 lm.     

半透膜阵列式平板显示设计

微型化、轻型化的穿透式平板显示是近年来穿透式头盔显示的主要发展方向.本文针对传统的45°倾角半透膜阵列式平板显示的鬼像产生问题进行分析,提出了一种改进型的平板显示结构.通过减小膜层倾角,不仅使得各视场光线在波导中的倾角增加从而保证出瞳扩展,而且改变了产生鬼像光线的传播路径.同时,采用有膜层和无膜层的双层波导结构以及特定的尺寸限制,有效地约束鬼像产生的条件.根据分析计算,设计了一种膜层倾角为24°的系统结构,实现了水平和垂直方向的二维扩展,以及20°水平视场和13.4°垂直视场的显示输出.     

High efficiency pocket-size projector with a compact projection lens and a light emitting diode-based light source system

We present a light emitting diode (LED)-based ultramini digital micromirror device projector with a size of 75 mm x 67 mm x 42 mm and a weight of 338 g. The LED illuminator inside this projector makes it possible to achieve a volume of 18 cm(3) by using a dichroic filter and a collimating lens. The illumination system consists of high uniformity of 93% through a microlens array as a homogenizer. A total internal reflection prism is also used to reduce the size of both the illumination system and the telecentric projection lens. A projection lens system with an ultrasmall track of 42 mm, including a high modulation transfer function value of 0.4 at 46.2 line pairs/mm, an optical distortion of only 0.25 %, and a television distortion of 0.01%, is designed. Through the above superior specification, we can produce a 20 in. (51 cm) color display comparable in brightness to a laptop with a contrast of 3700:1. The device is compact and suitable for personal use.     

Fast polarization switching panel with high brightness and contrast ratio for three-dimensional display

We propose a polarization switching device using optically compensated pi cell for polarization-glass-type three-dimensional display. This device shows good optical properties such as high transmittance and low cross-talk ratio because of its fast dynamic response characteristics. To improve the brightness and contrast ratio on the right- and left-hand sides, we attach optical retardation films on each side of the polarization glasses instead of attaching the films on the polarization switching panel. From the calculation and experiment, we obtain high contrast ratios, over 200:1, on both sides and a high brightness using only one film on each side.     

Optical screen for direct projection of integral imaging

We describe a way to display three-dimensional images by integral imaging using an ordinary projector. We first explain a method that uses a large-aperture converging lens, then we explain the proposed method that uses two sets of lens array. Based on the principle of this new approach, front projection as well as rear projection is possible. Only a proper viewing area can be formed on the optical screen by this method, which improves the brightness of images on the screen. The projector itself does not need an additional optical system. We report on the results of an experiment carried out to confirm the validity of the proposed method.     

Nonpolarizing and polarizing filter design

We describe a nonpolarizing filter design at oblique incidence and a polarizing filter design at normal incidence that use a uniaxially anisotropic layer. The phase thicknesses and the optical admittances of the layers are compensated for by the birefringent properties of a thin film at oblique incidence. This concept can be applied to the design of nonpolarizing bandpass and edge filters at oblique incidence and of polarizing beam splitters at normal incidence. Besides, the dependence of narrow-bandpass filters on normal incidence is discussed.     

Polarization Encoded Color Image Embedded in a Dielectric Metasurface

Optical metasurfaces have shown unprecedented capabilities in the local manipulation of the light's phase, intensity, and polarization profiles, and represent a new viable technology for applications such as high‐density optical storage, holography and display. Here, a novel metasurface platform is demonstrated for simultaneously encoding color and intensity information into the wavelength‐dependent polarization profile of a light beam. Unlike typical metasurface devices in which images are encoded by phase or amplitude modulation, the color image here is multiplexed into several sets of polarization profiles, each corresponding to a distinct color, which further allows polarization modulation‐induced additive color mixing. This unique approach features the combination of wavelength selectivity and arbitrary polarization control down to a single subwavelength pixel level. The encoding approach for polarization and color may open a new avenue for novel, effective color display elements with fine control over both brightness and contrast, and may have significant impact for high‐density data storage, information security, and anticounterfeiting.     

激光投影显示中二次散射散斑抑制方法

在光学投影系统中,激光经各种元件散射后在投影屏上形成一次散射散斑,投影屏对激光的再次散射,经人眼成像后在视网膜上形成二次散射散斑.在激光投影系统的照明系统中,引入位相调制器调制照明光束,可抑制在投影屏平面上的一次散射散斑,和投影屏二次散射后在人眼视网膜上的二次散射散斑.通过简化的投影系统,对二次散射散斑形成和抑制进行了理论分析,获得了二次散射散斑抑制的条件和抑制后散斑衬度的规律,并进行了仿真和实验验证.理论结果表明,在一定条件下,一次散射散斑与二次散射散斑的衬度变化规律相同.对简化模型进行了仿真计算,得到在投影屏上的像面散斑衬度为0.0096,在探测器平面上的二次散射散斑的衬度为0.019,表明位相板调制投影系统照明光束能抑制一次散射散斑和二次散射散斑.对理论和仿真结果进行了实验验证.