Contrast behavior of a field lens with a wire‐grid PBS

Abstract— In an LCOS projection system, a wire‐grid PBS may be used to improve contrast and a field lens may be employed to simplify the projection lens. However, the combination of the two leads to a reduced contrast, which decreases with increasing field angle in one direction. In a representative arrangement, measured contrast with a mirror varied from over 10,000 in the center to 450 at the left and right edges. Contrast loss can be improved with a weaker field lens. The problem arises due to a phase delay between polarization states introduced by the wire grid. Polarization measurements show a +53‐nm phase delay with an axis parallel to the wire‐grid surface normal. Various compensation schemes are proposed in order to reduce the aforementioned loss of contrast.     

Processes,characterizations, and system applications of color‐filter liquid‐crystal‐on‐silicon microdisplays

Abstract— A color‐filter liquid‐crystal‐on‐silicon (CF‐LCOS) microdisplay that integrates color filters on silicon for color will be presented. The color‐filter process on silicon was optimized to achieve fine resolution and precise alignment of the color filters on the pixel array, good adhesion to the silicon suface, and a flat surface for the liquid‐crystal cell assembly. Important optical and electrical parameters of the color filters were extracted to establish an electro‐optical model of the CF‐LCOS microdisplays for device simulation. Thermal, chemical, and light‐stability characterizations were performed to ensure the stabilty of the color filters and CF‐LCOS microdisplays. With color CF‐LCOS microdisplays already available, the projection or viewing optics is greatly simplified. This CF‐LCOS microdisplay is ideal for near‐to‐eye displays because of its low‐power consumption and compactness. The CF‐LCOS microdisplay could also withstand medium light illumination for medium‐sized projectors. A single‐panel projector based on one CF‐LCOS microdisplay of 1280 × 768 × RGB resolution was demonstrated.     

OLED‐based pico‐projection system

Abstract— Two pico‐projection systems, a monochrome green and a full‐color system, based on high‐efficiency OLED microdisplays (VGA; pixel size, 12 μm) are presented. Both optical systems are described by a numerical aperture of about 0.3, a magnification of 15x, and a working distance of 300–360 mm. The frequency limit of both systems is 42 cycles/mm at an image contrast of about 60%. The monochrome projection system with a volume smaller than 10 cm³ consists of one green OLED and a projection lens with five elements. The measured luminance in the image plane is about 0.061 lm. The image has a diagonal of 150 mm with a working distance of about 300 mm and has a considerable image contrast of 396:1. The second system combines three high‐brightness OLEDs, red, green, and blue colored, together with a projection lens and an image‐combining element, and an X‐Cube to achieve full‐color projection. The estimated luminance value for the three‐panel projection unit with an OLED luminance of 10,000 cd/m² for each display will be about Φcalculated = 0.147 lm. In this paper, the system concepts, the optical designs, and the realized prototypes of the monochrome and full‐color projection system are presented.     

Microdisplay‐based optical module for avionic display applications

Abstract— A new optical scheme for a LCOS‐based rear‐projection system utilizing an LED illumination source is presented. The proposed optical module could conveniently replace conventional aircraft panel instrumentation not only because it achieves major standard avionics application requirements, such as the capability to withstand mechanical shocks, high reliability, and weight and power‐consumption minimization, but also as a consequence of the fact that it allows the display image area to be properly matched to the shape of the instrument panel more easily than with conventional displays.     

Microdisplay‐based industrial 3‐D and microstructure measurement systems

Abstract— Microdisplays, whether they are of the liquid‐crystal‐on‐silicon (LCOS) or organic light‐emitting diode (OLED) type, have been, up until now, mainly used in multimedia applications or head‐mounted displays. Due to their interesting possibilities, these displays open more and more alternative applications; for example, in optical metrology. Projection lenses for this application area need to be specially designed because the requirements on these systems differ completely from those for multimedia applications. The lenses must have very low geometrical image distortion and they have to be adapted to small objects and/or image distances. On the other hand, they often work with light sources with small spectral bandwidths; consequently, they do not need to be corrected for chromatic aberrations. In addition, the numerical aperture has to be large enough to collect and transfer as much light as possible, but also the size of the projection lens has to be as small as possible to ensure compact measurement systems. All these requirements lead to a compromise in optical lens design. Three optical system designs and realizations — one with an OLED microdisplay and two with an LCOS microdisplay — are presented.     

Multi‐primary image projector using programmable spectral light source

This paper presents a practical prototype of a multi‐primary image projector system in which light source spectra can be programmable for suiting any purpose. Our multi‐primary projection system is mainly configured with a light source component and an image projection component. The programmable light source can reproduce any spectral curve. Spatial images are then generated using a digital mirror device chip that quickly controls the intensity of the light source spectra in 2D image plane. The multi‐primary images in our projection system are reproduced by multiplexing the time‐sequential images with different primary colors. Our multi‐primary image projector realizes not only wide gamut projection but also spectral projection. To achieve this, we also show how light source spectra of four or six primary colors are designed.     

Eye‐safety analysis of current laser‐based LCOS projection systems

Abstract— A laser safety analysis for liquid‐crystal—on—silicon (LCOS) based imaging projection systems utilizing laser light sources is presented. It is shown that a typical laser‐based imaging projector is capable of providing a D65 white‐balanced luminous flux in excess of 20 lm while remaining Class 1 eye safe. By considering a Class 2 classification, it is shown that the same architecture is capable of providing several hundred lumens, a performance level which could potentially be applicable to a new class of high‐brightness miniature projection systems.     

Designing multilayered wire‐grid polarizers using a monochromatic recursive convolution finite‐difference time‐domain algorithm

Multilayered wire‐grid polarizers (WGP) find application as low‐reflection polarizers in projection‐type liquid crystal display devices. A multilayered WGP is formed by adding thin layers on top of the metal ridges of an ordinary WGP. The ordinary WGP consists of a periodic array of parallel metal ridges, where the period of the array and the width of any individual metal ridge are typically less than the wavelength of the incident light. Such WGPs are often used as efficient polarizers. However, in certain applications, it is important to reduce the reflection from the WGP while preserving the polarization efficiency. One of the ways to achieve this goal is to add thin layers on top of the metal ridges of the ordinary WGP. The reduction in reflection from the multilayered WGP depends on the number and material of these additional layers. In this paper, we describe a design method for multilayered WGPs based on an effective medium theory, thin‐film computation method and a monochromatic recursive convolution finite‐difference time‐domain algorithm. The goal of design process is to identify suitable materials and thicknesses for the additional thin layers needed to lower the reflection appreciably. The design method is explained with the help of bilayered WGPs.     

Laser + LCOS projection‐technology revolution

Abstract— The advent of affordable direct‐diode lasers changes all the rules for optical designs and the associated technologies that generate the images from laser light. These new lasers are forseen as driving fundamental changes in the size, power consumption, cost, resolution, and even the uses for pico‐projectors. This paper discusses these topics from the perspective of laser‐light‐illuminated LCOS microdisplays.     

Bistable FLCOS devices for doubled‐brightness micro‐projectors

Abstract— Optical output is of paramount importance to emerging ultra‐miniature projector products. Experimental bistable ferroelectric liquid‐crystal‐on‐silicon (FLCOS) projection microdisplay devices using newly developed FLC materials aligned on obliquely deposited SiO₂ have been developed. These devices enable the doubling of the illumination duty cycle, and hence doubling of the achievable projector light output, while maintaining a DC‐balanced electrical drive.     

High‐efficiency light‐collection optics for lamp‐based projection TV

Abstract— A new light‐collection optics has been developed that enhances the luminance of projection TV which use lamps as the light source. The conventional optical system consists of an elliptical reflector and a flat‐surface front glass, but these systems cannot sufficiently collect the beams coming from the light source, and they cause loss in the coupling with the light pipe. To solve this problem, we devised a new optical system through a structure of an aspherical reflector and an aspherical front glass. This new optical system concentrates the beams coming from the light source to a smaller point which improves the coupling efficiency. Thus, we have successfully increased the luminance of the projection TV by approximately 10%. This paper reports the design principles of the new optical system and the results of a prototype experiment.     

Wire‐grid polarizers in modern LCOS light‐engine configurations

Abstract— Wire‐grid polarizers that have a very high transmission, reflection, polarized‐light optical performance, and opto‐mechanical packaging advantages compared to the older polarization technologies have been developed. The wire‐grid polarizer operation principles and performance data are reviewed. The power of using finite‐difference time‐domain (FDTD) modeling techniques to understand the interaction of the electromagnetic waves with the wires and improve the optical performance of the wire‐grid polarizers and ultimately the light‐engine optical performance is shown. The ability to ray trace through a complete digital projector light engine from light source to the screen, including the wire‐grid polarizers, will be discussed. The main focus is to present the modern LCOS light‐engine architectures that use the wire‐grid polarizers. One‐, two‐, and three‐panel LCOS light engines are covered.     

An optical design for reflective color STN‐LCDs

In reflective color STN‐LCDs, it is necessary to achieve achromatic representation in single‐polarizer STN‐LCD modes. We propose an optimization method for the optical components of single‐polarizer STN‐LCD modes in order to achieve achromatic representation. By applying this method, it is shown that a contrast ratio of more than 20 can be achieved in the normally black (NB) mode. Furthermore, we prove that the normally white (NW) mode can be realized as well as an NB mode which is usually used in current reflective color STN‐LCDs. Comparing the viewing‐angle characteristics of the NW and NB modes, it was found that those of the NW mode are better than those of the NB mode. Particularly, high reflectance can be realized even at larger viewing angles in the NW mode.     

Polarization‐independent liquid‐crystal‐etalon modulator

Abstract— The experimental demonstration of a polarization‐independent high‐transmission field‐sequential liquid‐crystal‐etalon modulator for portable projection‐display application is presented. Polarization independence allows for high transmission efficiency for laser illumination sources using the polarization diversity method of speckle reduction. These devices can also be considered for LED illumination sources; however, the spectral width of current LEDs does not allow for high efficiency, especially for the green channel. These devices demonstrate millisecond switching needed for field‐sequential‐color generation.     

Magnetically inert medical projection system with an image‐guiding optical fiber

Abstract— A visual system for stimulating specific human brain functions inside a clinical magnetoencephalography (MEG) measurement chamber was developed. This system is based on a three‐panel LCOS projection unit and uses a 4.5‐m‐long image‐guiding optical‐fiber bundle to transfer the image into the magnetically shielded MEG measurement chamber. In addition to a proper optical system design, special attention had to be paid to all materials used inside the magnetically shielded chamber. Here, no interfering fields due to electrics or ferromagnetic materials are allowed. The system concept, optical design, and the realized prototype are presented.     

Refractive‐reflective optical system for realizing an ultra‐thin rear projector

Abstract— This paper presents a new optical system used in an ultra‐thin rear projector with a 1500‐mm diagonal size and 260‐mm depth. A refractive‐reflective optical system was developed to achieve a large projection angle of 136° with a small optical distortion of 0.1%. The optics consists of a convex aspherical mirror and a refractive lens. In addition, a new Fresnel screen composed of hybrid blades of refractive‐TIR (total internal reflection) elements was developed to attain good uniformity of brightness and color within the image area.     

Design of a compact projection display for the visualization of 3‐D images using polarization sensitive eyeglasses

Abstract— A compact optical architecture of a three‐dimensional projection display that simultaneously generates two full‐color images with an orthogonal polarization state is presented. The minimal size of the optical engine was investigated and a compact illumination system using light‐emitting diodes as light sources was designed. The effect of dichroic mirrors in the illumination path on the stereoscopic images was also investigted.     

A YC‐separation‐type projector: High dynamic range with double modulation

Abstract— An experimental projector that features double modulation to obtain high‐resolution (4096 × 2160 pixels) and high‐dynamic‐range images has been developed. Although a conventional projector contains three modulators for red, green, and blue and outputs light after combining the modulated light from these three sources, our projector has an additional modulator for luminance that modulates the combined RGB modulated light. It can display high‐resolution color images by combining three low‐resolution panels for chrominance modulation and one high‐resolution panel for luminance modulation. In addition, the dynamic range is dramatically improved because the double‐modulation scheme minimizes black levels in projected images. The projector demonstrates an extremely high dynamic range of 1.1 million to 1 and 10‐bit tone reproduction.     

Green laser sources optimized for highly efficient microdisplay‐based field‐sequential mobile projectors

Abstract— A novel green laser source, based on a monolithic cavity microchip laser platform, has been developed. The laser is designed to be a part of a miniature and efficient RGB light source for microdisplay‐based mobile projector devices. The use of highly efficient, periodically poled MgO‐doped lithium niobate as the non‐linear frequency doubler allows for a significant increase in the overall efficiency of the green microchip laser. Specifically, a 50–150‐mW green output with a wall‐plug efficiency exceeding 10% in the temperature range of greater than 40°C has been demonstrated. A compact package for this laser source with a volume less than 0.33 cm³ is discussed and results of performance tests are presented.     

Image modification for immersive projection display based on pseudo‐projection models

This paper describes a practical method that enables actual images to be converted so that they can be projected onto an immersive projection display (IPD) screen. IPD screens are particularly unique in that their angle of view is extremely wide; therefore, the images projected onto them need to be taken on a special format. In practice, however, it is generally very difficult to shoot images that completely satisfy the specifications of the targeting IPD environment due to cost, technical problems or other reasons. To overcome these problems, we developed a method to modify the images by abandoning geometrical consistency. We were able to utilize this method by assuming that the given image was shot according to a special projection model. Because this model differed from the actual projection model with which the image was taken, we termed it the pseudo‐projection model. Since our method uses simple geometry, and can easily be expressed by a parametric function, the degree of modification or the time sequence for modification can readily be adjusted according to the features of each type of content. Copyright © 2003 John Wiley & Sons, Ltd.