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.     

MUTED: Multi‐user 3‐D display

Abstract— Research described in this paper encompasses the design and building of glasses‐free (autostereoscopic) displays that utilize a direct‐view liquid‐crystal display whose backlight is provided by a projector and novel steering optics. This is controlled by the output of a multi‐user head‐position tracker. As the displays employ spatial multiplexing on a liquid‐crystal‐display screen, they are inherently 2‐D/3‐D switchable with 2‐D being achieved by simply displaying the same image in the left and right channels. Two prototypes are described in this paper; one incorporating a holographic projector and the other a conventional LCOS projector. The LCOS projector version addresses the limitations of brightness, cross‐talk, banding in the images, and laser stability that occur in the holographic projector version. The future development is considered and a comparison between the prototypes and with other 3‐D displays is given.     

Multi‐viewer autostereoscopic display with dynamically addressable holographic backlight

Abstract— The Multi‐User 3‐D Television Display (MUTED), designed to provide three‐dimensional television (3‐D TV) by the display of autostereoscopic imagery to multiple viewers, each of whom should enjoy freedom of movement, is described. Such an autostereoscopic display system, which allows multiple viewers simultaneously by the use of head tracking, was previously demonstrated for TV applications in the ATTEST project. However, the requirement for a dynamically addressable, steerable backlight presented several problems for the illumination source. The MUTED system demonstrates significant advances in the realization of a multi‐user autostereoscopic display, partly due to the provision of a dynamic backlight employing a novel holographic laser projector. Such a technology provides significant advantages in terms of brightness, efficiency, laser speckle, and the ability to correct for optical aberrations compared to both imaging and scanned‐beam projection technologies.     

Eye‐safety analysis of current laser‐based scanned‐beam projection systems

Abstract— Scanned‐beam projection systems have attracted much interest recently, with claimed advantages including power efficiency and potential miniaturization consistent with embedding in mobile devices. However, the laser‐safety classification and concomitant performance implications, which are arguably the most important issues pertaining to this technology, remain widely misunderstood. In this paper, Class 1 and 2 laser‐safety radiometric image power limits for scanned‐beam systems are derived with reference to the IEC 60825‐1 standard. By calculating the equivalent photometric measure of luminous flux, it is possible to show that the brightness limits for scanned‐beam projection systems using current technology are approximately 1 and 1 7 lm for Class 1 and 2 safety classifications, respectively.     

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.     

Letter: Scaling rules for pico‐projector laser safety

Abstract— This letter presents a set of scaling rules for the laser safety analysis of scanned‐beam and LCOS projection systems, providing a simple method for determining the impact of changes in key system parameters upon the maximum eye‐safe luminous flux.     

High‐efficiency optical system for ultra‐short‐throw‐distance projector based on multi‐laser light source

Abstract— A novel laser‐light‐source projector having the three outstanding features of high brightness, ultra‐short throw distance, and high color reproduction has been developed.These features have recently come to be required in the high‐end projector market. The technologies for the laser‐light‐source projectors fully utilize the advantages of lasers, such as high luminance, small étendue, and high color purity. By integrating a triple‐rod illumination system with a multi‐laser light source and an ultra‐wide‐angle projection system, the developed high‐efficiency optical system has achieved a brightness of 7000 lm and a throw ratio of 0.28 with an image size of 100–150 in. Another new technology, laser color processing (LCP), has offered vivid color reproduction which has a color gamut that is up to 180% wider than the BT.709 standard without appearing unnaturally colored. Furthermore, a speckle suppression effect produced by the multi‐laser light source has been demonstrated. In this paper, an overview of these newly developed technologies that are used in the novel laser‐light‐source projector is presented, and solutions to the issues of speckle noise and safety are presented.     

Letter: On proximity detection systems for pico‐projectors

Abstract— Proximity detection systems have been proposed as a potentially beneficial method for increasing the eye‐safe luminous flux of laser‐based pico‐projectors. In this letter, it is shown that, while the benefit for panel‐based systems could be significant, the impact upon scanned‐beam projectors is far smaller.     

Optimizing flying‐spot display designs based on the upcoming edition of the laser safety standard

The evaluation concept of the laser safety standard International Electrotechnical Commission (IEC) 60825 for scanning‐beam projectors is presented. The most important parameters are identified as well as the impact of the expected changes to the laser safety standard IEC 60825 Edition 2. Vital differences are pointed out, and the maximum permissible output power for class 2 lasers is discussed in detail. In special focus is the case of two pulses falling into the aperture within the thermal confinement time, because this is one of the major scenarios being affected by the revision. It is shown that the upcoming third edition of the standard allows significantly more power for flying‐spot projectors, and finally, we provide a quick way to determine the permitted laser power depending on typical design parameters.     

Projection‐based head‐tracking 3‐D displays

Abstract— This paper describes the construction and operation of four 3‐D displays in which each display produces two images for each eye and thus fits into the category of projection‐based binocular stereoscopic displays. The four 3‐D displays described are pico‐projector‐based, liquid‐ crystal—on—silicon (LCOS) conventional projector‐based, 120‐Hz digital‐light‐processor (DLP) projector‐ based, and the HELIUM3D system. In the first three displays, images are produced on a direct‐view LCD whose conventional backlight is replaced with a projection illumination source that is controlled by a multi‐user head tracker; novel steering optics direct the projector output to regions referred to as exit pupils located at the viewers' eyes. In the HELIUM3D display, the image information is supplied by a horizontally scanned, fast, light valve whose output is controlled by a spatial light modulator (SLM) to direct images to the appropriate viewers' eyes. The current statu s and the multimodal potential of the HELIUM3D display are described.     

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.     

Laser‐based multi‐user 3‐D display

Abstract— The development of a multi‐user stereoscopic display that does not require the use of special glasses (autostereoscopic), and that enables a large degree of freedom of viewer movement and requires only the minimum amount of information (a stereo pair) for the displays described. The optics comprise an RGB holographic laser projector that is controlled by the output of a multi‐target head‐position head tracker, an optical assembly that converts the projector output into steerable exit pupils, and a screen assembly comprising a single liquid‐crystal display (LCD) and image multiplexing screen. A stereo image pair is produced on the LCD by simultaneously displaying left and right images on alternate rows of pixels. Novel steering optics that replace the conventional backlight are used to direct viewing regions, referred to as exit pupils, to the appropriate viewers' eyes. The results obtained from the first version of the display, where the illumination source consists of several thousand white LEDs, are given and the current status of the latest prototype being constructed on the basis of these results is described. The work indicates that a laser‐based head‐tracking display can provide the basis for the next generation of 3‐D display.     

Compensation of birefringence in lead‐free polarizing beam splitters for LCOS projectors

Abstract— Most optical designs for delivering light to LCOS imagers and then from the imagers to the projection lens use polarizing‐beam‐splitter (PBS) technology. Most of the PBSs used in commercial LCOS projectors contain glass with a significant amount of lead (Pb). Such glasses have inherently low stress birefringence, and therefore maintain the polarization state of light passing through them. However, Pb‐bearing glass is an expensive, difficult to process, and hazardous material with special disposal requirements and is therefore not desirable in consumer‐electronic products. On the other hand, Pb‐free wire‐grid plate PBSs require a longer back focal length than would be optimal. Data and modeling results show that uniform high‐contrast dark states may be obtained from lead‐free‐glass Cartesian PBS prisms when a quarter‐wave compensator is used between the imager and the PBS.     

Automatic Registration of Multi‐Projector Domes Using a Single Uncalibrated Camera

In this paper we present a novel technique for easily calibrating multiple casually aligned projectors on spherical domes using a single uncalibrated camera. Using the prior knowledge of the display surface being a dome, we can estimate the camera intrinsic and extrinsic parameters and the projector to display surface correspondences automatically using a set of images. These images include the image of the dome itself and a projected pattern from each projector. Using these correspondences we can register images from the multiple projectors on the dome. Further, we can register displays which are not entirely visible in a single camera view using multiple pan and tilted views of an uncalibrated camera making our method suitable for displays of different size and resolution. We can register images from any arbitrary viewpoint making it appropriate for a single head‐tracked user in a 3D visualization system. Also, we can use several cartographic mapping techniques to register images in a manner that is appropriate for multi‐user visualization. Domes are known to produce a tremendous sense of immersion and presence in visualization systems. Yet, till date, there exists no easy way to register multiple projectors on a dome to create a high‐resolution realistic visualizations. To the best of our knowledge, this is the first method that can achieve accurate geometric registration of multiple projectors on a dome simply and automatically using a single uncalibrated camera.     

Display grid: Ad‐hoc clusters of autonomous projectors

Abstract— Projectors, like computers, are becoming commoditized. Self‐contained computers are now being networked to create computing grids, allowing transparent access to a large computing resource or massive data storage. Image presentation devices can be similarly modified to support the concept of a “display grid” to create large seamless displays. Limiting ourselves to projector‐based display grids, we present techniques for creating multi‐projector displays via self‐configuring clusters of autonomous projectors. The ad‐hoc clustering approach avoids large monolithic installations. We show a low‐cost system that supports dynamic inclusion of new projectors, automatic geometric configuration, and seamless blending of overlapping projectors.     

Three‐panel LCOS projection systems

Abstract— Three‐panel liquid‐crystal‐on‐silicon (LCOS) projection systems are presented with an emphasis on the commercially successful shared retarder‐stack‐filter (RSF) polarizing‐beam‐splitter (PBS) architectures. The design and operation of the specific CQ90 projection core is presented in detail, and its contrast and transmission derived. alternative three‐PBS/X‐cube LCOS architectures are briefly introduced and their performance is compared to that of the CQ90.     

Advances towards high‐resolution pack‐and‐go displays: A survey

Abstract— Tiled displays provide high resolution and large scale simultaneously. Projectors can project on any available surface. Thus, it is possible to create a large high‐resolution display by simply tiling multiple projectors on any available regular surface. The tremendous advancement in projection technology has made projectors portable and affordable. One can envision displays made of multiple such projectors that can be packed in one's car trunk, carried from one location to another, deployed at each location easily to create a seamless high‐resolution display, and, finally, dismantled in minutes to be taken to the next location — essentially a pack‐and‐go display. Several challenges must be overcome in order to realize such pack‐and‐go displays. These include allowing for imperfect uncalibrated devices, uneven non‐diffused display surfaces, and a layman user via complete automation in deployment that requires no user invention. We described the advances we have made in addressing these challenges for the most common case of planar display surfaces. First, we present a technique to allow imperfect projectors. Next, we present a technique to allow a photometrically uncalibrated camera. Finally, we present a novel distributed architecture that renders critical display capabilities such as self‐calibration, scalability, and reconfigurability without any user intervention. These advances are important milestones towards the development of easy‐to‐use multi‐projector displays that can be deployed anywhere and by anyone.     

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.     

Thin‐film micromirror array (TMA) for large information‐display systems

Abstract— The thin‐film micromirror array (TMA) is a new reflective‐type spatial light modulator fabricated with microelectromechanical systems (MEMS) technology. Micromachined thin‐film piezoelectric actuators are used to control the tilt angle of each micromirror, which defines the gray scale of the matching screen pixel. A working projector prototype of 5400 true ANSI lumens was presented at Asian Display '98 with three VGA‐format TMAchips and a 1‐kW xenon lamp. The prototype showed a light‐transmitting efficiency of 22%. An XGA‐format TMA was developed for commercial purposes. The size of each micromirror of the XGA‐format TMA is reduced to half the size of the micromirror of a VGA‐format TMA. Efforts have been made to maintain the light‐transmitting efficiency of the XGA TMA projector over 21%, as well as to improve the uniformity.     

Spatially uniform colors for projectors and tiled displays

Abstract— A major issue when setting up multi‐projector tiled displays is the spatial non‐uniformity of the color throughout the display's area. Indeed, the chromatic properties do not only vary between two different projectors, but also between different spatial locations inside the displaying area of one single projector. A new method for calibrating the colors of a tiled display is presented. An iterative algorithm to construct a correction table which makes the luminance uniform over the projected area of one single projector is presented first. This so‐called intra‐projector calibration uses a standard camera as a luminance measuring device and can be processed in parallel for all projectors. Once the color inside each projector is spatially uniform, the set of displayable colors — the color gamut — of each projector is measured. On the basis of these measurements, the goal of the inter‐projector calibration is to find an optimal gamut shared by all the projectors. Finding the optimal color gamut displayable by n projectors in time O(n) is shown, and the color conversion from one specific color gamut to the common global gamut is derived. The method of testing it on a tiled display consisting of 48 projectors with large chrominance shifts was experimentally validated.