LED projection architectures for stereoscopic and multiview 3D displays

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1 LED projection architectures for stereoscopic and multiview 3D displays Youri Meuret a, Lawrence Bogaert a, Stijn Roelandt a, Jana Vanderheijden a, Aykut Avci b, Herbert De Smet b,c and Hugo Thienpont a a Vrije Universiteit Brussel, Faculty of Engineering Sciences, Brussels Photonics Team B-PHOT, Pleinlaan 2, 1050 Brussels, Belgium b Ghent University, Department of Electronics and Information Systems, Technologiepark 914, 9052 Ghent, Belgium c imec, Centre for Microsystems Technology, Technologiepark 914, 9052 Ghent, Belgium ABSTRACT LED-based projection systems have several interesting features: extended color-gamut, long lifetime, robustness and a fast turn-on time. However, the possibility to develop compact projectors remains the most important driving force to investigate LED projection. This is related to the limited light output of LED projectors that is a consequence of the relative low luminance of LEDs, compared to high intensity discharge lamps. We have investigated several LED projection architectures for the development of new 3D visualization displays. Polarization-based stereoscopic projection displays are often implemented using two identical projectors with passive polarizers at the output of their projection lens. We have designed and built a prototype of a stereoscopic projection system that incorporates the functionality of both projectors. The system uses high-resolution liquidcrystal-on-silicon light valves and an illumination system with LEDs. The possibility to add an extra LED illumination channel was also investigated for this optical configuration. Multiview projection displays allow the visualization of 3D images for multiple viewers without the need to wear special eyeglasses. Systems with large number of viewing zones have already been demonstrated. Such systems often use multiple projection engines. We have investigated a projection architecture that uses only one digital micromirror device and a LED-based illumination system to create multiple viewing zones. The system is based on the time-sequential modulation of the different images for each viewing zone and a special projection screen with micro-optical features. We analyzed the limitations of a LED-based illumination for the investigated stereoscopic and multiview projection systems and discuss the potential of a laser-based illumination. Keywords: LED, laser, projection display, 3D visualization, stereoscopic, multiview, micro-optics 1. INTRODUCTION The light flux of light emitting diodes (LEDs) has improved consistently since their invention. This progress made it recently interesting to use these devices as light sources for projection applications. LEDs are small light sources with a narrow spectral emission band and a low operating voltage, which makes them ideal light sources for compact projection displays. 1 Several examples of such systems have already been introduced to the market. LEDs as light sources for projection displays have many other interesting features compared to high-intensity discharge (HID) lamps: longer life time, more robust and a larger dimming ratio due to the high switching speed of LEDs. 2 However, especially the possibility to develop compact projection systems is the main driving force for the development of LED projectors. This is due to the fact that the optical power per unit of étendue (luminance) of a LED is still significantly lower than from a HID lamp. This means that it is very difficult to get much light through the optical system. 3 We could increase the acceptance étendue of the light valve(s) but this would also increase the total size of the projection system and is contradictory to the current trend of further Further author information: (Send correspondence to Youri Meuret) Youri Meuret: ymeuret@tona.vub.ac.be, Telephone: +32 (0)

2 miniaturizing the projection light valves. This currently sentences LED projection to applications with a low lumen output. We have tried to exploit the interesting features of LEDs for the development of several novel projection architectures for the visualization of three-dimensional (3D) information. We investigated both stereoscopic and multiview projection displays with LEDs as light sources. We looked at the opportunities offered by these relatively new projection light sources and investigated their limitations. 2. PROJECTION DISPLAYS FOR 3D VISUALIZATION 2.1. Stereoscopic projection systems Three-dimensional (3D) information can be displayed using a variety of techniques that rely on one or several eye mechanisms to induce a depth cue. Systems that require special eyeglasses use two different views of the real-life object that correspond with the left- and right-eye perspective, i.e., a stereoscopic image pair. The horizontal disparity between both images is interpreted by the human brain to induce a sense of depth. Stereoscopic displays are used in a wide variety of applications: medical imaging, scientific and technical visualization, and entertainment. The recent interest of the film industry in 3D digital cinema has provided a boost in development and public awareness of stereoscopic systems. There are several methods to create 3D stereoscopic images using projectors. These are based on color, shutters, or polarization. A first approach uses eyeglasses with two different color filters. The left-eye image is seen through a red-colored filter and the right-eye image is seen through a cyan-colored filter. One projector displays the image consisting of the two superimposed colors. The poor color perception can be overcome with a pair of color filters that transmit multiple narrow-wavelength bands. 4 The projector sequentially displays leftand right-eye images with a different set of primary colors. For instance, the red-colored left- and right-eye image components are modulated with a red that is orange tinted (shorter wavelength) and deep red tinted (longer wavelength), respectively. A second approach blocks the undesired image with a liquid-crystal based shutter incorporated into the eyeglasses. The eyeglasses are synchronized with the projection system via an infrared signal to close the right-eye shutter when the left eye image is shown and vice versa. One projector can be used to sequentially display full-color left- and right-eye images. A third approach uses polarization-sensitive eyeglasses in combination with a non-depolarizing screen. This approach enables the visualization of 3D images without expensive multilayer transmission coatings nor battery powered eyeglasses Multiview displays Multiview 3D displays show different views of an object or a scene in different directions. 5 At any viewing position, two different views are seen by both eyes stimulating depth perception. For this, the viewer does not have to wear any special eyeglasses. Because more than two different views are used, the viewer can see a different 3D view of the image depending on the viewing position. This enables look around capability, which increases the realism of the 3D sensation. Such displays can be implemented using a flat panel display in combination with a lenticular or parallax barrier. 6 In lenticular-based systems, each cylindrical lens is aligned with a specific number of sub pixels of the display such that the light emitted by each sub pixel is imaged toward a different direction. In such a configuration, the native resolution of the flat-panel display is decreased with a factor equal to the number of viewing zones that the display is capable of showing. Projection technology can be used to develop advanced multiview displays. To maintain the native resolution of the light modulator, the same number of projectors as viewing zones can be used. 7 Another approach uses the same number of light modulators as viewing zones and a common imaging system. 8 Both methods have the advantage that they can show a large number of views. When the viewing zone density is above a certain threshold, all human mechanisms for the observation of 3D visual information are addressed. This will solve the accommodation convergence conflict that leads to eye fatigue when viewing 3D images with negative parallax (images appear in front of the screen). 9 On the other hand, such systems are complex, large and expensive.

3 Figure 1. Optical engine to generate s- and p-polarized images with four LCOS panels. Illustrated on the left for the red and blue image components and on the right for the green image component. 3. NOVEL STEREOSCOPIC PROJECTION ARCHITECTURE WITH A LED-BASED ILLUMINATION SYSTEM A polarization-based stereoscopic projection architecture is often implemented using two identical projectors that are dedicated to displaying the left- and right-eye images respectively. Linear polarizers are placed at the output of both projectors to encode both images with an orthogonal polarization state. Another implementation uses one projector that sequentially displays both images and a polarization switch at the output of the projection system. 10 This single projector implementation necessitates a high field rate projector to avoid flicker and motion artifacts. This is not necessary in the dual projector system, since one projector is dedicated for each view. Furthermore, the brightness of the images is lower because they are sequentially projected instead of simultaneously, however, they exhibit a lower power consumption and are more compact. The main drawback of the dual projector system however is the cumbersome alignment of the left- and right-eye images on the projection screen. In order to solve this issue, we have designed a projection system that incorporates the functionality of both projectors and polarizers into one optical engine. Additionally, it does not halve the lumen throughput of the projection system when creating the polarized images, as is the case in all other polarization-based approaches Optical architecture The design of the optical engine consists of four polarizing beam splitters (PBSs) that are positioned with their polarization splitting surfaces in a cross shape. 11 It creates two polarized illumination light beams, modulates full-color s- and p-polarized images using liquid-crystal-on-silicon (LCOS) panels and combines both images into one light beam. A specific implementation of this optical architecture, shown in Figure 1, uses two LCOS panels to modulate each polarized image. One is continuously illuminated by green light while the other is color-sequentially illuminated by red and blue light. In this way, we optimize the light output of the green image component. For LEDs, the green color is typically the least efficient which means that the light output of the total system is optimized in this way. 12 To realize modulation of the image color components by two different LCOS panels, the green illumination light beam needs to have an orthogonal polarization state compared to the red and blue light beams. This is done with wavelength selective half-wave retarders for green light that are positioned after the first PBS. Additionally, such half-wave plates (HWP) are positioned before the final PBS to ensure that the green modulated light beam is recombined with the red and blue modulated light beams before they reach the projection lens. The necessity to use such HWPs is due to the fact that we need to position additional clean-up polarizers in between the PBSs to ensure a high contrast ratio. Otherwise we could illuminate the first PBS with a green light beam from one side and with the red and blue light beams from the other side.

4 Figure 2. Optical engine to generate s- and p-polarized images with two LCOS panels. Illustrated on the left for illumination by a first illumination system and on the right for illumination by a second illumination system. An alternative implementation of the optical architecture is shown in Figure 2. In this configuration only one LCOS panel is used to modulate each polarized image. The LCOS panel is time-sequentially illuminated by the different colors in a first and second illumination system that are located at different sides of the first PBS. To recombine the light beams from the two different illumination systems towards the LCOS panels, we make use of two switchable half-wave retarders. Additional clean-up polarizers are used in between the PBSs to ensure a high contrast ratio LED-based projection system with four liquid-crystal-on-silicon panels We built a demonstration system of the optical configuration with four LCOS panels, using a LED-based illumination system. 13 The LED-based illumination system is schematically illustrated in Figure 3 (left). We used amber (reddish color), true green, and blue OSTAR projection multi-chip LED modules from OSRAM as light sources. 14 The LED modules consist of a 2 2 matrix of semiconductor dies, with a square shape, that emit directly into air with a Lambertian radiation pattern. Their light output is collimated by a transparent compound parabolic concentrator with a square input facet that matches the dimensions of the LEDs. 15 To increase the available light we use two LED modules for each color. We designed a telecentric illumination system that is a succession of a position-to-angle and an angle-to-position transformation. Between both lens systems, required for the latter transformations, a lenslet integrator is placed. This component is crucial to ensure uniform illumination of the LCOS panels, resulting in system tolerance to a LED failure. 16,17 The full optical engine with the LED-based illumination system is shown in Figure 3 (right). Four QXGA (quad extended graphics array) resolution ( ) vertically aligned nematic LCOS microdisplays were used to modulate the images. 18 The LCOS panels have a diagonal of 0.82 inch and a 4:3 aspect ratio. The lenses in the illumination system are optimized such that these panels are uniformly illuminated with a 10% surface overfill and such that the incident angular light distribution corresponds to the acceptance angles of the projection lens (f -number of 2.3). Thus, we have a system étendue of 32.5 mm 2 sr. Due to the limited internal refresh rate of the high-resolution LCOS panels, we are only able to display red and blue images at 30 Hz as opposed to green images at 60 Hz. This limitation can be overcome by using faster LCOS panels at the cost of a lower resolution. To obtain a high-resolution LCOS projector, a three-panel approach is currently necessary. This means that a stereoscopic high-resolution LCOS projection system would normally consist of two stacked projectors with each three high-resolution panels. The presented four-panel approach is thus an interesting alternative to show high-resolution stereoscopic LCOS images with a more efficient and compact system. However, a slight increase of the internal refresh rate would be necessary. We experimentally characterized the optical efficiency of the optical engine and its contrast ratio. 13 We found an optical efficiency of the optical core of around 10% for red, green and blue light. The optical efficiency of the illumination system and projection lens should additionally be taken into account to find the total optical

5 Figure 3. (left) Illumination system using multiple red, green and blue LED light sources, ensuring telecentric illumination of the LCOS panels. (right) View of the demonstrated projection system with the illumination system and the optical engine located, respectively, at the left- and right-hand sides. efficiency of the system. It is clear that even with the current improved LED technology, it will be impossible to create a stereoscopic system with a fairly high light output. For the full-on/full-off contrast ratio we found values in the neighborhood of 100:1, with important differences between the contrast values for the three primary colors. Using a reliable optical simulation tool in ASAP 19 we found the two main reasons for this relative low contrast. It is vital that all used components in the optical core are well anti-reflection coated. Secondly, there should be a good match between the spectra of the LEDs and the polarization conversion characteristic of the wavelength-selective HWPs. To ensure a high contrast, it is necessary that all green wavelengths, that propagate in the optical core, are included in the wavelength region of the wavelength selective HWPs that fully convert the state-of-polarization (SOP) of the incident light rays. Additionally, the SOP of all red and blue wavelengths that propagate in the optical core should not be converted by the wavelength selective HWPs in any way. If we look at the spectra for red, green and blue light, of the state-of-the-art LED modules for projection applications, we see that these are not fully separated. The use of narrow bandpass filters to trim the LED spectra, could solve this problem but would add another component to the optical system LED-based projection system for extended color gamut We also built a demonstration system for the optical configuration with two LCOS panels, using two separate LED-based illumination systems. 20 The main goal of this system was to demonstrate the possibility to extend the color gamut of the projection system by working with more than three primary colors. When more than three LEDs with different primary colors have to be combined to form one illumination light beam, it is difficult to do so in an optically efficient way using only dichroic mirrors. This is because of the spectral overlap of the different LEDs. We can solve this problem by working with two illumination systems. To demonstrate the concept, we worked with one illumination system for red, green and blue and a second illumination system for cyan. This second illumination system can contain one or two additional primary colors. To realize the same modulation path for light coming from the first and second illumination system, switchable half-wave retarders are added between PBS 1 and 2 and between PBS 1 and 3 (see Figure 2). These components are synchronized with the illumination systems in order that they are only switched on when light from the second illumination system is modulated by the LCOS panels. Switchable retarders have already been reported in LED-based LCOS projectors with the goal to enhance the light output at the expense of a larger optical design. 21 We use them to extend the color gamut without increasing the size of the optical engine. We have designed an illumination system that telecentrically illuminates the LCOS panels in the optical engine. It is optimized for operation with the red, green and blue PT54 PhlatLight LEDs. 22 The light emitted by the PT54 PhlatLight LEDs is collimated by means of an optic based on PhotonVacuum technology. 23 Besides light collection, it also transforms the shape of the output beam and homogenizes it in an étendue preserving

6 Figure 4. (left) Simulation model of the illumination system for red, green and blue light. stereoscopic projector with extended color gamut. (right) Picture of the way. This means that the functionality of the lenslet integrator, lenses before the integrator and collimator of the illumination system in Figure 3 (left) is performed by a single component. Following this first optical component, we use an arrangement of lenses to image the rectangular-shaped angular distribution onto the surface of the LCOS panel. The LED-based illumination system for red, green and blue light is schematically illustrated in Figure 4 (left). It is clearly more compact than the previously described illumination system. The only drawback is that the dichroic filters to combine the primary colors into one illumination beam should be positioned in a region where there is no telecentric illumination, to make the illumination system compact. This can give rise to color non-uniformity on the projection screen. 24 If the étendue of the accepted light distribution by the projection lens is relatively small, these color non-uniformities will be very small and thus acceptable. The full optical engine with the LED-based illumination system is shown in Figure 4 (right). For this configuration we opted to use wire-grid PBSs in stead of MacNeille PBSs that were used for the first demonstrator. Wire-grid PBSs do not introduce geometric depolarization and the transmission and reflection of polarized light is more uniform for large angular apertures. The LCOS panels in the optical engine are from Holoeye Photonics AG (HEO 5216). They support 120 Hz modulation with a resolution by doubling a 60 Hz input sequence. We found an optical efficiency of the optical core of around 20% for red, green and blue light. We measured a full-on/full-off contrast ratio of 1000:1 for both polarized images. For the cyan light path similar values were obtained. We measured a total light output of 25 lm. The light output of the demonstrated prototype system can be increased by driving the LEDs at higher currents than specified at continuous drive conditions, or by providing a better match between the system and LED light source étendue. 4. NOVEL MULTIVIEW PROJECTION ARCHITECTURE WITH A LED-BASED ILLUMINATION SYSTEM As already mentioned, multiview projection displays with a large number of viewing zones and a high spatial resolution have already been demonstrated by other groups. Such systems use multiple projectors or light valves and are complex and expensive. A multiview system with only one projector can provide a solution for this. This is possible by placing a configurable slit aperture in the projection lens that transmits only one viewing direction With increasing number of viewing zones, more light is blocked by the aperture and therefore the emitted light flux of the system decreases. The light modulator has to be sufficiently fast to time-sequentially modulate all the images such that moving images can be seen in all viewing zones. This is possible with digital light processing technology of Texas Instruments. 28 A digital micromirror device (DMD) is used to modulate the images in which the pixels on the screen are represented by micromirrors. These have a very high switching speed. If the switching speed is not sufficient to modulate all images, the required modulation speed can still be obtained by decreasing the number of gray levels of the images. An alternative configuration to obtain different viewing zones with a single projector is to illuminate the DMD light modulator from different directions that correspond with the different viewing zones. A configurable

7 Figure 5. Horizontal cross-section of the optimized steering system for a 10-times (left) and 50-times (right) enlarged image. slit aperture in the illumination system is again an option but the optical efficiency of the system is enhanced considerably if we work with an array of LEDs that time-sequentially illuminate the DMD from different directions. For all mentioned approaches, the angular extent of the total viewing space is very limited. This can be solved by using a multitude of such projection systems 29 or by using a micro-optical component at the projection screen that enlarges the viewing angles. 30 Although the use of a LED array improves the optical efficiency, the total possible light output of such a system is still very limited. This is due to the fact that the different LEDs can only illuminate a small portion of the total acceptance étendue of the DMD light valve. We have investigated an approach that does not pose this problem. This is possible by creating viewing zones with a specific projection screen architecture that sequentially steers images towards different directions. In this way, the entire system étendue can be used to modulate each image Basic principle A DMD-projector with a LED-based illumination system projects the different color images for each viewing zone time-sequentially on the rear-projection screen. The projection screen consists of two parts. First, a large Fresnel lens is used to collimate the light rays. Next, an optical system with a movable component is used to control the propagation direction of the images. 31 This movable component has to be synchronized with the image information and it has to be periodically moved by an accurate mechanical translation system. We have investigated an optical system that uses three lens arrays to generate a collimated beam of light of which the horizontal propagation angle can be changed. These lens arrays have the same size as the projected image such that the entire image is steered toward a specific viewing zone. The footprint of each individual lens corresponds to the size of one enlarged pixel at the input of the projection screen. First, a lens with a square footprint is used to focus the collimated light by the Fresnel lens in both the horizontal and vertical direction. This is followed by two cylindrical lenses that simultaneously move in the plane perpendicular to the optical axis. Both lenses have the same focal length and are separated by a distance equal to their focal length. The second lens is placed at the back focal plane of the first lens. Light rays can be steered in different horizontal directions by controlling the horizontal position of the movable lenses with respect to the first lens. The movable lenses do not affect the vertical propagation direction because they do not focus light in this direction. We simulated the lens array configuration in ASAP to model the divergence of the light beams in all steering directions. We implemented the movable lens arrays as one component consisting of a PMMA sheet with cylindrical lenses on both sides. We optimized the thickness of the movable lens sheet to realize the same divergence in all steering directions and to minimize its overall value. We simulated the lens array configuration for two multiview projection systems: one that steers images that are enlarged 10 times by the projection lens and one that steers images that are enlarged 50 times. We used a DMD with a 0.7 inch diagonal, 4:3 aspect ratio

8 Figure 6. (left) Simulation model of the LED-based illumination system towards an integrating rod. (right) Top view of the LED-based DMD projector prototype. and a pixel pitch of µm ( pixels). The parameters of the lens sheets are shown in Figure 5. We simulate a half-angle divergence of 1.7 in all steering directions for the 10-times enlarged image and 0.35 for the 50-times enlarged image. We remark that the divergence is smaller for the system that steers the larger image. This is due to the divergence of the collimated light rays after the Fresnel lens that is smaller for the larger image. Furthermore we can add a vertical diffuser to the projection screen configuration, that spreads the light in the vertical direction and has no influence on the horizontal direction Practical implementation The main advantage of using a LED-based illumination system for the DMD-projector in the proposed multiview system is the high switching speed of the LEDs. This makes it perfectly possible to synchronize: the modulation of the RGB images by a single DMD, the light emission of the red, green and blue LEDs, and the mechanical translation inside the rear-projection screen configuration. We developed our own compact single-panel DMD projector with a LED-based illumination system to have complete control of the RGB illumination and the image modulation. This is necessary to allow the synchronization of the projection system with the beam steering projection screen. For our prototype we made use of a DMD Development Board. We created a LED-based illumination system to couple the light of three PT54 PhlatLight LEDs into a rod integrator. This rod integrator is part of the custom illumination configuration for the DMD Discovery Starter Kit. A simulation model of the illumination system can be seen in Figure 6 (left). To make the illumination system as compact as possible we also used PhotonVacuum technology to collimate the light of the LEDs. Possible color non-uniformity due to the positioning of the dichroic filters in a non-telecentric illumination region are averaged out by the integrating rod. 32 The final prototype is depicted in Figure 6 (right). We experimentally characterized the optical performance of the system. The projector produces 80 ANSI lumen and 75 center lumen. The system has a contrast ratio of 680:1. We are currently finalizing the integration of this projection system with the beam steering projection screen which will allow us to characterize the 3D image quality of the total multiview system. 5. THE POTENTIAL OF LASER ILLUMINATION Lasers as light sources for projection displays have all benefits that LEDs offer for this application. 33 But lasers posses two additional benefits which LEDs do not have when considering projection displays. The étendue of laser sources is very small and consequently their luminance can be extremely large. This means that the light output of projection displays can be very large if laser sources are used, even if the acceptance étendue of the system is rather small. This clearly is in large contrast with LEDs. Secondly, lasers have a much more narrow DMD Discovery XGA LVDS

9 spectral emission band than LEDs. This allows an even larger color gamut than when LEDs are used. When we analyze these considerations in the framework of the discussed stereoscopic and multiview projection systems we can draw several conclusions. For the stereoscopic projection system with four liquid-crystal-on-silicon panels, it is obvious that lasers could largely increase the light output compared to what is possible with a LED-based illumination system. Secondly, we noticed that the contrast ratio of the system strongly depends on a good match between the spectra of the LEDs and the polarization conversion characteristic of the used wavelength-selective HWPs. The more narrow spectral emission band of lasers would also prove beneficial in this respect. Finally, we indicated that the wavelength selective HWPs are necessary because we need to position additional clean-up polarizers in between the PBSs to ensure a high contrast ratio. If it is found that the acceptance étendue of the projection system can be strongly reduced while still allowing a high output of modulated laser light, these clean-up polarizers might become redundant and hence also the HWPs. This would allow a truly efficient stereoscopic projection system. We can think of different configurations with multiple colored lasers and two or four LCOS panels to obtain large stereoscopic high-resolution images with an extended color gamut and high brightness. The main question is thus if it is possible to have a laser illumination with a low étendue that is useful for light valve projection displays. An important phenomenon to consider in this respect is speckle. It is well known that laser illumination gives rise to speckle on the projection screen, which deteriorates the image quality. 34 Several solutions to reduce speckle have been proposed in the past. 35,36 Most of these methods however, increase the étendue of the original laser illumination. So the question remains if the full potential of the low étendue of laser illumination can be exploited without any limitations. For the multiview projection architecture, light output is not a crucial issue if the targeted screen sizes are relatively small. However, we have seen that the divergence of the light beams in all steering directions for such screen sizes is relative large. This will limit the angular density of the viewing zones. Increasing the angular density of the viewing zones is a method to address all human mechanisms for the observation of 3D visual information. The fundamental reason for this limitation can be brought down to the étendue of the illumination light beam that is too large. A speckle-free laser illumination with a low étendue could allow to increase this viewing zone density significantly. 6. CONCLUSIONS We have demonstrated two novel polarization-based stereoscopic projection systems. Both systems use a LEDbased illumination system and liquid-crystal-on-silicon light valves. The proposed optical architecture can be used to create high-resolution stereoscopic images with a compact and efficient system. The possibility to extend the color gamut beyond three primary colors was successfully demonstrated. We also investigated a multiview display with a single LED-based DMD projector. For this, we have designed a projection screen that time-sequentially steers images into different directions. Finally, we discussed the potential of a laser-based illumination and explained which advantages a laser illumination could add to the proposed 3D visualization displays. ACKNOWLEDGMENTS Our work reported in this paper was supported in part by Research Foundation - Flanders (FWO-Vlaanderen). The project is titled Compact LCOS projection displays for high-quality 3D images with high spatial and angular resolution. Lawrence Bogaert is indebted to FWO-Vlaanderen for a PhD grant. The work was supported in part by the IAP BELSPO VI-10, the Industrial Research Funding (IOF), Methusalem, VUB-GOA, and the OZR of the Vrije Universiteit Brussel. REFERENCES 1. Harbers, G., Keuper, M. and Paolini, S., High power LED illuminators for data and video projectors, Proc. IDW, (2002). 2. Geibler, E., Meeting the challenges of developing LED-based projection displays, Proc. SPIE 6169, Photonics in Multimedia, (2006).

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