DIGITAL IMAGE PROJECTION SYSTEM AND METHOD FOR 3-DIMENSIONAL STEREOSCOPIC

DISPLAY

Technical Field

[1] The present invention relates to digital image projection systems, and more par¬ ticularly, to a system and method for 3-dimensional(3-D) stereoscopic projection display using a single digital projector. Background Art

[2] 3-dimensional(3-D) Stereoscopic projection display systems are based on parallax between the left eye and the right eye, in other words, binocular disparity depth cue of human vision. In order to let the viewers feel the depth of the images projected onto a screen, a single projector or multiple projectors project two slightly different images, the left eye-images and right eye-images, onto a screen. The entire 3-D stereoscopic projection display system enables the left eye to see only the left-eye image, and the right eye to see only the right-eye image.

[3] There are several ways to implement 3-D stereoscopic images from projection displays. Anaglyphic method is very economic one to achieve the stereoscopic effect and it has been used since the early 20 century. Viewers wear anaglyph eyeglasses consisting of red and blue or complementary colored glasses to view complementary colored stereoscopic images projected onto a screen from a projector. This anaglyphic method causes change of image colors with dark display. Moreover, the com¬ plimentary colored glasses do not provide good stereo image selection because each color glass has the limit of color filtering between the left-eye images and the right-eye images. Consequently, anaglyph stereoscopic projection system partially allows the left eye to see the right-eye images and the right eye to see the left-eye images. This is serious cross talk problem for the stereoscopic display. Therefore, the stereoscopic image projection system, based on anaglyph glasses with the complementary colored images, is not regarded as good quality displays.

[4] FlG. 1 illustrates a conventional dual projectors displaying left-eye images 100 and right-eye images 101 onto a screen 102. A projector 103 projects left-eye images 100 and a projector 104 projects right-eye images 101. Polarizers 105 and 106 are disposed upstream of the projection lens of each projector 103 and 104 and positioned in order

to make the light beams from one projector orthogonally polarized relative to the others. The light beams from the projectors are linearly or circularly polarized. In order to get circularly polarized light, additional quarter wave plate can be disposed upstream of each linear polarizer 105 and 106. Viewers wear polarizing eyeglasses 107 and see the stereoscopic 3-D images from a screen 102. The polarizer disposed upstream of each projector and eyeglasses(analyzer in this system) are made of optical polarizing film, which has usually around 1,000:1 to 2,000:1 extinction ratio between the orthogonally polarized light beams in the visible light spectrum. Therefore, the extinction ratio, the contrast ratio in other words, is around 1,000:1 to 2,000: 1 between the left-eye images and the right-eye images. The extinction ratio is so good that there is virtually no visual cross talk problem between the left-eye images and right-eye images. Consequently, this method enables the stereoscopic projection system to have good left and right image selection property in order to provide viewers with good stereoscopic 3-D effect. The averaged light efficiency of a polarizer to produce polarized light out of non-polarized visible white light is about 45 % for each orthogonal polarization state. Each analyzer of eyeglasses has the transmission efficiency of 85 % for already polarized visible white light. Therefore, the total light efficiency of this system for each left-eye or right-eye image, n is theoretically; [5] n = 0.45 x 0.85 = 38 %. e

[6] Along with high performance and good light efficiency for 3-D stereoscopic display, this dual projector method for 3-D stereoscopic display is appropriate when there is a need for a large audience theater or a large venue display, because of the relatively cheap cost for each polarizing eyewear and the facts that the polarizing eyewear is lightweight, recyclable, and durable. Therefore, this typed 3-D stereoscopic projection system is commonly used by Disney, IMAX, Cinema Ride, Six Flags, and many others around the world. The shortcoming of this method is that the system requires two projectors.

[7] Because of very fast temporal refresh rate for the image frames displayed by the recent digital spatial light modulators and their supportive technologies, such as, TI(Texas Instrument Inc.)'s DMD(Digital Micro-mirror Device) and DLP (Digital Light Processing), it is feasible to realize more than 96 Hz image frame rate which is the minimum frame rate for the flicker-free stereoscopic displays from a single projector with an active eyewear. Although the standard FPS (frame per second) number for motion pictures for theaters is 24 FES, minimum 48 FPS for each left and right image is required for non flickering images empirically if a single digital

projector is used. The methods for faster refresh rate of the image displayed by DMD systems along with a digital image data formatter are described in U.S. Patent Ap¬ plication Publication No. U. S.2002/0021261 Al(Werner). FlG 2 illustrates the schematic diagram of a prior art single stereoscopic digital projector methods with the active eyewear. A digital image data processor and formatter, PROCESSOR/ FORMATTER 111 processes stereoscopic image data, and switches the displayed images onto the spatial light modulator(s) between the left-eye images 100 and the right-eye images 101 at a frame rate, more than 96 FPS and desirably about 120 FPS, converted by a digital frame rate converter and multiplexer, FRC/MUX. Stereoscopic left-eye and right-eye images 100 and 101 are projected through a single projector 109 synchronized with an image playback system & synchronization controller, IPS/ SYNCH-CON 110, and viewers see the time sequential stereoscopic images through an active eyewear 108. The active eyewear 108 consists of two LC(Liquid Crystal) eyeglasses that work as electro-optical shutters driven by electromagnetic field. A sensor attached to the active eyewear 108 receives a synchronization signal(normally, infrared signal) from IPS/SYNCH-CON 110, and switches the LC shutters on and off for each left and right eye. This synchronized system includes a single projector and active eyewear enables the left eye to see only the left-eye images and vice versa. This active eyewear technology is commonly called "active eyewear stereoscopic system"in the industry. Among examples of stereoscopic display systems that utilize the active eyewear are those disclosed in US4, 145,713 (White); US4,387,396(Tanaka et al.); US 4,772, 943(Nakagawa et al.); and US5,327,269(Tilton et al.). The light transmission efficiency of the synchronized LC shutter glasses is about 35 %, although the theoretical light efficiency, based on the duty cycle of sequential left-eye and right-eye images, is 50 %, the actual light efficiency for the duty cycle is about 45 % because about 10 % extra blank time between the sequential left and right images is required for ensuring good stereo image separation. Therefore, the total light efficiency for this stereoscopic projection system for each left-eye and right-eye image is theoretically; [8] n = 0.45 x 0.35 = 15.8 %. e

[9] The shortcoming of this system is the cost for each active eyewear. In year 2004, the usual price for the polarizing eyeglasses used in a dual projector passive stereoscopic system is about $ 1.50 to $ 10 from off-the-shelf market. However, the usual price for the active eyeglasses with synchronization electronics is normally $ 200 to $ 1,000 per pair depending on the size of each eyeglass, performance and quality related to the light efficiency and image quality. And this electronic LC shutter

eyeglasses are more fragile than the simple polarizing film eyeglasses. Therefore, the 3-D stereoscopic projection system with the active eyeglasses is much less practical and economical choice if there is a need for a display with a large audience or high foot traffic.

[10] Liquid crystal polarization modulators driven by electromagnetic field have fast switching speed between two orthogonal polarization states. Devices and methods for displaying stereoscopic 3-D image with an LC polarization modulator are known. Examples of such systems include the one disclosed in US4,792,850(Lipton et al.). FlG 3 illustrates the schematic configuration of a conventional digital stereoscopic projection display system with the LC polarization modulator. A projector 112 displays time sequential left-eye images and right-eye images 100 and 101 at minimum 96 FES, 48 FES for the left-eye images and the right-eye images. The light beams from the projector 112 are non-polarized before an LC polarization modulator 113. The LC polarization modulator 113 is disposed upstream of the projection lens of a single projector 112 and synchronized with IFS & SYNCH-CON 110 in order to display time sequential stereoscopic images 100 and 101 on a screen 102. The LC polarization modulator 113 polarizes the light beams from the projector 112 and switches the po¬ larization state between two orthogonal directions in accordance with the syn¬ chronizing signals from IFS & SYNCH-CON 110. Therefore, the light beams projected from the digital projector 112 for each left-eye and right-eye image 100 and 101 are polarized and the polarization state is switched between two orthogonal po¬ larization states, usually circularly left and right polarization states. Unlike the single projector method with the active eyewear as described previously, multiplexing of the time sequential stereoscopic images is done at the projector side, so viewers can see the stereo images wearing the conventional passive polarizing eyeglasses 107. There are two main shortcomings of this system. First, the light efficiency of this system is relatively lower than that of the other 2 methods. Light efficiency of the LC po¬ larization modulator is only 35 % for each eye image. As this kind of LC polarization modulator is even slower than the active eyeglasses, more blank time is needed between the left and right eye image, resulting in a duty cycle efficiency of about 40 %. The passive polarization eyeglasses have light efficiency of 85 % for each polarized light. Therefore, the total light efficiency for each left and right eye of this system is theoretically:

[11] n = 0.4 x 0.35 x 0.85 = 11.9 %. e

[12] Second the LC glass has very low extinction ratio between left and right po-

larization which is about 100:1. With this low extinction ratio cross talk between the left-eye image and the right-eye image is more distinct. In addition to these main shortcomings, the LC polarization modulator may be deteriorated if it is exposed to high intensity light flux from a projector during operation. The liquid crystal glass is very expensive comparing to the polarizer window. Therefore, this system is not ap¬ propriate for the high performance stereoscopic projection systems that need good stereoscopic image selection, and for large venue projection systems that require high flux illumination light for the display. Disclosure of Invention Technical Problem

[13] Therefore, the typed 3-D stereoscopic projection system is commonly used by

Disney, IMAX, Cinema Ride, Six Flags, and many others around the world. The shortcoming of this method is that the system requires two projectors.

[14] The 3-D stereoscopic projection system with the active eyeglasses is much less practical and economical choice if there is a need for a display with a large audience or high foot traffic.

[15] The system with the LC polarization modulator is not appropriate for the high performance stereoscopic projection systems that need good stereoscopic image selection, and for large venue projection systems that require high flux illumination light for the display.

[16] There is a need for an approach to the better economical digital stereoscopic projection display system and methods for good image quality and stereoscopic effect. The present invention fulfills this need and further provides related advantages and offset to the shortcomings and drawbacks of the conventional digital stereoscopic projection systems. Technical Solution

[17] The present invention has been made in view of the above-mentioned problems, and an aspect of the invention is to provide a digital image projection system for displaying time sequential stereoscopic image frames onto a screen from a single digital image projector, and for viewing 3-dmensional stereoscopic images through the passive polarizing eyeglasses.

[18] It is another aspect of the present invention to provide a method for displaying time sequential stereoscopic image frames onto a screen from a single digital image projector, and for viewing 3-dmensional stereoscopic images through the passive

polarizing eyeglasses.

[19] According to one aspect of the present invention, the present invention provides a digital image projection system includes a single digital image projector, a projection optical system, a polarizing and/or polarization modulating optical system, a mechanical or electro-optical shutter system, passive polarizing eyewear, and a digital image-processing unit.

[20] Preferably, the single digital image projector includes a single spatial light modulator or multiple spatial light modulators modulating the light beams in accordance with the image data sent from a digital image-processing unit. An il¬ lumination system in the digital image projector creates the light beams to be modulated.

[21] The projection optical system includes a common optical lens group and two separated optical lens groups for providing the digital image projection system with a method creating separated two optical paths for the left-eye images and right-eye images.

[22] The polarizing and/or polarization modulating optical system is disposed upstream of two separated optical lens groups or in each optical path of two separated front optical lens groups, makes the light beams passing through the optical path of the left- eye images orthogonally polarized relative to the light beams passing through the optical path of the right-eye images, and vice versa.

[23] The beam splitting optical system is disposed between the common optical lens group and two separated optical lens groups of the projection optical system, and splits incoming light beams in order to create two optical paths of two separated front optical lens groups. Advantageous Effects

[24] It is an advantage of the present invention that the method in accordance with the present invention is more economic when contrasted against dual projector method.

[25] It is a further advantage of the present invention that it provides a stereoscopic projection system using a single digital projector with higher stereoscopic light efficiency when contrasted against the conventional stereoscopic projection system using a single projector with an LC polarization modulator. Brief Description of the Drawings

[26] These and/or other aspects and advantages of the invention will become apparent and more readily appreciated from the following description of the embodiments,

taken in conjunction with the accompanying drawings in which: [27] FlG. 1 is a schematic diagram illustrating a conventional stereoscopic projection system including dual projectors and passive polarizing eyeglasses; [28] FTG. 2 is a schematic diagram illustrating a conventional stereoscopic system including a single digital image projector and active eyeglasses; [29] FIG. 3 is a schematic diagram illustrating a conventional stereoscopic system including a single digital image projector with a LC polarization modulator and passive eyeglasses; [30] FIG. 4 is a schematic diagram illustrating a conventional digital projection system TM based on a single DMD(Digital Micro Mirror) spatial light modulator and DLP technologies; [31] FlG. 5 is a schematic diagram illustrating a conventional digital projection system TM based on multiple DMD spatial light modulators and DLP technologies; [32] FlG. 6 is a schematic diagram illustrating a conventional digital projection system based on multiple LCOS spatial light modulators; [33] FIG. 7 is a schematic block diagram showing an image playback and processing process for time-sequential 3-D stereoscopic images in accordance with the present invention; [34] FlG. 8 is a schematic diagram illustrating a preferred embodiment of a digital stereoscopic image projection system based on 3 DMD spatial light modulators in accordance with the present invention; [35] FIG. 9 is a schematic diagram illustrating an alternative embodiment of a digital stereoscopic image projection system based on 3 LOCS spatial light modulators in accordance with the present invention; [36] FlG. 10 is a schematic diagram showing the exemplary process of image frame rate converting in accordance with the present invention; [37] FlG. 11 is a ray tracing diagram of a projection optical system example which has sufficient separation between the front optical lens group and the common optical lens group in accordance with the present invention; [38] FlG. 12 is a diagram of a projection optical system example with a cubic polarizing beam splitter in accordance with the present invention; [39] FlG. 13 is a diagram of a projection optical system example with a flat polarizing beam splitter or perforated polka-dot beam splitter in accordance with the present invention; [40] FlG. 14 is a diagram of a projection optical system example with a rotating disc

having a mirror section and a transparent or a blank section in accordance with the present invention;

[41] FlG. 15 is a diagram illustrating the perforated polka-dot beam splitter;

[42] FTG. 16 is a schematic diagram illustrating the process of mechanical shutter system disposed in the projection optical system according to the present invention;

[43] FIG. 17 is a diagram illustrating the rotating disc having a mirror section and a transparent or a blank section in accordance with the present invention;

[44] FlG. 18 is a ray tracing diagram of another projection optical system embodiment in accordance with the present invention;

[45] FlG. 19 is a schematic diagram illustrating projected image offset from the projection system in accordance with the present invention; and

[46] FlG. 20 is a schematic diagram illustrating the left-eye and right-eye image position offset of the projection system, and the left-eye and right-eye image compensated for the image position offset in accordance with the present invention. Best Mode for Carrying Out the Invention

[47] The polarizing and/or polarization modulating optical system and the beam splitting optical system and their methods in accordance with the present invention can be one of the following:

[48] (1) Non-polarized light beams incident from a single spatial light modulator or multiple spatial light modulators are split by a cubic polarizing beam splitter or flat polarizing beam splitter. For a high-end stereoscopic systems which typically require more than 1,000: 1, extinction ratio between the left-eye images and the right-eye images, additional polarizer is disposed in each optical path downstream of the cubic or flat polarization beam splitter. If circularly polarized light is preferred for a stereoscopic projection system a quarter wave plate converting the linearly polarized light into circularly polarized light is disposed in each separated optical path downstream of the cubic or flat polarization beam splitter, or downstream of the additional polarizer.

[49] (2) Non-polarized light beams incident from a single spatial light modulator or multiple spatial light modulators are time-sequentially split by a rotating disc which includes a mirror section and a transmitting section. The rotational speed of the disc is desirably more than 48 Hz to provide an appropriate stereoscopic image frame rate for flicker-less stereoscopic image from a single projector. Once split into two directional beams, beams follow separated optical paths in each separated optical lens group and polarized by polarizer disposed in each optical path to be P-polarized light beams or S-

polarized light beams. For circular polarization stereoscopic projection system, a quarter wave plate, converting the linear polarized light into circular polarized light, is disposed downstream of the polarizer in each separated optical path. If polarized light beams come from polarizing spatial light modulator(s), such as LCOS(s) or LCD(s) then a half wave plate disposed in one optical path converts the polarization state of the light beams into orthogonal polarization state relative to the polarization state of the other. In a circular polarization stereoscopic projection system, the quarter wave plate converting the linearly polarized light into circularly polarized light is disposed in each separated optical path.

[50] (3) Non-polarized light beams incident from a single spatial light modulator or multiple spatial light modulators are split by a 50:50 apodization plate beam splitter(it is sometimes called a polka dot beam splitter), disposed between the common optical lens group and the two separated optical lens groups. The polka dot beam splitter includes small sized reflector arrays(usually aluminum dots) and transparent array spaces on the beam splitting surface. Once split into two directional beams, beams follow separated optical path in each separated optical lens group, and are polarized by the polarizer disposed in each optical path to be P-polarized light beams or S-polarized light beams. In a circular polarization stereoscopic projection system, a quarter wave plate, converting the linear polarized light into circular polarized light, is disposed downstream of the polarizer. If polarized light beams come from polarizing spatial light modulator(s), such as LCOS(s) or LCD(s) then the half wave plate disposed in one optical path converts the polarization state of the light beams into orthogonal po¬ larization state relative to the polarization state of the others. In a circular polarization stereoscopic projection system, the quarter wave plate, which converts the linear polarized light into circular polarized light, is disposed in each separated optical path.

[51] The mechanical or electro-optical shutter system is disposed in each separated optical path, which belongs to the left-eye image and the right-eye image. In the digital projection system having a rotating disc for beam splitting in accordance with the present invention, the rotating disc works as a beam splitter and a shutter simul¬ taneously, and does not require additional shutter system. Shutters and the rotating disc are synchronized with the spatial light modulation system to open the optical path which belongs to the left-eye image and to block the optical path which belongs to the right-eye image while the spatial light modulation system displays the left-eye image, and vice versa.

[52] The passive polarizing eyewear includes linear or circular polarizers for the left-eye

and right-eye image selection for each eye of a viewer.

[53] The digital image-processing unit includes an image playback system and a syn¬ chronization controller(IE5 & SYNCH-CON), a frame rate converter and multiplexer(FRC/MUX), and a data processor and formatter(PROCESSOR/ FORMATTER).

[54] The IIS/SYNCH-CON reads the image frame data with an specific image frame rate, usually 24 FES(frames per second) for the left-eye and right-eye images, and generate the synchronization signals to control the mechanical or electro-optical shutters in accordance with the image frame rate converted by FRC/MUX.

[55] In order to achieve the flicker-less stereoscopic display, the FRC/MUX converts the image frame rate of the image frames from the IFS/SYNCH-CON into the faster frame rate, preferably more than 48 FIS for the left-eye and right-eye images. Viewers do not see notable flicker from the stereoscopic images projected by a single image projector with more than 48 FES for the left-eye and right-eye images, 96 FP3 for the sequential image frame rate.

[56] The PROCESSOR/FORMATTER processes stereoscopic image data and switches the displayed images onto the spatial light modulator(s) between the left-eye images and the right-eye images at a frame rate of more than 96 FFS converted by FRC/MUX. The PROCESSOR/FORMATTER according to the present invention also provides the function of image position matching between the left-eye and right-eye images. Because of the separation of the optical paths for the left-eye and right-eye images, the projected left-eye and right-eye images have position offset on the screen from a single projector in accordance with the present invention. This image location offset between the left-eye and right- eye image is digitally compensated by the PROCESSOR/ FORMATTER. The PROCESSOR/ FORMATTER according to the present invention has another function to match the size of the left-eye images to the size of the right-eye images onto a projection screen if two separated optical lens groups have different optical magnification due to system design or manufacturing tolerance.

[57] Unlike the stereoscopic projection systems and methods with LC polarization modulator previously described the projection systems and methods of the present invention require the conventional polarizing optics system. Therefore, the minimum averaged extinction ratio between the left-eye and right-eye images can be more than 1,000:1 to 2,000:1, whereas the LC polarization modulator typically has about 100:1 extinction ratio which will cause serious problems of cross talk between the left-eye and right-eye images. Moreover, the cost for the conventional polarizers disposed

upstream of each end of separated projection lens for the left-eye and right-eye images is much cheaper than the LC polarization modulator which can be easily deteriorated if it is exposed to high intensity light flux. Therefore, in order to be used as high-end or large venue stereoscopic system which requires good stereoscopic image quality and high flux light source the stereoscopic image projection system in accordance with the present invention is much better than that with LC polarization modulator.

[58] Unlike the stereoscopic projection systems and methods with active eyewear which is expensive, fragile, and requiring batteries, the method of the present invention requires simple passive polarizing eyeglasses which is cheap and durable. Therefore, the present invention is a much more practical and economic solution for the stereoscopic projection displays if there is a need for a large audience, a seminar with more than 10 viewers, or high foot traffic through a projection area.

[59] Unlike the system and the method with dual projectors for 3-D stereoscopic projection display, the method according to the present invention requires a single projector. Obviously, it is an advantage of the present invention that the method in accordance with the present invention is more economic when contrasted against dual projector method. In order to match the dual projector system in terms of the brightness or light efficiency, the 3-D stereoscopic projection system according to the present invention can use plural lamps.

[60] It is a feature of the present invention that it employs the projection optical system with separated two optical paths for creating stereoscopic left-eye and right-eye images from a single digital image projector, the polarization selection method along with the polarizing and/or polarization modulating optical systems and the shutter systems for stereoscopic viewing, and the image-processing method and unit to create flicker-less stereoscopic displays and to compensate the optical offset or optical magnification.

[61] It is a further advantage of the present invention that it provides a stereoscopic projection system using a single digital projector with higher stereoscopic light efficiency when contrasted against the conventional stereoscopic projection system using a single projector with an LC polarization modulator.

[62] According to one aspect of the present invention, the present invention provides a method of projecting time-sequential left-eye and right-eye images from a single digital image projector for displaying 3-dimensional stereoscopic images onto a screen, and consequently enabling the viewers wearing polarizing eyeglasses to see 3-dimensional stereoscopic images displayed onto the screen. The method includes the steps of (1) displaying the time-sequential stereoscopic left-eye and right-eye images

onto at least one spatial light modulator in a single digital image projector, (2) projecting time-sequential stereoscopic left-eye images from the spatial light modulator onto the screen through the separated optical path that belongs to the separated optical lens group of the left-eye images in a projection optical system, and projecting right-eye images from the spatial light modulator onto a screen through the separated optical path that belongs to the separated optical lens group of the right-eye images in a projection optical system, (3) polarizing the light beams and/or modulating the polarized light beams projected through each optical path of the left-eye images and the right-eye images, (4) shuttering the optical path for the left-eye images off and opening the optical path for the right-eye images when the right-eye image is displayed by the spatial light modulator and vice versa, (5) synchronizing a shutter system disposed in a projection optical system with the image playback system for enabling stereoscopic left-eye and right-eye images to be displayed time-sequentially and to be out-of-phase to each other onto a screen, and (6) compensating the image position offset between the projected left-eye images and the projected right-eye images onto a screen due to the separation between two separated optical lens groups, and/or com¬ pensating the optical magnification difference between two separated optical lens groups due to the optical design or manufacturing tolerance for the optical lens groups. Mode for the Invention

[63] Although particular embodiments and elements forming part of, or cooperating more directly with, apparatus and methods in accordance with the present invention have been described in detail for purposes of illustration, various modifications and en¬ hancements may be made without departing from the main spirit and scope of the present invention. It is also to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.

[64] FlG. 4 schematically illustrates a conventional digital projection system with single

DMD spatial light modulator. The system includes a light source having a lamp 114 and a reflector 115 for producing visible light beams. This light source is usually sorts of short arc lamps like UHP(Ultra High Pressure), metal halide, or xenon lamps. The light beams from the lamp 114 and the reflector 115 are homogenized, and beam shaped through the projection illumination optical system which includes a light integrator 116, a relay lenses 117, a mirror 118, and a prisms 119 in order to illuminate a DMD spatial light modulator 120. A rotational color disc 121 including different color sectors separates incident white beams into different color beams in time sequential way in order to display color images. A projection lens 122 projects the

light beams modulated by the DMD spatial light modulator 120 to display the image onto the screen.

[65] A spatial light modulation system can have single DMD spatial light modulator as illustrated in FlG. 4 or multiple DMDs for each R, G, B color as illustrated in FlG. 5. Usually, multiple DMDs 123R, 123G, and 123B are required for color display with higher resolution and higher brightness. In an projection system with multiple spatial light modulators, the white light generated by the light source having a lamp 124 and a reflector 125 homogenized and beam-shaped by an illumination system which includes a light integrator 126, a relay lenses 127, a mirror 128, and pass through a TIR (Total Internal Reflecting) prisms 129 that deflect the light beams and a color separating prisms 130 that divide the white light beams into red green and blue lights. The beams modulated and reflected by those DMD light modulators 123R, 123G, and 123B are then combined and passed through a projection lens 131 to form color image onto the screen.

[66] FlG. 6 schematically shows a conventional digital projection system with multiple

LCOS(Liquid Crystal On Silicon) spatial light modulators. Unpolarized white light from a lamp 132 and a reflector 133 is integrated by a light integrator 134 and separated into R, G, and B color light beams by a dchroic cubic beam splitter 135. Downstream of mirrors 136- A, 136-B, and 136-C and a relay lens optical system 137-G, 137-B, and 137-R, those R, G, and B light beams converted into the S- polarization state light beams, reflected by cubic PBSs(R)larizing Beam Splitter) 138-R, 138-G, and 138-B and homogenized and beam-shaped to illuminate each LCOS spatial light beam modulators 139-R, 139-G, and 139-B corresponding to each R, G, and B colors. Hxels of LCOS spatial light modulators 139-R, 139-G, and 139-B modulate the S-polarized light into P-polarized light when they are switched "on"and do not modulate the light beams when they are switched "off. Only light beams modulated by the LCOS 139-R, 139-G, and 139-B that are converted into the P- polarization state light beams can pass through the PBS 138-R, 138-G, and 138-B where color multiplexing is done spatially. Once R, G, and B colored beams are recombined by a color combing prisms 140, they are projected through a projection lens 141 to form color image onto the screen.

[67] FlG. 7 is a block diagram illustrating the process of a digital image-processing unit

145 including an image playback system and a synchronization controller(IPS/SYNCH-CON) 142, a frame rate converter and a multiplexer (FRC/MUX) 143, and a image data processor and

formatterCPROCESSOR/FORMATTER) 144 in accordance with the present invention. The IES/SYNCH-CON 142 reads video data and sends a synchronization signal to the projection system. If an image frame rate of the video data needs to be changed in order to achieve flicker-less display, the FRC/MUX 143 converts the image frame rate and multiplexes the stereo image data. The PROCESSOR/FORMATTER 144 processes and formats the image data in order to display the stereo-image onto the screen through the projection system according to the present invention with a pre¬ determined frame rate for each image. The PROCESSOR/FORMATTER 144 also compensates the offset between the position of the left-eye images and the right-eye images onto the screen due to the separation of two front optical lens groups. And it also compensates the discrepancy of optical magnification between two separated front optical lens systems because of the system design or manufacturing tolerance in the projection optical system according to the present invention.

[68] FlG. 8 and 9 schematically illustrate projection systems in accordance with the present invention. A digital stereoscopic projection system according to the present invention mainly includes a single digital image projector 146 having a single spatial light modulator or multiple spatial light modulators, a digital image-processing unit 145, a projection optical system 147, a shutter system having left-eye and right-eye image shutters 148-L and 148-R, and a polarizing and/or polarization modulating optical system having polarizers and/or polarization modulators for the left-eye and right-eye images 149-L and 149-R. A single digital image projector 146 shown in FIG.

8 is described above along with FIG. 5, the single digital projector 146 shown in FIG.

9 is described above along with FIG. 6.

[69] It is to be understood that the DMD with DLP ™ and the LCOS are specifically described above in order to illustrate the examples of the spatial light modulator(s) for the digital stereoscopic projection system according to the present invention. The system design in accordance with the present invention is not limited by the type of the spatial light modulators as long as the spatial light modulators can display time sequential stereoscopic left and right images with a proper image frame rate required by the system.

[70] A single spatial light modulator or multiple spatial light modulators in a single digital projector 146 display the stereoscopic left-eye and right-eye images time- sequentially with an image frame rate, more than 60 FPS (more than 30 FPS for the left-eye images and more than 30 FPS for the right-eye images). In order to avoid visible flicker from the time sequential stereoscopic display images, more than 96 FPS

for the sequential left-eye and right-eye images (more than 48 FES for the left-eye images and 48 FES for the right-eye images) is preferably required by the system. In cases of slower image frame rate inputted from the IES/SYNCH-CON 142, the FRC/ MUX 143 temporally duplicates the image data of the input frames in buffer memories in order to match up the requirement of frame rates explained above. FIG. 10 il¬ lustrates this process briefly, if the input video signal for the stereoscopic display carries the data of the image frame sequences of A ILJ , A LRJ , B LLJ , B [R] at 48 FES, the FRC/MUX 143 creates image frame sequences of A [L] , A [R] , A [ϋ , A [R] , B CL] , B [R] , B IL) and B [R] at 96FES, and at the same time the IES/SYNCH-CON 142 generates synchronization signal for syn¬ chronizing other modules of the projection system to treat it as conventional 96 FES input. A digital image-processing unit includes an IES/SYNCH-CON 142, an FR C/ MUX 143, and a PROCESSOR/ FORMATTER 144, and is designed to handle most common video standards for the image data at different frame-rates.

[71] FIG. 11 illustrates the ray tracing of a projection optical system 147 including a common optical lens group 151 and a front optical lens group 152 which is identical to separated front optical lens groups 152L and 152R in a projection optical system 147. The projection optical system 147 in FIG. 11 has been designed to have throw ratio(projection throw distance/screen width) of 1.6 and F/# of F/2.4 for the DMD light modulators 123R, 123G, and 123B with 2000 X 1000 pixels. The size of each pixel including the gap between the pixels is 13.8 mm. The projection optical system 147 in accordance with the present invention has sufficient lens offset capability for off-axis projection. The offset capability between the projection optical system 147 and the DMD light modulators 123R, 123G, and 123B is 90 % in the horizontal direction and 120 % in the vertical direction. A glass block 153 has the same optical path length as that of the TIR and color separating prisms. The glass block 154 is the window of the DMD light modulator 123R, 123G, and 123B. It is to be understood that the optical design for this projection system is more challenging. Therefore, it indicates that the optical design of projection lenses for the digital stereoscopic projection system in accordance with the present invention is highly feasible and practical with most digital projection systems along with the DMD or LCOS spatial light modulators.

[72] FIG. 12 illustrates specifically a projection optical system 147 which has two optical paths that belong to separated front optical lens groups 152L and 152R at rear side of a common optical lens group 151 and a cubic polarizing beam splitter 156. Here, diagram prisms 153 are also described as a glass block to facilitate the de-

scription of the functions of the projection optical system 147 according to the present invention. Beams coming from single light modulator or the multiple light modulators 123R, 123G, and 123B pass through the prisms 153 and the common optical lens group 151, and reach the cubic polarizing beam splitter 156. The cubic polarizing beam splitter 156 splits the incident beams and linearly polarizes them. Once the incϋent beams are split and polarized into two orthogonally polarized light beams, P- polarized light beams follow the transmitting direction of the cubic polarizing beam splitter 156 toward the front lens group 152R. S-polarized light beams follow the reflecting direction of the cubic polarizing beam splitter 156 toward the front lens group 152L. Two front lens group 152L and 152R are identical to the front lens group 152 shown in FIG. 11. Mechanical or electro-optical shutters 155L and 155R disposed upstream of the lens groups 152L and 152R is synchronized with the IPS/ SYNCH-CON 142. In this diagram, the mechanical shutters attached to rotating electronic motors 158L and 158R that are synchronized with the IPS/Synch-Con 142 are illustrated. The shutter 155L is open when the left-eye image is displayed on the spatial light modulator(s) in a single digital image projector 146, and the shutter 155L is closed when the right-eye image is displayed on the spatial light modulator(s) in the single digital image projector 146. Conversely, the shutter 155R is open when the right-eye image is displayed and closed when the left-eye image is displayed on the spatial light modulator(s).

[73] Extinction ratio between the P-polarized light and the S-polarized light for the viewers through passive polarizing eyeglasses which work as analyzer for the cubic polarizing beam splitter 156 is typically about 250 to 800:1. For the high-end stereoscopic systems that typically require more than 1,000:1 of extinction ratio between the left-eye images and the right-eye images, additional polarizers 158L and 158R are disposed in each optical path. The P-polarizer 158R can be disposed upstream of the front optical lens group 152R which belongs to the transmitting direction of the cubic polarizing beam splitter 156 and the S-polarizer 158L can be disposed upstream of the front optical lens group 152L which belongs to the reflecting direction of the cubic polarizing beam splitter 156. Usual extinction ratio from the polarizers 158L and 158R is more than 1,000 to 2,000:1 which is sufficient for very high-end stereoscopic displays.

[74] For the stereoscopic projection system which requires circular polarized light along with circular polarizing eye glasses, quarter wave plates can be disposed upstream of each front lens group 152L and 152R or upstream of the additional polarizers 158L

and 158R. The quarter wave plate needs to be aligned with the linear polarization axis of the cubic polarizing beam splitter 156 and/or the additional polarizers 158L and 158R in order to convert the linearly polarized light beams into the circularly polarized light beams.

[75] The cubic polarizing beam splitter 156 can be replaced by a flat polarizing beam splitter or an apodization plate beam splitter (it is sometimes called as polka dot beam splitter, hereinafter polka dot beam splitter). FlG. 13 illustrates the projection optical system 147 which uses a flat polarizing beam splitter or a polka dot beam splitter 159. The projection optical system 147 in accordance with the present invention is optimized to overcome the optical performance degradation due to a tilted flat polarizing beam splitter which causes astigmatism and coma aberration in the projection optical system for the transmitting optical path.

[76] FlG. 15 illustrates the typical polka dot beam splitter which has 50:50 apodization rate for transmitting and reflecting direction. The polka dot beam splitter includes small sized reflector arrays(usually aluminum dots) 162 and transparent array spaces 163 on the beam splitting surface. Incident beam is almost perfectly reflected by the reflector dots 162 and almost perfectly transmitted through the spaces 163. This polka dot beam splitter in the projection lens for splitting beam is disposed where the size of each beam belong to image conjugation point is maximized or relatively larger comparing to the size of the dots and the spaces of the polka dot beam splitter to avoid undesirable image artifact.

[77] When the polka dot beam splitter is used and if the spatial light modulators 123R,

123G, and 123B modulate the light beams based on non-polarizing process, such as DMD spatial light modulators, the polarizer 158L and 158R should be disposed upstream of each front optical lens group 152L and 152R. This is because the polka dot beam splitter does not polarize the light when it splits the incoming light beams. For optical projection system with the polka dot beam splitter and polarizing spatial light modulators such as the LCD or LOCS that modulate the light beams based on polarizing process, the polarizers 158L and 158R are optional depending on the re¬ quirement for system extinction ratio between the left-eye image and the right-eye image. An exemplary projection system for the embodiment of a projection system with the LCOS spatial light modulators and the polka dot beam splitter according to the present invention is depicted in FlGs. 9 and 13.

[78] The shutters 155L and 154R disposed in each front optical lens group 152L and

152R can be mechanical shutter attached to electric motor synchronized with the IPS/

SYNCH-CON 142 or an electro-optical shutter driven by electro-magnetic field syn¬ chronized with the IR3/SYNCH-CON 142.

[79] FlG. 16 illustrates a typical mechanical shutter mechanism of the shutters 155L and

155R disposed in the projection optical system 147 according to the present invention. The shutters 155L and 155R are attached to spindle motors 157L and 157R syn¬ chronized with the IPS/SYNCH-CON 142. During the same duration while the left-eye image is displayed by the spatial light modulator(s) 123R, 123G, and 123B, the shutter 155R located in the right-eye optical path shutters off the aperture of the front optical lens group 152R which belongs to the right-eye image and the shutter 155L located in the left-eye optical path opens the aperture of the front optical lens group 152L. While the right view image is displayed on the spatial light modulator(s), the shutter systems work conversely. In this embodiment, the time sequential stereoscopic image frame rate is 96 FES. The mechanical shutter system illustrated in FlG. 16 has the light efficiency depending on the aperture size of the projection optical system and the size of a blade of the shutter. The PROCESSOR/ FORMATTER 144 displays blank image switching off the all pixels on spatial light modulator(s) when the shutters 155L and 155R reaches transition state between the left-eye image state and the right-eye image state. In this embodiment, the shutter is designed to have about 80 % of stereoscopic light efficiency during the system operation. Typical stereoscopic light efficiency of this mechanical shutter system is 80 % ~ 90 %.

[80] There are various electro-optical shutters driven by the electro-magnetic field this electro-optical shutter is highly transparent when the electric field is switched off and highly opaque when the electric field is switched on. The electro-optical shutter function is based on the polarization property, scattering property of LC(Liquid Crystal) material, and electro-optical properties of a conductive layer, typically ITO(Indium tin Oxide) coating layer. Transmission ratio of this electro-optical shutter is typically about 80 %. Typical response time of these electro-optical shutters, from 5 ms to 20 ms, which is compatible stereoscopic image refresh rate of 50 FPS to 200 FPS from single digital projector. Typical light efficiency of these electro-optical shutters is 80 % to 85 %.

[81] FIG. 14 illustrates a projection optical system 147 which has two separated optical paths that belong to the separated front optical lens groups 152L and 152R downstream of the common optical lens group 151 and the rotating disc 160 which includes a mirror sector 164 and a transparent sector 165 thereon as illustrated in FlG. 17. The prisms 153 are described as a glass block to facilitate the description of the

functions of the projection optical system 147 according to the present invention. Beams incident from non polarizing single light modilator or the multiple light modulators 123R, 123G, and 123B, such as the DMD spatial light modulators, pass through the prisms 153 and the common optical lens group 151 and reach the rotating disc 160, which includes the mirror sector 164 occupying the half of the area of the rotating disc 160 and the blank or transparent sector 165 occupying the half of the area of the rotating disc 160. The rotating disc 160 with this configuration spins around the rotational axis with the appropriate speed such as minimum rotational speed of 2880 RFM(Revolutions Per Minute) to create minimum 96 HZ refresh rate for the stereoscopic projection system displaying minimum 48 FIS of the left-eye image and 48 FES of the right-eye image. The rotational speed of 3600 RPM of the rotating disc 160 is for the refresh rate of 120 Hz. The spindle motor 161 for rotating a rotational disc is synchronized with the IPS/SYNCH-CON 142 in order to provide the exact timing for reflection and transmission similar to the rotational mechanical shutters 155L and 155R depicted in FTG. 12. Once the incident beams are reflected by the mirror sector 164, they go to the front optical lens group 152L and polarized by the polarizer 158L. The transmitted beams pass through the blank area or transparent area 165 of the rotating disc 160 and go to the front optical lens group 152R and polarized by the polarizer 158R for converting the light beams into orthogonally polarized light beams relative to that polarized by the polarizer 158L.

[82] The polarizers 158L and 158R are optional to the system with the polarizing spatial light modulators, such as LCOS or LCD. The polarization modulator, such as a half wave plate is disposed at the position of the polarizers 158L and 158R or after the polarizers 158 and 158-R if those polarizers are presented in the system.

[83] For the stereoscopic projection system which requires the circular polarized light along with the circular polarizing eye glasses, the quarter wave plates can be disposed upstream of each polarizer 158L and 158R or the front optical lens groups 152L and 152R if the polarizers 158L and 158R are not used in the projection system with the polarizing spatial light modulator(s).

[84] FIG. 18 illustrates the ray tracing diagram of another projection optical system 147 and FIG. 11 illustrates the ray tracing of the projection optical system 147 including the common optical lens group 151 and the front optical lens groups 152L and 152R. The projection optical system 147 shown in FIG. 18 has been designed to have throw ratio of 0.8: 1 and F/# of F/2.4 for the DMD light modulators 123R, 123G, and 123B with 2000 x 1000 pixels. The size of each pixel including the gap between the pixels is

13.8 mm. The projection optical system in accordance with the present invention has sufficient lens offset capability for off axis projection. The maximum offset capability between the projection optical system 147 and the DMD light modulators 123R, 123G, and 123B is 90 % in the horizontal or 120 % in the vertical direction. The glass block 153 has the same optical path length as the optical path of the TIR prisms and the color separating prisms. The glass block 154 is the window of the DMD light modulators 123R, 123G, and 123B.

[85] FlG. 19 illustrates the ray tracing diagram from the projection optical system 147 to a screen 166 in accordance with the present invention. The screen 166 preserves the polarization of the projected light from the projection system. The left-eye image 167 on to the screen 166 is offset from the right-eye image 168 due to the separation between two the front optical lens groups 152L and 152R. This offset can be either the horizontal direction or the vertical direction depending on the direction of beam splitting by a beam splitting optical element. As previously described, this image position offset is compensated by the PROCESSOR/FORMATTER 144. FTG. 20 schematically illustrates the stereoscopic images 167 and 168 on the projection screen 166 before and after the compensation for the image position offset. The image position offset illustrated in FlG. 20 is in the vertical direction in accordance with the vertical separation between two separated optical lens groups 152L and 152R.

[86] Optical magnification of each front optical lens group 152L or 152R can be different to each other due to different optical design or manufacturing tolerance for each. Therefore, the left-eye images and the right-eye images from the projection system can be different to each other in terms of the size of the image due to the optical magnification difference. Rather than a fixed compensation for this difference, there is a display calibration and pre-processing process by the PROCESSOR/ FORMATTER 144 in accordance with the present invention. With embedded software, it is capable of displaying geometric shapes for adjustment, and processed input signals for compensating of geometric size discrepancy between the left-eye images and the right-eye images. In different projection environments, where changes in throw distance between the projection point(the end point of each separated optical lens group) and the screen, or other settings would cause a geometric size mismatch of the displays between the left-eye images and the right-eye images. The PROCESSOR/ FORMATTER144 displays a set of shapes to be used as calibration patterns and store the results including the image position offset and the size of the projected left-eye and right-eye images, generated from calibration, for the process. The calibration process is

done manually with references to the calibration patterns on the display. The PROCESSOR /FORMATTER 144 automatically processes the input signal based on the calibration results before feed it into the digital projector 146, which in turn, displays aligned and matched stereoscopic pairs on the screen. The simple algorithms involved in processing include cropping/overlay and resizing. The cropping/overlay is to compensate the displacements between two displays, where the resizing is to match the size differences. The processing speed will match that of the frame rate of the system. Two options will be implemented for the match. The first option is to apply processing on either the left-eye images or the right-eye images, so that it matches the other one. The other option is to apply different pre-processing parameters to both left- eye images and the right-eye images respectively. The goal is to preserve as much displayable area as possible, and as little loss in image information as possible. In cases of not being able to display the whole image in either of the eyes, resizing may be applied depending on whether higher usage (efficiency) in the light source is preferred or larger viewable area in the image is preferred in the latter, part of displayable area on the screen will be blacked off, due to overlaying a down-sized image on bigger image plane that fed into digital image device.

[87] Viewers wearing passive polarization eyeglasses can see the stereoscopic image through the digital stereoscopic projection system according to the present invention.

[88] The typical composite light efficiency for each eye of the stereoscopic projection system with the cubic or flat polarizing beam splitter and the mechanical shutter system according to the present invention is

[89] n = A (0.4) x B(0.45) x C(0.9) x D (0.9) = 15 % e

[90] where, A is the composite duty cycle efficiency of the shutter system and the projector displaying the left-eye and the right-eye images, [91] B is the polarization efficiency of the cubic or flat polarizing beam splitter, C is the transmitting efficiency of the additional polarizer for the pre-polarized light which has same polarization direction, and D is the transmitting efficiency of the eyeglasses for the pre-polarized light which has the same polarization direction as the polarizing direction of each eyeglass. [92] The typical composite light efficiency for each eye of the stereoscopic projection system with the cubic or flat polarizing beam splitter and the electro-optical shutter system according to the present invention is; [93] n = A (0.4) x B(0.45) x C(0.9) x D (0.9) = 15 % e

[94] where, A is the composite duty cycle efficiency of the shutter system and the

projector displaying the left-eye and the right-eye images, B is the polarization efficiency of the cubic or flat polarizing beam splitter, C is the transmitting efficiency of the additional polarizer for the pre-polarized light, and D is the transmitting efficiency of the eyeglasses for the pre-polarized light which has the same polarization direction as the polarizing direction of each eyeglass.

[95] The typical composite light efficiency for each eye of the stereoscopic projection system with the rotating mirror according to the present invention is;

[96] n = A (0.4) x B (0.98) x C (0.45) x D (0.9) = 16 % e

[97] where, A is the composite duty cycle efficiency of the shutter function of the rotating mirror and the projector displaying the left-eye and right-eye images, B is the beam splitting efficiency(reflection or transmission efficiency) of the rotating disc, C is the transmitting efficiency of the polarizer for non-polarized light, and D is the transmitting efficiency of the eyeglasses for pre-polarized light which has the same po¬ larization direction as the polarizing direction of each eyeglass.

[98] The composite light efficiency for each eye of the stereoscopic projection system with the polka-dot beam splitter and the mechanical shutter system according to the present invention is;

[99] n = A (0.4) x B(0.5) x C(0.45) x D (0.9) = 8 % e

[100] where, A is the composite duty cycle efficiency of the shutter system and the projector displaying the left-eye and right-eye images, B is the beam splitting efficiency of polka-dot beam splitter for each reflecting and transmitting direction, C is the polarization efficiency of the polarizer for non-polarized light, and D is the transmitting efficiency of the eyeglasses for pre-polarized light which has the same po¬ larization direction as the polarizing direction of each eyeglass.

[101] It must be understood that the stereoscopic light efficiency previously described can be significantly increased if pre-polarized light comes from the polarizing spatial light modulator(s), such as LCOS or LCD. For example, the composite light efficiency for each eye of the stereoscopic projection system with the rotating mirror and the LCOS or LCD light modulators according to the present invention is;

[102] n = A (0.4) x B (0.95) x C (0.9) = 34 % e

[103] where, A is the composite duty cycle efficiency of the shutter system and the projector displaying the left-eye and right-eye images, B is the transmitting efficiency of the polarization converter(quarter wave plate or half wave plate), and C is the transmitting efficiency of the eyeglasses for the pre-polarized light which has the same polarization direction as the polarizing direction of each eyeglass. If the polka-dot

beam splitter is used; [104] n = A (0.4) x B (0.5) x C (0.95) x D (0.9) = 17 % e

[105] where, A is the composite duty cycle efficiency of the shutter system and the projector displaying the left-eye and right-eye images, B is the efficiency of the polka dot beam splitter for reflecting and transmitting, C is the transmitting efficiency of the polarization converter (quarter wave plate or half wave plate), and D is the transmitting efficiency of the eyeglasses for pre-polarized light which has the same polarization direction as the polarizing direction of each eyeglass.

[106] It must be understood that the arrangement of the projection optical system in accordance with the present invention allows a number of different optical methods in order to provide the digital stereoscopic projection system with dual optical paths which belong to each left-eye images and right-eye images.

[ 107] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modi¬ fications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.