Abstract:
The present disclosure is a novel design of a polarization based stereoscopic display system that efficiently utilizes the optical energy from three primary color solid state light sources of random polarization, combines the three primary colors into a full color beam of a single polarization state to enable passive separation of the two image channels. The high optical energy efficiency is achieved by splitting each primary color light into two orthogonal polarization states. The single polarization state of the combined full color image beam is achieved by employing a spectrally selective light beam combiner or X-cube. By making the optical configuration of sub-module basically identical and sharing a number of optical components among color and image channels, the size and cost is reduced. By compensating the depolarization effect that is introduced by folding mirror(s), the cross talk between the two displayed stereo images is minimized.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of provisional patent application Ser. No 60/995,463, filed Sep. 27, 2007 by the present inventors.
   US patent   US2006/0007538   US2006/0291053   
 
     
    
     FIELD OF THE INVENTION 
       [0005]    The present invention relates to stereoscopic display and in particular, to the design of a stereoscopic display system employing solid state light sources. 
       BACKGROUND OF THE INVENTION 
       [0006]    Traditionally, the stereoscopic image display systems are based on two forms of projection technology. i.e. sequential and simultaneous. In both approaches, two images are generated from two micro-display devices and are projected to a special screen, on which one image is made to be seen only by the left eye and the other image by the right eye. The difference between the images yields depth information, and therefore resulting in strong stereoscopic sensation when they are seen by an observer. 
         [0007]    In the sequential approach, the two displayed images are alternated between the left and right eyes, but at a rate higher than most human can distinguish so that the images to the left and right eyes appear continuous. The image sequence can be generated by a special polarization modulation device placed in the light path and then observed through a pair of passive polarization filters, as discussed in publication US 2006/0291053A1. Another approach is to use special digital projectors running at twice the video frame rate and the projected images are then observed through a pair of active shutter glasses operating in synchronization with the projectors. However, these sequential approaches suffer from the “motion effect”, in which a slight movement of the target object in horizontal direction usually results in false stereoscopic perception by the observer, leaving the observer with the impression that the object is moving in-and-out of the monitor screen, which also often result in fatigue to the eye. 
         [0008]    In the simultaneous approach, the stereo image pairs, which are recorded with synchronized shutters of dual camera system, are projected through two separate optical projectors/channels at the same time, and viewed individually by left and right eye of the observer. Compared with the sequential approach, this approach does not produce the “motion effect” and is therefore more preferred. However, prior art of simultaneous stereoscopic display systems also have their limitations. For example, in most of commercially available stereoscopic displays, one pair of orthogonal optical polarizers, being either linear or circular, are placed in front of each projector of the two channels to encode the left and right images with two orthogonal polarization states. A pair of matching polarizers is worn by the observer to discriminate the two images between the eyes. This approach suffers from a relatively huge loss of light (up to 70%) and a relatively substantial image cross talk between the two images, with the crossed over images appearing as ghost images. In general, there are two types of projectors that are used for stereoscopic displays, namely, DLP (digital light processor) and liquid crystal based such as an LCOS (liquid crystal on silicon). The DLP projector sequentially projects the three primary colors at high speed. But due to the limited duty cycle, it also requires that the light source of the three primary colors running at higher peak power. This is the main limiting factor for the high brightness displays currently utilizing solid light sources, such as LEDs, which is regarded as the most suitable light source for light projection engines. 
         [0009]    On other hand, the liquid crystal based projector also faces challenges. The first one is that it requires the output light from light source to be linearly polarized. At the moment, relatively high power solid state light sources such as LEDs (light emitting diodes) are already available in the three primary colors and they offer high efficiency and long working lifetime. Unfortunately, these high power LEDs generally produce non-polarized or randomly polarized light. As a result, half of the light energy will be lost unless means of polarization recycling is employed. Several polarization re-cycling and color combination schemes have been proposed to combine these three primary colors into a white color (US2006/0007538A1). However, those approaches are complicated and/or inefficient because the combined light, after being injected into the projection engine, is divided again into the three primary colors. The second challenge is caused by the use of a special spectral beam combiner, called an X cube, which is used to combine the three primary color images into a full color image. The most common way to use a traditional X cube is to have the green light enter the cube p-polarized and to have the red and blue light enter the cube s-polarized. As the result, images from the majority of liquid crystal projectors on market currently are linearly polarized in vertical direction for the red and blue color, and in horizontal direction for the green color, as shown in  FIG. 1 . To generate a co-linearly polarized light from three primary color images, a polarizer can be placed in front of projector and with its polarization axes rotated 45 degrees from the horizontal or vertical direction. Unfortunately, this approach will introduce an additional light loss of at least 50% in theory, which can be as high as 80% in reality. To avoid this extra light loss, as shown in  FIG. 2 , it has been proposed (US2006/0007538A1) that a color selective half wave-plate  206  to be added behind the X-cube  204  and to selectively rotate only polarization direction of the green light by 90 degrees so that the three primary color light beams are co-linearly polarized before the polarization is further cleaned by an s-polarizer  208 , to result in an improved performance in image brightness and contrast. However, this solution, when combined with the polarization recycling scheme will make the projection engine even more complicated. 
         [0010]    In addition to the optical energy efficiency issue, another major issue is associated with cross talk between the left and right images. For polarization based stereoscopic displays, the leakage of light from one stereo channel to the other always exist. This often is caused by the depolarization effect of any optical component in the light path. For rear projection based stereoscopic displays, the depolarization introduced by the last folding mirror is almost impossible to be removed. 
         [0011]    There is a need for a compact stereoscopic display system that will most efficiently use the optical energy from the three primary color sources and meanwhile further minimize the cross talk between the two stereo channels. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0012]    The present invention discloses a novel design of a liquid crystal based stereoscopic display system that can highly efficiently use the optical energy from solid light sources and divide the three primary colors of random polarizations each into two orthogonal polarization states, one for the left channel and the other for the right channel. By using a special X cube, the combined full color image beam has a co-linear polarization. In addition, by sharing a number of optical components between the two stereoscopic channels, and by making the optical configuration for each color light path identical, except for the coatings that are designed for the specific color spectral band, the presently disclosed design is not only more compact but also of lower in system cost. Furthermore, by employing a depolarization compensation scheme in a rear projection stereoscopic display, the cross talk between two stereoscopic channels is substantially reduced. 
         [0013]    One object of the invention is to increase the optical energy efficiency of a stereoscopic display system. 
         [0014]    Another object of the invention is to reduce the size of a projection engine used in stereoscopic display. 
         [0015]    Another object is to lower the cost of the stereoscopic display system by sharing some of the optical components for both the left and the right channels and by making the optical layout of each sub-channel basically the same. 
         [0016]    Still another object is to reduce the cross talk between the two stereo channels. 
         [0017]    Additional objects, features and advantages of the various aspects of the present invention will become apparent from the following description of its preferred embodiment, which description should be taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  shows the polarization directions of the three primary colors (R, G, B) after a conventional X-cube that can combine the three colors but with the green color (G) polarized in an orthogonal direction as compared to that of the red (R) and blue (B) colors. With a 45 degree oriented linear polarizer (dotted line) placed in the light path further behind the X-cube, only the components of the light which are parallel to the orientation of the dotted line will pass through the polarizer L. 
           [0019]      FIG. 2  shows a prior art X-cube related architecture in which a color selective half wave-plate is added behind the X-cube to selectively rotate the polarization direction of only the green light (G) by 90 degrees so that the three color light (R, G, B) are co-linearly polarized before the polarization is further cleaned by an s-polarizer (out-plane-polarization). 
           [0020]      FIG. 3  shows a special spectrally selective beam splitter/combiner or X-cube that reflect light beams in red (R) and blue (B) colors and transmits light beam in green color (G) also in s-polarization (out-plane-polarization). 
           [0021]      FIG. 4   a  shows one side view of the stereoscopic projection engine, illustrating how light beams from two random polarization solid light sources, one being red and the other blue, are divided into two orthogonal linear polarizations to create the left and right images for the red and blue colors. 
           [0022]      FIG. 4   b  shows a side view of the stereoscopic projection engine normal to that of  FIG. 4   a,  illustrating how a green light beam from random polarized solid light source is divided into two orthogonal linear polarizations to create the left and right images for the green color. 
           [0023]      FIG. 4   c  shows a top view of the stereoscopic projection engine, in which the three primary color image paths for one of the two stereo image pair are combined by a special spectrally selective X-cube into full color to be projected onto a screen. 
           [0024]      FIG. 4   d  shows the orientation of ordinary (o) and extraordinary (e) axis of the two quarter wave plates with respect to the linear s-polarization direction, that are used to convert the originally co-linear s-polarization of the left and right channels into circular polarizations of opposite directions. 
           [0025]      FIG. 4   e  shows the top view of the optical layout of the three illumination sub-systems utilizing three primary color solid state light sources. 
           [0026]      FIG. 5  shows a front projection arrangement, in which the two projection lenses are displaced with a small distance toward the central axis of the screen from the optical axes of the two stereo imaging paths in order to laterally shift and hence superimpose the left and right images from the two sub-engines on the screen. 
           [0027]      FIG. 6  shows a rear projection system that uses the presently disclosed stereoscopic projection engine together with two projection lenses, two reflective mirrors arranged to enable polarization depolarization compensation, and a polarization display screen. 
           [0028]      FIG. 7  shows another rear projection embodiment with acute angle reflections to reduce the depth of the display unit and meanwhile maintaining the depolarization compensation arrangement. 
           [0029]      FIG. 8   a  shows the top view of a space saving embodiment for illumination of sub-engine, in which all of the solid light sources and the light homogenization devices are located on one side of the engine. 
           [0030]      FIG. 8   b  shows the side view of one of the color channels of the space saving embodiment of  FIG. 8   a,  in which all of the solid light sources and the light homogenization devices are located on one side of the engine. 
           [0031]      FIG. 9   a  shows the red (R) and blue (B) color channel side view of another embodiment of the stereoscopic display system that has the same optical design for light illumination engine but uses transmissive LCD micro-display chips to achieve the same goal of generating the same single polarization. 
           [0032]      FIG. 9   b  shows the green (G) color channel side view of the embodiment of  FIG. 9   a.    
           [0033]      FIG. 9   c  shows the top view of the embodiment of  FIGS. 9   a  and  9   b,  illustrating the imaging channels only. 
           [0034]      FIG. 10  an alternative special spectrally selective X-cube combiner for both p and s polarizations that can also be used for the present projection engine. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0035]    In this invention, a novel digital simultaneously stereoscopic image display is disclosed. The term “simultaneously stereoscopic image display” is referred to the display means in which the stereoscopic image pairs, which are recorded with synchronized shutters simultaneously, is displayed through two optical channels at same time, and viewed individually by left and right eye of the observer. In the present design, the optics of the stereoscopic display can use solid state light source efficiently and also enable the sharing of a number of optical components by both channels. As a result, the optical system is more compact, optically efficient and meanwhile less costly. In addition, the use of solid light sources not only increases the reliability and life time of the light source significantly, but also enlarges the color gamut of display. The solid state light sources discussed in this invention include, but not are limited to, light emitting diode (LED), super luminescent diode (SLD) and laser diode (LD). 
         [0036]    As one key feature of the present invention, a specially designed spectrally selective beam splitter/combiner or X-cube is combined with polarization based micro displays, which can be either reflective type LCOSs or transmissive type LCDs for achieving the optical energy efficiency as well as a single polarization for the combined full color image beam.  FIG. 3  shows one embodiment of such special spectrally selective beam splitter/combiner or X-cube for combining images formed in three primary colors into a full color image in one polarization direction. The X-cube  302  is special in sense that the light beams in red (R) and blue (B) colors and in s-polarization are reflected by the X-cube  302 , while the light beam in green color (G) and also in s-polarization is transmitted through the cube. 
         [0037]      FIGS. 4   a,    4   b,    4   c,    4   d,  and  4   e  show one embodiment of the presently disclosed stereoscopic projection engine design as seen in different views. In  FIGS. 4   a  and  4   b,  an identical optical configuration is used for each of the three primary color channels. By making optics in each of the primary color channels identical, the cost of the projection system can be reduced. The projection engine consists of two sub-modules; one is outlined in blocks  432  and  467  of  FIG. 4   a  and  FIG. 4   b  respectively, and the another in block  422  and  462  of  FIG. 4   a  and  FIG. 4   b  respectively. Of the two sub-modules, one generates a full color image for the left eye and another generates a full color image for the right eye. As will be explained shortly, preferably, solid state light sources  410 ,  430  and  450  with random polarization light output, non-overlapping and narrow spectral bandwidth, are used to generate the light in three primary colors; and the light from each of the three primary colors is associated with an illumination sub-system and is divided into two orthogonal polarization states to be used for the left and right channels respectively. The illumination sub-system is outlined in block  412  and  452  in  FIG. 4   a  and  FIG. 4   b  respectively. Meanwhile, each of the two sub-modules uses three identical liquid crystal based micro-display chips to generate the three primary color images in the RGB color space separately. 
         [0038]    In the illumination sub-system outlined by  412 , the light from source  410 , which often is in red color, is coupled into a light homogenization device  411  either through direct coupling (as shown in  FIG. 4   a ) or by a condensing optical lens (not shown). The output end of the device  411  is made with an aspect ratio similar to that of the micro-display image chip  418 , and is imaged onto (in conjugation with) the chip  418  by the condensing optical lens  413  through the optical components  414 ,  415   416  and  417 . The polarization beam splitter  414 , either in the form of a cube as shown in  FIG. 4  or in the form of a plate (not shown), reflects the input light beam with predominantly s-polarization (or out-of-plane polarization as shown) toward a linear polarizer  416 , through optical compensation module  415 . 
         [0039]    In the sub engine outlined by  432 , the compensation module  415 , with an optical thickness that is the same as that of the beam splitter  414  for the transmitted p-polarization, is used to ensure an equal optical path for the two sub-engines. Instead of a compensation component, a pure air space with equivalent optical distance can also be implemented. Note that the absorption type linear polarizer  416 , with its polarization axis aligned with the s-polarization direction of the beam splitter  414  or out of paper plane as shown in  FIG. 4   a,  is used to further remove the light in other polarization directions. Thus the further cleaned light beam, with a pure s-polarization, is then reflected by a second polarization beam splitter cube  417  onto the micro-display chip  418 . The reflective LCOS chip  418 , behaving as an active phase modulator, can rotate the polarization state of some of the pixels and return light in the orthogonal (p) polarization direction (in the paper plane of  FIG. 4   a ). Therefore, those pixelated sub-light beams that are now p-polarized will then be transmitted through the polarization splitter cube  417  and reach the optical beam combiner  429 . The remaining portion of those pixelated sub-light beams, which is reflected from chip  418  without polarization rotation, is s-polarized and hence is reflected by cube  417 . Afterwards, they will pass through the polarizer  416 ; get returned to the light homogenization device  411  and light source  410  through optics  415 ,  414  and  413 . Meanwhile, any p-polarization component that can be caused by the imperfectness of the polarization beam splitter  417  will be absorbed by polarizer  416 . If the p-polarization component is not absorbed, it could reach another optical channel through beam splitter  414  and cause undesired optical artifact in the displayed stereo images. 
         [0040]    Also in the illumination sub-system outlined by  412 , the polarization beam splitter  414  allows transmission of the linearly polarized light beam with p-polarization (in plane of paper on  FIG. 4   a ) from the light source  410 , which is about half of the output light power from the source  410 . In the sub-engine outlined by  422 , an optical quarter wave plate  420  with its axes orientated at 45 degree from the p-polarization direction converts the light into circular polarization. The light beam reflected from the mirror  421  exhibits circular polarization in opposite rotation direction, and becomes linearly polarized after passing through wave plate  420  a second time, but in orthogonal polarization direction (s polarization). The light beam is then reflected by the polarization beam splitter  414 . 
         [0041]    In the sub engine outlined by  422 , the s-polarized light beam, after passing through linear polarizer  423 , is reflected by the polarization beam splitter cube  425  to reach the micro-display chip  426 . There afterwards, the light beam will behave in a similar fashion as has been discussed for the sub-engine  432 . Similarly, the function of optical components  423 ,  425 ,  426  and  449  is identical to that of  416 ,  417 , 418  and  429 . It is understood that the output end of the device  411  is also imaged onto (in conjugation with) the chip  426  by the condensing optical lens  413  through the optical components  414 ,  420 ,  421 ,  423 , and  425 , due to the equal optical path in tow sub engines. 
         [0042]    Note that the light beams reaching micro-display chips  418  and  426  come from the same light source  410 , and have the same polarization direction and roughly the same light intensity. However, due to the polarization modulation by the LCOS chips  418  and  426 , different images will be displayed for the left and right channels. The reflected light beams from the two LCOS chips, when reach the optical beam combiner  429  and  449 , represent the intensity modulated red color component of full color images for the left eye and the right eye respectively. It is understood that the optical components described above are designed to work in correspondence with the narrow spectral bandwidth of the light source  410  in terms of optical properties and optical coatings on these components. 
         [0043]    Similarly, the two blue color channels of the stereoscopic projection engine have the same optical layout as for the two red channels and are implemented in a symmetrical configuration on the right side of  FIG. 4   a.  The light from the solid state light source  430 , often with a relatively narrow spectrum in blue color, is used to illuminate two identical micro-display chips  438  and  446 . All of the optical components,  431 ,  433 ,  434 ,  435 ,  436 ,  437 ,  440 ,  441 ,  443 , and  445  are designed to work in the corresponding spectral range of the light source  430 .  FIG. 4   b  shows the optical layout for the two green color channels, which is identical to that of the other two primary colors, with the difference that this layout is positioned normal to that for the other two primary colors; that is,  FIG. 4   c  shows side view normal to  FIG. 4   a.  All of the optical components,  451 ,  453 ,  454 ,  455 ,  456 ,  457 ,  460 ,  461 ,  463  and  465  are also designed to work in the corresponding narrow spectral range of the green light source  450 . The green color is combined with the red and blue colors at the two combiners  429  and  449 . When viewed from the top of  FIG. 4   a  and  FIG. 4   b,  as can be seen from  FIG. 4   c,  the optical combiner  429 , which is a special spectrally selective X-cube as shown in  FIG. 3 , functions to combine the three primary colors in s-polarization for one of the two stereo pairs, reflects the light beams from red and blue beam paths, but transmits the light beam from the green beam path. The three primary color images generated by the three micro-display chips  418 ,  438  and  458  will now form a full color image in a single linear s-polarization after the special X-cube  429  and can be projected to a screen by the optical projection lens  474  with or without any further polarization state change. To accomplish the task of overlapping the three primary color images, the three micro-display chips  418 ,  438  and  458  must be optically aligned precisely and matched to each other at the pixel-to-pixel level. Similarly a bottom view with respect to  FIGS. 4   a  and  4   b  can be envisioned but is not repeated here. In this case, a second full color image for the other channel of the stereo pair, which also comprises three primary color images generated from the three micro-display chips  426 ,  446  and  466 , will be combined by the special X-cube  449  and projected onto the same screen through projection lens  477 . Note that although we have drawn two projection lenses  474  and  477 , these two lenses can be a shared single lens if additional optical modules are used to combine the two light beams either together or physically very close to each other. 
         [0044]    The special spectrally selective X-cube  449  and  429  are made with optical properties as has been described in  FIG. 3 . Accordingly, the three light beams  480 ,  481 ,  482  have the same polarization direction, which is normal to the paper plane of  FIG. 4   c.  Similarly, a bottom view (not shown) would show that this polarization statement is also true for other full color image of the stereo pair. Therefore, both light beams of left and right image generated respectively by the two sub-engines have the same polarization direction, which for now is not ready for polarization based simultaneously stereoscopic display yet. Further polarization manipulation is still need to convert the two collinear polarization states into two orthogonal states and will be explained shortly. Note that  FIG. 4   c  also shows spectrum of the three primary color light beams  480 ,  481 , and  482 . It is preferred that the three spectral bands are relatively well separated so that there is no spectral overlap but meanwhile each spectral band is also not too narrow to cause optical speckles to appear in the display. 
         [0045]    As shown in  FIGS. 4   b  and  4   c,  two broadband absorption type linear polarizers  470 ,  475  are used respectively for each of the two stereo channels to clean up the linear polarization and hence get rid of the unwanted polarization components. This is preferred because if the polarization is not pure, cross talk may occur even if the depolarization effect of the screen and projection lenses  474  and  477  are considered non-existent. Nevertheless, in practice, there is a limit to the purity of a polarization state due to, for example, the limited extinction ratio of any polarization manipulation component, the existence of small alignment errors and the depolarization effect in each optical component. 
         [0046]    To create two passively distinguishable images for each of the two eyes of the observer, two broadband optical quarter wave plates  471  and  476  are inserted respectively into the two optical paths behind the two purification linear polarizers  470  and  475 , as shown in  FIG. 4   b  and  FIG. 4   c.  The orientations of the two quarter wave plates are arranged orthogonally and at 45° with respect to the linear s-polarization direction as shown in  FIG. 4   d,  where the notation “o” stands for the ordinary axis and “e” stands for the extraordinary axis of the quarter wave plate. After passing through the two quarter wave plates  471  and  476 , one image light beam will become circularly polarized in clock wise direction while the other will become circularly polarized in counter-clock wise direction, thus forming two passively distinguishable images of orthogonal polarizations on the screen. When a pair of broadband circular analyzer spectacles, which can be constructed using the same polarizer/quarter wave plate combinations as shown in  FIG. 4   d,  but arranged in reverse order, is worn by the observer, the left and right images will be demultiplexed and be seen by individual eyes separately. A benefit of using two orthogonal circular polarizations to distinguish the left and right images is that the cross talk will not be deteriorated by the rotation of the observer&#39;s head. However, this preference should not exclude the possibility that two orthogonal linear polarizations can also be used for passively distinguishing the two images. In this latter case, one only need to rotate the polarization direction of one of the two light beams by 90°, using for example a broadband half-wave plate. 
         [0047]    It is to be understood that the polarizer/quarter wave plate combinations,  470 / 471  and  475 / 476 , can be arranged after projection lens  474 / 477  to achieve same effect as shown in  FIG. 4   b  and  FIG. 4   c  with the benefit that the depolarization effect that may be introduced by the projection lens  474 / 477  can be further cleaned up. 
         [0048]    The brightness of images from two sub engines could be slightly different due to the imperfection of the optical components. However, the difference can be reduced by adjusting the optical aperture of one of the projection lens, inserting a neutral density filter into one light path, or adjusting image brightness electronically through the micro-display chip with its extra dynamic range. 
         [0049]      FIG. 4   e  shows the top view of the optical layout of the three illumination sub-systems utilizing three primary color solid state light sources. One advantage of the design is that the cost of the overall system can be reduced because the optics for each channel are made with same size, but coated with optical films that are best suitable for the spectral band of each individual light source. 
         [0050]    One aspect of the present invention is that the stereoscopic projection engine can be used to project images of any aspect ratio although an aspect ratio of 1:1 is used for the micro-display chips and other optical components as shown in the Figures. For example, it can be used to project images in the most commonly used image aspect ratios of 4:3 and 16:9. The image generating micro-display chips could be oriented in either the vertical or horizontal direction if it is not in the shape of a square. In order to reduce the size of the optical components and minimize optical distortion that can be introduced from the projection lens, the display chips  418 ,  438 ,  458  are preferably arranged so that the long side of the display chips is visible from the top view, as shown in  FIG. 4   c.    
         [0051]    As can be seen in  FIG. 4   b,  the optical axes of the two sub-engines  462 ,  467  are designed to be exactly parallel. While the projection lenses  474  and  477  can be aligned alone the optical axes of the two stereoscopic imaging paths, the projected images on the screen will be offset by a distance equal to that between the optical axes of two projection lenses  474  and  477 .  FIG. 5  shows an improvement over the arrangement of the two projection lenses as shown in  FIG. 4   b,  in which a small inward translation toward the central axis of the screen  501  by the projection lens axes, Δ L  and Δ R , from the optical axes O′ L  and O′ R  of the two imaging paths, can be introduced for the projection lenses  474  and  477 , in order to laterally shift and hence superimpose the left and right images from the two sub-engines on the screen  501 . The amount of translation Δ L  could be, but not necessarily needed to be equal to Δ R . 
         [0052]    The presently disclosed projection engine can be used in a front projection display system, as illustrated in  FIG. 5 . The display screen  501  is specially made so that the reflected (diffused) light from the screen maintains the polarization state of the incoming light with minimum depolarization effect. However, the same engine with proper adjustment for image offset can also be used in a rear projection display system. 
         [0053]    As another aspect of the present invention, when the disclosed stereoscopic projection engine is used in a rear projection configuration, a special beam folding arrangement is proposed to compensate the depolarization effect that can be introduced by beam folding mirror(s).  FIG. 6  shows such a rear projection system, consisting of a stereoscopic projection engine  610 , two projection lenses  607 ,  608 , two reflective mirrors  605 ,  602  arranged to enable polarization depolarization compensation and a special display screen  601  which is capable maintaining polarization state of passing light with minimum depolarization effect. For the convenience of description, an imaginary optical axis 0-0′ is drawn to represent the base line of the rear projection system. To reduce the depth of the display unit, a large reflective mirror  602 , which is preferably coated with metallic and dielectric coatings, is used to bend the light beam by 90 degrees to the screen  601 . However, the use of a mirror with even slightly different reflectivity in s and p polarization can cause significant geometric depolarization for the reflected light beams, especially for skew light rays. The depolarization effect will reduce the ANSI contrast of the stereoscopic images and increase the cross-talk between the left and right channels perceived by observers. To minimize the depolarization impact, a second mirror  605  with exactly the same mirror coating as that of the upper larger mirror  602  is introduced between the mirror  602  and projection engine  610 . As shown in  FIG. 6 , the plane of incidence for mirror  605 , which is formed by the base line O′-O″ and normal of the mirror  605 , is perpendicular to the plane of incidence for mirror  602 , which is formed by base line O-O′ and normal of the mirror  602 . This arrangement results in two orthogonal right angle turns of the base line from the projection system to the screen, one at the center of the mirror  605  and the other at the center of mirror  602 . With such an arrangement, the s-polarization at the turn of the lower mirror  605  will become p-polarized at the turn of the upper mirror  602  and the p-polarization at the turn of the lower mirror  605  will become s-polarized at the turn of the upper mirror  602 . Consequently, any depolarization terms from s to p (or p to s) at the first turn will be reversed from p to s (or s to p) at the second turn, resulting in reduction in depolarization effect. Since the projected images are also rotated by 90 degrees at the mirror  605 , the projection lenses  607  and  608  can be orientated accordingly to maintain the proper orientation of the final displayed image. 
         [0054]    In the optical layout shown in  FIG. 6 , the reflective surface of mirror  602  is placed at 45 degrees relative to the horizontal direction. Although this arrangement is simple to implement, the configuration also leads to boxy construction with a depth close to half of the screen height. To reduce the depth of the display unit,  FIG. 7  shows an alternative of a rear projection system, in which a smaller angle is used for the mirror  702 . To maintain the required relationship, in which the plane of incidence for mirror  702  is perpendicular to that for mirror  705  and the angle of incidence is same for tow mirrors, the projection engine  710  and its projection lenses  707 ,  708  are preferably also rotated in two directions as shown in  FIG. 7 . 
         [0055]      FIGS. 8   a  and  8   b  show another embodiment of the projection engine design, in which all of the solid light sources and the light homogenization devices are located on one side of the engine. This design reduces the overall size. In comparison with  FIG. 4   e,  the light source  810 ,  850 , and  830  are equivalent to  410 ,  450 , and  430  respectively. The optical layout and components used for generating the green images are identical as shown in  FIG. 4   b.  However, due to the rotation of the optical path for the red and blue color channels, additional optical components are added to maintain the proper polarization state for the micro-display chips.  FIG. 8   b  exemplifies the optical layout for red color channel, in which two optical half-wave plate  819  and  824  are used to rotate the polarization direction of the red light beam by 90° because the polarization splitter  814  is rotated by 90° compared with the layout shown in  FIG. 4   a.  The optical layout for the blue color channel is identical to that of the red color channel, except that the optical components are made for a different optical spectral band. As shown in  FIG. 8   c,  for the combination of the three primary colors, a similar special spectrally selective X-cube  829  as elaborated before can be used and the arrangement of the three polarization beam splitters  817 ,  837 ,  857  are same to that of  417 ,  437 , and  457 . The rest of the stereoscopic projection engine is similar to what has already been discussed. 
         [0056]    The same technology and design concept can also be applied to a stereoscopic projection engine based on transmissive LCD micro-display technologies.  FIG. 9   a  shows one side view of another embodiment of the stereo display system that has the same optical design for light illumination engine but uses transmissive LCD micro-display chips to achieve the same goal of generating light with same single polarization for the combined full color beam. Two sub-modules  932 ,  922  generate the same images as  432 ,  425 , while the light or optical energy is provided by the illumination sub-system  912 , similar to that of module  412 . The light sources  910 ,  930  and  950  in  FIGS. 9   a  and  9   b  correspond to the light sources  410 ,  430  and  450  in  FIGS. 4   a,    4   b  and  4   e.  Instead of a polarization beam splitter cube  417  ( 425 ,  437   445 ), a reflective surface  917  ( 925 ,  937 ,  945 ) is used to guide the light beam to the transmissive micro display chip  918  ( 926 ,  938 ,  946 ). Two absorption type linear polarizers  916  and  919  ( 923  and  928 ,  936  and  939 ,  943  and  948 ), with their polarization axes perpendicular to each other, are placed on the opposite sides of the LCD chip  918  ( 926 ,  938 ,  946 ). Similar to polarizer  416  ( 423 ,  436 ,  443 ), the polarizer  916  ( 923 ,  936 ,  943 ) is used to suppress the stray polarization light in the light beam.  FIG. 9   b  shows the green color channel side section view of the embodiment of  FIG. 9   a  and it is easy to understand when referenced to the similarity and difference of design shown in  FIG. 4   b.    FIG. 9   c  shows the top view of the embodiment of  FIGS. 9   a  and  9   b,  illustrating the imaging channels only. Due to the use of transmissive micro-displays, the polarization of the light beam is rotated by the display chip/polarizer combination module by 90° for all of the three color channels. Accordingly, the three primary color light beams  980 ,  981  and  982 , when exiting from the light engine after the special X-cube or optical beam combiner  929 , have the same optical polarization direction and form the same full color image as in the case of the light beams  480 ,  481  and  482   
         [0057]    The stereoscopic projection engine, described in  FIGS. 9   a,    9   b  and  9   c,  can be used in either front or rear projection display systems in an identical way as and virtually exchangeable with the one described before. 
         [0058]    It should be understood that the embodiment of the illumination sub-system discussed in  FIGS. 8   a  and  8   b  is also applicable to the projection engine described in  FIGS. 9   a,    9   b  and  9   c.  Additional optical components, like half-wave plates, are placed inside the red and blue channels of the engine similar to elaborated above, in order to maintain the required polarization states. 
         [0059]    Besides the special spectrally selective optical beam combiner (X-cube), described in  FIG. 3 , it should be understood that other spectrally selective X-cubes can also be used. For example, the X-cube can be one that works for the p-polarization instead of s-polarization as shown in  FIG. 3 . As another alternative,  FIG. 10  shows another X-cube design that is also suitable for the presently disclosed stereoscopic projection engines. The embodiment specified in  FIG. 3  only transmits green light beam in s-polarization, while reflects red and blue light beams in s-polarization. As shown in  FIG. 10 , the alternative optical beam combiner is polarization insensitive because it reflects red and blue light in both polarization and transmits green light in both polarization too. In  FIG. 10 , G, R, B represent green, red and blue light beams respectively. 
         [0060]    To implement the X-cube working in p-polarization for the engine elaborated in  FIGS. 4   a,    4   b,    4   c,    4   d  and  4   e,  the polarization beam splitter  417 ,  437 ,  425 ,  445 ,  457 , and  465  have to be rotated by 90° to allow the p-polarized light beam to pass through. Then a matching reflector for each beam splitter is added to its side to further guide the light beam from the polarizer  416 ,  436 ,  423 ,  443 ,  456 , and  463 . The embodiment of the illumination sub-system discussed in  FIGS. 8   a  and  8   b  is also applicable to the LCOS projection engine using the X-cube described in  FIG. 10  for p-polarization. Additional optical components, like half-wave plates, are placed inside the red and blue channels of the engine similar to elaborated above, in order to maintain the required polarization states. 
         [0061]    To implement the X-cube working in p-polarization for the engine elaborated in  FIGS. 9   a,    9   b  and  9   c,  a half-wave plate is inserted between beam splitter  914  and polarizer  916  to rotate the polarization direction by 90°. Accordingly the image modulation module, which consists of  916 ,  918  and  919 , is also rotated by 90°. As the result, the polarization for light beam exiting  919  is in p-polarization for the X-cube. Similar modifications are carried out for all of micro-displays in other channels and colors. The embodiment of the illumination sub-system discussed in  FIGS. 8   a  and  8   b  is also applicable to the transmissive LCD projection engine using the X-cube described in  FIG. 10  for p-polarization. Due to the rotation of the beam splitter  914  and  934  by 90°, only one half-wave plates is placed inside the green channel of the engine, between beam splitter  954  and polarizer  956 , to rotate the polarization direction by 90°. Accordingly, the image modulation modules for three primary colors, which for example, consist of  916 ,  918 ,  919  and  923 ,  926 ,  928  for red color, are also rotated by 90° to generated p-polarized light beam in X-cube. 
         [0062]    It is also to be understood that, although red, green and blue (RGB) based primary colors are used in the discussion for forming the full color image and architectural design of the projection engine, the embodiments are also suitable for projection engines using the complementary colors of RGB. Hence the term “three primary colors” should be interpreted as represent any three colors that can result in a full color display on a monitor when they are combined.