Abstract:
Projection systems and methods for providing stereoscopic images viewed through passive polarizing eyewear. The systems relate to projectors that create left and right eye images simultaneously and often as side-by-side images on the image modulator. The systems act to superimpose the spatially separated images on a projection screen with alternate polarization states. The embodiments are best suited to liquid crystal polarization based projection systems and use advanced polarization control.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation patent application and claims priority to U.S. patent application Ser. No. 12/826332, entitled “Stereoscopic projection system employing spatial multiplexing at an intermediate image plane,” to Schuck et al., filed Jun. 29, 2010 that relates to and claims priority of: 
         [0002]    (1) provisional patent application Ser. No. 61/221,482, entitled “Stereoscopic Projection System Employing Spatial Multiplexing at an Intermediate Image Plane,” to Robinson et al., filed Jun. 29, 2009 (“Robinson et al. Prov. Pat. App.”); 
         [0003]    (2) provisional patent application Ser. No. 61/221,516, entitled, “Stereoscopic Projection System Employing Spatial Multiplexing Near the Aperture Stop,” to Schuck et al., filed Jun. 29, 2009; 
         [0004]    (3) provisional patent application Ser. No. 61/224,416, entitled “Stereoscopic Projection System Employing Spatial Multiplexing at an Intermediate Image Plane,” to Schuck et al, filed Jul. 9, 2009 (“Schuck et al. Prov. Pat. App.”); 
         [0005]    (4) provisional patent application Ser. No. 61/249,018, entitled, “Stereoscopic projection system employing spatial multiplexing at an intermediate image plane,” to Schuck et al., filed Oct. 6, 2009; and 
         [0006]    (5) provisional patent application Ser. No. 61/256,854, entitled, “Stereoscopic projection system employing spatial multiplexing at an intermediate image plane,” to Schuck et al., filed Oct. 30, 2009; 
         [0007]    all of which are herein incorporated by reference for all purposes. 
     
    
     TECHNICAL FIELD 
       [0008]    The disclosed embodiments generally relate to projection systems and, more specifically, relate to projection systems that may selectively operate in a stereoscopic mode and a non-stereoscopic mode. 
       BACKGROUND 
       [0009]    Stereoscopic projection dates back to the early 20 th  century and was first seen in cinemas during the 1950s. These systems were film based and were limited mechanically to modest ˜24 Hz frame rate. As such, it was not possible to use temporal methods of providing flicker-free sequential left and right eye images for stereoscopy. Spatially multiplexed image display systems were therefore implemented. Some comprised separate projectors while others employed a single projector with each frame comprising spatially separate left and right eye images. Complex frame dividing optics was used in this latter case to successfully superimpose the images on the screen. Many systems were developed and several commercially successful, as discussed by L. Lipton in  Foundations of the Stereoscopic Cinema , Van Nostrand-Reinhold, Appendix 7, p. 260, 1982, which is hereby incorporated by reference. Unfortunately the quality of the stereoscopic experience was insufficient to draw customers leading to a reversal to 2D cinema in the latter half of the century. 
         [0010]    Stereoscopic projection has recently been revitalized with high quality advanced digital equipment encompassing capture, distribution and display. To date the most successful projection system has been developed and installed by RealD. Based on Texas instruments Digital Light Processing (DLP) technology, systems provide time sequential left and right eye images at flicker free rates. Incorporating a polarization switch in the projection path provides sequential left and right eye images for viewing through passive polarizing eyewear. While the system based on DLP technology may provide good quality stereoscopic imagery, alternative projection platforms, such as those based on liquid crystal (LC) modulation, can also be considered. Desirable features of an LC projector-based platform are potentially providing improved resolution, motion rendition, and optical polarization efficiencies. Presently, a single LC projector does not however provide time-sequential images with sufficient frame rate to allow temporal left eye/right eye polarization modulation. 
       SUMMARY 
       [0011]    Disclosed are stereoscopic projection systems and methods for stereoscopic projection. 
         [0012]    Generally, according to an aspect, a projection system is operable to selectively project stereoscopic and non-stereoscopic projection modes. The projection system includes a relay lens subsystem, a stereoscopic module, a non-stereoscopic module, and a projection lens subsystem. The relay lens subsystem is operable to receive input light from the projection subsystem and convey the input light toward an intermediate light path. The stereoscopic module is operable to receive the light from the intermediate light path and process the light for stereoscopic projection of left and right eye images having orthogonal polarization states. The non-stereoscopic module is operable to receive the light from the intermediate light path. The projection lens subsystem is operable to focus light from the stereoscopic module or the non-stereoscopic module toward a screen. When the projection system is in a stereoscopic projection mode, the stereoscopic module is located in the intermediate light path, and when the projection system is in a non-stereoscopic projection mode, the non-stereoscopic module is located in the intermediate light path. 
         [0013]    Generally, according to another aspect, the stereoscopic projection systems may include a relay lens subsystem, a light splitting subsystem, a light combining subsystem, and a projection lens subsystem. The relay lens subsystem is operable to receive a stereoscopic image frame from an input light path and convey the stereoscopic image frame to an intermediate image plane via a light directing element. The stereoscopic image frame has first image area light and second image area light. The light splitting subsystem is operable to receive the stereoscopic image frame at the intermediate image plane and split the first image area light from the second image area light. The light splitting subsystem is also operable to direct the first image area light on a first image light path, and to direct the second image area light on a second image light path. The light combining subsystem is operable to combine the first and second image area light, wherein the first image area light that is output from the light combining subsystem has a polarization state orthogonal to the second image area light. The projection lens subsystem is operable to direct the first and second image area light toward a screen. 
         [0014]    Other aspects, features and methods of stereoscopic and non-stereoscopic projection are apparent from the detailed description, the accompanying figures and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1A  is a schematic block diagram of an exemplary projection system in a stereoscopic projection mode, in accordance with the present disclosure; 
           [0016]      FIG. 1B  is a schematic block diagram of an exemplary projection system in a non-stereoscopic projection mode, in accordance with the present disclosure; 
           [0017]      FIG. 2  is a schematic diagram of an embodiment of a stereoscopic projection system, in accordance with the present disclosure; 
           [0018]      FIG. 3  is a schematic diagram of an enlarged view of the image splitting and combining section of  FIG. 2 ; 
           [0019]      FIGS. 4A and 4B  are drawings illustrating distorted side-by-side images as displayed on LC panels ( 4 A) and anamorphic superposition on a screen ( 4 B), in accordance with the present disclosure; 
           [0020]      FIG. 5  is a schematic diagram illustrating another embodiment of a stereoscopic projection system, in accordance with the present disclosure; 
           [0021]      FIG. 6  is a schematic diagram illustrating another embodiment of a stereoscopic projection system, in accordance with the present disclosure; 
           [0022]      FIG. 7  is a schematic diagram of an enlarged view of the image splitting and combining section of  FIG. 6 ; 
           [0023]      FIG. 8  is a schematic diagram of an enlarged view of another exemplary embodiment of an image splitting and combining section, in accordance with the present disclosure; 
           [0024]      FIG. 9  is a schematic diagram of a top down view of an embodiment of a stereoscopic projection system, in accordance with the present disclosure; 
           [0025]      FIG. 10  is an illumination footprint diagram at a screen for a projection lens with and without cylindrical elements, in accordance with the present disclosure; 
           [0026]      FIG. 11  is a schematic diagram illustrating an alternative technique for enhancing image brightness with cylindrical elements by anamorphically stretching an image to produce a brighter on-screen image, in accordance with the present disclosure; 
           [0027]      FIG. 12  is a schematic ray trace diagram illustrating a technique for converting a spatially multiplexed 3D projection system to a non-multiplexed full resolution 2D system, in accordance with the present disclosure; 
           [0028]      FIG. 13  is a schematic ray trace diagram illustrating another example of a technique for converting the optical system from 3D mode to 2D full resolution mode, in accordance with the present disclosure; 
           [0029]      FIG. 14  is a schematic diagram illustrating an embodiment of a system with an external anamorphic converter lens located in the light path after the projection lens, in accordance with the present disclosure; 
           [0030]      FIG. 15  is a schematic diagram illustrating another embodiment of a system with an external anamorphic converter lens, in accordance with the present disclosure; 
           [0031]      FIG. 16  is a schematic diagram illustrating another embodiment of a stereoscopic projection system, in accordance with the present disclosure; 
           [0032]      FIG. 17  is a schematic diagram illustrating another embodiment of a stereoscopic projection system, in accordance with the present disclosure; 
           [0033]      FIG. 18  is a close-up view of the image splitting and combining assembly in  FIG. 17 ; 
           [0034]      FIG. 19  is a schematic diagram of an enlarged view of another exemplary embodiment of an image splitting and combining section, in accordance with the present disclosure; and 
           [0035]      FIG. 20  is a schematic diagram illustrating another embodiment of a stereoscopic projection system, in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]      FIG. 1A  is a schematic block diagram of an exemplary projection system  100  in a stereoscopic projection mode, and  FIG. 1B  is a schematic block diagram of an exemplary projection system  100  in a non-stereoscopic projection mode.  FIGS. 1A and 1B  illustrate the principle that a stereoscopic module  150  or a non-stereoscopic module  180  may be selectively placed in a light path from a projector. A mechanical assembly  145  may contain both the stereoscopic module  150  and the nonstereoscopic module  180 , which may selectively slide back and forth between the stereoscopic mode and the nonstereoscopic mode according to whether the media type is stereoscopic imagery or not. Such a configuration may have practical application in cinematic environments, as well as home and office environments, requiring minimal technical skill from an operator to place in the stereoscopic or a nonstereoscopic mode. 
         [0037]    The exemplary projection system  100  includes a relay lens subsystem  130 , optional light directing element  140 , stereoscopic module  150 , non-stereoscopic module  180 , and projection lens subsystem  190 . Stereoscopic module  150  may include a light splitting subsystem  160  and a light combining subsystem  170 . Non-stereoscopic module  180  may include a 2D bypass subsystem  182 , which may have an optical path length similar to the stereoscopic module  150 . In an embodiment, the stereoscopic projection system  100  may also include an audio visual source  134 , a controller subsystem  132 , and a projection subsystem  110 . The projection subsystem  110  may include, but is not limited to, an LC projection system or a DLP projection system. 
         [0038]    Although an exemplary multi-mode stereoscopic/nonstereoscopic system is shown herein, it should be apparent that this disclosure is not limited to a multi-mode system. For example, the exemplary stereoscopic projection system architecture shown herein may be applied to a stereoscopic-only projection system that omits the nonstereoscopic module  180 . 
         [0039]    Referring to  FIG. 1A , in a stereoscopic mode of operation, the audio visual source  134  provides an audio visual signal  135  to the stereoscopic projection system  100 . The controller subsystem  132  may transmit a stereoscopic video signal  137  to the projection subsystem  110 . The projection subsystem  110  projects an image pair at the input light path  112 . The relay lens subsystem  130  receives the input light path  112  and outputs intermediate light  114 . An optional light directing element  140  may direct the intermediate light  114  toward the light splitting subsystem  160  in the stereoscopic module  150 . The light splitting subsystem  160  receives the intermediate light path  114  and outputs light on a first image light path  116  and on a second image light path  118 . The light combining subsystem combines the light from first and second image light paths  116 ,  118  and directs substantially overlapping first and second image light having orthogonal polarization states toward the projection lens subsystem  190 , which is focused on a screen  195 . Various different optical architectures are presented herein illustrating exemplary stereoscopic modules. 
         [0040]    Referring to  FIG. 1B , illustrating a nonstereoscopic mode of operation, the non-stereoscopic module  180  may be placed in the light path from the light directing element  140 , thus the 2D bypass subsystem  182  directs the intermediate light path  114  toward the projection lens subsystem  190 . Various different optical architectures are presented herein illustrating exemplary non-stereoscopic modules. 
         [0041]    In an embodiment, common to both modes of operation, controller subsystem  132  receives the audio visual signal  135  and outputs a control signal  136 . The controller subsystem  132  may be operatively coupled to the various subsystems, as shown. Controller subsystem  132  is operable to send control signals and receive feedback signals from any one of the various operatively coupled subsystems to adjust their respective optical characteristics. The controller subsystem  132  may take input from sensors, from the audio visual source  134 , and/or from user input to make adjustments (e.g., to focus or calibrate the stereoscopic projection equipment on screen  195 ). The controller subsystem  132  may also control and/or drive an actuator  145  that moves the stereoscopic/non-stereoscopic modules  150 / 180  between stereoscopic and non-stereoscopic configuration modes. Such an actuator  145  may be a precise driving mechanism known to those of ordinary skill in the art, such as a stepper motor, and the like. 
         [0042]    In another embodiment, the system  100  is a passive system and does not include active switching/control components. Thus, in such an embodiment, the system  100  does not include a control signal  136 . 
         [0043]    The relay lens subsystems (e.g.,  130  in  FIG. 1 , et cetera) disclosed herein are assumed to be polarization-preserving and are operable to work in parallel with the projection lens subsystem (e.g.,  190  in  FIG. 1 , et cetera) to provide approximately panel-sized intermediate images at a modest distance from the lens output. Although the relay lens subsystem is assumed to be a black box for all embodiments and its design is not specific to the disclosures herein, examples of relay systems may be found in commonly-assigned patent application Ser. No. 12/118,640, entitled “Polarization conversion system and method for stereoscopic projection,” filed May 9, 2008, which is herein incorporated by reference. In a similar manner, the projection optics used to relay the intermediate images onto the screen are assumed conventional and specific designs are not provided since they are not germane to the disclosure. In some embodiments, a polarization preserving projection lens may be used. An example of a polarization preserving lens is discussed by L. Sun et al. in  Low Birefringence Lens Design for Polarization Sensitive Systems , Proc. SPIE Vol. 6288, herein incorporated by reference. 
         [0044]    The polarization aspects of the disclosure generally include conditioning the light for efficient splitting and encoding of output images. Electronic aspects may generally include pre-distorting the images to accommodate optical aberrations and allow anamorphic imaging techniques to preserve aspect ratio of the original panel when only half of the area is allocated to a full screen image. Generally, electronic alignment techniques may be used for on-screen image alignment. Optical aspects of the disclosure generally cover techniques of physically separating optical paths for each of the left and right eye images (e.g., the light splitting subsystem  160  in  FIG. 1 ). In an embodiment, this splitting architecture is extended to enable superposition of the left and right eye images prior to projection. 
         [0045]    In an embodiment, it is assumed that the projection subsystem  110  provides circular polarized light with green light having the opposite handedness to red and blue. This is typical of three panel liquid crystal projectors that use a combining X-cube. The color dependent linear polarizations emanating from this element are routinely transformed into circular polarization to avoid back reflections from the projection lens which may affect ANSI contrast. The precise allocation of left handed or right handed polarization to the odd green wavelengths is arbitrary, but may be pre-conditioned correctly. It is assumed here that effective correction may use a crossed matching retarder, as this is the case for most commercial projectors on the market. Though geared toward the mixed circular output, the system embodiments should not be limited to the precise polarization states assumed to emanate from the projector. The concepts covered here can be applied to alternative projectors (e.g., DLP, etc.) since the creation of equivalent entrance polarizations can be easily provided by available components. For instance, ColorSelect® technology may map between defined wavelength dependent polarization states, and are described in commonly-assigned U.S. Pat. No. 5,751,384, herein incorporated by reference. 
         [0046]      FIG. 2  is a schematic diagram of an embodiment of a stereoscopic projection system  200 . Generally, the system  200  may include a projection subsystem  210 , relay lens  230 , light directing element  240 , stereoscopic subsystem  250 , and a projection lens  290 . In this exemplary system  200 , stereoscopic subsystem  250  may include light splitting subsystem  260 , first and second light directing elements  262 ,  264 , and light combining subsystem  270 . Light combining subsystem  270  may include a polarization beam splitter (PBS)  272  and an achromatic rotator  242  located on an input port of PBS  272 . The system  200  may also include matched waveplates  222   a,    222   b  and, wavelength-selective polarization filter  224  (e.g., a ColorSelect filter as taught in Pat. Nos. 5,751,384 and 5,953,083, herein incorporated by reference), both arranged as shown, located in the light path between projection subsystem  210  and relay lens  230 . Additionally, the system may include light directing element  240  to direct light from relay lens subsystem  230  toward the stereoscopic subsystem  250 . 
         [0047]    In operation, the relay lens subsystem  230  receives light from the projection subsystem  210  at the input light path  212 . In an embodiment, matched waveplates  222   a ,  222   b  and wavelength-selective polarization filter  224  are positioned on the input light path  212  between the projection subsystem  210  and the relay lens subsystem  230 . Alternatively, matched waveplates  222   a,    222   b  may be positioned between the relay lens subsystem  230  and the image splitting element  260 , near the intermediate image plane  255 . As another alternative, a first matched waveplate  222   a  is positioned between the projection subsystem  210  and the relay lens subsystem  230  (as shown) and a second matched waveplate  222   b  is positioned between the relay lens subsystem  230  and the light splitting element  260 , near the intermediate image plane  255 . The relay lens subsystem  230  outputs an intermediate light path  214  toward a light directing element  240 , that directs the light  214  toward an intermediate image plane  255  at the input of the light splitting element  260 . It should be noted that wherever the waveplate  222   b  is placed in the optical path, the wavelength selective filter  224  will follow it somewhere downstream in the following light path, before reaching the light combining subsystem  270 . 
         [0048]    Light directing element  240  is located in the light path  214  after the relay lens subsystem  230 . Light directing element  240  may be a fold mirror (as shown here) or a prism. The light directing element  240  redirects the light path  214  such that the optical axis of the projection lens subsystem  290  is parallel to the optical axis of the relay lens subsystem  230 . This improves system compatibility with existing projection engines and theater geometries. 
         [0049]    The light splitting subsystem  260  may be provided by highly reflective silver mirrors that are polarization preserving or a prism with mirrored or TIR surfaces. The light splitting subsystem  260  may alternatively be provided by any other device that can split the light, for example circularly polarizing optical gratings may be used. The light splitting element  260  is operable to split the intermediate light path  214  into a first image light path  216  and a second image light path  218 . In an embodiment, the first and second light directing elements  262 ,  264  includes first and second mirrors configured to reflect their respective first and second image light paths  216 ,  218  toward first and second input ports of light combining subsystem  270 . The PBS  272  is operable to combine the first and second image light paths  216 ,  218  into a third image light path  219 . The projection lens subsystem  290  receives the light on the third image light path  219  and projects output image light  292  toward a screen (not shown). 
         [0050]    The exemplary system  200  includes superposition of oppositely polarized left- and right-eye image paths (e.g., first and second image light paths  216 ,  218 ) carried out at the interface of a PBS  272  before being projected by a single lens  290 . By encoding the two images with orthogonal polarizations and directing them symmetrically into a polarizing beam splitting element  272  the two images appear to emanate from the same plane. A single polarization preserving projection lens  290  can then project the images onto a screen. 
         [0051]    In some embodiments, the polarization rotator element  274  may introduce an optical path mismatch which may in practice be matched with dummy material at the other input port to the PBS  272 . 
         [0052]    “Wobulation” is a technical term for spatially dithering an image to increase the perceived quality of the image. Spatial dithering involves presenting an image at one instance in time, and presenting a spatially shifted image the next instance in time. The spatial shift is typically a fraction of a pixel. The images from one instance to the next may be the same (for a smoother overall image), or they may be different (for a smoother and sharper image). Methods for implementing wobulation include vibrating a mirror in the optical path (e.g., light directing element  240 ) in synchronization with the two instances of the images as discussed in U.S. Pat. No. 7,330,298 to Bommerbach et al, which is herein incorporated by reference for all purposes. The mirror vibration is modulated to produce an offset in one image that is generally a fraction of a pixel relative to the other image. Another method is to use birefringent materials coupled with switching liquid crystal elements to induce the image shift, as discussed in U.S. Pat. No. 5,715,029 to Fergason, which is herein incorporated by reference for all purposes. 
         [0053]    In an embodiment, the stereoscopic projection systems discussed above are altered to include wobulation. For example, in  FIG. 2 , the light directing element  240  may be vibrated in synchronization with the image data to present spatially shifted images on-screen. Alternatively or additionally, an LC cell and birefringent plate may be added to the optical path prior to image splitting, to enable wobulation of both images together. An additional LC cell may be placed after the birefringent plate (prior to the PBS  272 ) to restore the desired orthogonal polarization states. Alternatively, the LC cells and birefringent plate may be added after image splitting (prior to the PBS  272 ) to enable wobulation separately for each image. Again, the additional LC cell would be used after the birefringent plate to restore the desired orthogonal polarization states prior to recombining the beams in the PBS  272 . 
         [0054]      FIG. 3  is a schematic diagram of an enlarged view of the image splitting and combining section of  FIG. 2 , including matched waveplate  222   b,  wavelength-selective filter  224 , light directing element  240 , light splitting subsystem (mirrored prism)  260 , light directing elements  262 ,  264 , a PBS  272 , and a rotator  274  for superimposing images. In this exemplary embodiment, the matched waveplate  222   b  and a Green/Magenta (G/M) wavelength-selective (Colorselect) filter  224  follow the relay lens  230  in a light path to provide higher contrast by substantially removing relay lens ghost reflections. The diagram also illustrates a ray trace analysis of the system. 
         [0055]    The light directing element (mirror)  240  is placed at an angle such that the projection lens (not shown) and relay lens optical axes are parallel. A mirrored prism  260  is placed at the intermediate image location  255  to split the two halves of the intermediate image. A mirrored prism  260  with very sharp corners (e.g., ˜50 um in width) may be selected to minimize the unusable area at the intermediate image. A V-mirror arrangement (the combination of two flat mirrors) might also be utilized for the image splitting subsystem  260 . Following the mirrored prism  260  are two folding mirrors  262 ,  264 . The two folding mirrors  262 ,  264  redirect the rays into the entrance surfaces of the PBS  272 . Prior to the PBS  272 , a rotator  274  is located in one path while an isotropic plate  276  (matched in optical thickness to the rotator) is placed in other path. The rotator  274  rotates one of the incident polarization states by 90 degrees such that the two states become orthogonal. A PBS  272  combines the two orthogonal polarization states along the same optical path prior to the projection lens (not shown). The polarizing beam splitter  272  is shown as a cube polarizing beam splitter. The PBS surface can include of dielectric coating layers, or a wire grid polarizer. Additionally, the PBS may be implemented with a plate in place of the cube, where the plate is coated with appropriate dielectric layers or wire grid coating. However, in this case the beam is diverging, and thick plates will induce astigmatism in the image path which may be corrected later in the system. 
         [0056]      FIGS. 4A and 4B  are drawings illustrating distorted side-by-side images as displayed on LC panels ( 4 A) and their anamorphic superposition on a screen ( 4 B). Though drawn side-by-side, this embodiment may also cover over and under formats. 
         [0057]    Anamorphic imaging could be carried out in the relay lens subsystem to provide an intermediate image with correct aspect for each of the left or right eye images. In this case, distortion expected in the complex relay system may utilize electronic correction, or relative inversion of the paired images about the optical axis. Rotation of one of the images would then be performed with use of rotating separating prisms, as discussed by L. Lipton in  Foundations of the Stereoscopic Cinema  referenced above. 
         [0058]    Another related embodiment uses non-ideal separating mirrors in which the geometry would dictate polarization mixing, particularly if using a total internal reflection (TIR) prism for redirecting circular polarized beams. For smaller systems, a TIR prism is preferred over mirrors for its higher reflectivity and smaller physical size. Its imparted phase delay on reflection between s and p polarization components rapidly transform polarization into a propagation dependent state. This leads in general to projected image non-uniformity that may be corrected by introducing intensity and bit depth loss. To reduce this problem to an acceptable level, linear polarization states can be created prior to entering the system. To a great extent, polarization is preserved since these states would closely resemble the s or p Eigen-states for the majority of rays present in the imaging system. 
         [0059]      FIG. 5  is a schematic diagram of an embodiment of a stereoscopic projection system  500  in which a delta prism  540  is used as a light directing element, to provide a more compact system. Generally, the system  500  may include a projection subsystem  510 , relay lens  530 , and projection lens  590 . The system  500  may also include matched waveplates  522   a,    522   b  and, wavelength-selective polarization filter  524 . 
         [0060]    A delta prism  540  includes a triangular prism, with one face  542  coated with a mirror coating. Light enters a transmissive face  544 , travels to the second transmissive face  546 , and totally internally reflects (TIRs) at the second transmissive face  546 . The reflected light then travels to the mirrored face  542 , reflects, and travels to the first transmissive face  544 . The light again TIRs at the input face  544  and travels to the second transmissive face  546 . The angles and refractive index of the prism are designed such that the light will exit the second face  546  on this pass. In this case, the light is now incident on the light splitting subsystem  560  at 45 degrees to the optical axis of the relay lens  530 , the same as in the case of the mirror system in  FIG. 2 . The delta prism  540  provides a more compact solution than the mirror  240  of  FIG. 2 . The delta prism  540  is desired to have low birefringence for efficiency and include anti-reflection coatings for the input face  544  and output face  546 . 
         [0061]    Wobulation is enabled in this exemplary embodiment by rotating the prism  540  about the optical axis of the relay lens  530 . This rotation induces a shift in image location on the screen. Alternatively, wobulation of each image might be enabled by vibrating the two re-directing mirrors  562 ,  564  prior to the PBS  572 . 
         [0062]      FIG. 6  is a schematic ray tracing diagram of projection system  600  showing the relay lens subsystem  630 , delta prism  640 , stereoscopic subsystem  650 , and projection lens subsystem  690 . The stereoscopic subsystem  650  includes a mirrored splitting prism  660 , re-directing mirrors  662 ,  664 , and the PBS  672  for combining the optical paths. The matched waveplate  622   b,  wavelength-selective filter  624 , and rotator  674  are also included for supporting the system operation. 
         [0063]      FIG. 7  is a schematic ray trace diagram illustrating an enlarged view of the stereoscopic subsystem  650  of  FIG. 6 . The stereoscopic subsystem  650  includes the mirrored splitting prism  660 , mirrors  662 ,  664 , PBS  672 , and rotator element  674 , as described above with reference to  FIG. 6 . As shown in this example, the matched waveplate  622   b  and wavelength-selective (or G/M) filter  624  are included after the relay lens  630 . Polarized light enters and exits the delta prism  640  with substantially the same polarization. The mirrored prism  660  splits the image, and the two flat mirrors  662 ,  664  redirect the images to the PBS  672 . Prior to the PBS  672 , a rotator  674  changes the polarization of one path while an isotropic plate  676  maintains the polarization in the other path. The PBS  672  combines the two paths into one path prior to projection. Again, wobulation may be enabled by rotating the delta prism  640  and/or vibrating the two redirection mirrors  662 ,  664 . 
         [0064]      FIG. 8  is a schematic diagram of another embodiment of a light directing element  840  and light splitting and combining subsystem  860 . Light splitting and combining subsystem  860  may include delta prisms  862 ,  864 , PBS  872 , and optional isotropic plate  876  and rotator  874 . In this exemplary embodiment, two delta prisms  862 ,  864  replace the reflective surfaces in the embodiment discussed in relation to  FIG. 7  ( 660 ,  662 ,  664 ). An optional rotator  854  may also be included for compensating skew ray phase differences induced in the sets of delta prisms. Use of the delta prisms  840 ,  862 ,  864 , as opposed to the redirection mirrors, result in compacting the system. This allows a rotator  854  to be inserted between delta prism  840  and the two following delta prisms  862 ,  864 . A rotator  854  allows for near perfect compensation of phase errors induced by geometry effects of skew rays in the delta prisms  840 ,  862 ,  864 . Since the prisms all have substantially same geometry relative to the ray paths, a rotator  854  between the prisms will optimally compensate for the skew ray polarization effects. Wobulation, in this case, is enabled by rotating the first delta prism  840  and/or rotating each of the following delta prisms  862 ,  864 . 
         [0065]      FIG. 9  is a schematic ray trace diagram of a top down view of an embodiment of a stereoscopic projection system  900 . This embodiment includes a relay lens  930 , light directing element  940 , light splitting subsystem  960 , light combining subsystem  970 , and projection lens  990 . The projection lens  990  includes cylindrical elements for enabling anamorphic imaging. Cylindrical elements have been included in the projection lens  990  to produce an anamorphically compressed image at the screen, as disclosed in U.S. Pat. No. 3,658,410 to Willey, which is herein incorporated by reference for all purposes. 
         [0066]      FIG. 10  is an illumination footprint diagram at the screen for the projection lens with and without the cylindrical elements. Cylindrical elements enable anamorphic functionality. Region  1002  includes anamorphic elements and regions  1004  are without anamorphic elements. In an embodiment, in the case of inclusion of the cylindrical elements, the overall screen brightness is estimated to be approximately 57.5% brighter due to the addition of the anamorphic functionality when compared to the standard projection lens case. The anamorphic elements alter the aspect ratio of the projected image on screen. 
         [0067]      FIG. 11  is a schematic diagram illustrating an alternative technique for enhancing image brightness with cylindrical elements by anamorphically stretching an image to produce a brighter on-screen image. The image is stretched in the vertical direction to substantially fill the existing screen area. Regions  1102  are the regions of anamorphic stretching. Stretching in the vertical direction allows the same projection lens to be utilized for 2D and 3D presentations. 
         [0068]      FIG. 12  is a schematic ray trace diagram illustrating a technique for converting a spatially multiplexed 3D projection system to a non-multiplexed full resolution 2D system. In this exemplary embodiment, the splitting and recombining optics near the aperture stop are removed from the optical path, and the projection lens  1290  is pivoted such that it is parallel with the relay lens  1230  optical axis. The projection lens  1290  may be moved such that the intermediate image  1255  is located near the back focal length of the projection lens  1290 . The projection lens  1290  can then be focused and zoomed for proper presentation. 
         [0069]      FIG. 13  is a schematic ray trace diagram illustrating another example of a technique for converting the optical system from 3D mode to 2D full resolution mode. In this embodiment, a portion of the splitting and recombining optics are removed (the mirrored prism, mirrors, and PBS) and a 2D bypass subsystem  1380  is inserted into the optical path. In this example, the 2D bypass subsystem  1380  is a second delta prism. The second delta prism  1380 , in combination with the first delta prism  1340 , vertically shifts the optical axis of the light path coming from the relay lens  1330  to align with the projection lens  1390  optical axis. The projection lens  1390  may move along the optical axis to re-focus the image. The prisms do not have to be delta prisms; rather, prisms or mirrors that redirect the optical axis at approximately 45 degrees may be used. Alternative prisms include the TIR prism type shown in  FIG. 16 . 
         [0070]      FIG. 14  is a schematic ray trace diagram illustrating an embodiment of a system  1400  with an external anamorphic converter lens  1495  located in the light path after the projection lens  1490 , see, e.g., U.S. Pat. No. 5,930,050 to Dewald (the magnification in  FIG. 14  has opposite polarity to Dewald). 
         [0071]      FIG. 15  is a schematic ray trace diagram illustrating another embodiment of a system  1500  with an external anamorphic converter lens  1595 . As shown, for 3D operation, the anamorphic converter  1595  (i.e. an anamorphic afocal converter) is put in place after the projection lens  1590  to produce a brighter 3D image from the multiplexed panel. For 2D full resolution operation, the anamorphic converter  1595  may be removed from the optical path to allow the non-multiplexed full 2D panel resolution to be presented without anamorphic distortion.  FIG. 14  depicts an anamorphic converter with magnification 0.5× in the horizontal direction, while  FIG. 15  depicts an anamorphic converter with magnification 2× in the vertical direction. 
         [0072]      FIG. 16  is a schematic ray trace diagram illustrating another embodiment of a stereoscopic projection system  1600 . System  1600  further includes a Bravais subsystem  1632  implemented at the output of the relay lens  1630  and prior to the image splitting subsystem  1660 . The TIR prism  1640  has been changed to accommodate the non-telecentric ray bundles emerging from the Bravais  1632 . An optional cylindrical field lens  1636  is also shown for creating telecentric bundles, should a telecentric projection lens be used. The anamorphic stretch has been implemented in the vertical direction, allowing the same projection lens  1690  to be utilized for both 2D and 3D presentations with little or no change in projection lens zoom setting. 
         [0073]    Bravais optical systems have been utilized to provide anamorphic stretch or compression along one direction of an image as disclosed by W. Smith in  Modern Optical Engineering , p. 272, McGraw-Hill 1990 (describing the use of Bravais optics in motion pictures work), which is herein incorporated by reference for all purposes. Bravais systems comprise a positive and negative cylindrical element separated by a finite distance and located in the finite conjugate of a lens system. 
         [0074]    A Bravais system might be inserted near the panel, close to the relay lens output, or close to the projection lens input. The polarization and color management optics make inserting Bravais optics near the panel difficult. The Bravais system shortens the projection lens back focal length (BFL), and a long BFL is preferred for inserting the PBS, splitting prism, and mirrors. 
         [0075]      FIG. 17  is a schematic ray trace diagram illustrating another embodiment of a stereoscopic projection system  1700  that is similar to the embodiment shown in  FIG. 16 , with a difference being that the mirrored prism and folding mirrors have been replaced with more compact delta prisms  1760 . System  1700  also includes a Bravais subsystem  1732  implemented at the output of the relay lens  1730 . 
         [0076]      FIG. 18  is a close-up view of the image splitting and combining assembly in  FIG. 17 , specifically showing a close-up view of the paraxial Bravais system  1732 , filter  1714 , field lens  1736 , and prisms  1764 ,  1762 . The Bravais anamorphic lens  1732  (depicted as a paraxial lens) follows the relay lens. A matched quarter-wave plate  1722   b  and wavelenth-selective filter  1714  follow the Bravais  1732 . A TIR turning prism  1734  is next, followed by a cylindrical field lens  1736  to provide for telecentricity at the intermediate image. Two modified delta prisms  1762 ,  1764  follow the cylindrical field lens  1736 . The delta prisms  1762 ,  1764  have cut corners near the intermediate image to facilitate image splitting while maintaining clear aperture through the prism for the marginal rays. A PBS  1772  follows the two delta prisms and combines the images. A rotator  1774  is included in one of the optical paths after the delta prisms, and optional cleanup polarizers (not shown) may also be implemented between the delta  1762 ,  1764  and PBS  1774 . The entire assembly is compact and affords a small back focal length in the projection lens. This aids in reducing cost and/or improving performance of the projection lens. 
         [0077]      FIG. 19  is a close-up view of another image combining assembly that is similar to the embodiment shown in  FIG. 18 , but is adapted for non-anamorphic systems (i.e. systems without the Bravais anamorphic lens). This embodiment includes filter  1914 , matched quarter-wave plate  1922   b,  TIR prism  1934 , prisms  1964 ,  1962 , PBS  1972 , and rotator  1774 . In this embodiment, the cylindrical field lens  1736  of  FIG. 17  is not included, and may be utilized in non-anamorphic systems (i.e. systems without the Bravais anamorphic lens). 
         [0078]      FIG. 20  is a schematic ray trace diagram illustrating another embodiment of a 3D lens system  2000 . In this embodiment, an anamorphic telecentric relay lens  2099  is inserted between the standard relay lens  2030  and the projection lens  2090 . The anamorphic relay lens  2099  creates a real image of the intermediate image produced by the standard relay lens  2030 . The projection lens  2090  then projects an image of the anamophic relay lens&#39;s  2099  image onto the screen  2095 . 
         [0079]    In this embodiment, the anamorphic telecentric relay lens  2099  may be a telecentric relay lens with an afocal anamorphic converter located near its aperture stop. The afocal anamorphic converter may be an afocal converter implemented with cylindrical lenses. The cylindrical lenses may change the magnification of the relay in one aspect (e.g. 2× magnification vertically) while having a unity magnification in the orthogonal aspect (e.g. 1× magnification horizontally). In any of the anamorphic implementations, the magnification in each aspect may be different to be considered anamorphic (i.e. the aspects can be magnifications other than unity magnification). If both aspects have magnification not equal to 1, then toric elements are desirable in the converter, or multiple cylindrical elements which have orthogonal axes of rotation may be used. The anamorphic relay is preferably telecentric to maintain light throughput and contrast. The telecentric anamorphic relay lens  2099  is shown between the prism assembly  2060  and projection lens  2090  in this exemplary embodiment, but it may also be implemented between the standard relay lens  2030  and prism assembly  2060 . 
         [0080]    Note that in this embodiment a cylindrical field lens is not included at the first intermediate image. When the anamorphic converter is placed near the aperture stop of a lens, it is operating on collimated beams, an advantage in terms of aberration correction. Telecentricity can thus be maintained without the use of a field lens. Additionally, the anamorphic converter may be implemented near the aperture stop of the first relay lens or the projection lens, moving the anamorphic function to one of those locations, which may allow for the lack of the anamorphic telecentric relay. An advantage of a system utilizing the anamorphic telecentric relay is that the anamorphic telecentric relay may be removed, and the system may operate with equal magnification in all directions (e.g. for 2D presentation using the full panel resolution). U.S. Pat. No. 6,995,920 describes a telecentric anamorphic relay lens for use with camera (image taking) lenses, and is herein incorporated by reference. 
         [0081]    It should be appreciated that a Bravais anamorphic lens may be added to the various embodiments disclosed herein in order to improve the lumen output of the system. The Bravais can be placed after the relay lens and before the splitting prisms. The Bravais magnifies the intermediate image by 2× in the vertical direction and 1× in the horizontal direction, allowing the full panel size to be utilized in 3D mode. If the Bravais is removed, and the splitting prisms and projection lenses are translated vertically such that the entire intermediate image passes through a single TIR prism and single projection lens, the full resolution image from the panel can be utilized for 2D presentations. 
         [0082]    Additionally, it should be appreciated that external anamorphic afocal converters may be applied to the various embodiments disclosed herein in order to improve the lumen output of the system. Such external anamorphic converters can be located after the projection lenses. Alternatively, the projection lenses themselves may be made anamorphic (e.g. as a single projection lens is made anamorphic in U.S. Pat. No. 5,930,050, herein incorporated by reference) to improve the lumen output. 
         [0083]    While various embodiments in accordance with the disclosed principles have been described above, it should be understood that they have been presented by way of example only, and are not limiting. Thus, the breadth and scope of the invention(s) should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages. 
         [0084]    Additionally, the section headings herein are provided for consistency with the suggestions under 37 C.F.R. 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” such claims should not be limited by the language chosen under this heading to describe the so-called technical field. Further, a description of a technology in the “Background” is not to be construed as an admission that technology is prior art to any invention(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the invention(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple inventions may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the invention(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings herein.