Abstract:
Proposed are various embodiments of projection systems that generally provide stereoscopic images. The projection systems act to split a spatially separated image in a stereoscopic image frame and superimpose the left- and right-eye images on a projection screen with orthogonal polarization states. The embodiments are generally well suited to liquid crystal polarization based projection systems and may use advanced polarization control.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation application of U.S. patent application Ser. No. 12/629,017, entitled “Stereoscopic projection systems for employing spatial multiplexing at an intermediate image plane,” to Schuck et al., filed Dec. 1, 2009, which is herein incorporated by reference and which relates and claims priority to: 1) provisional patent application 61/119,014, entitled “Methods and systems for stereoscopic projection,” to Robinson et al., filed Dec. 1, 2008; 2) provisional patent application 61/249,018, entitled “Stereoscopic projection system employing spatial multiplexing at an intermediate image plane,” to Schuck et al., filed Oct. 6, 2009; and 3) provisional patent application 61/256,854, entitled “Stereoscopic projection system employing spatial multiplexing at an intermediate image plane,” to Schuck et al., filed Oct. 30, 2009, each of which are also herein incorporated by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The disclosed embodiments generally relate to stereoscopic projection systems and, more specifically, relate to stereoscopic projection systems that output polarization encoded left and right eye images. 
       BACKGROUND 
       [0003]    Stereoscopic projection dates back to the early 20′ century and was first seen in cinemas during the 1950s. These systems were film based and were limited mechanically to modest ˜24Hz 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. 
         [0004]    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 
       [0005]    Disclosed are stereoscopic projection systems and methods for stereoscopic projection. 
         [0006]    Generally, according to an aspect, the stereoscopic projection systems may include a relay lens subsystem, a light splitting 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. The stereoscopic image frame may include 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 and second image area light, 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 projection lens subsystem is operable to direct the first and second image area light toward a screen. 
         [0007]    According to another aspect, a method of stereoscopic projection may include optically receiving a stereoscopic image frame from a projector, splitting the first image area light from the second image area light, directing the first image area light on a first light path, directing the second image area light on a second light path, and focusing the first and second light path light toward a screen. The stereoscopic image frame includes first image area light and second image area light. In another aspect, the first image area light substantially overlaps with the second image area light. 
         [0008]    Other aspects of the disclosure will be apparent with reference to the detailed description, the drawings, and the appended claims. 
     
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram of a stereoscopic projection system, in accordance with the present disclosure; 
           [0010]      FIG. 2  is a block diagram of a stereoscopic projection system, in accordance with the present disclosure; 
           [0011]      FIG. 3  is a schematic diagram of a stereoscopic projection system, in accordance with the present disclosure; 
           [0012]      FIG. 4A  is a schematic diagram of left/right side-by-side images, in accordance with the present disclosure; 
           [0013]      FIG. 4B  is a schematic diagram of the left/right side-by-side images anamorphically superpositioned on a screen, in accordance with the present disclosure; 
           [0014]      FIG. 5  is a schematic diagram of a stereoscopic projection system, in accordance with the present disclosure; 
           [0015]      FIG. 6  is a schematic diagram of a stereoscopic projection system, in accordance with the present disclosure; 
           [0016]      FIG. 7  is a schematic diagram of a stereoscopic projection system, in accordance with the present disclosure; 
           [0017]      FIG. 8  is a schematic diagram of a stereoscopic projection system, in accordance with the present disclosure; 
           [0018]      FIG. 9  is a schematic diagram of a stereoscopic projection system, in accordance with the present disclosure; 
           [0019]      FIG. 10  is a schematic diagram of a stereoscopic projection system, in accordance with the present disclosure; and 
           [0020]      FIG. 11  is a schematic diagram of a stereoscopic projection system, in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    To address the above shortcomings of using a single LC projector, a separate frame dividing subsystem in conjunction with sophisticated polarization, electronic and optical management may be used. Accordingly, the stereoscopic frame dividing system embodiments disclosed herein generally include three parts: a relay subsystem that forms a real intermediate image; a light splitting subsystem that separates two areas of the real image; and a projection subsystem which creates overlapping screen images with opposite polarizations. 
         [0022]      FIG. 1  is a schematic diagram of a stereoscopic projection system  100 . In an embodiment, the system  100  includes a relay lens subsystem  106 , light splitting subsystem  108 , and projection subsystem  110 . 
         [0023]    In operation, the system  100  receives light at the relay lens subsystem  106  from a projection subsystem  120  at the input light path  104 . The projection subsystem  120  may include, but is not limited to, an LC projection system or a DLP projection system. The light splitting subsystem  108  receives light from the relay lens subsystem  106  at the intermediate light path  107 . The light splitting subsystem is operable to split the intermediate light path  107  into a first image light path  105  and a second image light path  109 . The projection lens subsystem  110  receives light from the light splitting subsystem  108  at the first and second image light paths  105 ,  109  and outputs light to a screen  130 . 
         [0024]    In an embodiment, the projection lens subsystem  110  includes a first and second projection lens  111 ,  112 . The first and second projection lens  111 ,  112  output a first and second output light path  115 ,  119  onto the screen  130 . 
         [0025]    In another embodiment (not shown), the projection lens subsystem  110  may include a single lens. In another embodiment (not shown), the projection lens subsystem  110  may include more than two lenses. 
         [0026]      FIG. 2  is a block diagram of a stereoscopic projection system  200 . The stereoscopic projection system  200  includes a relay lens subsystem  202 , light splitting subsystem  204 , and a projection lens subsystem  206 . In an embodiment, the stereoscopic projection system  200  may also include an audio visual source  220 , a controller subsystem  222 , and a projection subsystem  224 . The projection subsystem  224  may include, but is not limited to, an LC projection system or a DLP projection system. 
         [0027]    In operation, the audio visual source  220  provides an audio visual signal  221  to the stereoscopic projection system  200 . The controller subsystem  222  sends the signal to the projection subsystem  224 . The projection subsystem  224  projects an image pair at the input light path  201 . The relay lens subsystem  202  receives the input light path  201  and outputs an intermediate light path  203 . The light splitting subsystem  204  receives the intermediate light path  203  and outputs a first image light path  205  and a second image light path  209 . The projection lens subsystem  206  receives the first and second image light paths  205 ,  209 . 
         [0028]    In an embodiment, the projection lens subsystem  206  includes a first projection lens  207  and a second projection lens  208 . The first projection lens  207  outputs a first output light path  215  onto a screen  230 . The second projection lens outputs a second output light path  219  onto the screen  230 . 
         [0029]    In another embodiment, the projection lens subsystem  206  may include an image combining element (not shown) and a single projection lens (not shown). The image combining element (not shown) receives the first and second image light paths  205 ,  209  and the single projection lens (not shown) projects the combined image light paths onto the screen  230 . 
         [0030]    In another embodiment, the projection lens subsystem  206  includes more than two projection lenses and may include at least one image combining element. 
         [0031]    In an embodiment, a controller subsystem  222  receives the audio visual signal  221  and outputs a control signal  223 . The controller subsystem  222  is operable to sync with the projection subsystem  224 , the relay lens subsystem  202 , the light splitting subsystem  204 , and the projection lens subsystem  206  via the control signal  223 . Controller subsystem  222  is operable to send control signals and receive feedback signals from any one of the various subsystems to adjust their respective optical characteristics. The controller may take input from sensors, from the audio visual source  220 , and/or from user input to make adjustments (e.g., to focus or calibrate the stereoscopic projection equipment on screen  230 ). 
         [0032]    In another embodiment, the system  206  is a passive system and does not include active switching components. Thus, in this embodiment, the system  206  does not include a sync signal. 
         [0033]    The relay lens subsystems (e.g.,  106  in  FIG. 1  or  202  in  FIG. 2 ) disclosed herein are assumed to be polarization-preserving and are operable to work in parallel with the projection lens subsystem (e.g.,  110  in  FIG. 1  or  206  in  FIG. 2 ) 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 (e.g., in embodiments with a single projection lens), a polarization preserving projection lens may be used. An example of a polarization preserving projection lens is discussed by L. Sun et al. in  Low Birefringence Lens Design for Polarkation Sensitive Systems,  Proc. SPIE Vol. 6288, herein incorporated by reference. 
         [0034]    The polarization aspects of the disclosure generally include conditioning the light for efficient splitting and encoding of output images. Electronic aspects 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  108  in  FIG. 1  or  204  in  FIG. 2 ). In an embodiment, this splitting architecture is extended to enable superposition of the left and right eye images prior to projection. 
         [0035]    In an embodiment, it is assumed that the projector 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. 
         [0036]      FIG. 3  is a schematic diagram of a stereoscopic projection system  300 . The stereoscopic projection system  300  may include a projector  302 , a relay lens  304 , an image splitting element  306 , a first projection lens  308 , and a second projection lens  310 . In this exemplary embodiment, the system may  300  also include mirrors  316  for redirecting the first and second image light paths  328 ,  326  such that they are parallel with the relay lens  304  and projection lenses  308 ,  310 . 
         [0037]    In operation, the relay lens  304  receives light from the projector  302  at the input light path  320 . The relay lens  304  outputs an intermediate light path  322  toward the image splitting element  306 . The image splitting element  306  is operable to split the intermediate light path  322  into a first image light path  328  and a second image light path  326 . The image splitting element  306  outputs the first and second image light paths  328 ,  326  toward the first and second projection lenses  308 ,  310 . The first and second projection lenses  308 ,  310  receive the first and second image light paths  328 ,  326  and output first and second output light paths  329 ,  330  toward the screen  350 . In this exemplary embodiment, a rotator  332  is positioned in the first output light path  329  between the first projection lens  308  and the screen  350 . The rotator  332  may comprise an achromatic polarization rotator element (e.g., an achromatic half wave plate oriented at 45°). The achromatic polarization rotator element  332  is operable to rotate the polarization of the first output light path  329  such that the first and second output light paths  329 ,  330  have opposite polarization. The left and right eye images associated with the two beams are then projected and superimposed onto a polarization preserving screen using separate lenses. 
         [0038]    In an embodiment, the input light path  320  relayed from the projector  302  may be circularly polarized image light. A circularly polarized intermediate image  334  relayed from the projector  302  may be optically split into two beams using a polarization preserving image splitting element  306  (e.g., highly reflective silver mirrors). The polarization of one beam is transformed using an achromatic polarization transformer  332 , such as an achromatic half wave plate oriented at 45°, into its orthogonal state. The left and right eye images associated with the two beams are then projected (at the first and second output light paths  329 ,  330 ) and superimposed onto a polarization preserving screen  350  using separate projection lenses  308 ,  310 . 
         [0039]    The advantages of the projection system  300  include minimal polarization management and minimal special optical hardware. With this approach, an image&#39;s magenta and green color components emanate from different beams and projection lenses. As such, the projection system  300  may use careful image convergence, such as using manual or electronic manipulation of the underlying image to obtain adequate image convergence. 
         [0040]    Using conventional optics, the projection system  300  may be inefficient. For example, the aspect ratios of the screen image to the separate left and right eye regions of the intermediate image may leave blank areas that nevertheless remain illuminated. This may be the case when the screen aspect closely matches the entire projection panel. To avoid this potential light loss, it may be better to distort the images on the projector&#39;s panels to fill the entire illuminated area and then to restore the required aspect ratio with anamorphic projection. 
         [0041]      FIG. 4A  is a schematic diagram of an effective side- by-side distorted image  400  as efficiently displayed on LC panels. Side-by-side distorted image  400  includes left eye image  402  and right eye image  404 . Though drawn side-by-side, it should be apparent to a person of ordinary skill in the art that this embodiment may also apply to over and under formats. 
         [0042]      FIG. 4B  is a schematic diagram of resulting superimposed images  450  achieved by parallel anamorphic projection of the left eye image  402  and the right eye image  404 . 
         [0043]    Referring back to  FIG. 3 , in an alternative embodiment, anamorphic imaging may be carried out in the relay lens  304  to provide an intermediate image  334  with correct aspect for each of the left or right eye images. In this case, distortion expected in the complex relay system may use electronic correction, or better, relative inversion of the paired images about the optical axis. Rotation of one of the images may then be done with the use of rotating separating prisms (not shown) as discussed in L. Lipton, Foundations of the Stereoscopic Cinema, Van Nostrand-Reinhold, Appendix 7, p. 260, 1982, herein incorporated by reference. 
         [0044]    Anamorphic imaging may also be carried out in the projection lenses  308 ,  310  or using anamorphicafocal converter attachments after the projection lenses  308 ,  310 . 
         [0045]      FIG. 5  is a schematic diagram of a stereoscopic projection system  500 . The stereoscopic projection system  500  may include a projector  502 , a relay lens  504 , an image splitting element  506 , a first projection lens  508 , and a second projection lens  510 . 
         [0046]    In operation, the relay lens  504  receives light from the projector  502  at the input light path  520 . The relay lens  504  outputs light on an intermediate light path  522  toward an intermediate image plane  534  at the input of the image splitting element  506 . The image splitting element  506  is operable to split the light on the intermediate light path  522  into first image light path  528  and second image light path  526 . In an embodiment, the image splitting element  506  may be total internal reflection (TIR) prisms  516 . The TIR prisms  516  are operable to split the light on the intermediate light path  522  between a first and second image light path  528 ,  526  and to redirect the first and second image light paths  528 ,  526  such that they are parallel relative to the relay lens  504  and projection lenses  508 ,  510 . The image splitting element  506  outputs light on the first and second image light paths  528 ,  526  toward the first and second projection lenses  508 ,  510 . The first and second projection lenses  508 ,  510  receive the light on the first and second image light paths  528 ,  526  and output light on the first and second output light paths  529 ,  530  toward the screen  550 . In an embodiment, a rotator  532  is positioned in the first output light path  529  between the first projection lens  508  and the screen  550 . The rotator  532  may comprise an achromatic polarization rotator element (e.g., an achromatic half wave plate oriented at 45°. In operation, the achromatic polarization rotator element  532  is operable to rotate the polarization of the light on the first output light path  529  such that the light on the first and second output light paths  529 ,  530  have opposite polarization. The left and right eye images associated with the two beams are then projected and superimposed onto a polarization preserving screen using separate lenses. 
         [0047]    In an embodiment, the stereoscopic projection system  500  may also include matched waveplates  512 , arranged as shown, between the projector  502  and the relay lens  504 . Alternatively, matched waveplates  512  may be positioned between the relay lens  504  and the image splitting element  506 , near the intermediate image plane  534 . As another alternative, a first matched waveplate  512  is positioned between the projector  502  and the relay lens  504  (as shown) and a second matched waveplate  512  is positioned between the relay lens  504  and the image splitting element  506 , near the intermediate image plane  534 . Linearly polarized light may be launched into the system for better preservation through the splitting elements. In an embodiment, the image splitting element  506  may include non-ideal separating mirrors. Here, the geometry may use polarization mixing, particularly if using a total internal reflection (TIR) prism  516  for redirecting circular polarized beams. In an embodiment, a TIR prism is preferred over mirrors for its higher reflectivity and smaller physical size. The 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 can be corrected by introducing intensity and bit depth loss. To reduce these effects, linear polarization states may be created prior to entering the system. Polarization would be preserved to a great extent since these states would closely resemble the s- or p-Eigen-states for the majority of rays present in the imaging system. 
         [0048]      FIG. 6  is a schematic diagram of a stereoscopic projection system  600  using linear polarization states created prior to entering the system  600 . The stereoscopic projection system  600  may include a projector  602 , a relay lens  604 , an image splitting element  606 , a first projection lens  608 , and a second projection lens  610 . 
         [0049]    In an embodiment, the stereoscopic projection system  600  may also include matched waveplates  612 , wavelength-selective polarization filter  618  (e.g., a ColorSelect filter as taught in U.S. Pat. Nos. 5,751,384 and 5,953,083, herein incorporated by reference), linear polarizers  640 , and/or 45° quarter wave plates  632 . 
         [0050]    In operation, the relay lens  604  receives light from the projector  602  at the input light path  620 . In an embodiment, matched waveplates  612  and wavelength-selective polarization filter  618  are positioned on the input light path  620  between the projector  602  and the relay lens  604 , arranged as shown. Alternatively, matched waveplates  612  may be positioned between the relay lens  604  and the image splitting element  606 , near the intermediate image plane  634 . As another alternative, a first matched waveplate  612  is positioned between the projector  602  and the relay lens  604  (as shown) and a second matched waveplate  612  is positioned between the relay lens  604  and the image splitting element  606 , near the intermediate image plane  634 . The relay lens  604  outputs an intermediate light path  622  toward an intermediate image plane  634  at the input of the image splitting element  606 . The image splitting element  606  is operable to split the intermediate light path  622  into a first image light path  628  and a second image light path  626 . In an embodiment, the image splitting element  606  may be total internal reflection (TIR) prisms  616 . The TIR prisms  616  are operable to split the intermediate light path  622  into a first and second image light path  628 ,  626  and to redirect the first and second image light paths  628 ,  626  such that they are parallel relative to the relay lens  604  and projection lenses  608 ,  610 . The image splitting element  606  outputs the first and second image light paths  628 ,  626  toward the first and second projection lenses  608 ,  610 . The first and second projection lenses  608 ,  610  receive the first and second image light paths  628 ,  626  and output first and second output light paths  629 ,  630  toward the screen  650 . In this exemplary embodiment, linear polarizers  640  are positioned in at least one of the first and second image light paths  628 ,  626  between the first and second projection lenses  608 ,  610  and the screen  650 ; and  45 ° quarter wave plates  632  are positioned in at least one of the first and second image light paths  628 ,  626  between the first and second projection lenses  608 ,  610  and the screen  650 . 
         [0051]    As discussed above, the stereoscopic projection system  600  may include wavelength-selective polarization filters  618  (e.g., ColorSelect polarization filters) to create a linearly polarized input beam  620 , which is preserved throughout the system, and which is cleaned up and circularly encoded at the system exit  629 ,  630  for good polarization fidelity. To be compatible with the head tilt tolerant circular polarization of incumbent systems, orthogonally oriented quarter-wave plates  632  may be introduced at the output of the projection lenses  608 ,  610  with the achromatic rotator element removed. Anamorphic imaging techniques may also be introduced for efficiency as described above. 
         [0052]    Polarization integrity, which is desired for low cross-talk stereoscopic systems, relies on the polarization preservation of substantially the entire optical system. While this might be sufficient for some systems, a clean-up step is preferred for high end performance. In an embodiment, neutral linear polarizers  640  may be introduced into the first and second output light paths  629 ,  630  to clean up the polarization. The stereoscopic projection system  600  selectively transforms the green polarization into that of the red and blue (thus, the optical beams have substantially uniform polarization) using wavelength-selective polarization filters  618 . An additional advantage of creating uniformly polarized white beams is that the final projected images for the left and right eyes do not use internal alignment via lens shift. In other words, the left and right eye images will have their color components aligned independently of the lens alignment. Only the much less sensitive relative alignments would be determined by mechanical or electronic manipulations. In an embodiment, the preferred position for a green-transforming filter would be earlier rather than later in the system  600  such that color components of either right- or left-eye images follow the same path through the optical system. Equivalent paths may avoid unwanted color-dependent distortions in any one image. 
         [0053]      FIG. 7  is a schematic diagram of a stereoscopic projection system  700  incorporating polarization transforming components within TIR prisms. The stereoscopic projection system  700  may include a projector  702 , a relay lens  704 , an image splitting element  706 , a first projection lens  708 , and a second projection lens  710 . 
         [0054]    In this exemplary embodiment, the stereoscopic projection system  700  may also include matched waveplates  712 , a wavelength-selective polarization filter  718 , polarization transformers  760 , prisms  716 , linear polarizers  740 , and/or  45 ° quarter wave plates  732 . 
         [0055]    In operation, the relay lens  704  receives light from the projector  702  at the input light path  720 . Matched waveplates  712  and wavelength-selective polarization filter  718  may be positioned on the input light path  720  between the projector  702  and the relay lens  704 . Alternatively, matched waveplates  712  may be positioned between the relay lens  704  and the image splitting element  706 , near the intermediate image plane  734 . As another alternative, a first matched waveplate  712  is positioned between the projector  702  and the relay lens  704  (as shown) and a second matched waveplate  712  is positioned between the relay lens  704  and the image splitting element  706 , near the intermediate image plane  734 . The relay lens  704  outputs an intermediate light path  722  toward an intermediate image plane  734  at the input of the image splitting element  706 . The image splitting element  706  is operable to split the intermediate light path  722  into a first image light path  728  and a second image light path  726 . In this exemplary embodiment, the image splitting element  706  may be prisms  716 . The prisms  716  are operable to split the intermediate light path  722  into a first image light path  728  and a second image light path  726  and to redirect the first and second image light paths  728 ,  726  such that they are parallel relative to the relay lens  704  and projection lenses  708 ,  710 . In another embodiment, the prisms  716  may further include polarization transformers  760 . The polarization transformers  760  are operable to transform the state of polarization (e.g., from p-polarization to s-polarization eigen states). The image splitting element  706  outputs the first and second image light paths  728 ,  726  toward the first and second projection lenses  708 ,  710 . The first and second projection lenses  708 ,  710  receive the first and second image light paths  728 ,  726  and output first and second output light paths  729 ,  730  toward the screen  750 . In an embodiment, linear polarizers  740  are positioned in at least one of the first and second image light paths  728 ,  726  between the first and second projection lenses  708 ,  710  and the screen  750 ; and 45° quarter wave plates  732  are positioned in at least one of the first and second image light paths  728 ,  726  between the first and second projection lenses  708 ,  710  and the screen  750 . 
         [0056]    The system  700  discussed above incorporates polarization compensating and polarization transforming components  760  within the Total Internal Reflection (TIR) prisms  716  resulting in improved polarization conservation within the TIR prisms. Even with linear polarization, TIR reflection may preserve polarization for rays in the plane of the diagram of  FIG. 6 . For all rays out of this plane, the s- and p-polarization axes are rotated geometrically with respect to the input polarization orientation. The significant phase difference on total internal reflection (TIR) may cause ellipticity. In this embodiment, where the two reflecting surfaces of a single prism  716  are substantially parallel, the orientation of the s- and p-polarization axes are substantially equivalent and the net phase difference is twice that imparted by the first reflection. This precise geometrical alignment of the s- and p-axes for all ray directions enables correction of this depolarization through axis swapping. It is possible to swap the s- and p-polarization components without affecting their relative phase by introducing a special polarization transformation element  760  in the optical path between the two reflections (i.e. inside the prism  716 ). Reflecting off the second TIR surface  762  then introduces the opposite phase difference between s- and p-returning linear polarization for substantially all rays. In an embodiment, the transforming component  760  is made of a stack of retarders that create a 90° polarization transformation independent of the component&#39;s orientation and is described in detail in Robinson et al., Polarization Engineering for LCD Projection, Ch. 6, Wiley &amp; Sons, 2004, herein incorporated by reference. 
         [0057]      FIG. 8  is a schematic diagram of a stereoscopic projection system  800  incorporating spatial polarization manipulations and polarization beam splitting. The stereoscopic projection system  800  may include a projector  802 , a relay lens  804 , an image splitting element  806 , a first projection lens  808 , and a second projection lens  810 . 
         [0058]    In an embodiment, the stereoscopic projection system  800  may also include matched waveplates  812 , a wavelength-selective polarization filter  818 , polarization transformer  860 , linear polarizers  840 , an achromatic rotator  834 , and/or 45° quarter wave plates  832 . 
         [0059]    In operation, the relay lens  804  receives light from the projector  802  at the input light path  820 . In an embodiment, matched waveplates  812  and wavelength-selective polarization filter  818  are positioned on the input light path  820  between the projector  802  and the relay lens  804 . Alternatively, matched waveplates  812  may be positioned between the relay lens  804  and the image splitting element  806 , near the intermediate image plane  834 . As another alternative, a first matched waveplate  812  is positioned between the projector  802  and the relay lens  804  (as shown) and a second matched waveplate  812  is positioned between the relay lens  804  and the image splitting element  806 , near the intermediate image plane  834 . The relay lens  804  outputs an intermediate light path  822  toward an intermediate image plane  834  at the input of the image splitting element  806 . The image splitting element  806  is operable to split the intermediate light path  822  into a first image light path  828  and a second image light path  826 . In an embodiment, the image splitting element  806  includes a polarization beam splitter  886 . The polarization beam splitter (PBS)  886  is positioned in part of the intermediate light path  822 . As a result, part of the intermediate light path  822  passes by the PBS  886  toward the second projection lens  810  (becoming the second image light path  826  with little or no optical transformation) and the other part of the intermediate light path  822  that passes through the PBS  886 , is reflected at reflector  816 , and output towards the first projection lens  808  (becoming the first image light path  828 ). Reflector  816  may be a mirror, PBS, a TIR prism surface, or any other suitable reflective element. 
         [0060]    In an embodiment, the system  800  further includes a polarization transformer  860  positioned on the part of the intermediate light path  822  between the relay lens  804  and the PBS  886 . The image splitting element  806  outputs the first and second image light paths  828 ,  826  toward the first and second projection lenses  808 ,  810 . The first and second projection lenses  808 ,  810  receive the first and second image light paths  828 ,  826  and output first and second output light paths  829 ,  830  toward the screen  850 . In an embodiment, linear polarizers  840  are positioned in at least one of the first and second image light paths  828 ,  826  between the first and second projection lenses  808 ,  810  and the screen  850 ; and 45° quarter wave plates  832  are positioned in at least one of the first and second image light paths  828 ,  826  between the first and second projection lenses  808 ,  810  and the screen  850 . An achromatic rotator  834  may be positioned in at least one of the first and second image light paths  828 ,  826  between the first and second projection lenses  808 ,  810  and the screen  850 . 
         [0061]    In this exemplary embodiment, polarization integrity is preserved by using the PBS  886  in place of the reflecting splitting elements (e.g., the reflecting splitting elements shown in  FIGS. 3 ,  5 ,  6 , and  7 ). By splitting the beams in this manner, a single pair of reflecting surfaces is used. In its simplest form, this imparts a path difference between the channels as shown in  FIG. 8 . Introducing extra glass between the intermediate image  834  and the bottom projection lens  810  may match the optical path to the lenses. The resultant difference in the path lengths to the screen  850  may be tolerated in cinema systems where the throw is large. De-magnifying correction optics may be used in shorter throw situations or, alternatively, electronic correction may be used at the panel. In an embodiment, the quarter wave plates  832  may be removed and the system  800  is thereby transformed into one that delivers linearly polarized projected light. Another adaptation to this exemplary embodiment is to introduce an additional PBS interface at the reflective surface  816  where the TIR reflection occurs. Having two PBS reflections in series may reduce the level of unwanted p-polarization leakage enough that lossy clean-up polarizers at the lens&#39; exits are not used. 
         [0062]      FIG. 9  is a schematic diagram of a stereoscopic projection system  900 . The system  900  may include a projection element  902 , a relay lens  904 , an image splitting element  906 , an image combining element  910  and a projection lens  908 . 
         [0063]    In operation, the relay lens  904  receives light from the projector  902  at the input light path  920 . In an embodiment, matched waveplates  912  and wavelength-selective polarization filter  918  are positioned on the input light path  920  between the projector  902  and the relay lens  904 . Alternatively, matched waveplates  912  may be positioned between the relay lens  904  and the image splitting element  906 , near the intermediate image plane  934 . As another alternative, a first matched waveplate  912  is positioned between the projector  902  and the relay lens  904  (as shown) and a second matched waveplate  912  is positioned between the relay lens  904  and the image splitting element  906 , near the intermediate image plane  934 . The relay lens  904  outputs an intermediate light path  922  toward an intermediate image plane  934  at the input of the image splitting element  906 . The image splitting element  906  is operable to split the intermediate light path  922  into a first image light path  928  and a second image light path  926 . In an embodiment, the image splitting element  906  includes mirrors  907 . The initial set of mirrors  917  (i.e., the two mirrors  907  receiving the intermediate light path  922 ) split the intermediate light path  922  into the first and second image light paths  928 ,  926 . The second set of mirrors  927  (i.e., the two mirrors  907  receiving the first and second image light paths  928 ,  926  from the initial set of mirrors  917 ) then reflect the first and second image light paths  928 ,  926  toward an image combining element  910 . In an embodiment, the image combining element includes a polarizing beam splitter. An achromatic rotator may be positioned in one or both of the first and second image light paths  928 ,  926  between one or both of the second set of mirrors  927  and the PBS  910 . The PBS  910  is operable to combine the first and second image light paths  928 ,  926  into a third image light path  938 . The projection lens  908  receives the third image light path  938  and projects an output image light path  948  toward a screen (not shown). 
         [0064]    In an embodiment, the system  900  includes superposition of oppositely polarized left- and right-eye image paths (e.g., first and second image light paths  928 ,  926 ) carried out at the interface of a PBS  910  before being projected by a single lens  908 . By encoding the two images with orthogonal polarizations and directing them symmetrically into a polarizing beam splitting element  910  the two images appear to emanate from the same plane. A single polarization preserving projection lens  908  can then project the images onto a screen. 
         [0065]    The projected beam  948  exits at 45° to the original projection direction. Extra folding mirrors and/or prisms may be introduced to avoid this but have been omitted from the diagram for better clarity. Also the polarization rotator element  934  may introduce an optical path mismatch which may in practice be matched with dummy material at the top entrance to the PBS  910 . Slight modifications to this embodiment could include using TIR reflecting glass prisms in place of mirrors for efficiency and size reasons. Also anamorphic imaging with suitable correction is assumed for efficiency purposes. This includes using a Bravais system in conjunction with the relay lens, or an anamorphic afocal converter following the projection lens. 
         [0066]    The advantages of this system concerns internal alignment of the images, which minimizes external lens manipulations. System size, cost and complexity of operation are also significant advantages. 
       Bravais Systems  
       [0067]    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. Generally, Bravais systems comprise a positive and negative cylindrical element separated by a finite distance and located in the finite conjugate of a lens system. 
         [0068]    A Bravais system may be inserted near the panel, close to the relay lens output, or close to the projection lens input. The polarization and color management optics may in some cases make inserting Bravais optics near the panel difficult. A Bravais system implemented close to the projection lens input may also be 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. 
         [0069]      FIG. 10  is a schematic diagram of a stereoscopic projection system  1000  incorporating Bravais optics  1008 . The system  1000  includes a panel  1002  (or is positioned after a panel  1002 ); a relay lens  1006 ; an image splitting subsection  1040 ; and a projection lens subsystem  1050 . The system  1000  may also include a quarter wave plate  1004  positioned adjacent to the relay lens  1006  and/or quarter wave plates  1014  positioned adjacent to at least one projection lens  1016 ,  1018  in the projection lens subsystem  1050 . 
         [0070]    In operation, the relay lens  1006  receives light from a panel  1002 . The relay lens  1006  outputs that light to the Bravais optics  1008 . The Bravais optics  1008  output the light to an intermediate image plane  1010 . In an embodiment, at the intermediate image plane  1010 , the light input by the panel  1002  has been magnified 2× vertically and 1× horizontally by the Bravais optics  1008 . Next, the intermediate image light is split by the image splitting subsystem  1040 . In an embodiment, the image splitting subsystem  1040  includes two prisms  1012 ,  1013  for splitting the intermediate image light. The two prisms  1012 ,  1013  output the light to the projection lens subsystem  1050 . In an embodiment, the projection lens subsystem  1050  includes two projection lenses  1016 ,  1018 . Each projection lens  1016 ,  1018  receives light from one of the prisms  1012 ,  1013  and each projection lens  1016 ,  1018  outputs a separate image. In another embodiment, the projection lens subsystem  1050  includes an image combining element (not shown) and only contains a single projection lens. The image combining element combines the light received from the prisms  1012 ,  1013  into one light beam and the single projection lens projects the light. 
         [0071]    In an embodiment, a quarter wave plate  1004  may be positioned between the panel  1002  and relay lens  1006 . In another embodiment, a quarter wave plate  1014  may be positioned between at least one prism  1012 ,  1013  and a projection lens  1016 ,  1018 . 
         [0072]    The Bravais  1008  may be placed after the relay lens  1006  and before the splitting prisms  1012 ,  1013 . In this exemplary embodiment, the Bravais  1008  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. In an embodiment, the Bravais  1008  is removed, and the splitting prisms  1012 ,  1013  and projection lenses  1016 ,  1018  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. 
       External Anamorphic Afocal Converters 
       [0073]      FIG. 11  is a schematic diagram of a stereoscopic projection system  1100 . The system  1100  may include a panel  1102  (or is positioned after a panel  1102 ); a relay lens  1106 ; an image splitting subsection  1140 ; and a projection lens subsystem  1150 . The system  1100  may also include a quarter wave plate  1104  positioned adjacent to the relay lens  1106  and/or quarter wave plates  1114  positioned adjacent to at least one projection lens  1116 ,  1118  in the projection lens subsystem  1150 . In an embodiment, the projection lens subsystem includes an anamorphic afocal converter  1120 . 
         [0074]    In operation, the relay lens  1106  receives light from a panel  1102 . The relay lens  1106  outputs that light to an intermediate image plane  1110 . The intermediate image light is split by the image splitting subsystem  1140 . In an embodiment, the image splitting subsystem  1140  may include two prisms  1112 ,  1113  for splitting the intermediate image light. The two prisms  1112 ,  1113  output the light to the projection lens subsystem  1150 . In an embodiment, the projection lens subsystem  1150  includes two projection lenses  1116 ,  1118 . Each projection lens  1116 ,  1118  receives light from one of the prisms  1112 ,  1113  and each projection lens  1116 ,  1118  outputs a separate image. In an embodiment, an anamorphic afocal converter  1120  is positioned after each projection lens  1116 ,  1118 . In an embodiment, a quarter wave plate  1104  may be positioned between the panel  1102  and relay lens  1106 . In another embodiment, a quarter wave plate  1114  may be positioned between at least one prism  1112 ,  1113  and a projection lens  1116 ,  1118 . 
         [0075]    The external anamorphic afocal converters  1120  may improve the lumen output of the system. As shown in  FIG. 11 , the converters  1120  can be located after the projection lenses  1116 ,  1118 . Alternatively, the projection lenses  1116 ,  1118  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. 
         [0076]    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. 
         [0077]    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.