Abstract:
A polarization conversion system separates light from an unpolarized image source into a first state of polarization (SOP) and an orthogonal second SOP, and directs the polarized light on first and second light paths. The SOP of light on only one of the light paths is transformed to an orthogonal state such that both light paths have the same SOP. A polarization modulator temporally modulates the light on the first and second light paths to first and second output states of polarization. First and second projection lenses direct light on the first and second light paths toward a projection screen to form substantially overlapping polarization encoded images. The polarization modulator may be located before or after the projection lenses. The polarization-encoded images may be viewed using eyewear with appropriate polarization filters.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application relates and claims priority to provisional patent application 60/916,970, entitled “Polarization conversion system for 3-D projection,” filed May 9, 2007; this patent application also relates and claims priority to provisional patent application 60/988,929, entitled “Polarization conversion system for 3-D projection,” filed Nov. 19 2007; and this patent application further relates and claims priority to provisional patent application 61/028,476, entitled “Polarization conversion system for stereoscopic projection,” filed Feb. 13, 2008, all of which are herein incorporated by reference for all purposes. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The disclosed embodiments relate generally to projection of polarization-encoded images and, more specifically, to a polarization conversion system and method for transmitting polarization-encoded imagery to a projection screen. 
         [0004]    2. Background 
         [0005]      FIG. 1  is a schematic diagram illustrating an exemplary polarization-preserving display system  100 . The display system  100  includes a projection screen  102  and polarization filtering eyewear  104 . Stereoscopic three-dimensional (3D) imagery is observed using a single polarization-preserving screen  102  sequentially displaying left and right perspective imagery, with polarization filtering eyewear  104 . The polarization filtering eyewear  104  contains two lenses  106  and  108  of alternately orthogonal polarization. 
         [0006]    3D imagery can be synthesized using polarization control at the projector to encode, and polarization filtering eyewear to decode the left and right perspective imagery (See, e.g., commonly-owned U.S. Pat. No. 4,792,850, entitled “Method and system employing a push-pull liquid crystal modulator,” to Lenny Lipton et al. and U.S. patent application Ser. No. 11/424,087 entitled “Achromatic Polarization Switches,” filed Jun. 14, 2006, both of which are herein incorporated by reference in their entirety for all purposes). 
         [0007]    A conventional implementation of polarization control after the projection lens is shown in  FIG. 2 . Nearly-parallel rays emerge from the output of the lens, appearing to originate from a pupil inside of the lens, and converge to form spots on a distant screen. Ray bundles A, B, and C in  FIG. 2  are bundles forming spots at the bottom, center, and top of a projection screen. The light emerging from the projection lens is randomly polarized, depicted in  FIG. 2  as both S- and P-polarized light. The light passes through a linear polarizer, resulting in a single polarization state after the polarizer. The orthogonal polarization state is absorbed (or reflected), and the light flux after the polarizer is less than 50% of the original flux (resulting in a dimmer final image). The polarization switch is synchronized with the image frame, and the polarization state emerging from the polarization switch is alternated, producing images of alternately orthogonal polarization at the screen. Polarization selective eyewear  104  allows images of one polarization to pass to the left eye, and images of the orthogonal polarization to pass to the right eye. By presenting different images to each eye, 3D imagery can be synthesized. 
         [0008]    This system is currently in use in movie theatres. However, typically, this system design suffers from having more than 50% of the light absorbed by the polarizer, and thus the resulting image is typically more than 50% dimmer than that of a typical 2D theatre. Moreover, time-sequential stereoscopic 3D further reduces the brightness by more than 50%. The dimmer image can therefore limit the size of the theatre used for 3D applications and/or provides a less desirable viewing experience for the audience. 
       SUMMARY 
       [0009]    The present disclosure addresses the aforementioned issues as well as others to provide a polarization conversion system and method for stereoscopic projection. Generally, a polarization conversion system separates light from an unpolarized image source into a first state of polarization (SOP) and an orthogonal second SOP, and directs the polarized light on first and second light paths. The SOP of light on only one of the light paths is transformed to an orthogonal state such that both light paths have the same SOP. A polarization modulator temporally modulates the light on the first and second light paths to first and second output states of polarization. First and second projection lenses direct light on the first and second light paths toward a projection screen to form substantially overlapping polarization encoded images, much brighter than the referenced prior art system. The polarization-encoded images may be viewed using eyewear with appropriate polarization filters 
         [0010]    According to an aspect, a polarization conversion system for transmitting polarization encoded imagery to a projection screen includes a first projection lens, a second projection lens, a polarization beam splitter (PBS), a reflecting element, and a polarization modulator. The PBS is operable to transmit light of a first polarization state toward the first projection lens on a first light path, and is further operable to reflect light of a second polarization state toward a second light path. The reflecting element is located on the second light path and is operable to reflect light toward the second projection lens. The polarization modulator may be located on the first and second light paths. The first and second projection lenses are operable to direct the polarization encoded images toward the projection screen. 
         [0011]    In some embodiments, the polarization modulator may be a single unit that is located on both the first and second light paths. In other embodiments, the polarization modulator may include a first polarization switch and a second polarization switch, each polarization switch being located on respective first and second light paths. The polarization switch(es) may be located before or after the projection lenses. 
         [0012]    According to another aspect, a method for projecting polarization-encoded stereoscopic images includes receiving unpolarized image source light at a polarization beam splitter. The method includes transmitting image source light of a first polarization state at the polarization beam splitter toward a projection lens located on a first light path. The method also includes reflecting image light of a second polarization state at the polarization beam splitter toward a second light path. The method further includes reflecting light on the second light path toward a second projection lens. The method additionally includes rotating the state of polarization of light on one of the first and second light paths light to an orthogonal state of polarization. The method further includes temporally modulating the state of polarization of the light on the first and second light paths between a first polarized output state and a second polarized output state. 
         [0013]    Other features will be apparent with reference to the foregoing specification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    Embodiments are illustrated by way of example in the accompanying figures, in which like reference numbers indicate similar parts, and in which: 
           [0015]      FIG. 1  is a schematic diagram illustrating an exemplary polarization-preserving display system, in accordance with the present disclosure; 
           [0016]      FIG. 2  illustrates a known implementation of polarization control in a cinematic 3D system utilizing a polarization switch; 
           [0017]      FIG. 3  is a schematic diagram illustrating a first embodiment of a polarization conversion system (PCS), in accordance with the present disclosure; 
           [0018]      FIG. 3B  is a schematic diagram illustrating a polarization converting and switching module, in accordance with the present disclosure; 
           [0019]      FIG. 4  is a schematic diagram illustrating a second embodiment of a PCS, in accordance with the present disclosure; 
           [0020]      FIG. 5  is a schematic diagram illustrating a third embodiment of a PCS, in accordance with the present disclosure; 
           [0021]      FIG. 6  is a schematic diagram illustrating a fourth embodiment of a PCS, in accordance with the present disclosure; 
           [0022]      FIG. 7  is a schematic diagram illustrating a fifth embodiment of a PCS, in accordance with the present disclosure; 
           [0023]      FIG. 8  is a schematic diagram illustrating a sixth embodiment of a PCS, in accordance with the present disclosure; 
           [0024]      FIG. 9  is a schematic diagram illustrating a seventh embodiment of a PCS, in accordance with the present disclosure; 
           [0025]      FIG. 10  is a schematic diagram illustrating an eighth embodiment of a PCS, in accordance with the present disclosure; 
           [0026]      FIG. 11  is a schematic diagram of a ninth embodiment of a PCS, in accordance with the present disclosure; and 
           [0027]      FIG. 12  is a schematic diagram of a tenth embodiment of a PCS, in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
     First Embodiment: 
       [0028]      FIG. 3  is a schematic diagram illustrating a first embodiment of a polarization conversion system (PCS)  300 . Generally, PCS  300  may include an image source  304  (e.g., from a light modulating panel or conventional film), initial relay lens  302 , polarizing beamsplitter (PBS)  310 , first and second relay lenses  306 ,  308 , polarization switch  312 , fold mirror  318 , polarization converting and switching module  320 , and first and second projection lenses  328 ,  330 , arranged as shown. As illustrated by  FIG. 3B , polarization converting and switching module  320  may include polarization converter  322  and polarization switch  324 , and may optionally include a pre-polarizer  326  to improve contrast, all arranged as shown. Polarization converter  322  is an optical component that is operable to transform an input state of polarization to an orthogonal state of polarization (e.g., a half wave plate). 
         [0029]    The first and second relay lenses  306  and  308  are preferably symmetric about respective aperture stops  301 ,  303 , respectively located after the polarization switch  312  and polarization converting and switching module  320 , providing substantially distortion-less images of the panel  304  at each image location  314  and  316 . In an alternative embodiment, the aperture stops  301 ,  303 , may be located on the respective light paths  305 ,  307 , immediately prior to the polarization switch  312  and polarization converting and switching module  320 . In another alternative embodiment,  FIG. 3  depicts an alternate location  332  for the polarization switch  312  in the first light path  306 , and an alternate location  334  for the polarization converting and switching module  320  in the second light path  308 . These locations may prove to be beneficial alternatives if birefringence through the lens elements  302  of the relay system  300  degrades the system contrast. As another alternative location, the polarization switches  312 ,  324  may instead be placed after the projection lens rather than prior to it. Such an embodiment may provide system contrast advantages. Note that it is not necessary that the half wave plate  322  is immediately adjacent the polarization switch  324 —the half wave plate  322  may be located anywhere in the light path between the PBS  310  and the polarization switch  324 . Indeed, in alternative embodiments, the positions of the polarization switch  312  and the polarization converting and switching module  320  may be reversed such that the polarization switch  312  is located on the second light path  307  and the polarization converting and switching module  320  is located on the first light path  305 . 
         [0030]    In operation, panel  304  (e.g., a Digital Light Processing (DLP) panel from Texas Instruments or conventional film) is illuminated with randomly polarized light from a light source (not shown) to provide unpolarized image source light. The light source may be, for example, a conventional UHP lamp, a xenon lamp, a light emitting diode light source, or in some embodiments, a light source taught in commonly-owned U.S. patent application Ser. No. 11/779,708, entitled “Light collector for projection systems,” filed Jul. 18, 2007, herein incorporated by reference. The unpolarized image source light from the panel  304  is directed toward PBS  310  by initial relay lens  302 . The PBS  310  may transmit P-polarized light on a first light path  305 , and reflect S-polarized light toward a second light path  307 . On the first light path  305 , the P-polarized light passes through the polarization switch  312 , which operates to rotate the light passing through the switch  312  in alternating frames, similar to the bundles A, B, and C in  FIG. 2 . 
         [0031]    On the second light path  307 , the S-polarized light reflected by the PBS  310  passes to a fold mirror  318  (or any optical component that serves to reflect light without changing the polarization state, e.g., a prism). The S-polarized light then passes through a polarization converting and switching module  320 . The polarization converter  322  (which may be a half wave plate) preferably transforms substantially all visible wavelengths to the orthogonal polarization (in this case, from S- to P-polarized light). The now-P-polarized light then passes through polarization switch  324 . In some embodiments, a pre-polarizer  326  may be added before or after module  320  for higher contrast. The polarization switch  324  included in the polarization and switching module  320  operates to create alternating orthogonal states in a manner substantially identical to the switch  312  in the first light path  305 . 
         [0032]    The polarization conversion system  300  may form two separate images  314  and  316  of the panel  304 , each with magnification 1× (i.e., the output images at  314  and  316  may be substantially the same size as the input image from panel  304 ). It should be appreciated that the magnification could be other than 1× in this and other embodiments and that this magnification is provided as an example. First and second projection lenses  328  and  330  respectively image the intermediate images  314  and  316  onto the projection screen  102 . The projection lenses  328  and  330  are allowed to move laterally, such that the images on the screen  102  from the two optical paths  305  and  307  are superimposed, substantially overlapping, preferably with minimal keystone distortion. Since nearly all of the randomly polarized light from the panel  304  is imaged at the screen  102  with a single polarization state, the resulting image of the system in FIG.  3  is approximately two times brighter than the image at the screen  102  for the system in  FIG. 2 . 
         [0033]    This system may also be applied to cinematic, professional and consumer applications such as home theatre and rear projection television (RPTV), assuming polarization-preserving screens  102  are utilized. 
       Second Embodiment 
       [0034]      FIG. 4  is a schematic diagram illustrating a second embodiment of a polarization conversion system (PCS)  400 . PCS  400  provides a similar relay system to that shown in  FIG. 3 , with an arrangement of components having substantially similar structure and function, except a glass prism  410  has been inserted into the second light path  407 , arranged as shown. Glass prism  410  may be a high index glass prism. 
         [0035]    In operation, the glass prism  410  allows the two images  414  and  416  of the panel  404  to be collocated substantially in a single plane, providing more convenient packaging and adjustment of the projection lenses  428  and  430 . It is preferable that the relay system  400  is designed such that rays from a single field point at the object (i.e., panel  404 ) produce a collimated bundle (all rays from a field point having the same angle) at the aperture stops  401  and  403 . This allows the insertion of the glass prism  410  at the aperture stop without affecting the lens  402  performance. The glass prism  410  allows the two images  414  and  416  to be collocated. Again, in alternate embodiments, the polarization converting and switching module  420  and polarization switch  412  may each have alternate locations  404  and  406  respectfully for each path, either before the projection lens or after the projection lens. 
       Third Embodiment 
       [0036]      FIG. 5  is a schematic diagram illustrating a third embodiment of a polarization conversion system (PCS)  500 .  FIG. 5  provides a similar PCS  500  to that shown in  FIG. 4 , except the polarization switch  412  of  FIG. 4  has been replaced by a spinning wheel  550  operable to convert the polarized input to a set of temporally alternating orthogonally polarized output states. In one embodiment, the spinning wheel  550  may contain segments that transmit alternating orthogonal polarizations from a non-polarized input. In another embodiment, the spinning wheel  550  may be preceded by a fixed polarizer. The spinning wheel  550  may then contain segments that represent unitary polarization transformations (e.g. from a stack of retardation films). 
         [0037]    An issue resulting from physical rotation of a polarizer (spinning wheel  550 ) is that the output varies in an analog fashion, unless each segment is patterned to compensate for this effect. Functionally, a binary polarization switching effect is desired, which according to this disclosure is optimally accomplished using elements with circular Eigenpolarizations. For instance, a true circular polarizer (versus, for example, a linear polarizer followed by a retarder, or retarder stack) will transmit a particular handedness (e.g. right or left) of circular state, regardless of wheel orientation. 
         [0038]    Alternatively, a fixed polarizer can be followed by a unitary polarization transforming element with circular Eigenpolarizations, or a pure circular retarder. For instance, a linear polarizer can be followed by a rotating wheel  550  that contains a combination of isotropic segments, as well as pure achromatic polarization rotating elements. A pure achromatic rotator has zero linear retardation (no optic axis), but has a desired amount of phase delay between orthogonal circular states. In this case, a π phase shift between circular Eigenstates will convert the input to the orthogonal linear output, regardless of wheel orientation. Thus, an analog wheel will provide binary switching between orthogonal linear polarizations. 
         [0039]    Pure achromatic polarization rotators may be fabricated using stacks of linear retarders. One design method is to pair stacks with a particular symmetry arrangement. For instance, a stack that produces a particular retardation and rotation can be paired with an identical stack with reverse-order, or reverse-order reflected symmetry (See, e.g., Chapter 5 of Robinson et. al., POLARIZATION ENGINEERING FOR LCD PROJECTION, Wiley &amp; Sons 2005, which is hereby incorporated by reference). A reverse order stack doubles the net retardation while eliminating rotation, while the addition of reflection has the effect of doubling rotation while eliminating retardation. A stack designed to convert a 0-oriented linear input to a π/4 oriented linear output (at all wavelengths of interest) can be designed, which in general contains linear retardation. However, when paired with the reverse-order-reflected stack, the net effect is zero retardation and the desired π/2 orientation transformation. Such transparent elements can be laminated as segments on an isotropic disk to produce binary polarization switching with spinning wheel  550 . 
         [0040]    Table 1 provides a design for an exemplary retarder stack exhibiting substantially achromatic rotation of π/2 having reverse-order-reflected symmetry. Note that this symmetry is a sufficient, but not necessary condition for achieving the desired polarization transformation. It is easily verified that the state of polarization after layer-6 is 45° linear, though the stack possesses linear retardation that is eliminated by the subsequent stack. In this example, all layers have a zero-order in-plane retardation of ½-wave (typically 240-270 nm to span the visible). It should be apparent that, in accordance with the present disclosure, other retarder combination designs may be employed that have different orientations and retardation profiles. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Layer Number 
                 Orientation 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 1 
                 −19.6° 
               
               
                   
                 2 
                 2.4° 
               
               
                   
                 3 
                 18.1° 
               
               
                   
                 4 
                 −65.6° 
               
               
                   
                 5 
                 −54.3° 
               
               
                   
                 6 
                 −15.0° 
               
               
                   
                 7 
                 15.0° 
               
               
                   
                 8 
                 54.3° 
               
               
                   
                 9 
                 65.6° 
               
               
                   
                 10 
                 −18.1° 
               
               
                   
                 11 
                 −2.4° 
               
               
                   
                 12 
                 19.6° 
               
               
                   
                   
               
             
          
         
       
     
         [0041]    Still referring to  FIG. 5 , in operation, light from the lower light path  505  is P-polarized and passes through the isotropic segment  504  of the wheel  550 . The light remains P-polarized through Image  2   516 , through the projection lens  530 , and onto the screen  102 . In this instance, light in the upper path  507  is S-polarized and passes through the rotator segment  506  of the wheel  550 . The S-polarized light is rotated to P-polarized light by the wheel  550 , and passes through the projection lens  528  and onto the screen  102  as P-polarized light. The wheel  550  is then synchronized with the video frames to produce images on a screen  102  with alternating polarization. Variations of the polarization states are also possible, with each path producing circular polarization by addition of quarter-wave plates in the optical paths, or rotated linear polarization states (e.g., +1-45 degrees) by addition of rotators in each path. 
       Fourth Embodiment 
       [0042]      FIG. 6  is a schematic diagram illustrating a fourth embodiment of a polarization conversion system (PCS)  600 .  FIG. 6  illustrates a PCS  600  where the magnification has been increased to 2× (versus 1 33  previously). In this case, the first half of the PCS  600 , including PBS  610  and path-matching glass prism  602  may be identical in structure and function to the components described in  FIG. 4 . However, the second half of the PCS  600  has been scaled by 2 to increase the focal length of the second half by 2. The PCS  600  produces an image that is twice the size of the panel  604 , yet maintains the same f-number (or numerical aperture). In this exemplary embodiment, a single relay lens  608  may be used to provide an intermediate image, and a single projection lens  630  (e.g., a 70 mm cinema lens) may be utilized to image the intermediate image to the screen  102 . The polarization converting and switching module  620  and its alternate location  625  are also shown. 
       Fifth Embodiment 
       [0043]      FIG. 7  is a schematic diagram illustrating a fifth embodiment of a PCS  700 .  FIG. 7  depicts a similar PCS  700  to that shown in  FIG. 6 , except the polarization converting and switching modules  620  of  FIG. 6  have been replaced by the segmented wheel  750  (similar to the segmented wheel  550  described in  FIG. 5 ). The segmented wheel  750  and an alternate segment wheel location  752  are also indicated. Once again, a single projection lens  730  can be utilized to image the intermediate image to the screen  102 . 
       Sixth Embodiment 
       [0044]      FIG. 8  is a schematic diagram illustrating a sixth embodiment of a PCS  800 . The PCS  800  may include a panel  804 , an initial relay lens  802 , a PBS  810 , a polarization switch  812  on a first light path, a glass prism  814  with a reflector (e.g., a mirrored angled surface)  816 , a polarization converting and switching module  818 , and first and second projection lenses  820  and  822 , all arranged as shown. Polarization converting and switching module  818  may have an optional pre-polarizer, a polarization rotator and a polarization switch, similar to the description of the polarization converting and switching module  320  of  FIG. 3B . The projection lens system  800  may form two separate images of the panel  304 , each with large magnification. This PCS  800  may also be applied to professional and consumer applications such as home theatre and RPTV, assuming polarization-preserving screens  102  are available. 
         [0045]    In operation, panel  804  (such as a Digital Light Processing, or DLP, panel from Texas Instruments) is illuminated with randomly polarized light. In this embodiment, light from the panel  804  is projected to a screen  102  by first and second projection lenses  820  and  822 , which may be of the reverse telephoto type. The PBS  810  transmits P-polarized light along a first light path, and reflects S-polarized light along a second light path. The P-polarized light passes through the polarization switch  812  and is rotated by the polarization switch  812  in alternating frames, similar to bundles A, B, and C in  FIG. 2 . 
         [0046]    The S-polarized light reflected by the PBS  810  (on the second light path) passes to a prism  814 . The prism  814  may contain an angled surface  816  that serves as a fold mirror. Reflection may be accomplished with total internal reflection, or by coating the hypotenuse with a mirror layer (e.g., silver). In order to insert such a prism  814  internal to the PCS  800  without creating excessive aberrations in the final image, it is preferable that rays from a field point at the object (panel  304 ) are collimated (i.e., the rays in the bundle have the same angle) at the aperture stop(s)  830  and  832 . In some embodiments, the aperture stop  830  may be located along the first light path before the polarization switch  812 , and/or along the second light path at some location (i.e.,  832 ) before the prism structure  814 . Thus, collimated rays pass through the prism structure  814  without the introduction of aberrations. The S-polarized light then passes out of the prism  814 , through polarization converting and switching module  818 , and is rotated to P-polarized light. The polarization switch within polarization converting and switching module  818  acts on P-polarized light, rotating the polarization of the ray bundles in alternating frames, in synchronization with the rotation of bundles in the non-mirror path. 
         [0047]    Two substantially identical second halves of the lenses  820  and  822  project the two images onto the screen  102 . To overlap the two images on the screen  102 , the polarizing beamsplitter  810  tilt may be adjusted and/or the prism  808  tilt may be adjusted. The projection lens assembly, may as a whole, be allowed to move laterally, such that the images on the screen  102  from the first and second optical paths can be offset vertically for various theatre configurations. The first half lenses  820  may be cut in the lower path to allow for light to pass clearly in the upper path, as is depicted in  FIG. 8 . 
         [0048]    Since nearly all of the randomly polarized light from the panel  804  is imaged at the screen  102  with a single polarization state, the resulting image of the system in  FIG. 8  is approximately two times brighter than the image at the screen  102  for the system in  FIG. 2 . 
       Seventh Embodiment 
       [0049]      FIG. 9  depicts a similar polarization conversion system  900  as in  FIG. 8 , except that the polarization switch  812  has been replaced by a spinning wheel  902 . The wheel  902  is segmented into two or more regions as described previously. In this instance, light from the lower path  904  is P-polarized and passes through the (e.g.) isotropic segment  901  of the wheel  902 . The light remains P-polarized through the rest of the projection lens system  900 , and onto the screen  102 . In this instance, light in the upper path  906  is S-polarized and passes through the (e.g.) rotator segment  903  of the wheel  902 . The S-polarized light is rotated to P-polarized light by the wheel  902 , and passes through the rest of the projection lens system  900  and onto the screen  102  as P-polarized light. The wheel  902  is then synchronized with the video frames to produce images on screen  102  with alternating polarization. Variations of the polarization states are also possible, with each path  904  and  906  producing circular polarization by addition of quarter wave plates (not shown) in the optical paths, or rotated linear polarization states (e.g. +/−45 degrees) by addition of rotators in each path. 
       Eighth Embodiment 
       [0050]      FIG. 10  depicts a similar polarization conversion system  1000  to that of  FIG. 9 . In this exemplary embodiment, the structure and function of the components of the PCS  1000  are substantially similar to that of the PCS  900 , except two rotator wheels  1002  and  1004  are implemented instead of one, in part, to ease packaging constraints near the prism  808 . The rotator wheels  1002  and  1004  may operate in synchronization with each other. 
       Ninth Embodiment 
       [0051]      FIG. 11  is a schematic diagram of an exemplary cinematic PCS system  1100  that implements zoom lenses. Cinematic PCS system  1100  may include a panel  1102 , a telecentric objective  1104  (i.e., an initial relay lens), a polarization beam splitter (PBS)  1106 , first and second aperture stops  1108 ,  1110 , first and second mechanically compensated afocal zooms  1112 ,  1132 , reflecting element  1130 , rotator  1136 , and first and second z-screens  1124 ,  1138 . 
         [0052]    In operation, s- and p-polarized light from panel  1102  passes through telecentric objective  1104  toward PBS  1106 . Telecentric objective  1104  is used to maintain collimated light at the PBS  1106  for all zoom settings. PBS  1106  may be a cube or wire grid plate, or any other PBS known in the art. In this embodiment, p-polarized light is transmitted through the PBS  1106  toward a first direction, while s-polarized light is reflected at the PBS  1106  toward a second direction. 
         [0053]    The p-polarized light passes through aperture stop  1108  toward a first mechanically compensated afocal zoom apparatus  1112 . Zoom  1112  may include various elements having positive and negative optical powers. The afocal zoom can be mechanically compensated or optically compensated, for instance, using techniques in zoom lens design from “Modern Optical Engineering” by Warren Smith, 1990, McGraw-Hill, herein incorporated by reference. Zoom  1112  in this exemplary embodiment may have, on a light path, a fixed optical element such as concave lens  1114 , followed by moving elements convex lens  1116  and concave lens  1118 , followed by another fixed element, convex lens  1120 . Generally in  FIG. 11 , convex lenses are represented by lines with dots at either end, and generally have positive optical power and may include single or multiple optical elements to provide such positive optical power. Conversely, concave lenses (represented by concave graphics) generally have negative optical power and may include single or multiple optical elements to provide such negative optical power. The moving elements  1122  may move along the optical axis to adjust the zoom of the image as desired. Light from zoom  1112  then passes through a first z-screen  1124  and then toward a screen  1150  to form a first image. 
         [0054]    S-polarized light from PBS  1106  that is reflected toward the second direction passes through aperture stop  1110 . Subsequently, the light is reflected by about 90 degrees by a reflecting element  1130 , such as a right angle prism with mirror  1130 . The s-polarized light then passes through second mechanically compensated afocal zoom  1132 . Zoom  1132  may employ a similar structure and operate in a similar way to the structure and operation described for zoom  1112 . Of course, the moving elements  1134  may be adjusted differently, to provide a different zoom, as desired. S-polarized light from zoom  1132  may then pass through rotator  1136 , which may be an achromatic half wave plate. Rotator  1136  functions to rotate the s-polarized light into p-polarized light. The p-polarized light on the second light path then passes through second z-screen  1138 , and then toward screen  1150 , to form a second image. The first and second images are overlaid at screen  1150 . 
         [0055]    The following discussion relates to further embodiments, components used in the disclosed embodiments, and variations of embodiments disclosed herein. 
         [0056]    Polarizing beamsplitter: The exemplary PBS shown in  FIG. 3  through  FIG. 12  is depicted as a PBS plate. This PBS plate may be constructed using a wire grid layer on glass (e.g., Proflux polarizer from Moxtek in Orem, Utah), polarization recycling film (e.g., Double Brightness Enhancing Film from 3M in St. Paul, Minn.), polarization recycling film on glass (for flatness), or a multi-dielectric layer on glass. The PBS could also be implemented as a glass cube (with wire grid, polarization recycling film, or dielectric layers along the diagonal). 
         [0057]    Adjustment of image location: In  FIG. 3 , the primary adjustment of image location for each path is lateral displacement of the projection lenses  328  and  330 . Additional adjustment of the image location may be achieved by adjusting the PBS  310  and/or the mirror  318 . In  FIG. 4  and  FIG. 5 , the primary adjustment of the image location for each path is the lateral displacement of the projection lenses  428 / 430  and  528 / 530 . Additional fine adjustment of the image location may be achieved by laterally displacing and tilting the prism structure  402 . In  FIG. 6  and  FIG. 7 , adjustment of the image overlay can be achieved by fine adjustment of the prism location and tilt. Adjustment of the image location on-screen may be accomplished by lateral displacement of the projection lens ( 630  or  730 ). In  FIG. 8  through  FIG. 10 , adjustment of the image overlay may be accomplished by tilting the polarizing beamsplitter ( 810 ,  910 , or  1010 ) and/or tilting the prism ( 814 ,  914 , or  1014 ). Adjustment of the aforementioned components (PBS, mirror and/or projection lenses) to control image location may be accomplished using electro-mechanical actuators. Feedback control systems and sensors may provide processing, control and drive instructions to the actuators in order to position the location of the first and second images on the screen  102 . 
         [0058]    Polarization switch: The polarization switch, as illustrated in disclosed embodiments, may be a circular polarization switch or a linear polarization switch (e.g., Z-screen of U.S. Pat. No. 4,792,850 to Lipton, or one of the Achromatic Polarization Switches as disclosed in U.S. patent application Ser. No. 11/424,087, all of which are previously incorporated by reference). Another technique disclosed herein for switching polarization includes using a rotating polarization wheel, as shown in the embodiments taught with reference to  FIGS. 5 ,  7 ,  9  and  10 . For that matter, the polarization switch  312  can be any switch that alternates between orthogonal polarization states, such that the eyewear  104  can decode the states and send the appropriate imagery to each eye. The polarization switch can be split between the two paths (e.g. to increase yield of the device). 
         [0059]    Transmission and stray light control: All transmissive elements may be anti-reflection coated to provide high transmission and low reflection. Reflections from transmissive elements can cause stray light in the system, which degrades contrast and/or produces disturbing artifacts in the final image. Non-optical surfaces (e.g., the prism sides) can be painted black to enhance contrast. Additional absorptive polarizers may be placed after the PBS  310  in either path to control polarization leakage and improve the final image contrast. 
         [0060]    Fold mirror and polarization purity: The fold mirror may be replaced with a PBS element (e.g., wire grid plate) in  FIG. 3  through  FIG. 10 . In this case, a purer polarization may be maintained after the folding element and could negate the need for an input polarizer on the polarization switch. Additionally, light reflected at the angled face of the prism may use total internal reflection for the reflecting mechanism. Dielectric and metal layers may also be added to the prism at the angled face to enhance reflection and preserve polarization purity. 
         [0061]    Projection Lenses: Although the embodiments of  FIGS. 3-10  illustrate the use of projection lenses with reverse telephoto construction, the polarization conversion systems disclosed herein are not limited to using such projection lenses. A reverse telephoto lens in a compact form is described in U.S. Pat. No. 6,473,242 (242 patent), which is hereby incorporated by reference. For instance,  FIG. 12  illustrates a tenth embodiment of the polarization conversion system  1200  that provides a polarization beam splitter internal to the projection lens, differing from the reverse telephoto lens design of the &#39;242 patent. In this embodiment, the polarizing beamsplitter  1210  is incorporated into the lens ( 1230   a,    1230   b  and  1230   c ) at the aperture stop, and two optical paths  1212  and  1214  exist for overlaying the two polarization states out of the projector. In this example, the mirror  1216 , rotator  1218  and polarization switches  1220  and  1222  are located after the second half of the projection lens ( 1230   b  and  1230   c ), between the lens  1230  and silver screen. The lens prescription has been modified to produce collimated rays from each field point at the aperture stop. This modification results two particular differences from the lens described in the &#39;242 patent. First, whereas the lens of the &#39;242 patent satisfies the conciliation “0.18&lt;r4/f&lt;0.45,” the lens described herein has no such restriction on r4 (e.g., r4/f could be 0.6 in this instance). Second, whereas the lens of the &#39;242 patent includes a “third lens group having a positive refractive power,” the lens described herein may include a third lens having negative refractive power. As a consequence of the modification, the lens described herein is no longer reverse telephoto. A PBS  1210 , mirror  1216 , and polarization switch(es)  1220 ,  1222  are included for the PCS function. The mirror  1216  can be tilted to align the two images at the screen. In some embodiments, a right-angle glass prism may substitute the mirror  1216 . In some embodiments, the PBS  1210  can be replaced with a PBS cube. In the diagram, the polarization switches are placed at the output of the lens for highest polarization purity. One or two polarization switches may be used at the output. One path may include an achromatic rotator prior to the switch. 
         [0062]    While various embodiments in accordance with the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. 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 any 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. 
         [0063]    Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 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,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background” is not to be construed as an admission that certain technology is prior art to any invention(s) in this disclosure. Neither is the “Brief 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 set forth herein.