# Patent

## Title
Polarization conversion system and method for projecting polarization encoded imagery

## 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.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
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
Technical Field
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.
Background
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.
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).
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.
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
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.
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.
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.
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.
Other features will be apparent with reference to the foregoing specification.



BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments are illustrated by way of example in the accompanying figures, in which like reference numbers indicate similar parts, and in which:
FIG. 1 is a schematic diagram illustrating an exemplary polarization-preserving display system, in accordance with the present disclosure;
FIG. 2 illustrates a known implementation of polarization control in a cinematic 3D system utilizing a polarization switch;
FIG. 3 is a schematic diagram illustrating a first embodiment of a polarization conversion system (PCS), in accordance with the present disclosure;
FIG. 3B is a schematic diagram illustrating a polarization converting and switching module, in accordance with the present disclosure;
FIG. 4 is a schematic diagram illustrating a second embodiment of a PCS, in accordance with the present disclosure;
FIG. 5 is a schematic diagram illustrating a third embodiment of a PCS, in accordance with the present disclosure;
FIG. 6 is a schematic diagram illustrating a fourth embodiment of a PCS, in accordance with the present disclosure;
FIG. 7 is a schematic diagram illustrating a fifth embodiment of a PCS, in accordance with the present disclosure;
FIG. 8 is a schematic diagram illustrating a sixth embodiment of a PCS, in accordance with the present disclosure;
FIG. 9 is a schematic diagram illustrating a seventh embodiment of a PCS, in accordance with the present disclosure;
FIG. 10 is a schematic diagram illustrating an eighth embodiment of a PCS, in accordance with the present disclosure;
FIG. 11 is a schematic diagram of a ninth embodiment of a PCS, in accordance with the present disclosure; and
FIG. 12 is a schematic diagram of a tenth embodiment of a PCS, in accordance with the present disclosure.



DETAILED DESCRIPTION
First Embodiment:
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).
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.
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.
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.
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.
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:
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.
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:
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).
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.
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.
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 & 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.
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 1⁄2-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.










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°










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., +/−45 degrees) by addition of rotators in each path.
Fourth Embodiment:
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 133  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:
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:
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.
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.
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.
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.
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:
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:
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:
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.
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.
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.
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.
The following discussion relates to further embodiments, components used in the disclosed embodiments, and variations of embodiments disclosed herein.
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).
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.
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).
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.
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.
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 '242 patent. In this embodiment, the polarizing beamsplitter 1210 is incorporated into the lens (1230a, 1230b and 1230c) 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 (1230b and 1230c), 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 '242 patent. First, whereas the lens of the '242 patent satisfies the conciliation “0.18<r4/f<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 '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.
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.
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.

## Claims

1. A polarization conversion system for transmitting polarization encoded imagery to a projection screen, comprising:
a first projection lens;
a second projection lens;
a polarization beam splitter operable to receive image light from an image source, operable to transmit image light of a first polarization state toward the first projection lens on a first image light path, and operable to reflect image light of a second polarization state toward a second image light path;
a reflecting element located on the second image light path operable to reflect the image light of the second image light path toward the second projection lens; and
a polarization switch located on at least one of the first image light path and the second image light path, the polarization switch operable to selectively alternate the polarization of image light passing therethrough between a first output state of polarization and a second output state of polarization so that the image light passing therethrough has sequentially alternating polarizations;
wherein the first projection lens is operable to project the image light of the first image light path onto the projection screen as a first diverging light beam between the first projection lens and the projection screen;
wherein the second projection lens is operable to project the image light of the second path image light onto the projection screen as a second diverging light beam between the second projection lens and the projection screen;
wherein the first and second projection lenses are further operable to direct the first diverging light beam and the second diverging light beam to superimpose and substantially overlap with each other on the projection screen;
wherein the polarization conversion system is positioned between the image source and the projection screen; and
wherein the polarization switch operates such that the first and second diverging light beams have the same state of polarization as they simultaneously reach the projection screen.

2. The polarization conversion system of claim 1, wherein the polarization switch is located on the first image light path and between the polarization beam splitter and the first projections lens; or
the polarization switch is located on the second image light path and between the polarization beam splitter and the second projections lens.

3. The polarization conversion system of claim 1, further comprising an initial relay lens operable to direct image light from an image source toward the polarization beam splitter.

4. The polarization conversion system of claim 1, further comprising a first relay lens located on the first image light path and operable to direct image light toward the first projection lens, and a second relay lens located on the second image light path and operable to direct image light toward the second projection lens.

5. The polarization conversion system of claim 1, further comprising a pre-polarizer located before the polarization switch.

6. The polarization conversion system of claim 1, wherein the reflecting element comprises one of a glass prism, a mirror, and a second polarization beam splitter.

7. The polarization conversion system of claim 1, wherein the polarization switch comprises a half wave plate.

8. The polarization conversion system of claim 1, wherein the image light from the image source comprises unpolarized light.

9. A method for projecting polarization-encoded images, comprising:
receiving image source light from a projection panel at a polarization beam splitter;
transmitting image source light of a first polarization state at the polarization beam splitter toward a first projection lens located on a first image light path;
reflecting image source light of a second polarization state at the polarization beam splitter toward a second image light path;
reflecting the image source light of the second polarization state toward a second projection lens;
sequentially alternating the state of polarization of light on at least one of the first and second image light paths between a first output state of polarization and a second output state of polarization so that the image light passing therethrough has sequentially alternating polarizations; and
projecting, using the first projection lens, the image light of the first image light path as a first diverging light beam between the first projection lens and a projection screen;
projecting, using the second projection lens, the image light of the second image light path as a second diverging light beam between the second projection lens and the projection screen;
wherein the first and second projection lenses further direct the first and second diverging light beams respectively toward the projection screen such that the first and second diverging light beams are superimposed and substantially overlapping on the projection screen; and
wherein the first and second diverging light beams have the same state of polarization as they simultaneously reach the projection screen.

10. The method for projecting polarization-encoded images of claim 9, wherein the first and second polarization output states comprise a substantially similar state of circularly polarized image light.

11. The method for projecting polarization-encoded images of claim 9, wherein the light on the first image light path passes through the first projection lens before being rotated.

12. The method for projecting polarization-encoded images of claim 9, wherein the light on the first image light path passes through the first projection lens after being rotated.

13. The method for projecting polarization-encoded images of claim 9, further comprising:
temporally rotating the state of polarization of the image light on the first and second image light paths to one of a first polarized output state and a second polarized output state.

14. The method for projecting polarization-encoded images of claim 9, wherein the image source light comprises unpolarized image light.

15. The polarization conversion system of claim 1, wherein the first and second image light paths substantially overlap with minimal keystone distortion at the projection screen to form the polarization-encoded images.

16. The method for projecting polarization-encoded images of claim 9, wherein the first and second image light paths substantially overlap with minimal keystone distortion at the projection screen to form the polarization-encoded images.