Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application is a continuation application of and claims priority to U.S. patent application Ser. No. 13/047,763, now U.S. Pat. No. 8,220,934, entitled “Polarization conversion system for stereoscopic projection”, filed Mar. 14, 2011, which is a continuation application of U.S. patent application Ser. No. 11/864,198, now U.S. Pat. No. 7,905,602, entitled “Polarization conversion system for stereoscopic projection”, filed Sep. 28, 2007, which relates and claims benefit of: (a) U.S. provisional patent application No. 60/827,657, entitled “Polarization Conversion System for Cinematic Projection,” filed Sep. 29, 2006; (b) U.S. provisional patent application No. 60/911,043, entitled “Polarization conversion system for 3-D projection,” filed Apr. 10, 2007; and (c) U.S. provisional patent application No. 60/950,652, entitled “Polarization conversion system for 3-D projection,” filed Jul. 19, 2007. All applications referenced above are herein incorporated by reference in their entirety. 
     
    
     TECHNICAL FIELD  
       [0002]    This disclosure relates to a projection system for projecting images for a three-dimensional viewing experience, and more in particular to a polarization conversion system utilizing polarized light for encoding stereoscopic images. 
       BACKGROUND  
       [0003]    Three-dimensional (3D) imagery can be synthesized using polarization control following the projector and polarization controlling eyewear (see, e.g., U.S. Pat. No. 4,792,850 to Lipton, which is hereby incorporated by reference herein). 
         [0004]    A conventional implementation of polarization control at the projector is shown in  FIG. 1 . In this implementation, nearly parallel rays emerge from the output of the lens  10 , appearing to originate from a pupil  12  inside of the lens  10 , and converge to form spots on a screen  14 . Ray bundles A, B, and C in  FIG. 1  are bundles forming spots at the bottom, center, and top of a screen  14 , respectively. The light  20  emerging from the projection lens is randomly polarized, depicted in  FIG. 1  as both s- and p-polarized light [s-polarized light is conventionally represented as ‘o’; p-polarized light is represented with a double arrow-ended line]. The light  20  passes through a linear polarizer  22 , resulting in a single polarization state after the polarizer  22 . The orthogonal polarization state is absorbed (or reflected), and the light flux after the polarizer  22  is typically less than half of the original flux, thus resulting in a dimmer final image. The polarization switch  30  is synchronized with the image frame, and the polarization state  24  emerging from the polarization switch is alternated, producing images of alternately orthogonal polarization at the screen. Polarization-selective eyewear 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. 
         [0005]    This conventional system has been used in theatres. However, the conventional system requires that greater than 50% of the light is absorbed by the polarizer, and the resulting image is greater than 50% dimmer than that of a typical 2D theatre. The dimmer image can limit the size of theatre used for 3D applications and/or provides a less desirable viewing experience for the audience. 
       SUMMARY 
       [0006]    Addressing the aforementioned problems, various embodiments of polarization conversion systems that receive light from a projector are described. The polarization conversion systems present a brighter screen image in cinematic applications utilizing polarized light for three-dimensional viewing. 
         [0007]    In an embodiment, a polarization conversion system includes a polarization beam splitter (PBS), a polarization rotator, and a polarization switch. The PBS is operable to receive randomly-polarized light bundles from a projector lens, and direct first light bundles having a first state of polarization (SOP) along a first light path. The PBS is also operable to direct second light bundles having a second SOP along a second light path. The polarization rotator is located on the second light path, and is operable to translate the second SOP to the first SOP. The polarization switch is operable to receive first and second light bundles from the first and second light paths respectively, and to selectively translate the polarization states of the first and second light bundles to one of a first output SOP and a second output SOP. First light bundles are transmitted toward a projection screen. A reflecting element may be located in the second light path to direct second light bundles toward a projection screen such that the first and second light bundles substantially overlap to form a brighter screen image. 
         [0008]    In accordance with another aspect of the disclosure, a method for stereoscopic image projection includes receiving randomly-polarized light from a projector, directing first state of polarization (SOP) light on a first light path, and directing second SOP light on a second light path. The method also includes transforming the second SOP light on the second light path to first SOP light, and selectively translating the first SOP light on both light paths to one of a first output SOP and a second output SOP. 
         [0009]    Other aspects and embodiments are described below in the detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic diagram of a conventional polarization switch for stereoscopic projection; 
           [0011]      FIG. 2  is a schematic diagram of a polarization conversion system (PCS) for cinematic projection in accordance with the present disclosure; 
           [0012]      FIG. 3  is a schematic diagram of another embodiment of a PCS for cinematic projection in accordance with the present disclosure; 
           [0013]      FIG. 4  is a schematic diagram of another embodiment of a PCS for cinematic projection, including a telephoto lens along an optical path and with the field of view centered on the optical axis, in accordance with the present disclosure; 
           [0014]      FIG. 5  is a schematic diagram of another embodiment of a PCS for cinematic projection, including a telephoto lens along an optical path and with the field of view not centered on the optical axis, in accordance with the present disclosure; 
           [0015]      FIG. 6  is a schematic diagram of another embodiment of a PCS for cinematic projection to provide a circularly-polarized output, including a telephoto lens along an optical path and with field of view centered on an optical axis, in accordance with the present disclosure; 
           [0016]      FIG. 7  is a schematic diagram of another embodiment of a PCS for cinematic projection to provide a linearly-polarized output, including a telephoto lens along an optical path and with field of view centered on an optical axis, in accordance with the present disclosure; and 
           [0017]      FIG. 8  is a schematic diagram of another embodiment of a PCS for cinematic projection in accordance with the present disclosure. 
       
    
    
     DESCRIPTION  
       [0018]    Various embodiments of polarization conversion systems that receive light from a projector are described. The polarization conversion systems present a brighter screen image in cinematic applications utilizing polarized light for three-dimensional viewing. 
         [0019]      FIG. 2  is a schematic diagram showing a polarization conversion system (PCS)  100  for cinematic projection. An embodiment of the polarization conversion system  100  includes a polarizing beam splitter (PBS)  112 , a polarization rotator  114  (e.g., a half-wave plate), a relecting element  116  (e.g., a fold mirror), and a polarization switch  120 , arranged as shown. The polarization conversion system  100  may receive images from a conventional projector with a projection lens  122 . 
         [0020]    In operation, ray bundles A, B, and C emerge randomly polarized from the lens  122  and are projected toward a screen  130  to form an image. In this embodiment, a PBS  112  is inserted in place of the polarizer  22  shown in  FIG. 1 . The PBS  112  transmits P-polarized light  124 , and reflects S-polarized light  126 . The P-polarized light  124  passes through the polarization switch (bundles A, B, and C) and is rotated by the polarization switch in alternating frames, same as bundles A, B, and C in  FIG. 1 . 
         [0021]    The S-polarized light  126  reflected by the PBS  112  passes through a polarization rotator  114  (e.g., a half-wave plate, preferably achromatic in some embodiments) and is rotated to p-polarized light  128 . The new p-polarized light  128  passes to a fold mirror  116 . The fold mirror  116  reflects the new p-polarized light  128  and passes it to polarization switch  120 . The polarization switch  120 , acting on p-polarized ray bundles A′, B′, and C′, rotates the polarization of the ray bundles in alternating frames, in synchronization with the rotation of bundles A, B, and C. The position of bundles A′, B′, and C′ at the screen may be adjusted (e.g., by adjusting the tilt of the fold mirror  116 ) to closely or exactly coincide with the positions of bundles A, B, and C at the screen. Since nearly all of the randomly polarized light  106  from the projection lens  122  is imaged at the screen  130  with a single polarization state, the resulting image of the system in  FIG. 2  is approximately two times brighter than the image at the screen for the system in  FIG. 1 . 
         [0022]    In this exemplary embodiment, the PBS  112  in  FIG. 2  is depicted as a plate. However, various types of PBSs may be used. For example, the 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  112  in  FIG. 2  could alternatively be implemented as a glass cube (with wire grid, polarization recycling film, or dielectric layers along the diagonal) to reduce astigmatism in the final image associated with light passing through a tilted plate. Alternatively, the tilted plate PBS  112  in  FIG. 2  may, in various embodiments, be implemented with spherical, aspheric, cylindrical or toroidal surfaces to reduce astigmatism in the final image at the screen  130 . De-centered spherical, aspheric, cylindrical or toroidal surfaces on the plate, and/or additional de-centered spherical, aspheric, cylindrical or toroidal elements in the optical path after the plate can be implemented to reduce astigmatism in the final image. See, e.g., “Simple method of correcting the aberrations of a beamsplitter in converging light,” V. Doherty and D. Shafer, Proc. SPIE, Vol. 0237, pp. 195-200, 1980, which is hereby incorporated by reference. It should also be noted that a second flat plate may be inserted into the system after the tilted PBS plate  112  and its tilt adjusted to reduce or correct astigmatism in the final image. 
         [0023]    In some embodiments, the polarization rotator  114  in  FIG. 2  may be an achromatic half-wave plate. The half-wave plate may be implemented with polymer films (e.g., Achromatic Retardation Plate from ColorLink, Inc., Boulder, Colo.), quartz plates, or a static liquid crystal device optionally patterned to account for geometric polarization alteration. The half-wave plate  114  may be positioned as shown in  FIG. 2 , or in other embodiments, it may be positioned between the fold mirror  116  and polarization switch  120 , intersecting ray bundles A′, B′, and C′. This implementation may be desirable, as bundles A′, B′, and C′ reflect from the fold mirror  116  in s-polarization state and mirrors often have a higher reflection for s-polarized light. However, with such an implementation, the half-wave plate  114  should be located such that bundles A′ and C do not overlap at the plate. Although in most described embodiments herein, the polarization rotator  114  is located in the second light path, it may alternatively be placed in the first light path instead, and the polarization conversion system will operate in a similar manner in accordance with the principles of the present disclosure. 
         [0024]    In some embodiments, the fold mirror  116  may be replaced with a PBS element (e.g., wire grid plate). In this case, a purer polarization may be maintained after the PBS element. 
         [0025]    Polarization switch  120  may be a switch as taught by U.S. Pat. No. 4,792,850; a switch as taught by any of the switches of commonly-assigned U.S. patent application Ser. No. 11/424,087 entitled “Achromatic Polarization Switches”, filed Jun. 14, 2006; both of which are incorporated by reference in their entirety for all purposes, or any other polarization switch known in the art that selectively transforms an incoming state of polarization. In some embodiments, the polarization switch  120  can be split (i.e., to increase yield of the device). If the polarization switch  120  is split, it is desirable that the two devices are located such that there is no overlap of bundles A′ and C in  FIG. 2 . Splitting the polarization switch  120  allows one portion to be relocated in the A′, B′, C′ optical path between the half-wave plate  114  and fold mirror  116 . Placing the polarization switch  120  here may call for the fold mirror  116  to have better polarization preserving properties (e.g., a Silflex coating from Oerlikon in Golden, Colo.) as this may be the last element in the A′, B′, C′ optical path prior to the screen. 
         [0026]    In the polarization conversion system  100  of  FIG. 2 , the optical path of ray bundle A′ is longer than that of ray bundle A (similarly B′-B and C′-C) resulting in a magnification difference between the images produced by A′, B′, C′ and A, B, C. This magnification difference may be unacceptable to an audience, especially for wide angle and short-throw projection systems. Some techniques for correcting this magnification difference may include (1) providing a curved surface on the fold mirror  116  with optical power that compensates for the magnification difference; this solution is achromatic, which is desirable; (2) adding a fresnel or diffractive surface with optical power to the fold mirror  116  to compensate for the magnification difference (which may or may not be achromatic); (3) adding a refractive element (lens) between the fold mirror  116  and polarization switch  120 , or between the PBS  112  and fold mirror  116 ; a singlet lens is unlikely to be achromatic, but a doublet solution can be achromatic; (4) addition of a telephoto lens as illustrated in  FIGS. 3 and 4 ; or (5) a combination of at least two of the above four techniques. 
         [0027]    Although as described, p-polarized light is transmitted toward the polarization switch  120 , while s-polarized light is directed toward half-wave plate  114 , it should be apparent to a person of ordinary skill in the art that an alternative configuration may be employed in which s-polarized light is transmitted toward the polarization switch  120 , while p-polarized light is directed toward the half-wave plate  114 . 
         [0028]      FIG. 3  is a schematic diagram showing another embodiment of a PCS for cinematic projection  200 . The elements of PCS  200  may be of similar type and function for those shown with respect to PCS  100  of  FIG. 2 . For instance, elements  2 xx are similar to elements  1 xx, where xx are the last two digits of the respective elements. In this embodiment, ray bundles A, B, and C may be directed through an additional set of fold mirrors  232 ,  234  operable to equalize the optical path lengths of bundles A and A′, B and B′, C and C′ as shown in  FIG. 3 . [Note: bundles A′ and C′ are present, but not illustrated. They follow a similar path to the A′, B′, C′ bundles shown in  FIG. 2 ]. Note that although the PBS and fold mirrors are shown here to be orientated at 45 degrees to the optical axis, the PBS  212  and fold mirrors  216 ,  232 ,  236  may have other orientations in accordance with the present teachings. Additionally, glass may be inserted into the optical path of A′, B′, and C′ (e.g., by replacing the fold mirror  216  with a right angle prism and/or using a glass cube PBS in place of a plate PBS) to reduce or eliminate the optical path difference between the A, B, C and A′, B′, C′ bundles, respectively. 
         [0029]    With reference to  FIGS. 2 and 3 , the image from bundles A′, B′, and C′ should substantially overlap the image from bundles A, B, and C for viewing comfort (although perfect overlap is not necessarily required). Some techniques of adjusting one image location relative to the other include (1) using thumb screws or a similar mechanical techniques to tilt the fold mirror, PBS plate, or PBS cube; (2) mechanically de-centering a lens or element with optical power (e.g. curved mirror); (3) utilizing a feedback system to automatically adjust image position via one of the aforementioned image adjustment techniques; or (4) a combination of at least two of the above three techniques. 
         [0030]    Optical transmission and stray light control may be optimized on optically transmissive elements by providing an anti-reflection coat thereon for 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. In some embodiments, additional absorptive polarizers may be placed after the half-wave plate  114  in the A′, B′, C′ path and/or after the PBS  112  in either path to control polarization leakage and improve the final image contrast. 
         [0031]      FIG. 4  is a schematic diagram showing another embodiment of a PCS for cinematic projection  300 . The elements of PCS  300  may be of similar type and function for those shown with respect to PCS  100  of  FIG. 2 . For instance, elements  3 xx are similar to elements  1 xx, where xx are the last two digits of the respective elements. 
         [0032]    In this exemplary embodiment, a telephoto lens pair  340  may be implemented in the optical path where light transmits through the PBS  312 . Here, telephoto lens pair  340  is located along an optical path and with the field of view centered on the optical axis. Typically, telephoto lens  340  allows control of magnification, distortion, and imaging properties with two elements such that the two images overlay relatively close, i.e., within 1-4 pixels of each other, while maintaining spots sizes on the order of a fraction of a pixel and lateral color on the order of a pixel. Alternatively, a reverse telephoto lens (not shown) may be implemented in the optical path where light reflects from the PBS  312  (located between the polarization switch  320  and fold mirror  316 , or after the fold mirror  316 ). If a telephoto or reverse telephoto lens is used for controlling magnification in one optical path, the radial distortion and keystone distortion of the final image can be tuned by laterally displacing the individual elements or pair of elements from the optical axis. 
         [0033]      FIG. 5  is a schematic diagram showing another embodiment of a PCS for cinematic projection  400 . The elements of PCS  400  may be of similar type and function for those shown with respect to PCS  100  of  FIG. 2 . For instance, elements  4 xx are similar to elements  1 xx, where xx are the last two digits of the respective elements. In this exemplary embodiment, a telephoto lens pair  440  may be implemented in the optical path where light transmits through the PBS  412 . Here, telephoto lens pair  440  is located along an optical path and with the field of view decentralized from the optical axis. Just as described above, the radial distortion and keystone distortion of the final image can be tuned by laterally displacing the individual elements or pair of elements  440  from the optical axis. 
         [0034]      FIG. 6  is a schematic diagram of another embodiment of a PCS for cinematic projection  500  that provides a circularly polarized output. PCS  500  includes a telephoto lens pair  540  along an optical path, with field of view centered on an optical axis. In this case, each polarization switch  520  is a circular polarization switch (or Z-screen), e.g., as described in U.S. Pat. No. 4,792,850. The cleanup polarizers  542 ,  544  in each path are optional, depending on the level of contrast desired from the system. For example, including one or both cleanup polarizers may enhance the system contrast. 
         [0035]      FIG. 7  is a schematic diagram of another embodiment of a PCS for cinematic projection  600  that provides a linearly polarized output. Here, each polarization switch  620  is an achromatic linear polarization switch, as described in U.S. patent application Ser. No. 11/424,087 entitled “Achromatic Polarization Switches”, filed Jun. 14, 2006; also manufactured by ColorLink, Inc., of Boulder, Colo. Similar to the example in  FIG. 6 , cleanup polarizers  642 ,  644  in each path are optional, depending on the level of contrast desired from the system. For example, including one or both cleanup polarizers may enhance the system contrast. Additionally, the achromatic rotator  648  is optional, depending on the achromatic properties of the polarization switch  620 . 
         [0036]      FIG. 8  is a schematic diagram of another embodiment of a PCS for cinematic projection  700 , showing an alternative configuration in which the polarizers  746 , achromatic rotator  714 , and polarization switches  720  are located after other optical components. The elements of PCS  700  may be of similar type and function for those shown with respect to PCS  100  of  FIG. 2 . For instance, elements  7 xx are similar to elements  1 xx, where xx are the last two digits of the respective elements. 
         [0037]    In operation, light exits projection lens  722  toward PBS  712 . P-polarized light passes through PBS  712  toward telephoto lens pair  740 , then toward polarization switch  720 . An optional cleanup polarizer  746  may be located between telephoto lens pair  740  and polarization switch  720  to further enhance contrast. The s-polarized light reflected by PBS  712  is directed toward fold mirror  716 , where it reflects toward an achromatic rotator  714  that transforms the s-polarized light into p-polarized light, then it passes through an optional cleanup polarizer  746 . Next, the p-polarized light from achromatic rotator  714  passes through polarization switch  720 . In this configuration, the s-polarized light reflected by the PBS  716  is efficiently reflected, with polarization maintained by the fold mirror  716 . This relaxes any want for polarization preservation from the fold path and maximizes brightness. An achromatic 90° rotator  714  (probably retarder stack based) can be used to convert light from the fold mirror to the orthogonal state. In order to eliminate P-reflection from the PBS  712 , a clean up polarizer  746  is likely desirable. This preferably follows the achromatic rotator  714 , thereby reducing polarization conversion efficiency as a factor in system level contrast. 
         [0038]    PCS  700  provides a high contrast image on the screen. In this exemplary embodiment, the final screen image has a center located on the optical axis of the projection lens. In some other embodiments, the final screen image may be located off-center from the optical axis—for example, a half screen height below the optical axis of the projection lens. In such embodiments, the polarizing beamsplitter  712  may be relocated to intercept the full illumination from the projection lens  722 , and the fold mirror  716  may be tilted to properly overlay the two images on the screen. The polarization switch  720  in this embodiment has been split into two elements (one for each path) to increase fabrication yield; although, as previously discussed, it could alternatively be a single unit. 
         [0039]    As used herein, the term “cinematic projection” refers to the projection of images using front and/or rear projection techniques, and includes, but is not limited to, applications for cinema, home theatre, simulators, instrumentation, head-up displays,. and other projection environments where stereoscopic images are displayed. 
         [0040]    While several embodiments and variations of polarization conversion systems for stereoscopic projection 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. 
         [0041]    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,” 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 “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.

Technology Category: 3