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`ATTORNEY DOCKET NO: 229501
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`Polarization conversion systems for stereoscopic
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`projection
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`Cross-Reference to Related Applications
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`[0001] This patent application is a continuation application Of and claims priority to US.
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`Patent Application NO. 13/047,763, now US. Patent NO. 8,220,934, entitled “Polarization
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`conversion system for stereoscopic projection”,
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`filed March 14, 2011, which is a
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`continuation application Of US. patent Application NO. 11/864,198, now US. Patent NO.
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`7,905,602, entitled “Polarization conversion system for stereoscopic projection”, filed
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`September 28, 2007, which relates and claims benefit Of: (a) US. provisional patent
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`application number 60/827,657, entitled “Polarization Conversion System for Cinematic
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`Projection,” filed 9/29/2006, (b) US. provisional patent application number 60/91 1,043,
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`entitled “Polarization conversion system for 3-D projection,” filed 4/10/2007, and (c)
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`US. provisional patent application number 60/950,652, entitled “Polarization conversion
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`system for 3-D projection,” filed 7/ 19/2007. All applications referenced above are herein
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`incorporated by reference in their entirety.
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`Technical Field
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`[0002] This disclosure relates to a projection system for projecting images for a three-
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`dimensional Viewing experience, and more in particular to a polarization conversion
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`system utilizing polarized light for encoding stereoscopic images.
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`Background
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`[0003] Three-dimensional (3D) imagery can be synthesized using polarization control
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`following the projector and polarization controlling eyewear (see, e.g., US. Pat. NO.
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`4,792,850 to Lipton, which is hereby incorporated by reference herein).
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`UTILITY PATENT APPLICATION
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`ATTORNEY DOCKET NO: 229501
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`[0004] A conventional implementation of polarization control at the projector is shown in
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`Figure 1.
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`In this implementation, nearly parallel rays emerge from the output of the lens
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`10, appearing to originate from a pupil 12 inside of the lens 10, and converge to form
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`spots on a screen 14. Ray bundles A, B, and C in Figure l are bundles forming spots at
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`the bottom, center, and top of a screen 14, respectively. The light 20 emerging from the
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`projection lens is randomly polarized, depicted in figure 1 as both s- and p-polarized light
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`[s-polarized light is conventionally represented as ‘o’, p-polarized light is represented
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`with a double arrow-ended line]. The light 20 passes through a linear polarizer 22,
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`resulting in a single polarization state after the polarizer 22. The orthogonal polarization
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`state is absorbed (or reflected), and the light flux after the polarizer 22 is typically less
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`than half of the original flux, thus resulting in a dimmer final image. The polarization
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`switch 30 is synchronized with the image frame, and the polarization state 24 emerging
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`from the polarization switch is alternated, producing images of alternately orthogonal
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`polarization at
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`the screen.
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`Polarization-selective eyewear allows images of one
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`polarization to pass to the left eye, and images of the orthogonal polarization to pass to
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`the right eye.
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`By presenting different
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`images to each eye, 3D imagery can be
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`synthesized.
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`[0005] This conventional system has been used in theatres. However, the conventional
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`system requires that greater than 50% of the light is absorbed by the polarizer, and the
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`resulting image is greater than 50% dimmer than that of a typical 2D theatre. The
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`dimmer image can limit the size of theatre used for 3D applications and/or provides a less
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`desirable viewing experience for the audience.
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`My
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`[0006] Addressing the aforementioned problems, various embodiments of polarization
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`conversion systems that receive light from a projector are described. The polarization
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`conversion systems present a brighter screen image in cinematic applications utilizing
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`polarized light for three-dimensional viewing.
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`[0007] In an embodiment, a polarization conversion system includes a polarization beam
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`splitter (PB S), a polarization rotator, and a polarization switch. The PBS is operable to
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`UTILITY PATENT APPLICATION
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`ATTORNEY DOCKET NO: 229501
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`receive randomly-polarized light bundles from a projector lens, and direct first light
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`bundles having a first state of polarization (SOP) along a first light path. The PBS is also
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`operable to direct second light bundles having a second SOP along a second light path.
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`The polarization rotator is located on the second light path, and is operable to translate
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`the second SOP to the first SOP. The polarization switch is operable to receive first and
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`second light bundles from the first and second light paths respectively, and to selectively
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`translate the polarization states of the first and second light bundles to one of a first
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`output SOP and a second output SOP. First light bundles are transmitted toward a
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`projection screen. A refiecting element may be located in the second light path to direct
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`second light bundles toward a projection screen such that the first and second light
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`bundles substantially overlap to form a brighter screen image.
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`[0008] In accordance with another aspect of the disclosure, a method for stereoscopic
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`image projection includes receiving randomly-polarized light from a projector, directing
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`first
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`state
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`of
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`polarization
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`(SOP)
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`light
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`on
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`a
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`first
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`light
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`path,
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`and
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`directing second SOP light on a second light path.
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`The method also includes
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`transforming the second SOP light on the second light path to first SOP light, and
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`selectively translating the first SOP light on both light paths to one of a first output SOP
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`and a second output SOP.
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`[0009] Other aspects and embodiments are described below in the detailed description.
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`Brief Description of the Drawings
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`[0010] Figure l
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`is a schematic diagram of a conventional polarization switch for
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`stereoscopic proj ection,
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`[0011] Figure 2 is a schematic diagram of a polarization conversion system (PCS) for
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`cinematic projection in accordance with the present disclosure,
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`[0012] Figure 3 is a schematic diagram of another embodiment of a PCS for cinematic
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`projection in accordance with the present disclosure,
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`[0013] Figure 4 is a schematic diagram of another embodiment of a PCS for cinematic
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`projection, including a telephoto lens along an optical path and with the field of view
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`centered on the optical aXis, in accordance with the present disclosure,
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`UTILITY PATENT APPLICATION
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`ATTORNEY DOCKET NO: 229501
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`[0014] Figure 5 is a schematic diagram of another embodiment of a PCS for cinematic
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`projection, including a telephoto lens along an optical path and with the field of view not
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`centered on the optical aXis, in accordance with the present disclosure;
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`[0015] Figure 6 is a schematic diagram of another embodiment of a PCS for cinematic
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`projection to provide a circularly-polarized output, including a telephoto lens along an
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`optical path and with field of view centered on an optical aXis, in accordance with the
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`present disclosure,
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`[0016] Figure 7 is a schematic diagram of another embodiment of a PCS for cinematic
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`projection to provide a linearly-polarized output,
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`including a telephoto lens along an
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`optical path and with field of view centered on an optical aXis, in accordance with the
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`present disclosure, and
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`[0017] Figure 8 is a schematic diagram of another embodiment of a PCS for cinematic
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`projection in accordance with the present disclosure.
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`mm
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`[0018] Various embodiments of polarization conversion systems that receive light from a
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`projector are described. The polarization conversion systems present a brighter screen
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`image in cinematic applications utilizing polarized light for three-dimensional viewing.
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`[0019] Figure 2 is a schematic diagram showing a polarization conversion system (PCS)
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`100 for cinematic projection. An embodiment of the polarization conversion system 100
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`includes a polarizing beam splitter (PBS) 112, a polarization rotator 114 (e.g., a half-
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`wave plate), a relecting element 116 (e.g., a fold mirror), and a polarization switch 120,
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`arranged as shown. The polarization conversion system 100 may receive images from a
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`conventional projector with a projection lens 122.
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`[0020] In operation, ray bundles A, B, and C emerge randomly polarized from the lens
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`122 and are projected toward a screen 130 to form an image.
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`In this embodiment, a PBS
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`112 is inserted in place of the polarizer 22 shown in Figure 1. The PBS 112 transmits P-
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`polarized light 124, and reflects S-polarized light 126. The P-polarized light 124 passes
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`ATTORNEY DOCKET NO: 229501
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`through the polarization switch (bundles A, B, and C) and is rotated by the polarization
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`switch in alternating frames, same as bundles A, B, and C in Figure 1.
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`[0021] The S-polarized light 126 reflected by the PBS 112 passes through a polarization
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`rotator 114 (e.g., a half-wave plate, preferably achromatic in some embodiments) and is
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`rotated to p-polarized light 128. The new p-polarized light 128 passes to a fold mirror
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`116.
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`The fold mirror 116 reflects the new p-polarized light 128 and passes it to
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`polarization switch 120. The polarization switch 120, acting on p-polarized ray bundles
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`A’, B’, and C’, rotates the polarization of the ray bundles in alternating frames,
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`in
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`synchronization with the rotation of bundles A, B, and C. The position of bundles A’, B’,
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`and C’ at the screen may be adjusted (e. g., by adjusting the tilt of the fold mirror 116) to
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`closely or exactly coincide with the positions of bundles A, B, and C at the screen. Since
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`nearly all of the randomly polarized light 106 from the projection lens 122 is imaged at
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`the screen 130 with a single polarization state, the resulting image of the system in Figure
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`2 is approximately two times brighter than the image at the screen for the system in
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`Figure 1.
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`[0022] In this exemplary embodiment, the PBS 112 in Figure 2 is depicted as a plate.
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`However, various types of PBSs may be used. For example, the PBS plate may be
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`constructed using a wire grid layer on glass (e.g., Proflux polarizer from Moxtek in
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`Orem, UT), polarization recycling fllm (e.g., Double Brightness Enhancing Film from
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`3M in St. Paul, MN), polarization recycling film on glass (for flatness), or a multi-
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`dielectric layer on glass. The PBS 112 in Figure 2 could alternatively be implemented as
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`a glass cube (with wire grid, polarization recycling film, or dielectric layers along the
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`diagonal) to reduce astigmatism in the final image associated with light passing through a
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`tilted plate. Alternatively,
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`the tilted plate PBS 112 in Figure 2 may,
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`in various
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`embodiments, be implemented with spherical, aspheric, cylindrical or toroidal surfaces to
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`reduce astigmatism in the final image at the screen 130. De-centered spherical, aspheric,
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`cylindrical or toroidal surfaces on the plate, and/or additional de-centered spherical,
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`aspheric, cylindrical or toroidal elements in the optical path after the plate can be
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`implemented to reduce astigmatism in the final image. See, e.g., “Simple method of
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`correcting the aberrations of a beamsplitter in converging light,” V. Doherty and D.
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`Shafer, Proc. SPIE, Vol. 0237, pp. 195-200, 1980, which is hereby incorporated by
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`UTILITY PATENT APPLICATION
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`ATTORNEY DOCKET NO: 229501
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`reference.
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`It should also be noted that a second flat plate may be inserted into the system
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`after the tilted PBS plate 112 and its tilt adjusted to reduce or correct astigmatism in the
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`final image.
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`[0023] In some embodiments,
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`the polarization rotator 114 in Figure 2 may be an
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`achromatic half-wave plate. The half-wave plate may be implemented with polymer
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`films (e.g., Achromatic Retardation Plate from ColorLink, Inc., Boulder, CO), quartz
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`plates, or a static liquid crystal device optionally patterned to account for geometric
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`polarization alteration. The half-wave plate 114 may be positioned as shown in Figure 2,
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`or in other embodiments,
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`it may be positioned between the fold mirror 116 and
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`polarization switch 120, intersecting ray bundles A’, B’, and C’. This implementation
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`may be desirable, as bundles A’, B’, and C’ reflect from the fold mirror 116 in s-
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`polarization state and mirrors often have a higher reflection for s-polarized light.
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`However, with such an implementation, the half-wave plate 114 should be located such
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`that bundles A’ and C do not overlap at
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`the plate. Although in most described
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`embodiments herein, the polarization rotator 114 is located in the second light path, it
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`may alternatively be placed in the first light path instead, and the polarization conversion
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`system will operate in a similar manner in accordance with the principles of the present
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`disclosure.
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`[0024] In some embodiments, the fold mirror 116 may be replaced with a PBS element
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`(e.g., wire grid plate).
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`In this case, a purer polarization may be maintained after the PBS
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`element.
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`[0025] Polarization switch 120 may be a switch as taught by US. Pat. No. 4,792,850, a
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`switch as taught by any of the switches of commonly-assigned US. Pat. App. No.
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`ll/424,087 entitled “Achromatic Polarization Switches”, filed June 14, 2006, both of
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`which are incorporated by reference in their entirety for all purposes, or any other
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`polarization switch known in the art that selectively transforms an incoming state of
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`polarization.
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`In some embodiments, the polarization switch 120 can be split (i.e., to
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`increase yield of the device). If the polarization switch 120 is split, it is desirable that the
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`two devices are located such that there is no overlap of bundles A’ and C in Figure 2.
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`Splitting the polarization switch 120 allows one portion to be relocated in the A’, B’, C’
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`optical path between the half-wave plate 114 and fold mirror 116.
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`Placing the
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`UTILITY PATENT APPLICATION
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`ATTORNEY DOCKET NO: 229501
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`polarization switch 120 here may call for the fold mirror 116 to have better polarization
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`preserving properties (e. g., a SilfleX coating from Oerlikon in Golden, CO) as this may be
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`the last element in the A’, B’, C’ optical path prior to the screen.
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`[0026] In the polarization conversion system 100 of Figure 2, the optical path of ray
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`bundle A’ is longer than that of ray bundle A (similarly B’-B and C’-C) resulting in a
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`magnification difference between the images produced by A’, B’, C’ and A, B, C. This
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`magnification difference may be unacceptable to an audience, especially for wide angle
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`and short-throw projection systems. Some techniques for correcting this magnif1cation
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`difference may include (1) providing a curved surface on the fold mirror 116 with optical
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`power that compensates for the magnification difference, this solution is achromatic,
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`which is desirable, (2) adding a fresnel or diffractive surface with optical power to the
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`fold mirror 116 to compensate for the magnification difference (which may or may not be
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`achromatic), (3) adding a refractive element (lens) between the fold mirror 116 and
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`polarization switch 120, or between the PBS 112 and fold mirror 116, a singlet lens is
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`unlikely to be achromatic, but a doublet solution can be achromatic, (4) addition of a
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`telephoto lens as illustrated in Figures 3 and 4, or (5) a combination of at least two of the
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`above four techniques.
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`[0027] Although as described, p-polarized light is transmitted toward the polarization
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`switch 120, while s-polarized light is directed toward half-wave plate 114, it should be
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`apparent to a person of ordinary skill in the art that an alternative configuration may be
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`employed in which s-polarized light is transmitted toward the polarization switch 120,
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`while p-polarized light is directed toward the half-wave plate 114.
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`[0028] Figure 3 is a schematic diagram showing another embodiment of a PCS for
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`cinematic projection 200. The elements of PCS 200 may be of similar type and function
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`for those shown with respect to PCS 100 of Figure 2. For instance, elements 2);); are
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`similar to elements lXX, where XX are the last two digits of the respective elements.
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`In
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`this embodiment, ray bundles A, B, and C may be directed through an additional set of
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`fold mirrors 232, 234 operable to equalize the optical path lengths of bundles A and A’, B
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`and B’, C and C’ as shown in Figure 3.
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`[Note: bundles A’ and C’ are present, but not
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`illustrated. They follow a similar path to the A’, B’, C’ bundles shown in Figure 2].
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`Note that although the PBS and fold mirrors are shown here to be orientated at 45 degrees
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`ATTORNEY DOCKET NO: 229501
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`to the optical axis,
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`the PBS 212 and fold mirrors 216, 232, 236 may have other
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`orientations in accordance with the present teachings. Additionally, glass may be
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`inserted into the optical path of A’, B’, and C’ (e. g., by replacing the fold mirror 216 with
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`a right angle prism and/or using a glass cube PBS in place of a plate PBS) to reduce or
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`eliminate the optical path difference between the A, B, C and A’, B’, C’ bundles,
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`respectively.
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`[0029] With reference to Figures 2 and 3, the image from bundles A’, B’, and C’ should
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`substantially overlap the image from bundles A, B, and C for viewing comfort (although
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`perfect overlap is not necessarily required). Some techniques of adjusting one image
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`location relative to the other include (1) using thumb screws or a similar mechanical
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`techniques to tilt the fold mirror, PBS plate, or PBS cube, (2) mechanically de-centering a
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`lens or element with optical power (e. g. curved mirror), (3) utilizing a feedback system to
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`automatically adjust image position via one of the aforementioned image adjustment
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`techniques, or (4) a combination of at least two of the above three techniques.
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`[0030] Optical
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`transmission and stray light control may be optimized on optically
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`transmissive elements by providing an anti-reflection coat thereon for high transmission
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`and low reflection. Reflections from transmissive elements can cause stray light in the
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`system which degrades contrast and/or produces disturbing artifacts in the final image. In
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`some embodiments, additional absorptive polarizers may be placed after the half-wave
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`plate 114 in the A’, B’, C’ path and/or after the PBS 112 in either path to control
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`polarization leakage and improve the final image contrast.
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`[0031] Figure 4 is a schematic diagram showing another embodiment of a PCS for
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`cinematic projection 300. The elements of PCS 300 may be of similar type and function
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`for those shown with respect to PCS 100 of Figure 2. For instance, elements 3xx are
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`similar to elements 1xx, where xx are the last two digits of the respective elements.
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`[0032] In this exemplary embodiment, a telephoto lens pair 340 may be implemented in
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`the optical path where light transmits through the PBS 312. Here, telephoto lens pair 340
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`is located along an optical path and with the field of view centered on the optical axis.
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`Typically, telephoto lens 340 allows control of magnif1cation, distortion, and imaging
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`properties with two elements such that the two images overlay relatively close, 1'.e.,
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`within 1-4 pixels of each other, while maintaining spots sizes on the order of a fraction of
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`ATTORNEY DOCKET NO: 229501
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`a pixel and lateral color on the order of a pixel. Alternatively, a reverse telephoto lens
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`(not shown) may be implemented in the optical path where light reflects from the PBS
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`312 (located between the polarization switch 320 and fold mirror 316, or after the fold
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`mirror 316). If a telephoto or reverse telephoto lens is used for controlling magnification
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`in one optical path, the radial distortion and keystone distortion of the final image can be
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`tuned by laterally displacing the individual elements or pair of elements from the optical
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`axis.
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`[0033] Figure 5 is a schematic diagram showing another embodiment of a PCS for
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`cinematic projection 400. The elements of PCS 400 may be of similar type and function
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`for those shown with respect to PCS 100 of Figure 2. For instance, elements 4xx are
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`similar to elements lxx, where xx are the last two digits of the respective elements.
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`In
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`this exemplary embodiment, a telephoto lens pair 440 may be implemented in the optical
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`path where light transmits through the PBS 412. Here, telephoto lens pair 440 is located
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`along an optical path and with the field of view decentralized from the optical axis. Just
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`as described above, the radial distortion and keystone distortion of the final image can be
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`tuned by laterally displacing the individual elements or pair of elements 440 from the
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`optical axis.
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`[0034] Figure 6 is a schematic diagram of another embodiment of a PCS for cinematic
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`projection 500 that provides a circularly polarized output. PCS 500 includes a telephoto
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`lens pair 540 along an optical path, with field of view centered on an optical axis. In this
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`case, each polarization switch 520 is a circular polarization switch (or Z-screen), e. g., as
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`described in US. Pat. No. 4,792,850. The cleanup polarizers 542, 544 in each path are
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`optional, depending on the level of contrast desired from the system. For example,
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`including one or both cleanup polarizers may enhance the system contrast.
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`[0035] Figure 7 is a schematic diagram of another embodiment of a PCS for cinematic
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`projection 600 that provides a linearly polarized output. Here, each polarization switch
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`620 is an achromatic linear polarization switch, as described in US. Pat. App. No.
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`11/424,087 entitled “Achromatic Polarization Switches”,
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`filed June 14, 2006, also
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`manufactured by ColorLink, Inc., of Boulder, Colorado. Similar to the example in Figure
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`6, cleanup polarizers 642, 644 in each path are optional, depending on the level of
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`contrast desired from the system. For example, including one or both cleanup polarizers
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`may enhance the system contrast. Additionally, the achromatic rotator 648 is optional,
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`depending on the achromatic properties of the polarization switch 620.
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`[0036] Figure 8 is a schematic diagram of another embodiment of a PCS for cinematic
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`projection 700, showing an alternative configuration in which the polarizers 746,
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`achromatic rotator 714, and polarization switches 720 are located after other optical
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`components. The elements of PCS 700 may be of similar type and function for those
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`shown with respect to PCS 100 of Figure 2. For instance, elements 7xx are similar to
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`elements lxx, where xx are the last two digits of the respective elements.
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`[0037] In operation, light exits projection lens 722 toward PBS 712. P-polarized light
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`passes through PBS 7l2 toward telephoto lens pair 740, then toward polarization switch
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`720. An optional cleanup polarizer 746 may be located between telephoto lens pair 740
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`and polarization switch 720 to further enhance contrast. The s-polarized light reflected
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`by PBS 712 is directed toward fold mirror 716, where it reflects toward an achromatic
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`rotator 714 that transforms the s-polarized light into p-polarized light, then it passes
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`through an optional cleanup polarizer 746. Next, the p-polarized light from achromatic
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`rotator 7l4 passes through polarization switch 720.
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`In this configuration, the s-polarized
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`light reflected by the PBS 716 is efficiently reflected, with polarization maintained by the
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`fold mirror 716. This relaxes any want for polarization preservation from the fold path
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`and maximizes brightness. An achromatic 9Oo rotator 7l4 (probably retarder stack
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`based) can be used to convert light from the fold mirror to the orthogonal state.
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`In order
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`to eliminate P-reflection from the PBS 712, a clean up polarizer 746 is likely desirable.
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`This preferably follows the achromatic rotator 7l4,
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`thereby reducing polarization
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`conversion efficiency as a factor in system level contrast.
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`[0038] PCS 700 provides a high contrast
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`image on the screen.
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`In this exemplary
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`embodiment,
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`the final screen image has a center located on the optical axis of the
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`projection lens.
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`In some other embodiments, the final screen image may be located off-
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`center from the optical axis — for example, a half screen height below the optical axis of
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`the projection lens.
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`In such embodiments,
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`the polarizing beamsplitter 712 may be
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`relocated to intercept the full illumination from the projection lens 722, and the fold
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`mirror 716 may be tilted to properly overlay the two images on the screen.
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`The
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`polarization switch 720 in this embodiment has been split into two elements (one for each
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`path)
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`to increase fabrication yield;
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`although,
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`as previously discussed,
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`it could
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`alternatively be a single unit.
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`[0039] As used herein, the term “cinematic projection” refers to the projection of images
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`using front and/or rear projection techniques, and includes, but
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`is not
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`limited to,
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`applications for cinema, home theatre, simulators, instrumentation, head-up displays,. and
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`other projection environments where stereoscopic images are displayed.
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`[0040] While several embodiments and variations of polarization conversion systems for
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`stereoscopic projection have been described above, it should be understood that they have
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`been presented by way of example only, and not limitation. Thus, the breadth and scope
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`of the invention(s) should not be limited by any of the above-described exemplary
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`embodiments, but should be defined only in accordance with any claims and their
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`equivalents issuing from this disclosure. Furthermore, the above advantages and features
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`are provided in described embodiments, but shall not limit the application of such issued
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`claims to processes and structures accomplishing any or all of the above advantages.
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`[0041] Additionally, the section headings herein are provided for consistency with the
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`suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These
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`headings shall not limit or characterize the invention(s) set out in any claims that may
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`issue from this disclosure. Specif1cally and by way of example, although the headings
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`refer to a “Technical Field,” such claims should not be limited by the language chosen
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`under this heading to describe the so-called technical field. Further, a description of a
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`technology in the “Background” is not to be construed as an admission that technology is
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`prior art to any invention(s) in this disclosure. Neither is the “Brief Summary” to be
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`considered as
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`a characterization of the invention(s)
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`set
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`forth in issued claims.
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`Furthermore, any reference in this disclosure to “invention” in the singular should not be
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`used to argue that there is only a single point of novelty in this disclosure. Multiple
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`inventions may be set forth according to the limitations of the multiple claims issuing
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`from this disclosure, and such claims accordingly define the invention(s), and their
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`equivalents, that are protected thereby.
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`In all instances, the scope of such claims shall be
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`considered on their own merits in light of this disclosure, but should not be constrained
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`by the headings set forth herein.
`
`ll
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`
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