Polarization conversion systems for stereoscopic projection

A polarization conversion system (PCS) is located in the output light path of a projector. The PCS may include a polarizing beam splitter, a polarization rotating element, a reflecting element, and a polarization switch. Typically, a projector outputs randomly-polarized light. This light is input to the PCS, in which the PCS separates p-polarized light and s-polarized light at the polarizing beam splitter. P-polarized light is directed toward the polarization switch on a first path. The s-polarized light is passed on a second path through the polarization rotating element (e.g., a half-wave plate), thereby transforming it to p-polarized light. A reflecting element directs the transformed polarized light (now p-polarized) along the second path toward the polarization switch. The first and second light paths are ultimately directed toward a projection screen to collectively form a brighter screen image in cinematic applications utilizing polarized light for three-dimensional viewing.

TECHNICAL FIELD

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

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).

A conventional implementation of polarization control at the projector is shown inFIG. 1. In this implementation, nearly parallel rays emerge from the output of the lens10, appearing to originate from a pupil12inside of the lens10, and converge to form spots on a screen14. Ray bundles A, B, and C inFIG. 1are bundles forming spots at the bottom, center, and top of a screen14, respectively. The light20emerging from the projection lens is randomly polarized, depicted inFIG. 1as 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 light20passes through a linear polarizer22, resulting in a single polarization state after the polarizer22. The orthogonal polarization state is absorbed (or reflected), and the light flux after the polarizer22is typically less than half of the original flux, thus resulting in a dimmer final image. The polarization switch30is synchronized with the image frame, and the polarization state24emerging 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.

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

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.

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.

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.

Other aspects and embodiments are described below in the detailed description.

DESCRIPTION

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.

FIG. 2is a schematic diagram showing a polarization conversion system (PCS)100for cinematic projection. An embodiment of the polarization conversion system100includes a polarizing beam splitter (PBS)112, a polarization rotator114(e.g., a half-wave plate), a relecting element116(e.g., a fold mirror), and a polarization switch120, arranged as shown. The polarization conversion system100may receive images from a conventional projector with a projection lens122.

In operation, ray bundles A, B, and C emerge randomly polarized from the lens122and are projected toward a screen130to form an image. In this embodiment, a PBS112is inserted in place of the polarizer22shown inFIG. 1. The PBS112transmits P-polarized light124, and reflects S-polarized light126. The P-polarized light124passes 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 inFIG. 1.

The S-polarized light126reflected by the PBS112passes through a polarization rotator114(e.g., a half-wave plate, preferably achromatic in some embodiments) and is rotated to p-polarized light128. The new p-polarized light128passes to a fold mirror116. The fold mirror116reflects the new p-polarized light128and passes it to polarization switch120. The polarization switch120, 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 mirror116) to closely or exactly coincide with the positions of bundles A, B, and C at the screen. Since nearly all of the randomly polarized light106from the projection lens122is imaged at the screen130with a single polarization state, the resulting image of the system inFIG. 2is approximately two times brighter than the image at the screen for the system inFIG. 1.

In this exemplary embodiment, the PBS112inFIG. 2is 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 PBS112inFIG. 2could 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 PBS112inFIG. 2may, in various embodiments, be implemented with spherical, aspheric, cylindrical or toroidal surfaces to reduce astigmatism in the final image at the screen130. 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 plate112and its tilt adjusted to reduce or correct astigmatism in the final image.

In some embodiments, the polarization rotator114inFIG. 2may 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 plate114may be positioned as shown inFIG. 2, or in other embodiments, it may be positioned between the fold mirror116and polarization switch120, intersecting ray bundles A′, B′, and C′. This implementation may be desirable, as bundles A′, B′, and C′ reflect from the fold mirror116in s-polarization state and mirrors often have a higher reflection for s-polarized light. However, with such an implementation, the half-wave plate114should be located such that bundles A′ and C do not overlap at the plate. Although in most described embodiments herein, the polarization rotator114is 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.

In some embodiments, the fold mirror116may be replaced with a PBS element (e.g., wire grid plate). In this case, a purer polarization may be maintained after the PBS element.

Polarization switch120may 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 switch120can be split (i.e., to increase yield of the device). If the polarization switch120is split, it is desirable that the two devices are located such that there is no overlap of bundles A′ and C inFIG. 2. Splitting the polarization switch120allows one portion to be relocated in the A′, B′, C′ optical path between the half-wave plate114and fold mirror116. Placing the polarization switch120here may call for the fold mirror116to 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.

In the polarization conversion system100ofFIG. 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 mirror116with 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 mirror116to compensate for the magnification difference (which may or may not be achromatic); (3) adding a refractive element (lens) between the fold mirror116and polarization switch120, or between the PBS112and fold mirror116; a singlet lens is unlikely to be achromatic, but a doublet solution can be achromatic; (4) addition of a telephoto lens as illustrated inFIGS. 3 and 4; or (5) a combination of at least two of the above four techniques.

Although as described, p-polarized light is transmitted toward the polarization switch120, while s-polarized light is directed toward half-wave plate114, 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 switch120, while p-polarized light is directed toward the half-wave plate114.

FIG. 3is a schematic diagram showing another embodiment of a PCS for cinematic projection200. The elements of PCS200may be of similar type and function for those shown with respect to PCS100ofFIG. 2. For instance, elements2xx are similar to elements1xx, 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 mirrors232,234operable to equalize the optical path lengths of bundles A and A′, B and B′, C and C′ as shown inFIG. 3. [Note: bundles A′ and C′ are present, but not illustrated. They follow a similar path to the A′, B′, C′ bundles shown inFIG. 2]. Note that although the PBS and fold mirrors are shown here to be orientated at 45 degrees to the optical axis, the PBS212and fold mirrors216,232,236may 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 mirror216with 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.

With reference toFIGS. 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.

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 plate114in the A′, B′, C′ path and/or after the PBS112in either path to control polarization leakage and improve the final image contrast.

FIG. 4is a schematic diagram showing another embodiment of a PCS for cinematic projection300. The elements of PCS300may be of similar type and function for those shown with respect to PCS100ofFIG. 2. For instance, elements3xx are similar to elements1xx, where xx are the last two digits of the respective elements.

In this exemplary embodiment, a telephoto lens pair340may be implemented in the optical path where light transmits through the PBS312. Here, telephoto lens pair340is located along an optical path and with the field of view centered on the optical axis. Typically, telephoto lens340allows 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 PBS312(located between the polarization switch320and fold mirror316, or after the fold mirror316). 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.

FIG. 5is a schematic diagram showing another embodiment of a PCS for cinematic projection400. The elements of PCS400may be of similar type and function for those shown with respect to PCS100ofFIG. 2. For instance, elements4xx are similar to elements1xx, where xx are the last two digits of the respective elements. In this exemplary embodiment, a telephoto lens pair440may be implemented in the optical path where light transmits through the PBS412. Here, telephoto lens pair440is 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 elements440from the optical axis.

FIG. 6is a schematic diagram of another embodiment of a PCS for cinematic projection500that provides a circularly polarized output. PCS500includes a telephoto lens pair540along an optical path, with field of view centered on an optical axis. In this case, each polarization switch520is a circular polarization switch (or Z-screen), e.g., as described in U.S. Pat. No. 4,792,850. The cleanup polarizers542,544in 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.

FIG. 7is a schematic diagram of another embodiment of a PCS for cinematic projection600that provides a linearly polarized output. Here, each polarization switch620is 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 inFIG. 6, cleanup polarizers642,644in 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 rotator648is optional, depending on the achromatic properties of the polarization switch620.

FIG. 8is a schematic diagram of another embodiment of a PCS for cinematic projection700, showing an alternative configuration in which the polarizers746, achromatic rotator714, and polarization switches720are located after other optical components. The elements of PCS700may be of similar type and function for those shown with respect to PCS100ofFIG. 2. For instance, elements7xx are similar to elements1xx, where xx are the last two digits of the respective elements.

In operation, light exits projection lens722toward PBS712. P-polarized light passes through PBS712toward telephoto lens pair740, then toward polarization switch720. An optional cleanup polarizer746may be located between telephoto lens pair740and polarization switch720to further enhance contrast. The s-polarized light reflected by PBS712is directed toward fold mirror716, where it reflects toward an achromatic rotator714that transforms the s-polarized light into p-polarized light, then it passes through an optional cleanup polarizer746. Next, the p-polarized light from achromatic rotator714passes through polarization switch720. In this configuration, the s-polarized light reflected by the PBS716is efficiently reflected, with polarization maintained by the fold mirror716. This relaxes any want for polarization preservation from the fold path and maximizes brightness. An achromatic 90° rotator714(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 PBS712, a clean up polarizer746is likely desirable. This preferably follows the achromatic rotator714, thereby reducing polarization conversion efficiency as a factor in system level contrast.

PCS700provides 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 beamsplitter712may be relocated to intercept the full illumination from the projection lens722, and the fold mirror716may be tilted to properly overlay the two images on the screen. The polarization switch720in 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.

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.