Distortion matching polarization conversion systems and methods thereof

Abstract

Optical systems having at least one polarization beam splitter (PBS) are operable to receive randomly-polarized light bundles from a projector lens. The PBS is further operable to direct light bundles having a state of polarization (SOP) along a light path and operable to direct other light bundles having a different SOP along different light paths. The light paths have optical path lengths which may differ. Each light path produces an image having a distortion which may differ from the distortion of an image produced by a different light path. A compensation in a light path is operable to convert a non-compensated distortion of an image into a compensated distortion that more closely matches the distortion of images in other light paths.

Description

TECHNICAL FIELD

This disclosure generally relates to polarization systems, and more particularly to distortion matching in polarization conversion systems.

BACKGROUND

Three-dimensional imagery can be synthesized using polarization control following the projector and also employing polarization controlling eyewear, as generally discussed in U.S. Pat. No. 4,792,850 entitled “Method and system employing a push-pull liquid crystal modulator” filed Nov. 25, 1987, and in U.S. Pat. No. 7,905,602 entitled “Polarization conversion system for stereoscopic projection” filed Sep. 28, 2007, both of which are herein incorporated by reference in their entireties.

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.

This conventional system has been used in theaters. 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 theater. The dimmer image may limit the size of theater used for 3D applications and/or may provide a less desirable viewing experience for the audience.

BRIEF SUMMARY

According to an aspect of the present disclosure, an optical system may include a polarization conversion system. The polarization conversion system may include at least a polarization beam splitter (PBS) 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, and direct second light bundles having a second SOP along a second light path, a polarization rotator located on the first light path, the polarization rotator being operable to translate the first SOP to the second SOP and a polarization switch 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. The polarization switch may include first and second polarization switch panels, the first polarization switch panel receiving light from the first light path, and the second polarization switch panel receiving light from the second light path. The polarization conversion system may also include at least one of the following: providing a curved surface on the fold mirror with optical power that compensates for the magnification difference; (2) adding a Fresnel or diffractive surface with optical power to the reflecting element to compensate for the magnification difference; (3) adding a refractive element (lens) between the reflecting element and polarization switch, or between the PBS and reflecting element; or (4) addition of a telephoto lens.

Disclosed herein are optical systems having at least one polarization beam splitter (PBS). The PBS is operable to receive randomly-polarized light bundles from a projector lens. The PBS is further operable to direct light bundles having a state of polarization (SOP) along a light path and operable to direct other light bundles having a different SOP along different light paths. The light paths have optical path lengths which may differ. Each light path produces an image having a distortion which may differ from the distortion of an image produced by a different light path. A compensation in a light path is operable to convert a non-compensated distortion of an image into a compensated distortion that more closely matches the distortion of images in other light paths.

In some embodiments, a compensation may remove distortion from a light path, and in some embodiments, a compensation may add distortion to a light path. In some embodiments, the non-compensated distortion and/or the compensated distortion in a light path may be equivalent to substantially no distortion.

In some embodiments, a compensation may create a vertical tilt in a lens, a half-lens, or a lens pair in a light path. A compensation may be achieved by adjusting a mounting location associated with the optical system, by a mechanical device such as a shim or a hinge, or by some other process or device.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanying figures, in which like reference numbers indicate similar parts, and in which:

FIG. 1 is a schematic diagram illustrating a polarization conversion system, in accordance with the present disclosure;

FIG. 2 is a schematic diagram illustrating a polarization conversion system, in accordance with the present disclosure;

FIG. 3 is a schematic diagram illustrating a polarization conversion system with no lens tilt, in accordance with the present disclosure;

FIG. 4A is a diagram illustrating reflected light path distortion on-screen in a polarization conversion system with no lens tilt, in accordance with the present disclosure;

FIG. 4B is a diagram illustrating transmitted light path distortion on-screen in a polarization conversion system with no lens tilt, in accordance with the present disclosure;

FIG. 5 is a schematic diagram illustrating a polarization conversion system with lens tilt, in accordance with the present disclosure;

FIG. 6A is a diagram illustrating reflected light path distortion on-screen in a polarization conversion system with lens tilt, in accordance with the present disclosure;

FIG. 6B is a diagram illustrating transmitted light path distortion on-screen in a polarization conversion system with lens tilt, in accordance with the present disclosure;

FIG. 7 is a schematic diagram illustrating a stacked polarization system with tilted lens pairs for distortion matching, in accordance with the present disclosure;

FIG. 8A is a diagram illustrating light path distortion on-screen in a stacked polarization system with no distortion matching, in accordance with the present disclosure; and

FIG. 8B is a diagram illustrating light path distortion on-screen in a stacked polarization system with tilted lens pairs for distortion matching, in accordance with the present disclosure.

DETAILED DESCRIPTION

According to an aspect of the present disclosure, an optical system may include a polarization conversion system. The polarization conversion system may include at least a polarization beam splitter (PBS) 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, and direct second light bundles having a second SOP along a second light path, a polarization rotator located on the first light path, the polarization rotator being operable to translate the first SOP to the second SOP and a polarization switch 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. The polarization switch may include first and second polarization switch panels, the first polarization switch panel receiving light from the first light path, and the second polarization switch panel receiving light from the second light path. The polarization conversion system may also include at least one of the following: providing a curved surface on the fold mirror 116 with optical power that compensates for the magnification difference; (2) adding a Fresnel or diffractive surface with optical power to the reflecting element to compensate for the magnification difference; (3) adding a refractive element (lens) between the reflecting element and polarization switch, or between the PBS and reflecting element; or (4) addition of a telephoto lens.

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.

Polarization-preserving stereoscopic cinema systems have been generally described in several patents and patent applications, for example U.S. Pat. No. 7,905,602 entitled “Polarization conversion system for stereoscopic projection” filed Sep. 28, 2007, U.S. Pat. No. 7,857,455 entitled “Combining P and S rays for bright stereoscopic projection” filed Oct. 18, 2006, U.S. Pat. No. 8,727,536 entitled “Polarization conversion system and method for projecting polarization encoded imagery” filed May 9, 2008, WO 2013/010167 entitled “Optical systems with compact back focal lengths” filed Jul. 16, 2012, and U.S. Pat. No. 8,220,934 entitled “Polarization conversion system for stereoscopic projection” filed Mar. 14, 2011, all of which are herein incorporated by reference in their entireties.

Randomly polarized light from a projector is split into orthogonal polarization states, re-directed and manipulated to produce matching polarization states, and overlaid on-screen for viewing. These systems may actively switch polarization states, for example with liquid-crystal switches or use passive components in dual projector configurations or one projector per eye which may provide greater image brightness.

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 herein incorporated by reference in its entirety).

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.

This conventional system has been used in theaters. 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 theater. The dimmer image may limit the size of theater used for 3D applications and/or may provide a less desirable viewing experience for the audience.

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, for example a half-wave plate, a reflecting element 116, for example 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.

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.

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.

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, aspherical, cylindrical or toroidal surfaces to reduce astigmatism in the final image at the screen 130. De-centered spherical, aspherical, cylindrical or toroidal surfaces on the plate, and/or additional de-centered spherical, aspherical, 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 in its entirety. 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.

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.

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.

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. Pat. No. 7,528,906 entitled “Achromatic Polarization Switches” filed Jun. 14, 2006; hereby incorporated by reference in its entirety, 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, for example, 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.

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) any combination of at least two of the above techniques.

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.

FIG. 3 depicts a polarization conversion system which may be used for creating stereoscopic imagery in digital cinema. The two optical paths (a reflected path 320 and a transmitted path 330) may have different optical path lengths. Without some form of compensation, the two images produced by the two optical paths may not overlay well on-screen. A lens pair 350 may be included in the transmitted path 330, for example, to change the magnification of the transmitted path image to better match the reflected path image.

For situations when the throw distance (i.e., the distance from projector to screen) is long relative to the distance between reflected and transmitted path optical axes, the image overlay with the art in FIG. 3 is satisfactory. However, for decreasing throw distances, the reflected path image may become more trapezoidal in nature, and overlay of the two images at the corners becomes more difficult. FIG. 4A and FIG. 4B depict the image distortions, magnified 100×, in the two optical paths when the throw distance becomes very short. FIG. 4A shows the trapezoidal distortion of the image in the reflected path 320, whereas FIG. 4B shows the relatively undistorted nature of the image in the transmitted path 330.

Additionally, for situations where the throw ratio decreases, in which the throw ratio may be defined as the ratio of throw distance to screen width, the reflected path image again can be excessively trapezoidal resulting in overlay difficulties.

The mismatch in distortion of the two optical paths can be overcome in short throw and low throw ratio situations by introducing a matching trapezoidal distortion in the transmitted optical path 330 that substantially matches the distortion in the reflected optical path 320. Alternatively, trapezoidal distortion may be removed from the reflected optical path 320 to better match the transmitted path 330. In other words, a compensation may be included in one or both of the light paths to convert the native (non-compensated) distortion into compensated distortions that more closely match.

Introducing trapezoidal distortion in the transmitted optical path can be accomplished by introducing a tilt in the vertical direction of one or both lenses in the lens pair. In this case, the lens tilt may create varying magnification from top to bottom of the transmitted path image. FIG. 5 is similar to FIG. 3, except that in FIG. 5 a single lens 520 in the lens pair 350 is tilted by approximately 1.5 degrees in the transmitted path 330. The lens may be tilted by a mechanical shim, by a hinge, by an adjustment of mechanical mounting locations, or by any number of mechanical means. For example, a 2.9 millimeter shim may be placed at location 510 to achieve the tilt.

FIG. 6A and FIG. 6B depict the image distortions, magnified 100×, in the two optical paths when the throw distance becomes very short and a single lens 520 in a lens pair 350 is tilted by approximately 1.5 degrees. FIG. 6A is similar to FIG. 4A and shows the trapezoidal distortion of the image in the reflected path 320, whereas FIG. 6B shows the compensated distortion of the image in the transmitted path 330. The images in the reflected path 320 and the transmitted path 330 are now better overlaid because compensating trapezoidal distortion has been introduced in the transmitted path 330 that is similar to the distortion in the reflected path 320, resulting in significant image overlay improvement in the corners in a short throw environment.

This type of distortion matching can be used with single or multiple projector systems to improve image overlay at the screen.

For polarization conversion systems utilizing a stacked configuration, in which two polarization conversion systems are stacked on top of each other to produce overlaid images on-screen, tilted lenses may again be used to improve the image overlay at the screen. FIG. 7 depicts such a system where two lens pairs 710 are included, one for each reflected path 320, to reduce the trapezoidal distortion in each of the reflected paths 320. Additionally depicted is an optional lens pair 720 in the transmitted path 330 for further magnification matching. Note that the lens pairs 710 in the reflected paths 320 may be lens halves, as depicted in FIG. 7, or the lens pairs 710 may be full lenses. FIG. 8A illustrates light path distortion on-screen in a stacked polarization system with no distortion matching, while FIG. 8B illustrates light path distortion on-screen in a stacked polarization system with tilted lens pairs for distortion matching, as depicted in FIG. 7.

As may be used herein, the terms “substantially” and “approximately” provide an industry-accepted tolerance for its corresponding term and/or relativity between items. Such an industry-accepted tolerance ranges from zero to ten percent and corresponds to, but is not limited to, component values, angles, et cetera. Such relativity between items ranges between approximately zero percent to ten percent.

While various embodiments in accordance with the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of this disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages.

Additionally, the section headings herein are provided for consistency with the suggestions under 37 CFR 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the embodiment(s) set out in any claims that may issue from this disclosure. Specifically and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology in the “Background” is not to be construed as an admission that certain technology is prior art to any embodiment(s) in this disclosure. Neither is the “Summary” to be considered as a characterization of the embodiment(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 embodiments may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the embodiment(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.

Claims

1. An optical system, comprising: at least one polarization beam splitter (PBS) operable to receive randomly-polarized light bundles from a projector lens, further operable to direct first light bundles having a first state of polarization (SOP) along a first light path having a first optical path length to produce a first image having a first non-compensated distortion, further operable to direct second light bundles having a second SOP along a second light path having a second optical path length to produce a second image having a second distortion, and further operable to direct third light bundles having the first SOP along a third light path having a third optical path length to produce a third image having a third non-compensated distortion, the first non-compensated distortion and the second distortion having differing amounts of trapezoidal distortion;a compensation in the first light path, the compensation operable to convert the first non-compensated distortion into a first compensated distortion, the first compensated distortion more closely matching the second distortion than the first non-compensated distortion;a second compensation in the third light path, the second compensation operable to convert the third non-compensated distortion into a third compensated distortion, the third compensated distortion more closely matching the second distortion than the third non-compensated distortion; anda first half-lens pair in the first light path and a second half-lens pair in the third light path, wherein the compensation creates a vertical tilt in at least one of the half-lenses in the first half-lens pair, and wherein the second compensation creates a vertical tilt in at least one of the half-lenses in the second half-lens pair.

2. The optical system of claim 1, wherein the first non-compensated distortion is more trapezoidal than the second distortion, and wherein the compensation removes trapezoidal distortion from the first light path.

3. The optical system of claim 1, wherein the first non-compensated distortion is less trapezoidal than the second distortion, and wherein the compensation introduces additional trapezoidal distortion to the first light path.

4. The optical system of claim 1, wherein the compensation is achieved by adjusting a mounting location associated with the optical system.

5. The optical system of claim 1, wherein the compensation is achieved by a mechanical device.

6. The optical system of claim 5, wherein the mechanical device is selected from the group consisting of a shim and a hinge.

7. The optical system of claim 5, wherein the mechanical device is a 2.9 millimeter shim.

8. The optical system of claim 1, wherein the first light path is a transmitted light path, and wherein the second light path is a reflected light path.

9. The optical system of claim 1, wherein the first optical path length is substantially similar to the third optical path length, wherein the first non-compensated distortion is substantially similar to the third non-compensated distortion, and wherein the first compensated distortion is substantially similar to the third compensated distortion.

10. The optical system of claim 1, further comprising: a polarization rotator located on the first light path, the polarization rotator being operable to translate the first SOP to the second SOP.

11. The optical system of claim 10, further comprising: a polarization switch operable to receive first and second light bundles from the first and second light paths respectively, and further operable 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, wherein the polarization switch comprises first and second polarization switch panels, the first polarization switch panel receiving light from the first light path, and the second polarization switch panel receiving light from the second light path.

12. The optical system of claim 1, wherein the first non-compensated distortion is substantially no distortion.