WO2017091538A1

WO2017091538A1 - Optical system, method, and applications

Info

Publication number: WO2017091538A1
Application number: PCT/US2016/063244
Authority: WO
Inventors: Yun-Han Lee; Shin-Tson Wu
Original assignee: University Of Central Florida Research Foundation, Inc.
Priority date: 2015-11-23
Filing date: 2016-11-22
Publication date: 2017-06-01

Abstract

An electrically controllable optical imaging system (and associated methods) in which a liquid crystal (LC) panel (without polarizers or color filters) is utilized to control the input light polarization from a polarized display device. Different polarization states lead to different path lengths in the proposed optical system that includes a polarization dependent component, which in turn results in different focal lengths or different image distances and magnifications upon electrically controlling the LC panel.

Description

OPTICAL SYSTEM, METHOD, AND APPLICATIONS

PRIORITY DATA

The instant application claims priority to us provisional application s/n 62/258,707 filed November 23, 2015, the subject matter of which is incorporated by reference herein in its entirety.

FEDERALLY SPONSORED RESEARCH

The invention was made with funding from the AFOSR under project #6501- 6269. The U.S. government has certain rights in the invention.

FIELD OF THE INVENTION

Aspects and embodiments of the invention relate to an electrically controllable optical imaging system and associated methods and applications thereof and, more particularly, to an optical system, and associated methods and applications thereof, in which the resultant images' distances and magnifications can be controlled upon applying voltage to a liquid crystal cell.

BACKGROUND

Virtual reality and augmented reality are emerging wearable display technologies for immersive video games and interactive 3D graphics. A critical issue with these display devices is distance matching. For stereoscopic 3D displays that are based on sending different images to different eyes, e.g. Oculus Rift™, the perceived image may locate at a distance different from the eye's focal length, thus causing eye-brain conflict and eye strain. For devices such as Google Glass™, since only one eye is receiving information the above mentioned problem is mitigated. Instead, its major problem is the mismatch between the distances of a displayed image and the surrounding image because the displayed image remains in a certain plane. In this case, the viewer cannot focus on the image from the device and the surrounding objects simultaneously. In either case mentioned above, the need for a tunable/switchable lens is apparent. Even though several types of tunable/switchable lenses have been proposed before, in order to achieve a depth- fused 3D display, which forms the sense of 3D through fast switching between different focuses, it is required to switch between at least six image planes while keeping the same angular size. For a display with 60 frames per second, when switching between six focal lengths in a field sequential manner, the response time should be less than 3 ms. Among many proposed tunable/switchable lenses/ mirrors, only a few candidates can achieve such a fast response time. Microelectromechanical systems (MEMS), in particular, a deformable membrane mirror device is a promising candidate, while a current-induced shape-changing lens is another option. However, the former is relatively difficult to fabricate for a large aperture and it is a reflective device, while the latter is essentially a current device, which consumes more power.

SUMMARY

We disclose here an electrically controllable optical imaging system (and associated methods) in which a liquid crystal panel (without polarizers or color filters), advantageously a twisted nematic (TN) panel, is utilized to control the input light polarization from a polarized display device. Different polarization states lead to different path lengths in the proposed optical system that includes either a polarizing beam splitter, a wire-grid polarizer, a dual-brightness enhancement film, a uniaxial plate or a biaxial plate, or the combination of these components, which in turn results in different focal lengths or different image distances and magnifications upon controlling the TN panel electrically. This system has advantages in fast response time, low chromatic aberration, and low operation voltage. Using a pixelated TN panel, we can create depth information to the selected pixels and thus add depth information to a 2D image. By cascading three or more such device structures together, we can generate 8 or more different focuses for 3D displays, wearable virtual/augmented reality, and other head mounted display devices.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 shows one embodiment of the proposed device to illustrate the operation principle. FIG. 2 shows another embodiment of the proposed device.

FIG. 3 shows yet another embodiment of the proposed device.

FIG. 4 shows another possible embodiment of the proposed device.

FIG. 5 shows another possible embodiment using a uniaxial or biaxial plate.

FIG. 6 shows another embodiment of the proposed device using only one lens.

FIG. 7 shows yet another embodiment of the proposed device using only one lens.

FIG. 8 shows another possible embodiment of the proposed device using only one lens. FIG. 9 shows another possible embodiment using a uniaxial or biaxial plate using only one lens.

FIG. 10 shows one possible embodiment for a multi-focal device.

FIG. 11 shows another possible embodiment for a multi-focal device using a uniaxial or biaxial plate.

FIG. 12 shows another possible embodiment for a multi-focal device using both a uniaxial or biaxial plate and a polarizing beam splitter.

DESCRIPTION OF NON-LIMITING. EXEMPLARY EMBODIMENTS

We propose apparatus and methods to obtain fast focus-control through the use of a 90° twisted nematic (TN) LC cell. The TN cell is known as an electrically controllable achromatic half-wave (λ/2) plate. For a linearly polarized incident light, say p-wave, the output light can be converted to s-wave if the applied voltage is off (V=0) or, it can remain p-wave if V»V_th (threshold voltage). In most practical devices, V_th lies in the range from 2.5 Vrms to 15 Vrms- For a commercial TN display, the response time can be as fast as 2 ms, or sub-millisecond if the cell gap is reduced to 1.6 μπι, or if a dual-frequency LC is employed while keeping the driving voltage below 10 Vrms- Also, due to its achromatic nature, a TN-assisted lens system has very little chromatic aberration. With these advantages, here we incorporate a TN cell as a polarization control in our optical system. The outgoing beam will travel along different paths, depending on the p- or s- wave, when entering a polarization dependent component such as a polarizing beam splitter (PBS), a wire-grid polarizer (WGP), a dual brightness enhancement film (DBEF), or a uniaxial/biaxial plate (e.g. calcite, calomel, etc.). Upon properly controlling and recombining the output waves (with, e.g., retardation films, lenses, and/or mirrors), this path difference can be exploited to change the effective focal distance or image planes and thus effectively forms a high-speed controllable lens with 0.1-3 ms switching time. Following this principle, different setups, such as the number of lenses/mirrors, focal length of each lens/mirror, the distance between the lens/mirror, the image distance, magnification, and the direction of outgoing light can be utilized for different

applications. A special advantage of this device is that it can create depth information to a 2D image when the TN panel is pixelated.

Figure 1 shows one possible system embodiment for illustrating the operation principle. In this embodiment, a 90° TN cell 12 is used to control the polarization of the input light of a display 11 between s-wave and p-wave. After passing through a first lens 13, one polarization (dashed arrow, say, p-wave) passes straight through PBS 14, a first quarter-wave retarder 15, and reaches a first mirror 16. Upon reflection, this p-wave beam again passes through the quarter-wave retarder 15. The first quarter-wave retarder 15 is aligned such that the p-wave polarization state converts to s-wave. It is thus reflected by the PBS 14 toward a second lens 17 and directed towards an observer's eye 18. The other polarization (solid arrow, say s-wave) is reflected first by the PBS 14 toward a second quarter- wave retarder 19 and a second mirror 20. Upon reflection, the s-wave beam passes through the second quarter-wave retarder 19 one more time and converts to p- wave. As a result, it passes through the PBS 14 and the second lens 17 towards the eye. Since one mirror (e.g., 16) is placed closer to PBS 14 than the other mirror 20, its path is shorter. The resultant images from the display have different sizes and locations in the observer's eye 18 when switching between different paths through the TN cell 12.

Figure 2 shows another possible embodiment that also uses two lenses 21 and 25, two quarter- wave retarders 24 and 26, two mirrors 23 and 27, and one PBS 22. In this embodiment the polarized light from the display 28 passes through the TN panel 29 with one polarization (dashed arrow) passing right through the PBS 22, while the other polarization (solid arrow) is twice reflected before exiting the PBS. The outgoing light and the incoming light are along the same direction. The display 28 may be, but is not limited to, a micro light-emitting-diode display, a liquid crystal display, a

microelectromechanical display, or an organic light-emitting-diode display panel. The TN panel 29 may be pixelated such that in one frame of the display, different sizes and locations can be applied to different display pixels of the display 28. The lenses 21, 25 and mirrors 23, 27 can be at arbitrary distances with respect to the PBS and they can have arbitrary curvatures depending on the application. In a simple example, the curvatures of the mirrors is zero, i.e., flat mirrors. The focal length of lens 21 is 1.0-30.0 cm, the focal length of lens 25 is 1.0-30 cm, and the width of the PBS is 0.25-5.0 cm. In this manner, for a display locating within 20.0 cm before the first lens, the resultant image perceived at within 20.0 cm after the second lens will be at different distances depending on the polarization. More lenses and mirrors may be added to provide better image quality. For example, one can add a second lens adjacent to the existing lenses to form an achromat for the reduction of chromatic aberration. Likewise, one can add a correcting lens for reduction of achromatic aberration. The PBS could be replaced with a wire-grid polarizer, or a dual brightness enhancement film, or other polarization dependent component as known in the art. The mirrors could be replaced with a wire-grid polarizer, or a dual brightness enhancement film for see-through devices as known in the art. These same options for design freedom holds for the other disclosed embodiments.

Figure 3 shows another possible embodiment which also exploits two lenses 31 and 33, two quarter- wave retarders 34 and 36, two mirrors 35 and 37, and one PBS 32. In this embodiment the polarized light from the display 38 passes through the TN panel 39 with one polarization (solid arrow) passing right through the PBS 32 and the other (dashed arrow) reflecting twice before exiting the PBS. The outgoing light experiences a 90° turn in direction with respect to the incoming light.

Figure 4 shows another possible embodiment which also exploits two lenses 41 and 43, two quarter- wave retarders 44 and 46, two mirrors 45 and 47, and one PBS 42. In this embodiment the polarized light from the display 48 passes through the TN panel 49 with both polarizations reflecting once at the PBS. The outgoing light experiences a 90° turn in direction with respect to the incoming light.

Figure 5 shows yet another possible embodiment which exploits two lenses 51 and 53, and one uniaxial plate 52. In this embodiment the polarized light from the display 54 passes through the TN panel 55 with the uniaxial plate being placed such that the refractive index experienced by each polarization is different and hence the optical paths are different. This uniaxial plate can be made of calcite crystal, calomel crystal, polyethylene naphthalate (PEN) or polyethylene terephthalate (PET), or other uniaxial material with proper birefringence. This uniaxial plate can also be replaced with a biaxial plate. Same arguments for the design freedom hold for other embodiments. The above mentioned embodiments all exploit two lenses. Below we disclose some possible embodiments using only one lens. In cases with only one lens, the object- to-lens distance or the lens-to-image distance can be controlled through the TN panel.

Figure 6 shows another possible embodiment which exploits one lens 64, two quarter-wave retarders 63 and 65, two mirrors 62 and 66, and one PBS 61. In this embodiment the polarized light from the display 67 passes through the TN panel 68 with one polarization (dashed arrow) passing right through the PBS 61, while the other (solid arrow) reflects twice before exiting the PBS. The outgoing light and the incoming light are along the same direction.

Figure 7 shows another possible embodiment which also exploits one lens 72, two quarter- wave retarders 73 and 75, two mirrors 74 and 76, and one PBS 71. In this embodiment the polarized light from the display 77 passes through the TN panel 78 with one polarization (solid arrow) passing right through the PBS 71 yet the other (dashed arrow) reflecting twice before exiting the PBS. The outgoing light experiences a 90° turn in direction with respect to the incoming light.

Figure 8 shows another possible embodiment which also exploits one lens 82, two quarter- wave retarders 83 and 85, two mirrors 84 and 86, and one PBS 81. In this embodiment the polarized light from the display 87 passes through the TN panel 88 with both polarizations reflected once at the PBS. The outgoing light experiences a 90° turn in direction with respect to the incoming light.

Figure 9 shows yet another possible embodiment which exploits one lens 92, and one uniaxial plate 91. In this embodiment the polarized light from the display 93 passes through the TN panel 94 with uniaxial plate being placed such that the refractive index experienced by each polarization is different and hence the optical paths are different. The above mentioned possible embodiments are capable of controlling two focal distances or two image distances since each polarization contributes to only one focal distance. Next we describe some possible embodiments that provide more than two controllable distances.

Figure 10 shows a possible embodiment which also exploits one lens 105, two quarter-wave retarders 107 and 108, two mirrors 102 and 103, three PBSs 104 and three TN panels 101. The three TN panels may be controlled separately to provide more than two different distances from the display 106 to the lens 105.

Figure 11 shows yet another possible embodiment which exploits one lens 113, three uniaxial plates 112, and three TN panels 111. Three TN panels may be controlled separately to provide more than two different optical distances from the display 114 to the lens 113. The thickness of each plate can be made differently so that different

combination of focal lengths or image distances may be obtained.

It is also possible to combine both PBS and uniaxial/biaxial plates to form multi- focal or multi-distance switching devices.

Figure 12 shows one possible embodiment which exploits both PBS and uniaxial/biaxial plates for image distance controlling. Similar to the embodiment shown in Figure 8, it also exploits one lens 130, two quarter-wave retarders 124 and 126, two mirrors 125 and 127, and one PBS 123. In this embodiment the polarized light from the display 121 passes through a TN panel 122 with both polarizations reflected once at the PBS. A pair of TN 128 and uniaxial/biaxial plate 129 is placed after the PBS 127 and before the lens 130 so that two more image distances can be obtained.

Claims

We claim:

1. An electrically controllable optical imaging system, comprising:

a liquid crystal cell having an input side and an output side, a thickness, d (microns), an ordinary and an extraordinary refractive index having a difference, Δη, and, liquid crystals (LCs) on the input side of said cell being oriented with their directors at an angle between 45-135° to the directors of the LCs on the output side of said cell;

a multiply-polarized light beam-emitting display disposed on the input side of the liquid crystal cell positioned so that the polarizations of the light beams emitted from the display pass through and can be controlled by the liquid crystal cell;

a polarization dependent component disposed on the output side of the liquid crystal cell in the path of the multiply-polarized emitted light beams from the liquid crystal cell through which the multiply-polarized light beams can propagate, wherein the differently polarized beams have different optical paths; a mirror and a lens disposed in each of the optical paths of the output beams from the polarization dependent component, wherein the different optical paths result in different final image distances and a dAn in the range of 0.1-1.0 microns; and

electrodes disposed so as to establish an electric field through the liquid crystal cell that can modulate the polarization rotation of the light in accordance with an electrical signal applied to said electrodes.

2. The electrically controllable optical imaging system of 1, wherein said display is a liquid crystal display.

3. The electrically controllable optical imaging system of claim 1, wherein said display is an organic light-emitting-diode display.

4. The electrically controllable optical imaging system of claim 1, wherein said display is a micro light-emitting-diode display.

5. The electrically controllable optical imaging system of claim 1, wherein said display is a microelectromechanical display.

6. The electrically controllable optical imaging system of claim 1, wherein the liquid crystals on the input side of said cell are oriented with their directors at approximately 90° to the directors of the LCs on the output side of said liquid crystal cell.

7. The electrically controllable optical imaging system of claim 1, wherein said liquid crystal cell is pixelated.

8. The electrically controllable optical imaging system of claim 1, wherein the

polarization dependent component is a wire-grid polarizer.

9. The electrically controllable optical imaging system of claim 1, wherein the

polarization dependent component is a polarizing beam splitter.

10. The electrically controllable optical imaging system of claim 1, wherein the

polarization dependent component is a dual brightness enhancement film.

11. The electrically controllable optical imaging system of claim 1, further comprising a quarter-wave retarder disposed intermediate the polarization dependent component and at least one of the mirrors.

12. The electrically controllable optical imaging system of claim 11, wherein the quarter-wave retarder is laminated onto the mirror.

13. The electrically controllable optical imaging system of claim 1, comprising a plurality of mirrors.

14. The electrically controllable optical imaging system of claim 13, wherein the mirrors are disposed equal to or less than 30.0 cm from the polarization dependent component.

15. The electrically controllable optical imaging system of claim 13, wherein the mirrors are laminated onto the polarization dependent component.

16. The electrically controllable optical imaging system of claim 13, wherein one or more of the mirrors are flat mirrors.

17. The electrically controllable optical imaging system of claim 13, wherein one or more of the mirrors are concave mirrors with a focal length from 1.0 to 30.0 cm.

18. The electrically controllable optical imaging system of claim 13, wherein one or more of the mirrors are convex mirrors with a focal length from -30.0 to -1.0 cm.

19. The electrically controllable optical imaging system of claim 13, wherein one or more of the mirrors are free-form mirrors.

20. The electrically controllable optical imaging system of claim 1, comprising a plurality of lenses.

21. The electrically controllable optical imaging system of claim 20, wherein the lenses are disposed equal to or less than 30.0 cm from the polarization dependent component.

22. The electrically controllable optical imaging system of claim 20, wherein the lenses are laminated onto the polarization dependent component.

23. The electrically controllable optical imaging system of claim 20, wherein one or more of the lenses are concave lenses with a focal length from 1.0 to 30.0 cm.

24. The electrically controllable optical imaging system of claim 20, wherein one or more of the lenses are convex lenses with a focal length from -30.0 to -1.0 cm.

25. The electrically controllable optical imaging system of claim 20, wherein one or more of the lenses are achromatic lenses.

26. The electrically controllable optical imaging system of claim 20, wherein one or more of the lenses are aspherical lenses.

27. The electrically controllable optical imaging system of claim 20, wherein one or more of the lenses are free-form lenses.

28. The electrically controllable optical imaging system of claim 1, further

comprising:

one or more repeated sets of the liquid crystal cells and the polarization dependent components positioned on the output side of the liquid crystal cell, with a dAn in the range of 0.1-1.0 microns.

29. An electrically controllable optical imaging system, comprising:

at least one of a uniaxial and a biaxial plate disposed on the output side of the liquid crystal cell in the path of the multiply-polarized emitted light beams from the liquid crystal cell through which the multiply- polarized light beams can propagate, wherein the differently polarized beams have different optical paths; and at least one lens disposed at least on an output side of the at least one uniaxial and biaxial plate.

30. The electrically controllable optical imaging system of claim 29, wherein the uniaxial crystal has a birefringence in the range from -0.6 to 0.6.

31. The electrically controllable optical imaging system of claim 29, wherein the biaxial crystal has a birefringence in the range from -0.6 to 0.6 between two of its optical axes.