Optical system for display

Abstract

An optical system (100, 111, 125, 126, 127, 128) includes an aperture expanding light guide optical element (LOE) (106) having major surfaces (103a, 103b) separated by a first thickness T1. The LOE (106) includes a redirection configuration for progressively redirecting light within the LOE and coupling the light out towards the observer. The coupling-in device comprises a coupling light guide element (CLE) (104) having mutually parallel surfaces separated by a second thickness T2 not greater than half the first thickness T1. The CLE (104) is bonded to the major surface (103a) at an interface (105) provided with a beam splitter coating having a reflectivity of at least 50%. The coupling device also includes an input coupler deployed to couple light corresponding to the collimated image into the CLE.

Description

[0001] Technical field and background of the present invention

[0002] The present invention relates to optical systems for displays, and in particular, the present invention relates to arrangements for injecting an image into a light-guiding optical element having an expanded optical aperture.

[0003] Lightguide-based displays use a lightguide, typically in the form of a plate with mutually parallel front and back surfaces, to direct an image in front of a user's eye for outcoupling toward the eye for viewing. In some cases, a lightguide can achieve one- or two-dimensional optical aperture expansion by progressively redirecting light within the lightguide and / or during outcoupling. Progressive redirection of light is typically performed by a set of embedded partial reflectors or by a diffractive optical element.

[0004] Coupling an image into a light guide presents a design challenge. Optimal image uniformity is achieved when the image light "fills" the light guide thickness—that is, when all of the image's light rays and their reflections are present at every point within the light guide's thickness. This would require a relatively large projector and coupling configuration, which defeats the practical purpose of minimizing the system's size and weight. Summary of the Invention

[0005] The present invention is an optical system.

[0006] According to the teachings of an embodiment of the present invention, an optical system is provided, the optical system comprising: (a) an aperture expansion lightguide optical element (LOE) having a pair of mutually parallel major surfaces separated by a first thickness, the LOE supporting propagation of light by internal reflection at the major surfaces, the LOE having: (i) a first redirection configuration disposed in a first region of the LOE for progressively redirecting light propagating in a first in-plane direction to propagate in a second in-plane direction toward a second region of the LOE, and (ii) a second redirection configuration disposed in a second region of the LOE for progressively redirecting light propagating in the second in-plane direction out of the LOE for observation by an observer; and (b) a coupling device comprising: (i) a coupling lightguide element (LFE) and (ii) an input coupler disposed to couple light corresponding to the collimated image into the CLE.

[0007] According to further features in embodiments of the invention, the input coupler is a coupling prism presenting an input surface that is substantially perpendicular to the chief ray of the collimated image coupled into the CLE.

[0008] According to further features of embodiments of the present invention, the first redirection configuration includes a first set of mutually parallel internal partially reflective surfaces that are not parallel to the main surface, and wherein the second redirection configuration includes a second set of mutually parallel internal partially reflective surfaces that are at an oblique angle to the main surface.

[0009] According to further features of embodiments of the present invention, the CLE further comprises a preliminary aperture expansion device.

[0010] According to further features of embodiments of the present invention, the preliminary aperture expansion device comprises a set of mutually parallel internal partially reflective surfaces within the CLE for progressively redirecting the light coupled in by the input coupler.

[0011] According to further features of embodiments of the invention, the input coupler is arranged such that a chief ray of the collimated image coupled into the CLE propagates in a first in-plane direction after a single reflection from one of the internal partially reflective surfaces of the CLE.

[0012] According to further features of embodiments of the invention, the input coupler is arranged such that a chief ray of the collimated image coupled into the CLE propagates in a first in-plane direction after reflecting twice from an internal partially reflective surface of the CLE.

[0013] According to further features in embodiments of the invention, the preliminary aperture expansion means comprises a diffractive optical element associated with the CLE.

[0014] According to a further feature of an embodiment of the present invention, the input coupler is deployed so that the main light of the collimated image coupled into the CLE propagates in a first in-plane direction after being redirected twice by diffraction at the diffractive optical element to offset the dispersion generated by the first diffraction at the diffractive optical element.

[0015] According to further features of embodiments of the invention, the first redirecting arrangement of the LOE is a diffractive optical element configured to match the diffractive optical element of the CLE to cancel the dispersion generated by the diffractive optical element of the CLE.

[0016] According to further features of embodiments of the present invention, a portion of the interface between the CLE and the LOE located below the preliminary aperture expansion device is provided with a high reflective coating.

[0017] According to further features of embodiments of the present invention, the input coupler is a first diffractive optical element associated with a surface of the CLE, and wherein the second redirection configuration is a second diffractive optical element configured to match the first diffractive optical element to offset the dispersion generated by the first diffractive optical element.

[0018] According to further features of embodiments of the present invention, the beam splitter coating has a reflectivity between 55% and 95%, and in certain preferred cases, between 65% and 90%.

[0019] According to further features in embodiments of the present invention, the beam splitter coating has a reflectivity that decreases progressively along the first in-plane direction.

[0020] According to further features in embodiments of the present invention, the second thickness is between 20% and 40% of the first thickness. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention is herein described, by way of example only, with reference to the accompanying drawings, in which:

[0022] Figure 1A is a schematic isometric view of an optical system constructed and operative in accordance with the teachings of the present invention, the optical system employing a coupling light-guiding element (CLE) for directing a collimated image into a light-guiding optical element (LOE) for delivery to an observer's eye;

[0023] Figure 1B yes Figure 1A A schematic plan view of an optical system;

[0024] Figure 2A and Figure 2B They are respectively Figure 1A and Figure 1B Similar views showing a first variant implementation of the optical system of the invention using a CLE with preliminary aperture expansion achieved by an even number of reflections;

[0025] Figure 3A and Figure 3B They are respectively Figure 1A and Figure 1B Similar views showing a second variant implementation of the optical system of the invention employing a CLE with preliminary aperture expansion achieved by an even number of reflections;

[0026] Figure 4A is with Figure 3A similar schematic isometric view showing a modified implementation in which the CLE is subdivided into two parts for sequential aperture expansion and coupling into the LOE;

[0027] Figure 4B yes Figure 4A A schematic side view of

[0028] Figure 5A and Figure 5B They are respectively Figure 4A and Figure 4B Similar views showing another variant implementation of the optical system of the present invention employing a pair of optically matched diffractive optical elements (DOEs)—one in the CLE and the other in the LOE; and

[0029] Figure 6 is with Figure 4A A similar schematic isometric view shows another alternative implementation of the optical system of the present invention in which light coupling into the CLE and light coupling out of the LOE are achieved by a pair of optically matched DOEs.

[0030] DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] The present invention is an optical system and a corresponding method of producing an optical system.

[0032] The principles and operation of the optical system and corresponding methods according to the present invention may be better understood with reference to the drawings and accompanying description.

[0033] Referring now to the accompanying drawings, Figures 1A to 6 Various implementations of optical systems constructed and operated in accordance with the teachings of the present invention are schematically illustrated. Generally, the optical system includes an aperture-expanding light-guiding optical element (LOE) 106 having a pair of mutually parallel major surfaces 103a, 103b separated by a first thickness T1, the aperture-expanding light-guiding optical element (LOE) 106 supporting propagation of light by internal reflection at the major surfaces. The LOE 106 includes: a first redirecting configuration disposed in a first region 107 of the LOE for progressively redirecting light propagating in a first in-plane direction D1 to propagate in a second in-plane direction D2 toward a second region 109 of the LOE; and a second redirecting configuration disposed in the second region 109 of the LOE for progressively redirecting light propagating in the second in-plane direction D2 out of the LOE (direction D3) for observation by an observer.

[0034] The coupling device is arranged to couple a projected image from a projector (not shown) into the LOE 106. The coupling device includes a coupling light-guiding element (CLE) 104 having a pair of mutually parallel surfaces separated by a second thickness T2 that is not greater than half of the first thickness T1, and in certain particularly preferred cases, the second thickness T2 is between 20% and 40% of the first thickness T1. The area of ​​the parallel surfaces of the CLE 104 is not greater than 20% of the area of ​​the main surfaces 103a, 103b and preferably less than 10% of the area of ​​the main surfaces 103a, 103b. One of the parallel surfaces of the CLE 104 is joined to a main surface 103a of the LOE 106 at an interface 105, at least a portion of which is provided with a beam splitter coating having a reflectivity of at least 50%. The coupling device also includes an input coupler arranged to couple light corresponding to the collimated image into the CLE. In Figures 1A to 5B In a first set of particularly preferred implementations shown, the input coupler for inputting the chief ray D0a is a coupling prism presenting an input surface that is substantially perpendicular to the chief ray of the collimated image coupled into the CLE. Alternatively, the prism can be a reflecting prism, for example, coated with a 100% reflector, for coupling in an input image having a chief ray D0b irradiated from the opposite side of the light guide 106. In the subsequent discussion, for simplicity of presentation, only the option of input direction D0a will be mentioned.

[0035] You can refer to Figure 1A and Figure 1B To understand the basic functionality of this aspect of the invention, Figure 1A and Figure 1B An optical system 100 constructed and operative in accordance with a first implementation of the present invention is shown. Figure 1A In , the general propagation of the light beam (corresponding to the chief ray of the collimated image and showing the "in-plane component" within the light guide) is shown as square arrows, while specific rays are shown as black arrows. Figure 1B In a plan view of the CLE 104, this distinction is not visible. Light impinges on prism 102a, which couples the light into CLE 104. Because CLE 104 is thinner than LOE 106, the coupling interface can be smaller, allowing the use of a smaller projector (not shown) than required for the LOE. The partially reflective interface 105 between CLE 104 and LOE 106 is a partial reflector that causes light to progressively "leak" from CLE 104 into LOE 106 as light is guided within CLE 104, resulting in relatively uniform illumination across the thickness of LOE 106, effectively expanding the aperture along the thickness of the light guide and helping to "fill" the thickness of the LOE with the projected image.

[0036] In the preferred implementation shown here, a first redirection configuration is implemented as a first set of mutually parallel internal partially reflective surfaces 108 that are non-parallel to the main surface 103a, and a second redirection configuration is implemented as a second set of mutually parallel internal partially reflective surfaces 110 that are at an oblique angle to the main surface 103a. Partially reflective surfaces 108 progressively reflect and redirect guided light propagating in a first in-plane direction D1 within a first region 107 toward a second region 109, thereby achieving a first dimension of aperture expansion along direction D1. Partially reflective surfaces 110 progressively couple image light out of the lightguide within the second region 109 for viewing by a user, thereby achieving a second dimension of aperture expansion along direction D2.

[0037] In addition to reducing the thickness of the entrance pupil, the coupling arrangement described herein is also advantageous due to ease of manufacture. The coupling arrangement is a small component that is bonded to a major surface of the light guide (an easily accessible attachment surface) and does not require high precision alignment.

[0038] The CLE 104 is preferably deployed over an area of ​​the LOE 106 that is outside the user's viewing area, or at least at the periphery of the user's viewing area, such that the edge of the CLE 104 does not disrupt the user's field of view. Compared to the LOE 106, the relatively small size of the CLE 104, covering less than 20% and preferably less than 10% of the LOE area, facilitates unobtrusive CLE deployment. If the CLE 104 is located at the periphery of the viewing area, the beam splitter coating at the interface 105 can be implemented using a multilayer dielectric coating with angle-dependent reflectivity that is transparent (over 90% transmittance) at small angles of incidence and has a desired reflectivity, as defined below, for larger angles relevant to the propagation of images within the LOE. If the CLE 104 is located outside the viewing area, a simple dielectric or metallic partially reflective coating can be used. The CLE 104 is preferably a "slab" type light guide, meaning that two in-plane dimensions of the CLE 104 are at least an order of magnitude larger than its thickness.

[0039] The reflectivity of the beam splitter coating at interface 105 is preferably selected as a function of the angular range of the injected image and the thickness and length of CLE 104 so that a large proportion of the image light intensity undergoes multiple internal reflections within CLE 104 while progressively "leaking" into LOE 106, and so that most of the image light intensity escapes from CLE 104 at the end of the CLE. The preferred reflectivity is typically at least 50%, more preferably between 55% and 95%, and most preferably between 65% and 90%. In some cases, the beam splitter coating has a reflectivity that progressively decreases along the first in-plane direction, for example, as two or more strips of reflective coating with different reflectivities.

[0040] Now go to Figure 2A and Figure 2B , which both illustrate another optical system, generally designated 111, constructed and operative in accordance with another implementation of the present invention. Optical system 111 is generally similar in structure and function to optical system 100, with like-numbered equivalent elements. Optical system 111 differs from optical system 100 in that CLE 104 includes a preliminary aperture expansion arrangement, here implemented as a set of mutually parallel internal partially reflective surfaces 112 within CLE 104, for progressively redirecting the light coupled in by the input coupler. This allows the use of an entrance pupil defined by prism 102b that is smaller than the entrance pupil of prism 102a in the previous implementation.

[0041] In the implementation shown here, the image is injected with an in-plane component parallel to the direction D1, and the redirected light rays entering the LOE 106 propagate in the first in-plane direction D1 after reflecting twice (or some other even number of times) from the surface 112 of the CLE 104. Optionally, similar to the following with reference to Figure 4A and Figure 4B As discussed, at least a portion of the interface 105 below the region of the partially reflective surface 112 can have a high reflectivity to minimize the loss of light after an odd number of reflections in the region of the partially reflective surface 112 and to limit outcoupling into the light guide 106 to the region of the CLE 104 beyond the surface 112. Additionally or alternatively, efficiency can be improved by providing a first surface 112a having a high reflectivity, positioned so that the incoupled light does not pass through the high reflectivity surface, but so that all light reflected an odd number of times will reach the first surface and return toward the direction D1.

[0042] The effect of the preliminary aperture expansion device is to achieve an initial widening of the width of the image entrance pupil, thereby facilitating the use of a narrower entrance pupil for prism 102b and a correspondingly smaller image projector. Additionally or alternatively, this preliminary in-plane expansion can relax design requirements for the first aperture expansion device, for example, allowing the use of a larger spacing between successive partially reflective surfaces 108. In all other respects, the structure and function of optical system 111 are similar to those of optical system 100 and will be understood by reference to the above description.

[0043] Now go to Figure 3A and Figure 3B, shows another optical system, generally designated 125, constructed and operative in accordance with another implementation of the present invention. Optical system 125 is generally similar in structure and function to optical systems 100 and 111, with like-labeled equivalent elements. Similar to optical system 111, CLE 104 of optical system 125 includes a preliminary aperture expansion device implemented as a set of mutually parallel internal partially reflective surfaces 112 for progressively redirecting light coupled in by the input coupler. However, in this case, the image is injected via prism 102c with an in-plane component that is non-parallel to direction D1, and the redirected light entering LOE 106 propagates in a first in-plane direction D1 after reflecting once (or another odd number of times) from surfaces 112 of CLE 104. In this case, efficiency can be improved by providing successively increasing reflectivity of surfaces 112 along the optical path.

[0044] In this implementation, the direction of injection of the image light is shown here as being perpendicular to the first in-plane direction D1 , with the partially reflective surface 112 at 45 degrees to D1 . However, other injection angles and corresponding orientations of the surface 112 may be chosen.

[0045] Surface 112 can be perpendicular (orthogonal) to the main surface of the LOE, or can be tilted diagonally. Alternatively, a set of obliquely angled surfaces 112 can be used so that the injected image spans a first range of angles and the reflected image (after one or other odd number of reflections) spans a second range of angles. In this case, the partially reflective coating of interface 105 can be designed to be angle-selective to partially reflect within the second range of angles to allow the reflected image to be coupled out, while having a higher reflectivity for the first range of angles to minimize the loss of light that does not contribute to the output image.

[0046] Now go to Figure 4A and Figure 4B , these figures show an implementation of an optical system 126 similar to the optical system 125 with modified interfaces to improve efficiency. Specifically, in this case, a portion 105a of the interface between the CLE 104 and the LOE 106 located below the preliminary aperture expansion device is provided with a highly reflective coating, i.e., with a reflectivity of more than 90% and preferably at least 95%, while the aforementioned beam splitter coating is applied to a second portion 105b of the interface beyond the preliminary aperture expansion device (e.g., surface 112). This helps to ensure that light propagating in undesired directions (e.g., in the direction of the beam splitter) is not reflected. Figure 4AIn the case of , after zero or even reflections, the light is not coupled out into the light guide 106, but is confined in the CLE 104 until it encounters the additional partially reflecting surface 112, thereby increasing the amount of light that ends up in the correct orientation. The light that has undergone an odd number of reflections propagates in the desired direction D1 and enters the second part of the CLE 104, where it is progressively coupled out through the second part 105b of the interface as previously described. The highly reflective coating preferably also extends under the coupling prism 102c. This method is also applicable to the use of Figure 2A and Figure 2B An implementation of a geometry in which the internal partially reflective surface 112 can also be positioned to a first portion of the CLE 104 having a highly reflective interface, and outcoupling occurs at a different second portion of the CLE 104.

[0047] In all other respects, the structure and operation of optical system 126 are identical to those of optical system 125 and will be understood by reference to the corresponding description above.

[0048] The present invention has been described so far with reference to an optical system in which the redirection of light within the LOE 106 and any preliminary aperture expansion within the CLE 104 are achieved by reflective elements. However, it should be noted that one or more of the redirection components may alternatively be implemented using diffractive optical elements, such as surface gratings, volume gratings, or holographic elements, as are known in the art. In general, to avoid significant dispersion effects that are characteristic of diffractive optical elements, the system design should preferably employ two equal but opposite redirections performed by the diffractive element, such that the second redirection compensates for the dispersion effects introduced by the first redirection.

[0049] Can refer to Figure 2A It is understood that one such embodiment would replace the set of partially reflective surfaces 112 in the CLE 104 with a diffractive optical element associated with the CLE. Such an embodiment should be configured so that the chief ray of the collimated image coupled into the CLE propagates in a first in-plane direction after being redirected twice by diffraction at the diffractive optical element to cancel the dispersion generated by the first diffraction at the diffractive optical element. All other aspects of the design would be as described above with reference to Figure 2A and Figure 2B As described.

[0050] Figure 5A and Figure 5B The alternative method shown provides an optical system 127 that is connected to Figure 4A and Figure 4BThe optical system 126 is geometrically similar but employs a diffractive optical element 130a associated with the CLE 104 for preliminary aperture expansion within the CLE 104 and a second diffractive optical element 130b as a first redirection configuration of the LOE 106. The DOE 130b is configured to match the DOE 130a to cancel the dispersion generated by the diffractive optical element 130a of the CLE 104. For this embodiment, the direction of light injection into the CLE 104 should be parallel to the second in-plane direction D2 so that after two redirections, the light exits the DOE 130b again along the in-plane direction D2. As described above with reference to the optical systems 125 and 126, the image light is coupled into the CLE 104 via the prism 102c, and outcoupling of the image toward the user's eye is achieved by the partially reflective surface 110.

[0051] Now go to Figure 6 , which shows another optical system, generally designated 128, which illustrates an alternative diffractive (or hybrid "hybrid") implementation. In this case, the input coupler is implemented as a first diffractive optical element 132a associated with a surface of the CLE 104, and the second redirection configuration is a second diffractive optical element 132b, which is configured to match the first diffractive optical element to cancel the dispersion generated by the first diffractive optical element. In this case, the image is injected generally substantially perpendicular to the primary surfaces of the CLE and LOE, with the primary ray shown here as D0c.

[0052] Another implementation not shown may be Figure 5A The diffraction element 130a and the diffraction element 130b are Figure 6 The diffraction element 132a and the diffraction element 132b in the embodiment of the present invention are combined to realize a full diffraction implementation with two pairs of diffraction elements that cancel out their respective dispersion effects.

[0053] In all of the above cases, the LOE 106 is schematically shown as a rectangular element. In a typical practical implementation, the LOE is shaped to fit into a suitable support structure, such as an eyeglass frame, which supports the LOE in a properly aligned relationship facing the user's eyes. A display device may typically include two such optical devices, each fed with a collimated image by a micro-image projector and also including on-board components such as processing components, power supplies, and various communication subsystems, all of which are required for each application and as generally known in the art. These additional components are not themselves part of the present invention and, therefore, are not described in detail herein.

[0054] It will be appreciated that the above description is intended to serve as examples only and that many other embodiments are possible within the scope of the invention as defined by the appended claims.

Claims

1. An optical system comprising: (a) an aperture-expanding light-guiding optical element (LOE) having a pair of mutually parallel major surfaces separated by a first thickness, the LOE supporting propagation of light by internal reflection at the major surfaces, the LOE having: (i) a first redirecting arrangement disposed in a first region of the LOE for progressively redirecting light propagating in a first in-plane direction to propagate in a second in-plane direction toward a second region of the LOE, and (ii) a second redirecting arrangement disposed in a second region of the LOE for progressively redirecting light propagating in the second in-plane direction out of the LOE for observation by an observer; as well as (b) a coupling-in device comprising: (i) a coupling lightguide element (CLE) having a pair of mutually parallel surfaces separated by a second thickness no greater than half the first thickness, the parallel surfaces of the CLE having an area no greater than 10% of the area of ​​the major surface of the LOE, one of the surfaces of the CLE being joined to one of the major surfaces of the LOE at an interface, at least a portion of the interface being provided with a beam splitter coating having a reflectivity of at least 50%, and (ii) an input coupler disposed to couple light corresponding to the collimated image into the CLE.

2. The optical system according to claim 1, wherein The input coupler is a coupling prism presenting an input surface that is substantially perpendicular to the chief ray of the collimated image coupled into the CLE.

3. The optical system according to claim 1, wherein: The first redirecting configuration includes a first set of mutually parallel internal partially reflecting surfaces that are non-parallel to the major surface, and wherein the second redirecting configuration includes a second set of mutually parallel internal partially reflecting surfaces at an oblique angle to the major surface.

4. The optical system according to claim 1, wherein: The CLE also includes a preliminary aperture expansion device.

5. The optical system according to claim 4, wherein: The preliminary aperture expansion device comprises a set of mutually parallel internal partially reflective surfaces within the CLE for progressively redirecting the light coupled in by the input coupler.

6. The optical system according to claim 5, wherein: The input coupler is arranged such that a chief ray of the collimated image coupled into the CLE propagates in the first in-plane direction after a single reflection from one of the internal partially reflective surfaces of the CLE.

7. The optical system according to claim 5, wherein: The input coupler is disposed such that a chief ray of the collimated image coupled into the CLE propagates in the first in-plane direction after reflecting twice from the internal partially reflective surface of the CLE.

8. The optical system according to claim 4, wherein: The preliminary aperture expansion device comprises a diffractive optical element associated with the CLE.

9. The optical system according to claim 8, wherein: The input coupler is disposed such that the main light of the collimated image coupled into the CLE propagates in the first in-plane direction after being redirected twice by diffraction at the diffractive optical element to offset the dispersion generated by the first diffraction at the diffractive optical element.

10. The optical system according to claim 8, wherein: The first redirection configuration of the LOE is a diffractive optical element configured to match the diffractive optical element of the CLE to cancel the dispersion generated by the diffractive optical element of the CLE.

11. The optical system according to claim 4, wherein: A portion of the interface between the CLE and the LOE located below the preliminary aperture expansion device is provided with a high reflective coating.

12. The optical system according to claim 1, wherein: The input coupler is a first diffractive optical element associated with a surface of the CLE, and wherein the second redirection configuration is a second diffractive optical element configured to match the first diffractive optical element to cancel the dispersion generated by the first diffractive optical element.

13. The optical system according to claim 1, wherein: The beam splitter coating has a reflectivity between 55% and 95%.

14. The optical system according to claim 1, wherein: The beam splitter coating has a reflectivity between 65% and 90%.

15. The optical system according to claim 1, wherein: The beam splitter coating has a reflectivity that decreases progressively along the first in-plane direction.

16. The optical system according to claim 1, wherein The second thickness is between 20% and 40% of the first thickness.

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