Light guide based display using light recycling

Abstract

The display includes a light guide array in which at least a portion of the image light is not coupled out of the light guide during a first pass through the coupling-out array, but is recycled for repeated passes through the coupling-out array. In one set of embodiments, the recycling of the light is performed via separate light guides. In another set of embodiments, the light is recycled within a single light guide, employing polarization management to avoid undesired interactions between the light and the coupling-out array.

Description

[Technical Field]

[0001] The present invention relates to light guide based displays, and in particular to light guide based displays in which light reaching the end of the light guide is recycled to pass through the light guide again. [Background technology]

[0002] It is known to use light guides to transmit an image in front of an observer's eye and to couple out towards the observer for viewing. The image is coupled out by a suitable coupling out array, which may be a set of partially reflective embedded reflectors, pin mirrors, or diffractive optical elements. In some cases, the light guide is configured to achieve two-dimensional aperture expansion by, for example, using additional sets of embedded reflectors or diffractive optical elements to progressively deflect the light propagating within the light guide and redirect it towards the coupling out array.

[0003] A schematic illustration of a two-dimensional aperture-expanding light guide 100 is shown in plan view in FIG. 1. An image is typically injected through a coupling input prism or reflective coupling input, as represented schematically by arrow 102, to propagate by internal reflection from the top and bottom surfaces (parallel to the plane of the image) in a first direction D1, encountering a first set of mutually parallel partially reflective internal surfaces (or "facets") 104 that progressively redirect (reflect) a portion of the image light in a second direction D2. This light then encounters a second set of mutually parallel partially reflective internal surfaces (or "facets") 106 that progressively redirect (reflect) a portion of the image light outward from the light guide to couple out toward a viewer for viewing the image. To reduce non-uniformity in the output image, the light guide may include a homogenizing element or "mixer" 108, implemented as a partial reflector disposed between and parallel to the major surfaces of the light guide. One or both of the facets may be replaced by suitable surface or volume diffractive optical elements embedded within or on the surface of the light guide to redirect the image light, all as known in the art.

[0004] To minimize disturbance to the scene viewed by an observer through the light guide, the reflectivity (or diffraction efficiency) of at least the second set of facets, and possibly both sets of facets, is preferably relatively low. As a result, a significant proportion of the image light reaches the ends of the light guide, particularly in the shaded region designated 110, resulting in a loss of efficiency and image brightness. Summary of the Invention

[0005] The present invention is a display for delivering image light to a viewer.

[0006] In accordance with the teachings of one embodiment of the present invention, a display for delivering image light to a viewer is provided, the display comprising: (a) a first light guide comprising a block of transparent material having major surfaces for supporting propagation of the image light by internal reflection at two mutually parallel major surfaces, the first light guide including at least one set of internal, mutually parallel partially reflective surfaces angled obliquely relative to the major surfaces to progressively couple out a portion of the image light propagating in the first direction to be reflected towards the viewer while transmitting a portion of the image light to continue propagating in the first direction within the first light guide; and (b) a second light guide comprising a block of transparent material having major surfaces for supporting propagation of the image light by internal reflection at the two mutually parallel major surfaces, the second light guide being stacked relative to the first light guide. (c) a first reflective coupling arrangement comprising at least one planar mirror, the first reflective coupling arrangement being arranged to reflect image light propagating in the first light guide in a first direction that has been transmitted through the set of partially reflective surfaces to propagate in the second light guide in a second direction having an in-plane component opposite to the first direction; and (d) a second reflective coupling arrangement comprising at least one planar mirror, the second reflective coupling arrangement being arranged to reflect image light that has traversed the second light guide in the second direction to be reintroduced for propagation in the first direction within the first light guide, such that at least a portion of the image light that was transmitted through the set of partially reflective surfaces on its first pass through the partially reflective surfaces is coupled out to be reflected towards the viewer when it re-incident on the partially reflective surfaces.

[0007] Also in accordance with the teachings of one embodiment of the present invention, there is provided a display for delivering image light to a viewer, the display comprising: (a) a first light guide comprising a block of transparent material having major surfaces for supporting propagation of the image light by internal reflection at two mutually parallel major surfaces, the first light guide including at least one set of internal, mutually parallel partially reflective surfaces non-parallel to the major surfaces to progressively redirect a portion of the image light propagating in a first direction to propagate within the first light guide by internal reflection in a second direction while transmitting a portion of the image light to continue propagating in the first direction within the first light guide; and (b) a second light guide comprising a block of transparent material having major surfaces for supporting propagation of the image light by internal reflection at the two mutually parallel major surfaces, the second light guide being stacked relative to the first light guide. (c) a first reflective coupling arrangement comprising at least one planar mirror, the first reflective coupling arrangement being arranged to reflect image light that has been transmitted through the set of partially reflective surfaces and propagating in the first light guide in a first direction to propagate in the second light guide in a third direction having an in-plane component opposite to the first direction; and (d) a second reflective coupling arrangement comprising at least one planar mirror, the second reflective coupling arrangement being arranged to reflect image light that has traversed the second light guide in a second direction to be reintroduced for propagation in the first direction in the first light guide, whereby at least a portion of the image light that was transmitted through the set of partially reflective surfaces on its first pass through the partially reflective surfaces is redirected to propagate in the second direction when it re-incident on the partially reflective surfaces.

[0008] According to a further feature of an embodiment of the invention, each of the first and second reflective coupling arrangements comprises a single plane mirror and a light transmitting region adjacent to the single plane mirror, in which an optical continuum exists between the first and second light guides for transmitting image light between the first and second light guides.

[0009] According to a further feature of an embodiment of the invention, each of the single plane mirrors is oriented perpendicular to a major surface of the first and second light guides.

[0010] According to a further feature of an embodiment of the invention, the single plane mirror of the first reflective coupling arrangement is oriented at a first tilt relative to the major surfaces of the first and second light guides, and the single plane mirror of the second reflective coupling arrangement is oriented at a second tilt relative to the major surfaces of the first and second light guides that is equal but opposite to the first tilt.

[0011] According to a further feature of an embodiment of the invention, the optical continuum is implemented by connecting the first light guide to the second light guide using an index-matched adhesive.

[0012] According to a further feature of an embodiment of the invention, the optical continuum is implemented by connecting a continuous block of transparent material to edge surfaces of the first light guide and the second light guide.

[0013] According to a further feature of an embodiment of the present invention, each of the first and second reflective coupling arrangements comprises a pair of mutually perpendicular plane mirrors.

[0014] According to a further feature of an embodiment of the invention, the second light guide is attached to the first light guide by a layer of adhesive having a lower refractive index than the transparent material of the first and second light guides.

[0015] According to a further feature of an embodiment of the invention, the second light guide is separated from the first light guide by an air gap.

[0016] Also in accordance with the teachings of one embodiment of the present invention, there is provided a display for delivering image light to an observer, the display comprising: (a) a light guide comprising a block of transparent material having major surfaces for supporting propagation of the image light by internal reflection at two mutually parallel major surfaces, the light guide including at least one set of internal, mutually parallel, polarization-selective partially reflective surfaces angled obliquely with respect to the major surfaces to progressively couple out a portion of the image light of a first polarization propagating in the first direction to be reflected towards the observer while transmitting a portion of the image light of the first polarization to continue propagating in the first direction within the light guide, the partially reflective surfaces being substantially transparent to light of a second polarization orthogonal to the first polarization; and (b) a quarter-wave phase plate and a plane mirror. and (c) a second reflecting arrangement comprising a quarter wave retarder and a planar mirror, the second reflecting arrangement being arranged to reflect image light that has traversed the light guide in the second direction to propagate within the light guide in a direction parallel to the first direction, whereby at least a portion of the image light that was transmitted through the set of partially reflective surfaces on its first pass through the partially reflective surfaces is coupled out to be reflected towards a viewer when it re-incident on the partially reflective surfaces.

[0017] According to a further feature of an embodiment of the invention, there is also provided an image injection aperture for injecting image light into the light guide to propagate the image in a first direction by internal reflection at the major surfaces, the first direction being oblique to the first and second reflecting configurations, such that image light introduced into the image injection aperture is incident on a first region of the partially reflective surface and, after reflection from the first and second reflecting configurations, is incident on a second region of the partially reflective surface, the second region at least partially non-overlapping with the first region.

[0018] According to a further feature of an embodiment of the invention, the reflectivity of the partially reflective surfaces in at least the first region is such that a majority of image light injected into the image injection aperture is transmitted through the set of partially reflective surfaces and recycled by reflection in the first and second reflective configurations.

[0019] According to a further feature of an embodiment of the present invention, the quarter wave phase plate is part of a graduated phase plate.

[0020] According to a further feature of an embodiment of the invention, there is also provided a partially reflective surface disposed within the light guide, perpendicular to the major surface and parallel to the first and second reflective arrangements.

[0021] Also in accordance with the teachings of one embodiment of the present invention, there is provided a display for delivering image light to a viewer, the display comprising: (a) a light guide comprising a block of transparent material having major surfaces for supporting propagation of the image light by internal reflection at two mutually parallel major surfaces, the light guide including at least one set of internal, mutually parallel, polarization-selective partially reflective surfaces that are not parallel to the major surfaces to progressively redirect a portion of the image light in a first direction to propagate in a second direction by internal reflection within the light guide while transmitting a portion of the image light to continue propagating in the first direction within the light guide, the partially reflective surfaces being substantially transparent to light of a second polarization that is orthogonal to the first polarization; and (b) a quarter-wave retarder and a planar mirror. and (c) a second reflecting arrangement comprising a quarter wave retarder and a planar mirror, the second reflecting arrangement being arranged to reflect image light that traversed the light guide in a third direction to propagate within the light guide in a direction parallel to the first direction, whereby at least a portion of the image light that was transmitted through the set of partially reflective surfaces on its first pass through the partially reflective surfaces is redirected to propagate in the second direction when it re-incident on the partially reflective surfaces. [Brief explanation of the drawings]

[0022] The invention is herein described, by way of example only, with reference to the accompanying drawings. [Figure 1] FIG. 1 is a schematic plan view of a display light guide providing two-dimensional aperture expansion, as described above. [Figure 2] 1 is a simplified plan view of a display light guide arrangement constructed and operative in accordance with one embodiment of the present invention; [Figure 3A] 3 is a cross-sectional view taken along line III-III in FIG. 2 according to a modified implementation of the light guide arrangement. [Figure 3B] 3 is a cross-sectional view taken along line III-III in FIG. 2 according to a modified implementation of the light guide arrangement. [Figure 3C] 3 is a cross-sectional view taken along line III-III in FIG. 2 according to a modified implementation of the light guide arrangement. [Figure 3D] 3 is a cross-sectional view taken along line III-III in FIG. 2 according to a modified implementation of the light guide arrangement. [Figure 3E] 3 is a cross-sectional view taken along line III-III in FIG. 2 according to a modified implementation of the light guide arrangement. [Figure 4A] 1 is a schematic plan view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention; [Figure 4B] FIG. 4B is a cross-sectional view taken along line IV-IV of FIG. 4A. [Figure 5A] 3B is a cross-sectional view similar to FIG. 3A illustrating an alternative construction of the light guide. [Figure 5B] 4C is a cross-sectional view similar to FIG. 4B illustrating an alternative construction of the light guide. [Figure 6] 1 is a schematic side view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention; [Figure 7A] 1 is a schematic plan view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention, employing image light recycling to achieve an expanded aperture; [Figure 7B] FIG. 7B is similar to FIG. 7A, but employs a different arrangement of partially reflective coupling output surfaces. [Figure 7C] FIG. 7B is similar to FIG. 7A, but employs recycling of image light along the X-axis direction. [Figure 8] FIG. 7B is similar to FIG. 7A, but employing a segmented partially reflective coupling output surface. [Figure 9] 7B is a diagram similar to FIG. 7A illustrating parameters that affect the image filling of the light guide, as a function of the distance between the two mirrors. [Figure 10]7B is a diagram similar to FIG. 7A illustrating parameters that affect the image filling of the light guide, as a function of the distance between the two mirrors. [Figure 11] FIG. 7B is a diagram similar to FIG. 7A illustrating a display light guide arrangement employing an additional mixer element. [Figure 12] FIG. 7B is a diagram similar to FIG. 7A illustrating a display light guide arrangement employing a phase plate with gradually varying efficiency. [Figure 13A] FIG. 1 is a schematic plan view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention, employing recycling of image light along two perpendicular directions having different initial propagation directions. [Figure 13B] FIG. 1 is a schematic plan view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention, employing recycling of image light along two perpendicular directions having different initial propagation directions. [Figure 14A] FIG. 1 is a schematic plan view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention, employing recycling of image light along two perpendicular directions (first within a first light guide and second through a second light guide). [Figure 14B] 14B is a cross-sectional view taken along line XIV-XIV of FIG. 14A according to an alternative implementation of the display light guide arrangement. [Figure 14C] 14B is a cross-sectional view taken along line XIV-XIV of FIG. 14A according to an alternative implementation of the display light guide arrangement. [Figure 15A] 1 is a schematic plan view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention; [Figure 15B] FIG. 15B is a cross-sectional view taken along line XV-XV in FIG. 15A. [Figure 15C] 15B is a schematic plan view of an alternative implementation of the display light guide arrangement of FIG. 15A. FIG. [Figure 16A] 1 is a schematic plan view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention; [Figure 16B] FIG. 16B is a schematic cross-sectional view taken along line XVI-XVI in FIG. 16A. [Figure 16C] FIG. 16B is a schematic cross-sectional view taken along line XVII-XVII in FIG. 16A. [Figure 17] FIG. 10 is a schematic plan view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention, employing eight containment reflectors; [Figure 18A] FIG. 10 is a schematic plan view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention, illustrating the use of selective light absorbing elements to restrict selected light paths; [Figure 18B] FIG. 10 is a schematic plan view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention, illustrating the use of selective light absorbing elements to restrict selected light paths; [Figure 19A] 1 is a schematic plan view of a display light guide arrangement constructed and operative in accordance with a further embodiment of the present invention; [Figure 19B] FIG. 19B is a cross-sectional view taken along line XIX-XIX in FIG. 19A. [Figure 19C] FIG. 19C is a schematic close-up view of the coupling input configuration from the display light guide array of FIG. 19B. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention is a display for delivering image light to a viewer.

[0024] The principles and operation of a display according to the present invention may be better understood with reference to the drawings and accompanying description.

[0025] By way of introduction, various embodiments of the present invention are based on two families of solutions for light recycling within a light guide. According to a first set of embodiments described herein with reference to FIGS. 2-5B, light reaching the edge of a first light guide is coupled into a second, parallel light guide where it is directed to re-traverse the second light guide for reintroduction into the first light guide. In a second set of embodiments illustrated with reference to FIGS. 6-13B, light is recycled within a single light guide and employs polarization management to avoid undesired interactions between the light and the combined output array. The remaining figures relate to further variations, combinations, and applications of these approaches, including light recycling along two or more in-plane axes.

[0026] The present invention is presented herein as a display. More specifically, the present invention relates primarily to various light guide configurations for transmitting images from an image projector to deliver them to a user's eyes, typically in the context of an augmented reality display where the user simultaneously views the real world through the light guide configuration. The present invention is applicable to all sizes and types of displays, from near-eye and / or head-mounted displays to automotive head-up displays and beyond. Full display implementations include additional components, including an image projector for generating the desired image and a coupling arrangement for coupling the projected image into the light guide for propagation within the light guide by internal reflection. The image projector itself typically includes an image generator, which may be based on an LCD element, a scanned modulated laser beam, a digital light processor, an active matrix display, or any other image generator, and collimating optics for collimating the image and delivering it to the projector output aperture. The coupling input arrangement may employ a coupling prism, a diffractive optical element, and / or one or more reflectors arranged to introduce the image into the light guide within a desired range of propagation angles. To this is added power components, processing components to drive the image generator, and mechanical components to support all of the above in deployment as needed for use, such as on a user's head, in association with a vehicle, or as needed for any other application, all of which are well known in the art and will not be treated further herein for the sake of brevity of presentation.

[0027] Additionally, although presented as displays, various implementations of the light guides described herein may also be advantageously used in various lighting applications where the light guide itself does not transmit an image but rather provides illumination, such as to illuminate an LCOS image generator as part of a display.

[0028] Light recycling with additional light guides Thus, according to certain embodiments of the present invention, light that would otherwise be lost upon reaching the end of the light guide can be reinjected into the waveguide to increase the overall efficiency of the waveguide. One embodiment is shown in Figures 2 and 3A, where LOE losses are reduced by adding another inert light guide 10B below the light guide 10A and two parallel mirror plates 20 on either side of the light guide pair, forming an optical cavity and allowing recovery of light that would otherwise be lost at the far right side of the waveguide.

[0029] In the non-limiting example illustrated herein, light guide 10A is a two-dimensional aperture-expanding light guide employing two sets of mutually parallel internal partially reflective surfaces. An image is injected through the image injection aperture, typically via a coupling input prism or reflective coupling input, as represented schematically by arrow 12 to propagate by internal reflection from the top and bottom surfaces (parallel to the plane of the image) in a first direction D1, encountering a first set 14 of mutually parallel partially reflective internal surfaces (or "facets") that progressively redirect (reflect) a portion of the image light in a second direction D2. This light then encounters a second set 16 of mutually parallel partially reflective internal surfaces (or "facets") that progressively redirect (reflect) a portion of the image light outward from the light guide to couple out toward a viewer for viewing. To reduce non-uniformity in the output image, the light guide may include a homogenizing element or "mixer" 18 implemented as a partial reflector disposed between and parallel to the major surfaces of the light guide. One or both of the facets may be replaced by suitable surface or volume diffractive optical elements embedded within or on the surface of the light guide to redirect the image light, as is known in the art. Also, the present invention is not limited to light guides with two-dimensional aperture expansion.

[0030] Here, and throughout the description and claims, whenever a direction of propagation is referenced, this refers to the in-plane direction of propagation, i.e., the component of the ray direction parallel to the major surfaces of the light guide, unless expressly stated otherwise. Although propagation within a light guide occurs by repeated internal reflections at the major surfaces, as illustrated in some specific side views, the "propagation direction" within a light guide is considered to be an in-plane component. Additionally, unless otherwise indicated, "direction" refers to all light rays propagating in the same or parallel in-plane directions, even if they are displaced / offset from one another.

[0031] In a first set of embodiments, light recycling is achieved through the use of a second light guide 10B implemented as a block of transparent material having two mutually parallel major surfaces to support propagation of image light by internal reflection at the two mutually parallel major surfaces. The second light guide 10B is disposed in a stacked relationship with the first light guide 10A. A first reflective coupling arrangement including at least one planar mirror 20A is disposed to reflect image light propagating in the first light guide 10A in a second direction D2 through the second set of partially reflective surfaces 16 to propagate in the second light guide in a reflective direction D3 having an in-plane component opposite to direction D2 (opposite to the component perpendicular to the planar mirror 20B). A second reflective coupling arrangement including at least one planar mirror 20B is arranged to reflect image light that has traversed the second light guide in a reflective direction to be reintroduced for propagation in the first light guide 10A in the first direction, whereby at least a portion of the image light that was transmitted through the set of partially reflective surfaces in its first pass through the partially reflective surfaces is coupled out to be reflected towards the observer when it re-enters the partially reflective surfaces.

[0032] This is further explained with reference to FIG. 3A. Injected ray i enters through major surface (S1), propagates by total internal reflection (TIR) ​​until it hits far mirror 20A, returns to the light guide as ray j, then hits second mirror 20B and returns to the light guide as ray k. Note that rays i and k are parallel as seen in FIG. 3A. To maintain TIR conditions within the first and second set of regions of facets 14, 16, and mixer 18 and allow light to pass between light guide 10A and inert glass 10B, a low refractive index (RI) adhesive should be employed between the light guides in region 22, and an index-matched adhesive should be employed in region 24. An alternative implementation may use an air gap in region 22, which is maintained by inserting a thin index-matched plate in region 24. Yet another option is to provide the internal reflection properties of region 22 by employing a multi-layer dielectric coating designed to provide angle-selective properties that mimic TIR properties, with high reflectivity at high angles and high transparency at small angles (i.e., near normal to the light guide major surfaces), as is known in the art.

[0033] In a first set of implementations, each of the first and second reflective coupling configurations is implemented using a single planar mirror 20A, 20B and a light-transmitting region 24 adjacent to the single planar mirror, in which an optical continuum exists between the first and second light guides for transmitting image light between the first and second light guides. In the case of FIGS. 3A and 3B, the planar mirrors are oriented perpendicular to the major surfaces (e.g., the top surfaces labeled S1) of the first and second light guides. As a result, the propagation angle of light rays in the second light guide 10B is the same as the propagation angle of light rays in the first light guide 10A. FIG. 3B illustrates a structurally and functionally equivalent configuration to FIG. 3A, except that the first light guide 10A can be implemented further away from the user's eye, and the coupled-out light passes through the second light guide 10B before reaching the eye.

[0034] 3C and 3D, the single plane mirror 20A of the first reflective coupling configuration is oriented at a first tilt α relative to the light guide's major surface S1, and the single plane mirror 20B of the second reflective coupling configuration is oriented at a second tilt equal to the first tilt α but opposite to the light guide's major surface S1. This tilt results in light rays propagating in the second light guide at a different ray angle (either a shallower or steeper angle relative to the main light guide surface) compared to the light rays in the first light guide. After reflecting off the second reflective coupling configuration, the angle is restored to the original ray angle for propagation along the first light guide.

[0035] 3E illustrates a further variation of the reflective coupling configuration, according to which each of the first and second reflective coupling configurations includes a pair of planar mirrors 20A1, 20A2, 20B1, and 20B2, preferably aligned perpendicular to one another to form right-angle reflectors, each reflector preferably at 45 degrees relative to the major surfaces of the light guides. In this case, image light is transmitted between the first and second light guides after reflection from the first of these mirrors 20A1 or 20B1 and before reflection from the second of these mirrors 20A2 or 20B2, while the light rays are angled relatively small relative to the normal to the major surfaces. This allows the entire interface between the first and second light guides to be implemented using a low-index adhesive, since light is transmitted between the light guides at an angle of incidence smaller than the critical angle.

[0036] While Figures 2-3E show recycling of image light in the "X" direction, i.e., light passing through set of facets 16, in the case of a two-dimensional aperture-expanding light guide as illustrated, the same principle can be implemented to recirculate light passing through facet 14 (or a diffractive optical element that performs an equivalent function of progressively redirecting the image light within the light guide before coupling out). This option is illustrated in Figures 4A and 4B. The same reference numerals as above are used throughout.

[0037] In an alternative implementation, the optical continuum between light guides 10A and 10B is implemented by connecting a continuous block 28 of transparent material to the edge surfaces of the first and second light guides 10A, 10B and employing an index-matched adhesive at the interfaces between the light guides and block 28 perpendicular to the major surfaces of the light guides. Figures 5A and 5B illustrate examples of such implementations in which image light recycling is in the X and Y directions, respectively. These configurations are optically substantially equivalent to the implementations of Figures 3A and 4B, respectively, but may have manufacturing advantages.

[0038] Light recycling with polarization management 6, which illustrates an alternative operating principle for recycling light along a light guide, where light is recycled within a single light guide, and undesired interactions between the image light and the combining output array (and / or redirecting array for one dimension of aperture expansion) are avoided by suitable polarization management.

[0039] Specifically, in this case, and with reference to a facet-based light guide as a non-limiting example, the light guide 10 includes at least one set of internal, mutually parallel, polarization-selective partially reflective surfaces 16 angled obliquely relative to the major surfaces to progressively couple out a portion of the image light of a first polarization (e.g., S-polarized) propagating in the forward direction to be reflected toward an observer, while transmitting a portion of the image light of the first polarization to continue propagating in the forward direction within the light guide. The partially reflective surfaces are implemented to be substantially transparent to light of a second polarization (e.g., P-polarized) perpendicular to the first polarization. Recycling of the image light reaching the end of the light guide is achieved by providing a first reflecting arrangement comprising a quarter-wave phase plate 26 and a planar mirror 20A. This first reflecting arrangement is arranged to reflect the image light transmitted through the set of partially reflective surfaces 16 to propagate within the light guide in a reflected direction having an in-plane component and a circular polarization (S to P) opposite to the forward direction for transmission through the partially reflective surfaces 16. A second reflective arrangement including a quarter-wave phase plate 26 and a flat mirror 20B is arranged to reflect image light that has traversed at least a portion of the light guide in the reflective direction so that it propagates forward within the light guide with a circular polarization (P to S), thereby coupling out at least a portion of the image light that was transmitted through a set of partially reflective surfaces on its first pass through the partially reflective surfaces to be reflected back towards the observer when it re-enters the partially reflective surfaces.

[0040] In certain applications, such as the illustrated reflective two-dimensional aperture-expanding light guide, polarization requirements may vary along the length of the light guide. Specifically, referring back to FIG. 2, if the first set of partially reflective surfaces 14 are partially reflective for S-polarized light and transparent for P-polarized light, the different orientations of surfaces 14 and 16 cause S-polarized light reflected from surface 14 to appear as P-polarized light to surface 16. To present S-polarized light to surface 16, a half-wave phase plate 30 (FIG. 6) is advantageously interposed between surfaces 14 and 16 to rotate the polarization. Phase plate 30 complements the function of quarter-wave phase plate 26 in this implementation to keep the polarization of light propagating in the "reverse" direction (from right to left as illustrated) properly aligned to avoid undesired interactions between the surfaces and the returning image light.

[0041] Light recycling applications The configurations described thus far can be advantageously used to enhance the energy efficiency of light guide-based displays by recycling light that would otherwise be lost. This can facilitate achieving enhanced display brightness even when employing a low-visibility coupling-out array with low reflectivity or low diffraction efficiency, for example, when the visibility of the coupling-out array needs to be minimized. Passing the image light repeatedly through a set of facets with low reflectivity or through a diffractive optical element with low coupling efficiency can be used to compensate for the low coupled-out power from each pass of the image light through the coupling-out array.

[0042] Additionally, various schemes for recycling the image light described above are also believed to enable the technology for a variety of applications. In one important subset of additional applications, the recycled image light does not follow the same path as the light originally injected into the light guide. One such example is illustrated schematically in Figures 7A-7C.

[0043] Specifically, FIG. 7A shows a light guide 10 with a set of coupling-out partially reflective surfaces 16 having image injection apertures, represented generally by arrows 12, for propagating in a first direction by internal reflection at the major surfaces (the front and back surfaces in plan view, as illustrated) to inject image light from a suitable projector into the light guide. The first direction is tilted relative to the first and second reflecting configurations (mirrors M1 and M2 and quarter-wave retarder 26) so that image light introduced at the image injection apertures enters a first region of the partially reflective surface 16, and after reflection from the first and second reflecting configurations, enters a second region of the partially reflective surface 16, the second region at least partially not overlapping the first region. Thus, successive recycling of the image light illuminates successive regions of each partially reflective surface 16, thereby achieving aperture expansion in the X direction (in addition to the Y direction expansion achieved by partial reflection at successive surfaces). This arrangement can therefore achieve two-dimensional aperture expansion.

[0044] When multiple cycles of light recycling are used, it is preferable to select the reflectivity of the partially reflective surfaces in at least the first region to be sufficiently low so that a majority of the image light injected into the image injection aperture is transmitted through the set of partially reflective surfaces and recycled by reflection in the first and second reflective configurations.

[0045] Thus, as shown in FIG. 7A, S-polarized ray i is injected into the light guide at P1 and trapped due to the parallel mirrors (M1 and M2) in the y-dimension. Ray i is also confined in the z-dimension (TIR-in-page, not shown). In the x-dimension, ray i is eventually absorbed and lost at the edges of the light guide. When ray i strikes the bottom quarter-wave plate (QWP) 26 and M2, it is reflected back into the light guide, changing its polarization to P-polarized. When ray i strikes the top QWP and M1, it is reflected back into the light guide, changing its polarization back to S-polarized. Assuming that the partially reflective surface 16 is implemented with a multilayer dielectric coating designed to partially reflect and couple out only a portion of the S-polarized light and be transparent to P-polarized light, only the top-to-bottom ray will be partially reflected, and its path will be directed toward the user's eye. FIG. 7B is similar to FIG. 7A but illustrates that the partially reflective facets need not cover the entire light guide aperture but can be limited to only a portion of it. FIG. 7C illustrates a similar case to FIG. 7A, but with additional 2D constraints in the x and z dimensions.

[0046] In the above examples, the partially reflective facets are shown parallel to the mirrors M1 and M2, but this is merely a non-limiting example and the facets may be implemented at any desired angle, such as perpendicular to the propagation direction of the chief ray of the image light. Furthermore, while the mirrors and QWPs are illustrated as being at the edges of the waveguide, this is merely a non-limiting example and these elements may be located at other locations within the light guide.

[0047] As light propagates through the waveguide and bounces back and forth between mirrors M1 and M2, it is gradually coupled out of the waveguide and into the eye-motion box ("EMB"—the area where the user's eyes are expected to be to view the projected image). As a result, the intensity of the image light propagating through the waveguide gradually decreases, potentially creating an intensity gradient across the field of view (FoV) and across the eye-motion box. To overcome this effect, the facet coating can be implemented to provide a gradual increase in reflectivity along the x-axis of FIG. 7A, either as a gradual increase or as steps of different reflectivity. In one such implementation, the facet can be composed of several sections that are not necessarily aligned in a continuous surface, where the reflectivity of the sections increases according to their position along the x-axis. This at least partially compensates for the decrease in image light intensity propagating through the light guide during continuous recycling of the light. This option is illustrated in FIG. 8.

[0048] To achieve a uniform output image, light propagating within the waveguide should uniformly cover the entire output area, i.e., the area where the facets must output light to the eye motion box. This condition is called aperture filling. Figure 9 shows the ray trajectories of a single field (i.e., a single parallel ray of light that ultimately corresponds to a pixel of the projected image on the observer's retina) injected into the waveguide. The injected field has an initial width D in the x-axis and an angle α with respect to the norm of the extension direction of the parallel mirrors M1 and M2. The height of the waveguide is denoted by H. It is clear that aperture filling is achieved when the projection of the injected ray onto the xy plane makes an angle α with respect to the normal to the mirrors M1 and M2.

[0049]

number

[0050] Fields with angles α<α0 overlap the entrance aperture of the light guide, which can lead to energy losses. Fields with angles α>α0 are not completely filled. However, because the diameter of the user's eye pupil is several millimeters, the eye averages out local non-uniformities to some extent. Therefore, satisfactory results can be achieved even when the aperture is partially filled. In general, the ratio of aperture filling η for fields with α>α0 can be quantified as η=α0 / α.

[0051] Minimum level of opening filling η min should be determined by the requirements of the specific system (e.g., the expected size of the eye pupil, the required intensity uniformity). min The value of determines the maximum field of view in the xy plane that can be supported by the waveguide.

[0052]

number

[0053] waveguide FOV max The maximum field of view supported by mirrors M1 and M2 increases linearly with the input aperture size D and inversely with the distance H between mirrors M1 and M2. Since projector size typically scales with input size, increasing the size of aperture D may not be a desirable option, especially in compact systems. Therefore, in practice, it may be preferable to have a short distance between mirrors M1 and M2 so that a large field of view can be induced. This is illustrated schematically in FIG. 10.

[0054] However, it should be noted that the size of the FoV and the eye-motion box (both in the y-axis direction) determines the minimum spacing between mirrors M1 and M2, which may impose design constraints that limit the extent to which this approach can extend the FoV in the x-axis direction. An additional approach to address these constraints is to add a partially reflective surface, or “mixer,” 32, located inside the light guide, perpendicular to the major surface and parallel to the first and second reflective structures (mirrors M1 and M2), as illustrated in FIG. 11 . The mixer preferably has a reflectivity of approximately 50%. The mixer supports a large active area while reducing the effective distance between the mirrors (here, the distance between the mirror and the mixer), thus supporting a large eye-motion box and a large FoV. As a result, the mixer helps fill the light guide aperture and improve overall uniformity. Optionally, multiple mixers can be added at different locations within the light guide. An orthogonal mixer (a mixer with the same orientation as mixer 18 in FIG. 2) can also be added.

[0055] The various aperture-expanding architectures of Figures 7A-11 can be implemented using either a separate light guide for return propagation or polarization management within a single light guide, using any of the recycling principles described above. In the latter case, when a mixer 32 is used, it is preferably combined with a quarter-wave plate on one side and a quarter-wave plate with the opposite sign on the other side. This ensures that any light transmitted by the mixer does not change its polarization state, and any light reflected rotates its polarization state, switching P to S and S to P.

[0056] 2D confinement is expected to be more efficient than typical 1D confinement waveguides because it conserves energy on two axes. For good image quality, the mirrors (M1 and M2) should be of high optical quality and parallel within close manufacturing tolerances. Also, the mirrors are preferably embedded or otherwise covered inside the eyeglass frames to prevent unwanted world ghosts from reaching the user's eyes.

[0057] Another approach to enhancing image uniformity is illustrated in Figure 12. In this example, one of the mirrors is equipped with a phase plate 26' with a graded efficiency that increases along the x-axis, and an impure P-polarized beam is injected into the light guide at point P1. When ray i reaches the graded efficiency phase plate, only a portion of the P-polarized light is converted to S-polarized light. As the beam progresses along the x-direction, the efficiency of the phase plate increases, gradually approaching its performance as a perfect QWP, converting all P-polarized light to S-polarized light and resulting in maximum reflected intensity. In this way, the intensity (which is a function of the S-polarized light reflected by the facet) can be controlled along the x-axis.

[0058] More generally, while quarter-wave plates are believed to be an appropriate choice for various implementations of the invention described herein, it should be noted that some architectures, particularly those involving more complex geometries, may employ other branching phase plates associated with mirrors around the light guide and / or otherwise arranged in an optical arrangement.

[0059] Another parameter that can be varied to help address non-uniformity is a step variation in the reflectivity of one of the mirrors M2, in this case the mirror reflectivity would increase with progression along the x-axis.

[0060] 3D confined waveguide Another embodiment of the present invention can be seen in Figures 13A and 13B. In Figure 13A, there are four mirrors on all edges of the waveguide (M1 parallel to M2, M3 parallel to M4) and two splitting phase plates 26 on the inside of the top and bottom edges. In Figure 13B, there are four mirrors and four splitting phase plates 26 on all edges of the waveguide. Due to the combination of TIR and parallel mirrors, the injected image light is captured in all three dimensions in both cases.

[0061] In FIG. 13A, S-polarized ray i is injected into the waveguide at P1. As in FIG. 7A above, the polarization of ray i is repeatedly switched between S and P polarization as the ray propagates in the x-direction. When ray i strikes M3, it is reflected back into the light guide as S-polarized. Because the returning ray i at P6 has a strange reflection compared to the propagating ray i at P2, at P3 the returning image is rotated compared to the propagating image. This can be mitigated by controlling the LOE facet angle and coating reflection as a function of incident ray angle to the extent that only the propagating ray i is coupled out of the waveguide. Polarization selectivity for two of the four images propagating within the light guide 10, along with angular selectivity, provides more design flexibility for selecting the desired source image to couple out.

[0062] Another method of combining out only the propagating image is illustrated in Figures 14A and 14B, which combines the dual waveguide approach of Figures 2-5B for including one dimension with the polarization management approach of Figure 6 for the second dimension. The various components are labeled using the same reference numbers used throughout this document for each equivalent function.

[0063] The size of the coupling region (where an index-matched adhesive is used to avoid TIR and allow transmission of image light between the light guides) is typically adjusted for a single field, which can result in a "hole" in the aperture fill, where light intended to be coupled into the lower waveguide 10B is reflected back into waveguide 10A. To address this effect, it can be advantageous to use a configuration such as that illustrated in FIG. 14C, similar to the configuration of FIG. 3E above, in which light is reflected by two 45-degree reflectors 20A1, 20A2, 20B1, and 20B2 (e.g., right-angle prisms), allowing the entire interface of the bond between waveguides 10A and 10B to be implemented by a low-index adhesive (or air space) that maintains the TIR condition for light rays incident beyond the critical angle.

[0064] Another approach for coupling out only the propagating image is illustrated in Figures 15-15C. In this case, mirrors M3 and M4 are nonparallel. As discussed above in the context of Figures 3C and 3D, the angles between mirrors M3 and M4 and the major surfaces must be equal in magnitude but have reflection symmetry about a plane perpendicular to both waveguide faces. The use of angled mirrors M3 and M4 renders the laterally recycled light rays into different ranges of propagation angles, which can facilitate the design of angle-selective partially reflective coatings for the facets to select the desired image.

[0065] In Figures 15A and 15B, S-polarized light is injected into the waveguide at P1. As in Figure 13A, the polarization of ray i will switch between S and P polarization as the ray propagates in the x-direction. In this case, when ray i strikes M3 at P2, the ray is reflected back into the light guide as S-polarized light, but at a different angle relative to the major surfaces S1 and S2, as seen in Figure 15B. Returning ray i strikes M4 at P3 and returns to the waveguide at the same angle relative to the major surfaces S1 and S2 as the first propagating ray i. Because the non-parallel mirror switches between the two different angular regimes relative to the major surfaces, specific coatings can be chosen to reflect only the propagating image from the waveguide while completely transmitting the returning rotated image.

[0066] The use of four non-parallel mirrors to achieve 3D light guide confinement is illustrated in Figure 15C. In this case, the angles between mirrors M1 and M2 and the major surfaces must be equal in magnitude, but have reflection symmetry about a plane normal to the waveguide surface. The angles between mirrors M3 and M4 and the major surfaces must be equal in magnitude, but have reflection symmetry about a plane normal to both waveguide surfaces.

[0067] A further approach for selectively coupling out only the desired propagated image is illustrated in Figures 16A-16C. In this case, three waveguides 10A, 10B, and 10C are separated by an air gap or a low-index adhesive. Light guide 10A has a coupling-out facet, while light guides 10B and 10C are transparent light guides in the present non-limiting example. Light guide 10A is coupled to light guide 10B by high-reflection mirrors (M5-M8) with complementary angles between them (Figure 16C). Light guide 10A is coupled to light guide 10C by another set of high-reflection mirrors (M1-M4) with complementary angles between them (Figure 16B). Figure 8 shows, by way of example, such a configuration, where the mirrors are oriented at 45 degrees relative to the waveguides and the major surfaces of the waveguides are parallel to each other. Mirrors M9-M12 are additional high-reflection mirrors positioned to confine light to all three light guides.

[0068] In this example, ray i is injected into light guide 10A at P1. A somewhat similar case was already presented above in Figure 3E for two waveguides. The orientation and reflection of the facets as a function of angle are specifically chosen to combine the propagating images out while minimizing undesired ghosting.

[0069] The system is expected to be more efficient than the prior art 1D and 2D confined waveguides mentioned above, since the 3D confinement conserves energy in all three axes, and therefore the mixer elements in the 3D case may be less efficient.

[0070] While the embodiments illustrated so far for 3D and 2D confinement involve rectangular confinement between two pairs of parallel reflectors arranged orthogonally to one another and a set of partially reflecting facets, other confinement shapes, e.g., triangular, circular, hexagonal, etc., can be created and additional sets of partially reflecting facets can be used. One alternative embodiment employing additional design freedom is shown in FIG. 17, which employs 3D confinement using four pairs of parallel mirrors, labeled M1-M8, and includes two sets of partially reflecting facets 16, 16' arranged at different angles. Quarter-wave, eighth-wave, or any other fractional wave plate 26 can be associated with any or all of the mirrors.

[0071] Another degree of freedom is illustrated in Figures 18A and 18B, where an absorbing layer 40 is placed on the side of the light guide (Figure 18A) or inside the light guide (Figure 18B) to stop unwanted optical paths.

[0072] The embodiments illustrated herein employ reflective light guides based on an array of partially reflective internal surfaces to illustrate various aspects of the invention, however, the invention is not limited to such implementations and may equally be implemented in light guides employing pin-mirror light guides, diffractive light guides employing surface or volume diffractive optical elements, etc.

[0073] Another degree of freedom is to use active half wave plates on one or all of the confinement mirrors, with or without the use of a separate light guide.

[0074] A final example, using the degrees of freedom already described, in a slightly different configuration, is illustrated in Figures 19A-19C. In this case, ray i is injected into the light guide using coupling input element A. Ray i is guided through the respective QWPs between mirrors M1 and M2 and the upper light guide between major surfaces S1 and S2 until it reaches mirror M3, where the image light is reflected back through the non-faceted light guides guided between M1 and M2, their respective QWPs, and major surfaces S2 and S3, until it reaches mirror M4 and returns to the faceted light guide as ray j. The propagation angle of ray i is the angle between ray j and ray j n can be selected so that it does not hit the coupling input element.

[0075] Alternatively, in another case, ray j or ray j n is incident on the coupling input within the element. In this case, the coupling input element is a retro coupling input prism, which is a specially cut prism that causes the incident ray j to exit at the required complementary angle (the same angular orientation as ray i). The coupling input element may employ a partial mirror (e.g., 10% transmission and 90% reflection, or other ratios).

[0076] It will be understood that the above description is intended to serve as an example only, and that many other embodiments are possible within the scope of the invention as defined in the appended claims.

Claims

1. 1. A display for delivering image light to a viewer, said display comprising: (a) a first light guide comprising a block of transparent material having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the major surfaces, the first light guide including at least one set of internal, mutually parallel partially reflective surfaces obliquely angled with respect to the major surfaces to progressively couple out a portion of the image light propagating in the first direction to be reflected towards a viewer while transmitting a portion of the image light to continue propagating in the first direction within the first light guide; (b) a second light guide comprising a block of transparent material having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the surfaces, the second light guide being disposed in a stacked relationship relative to the first light guide; (c) a first reflective coupling arrangement comprising at least one planar mirror, the first reflective coupling arrangement being arranged to reflect the image light transmitted through the set of partially reflective surfaces and propagating in the first direction within the first light guide to propagate in the second light guide in a second direction having an in-plane component opposite to the first direction; and (d) a second reflective coupling arrangement comprising at least one planar mirror, the second reflective coupling arrangement being arranged to reflect the image light that traversed the second light guide in the second direction to be reintroduced for propagation within the first light guide in the first direction, whereby at least a portion of the image light that was transmitted through the set of partially reflective surfaces on its first pass through the partially reflective surfaces is coupled out to be reflected towards the viewer when it re-incident on the partially reflective surfaces.

2. 1. A display for delivering image light to a viewer, said display comprising: (a) a first light guide comprising a block of transparent material having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the major surfaces, the first light guide including at least one set of internal, mutually parallel partially reflective surfaces non-parallel to the major surfaces to progressively redirect a portion of the image light in the first direction to propagate by internal reflection within the first light guide in a second direction while transmitting a portion of the image light to continue propagating in the first direction within the first light guide; (b) a second light guide comprising a block of transparent material having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the surfaces, the second light guide being disposed in a stacked relationship relative to the first light guide; (c) a first reflective coupling arrangement comprising at least one planar mirror, the first reflective coupling arrangement being arranged to reflect the image light transmitted through the set of partially reflective surfaces and propagating in the first direction within the first light guide to propagate in the second light guide in a third direction having an in-plane component opposite to the first direction; and (d) a second reflective coupling arrangement comprising at least one planar mirror, the second reflective coupling arrangement being arranged to reflect the image light that traversed the second light guide in the second direction to be reintroduced for propagation in the first direction within the first light guide, whereby at least a portion of the image light that was transmitted through the set of partially reflective surfaces on a first pass through the partially reflective surfaces is redirected to propagate in the second direction when re-incident on the partially reflective surfaces.

3. 3. A display as described in claim 1 or 2, wherein each of the first and second reflective coupling arrangements comprises a single plane mirror and a light transmitting region adjacent to the single plane mirror, within which an optical continuum exists between the first and second light guides for transmitting image light between the first and second light guides.

4. 4. The display of claim 3, wherein each of the single plane mirrors is oriented perpendicular to the major surfaces of the first and second light guides.

5. 4. The display of claim 3, wherein the single plane mirror of the first reflective combining configuration is oriented at a first tilt relative to the major surfaces of the first and second light guides, and the single plane mirror of the second reflective combining configuration is oriented at a second tilt relative to the major surfaces of the first and second light guides that is equal but opposite to the first tilt.

6. 4. The display of claim 3, wherein the optical continuum is implemented by connecting the first light guide to the second light guide using an index-matched adhesive.

7. 4. The display of claim 3, wherein the optical continuum is implemented by connecting a continuous block of transparent material to edge surfaces of the first light guide and the second light guide.

8. 3. A display as claimed in claim 1 or 2, wherein the first reflective combining arrangement and the second reflective combining arrangement each comprise a pair of mutually perpendicular plane mirrors.

9. 3. A display as claimed in claim 1 or 2, wherein the second light guide is attached to the first light guide by a layer of adhesive having a lower refractive index than the transparent material of the first and second light guides.

10. 3. A display as claimed in claim 1 or 2, wherein the second light guide is separated from the first light guide by an air gap.

11. 1. A display for delivering image light to a viewer, said display comprising: (a) a light guide comprising a block of transparent material having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the major surfaces, the light guide including at least one set of internal, mutually parallel, polarization-selective partially reflective surfaces angled obliquely with respect to the major surfaces to progressively couple out a portion of the image light of a first polarization propagating in the first direction to be reflected towards an observer while transmitting a portion of the image light of the first polarization to continue propagating in the first direction within the light guide, the partially reflective surfaces being substantially transparent to light of a second polarization that is orthogonal to the first polarization; (b) a first reflecting arrangement comprising a quarter wave phase plate and a plane mirror, the first reflecting arrangement being arranged to reflect the image light transmitted through the set of partially reflective surfaces to propagate within the light guide in a second direction having an in-plane component and a circular polarization opposite to the first direction for transmission through the partially reflective surfaces; and (c) a second reflecting arrangement comprising a quarter-wave phase plate and a plane mirror, the second reflecting arrangement being arranged to reflect the image light that traversed the light guide in the second direction so as to propagate within the light guide in a direction parallel to the first direction, whereby at least a portion of the image light that was transmitted through the set of partially reflective surfaces on its first pass through the partially reflective surfaces is coupled out to be reflected towards the viewer when it re-incident on the partially reflective surfaces.

12. 12. The display of claim 11, further comprising an image injection aperture for injecting the image light into the light guide so as to propagate an image in the first direction by internal reflection at the major surfaces, the first direction being oblique to the first and second reflecting configurations, such that the image light introduced at the image injection aperture is incident on a first region of the partially reflective surface and, after reflection from the first and second reflecting configurations, is incident on a second region of the partially reflective surface, the second region at least partially non-overlapping with the first region.

13. 13. The display of claim 12, wherein the reflectivity of the partially reflective surfaces in at least the first region is such that a majority of the image light injected into the image injection aperture is transmitted through the set of partially reflective surfaces and recycled by reflection in the first and second reflective configurations.

14. 13. The display of claim 12, wherein the quarter wave retarder is part of a graduated retarder.

15. 13. The display of claim 12, further comprising a partially reflective surface disposed within the light guide, the partially reflective surface being perpendicular to the major surface and parallel to the first and second reflective features.

16. 1. A display for delivering image light to a viewer, said display comprising: (a) a light guide comprising a block of transparent material having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the surfaces, the light guide including at least one set of internal, mutually parallel, polarization-selective partially reflective surfaces that are not parallel to the major surfaces to progressively redirect a portion of the image light in a first direction to propagate in a second direction within the light guide by internal reflection while transmitting a portion of the image light to continue propagating in the first direction within the light guide, the partially reflective surfaces being substantially transparent to light of a second polarization that is orthogonal to the first polarization; (b) a first reflecting arrangement comprising a quarter wave plate and a plane mirror, the first reflecting arrangement being arranged to reflect the image light transmitted through the set of partially reflective surfaces to propagate within the light guide in a third direction having an in-plane component opposite to the first direction and a circular polarization that is transmitted through the partially reflective surfaces; and and (c) a second reflecting arrangement including a quarter-wave phase plate and a plane mirror, the second reflecting arrangement being arranged to reflect the image light that traversed the light guide in the third direction to propagate within the light guide in a direction parallel to the first direction, such that at least a portion of the image light that was transmitted through the set of partially reflective surfaces on a first pass through the partially reflective surfaces is redirected to propagate in the second direction upon re-incident on the partially reflective surfaces.

Images