Lightguide display with ray-blocking ghost suppression elements

Abstract

An optical display apparatus includes a lightguide optical element (12) with major surfaces (12a, 12b) that guide image light by total internal reflection. A projector (14) injects image light; a first region (16) redirects the guided light from a first to a second direction using reflective or diffractive arrangements, and a second region (18) out-couples the guided light toward an eye- motion box (102). To suppress real-world ghosts from external light that follows a path through the LOE including a single reflection at an internal partially reflective surface (26, 27) or other interface (25), a plurality of ray-blocking elements are formed as elongate strips deployed on one or both of the lightguide surfaces running adjacent to the internal surfaces or interfaces. The ray- blocking strips may be lines of ink. A low-index layer may underlie the ray-blocking elements if needed to preserve lightguide internal reflection.

Description

[0001] Lightguide Display with Ray-Blocking Ghost Suppression Elements

[0002] FIELD AND BACKGROUND OF THE INVENTION

[0003] The present invention relates to near-eye and head-mounted display systems that employ a lightguide optical element (LOE) to guide image light by total internal reflection. More particularly, it concerns suppression of ghost images produced by light from external sources that, after entering the LOE, follow a path including a single reflection at internal partially reflective surfaces of a first region of the LOE and / or at an interface between first and second regions of the LOE.

[0004] Waveguide -based near-eye displays commonly employ an LOE having opposed major surfaces that confine and convey image light by total internal reflection (TIR). In representative architectures, a first region of the LOE includes a set of mutually-parallel internal partially reflective surfaces (sometimes referred to herein, for brevity, as “facets”) that progressively redirect the guided image light from a first guided propagation direction to a second guided propagation direction. A second region of the LOE then out-couples the guided image light toward locations from which the image is intended to be viewed, referred to as an eye-motion box (EMB), while a support structure fixes the LOE’s position relative to the user’s head, thereby defining the geometry of the EMB relative to the LOE.

[0005] In practical use, bright external illumination, e.g., sunlight or overhead lighting, can give rise to objectionable ghost images within the user’s field of view. A particularly troublesome mechanism occurs when external light enters the LOE through a major surface, undergoes a single reflection at one of the internal partially reflective surfaces of the first region (which surfaces are typically steep relative to the major surfaces), and then exits the LOE toward the EMB. A similar single-bounce path can originate at a plane of connection between the first and second regions of the LOE, for example where an optical element such as a retarder is disposed at that interface. These ghosts may be especially prominent for above-horizontal, forward-field illumination and can degrade image contrast and user comfort in see-through applications.

[0006] SUMMARY OF THE INVENTION

[0007] The present invention is a display apparatus with suppression of real-world ghost images.

[0008] According to the teachings of an embodiment of the present invention there is provided, a display apparatus for displaying an image to a user, the display apparatus comprising: (a) a lightguide optical element (LOE) having a pair of mutually parallel major surfaces for guiding light by internal reflection at the major surfaces; (b) an image projector optically coupled to the LOE so as to introduce into the LOE image light corresponding to a collimated image to propagate within the LOE in a first guided propagation direction by internal reflection at the major surfaces; and (c) a support structure configured to support the LOE relative to the user’s head so that an eye of the user is located within an eye-motion box (EMB) relative to the LOE, wherein the LOE comprises: (i) a first region including a first set of mutually -parallel internal partially reflective surfaces configured to progressively redirect the image light propagating in the first guided propagation direction to propagate in a second guided propagation direction, and (ii) a second region including an out-coupling arrangement configured to progressively couple out the image light propagating in the second guided propagation direction toward the EMB, and wherein, for a plurality of the internal partially reflective surfaces of the first set, a respective ray-blocking element is disposed on at least one of the major surfaces and is formed as an elongate strip extending along and adjacent to a line of intersection between a plane of the corresponding internal partially reflective surface and the major surface, the strip extending along at least a portion of a length of the internal partially reflective surface along the line of intersection, the ray-blocking elements having absorbing and / or scattering optical properties sufficient to attenuate or eliminate an intensity of light that, in the absence of the ray-blocking elements, would reach the EMB from a light source outside the LOE along a light path passing through the LOE that includes a single reflection at the corresponding internal partially reflective surface.

[0009] According to a further feature of an embodiment of the present invention, each rayblocking element extends along a majority of the length of the internal partially reflective surface.

[0010] According to a further feature of an embodiment of the present invention, each rayblocking element extends along substantially an entirety of the length of the internal partially reflective surface.

[0011] According to a further feature of an embodiment of the present invention, ray-blocking elements are provided for at least a majority of the internal partially reflective surfaces of the first set.

[0012] According to a further feature of an embodiment of the present invention, ray-blocking elements are provided for all of the internal partially reflective surfaces of the first set.

[0013] According to a further feature of an embodiment of the present invention, each rayblocking element has a width within the range 150-300 pm.

[0014] According to a further feature of an embodiment of the present invention, a width of a given ray-blocking element is between 5% and 30% of a spacing between adjacent ones of the internal partially reflective surfaces.

[0015] According to a further feature of an embodiment of the present invention, the ray -blocking elements collectively obscure less than one-half of a surface area between adjacent ones of the internal partially reflective surfaces. According to a further feature of an embodiment of the present invention, the ray -blocking elements collectively obscure less than one-third of the surface area between adjacent ones of the internal partially reflective surfaces.

[0016] According to a further feature of an embodiment of the present invention, the ray -blocking elements collectively obscure less than one-quarter of the surface area between adjacent ones of the internal partially reflective surfaces.

[0017] According to a further feature of an embodiment of the present invention, widths of the ray-blocking elements differ among the internal partially reflective surfaces according to angles of respective incident ray paths that, with a single reflection at the corresponding internal partially reflective surface, would reach the EMB.

[0018] According to a further feature of an embodiment of the present invention, a width of at least one ray -blocking element varies along the length of the internal partially reflective surface.

[0019] According to a further feature of an embodiment of the present invention, ray-blocking elements are disposed on both of the major surfaces of the LOE.

[0020] According to a further feature of an embodiment of the present invention, ray-blocking elements are disposed only on one of the major surfaces of the LOE.

[0021] According to a further feature of an embodiment of the present invention, at least one rayblocking element comprises an ink composition.

[0022] According to a further feature of an embodiment of the present invention, there is also provided a film bonded to the LOE with an adhesive having refractive-index and thickness selected to maintain total internal reflection for guided display light, wherein the ray -blocking elements are formed on or within the film.

[0023] According to a further feature of an embodiment of the present invention, there is also provided an angularly selective multilayer dielectric coating disposed on at least one of the major surfaces, wherein the ray-blocking elements are disposed on or within the dielectric coating.

[0024] According to a further feature of an embodiment of the present invention, there is also provided a low-index overlayer disposed on at least one of the major surfaces, the overlayer having refractive index lower than that of a core of the LOE and a thickness selected to maintain total internal reflection for guided display light, wherein the ray-blocking elements are disposed on or within the overlayer.

[0025] According to a further feature of an embodiment of the present invention, the second region comprises a diffractive out-coupling arrangement.

[0026] According to a further feature of an embodiment of the present invention, the second region comprises a reflective out-coupling arrangement. According to a further feature of an embodiment of the present invention, the ray -blocking elements are configured to attenuate external-source light arriving predominantly from an upperforward field relative to a gaze direction of the user.

[0027] According to a further feature of an embodiment of the present invention, at least one rayblocking element extends on both sides of the line of intersection.

[0028] According to a further feature of an embodiment of the present invention, at least one rayblocking element extends only on a side of the line of intersection that is closer to the EMB.

[0029] According to a further feature of an embodiment of the present invention, the LOE further comprises an interface between the first region and the second region including an optical element configured to modify polarization of light propagating from the first region to the second region, and further comprising a ray-blocking element disposed on at least one of the major surfaces and formed as an elongate strip extending along and immediately adjacent to a line of intersection between a plane of the interface and the major surface, the strip being located on a side of the line closer to the EMB and extending along at least a portion of a length of the interface along the line of intersection.

[0030] According to a further feature of an embodiment of the present invention, the optical element comprises a retarder.

[0031] There is also provided according to the teachings of an embodiment of the present invention, a display apparatus for displaying an image to a user, the display apparatus comprising: (a) a lightguide optical element (LOE) having a pair of mutually parallel major surfaces for guiding light by internal reflection at the major surfaces; (b) an image projector optically coupled to the LOE so as to introduce into the LOE image light corresponding to a collimated image to propagate within the LOE in a first guided propagation direction by internal reflection at the major surfaces; and (c) a support structure configured to support the LOE relative to the user’s head so that an eye of the user is located within an eye-motion box (EMB) relative to the LOE, wherein the LOE comprises: (i) a first region including a reflective or diffractive arrangement configured to progressively redirect the image light propagating in the first guided propagation direction to propagate in a second guided propagation direction, (ii) a second region including an out-coupling arrangement configured to progressively couple out the image light propagating in the second guided propagation direction toward the EMB, (iii) an interface between the first region and the second region, and (iv) a ray-blocking element disposed on at least one of the major surfaces and formed as an elongate strip extending along and immediately adjacent to a line of intersection between a plane of the interface and the major surface, the strip being located on a side of the line closer to the EMB and extending along at least a portion of a length of the interface along the line of intersection, the ray-blocking element having absorbing and / or scattering optical properties sufficient to attenuate or eliminate an intensity of light that, in the absence of the ray-blocking element, would reach the EMB from a light source outside the LOE along a light path passing through the LOE that includes a single reflection at the interface.

[0032] According to a further feature of an embodiment of the present invention, the interface includes an optical element configured to modify polarization of light propagating from the first region to the second region.

[0033] BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0035] FIGS. 1 A and IB are schematic isometric views of an optical system including two displays each implemented using a lightguide optical element (LOE), constructed and operative according to the teachings of an embodiment of the present invention, illustrating a top-down and a sideinjection configuration, respectively.

[0036] FIG. 2 is a schematic of the LOE showing an image-ray path propagating by internal reflection, reflecting at an internal partially reflective surface in a first region, passing an interface between regions, and entering a second region for out-coupling.

[0037] FIG. 3 is the schematic of FIG. 2 with an external- source ray path illustrated, showing a single-reflection ghost path toward the EMB.

[0038] FIG. 4A is an extracted detail of FIG. 3 isolating a first-region internal partially reflective surface and the associated ghost-producing ray path.

[0039] FIG. 4B is a sectional view in a plane containing the ray path of FIG. 4A.

[0040] FIG. 5 is analogous to FIG. 3, illustrating a ghost path from a lower-quadrant external source.

[0041] FIG. 6A is an extracted detail of FIG. 5 isolating the surface and ray path.

[0042] FIG. 6B is a sectional view in a plane containing the ray path of FIG. 6A.

[0043] FIG. 7 is analogous to FIGS. 3 and 5, illustrating a ghost path reflecting from an interface between first and second regions.

[0044] FIG. 8 A is a sectional view like FIG. 4B with ray-blocking elements on the eye- side and far-side major surfaces for an upper-source scenario.

[0045] FIG. 8B is a sectional view like FIG. 6B with ray -blocking elements for a lower- source scenario.

[0046] FIG. 8C is a sectional view of the interface-reflection scenario with ray-blocking elements on both major surfaces along the interface line. FIG. 9A is a detailed sectional view of an upper-source scenario without ray-blocking elements, showing a bundle of rays and their refractions and reflections.

[0047] FIG. 9B is the view of FIG. 9A with relatively short ray-blocking elements on both major surfaces.

[0048] FIG. 9C is the view of FIG. 9A with a longer ray-blocking element on the external-side major surface.

[0049] FIG. 9D is the view of FIG. 9A with a larger ray-blocking element on the eye-side major surface.

[0050] FIG. 10 is a system-level schematic showing ray -blocking elements along internal partially reflective surface lines and along an inter-region interface.

[0051] FIG. 11 is a schematic showing external lenses or films bonded to the LOE, compatible with the ray -blocking implementations.

[0052] DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] According to certain preferred embodiments of the present invention, there is provided a near-eye display apparatus 10 that includes a lightguide optical element 12 having opposed major surfaces 12a, 12b that guide image light by total internal reflection. An image projector assembly 14 is optically coupled to the lightguide so as to introduce image light corresponding to a collimated image that propagates within the lightguide in a first guided propagation direction by internal reflection at the major surfaces. A support structure 20 positions the lightguide relative to the user’ s head so that a viewer’ s eye 100 is located within an eye-motion box 102 defined spatially in relation to the lightguide.

[0054] The lightguide includes a first region 16 that includes an image redirection arrangement that progressively redirects the image light from the first direction to a second direction while expanding the optical aperture of the image in a first dimension. In representative implementations, the image redirection arrangement of the first region includes a set of mutually parallel internal partially reflective surfaces, labeled 26 or 27 according to criteria which will be detailed below. These surfaces progressively redirect the guided image light from the first guided propagation direction to a second guided propagation direction. The lightguide further includes a second region 18 with an out-coupling arrangement, which may include internally reflective out-coupling surfaces 28 or a diffractive out-coupler, that progressively couples out the guided image light toward the eye-motion box.

[0055] Displays of this type sometimes suffer from real-world ghost images. One class of real- world ghost images that is of particular concern can arise from external illumination entering the LOE 12 through a major surface and following a light path passing through the LOE that includes a single reflection at an internal partially reflective surface of the first region 16 or at an interface 25 between the first region 16 and the second region 18, and then exiting toward the eye-motion box 102. Such paths are common for bright overhead or off-axis sources, and they are distinct from any stray light originating within the image projector assembly 14.

[0056] According to particularly preferred embodiments of the present invention, such ghosts are attenuated or eliminated by disposing a plurality of ray-blocking elements on at least one of the major surfaces 12a, 12b. For a plurality of the internal partially reflective surfaces of the first region 16, a respective ray-blocking element is formed as an elongate strip that extends along and adjacent to a line of intersection between a plane of the corresponding internal partially reflective surface and the major surface, located on a side of the line that is closer to the eye-motion box 102, and extending along at least a portion of a length of that internal partially reflective surface. In certain embodiments, as an addition or as an alternative to ray-blocking elements in the first region 16, ray-blocking elements are provided along the interface 25 on the front and back major surfaces of the LOE 12, each placed only on the side of the interface line that is closer to the eye-motion box 102. The ray-blocking elements can be implemented as ink lines directly on the LOE 12, the ink optionally formulated to have low refractive index so as to maintain total internal reflection for guided image light. Alternatively, the ray-blocking elements can be carried on external films or lenses 110, 111 bonded with adhesive 112, or formed on or within angularly selective dielectric coatings 114 or low-index overlayers 116 disposed on a major surface. The materials and layer stacks are selected to attenuate or eliminate the intensity of external-source light associated with the single-reflection paths while maintaining total internal reflection for guided image light, and the approach is compatible with reflective or diffractive out-coupling in the second region 18.

[0057] The following terms are used for clarity in the detailed description. They are intended to be consistent with the claims and figures.

[0058] • Lightguide optical element (LOE, 12): a waveguide structure with opposed major surfaces (12a, 12b) configured to guide image light by total internal reflection.

[0059] • Internal partially reflective surface: one or more internal surfaces within the LOE that exhibit partial reflectance, typically dependent on angle of incidence and / or polarization, used to redirect light propagating within the LOE. Depending on the angle of these surfaces, the redirection may be toward a second direction of guided propagation within the LOE or toward out-coupling to direct image light to the viewer’ s eye.

[0060] • Interface (25): a plane of connection between the first region and the second region. The interface can include an optical element such as a retarder. For the drawings and description, the interface is referenced as 25 without separately numbering the optical element.

[0061] • Eye-motion box (EMB, 102): locations from which the projected image is intended to be viewed, defined spatially in relation to the LOE.

[0062] • Line of intersection: the line defined by the intersection of a plane of an internal partially reflective surface, or of the interface, with a major surface of the LOE.

[0063] • Substantially an entire length: at least ninety percent of the length of the corresponding internal partially reflective surface along the relevant line of intersection.

[0064] • Majority: when referring to a collection of elements, more than one half in number of the elements in that collection; when referring to a continuous quantity, more than one half of the magnitude of that quantity, for example a majority of a length.

[0065] • Width and thickness: width refers to the in-plane dimension of a ray-blocking element measured along the major surface and perpendicular to the line of intersection; thickness refers to the dimension of a ray-blocking element perpendicular to the major surfaces, the same axis as the thickness of the LOE.

[0066] • Opacity: one minus transmittance across a visible spectral band, optionally averaged over polarization (for example, opacity of at least ninety percent).

[0067] • External light source: any light source outside the LOE that emits light which can enter the LOE and follow a path that includes a single reflection at an internal partially reflective surface or at the interface.

[0068] • Ray-blocking element: an elongate feature disposed on a major surface of the LOE and used to attenuate or eliminate external-source light associated with single-reflection ghost paths. Examples and structural details are provided elsewhere in this description.

[0069] FIGS. 1A and IB are schematic isometric views of an optical system including two displays, each implemented using a lightguide optical element (LOE), constructed and operative according to an embodiment disclosed herein, illustrating a top-down configuration and a sideinjection configuration, respectively.

[0070] A near-eye display 10 employs a compact image projector assembly 14 integrated so as to inject an image into a lightguide optical element (LOE, also referred to as a waveguide, a substrate or a slab) 12 within which the image light is trapped by internal reflection at a pair of mutually parallel major external surfaces. The LOE 12 typically includes an arrangement for expanding the optical aperture of the injected image in one or two dimensions, and for coupling out the image illumination toward the eye of an observer. In representative implementations, these functions are based on internal partially reflecting surfaces or on diffractive optical elements. In one non-limiting set of implementations, the light injected into LOE 12 by image projector assembly 14 impinges on a set of partially reflecting surfaces that are parallel to each other and inclined obliquely to a direction of propagation of the image light. Each successive surface deflects a proportion of the image light into a deflected direction that remains guided by internal reflection within the substrate. These surfaces, which are not illustrated individually in FIGS. 1A and IB, are located in a first region 16. This partial reflection at successive surfaces progressively redirects the direction of guided propagation while expanding the optical aperture in a first dimension. The redirected image illumination then passes into a second region 18, which may be implemented as an adjacent distinct substrate or as a continuation of a single substrate, in which a coupling-out arrangement, either a further set of partially reflective surfaces or a diffractive optical element, progressively couples out a proportion of the image illumination toward an eye located within an eye-motion box 102, thereby achieving a second dimension of optical aperture expansion.

[0071] The overall device may be implemented separately for each eye, and is preferably supported relative to the head of a user with each LOE 12 facing a corresponding eye 100. A support arrangement 20 may be implemented as an eyeglass frame with sides for supporting the device relative to the user’s ears. Other forms of support arrangement may also be used, including head bands, visors, or devices suspended from helmets.

[0072] Reference is made to an X axis which extends in the general extensional direction of the first region of the LOE, and to a Y axis which extends perpendicular thereto. In FIG. 1A the X axis is horizontal and the Y axis is vertical. In FIG. IB the X axis is vertical and the Y axis is horizontal. In approximate terms, the first region 16 may be considered to achieve aperture expansion in the X direction while the second region 18 achieves aperture expansion in the Y direction. For simplicity of presentation in subsequent drawings, the invention will be exemplified in the context of an LOE with orientation similar to FIG. IB, with the understanding that the same principles apply to aperture-expansion lightguides with other orientations or designs.

[0073] The image injected into the lightguides is preferably a collimated image, in which light of each image pixel is a parallel beam, collimated to infinity, with an angular direction corresponding to pixel position so that the image illumination spans an angular field of view in two dimensions. The near-eye display 10 may include additional components such as a controller 22 for actuating image projector 14, powered by an onboard battery or another suitable power source. The projector technology and coupling approach are not limiting, and additional options are described later in the detailed description.

[0074] FIG. 2 is a schematic illustration of LOE 12 corresponding to the side-injection arrangement of FIG. IB illustrating a typical ray path followed by image light. Image light from projector 14 enters the LOE 12, is guided by internal reflection between major surfaces 12a, 12b, and impinges on an internal partially reflective surface in the first region 16 (illustrated as 26) which progressively redirects the guided image light from a first guided propagation direction to a second guided propagation direction. The redirected image light passes through an interface 25 and into the second region 18, where an out-coupling arrangement, for example reflective out- coupling surfaces 28 or a diffractive out-coupler, directs the image light toward the viewer’ s eye 100 located within the EMB designated by rectangle 102.

[0075] The internal structures of LOE 12 may give rise to various real-world ghost images that are addressed by various aspects of the present invention. For clarity in the following figures, reference will be made to three groups of real-world ghost paths:

[0076] 1. Upper-incident facet ghosts: external light entering a major surface of the LOE 12, following a path that includes a single reflection at an internal partially reflective surface in the first region 16, then exiting toward the EMB 102 from above-horizontal or upperforward directions. In the drawings these first-region surfaces are labeled 26.

[0077] 2. Lower-incident facet ghosts: analogous single-reflection paths arising from below- horizontal incidence, involving first-region surfaces labeled 27.

[0078] 3. Interface ghosts: paths including reflection at an interface 25 between the first region 16 and the second region 18.

[0079] These groups of ghost image ray paths will be illustrated below as they occur in the absence of the ghost-blocking elements of the present invention, and then, a preferred deployment of ghostblocking elements for each will be described.

[0080] Turning now to FIG. 3, this presents the same schematic view as FIG. 2 but shows an external-source ghost path. Light from an external source incident in certain particular directions enters the LOE 12 through a major surface (somewhat bent by refraction, not shown here), and follows a path that includes a single reflection at an internal partially reflective surface in the first region 16 (labeled 26 in this upper-incident case). The reflected light then exits the LOE toward the EMB 102, generating a real-world ghost. This view establishes the geometry of the upperincident facet ghost family.

[0081] FIG. 4A isolates the elements of FIG. 3 that participate in the ghost path. A single internal partially reflective surface 26 of the first region 16 is shown together with the incident external light and the resulting single-reflection path toward the EMB 102. The surrounding portions of the LOE 12 are omitted for clarity.

[0082] FIG. 4B shows a sectional view through FIG. 3 taken in a plane that contains the ghostforming rays of FIG. 4A. The external source is depicted to the left. A representative ray enters through the left major surface of the LOE 12, impinges on an upper side of the internal partially reflective surface 26 (drawn horizontally in this view), and reflects upward and to the right to exit the LOE toward the eye 100 and the EMB 102. This sectional representation makes the singlereflection path of the upper-incident facet ghost explicit.

[0083] Turning now to FIG. 5, this view is analogous to FIG. 3 but illustrates a lower-incident facet ghost. Light from an external source in a lower quadrant enters the LOE 12 through a major surface (somewhat bent by refraction, not shown here), and follows a path that includes a single reflection at an internal partially reflective surface in the first region 16 (labeled 27 for this lower- incident case). The reflected light then exits the LOE toward the EMB 102, establishing the geometry of the lower-incident facet ghost family.

[0084] FIG. 6A isolates the elements of FIG. 5 that participate in the ghost path. A single internal partially reflective surface 27 of the first region 16 is shown together with the incident external light and the resulting single-reflection path toward the EMB 102. Surrounding portions of the LOE 12 are omitted for clarity.

[0085] FIG. 6B shows a sectional view through FIG. 5 taken in a plane that contains the ghostforming rays of FIG. 6A. The external source is depicted to the left. A representative ray enters through the left major surface of the LOE 12, impinges on a lower side of the internal partially reflective surface 27 (drawn horizontally in this view), and reflects downwards and to the right to exit the LOE toward EMB 102. This sectional representation makes the single-reflection path of the lower-incident facet ghost explicit.

[0086] Turning now to FIG. 7, this view is analogous to FIGS. 3 and 5 but illustrates an interface ghost. Light from an external source above the LOE enters through a major surface 12b (somewhat bent by refraction, not shown here), and follows a path that includes a single reflection at the interface 25 between the first region 16 and the second region 18, after which the light exits the LOE toward the EMB 102. This establishes the geometry of the interface-reflection ghost family.

[0087] Turning now to FIG. 8A, this view corresponds to FIG. 4B but shows ray-blocking elements added on both major surfaces of the LOE 12 for the upper-incident facet-ghost case. An elongate ray-blocking element 261 is shown on the eye-side surface 12a and a corresponding element 262 on the far-side surface 12b. Each element extends along and adjacent to the line of intersection of the plane of the relevant internal partially reflective surface 26 with the respective major surface, placed on the side of the line closer to the EMB, to block the single-reflection ghost path. The direction of elongation is therefore into the page in this view.

[0088] Similarly, turning now to FIG. 8B, this view corresponds to FIG. 6B but for the lower- incident facet-ghost case. Ray-blocking elements 271 and 272 are shown on 12a and 12b, respectively, extending along and adjacent to the lines of intersection for the relevant internal partially reflective surface 27, and located on the side of each line closer to the EMB, thereby interrupting the single-reflection ghost path. In this case, since the ghost ray path to be blocked is via the underside of the facet, ray -blocking elements 271 and 272 extend downwards from the edges of the facet, as shown.

[0089] Turning now to FIG. 8C, this view illustrates the interface-reflection case corresponding to FIG. 7 with ray-blocking elements extending along and adjacent to interface 25. Elements 251 and 252 are shown on 12a and 12b, respectively, each extending along and immediately adjacent to the line of intersection of the plane of the interface 25 with the corresponding major surface, and positioned on the side of the line closer to the EMB, i.e., towards second region 18.

[0090] FIGS. 9A-9D provide a more detailed and precise sectional representation paralleling the case of FIGS. 4B and 8 A, and by analogy for the other groups of ghost light paths, explicitly showing refraction at entry into and exit from the LOE 12, for a bundle of five rays labelled 31a- 31e (top to bottom) from the external source 50. FIG. 9A illustrates this scenario in the absence of ray-blocking elements. Uppermost ray 31a passes above the internal partially reflective surface (facet) 26 and exits without contributing to a ghost. Three rays 31b, 31c and 31d, entering at the same angle, are refracted within the LOE and impinge on internal partially reflecting surface 26 at three different locations across the width of facet 26, which spans the thickness of the LOE: one near surface 12b, one near mid-thickness, and one near surface 12a. Each of these three rays reflects at facet 26 and exits toward the eye 100 within the EMB 102, forming the ghost. A fifth ray 31e enters the LOE slightly lower, missing facet 26, refracts, and exits at a direction parallel to the uppermost ray without contributing to the ghost.

[0091] FIGS. 9B-9D illustrate how this ghost can be eliminated, or at least attenuated, by various different deployments of ray -blocking elements. In FIG. 9B, the same incident-ray bundle and geometry are shown with relatively short ray-blocking elements on both major surfaces. A short element 262 on surface 12b prevents two of the ghost-forming rays from entering the LOE. The remaining ghost-forming ray passes the external-side element but, after reflection at 26, is intercepted by a short element 261 on surface 12a before it can exit toward the viewer, thereby preventing the ghost image from reaching the user’s eye.

[0092] In FIG. 9C, a longer ray-blocking element 262 is shown on surface 12b. This element blocks all three of the rays that would have produced the ghost before they enter the LOE, so no ghost path is formed.

[0093] Conversely, in FIG. 9D, no element is shown on surface 12b, so all three ghost-forming rays enter the LOE. A larger ray -blocking element 261 on surface 12a obstructs and absorbs each ray after reflection at facet 26, preventing exit toward the viewer and thereby eliminating the ghost.

[0094] The geometry and optical properties of the ray-blocking elements are selected according to the families of external- source rays that would otherwise reach the EMB 102 along single- reflection paths. In general, the width of each element is determined primarily by the range of incident angles at which external rays, once refracted into the LOE 12, impinge on the relevant internal partially reflective surface and then exit toward the EMB. This geometry is typically a function of the EMB size, the eye relief (distance from the EMB to the lightguide), and the off- axis distance of each facet from the optical axis of the viewer. By way of example only, eye relief may be about 20-25 mm, the first-region surfaces may be located about 20-30 mm off an optical axis, and the eye-motion box may be about 10 mm by 10 mm, although all of these values may vary from implementation to implementation.

[0095] It should be noted, however, that the width of the ray-blocking elements need not necessarily block all potential ghost-forming ray paths for the corresponding facet for the entire EMB. In certain cases, it may be preferable to implement ray -blocking elements that attenuate ghost images by blocking a majority of the relevant ray paths, or to block all potential ghostforming rays for at least part of the EMB. For a representative LOE thickness on the order of 1-2 millimeters and a spacing between adjacent internal partially reflective surfaces in the range of about 1-3 millimeters, suitable widths for the ray-blocking elements may fall in the range of about 150-300 micrometers for pairs of ray-blocking elements on both faces of the LOE, or 300-600 micrometers for elements on only one major LOE surface. Expressed relative to spacing, the width of the ray-blocking elements is typically between about five percent and thirty percent of the spacing between adjacent internal partially reflective surfaces. To preserve peripheral vision for the user and minimize conspicuity to an outside observer, the elements should obscure less than half of the surface area between adjacent internal partially reflective surfaces, more preferably less than one third, and most preferably less than one quarter.

[0096] The length of a ray-blocking element along the line of intersection may be chosen to cover only the portion of the internal partially reflective surface that contributes to ghost formation, may extend along a majority of the length of the internal partially reflective surface, or may extend along substantially an entire length where warranted by the optical geometry. The width of a given element may also vary along its length to match position-dependent ray families. Elements may be provided on both major surfaces 12a and 12b, or only on one of them, and may be provided for a plurality of the internal partially reflective surfaces, for at least a majority in number, or for all of them. Widths may differ from surface to surface according to the local angles associated with the corresponding single-reflection paths that can reach the EMB.

[0097] Although many implementations place each element only on the side of the line of intersection that is closer to the EMB 102, certain viewing geometries may benefit from elements that extend on both sides of the line of intersection. For example, where the plane of an internal partially reflective surface intersects the EMB, a top-reflected ghost can occur when the eye is at an upper location in the EMB and a bottom-reflected ghost can occur from the underside of the same surface when the eye is lower within the EMB. Although the angular footprint of a facet at grazing incidence is small, its reflectance can increase rapidly at near grazing angles, so provision of ray-blocking elements may be valuable. A narrow element that extends across the line of intersection, with portions on both sides of the line, may be used to attenuate these ghosts. Where such straddling elements are used, the total width can remain small to minimize obscuration.

[0098] The elements may function primarily by absorption, by scattering, or by a combination of both. In some embodiments, the materials and geometry are selected to provide an opacity of at least fifty percent in a visible spectral band, preferably at least eighty percent, and most preferably at least ninety percent. For scattering implementations, the elements are selected so that a sufficient fraction of the power that would otherwise reach the EMB 102 along the single-reflection path is redirected away from the EMB. By choice of suitable materials and / or structures, the ray-blocking elements are implemented so that guidance of image light by total internal reflection is maintained for the range of guided angles used by the display.

[0099] In one set of preferred but non-limiting implementations, the elements are implemented as ink lines deposited directly on the LOE 12. In certain embodiments, the ink is formulated to have a sufficiently low refractive index relative to the bulk material of the LOE that the guided image light remains confined by total internal reflection. As alternatives, the elements can be formed on an external film or a layer of low-index adhesive at the surface of the LOE, where the refractive index of the film or adhesive is selected to maintain total internal reflection for guided rays. In other embodiments, the elements are formed on an angularly selective dielectric coating disposed on a major surface. These alternatives can be used individually or in combination. The same material and placement options are applicable to the elements used for the interface-reflection case along the interface 25.

[0100] In some embodiments, the ray -blocking elements are formed from inks or other compositions selected to reduce visual conspicuity to an outside observer while maintaining attenuation of ghost-forming rays. Suitable appearances can include bright tones and / or skin-tone ranges as may be desired for a given product. Additionally, or alternatively, in some cases, conspicuity may be reduced by forming the ray-blocking elements with “soft edges,” i.e., where the opacity reduces gradually over a transition region at the edge of the strip, making the edge less apparent.

[0101] Thus, the invention encompasses implementations in which element width is set according to ray angles, in which elements extend along a majority or substantially an entire length of the internal partially reflective surface, in which widths differ from facet to facet and / or between front and back surfaces of the LOE, in which obscuration limits are respected, and in which elements are realized as ink directly on the LOE 12, as features on bonded film, carriers and / or adhesive, or as features on or within coatings or overlayers, all while maintaining total internal reflection for guided image light.

[0102] Turning now to FIG. 10, this view corresponds generally to the schematic of FIG. 2 but illustrates ray-blocking elements deployed for each of the three ghost mechanisms in a single example. In the first region 16, internal partially reflective surfaces 26 that are candidates for upper-incident ghosts are shown with ray-blocking elements 261 on surface 12a and 262 on surface 12b, each extending along and adjacent to the corresponding line of intersection on the side closer to the EMB. Internal partially reflective surfaces 27 that are candidates for lower- incident ghosts are shown with ray-blocking elements 271 and 272 on 12a and 12b, respectively, placed along and adjacent to the corresponding lines of intersection on the side closer to the EMB. Along the interface 25 between the first and second regions, ray-blocking elements 251 and 252 are shown on 12a and 12b, respectively, positioned along and adjacent to the interface line on the side closer to the EMB. Although FIG. 10 shows these configurations together, each mechanism can be used independently according to the needs of a particular implementation. Suppression of interface-reflection ghosts at 25 can be of particular importance even where diffractive elements are used in both the first region 16 and the second region 18.

[0103] Turning now to FIG. 11, this view illustrates that ray-blocking elements can be carried on external optical elements bonded to the LOE 12. In one example, a lens 110 is bonded on surface 12a and optionally a lens 111 on surface 12b. These may be, for example, “push-pull lenses” of opposite optical power that may be used to achieve a desired perceived viewing distance of the display. The ray-blocking elements that correspond in position to elements 261 and 262 (shown schematically), and 271, 272, 251, and 252 (not shown here) are formed on the external optical elements, typically on the surface facing the LOE, and are aligned to the respective lines of intersection when the external elements are bonded in place. Here too, attachment is achieved in a manner that preserves TIR within the lightguide, either using an air gap, low index adhesive, or providing angularly- selective reflective coatings which mimic TIR behavior. This arrangement is compatible with the implementations described above and can be used in conjunction with reflective or diffractive out-coupling in the second region 18.

[0104] It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A display apparatus for displaying an image to a user, the display apparatus comprising:(a) a lightguide optical element (LOE) having a pair of mutually parallel major surfaces for guiding light by internal reflection at said major surfaces;(b) an image projector optically coupled to said LOE so as to introduce into said LOE image light corresponding to a collimated image to propagate within said LOE in a first guided propagation direction by internal reflection at said major surfaces; and(c) a support structure configured to support said LOE relative to the user’s head so that an eye of the user is located within an eye-motion box (EMB) relative to said LOE, wherein said LOE comprises:(i) a first region including a first set of mutually-parallel internal partially reflective surfaces configured to progressively redirect the image light propagating in the first guided propagation direction to propagate in a second guided propagation direction, and(ii) a second region including an out-coupling arrangement configured to progressively couple out the image light propagating in the second guided propagation direction toward the EMB, and wherein, for a plurality of the internal partially reflective surfaces of said first set, a respective ray-blocking element is disposed on at least one of said major surfaces and is formed as an elongate strip extending along and adjacent to a line of intersection between a plane of the corresponding internal partially reflective surface and said major surface, the strip extending along at least a portion of a length of said internal partially reflective surface along said line of intersection, the ray-blocking elements having absorbing and / or scattering optical properties sufficient to attenuate or eliminate an intensity of light that, in the absence of said ray-blocking elements, would reach the EMB from a light source outside the LOE along a light path passing through said LOE that includes a single reflection at the corresponding internal partially reflective surface.

2. The display apparatus of claim 1, wherein each ray-blocking element extends along a majority of the length of said internal partially reflective surface.

3. The display apparatus of claim 1, wherein each ray-blocking element extends along substantially an entirety of the length of said internal partially reflective surface.

4. The display apparatus of claim 1, wherein ray-blocking elements are provided for at least a majority of said internal partially reflective surfaces of said first set.

5. The display apparatus of claim 1, wherein ray-blocking elements are provided for all of said internal partially reflective surfaces of said first set.

6. The display apparatus of claim 1 , wherein each ray -blocking element has a width within the range 150-300 pm.

7. The display apparatus of claim 1, wherein a width of a given ray-blocking element is between 5% and 30% of a spacing between adjacent ones of said internal partially reflective surfaces.

8. The display apparatus of claim 1, wherein the ray -blocking elements collectively obscure less than one-half of a surface area between adjacent ones of said internal partially reflective surfaces.

9. The display apparatus of claim 1, wherein the ray -blocking elements collectively obscure less than one-third of the surface area between adjacent ones of said internal partially reflective surfaces.

10. The display apparatus of claim 1, wherein the ray -blocking elements collectively obscure less than one-quarter of the surface area between adjacent ones of said internal partially reflective surfaces.

11. The display apparatus of claim 1, wherein widths of the ray -blocking elements differ among said internal partially reflective surfaces according to angles of respective incident ray paths that, with a single reflection at the corresponding internal partially reflective surface, would reach the EMB.

12. The display apparatus of claim 1, wherein a width of at least one ray-blocking element varies along the length of said internal partially reflective surface.

13. The display apparatus of claim 1, wherein ray-blocking elements are disposed on both of said major surfaces of said LOE.

14. The display apparatus of claim 1, wherein ray-blocking elements are disposed only on one of said major surfaces of said LOE.

15. The display apparatus of claim 1, wherein at least one ray-blocking element comprises an ink composition.

16. The display apparatus of claim 1, further comprising a film bonded to said LOE with an adhesive having refractive-index and thickness selected to maintain total internal reflection for guided display light, wherein said ray-blocking elements are formed on or within said film.

17. The display apparatus of claim 1, further comprising an angularly selective multilayer dielectric coating disposed on at least one of said major surfaces, wherein said ray-blocking elements are disposed on or within said dielectric coating.

18. The display apparatus of claim 1, further comprising a low-index overlayer disposed on at least one of said major surfaces, the overlayer having refractive index lower than that of a core of said LOE and a thickness selected to maintain total internal reflection for guided display light, wherein said ray-blocking elements are disposed on or within said overlayer.

19. The display apparatus of claim 1, wherein said second region comprises a diffractive out-coupling arrangement.

20. The display apparatus of claim 1, wherein said second region comprises a reflective out-coupling arrangement.

21. The display apparatus of claim 1, wherein the ray-blocking elements are configured to attenuate external- source light arriving predominantly from an upper- forward field relative to a gaze direction of the user.

22. The display apparatus of claim 1, wherein at least one ray -blocking element extends on both sides of the line of intersection.

23. The display apparatus of claim 1, wherein at least one ray -blocking element extends only on a side of the line of intersection that is closer to the EMB.

24. The display apparatus of claim 1, wherein said LOE further comprises an interface between said first region and said second region including an optical element configured to modify polarization of light propagating from said first region to said second region, and further comprising a ray-blocking element disposed on at least one of said major surfaces and formed as an elongate strip extending along and immediately adjacent to a line of intersection between a plane of said interface and said major surface, the strip being located on a side of said line closer to the EMB and extending along at least a portion of a length of said interface along said line of intersection.

25. The display apparatus of claim 24, wherein said optical element comprises a retarder.

26. A display apparatus for displaying an image to a user, the display apparatus comprising: (a) a lightguide optical element (LOE) having a pair of mutually parallel major surfaces for guiding light by internal reflection at said major surfaces;(b) an image projector optically coupled to said LOE so as to introduce into said LOE image light corresponding to a collimated image to propagate within said LOE in a first guided propagation direction by internal reflection at said major surfaces; and(c) a support structure configured to support said LOE relative to the user’s head so that an eye of the user is located within an eye-motion box (EMB) relative to said LOE, wherein said LOE comprises:(i) a first region including a reflective or diffractive arrangement configured to progressively redirect the image light propagating in the first guided propagation direction to propagate in a second guided propagation direction,(ii) a second region including an out-coupling arrangement configured to progressively couple out the image light propagating in the second guided propagation direction toward the EMB,(iii) an interface between said first region and said second region, and(iv) a ray -blocking element disposed on at least one of said major surfaces and formed as an elongate strip extending along and immediately adjacent to a line of intersection between a plane of said interface and said major surface, the strip being located on a side of said line closer to the EMB and extending along at least a portion of a length of said interface along said line of intersection, the ray-blocking element having absorbing and / or scattering optical properties sufficient to attenuate or eliminate an intensity of light that, in the absence of said ray -blocking element, would reach the EMB from a light source outside the LOE along a light path passing through said LOE that includes a single reflection at said interface.

27. The display apparatus of claim 26, wherein said interface includes an optical element configured to modify polarization of light propagating from said first region to said second region.

Images