Compound light-guide optical elements

Abstract

To provide a light-guide optical element (LOE) for achieving optical aperture expansion.SOLUTION: An optical system for directing image illumination injected at a coupling-in region towards a user for viewing includes a light-guide optical element (LOE) (12) with a pair of parallel major external surfaces (24). A first region (16) of the LOE contains a first set of partially reflecting surfaces (17) oriented to redirect image illumination propagating within the LOE towards a second region (18) of the LOE. The second region (18) contains a second set of partially reflecting surfaces (19) oriented to externally couple the image illumination towards the user. The first set of partially reflecting surfaces (17) extends across at least 95% of a thickness of the LOE while the second set of partially reflecting surfaces (19) are contained within a subsection of the thickness spanning less than 95% of the thickness so that the second set of partially reflecting surfaces (19) are excluded from one or both surface layers of the second region (18).SELECTED DRAWING: Figure 6B

Description

[Technical Field]

[0001] The present invention relates to optical systems, and more particularly to optical systems that include light-directing optical elements (LOEs) to achieve optical aperture expansion. [Background technology]

[0002] Composite light-guiding optical elements (LOEs) or “two-dimensionally expanded waveguides” have been described in previous publications by Lumus Ltd. (Israel). Examples of such composite LOEs can be found, for example, in PCT Publication No. WO2020 / 049542. Generally, these composite LOEs employ two regions, each of which is a parallel-sided block of transparent material to aid in the propagation of light carrying a collimated image by internal reflection at its major surfaces, and includes a set of mutually parallel internal partially reflective surfaces or “facets” that gradually redirect portions of the collimated image, achieving an expanded optical aperture. By combining two such elements with different facet orientations, it is possible to achieve a two-dimensional expansion of the optical aperture within a single element, thereby magnifying an input image from an image projector and outputting it over a larger area toward the viewer's eye.

[0003] For ease of reference, the light-directing optical element (LOE) region responsible for the first stage of expansion within the composite element is referred to herein as the "first LOE" or "LOE1," while the LOE region responsible for outcoupling the once-deflected image toward the viewer is referred to herein as the "second LOE" or "LOE2." Summary of the Invention

[0004] The present invention is an optical system for directing image illumination injected at the incoupling region towards a user for viewing.

[0005] In accordance with the teachings of embodiments of the present invention, an optical system is provided for directing image illumination injected at an internal coupling region toward a user for viewing, the optical system comprising: a light-directing optical element (LOE) formed from a transparent material, the LOE comprising: (a) a first region including a first set of planar, mutually parallel partially reflective surfaces having a first orientation; (b) a second region including a second set of planar, mutually parallel partially reflective surfaces having a second orientation non-parallel to the first orientation; and (c) a set of mutually parallel outer major surfaces extending across the first region and the second region such that both the first set of partially reflective surfaces and the second set of partially reflective surfaces are located between the major outer surfaces; The second set of reflective surfaces is at an oblique angle to the major outer surfaces such that a portion of the image illumination propagating within the LOE from the first region into the second region due to internal reflection at the major outer surfaces is outcoupled from the LOE toward a user, and the first set of partially reflective surfaces is oriented such that a portion of the image illumination propagating within the LOE from the internal coupling region due to internal reflection at the major outer surfaces is deflected toward the second region, the LOE having a thickness between the major outer surfaces, the first set of partially reflective surfaces extending through at least 95% of the thickness, and the second set of partially reflective surfaces within the second region being contained within a subsection of the thickness spanning less than 95% of the thickness such that the second set of partially reflective surfaces is excluded from at least one surface layer of the second region.

[0006] According to a further feature of embodiments of the present invention, the second set of partially reflective surfaces is excluded from both surfaces of the major exterior surface in a second region.

[0007] According to further features of embodiments of the present invention, the total thickness of at least one surface of the second region, from which the second set of partially reflective surfaces is excluded, is between 6% and 33% of the thickness.

[0008] According to further features of embodiments of the present invention, the spacing between adjacent surfaces of the second set of partially reflective surfaces in a direction parallel to the outer major surface is at least 1 mm, and the total thickness of at least one surface layer in the second region from which the second set of partially reflective surfaces is excluded is at least 10% of the thickness.

[0009] According to further features in embodiments of the present invention, the first set of partially reflective surfaces extends through at least 96% of the thickness.

[0010] According to further features in embodiments of the present invention, the first set of partially reflective surfaces extends through at least 98% of the thickness.

[0011] According to further features of embodiments of the present invention, the first set of partially reflective surfaces extends through the entire thickness.

[0012] According to a further feature of embodiments of the present invention, the first orientation of the first set of partially reflective surfaces is perpendicular to the outer major surface. [Brief explanation of the drawings]

[0013] The invention is herein described, by way of example only, with reference to the accompanying drawings. [Figure 1] 1A and 1B are schematic isometric views of an optical system implemented using a light-directing optical element (LOE), constructed and operative in accordance with the teachings of the present invention, showing top-down and side-looking configurations, respectively; [Figure 2A] FIG. 2 is an enlarged schematic isometric view of the LOE of FIG. 1A or FIG. 1B showing the optical paths for the two extreme regions of the image. [Figure 2B] FIG. 2 is an enlarged schematic isometric view of the LOE of FIG. 1A or FIG. 1B showing the optical paths for the two extreme regions of the image. [Figure 3] 2C is a schematic front view of the LOE of FIG. 2A and FIG. 2B showing the path of a chief ray propagating through the LOE and experiencing a first redirection within the LOE and a second redirection for outcoupling of the LOE to a viewer. [Figure 4] 4A and 4B are partial schematic cross-sectional views along line IV-IV in FIG. 3 showing the ray paths of light rays propagating within the second region of the LOE and encountering internal partially reflective surfaces that extend and do not extend into the thickness of the LOE, respectively. [Figure 5]5A, 5B, and 5C are partial schematic cross-sectional views along line VV in FIG. 3 showing the ray paths of light rays propagating within a first region of the LOE and encountering internal partially reflective surfaces that span the entire thickness of the LOE, or that are spaced apart from both major surfaces, or from only one major surface, respectively. [Figure 6A] 2C is a schematic side view of the LOE of FIGS. 2A and 2B in an embodiment in which both the first and second regions of the LOE have a surface, i.e., a cover plate, from which the partially reflective inner surface is excluded. [Figure 6B] FIG. 6B is a view similar to FIG. 6A in an embodiment in which only the second region of the LOE has a cover plate. [Figure 6C] FIG. 6B is a view similar to FIG. 6A, but showing a second region of the LOE having a cover plate and a first region of the LOE having a thinner cover plate than the second region. [Figure 6D] FIG. 6D is a view similar to FIG. 6C, but showing a cover plate on only one side of the first region of the LOE. [Figure 7] 10 is a schematic graph illustrating the relationship between the spatial density of the LOE partially reflective surfaces and the preferred minimum thickness of the cover plate in the second region of the LOE. [Figure 8] 1A and 1B are side and isometric views, respectively, of an intermediate work product in the manufacturing process for an LOE, according to a specific embodiment of the present invention. [Figure 9A] 10A-10C are schematic side views illustrating stages in the manufacturing process in which a thin cover plate is applied over an LOE in which a first region is initially formed without a cover plate and a second region is formed with a cover plate having only a portion of the desired cover plate thickness. [Figure 9B] 9B is a schematic side view illustrating the structure resulting from the manufacturing process of FIG. 9A. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention is an optical system for directing image illumination injected at the incoupling region towards a user for viewing.

[0015] Certain embodiments of the present invention provide an optical system including a light-directing optical element (LOE) for achieving optical aperture expansion for the purposes of a head-up display, most preferably a near-eye display, which may be a virtual reality display, or more preferably an augmented reality display.

[0016] An exemplary implementation of a device in the form of a near-eye display, generally designated 10, employing an LOE 12 according to the teachings of an embodiment of the present invention is shown schematically in FIGS. 1A and 1B. Near-eye display 10 employs a compact image projector (or “POD”) 14 optically coupled to inject an image into LOE 12 (interchangeably referred to as a “waveguide,” “substrate,” or “slab”), where image light is captured in one dimension by internal reflection at a set of mutually parallel, planar exterior surfaces. The light impinges on a set of partially reflective surfaces (interchangeably referred to as “facets”) that are parallel to one another and obliquely tilted relative to the propagation direction of the image light, with each successive facet deflecting a percentage of the image light that is also trapped / guided by internal reflection within the substrate in a deflected direction. This first set of facets, not individually shown in FIGS. 1A and 1B, is located in a first region of the LOE, designated 16. This partial reflection at successive facets achieves optical aperture expansion in the first dimension.

[0017] In a first set of preferred, but non-limiting, embodiments of the present invention, the facets of said set are orthogonal to the outer major surfaces of the substrate. In this case, both the injected image and its conjugate, which undergoes internal reflection as it propagates within region 16, are polarized, resulting in conjugate images propagating in the polarized direction. In an alternative set of preferred, but non-limiting, embodiments, the partially reflective surfaces of the first set are angled obliquely relative to the outer major surfaces of the LOE. In the latter case, either the injected image or its conjugate forms the desired polarized image propagating within the LOE, while other reflections can be minimized, for example, by employing angle-selective coatings on the facets that make them relatively transparent for the range of incident angles represented by images for which reflection is not desired.

[0018] The first set of partially reflective surfaces deflects the image illumination from a first direction of propagation that is captured within the substrate by total internal reflection (TIR) ​​to a second direction of propagation that is also captured within the substrate by TIR.

[0019] The deflected image illumination then enters a second substrate region 18, which may be implemented as an adjacent, separate substrate or as an extension of a single substrate, in which an outcoupling arrangement (typically an additional set of partially reflective facets) progressively outcouples a percentage of the image illumination toward the observer's eyes located within an area defined as the eye movement box (EMB), thereby achieving a second dimension of optical aperture expansion. The entire device may be implemented separately for each eye and is preferably supported against the user's head, with each LOE 12 facing the user's corresponding eye. In one particularly preferred option, as shown here, the support structure is implemented as an eyeglass frame having sides 20 for supporting the device against the user's ears. Other forms of support structure may also be used, including, but not limited to, a headband, a sun visor, or a device suspended from a helmet.

[0020] In the drawings and claims, reference is made herein to an X-axis extending horizontally (FIG. 1A) or vertically (FIG. 1B) in the general direction of extension of the first region of the LOE, and a Y-axis extending perpendicular thereto, i.e., vertically in FIG. 1A and horizontally in FIG. 1B.

[0021] Very broadly speaking, the first LOE, or first region 16 of LOE 12, can be considered to achieve aperture expansion in the X direction, while the second LOE, or second region 18 of LOE 12, achieves aperture expansion in the Y direction. The details of the angular spread through which different portions of the field of view propagate are discussed more precisely below. Note that an orientation such as that shown in FIG. 1A can be considered a “top-down” implementation, in which image illumination entering the main portion (second region) of the LOE enters from the top edge, while the orientation shown in FIG. 1B can be considered a “side-looking” implementation, in which an axis, referred to herein as the Y axis, is horizontally extended. In the remaining figures, various features of certain embodiments of the present invention are shown in the context of a “top-down” orientation similar to FIG. 1A. However, it should be understood that all of these features are equally applicable to side-looking implementations, which are also within the scope of the invention. In certain cases, other intermediate orientations are also applicable and, unless expressly excluded, are included within the scope of the present invention.

[0022] The PODs employed in the devices of the present invention are preferably configured to produce collimated images, i.e., images in which the light for each image pixel is a parallel beam collimated to infinity with the angular direction corresponding to the pixel location, and the image illumination therefore spans an angular range corresponding to the two-dimensional field of view angle.

[0023] The image projector 14 typically includes at least one light source arranged to illuminate a spatial light modulator, such as an LCOS chip. The spatial light modulator modulates the projected intensity of each pixel of the image, thereby generating the image. Alternatively, the image projector may include a scanning configuration, typically implemented using a high-speed scanning mirror, in which illumination from a laser light source is scanned across the projector's image plane while the beam intensity is varied pixel-by-pixel in synchronization with the movement, thereby projecting the desired intensity at each pixel. In either case, collimating optics are provided to generate an output projected image that is collimated to infinity. Some or all of the above components are typically disposed on the surface of one or more polarizing beam splitter (PBS) cubes or other prism configurations, as is well known in the art.

[0024] Optical coupling of image projector 14 to LOE 12 may be achieved by any suitable optical coupling, such as via a side edge and / or one of the major outer surfaces of the LOE, via a coupling prism with an obliquely angled input face, or via a reflective coupling arrangement. The details of the coupling input arrangement are not important to the invention and are shown here schematically as a non-limiting example wedge prism 15 applied to one of the major outer surfaces of the LOE.

[0025] It will be understood that near-eye display 10 includes various additional components, including a controller 22 for operating image projector 14, typically employing power from a small on-board battery (not shown) or some other suitable power source. It will be understood that controller 22 includes all necessary electronic components, such as at least one processor or processing circuitry, for driving the image projector, all as is well known in the art.

[0026] 2A and 2B, the optical properties of an embodiment of a near-eye display are illustrated in more detail. Specifically, a more detailed view of a light-directing optical element (LOE) 12 formed from a transparent material is shown, including a first region 16, also referred to herein as "LOE 1," that includes a first set of planar, mutually parallel partially reflective surfaces 17 having a first orientation, and a second region 18, also referred to herein as "LOE 2," that includes a second set of planar, mutually parallel partially reflective surfaces 19 having a second orientation that is non-parallel to the first orientation. A set of mutually parallel outer major surfaces 24 extends across first and second regions 16 and 18 such that both first set of partially reflective surfaces 17 and second set of partially reflective surfaces 19 are located between the outer major surfaces 24. Most preferably, the sets of outer major surfaces 24 are each a pair of surfaces that are continuous throughout the entire first and second regions 16 and 18. The option of having a set-down or step-up in thickness between regions 16 and 18 is also within the scope of the present invention. Regions 16 and 18 may be directly juxtaposed to meet at a boundary, which may be a straight boundary or some other shape, or there may be one or more additional LOE regions interposed between them to provide various additional optical or mechanical functions, depending on the particular application. While the present invention is not limited to any particular manufacturing technique, in certain particularly preferred implementations, a particularly high quality primary outer surface is achieved by employing continuous outer plates between separately formed regions 16 and 18 that are sandwiched therebetween to form a composite LOE structure. This option and considerations regarding the thickness of these plates are discussed further below.

[0027] The optical properties of the LOE can be understood by tracing the image illumination path backward. The second set of partially reflective surfaces 19 are at an oblique angle to the outer major surface 24 so that a portion of the image illumination propagating within LOE 12 from first region 16 to second region 18 due to internal reflections at the outer major surface is coupled out of the LOE toward eye movement box 26. The first set of partially reflective surfaces 17 are oriented so that a portion of the image illumination propagating within LOE 12 from the coupling input region (combining prism 15) is deflected toward second region 18 due to internal reflections at the outer major surface.

[0028] One dimension of the angular spread of the projected image from image projector 14 is represented in FIG. 2A by the cone of illumination extending from the POD opening on the right side of the LOE toward the left side of the LOE. In the non-limiting example shown here, the central optical axis of the POD defines a propagation direction within the LOE aligned with the X-axis, and the angular spread (within the LOE) is approximately ±16°. (Note that the FOV angle is larger in air due to changes in refractive index.) A first set of partially reflective surfaces 17 is shown in first region 16, and a second set of partially reflective surfaces 19 is shown in second region 18.

[0029] The near-eye display is designed to provide a full field of view of the projected image to a user's eye, positioned within a range of permitted positions specified by an "eye movement box" (EMB) 26 (i.e., a shape, typically represented as a rectangle, away from the plane of the LOE where the eye's pupil will see the projected image). To reach the eye movement box, light must be coupled out from the second region 18 toward the EMB 26 by a second set of partially reflective surfaces 19. To provide the full image field, each point within the EMB must receive a full angular range of images from the LOE. Tracing back the field of view from the EMB suggests a larger rectangle 28 into which the relevant illumination is coupled out from the LOE toward the EMB.

[0030] FIG. 2A shows the first end of the field of view, corresponding to the bottom-left pixel of the projected image. A beam of width corresponding to the projector's optical aperture when coupled into the LOE is shown propagating upward and left from the POD and being partially reflected from a series of partially reflective surfaces 17. As shown here, only a subset of the facets produce reflections useful for providing the corresponding pixel in the image viewed by the user, and only a subregion of those facets contributes to the observed image of this pixel. The relevant region is indicated by a thick black line, showing the light rays corresponding to this pixel in the redirected image reflected from facet 17 and then coupled out by facet 19 to reach the four corners of the EMB 26. Note that here, and throughout the description, only the in-plane propagation direction of the ray is shown here as it propagates within the LOE, but the ray actually follows a zigzag path of repeated internal reflections from the two outer major surfaces, and the overall image field in one dimension is encoded by the angle of inclination of the ray with respect to the outer major surfaces, which corresponds to the pixel location in the Y dimension. As an additional example, the deflected and combined out ray is shown in dashed lines, corresponding to the upper left extremity of the image, as seen in the upper left corner of the EMB.

[0031] FIG. 2B shows the same configuration as FIG. 2A, but now shows rays corresponding to the pixel at the bottom right of the field of view reaching the four corners of the EMB, again with the relevant areas of the associated partially reflective surfaces 17 shown in bold.

[0032] By additionally tracing the corresponding ray paths for every region (direction or pixel) of the image that reaches every region of the EMB, it is possible to map the envelope of all ray paths that propagate from the combined input region, through the LOE, are deflected by one of the first set of partially reflective surfaces, and are combined out by one of the second set of partially reflective surfaces in a direction that reaches the eye movement box; it will be clear that this envelope defines the "imaging region" of each facet 17 required to deflect the portion of the image illumination that contributes to the image reaching the EMB, while the remainder of the facets 17 outside the envelope are "non-imaging regions" that do not contribute to the required image. Optionally, the in-plane extent of the facets may be truncated to cover only the imaging region. This and other variant embodiments, employing different imaging implant locations and geometries, non-uniform facet spacing, or introducing additional (e.g., a third) sets of partially reflective internal surfaces, have been discussed in detail in previous publications by Lumus Ltd. (Israel), particularly the aforementioned PCT Publication No. WO2020 / 049542, as well as PCT Publication No. WO2020 / 152688 and PCT Application No. PCT / IL2020 / 051354, all of which were unpublished as of the priority date of this application and are not considered prior art. While all of these additional features may be implemented in the context of the present invention, for brevity, they will not be addressed in detail herein.

[0033] It is a particularly preferred feature of certain embodiments of the present invention that the first set of partially reflective surfaces 17 extend across at least 95% of the thickness of the LOE such that the second set of partially reflective surfaces 19 are excluded from at least one surface layer of the second region 18, while the second set of partially reflective surfaces 19 within the second region 18 are contained within a subsection of the thickness that spans less than 95% of the thickness. We will now present the advantages of this combination.

[0034] As a matter of terminology, the term “cover plate” is used herein to generally refer to any embodiment of a layer of a certain depth adjacent one or both of the LOE's major surfaces, from which the set of internal partially reflective surfaces of the LOE is excluded. One approach to forming such a layer is by attaching a sheet of transparent material, i.e., a physically separate cover plate, to the LOE component. However, other manufacturing techniques are possible, such as by creating an LOE structure from a stack of plates in which a partially reflective coating is applied only to the area corresponding to the active LOE layer, and the area that will be adjacent to the LOE's major outer surfaces is bonded together using an index-matching adhesive without a reflective coating. Regardless of the manufacturing technique, the term “cover plate” is used to refer to a functional structure in which a surface layer functions as a cover plate without facets, regardless of how the layer is formed.

[0035] Referring to FIG. 3, this illustrates that as the path of a single ray 30, here the chief ray corresponding to the center of the image field in the X dimension, traverses a portion of LOE 1, it is redirected by reflection at one of the partially reflective surfaces 17 toward LOE 2 (ray 30') and by reflection at one of the partially reflective surfaces 19 and outcouples toward the viewer (ray 30"). FIGS. 4A and 4B show the second redirection / outcoupling geometry without and with the addition of a face plate to the LOE, and FIGS. 5A and 5B show the first redirection geometry.

[0036] In LOE2 (region 18), where the image is outcoupled toward the viewer, obliquely angled facets are used. If the facets are angled (e.g., at 25 degrees to the major exterior surface), a ray of light can be reflected twice from the same facet, as illustrated in Figure 4A. This results in an uneven beam exiting the waveguide. Darker regions are generated by the second reflection. Dark fringes then appear at the exit pupil of the waveguide. To the viewer, this results in dark fringes in the far-field image.

[0037] Figure 4B illustrates how this double reflection can be avoided, effectively spacing the facets away from the outer surface of the waveguide, by adding a cover plate 32 to the waveguide on one or both of its outer surfaces. In this way, after reflecting off a facet, the transmitted portion of the light ray jumps above or below it and propagates directly to the next facet, improving image uniformity.

[0038] However, with regard to the use of cover plates, it has been found that the considerations for achieving image uniformity in LOE1 are significantly different from those for LOE2. The partially reflective surfaces used to redirect image illumination from one guided direction within the waveguide to another are necessarily very steep, and in some embodiments, perpendicular to the waveguide's major exterior surface, so that light rays are not reflected twice by a single facet. In this case, optimal image uniformity would be achieved by using facets that span the entire thickness of the substrate (FIG. 5A), but facets that do not reach the surface have been found to allow certain light rays to skip the facet entirely (FIG. 5B), resulting in dark lines in the output image. The overall result of providing a structure with cover plates across regions 16 and 18 is illustrated schematically in FIG. 6A, where a uniformly illuminated input aperture 34 from the image projector propagates through the LOE and is outcoupled toward the eye movement box (EMB) 26 as image region 36 disrupted by dark lines 38 in the output. In contrast, the structure of Figure 6B employs a cover plate only on LOE2 18, while the facets of LOE1 16 extend to the main exterior surface of the device. In this case, a uniformly injected image 34 from the image projector preferably results in a relatively uniform image 36 as perceived by the viewer.

[0039] Although the presence of a cover plate within the LOE1 region 16 adversely affects the quality of the output image, practical considerations may favor the use of a cover plate on one or both major surfaces of the LOE1 region 16. For example, the absence of any adhesive joints extending to the outer surface can facilitate achieving a high-quality, planar outer surface of the waveguide. The presence of a cover plate can be tolerated if the cover plate is thin enough that any resulting image artifacts are not disturbing to the human eye (Figure 6C). A thin cover plate generates thin dark fringes where the output image is missing. The spatial frequency and width of the dark fringes determine their appearance and impact on the image, as perceived by the human eye. To properly assess the severity of unfilled fringes to the human eye, convolving the pupil of the human eye over the waveguide exit pupil reveals the acceptable cover plate thickness. If the spatial frequency of the dark fringes is significantly higher than the diameter of the eye's pupil, the fluctuations are essentially averaged out by the eye. At lower spatial frequencies, stripes that are thin enough that the intensity averaged over the size of the pupil does not change significantly may be acceptable.

[0040] In practice, the thickness range of the cover plate in the LOE1 region, if present, should be 1 to 100 microns, most preferably less than 50 microns. As a percentage of the LOE thickness, the total thickness of the cover plate is preferably less than 5% of the thickness, preferably 4% or less of the thickness, and most preferably 2% or less of the thickness. This corresponds to the first set of partially reflective surfaces extending through at least 95% of the thickness, more preferably at least 96% of the thickness, and most preferably at least 98% of the thickness. As illustrated schematically in Figure 6D, the problem of dark fringes can be ameliorated by using a cover plate on only one side of the LOE1 region 16.

[0041] Despite the possible advantages of employing a cover plate, in certain particularly preferred embodiments of the present invention, the first set of partially reflective surfaces 17 extend throughout the entire thickness of LOE1 16, i.e., without a cover plate, as illustrated schematically in FIG. 6B.

[0042] As discussed above, with respect to LOE2 in region 18, the cover plate in this region contributes to reducing illumination non-uniformity, thereby improving the quality of the viewed image. The second set of partially reflective surfaces 19 is preferably excluded from both major exterior surface layers in the second region, meaning that both major surfaces have a "cover plate." The total thickness of the surfaces in second region 18 from which the second set of partially reflective surfaces 19 are excluded is preferably between 6% and 33% of the total thickness of LOE2.

[0043] Again, the degree to which illumination nonuniformities are perceived by the human eye depends on the spatial frequency of the intensity variations, their dynamic range, and their width, which in turn determines the preferred thickness of the cover plate effective in ameliorating these variations. In the case of the outcoupling facets 19, the spatial frequency arises directly from the spacing between adjacent faces of the second set of partially reflective surfaces 19 in a direction parallel to the major exterior surface. Figure 7 illustrates the preferred minimum cover plate thickness (total cover plate thickness as a percentage of the LOE2 total thickness) for various facet densities, defined herein as the number of facets that overlap the pupil diameter, here approximately 3 millimeters. At high facet densities, a relatively thin cover plate proves sufficient, as intensity variations are essentially averaged over the pupil region and are therefore less perceptible to the viewer. As facet spacing increases, the spatial frequency of the intensity variations decreases, requiring a thicker cover plate to compensate for these variations.

[0044] As a useful reference point, indicated by the horizontal dashed line in FIG. 7, when the spacing between adjacent surfaces of the second set of partially reflective surfaces 19 in a direction parallel to the major outer surface is at least 1 mm (corresponding to a density of three facets per 3 mm pupil diameter), the total thickness of the surface layer(s) in the second region from which the second set of partially reflective surfaces are excluded is preferably at least 10% of the total thickness.

[0045] Optical systems according to the present invention can be fabricated by a variety of processes based on standard manufacturing techniques employed in the art, as will be apparent to those skilled in the art. Each LOE region is typically formed by bonding together a stack of thin plates coated on one or both sides (usually all coated on one side, or alternating plates coated on both sides) to provide the desired partially reflective properties at each interface. The partially reflective properties are typically provided by multilayer dielectric coatings that can provide angle-selective reflectivity, as is well known in the art. These stacks are then sliced ​​at the required angle to produce LOE sections / regions with properly oriented internal partially reflective surfaces. Cover plates of the appropriate thickness are then added to each region, if necessary, and the edge surfaces of the LOE sections are polished and then glued together to form the final composite LOE.

[0046] Optionally, if cover plates are provided on one or both major surfaces of LOE1, it may be advantageous to create LOE2 with partial-thickness cover plates corresponding to the desired final cover plate thickness minus the cover plate thickness required for LOE1. A single continuous cover plate can then be added during assembly of the composite LOE, providing LOE1 with the total desired cover plate thickness and complementing the cover plate thickness of LOE2 to the desired thickness. This option is further described below with reference to Figures 9A and 9B.

[0047] Alternatively, in some cases, it may be desirable to manufacture a stepped cover plate having a first portion with a first thickness appropriate for LOE 1 and a second portion with a second (larger) thickness appropriate for LOE 2. The step between the two portions can then be used as an alignment feature for assembly of the two LOE sections.

[0048] A further option for fabricating the composite LOEs of the present invention is illustrated schematically in FIGS. 8A and 8B. In this case, a stack of plates for forming LOE 1 is cut to form block 80 of dimensions corresponding to multiple LOEs. A second block 82 is formed by combining multiple active layers 84 (i.e., sections of the LOE that include partially reflective surfaces) of LOE 2, which are bonded together with an intermediate transparent plate 86. The first and second blocks 80 and 82 are then bonded together to form intermediate work product 81, as illustrated in FIGS. 8A and 8B, which can be sliced ​​along slice plane 88 and polished to produce multiple composite LOEs, with a portion of the thickness of intermediate transparent plate 86 becoming a cover sheet for second LOE region 18 of each composite LOE.

[0049] The fabrication techniques of Figures 8A and 8B, and variations thereof, are discussed in further detail in a co-pending PCT application entitled "Method of Fabrication of Compound Light-Guide Optical Elements," filed on the same day as this application, which takes priority from U.S. Provisional Patent Application No. 63 / 029,500, filed May 24, 2020.

[0050] Again, if it is desired to also have a cover plate over first LOE region 16, it may be advantageous to create a composite LOE structure according to Figures 8A and 8B, in which cover plate 32a over region 18 is less than the desired thickness by an amount equal to the thickness desired for region 16, as shown in Figure 9A. Both cover plates can then be brought to their intended total thickness by adding plate 32b of uniform thickness bonded to the waveguides throughout the structure, thereby creating the final structure as illustrated in Figure 9B.

[0051] 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. an optical system for directing image illumination injected into the internal coupling region toward a user for viewing, the optical system comprising: a light-directing optical element (LOE) formed from a transparent material, the LOE comprising: (a) a first region including a first set of planar, mutually parallel partially reflective surfaces having a first orientation; (b) a second region including a second set of planar, mutually parallel partially reflective surfaces having a second orientation non-parallel to the first orientation; (d) a set of mutually parallel outer major surfaces that extend across the first and second regions such that both the first set of partially reflective surfaces and the second set of partially reflective surfaces are located between the outer major surfaces; the second set of partially reflective surfaces are at an oblique angle with respect to the outer major surfaces such that a portion of image illumination propagating within the LOE by internal reflection at the outer major surfaces from the first region to the second region is outcoupled from the LOE toward the user, and the first set of partially reflective surfaces are oriented such that a portion of image illumination propagating within the LOE by internal reflection at the outer major surfaces from the incoupling region is deflected toward the second region; 10. The optical system of claim 1, wherein the LOE has a thickness between the outer major surfaces, the first set of partially reflective surfaces extending through at least 95% of the thickness, and a second set of partially reflective surfaces within the second region being included within a subsection of the thickness that spans less than 95% of the thickness, such that the second set of partially reflective surfaces is excluded from at least one surface of the second region.

2. The optical system of claim 1 , wherein the second set of partially reflective surfaces is excluded from both surfaces of the outer major surface within the second region.

3. 10. The optical system of claim 1, wherein a total thickness of the at least one surface layer in the second region, excluding the second set of partially reflective surfaces, is between 6% and 33% of the thickness.

4. 2. The optical system of claim 1, wherein a spacing between adjacent surfaces of the second set of partially reflective surfaces in a direction parallel to the outer major surface is at least 1 mm, and a total thickness of the at least one surface layer in the second region excluding the second set of partially reflective surfaces is at least 10% of the thickness.

5. The optical system of claim 1 , wherein the first set of partially reflective surfaces extends through at least 96% of the thickness.

6. The optical system of claim 1 , wherein the first set of partially reflective surfaces extends through at least 98% of the thickness.

7. The optical system of claim 1 , wherein the first set of partially reflective surfaces extends throughout the thickness.

8. The optical system of claim 1 , wherein the first orientation of the first set of partially reflective surfaces is perpendicular to the outer major surface.

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