Non-polarized lightguide-based display system employing polarizing beam splitter

Abstract

A lightguide-based display system includes a lightguide (82). An image projector (50, 50', 52, 58, 60, 62, 64, 130, 150) is coupled to the lightguide (82) and includes: at least one active¬ pixel array (70); a polarizing beam splitter (PBS) (76); first reflective back-collimating optics (80a); and second reflective back-collimating optics (80b). The PBS (76) splits the unpolarized image light from the active-pixel array (70) and recombines reflected polarized images from the back-collimating optics (80a, 80b) to inject a superposition of the images into the lightguide (82) as a mixed polarization image.

Description

[0001] Non-Polarized Lightguide-Based Display System employing Polarizing Beam Splitter

[0002] FIELD AND BACKGROUND OF THE INVENTION

[0003] This invention relates to optical systems and, more particularly, to lightguide-based display systems for use in applications such as near-eye displays, head-up displays, and augmented reality devices.

[0004] Lightguide-based displays typically employ an image projector that injects image light into a lightguide. When using active-pixel arrays that generate unpolarized light, achieving sufficient brightness can be challenging. This challenge is further compounded by the inherent loss of 50% of the light intensity when using polarizing optical components, such as a polarizing beam splitter.

[0005] SUMMARY OF THE INVENTION

[0006] The present invention is a lightguide-based display system.

[0007] According to the teachings of an embodiment of the present invention there is provided, a lightguide-based display system comprising: (a) a lightguide formed from transparent material and having a pair of mutually-parallel major surfaces that support propagation of image light by internal reflection, the lightguide preferably containing a set of mutually-parallel partially reflecting internal surfaces deployed to couple out light propagating within the lightguide so as to exit from the lightguide; and (b) an image projector coupled to the lightguide so as to inject a collimated image into the lightguide, the image projector comprising: (i) at least one active -pixel array configured to generate unpolarized light corresponding to an image, (ii) a polarizing beam splitter (PBS) prism deployed for receiving the unpolarized light from the active-pixel array, the PBS prism including a PBS surface configured to split the unpolarized light into a first polarized light path and a second polarized light path, (iii) first back-reflective collimating optics associated with the first polarized light path, and (iv) second back-reflective collimating optics associated with the first polarized light path, wherein back-reflected polarized images received from the first and second back-reflective collimating optics are recombined by the PBS surface so as to be injected in superposition into the lightguide as a mixed polarization image.

[0008] According to a further feature of an embodiment of the present invention, the polarizing beam splitter prism provides an angularly-selective reflecting surface coplanar with or parallel to one of the major surfaces of the lightguide, the angularly-selective reflecting surface being traversed by at least part of the light corresponding to an image prior to the recombining, the angularly-selective reflecting surface reflecting at least part of the mixed polarization image prior to reaching an entrance aperture of the lightguide. According to a further feature of an embodiment of the present invention, the angularly- selective reflecting surface is provided by a layer of low-index adhesive or an air gap, a path of at least one polarization component of the image crossing the angularly-selective reflecting surface and at least part of the mixed polarization image undergoing internal reflection at the angularly- selective reflecting surface of the lightguide within the PBS prism.

[0009] According to a further feature of an embodiment of the present invention, there are also provided anti -reflective coatings at surfaces adjacent to the low-index adhesive or air gap to minimize losses in light paths crossing the angularly-selective reflecting surface.

[0010] According to a further feature of an embodiment of the present invention, the polarizing beam splitter prism comprises a first portion and a second portion attached to the mutually -parallel major surfaces of the lightguide aligned in opposing relation.

[0011] According to a further feature of an embodiment of the present invention, the image projector is one of a plurality of similar image projectors deployed sequentially along the lightguide, wherein a first of the image projectors projects an image of a first color and wherein a second of the image projectors projects an image of a second color different from the first color, wherein a major portion of the PBS prism of the second image projector is attached to the lightguide by index-matched adhesive overlying a dichroic layer that is transparent to the second color and reflective to the first color.

[0012] According to a further feature of an embodiment of the present invention, there is also provided a transmissive or reflective coupling prism configured to couple the mixed polarization image into the lightguide.

[0013] According to a further feature of an embodiment of the present invention, there is also provided a refractive lens interposed between the PBS prism and the coupling prism, the refractive lens supplementing an optical power of the first and second back-reflective collimating optics to achieve collimation of the mixed polarization image.

[0014] According to a further feature of an embodiment of the present invention, light paths from an entrance to the PBS prism via the first and second back-reflective collimating optics and the coupling prism do not cross any air gap before entering the lightguide.

[0015] According to a further feature of an embodiment of the present invention, the polarizing beam splitter prism is configured such that the first back-reflecting collimating optics is inclined relative to axes of the lightguide to direct the mixed polarization image so as to propagate within the lightguide.

[0016] According to a further feature of an embodiment of the present invention, the at least one active-pixel array comprises a color micro-LED array. According to a further feature of an embodiment of the present invention, the active -pixel array is spaced from a surface of the polarizing beam splitter prism, and a field lens is attached to the active-pixel array.

[0017] According to a further feature of an embodiment of the present invention, the at least one active-pixel array comprises three monochrome arrays combined on faces of a dichroic X-cube prism to provide the unpolarized image light to the polarizing beam splitter prism.

[0018] According to a further feature of an embodiment of the present invention, there is also provided a lens interposed between the dichroic X-cube prism and the polarizing beam splitter prism, the lens providing a part of a collimating optical power.

[0019] According to a further feature of an embodiment of the present invention, the lightguide further comprises, perpendicular to the pair of mutually-parallel major surfaces, a second pair of mu tu ally-parallel major surfaces, thereby defining a rectangular cross-section lightguide that supports propagation of light by four-fold internal reflection, and wherein the polarizing beam splitter prism is configured such that the first back-reflecting collimating optics is inclined relative to the rectangular cross-section of the lightguide so as to direct the mixed polarization image to propagate within the lightguide by four-fold internal reflection.

[0020] According to a further feature of an embodiment of the present invention, an optical power of the first and second back-reflective collimating optics is supplemented by refractive optics at an entrance or an exit of the polarizing beam splitter prism to achieve collimation of the mixed polarization image.

[0021] According to a further feature of an embodiment of the present invention, there is also provided a depolarizer or other polarization modifying element deployed between the PBS prism and the lightguide.

[0022] There is also provided according to the teachings of an embodiment of the present invention, a method for projecting an image in a lightguide-based display system, the method comprising: (a) generating, by at least one active-pixel array, unpolarized image light; (b) receiving, by a polarizing beam splitter prism, the unpolarized image light; (c) splitting, by a PBS surface within the polarizing beam splitter prism, the received unpolarized image light into a first polarization component directed towards first back-reflective collimating optics and a second polarization component directed towards second back-reflective collimating optics; (d) recombining, by the PBS surface within the polarizing beam splitter prism, at least partially collimated polarized images received by reflection of the first and second polarization components from the first and second back-reflective collimating optics, respectively, to generate a superposition of the collimated polarized images as a mixed polarization image; and (e) coupling the mixed polarization image into a lightguide formed from transparent material and having a pair of mutually-parallel major surfaces so that the mixed polarization image propagates within the lightguide by internal reflection.

[0023] According to a further feature of an embodiment of the present invention, an optical power of the first back-reflective collimating optics and the second back-reflective collimating optics is supplemented by at least one refractive lens deployed in a light path before or after the PBS prism so that the mixed polarization image coupled into the lightguide is a collimated image.

[0024] According to a further feature of an embodiment of the present invention, the first back- reflective collimating optics and the second back-reflective collimating optics have sufficient optical power to collimate the first polarization component and the second polarization component, respectively, such that the mixed polarization image coupled into the lightguide is a collimated image.

[0025] BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0027] FIGS. 1A and IB are side views of optical components of a lightguide-based display system, constructed and operative according to an embodiment of the present invention, showing light paths from an active-pixel array image generator through a polarizing beam splitter (PBS) prism and into a lightguide, where, to facilitate understanding, light paths for a first polarization component are illustrated by arrows in FIG. 1 A and light paths of a second polarization component are illustrated by arrows in FIG. IB, both components being recombined in superposition for injection into the lightguide;

[0028] FIGS. 2A and 2B are optically simplified schematic side views of the lightguide-based display system of FIGS. 1A and IB illustrating first and second architectures, respectively, for integrating the PBS prism and its associated components with the lightguide;

[0029] FIGS. 2C and 2D are schematic side views similar to FIGS. 2A and 2B, respectively, illustrating variant implementations with non-orthogonal PBS prism geometry;

[0030] FIGS. 3A-3D are schematic side views similar to FIG. 2A illustrating four different options for implementing degrees of freedom, with or without a corresponding structural modification, for achieving optical alignment and focal matching between two optical channels during assembly of the display system;

[0031] FIG. 4 is a side view of optical components of a lightguide-based display system, constructed and operative according to a further embodiment of the present invention, showing light paths from an active -pixel array image generator through a PBS prism and into a lightguide, where light is coupled into the lightguide via a coupling prism;

[0032] FIGS. 5A-5C are schematic side views of optical components of a lightguide-based display system, constructed and operative according to a further embodiment of the present invention, employing a reflective polarization-modifying spatial light modulator (SLM) illuminated by light that has passed through a major surface of the lightguide, where FIG. 5A illustrates sample light paths throughout the optical components and, to facilitate understanding, FIGS. 5B and 5C illustrate sample light paths, respectively from the illumination source to the SLM and from the SLM to the lightguide;

[0033] FIG. 5D is a schematic side view similar to FIG. 5A illustrating an alternative illumination arrangement;

[0034] FIGS. 6A and 6B are schematic side views similar to FIGS. 4 and 2A, respectively, where the active-pixel array is replaced by an X-cube dichroic combiner that combines image light from three monochrome active-pixel array image sources;

[0035] FIG. 6C is a schematic side view similar to FIG. 6B with addition of a refractive lens between the X-cube dichroic combiner and the PBS prism and illustrating sample light paths through the display system;

[0036] FIG. 7 A is a schematic front view of a display system, constructed and operative according to an embodiment of the present invention, illustrating an image projector arrangement similar to that of FIG. 2A expanded into a three-color image projector with side-by-side assemblies sharing a common internally-partitioned PBS prism, shown in the context of a two-dimensional optical aperture expansion lightguide;

[0037] FIG. 7B is a schematic isometric representation of the three-color image projector of FIG. 7A;

[0038] FIG. 7C is a schematic front view similar to FIG. 7A modified to project multiple partial images along non-parallel optical axes;

[0039] FIGS. 8A and 8B are schematic side views illustrating alternative structural implementations of a display system optically equivalent to that of FIG. 2A;

[0040] FIG. 9 is a schematic side view of a lightguide-based display system, constructed and operative according to a further embodiment of the present invention, where a PBS prism is implemented using prism portions attached to the major surfaces of a lightguide in opposing relation;

[0041] FIG. 10 is a schematic side view illustrating implementation of a display system employing multiple projectors each similar to that of FIG. 9 attached to a common lightguide; FIG. 11A is a schematic plan view of a group of projectors each similar to that of FIG. 9 deployed in staggered relation with parallel optical axes;

[0042] FIG. 1 IB is a schematic front view of a display system employing the group of projectors of FIG. 11 A deployed on a lightguide arrangement;

[0043] FIG. 11C is a schematic plan view of a group of projectors each similar to that of FIG. 9 deployed in staggered relation with non-parallel optical axes;

[0044] FIG. 1 ID is a schematic front view of a display system employing the group of projectors of FIG. 11C deployed on a lightguide arrangement; and

[0045] FIG. 12 is a schematic isometric view of a display system, constructed and operative according to a further embodiment of the present invention, for coupling an image into a rectangular cross-section lightguide so as to undergo fourfold internal reflection.

[0046] DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] The present invention relates to lightguide-based display systems, and particularly to systems in which image light is injected into the lightguide from a projector which is coupled to the lightguide.

[0048] The principles and operation of lightguide-based display systems according to the present invention may be better understood with reference to the drawings and the accompanying description.

[0049] Non-Polarized Lightguide-Based Display System employing a PBS

[0050] In general terms, referring collectively to FIGS. 1A-4 and 6A-11, certain particularly preferred embodiments of the present invention provide a lightguide-based display system including a lightguide 82 formed from transparent material and having a pair of mutually-parallel major surfaces 86, 87 that support propagation of image light by internal reflection. An image projector, variously identified as 50, 50', 52, 58, 60, 62, 64, 130 or 150, is coupled to lightguide 82 so as to inject a collimated image into the lightguide. The image projector includes at least one active-pixel array 70 configured to generate unpolarized light corresponding to an image, and a polarizing beam splitter (PBS) prism (typically made up of a number of prism portions 75a, 75b, 88) and a PBS surface 76 deployed for receiving the unpolarized light from the active-pixel array and splitting it into a first polarized light path and a second polarized light path. First back- reflective collimating optics 80a are associated with the first polarized light path and second back- reflective collimating optics 80b are associated with the second polarized light path.

[0051] Unpolarized image light from the active pixel array enters the PBS prism where it is split by a polarizing beam splitter surface 76 into a first polarization component directed along the first polarized light path towards first reflective collimating optics 80a and a second polarization component directed along the second polarized light path towards second reflective collimating optics 80b. Polarized images back-reflected from the first and second back-reflective collimating optics are recombined at PBS surface 76 so as to be injected in superposition into lightguide 82 as a mixed polarization image.

[0052] In order to achieve this particularly compact configuration in which splitting and recombining are performed by a single PBS surface 76, the first and second reflective collimating optics are preferably “back-reflective,” meaning that they are each deployed substantially on axis, i.e., with their optical axis within 10 degrees of the chief ray of the incoming and reflected images. Most preferably, the back-reflective optics are deployed on axis, so that the chief ray of the received and reflected images both coincide with the optical axis of the optics.

[0053] The active pixel array is typically a micro-LED array, which may be either a color microLED array 70 or a monochrome array. In certain cases, a set of three arrays providing red, green and blue images, designated 70R, 70G, 70B, respectively, are used, as will be exemplified below. Although the description will refer throughout to micro-LED arrays as a preferred non-limiting example, it should be noted that other active-pixel arrays using other technology, such as OLED arrays, also fall within the scope of the description and claims.

[0054] This arrangement achieves high efficiency and brightness by capturing a high proportion of the non-polarized image light emitted by the micro-LED array while benefiting from the compactness and structural advantages of optics based on a PBS prism. The first and second reflective collimating optics may have sufficient optical power to achieve the collimation or may have lower power and be supplemented by refractive optics either at the entrance or at the exit from the PBS prism. Use of a combination of reflective and refractive optics may be advantageous since they typically introduce opposite optical aberrations, which may be designed to cancel each other out.

[0055] The PBS-based image projector may be coupled to the lightguide via a coupling prism with an obliquely- angled coupling-in surface or via a lightguide surface with a reflective prism on the opposite lightguide surface. In these cases, a refractive lens may be interposed before coupling in, providing part of the collimating optical power and thereby canceling out some aberrations due to the reflective optics.

[0056] Alternatively, the projector may be integrated with the lightguide. In this case, the PBS prism can be inclined relative to the lightguide axes so that the combined light leaving the PBS plane is correctly inclined relative to the lightguide surfaces for the entire field of the image to propagate within the lightguide. The lightguide surface preferably bisects the PBS prism, which may include a wedge-shaped prism located below the lightguide surface and bonded thereto by a low-index adhesive so that the injected image passes through the surface whereas the exiting collimated image light undergoes total internal reflection (TIR) at that surface. Anti-reflective coatings at both surfaces adjacent to the low-index adhesive may be helpful to minimize losses in the light paths crossing that plane. A further variant option for coupling to the lightguide splits the PBS prism into two portions which are aligned in opposing relation attached to upper and lower surfaces of the lightguide.

[0057] Where a single, e.g., color, micro- LED array is used, it may be slightly spaced from the PBS prism surface and a field lens may be attached to the array. In some implementations, three monochrome micro-LED arrays are combined on faces of a dichroic X-cube prism to provide the input image into the PBS prism. If a lens is interposed between the X-cube prism and the PBS prism, it may provide part of the collimating optical power. These options will be exemplified by the embodiments described below.

[0058] Lightguide-Integrated Projector Implementation

[0059] Referring now more specifically to the embodiment of FIGS. 1A-3D, these illustrate a first particularly preferred but non-limiting embodiment of the display system of the present invention employing a projector 50 that is integrated with lightguide 82. The term “integrated” in this context refers to a configuration in which the polarizing beam splitter prism provides an angularly- selective reflecting surface 84, coplanar with or parallel to major surface 86 of the lightguide, which is traversed by at least one light path within the PBS prism as part of the projector functionality and which reflects at least part of the mixed polarization image prior to injection into the lightguide. Surface 84 serves to reflect part of the coupled-in image so as to “fill” an entrance aperture of lightguide 82 defined by cutoff edge 6a and the image of 6a in the plane of major surface 86. In particularly preferred implementations of the present invention, this means that at least one polarization component of the image light passes through internal surface 84 as part of the light paths defined by the PBS prism while the collimated image light of the mixed polarization output image is angled so as to undergo internal reflection at internal surface 84. The angularly- selective reflection is preferably achieved by attachment of a lower wedge prism portion 88 of the PBS prism, or in some cases second back-reflective collimating optics 80b (see FIGS. 2C and 2D below), via low-index adhesive, thereby providing conditions for total internal reflection of the image light at the range of angles relevant for the part of the field which must be reflected from surface 84. In some cases, where the steepest part of the field is at angles of incidence below the critical angle for the glass-adhesive interface, the geometry of the design may ensure that steep rays below the critical angle are incident on surface 84 beyond the edge of prism portion 88, thereby encountering a glass-air boundary with a smaller critical angle. To minimize losses in the light paths crossing the boundary at surface 84, one or both surfaces facing the low-index adhesive may be provided with anti-reflective coatings. As an addition, or alternative, to the use of low- index adhesive, angularly-selective reflective properties may be provided by a suitable multilayer dielectric coating on surface 84. The required functionality may also be provided by an air gap (with antireflective coatings on both surfaces), although implementations without an air gap are typically preferred for their superior structural integrity and ease of manufacture.

[0060] The PBS prism is preferably inclined relative to the lightguide axes so that the combined light leaving the PBS plane is at an angle relative to the lightguide surfaces that is appropriate for the entire field of the image to propagate within the lightguide 82 by internal reflection at the lightguide major surfaces 86 and 87.

[0061] FIGS. 1A and IB illustrate the optical paths of the S-polarized and P-polarized light, respectively, through the system. The solid arrows represent the unpolarized light emitted from color micro-LED array 70. In FIG. 1A, the dot-dashed arrows represent the S-polarized light that is reflected by the PBS surface 76 and converted to circularly polarized light (dotted arrows) by a quarter-wave plate 78a associated with first reflective collimating optics 80a. The circularly polarized light is then reflected by the reflective optics 80a, which may be a doublet or other embedded optics to improve optical quality. The reflected light passes back through quarter-wave plate 78a, becoming P-polarized, and then passes through PBS surface 76 to enter lightguide 82.

[0062] In FIG. IB, the dashed arrows represent the P-polarized light that passes through PBS surface 76 and a quarter-wave plate 78b before being reflected by the reflective optics 80b. The reflected light passes back through quarter-wave plate 78b, becoming S-polarized, and is then reflected by PBS surface 76 to become co-aligned and combined with the other polarization component illustrated in FIG. 1A. The combined beams from both polarizations enter lightguide 82 as a combined polarization image, either directly or after internal reflection from surface 84.

[0063] Internal surface 84 is described in this embodiment as a portion of lightguide surface 86 which extends into and bisects the PBS prism, thereby subdividing the PBS prism into a portion 75b between surface 84 and the PBS surface 76 and additional wedge portion 88 which is located below (in the orientation illustrated) surface 84. It should be noted that PBS prism portion 75b is defined according to its optical function, and it may be implemented in structurally in various ways, either contiguous with the material of lightguide 82, distinct therefrom, or including an extension of the lightguide combined with one or more additional component.

[0064] By way of non-limiting examples, FIGS. 8A and 8B illustrate alternative structural implementations of a display system with an integrated projector and address structural solutions which are illustrated in the context of projector 50 as described above, but will be equally applicable to the various other implementations described below.

[0065] FIG. 8A illustrates an implementation in which the PBS prism is formed from prism portions 75a, 75b and 88 corresponding structurally to the functional parts of the PBS prism described above, and the PBS prism is bonded to a perpendicular edge surface of lightguide 82.

[0066] In some embodiments, the lightguide 82 may be formed from a material with a lower refractive index (nl) than the PBS prism. This allows for total internal reflection (TIR) at the lightguide's interface with air. The PBS prism, on the other hand, may be formed from a higher refractive index material (n2) to achieve TIR over the required range of angles at its interface with the adhesive layer at surface 84.

[0067] When implementing such an arrangement where nl < n2, the interface between the lightguide 82 and the PBS prism (portion 75b) is advantageously perpendicular to the lightguide major surfaces and surface 84. This perpendicular orientation ensures proper light transmission even with the refractive index difference without introducing distortion.

[0068] Furthermore, the difference between nl and n2 can be strategically chosen to compensate for chromatic dispersion effects that may arise if the light output from the lightguide is coupled at a slanted angle.

[0069] The adhesive layer 222 at the interface between the lightguide 82 and the prism portion 75b may advantageously be chosen to have a low refractive index compared to both nl and n2. This applies whether nl = n2 or nl n2. In cases where n2 = nl, the adhesive 222 may have the same refractive index as the “low-index” adhesive used on internal surface 84, since this minimizes any perturbations to TIR within lightguide 82 caused by adhesive spreading (overspilling) beyond the interface onto the lightguide surfaces. The use of low-index adhesive, i.e., with a refractive index lower than that of either of the adjacent elements, is applicable to advantage more generally whenever bonding a projector or coupling prism to a lightguide in the context of a display, and is not limited to the context of the other features of the projectors described herein.

[0070] The location designated 222 in FIG. 8A, in addition to an adhesive layer, may also accommodate a depolarizer or other polarization modifying element deployed between the PBS prism and the lightguide. The polarization modifying element may be a polarization rotator (halfwave plate), a quarter-wave plate or a birefringent depolarizer of sufficient thickness relative to the bandwidth of each color to achieve effective depolarization. These components may further enhance uniformity of the output image.

[0071] To enhance the mechanical strength of the bond between lightguide 82 and prism portion 75b, a plate 224a and / or 224b can be placed at the interface above and / or below the joint. The plate is preferably bonded to the structure using a low refractive index adhesive, typically the same as used at internal surface 84, to avoid disrupting TIR.

[0072] In embodiments where the lightguide 82 is made of a low refractive index material such that the available low-index adhesives do not support TIR over the required range of propagation angles, a reflective coating 226a and / or 226b, formed from either dielectric or metallic material, may be applied beneath the adhesive layer to maintain high reflectivity within the lightguide.

[0073] Turning now to FIG. 8B, this illustrates an alternative construction which avoids the potential region of mechanical weakness that may occur at a small-area perpendicular joint between the lightguide and the PBS prism. It also circumvents the challenges of assembling the system with exact parallelism between surface 84 and lightguide surface 86. In this case, instead of such a joint, the lightguide 82 extends into the prism structure where its thickness is supplemented by bonding thereto of a prism portion 75c. In this case, the prism portions making up the PBS prism as well as the adhesive used to bond portion 75c to the lightguide should all be index matched to the lightguide. The bonded combination of the lightguide end and prism portion 75c is then polished at the required angle to create an effective prism portion optically and functionally equivalent to portion 75b of FIG. 8A. A second prism portion 75a (equivalent to that of FIG. 8A) is placed on top, with the PBS surface 76 deployed at the interface, to complete the PBS arrangement. This over-waveguide configuration offers another approach to integrating the components.

[0074] Although surface 84 is shown here as coplanar with the lightguide surface 86, it may alternatively be parallel thereto with a small step between them (not shown) which acts as an optical cut-off, preventing entry of light which might be scattered from imperfections at the interface between the optical elements.

[0075] The PBS surface 76 may be implemented as a reflective polarizing beam splitter surface of any suitable type, including but not limited to a structural polarizer such as a wire grid polarizer, or a dielectric polarizing beam splitter. Each option has its own advantages and disadvantages, as is known in the art. In each case, the subsequent recombination of the two channels results in high efficiency. The above description that referred to the S -polarization being deflected and the P- polarization being transmitted refers to an implementation using a dielectric PBS or a structural polarizer correspondingly oriented, while a structural polarizer may alternatively be deployed to reverse this functionality or to split the channels according to some other selected polarization orientation is desired.

[0076] In this embodiment, if a color display is required, micro-LED array 70 is preferably a colored micro-LED array, which may employ effective pixels which each include a group of red, green and blue micro-LEDs, or in which each pixel cycles between multiple colors in sequence. The micro-LED array is preferably slightly spaced from the PBS prism surface with a field lens 74 preferably attached to the array, as illustrated in FIGS. 1A and IB.

[0077] The reflective optics 80a, 80b are preferably identical in optical design in order to achieve identical images of both polarizations that can be superimposed in alignment into a single perceived image with identical content to each individual image. Although described functionally as “reflective optics,” they may each include elements with both reflective and refractive optical power. Thus, in the implementation illustrated here, reflective optics 80a, 80b are each illustrated as a doublet with refractive power in addition to the reflective lens surface. The use of such a doublet structure provides additional degrees of freedom for minimizing optical aberration in the system. Many of the subsequent drawings represent the reflective optics more schematically without details of internal components, but it should be noted that the doublet implementation is equally relevant and applicable to all embodiments.

[0078] In this non-limiting implementation, the optical power of reflective optics 80a and 80b is sufficient alone to achieve collimation of the image light from micro-LED array 70. In other words, the effective focal length of the optics is equal to the light path length from the optics to array 70.

[0079] For simplicity of presentation, the drawings presented here and throughout this document focus on the structures of the projector 50 and its coupling to lightguide 82, since these convey the novel features of the lightguide-based display systems of the present invention. The display systems of the present invention are applicable particularly, although not exclusively, to augmented reality displays, such as head-up displays (HUDs) or head-mounted displays (HMDs), and include numerous additional components that are, per se, well-known in the art. These include a support structure for the display which, in the case of HMDs, may be a housing with the form factor of eyeglasses or a visor. In such displays, the lightguide may achieve one-dimensional or two-dimensional aperture expansion and then couple out the image towards the eye of the user, typically using an arrangement of partially-reflecting inclined surfaces internal to the lightguide or through a diffractive coupling-out arrangement. In addition to the components of the lightguidebased display systems described above, such augmented reality displays may also include: a power source; microprocessors; electronic display drivers; and communication components, all as required by each particular application. These features are all well-known and will not be described further herein.

[0080] Turning now to FIGS. 2A-2D, these illustrate that integration of the projector with the lightguide may be achieved using various architectures. The configuration of FIG. 2A corresponds to that of FIGS. 1 A and IB (simplified by omitting the field lens and details of the reflective optics) and illustrates an architecture in which micro-LED array 70 and first reflective optics 80a are located on one side of surface 84 and second reflective optics 80b are located on the other side of surface 84 attached to wedge prism 88. In this case, one of the polarized channels passes twice through surface 84. Here and throughout the rest of this document, the presence of quarter-wave plates 78a and 78b is assumed as part of the PBS architecture and will not be explicitly indicated or further discussed.

[0081] FIG. 2B illustrates an alternative architecture of the projector, here labelled 50', in which the image generating array 70 is located beneath the lightguide (in the orientation illustrated), attached to wedge prism 88, and both reflective optics 80a, 80b are located on faces of the PBS prism above surface 84. In this case, all of the unpolarized image light is introduced from below and passes once through surface 84. The functionality of projector 50' is essentially equivalent to that of projector 50 described above, simply with the direction of the light paths from the image generating array 70 to second reflective optics 80b reversed and the orientation of the PBS surface 76b flipped relative to surface 76 of projector 50.

[0082] FIG. 2C shows an alternative implementation employing a projector 50'', which is a modification of projector 50 of FIG. 2A which may simply manufacture. The angle of the PBS surface 76c differs from that of PBS surface 76, allowing active-pixel array 70 and second back- reflective collimating optics 80b to be mounted on surfaces that are parallel to the lightguide major surfaces. The wedge prism 88 of the previous implementations is here replaced by a flat window 89 or, in some cases, no window at all. A thickness of window 89 is chosen to keep the optical path from active-pixel matrix 70 to first back-reflective collimating optics 80a equal to that to second back-reflective collimating optics 80b so that the images are of the same size and overlapping. The PBS prism in this case has non-orthogonal surfaces, with first back-reflecting collimating optics 80a preferably being mounted on a surface with an inclination that aligns the optical axis of the optics with the desired inclination of the chief ray of the collimated image for injection into the lightguide (as is common to all of the integrated embodiments of the present invention).

[0083] FIG. 2D shows a similar modification employing a projector 50''' that has the active -pixel array 70 deployed beneath the lightguide, similar to FIG. 2B. In this case, PBS surface 76d is oriented so that active-pixel array 70 can be deployed against a flat window 89 adjacent to surface 84 that maintains total internal reflection (TIR). Other cases may include a field lens or may have no window present between the array and surface 84. Alignment Method

[0084] Turning now to FIGS. 3A-3D, in order to recombine the images returned from the two polarization channels into a single projected image without degradation of the image quality, precise spatial and focal alignment must be achieved between the two channels. Where sufficiently low manufacturing tolerances can be achieved both in the component manufacture and in device assembly, acceptable image quality may be achieved directly. In some cases, however, an alignment process is performed during assembly. FIGS 3A-3D illustrate four different options for implementing degrees of freedom, with or without a corresponding structural modification, for achieving optical alignment and focal matching between two optical channels during assembly of the display system.

[0085] In FIG. 3 A, optics 80b may be shifted laterally (arrow 90a) so that the optical axis of the reflected beams is parallel to that of 80a (alternatively, 80a may be shifted laterally), while the relative focus may be equalized by shifting prism 88 (indicated by arrow 90b). The drawing illustrates only the alignment adjustments in the plane of the drawing but shift 90a also allows adjustment in the direction into the page.

[0086] In a practical procedure, the components other than those to be moved for alignment are first bonded together, and the interfaces at which adjustment is to be made are provided with not- yet-cured adhesive and held in place. The device is arranged so that an output image can be viewed, either directly or by a camera, and the micro-LED array is actuated to generate a test pattern. Before alignment adjustment, it should be ensured that the non-adjusted optics 80a is generating a correctly focused (collimated) output image, typically by adjusting a spacing of array 70 from the input surface of the PBS prism, as is known in the art. Prism 88 is then moved according to arrow 90b until the second component is also precisely collimated, and then optics 80b is moved according to arrow 90a (including into the page) until the two components of the test align into a single image. These positions are then fixed by UV curing of the adhesive. Optionally, these alignment adjustments may be performed automatically by a computerized robotic system under control of image processing performed on images from a camera monitoring the output image.

[0087] FIG. 3B shows that first reflective optics 80a and first PBS prism component 75a are modified to interface at a surface that is tilted relative to the optical axis of the optics. As a result, adjustment in the direction of arrow 90b varies both lateral alignment and axial (focus) shift. Adjustment 90b is thus first used to adjust the focus (collimation) and then adjustment 90a can be used to align the optical axes, as before, without impacting focus. The practical process is analogous to that described above, where pre-focusing is performed for the image component directed to second reflective optics 80b. FIG. 3C shows an arrangement similar to FIG. 3B but where an extra degree of freedom is introduced by adding a wedge prism 94 at the interface between first reflective optics 80a and first PBS prism component 75a. This provides two non-parallel directions of adjustment 90a, 90b in the plane of the drawing (in addition to the adjustment into the page), thereby allowing both focal adjustment and lateral alignment to be adjusted at first reflective optics 80a.

[0088] In FIG. 3D, the relative focus may be equalized by shifting prism portions 75a and 75b relative to each other along the plane of PBS surface 76 (indicated by arrow 90b), after which alignment may be achieved my adjusting one or both of reflective optics 80a and 80b in a plane perpendicular to their respective optical axes (arrows 90a).

[0089] The above are non-limiting examples of the available options for implementing such an alignment and focus adjustment. In more general terms, at least two adjustments are needed in which a first adjustment 90a allows alignment at least in a plane orthogonal to the respective optical axis of at least one of the reflective optics and a second adjustment 90b includes at least a component parallel to one of the optic axes. In some cases, such as in FIG. 3C, both adjustments impact both lateral alignment and axial position, requiring simultaneous adjustment of both. In certain preferred implementations, such as in FIGS. 3A, 3B and 3D, one of the adjustments is purely orthogonal to the corresponding optical axis, facilitating sequential adjustment of focus followed by lateral alignment.

[0090] Non-Integrated Projector Implementation

[0091] The implementations of the present invention described above employ projector 50 integrated with lightguide 82 to achieve a highly compact configuration. In some cases, however, it may be preferable to employ a distinct projector architecture, which may optionally be used in other applications where a highly miniaturized projector is needed. The projector can then be coupled to a lightguide using a coupling prism, potentially rendering the device modular and simplifying manufacture and assembly. FIG. 4 illustrates an example of such an implementation.

[0092] Specifically, FIG. 4 illustrates a compact projector 52 that operates on the same principles as projector 50 described above, with an active pixel array 70 injecting unpolarized light corresponding to an image into polarizing a beam splitter (PBS) prism (prism portions 75a, 75b). The image generating array is shown without a field lens, but, in other preferred configurations, a field lens may be included. First reflective collimating optics 80a are associated with a first face of the PBS prism, and second reflective collimating optics 80b are associated with a second face of the PBS prism. Unpolarized image light from the active pixel array enters the PBS prism where it is split by a polarizing beam splitter surface 76 into a first polarization component directed towards first reflective collimating optics 80a and a second polarization component directed towards second reflective collimating optics 80b. The polarized images reflected from the first and second reflective collimating optics are recombined at PBS surface 76 to generate a mixed polarization projected image. This image is coupled to lightguide 82 via a coupling prism 102. In the example illustrated here, coupling prism 102 is a transmissive coupling prism which presents an obliquely angled coupling-in surface oriented roughly perpendicular to the chief ray of the projected image correctly oriented for propagating within lightguide 82. In alternative embodiments (not shown), the projector can be directly attached to one of the major surfaces of the lightguide and a reflective prism may be positioned in opposing relation on the other side of the lightguide, thereby achieving reflective coupling in of the projected image, as is known in the art.

[0093] The use of distinct components for projector 52 and coupling prism 102 makes it convenient to interpose a refractive lens 100 between them. Refractive lens 100 preferably provides part of the collimating optical power and thereby lowers the optical power requirements on reflective collimating optics 80a and 80b. The use of lower power reflective optics together with a combination of reflective and refractive optical components facilitates optical design with reduced optical aberration.

[0094] In certain particularly preferred implementations, air spaces are avoided in the light path from the point that the image light enters the PBS prism through to entering the lightguide. This preferred requirement can be combined with use of refractive lens 100 by implementing refractive lens 100 with planar attachment surfaces on both sides. To this end, lens 100 may advantageously be implemented as a doublet (as shown) with parallel planar outer surfaces or using a graduated index (“GRIN”) lens. Other optical components may optionally be included between the projector and the coupling prism, either in addition to or instead of lens 100. In particular, these may include a polarization rotator (half-wave plate), a quarter-wave plate, or a birefringent depolarizer of sufficient thickness relative to the bandwidth of each color to achieve effective depolarization. These components may further enhance uniformity of the output image.

[0095] One advantage of projector 52 is that multi-stage alignment processes described above with reference to FIGS. 3A-3D are not typically required. Regular cubic PBS prisms can be manufactured to high precision and provide convenient reference planes for accurate deployment of the components on each face of the cube. It is therefore often feasible to achieve precise alignment of the reflective optics 80a and 80b during manufacture without an adjustment process. Even if an adjustment process is required, it will typically be limited to a single step of lateral adjustment of one of the reflective optics components across the corresponding plane of the cubic prism. This configuration also allows the projector 52 to be rotated about the optical axis of the projected image to any required angular position in order to address requirements for the orientation of the image and / or to address structural limitations.

[0096] Although the description thus far has referred to use of a PBS prism, particularly in implementations in which the projector is not integrated with lightguide 82, the same optical architecture may be implemented using a “window PBS” (also referred to as a “cage cube-mounted beam splitter”) in which the PBS surface 76 is a flat sheet deployed across a 45-degree diagonal plane (sometimes referred to as a (110) plane borrowing from Miller indices notation) within a cubic open frame. The remaining optical elements are attached to the cage / frame in the same spatial relation as described thus far, and the functionality remains as described above.

[0097] Color Displays employing X-Cube Dichroic Combiner

[0098] The structures described above employ a single active pixel array to generate the projected image, which may be a monochrome image or a color image generated by a color micro-LED array. In some cases, however, it may be desired to generate a color image by combining images from a number of separate monochrome arrays. FIGS. 6A-6C illustrate such implementations.

[0099] Specifically, according to an aspect of the present invention, the active pixel (micro-LED) array image generator of the previous embodiments is replaced by an X-cube dichroic (or “trichroic”) combiner 110 that combines image light from three monochrome active-pixel array image sources. FIG. 6A illustrates an embodiment corresponding to that of FIG. 4, and FIG. 6B illustrates an embodiment corresponding to that of FIG. 2A.

[0100] In these embodiments, projectors 58 and 60 employ an X-cube dichroic combiner 110 to combine the optical paths of unpolarized light from panels 70R, 70G, and 70B (as shown by arrows) onto a single unpolarized beam impinging on PBS surface 76. Only a single central ray for a central pixel of each array is shown for clarity, but each pixel generates a diverging beam which is combined by the dichroic combiner with the beams of corresponding pixels of the other two colors and directed into the PBS prism. Beam propagation from that point onwards is as described above for each corresponding embodiment.

[0101] The structure of an X-cube dichroic combiner of this sort is, per se, known, and employs a first diagonal dichroic reflector which reflects one color and transmits the other two colors and a second diagonal dichroic reflector which reflects a second color and transmits the other two colors. The overall effect is that of superimposing the light corresponding to an image generated by each of the arrays.

[0102] The optical design of projectors 58 and 60 are essentially similar to that of projectors 50 and 52 described above, modified by the addition of the length of a side of the X-cube dichroic combiner to the light path from the active pixel arrays to the reflective optics. While this might be expected to render the projectors more bulky, this is typically offset by the higher pixel density of the monochrome arrays compared to a color array which requires multiple micro-LEDs per effective pixel. This allows the dimensions of the entire projector assembly to be scaled down so that the apparently more bulky structures of projectors 58 and 60 may in practice be more compact than projectors 50 and 52.

[0103] One or more refractive lens may be introduced between each image generating array 70R, 70G, and 70B and X-cube combiner 110 (serving as a field lens) and / or between the X-cube combiner 110 and the PBS prism input surface. FIG. 6C illustrates an embodiment similar to FIG. 6B in which a refractive lens 700 interposed between the X-cube combiner 110 and the PBS prism input surface. A lens in this position, optically spaced from the image generating arrays by the relatively small dimension of the X-cube, may serve dual roles, serving to modify the field from the micro-EED array while also contributing optical power towards partially collimating the image, thereby reducing the optical power required of the reflective optics. The positive field curvature introduced by refractive element 700 may also contribute to cancelling out the typically negative field curvature of the reflective collimating optics 80a, 80b, thereby further facilitating effective optical design to generate high quality image output.

[0104] Although the X-cube configuration is believed to be particularly advantageous due to its compactness and symmetry, it should be noted that alternative designs of trichroic beam combiners could be used to provide equivalent functionality. Another such option is the trichroic beamsplitter prism design best known for its use in 3CCD cameras. Such a design may avoid the complexity of implementing high-quality intersecting dichroic filters.

[0105] Alternative Multi- Array Projector Architectures

[0106] In certain preferred embodiments, the image projector may include a set of side-by-side combined image generators having different colors to generate multi-color images from separate monochrome active-pixel arrays of different colors. FIGS. 7A and 7B illustrate such an embodiment.

[0107] FIG. 7A illustrates an embodiment in which a multi-color image is projected into a two- dimensional optical aperture expansion lightguide. In this embodiment, a three-color image projector 62 includes side-by-side assemblies sharing a common internally-partitioned PBS prism.

[0108] FIG. 7B illustrates the structural implementation of the three-color image projector 62 of FIG. 7A. In this embodiment, a single PBS prism with a shared PBS surface 76 is used. The PBS prism may have a single contiguous top and bottom prism portion shared by all channels or may include separators 96a in one or both prisms to minimize cross-talk between the channels. The bottom prism portion 88 may also have separators 96b. The “separators” are typically an absorbing surface, such as black paint, applied between the sections before bonding together. Most preferably, prism portion 75b that is closest to the exit aperture is implemented as a continuous single prism without any internal separators, thereby allowing for overlap of the exit apertures for the different colors.

[0109] Separate illuminating active-pixel arrays 70R, 70G, 70B deliver image light into the PBS prism, and separate reflective collimating optics 80aR, 80aG, 80aB, respectively, reflect and collimate one polarization while another set of reflective collimating optics 80bR, 80bG, 80bB, respectively, (visible in FIG. 7B) reflect and collimate the other polarization. Collimated beams of both polarizations for each color are recombined at PBS surface 76 and the combined polarization image beams from all channels (R, G, B) enter the lightguide side-by-side as shown in FIG. 7A. The use of single PBS surface 76 minimizes deviation between the channels, simplifies integration, and reduces cost. Furthermore, in such a configuration (assuming separators are not close to output), the exit apertures may overlap and thereby assure high coupling efficiency with reduced size.

[0110] Parenthetically, FIG. 7A illustrates projector 62 in the context of a two-dimensional aperture expansion lightguide which includes a first lightguide portion 82a which contains a redirecting configuration for progressively redirecting light propagating within lightguide portion 82a in a first direction (towards the left as shown) so as to propagate within the lightguide towards a second lightguide portion 82b (downwards as shown) while expanding the effective optical aperture horizontally. Second lightguide portion 82b includes a coupling-out configuration with progressively couples out the propagating towards the eye of the user while expanding the effective optical aperture vertically. In the non-limiting example illustrated here schematically, the redirecting configuration is implemented as a first set of partially reflecting internal surfaces 212a and the coupling-out configuration is implemented as a second set of partially reflecting internal surfaces 212b obliquely angled relative to the major lightguide surfaces. One or both of these sets of internal reflective elements may be substituted by one or more diffractive optical elements to achieve similar functions, as is known in the art.

[0111] FIG. 7C illustrates an embodiment with a projector 64 generally similar to projector 62 in which three pixel arrays and three pairs of reflective collimating optics are combined to use a common PBS prism with a shared PBS surface 76. In this case, the first reflective collimating optics 80al, 80a2, 80a3 (and the corresponding second reflective collimating optics, not visible in this view) for each channel are shifted relative to the corresponding pixel array 70 so that the output beams 216, 218 and 220, respectively, are not parallel to each other. In such a configuration, each panel projects a different segment of the horizontal field of view of the final image. The panels may be rotated, tilted, and have field lenses (not shown) at off-axis in order to generate minimal aberrations and distortion to each image section. In the particularly preferred configuration shown here, the lowest projected image 220 needs to propagate the furthest within the lightguide while the uppermost beam 216 propagates the shortest distance across the lightguide. Consequently, the lightguide section 82a including reflectors 212a may be reduced in size as shown in this figure.

[0112] Split Integrated Projector Embodiments

[0113] The embodiments described above with reference to FIGS. 1A-2B employ an image projector 50 or 50' that is integrated with lightguide 82 at an edge of the lightguide. A variant implementation of this embodiment, illustrated in FIG. 9, provides a projector 130 in which the polarizing beam splitter prism is split into two portions attached, respectively, to the upper and lower mutually-parallel major surfaces of the lightguide aligned in opposing relation.

[0114] Specifically, as shown in FIG. 9, the PBS prism is in this case formed from a first prism portion 120a above the PBS surface 76 (in the orientation as illustrated), a second prism portion 120b below the PBS surface 76, the thickness of lightguide 82 itself, and lower prism portion 120c deployed below the lightguide (in the orientation shown here). Second prism portion 120b is bonded to lightguide surface 87 with index-matched adhesive to allow injection of the image from the projector into the lightguide while lower prism 120c is bonded to lightguide surface 86 by low- index adhesive in order to provide the angularly selective reflecting properties of internal surface 84, thereby providing functionality equivalent to the embodiments described above.

[0115] The optical performance of projector 130 is fully analogous to that of projector 50 described above. Unpolarized image light from active pixel array 70 passes through field lens 74 and enters the upper PBS prism portion 120a where it is split by polarizing beam splitter surface 76 into a first polarization component directed towards first reflective collimating optics 80a and a second polarization component that passes through second prism portion 120b, the thickness of lightguide 82 and lower prism portion 120c towards second reflective collimating optics 80b. The polarized images reflected from the first and second reflective collimating optics are recombined at PBS surface 76 to generate a mixed polarization projected image that is directed through second prism portion 120b and the index-matched adhesive at the boundary with the lightguide so as to be coupled into lightguide 82 to propagate within the lightguide.

[0116] It should be noted that the configuration of projector 130 is advantageous for a range of applications that are not limited to unpolarized image sources and / or parallel processing of two images of orthogonal polarization. Thus, in some cases, only the first or the second optical path exists, and can be used to advantage in configurations which are, for example, otherwise similar to those discussed below with reference to FIGS. 10 and 11A-11D. In other applications, dual polarization optical processing and recombination are used, as shown.

[0117] The ability to deploy projector 130 on the major surfaces, i.e., not limited to deployment at the extreme edge of the lightguide, opens up the possibility of deploying groups of projectors in various geometrical configurations. Examples of possible deployments will now be discussed with reference to FIGS. 10-1 ID.

[0118] Turning now to FIG. 10, in one application, two or more similar image projectors 130 are deployed sequentially along the lightguide, with each image projector projecting a monochrome image of a different color. Thus, in the example illustrated here, a first image projector 130R has an active-pixel matrix 70R generating a red image, a second image projector 130G has an activepixel matrix 70G generating a green image and a third image projector 130B has an active -pixel matrix 70B generating a blue image. The three color images propagate along the lightguide and generate together a color image to be viewed by a user.

[0119] As before, internal surface 84 for each image projector is implemented using low-index adhesive which maintains internal reflection for the images propagating within the lightguide, so no leakage will occur at surface 84 through successive projectors. Regarding the upper attachment surface, surface 122G must here be provided with a dichroic coating which is transparent to green light and reflects blue light, thereby preventing “leakage” of the blue image injected by projector 130B. Similarly, in the sequential deployment illustrated here, upper attachment surface 122R must be provided with a dichroic coating that is transparent to red light but reflects both green and blue light. Upper attachment surface 122B, being the first in the row, can be implemented using simple index-matched adhesive without coatings.

[0120] As before, although the sequential deployment may appear bulky, monochrome projectors can be implemented using display arrays with high pixel density, which may facilitate miniaturization of the components.

[0121] In some cases, instead of a single row deployment, a more compact arrangement of projectors 130R, 130G and 130B may be achieved by using a staggered configuration as illustrated in FIGS. 11 A and 1 IB. In each case, the housing of the projector is wider than the output aperture, which limits how closely the projectors can be juxtaposed in a row. By staggering the projectors, the spacing between the apertures for the injection of the different colors can be brought closer together than could otherwise be achieved, resulting in a smaller lightguide. In the examples illustrated here, staggering is achieved by positioning the projectors with two adjacent and the third aligned between them and positioned behind or in front of the others. In alternative implementations (not shown), three or more projectors can be staggered in a row, one behind the next in partially overlapping relation. Depending on the size of the apertures through which the images are introduced and the in-plane angular spread of the projected field, it may be possible to avoid overlap between the propagating images within the lightguide and the aperture of other projectors, thereby circumventing the need for the dichroic filters described in FIG. 10. If there is overlap, relatively simple dichroic coatings which need only separate between two colors will be sufficient. FIG. 11B is thus functionally similar to FIG. 7A described above, but employing three separate projectors 130R, 130G and 130B.

[0122] FIGS. 11C and 11D illustrate a similar staggered deployment of multiple projectors 130a, 130b, 130c which are oriented with non-parallel projection directions so as to project different regions of the projected image. This is functionally analogous to FIG. 7C described above and, as described there, allows for a reduction in the size of the first lightguide portion 82a.

[0123] Rectangular Lightguide Implementation

[0124] The various embodiments discussed thus far all relate to injection of an image so as to propagate within a slab-type lightguide defined by one pair of mutually-parallel major surfaces. The same principles can, however, be applied to injecting an image into a lightguide with rectangular cross-section which supports propagation of an image by four- fold internal reflection, as will now be described with reference to FIG. 12. Rectangular cross-section lightguides are discussed extensively in PCT Patent Application Publication No. WO 2018 / 065975 Al, and may be used to advantage for a first dimension of optical aperture expansion prior to injecting the image into a second slab-type lightguide.

[0125] Referring to FIG. 12, this illustrates an application of the present invention to a lightguide 83 which, in addition to the first pair of mutually-parallel major surfaces 86 and 87, has a second pair of mutually-parallel major surfaces 89 and 91 perpendicular thereto. This defines a rectangular cross-section lightguide that supports propagation of light by four-fold internal reflection. A projector 150 is provided to inject an image into lightguide 83. Projector 150 is essentially similar to projector 50 described above, based on a PBS prism which has image light injected from an active-matrix array 70, split into two polarization components by polarizing beam splitter surface 76, and the two channels separately collimated by first and second reflective collimating optics 80a and 80b. The two polarized images are then recombined by PBS surface 76 for injection into the lightguide 83, which is achieved in part through reflection at internal surface 84, all analogous to the description of projector 50 above. In this case, the PBS prism is additionally inclined relative to axes of the lightguide such that the mixed polarization image leaving the polarizing beam splitter prism is inclined relative to the rectangular cross-section of the lightguide so as to propagate within the lightguide by four-fold internal reflection. To specify the PBS inclination more clearly, the inclination for injection into the rectangular lightguide requires an inclination of the chief ray of the projected image relative to the planes of both surface 86 (back surface as shown) and surface 89 (bottom surface as shown). The required inclination of the chief ray relative to surface 86 is achieved by a geometry analogous to FIGS. 1 A-2A, as would be seen by viewing FIG. 12 from the top. The inclination relative to surface 89 is achieved by an additional upward tilt of the PBS prism of the projector, as would be seen by viewing FIG. 12 from the front. The injected image is trimmed by edges 6a and 6b, which may be staggered as shown, or may be coplanar.

[0126] In all other respects, the embodiment of FIG. 12 is structurally and functionally similar to that of FIG. 1 A, and can be fully understood by analogy to the description above.

[0127] Polarization-Modifying Reflective SLM Embodiments

[0128] Turning finally to FIGS. 5A-5D, although the embodiments discussed thus far have related to injection of non-polarized image light from an active-pixel array, and have preferably used recombination of two polarized channels, certain aspects of the optical architecture described herein may also be used to advantage in the context of polarized image sources, and most notable, polarization-modifying spatial light modulators (SLM), such as a liquid crystal on silicon (LCOS) display chip. FIGS. 5A-5D illustrate such embodiments. These embodiments are included here due to various structural similarities with the embodiments of FIGS. 1A-4. However, in the embodiments of FIGS. 5A-5D, the image generation process itself generates a polarized image; therefore, there is no polarization recycling or recombination. Instead, the second optical channel is used for illumination.

[0129] FIGS. 5A-5C illustrate a single LCOS-based embodiment, where FIG. 5A shows sample rays illustrating the overall light path of both the illumination into the LCOS 106 and the reflected image (light modulated by the LCOS) into lightguide 82. For clarity, FIG. 5B shows only the illumination light ray paths and FIG. 5C shows only the reflected image ray paths into lightguide 82.

[0130] The illumination is injected into the projector from an illumination channel 104, which provides, directly or indirectly, polarized illumination. This illumination channel may include a light pipe, diffuser, Fresnel lens, dichroic combiner, mirror, laser, or other optical elements or sources. The illumination entrance aperture is located adjacent to the exit aperture for injection of the image into the lightguide. The injected light from the illumination channel 104 is polarized so as to be reflected by polarizing beam splitter surface 76. The reflected polarized light propagates onto a tilted reflective lens 108 (see FIG. 5B). This lens incorporates a waveplate that rotates the reflected light polarization to be orthogonal. The reflected orthogonally polarized light passes through PBS surface 76 to illuminate LCOS 106. The reflected light from the LCOS is modulated to generate an image, then follows the optical path shown in FIG. 5C, including reflection at PBS surface 76, reflection, collimation and polarization rotation (through a quarter wave plate) at reflective collimating optics 80a, transmission through PBS surface 76 and coupling into lightguide 82 including at least part of the light being reflected by internal surface 84. With proper choice of the properties of reflective illumination lens 108, illumination aperture 104 is imaged onto the exit aperture (entrance to lightguide), thereby reducing light loss.

[0131] FIG. 5D illustrates an alternative illumination arrangement. In this embodiment, the illumination passes through surface 84, which supports TIR. Light from source 200 (e.g., a LED) expands through lenses or prisms or a light pipe 201 to be reflected by surface 202 towards LCOS 106. A polarizer is provided at surface 84 to ensure that only the polarization component which will be transmitted through PBS surface 76 enters the PBS prism. That light then illuminates LCOS 106. The remainder of the light path is the same as described above with reference to FIG. 5C.

[0132] 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 lightguide-based display system comprising:(a) a lightguide formed from transparent material and having a pair of mutually- parallel major surfaces that support propagation of image light by internal reflection, the lightguide containing a set of mutually-parallel partially reflecting internal surfaces deployed to couple out light propagating within the lightguide so as to exit from the lightguide; and(b) an image projector coupled to the lightguide so as to inject a collimated image into the lightguide, the image projector comprising:(i) at least one active-pixel array configured to generate unpolarized light corresponding to an image,(ii) a polarizing beam splitter (PBS) prism deployed for receiving the unpolarized light from the active-pixel array, the PBS prism including a PBS surface configured to split the unpolarized light into a first polarized light path and a second polarized light path,(iii) first back-reflective collimating optics associated with the first polarized light path, and(iv) second back-reflective collimating optics associated with the first polarized light path, wherein back-reflected polarized images received from the first and second back-reflective collimating optics are recombined by the PBS surface so as to be injected in superposition into the lightguide as a mixed polarization image.

2. The lightguide-based display system of claim 1 or claim 21, wherein the polarizing beam splitter prism provides an angularly-selective reflecting surface coplanar with or parallel to one of the major surfaces of the lightguide, the angularly-selective reflecting surface being traversed by at least part of the light corresponding to an image prior to the recombining, the angularly-selective reflecting surface reflecting at least part of the mixed polarization image prior to reaching an entrance aperture of the lightguide.

3. The lightguide-based display system of claim 2, wherein the angularly-selective reflecting surface is provided by a layer of low-index adhesive or an air gap, a path of at leastone polarization component of the image crossing the angularly-selective reflecting surface and at least part of the mixed polarization image undergoing internal reflection at the angularly- selective reflecting surface of the lightguide within the PBS prism.

4. The lightguide-based display system of claim 3, further comprising anti-reflective coatings at surfaces adjacent to the low-index adhesive or air gap to minimize losses in light paths crossing the angularly-selective reflecting surface.

5. The lightguide-based display system of any one of claims 1-4, wherein the polarizing beam splitter prism comprises a first portion and a second portion attached to the mutually- parallel major surfaces of the lightguide aligned in opposing relation.

6. The lightguide-based display system of claim 5, wherein the image projector is one of a plurality of similar image projectors deployed sequentially along the lightguide, wherein a first of the image projectors projects an image of a first color and wherein a second of the image projectors projects an image of a second color different from the first color, wherein a major portion of the PBS prism of the second image projector is attached to the lightguide by index- matched adhesive overlying a dichroic layer that is transparent to the second color and reflective to the first color.

7. The lightguide-based display system of claim 1 , further comprising a transmissive or reflective coupling prism configured to couple the mixed polarization image into the lightguide.

8. The lightguide-based display system of claim 7, further comprising a refractive lens interposed between the PBS prism and the coupling prism, the refractive lens supplementing an optical power of the first and second back-reflective collimating optics to achieve collimation of the mixed polarization image.

9. The lightguide-based display system of claim 7, wherein light paths from an entrance to the PBS prism via the first and second back-reflective collimating optics and the coupling prism do not cross any air gap before entering the lightguide.

10. The lightguide-based display system of any one of claims 1, 2 and 7, wherein the polarizing beam splitter prism is configured such that the first back-reflecting collimating opticsis inclined relative to axes of the lightguide to direct the mixed polarization image so as to propagate within the lightguide.

11. The lightguide-based display system of any one of claims 1, 2 and 7, wherein the at least one active -pixel array comprises a color micro-LED array.

12. The lightguide-based display system of any one of claims 1, 2 and 7, wherein the active-pixel array is spaced from a surface of the polarizing beam splitter prism, and a field lens is attached to the active-pixel array.

13. The lightguide-based display system of any one of claims 1, 2 and 7, wherein the at least one active-pixel array comprises three monochrome arrays combined on faces of a dichroic X-cube prism to provide the unpolarized image light to the polarizing beam splitter prism.

14. The lightguide-based display system of claim 13, further comprising a lens interposed between the dichroic X-cube prism and the polarizing beam splitter prism, the lens providing a part of a collimating optical power.

15. The lightguide-based display system of any one of claims 1, 2 and 7, wherein the lightguide further comprises, perpendicular to the pair of mutually-parallel major surfaces, a second pair of mutually-parallel major surfaces, thereby defining a rectangular cross-section lightguide that supports propagation of light by four-fold internal reflection, and wherein the polarizing beam splitter prism is configured such that the first back-reflecting collimating optics is inclined relative to the rectangular cross-section of the lightguide so as to direct the mixed polarization image to propagate within the lightguide by four-fold internal reflection.

16. The lightguide-based display system of any one of claims 1, 2 and 7, wherein an optical power of the first and second back-reflective collimating optics is supplemented by refractive optics at an entrance or an exit of the polarizing beam splitter prism to achieve collimation of the mixed polarization image.

17. The lightguide-based display system of any one of claims 1, 2 and 7, further comprising a depolarizer or other polarization modifying element deployed between the PBS prism and the lightguide.

18. A method for projecting an image in a lightguide-based display system, the method comprising:(a) generating, by at least one active-pixel array, unpolarized image light;(b) receiving, by a polarizing beam splitter prism, the unpolarized image light;(c) splitting, by a PBS surface within the polarizing beam splitter prism, the received unpolarized image light into a first polarization component directed towards first back-reflective collimating optics and a second polarization component directed towards second back-reflective collimating optics;(d) recombining, by the PBS surface within the polarizing beam splitter prism, at least partially collimated polarized images received by reflection of the first and second polarization components from the first and second back-reflective collimating optics, respectively, to generate a superposition of the collimated polarized images as a mixed polarization image; and(e) coupling the mixed polarization image into a lightguide formed from transparent material and having a pair of mutually-parallel major surfaces so that the mixed polarization image propagates within the lightguide by internal reflection.

19. The method of claim 18, wherein an optical power of the first back-reflective collimating optics and the second back-reflective collimating optics is supplemented by at least one refractive lens deployed in a light path before or after the PBS prism so that the mixed polarization image coupled into the lightguide is a collimated image.

20. The method of claim 18, wherein the first back-reflective collimating optics and the second back-reflective collimating optics have sufficient optical power to collimate the first polarization component and the second polarization component, respectively, such that the mixed polarization image coupled into the lightguide is a collimated image.

21. A lightguide-based display system comprising:(a) a lightguide formed from transparent material and having a pair of mutually- parallel major surfaces that support propagation of image light by internal reflection, the lightguide containing a set of mutually-parallel partially reflecting internal surfaces deployed to couple out light propagating within the lightguide so as to exit from the lightguide; and(b) an image projector coupled to the lightguide so as to inject a collimated image into the lightguide, the image projector comprising:(i) at least one active-pixel array configured to generate unpolarized light corresponding to an image,(ii) a polarizing beam splitter (PBS) prism deployed for receiving the unpolarized light from the active-pixel array, the PBS prism including a PBS surface configured to split the unpolarized light into a first polarized light path and a second polarized light path,(iii) first back-reflective collimating optics associated with the first polarized light path, and(iv) second back-reflective collimating optics associated with the first polarized light path, wherein back-reflected polarized images received from the first and second back-reflective collimating optics are recombined by the PBS surface so as to be injected in superposition into the lightguide as a mixed polarization image.

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