Display with chromatic dispersion compensation in a lightguide architecture

Abstract

A display includes a lightguide (12) formed from a transparent material having parallel major surfaces for supporting propagation of collimated image light by internal reflection. An image generator (30) forms an image at an image plane, and a collimating optical arrangement includes a reflective lens and a quarter-wave plate. A polarizing beam splitter arrangement includes a PBS surface (36) at an interface between a first PBS prism component (38) and a second PBS prism component (40), the latter being optically bonded to the lightguide (12). The image light follows a folded path through the PBS and collimating optics before entering the lightguide (12). The lightguide and at least one of the PBS prism components are formed from materials with different Abbe numbers, introducing a compensatory chromatic dispersion that at least partially offsets linear chromatic aberration caused by the geometry of the lightguide with respect to the viewing direction.

Description

[0001] Display with Chromatic Dispersion Compensation in a Lightguide Architecture

[0002] FIELD AND BACKGROUND OF THE INVENTION

[0003] The present invention relates to optical display systems, and more particularly to near-eye displays employing lightguide -based image delivery, and amelioration of chromatic dispersion within such systems.

[0004] Lightguide -based display architectures are widely used in near-eye systems such as augmented reality (AR) and virtual reality (VR) headsets. These systems typically employ a transparent lightguide to convey and expand a projected image in front of the eye, propagating by internal reflection, where it is coupled out towards the eye for viewing. A collimated image is introduced into the lightguide from a projector, which may employ a polarizing beam splitter (PBS) prism to define a light path between the various components with a compact form factor.

[0005] SUMMARY OF THE INVENTION

[0006] The present invention is a display with chromatic dispersion compensation in a lightguide architecture.

[0007] Accordingly, according to the teachings of an embodiment of the present invention there is provided a display comprising: (a) a lightguide formed from transparent material having mutually-parallel first and second major surfaces for supporting propagation of image light by internal reflection at the major surfaces; (b) an image generator for generating an image at an image plane; (c) a collimating optical arrangement including a reflective lens and an associated quarter-wave phase plate; and (d) a polarizing beam splitter (PBS) arrangement comprising a PBS surface at an interface between a first PBS prism component and a second PBS prism component, the second PBS prism component being optically bonded to the first major surface, the PBS arrangement defining a light path from the image generator to the collimating optical arrangement and from the collimating optical arrangement through at least the second PBS prism into the lightguide at a propagation angle that undergoes internal reflection at the major surfaces, wherein the lightguide is formed from a material having a first Abbe number, and wherein at least one of the first and second PBS prism components is formed from a material having a second Abbe number different from the first Abbe number so as to introduce a compensatory chromatic dispersion into the image light propagating within the lightguide. According to a further feature of an embodiment of the present invention, the collimating optical arrangement is associated with the second major surface of the lightguide and the light path passes from the PBS surface through a thickness of the lightguide to the collimating optical arrangement, back through the thickness of the lightguide and is reflected at the PBS surface to reenter the lightguide at an oblique angle for propagation within the lightguide.

[0008] According to a further feature of an embodiment of the present invention, the collimating optical arrangement is associated with the second major surface of the lightguide via an internal-reflection-preserving interface.

[0009] According to a further feature of an embodiment of the present invention, both the first PBS prism component and the second PBS prism component are formed from the material with the second Abbe number.

[0010] According to a further feature of an embodiment of the present invention, the first PBS prism component presents an outer surface parallel to the first major surface and the image plane is associated with the outer surface.

[0011] According to a further feature of an embodiment of the present invention, a field lens is interposed between the image plane and the outer surface.

[0012] According to a further feature of an embodiment of the present invention, the image generator comprises a reflective polarization-modifying spatial light modulator (SLM) located at the image plane and an illumination arrangement for delivering polarized illumination to the SLM via reflection at the PBS surface.

[0013] According to a further feature of an embodiment of the present invention, the illumination arrangement is configured to deliver the polarized illumination via internal reflection at the outer surface prior to reflection at the PBS surface.

[0014] According to a further feature of an embodiment of the present invention, the first PBS prism component or an adjacent optical element provides a reflective surface, the illumination arrangement being configured to deliver the polarized illumination via reflection at the reflective surface prior to internal reflection at the outer surface and at the PBS surface.

[0015] According to a further feature of an embodiment of the present invention, the first PBS prism component presents an image input surface with which the image generator is associated and an optics interface surface with which the collimating optical arrangement is associated, the light path passing from the image generator via reflection at the PBS surface to the collimating optical arrangement and then via transmission at the PBS surface through the second PBS prism component into the lightguide, wherein the optics interface surface is oriented so that an optical axis of the collimating optical arrangement is obliquely angled to the first major surface of the lightguide.

[0016] According to a further feature of an embodiment of the present invention, at least the second PBS prism component is formed from the material having the second Abbe number.

[0017] According to a further feature of an embodiment of the present invention, the second PBS prism component is formed from the same material as the lightguide and the first PBS prism component is formed from the material having the second Abbe number.

[0018] According to a further feature of an embodiment of the present invention, a halfwave retarder plate is deployed between the PBS surface and the first major surface.

[0019] According to a further feature of an embodiment of the present invention, the halfwave retarder plate is interposed between the PBS surface and the second PBS prism component.

[0020] According to a further feature of an embodiment of the present invention, the halfwave retarder plate is interposed between the second PBS prism component and the first major surface of the lightguide.

[0021] According to a further feature of an embodiment of the present invention, the image generator comprises a micro-LED matrix deployed at the image plane.

[0022] BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0024] FIGS. 1A and IB are schematic isometric views of an optical system including two displays each implemented using a lightguide optical element (LOE), constructed and operative according to the teachings of an embodiment of the present invention, illustrating a top-down and a side-injection configuration, respectively.

[0025] FIGS. 2A and 2B are a schematic top view and side view, respectively, of one of the displays of FIGS. 1A and IB relative to the eye of a viewer, illustrating the linear chromatic aberration that is generated by the uncorrected system. FIG. 3A is a schematic side view of a preferred implementation of a display according to an embodiment of the present invention.

[0026] FIG. 3B is a schematic representation illustrating the principle of chromatic compensation implemented in the system of FIG. 3A.

[0027] FIG. 4A is a view similar to FIG. 3A, illustrating a first variant configuration.

[0028] FIG. 4B is a view similar to FIG. 3A, illustrating a second variant configuration.

[0029] FIG. 5 is a view similar to FIG. 3A, illustrating a further variant configuration including an alternative illumination arrangement.

[0030] FIG. 6 is a schematic side view of an alternative embodiment of a display according to the invention.

[0031] DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] The present invention is a display with chromatic dispersion compensation in a lightguide architecture.

[0033] The principles and operation of displays according to the present invention may be better understood with reference to the drawings and the accompanying description.

[0034] An exemplary implementation of a device in the form of a near-eye display, generally designated 10, employing a lightguide optical element (LOE) 12, is illustrated schematically in FIGS. 1A and IB. This is a non- limiting example of a system in which context the compensation element of the present invention is used to advantage, as detailed below. The near-eye display 10 employs a compact image projector assembly 14 integrated so as to inject an image into LOE (interchangeably referred to as a “waveguide,” a “substrate” or a “slab”) 12 within which the image light is trapped in one dimension by internal reflection at a set of mutually-parallel planar external surfaces.

[0035] The LOE typically includes an arrangement for expanding the optical aperture of the injected image in one or two dimensions, and for coupling-out the image illumination towards the eye of the observer, typically based either on the use of internal partially- reflecting surfaces or on diffractive optical elements. In one non-limiting set of implementations further illustrated schematically in FIGS. 2A and 2B, the light injected into LOE 12 by image projector assembly 14 impinges of a set of partially-reflecting surfaces (interchangeably referred to as “facets”) 17 that are parallel to each other, and inclined obliquely to the direction of propagation of the image light, with each successive facet deflecting a proportion of the image light into a deflected direction, also trapped / guided by internal reflection within the substrate. This first set of facets 17 are not illustrated individually in FIGS. 1A and IB, but are located in a first region of the LOE designated 16 and are shown schematically in FIG. 2B. This partial reflection at successive facets achieves a first dimension of optical aperture expansion. In a first set of preferred but non-limiting examples of the present invention, the aforementioned set of facets 17 are orthogonal to the major external surfaces of the substrate. In this case, both the injected image and its conjugate (inverted form) undergoing internal reflection as it propagates within region 16 are deflected and become conjugate images propagating in a deflected direction. In an alternative set of preferred but non-limiting examples, the first set of partially -reflecting surfaces 17 are obliquely angled relative to the major external surfaces of the LOE. In the latter case, either the injected image or its conjugate forms the desired deflected image propagating within the LOE, while the other reflection may be minimized, for example, by employing angularly-selective coatings on the facets which render them relatively transparent to the range of incident angles presented by the image whose reflection is not needed.

[0036] The first set of partially-reflecting surfaces deflect the image illumination from a first direction of propagation trapped by total internal reflection (TIR) within the substrate to a second direction of propagation, also trapped by TIR within the substrate.

[0037] The deflected image illumination then passes into a second substrate region 18, which may be implemented as an adjacent distinct substrate or as a continuation of a single substrate, in which a coupling-out arrangement (either a further set of partially reflective facets 19 or a diffractive optical element) progressively couples out a proportion of the image illumination towards the eye of an observer located within a region defined as the eye-motion box (EMB), thereby achieving a second dimension of optical aperture expansion. The overall device may be implemented separately for each eye, and is preferably supported relative to the head of a user with the each LOE 12 facing a corresponding eye of the user. In one particularly preferred option as illustrated here, a support arrangement is implemented as an eye glasses frame with sides 20 for supporting the device relative to ears of the user. Other forms of support arrangement may also be used, including but not limited to, head bands, visors or devices suspended from helmets.

[0038] Reference is made herein in the drawings and claims to an X axis which extends horizontally (FIG. 1A) or vertically (FIG. IB), in the general extensional direction of the first region of the LOE, and a Y axis which extends perpendicular thereto, i.e., vertically in FIG. 1 A and horizontally in FIG. IB.

[0039] In very approximate terms, the first LOE, or first region 16 of LOE 12, may 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. It should be noted that the orientation as illustrated in FIG. 1A may be regarded as a “top-down” implementation, where the image illumination entering the main (second region) of the LOE enters from the top edge, whereas the orientation illustrated in FIG. IB may be regarded as a “side-injection” implementation, where the axis referred to here as the Y axis is deployed horizontally. In the remaining drawings, the various features of certain embodiments of the present invention will be illustrated in the context of a “top-down” orientation, similar to FIG. 1A. However, it should be appreciated that all of those features are equally applicable to side-injection implementations, which also fall within the scope of the invention. In certain cases, other intermediate orientations are also applicable, and are included within the scope of the present invention except where explicitly excluded. Although illustrated herein in the context of an LOE which achieves two-dimensional expansion, it should be noted that the present invention is also applicable to devices in which an LOE performs only a single dimension of expansion. For simplicity of presentation in the drawings presented below, only a single dimension of expansion will be shown, but it will be understood in each case that the principles described apply equally to two-dimensional aperture expansion lightguides.

[0040] The image injected into the lightguides of the present invention is preferably a collimated image, i.e., in which the light of each image pixel is a parallel beam, collimated to infinity, with an angular direction corresponding to the pixel position. The image illumination thus spans a range of angles corresponding to an angular field of view in two dimensions.

[0041] It will be appreciated that the near-eye display 10 includes various additional components, typically including a controller 22 (FIG. 1A-1B) for actuating the image projector 14, typically employing electrical power from a small onboard battery (not shown) or some other suitable power source. It will be appreciated that controller 22 includes all necessary electronic components such as at least one processor or processing circuitry to drive the image projector, all as is known in the art. For aesthetic reasons, the waveguide of an augmented reality (AR) near-eye display often has non-normal orientation relative to the user’s eye. It is desirable to design AR glasses that are as similar as possible to conventional eye glasses. Consequently, the out- coupled projected image optical axis is not normal to the waveguide surface. This generates linear chromatic aberrations along the projected image, causing the colors of the image to appear shifted so that the user sees a shifted image duplicated in different colors. Where each color is generated with a spread of wavelengths, the linear chromatic aberration also increases the point spread function (PSF) of each color, reducing image quality.

[0042] The two sources of linear chromatic aberration resulting from a typical deployment of a near-eye display on the face of a user are illustrated schematically in FIGS. 2 A and 2B. Firstly, as shown in the top view of FIG. 2 A, fitting of a near-eye display to the curvature of the human face typically requires the LOE 12 to be arranged at a “face-curve tilt” relative to the primary viewing direction (center field) of the projected image. This dictates that a projected image must exit from the LOE surface at an oblique (nonperpendicular) angle, thereby giving rise to chromatic dispersion at the LOE-air boundary.

[0043] Additionally, as illustrated in the side view of FIG. 2B, a near-eye display is often deployed with a pantoscopic tilt, such that the lower edge of the LOE is closer to the face than the upper edge. This too dictates an oblique (non-perpendicular) exit angle of the center field of the projected image exiting from the LOE surface, thereby giving rise to chromatic dispersion at the LOE-air boundary. These two effects may occur separately, or may be combined, giving rise to an overall linear chromatic aberration which varies both horizontally and vertically across the field of view of the image.

[0044] An aspect of the present invention relates to a particularly compact optical architecture, illustrated below with reference to FIGS. 3A-6, which allows integration of a projector assembly with the lightguide in such a way as to allow introduction of a compensatory chromatic aberration without an increase in size or addition of components. By introducing such compensatory chromatic aberrations intentionally into the image propagating within the lightguide, chromatic aberrations generated on exit from the waveguide are at least partially cancelled, resulting in an out-coupled image reaching the user’s eye with reduced chromatic aberration.

[0045] The image projector assembly 14, further detailed in various implementations below, may be implemented using a wide range of image generating technologies. In one subset of implementations, the projector assembly includes at least one light source (not shown), typically deployed to illuminate a spatial light modulator 30, such as a front-lit LCOS chip or a back-lit LCD panel. The spatial light modulator modulates the projected intensity of each pixel of the image, either directly or by modifying a polarization property which modulates the intensity delivered by a polarizing beam splitter, thereby generating an image. Another option is the use of a light-generating display, such as an OLED microdisplay or a micro-LED array. Alternatively, the image projector may include a scanning arrangement, typically implemented using a fast-scanning mirror, which scans illumination from a laser light source across an image plane of the projector while the intensity of the beam is varied synchronously with the motion on a pixel-by-pixel basis, thereby projecting a desired intensity for each pixel. Collimating optics 32 are provided to generate an output projected image which is collimated to infinity. A field lens 34 may be provided adjacent to the image generator. In preferred implementations of the present invention, some or all of the above components are associated with surfaces of one or more polarizing beamsplitter (PBS) prism arrangement including a PBS surface 36 deployed at the interface between a first PBS prism component 38 and a second PBS prism component 40. Various configurations of these components according to corresponding implementations of the present invention will be discussed below.

[0046] Turning now to a preferred but non-limiting implementation of a display according to the teachings of an embodiment of the present invention, reference is made to FIG. 3A, which illustrates a schematic side view of the core optical architecture. The display includes a lightguide 12 formed from a transparent material having mutually -parallel major surfaces, within which image light is supported for propagation by internal reflection. An image generator is provided for generating an image at an image plane. In the non-limiting exemplary implementation shown here, the image generator is based on a polarizationmodifying reflective spatial light modulator (SLM) 30 such as a liquid crystal on silicon (LCOS), although other technologies may be used, as already mentioned and as further described below.

[0047] A collimating optical arrangement including a reflective lens 32 and an associated quarter- wave phase plate 31 is deployed so as to project the generated image as a collimated beam. The light path between the image generator and the lightguide is defined using a polarizing beam splitter (PBS) arrangement. The PBS arrangement includes a PBS surface 36 deployed at an interface between a first PBS prism component 38 and a second PBS prism component 40, where the second PBS prism component 40 is optically bonded, preferably by index-matched optical adhesive and / or anti-reflective coatings 33, to one of the major surfaces of the lightguide 12. In the first preferred implementation illustrated here, an image input surface 60 of the first PBS prism component 38, i.e., the surface with which the image plane of the image generator is associate, is parallel to the major surface of the lightguide 12.

[0048] The image light from the image generator (here SLM 30) enters image input surface 60 and passes through PBS surface 36 and a thickness of lightguide 12 to reach the collimating optical arrangement, where it is collimated and reflected by lens (or lens group) 32 and its polarization is switched by passing twice through quarter-wave plate 31. The reflected collimated image then passes back through the thickness of the lightguide and through second PBS prism component 40, this time being reflected at PBS surface 36 to an oblique angle so as to enter lightguide 12 from second PBS prism component 40 at an oblique angle that satisfies the conditions for total internal reflection, thus propagating within the lightguide for subsequent coupling out of the lightguide by a suitable coupling- out configuration (exemplified here by partially-reflecting internal surfaces or “facets” 19) towards an eye motion box 50 for viewing by a user.

[0049] In this context, it should be noted that the term “lightguide” refers to a transparent optical element supporting internal reflection between generally planar major surfaces. Other terms such as “waveguide,” “substrate,” or “slab” may be used in the field, and are considered generally equivalent unless a specific distinction is made. For clarity and consistency, the present description uses the term “lightguide” throughout.

[0050] In this example and subsequent examples, the image light propagation within lightguide 12 is shown simply as one-dimensional expansion, without redirection within the lightguide prior to coupling out. This representation has been chosen to simplify the visual representation and facilitate an understanding on the principles of the present invention, but it should be understood that embodiments of the present invention may equally, and in fact primarily, be implemented with a lightguide configuration which achieves two-dimensional aperture expansion, as was illustrated above with reference to FIGS. 1A and IB. According to certain preferred embodiments, the lightguide is formed from a material having a first Abbe number, and at least one of the first PBS prism component 38 and the second PBS prism component 40 is formed from a material having a second Abbe number that differs from the first Abbe number. The orientation and interface between the components are selected to introduce a chromatic dispersion that at least partially compensates for the wavelength-dependent deviation introduced upon non-orthogonal exit from the lightguide. In particular, the geometry shown in FIG. 3A is configured to introduce a linear chromatic dispersion into the image light within the lightguide, in a direction chosen to oppose the chromatic aberration associated with non-normal exit from the lightguide surface, as described above with reference to FIGS. 2 A and 2B.

[0051] In this configuration, the PBS arrangement serves multiple roles: directing the image light between components of the image projector assembly, controlling polarization, and coupling the image into the lightguide. The PBS surface 36 in this case may be implemented using various different PBS technologies, including but not limited to, a dielectric PBS and a wire-grid or other structural polarizing reflector. In the case of a dielectric PBS, the arrangement will typically provide high transmission of P-polarization and high reflection of S -polarization. In the case of a structural polarizer, the arrangement may be designed to work with transmission of P-polarization and reflection of S, or the reverse, according to other design considerations. The orientation of the PBS surface is chosen according to the desired range of propagation angles of the image light within the lightguide, bearing in mind that the chief ray of the image and the entire range of the field about that chief ray should undergo internal reflection within the lightguide and should not cross the central plane of the lightguide. The PBS surface inclination to the lightguide surface, corresponding to the apex angle of second PBS prism component 40 (referred to below as A2), is preferably in the range of 25-40 degrees, and typically in the range of 27- 35 degrees. One typical example is within a couple of degrees either side of 30 degrees.

[0052] The physical configuration of the optical materials and the relative Abbe numbers are chosen so that the net chromatic dispersion imparted by the PBS prisms and the bonded interface introduces a pre-compensation of the wavelength-dependent deviation experienced at the exit surface of the lightguide.

[0053] In the first preferred but non-limiting implementation illustrated in FIG. 3A, the collimating optical arrangement is associated with the opposite major surface of the lightguide 12 from that to which the PBS assembly is bonded. In this configuration, the light path passes from the PBS surface 36 through the thickness of the lightguide to the collimating optical arrangement, and then returns back through the thickness of the lightguide for a second interaction with the PBS surface. The image light is thereby redirected into the second PBS prism component 40 and enters the lightguide 12 at an oblique angle, suitable for propagation by total internal reflection between the major surfaces of the lightguide.

[0054] An interface 35 between collimating lens 32 and the adjacent surface of the lightguide is configured to preserve internal reflection. In some preferred but non-limiting implementations, interface 35 may be implemented using an air gap, a low-index adhesive, a structured “moth-eye” surface (micro-structure with tiny projecting features which maintains a small effective airgap), an aerogel layer, or a multilayer dielectric coating designed to exhibit angular selectivity that approximates to TIR properties. Each of these options enables efficient transmission of the image light reaching and reflected from the collimating optical arrangement (at small incident angles) while ensuring internal reflection at the interface for a first reflection of the light coupled into the lightguide at angles suitable for propagation by internal reflection, thereby maintaining the intended beam path and preserving optical efficiency.

[0055] In some implementations, both the first PBS prism component 38 and the second PBS prism component 40 are formed from the same material having the second Abbe number. In the implementation of FIG. 3A, matching of properties of the two prism components as well as orthogonal entry into the lightguide surface are important to avoid introducing chromatic aberrations in the light path prior to collimation of the image light. In other implementations exemplified below, only one of the PBS prism components may differ in Abbe number from the lightguide, or both may differ from both the lightguide and from each other, as discussed further below.

[0056] The first PBS prism component 38 preferably presents an outer surface that is parallel to the adjacent major surface of the lightguide, and the image plane of the image generator is optically associated with this outer surface. In the reflective SLM-based implementation shown, the SLM 30 is deployed adjacent to this surface, and a field lens 34 may be interposed between the image plane and the outer surface to improve optical performance. The field lens helps shape the angular distribution of the projected rays, enhancing collimation and optimizing coupling into the downstream optics.

[0057] The image generator, in this case, includes a reflective polarization-modifying spatial light modulator 30, illuminated by an optical path that directs polarized illumination via the PBS surface 36. The polarization state is selected to be reflected at the PBS surface during the inbound path, and is modulated by the SLM so that it is transmitted through the PBS surface on the return path toward the collimating optics.

[0058] An illumination arrangement is configured to deliver polarized illumination of an orientation that is reflected by the PBS surface unless modified by SLM 30 to form part of the image. To achieve the correct direction of illumination in view of the PBS surface orientation, the illumination preferably undergoes an internal reflection at the outer surface (image input surface) 60 of first PBS prism component 38, prior to reflection at PBS surface 36.

[0059] In some cases, one or more additional reflective surface 46 may be used to guide the illumination path into the desired geometry, as exemplified in FIG. 5 below. In each of these cases, the illumination beam is typically expanded to match the active area of the SLM by illumination optics (not shown) as is known in the art. Reflective surface 46 may be provided integrally by a suitably shaped first PBS prism component 38 or may be provided by an adjacent optical element. In either case, the geometry is selected such that the illumination beam undergoes one or more reflections, including at least one internal reflection, before reaching the PBS surface and being directed toward the image generator. These alternative options for folded illumination geometry provide design flexibility for various form-factors of implementation, supporting a compact and ergonomic design of the projector assembly while satisfying polarization conditions and maintaining optical efficiency.

[0060] Reference is now made to FIG. 3B, which provides a schematic representation of the chromatic compensation principle implemented in the display architecture of FIG. 3A. The geometry illustrated in FIG. 3B simplifies the relevant components to an optically equivalent pair of oppositely-oriented wedge prisms traversed by the image light. A first prism of a material with a second Abbe number, corresponding to the second PBS prism component 40, receives the collimated image light approximately perpendicularly. The beam then crosses an interface into a second prism of material with a first Abbe number, corresponding to the lightguide 12, and exits at an oblique angle. The angle of the interface is governed by prism apex angle A2 so as to correspond to the angle at which the chief ray of the image passes through interface 33 of FIG. 3A, corresponding to twice the wedge angle of prism component 40 (not shown here to scale). Angle Al is determined by the propagation angle and the facet angle so that the chief ray exit angle corresponds to the desired exit angle from the lightguide surface towards the EMB 50.

[0061] This configuration introduces a controlled chromatic dispersion into the image light that propagates within the lightguide. By appropriate selection of prism materials and angles, the induced chromatic deviation may be used to at least partially compensate for the linear chromatic aberration generated when the image exits from the lightguide surface at an oblique angle, as illustrated in FIGS. 2A and 2B. The net effect is a reduction in perceived chromatic shift and color fringing in the final projected image.

[0062] The design of such a system, including the appropriate choice of material properties and wedge angle for the PBS prism components for a given lightguide material and coupling-out geometry, may be approached using various levels of approximation or numerical precision, depending on the accuracy required and the specific implementation constraints. Three exemplary approaches are summarized below. It should be noted that the efficacy of the present invention is not dependent on the technical accuracy of the analysis presented below or of any of these exemplary approaches alone. Even if the theoretical analysis presented below is found to be in error, optimization of materials selection and other design parameters may be optimized according to the teachings of the present invention by numerical methods, such as along the lines of approach (3) below, or by alternative numerical or empirical approaches.

[0063] (1) Paraxial approximation:

[0064] In the case of small prism angles and near-normal incidence, an achromatic prism pair may be designed using the simplified relation: where in, n are the refractive indices of the prism materials, Vi, V2 are their Abbe numbers, and Ai, A2 are the prism apex angles. This relation provides a first-order approximation for balancing the chromatic dispersion contributions of the two materials.

[0065] (2) Exact angular deviation formulation: A more precise formulation takes into account the full ray-tracing equations, including the angle of incidence. The chromatic deviation 8(1, i) through a single prism of apex angle A, refractive index n( ), and incident angle z is calculated as:

[0066] /

[0067] <5( A, ?') ~ i ■■■■ .4 4- arcsin nl Al * sin A -- arcsln

[0068] For an achromatic combination of two prisms, the condition for compensation requires that the wavelength derivatives of the respective deviations cancel:

[0069] This approach accounts for higher-order dispersion and angular effects and is suitable for non-paraxial configurations, but may be complex to solve analytically. Numerical solution techniques may be employed.

[0070] (3) Numerical optimization:

[0071] In most practical scenarios, an achromatic system is best designed using numerical simulation and optimization. This typically involves:

[0072] • Defining candidate materials and geometries (refractive indices, Abbe numbers, apex angles, and incidence angles),

[0073] • Simulating wavelength-dependent deviation across the optical band of interest,

[0074] • Iteratively adjusting parameters to minimize residual chromatic dispersion, often using commercial optical design software.

[0075] This method enables accurate tuning of the chromatic correction for specific application constraints, including off-axis geometries and wide spectral ranges.

[0076] By way of non-limiting example, for a lightguide formed from BK7 glass, a suitable compensating material for the PBS prism may be selected from a class of glasses such as N-BK10, depending on the chief ray incidence angle and desired level of compensation. The geometrical configuration, including wedge angles and bonded interface orientation, is then optimized accordingly.

[0077] Reference is now made to FIGS. 4A and 4B, which illustrate two variant configurations of the display architecture of FIG. 3A, differing in the placement of a polarization-modifying element between the PBS arrangement and the lightguide. These implementation options support additional design flexibility for polarization management and are particularly useful in configurations where the PBS operates with a specific polarization state (e.g., high / .s and Tp operation) which does not match the optimal polarization input state for the lightguide design.

[0078] In FIG. 4A, a halfwave retarder plate 37 is interposed between the PBS surface and the second PBS prism component 40. This configuration allows rotation of the polarization state of the image light after modulation by the image generator and before entry into the second PBS prism component so that p-polarization rather than s-polarization is injected into the lightguide. It will be noted that the halfwave retarder plate does not impact the functionality of the light path to and from the collimating optics since the light passes twice through the halfwave plate before interacting with the PBS surface, cancelling out the effect of this element.

[0079] In FIG. 4B, an alternative configuration is shown in which the halfwave retarder plate 37 is deployed between the second PBS prism component 40 and the adjacent surface of the lightguide 12. This arrangement similarly rotates the polarization state of the image light prior to entry into the lightguide, but provides an alternative optical geometry which may be beneficial in certain cases.

[0080] In both configurations, the option of including a halfwave retarder plate provides an additional degree of design flexibility to optimize integration of the projector components with the lightguide in view of various design considerations, such as the optimal polarization for various reflective coatings according to a given lightguide architecture. The two implementations illustrated here are non-exclusive of other possible options for deploying a halfwave retarder interposed between the PBS surface and the first major surface of the lightguide.

[0081] As already mentioned above, FIG. 5 illustrates a further variant configuration of the display architecture of FIG. 3A, distinguished by a modified illumination geometry. In this implementation, the polarized illumination is directed to the image generator 30 via an additional reflection prior to reaching the outer surface of the first PBS prism component 38. This additional reflection may be implemented using either a mirror or a surface 46 configured for total internal reflection (TIR), depending on the optical layout and polarization requirements.

[0082] The introduction of this additional reflection allows for greater flexibility in folding the illumination path and may support a more compact arrangement of the light source relative to the rest of the projector assembly. The image modulation and coupling path remains unchanged from that described above with reference to FIG. 3A, and the chromatic compensation mechanism is maintained.

[0083] Reference is now made to FIG. 6, which illustrates an alternative embodiment of a display 10 according to the teachings of an embodiment of the present invention. In this configuration, the image generator and collimating optics are both associated with surfaces of the PBS prism arrangement which are located on the same side of the lightguide and obliquely angled to the lightguide major surfaces, resulting in an optical axis that is obliquely angled to the major surfaces of the lightguide 12.

[0084] The display includes a first PBS prism component 38 having an image input surface 60 and an optics interface surface 62, which are non-parallel. The image generator 70 is deployed to introduce image light through image input surface 60, optionally via a field lens 34, and a collimating optical arrangement (reflective collimating lens 32 and quarterwave plate 31) is associated with the optics interface surface 62. PBS surface 36 is oriented so as to direct the image light from image generator 70 towards the collimating optical arrangement and to reflect a collimated image with rotated polarization along an optical axis 74. This optical axis is angled obliquely with respect to the plane of the lightguide, and the resulting beam path proceeds via transmission through PBS surface 36 and second PBS prism component 40 into the lightguide 12, as described above.

[0085] This architecture is particularly suited for use with an active image generator matrix, such as a micro-LED matrix 70, deployed at the image plane, thereby avoiding any need for a separate illumination arrangement.

[0086] The use of this optical architecture achieves the desired image ray injection angles through the oblique alignment of the optical axis 74, thereby allowing greater design freedom in the choice of the PBS surface orientation and the corresponding wedge angles of the PBS prism components. Additionally, since the light path prior to collimation does not enter the second PBS prism component 40, this configuration provides additional design flexibility of allowing the two PBS prism components to be of different materials with differing Abbe numbers.

[0087] Thus, in some cases, second PBS prism component 40 may here be formed from the same material as the lightguide 12, while the first PBS prism component 38 is formed from a material having a different (i.e., “the second”) Abbe number. In other cases, the second PBS prism component 40 may be implemented from a material having a different Abbe number from the lightguide, which may be the same as that of first PBS prism component 38 or may differ therefrom. These multiple design options support different compensation strategies, and the choice between them may be made based on available materials, required dispersion profiles, and manufacturing considerations. As in previously described embodiments, the various optical elements are selected and configured so that the image light undergoes an intentional chromatic dispersion within the lightguide that at least partially offsets the chromatic aberration resulting from oblique exit at the out-coupling surface.

[0088] The embodiments of FIGS. 3 A and 6 together exemplify the principles of the present invention may be applied in a range of different architectures based on a compact PBS prism arrangement integrated with a lightguide to achieve at least partial compensation for chromatic aberration. In particular, the range of possible implementations include both cases in which the last interaction of the image illumination before entering the lightguide is reflection (FIG. 3A) and transmission (FIG. 6). It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A display comprising:(a) a lightguide formed from transparent material having mutually-parallel first and second major surfaces for supporting propagation of image light by internal reflection at said major surfaces;(b) an image generator for generating an image at an image plane;(c) a collimating optical arrangement including a reflective lens and an associated quarter- wave phase plate; and(d) a polarizing beam splitter (PBS) arrangement comprising a PBS surface at an interface between a first PBS prism component and a second PBS prism component, said second PBS prism component being optically bonded to said first major surface, said PBS arrangement defining a light path from said image generator to said collimating optical arrangement and from said collimating optical arrangement through at least said second PBS prism into said lightguide at a propagation angle that undergoes internal reflection at said major surfaces, wherein said lightguide is formed from a material having a first Abbe number, and wherein at least one of said first and second PBS prism components is formed from a material having a second Abbe number different from said first Abbe number so as to introduce a compensatory chromatic dispersion into the image light propagating within said lightguide.

2. The display of claim 1, wherein said collimating optical arrangement is associated with said second major surface of said lightguide and wherein said light path passes from said PBS surface through a thickness of said lightguide to said collimating optical arrangement, back through the thickness of said lightguide and is reflected at said PBS surface to reenter said lightguide at an oblique angle for propagation within said lightguide.

3. The display of claim 2, wherein said collimating optical arrangement is associated with said second major surface of said lightguide via an internal-reflectionpreserving interface.

4. The display of claim 2, wherein both said first PBS prism component and said second PBS prism component are formed from said material with said second Abbe number.

5. The display of claim 4, wherein said first PBS prism component presents an outer surface parallel to said first major surface and wherein said image plane is associated with said outer surface.

6. The display of claim 5, further comprising a field lens interposed between said image plane and said outer surface.

7. The display of claim 5, wherein said image generator comprises a reflective polarization-modifying spatial light modulator (SLM) located at said image plane and an illumination arrangement for delivering polarized illumination to said SLM via reflection at said PBS surface.

8. The display of claim 7, wherein said illumination arrangement is configured to deliver said polarized illumination via internal reflection at said outer surface prior to reflection at said PBS surface.

9. The display of claim 8, wherein said first PBS prism component or an adjacent optical element provides a reflective surface, said illumination arrangement being configured to deliver said polarized illumination via reflection at said reflective surface prior to internal reflection at said outer surface and at said PBS surface.

10. The display of claim 1, wherein said first PBS prism component presents an image input surface with which said image generator is associated and an optics interface surface with which said collimating optical arrangement is associated, the light path passing from said image generator via reflection at said PBS surface to said collimating optical arrangement and then via transmission at said PBS surface through said second PBS prism component into said lightguide, wherein said optics interface surface is oriented sothat an optical axis of said collimating optical arrangement is obliquely angled to said first major surface of said lightguide.

11. The display of claim 10, wherein at least said second PBS prism component is formed from said material having said second Abbe number.

12. The display of claim 10, wherein said second PBS prism component is formed from the same material as said lightguide and wherein said first PBS prism component is formed from said material having said second Abbe number.

13. The display of claim 1, further comprising a halfwave retarder plate deployed between said PBS surface and said first major surface.

14. The display of claim 13, wherein said halfwave retarder plate is interposed between said PBS surface and said second PBS prism component.

15. The display of claim 13, wherein said halfwave retarder plate is interposed between second PBS prism component and said first major surface of said lightguide.

16. The display of claim 1, wherein said image generator comprises a micro-LED matrix deployed at said image plane.

Images