Optical system for conveying an image to an eye of a viewer

Abstract

An optical system (100) for conveying light corresponding to an image to an eye of a viewer has a first lightguide portion (120) including a progressive redirection configuration (121) for progressively redirecting light propagating within the first lightguide portion, a polarization modifying element (131) and a second lightguide portion (110) including a progressive coupling-out configuration (111) for progressively coupling-out light propagating within the second lightguide portion towards the eye of the viewer. A pair of major surfaces (101b, 102b) of the second lightguide portion (110) are parallel to, but non-coplanar with, a corresponding pair of major surfaces (101a, 102a) of the first lightguide portion (120). A dimension (T3) of the polarization-modifying element (131) perpendicular to the first and second pairs of major surfaces overlaps an entirety of both an optical thickness of the first lightguide portion (T 1) and an optical thickness of the second lightguide portion (T2).

Description

[0001] Optical System for Conveying an Image to an Eye of a Viewer

[0002] FIELD AND BACKGROUND OF THE INVENTION

[0003] The present invention relates to optical systems and, in particular, it concerns an optical system for conveying light corresponding to an image to an eye of a viewer.

[0004] Many near-eye display systems include a transparent lightguide optical element (LOE) or “waveguide” placed before the eye of the user, which conveys an image within the LOE by internal reflection and then couples out the image by a suitable output coupling mechanism towards the eye of the user. The output coupling mechanism may be based on embedded partial reflectors (also referred to as “facets”) or may employ a diffractive optical element. The description below will refer primarily to a facet-based coupling-out arrangement, but it should be appreciated that various features of the invention are also applicable to diffractive arrangements.

[0005] In some cases, the lightguide optical element may be designed to achieve two- dimensional aperture expansion. This is typically achieved by providing a first portion of the lightguide with a progressive redirection configuration (partially reflecting surfaces or a diffractive optical element) for progressively redirecting light from a first in-plane direction to a second in-plane direction and providing a second portion of the lightguide with a progressive coupling-out configuration for progressively coupling-out light propagating in the second inplane direction so as to be coupled out of the second lightguide portion towards an eye of a viewer.

[0006] SUMMARY OF THE INVENTION

[0007] The present invention is an optical system for conveying light corresponding to an image to the eye of a viewer.

[0008] According to the teachings of an embodiment of the present invention there is provided, an optical system for conveying light corresponding to an image to an eye of a viewer, the optical system comprising: (a) a first lightguide portion formed from transparent material and having a first pair of mutually-parallel major surfaces, separated by a first optical thickness, for supporting propagation of light within the first lightguide portion by internal reflection at the first pair of major surfaces and an edge, the first lightguide portion including a progressive redirection configuration for progressively redirecting light propagating within the first lightguide portion in a first in-plane direction so as to propagate by internal reflection at the first pair of major surfaces in a second in-plane direction non-parallel to the first direction towards the edge; (b) a polarization-modifying element optically connected at the edge of the first lightguide portion; and (c) a second lightguide portion formed from transparent material and having a second pair of mutually-parallel major surfaces, separated by a second optical thickness, for supporting propagation of light within the second lightguide portion by internal reflection at the second pair of major surfaces, the second lightguide portion being optically connected to the polarizationmodifying element so that a majority of the light propagating in the second in-plane direction in the first lightguide portion passes through the polarization-modifying element and continues to propagate within the second lightguide portion in the second in-plane direction, the second lightguide portion including a progressive coupling-out configuration for progressively coupling-out light propagating within the second lightguide portion in the second in-plane direction so as to be coupled out of the second lightguide portion towards the eye of the viewer, wherein each of the second pair of major surfaces is parallel to, but non-coplanar with, a corresponding one of the first pair of major surfaces, and wherein a dimension of the polarization-modifying element perpendicular to the first and second pairs of major surfaces overlaps an entirety of both the first optical thickness and the second optical thickness.

[0009] According to a further feature of an embodiment of the present invention, the second optical thickness is less than the first optical thickness such that light propagating from the first lightguide portion to the second lightguide portion encounters an inward step at both major surfaces such that an entrance to the second lightguide portion trims the light propagating in the first direction from the first lightguide portion, excluding from the second lightguide portion light which is internally reflected adjacent to the polarization-modifying element.

[0010] According to a further feature of an embodiment of the present invention, there are also provided thickness-equalizing face plates attached to the second lightguide portion so as to provide an external surface substantially coplanar with the major surfaces of the first lightguide portion, the thickness-equalizing face plates being attached to the second lightguide portion by low-index adhesive that preserves internal reflection at the second pair of major surfaces.

[0011] According to a further feature of an embodiment of the present invention, the second optical thickness is greater than the first optical thickness such that light propagating from the first lightguide portion to the second lightguide portion encounters an outward step at both major surfaces and an exit from the first lightguide portion defines a location of first internal reflection within the second lightguide portion that is beyond the polarization-modifying element.

[0012] According to a further feature of an embodiment of the present invention, there are also provided thickness-equalizing face plates attached to the first lightguide portion so as to provide an external surface substantially coplanar with the major surfaces of the second lightguide portion, the thickness-equalizing face plates being attached to the first lightguide portion by low- index adhesive that preserves internal reflection at the first pair of major surfaces.

[0013] According to a further feature of an embodiment of the present invention, the second lightguide portion is offset from the first lightguide portion in a direction perpendicular to the first and second major surfaces such that light propagating from the first lightguide portion to the second lightguide portion encounters an inward step at one of the major surfaces and an outward step at another of the major surfaces.

[0014] According to a further feature of an embodiment of the present invention, there are also provided a first thickness-equalizing face plate attached to the first lightguide portion and a second thickness-equalizing face plate attached to the second lightguide portion so as to provide substantially planar continuous external surfaces across the first and second lightguide portions, the thickness-equalizing face plates being attached to the first and second lightguide portions by low-index adhesive that preserves internal reflection at the first and second pairs of major surfaces.

[0015] According to a further feature of an embodiment of the present invention, the polarization-modifying element is a halfwave retarder plate deployed to convert p-polarization propagating within the first lightguide portion into s-polarization propagating within the second lightguide portion.

[0016] According to a further feature of an embodiment of the present invention, the polarization-modifying element has a layer thickness and wherein an offset between each of the second pair of major surfaces and a corresponding one of the first pair of major surfaces is at least half of the layer thickness and no more than twice the layer thickness.

[0017] According to a further feature of an embodiment of the present invention, the progressive redirection configuration comprises a first plurality of mutually parallel partially reflecting surfaces deployed within the first lightguide portion non-parallel to the first pair of major surfaces.

[0018] According to a further feature of an embodiment of the present invention, the progressive coupling-out configuration comprises a second plurality of mutually parallel partially reflecting surfaces deployed within the second lightguide portion obliquely to the second pair of major surfaces.

[0019] In a matter of terminology, the phrase “in-plane direction” is used to describe a direction of propagation of a ray within a lightguide corresponding to the in-plane component of the ray as it is repeatedly internally reflected at the major surfaces. At each reflection, the in-plane component parallel to the major surfaces remains unchanged while the component perpendicular to the major surfaces is reversed. The in-plane direction also corresponds to the ray path as viewed in plan view along a line of sight perpendicular to the major surfaces.

[0020] BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0022] FIGS. 1 A and IB are schematic isometric views of a display, constructed and operative according to the teachings of the present invention, employing an optical system having first and second lightguide portions for two-stage optical aperture expansion illustrating a top-down and a side-injection configuration, respectively;

[0023] FIG. 2A is a schematic isometric view illustrating a sample ray path through the optical system of FIG. IB (or a rotated view of that of FIG. 1A);

[0024] FIG. 2B is a schematic edge view of the optical system of FIG. 2A implemented according to a conventional approach;

[0025] FIG. 2C is an enlarged view of the region of FIG. 2B designated I;

[0026] FIG. 3A is a schematic edge view of the optical system of FIG. 2A implemented according to a first aspect of the present invention;

[0027] FIG. 3B is an enlarged view of the region of FIG. 3 A designated II;

[0028] FIG. 3C is a schematic edge view similar to FIG. 3 A illustrating a variant implementation of the optical system including thickness-equalizing face plates;

[0029] FIG. 3D is an enlarged view of the region of FIG. 3C designated III;

[0030] FIG. 4A is a schematic edge view of the optical system of FIG. 2A implemented according to a second aspect of the present invention;

[0031] FIG. 4B is an enlarged view of the region of FIG. 4 A designated IV;

[0032] FIG. 4C is a schematic edge view similar to FIG. 4A illustrating a variant implementation of the optical system including thickness-equalizing face plates;

[0033] FIG. 4D is an enlarged view of the region of FIG. 4C designated V;

[0034] FIG. 4E is a schematic edge view similar to FIG. 4C illustrating a further variant implementation integrating a homogenizing internal partial reflector;

[0035] FIG. 5A is a schematic edge view of the optical system of FIG. 2A implemented according to a third aspect of the present invention;

[0036] FIGS. 5B and 5C are enlarged views of the regions of FIG. 5A designated VI and VII, respectively; FIG. 5D is a schematic edge view similar to FIG. 5A illustrating a variant implementation of the optical system including thickness-equalizing face plates; and

[0037] FIGS. 5E and 5F are enlarged views of the regions of FIG. 5D designated VIII and IX, respectively.

[0038] DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] The present invention is an optical system for conveying light corresponding to an image to the eye of a viewer.

[0040] The principles and operation of optical systems according to the present invention may be better understood with reference to the drawings and the accompanying description.

[0041] Referring now to the drawings, FIGS. 1A and IB illustrate a head mounted display, generally designated 10, constructed and operative according to the teachings of an embodiment of the present invention, employing a pair of optical systems (interchangeably referred to as “lightguide optical elements” and “slab optical waveguide structures”) 100 each having a first lightguide portion 120 and a second lightguide portion 110 for two-stage optical aperture expansion in an X direction followed by a Y direction. In the case of FIG. 1A, first lightguide portion 120 is deployed along the top of the optical system for horizontal aperture expansion while redirecting the image light downwards and second lightguide portion 110 achieves vertical aperture expansion while coupling out of the image light towards an “eye motion box” (EMB), corresponding to the range of eye positions from which the displayed image is to be viewed. In FIG. IB, the sequence of these directions of optical aperture expansion is switched.

[0042] Head mounted display 10 typically includes a projector 200 (for each single eye display) that generates an image which is coupled into lightguide optical element (LOE) 100. Various coupling configurations may be used for coupling the image light into the LOE, typically using a coupling prism and / or a coupling reflector. The choice of coupling configuration and details of its implementation are not critical to the present invention and will not be described here in detail. The projector is controlled by an electronic controller 300. The entire display is preferably supported by a support structure which, in this case, is illustrated as a glasses-frame form factor support including side supports 400 for supporting the display relative to the head (e.g., nose and ears) of the user.

[0043] FIG. 2A illustrates schematically the internal structure and operation of LOE 100 according to one non-limiting exemplary implementation of the present invention. First lightguide portion 120, formed from transparent material, has a first pair of mutually parallel major surfaces (front and back surfaces in FIG. 2A which are labeled 101 and 102 or 101a and 102a in the subsequent drawings), separated by a first optical thickness Tl. These surfaces support propagation of light within the first lightguide portion by internal reflection. First lightguide portion 120 includes a progressive redirection configuration, here implemented as a set of mutually -parallel internal partially reflecting surfaces 121 non-parallel to the major surfaces, for progressively redirecting light propagating within the first lightguide portion in a first in-plane direction so as to propagate by internal reflection at the first pair of major surfaces in a second in-plane direction non-parallel to the first direction, towards an edge of first lightguide portion.

[0044] Second lightguide portion 110, formed from transparent material, has a second pair of mutually parallel major surfaces (front and back surfaces in FIG. 2A which are labeled 101 and 102 or 101b and 102b in the subsequent drawings), separated by a second optical thickness T2. These surfaces support propagation of light within the second lightguide portion by internal reflection. Second lightguide portion 110 includes a progressive coupling-out configuration, here implemented as a set of mutually-parallel internal partially reflecting surfaces 111 obliquely angled to the major surfaces, for progressively coupling-out light propagating within the second lightguide portion in the second in-plane direction so as to be coupled out of the second lightguide portion towards the eye of the viewer (EMB).

[0045] In many cases, the progressive redirection configuration (e.g., internal partially reflecting surfaces) 121 and the progressive coupling-out configuration (e.g., partially reflecting surfaces) 111 are polarization dependent, and the polarization most suited for one may be problematic, or at least non-optimal, for the other. For example, in many cases, it is preferable to work with s- polarization for reflective elements in order to avoid limitations on reflectance of p-polarization around Brewster’s Angle. However, due to the differing orientations of the partially reflecting surfaces 121 and 111, a representative ray 1001 illustrated in FIG. 2A as introduced by projector 200 as s-polarized relative to surfaces 121 would generally correspond to a mixed s and p- polarization relative to surfaces 111. To address this, a polarization-modifying element 131, typically implemented as a halfwave retarder (waveplate), is optically connected between the adjacent edges of first lightguide portion 120 and second lightguide portion 110. The result is illustrated in FIG. 2A by dots for polarization into the page and arrows for polarization in the plane of the page. The ray propagating within lightguide portion 120 is p-polarized relative to the major surfaces, which corresponds to mostly s-polarization relative to partially reflecting surfaces 121. After redirection by reflection at one of surfaces 121 towards the edge of first lightguide portion 120, the ray passes through retarder 131 which converts the ray to s- polarization relative to partially reflecting surfaces 111. A default implementation of this structure, as illustrated in FIGS. 2B and 2C, would be to implement first lightguide portion 120, retarder 131 and second lightguide portion 110 with continuous planar major surfaces 101 and 102 that extend across the entire LOE 100. It has been found, however, that such an implementation presents challenges. Since the waveplate 131 typically has mechanical properties that are different than the surrounding glass, mechanical processing of the glass often results in sudden discontinuities (such as different thickness of 131 as compared to the surrounding glass) or rough surfaces. This results in a low optical quality surface 132 on the boundary between the waveplate 131 and air, which may cause deleterious artifacts that reduce output image quality.

[0046] To address this issue, according to an aspect of the present invention, each of the second pair of major surfaces 101b and 102b is parallel to, but non-coplanar with, a corresponding one of the first pair of major surfaces 101a and 102a. In other words, a step is formed between the major surface of the first lightguide portion and the major surface of the second lightguide portion. This step may be an inwards step (in the direction of propagation), as will be exemplified with reference to FIGS. 3 A-3D and 5B, or an outwards step, as will be exemplified with reference to FIGS. 4A-4E and 5C. In each case, ray paths of light propagating via waveplate 131 are modified by the presence of the step so as to avoid any contribution to the output image from rays internally reflected at low optical quality surface 132. The optical mechanism for each case will be explained below. A dimension T3 of the polarization-modifying element perpendicular to the first and second pairs of major surfaces preferably overlaps an entirety of both the first optical thickness T1 and the second optical thickness T2.

[0047] Referring now to the non-limiting exemplary implementation of FIGS. 3 A and 3B, in this case, second optical thickness T2 is less than first optical thickness T1 such that light propagating from first lightguide portion 120 to second lightguide portion 110 encounters an inward step at both major surfaces. The result of this geometry, best understood with reference to FIG. 3B, is that an entrance to the second lightguide portion (defined by the beginning of major surfaces 101b, 102b) trims the light propagating in the first direction from first lightguide portion 120, thereby excluding light (e.g., sample ray 1031) which is internally reflected adjacent to the polarization-modifying element from second lightguide portion 110. The remaining rays, such as ray 1031', propagate freely through waveplate 131 and continue to provide the coupled out image, all as described above.

[0048] The light which is trimmed typically escapes the lightguide and is lost. The direction of the lost light is typically at angles that are unlikely to reach the user’s eye and will therefore not adversely affect the quality of the viewed image or otherwise disturb the user. Trimming of the image light results in some reduction in efficiency, but depending on the size of the step, the losses are typically relatively small, for example, less than 5%.

[0049] The preferred size AT of the step (offset) at each face (which in symmetrical implementations corresponds to |Tl-T2| / 2) is determined by geometrical considerations to ensure that the steepest rays propagating along the LOE and reflecting internally from surface 132 are trimmed so as not to enter second lightguide portion 110. Thus, AT is a function of a layer thickness d of the waveplate 131, i.e., the thickness in the in-plane direction of propagation, and the angle of the steepest guided ray 1031 with relation to major surfaces of the waveguide. In a range of implementations, an offset AT between each major surface 101b, 102b and its corresponding major surface 101a, 102a is at least half of layer thickness d. The offset AT is also preferably no more than twice the layer thickness d. Larger steps are typically undesirable since they are unnecessary for the trimming function and simply result in excess trimming of the propagating image light and consequent inefficiency. A typical preferred choice for the step size is roughly equal (±15%) to the layer thickness of the waveplate. The thickness d of the waveplate will depend on the optical design and the choice of material of the waveplate. In the case of a quartz halfwave retarder, a thickness of roughly 30 microns is typically used. For a polymer retarder, a thickness of around 60 microns may be used.

[0050] FIGS. 3C and 3D illustrate a variation of the implementation of FIGS. 3A and 3B. In certain cases, it may be desirable to avoid a step in the external surfaces of the LOE, which may cause artifacts in the user’s view of the real world scene through the LOE and / or may collect dirt and be difficult to clean properly. To achieve a smooth external surface, certain preferred implementations further include a thickness-equalizing face plate 140a, 140b attached to one or both major surfaces 101b, 102b of second lightguide portion 110 so as to provide an external surface 101c, 102c substantially coplanar with major surfaces 101a, 101b of first lightguide portion 120. Thickness-equalizing face plates 140a, 140b are attached to the second lightguide portion by low-index adhesive, typically with a refractive index of no more than 1.4, that preserves internal reflection at second pair of major surfaces 101b, 102b. Equivalent to this is a coating of low-index material underlying an adhesive. The effective optical thickness of second lightguide portion 110 therefore remains T2 and trimming of the light propagating from first lightguide portion 120 to second lightguide portion 110 remains as in FIGS. 3A and 3B. In this case, instead of being released from the LOE, the trimmed light is typically trapped within the face plate by internal reflection, to be absorbed at the edge of the LOE.

[0051] From a manufacturing perspective, handling of a face plate of thickness AT may not be practical. In this case, the face plates may be applied as thicker elements of dimensions that are more readily handled and bonded, and the assembled LOE precursor may then be thinned, by cutting, grinding and / or polishing, until the desired final thickness is achieved.

[0052] Turning now to a further family of implementations, FIGS. 4A and 4B illustrate an implementation in which second optical thickness T2 is greater than first optical thickness T1 such that light propagating from first lightguide portion 120 to second lightguide portion 110 encounters an outward step at both major surfaces. An exit aperture from between major surfaces 101a, 102a of first lightguide portion 120 defines a location of first internal reflection within second lightguide portion 110 that is beyond polarization-modifying element 131 (see ray 1031).

[0053] As before, in a variant implementation illustrated in FIGS. 4C and 4D, thicknessequalizing face plates 140a, 140b may be provided to avoid leaving a step in the major surfaces of the LOE. In this case, the face plates are attached to first lightguide portion 120 so as to provide an external surface substantially coplanar with major surfaces 101b, 102b of second lightguide portion 110. As before, a low-index adhesive (preferably with R.I. of 1.4 or less) or a low-index coating beneath the adhesive preserves internal reflection at major surfaces 101a and 102a. This, together with a suitable coupling-in arrangement from the projector, ensures that the implementation of FIGS. 4C and 4D is optically equivalent to the implementation of FIGS. 4A and 4B.

[0054] The considerations as to the required size of the outward offset / step generally parallel the discussion above in the context of an inward offset / step. In the case of an outward step, there is no loss in efficiency through trimming. Instead, there may be incomplete filling of the second lightguide portion with the projected image. For a step which is a small proportion of the lightguide thickness, this incomplete filling may not result in any discernable deterioration in the observed image quality. In a case in which it is desired to compensate for this incomplete filling, this may be done by addition of homogenizer (or “mixer”), preferably implemented as a midplane 50% reflector 141 as shown in FIG. 4E.

[0055] Turning finally to FIGS. 5A-5F, these illustrate that the options of FIGS. 3 A-3D and 4A- 4E are not mutually exclusive and can in fact be combined. Thus, as shown in FIGS. 5A-5C, there is shown an implementation of LOE 100 in which second lightguide portion 110 is offset from first lightguide portion 120 in a direction perpendicular to the major surfaces such that light propagating from first lightguide portion 120 to second lightguide portion 110 encounters an inward step at one of the major surfaces (top as illustrated) and an outward step at the other of the major surfaces (bottom as illustrated). Optionally, although not necessarily, the first and second thicknesses T1 and T2 are the same, so that the inward step and the outward step have the same offset height. The inward step detailed in FIG. 5B functions identically to that of FIG. 3B while the outward step detailed in FIG. 5C functions identically to that of FIG. 4B. This “hybrid” implementation may have particular advantages as a good compromise solution between the two options illustrated above: efficiency loss is halved by employing only one inward step rather than two, and incomplete filling of the lightguide is also reduced in significance.

[0056] As in the previous cases, a further optional variant implementation includes a first thickness-equalizing face plate 140b attached to the first lightguide portion 120 and a second thickness-equalizing face plate 140a attached to second lightguide portion 110 so as to provide substantially planar continuous external surfaces across the first and second lightguide portions 120, 110. The thickness-equalizing face plates are attached to first and second lightguide portions 120, 110 by low-index adhesive or a low-index coating that preserves internal reflection at the first and second pairs of major surfaces.

[0057] Although illustrated herein in particularly preferred implementations in which both the progressive redirection configuration and the progressive coupling-out configuration are implemented as sets of mutually parallel partially reflecting surfaces deployed within the respective lightguide portions, it should be noted that either one or both of these elements may be replaced by diffractive optical elements and / or any other suitable elements.

[0058] 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. An optical system for conveying light corresponding to an image to an eye of a viewer, the optical system comprising:(a) a first lightguide portion formed from transparent material and having a first pair of mutually-parallel major surfaces, separated by a first optical thickness, for supporting propagation of light within the first lightguide portion by internal reflection at the first pair of major surfaces and an edge, the first lightguide portion including a progressive redirection configuration for progressively redirecting light propagating within the first lightguide portion in a first in-plane direction so as to propagate by internal reflection at the first pair of major surfaces in a second in-plane direction non-parallel to the first direction towards the edge;(b) a polarization-modifying element optically connected at the edge of the first lightguide portion; and(c) a second lightguide portion formed from transparent material and having a second pair of mutually-parallel major surfaces, separated by a second optical thickness, for supporting propagation of light within the second lightguide portion by internal reflection at the second pair of major surfaces, the second lightguide portion being optically connected to the polarization-modifying element so that a majority of the light propagating in the second in-plane direction in the first lightguide portion passes through the polarization-modifying element and continues to propagate within the second lightguide portion in the second in-plane direction, the second lightguide portion including a progressive coupling-out configuration for progressively coupling-out light propagating within the second lightguide portion in the second in-plane direction so as to be coupled out of the second lightguide portion towards the eye of the viewer, wherein each of the second pair of major surfaces is parallel to, but non-coplanar with, a corresponding one of the first pair of major surfaces, and wherein a dimension of the polarization-modifying element perpendicular to the first and second pairs of major surfaces overlaps an entirety of both the first optical thickness and the second optical thickness.

2. The optical system of claim 1, wherein the second optical thickness is less than the first optical thickness such that light propagating from the first lightguide portion to the secondlightguide portion encounters an inward step at both major surfaces such that an entrance to the second lightguide portion trims the light propagating in the first direction from the first lightguide portion, excluding from the second lightguide portion light which is internally reflected adjacent to the polarization-modifying element.

3. The optical system of claim 2, further comprising thickness-equalizing face plates attached to the second lightguide portion so as to provide an external surface substantially coplanar with the major surfaces of the first lightguide portion, the thickness-equalizing face plates being attached to the second lightguide portion by low-index adhesive that preserves internal reflection at the second pair of major surfaces.

4. The optical system of claim 1, wherein the second optical thickness is greater than the first optical thickness such that light propagating from the first lightguide portion to the second lightguide portion encounters an outward step at both major surfaces and an exit from the first lightguide portion defines a location of first internal reflection within the second lightguide portion that is beyond the polarization-modifying element.

5. The optical system of claim 4, further comprising thickness-equalizing face plates attached to the first lightguide portion so as to provide an external surface substantially coplanar with the major surfaces of the second lightguide portion, the thickness-equalizing face plates being attached to the first lightguide portion by low-index adhesive that preserves internal reflection at the first pair of major surfaces.

6. The optical system of claim 1, wherein the second lightguide portion is offset from the first lightguide portion in a direction perpendicular to the first and second major surfaces such that light propagating from the first lightguide portion to the second lightguide portion encounters an inward step at one of the major surfaces and an outward step at another of the major surfaces.

7. The optical system of claim 6, further comprising a first thickness-equalizing face plate attached to the first lightguide portion and a second thickness-equalizing face plate attached to the second lightguide portion so as to provide substantially planar continuous external surfaces across the first and second lightguide portions, the thickness-equalizing face plates being attached to the first and second lightguide portions by low-index adhesive that preserves internal reflection at the first and second pairs of major surfaces.

8. The optical system of claim 1, wherein the polarization-modifying element is a halfwave retarder plate deployed to convert p-polarization propagating within the first lightguide portion into s-polarization propagating within the second lightguide portion.

9. The optical system of claim 1 , wherein the polarization-modifying element has a layer thickness and wherein an offset between each of the second pair of major surfaces and a corresponding one of the first pair of major surfaces is at least half of the layer thickness and no more than twice the layer thickness.

10. The optical system of claim 1, wherein the progressive redirection configuration comprises a first plurality of mutually parallel partially reflecting surfaces deployed within the first lightguide portion non-parallel to the first pair of major surfaces.

11. The optical system of claim 10, wherein the progressive coupling-out configuration comprises a second plurality of mutually parallel partially reflecting surfaces deployed within the second lightguide portion obliquely to the second pair of major surfaces.

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