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

Abstract

An optical system (100) for transmitting light corresponding to an image to the eyes of a viewer has a first light guide portion (120) comprising a progressive redirection arrangement (121) for progressively redirecting light propagating within the first light guide portion; a polarization modifying element (131); and a second light guide portion (110) comprising a progressive out-coupling arrangement (111) for progressively out-coupling light propagating within the second light guide portion towards the eyes of the viewer. A pair of main surfaces (101b, 102b) of the second light guide portion (110) is parallel to, but not coplanar with, a corresponding pair of main surfaces (101a, 102a) of the first light guide 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 (T1) of the first light guide portion and an optical thickness (T2) of the second light guide portion.

Description

[0001] TECHNICAL FIELD AND BACKGROUND

[0002] The present invention relates to optical systems, and in particular the present invention relates to optical systems for conveying light corresponding to an image to a viewer's eye.

[0003] Many near-eye display systems include a transparent light guide optical element (LOE) or "waveguide" placed in front of the user's eye, which conveys an image within the LOE by internal reflection, and then couples the image out towards the user's eye by a suitable out-coupling mechanism. The out-coupling mechanism can be based on embedded partial reflectors (also known as "facets"), or can employ a diffractive optical element. The following description will primarily relate to facet-based coupling-out devices, but it will be understood that various features of the invention can also be applicable to diffractive devices.

[0004] In some cases, the light guide optical element can be designed to achieve two-dimensional aperture expansion. This is typically achieved by providing a first portion of the light guide having a progressive redirection configuration (partial reflective surface or 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 light guide having a progressive coupling-out configuration for progressively coupling out light propagating in the second in-plane direction to couple out the second light guide portion towards the viewer's eye. SUMMARY

[0005] The present invention relates to optical systems, and in particular the present invention relates to optical systems for conveying light corresponding to an image to a viewer's eye.

[0006] According to the teachings of embodiments of the present application, there is provided an optical system for delivering light corresponding to an image to an eye of a viewer, the optical system comprising: (a) a first light guide portion formed of a 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 light guide portion by internal reflection at the first pair of major surfaces, and an edge; the first light guide portion comprising a progressive redirection configuration for progressively redirecting light propagating within the first light guide portion in a first planar in-plane direction to propagate by internal reflection at the first pair of major surfaces in a second planar 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 light guide portion; and (c) a second light guide portion formed of a 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 light guide portion by internal reflection at the second pair of major surfaces, the second light guide portion being optically connected to the polarization modifying element such that a substantial portion of the light propagating in the second planar in-plane direction in the first light guide portion passes through the polarization modifying element and continues to propagate within the second light guide portion in the second planar in-plane direction, the second light guide portion comprising a progressive out-coupling configuration for progressively out-coupling light propagating within the second light guide portion in the second planar in-plane direction to out-couple the second light guide portion towards the eye of the viewer, wherein each of the second pair of major surfaces is parallel but not coplanar to 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 pair of major surfaces overlaps all of the first and second optical thicknesses.

[0007] According to further features in the teachings of embodiments of the present application, the second optical thickness is smaller than the first optical thickness, such that light propagating from the first light guide portion to the second light guide portion encounters an inward step at both major surfaces, such that the entrance to the second light guide portion trims the light propagating in the first direction from the first light guide portion, thereby excluding internally reflected light adjacent to the polarization modifying element from the second light guide portion.

[0008] According to still further features in the teachings of embodiments of the present application, there is also provided a thickness equalization panel attached to the second light guide portion to provide an outer surface substantially coplanar to the major surfaces of the first light guide portion, the thickness equalization panel being attached to the second light guide portion by a low refractive index adhesive that maintains internal reflection at the second pair of major surfaces.

[0009] According to a further feature of the implementation of the present application, the second optical thickness is greater than the first optical thickness such that light propagating from the first light guide portion to the second light guide portion encounters an outward step at both major surfaces and the exit from the first light guide portion defines a location within the second light guide portion beyond the first internal reflection of the polarization modifying element.

[0010] According to a further feature of the implementation of the present application, there is also provided a thickness equalization panel attached to the first light guide portion to provide an outer surface that is substantially coplanar with the major surfaces of the second light guide portion, the thickness equalization panel being attached to the first light guide portion by a low refractive index adhesive that maintains internal reflection at the first pair of major surfaces.

[0011] According to a further feature of the implementation of the present application, the second light guide portion is offset relative to the first light guide portion in a direction perpendicular to the first and second major surfaces such that light propagating from the first light guide portion to the second light guide portion encounters an inward step at one of the major surfaces and an outward step at the other of the major surfaces.

[0012] According to a further feature of the implementation of the present application, there is also provided a first thickness equalization panel attached to the first light guide portion and a second thickness equalization panel attached to the second light guide portion to provide a substantially planar continuous outer surface across the first and second light guide portions, the thickness equalization panels being attached to the first and second light guide portions by a low refractive index adhesive that maintains internal reflection at the first and second pairs of major surfaces.

[0013] According to a further feature of the implementation of the present application, the polarization modifying element is a half-wave retarder sheet disposed to convert p-polarization propagating within the first light guide portion to s-polarization propagating within the second light guide portion.

[0014] According to a further feature of the implementation of the present application, the polarization modifying element has a layer thickness, and wherein the 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 the layer thickness and no more than twice the layer thickness.

[0015] According to a further feature of the implementation of the present application, the progressive redirection configuration includes a first plurality of mutually parallel partially reflective surfaces disposed within the first light guide portion non-parallel to the first pair of major surfaces.

[0016] According to a further feature of the implementation of the present application, the progressive out-coupling configuration includes a second plurality of mutually parallel partially reflective surfaces disposed within the second light guide portion obliquely relative to the second pair of major surfaces.

[0017] In terms, the phrase "in-plane direction" is used to describe the propagation direction of a light ray within the light guide that corresponds to the in-plane component of the light ray as it repeatedly internally reflects at the major surface. At each reflection, the in-plane component parallel to the major surface remains unchanged, while the component normal to the major surface is reversed. The in-plane direction also corresponds to the light ray path viewed along a line of sight normal to the major surface in a plan view. BRIEF DESCRIPTION OF DRAWINGS

[0018] The application is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0019] FIG. 1A and FIG. 1B is a schematic isometric view of a display constructed and operative in accordance with the teachings of the present application, employing an optical system having first and second light guide portions for respectively illustrating two-stage optical aperture expansion in top-down and lateral injection configurations;

[0020] FIG. 2A is a schematic isometric view of sample light ray paths through the optical system of FIG. 1B (or the optical system of FIG. 1A );

[0021] FIG. 2B is a schematic edge view of the optical system of FIG. 2A implemented in accordance with conventional methods;

[0022] FIG. 2C is a magnified view of the region designated I in FIG. 2B ;

[0023] FIG. 3A is a schematic edge view of the optical system of FIG. 2A implemented in accordance with the first aspect of the present application;

[0024] FIG. 3B is a magnified view of the region designated II in FIG. 3A ;

[0025] FIG. 3C is a schematic edge view similar to FIG. 3A , illustrating a variant implementation of the optical system including a thickness equalization panel;

[0026] FIG. 3D is a magnified view of the region designated III in FIG. 3C ;

[0027] FIG. 4A is a schematic edge view of the optical system of FIG. 2A implemented in accordance with the second aspect of the present application;

[0028] FIG. 4B is FIG. 4A an enlarged view of the area designated IV in

[0029] FIG. 4C is a schematic edge view showing a variant implementation of an optical system including a thickness equalization panel, similar to FIG. 4A

[0030] FIG. 4D is FIG. 4C an enlarged view of the area designated V in

[0031] FIG. 4E is a schematic edge view showing a further variant implementation of an optical system integrating a homogenizing internal partial reflector, similar to FIG. 4C

[0032] FIG. 5A is a schematic edge view of an optical system according to a third aspect of the application, FIG. 2A

[0033] FIG. 5B and FIG. 5C are enlarged views of the areas designated VI and VII in FIG. 5A

[0034] FIG. 5D is a schematic edge view showing a variant implementation of an optical system including a thickness equalization panel, similar to FIG. 5A

[0035] FIG. 5E and FIG. 5F are enlarged views of the areas designated VIII and IX in FIG. 5D

[0036] Description of preferred embodiments

[0037] The present application is an optical system for delivering light corresponding to an image to an eye of a viewer.

[0038] The principles and operation of an optical system according to the present application can be better understood with reference to the drawings and accompanying descriptions.

[0039] Now referring to the drawings, FIG. 1A and FIG. 1B a head-mounted display (generally designated 10) constructed and operative according to the teachings of embodiments of the present application is shown, employing a pair of optical systems (interchangeably referred to as "light-guide optical elements" and "flat panel lightwave guide structures") 100, each having a first light guide portion 120 and a second light guide portion 110, for two-stage optical aperture expansion in the X direction and then in the Y direction.​​​​​​FIG. 1A In this case, the first light guide portion 120 is deployed along the top of the optical system for horizontal aperture expansion while redirecting image light downwards; and the second light guide portion 110 achieves vertical aperture expansion while coupling image light toward the "eye motion box" (EMB), which corresponds to the range of eye positions to be viewed in the displayed image. FIG. 1B In this process, the order of the directions of these optical aperture expansions was switched.

[0040] The head-mounted display 10 typically includes a projector 200 (for each monocular display) that generates an image coupled to a light-guiding optical element (LOE) 100. Various coupling configurations—typically using coupling prisms and / or coupling reflectors—can be used to couple image light into the LOE. Details of the choice of coupling configuration and its implementation are not critical to the present invention and will not be described in detail here. The projector is controlled by an electronic controller 300. The entire display is preferably supported by a support structure, in this case illustrated as an eyeglass frame shape factor support including side supports 400 for supporting the display relative to the user's head (e.g., nose and ears).

[0041] FIG. 2A The internal structure and operation of a LOE 100 according to a non-limiting exemplary implementation of the present invention are schematically illustrated. A first light guide portion 120, formed of a transparent material, has a first pair of mutually parallel main surfaces separated by a first optical thickness T1. FIG. 2A The front and rear surfaces, referred to as 101 and 102 (or 101a and 102a) in subsequent figures, support the propagation of light within the first light guide portion via internal reflection. The first light guide portion 120 includes a progressive redirection configuration, here implemented as a set of mutually parallel internal partial reflective surfaces 121, not parallel to the main surfaces, for progressively redirecting light propagating within the first light guide portion in a first plane direction to propagate towards the edge of the first light guide portion in a second plane direction, not parallel to the first direction, via internal reflection at the first pair of main surfaces.

[0042] The second light guide portion 110, formed of a transparent material, has a second pair of mutually parallel main surfaces separated by a second optical thickness T2. FIG. 2Afront and back surfaces (labeled 101 and 102 or 101b and 102b in subsequent figures) that support the propagation of light within the second light guide portion by internal reflection. The second light guide portion 110 includes a progressive out-coupling configuration, which is here implemented as a set of mutually parallel internal partial reflection surfaces 111 at an oblique angle with respect to the major surfaces, for progressively coupling out light propagating within the second light guide portion in the second plane of directions, to out-couple the second light guide portion towards the viewer’s eye (EMB).

[0043] In many cases, the progressive redirection configuration (e.g., internal partial reflection surfaces) 121 and the progressive out-coupling configuration (e.g., partial reflection surfaces) 111 are polarization dependent, and the polarization that is optimal for one can be problematic, or at least suboptimal, for the other. For example, in many cases it is preferable for the reflective elements to operate with s-polarization, to avoid the limitations on reflectivity for p-polarization around the Brewster angle. However, due to the different orientations of the partial reflection surfaces 121 and the partial reflection surfaces 111, FIG. 2A The representative ray 1001 of s-polarization introduced by the projector 200 with respect to the surfaces 121 shown typically corresponds to a mixed s- and p-polarization with respect to the surfaces 111. To address this issue, a polarization modification element 131, typically implemented as a half-wave retarder (waveplate), is optically connected between the adjacent edges of the first light guide portion 120 and the second light guide portion 110. In FIG. 2A The result is shown in by the point for the polarization entering the page and the arrow for the polarization in the plane of the page. The ray propagating within the light guide portion 120 is p-polarized with respect to the major surfaces, which corresponds to mostly s-polarized with respect to the partial reflection surfaces 121. After redirection by reflection at one of the surfaces 121 towards the edge of the first light guide portion 120, the ray passes through the retarder 131, which converts the ray to s-polarization with respect to the partial reflection surfaces 111.

[0044] As FIG. 2B and FIG. 2C shown, the default implementation of this structure is to implement the first light guide portion 120, the retarder 131, and the second light guide portion 110 with continuous planar major surfaces 101 and 102 extending across the entire LOE 100. However, it has been found that such an implementation presents challenges. Since the waveplate 131 typically has different mechanical properties than the surrounding glass, mechanical handling of the glass often results in abrupt discontinuities (e.g., different thickness of 131 compared to the surrounding glass) or rough surfaces. This results in a low optical quality surface 132 at the boundary between the waveplate 131 and air, which can cause deleterious artifacts that degrade the output image quality.

[0045] To address this problem, according to an aspect of the present application, each of the second pair of major surfaces 101b and 102b is parallel to, but not 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 surfaces of the first light guide portion and the major surfaces of the second light guide portion. This step can be an inward step (in the direction of propagation), as will be illustrated with reference to FIG. 3A to FIG. 3D and FIG. 5B illustrated, or an outward step, as will be illustrated with reference to FIG. 4A to FIG. 4E and FIG. 5C illustrated. In each case, the ray path of the light propagating via the waveplate 131 is modified by the presence of the step to avoid any contribution to the output image of the light rays that are internally reflected at the low optical quality surface 132. The optical mechanism of each case will be explained below. The size T3 of the polarization modifying element perpendicular to the first and second pairs of major surfaces preferably overlaps all of the first and second optical thicknesses T1 and T2.

[0046] Referring now to the non-limiting exemplary implementation of FIG. 3A and FIG. 3B in this case, the second optical thickness T2 is less than the first optical thickness T1, so that the light propagating from the first light guide portion 120 to the second light guide portion 110 encounters an inward step at both major surfaces. Referring to FIG. 3B It is best understood that the result of this geometry is that the entrance to the second light guide portion, which is defined by the major surface 101b, the beginning portion of the major surface 102b, trims the light propagating in the first direction from the first light guide portion 120, thereby excluding internally reflected light (e.g., sample ray 1031) adjacent to the polarization modifying element from the second light guide portion 110. The remaining rays, e.g., ray 1031', propagate freely through the waveplate 131 and continue to provide a coupled-out image, all as described above.

[0047] The trimmed light generally escapes from the light guide and is lost. The direction of the lost light is generally at an angle that is unlikely to reach the user's eye, and thus will not adversely affect the quality of the viewed image or otherwise interfere with the user. The trimming of the image light results in some reduction in efficiency, but depending on the size of the step, the loss is generally relatively small, e.g., less than 5%.

[0048] The preferred size of the step (offset) at each face, which in a symmetric implementation corresponds to |T1-T2| / 2, is determined by geometric considerations to ensure that the steepest light rays propagating along the LOE and internally reflecting from the surface 132 are trimmed so as not to enter the second light guide portion 110. Thus, AT is a function of the layer thickness d of the waveplate 131 (i.e., the thickness in the in-plane propagation direction) and the angle of the steepest guided light ray 1031 relative to the major surface of the waveguide. In implementations, the offset AT between each major surface 101b, 102b and its corresponding major surface 101a, 102a is at least half the layer thickness d. The offset AT is also preferably no more than twice the layer thickness d. Larger steps are generally undesirable as they are unnecessary for the trimming function and only result in excessive trimming of the propagating image light and consequent inefficiency. A typical preferred choice for the step size is approximately equal to (±15%) the layer thickness of the waveplate. The thickness d of the waveplate will depend on the optical design and the choice of waveplate material. In the case of a quartz half-wave retarder, a thickness of approximately 30 microns is typically used. For a polymer retarder, a thickness of about 60 microns can be used.

[0049] FIG. 3C and FIG. 3D Variations of the implementation of FIG. 3A and FIG. 3B are shown. In some cases, it can be desirable to avoid the step in the outer surface of the LOE, which can cause artifacts when the user sees the real-world scene through the LOE, and / or can collect dust and be difficult to clean properly. To achieve a smooth outer surface, some preferred implementations further include a thickness equalization panel 140a, 140b attached to one or both major surfaces 101b, 102b of the second light guide portion 110 to provide an outer surface 101c, 102c that is substantially coplanar with the major surfaces 101a, 101b of the first light guide portion 120. The thickness equalization panel 140a and the thickness equalization panel 140b are attached to the second light guide portion by a low-index adhesive, typically having an index of refraction of no more than 1.4, which maintains internal reflection at the second pair of major surfaces 101b, 102b. Equivalently to this is a coating of low-index material underneath the adhesive. Thus, the effective optical thickness of the second light guide portion 110 remains T2, and the trimming of light propagating from the first light guide portion 120 to the second light guide portion 110 remains as in FIG. 3A and FIG. 3B In this case, instead of being released from the LOE, the trimmed light is typically trapped within the panel by internal reflection to be absorbed at the edge of the LOE.

[0050] From a manufacturing perspective, it can not be practical to handle a panel of thickness AT. In this case, the panel can be applied as a thicker element having dimensions that are easier to handle and to bond, and the assembled LOE precursor can then be thinned by cutting, grinding and / or polishing until the desired final thickness is achieved.

[0051] Turning now to a further series of implementations, FIG. 4A and FIG. 4B An implementation is shown in which the second optical thickness T2 is greater than the first optical thickness T1 such that light propagating from the first light guide portion 120 to the second light guide portion 110 encounters an outward step at both major surfaces. The exit aperture between the major surface 101a, the major surface 102a out of the first light guide portion 120 defines the location of the first internal reflection beyond the polarization modifying element 131 within the second light guide portion 110 (see ray 1031).

[0052] As mentioned previously, in FIG. 4C and FIG. 4D In the variant implementation shown, thickness equalizing panels 140a, 140b can be provided to avoid leaving a step in the major surface of the LOE. In this case, the panels are attached to the first light guide portion 120 to provide an outer surface that is substantially coplanar with the major surface 101b, the major surface 102b of the second light guide portion 110. As mentioned previously, a low refractive index adhesive (preferably having an R.I. of 1.4 or less) or a low refractive index coating under the adhesive maintains the internal reflection at the major surface 101a and the major surface 102a. This, together with a suitable in-coupling arrangement from the projector, ensures that FIG. 4C and FIG. 4D The implementation of FIG. 4A and FIG. 4B is optically equivalent to the implementation of

[0053] Considerations regarding the required size of the outward offset / step are generally similar to the discussion above in the context of the inward offset / step. In the case of an outward step, there is no efficiency loss through truncation. Instead, there can be an incomplete filling of the second light guide portion with the projected image. For a step that is a small fraction of the thickness of the light guide, this incomplete filling can not result in any discernible degradation of the observed image quality. In cases where it is desired to compensate for such incomplete filling, this can be done by adding a homogenizer (or "mixer"), preferably implemented as an intermediate plane 50% reflector 141 as shown in FIG. 4E

[0054] Turning finally to FIG. 5A to FIG. 5F these figures show that FIG. 3A to FIG. 3D and FIG. 4A to FIG. 4E the options of FIG. 5A to FIG. 5C ​As shown, an implementation of the LOE 100 is shown in which the second light guide portion 110 is offset relative to the first light guide portion 120 in a direction perpendicular to the major surfaces, such that light propagating from the first light guide portion 120 to the second light guide portion 110 encounters an inward step at one of the major surfaces (the top as shown) and an outward step at the other of the major surfaces (the bottom as shown). Optionally, although not necessarily, the first thickness T1 and the second thickness T2 are the same, such that the inward step and the outward step have the same offset height. FIG. 5B The function of the inward step detailed in FIG. 3B is the same as the step in FIG. 5C The function of the outward step detailed in FIG. 4B is the same as the step in This “hybrid” implementation can have particular advantages as a good compromise between the two options described above: by employing only one inward step instead of two, the efficiency loss is halved, and the incomplete filling of the light guide is also significantly reduced.

[0055] As in the previous cases, a further optional variant implementation includes a first thickness equalization panel 140b attached to the first light guide portion 120 and a second thickness equalization panel 140a attached to the second light guide portion 110 to provide a substantially planar continuous outer surface across the first light guide portion 120 and the second light guide portion 110. The thickness equalization panels are attached to the first light guide portion 120 and the second light guide portion 110 by a low-index adhesive or a low-index coating, which maintains internal reflection at the first and second pairs of major surfaces.

[0056] Although shown herein in particularly preferred implementations in which both the progressive redirection configuration and the progressive out-coupling configuration are implemented as sets of mutually parallel partially reflective surfaces deployed within respective light guide portions, it should be noted that one or both of these elements can be replaced by diffractive optical elements and / or any other suitable elements.

[0057] It will be understood that the above description is intended to be illustrative only and that many other embodiments are possible within the scope of the present application as defined in the appended claims.

Claims

1. An optical system for delivering light corresponding to an image to an eye of a viewer, the optical system comprising: (a) a first light guide portion formed of 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 light guide portion by internal reflection at the first pair of major surfaces, and an edge; the first light guide portion including a progressive redirection configuration for progressively redirecting light propagating within the first light guide portion in a first planar in-plane direction to propagate by internal reflection at the first pair of major surfaces in a second planar in-plane direction non-parallel to the first direction toward the edge; (b) a polarization modifying element optically connected at the edge of the first light guide portion; and (c) a second light guide portion formed of 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 light guide portion by internal reflection at the second pair of major surfaces, the second light guide portion optically connected to the polarization modifying element so that a substantial portion of light propagating in the second planar in-plane direction in the first light guide portion passes through the polarization modifying element and continues to propagate within the second light guide portion in the second planar in-plane direction, the second light guide portion including a progressive out-coupling configuration for progressively out-coupling light propagating within the second light guide portion in the second planar in-plane direction to out-couple the second light guide portion toward the eye of the viewer, wherein each of the second pair of major surfaces is parallel to but not co-planar 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 all of both the first and second optical thicknesses. The second optical thickness is less than the first optical thickness so that light propagating from the first light guide portion to the second light guide portion encounters an inward step at both major surfaces so that an entrance to the second light guide portion trims light propagating in the first direction from the first light guide portion so as to exclude internally reflected light adjacent to the polarization modifying element from the second light guide portion.

2. The optical system of claim 1, wherein, 3. The optical system of claim 2, further comprising a thickness equalization panel attached to the second light guide portion so as to provide an outer surface substantially co-planar with the major surfaces of the first light guide portion, the thickness equalization panel being attached to the second light guide portion by a low index adhesive that maintains internal reflection at the second pair of major surfaces. The second optical thickness is greater than the first optical thickness so that light propagating from the first light guide portion to the second light guide portion encounters an outward step at both major surfaces and an exit from the first light guide portion defines a location within the second light guide portion beyond the first internal reflection of the polarization modifying element.

4. The optical system of claim 1, wherein, ​ 5. The optical system of claim 4, further comprising a thickness equalization panel attached to the first light guide portion so as to provide an outer surface that is substantially coplanar with the major surfaces of the second light guide portion, the thickness equalization panel being attached to the first light guide portion by a low index adhesive that maintains internal reflection at the first pair of major surfaces.

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

7. The optical system of claim 6, further comprising a first thickness equalization panel attached to the first light guide portion and a second thickness equalization panel attached to the second light guide portion so as to provide a substantially planar continuous outer surface across the first and second light guide portions, the thickness equalization panels being attached to the first and second light guide portions by a low index adhesive that maintains internal reflection at the first and second pairs of major surfaces.

8. The optical system of claim 1, wherein, The polarization modification element is a half-wave retarder sheet disposed to convert p-polarization propagating within the first light guide portion to s-polarization propagating within the second light guide portion.

9. The optical system of claim 1, wherein, The polarization modification 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 the layer thickness and no more than twice the layer thickness.

10. The optical system of claim 1, wherein, The progressive redirection configuration includes a first plurality of mutually parallel partially reflective surfaces disposed within the first light guide portion non-parallel to the first pair of major surfaces.

11. The optical system of claim 10, wherein, The progressive out-coupling configuration includes a second plurality of mutually parallel partially reflective surfaces disposed within the second light guide portion obliquely relative to the second pair of major surfaces. The polarization modification element is a half-wave retarder sheet disposed to convert p-polarization propagating within the first light guide portion to s-polarization propagating within the second light guide portion. The polarization modification 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 the layer thickness and no more than twice the layer thickness. The progressive redirection configuration includes a first plurality of mutually parallel partially reflective surfaces disposed within the first light guide portion non-parallel to the first pair of major surfaces. The progressive out-coupling configuration includes a second plurality of mutually parallel partially reflective surfaces disposed within the second light guide portion obliquely relative to the second pair of major surfaces.

Images