Light guide based display with light recirculation

Abstract

A display includes a light guide device in which at least a portion of image light is not coupled out of the light guide during a first pass through a coupling-out device, but is recycled to repeatedly pass through the coupling-out device. In one set of embodiments, recirculation of light is performed via separate light guides. In another set of embodiments, light is recirculated within a single light guide, employing polarization management to avoid undesirable interaction between the light and the outcoupling device.

Description

Technical Field

[0001] The present invention relates to light guide based displays and, in particular, to light guide based displays that recycle light reaching the end of a light guide to pass through the light guide again. Background Art

[0002] It is known to employ a light guide to deliver an image in front of an observer's eye and couple the image out toward the observer for viewing. The image is coupled out by a suitable outcoupling device, which may be a set of partially reflecting embedded reflectors, a pin mirror, or a diffractive optical element. In some cases, the light guide is configured to achieve a two-dimensional aperture expansion, for example by using an additional set of embedded reflectors or diffractive optical elements to gradually deflect light propagating within the light guide to redirect it toward the outcoupling device.

[0003] A schematic diagram of a two-dimensional aperture expansion lightguide 100 is shown in plan view in FIG1 . As schematically represented by arrows 102, an image is typically injected via an in-coupling prism or via reflective in-coupling to propagate in a first direction D1 from the top and bottom surfaces (parallel to the plane of the image) by internal reflection and encounters a first set of mutually parallel internal partially reflective surfaces (or "facets") 104 that gradually redirect (reflect) a portion of the image light in a second direction D2. The light encounters a second set of mutually parallel internal partially reflective surfaces (or "facets") 106 that gradually redirect (reflect) a portion of the image light outward from the lightguide to couple the image out toward an observer for viewing. A homogenizing element or "mixer" 108 implemented as a partial reflector disposed between and parallel to the major surfaces of the lightguide may be included to reduce non-uniformity of the output image. One or both of the sets of facets may be replaced by suitable surface or volume diffractive optical elements embedded in or on the surface of the light guide to redirect the image light, all of which are known in the art.

[0004] In order to minimize interference with the scene viewed by an observer through the light guide, the reflectivity (or diffraction efficiency) of at least the second set of facets and possibly both sets of facets is preferably relatively low. As a result, a significant portion of the image light reaches the end of the light guide (particularly the marked shadow area 110), resulting in a loss of efficiency and image brightness. Summary of the invention

[0005] The present invention is a display for delivering image light to a viewer.

[0006] According to the teachings of an embodiment of the present invention, there is provided a display for delivering image light to an observer, the display comprising: (a) a first light guide, the first light guide comprising a block of transparent material, the first light guide having two mutually parallel main surfaces for supporting the propagation of image light by internal reflection at the main surfaces, the light guide comprising at least one set of mutually parallel internal partially reflecting surfaces, the at least one set of mutually parallel internal partially reflecting surfaces being inclined at angles relative to the main surfaces so as to gradually couple out a portion of the image light propagating in a first direction so that it is reflected toward the observer, while transmitting a portion of the image light so that it continues to propagate in the first direction within the first light guide; and (b) a second light guide, the second light guide comprising a block of transparent material, the second light guide having two mutually parallel main surfaces for supporting the propagation of image light by internal reflection at the main surfaces, the second light guide being partially (c) a first reflective coupling configuration comprising at least one plane reflector disposed to reflect image light that has passed through the second lightguide along the second direction to reintroduce the image light to propagate along the first direction in the first lightguide, such that at least a portion of the image light that was transmitted by the set of partially reflective surfaces when first passing through the set of partially reflective surfaces is coupled out to be reflected toward an observer when it is incident on the partially reflective surfaces again.

[0007] According to the teachings of an embodiment of the present invention, there is also provided a display for delivering image light to an observer, the display comprising: (a) a first light guide, the first light guide comprising a block of transparent material, the first light guide having two mutually parallel main surfaces for supporting the propagation of image light by internal reflection at the main surfaces, the first light guide comprising at least one set of mutually parallel internal partially reflective surfaces, the at least one set of mutually parallel internal partially reflective surfaces being non-parallel to the main surfaces, so as to gradually redirect a portion of the image light propagating in a first direction so that it propagates in a second direction in the first waveguide by internal reflection, while transmitting a portion of the image light so that it continues to propagate in the first direction in the first lightguide; (b) a second light guide, the second light guide comprising a block of transparent material, the second light guide having two mutually parallel main surfaces for supporting the propagation of image light by internal reflection at the main surfaces, the first light guide comprising at least one set of mutually parallel internal partially reflective surfaces, the at least one set of mutually parallel internal partially reflective surfaces being non-parallel to the main surfaces The second light guide is arranged in a stacked relationship with the first light guide; (c) a first reflective coupling configuration, the first reflective coupling configuration comprising at least one plane reflector, the first reflective coupling configuration being arranged to reflect image light transmitted by a set of partially reflective surfaces and propagating in a first direction in the first light guide so that the image light propagates in a third direction in the second light guide, the third direction having an in-plane component reversed relative to the first direction; and (d) a second reflective coupling configuration, the second reflective coupling configuration comprising at least one plane reflector, the second reflective coupling configuration being arranged to reflect image light that has passed through the second light guide in a second direction so as to reintroduce the image light to propagate in the first direction in the first light guide, so that at least a portion of the image light transmitted by the set of partially reflective surfaces when first passing through the set of partially reflective surfaces is redirected to propagate in the second direction when it is incident on the partially reflective surface again.

[0008] According to another feature of an embodiment of the present invention, each of the first reflection coupling configuration and the second reflection coupling configuration includes a single plane reflector and a light transmission area adjacent to the single plane reflector, wherein within the light transmission area, an optical continuum exists between the first light guide and the second light guide for transmitting image light between the first light guide and the second light guide.

[0009] According to another feature of embodiments of the invention, each of the single plane mirrors is oriented perpendicular to a major surface of the first light guide and the second light guide.

[0010] According to another feature of an embodiment of the present invention, a single plane mirror of a first reflection coupling configuration is oriented to have a first inclination angle relative to major surfaces of the first light guide and the second light guide, and wherein a single plane mirror of a second reflection coupling configuration is oriented to have a second inclination angle relative to major surfaces of the first light guide and the second light guide, the second inclination angle being equal to the first inclination angle but opposite to the first inclination angle.

[0011] According to another feature of embodiments of the invention, the optical continuum is achieved by connecting the first light guide to the second light guide using an index matching adhesive.

[0012] According to another feature of embodiments of the invention, the optical continuum is realized by connecting a continuous block of transparent material to edge surfaces of the first light guide and the second light guide.

[0013] According to another feature of an embodiment of the present invention, each of the first reflection coupling configuration and the second reflection coupling configuration comprises a pair of mutually perpendicular plane mirrors.

[0014] According to another feature of an embodiment of the invention, the second light guide is attached to the first light guide by an adhesive layer having a refractive index lower than the refractive index of the transparent material of the first and second light guides.

[0015] According to another feature of embodiments of the invention, the second light guide is separated from the first light guide by an air gap.

[0016] According to the teachings of the embodiments of the present invention, there is also provided a display for delivering image light to an observer, the display comprising: (a) a light guide comprising a block of transparent material, the light guide having two mutually parallel main surfaces for supporting the propagation of image light by internal reflection at the main surfaces, the light guide comprising at least one set of mutually parallel polarization-selective internal partially reflective surfaces, the at least one set of mutually parallel polarization-selective internal partially reflective surfaces being inclined at angles relative to the main surfaces so as to gradually couple out a portion of the image light of a first polarization propagating in a first direction so that it is reflected toward the observer, while transmitting a portion of the image light of the first polarization so that it continues to propagate in the light guide along the first direction, the partially reflective surfaces being substantially transparent to light of a second polarization orthogonal to the first polarization; and (b) a second polarization-selective internal partially reflective surface having two mutually parallel main surfaces for supporting the propagation of image light by internal reflection at the main surfaces, the light guide comprising at least one set of mutually parallel polarization-selective internal partially reflective surfaces being inclined at angles relative to the main surfaces so as to gradually couple out a portion of the image light of a first polarization propagating in a first direction so that it is reflected toward the observer, while transmitting a portion of the image light of the first polarization so that it continues to propagate in the light guide along the first direction, the partially reflective surfaces being substantially transparent to light of a second polarization orthogonal to the first polarization; A reflective configuration, the first reflective configuration comprising a quarter wave phase plate and a plane mirror, the first reflective configuration being arranged to reflect image light transmitted by a set of partially reflective surfaces so that the image light propagates in a second direction with a rotated polarization within the light guide to be transmitted by the partially reflective surfaces, the second direction having an in-plane component reversed relative to the first direction; and (c) a second reflective configuration, the second reflective configuration comprising a quarter wave phase plate and a plane mirror, the second reflective configuration being arranged to reflect image light that has passed through the light guide in the second direction so that the image light propagates in a direction parallel to the first direction within the light guide, so that at least a portion of the image light transmitted by the set of partially reflective surfaces when passing through the set of partially reflective surfaces for the first time is coupled out to be reflected toward an observer when it is incident on the partially reflective surfaces again.

[0017] According to another feature of an embodiment of the present invention, an image injection aperture is also provided, which is used to inject image light into the light guide to propagate along a first direction by internal reflection at the main surface, and wherein the first direction is inclined relative to the first reflection configuration and the second reflection configuration, so that the image light introduced at the image injection aperture is incident on a first area of ​​the partially reflective surface, and after reflecting from the first reflection configuration and the second reflection configuration, is incident on a second area of ​​the partially reflective surface, and the second area does not at least partially overlap with the first area.

[0018] According to another feature of an embodiment of the invention, the reflectivity of the partially reflective surfaces in at least the first region is such that most of the image light injected at the image injection aperture is transmitted through a set of partially reflective surfaces and recycled by reflection at the first and second reflective configurations.

[0019] According to another feature of embodiments of the invention, the quarter wave phase plate is part of a gradually varying phase plate.

[0020] According to another feature of an embodiment of the present invention, there is further provided a partially reflective surface disposed inside the light guide, perpendicular to the main surface and parallel to the first and second reflective configurations.

[0021] According to the teachings of the embodiments of the present invention, there is also provided a display for delivering image light to an observer, the display comprising: (a) a light guide comprising a block of transparent material, the light guide having two mutually parallel main surfaces for supporting the propagation of image light by internal reflection at the main surfaces, the light guide comprising at least one set of mutually parallel polarization-selective internal partially reflective surfaces, the at least one set of mutually parallel polarization-selective internal partially reflective surfaces being non-parallel to the main surfaces so as to gradually redirect a portion of the image light propagating in a first direction so that it propagates in a second direction in the light guide by internal reflection, while transmitting a portion of the image light so that it continues to propagate in the first direction in the light guide, the partially reflective surfaces being substantially transparent to light of a second polarization orthogonal to the first polarization; and (b) the first reflective The invention relates to a light guide comprising a first reflecting configuration, the first reflecting configuration comprising a quarter wave phase plate and a plane mirror, the first reflecting configuration being arranged to reflect image light transmitted by a set of partially reflecting surfaces so that the image light propagates in a third direction within the light guide with a rotated polarization to be transmitted by the partially reflecting surfaces, the third direction having an in-plane component reversed relative to the first direction; and (c) a second reflecting configuration, the second reflecting configuration comprising a quarter wave phase plate and a plane mirror, the second reflecting configuration being arranged to reflect image light that has passed through the light guide along the third direction so that the image light propagates in a direction parallel to the first direction within the light guide, so that at least a portion of the image light transmitted by the set of partially reflecting surfaces when passing through the set of partially reflecting surfaces for the first time is redirected to propagate in the second direction when it is incident on the partially reflecting surfaces again. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention is herein described, by way of example only, with reference to the accompanying drawings, in which:

[0023] As mentioned above, FIG1 is a schematic plan view of a display light guide providing two-dimensional aperture expansion;

[0024] Figure 2 is a schematic plan view of a display light guide constructed and operative in accordance with an embodiment of the present invention;

[0025] FIG. 3A to FIG. 3E According to the variant implementation of the light guide device Figure 2 A cross-sectional view taken along line III-III of ;

[0026] Figure 4A is a schematic plan view of a display light guide constructed and operative in accordance with another embodiment of the present invention;

[0027] Figure 4B is along Figure 4A A cross-sectional view taken along line IV-IV of ;

[0028] Figure 5A and Figure 5BThey are respectively Figure 3A and Figure 4B A similar cross-sectional view showing an alternative configuration of the light guide;

[0029] Figure 6 is a schematic side elevational view of a display light guide constructed and operative in accordance with another embodiment of the present invention;

[0030] Fig. 7A is a schematic plan view of a display light guide constructed and operative in accordance with another embodiment of the present invention that employs recycling of image light to achieve aperture expansion;

[0031] Figure 7B is with Fig. 7A Similar figure, but with a different deployment of the partially reflecting outcoupling surface;

[0032] Figure 7C is with Fig. 7A A similar diagram, but with recycling of image light along the x-axis;

[0033] Figure 8 is with Fig. 7A Similar figure, but with subdivided partially reflecting outcoupling surface;

[0034] Fig. 9 and Fig.10 is with Fig. 7A Similar diagrams showing parameters that influence the degree to which the light guide is filled with an image and that vary with respect to the distance between the two mirrors;

[0035] Fig.11 is with Fig. 7A A similar figure showing a display light guide employing an additional mixer element;

[0036] Fig.12 is with Fig. 7A A similar figure shows a display light guide using a phase plate with gradually varying efficiency;

[0037] Fig.13A and Fig. 13B is a schematic plan view illustrating a display light guide constructed and operative in accordance with another embodiment of the present invention, the display light guide employing recycling of image light in two perpendicular directions, the two directions having different initial directions of propagation;

[0038] Fig.14A is a schematic plan view of a display light guide constructed and operative in accordance with another embodiment of the present invention, the display light guide employing recycling of image light in two perpendicular directions, one direction within a first light guide and a second direction via a second light guide;

[0039] Fig. 14B and Fig. 14C According to two variant implementations of the display light guide device, Fig.14A A cross-sectional view along line XIV-XIV in FIG.

[0040] Fig.15A is a schematic plan view of a display light guide constructed and operative in accordance with another embodiment of the present invention;

[0041] Fig. 15B is along Fig.15A A cross-sectional view along line XV-XV in FIG.

[0042] Fig. 15C yes Fig.15A A schematic plan view showing a variant implementation of a display light guide device;

[0043] Fig.16A is a schematic plan view of a display light guide constructed and operative in accordance with another embodiment of the present invention;

[0044] Fig. 16B and Fig. 16C Along Fig.16A Schematic cross-sectional view taken along line XVI-XVI and line XVII-XVII in FIG.

[0045] Fig.17 is a schematic plan view of a display light guide constructed and operative in accordance with another embodiment of the present invention, the display light guide employing eight housing reflectors;

[0046] Fig.18A and Fig.18B is a schematic plan view of a display light guide constructed and operative in accordance with another embodiment of the present invention, illustrating the use of selective light absorbing elements to restrict selected light paths;

[0047] Fig.19A is a schematic plan view of a display light guide constructed and operative in accordance with another embodiment of the present invention;

[0048] Fig.19B is along Fig.19A A cross-sectional view along line XIX-XIX in FIG. 1 ; and

[0049] Fig.19C Is from Fig.19B A schematic enlarged view showing the coupling configuration of the light guide device. DETAILED DESCRIPTION

[0050] The present invention is a display for delivering image light to a viewer.

[0051] The principles and operations of a display according to the present invention may be better understood with reference to the drawings and accompanying description.

[0052] By way of introduction, various embodiments of the present invention are based on two families of solutions for light recycling in light guides. Figures 2 to 5B In the first set of embodiments described, light reaching the edge of the first light guide is coupled into a second parallel light guide to be guided back and reintroduced into the first light guide to pass through the second light guide again. Figures 6 to 13B In a second set of embodiments shown, light is recycled within a single light guide, with polarization management to avoid undesired interactions between the light and the outcoupling device. The remaining figures relate to further variations, combinations and applications of these methods, including recycling of light along more than one in-plane axis.

[0053] The present invention is presented herein as a display. More specifically, the present invention is generally directed to various light guide configurations for transmitting images from an image projector to deliver the images to the eyes of a user, typically in the context of an augmented reality display, wherein the user can simultaneously view the real world via the light guide configuration. The present invention is applicable to displays of all sizes and types, from near-eye displays and / or head-mounted displays to automotive head-up displays or larger displays. A full display implementation includes additional components, including an image projector for generating a desired image, and a coupling device for coupling the projected image into a light guide so that the image propagates within the light guide by internal reflection. The image projector itself typically includes an image generator, which may be based on a liquid crystal display (LCD) element, a scanned modulated laser beam, a digital light processor, an active matrix display, or any other image generator; and a collimating optical device for collimating the image and delivering the image to the projector output aperture. The coupling device may employ one or more reflectors, coupling prisms, and / or diffractive optical elements deployed to introduce the image into the light guide within a desired range of propagation angles. In addition, power supply components, processing components for driving the image generator, and mechanical components for supporting all of the above components in a desired deployment for use relative to the user's head, vehicle, or any other application requirements are included. All of the above components are known in the art and will not be discussed again in this article for simplicity of presentation.

[0054] In addition, although presented as a display, the various implementations of the light guides described herein can also be advantageously used in various lighting applications in which the light guide itself does not convey an image but rather provides illumination, such as for illuminating a Liquid Crystal On Silicon (LCOS) image generator that is part of a display.

[0055] Light recycling via additional light guides

[0056] Therefore, according to certain embodiments of the present invention, light that would otherwise be lost upon reaching the end of the waveguide may be re-injected into the waveguide to improve the overall efficiency of the waveguide. Figure 2 and Figure 3A An embodiment is shown in which the loss of light in a lightguide optical element (LOE) is reduced by adding another inert lightguide 10B below the lightguide 10A, and adding two parallel mirror plates 20 on both sides of the lightguide pair to form an optical cavity, so that the light originally lost on the rightmost side of the waveguide can be recovered.

[0057] In the non-limiting example shown here, the light guide 10A is a two-dimensional aperture expansion light guide that employs two sets of mutually parallel internal partially reflective surfaces. As schematically represented by arrow 12, an image is injected via an image injection aperture (typically via a coupling prism or via reflective coupling) to propagate along a first direction D1 by internal reflection from the top and bottom surfaces (parallel to the plane of the image) and encounters a first set of mutually parallel internal partially reflective surfaces (or "facets") 14 that gradually redirect (reflect) a portion of the image light along a second direction D2. The light encounters a second set of mutually parallel internal partially reflective surfaces (or "facets") 16 that gradually redirect (reflect) a portion of the image light outward from the light guide to couple the image out toward an observer for viewing. A homogenizing element or "mixer" 18 implemented as a partial reflector disposed between and parallel to the major surfaces of the light guide may be included to reduce non-uniformity of the output image. One or both of the sets of facets may be replaced by suitable surface or volume diffractive optical elements embedded in or on the surface of the light guide to redirect the image light, all as is known in the art.The invention is also not limited to light guides with two-dimensional aperture expansion.

[0058] Herein and throughout the specification and claims, unless explicitly stated otherwise, references to propagation directions relate to in-plane propagation directions, i.e., components of the light ray directions that are parallel to the major surfaces of the light guide. As will be shown in some side views, propagation within the light guide occurs by repeated internal reflections at the major surfaces, but the "propagation direction" within the light guide is considered to be the in-plane component. Furthermore, unless otherwise indicated, "direction" refers to all light rays propagating in the same or parallel in-plane directions, even if displaced / offset from each other.

[0059] In a first group of embodiments, light recycling is achieved by using a second light guide 10B, which is implemented as a block of transparent material and has two mutually parallel major surfaces for supporting the propagation of image light by internal reflection at the major surfaces. The second light guide 10B is arranged in a stacked relationship with the first light guide 10A. A first reflective coupling configuration including at least one plane reflector 20A is arranged to reflect image light transmitted by the second set of partially reflective surfaces 16 and propagating in the first light guide 10A along a second direction D2, so that the image light propagates in the second light guide along a reflected direction D3, which has an inverted in-plane component relative to the direction D2 (the inversion of the component perpendicular to the plane reflector 20B). A second reflective coupling configuration comprising at least one plane reflector 20B is deployed to reflect image light that has passed through the second light guide along a reflective direction to reintroduce the image light so that it propagates along the first direction in the first light guide 10A, so that at least a portion of the image light transmitted by a group of partially reflective surfaces when passing through the group of partially reflective surfaces for the first time is coupled out to be reflected toward an observer when it is incident on the group of partially reflective surfaces again.

[0060] Reference Figure 3A To further illustrate this. An incident ray i enters via the primary surface (S1) and propagates via total internal reflection (TIR) ​​until it strikes the far reflector 20A and returns to the light guide as ray j, which then strikes the second reflector 20B and returns to the light guide as ray k. Note that, as in Figure 3A , ray i and ray k are parallel. In order to maintain TIR conditions in the region of the first and second sets of facets 14, 16 and the mixer 18, and to enable light to pass between the light guide 10A and the inert glass 10B, a low refractive index (RI) glue should be used between the light guides in region 22, and a refractive index matching glue should be used in region 24. An alternative implementation may use an air gap in region 22, which is maintained by inserting a thin refractive index matching plate in region 24. Yet another option is to provide internal reflection properties in region 22 by using a multilayer dielectric coating designed to provide angle selective properties that mimic TIR properties, having high reflectivity at large angles and high transparency at small angles (i.e., close to perpendicular to the main surface of the light guide) (as known in the art).

[0061] In a first set of implementations, each of the first and second reflective coupling configurations is implemented using a single plane reflector 20A, 20B and a light transmission region 24 adjacent to the single plane reflector, within which an optical continuum exists between the first light guide and the second light guide for transmitting image light between the first light guide and the second light guide. Figure 3A and Figure 3B In this case, these plane mirrors are oriented perpendicular to the major surfaces of the first and second light guides (eg, the top surfaces labeled S1). Therefore, the propagation angle of the light in the second light guide 10B is the same as the propagation angle in the first light guide 10A. Figure 3B Structurally and functionally equivalent to Figure 3A ,but Figure 3B It is shown that the first light guide 10A may be implemented on a side facing away from the user's eye, wherein the outcoupled light is transmitted through the second light guide 10B before reaching the eye.

[0062] according to Figure 3C and Figure 3D Another option shown, a single plane reflector 20A of a first reflective coupling configuration is oriented to have a first tilt angle α relative to the main surface S1 of the light guide, and a single plane reflector 20B of a second reflective coupling configuration is oriented to have a second tilt angle relative to the main surface S1 of the light guide, the second tilt angle being equal in magnitude to the first tilt angle α but relatively tilted. This tilt angle causes light rays to propagate in the second light guide at a different ray angle (a shallower angle or a steeper angle relative to the main light guide surface) than the light rays in the first light guide. After reflection at the second reflective coupling configuration, the angle is restored to the original ray angle to propagate along the first light guide.

[0063] Figure 3E Another variation of the reflective coupling configuration is shown, according to which each of the first and second reflective coupling configurations comprises a pair of plane mirrors 20A1, 20A2 and 20B1, 20B2, each pair of plane mirrors being preferably arranged perpendicular to each other to form right-angle reflectors, wherein each reflector is preferably 45 degrees to a major surface of the light guide. In this case, image light is transmitted between the first and second light guides after being reflected at a first of these reflectors 20A1 or 20B1 and before being reflected in a second of these reflectors 20A2 or 20B2, while the light is at a relatively small angle relative to the normal to the major surfaces. This enables the use of a low refractive index adhesive to realize the entire interface between the first and second light guides, since the light is transmitted between the light guides at an incident angle that is smaller than the critical angle.

[0064] Although Figures 2 to 3ERecycling of image light along the "X" direction (i.e., light passing through a set of facets 16) has been shown, but in the case of a two-dimensional aperture expansion light guide as shown, the same principle can be implemented to recycle light passing through a set of facets 14 (or a diffractive optical element within the light guide that performs the equivalent function of gradually redirecting the image light before outcoupling). Figure 4A and Figure 4B This option is shown. The same reference numerals as above are used throughout.

[0065] In an alternative implementation, the optical continuum between the light guide 10A and the light guide 10B is achieved by connecting a continuous block 28 of transparent material to the edge surfaces of the first light guide 10A and the second light guide 10B, and using a refractive index matching adhesive at the interface between the light guide and the block 28 perpendicular to the major surfaces of the light guides. Figure 5A and Figure 5B An example of such an implementation is shown, where the recycling of image light is in the X direction and in the Y direction respectively. These configurations are essentially equivalent to Figure 3A and Figure 4B implementation, but may have production advantages.

[0066] Light recycling through polarization management

[0067] Now go to Figure 6 , Figure 6 An alternative operating principle for the recycling of light along a light guide is shown. In this case, the light is recycled within a single light guide and undesired interactions between the image light and the outcoupling means (and / or redirection means for aperture expansion in the first dimension) are avoided by appropriate polarization management.

[0068] Specifically, in this case, and by way of non-illustration with reference to a facet-based light guide, the light guide 10 includes at least one set of mutually parallel polarization-selective internal partially reflective surfaces 16 inclined at an angle to the main surface to gradually couple out a portion of image light of a first polarization (e.g., S polarization) propagating in a forward direction to be reflected toward an observer, while transmitting a portion of the image light of the first polarization to continue propagating in the forward direction within the light guide. The partially reflective surfaces are implemented to be substantially transparent to light of a second polarization (e.g., P polarization) perpendicular to the first polarization. Recycling of image light arriving at the end of the light guide is achieved by providing a first reflective configuration including a quarter-wave phase plate 26 and a plane mirror 20A. The first reflective configuration is arranged to reflect image light transmitted by a set of partially reflective surfaces 16 so that the image light propagates in a rotated polarization (from S to P) within the light guide in a reflection direction to be transmitted by the partially reflective surfaces 16, the reflection direction having an in-plane component reversed relative to the forward direction. A second reflective configuration including a quarter-wave phase plate 26 and a plane mirror 20B is deployed to reflect at least a portion of the image light that has passed through the light guide along the reflection direction so that the image light propagates in a forward direction within the light guide with a rotated polarization (from P to S), so that at least a portion of the image light that was transmitted by the set of partially reflective surfaces when first passing through the set of partially reflective surfaces is coupled out to be reflected toward an observer when it is incident on the partially reflective surfaces again.

[0069] In some applications, such as in the reflective two-dimensional aperture expansion light guide shown, the polarization requirements may vary along the length of the light guide. Figure 2 , if the first set of partially reflective surfaces 14 is partially reflective for S polarization and transparent for P polarization, the different orientations of surfaces 14 and 16 cause the S polarization reflected from surface 14 to appear as P polarization to surface 16. In order to appear as S polarization to surface 16, it is advantageous to place a half-wave phase plate 30 ( Figure 6 ) is inserted between surfaces 14 and 16 to rotate the polarization. In this implementation, phase plate 30 complements the function of quarter wave phase plate 26 to keep the polarization of light propagating in the "reverse" direction (from right to left as shown) properly regulated, thereby avoiding undesirable interactions between the surfaces and the returning image light.

[0070] Applications of light recycling

[0071] The configurations described thus far can be used to advantageously improve the energy efficiency of a lightguide-based display by recycling light that would otherwise be lost. Recycling light that would otherwise be lost can, for example, help achieve enhanced display brightness even when low-visibility outcoupling devices with low reflectivity or low diffraction efficiency are employed, such as when it is desired to minimize the visibility of the outcoupling device. Repeated passage of image light through a set of facets with low reflectivity or through a diffractive optical element with low coupling efficiency can be used to compensate for the low outcoupled power from each transmission of the image light through the outcoupling device.

[0072] Furthermore, the various schemes presented above for recycling image light are also considered enabling technologies for a range of applications. In an important subset of additional applications, the recycled image light does not follow the same path as the light originally injected into the light guide. 7A to 7C One such example is schematically shown in .

[0073] Specifically, Fig. 7A A light guide 10 is shown having a set of out-coupling partially reflective surfaces 16 having an image injection aperture (schematically indicated by arrows 12) to inject image light from an appropriate projector into the light guide so that the image light propagates in a first direction by internal reflection at the major surfaces (the front and rear surfaces in the plan view shown). The first direction is inclined relative to the first and second reflective configurations (mirrors M1 and M2 and quarter-wave phase plate 26) so that the image light introduced at the image injection aperture is incident on a first area of ​​the partially reflective surface 16 and, after reflection from the first and second reflective configurations, is incident on a second area of ​​the partially reflective surface 16, the second area at least partially not overlapping the first area. Thus, continuous recycling of the image light illuminates continuous areas of each partially reflective surface 16, thereby achieving aperture expansion in the X direction (in addition to the expansion in the Y direction achieved by partial reflection at a series of surfaces). Thus, the device can achieve two-dimensional aperture expansion.

[0074] In cases where multiple cycles of light recycling are to be used, it is preferred that the reflectivity of the partially reflective surfaces, at least in the first region, be selected to be sufficiently low so that most of the image light injected at the image injection aperture is transmitted through the set of partially reflective surfaces and recycled by reflection at the first reflective configuration and the second reflective configuration.

[0075] Therefore, if Fig. 7AAs shown, S polarized light ray i is injected into the light guide at P1 and is captured in the y dimension due to the parallel reflectors (M1 and M2). Light ray i is also captured in the z dimension (TIR - not shown in the page). In the x dimension, light ray i will eventually be absorbed and lost on the edge of the light guide. When light ray i hits the lower quarter-wave plate (Quarter-Wave Plate, QWP) 26 and M2, it will be reflected back into the light guide and the polarization of light ray i will become P polarization. When light ray i hits the QWP and M1, it will be reflected back into the light guide and the polarization of light ray i will change back to S polarization. Assuming that the partially reflective surface 16 is implemented as a multilayer dielectric coating, which is designed to partially reflect and couple out only some of the S polarized light and is transparent to P polarized light, only the light from the top to the bottom will be partially reflected to guide their path to the user's eyes. Figure 7B and Fig. 7A Similar, but illustrating that the partially reflective facets do not necessarily have to cover the entire light guide aperture, but can be constrained to cover only a portion of the entire light guide aperture. Figure 7C Shown with Fig. 7A Another situation similar to the situation in Figure 7C In , the 2D restrictions are in the x and z dimensions.

[0076] In the above examples, the partially reflective facets are shown parallel to the reflectors M1 and M2, but this is only a non-limiting example, and the facets can be implemented at any desired angle, for example, perpendicular to the propagation direction of the main ray of the image light. In addition, the reflectors and QWPs are shown at the edge of the waveguide, but this is only a non-limiting example, and these elements can be positioned at other locations within the light guide.

[0077] As light propagates within the waveguide and bounces back and forth between the reflectors M1 and M2, the light is gradually coupled out of the waveguide and into the eye box ("EMB" - the area where the user's eyes are assumed to view the projected image). As a result, the intensity of the image light propagating within the waveguide gradually decreases, potentially resulting in intensity gradients across the field of view (FoV) and across the eye box. To overcome this effect, the facet coating can be implemented such that its reflectivity decreases along the waveguide. Fig. 7A The x-axis in the light guide gradually increases, i.e., gradually increases or step-wise increases of different reflectivities. In one such implementation, the facet may include multiple portions, which are not necessarily aligned into a continuous surface, wherein the reflectivity of each portion increases according to their position along the x-axis. This at least partially compensates for the reduced image light intensity propagating within the light guide during the sequential recycling of light. Figure 8 This option is shown in .

[0078] To achieve a uniform output image, the light propagating in the waveguide should uniformly cover the entire output area, i.e. the area where the facets must output light to the eyebox. This is called aperture filling. Fig. 9 The ray trace of a single field (i.e. a single collimated ray that ultimately corresponds to a pixel of the projected image on the retina of the observer) injected into the waveguide is shown. The injected field has an initial width D in the x-axis and makes an angle α with respect to the normal to the extension direction of the parallel mirrors M1 and M2. The height of the waveguide is denoted by H. It is evident that aperture filling is achieved if the projection of the injected ray on the xy plane results in an angle α0 with respect to the normal to the mirrors M1 and M2, where:

[0079]

[0080] Fields with angle α < α0 will overlap with the entrance aperture of the light guide, which may result in energy losses. Fields with angle α > α0 will not be completely filled. However, since the diameter of the user's eye pupil is several millimeters, the eye will average out local inhomogeneities to a certain extent. Therefore, even with partial aperture filling, satisfactory results can be achieved. In general, the aperture filling ratio η for fields with α > α0 can be quantified as η = α0 / α.

[0081] Minimum level of pore filling η min Should be determined by the requirements of the specific system (e.g., assumed size of the eye pupil, required intensity uniformity). min The value of determines the maximum field of view in the xy plane that can be supported by the waveguide:

[0082]

[0083] Maximum field of view FOV supported by waveguide max It grows linearly with the input aperture size D, and inversely with the distance H between the mirrors M1 and M2. The size of the projector is usually proportional to the input size, so increasing the size of the aperture D may not be a desirable option, especially in compact systems. Therefore, in practice, it may be preferred to have a short distance between the mirrors M1 and M2 so that a large field of view can be directed. This is in Fig.10 It is schematically shown in FIG.

[0084] However, it is important to note that the size of the FoV and the eyebox (both in the y-direction) dictate the minimum spacing between mirrors M1 and M2, which may impose design constraints that limit the extent to which the FoV can be extended in the x-direction by this approach. Fig.11As shown, an additional approach to addressing these limitations is to add a partially reflective surface or "mixer" 32, which is deployed inside the light guide, perpendicular to the main surface and parallel to the first and second reflective configurations (mirrors M1 and M2). The mixer preferably has a reflectivity of about 50%. The mixer is used to reduce the effective distance between the mirrors (which here is the distance between the mirrors and the mixer) while still supporting a large active area and therefore also a large eye box and a large FoV, so the mixer helps fill the light guide aperture and improve overall uniformity. Optionally, multiple mixers can be added at different locations in the light guide. Orthogonal mixers (with Figure 2 The mixers 18 in the embodiment have the same orientation of the mixers).

[0085] FIG. 7A to FIG. 11 Various aperture expansion architectures can be implemented using any of the above-described recycling principles, either with separate light guides for return propagation or using polarization management within a single light guide. In the latter case, where a hybrid 32 is used, it is preferably combined with a quarter wave plate on one side and a quarter wave plate of opposite sign on the other side. This ensures that any light transmitted through the hybrid has its polarization state unchanged, while any light reflected has its polarization state rotated to switch P to S and S to P.

[0086] Since 2D confinement preserves energy on both axes, it is expected to be more efficient than a typical 1D confinement waveguide. For good image quality, the mirrors (M1 and M2) should be of high optical quality and parallel to within high manufacturing tolerances. Furthermore, it is preferred that the mirrors will be embedded in the eyeglass frame or otherwise covered to avoid unwanted double images of the world reaching the user's eyes.

[0087] exist Fig.12 Another method of enhancing image uniformity is shown in . In this example, one of the reflectors is provided with a phase plate 26' having a gradually varying efficiency that increases along the x-axis, and non-pure P-polarized light is injected into the light guide at point P1. When the light ray i reaches the gradient efficiency phase plate, only a portion of the P-polarized light is converted to S-polarized light. As the light ray proceeds in the x-direction, the efficiency of the phase plate increases, becoming gradually closer to the performance of a full QWP that converts all P-polarized light to S-polarized light, thereby producing maximum reflected intensity. In this way, the intensity (which is a function of the S-polarized light reflected by the facet) can be controlled along the x-axis direction.

[0088] More generally, it should be noted that while quarter wave plates are considered an appropriate choice for various implementations of the invention described herein, some architectures, and particularly where more complex geometries are involved, may employ other fractional wave phase plates associated with mirrors at the periphery of the light guide and / or otherwise placed in the optical device.

[0089] Another parameter that can be varied to help account for non-uniformity is a gradual change in the reflectivity of one of the mirrors, M2, where the reflectivity of the mirror will increase as one proceeds along the x-axis.

[0090] 3D confinement waveguide

[0091] exist Fig.13A and Fig. 13B Another embodiment of the present invention can be seen in Fig.13A In the case of , there are four mirrors on all edges of the waveguide (M1 is parallel to M2, and M3 is parallel to M4), and there are two fractional phase plates 26 at the top and bottom edges. Fig. 13B In the case of , there are four mirrors and four fractional phase plates 26 on all waveguide edges. Due to the combination of TIR and parallel mirrors, in both cases the injected image light is captured in all three dimensions.

[0092] exist Fig.13A In the example above, S polarized light i is injected into the waveguide at P1. Fig. 7A Similar to the case of , the polarization of ray i repeatedly switches between S polarization and P polarization as the light propagates in the x-direction. When ray i impinges on M3, it will be reflected to the light guide as S polarization. Since the returning ray i at P6 has an odd reflection compared to the ray i propagating at P2 and P3, the returned image will be rotated compared to the propagated image. This can be mitigated by controlling the coating reflection and LOE facet angle as a function of the incident light angle to the extent that only the propagated ray i will be coupled out of the waveguide. The polarization selectivity and angle selectivity associated with two of the four images propagating in the light guide 10 provide more design flexibility to select the desired unique image for coupling out.

[0093] exist Fig.14A and Fig. 14B Another method of coupling out only the propagation image is shown in Figures 2 to 5B A dual waveguide method for including one dimension and Figure 6 Various components are labeled with the same reference numerals as used throughout this document to identify equivalent features.

[0094] The size of the coupling region (where an index-matched adhesive is used to avoid TIR and allow transmission of image light between the lightguides) is typically tailored for a single field and can result in a "hole" in the aperture filling where light intended to be coupled into the lower waveguide 10B is reflected back into waveguide 10A. To address this effect, it may be advantageous to use Fig. 14C The configuration shown is similar to the above Figure 3E A configuration in which light is reflected by two 45 degree reflectors 20A1 and 20A2, 20B1 and 20B2 (e.g., right angle prisms) so that the entire bonding interface between the waveguide 10A and the waveguide 10B can be realized using a low refractive index adhesive (or air space) that maintains the TIR condition for light incident above the critical angle.

[0095] exist FIG. 15A to FIG. 15C Another method for coupling out only the propagation image is shown in FIG. In this case, the reflector M3 and the reflector M4 are not parallel. As above in Figure 3C and Figure 3D As discussed in the context of , the angles between reflectors M3 and M4 and the major surfaces must be equal in magnitude, but with reflection symmetry about a plane perpendicular to the two surfaces of the waveguide. The use of tilted reflectors M3 and M4 allows the laterally recycled light to enter a different range of propagation angles, which can aid in the design of angle-selective partially reflective coatings for the facets to select the desired image.

[0096] exist Fig.15A and Fig. 15B In the example, S polarized light is injected into the waveguide at P1. Fig.13A Similar to the case of , when the light propagates along the x direction, the i polarization of the light will switch between S polarization and P polarization. In the current case, if Fig. 15B As shown, when ray i strikes M3 at P2, it will be reflected back into the waveguide as S polarization, but at a different angle relative to major surface S1 and major surface S2. The returning ray i strikes M4 at P3 and returns to the waveguide only at the same angle relative to major surface S1 and major surface S2 as the first propagating ray i. Since the non-parallel mirror switches between two different angle zones relative to the major surfaces, a specific coating can be selected to reflect only the propagating image out of the waveguide while fully transmitting the returned rotated image.

[0097] exist Fig. 15C3D lightguide confinement using four non-parallel mirrors is shown in FIG. In this case, the angles between mirrors M1 and M2 and the main surface must be equal, but with reflection symmetry about a plane perpendicular to the waveguide surface. The angles between mirrors M3 and M4 and the main surface must be equal, but with reflection symmetry about a plane perpendicular to the waveguide surface.

[0098] exist FIG. 16A to FIG. 16C Another method for selectively coupling out only the desired propagating image is shown in FIG. In this case, three waveguides 10A, 10B and 10C are separated by air gaps or low refractive index adhesives. Lightguide 10A has an outcoupling facet, while lightguides 10B and 10C are transparent lightguides in the present non-limiting example. Lightguide 10A is coupled to lightguide 10B by high reflectors (M5 to M8) with complementary angles ( Fig. 16C The light guide 10A is coupled to the light guide 10C through another set of high reflective mirrors (M1 to M4) with complementary angles ( Fig. 16B ). FIG. 16A to FIG. 16C Such a configuration is shown by way of example, where the mirrors are oriented at 45 degrees compared to the waveguides, and the major surfaces of the waveguides are parallel to each other. Mirrors M9 to M12 are additional high reflectors positioned to confine light in all three lightguides.

[0099] In this example, light ray i is injected into the light guide 10A at P1. Figure 3E A somewhat similar situation for two waveguides has been presented in . The reflections and facet orientations as a function of angle are specifically chosen to couple out the propagating image while minimizing undesirable ghost images.

[0100] Since 3D confinement preserves energy on all three axes, the system is expected to be more efficient than prior art 1D and the above-mentioned 2D confined waveguides. Therefore, the mixer elements for the 3D case may be less efficient.

[0101] Although the examples shown so far for 3D and 2D confinement involve rectangular confinement between two pairs of parallel reflectors arranged orthogonally to each other and a set of partially reflecting facets, other confinement shapes may be formed, such as triangular, circular, hexagonal, etc., and additional sets of partially reflecting facets may be used, whether or not with reflection or rotational symmetry about the normal to the waveguide major surface. Fig.17An alternative example of additional design freedom is shown in , which uses 3D confinement using four pairs of parallel mirrors labeled M1 to M8 and includes two sets of partially reflecting facets 16, 16' arranged at different angles. A quarter wave plate, an eighth wave plate or any other fractional wave plate 26 can be associated with any or all of the mirrors.

[0102] exist Fig.18A and Fig.18B Another degree of freedom is shown in . In this case, the absorbing layer 40 is positioned on the side of the light guide ( Fig.18A ) or the interior of the light guide ( Fig.18B ) to block unwanted light paths.

[0103] The examples shown herein have employed reflective light guides based on arrays of internal partially reflective surfaces to illustrate aspects of the invention. However, the invention is not limited to such implementations and may equally be implemented in light guides employing pin mirror light guides and in diffractive light guides employing surface or volume diffractive optical elements, etc.

[0104] Another degree of freedom would be to use an active half wave plate on one or all of the limiting mirrors with or without another light guide.

[0105] exist FIG. 19A to FIG. 19C A final example is shown in , which uses the degrees of freedom already described in a slightly different configuration. In this case, a ray i is injected into the lightguide using an incoupling element A. The ray i is guided through the top lightguide between the reflectors M1 and M2 and their respective QWPs and the main surfaces S1 and S2 until it reaches the reflector M3, where the image light is reflected to return through the lightguide without facets, is guided between M1 and M2 and their respective QWPs and the main surfaces S2 and S3 until it reaches the reflector M4 and returns to the lightguide with facets as ray j. The propagation angle of the ray i can be chosen so that the ray j and the ray j n The coupling element is not impacted.

[0106] Alternatively, in another case, ray j or ray j n will hit the coupling element. In this case, the coupling element will be an anti-coupling prism, which is a specially cut prism that causes the incoming ray j to exit at a complementary desired angle (the same angular orientation as ray i). The coupling element can be a partial mirror (e.g., 10% transmission mirror and 90% reflection mirror, or other ratio mirrors).

[0107] It will be appreciated that the above description is intended to be exemplary only and that many other embodiments are possible within the scope of the invention as defined in the appended claims.

Claims

1. A display for delivering image light to an observer, the display comprising: (a) a first light guide, the first light guide comprising a block of transparent material, the first light guide having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the major surfaces, the light guide comprising at least one set of mutually parallel internal partially reflective surfaces, the at least one set of mutually parallel internal partially reflective surfaces being inclined relative to the major surfaces to gradually couple out a portion of the image light propagating in a first direction so that the portion of the image light is reflected toward an observer, while transmitting a portion of the image light so that the portion of the image light continues to propagate in the first direction within the first light guide; (b) a second light guide comprising a block of transparent material, the second light guide having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the major surfaces, the second light guide being disposed in a stacked relationship with the first light guide; (c) a first reflective coupling configuration, the first reflective coupling configuration comprising at least one plane mirror, the first reflective coupling configuration being arranged to reflect the image light transmitted by the set of partially reflective surfaces and propagating in the first direction in the first light guide, so that the image light propagates in a second direction within the second light guide, the second direction having an in-plane component that is reversed relative to the first direction; as well as (d) a second reflective coupling configuration, the second reflective coupling configuration comprising at least one plane reflector, the second reflective coupling configuration being arranged to reflect image light having passed through the second light guide along the second direction to reintroduce the image light to propagate along the first direction in the first light guide, so that at least a portion of the image light transmitted by the set of partially reflective surfaces when first passing through the set of partially reflective surfaces is coupled out to be reflected toward the observer when it is incident on the partially reflective surfaces again.

2. A display for delivering image light to an observer, the display comprising: (a) a first light guide, the first light guide comprising a block of transparent material, the first light guide having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the major surfaces, the first light guide comprising at least one set of mutually parallel internal partially reflective surfaces, the at least one set of mutually parallel internal partially reflective surfaces being non-parallel to the major surfaces for gradually redirecting a portion of the image light propagating in a first direction so that the portion of the image light propagates in a second direction within the first waveguide by internal reflection, while transmitting a portion of the image light so that the portion of the image light continues to propagate in the first direction within the first light guide; (b) a second light guide comprising a block of transparent material, the second light guide having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the major surfaces, the second light guide being disposed in a stacked relationship with the first light guide; (c) a first reflective coupling configuration, the first reflective coupling configuration comprising at least one plane mirror, the first reflective coupling configuration being arranged to reflect the image light transmitted by the set of partially reflective surfaces and propagating in the first direction in the first light guide, so that the image light propagates in a third direction within the second light guide, the third direction having an in-plane component that is reversed relative to the first direction; as well as (d) a second reflective coupling configuration, the second reflective coupling configuration comprising at least one plane mirror, the second reflective coupling configuration being arranged to reflect image light having passed through the second light guide along the second direction to reintroduce the image light to propagate along the first direction in the first light guide, so that at least a portion of the image light transmitted by the set of partially reflective surfaces when first passing through the set of partially reflective surfaces is redirected to propagate along the second direction when it is incident on the partially reflective surfaces again.

3. The display according to claim 1 or claim 2, wherein: Each of the first reflective coupling configuration and the second reflective coupling configuration includes a single plane reflector and a light transmission region adjacent to the single plane reflector, wherein an optical continuum exists between the first light guide and the second light guide within the light transmission region for transmitting image light between the first light guide and the second light guide.

4. The display according to claim 3, wherein: Each of the single planar mirrors is oriented perpendicular to the major surfaces of the first and second light guides.

5. The display according to claim 3, wherein: The single plane reflector of the first reflection coupling configuration is oriented to have a first tilt angle relative to the major surfaces of the first light guide and the second light guide, and wherein the single plane reflector of the second reflection coupling configuration is oriented to have a second tilt angle relative to the major surfaces of the first light guide and the second light guide, and the second tilt angle is equal to the first tilt angle but opposite to the first tilt angle.

6. The display according to claim 3, wherein: The optical continuum is achieved by connecting the first light guide to the second light guide using an index matching adhesive.

7. The display according to claim 3, wherein: The optical continuum is achieved by connecting a continuous block of transparent material to edge surfaces of the first and second light guides.

8. The display according to claim 1 or claim 2, wherein: Each of the first reflective coupling configuration and the second reflective coupling configuration includes a pair of mutually perpendicular plane mirrors.

9. The display according to claim 1 or claim 2, wherein: The second light guide is attached to the first light guide by an adhesive layer having a refractive index lower than the refractive index of the transparent material of the first and second light guides.

10. The display according to claim 1 or claim 2, wherein: The second light guide is separated from the first light guide by an air gap.

11. A display for delivering image light to an observer, the display comprising: (a) a light guide comprising a block of transparent material, the light guide having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the major surfaces, the light guide comprising at least one set of mutually parallel polarization-selective internal partially reflective surfaces, the at least one set of mutually parallel polarization-selective internal partially reflective surfaces being inclined relative to the major surfaces to gradually couple out a portion of image light of a first polarization propagating in a first direction so that the portion of the image light of the first polarization is reflected toward an observer, while transmitting a portion of the image light of the first polarization so that the portion of the image light of the first polarization continues to propagate within the light guide along the first direction, the partially reflective surfaces being substantially transparent to light of a second polarization orthogonal to the first polarization; (b) a first reflective configuration comprising a quarter wave phase plate and a plane mirror, the first reflective configuration being arranged to reflect the image light transmitted by the set of partially reflective surfaces so that the image light propagates within the light guide in a second direction with a rotated polarization to be transmitted by the partially reflective surfaces, the second direction having an in-plane component reversed relative to the first direction; as well as (c) a second reflection configuration, the second reflection configuration comprising a quarter wave phase plate and a plane mirror, the second reflection configuration being arranged to reflect the image light having passed through the light guide along the second direction so that the image light propagates within the light guide along a direction parallel to the first direction, so that at least a portion of the image light transmitted by the set of partially reflective surfaces when passing through the set of partially reflective surfaces for the first time is coupled out to be reflected toward the observer when it is incident on the partially reflective surfaces again.

12. The display of claim 11 further comprising an image injection aperture for injecting the image light into the light guide to propagate in the first direction by internal reflection at the major surface, and wherein, The first direction is inclined relative to the first reflection configuration and the second reflection configuration, so that the image light introduced at the image injection aperture is incident on a first area of ​​the partially reflective surface, and after reflecting from the first reflection configuration and the second reflection configuration, is incident on a second area of ​​the partially reflective surface, and the second area does not at least partially overlap with the first area.

13. The display according to claim 12, wherein: The reflectivity of the partially reflective surfaces in at least the first region is such that a majority of the image light injected at the image injection aperture is transmitted through the set of partially reflective surfaces and recycled by reflection at the first and second reflective configurations.

14. The display according to claim 12, wherein: The quarter wave phase plate is part of a gradually varying phase plate.

15. The display of claim 12, further comprising a partially reflective surface disposed inside the light guide, perpendicular to the major surface and parallel to the first and second reflective configurations.

16. A display for delivering image light to an observer, the display comprising: (a) a light guide comprising a block of transparent material, the light guide having two mutually parallel major surfaces for supporting propagation of image light by internal reflection at the major surfaces, the light guide comprising at least one set of mutually parallel polarization-selective internal partially reflective surfaces, the at least one set of mutually parallel polarization-selective internal partially reflective surfaces being non-parallel to the major surfaces for gradually redirecting a portion of the image light propagating in a first direction so that the portion of the image light propagates in a second direction within the light guide by internal reflection, while transmitting a portion of the image light so that the portion of the image light continues to propagate in the first direction within the light guide, the partially reflective surfaces being substantially transparent to light of a second polarization orthogonal to the first polarization; (b) a first reflective configuration comprising a quarter wave phase plate and a plane mirror, the first reflective configuration being arranged to reflect the image light transmitted by the set of partially reflective surfaces so that the image light propagates within the light guide in a third direction with a rotated polarization to be transmitted by the partially reflective surfaces, the third direction having an in-plane component reversed relative to the first direction; as well as (c) a second reflection configuration, the second reflection configuration comprising a quarter wave phase plate and a plane mirror, the second reflection configuration being arranged to reflect the image light having passed through the light guide along the third direction so that the image light propagates within the light guide along a direction parallel to the first direction, so that at least a portion of the image light transmitted by the set of partially reflective surfaces when first passing through the set of partially reflective surfaces is redirected to propagate along the second direction when it is incident on the partially reflective surfaces again.

Images