Lightguide-based display

Abstract

A lightguide-based display with improved polarization control is disclosed. The display includes a lightguide and an image projector. The lightguide includes partially-reflecting surfaces configured to deflect an image injected into the lightguide by the image projector so that it propagates in a second direction within the lightguide and is then coupled out of the lightguide. The partially-reflecting surfaces may be implemented as multilayer dielectric coatings configured to have a partial reflectance to p-polarization at a range of angles of incidence greater than a Brewster angle and to have a reduced reflectance at a lower range of angles of incidence.

Description

[0001] Lightguide-Based Display

[0002] FIELD AND BACKGROUND OF THE INVENTION

[0003] The present invention relates to optical displays, and more particularly, to lightguide-based displays with improved polarization management and coupling-out configurations.

[0004] Lightguide-based displays are used in various applications, such as near-eye displays and head-up displays. These displays often utilize partially reflecting surfaces within the lightguide to direct and expand the image towards the viewer. However, managing the polarization of light within the lightguide can be challenging due to the angularly varying polarization-dependence of the properties of multilayer dielectric coatings. In particular, the Brewster angle effect can lead to near-zero reflection for certain polarization states and angles of incidence, making it difficult to achieve efficient light extraction.

[0005] SUMMARY OF THE INVENTION

[0006] The present invention relates generally to optical displays, and more particularly to lightguide-based displays with improved polarization management and coupling-out configurations.

[0007] According to the teachings of an embodiment of the present invention there is provided, a display for displaying an image to an eye of a user, the display comprising: (a) a lightguide configuration formed from transparent material, the lightguide configuration having first and second mutually-parallel major surfaces for supporting propagation of light within the lightguide configuration by internal reflection at the major surfaces; and (b) an image projector optically coupled to the lightguide configuration so as to introduce light corresponding to a collimated image to propagate in a first direction within the lightguide configuration by internal reflection at the major surfaces, wherein the lightguide configuration includes a first set of partially-reflecting internal surfaces deployed to progressively deflect the light propagating in the first direction so as to propagate in a second direction within the lightguide configuration by internal reflection at the major surfaces, and a second set of partially-reflecting internal surfaces deployed to progressively deflect the light propagating in the second direction so as to be coupled out through the second major surface of the lightguide configuration towards the eye of the user, and wherein the light injected by the image projector into the lightguide configuration includes image light rays with p- polarization relative to the major surfaces, and wherein the first set of partially-reflecting internal surfaces are oriented such that the image light rays are primarily s-polarized relative to those surfaces, and wherein a first set of the image light rays propagating in the second direction and reflected off the first major surface spans a first range of propagation angles relative to the major surfaces and a second set of the image light rays reflected off the second major surface spans a second range of propagation angles, opposite to the first range of propagation angles, relative to the major surfaces, the second set of partially-reflecting internal surfaces being implemented as multilayer dielectric coatings configured and oriented so as to have a partial reflectance to p- polarization within a first range of angles of incidence corresponding to the first range of propagation angles so as to progressively couple out the first set of image light rays towards the eye of the user and to have a reduced reflectance no greater than half of the partial reflectance to p-polarization within a second range of angles of incidence corresponding to the second range of propagation angles, wherein the first range of angles of incidence is greater than the second range of angles of incidence and greater than the Brewster angle for the second set of partially-reflecting surfaces.

[0008] According to a further feature of an embodiment of the present invention, the second range of angles of incidence includes the Brewster angle for the second set of partially-reflecting surfaces.

[0009] According to a further feature of an embodiment of the present invention, the second range of angles of incidence is below the Brewster angle for the second set of partially-reflecting surfaces.

[0010] According to a further feature of an embodiment of the present invention, the reduced reflectance is less than one third of the partial reflectance.

[0011] According to a further feature of an embodiment of the present invention, the image light rays injected by the image projector into the lightguide configuration are entirely plane polarized with p-polarization relative to the major surfaces.

[0012] According to a further feature of an embodiment of the present invention, the image light rays injected by the image projector into the lightguide configuration include a mixture of p- polarization and s-polarization relative to the major surfaces.

[0013] According to a further feature of an embodiment of the present invention, the second set of partially-reflecting internal surfaces define a coupling-out arrangement having an active area, the active area having a first distal corner and a second distal corner illuminated by laterally outermost rays from the first set of partially-reflecting internal surfaces, a coupling-out- arrangement entrance plane being defined as a plane perpendicular to a chief ray of the collimated image, located at a proximal boundary of the coupling-out arrangement, a distal baseline of the active area being a straight line between the first and second distal corners, wherein a first extreme ray propagates from the coupling-out-arrangement entrance plane to the first distal corner of the active area, the first extreme ray together with a line passing through the first distal corner perpendicular to the distal baseline and the coupling-out-arrangement entrance plane defining a first peripheral triangle, and wherein a second extreme ray propagates from the coupling-out- arrangement entrance plane to the second distal corner of the active area, the second extreme ray together with a line passing through the second distal corner perpendicular to the distal baseline and the coupling-out-arrangement entrance plane defining a second peripheral triangle, the active area extending over more than 50% of the area of at least one of the peripheral triangles so as to provide a part of the image from the at least one of the peripheral triangles to the EMB.

[0014] There is also provided according to the teachings of an embodiment of the present invention, a display for displaying an image to an eye of a user located at an eye-motion box (EMB), the display comprising: (a) a lightguide configuration formed from transparent material, the lightguide configuration having a pair of mutually-parallel major surfaces for supporting propagation of light within the lightguide configuration by internal reflection at the major surfaces, the lightguide configuration having a coupling-out arrangement configured for coupling out light propagating within the lightguide configuration towards the EMB; and (b) a projecting arrangement comprising an image projector optically coupled to an optical aperture expander, the projecting arrangement being deployed to introduce image light corresponding to a collimated image to propagate within the lightguide configuration by internal reflection at the major surfaces, wherein a coupling-out-arrangement entrance plane is defined as a plane perpendicular to a chief ray of the collimated image, located at a proximal boundary of the coupling-out arrangement, wherein the coupling-out arrangement has an active area from which image light is coupled out towards the EMB, the active area having a first distal corner and a second distal corner illuminated by laterally outermost rays from the coupling-out- arrangement entrance plane, a distal baseline of the active area being a straight line between the first and second distal corners, wherein a first extreme ray propagates from the coupling-out-arrangement entrance plane to the first distal corner of the active area, the first extreme ray together with a line passing through the first distal corner perpendicular to the distal baseline and the coupling-out-arrangement entrance plane defining a first peripheral triangle, and wherein a second extreme ray propagates from the coupling-out- arrangement entrance plane to the second distal corner of the active area, the second extreme ray together with a line passing through the second distal corner perpendicular to the distal baseline and the coupling-out-arrangement entrance plane defining a second peripheral triangle, the active area extending over more than 50% of the area of at least one of the peripheral triangles so as to provide a part of the image from the at least one of the peripheral triangles to the EMB. According to a further feature of an embodiment of the present invention, the coupling-out arrangement comprises a set of mutually-parallel partially reflecting surfaces internal to the lightguide and obliquely angled to the major surfaces.

[0015] According to a further feature of an embodiment of the present invention, the coupling-out arrangement comprises a diffractive optical element deployed on or in the lightguide.

[0016] According to a further feature of an embodiment of the present invention, the active area extends over more than 50% of the area of both of the peripheral triangles.

[0017] According to a further feature of an embodiment of the present invention, the active area extends over more than 90% of the area of at least one of the peripheral triangles.

[0018] According to a further feature of an embodiment of the present invention, the active area extends over more than 90% of the area of both of the peripheral triangles.

[0019] According to a further feature of an embodiment of the present invention, the optical aperture expander comprises a set of mutually-parallel partially reflecting surfaces located within the lightguide.

[0020] According to a further feature of an embodiment of the present invention, the optical aperture expander comprises an optical arrangement including one or more refractive or reflective lenses.

[0021] According to a further feature of an embodiment of the present invention, the active area is substantially trapezoid.

[0022] According to a further feature of an embodiment of the present invention, the display is a near-eye display.

[0023] According to a further feature of an embodiment of the present invention, the display is a head-up display.

[0024] According to a further feature of an embodiment of the present invention, the head-up display is configured for use in a vehicle and wherein the projecting arrangement is configured to project the collimated image from a lower portion of the lightguide configuration towards an upper portion of the lightguide configuration.

[0025] According to a further feature of an embodiment of the present invention, the active area is asymmetric, extending over a different percentage of each of the peripheral triangles.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: FIG. 1A is a schematic side view of a lightguide-based display according to a first embodiment of the present invention.

[0028] FIG. IB is an enlarged view of a region of the lightguide-based display of FIG. 1A designated “I”.

[0029] FIG. 2A is a schematic front view of the lightguide-based display of FIG. 1A.

[0030] FIG. 2B is a schematic view of a region of the lightguide-based display of FIG. 1A, taken along the direction of an inclined partially reflecting surface.

[0031] FIG. 3A is a graph showing the reflectivity profile of a typical multilayer dielectric coating employed in conventional reflective lightguides as a function of angle of incidence for s- polarization and p-polarization.

[0032] FIG. 3B is a graph showing the reflectivity profile of a modified multilayer dielectric coating according to an aspect of the present invention as a function of angle of incidence for s- polarization and p-polarization for use in the lightguide-based display of FIG. 1 A.

[0033] FIG. 4A is a schematic view of a second embodiment of a lightguide-based display according to the present invention, with p-polarized light injected into the waveguide.

[0034] FIG. 4B is a schematic view of a third embodiment of a lightguide-based display according to the present invention, with s-polarized light injected into the waveguide.

[0035] FIG. 5A is a schematic side view of a display according to a further aspect of the present invention employing a reflective coupling-out arrangement.

[0036] FIG. 5B is a schematic side view of a display according to the further aspect of the present invention employing a diffractive coupling-out arrangement.

[0037] FIG. 6A is a schematic front view of a lightguide with a rectangular coupling-out area.

[0038] FIG. 6B is a schematic front view of a lightguide according to the further aspect of the present invention employing a trapezoidal coupling-out active area.

[0039] FIG. 6C is a schematic representation of the region of an image generator which takes part in generating the image viewed in the system of FIG. 6A.

[0040] FIG. 6D is a schematic representation of the region of an image generator which takes part in generating the image viewed in the system of FIG. 6B.

[0041] FIG. 7 is a schematic diagram of a first alternative implementation of a lightguide-based display similar to the display of FIG. 6B with an asymmetric trapezoidal active area.

[0042] FIG. 8A is a schematic diagram of a second alternative implementation of a lightguidebased display similar to the display of FIG. 6B with a rounded active area.

[0043] FIG. 8B is an enlarged view of a region of the lightguide of FIG. 8A designated “III”. DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] The present invention relates generally to optical displays, and more particularly to lightguide-based displays with improved polarization management and coupling-out configurations.

[0045] Lightguide-based displays are used in various applications, such as near-eye displays and head-up displays. These displays often utilize partially reflecting surfaces within the lightguide to expand the image and direct it towards the viewer. However, managing the polarization of light within the lightguide can be challenging due to the angularly varying polarization dependence of the properties of multilayer dielectric coatings. In particular, the Brewster angle effect can lead to zero reflection for certain polarization states and angles of incidence, making it difficult to achieve efficient light extraction.

[0046] Two aspects of the present invention provide alternative solutions to this issue. A first aspect of the invention relates to a specific choice of orientations and properties of the partially reflecting surfaces so that a first set of partially reflecting surfaces operate with s-polarization relative to those surfaces while a second set operate with p-polarization but at a range of incident angles relative to those surfaces which are greater than the Brewster angle and with coatings designed to provide partial reflection at these high incident angles. A second aspect of the invention relates to the introduction of a thin metallic layer in the coatings of the partially reflecting surfaces to enhance reflectivity for p-polarization.

[0047] A further aspect of the invention, which may be used to advantage together with the first aspects but is not limited to such applications, relates to enhancing the field of view that can be projected for a given size of lightguide. This aspect of the invention relates to an optimization of the geometry of a lightguide having a coupling-out arrangement for coupling out an image from the lightguide, wherein an active area of the coupling-out arrangement is defined according to specified geometrical criteria.

[0048] Referring to FIGS. 1A and IB, a first embodiment of a lightguide-based display according to the present invention is shown. The display includes a lightguide configuration 102 formed of a transparent material, such as glass. Lightguide configuration 102 has a pair of mutually parallel major surfaces 9. An image projector 104 is optically coupled to the lightguide configuration 102 so as to introduce a collimated image 108 into a first section 10 of the lightguide. Only the central beam (“chief ray”) of the collimated image is shown in FIG. 1A for simplicity.

[0049] As best see in FIG. 2 A, the lightguide configuration 102 further includes a first set of partially-reflecting internal surfaces 13, deployed within first lightguide section 10, and a second set of partially-reflecting internal surfaces 14, deployed within a second lightguide section 12. The first set of partially-reflecting surfaces 13 is deployed to progressively deflect the collimated image 108 so that it propagates in a second direction within the lightguide configuration 102 by internal reflection at the major surfaces 9. The second set of partially-reflecting surfaces 14 is deployed to progressively couple out the image light towards an observer's eye 32 (FIG. 1A).

[0050] The partial reflectors are described as “progressively reflecting” the image light propagating along the lightguide sections in the sense that each surface (also referred to herein as a “facet”) reflects part of the image light and transmits part, with the transmitted part in turn being partially reflected at the next facet. This has the effect of expanding the effective optical aperture injected by projector 104. The properties of the multilayer dielectric coatings of facets 13 and 14 are polarization dependent. In the drawings, the orientation of the plane of polarization is shown as two-headed arrows when polarization (electric field of the light-beam) is parallel to the drawing plane and as circles when perpendicular to the drawing plane. The polarization is also defined as s-polarization when oriented parallel to any given incident plane and p-polarization when perpendicular.

[0051] The polarized light 108 propagates within lightguide section 10 until being reflected by partial reflector (facet) 13, as shown in FIG. 2A and in more detail in FIG. 2B. The light is s- polarized relative to facet 13, that is typically perpendicular to waveguide faces 9. Some of the light passes through as beam 22 and part is reflected towards section 12, as beam 24. Different parts of the field of view are carried by rays at different angles of inclination to the lightguide major surfaces and in different in-plane directions. For simplicity of presentation, only the chief ray and two extreme rays corresponding to lower corners of the field of view are illustrated in FIG. 2A. The various rays are coupled out by facets 14 as rays 30 (dotted lines - uppermost and lowest parts of field illustrated in FIG. 1A) which are directed towards an “eye motion box” (EMB) 18 (corresponding to the range of eye positions from which the display is designed to be viewed) for viewing by the eye 32 of the user. Although illustrated here schematically as a region external to the eye, a more precise analytical definition for defining an EMB is to ensure that the entirety of the projected image is visible for a given desired location of the eyeball center, and to add a region of tolerance around that eyeball center.

[0052] In a first preferred implementation of this embodiment, the light injected by the image projector 104 into the lightguide configuration 102 is p-polarized relative to the major surfaces 9 of the lightguide, i.e., the electric field of the light is mostly perpendicular to the major surfaces of the lightguide. The first set of partially-reflecting surfaces 13 is typically oriented at 90° to the major surfaces 9, or is at least at a relatively steep angle, so that the image light is primarily s- polarized relative to the surfaces 13. The second set of partially-reflecting surfaces 14 is oriented at an oblique angle to the major surfaces 9 and transverse to the (second) direction of propagation, so that the image light is p-polarized relative to the surfaces 14.

[0053] The second set of partially-reflecting surfaces 14 is implemented using multilayer dielectric coatings. The coatings are configured and oriented so as to have a partial reflectance to p-polarization within a range of angles of incidence greater than the Brewster angle for progressively coupling-out the projected image reflected from the surface further from the user’s eye towards the user while having a reduced reflectance within a lower range of angles of incidence, less than or overlapping the Brewster angle, within which rays of the inverted image reflected from the lightguide surface closer to the eye are incident on surfaces 14. These properties are discussed further below with reference to FIG. 3B.

[0054] In other embodiments, the image light injected into the lightguide may be unpolarized, circularly polarized or otherwise include a mixture of p-polarization and s-polarization relative to the major surfaces of the waveguide. In these cases, at least the p-polarized component of the light injected into the lightguide performs as described above.

[0055] Thus, this aspect of the present invention strategically manages the polarization of light within the lightguide to overcome the limitations of conventional coatings. By using p-polarized input light and carefully orienting the first set of reflectors, we ensure efficient reflection. The second set of reflectors is then designed to work with p-polarized light at angles where reflection is still effective, outside the region impacted by the Brewster angle effect. This arrangement allows for efficient light extraction and a brighter, more uniform image.

[0056] FIGS. 3A and 3B illustrate the reflectivity profiles of typical and modified multilayer dielectric coatings, respectively. The Brewster angle effect can be seen in both figures, where the reflectivity of p-polarized light drops to zero at a certain angle of incidence.

[0057] It should be noted that, in the context of multilayer coatings, a precise analytical definition of the Brewster angle may be non-trivial to calculate. Practically, for the purpose of the present invention, the multilayer coatings exhibit a minimum reflectivity for p-polarization as a function of angle, which may be zero or near- zero at some angle of incidence, and this angle may be taken empirically as the (effective) Brewster angle for our definitions. The effective Brewster angle may also be determined from the design software employed to design the coatings, where the Brewster angle is identified by the inability of the software to provide a design with a required reflectivity at that angle.

[0058] In FIG. 3A, the reflectivity profile of a typical multilayer dielectric coating is shown. The angular reflectivity profile of the s-polarization (dashed curve) denoted “S” and p-polarization (solid curve) denoted “P” is low at zero angle of incidence and high close to 90 degrees. The p- polarized light has zero reflectivity at the Brewster angle, which is approximately 45 degrees (but may vary according to coating design), while the s-polarization reflectivity does not have such a zero reflectivity angle.

[0059] The typical coating profile has low reflectivity at the medium-high angle of incidence range 40a, which is not used for coupling-out an image. Angular range 26a in FIG. 3A corresponds to the typical angular range that rays 108 impinge on facets 13 within lightguide section 10. It is apparent that the Brewster angle within this range prevents the use of p-polarization for facets 13, and only s-polarization can generate uniform reflectivity across the relevant angular range 26a. Consequently, it is preferable that projector 104 injects s-polarization relative to facets 13 (p- polarization relative to waveguide faces 9). If the low reflectivity range 40a does not overlap the required reflectivity range 26a (depending on specific system parameters), then this typical coating profile may be used for these facets.

[0060] In most conventional implementations of lightguide configurations of this type employing two sets of partially-reflecting surfaces for aperture expansion and coupling out, the range of incident angles used for coupling out of the image would also overlap the Brewster angle so that it would not be feasible to use p-polarized light relative to facets 14 for coupling out the image to the eye of the user. For this reason, some lightguide-based displays employ a retarder (waveplate) deployed between lightguide sections 10 and 12 to rotate the polarization to s-polarization relative to the lightguide and facets 14. However, the inclusion of a retarder extending across the lightguide configuration typically complicates manufacturing processes, particularly due to the dissimilar mechanical properties of materials typically used for the lightguide and for retarder elements, and may therefore reduce lightguide quality and / or significantly increase production costs. In contrast, according to this aspect of the present invention, by selection of a suitable range of propagation angles for the injected image relative to the lightguide surfaces and suitable design of the angle and properties of facets 14, the present invention achieves good uniformity of an output image while employing p-polarized image light relative to facets 14.

[0061] In FIG. 3B, the reflectivity profile of a modified multilayer dielectric coating is shown. This modified coating is designed to have a higher reflectivity for p-polarized light at higher angles of incidence, as shown by curve “P”. This allows for efficient reflection of p-polarized light by operating at angular ranges which are beyond the Brewster angle for the required partial reflection.

[0062] The modified coating profile in FIG. 3B preferably has a partial reflectivity at the medium- high angle of incidence range 40b, rendering this range useful for coupling out the projected image towards the eye of the user. Angular range 26b in FIG. 3B corresponds to the typical angular range at which rays 108 impinge on the first set of partially-reflecting surfaces 13 within lightguide section 10. As before, it is apparent that the Brewster angle within this range prevents the use of p-polarization for facets 13, and only s-polarization can generate uniform reflectivity across the angular range. Consequently, it is preferable that projector 104 injects s-polarization relative to facets 13 (p-polarization relative to waveguide faces 9). By designing the lightguide configuration so that the angular range of incident angles of the relevant image reaching partially-reflecting surfaces 14 is within range 40b, it becomes possible to employ image light which is p-polarized relative to facets 14 for coupling out the image towards the eye of the user. In certain particularly preferred implementations, the reflectivity within range 40b is within the range of 3%-15%. For any given implementation, the reflectivity preferably varies across the range 40b used for coupling out of the image by no more than 40% of its maximum value. The reflectivity in the range 42b (relevant for the inverted image) is preferably significantly lower so as to minimize the risk of troublesome ghost images and to maximize efficiency. Reflectivity within range 42b is preferably below 50% of the maximum reflectivity of range 40b throughout, and more preferably below 33% thereof. In certain non-limiting but preferred cases, range 42b overlaps the Brewster angle, resulting in zero reflectivity at that angle and low reflectivity values adjacent thereto.

[0063] Thus, the use of the coating profile of FIG. 3B for second set of partially-reflecting surfaces 14 together with an angular implementation in which coupling-out of the image occurs in range 40b, above the Brewster angle range of 26b, circumvents the limitations otherwise associated with using p-polarized image light for coupling out. To achieve the desired range of angles, the image that was last reflected from major surface 9 further from the eye of the user is that which is coupled- out towards the eye-motion box. The complementary inverted image, i.e., reflected from the surface closer to the eye of the user, falls within an angular range 42b relative to the partially- reflecting surfaces 14, within which the reflectivity is preferably no more than half, and more preferably no more than a third, of the reflectivity within range 40b. Angular range 42b may be entirely at angles of incidence smaller than the Brewster angle or, in some cases, may overlap the Brewster angle.

[0064] By way of one typical, but non-limiting, example of this implementation, the range of incident angles 40b partially reflected so as to be coupled out by facets 14 may cover the range 50-70 degrees. In an implementation where the central axis of the coupled-out image is perpendicular to the lightguide, this would correspond to a facet angle for facets 14 of about 60 degrees relative to the lightguide surfaces 9, and the corresponding propagation angles of the image light within the lightguide section 12 would be inclined at between 20-40 degrees to the major surfaces (i.e., angles of incidence of 50-70 degrees to a normal to the lightguide surfaces). These parameters may vary considerably depending on particular design considerations, including cases in which the coupling-out direction of the chief ray is inclined significantly relative to a perpendicular to the lightguide. In various preferred cases, the angle of facets 14 lies somewhere in the range of 45-70 degrees inclination to the lightguide surfaces, and the range of propagation angles for the shallow-angle rays of the image within the lightguide in certain preferred cases extends down to about 10 degrees, or even 5 degrees, inclination to the surfaces.

[0065] The architecture described herein with reference to FIGS. 1A-2B and 3B is designed to operate particularly with plane-polarized image light introduced with p-polarization relative to the lightguide major surfaces, as described thus far. It has been found, however, that high quality results can also be achieved from such an architecture by introducing unpolarized or mixed- polarization image light. In this case, the differing properties of the various facets to each polarization tend to compensate one for the other, typically leading to relatively uniform results. Nevertheless, to prevent a situation in which both polarizations are not illuminated for a certain part of the field, as might occur if injected s polarization is not reflected by surfaces 13 due to the Brewster angle effect and the injected p polarization not reflected surfaces 14, also due to the Brewster angle effect, it is preferable to maintain the aforementioned architecture in which reflection from partially-reflecting surfaces 14 is within angular range 40b also when using an unpolarized or mixed polarization injected image.

[0066] Turning now to the second aspect of the present invention, as an alternative to avoiding the Brewster angle for p-polarized light, this second aspect of the present invention provides an alternative approach in which the Brewster angle limitation is circumvented by adding a thin layer of metal to the partially reflecting surfaces. The metal layer ensures that there is some reflection of p-polarization even at the Brewster angle.

[0067] This approach may be implemented in a lightguide arrangement which appears generally similar to that of FIGS. 1A and 2 A, but relaxes the aforementioned limitations on the choices of propagation angles and corresponding facet angles. In certain preferred cases, the lightguide is further modified by addition of a mixer element. Referring now to FIGS. 4A and 4B, there are shown two distinct implementations of a lightguide-based display according to this second approach. The structure is similar to that of FIGS. 1A and IB, but includes a further partially reflecting surface 100 configured as a mixer for enhancing uniformity of the image. In FIG. 4A, image projector 104 injects an image with p-polarization relative to the lightguide major surfaces 9, while in FIG. 4B, the image projector injects an image with s-polarization relative to the lightguide major surfaces 9.

[0068] The layer of metal is preferably very thin, typically less than 10 nm and ideally around 2- 4 nm, and complements rather than replacing the multilayer dielectric coating. The metallic layer is typically formed as a layer of silver or aluminum, which can be applied during manufacture of the multilayer coatings using conventional techniques. This allows use of architectures in which either the first or the second set of facets works with p-polarization with a field that comes close to, or even overlaps, the Brewster angle without the reflected intensity dropping away to zero at those angles.

[0069] In FIG. 4A, image projector 104 injects an image with p-polarization relative to the lightguide major surfaces 9. First set of partially reflecting surfaces 13 are typically oriented either at 90° to the major surfaces 9 or at some other relatively steep angle so that the image light is primarily s-polarized relative to the partially-reflecting surfaces (facets) 13. Facets 13 can therefore be implemented using conventional multilayer dielectric coatings designed to provide the desired partial reflectivity for s-polarization within the relevant range of incident angles. The second set of partially-reflecting surfaces 14 are oriented at an oblique angle to the major surfaces 9 so that the image light is p-polarized relative to the surfaces 14. In this implementation, at least partially- reflecting surfaces 14 are implemented with addition of the aforementioned metallic layer so as to eliminate the zero reflectivity region around the Brewster angle.

[0070] A mixer 100, implemented as a partial reflector internal to the lightguide and parallel to the major surfaces, is in some cases provided at a location between the first set of partially reflecting surfaces 13 and the second set of partially reflecting surfaces 14. In the case of FIG. 4A, mixer 100 is particularly designed to provide partial reflection for p-polarized image light relative to the major surfaces 9 of the lightguide 106. Mixer 100 is preferably configured to exhibit roughly 50% reflectivity (e.g., between 40 and 60%) for p-polarization propagating within the range of propagation angles covered by the image light. The orientation of the mixer is parallel to the major surfaces, so the relevant ranges of angles typically do not include the Brewster angle, making it feasible to design mixer 100 using conventional multilayer dielectric coating design tools, and without any metallic layer. Nevertheless, the design will be distinct from the typical s-polarization mixer used in various conventional designs due to being optimized specifically for p-polarization.

[0071] In FIG. 4B, the image projector 104 injects an image with s-polarization relative to the lightguide major surfaces 9. In this case, the first set of partially reflecting surfaces 13, which are oriented either at 90° to the major surfaces 9 or at some other relatively steep angle to the major surfaces 9, receive the image light as primarily p-polarized relative to the surfaces 13. Thus, in this implementation, it is facets 13 which are implemented with the supplemental metallic layer to ensure non-zero reflectivity at the Brewster angle. The second set of partially-reflecting surfaces 14 are oriented at an oblique angle to the major surfaces 9 so that the image light is s-polarized relative to the surfaces 14, making it possible to employ conventional multilayer coatings without an additional metallic layer. In this case, mixer 100 is designed to provide partial reflectivity, preferably roughly 50%, for s-polarization.

[0072] In each of these cases, at least the set of partially-reflecting surfaces which encounter image light which is p-polarized are formed with the aforementioned additional metallic layer, thereby providing continuity of the partial reflection even where the angular range of the image illumination come close to, or even overlaps, the Brewster angle, resulting in improved image quality.

[0073] In all other respects, the embodiments of FIGS. 4 A and 4B are similar to the first embodiment described above, with equivalent elements labelled similarly.

[0074] Turning now to a third aspect of the present invention, this relates to an optimization of the geometry of a lightguide having a coupling-out arrangement for coupling out an image from the lightguide. This aspect of the invention is applicable to a lightguide-based display in which the coupling-out arrangement comprises partially reflecting surfaces, such as those described above with reference to the first aspect of the invention, as well as to other coupling-out arrangements, such as arrangements based on diffractive optics.

[0075] Referring to FIGS. 5 A and 5B, there are shown lightguide-based displays including a lightguide configuration 102 having a pair of mutually parallel major surfaces 9 and a coupling- out arrangement configured for coupling out an image propagating as rays 24 by internal reflection within the lightguide so as to be coupled out as rays 30 towards a user’ s eye 32 at an eye-motion box (EMB) 18. The image is provided by a collimated image projector 104 as image light 108. In the non-limiting examples illustrated here, lightguide configuration 102 has a first section 10 containing an optical aperture expansion configuration and a second section 12. (In other implementations, the optical aperture expansion configuration may be an optical arrangement external to the lightguide, as discussed further below.) The coupling-out arrangement is located in the second section 12 of the lightguide configuration 102. In the example of FIG. 5A, the optical aperture expansion arrangement and the coupling-out arrangement are implemented as sets of partially-reflecting internal surfaces 13 and 14, which may advantageously be according to one of the implementations described above, or may be any other implementation of a reflective lightguide arrangement for a display. In the example of FIG. 5B, coupling in and / or optical aperture expansion in lightguide section 10 are achieved by one or more diffractive optical element 25a, while coupling out is performed by a diffractive coupling-out arrangement 25b. This aspect of the present invention is also applicable to various hybrid combinations of reflective and diffractive elements, as are known in the art, which will not be individually described herein. In the subsequent drawings, only reflective lightguide implementations will be explicitly illustrated, but the design considerations described are equally relevant to diffractive and combination (hybrid) implementations, as will be self-evident to one ordinarily skilled in the art.

[0076] The coupling-out arrangement, whether reflective or diffractive, has an active area 44 from which image light is coupled out towards the EMB 18 so as to contribute to the projected image visible to the user at the EMB. The term “active area” is defined herein as the area of the lightguide containing a coupling-out arrangement which, in the overall system design, contributes to part of the projected image visible to the user’s eye at the designed eyeball center location, or within a predefined eye-motion box around that location. This aspect of the invention addresses an inefficiency in use of the lightguide which is inherent to the conventional rectangular active area 44, as will now be described with reference to FIG. 6A.

[0077] Specifically, in the case of a rectangular coupling-out active area 44, the active area has a base line 39 on the edge furthest from the direction of input of the image light (referred to as the “distal” edge) and two lateral edges 40al and 40a2 that are perpendicular to that base line 39. The intersection of base line 39 with lateral edges 40al and 40a2 define, respectively, a first distal corner 41a and a second distal corner 41b of active area 44. These corners provide the bottom lateral corners (in the non-limiting orientation illustrated here) of the field of view of the display reaching the EMB.

[0078] Guided beam 108 represents a beam in the center of the projected field. This beam propagates guided within section 10 to be reflected by facets 13 to beam 24 within section 12 where it is reflected by facets 14 towards the eye (dotted arrow, perpendicular to drawing plan). In addition to this chief ray path, the ray paths providing all four corners of the rectangular active area and corresponding EMB are here illustrated separately, where the rays in lightguide section 10 for the four corners of the field are labeled 36a2, 36a3, 36a4 and 36a5 and the corresponding reflected rays directed towards the corners of active area 44 are labeled 36b2, 36b3, 36b4 and 36b5, respectively.

[0079] Thus, the image light rays for the parts of an image to be displayed via corners 41a and 41b are provided by rays 36b2 and 36b5, respectively. These rays dictate the size of lightguide section 10 to be substantially wider than the active area 44. The result is an active area 44, and hence also an angular field of view at the EMB, which is relatively small compared to the overall dimensions of lightguide configuration 102.

[0080] Before specifying the modification of this structure according to the third aspect of the present invention, it will be useful to introduce a reference plane which we will call the “coupling- out-arrangement entrance plane” 45. The “coupling-out-arrangement entrance plane” is defined as a plane perpendicular to a chief ray 24 of the collimated image, located at a proximal boundary of the coupling-out arrangement active area 44. The terms “proximal” and “distal” are used herein the description and claims to identify elements relative to the direction of propagation of light through the system, with “proximal” referring to elements reached earlier along the paths of propagation, and “distal” referring to elements reached further along those paths. All rays of image light that contribute to the image viewed by the user at the EMB 18 pass through this entrance plane. The coupling-out-arrangement entrance plane is not a structure but a reference plane for the purpose of the geometrical definition of the present invention.

[0081] Having defined entrance plane 45, we can now define two lateral (peripheral) triangles of the lightguide portion 12. A first peripheral triangle 128 is delimited by the first lateral side 40a 1 of the rectangular active area, extreme ray 36b2 and the entrance plane 45. A second peripheral triangle 130 is delimited by the second lateral side 40a2, extreme ray 36b5 and the entrance plane 45. These two triangles define, for the case of a rectangular active area, a region of the lightguide section 12 which is needed for conveying rays of the image, but which is not used for coupling out of the image. It is a particular feature of this aspect of the present invention that some or all of one or both of these triangles is utilized to project part of the image towards the EMB, thereby achieving a higher proportion of the lightguide area that is useful for projecting an image than could be achieved using a rectangular active area.

[0082] Turning now to FIG. 6B, this illustrates a first non-limiting implementation of this aspect of the present invention. As before, active area 44 has a first distal corner 41a and a second distal corner 41b which are illuminated, respectively, by laterally outermost rays 36b2 and 36b5 from coupling-out-arrangement entrance plane 45. A distal baseline 39 of the active area is taken to be a straight line between first and second distal corners 41a and 41b.

[0083] In contrast to FIG. 6A, the active area 44 of FIG. 6B is expanded towards the entrance plane 45, thereby significantly increasing the area from which an image is coupled out towards the EMB but without requiring any additional lightguide area to conduct the image. This results in more efficient use of the lightguide area, with a larger proportion of the lightguide area being effective to deliver image light to the EMB.

[0084] Quantitatively, the architecture of this aspect of the present invention can be defined by referring again to peripheral triangles 128 and 130, which are defined by extreme rays 36b2 and 36b5, the entrance plane 45 and reference lines 40al and 40a2 perpendicular to distal baseline 39 that pass through distal corners 41a and 41b, respectively. Active area 44 preferably extends over more than 50% of the area of at least one, and preferably both, of the peripheral triangles 128 and 130 so as to provide a part of the image from the peripheral triangles to the EMB. In the example of FIG. 6B, the actual edges of the active area are designated 40b 1 and 40b2, forming a trapezoidal display area. The resulting required size of lightguide configuration 102 as shown here is clearly much more compact than that of FIG. 6 A for an equivalent total active area 44.

[0085] In the present example, the coupling-out arrangement employs a set of partially reflecting surfaces 14, although this aspect of the present invention is equally applicable to displays employing diffractive optical elements for coupling out of the image. A further set of partially reflecting surfaces 13 is provided for lateral expansion of the image. The coupling-out- arrangement entrance plane 45 is located between the two sets of partially reflecting surfaces 13 and 14.

[0086] In a near-eye display implementation, the trapezoidal form of the displayed field is typically implemented as a top-down image injection arrangement, where some cropping of a field of view towards the lower lateral sides of the field of view is generally acceptable, noting that the human peripheral vision / awareness tends to drop off towards the lower lateral areas, making them less noticed. A trapezoid coupling-out area 44 is thus ergonomically appropriate and allows, for a given size of coupling-out-arrangement entrance plane, to achieve a significantly larger coupling- out area 44. Conversely, for a given required area of image coupling out, a significantly smaller lightguide configuration (both sections 10 and 12) may be used.

[0087] In the present particularly preferred example, the active area 44 is substantially the entirety of at least one, and preferably both, of the peripheral triangles 128, 130. The term "substantially" in this context is defined as more than 90% of the area of each of the peripheral triangles 128, 130.

[0088] FIG. 6C is a schematic representation of the region of an image generator which takes part in generating the image viewed in the system of FIG. 6A. The entire image generator matrix is shown, with region 2a of the matrix which contributes to the projected image highlighted.

[0089] FIG. 6D is a schematic representation of the region of an image generator which takes part in generating the image viewed in the system of FIG. 6B. The entire image generator matrix is shown, but only region 2b of the matrix contributes to the projected image highlighted. The image projector and / or the coupling-in arrangement are configured to project a wider range of in-plane angles so that the projected image spans the increased dimension of the upper part of the active area (“upper” in the orientation illustrated here), thereby delivering image light to the active area within triangles 128 and / or 130. It is apparent that the lower corner regions of the matrix (in the orientation shown here) are not used for the projected image, and therefore do not need to be actuated. Depending upon the application, the image input may take into consideration the formfactor of the projected image and provide image data customized to the display shape. In other cases, where image data with rectangular frames is provided, the corners are typically truncated according to the trapezoidal displayable field. These truncated extremities of the field are in the periphery of the viewer’s field of view (FOV), and do not usually contain important content. In certain alternative implementations (not shown), it may be appropriate to employ either graduated optics or a modified image generator array to achieve varying effective pixel size so that the shorter side of the trapezoid has more closely spaced pixels than the longer side of the trapezoid.

[0090] FIG. 7 is a schematic diagram of a lightguide with an asymmetric trapezoidal active area 12b for use in a head-up display. The architecture of FIG. 7 is equivalent to that of FIG. 6B, with the exception that here the trapezoidal image 2b is asymmetric, i.e., the angle between side 40b 1 and lower side 48b is different from the angle between side 40b2 and lower side 48b. Such asymmetric trapezoid shapes provide additional design flexibility for certain applications, for example, where image information near one of the distal corners is of particular importance. This may arise, for example, in certain binocular applications (employing two such displays for two eyes), where cropping of the region of binocular overlap may in some cases need to be minimized while the outer peripheral corners may acceptably be cropped. In each such case, the asymmetry of the trapezoid 2b preferably matches the asymmetry of the field of view, thereby reducing the overall size of the waveguide 30b.

[0091] The implementation of FIG. 7 also illustrates that the configurations of the present invention may be used also in a bottom-up design, where the image is injected from the bottom of the lightguide configuration, and the trapezoidal field of view narrows upwards. This configuration may be particularly suited to certain applications such as, for example, a vehicular heads-up display, where the upwards narrowing of the display may match the perspective view of a road extending ahead of the vehicle and / or may be partially compensated for by an inclination of the lightguide, where the upper part of the lightguide is closer to the user and therefore subtends a larger angle than an equivalent width at the base of the display.

[0092] FIG. 8 A is a schematic diagram of a lightguide with a rounded active area 12b for use in a head-up display. The architecture of FIG. 8 A is equivalent to that of FIG. 6B, with the exception that here the output coupled beams 36b2 and 36b5 are tangential to the curved sides 40bl and 40b2 of the section of waveguide 12b. Additionally, the implementation of FIG. 8A illustrates the use of an optical arrangement including one or more refractive and / or reflective lens 105 to provide optical aperture expansion for injection into lightguide section 12, as an alternative to the lightguide-based aperture expansion described thus far.

[0093] FIG. 8B is an enlarged view of a portion of the lightguide of FIG. 8A, showing the relationship between the guided beams 36b2 and 36b5 and the curved sides 40bl and 40b2 of the section of lightguide 12. These beams are used to define the “distal corners” 41a and 41b and hence also distal baseline 39 as a straight line between these corners. This terminology allows the above definitions to be applied to rounded and otherwise irregularly shaped active areas.

[0094] In summary, the invention described with reference to FIGS. 5A-8B provides a method for enhancing the field of view that can be projected for a given size of lightguide. By utilizing a trapezoidal or other non-rectangular active area for the coupling-out arrangement, the invention allows for a more efficient use of the lightguide's surface area, particularly in near-eye display and head-up display applications.

[0095] The geometry optimization of the further aspect of the invention, as described above, may be implemented in combination with the polarization management techniques of the first aspect of the invention. In such a combined embodiment, the second set of partially reflecting surfaces 14 would serve as the coupling-out arrangement 38, and the active area 44 would be defined according to the geometric criteria described above. This combination allows for efficient light extraction and an enhanced field of view, particularly in near-eye display and head-up display applications.

[0096] Figures 5A and 6B can be taken as descriptive of such combined embodiments, where the polarization architecture, coatings, and propagation angles can be chosen to match the descriptions provided for the first and further aspects of the invention.

[0097] It will be appreciated that the above descriptions are intended only to serve as examples, and that many other embodiments are possible within the scope of the present invention as defined in the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A display for displaying an image to an eye of a user, the display comprising:(a) a lightguide configuration formed from transparent material, said lightguide configuration having first and second mutually-parallel major surfaces for supporting propagation of light within said lightguide configuration by internal reflection at said major surfaces; and(b) an image projector optically coupled to said lightguide configuration so as to introduce light corresponding to a collimated image to propagate in a first direction within said lightguide configuration by internal reflection at the major surfaces, wherein said lightguide configuration includes a first set of partially-reflecting internal surfaces deployed to progressively deflect the light propagating in the first direction so as to propagate in a second direction within said lightguide configuration by internal reflection at said major surfaces, and a second set of partially-reflecting internal surfaces deployed to progressively deflect the light propagating in the second direction so as to be coupled out through said second major surface of the lightguide configuration towards the eye of the user, and wherein the light injected by said image projector into said lightguide configuration includes image light rays with p-polarization relative to said major surfaces, and wherein said first set of partially-reflecting internal surfaces are oriented such that the image light rays are primarily s- polarized relative to those surfaces, and wherein a first set of the image light rays propagating in the second direction and reflected off said first major surface spans a first range of propagation angles relative to said major surfaces and a second set of the image light rays reflected off said second major surface spans a second range of propagation angles, opposite to said first range of propagation angles, relative to said major surfaces, said second set of partially-reflecting internal surfaces being implemented as multilayer dielectric coatings configured and oriented so as to have a partial reflectance to p- polarization within a first range of angles of incidence corresponding to said first range of propagation angles so as to progressively couple out said first set of image light rays towards the eye of the user and to have a reduced reflectance no greater than half of said partial reflectance to p-polarization within a second range of angles of incidence corresponding to said second range of propagation angles, wherein said first range of angles of incidence is greater than said secondrange of angles of incidence and greater than the Brewster angle for the second set of partially- reflecting surfaces.

2. The display of claim 1 , wherein said second range of angles of incidence includes the Brewster angle for the second set of partially-reflecting surfaces.

3. The display of claim 1 , wherein said second range of angles of incidence is below the Brewster angle for the second set of partially-reflecting surfaces.

4. The display of claim 1 , wherein said reduced reflectance is less than one third of said partial reflectance.

5. The display of claim 1, wherein the image light rays injected by said image projector into said lightguide configuration are entirely plane polarized with p-polarization relative to said major surfaces.

6. The display of claim 1, wherein the image light rays injected by said image projector into said lightguide configuration include a mixture of p-polarization and s-polarization relative to said major surfaces.

7. The display of claim 1, wherein said second set of partially -reflecting internal surfaces define a coupling-out arrangement having an active area, said active area having a first distal corner and a second distal corner illuminated by laterally outermost rays from said first set of partially-reflecting internal surfaces, a coupling-out-arrangement entrance plane being defined as a plane perpendicular to a chief ray of said collimated image, located at a proximal boundary of said coupling-out arrangement, a distal baseline of said active area being a straight line between said first and second distal corners, wherein a first extreme ray propagates from said coupling-out-arrangement entrance plane to said first distal corner of said active area, said first extreme ray together with a line passing through said first distal corner perpendicular to said distal baseline and said coupling-out- arrangement entrance plane defining a first peripheral triangle, and wherein a second extreme ray propagates from said coupling-out-arrangement entrance plane to said second distal corner of said active area, said second extreme ray together with a line passing through said second distal corner perpendicular to said distal baseline and said coupling-out- arrangement entrance plane defining a second peripheral triangle,said active area extending over more than 50% of the area of at least one of said peripheral triangles so as to provide a part of the image from said at least one of said peripheral triangles to the EMB.

8. A display for displaying an image to an eye of a user located at an eye-motion box (EMB), the display comprising:(a) a lightguide configuration formed from transparent material, said lightguide configuration having a pair of mutually-parallel major surfaces for supporting propagation of light within said lightguide configuration by internal reflection at said major surfaces, said lightguide configuration having a coupling-out arrangement configured for coupling out light propagating within said lightguide configuration towards the EMB; and(b) a projecting arrangement comprising an image projector optically coupled to an optical aperture expander, said projecting arrangement being deployed to introduce image light corresponding to a collimated image to propagate within said lightguide configuration by internal reflection at the major surfaces, wherein a coupling-out-arrangement entrance plane is defined as a plane perpendicular to a chief ray of said collimated image, located at a proximal boundary of said coupling-out arrangement, wherein said coupling-out arrangement has an active area from which image light is coupled out towards the EMB, said active area having a first distal corner and a second distal corner illuminated by laterally outermost rays from said coupling-out-arrangement entrance plane, a distal baseline of said active area being a straight line between said first and second distal corners, wherein a first extreme ray propagates from said coupling-out-arrangement entrance plane to said first distal corner of said active area, said first extreme ray together with a line passing through said first distal corner perpendicular to said distal baseline and said coupling-out- arrangement entrance plane defining a first peripheral triangle, and wherein a second extreme ray propagates from said coupling-out-arrangement entrance plane to said second distal corner of said active area, said second extreme ray together with a line passing through said second distal corner perpendicular to said distal baseline and said coupling-out- arrangement entrance plane defining a second peripheral triangle, said active area extending over more than 50% of the area of at least one of said peripheral triangles so as to provide a part of the image from said at least one of said peripheral triangles to the EMB.

9. The display of claim 8, wherein said coupling-out arrangement comprises a set of mutually-parallel partially reflecting surfaces internal to said lightguide and obliquely angled to said major surfaces.

10. The display of claim 8, wherein said coupling-out arrangement comprises a diffractive optical element deployed on or in said lightguide.

11. The display of claim 8, wherein said active area extends over more than 50% of the area of both of said peripheral triangles.

12. The display of claim 8, wherein said active area extends over more than 90% of the area of at least one of said peripheral triangles.

13. The display of claim 12, wherein said active area extends over more than 90% of the area of both of said peripheral triangles.

14. The display of claim 8, wherein said optical aperture expander comprises a set of mutually-parallel partially reflecting surfaces located within said lightguide.

15. The display of claim 8, wherein said optical aperture expander comprises an optical arrangement including one or more refractive or reflective lenses.

16. The display of claim 8, wherein said active area is substantially trapezoid.

17. The display of claim 8, wherein said display is a near-eye display.

18. The display of claim 8, wherein said display is a head-up display.

19. The display of claim 18, wherein said head-up display is configured for use in a vehicle and wherein said projecting arrangement is configured to project said collimated image from a lower portion of said lightguide configuration towards an upper portion of said lightguide configuration.

20. The display of claim 8, wherein said active area is asymmetric, extending over a different percentage of each of said peripheral triangles.

Images