Compound light-guide optical elements

Abstract

To make design of an optical system (100) for directing an image towards a user for viewing compact.SOLUTION: The optical system includes: a light-guide optical element (LOE) (10) having parallel major external surfaces (11a, 11b) for supporting propagation of an image by internal reflection; a coupling-out arrangement for coupling out the image towards an eye of a user, and a coupling-in aperture. An image projector (114) includes an image generator (32) for generating an image, collimating optics (31) for collimating the image, and an image conjugate generator (20, 33, 34). The image projector is coupled to the coupling-in aperture so as to introduce both the collimated image and its conjugate image into the LOE prior to the images impinging on either of major external surfaces. The image conjugate generator may be a second image generator (33), or may employ one or more reflecting surface (22, 23, 24, 34) non-contiguous with the major external surfaces of the LOE.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to optical systems, and more particularly to optical systems for displaying images to a user. [Background technology]

[0002] Various types of displays, particularly near-eye displays (NEDs), typically employ one or more waveguides through which an image is emitted from an image projector so as to propagate by total internal reflection (TIR) ​​and then coupled out toward the viewer's eye via one or more coupling-out elements (e.g., partially reflective internal surfaces ("facets"), diffraction gratings, etc.). Such waveguides are fabricated from transparent substrates having a pair of parallel outer major surfaces that extend along the length of the waveguide, between which the image and its conjugates are reflected. The image is preferably a collimated image, and the waveguide is preferably planar. For optimal performance, both the image and its conjugates should completely fill the waveguide, such that illumination corresponding to each pixel of the image and each pixel of the conjugate image is present at every point within the thickness of the waveguide (relative to the region of the waveguide that contributes to the output image that can reach the user's eye).

[0003] Filling the waveguide can be achieved by providing a coupling-in prism with a coupling-in face oriented nearly perpendicular to the principal ray of the incident image, thereby allowing the image to land on an elongated region on one surface of the waveguide to generate a conjugate image. However, particularly in implementations where the image is incident at a relatively shallow angle to the major outer surface (i.e., close to 90 degrees to the surface normal), it is important to add to the dimensions of the waveguide the length of the coupling-in region necessary to fill the waveguide with the conjugate image. This is illustrated in FIG. 2A, which shows a typical coupling input into a waveguide 10. A coupling-in prism 14, cut from or attached to the waveguide substrate, is used to direct rays 40 and 41 into the waveguide at a shallow angle. As rays 40 and 41 propagate through the waveguide, ray 41 is reflected from the top surface of the waveguide, thereby becoming a conjugate of ray 40. As is evident from FIG. 1, even with a coupling input prism, a relatively large input aperture (and therefore a larger projector) is required to create a shallow ray conjugate in the waveguide.

[0004] An alternative approach to filling the waveguide, shown in Figure 2B, employs a 50% beam splitter (or "mixer") 13 inside the waveguide 10 near the midpoint that subdivides the thickness of the waveguide between the outer major surfaces and extends at least partially along the length of the waveguide parallel to the outer surfaces. The beam splitter is effective at partially reflecting a ray to create its conjugate (e.g., ray 41) within the waveguide, allowing for a smaller input aperture and wedge prism 14, as shown in Figure 2A.

[0005] The presence of the mixer 13 allows for the use of smaller projector apertures and coupling prisms, but importantly the mixer itself adds to the dimensions of the waveguide. The minimum length required for the mixer 13 is given by the formula l mini= w tan(Φ), where w is the width of the waveguide and Φ is the field of view propagation (relative to the normal to the LOE major surface). Therefore, the above constraint on the minimum length of the mixer requires the waveguide to be longer to accommodate the mixer. Furthermore, incorporating the mixer within the waveguide requires greater precision in the fabrication of the waveguide due to the required parallelism with the waveguide surface. Summary of the Invention

[0006] The present invention is an optical system for directing an image towards a user for viewing.

[0007] In accordance with the teachings of embodiments of the present invention, an optical system for directing an image toward a user for viewing is provided, the optical system comprising: (a) a light-directing optical element (LOE) formed from a transparent material and having first and second outer major surfaces that are parallel to one another, the first and second outer major surfaces for assisting propagation of the image by internal reflection at the first and second outer major surfaces, the LOE having a coupling output arrangement for coupling the image toward an eye of the user, the LOE having a coupling input aperture; and (b) an image projector comprising an image generator for generating an image, a collimating optic for collimating the image, and an image conjugate generator, the image projector coupled to the input aperture to introduce the collimated image and its conjugate image into the coupling input aperture before the collimated image and its conjugate image impinge on either the first or second outer major surfaces.

[0008] According to a further feature of an embodiment of the invention, the image conjugate generator comprises a second image generator.

[0009] According to a further feature of an embodiment of the invention, the image conjugate generator comprises at least one reflective surface discontinuous with the first and second exterior major surfaces.

[0010] According to a further feature of an embodiment of the invention, the image conjugate generator comprises at least one reflective surface that is non-parallel to the first and second outer major surfaces.

[0011] According to a further feature of an embodiment of the invention, the image conjugate generator comprises a beam multiplier comprising at least one beam splitter disposed between and parallel to two reflective surfaces.

[0012] According to a further feature of an embodiment of the invention, the beam multiplier comprises at least two of the beam splitters interposed between at least three of the reflective surfaces.

[0013] According to a further feature of an embodiment of the present invention, the beam multiplier has an outer thickness that is different from the thickness of the LOE.

[0014] According to a further feature of an embodiment of the invention, the reflective surfaces of the beam multiplier are reflective surfaces at interfaces between layers of the layered structure, and the outer surfaces of the layered structure are optically non-functional surfaces of the beam multiplier.

[0015] According to a further feature of an embodiment of the invention, the LOE further comprises a coupling input reflector disposed at an angle on the first and second outer major surfaces, the coupling input reflector being disposed to redirect the collimated image to impinge on the first outer major surface and to redirect the conjugate image to impinge on the second outer major surface.

[0016] According to a further feature of an embodiment of the invention, the coupling input reflectors are disposed at a 45 degree angle on the first and second outer major surfaces.

[0017] According to a further feature of an embodiment of the invention, the image conjugate generator comprises a reflective surface across the LOE adjacent to the coupling input reflector, a portion of the reflective surface across the LOE being an angle-selective reflective surface.

[0018] According to a further feature of an embodiment of the present invention, the angle-selective reflective surface is mounted using an optical adhesive having a refractive index lower than the refractive index of the LOE adjacent to the coupling input reflector.

[0019] According to one aspect of the present invention, there is also provided an optical beam multiplier including a stack of transparent plates defining a plurality of parallel interfaces with coatings that define: (a) a set of N reflectors, where N is at least 3; and (b) a set of at least N−1 partially reflective beam splitters, each beam splitter interposed between two adjacent reflectors of the set. [Brief explanation of the drawings]

[0020] The invention is herein described, by way of example, with reference to the accompanying drawings. [Figure 1A] 1A-1C are schematic isometric views of optical systems implemented using light-guiding optical elements (LOEs), constructed and operative in accordance with the teachings of the present invention, showing top-down and side-entry configurations, respectively; [Figure 1B] 1A-1C are schematic isometric views of optical systems implemented using light-guiding optical elements (LOEs), constructed and operative in accordance with the teachings of the present invention, showing top-down and side-entry configurations, respectively; [Figure 2A] FIG. 1 is a schematic side view showing conventional coupling of an image into an LOE via a coupling input prism (described above). [Figure 2B] FIG. 1 is a schematic side view showing a conventional coupling input of an image into an LOE with an integrated beam multiplier (described above). [Figure 3] FIG. 1C is a schematic side view of a portion of the optical system of FIGS. 1A and 1B showing the combined input of an image and a conjugate image pair to the LOE. [Figure 4A] 1C is a schematic side view of a portion of the optical system of FIGS. 1A and 1B illustrating an alternative implementation of the present invention employing a beam multiplier. FIG. [Figure 4B] FIG. 4B is an enlarged schematic diagram of the beam multiplier of FIG. 4A. [Figure 5] 1C is a schematic side view of a portion of the optical system of FIGS. 1A and 1B illustrating an alternative implementation of the present invention employing a beam multiplier and an oblique angled coupling input reflector. FIG. [Figure 6] 6 is a ray tracing diagram for an implementation of the invention according to FIG. 5, showing the ray paths of different parts of an image directed towards the user's eyes. [Figure 7A] 7A-7C are enlarged partial views of the combined input regions of the LOEs from FIG. 6, each showing only half of the ray paths for a single image pixel that contribute to filling the LOE with the corresponding image illumination for that pixel. [Figure 7B] 7A-7C are enlarged partial views of the combined input regions of the LOEs from FIG. 6, each showing only half of the ray paths for a single image pixel that contribute to filling the LOE with the corresponding image illumination for that pixel. [Figure 8] 1C is a schematic side view of a portion of the optical system of FIGS. 1A and 1B illustrating an alternative implementation of the present invention employing a reflector surface across the thickness of the LOE as an angle-selective reflector. FIG.

[0021] EMBODIMENTS FOR CARRYING OUT THE INVENTION The present invention is an optical system for directing an image towards a user for viewing.

[0022] In certain embodiments of the present invention, an optical system is provided that includes a light-guiding optical element (LOE) to achieve optical aperture expansion for the purposes of a head-up display, most preferably a near-eye display, which may be a virtual reality display, or more preferably an augmented reality display.

[0023] An exemplary implementation of a device in the form of a near-eye display employing an LOE 10 and in accordance with the teachings of one embodiment of the present invention, generally designated 100, is shown schematically in Figures 1A and 1B. Near-eye display 100 employs a miniature image projector (or "POD") 114 optically coupled to an LOE (interchangeably referred to as a "waveguide," "substrate," or "slab") 10 such that the image light is captured in one dimension by internal reflections at a set of mutually parallel, planar exterior surfaces.

[0024] Optical aperture expansion is achieved within LOE 10 by one or more arrangements for continuously redirecting image illumination, typically employing a set of partially reflective surfaces (interchangeably referred to as “facets”) parallel to one another and obliquely tilted toward the propagation direction of the image light, with each successive facet deflecting a portion of the image light in a deflected direction. To expand the aperture in one dimension, the facets also couple the image light toward the user's eye. In some cases, as shown here, two-dimensional aperture expansion is achieved by employing a first set of facets in region 116 to continuously redirect the captured / guided image illumination into the LOE by internal reflection. The deflected image illumination then enters second substrate region 118, which may be implemented as an adjacent, distinct substrate or as a continuation of a single substrate, within which a coupling arrangement (e.g., a further set of partially reflective facets) continuously couples out a portion of the image illumination toward the eye of an observer located within an area defined as the eye motion box (EMB), thereby achieving two-dimensional optical aperture expansion. Similar functionality can be obtained using a diffractive optical element (DOE) to redirect and / or couple out image illumination within one or both of regions 116 and 118, as is well known in the art.

[0025] The entire device may be mounted individually for each eye and is preferably supported against the user's head, with each LOE 10 facing the user's corresponding eye. In one particularly preferred option as shown here, the support arrangement is implemented as an eyeglass frame having sides 120 for supporting the device against the user's ears. Other forms of support arrangement may also be used, including, but not limited to, a headband, a sun visor, or a device suspended from a helmet.

[0026] In the drawings and claims herein, reference is made to an X-axis extending horizontally ( FIG. 1A ) or vertically ( FIG. 1B ) in the general direction of extension of the first region of the LOE, and a Y-axis extending perpendicular thereto, i.e., vertically in FIG. 1A and horizontally in FIG. 1B . Very roughly, the first LOE, i.e., first region 116 of LOE 10, can be considered to achieve aperture expansion in the X-direction, while the second LOE, i.e., second region 118 of LOE 10, achieves aperture expansion in the Y-direction. Details of the angular spread through which different portions of the field of view propagate are discussed more precisely below. Note that an orientation such as that shown in FIG. 1A can be considered a “top-down” implementation, in which image illumination entering the main portion (second region) of the LOE enters from the top edge, while the orientation shown in FIG. 1B can be considered a “side-incidence” implementation, in which the axis referred to herein as the Y-axis is horizontally oriented. In the remaining figures, various features of certain embodiments of the present invention are shown in the context of a "top-down" orientation similar to that of FIG. 1A. However, it should be understood that all of these features are equally applicable to side-entry implementations, which are also within the scope of the invention. In certain cases, other intermediate orientations are also applicable and, unless expressly excluded, are included within the scope of the present invention. While the two-dimensional magnification embodiments shown herein are merely exemplary, the present invention is also applicable to embodiments in which only a single dimension of aperture magnification is performed by the LOE.

[0027] It will be appreciated that near-eye display 100 typically employs power from a small on-board battery (not shown) or other suitable power source and typically includes various additional components, including a controller 122 for operating image projector 114. It will be appreciated that controller 122 includes all necessary electronic components, such as at least one processor or processing circuitry, for driving the image projector, all as is well known in the art.

[0028] One aspect of the present invention relates to implementations of image projector 114 that include an image conjugate generator positioned such that the image projector projects both a collimated image and its conjugate image into LOE 10. Various non-limiting examples of image conjugate generators are presented herein below with reference to Figures 3-8.

[0029] Accordingly, referring to Figure 3, there is shown an enlarged, schematic, partial view of the optical system of Figure 1 for directing an image toward a user for viewing. The optical system includes LOE 10, formed from a transparent material and having first and second outer major surfaces 11a and 11b that are parallel to one another. First and second outer major surfaces 11a and 11b are for supporting propagation of the image by internal reflection at those surfaces. LOE 10 also has a coupling output arrangement (as described above and within region 118 of Figure 1, not shown here) for coupling the image out toward the user's eye, and a coupling input aperture 15, shown in this case as a side edge of LOE 10.

[0030] Instead of relying on structures integrated with LOE 10 to generate the image conjugate pair, image projector 114 according to this aspect of the invention includes an image conjugate generator that generates the image conjugate pair before either the collimated image or the conjugate image strikes either of LOE 10's outer major surfaces 11a and 11b.

[0031] 3, image projector 114 includes image generator 32 for generating an image, collimating optics 31 for collimating the image, and an image conjugate generator (here implemented as second image generator 33 that generates a conjugate image). In the embodiment shown, image generators 32 and 33 share common collimating optics 31. Image projector 114 is coupled to coupling input aperture 15 to introduce the collimated image and its conjugate image directly into LOE 10 before the collimated image or its conjugate image impinges on either of LOE 10's outer major surfaces 11a and 11b.

[0032] It will be appreciated that this solution contrasts sharply with the coupling input arrangement of Figures 2A and 2B, in which a conjugate image is generated within the LOE itself by reflection from the major outer surfaces (or surfaces of the coupling prism that are contiguous with these surfaces and are defined for this purpose herein as being part of the major outer surfaces of the LOE).

[0033] The two image generators 32 and 33 are driven to generate the same image, one mirrored, so that each field is shown identically from both fields. During device assembly, active alignment is preferably used, either by mechanical adjustment or, more preferably, by digital correction of the image display position, to move the two images on the image generators so that they are aligned as complementary conjugate images within the LOE. Thus, the LOE is "filled" with both the main image and its conjugate from the combined input aperture across the entire LOE, without requiring expansion of the LOE to achieve such filling.

[0034] In this and all other implementations of the present invention, the image generator may be any type of microdisplay image generator known in the art. Suitable examples include, but are not limited to, spatial light modulators (SLMs), including transmissive SLMs such as LCD displays, reflective SLMs such as LCOS displays, and active light-emitting displays such as OLED displays. Also, scanning image generators in which a fast-scanning laser beam is modulated synchronously with its scanning motion may be used as image generators in accordance with the present invention.

[0035] In another implementation of the present invention, the image conjugate generator is implemented as at least one reflective surface that is discontinuous with the major exterior surface to generate a conjugate image, as an alternative to the second image generator 33. Various examples of such implementations are presented with reference to Figures 4A-8.

[0036] Figure 4A shows an implementation in which the image conjugate generator is a beam multiplier or "mixer" arrangement 20 that is external to the waveguide 10. One particularly preferred implementation of the mixer 20, which is itself believed to be patentable, is shown in more detail in Figure 4B.

[0037] Conceptually, mixer 20 functions similarly to mixer 13 of FIG. 2B, but in this case, rather than being part of waveguide 10, it is part of an image projector assembly 114 interposed between LOE 10 and projector unit 30, which includes the image generator and collimating optics. In this case, projector unit 30 has a single image generator 32 that generates one image, while mixer 20 creates an image-conjugate pair with partially reflected ray 40 to create a conjugate (ray 41). The image pair is then injected into the waveguide. Because mixer 20 is external to the waveguide, it can be actively aligned with the waveguide during assembly without the manufacturing constraints of the waveguide. Mixer 20 can include multiple layers of mirrors 22, 24, beamsplitter 23, and an exterior surface 21 that need not be optical quality or coplanar with the LOE exterior surface. This significantly simplifies the manufacturing constraints of the structure.

[0038] With respect to the structure of the beam multiplier 20, the beam multiplier of the present invention is distinguished from those described in previous publications by the presence of at least one intermediate highly reflective layer, which effectively subdivides the mixer into two independent mixers stacked one on top of the other. Accordingly, the optical beam multiplier 20 is preferably formed from a stack of transparent plates defining a plurality of parallel interfaces that are coated to define: (a) A set of N reflectors (N is at least 3) (b) a set of at least N-1 partially reflecting beam splitters, each of the beam splitters being interposed between two adjacent reflectors of the set;

[0039] The "reflector" in this case is preferably highly reflective, meaning that it reflects at least 85%, more preferably at least 90%, and typically at least 95% of the incident light, at least within the range of angles relevant to propagation along the LOE. The partially reflective beam splitter is preferably an approximately 50% reflector (50% ±10%). In applications where the beam multiplier is outside the user's field of view, both the reflector and the beam splitter can advantageously be implemented using metallic coatings. When transparency is required to view a scene through the beam multiplier, multilayer dielectric coatings are used to provide the necessary level of reflectivity at high angles while offering relatively high transparency at small (near-orthogonal) angles, as is known in the art.

[0040] The intermediate reflectors effectively subdivide the mixer into two (or more) submixers. This reduces the length required by the mixer to fill the waveguide with the image and its conjugate by a factor of two. The input and output apertures of mixer 20 according to one embodiment are shown as dark lines in FIG. 4B. These apertures need not fill the entire width of the mixer. In fact, it may be advantageous for all of the reflectors and beam splitters to be implemented at the internal interfaces between transparent plates, while the external surfaces of the layered structure are optically non-functional. Here, "optically non-functional" refers to surfaces that are not reached by image light, or surfaces where any image light that reaches them does not subsequently enter the LOE. In such cases, the external surfaces need not be polished or parallel to other elements. Thus, as shown in FIG. 4A, the external thickness of mixer 20 may be different from, and typically greater than, the thickness of LOE 10 (i.e., the distance between first external major surface 11a and second external major surface 11b). To fill the LOE, the distance between the outermost reflectors 22 needs to match or be slightly longer than the LOE thickness.

[0041] 5-8, in some implementations, the use of an external image conjugate generator facilitates the use of a folded optical path, thereby removing bulk from the sides of the assembly. Thus, according to certain embodiments of the present invention, LOE 20 further includes coupling-in reflectors 12 disposed at an angle on the first and second outer major surfaces to redirect the collimated image to impinge on first outer major surface 11a and the conjugate image to impinge on second outer major surface 11b. While coupling-in reflectors 12 can be implemented at a range of angles, they are most preferably disposed at 45 degrees on the first and second outer major surfaces, thereby effectively folding the optical axis of the image projector by 90 degrees. Unlike certain conventional coupling-in configurations, reflectors 12 are specifically positioned within the thickness of LOE 20, thereby deflecting both the primary image and its conjugate image toward their respective upward and downward propagation directions.

[0042] FIG. 5 shows one such configuration, in which an external mixer 20 is positioned perpendicular to the waveguide. In this case, a reflective coupling input surface 12 with a 45-degree angle folds the image output by the mixer into the waveguide. A wedge prism 25 is preferably used to couple the central FOV light into the mixer at an angle perpendicular to the prism surface. To achieve this folding, the width of the mixer aperture must be larger than the width of the waveguide aperture. The exact size of the aperture depends on the angular FOV of the display light and the folding angle of surface 12. In this example, the width is increased by 66%.

[0043] Note that in this case, the mixer 20 is split into three sub-mixers. As a result, there is no need to increase the length of the mixer (as mentioned above, l mini = w tan(Φ) / 2, which increases the width (w) by 60% after folding. The mixer is therefore split into three sub-mixers by two internal mirror facets 24 between the external mirror facets 22. A beam splitter 23 is provided at the central plane of each sub-mixer.

[0044] FIG. 6 shows a ray diagram of the overall optical system incorporating the mixer of FIG. 5. Three exemplary points (pixels) on the image generator 32 are collimated by lens 31, exit the projector unit 30, and enter the coupling wedge prism 25 and mixer 20. The mixer generates conjugate fields such that the entire image and its conjugate are coupled into the waveguide 10 by the coupling input reflector 12. In this example, light is coupled from the waveguide to the eye motion box (EMB) 200 by a set of parallel partially reflective facets 11. Note that the example of facets 11 is one non-limiting example; other mechanisms for coupling outputs, such as holographic or dichroic gratings, are also possible and are within the scope of this specification. Finally, it can be seen that the propagation angles of the different fields can be very shallow, yet the input aperture between the mixer 25 and the projector unit 30 remains relatively small.

[0045] 7A and 7B show two cross-sectional views of an image (FIG. 7A) and its conjugate (FIG. 7B) propagating within a waveguide. Superimposing FIGS. 7A and 7B shows how the waveguide is completely filled with the image and its conjugate. Thus, optical homogenization is achieved within the waveguide. While this diagram subdivides illumination into "image" and "conjugate" according to a partial arbitrary position along the LOE, it should be noted that light constantly swaps between the image and the conjugate as it propagates, reflecting from the first and second outer major surfaces of the LOE. A defining feature of one particularly preferred embodiment of the present invention is that illumination entering the LOE's coupling input aperture and reaching the coupling input reflector 12 already fills the reflector with both the image and the conjugate image, one of which is directed upward as shown, striking the first outer major surface 11a first, and the other is directed downward as shown, striking the second outer major surface 11b first. The definition of which image is the "main image" and which image is the "conjugate image," or which surfaces are referred to as the "first" or "second" main exterior surface, is arbitrary, and it is generally not important whether the "main image" generated by the image generator is the image to be viewed by the user or an inverted version of that image, depending solely on various design considerations.

[0046] Note that, as seen in FIG. 7B , a portion of the light rays reflected and directed downward from the coupling input reflector 12 strikes the second outer major surface 11b in the overlap region with the beam multiplier 20. To maintain TIR in that overlap region, the device is preferably assembled with a small air gap between elements, or more preferably, with a low-index adhesive between the components. Particularly for propagating light rays at shallow angles, a relatively small index difference between the LOE material and the adhesive is sufficient to define a critical angle that preserves propagating image illumination through TIR. Alternatively, an angle-selective multilayer dielectric coating may be applied to the overlap region with the LOE to provide suitable internal reflection characteristics.

[0047] In all of the above embodiments employing mixer 20, the mixer is arbitrarily defined herein as being part of projector 114, since it forms part of the optical system before the image is incident on LOE 10 and does not include any surface extension of the LOE. In practical constructions of the product, the mixer is not necessarily integrated with projector unit 30, which combines the image generator and collimating optics, and in some cases may be more conveniently assembled by being attached to the LOE prior to positioning the projector unit.

[0048] FIG. 8 illustrates a further feature of a particular, particularly preferred implementation of the present invention. According to this feature, the image conjugate generator includes a reflective surface 34 that traverses the thickness of the LOE 10 adjacent to the coupling input reflector 12. A portion 121 of the reflective surface 34 that traverses the LOE is implemented as an angle-selective reflective surface to reflect light rays that enter the LOE from the projector 114 before being reflected by the reflector 12, while transmitting light rays that have already been reflected by the reflector 12, i.e., at angles relevant to image propagation along the LOE 20. Again, the angle-selective reflective surface can be most conveniently implemented using an optical adhesive with a refractive index lower than that of the LOE adjacent to the coupling input reflector, thereby providing a critical angle between the incidence angles of the light rays before and after reflection by the reflector 12. Other options discussed above, such as the use of angle-selective multilayer dielectric coatings or the inclusion of air gaps, may also be used.

[0049] In the non-limiting example shown in Figure 8, projector 114 generates the conjugate image not via an external mixer but by using a single elongated reflective surface 34 perpendicular to the LOE's outer major surfaces. A portion of the projected and collimated image, corresponding to the primary image deflected upward toward first outer major surface 11a, is projected directly onto coupling input reflector 12. Another portion of the image reflects from surface 34, thereby generating a conjugate image, which is deflected downward toward second outer major surface 11b by coupling input reflector 12. Reflective surface region 121 contributes to filling coupling input reflector 12 with the conjugate image, while overlap region 122 is similarly treated to provide angle-selective reflection (as described above) to avoid leakage of downwardly reflected conjugate image light within the overlap region.

[0050] Other aspects of the structure of projector 114 in FIG. 8 are based on principles employed in conventional reflective SLM image projectors based on polarizing beamsplitter prisms. Specifically, illumination source 40 introduces illumination into PBS prism 35, from which light is reflected toward reflective SLM 32, such as LCOS or DLP. The reflected image illumination passes through the PBS to reflective collimating lens 310 associated with a quarter-wave plate (not shown), resulting in a collimated and reflected image being reflected from the PBS toward the LOE coupling input aperture. Image doubling to provide both the image and its conjugate is achieved as described above by projecting a portion of the image directly onto reflector 12, while other image illumination is first reflected from surface 34. It is also advantageous to provide an angle-selective reflective coating, such as a low-index adhesive, on the region of reflective lens 310 below surface 34 to achieve TIR at relevant angles after reflection from the PBS. Additional lenses, such as field lens 313, may be added to improve optical performance.

[0051] In this implementation, it is particularly advantageous that prism face 34 is orthogonal to the major surface of waveguide 10, so that the two parallel light rays (as shown here) exiting reflective lens 310 become conjugate before entering the waveguide. Furthermore, it can be seen that the required input direction of illumination from source 40 is approximately 110 degrees relative to the major surface of the waveguide, thereby lending itself to a highly ergonomic design with little divergence between components to be integrated on either side of the device to properly fit the form factor of an eyeglass frame.

[0052] The use of a reflective surface 34 that traverses the thickness of the LOE in a region 121 with angularly selective reflective properties is also applicable to other implementations of the present invention described above. For example, if one of the reflectors of the beam multiplier 20 of Figures 5-7B is implemented as a surface that traverses the thickness of the LOE 20, the required optical input / output aperture of the beam multiplier 20 can be significantly reduced, resulting in a more compact design.

[0053] It will be understood that the above description is intended to serve as an example only, and that many other embodiments are possible within the scope of the invention as defined in the appended claims.

Claims

1. 1. An optical system for directing an image towards a user for viewing, said optical system comprising: (a) a light-directing optical element (LOE) formed from a transparent material and having first and second outer major surfaces that are parallel to one another, the first and second outer major surfaces for assisting propagation of an image by internal reflection at the first and second outer major surfaces, the LOE having an output coupling arrangement for coupling the image out toward an eye of the user, the LOE having an input coupling aperture; (b) an image projector, the image projector comprising: an image generator for generating an image; a collimating optic for collimating the image; and an image conjugate generator, the image projector coupled to the combining input aperture such that the collimated image and its conjugate image are introduced into the combining input aperture before the collimated image and its conjugate image impinge on either the first or second outer major surfaces.

2. The optical system of claim 1 , wherein the image conjugate generator comprises a second image generator.

3. The optical system of claim 1 , wherein the image conjugate generator comprises at least one reflective surface that is discontinuous with the first and second exterior major surfaces.

4. The optical system of claim 1 , wherein the image conjugate generator comprises at least one reflective surface that is non-parallel to the first and second outer major surfaces.

5. The optical system of claim 1 , wherein the image conjugate generator comprises a beam multiplier comprising at least one beam splitter disposed between and parallel to two reflective surfaces.

6. The optical system of claim 5 , wherein the beam multiplier comprises at least two of the beam splitters interposed between at least three of the reflective surfaces.

7. The optical system of claim 5 , wherein the beam multiplier has an outer thickness that is different from a thickness of the LOE.

8. The optical system of claim 5 , wherein the reflective surface of the beam multiplier is a reflective surface at an interface between layers of a layered structure, and an outer surface of the layered structure is an optically non-functional surface of the beam multiplier.

9. 2. The optical system of claim 1, wherein the LOE further comprises a coupling input reflector disposed at an angle to the first and second outer major surfaces, the coupling input reflector being disposed to redirect the collimated image to impinge on the first outer major surface and to redirect the conjugate image to impinge on the second outer major surface.

10. 10. The optical system of claim 9, wherein the coupling input reflector is disposed at a 45 degree angle to the first and second outer major surfaces.

11. 10. The optical system of claim 9, wherein the image conjugate generator comprises a reflective surface across the LOE adjacent the coupling input reflector, a portion of the reflective surface across the LOE being an angle-selective reflective surface.

12. The optical system of claim 11 , wherein the angle-selective reflective surface is mounted using an optical adhesive having a refractive index lower than the refractive index of the LOE adjacent the coupling input reflector.

13. 1. An optical beam multiplier comprising: a stack of transparent plates defining a plurality of parallel interfaces, said plurality of parallel interfaces comprising: (a) a set of N reflectors, where N is at least 3; (b) a set of at least N-1 partially reflective beam splitters, each of the beam splitters being interposed between two adjacent reflectors of the set of reflectors.

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