An optical system for orienting an image for viewing

Abstract

The optical system includes a partially internally reflective rectangular light guide (PRLG) (10) having three surfaces that support internal reflection and a partially reflective fourth surface (34) with which a second light guide portion (30) is associated. Typically, a light beam redirecting arrangement including partially reflective surfaces (12) embedded in a set of light guide portions (30) redirects light emerging from the PRLG towards a third light guide portion (20) that includes a combining arrangement (122), such as a further set of partially reflective surfaces (28), to combine out an image light beam towards a user's eye.

Description

[Technical field]

[0001] The present invention relates to optical systems, and more particularly to optical systems for directing a light beam corresponding to an image from an image projector towards a user for viewing.

[0002] Many near-eye display systems include a transparent light guide (LG) or "waveguide" placed in front of the user's eye, which transmits the image by internal reflection and then couples the image out for directing to the user's eye by a suitable coupling-out configuration. The coupling-out configuration may be based on embedded partial reflectors or "facets," or may employ a diffractive pattern. In either case, the coupling-out configuration progressively couples out the light beams that make up the image, thereby achieving an expansion of the optical aperture in one direction.

[0003] To enable the use of miniature image projectors, some near-eye display systems provide a two-dimensional extension of the optical aperture from the image projector. One subset of such solutions is described in U.S. Pat. No. 10,133,070, in which the first dimension of aperture extension is achieved using a rectangular cross-section waveguide with an embedded partial reflector. Summary of the Invention

[0004] The present invention is an optical system for directing a light beam corresponding to an image from an image projector towards a user for viewing.

[0005] In accordance with the teachings of one embodiment of the present invention, there is provided an optical system for directing a light beam corresponding to an image from an image projector towards a user for viewing, the optical system comprising: (a) a partially internally reflecting rectangular light guide (PRLG) formed from a transparent material and having first and second mutually parallel major surfaces, a third major surface perpendicular to the first and second major surfaces, the first, second, and third major surfaces supporting internal reflection for a range of angles of incidence, and a fourth major surface parallel to the third major surface, at least a portion of the fourth major surface comprising a partially reflective non-diffractive coating; and (b) a second light guide portion having a pair of mutually parallel major surfaces for transmitting the light beam by internal reflection, the second light guide portion optically coupled to at least a portion of an area of ​​the partially reflective coating, the second light guide portion comprising a set of planar mutually parallel portions located between and non-parallel to the pair of major surfaces. (c) a second light guide portion formed as a continuation of or adjacent to the second light guide portion, the third light guide portion including a coupling output configuration deployed for coupling out a light beam propagating in the third light guide portion by internal reflection to direct the light beam towards a user; and (d) a coupling input arrangement for coupling a light beam corresponding to a collimated image from the image projector into the PRLG to propagate in the PRLG by four internal reflections at the first, second, third and fourth major surfaces, whereby the light beam from the image projector propagating by four internal reflections in the PRLG is progressively emitted from the PRLG through the partially reflective coating, enters the second light guide portion, is redirected by reflection at the set of partially reflective surfaces to propagate in the third light guide portion, and is coupled out of the third light guide portion towards the user by the coupling output configuration.

[0006] According to a further feature of an embodiment of the invention, the PRLG has a length along a direction parallel to a line of intersection between the first major surface and the third major surface, and a maximum length of an optical path of the optical beam from being emitted from the PRLG until being redirected by one of the set of partially reflective surfaces is longer than the length of the PRLG.

[0007] According to a further feature of an embodiment of the invention, the PRLG has a length along a direction parallel to a line of intersection between the first major surface and the third major surface, and a maximum length of an optical path of the optical beam from being emitted from the PRLG until being redirected by one of the set of partially reflective surfaces is less than the length of the PRLG.

[0008] According to a further feature of an embodiment of the invention, the second light guide portion has a second pair of mutually parallel major surfaces perpendicular to the pair of mutually parallel major surfaces, whereby the second light guide portion transmits the light beam by four internal reflections and a set of partially reflective surfaces couples the light beam exiting the second light guide portion to a third light guide portion.

[0009] According to a further feature of an embodiment of the invention, the first and second major surfaces of the PRLG are parallel or coplanar with a pair of mutually parallel major surfaces of the second light guide portion.

[0010] According to a further feature of an embodiment of the invention, the second light guide portion and the third light guide portion are portions of a single light guide such that a pair of mutually parallel major surfaces extends continuously across the second and third light guide portions.

[0011] According to a further feature of an embodiment of the invention, the output coupling arrangement comprises a second set of mutually parallel partially reflective internal surfaces deployed within the third light guide section.

[0012] According to a further feature of an embodiment of the invention, the output coupling arrangement comprises a diffractive optical element associated with the third light guide portion.

[0013] According to a further feature of an embodiment of the invention, the PRLG does not include an internal reflector.

[0014] According to a further feature of an embodiment of the invention, the majority of the length of the PRLG does not include an internal reflector.

[0015] According to a further feature of an embodiment of the invention, the PRLG includes at least one partially reflective interior surface parallel to the set of partially reflective surfaces of the second light guide portion.

[0016] According to a further feature of an embodiment of the invention, the at least one partially reflective internal surface includes a surface having a reflectivity for at least one range of angles of incidence greater than 10%, the surface being located one-third of the length of the PRLG distal to the coupling input arrangement.

[0017] According to a further feature of an embodiment of the invention, the at least one partially reflective internal surface includes a surface having a reflectivity for at least one range of angles of incidence that is less than 10%, the surface being located one-third of the length of the PRLG proximal to the coupling input arrangement.

[0018] According to a further feature of an embodiment of the invention, the third major surface of the PRLG is coated with a reflective coating to support reflection at angles of incidence less than the critical angle of the material that is transparent in air.

[0019] In accordance with the teachings of one embodiment of the present invention, there is also provided an optical system for directing a light beam corresponding to an image from an image projector towards a user for viewing, the optical system including: (a) a partially internally reflecting rectangular light guide (PRLG) formed from a transparent material and having first and second mutually parallel major surfaces, a third major surface perpendicular to the first and second major surfaces, the first, second, and third major surfaces supporting internal reflection for a range of angles of incidence, and a fourth major surface parallel to the third major surface, at least a portion of the fourth major surface being provided with a partially reflective, non-diffractive coating; and (b) a second light guide portion having a pair of mutually parallel major surfaces for transmitting the light beam by internal reflection, the second light guide portion optically coupled to at least a portion of an area of ​​the partially reflective coating, the second light guide portion having a diffractive coating deployed to deflect the light beam propagating within the second light guide portion. (c) a third light guide portion formed as a continuation of or adjacent to the second light guide portion, the third light guide portion including a coupling output configuration deployed for coupling out a light beam propagating in the third light guide portion by internal reflection to direct the light beam towards a user; and (d) a coupling input arrangement for coupling a light beam corresponding to a collimated image from the image projector into the PRLG for propagation in the PRLG by four internal reflections at the first, second, third and fourth major surfaces, whereby the light beam from the image projector propagating by four internal reflections in the PRLG is progressively emitted from the PRLG through the partially reflective coating, enters the second light guide portion, is redirected by the diffractive optical element to propagate in the third light guide portion, and is coupled out of the third light guide portion towards the user by the coupling output configuration.

[0020] In accordance with the teachings of one embodiment of the present invention, there is also provided an optical system for directing a light beam corresponding to an image from an image projector towards a user for viewing, the optical system including: (a) a first major surface formed from a transparent material and having first and second mutually parallel major surfaces and a third major surface perpendicular to the first and second major surfaces, the first, second, and third major surfaces supporting internal reflection for angles of incidence greater than a critical angle, and at least the third major surface being transmissive for angles of incidence less than the critical angle; (b) a partially internally reflecting rectangular light guide (PRLG) having a fourth major surface parallel to the surface, at least a portion of the fourth major surface being provided with a partially reflective coating that is partially reflective for angles of incidence greater than a critical angle and transparent for angles of incidence less than the critical angle; and (b) a second light guide having a pair of mutually parallel major surfaces for transmitting a light beam by internal reflection, the second light guide being optically coupled to at least a portion of an area of ​​the partially reflective coating of the fourth major surface, the second light guide being optically coupled to at least a portion of an area of ​​the partially reflective coating of the fourth major surface, the second light guide being optically coupled to the pair of major surfaces. (c) a third light guide adjacent the third major surface, the third light guide including a coupling output configuration deployed to couple out a light beam propagating within the third light guide by internal reflection to direct the light beam towards a user; and (d) a light beam corresponding to a collimated image from an image projector into the PRLG to couple the light beam corresponding to a collimated image from the image projector into the PRLG and direct the light beam through the PRLG to a set of planar, mutually parallel partially reflective surfaces located between the first, second, third and fourth major surfaces. a coupling input arrangement for propagating by four internal reflections within the PRLG, whereby a light beam from the image projector propagating by four internal reflections within the PRLG is incident on the fourth major surface at an angle of incidence greater than the critical angle, progressively emerges from the PRLG through the partially reflective coating, enters the second light guide portion, is redirected by reflection off the set of partially reflective surfaces to be incident on the fourth major surface at an angle of incidence less than the critical angle, traverses the PRLG, passes through the third major surface to enter the third light guide, and propagates within the third light guide by internal reflection;The coupled output arrangement couples the light out of the third light guide section towards the user.

[0021] According to a further feature of an embodiment of the invention, a pair of major surfaces of the second light guide are perpendicular to a fourth major surface of the PRLG.

[0022] According to a further feature of an embodiment of the invention, the second light guide further comprises an additional major surface parallel to the fourth major surface, the additional major surface supporting internal reflection of the light beam at angles of incidence larger than the critical angle. [Brief description of the drawings]

[0023] The invention is herein described, by way of example only, with reference to the accompanying drawings.

[0024] [Figure 1] FIG. 1 is a schematic isometric view of a display device including an optical system constructed and operative in accordance with one embodiment of the present invention that employs a partially internally reflecting rectangular light guide (PRLG) for directing a light beam corresponding to an image from an image projector towards a user for viewing. [Figure 2A] FIG. 2 is a schematic front view of a first embodiment of the optical system of FIG. 1; [Figure 2B] FIG. 2B is a cross-sectional view taken along line AA in FIG. 2A. [Diagram 3] 2B is a view similar to FIG. 2A illustrating an alternative implementation of the embodiment of FIG. 2A. [Figure 4] 2B is a view similar to FIG. 2A illustrating an alternative implementation of the embodiment of FIG. 2A. [Diagram 5] FIG. 2B is a schematic isometric view of the optical system of FIG. 2A. [Figure 6] FIG. 6 is a view similar to FIG. 5 illustrating the incorporation of an internal beam splitter within the PRLG. [Figure 7] 6 is a view similar to FIG. 5 illustrating an implementation in which the PRLG is tilted relative to other light guide portions of the optical system. [Figure 8A]2B is a view similar to FIG. 2A illustrating an alternative embodiment in which the second light guide portion of the optical system is implemented as a rectangular light guide. [Figure 8B] 2B, illustrating an alternative embodiment in which the second light guide portion of the optical system is implemented as a rectangular light guide. FIG. [Figure 9A] FIG. 2 is a front view of an alternative embodiment of the optical system of FIG. [Figure 9B] 2 is a side view of an alternative embodiment of the optical system of FIG. 1. [Figure 10A] FIG. 9B is a view similar to FIG. 9A employing diffractive optical elements for the input coupling arrangement and the output coupling configuration. [Figure 10B] FIG. 9C is a view similar to FIG. 9B employing diffractive optical elements for the input coupling arrangement and the output coupling configuration. [Figure 11] 11A-11C, a schematic representation in angular notation for various stages of propagation of a collimated image through the optical system of FIGS. 9A and 9B when a set of partially reflective surfaces in the second light guide portion is perpendicular to the main surface of the light guide. [Figure 12] 12A-12C, a schematic representation in angular notation for various stages of propagation of a collimated image through the optical system of FIGS. 9A and 9B when a set of partially reflective surfaces in the second light guide portion is oriented obliquely with respect to the main surface of the light guide. [Figure 13A] 9C is a schematic isometric ray plot of light rays corresponding to the upper right corner of the image displayed by the optical system of FIGS. 9A and 9B. [Figure 13B] 9C is a schematic isometric ray plot of light rays corresponding to the lower left corner of the image displayed by the optical system of FIGS. 9A and 9B. [Figure 14] FIG. 9B is a view similar to FIG. 9A illustrating an alternative implementation and detailing a polarization management scheme. [Figure 15A] 9C is a schematic side view similar to FIG. 9B and illustrating an alternative implementation of the optical system. [Figure 15B]9C is a schematic side view similar to FIG. 9B and illustrating an alternative implementation of the optical system. [Figure 15C] 9C is a schematic side view similar to FIG. 9B and illustrating an alternative implementation of the optical system. [Figure 15D] 9C is a schematic side view similar to FIG. 9B and illustrating an alternative implementation of the optical system. [Figure 15E] 9C is a schematic side view similar to FIG. 9B and illustrating an alternative implementation of the optical system. [Figure 15F] 9C is a schematic side view similar to FIG. 9B and illustrating an alternative implementation of the optical system. [Figure 16A] 9B is a view similar to FIG. 9A illustrating an alternative embodiment in which the second light guide portion of the optical system is implemented as a rectangular light guide. [Figure 16B] 16B is a side view of the optical system of FIG. 16A according to two implementation variations. [Figure 16C] 16B is a side view of the optical system of FIG. 16A according to two implementation variations. [Figure 16D] 16B is a schematic plot illustrating the relative light intensity between the PRLG and the second light guide portion as a function of distance along the light guide of FIG. 16A; [Figure 17A] 2 is a front view of a further alternative embodiment of the optical system of FIG. 1. [Figure 17B] 2 is a side view of a further alternative embodiment of the optical system of FIG. 1. [Figure 17C] 17C is a schematic plot of reflectivity as a function of incidence angle (relative to the normal to the surface) for a coating of the surface between the PRLG and the second light guide in the optical system of FIGS. 17A and 17B. [Figure 18] 18A-18C are schematic representations in angular notation for various stages of propagation of a collimated image through the optical system of FIGS. 17A and 17B, with FIGS. 18B1 and 18B2 respectively illustrating the case where a set of partially reflective surfaces in the second light guide portion are orthogonal to the main surface of the light guide. [Figure 19] FIG. 17B is a schematic front view similar to FIG. 17A illustrating the geometric requirements for visual continuity of the display. [Figure 20] 17B is a schematic front view similar to FIG. 17A illustrating the deployment of partially reflective surfaces in a second light guide along a curved profile using non-overlapping and overlapping deployments of partially reflective surfaces. [Figure 21] 17B is a schematic front view similar to FIG. 17A illustrating the deployment of partially reflective surfaces in a second light guide along a curved profile using non-overlapping and overlapping deployments of partially reflective surfaces. [Figure 22] 17B is a view similar to FIG. 17A illustrating an alternative embodiment in which the second light guide of the optical system is implemented as a rectangular light guide. [Figure 23A] 17B is a front view similar to FIG. 17A illustrating an alternative embodiment having limited use of internal partially reflective surfaces within the PRLG. [Figure 23B] 9B is a front view similar to FIG. 9A illustrating an alternative embodiment having limited use of internal partially reflective surfaces within the PRLG. [Figure 24A] A schematic front view of an optical system for two-dimensional optical aperture expansion employing a rectangular light guide extending across a first portion of the width of the light guide and with light emerging from an end of the rectangular light guide into a slab light guide. [Figure 24B] FIG. 2B is a schematic front view of an optical system for two-dimensional optical aperture expansion employing a PRLG similar to that of FIG. 2A. [Figure 24C] FIG. 25 is a schematic front view of an optical system for two-dimensional optical aperture expansion employing a combination of features from FIGS. 24A and 24B. [Diagram 25] 11B is a schematic representation in angular notation corresponding to FIG. 11A, illustrating an image having an expanded angular dimension that can propagate through a system using a reflective coating applied to the PRLG. [Figure 26] 12B is a schematic representation in angular notation corresponding to FIG. 12A, illustrating an image having an expanded angular dimension that can propagate through a system using a reflective coating applied to the PRLG. [Figure 27A] 10B is a view similar to FIG. 10A illustrating a further embodiment in which the partially reflective surface of the second light guide portion is replaced by a diffractive optical element. [Figure 27B] 10C is a view similar to FIG. 10B illustrating a further embodiment in which the partially reflective surface of the second light guide portion is replaced by a diffractive optical element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] The present invention is an optical system for directing a light beam corresponding to an image from an image projector towards a user for viewing.

[0026] The principles and operation of an optical system according to the present invention may be better understood with reference to the drawings and the accompanying description.

[0027] By way of introduction, aspects of the present invention provide an optical system for directing a light beam corresponding to an image from an image projector towards a user for viewing, the optical system including a partially internally reflecting rectangular light guide (PRLG). FIG. 1 is a schematic diagram of a near-eye display 300, each employing two optical systems according to an embodiment of the present invention to provide a binocular display to a user's eyes. In each optical system, the PRLG 10, the second light guide portion 30, and the third light guide portion 20 are shown only diagrammatically as if they were combined into a common light guide optics 312 deployed facing each eye. In various implementations, the PRLG 10, and possibly the second light guide portion 30, may actually be incorporated into a housing that is outside the field of view of the user's eyes. Details of various implementations of these elements are discussed herein with reference to FIGS. 2A-26.

[0028] Conceptually, a PRLG is a rectangular light guide that employs a partial reflector on one major surface of the light guide to allow gradual emission ("leakage") of the light beam as it propagates along the length of the light guide, instead of relying on embedded facets for coupling out the image light beam. The direction of the emitted beam is a continuation of the internal beam path incident on the partially reflective major surface (subject to refractive deflection due to differences in the refractive indices of the materials, but without diffraction). A secondary set of embedded partially reflective surfaces is provided in an adjacent light guide to redirect the emitted light to propagate within the light guide or an adjacent light guide towards a coupling out arrangement that directs the image light beam towards the user for viewing.

[0029] Thus, as seen in Figures 2A and 2B, the PRLG 10 is formed from a transparent material and has first and second mutually parallel major surfaces 31 and 33, and a third major surface 32 perpendicular to the first and second major surfaces, where the first, second and third major surfaces support internal reflection for a range of angles of incidence, such as through total internal reflection (TIR) ​​or through the provision of a reflective coating. The rectangular cross-sectional shape is completed by a fourth major surface 34 parallel to the third major surface 32. At least a portion, and preferably all, of the fourth major surface 34 is provided with a partially reflective non-diffractive coating. A second light guide portion 30 having a pair of mutually parallel major surfaces 22a and 22b for transmitting a light beam by internal reflection is optically coupled to at least a portion, and preferably all, of the area of ​​the partially reflective coating on the fourth major surface 34 of the PRLG 10. The second light guide portion 30 includes a set of planar, mutually parallel partially reflective surfaces 12 located between and non-parallel to the pair of major surfaces 22a and 22b. These embedded partially reflective surfaces are illustrated diagrammatically in the isometric views of Figs. 5-7 below, and represented as lines elsewhere in the front view. The third light guide portion 20, formed as a continuation of or adjacent to the second light guide portion 30, includes a coupling output arrangement deployed to couple out light beams propagating within the third light guide portion by internal reflections to direct the light beams towards a user. The coupling input arrangement couples a light beam corresponding to a collimated image from an image projector 314 into the PRLG 10 for propagation within the PRLG by four internal reflections at the first, second, third and fourth major surfaces.

[0030] As a result of this structure, a light beam from the image projector 314 propagating by four internal reflections within the PRLG 10 is progressively emitted from the PRLG through the partially reflective coating, enters the second light guide portion 30, is redirected by reflection off a set of partially reflective surfaces 12 to propagate within the third light guide portion 20, and then is coupled out of the third light guide portion towards the user by the coupling output configuration.

[0031] This approach of employing a PRLG with one partially reflective surface in combination with a set of partially reflective surfaces in the light guide adjacent to the PRLG to redirect the image light beam toward a combined output configuration offers significant advantages over conventional optical aperture expansion arrangements employing rectangular light guides. For example, see the design disclosed in the aforementioned U.S. Pat. No. 10,133,070 ("the '070 patent"), where the rectangular light guide described therein relies entirely on embedded partially reflective surfaces to combine out the image light beam. The combination of increasing miniaturization of devices on the one hand and the demand for low-cost mass production manufacturing processes on the other hand presents challenges for such designs. The light guide of the '070 patent must be constructed by cutting and polishing a stack of thin plates coated with a partially reflective coating, while minimizing scattering defects at the cut edges and maintaining high-precision parallelism of the partially reflective coating and the sides of the rectangular light guide. In contrast, PRLGs, as discussed further below, may be largely or entirely free of embedded facets, allowing them to be manufactured as simple rectangular glass blocks, either separately or by slicing the structure from a stack of bonded plates, all of which can be done relatively inexpensively and with high precision. At the same time, the use of a set of partially reflective surfaces external to the rectangular light guide to deflect the light beams toward the combined output configuration frees the system from geometric limitations that would otherwise result from the use of PRLGs. The PRLG and the set of partially reflective surfaces together achieve the optical aperture expansion in a first dimension, while the combined output configuration achieves the optical aperture expansion in a second dimension.

[0032] The coupling-out arrangement as illustrated in most of the figures is implemented as a second set of mutually parallel partially reflective internal surfaces 28 deployed within the third light guide portion 20 and angled relative to the major surfaces of the third light guide portion, thereby redirecting the image light beams out of the light guide towards the user's eye. An alternative implementation illustrated below with reference to Figures 10A and 10B employs a coupling-out arrangement implemented as a diffractive optical element associated with the third light guide portion.

[0033] The image projector 314 employed in the device of the present invention is preferably configured to generate a collimated image, i.e., an image in which the light of each image pixel is a parallel beam collimated to infinity, with the angular direction corresponding to the pixel location. The image illumination thus spans an angular range corresponding to the two-dimensional viewing angle. The representative propagation direction is taken as the central direction corresponding to the image's "chief ray" (central pixel).

[0034] The image projector 314 includes at least one light source, typically deployed to illuminate a spatial light modulator, such as a liquid crystal over silicon (LCOS) chip. The spatial light modulator modulates the projection intensity of each pixel of the image, thereby generating the image. Alternatively, the image projector may include a scanning arrangement, typically implemented with one or more fast scanning mirrors, that scans illumination from a laser light source across the image plane of the projector while varying the intensity of the beam pixel by pixel in synchronism with the motion, thereby projecting the desired intensity for each pixel. A further possibility is the use of an active light generating image source, such as a micro LED array. In all the above cases, a collimating optical system is provided to generate an output projected image that is collimated to infinity. Some or all of the above components may be arranged on the surface of one or more polarizing beam splitter (PBS) cubes or other prism arrangements, all as known in the art.

[0035] Optical coupling of the image projector 314 to the PRLG 10 may be accomplished by any suitable optical coupling, such as, for example, via a coupling prism with an angled input face, or via a reflective coupling arrangement, via one of the side edges and / or major outer faces of the LOE. Alternatively, a diffractive optical element (DOE) may be used to couple the image into the substrate, particularly if the coupling output configuration is diffractive, to cancel chromatic dispersion. Details of the coupling input configuration are typically not important to the invention, except as specified in the specific examples below, and are otherwise only shown generally here.

[0036] It will be appreciated that the near-eye display 300 includes various additional components, including a controller 322 for operating the image projector 314, typically employing power from a small on-board battery (not shown) or some other suitable power source. It will be appreciated that the controller 322 includes all the necessary electronic components, such as at least one processor or processing circuitry, for driving the image projector, all as is known in the art.

[0037] The display 300 is preferably supported against the user's head, with each LOE 312 facing a corresponding eye of the user. In one particularly preferred option as illustrated herein, the support arrangement is implemented as an eyeglass frame having sides 320 for supporting the device against the user's ears. Other forms of support arrangements may also be used, including, but not limited to, a headband, a sun visor, or a device suspended from a helmet.

[0038] Turning now to specific but non-limiting exemplary implementations of the optical system of the present invention, a first group of implementations is illustrated in Figures 2A-8B, in which the maximum length of the optical path D of the light beam from emission from the PRLG 10 until it is redirected by one of the set of partially reflective surfaces 12 is longer than the length L of the PRLG 10 along a direction parallel to the intersection line between the first major surface 31 and the third major surface 32. In this case, the PRLG 10 is shorter than the desired horizontal aperture extension. (Throughout this specification, the terms "horizontal" and "vertical" are used intuitively to refer to the orientation illustrated in the drawings, while understanding that the orientation of the entire structure may be arbitrarily selected according to design considerations of the required form factor of the device.) Thus, this structure can be considered to be a three-stage aperture extension. The first stage is the PRLG 10. The light beams from the image projector 314 are coupled into the PRLG 10 as four reflections from the first and second major surfaces 31 and 33, the third major surface 32, and the partially reflective fourth major surface 34. As the light beams propagate along the PRLG 10, a portion of the light intensity of two of the reflected beams passes through the surface 34 and "leaks" into the second light guide portion 30 as shown in cross section AA, while the remainder of the light continues to propagate along the PRLG 10. This results in a vertical aperture expansion such that the input to the second light guide portion 30 is significantly wider than the output aperture of the image projector 314. As the light beams propagate along the second light guide portion 30 by two internal reflections, they are progressively partially reflected by the partially reflective surface 12, which redirects them to propagate downwards as shown, expanding the effective aperture of the beams in the horizontal direction. The light beams then propagate downwards and pass from the second light guide portion 30 to the third light guide portion 20. The light beam is then progressively coupled out by a coupling-out arrangement 122 implemented as a set of mutually parallel partially reflective surfaces 28, which progressively deflects the light beam out of the light guide towards the user's eye (not shown).

[0039] The PRLG 10 facilitates the use of a very compact image projector 314, preferably having a narrow, small output optical aperture.

[0040] In the preferred implementation illustrated herein, the first major surface 31 and the second major surface 33 of PRLG 10 are coplanar with the external parallel faces 22a and 22b of the second light guide portion 30, while the third major surface 32 and the fourth major surface 34 are perpendicular thereto. Preferably, for ease of manufacture, surfaces 32 and 34 are also implemented to be parallel to the embedded partially reflective surface 12. This allows surfaces 32 and 34 to be manufactured as coated plates and laminated with an additional coated plate to form surface 12, followed by slicing and polishing the laminate, thereby allowing PRLG 10 and the second light guide portion 30 to be formed simultaneously using a single manufacturing process.

[0041] Preferably, light injected into PRLG 10 is S-polarized with respect to fourth surface 34 , thereby matching the preferred polarization for facet 12 and maintaining the polarization orientation along light guide portion 30 .

[0042] A protective cover 36 may be advantageously provided over surface 32 to protect the structural integrity of surface 32. Protective cover 36 may be a low index material to maintain total internal reflection (TIR) ​​conditions at surface 32. Alternatively, the reflectivity of surface 32 may be provided by a reflective coating, such as a metallic coating, thereby making the device insensitive to the refractive index of the protective cover.

[0043] The partial reflectivity of the fourth major surface 34 may be provided by a metallic coating, making it relatively insensitive to the angle of incidence. Alternatively, the angular reflectivity of the fourth major surface 34 may be set to have different aperture expansions for different angular distribution beams by making its reflectivity dependent on the angle of incidence of the beam. The angular transmittance (complementary to the reflectivity) is optimized considering two opposing parameters depending on the location of the transmittance required to reach the viewing position (eye motion box) along the PRLG. A high transmittance allows efficient output coupling at the desired location, but may also cause too much light to be output coupled before the required location. The maximum total efficiency is achieved by optimizing these two considerations. Preferably, this optimization is performed up to the furthest point along the PRLG where the lowest power is present. An angular dependent reflectivity with a given desired dependency may be achieved using a multi-layer dielectric coating with layer thicknesses designed using standard software tools, all as known in the art.

[0044] Although illustrated herein in an implementation in which surfaces 32 and 34 are parallel to partially reflective surface 12, PRLG 10 may be generated separately from second light guide portion 30, in which case it may have a different orientation than surface 12 (as illustrated below with reference to FIG. 24B).

[0045] Figure 3 is similar to Figure 2A, with equivalent features similarly labeled. Figure 3 illustrates that the image projector 314 can be oriented at different angles and still produce the same propagation angle within the light guide. If a combining prism is used (not shown in detail, but implied in the illustrated structure), the prism can be applied to the front or back (first or second) major surface (31 or 33). Alternatively, the combining prism can be applied to surface 32 (or protective cover 36) in the area where the reflective coating has an opening.

[0046] Figure 4 is similar to Figure 3, but illustrates that the length of the interface between the PRLG 10 and the second light guide portion 30 can be varied according to the needs of each design. This is controlled by the reflectivity of the surface 34, where lower reflectivity and higher transmittance results in a shorter length of the region where effective aperture expansion occurs. In this illustrative view, the expansion of the facets 12 is also selective, according to a determination of which areas of the facets are required to provide the portion of the field of view that reaches the "eye motion box" in which the image is displayed. In all other aspects, Figures 3 and 4 are equivalent in structure and function to Figures 2A and 2B described above.

[0047] Figure 5 is an isometric view corresponding to the light guide arrangement from the embodiment of Figures 2A and 2B. Figure 6 is a similar view, but additionally illustrates a longitudinal partial reflector 38 disposed within PRLG 10, parallel to surfaces 31 and 33, to further improve vertical aperture expansion and allow for a smaller projector optical system. Partial reflector 38, or another similar reflector, can also be introduced elsewhere in the light guide.

[0048] 7 illustrates a further option in which the PRLG (here shown as 10T) is tilted so that only one of the two sets of beams emerging from surface 34 enters the second light guide portion 30. Again, an internal partial reflector (not shown), similar to partial reflector 38 but positioned within the second light guide portion 30, may be provided to ensure that the light guide is filled with the projected image.

[0049] 8A and 8B illustrate a variation of the embodiment of FIG. 3, in which the second light guide portion itself is implemented as a rectangular light guide 30R. Thus, in addition to a first pair of mutually parallel major surfaces 22a and 22b, the second light guide portion 30R has a second pair of mutually parallel major surfaces 22c and 22d perpendicular to the surfaces 22a and 22b, as illustrated diagrammatically by the arrows in FIG. 8B, such that the second light guide portion 30R transmits light beams by four internal reflections. In this case, the set of partially reflective surfaces 12 combines the light beams exiting the second light guide portion 30R and directs them to the third light guide portion 20, where they propagate by two internal reflections and are combined out by the embedded partial reflector 28, as described above. To support internal reflection at surface 22d, a small air gap, or a layer of low-index adhesive, is preferably interposed between the light guides 30R and 20. Alternatively, a multi-layer dielectric coating may be applied to surface 22d designed to mimic TIR behavior.

[0050] Protectively, and best understood with reference to the side view of Figure 8B, and as pertinent to several embodiments described herein, coupling out of the image light beams by the partially reflective surface 28 preferably occurs selectively for one of the propagating images by two internal reflections within the third light guide portion 20, while the surface 28 is preferably substantially transparent to the second image light beam that would form an undesirable "ghost" image. Such angle-selective reflectivity is achieved by suitable multi-layer dielectric coatings designed using standard software tools and implemented using standard coating techniques, all as known in the art.

[0051] Turning now to a further group of implementations illustrated in Figures 9A-16D, these relate to optical systems in which the extension direction of the PRLG 10 is not significantly parallel to the direction of the image's chief ray propagating towards the coupling output configuration. As with the above-mentioned "length", the extension direction of the PRLG 10 may be considered to be defined by the intersection line between (for example) the first major surface 31 and the third major surface 32, while the image's chief ray is the direction of the light beam corresponding to the image's central pixel on its path through the third light guide to the coupling output configuration. In this group of implementations, the extension direction is not significantly parallel to this chief ray, meaning that the extension direction forms an angle with the chief ray of at least 60 degrees, more preferably at least 70 degrees, with certain particularly preferred implementations being substantially orthogonal, meaning within the range of 90 ± 10 degrees, as illustrated here. As an alternative definition, particularly for coupling output configurations based on partially reflecting facets, the extension direction of the PRLG 10 may be substantially parallel (± 20 degrees, and possibly ± 10 degrees) to the extension direction of the coupling output facet 28. This orientation of the PRLG 10 makes the architecture much more similar to that of the aforementioned '070 patent, but with the advantages of a rectangular light guide that does not rely on embedded reflectors, as explained above. Instead of embedded reflectors, the set of partially reflective surfaces 12 in the second light guide portion 30 are external to the rectangular light guide, thereby relaxing many of the stringent requirements for precision that are typical of rectangular light guides with embedded reflectors.

[0052] In this case, the length of the optical path from leaving the PRLG 10 to the furthest point where the light beam is deflected by the partially reflective surface 12 is relatively short compared to previously described embodiments. In some cases, the maximum length of the optical path D of the light beam from leaving the PRLG until it is redirected by one of the set of partially reflective surfaces is less than the length L of the PRLG 10 along a direction parallel to the intersection line between the first major surface 31 and the third major surface 32.

[0053] 9A and 9B specifically, as previously discussed, PRLG 10 here defines exterior parallel surfaces 31 and 33, a top reflector 32, and a bottom partial reflector 34 (referred to above as a first surface, a second surface, a third surface, and a fourth surface, respectively). The partially reflective surface 34 is optically coupled with second light guide portion 30 and third light guide portion 20, which are preferably integrated into a single light guide optical element (LOE) 312. An optical coupling input arrangement, shown here as a coupling prism 100, couples image 60 into PRLG 10.

[0054] In the illustrated embodiment, LOE 312 is a thin slab waveguide having a pair of parallel faces 22a, 22b that are advantageously coplanar with surfaces 31 and 33, respectively. The set of mutually parallel partially reflective surfaces 12 in second light guide portion 30 function as a light redirecting arrangement that redirects (deflects) light coupled into second light guide portion 30 towards an additional set of partially reflective internal surfaces (or "facets") 28 in third light guide portion 20 toward the coupling output arrangement implemented herein.

[0055] The second lightguide portion 30 and the third lightguide portion 20 may be non-overlapping regions, such that the third lightguide portion 20 occupies a lower portion of the LOE 312 not occupied by the second lightguide portion 30 (as shown). Alternatively, in certain embodiments, the two portions 30 and 20 may partially overlap (along the "y" direction).

[0056] The partially reflective surface 12 is non-parallel to the coupling-out partially reflective surface 28. Specifically, surface 28 is inclined at an oblique angle relative to major surfaces 22a, 22b to effect coupling-out of light propagating within LOE 312, while the partially reflective surface 12 is orthogonal to major surfaces 22a, 22b in many implementations. Additionally, the extension directions of facets 12 and 28, defined by the intersection line of the plane of each surface with one of major surfaces 22a, 22b, are also non-parallel, and the orientation of facet 12 is selected to deflect the light beam emitted from PRLG 10 downward (along the "y" axis) toward the extension direction of facet 28 and typically generally perpendicular to facet 28.

[0057] The configuration of facets 12 and 28 is such that when the partially reflective surface 34 couples an image propagating within PRLG 10 into LOE 312 by four internal reflections (four images represented generally by chief ray paths 62a, 62b, 62c, 62d), the combined input image propagates within second light guide portion 30 in the first guide direction by internal reflection at major surfaces 22a, 22b (paths 63a, 63b) with a portion of the intensity of the image being reflected (deflected) at facet 12 so as to be redirected toward third light guide portion 20 in the second guide direction, and then propagates within LOE 312 by internal reflection at faces 22a, 22b (images 64a, 64b) with a proportion of the intensity of the image reflected (deflected) at partially reflective surface 28 so that the image is directed outward from one of parallel faces 22a as a visible image 66 seen by the observer's eye 2. The first guide direction is generally oblique to the direction of expansion of the PRLG 10 (ie, oblique to the surface 34).

[0058] 10A and 10B show an embodiment similar to that illustrated in FIGS. 9A and 9B, except that here the coupling input arrangement and the coupling output configuration are implemented using a diffractive element. In particular, an optical coupling input configuration 110 is used to couple the image 60 into the PRLG in the form of a diffractive element (e.g., a diffraction grating) deployed on one of the surfaces of the PRLG 10. The coupling input diffractive element is shown deployed in relation to a side surface to accommodate a preferred, but non-limiting deployment of an optical image generator on the back of the optical aperture multiplier. The output coupler is also a diffractive element 29, which couples out the light beam of the image 66 towards the eye 2. The combination of the two diffractive elements 110 and 29 allows for the cancellation of chromatic dispersion caused by the diffraction process (after taking into account the rotation introduced by the facet 12).

[0059] In an alternative embodiment, the beam redirecting arrangement of the second light guide portion 30 may additionally or alternatively be implemented using a diffractive optical element. In this case, cancellation of chromatic aberration introduced by the diffractive element is typically achieved by ensuring that each output beam is parallel to the corresponding beam input to the first diffractive optical element. Figures 27A and 27B illustrate such a variation of Figures 10A and 10B. In this case, the second light guide portion 30 uses a diffractive optical element (DOE), such as a surface grating 400 as shown or an embedded grating, to redirect the light beams exiting through the partially reflective surface 34. Dispersion is compensated for by ensuring that the entrance beam orientation is parallel to the exit beam, as is commonly done. By implementing the DOE in the second light guide portion 30 (i.e., not associated with any surface of the PRLG 10), the structure of the PRLG 10 is kept as simple as possible, thereby providing similar manufacturability and image quality advantages as described in connection with the reflective implementation described above.

[0060] 11A-11C illustrate the beam propagation shown in FIGS. 9A and 9B in a smoothed angular space (using a representation further detailed in the aforementioned '070 patent). Circles 200a and 200b represent the TIR angles of surfaces 22a and 22b, respectively, where the angles falling within the circles correspond to the ray directions escaping through those surfaces. Circle 202 represents the TIR angle of surface 32, while dashed circle 204 represents the partially transmitted region of partial reflector 34. The four images are marked as a1, a2, a3, and a4 (equivalent to 62a-62d above). FIG. 11A illustrates the distribution within PRLG 10. Here, only images a3 and a4 are within circle 204, which transfers some of their energy to LOE 320.

[0061] 11B shows the angular distribution in the second light guide portion 30, where line 206 represents the angle of facets 12 perpendicular to surfaces 22a and 22b. These facets reflect images a3 and a4 (equivalent to 63a, 63b) into different propagation directions marked as b1 and b2 (equivalent to 64a, 64b).

[0062] FIG. 11C shows the angular distribution within the third light guide portion 20, where facet 28 (angularly represented as line 208) reflects image b2 or b1 out of the light guide as image c (equivalent to 66).

[0063] In an alternative implementation, facets 12 may be oriented at an angle relative to outer surfaces 22a, 22b (not parallel to facets 28) as illustrated in the angle space of Figures 12A-C. In Figure 12A, the distribution is similar to 11A, except that the image orientation is angled, resulting in a straighter final output image.

[0064] In FIG. 12B, the orientation of the oblique facet 12 is shown as a diagonal line 210. This facet reflects image a3 (equivalent to 63a or 63b) only to b1 (equivalent to 64a or 63b). Propagation of b1 in the LOE 312 also produces its conjugate image b2 (so 64a and 64b coexist). The coating of the oblique facet 12 should be optimized to be transparent (minimal reflection) even at the incidence angles corresponding to the rays of images a4 and b2 to prevent loss of light in undesired directions (ghosting). The advantage of such oblique facets is that they scramble the light polarization, thereby reducing non-uniformity, and the manufacture of such facets does not require strict angular tolerances.

[0065] In the above configuration, where facet 12 reflects one or more downwardly propagating images that escape the PRLG, the light rays propagating at the steepest angles to surface 34 (i.e., small "angles of incidence" relative to the normal to surface 34) correspond to portions of the image field that are closer to the image projector, while the light rays propagating at shallower angles (i.e., larger "angles of incidence" relative to the normal to surface 34) correspond to portions of the image field that are furthest from the image projector. This is advantageous because the light rays with the longest propagation paths experience relatively fewer internal reflections within the light guide and are therefore less susceptible to image degradation due to any imperfections in the light guide.

[0066] Figures 13A and 13B show ray tracings from the combining prism 100 to the eye 2. For clarity, one particular ray in each drawing is highlighted as a heavy line and its reflection point is highlighted with an arrow. As can be seen, the furthest propagating beam (Figure 13B) propagates at the shallowest angle (it undergoes a similar number of reflections as the steeper beams, despite having a longer propagation distance), and therefore reduces power loss and / or image degradation.

[0067] FIG. 14 is equivalent to FIG. 9 with the addition of a schematic illustration of the polarization of one of the beams. The illumination beam 60 is here S-polarized (the circle represents the polarization vector perpendicular to the plane of the drawing). The coating on the plane 34 is convenient to design for this polarization (to avoid having to deal with the inherent 100% transmission at the Brewster angle). As the beam propagates along the PRLG 10, it generally maintains its polarization orientation along the symmetry of the light guide. This beam subsequently strikes the vertical facets 12 (shown in FIGS. 9 and 11) with the optimal orientation (S-polarization) for the coatings of these facets. In certain preferred embodiments, a polarization rotation element (half-wave plate) 130 (or depolarizer) can be introduced after the second light guide section 30 to rotate the orientation (shown as a double-headed arrow) so that these beams strike the facets 28 (rotated with respect to facets 12) with S-polarization also for these facets.

[0068] If facet 12 is tilted relative to the LOE surface (as in FIG. 12), polarization rotation element 130 may not be necessary, or it may be advantageously replaced with a depolarizer.

[0069] 14 also illustrates the option of the second light guide portion 30 (where the facets 12 are located) being "tilted" with respect to the extension direction of the PRLG 10 to improve the focusing of the beams emerging from the PRLG 10 by reducing the mutual obscuration of successive facets. Depending on the beam angle, other configurations are also possible.

[0070] The LOE side view of FIG. 9B shows that the PRLG 10, the second light guide portion 30, and the third light guide portion 20 are all stacked vertically, one above the other, and preferably have coplanar (shared) outer surfaces. This option may be preferred in many cases due to advantages in design simplicity and manufacturing process, and the relevant components for multiple such products can be manufactured separately and combined as a stack, then sliced ​​and polished to form multiple optical systems from a single stack. However, the system is not limited to such a design. FIGS. 15A-15F illustrate several alternative implementations in which angles and / or dimensions may vary between the various components.

[0071] 15A shows an alternative example where the PRLG 10 is tilted relative to the second light guide portion 30. In this case, only one image (a3 or a4 in FIGS. 11A-11C) is coupled into the second light guide portion 30.

[0072] In FIG. 15B, a tilt is introduced between the second light guide portion and the third light guide portion, such that only one image b1 or b2 propagating within the second light guide portion 30 is coupled from the second light guide portion 30 to the third light guide portion 20.

[0073] 15C, the PRLG 10 is coupled to the second light guide portion 30 via one of the major surfaces of the second light guide portion 30. This implementation may employ only one emitted image from the PRLG 10, or a second image may be recovered by implementing a top edge of the second light guide portion 30 with a mirror surface.

[0074] In Fig. 15D, the PRLG 10 is wider than the other sections. This arrangement is possible while taking into account that unless the protruding area of ​​the bottom surface has a reflective coating or meets the conditions of TIR, some of the light coupled in at the bottom surface may be lost (shown as arrows), while the area of ​​coupling with the second light guide portion 30 is implemented with a partially reflective coating as explained before. Fig. 15E shows that the second light guide portion 30 can be wider than the third light guide portion 20. Again, the extra width may cause light loss (shown as arrows).

[0075] 15F illustrates the case where the PRLG 10 is narrower than the second light guide portion 30 (alternatively, there could be a step-up in thickness between the second light guide portion 30 and the third light guide portion 20). In such a case, it is preferable to introduce a longitudinal partial reflector 38 (as described above with reference to FIG. 6 ) that acts as a beam splitter (indicated by an arrow), thereby helping to improve the uniformity of the image light beam within the third light guide portion 20.

[0076] Any and all of the configurations described herein (above and below) may be applicable to head-up displays as well as near-eye displays.

[0077] Reference is now made to Figure 16, which shows a similar optical system to that of Figure 9, but with a surface or coating 340 that supports total internal reflection (TIR) ​​added below the second light guide portion 30, such that the second light guide portion 30 itself becomes a rectangular light guide that supports propagation by four internal reflections. Thus, light that is transmitted through the partially reflective surface 12 and would otherwise be lost in a direction that does not contribute to the output image is instead recycled by internal reflection until it reaches the additional surface 12.

[0078] In the front view of Fig. 16A, an image projector with a narrow optical aperture 314 injects a light beam corresponding to a collimated image into PRLG 10 (corresponding to PRLG 10 in Fig. 9). The beam propagates in this light guide by four internal reflections, being reflected from the top and sides by TIR or reflective coatings, while being partially transmitted and reflected by surface 34 (corresponding to surface 34 in Fig. 9). Due to the presence of TIR surface 340, second light guide 30 also functions as a rectangular light guide, guiding the light beam through four internal reflections, where the sides are reflected by TIR, top surface 34 is reflected by partial reflection, and bottom surface 340 is reflected by TIR.

[0079] Since the light beams are incident on the surfaces / coatings 34 and 340 at equivalent angles, the choice of materials for their respective coatings and light guide portions must be such as to achieve partial reflection 34 and TIR at 340. TIR is most simply maintained by using a layer of low index adhesive or other low index material. As discussed above, the coating of the partially reflective surface 34 is preferably implemented using a multi-layer dielectric coating.

[0080] 16B shows a side view of a first implementation in which the PRLG 10 has the same width as the rectangular light guide 30, while FIG. 16C shows an alternative implementation in which the PRLG 10 is narrower than the rectangular light guide 30. In this second case, an additional rectangular light guide element 311 can be added adjacent to the PRLG 10 with a partial reflector between them so that light from the PRLG 10 can couple directly into the rectangular light guide 30, or indirectly, first into the additional rectangular light guide element 311 and then into the rectangular light guide 30.

[0081] FIG. 16D shows a schematic of a typical plot of intensity distribution along PRLG 10 (line 341) and rectangular light guide 30 (line 342). After light injection, the maximum power is initially contained within PRLG 10, and no power is present within rectangular light guide 30 (at the location marked 350 in FIG. 16A and plot FIG. 16D). Coupling between the light guides (through surface 34) causes a "diffusion" of intensity from PRLG 10 to rectangular light guide 30, which proceeds as shown at the intermediate location marked 352, and reaches approximate equilibrium further along the light guide (location marked 354). At equilibrium, the intensity is spread over the larger cross-sectional area of ​​PRLG 10+rectangular light guide 30, and since rectangular light guide 30 preferably has a larger cross-sectional area than PRLG 10, most of the light energy is located within rectangular light guide 30. Different angles of the field achieve equilibrium at different distances along the light guide.

[0082] The continuous "leakage" from PRLG 10 into rectangular light guide 30 creates an effective expansion of the injected aperture so that light guide rectangular light guide 30 is fully illuminated despite having a larger cross section than PRLG 10.

[0083] Optionally, one or both of coatings 34 and 340 may be angularly selective to ensure that the light beam is coupled out from PRLG 10 to rectangular light guide 30 and from there to third light guide portion 20 according to the desired optical design and observer eye position (not shown).

[0084] Figures 17A-22 relate to a further set of embodiments of the invention which are conceptually equivalent to the embodiments of Figures 9A-16D, but in which the sequence of components has been switched. In particular, instead of employing a second light guide portion interposed between the PRLG 10 and the third light guide portion 20, the second light guide is now provided "above" the PRLG 10, i.e. on the side remote from the third light guide portion 20.

[0085] 17A and 17B, an optical system for directing a light beam corresponding to an image from an image projector towards a user for viewing is shown, the optical system including a partially internally reflecting rectangular light guide (PRLG) 10 formed from a transparent material and having first and second major surfaces 31 and 33 parallel to each other, and a third major surface 230 perpendicular to the first and second major surfaces. The first major surface 31, the second major surface 33, and the third major surface 230 support internal reflection for angles of incidence greater than a critical angle, and at least the third major surface is transmissive for angles of incidence less than the critical angle. The PRLG 10 also has a fourth major surface 232 parallel to the third major surface 230. At least a portion of the fourth major surface 232 is provided with a partially reflective coating that is partially reflective for angles of incidence greater than the critical angle and transparent for angles of incidence less than the critical angle.

[0086] A second light guide 221 having a pair of mutually parallel major surfaces (front and back appearances) preferably perpendicular to a fourth major surface 232 for transmitting a light beam by internal reflection. The second light guide 221 includes a set of planar, mutually parallel partially reflective surfaces (or "facets") 219 optically coupled to at least a portion of the area of ​​the partially reflective coating of the fourth major surface 232 and positioned between and non-parallel to the pair of major surfaces.

[0087] The third light guide 20 adjacent the third major surface 230 includes a coupling output arrangement 122 deployed to couple out a light beam propagating in the third light guide by internal reflection to direct the light beam towards a user. The coupling input arrangement 100 is deployed to couple a light beam corresponding to a collimated image from an image projector into the PRLG 10, propagating in the PRLG 10 by four internal reflections at the first, second, third and fourth major surfaces.

[0088] The light beams from the image projector propagating by four internal reflections in the PRLG 10 are incident on the fourth major surface 232 at an angle of incidence greater than the critical angle and are progressively emitted from the PRLG through the partially reflective coating to enter the second light guide 221, where they are redirected by reflections at the set of partially reflective surfaces 219 to be incident on the fourth major surface 232 at an angle of incidence less than the critical angle. The downwardly deflected light rays are shown here as dashed arrows for clarity. The deflected light waves then traverse the PRLG 10, pass through the third major surface 230, enter the third light guide 20, propagate within the third light guide by internal reflections, and are coupled out of the third light guide 20 towards the user by the coupling out arrangement 122. The nomenclature used to label the light rays is similar to that of FIG. 9B.

[0089] The coating on the fourth major surface 232 may have an angular reflectivity profile as shown in Figure 17C. At large angles (relative to the normal to the surface) 222a, the coating is partially reflective, while at small angles 222b, the coating is transmissive, thereby allowing reflected light to pass through the surface 232 and traverse the PRLG 10 into the second light guide 20. As in Figure 9 above, the coupling output arrangement 122 may be implemented as a set of mutually parallel partially reflective surfaces 28.

[0090] Figures 18A-C illustrate the angular spatial progression of a light beam corresponding to a collimated image through the system of Figure 17. In Figure 18A, the TIR limit of coating 230 is represented as circle 231, and the angular spectrum of partial reflector 232 is shown as 233. As in Figure 11A above, circles 200a and 200b represent the TIR limits of the front and back surfaces. In this case, images a1 and a2 (of the four images propagated by the fourfold reflection of PRLG 10) are coupled into a second light guide 221 shown in Figures 18B1 and 18B2. Figure 18B1 shows the case where facets 219 are orthogonal to the major surface of second light guide 221 and are represented in angle space as lines 237, while lines 239 in Figure 18B2 show the case where facets 219 are oblique to the light guide surface according to an alternative implementation where they are implemented using a coating that is transparent to a1 and b2 but partially reflects a2 onto b1. Figure 18C shows the coupling output from light guide 20 by facet 28 (as in Figure 9B), where facet 28 is represented in angle space by line 208.

[0091] It should be noted that, as illustrated by Figures 10A and 10B and Figures 27A and 27B above with respect to Figures 9A and 9B, variations of the embodiments of Figures 17A and 17B can also be implemented using diffractive optical elements to replace the input coupling arrangement, the output coupling configuration and / or facet 219, as would be understood by one of ordinary skill in the art.

[0092] The light beam injected through the prism 100 is a multiplicity of collimated beams at various angles corresponding to an image at infinity (collimated image). FIG. 19 illustrates that the spacing between facets 219 should optimally vary to continuously illuminate toward the center of the eye motion box (not shown). Facets closer to the projector should be spaced closer, while facets further from the projector should be spaced at a greater distance. The dashed arrows (associated with different beams) indicate that the edges of the facet angles should overlap to create uniform illumination (all field propagation within the light guide 10 is not shown for clarity). The variable facet spacing configuration is applicable to all facet configurations disclosed herein.

[0093] FIG. 20 shows that the second light guide 221 (of FIG. 19) can be reformulated as a curved light guide 223, where the facets 225 (corresponding to facets 219) are parallel and distributed along an arc. As in FIG. 19, the facets need to be distributed at intervals that produce uniform illumination on the eye motion box (dashed arrow). This configuration allows for minimal shading of the more distant facets, resulting in less power loss and better illumination. This configuration also provides clearer viewing through the light guide with minimal facets obscuring the view, and may be of particular interest where the rounded overall form factor of a spectacle lens type design is required.

[0094] 21 shows an "overlapping facet" configuration in light guide 223 in which the density of facets is increased beyond that required to give continuous coverage at the EMB, such that reflections from more than one facet contribute to each ray that reaches the EMB. This configuration may provide improved image quality and / or higher efficiency, particularly in designs employing angled facets (as described above with reference to FIG. 18B2).

[0095] 22, according to a further variation of the embodiment of FIG. 17A, the second light guide 221 may have an additional major surface 40 parallel to the fourth major surface 34 that supports internal reflection of the light beam at angles of incidence above the critical angle. This makes the second light guide 221 a rectangular light guide that supports propagation by four internal reflections, thereby recycling the light beam transmitted by the partially reflective surface 219 and reducing energy losses. In all other aspects, the structure and operation of the optical system of FIG. 22 is similar to that of FIG. 17A.

[0096] 23A and 23B, in all embodiments illustrated thus far, the PRLG 10 is instead implemented without an internal reflector, relying on a partially reflective surface 34 for coupling out from the light beam, plus an additional beam redirecting arrangement (typically a set of facets 12, 219 or 225) for directing the light beam to the coupling out configuration. This approach has various advantages, as discussed above. Notwithstanding these advantages, while maintaining the primary functionality of coupling out via the partially reflective surface 34, certain preferred implementations may employ a PRLG 10 that includes one or more partially reflective internal surfaces 266 that are parallel to a set of partially reflective surfaces of the second light guide portion (corresponding to facets 12, 219 or 225 in any of the above embodiments). Even in such cases, a large portion of the length of the PRLG 10 (not shown to scale here) may advantageously be free of internal reflectors. The inclusion of several facets within the PRLG 10 may contribute to achieving a compact size and / or improved power efficiency, as will be described in more detail below.

[0097] The internal facets in the PRLG 10, if present, preferably fall into one of two categories. The first category is one or more partially reflective surfaces located toward the end of the PRLG 10, i.e., one third of the length of the PRLG distal to the coupling input arrangement 100. In this case, the surface preferably has a reflectivity for at least one range of incidence angles that exceeds 10%, and in some cases may exceed 50%, or in the case of the last facet, 100%. Such distal facets serve to couple out any remaining light intensity that is still propagating within the PRLG and would otherwise be lost without reaching the viewer.

[0098] The second category is one or more partially reflective internal surfaces located near the beginning of the PRLG 10, i.e., one third of the length of the PRLG proximate the coupling input arrangement 100. In this case, the surface preferably has a low reflectivity of less than 10%, in most cases less than 5%, and typically less than 3%, for a light beam in the range of angles relevant to the combined output of the desired image. These proximal facets are useful for compact implementations where the exit angle of the light beam through the partially reflective surface 34 at the angle of propagation within the PRLG 10 would otherwise require a larger overall system footprint.

[0099] As illustrated herein, these two categories of internal facets may be implemented in both embodiments employing facet 219 above the PRLG and in embodiments employing facet 12 below the PRLG.

[0100] In FIG. 23A, the first proximal section 260 of PRLG 10 includes facets 266, thereby allowing the coupling out of the light beams at the beginning of the light guide. The facets of this section must partially reflect images a1 and a2 (or only one of the images in the case of front and back angled facets) while simultaneously transmitting images a3 and a4. The second section 262 (equivalent to the second light guide 221 or 223 described above) must be angle selective to reflect images a1 and a2 (or one of them) and only those images if the second section 262 is implemented as a rectangular light guide. This section does not need to start with an input coupling prism. The last distal section 264 of PRLG 10 can have highly reflective facets. It is possible to implement only section 260 with 262 or 262 with 264.

[0101] Fig. 23B shows an implementation based on Fig. 9. Section 270 has facet 266 that partially reflects a3 and a4 (or one of them) while transmitting a1 and a2 in PRLG 10. Section 272 (based on second light guide portion 30) includes facet 12 that partially reflects a3 and a4 (or one of them) without the need to transmit a1 and a2. Facet 266 of section 274 can have a high degree of reflectivity. Again, it is possible to implement only section 270 with 272 or section 272 with 274, i.e. including an internal distal facet without a proximal internal facet, or vice versa.

[0102] Another group of configurations that use a combination of a rectangular light guide and facets on the exterior of the rectangular light guide to achieve lateral aperture expansion are shown in Figures 24A-24C.

[0103] 24A, a rectangular light guide 310 includes facets that couple the beam out into a light guide 20 that includes a coupling out arrangement 122 that couples out the light beam (typically with embedded partially reflective facets, not shown for clarity). Here, the rectangular light guide 310 is bounded both above and below by TIR surfaces 330 (in contrast to the PRLG embodiment described above).

[0104] The end of the rectangular light guide 310 opens onto the light guide section 321 so that the remaining light beams emerge from the rectangular light guide 310 and propagate through the light guide section 321, either directly or after an additional reflection on an extended parallel reflector 331, which is an extension of one of the faces 330 of the rectangular light guide. The beams emerging from the rectangular light guide 310 into the light guide section 321 are guided by two internal reflections, but are no longer guided in the second dimension (up and down as illustrated) so that the light beams spread within the plane of the light guide section 321. The beams are deflected by facets 266 in the rectangular light guide 310 and facets 12 in the light guide section 321 and are guided within the light guide 20 to the coupling output arrangement 122 and the viewer's eye.

[0105] In this configuration, the center of the light guide is well defined other than in the combined output configuration, as is preferred in augmented reality (AR) glasses. Furthermore, the fact that the rectangular light guide extends across only a portion of the optical system's dimensions reduces the number of four reflections in the light path, making the system less susceptible to image degradation during propagation due to imperfections in the rectangular light guide.

[0106] Figure 24B shows an alternative configuration in which the beams of PRLG 311 are coupled out through surface 34 in a manner entirely similar to the embodiment of Figures 2A and 9A. In this case, PRLG 311 is short and angled so that the beams emerging from it and entering light guide 20 expand as they propagate through section 322. Facets 12 in expanded section 322 redirect the beams towards coupling-out configuration 122.

[0107] FIG. 24C shows a combination of features from the embodiments of FIGS. 24A and 24B in which a partially internally reflective rectangular light guide 312 includes internal facets 266 along at least a portion of its length, but also features a partially reflective lower surface, allowing a portion of the propagating light intensity to escape and continue in its propagation direction into a light guide section 323 that includes facets 12 positioned to deflect both the light beam that escapes through the partially reflective surface 34 and the light beam that exits the end of the PRLG 312.

[0108] Certain implementations of the present invention allow for an extension of the field of view of the output image beyond the limits that can normally be conveyed by a rectangular light guide that is purely limited by TIR. Specifically, in at least the embodiments of Figures 2A-14 and 23B, the internal reflections from surface 34 are governed by the properties of the partially reflective coating and are not limited by TIR, and other parallel surfaces 32 of PRLG 10 can also be made to reflect at angles beyond the limits of TIR with the application of reflective coatings. This is illustrated in the angle space of Figures 25 and 26.

[0109] By way of introduction, in the angular representations of Figures 11A and 12A, the angular size of the projected image is limited by the TIR angle of the light guide, including in particular the TIR condition represented by circle 202 for the top surface of PRLG 10. Thus, in Figures 11A and 12A, the width of the projected image is limited in angular size so that images a1 and a2 do not overlap circle 202, while the TIR limitations of the front and back surfaces of light guides 200a and 200b limit the vertical angular size of the image. Figures 25 and 26 illustrate how this limitation can be overcome to project a wider image field by coating the third major surface 32 of PRLG 10 with a reflective coating to support reflection at angles of incidence less than the critical angle of the transparent material in air.

[0110] Figure 25 shows an optical architecture in angle space equivalent to that of Figure 11A. By coating surface 32 (relevant to all configurations with rectangular light guides) with a reflective coating (which can be a dielectric coating, silver, aluminum or other), the TIR limit represented by circle 202 is no longer relevant (and is therefore marked as a dashed line for reference) and images a1 and a2 (and thereby also their conjugates a3 and a4) can become wider so as to overlap circle 202, but still continue to propagate within the light guide. The remaining reflection process of the light path, via reflection by facet 12 and subsequent light paths, still has a wider image, as explained in Figures 11B and 11C.

[0111] Figure 26 shows a corresponding magnified image of an implementation similar to that of Figure 12A, where a reflective coating is implemented on surface 32 in the case of tilted facets. Again, a wider field can be achieved.

[0112] All the above descriptions refer for clarity to "lateral" and "vertical" directions as shown in the figures. In practice, these orientations may vary according to the arrangement of the light guide.

[0113] 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 a light beam corresponding to an image from an image projector towards a user for viewing, the optical system comprising: (a) a partially internally reflecting rectangular light guide (PRLG) formed from a transparent material and having first and second mutually parallel major surfaces, a third major surface perpendicular to the first and second major surfaces, the first, second, and third major surfaces supporting internal reflection for a range of angles of incidence, and a fourth major surface parallel to the third major surface, at least a portion of the fourth major surface comprising a partially reflective, non-diffractive coating; (b) a second light guide portion having a pair of mutually parallel major surfaces for transmitting the light beam by internal reflection, the second light guide portion being optically coupled to at least a portion of an area of ​​the partially reflective coating, the second light guide portion including a set of planar, mutually parallel partially reflective surfaces located between and non-parallel to the pair of major surfaces; (c) a third light guide portion formed as a continuation of or adjacent to the second light guide portion, the third light guide portion including a coupling out arrangement deployed to couple out the light beam propagating within the third light guide portion by internal reflection to direct the light beam towards the user; (d) a coupling input arrangement for coupling a light beam corresponding to a collimated image from the image projector into the PRLG for propagation within the PRLG by four internal reflections at the first, second, third, and fourth major surfaces; This optical system comprises a light beam from the image projector, which propagates by four internal reflections within the PRLG, progressively emitted from the PRLG through the partially reflective coating, enters the second light guide portion, is redirected by reflection off the set of partially reflective surfaces to propagate within the third light guide portion, and is coupled out of the third light guide portion towards the user by the coupling output configuration.

2. 2. The optical system of claim 1, wherein the PRLG has a length along a direction parallel to a line of intersection between the first major surface and the third major surface, and a maximum length of an optical path of the light beam from being emitted from the PRLG until being redirected by one of the set of partially reflective surfaces is longer than the length of the PRLG.

3. 2. The optical system of claim 1, wherein the PRLG has a length along a direction parallel to a line of intersection between the first major surface and the third major surface, and a maximum length of an optical path of the light beam from being emitted from the PRLG until being redirected by one of the set of partially reflective surfaces is less than the length of the PRLG.

4. 2. The optical system of claim 1 , wherein the second light guide portion has a second pair of mutually parallel major surfaces perpendicular to the pair of mutually parallel major surfaces, whereby the second light guide portion transmits the light beam by four internal reflections, and the set of partially reflective surfaces couples the light beam exiting the second light guide portion to the third light guide portion.

5. The optical system of claim 1 , wherein the first and second major surfaces of the PRLG are parallel or coplanar with the pair of mutually parallel major surfaces of the second light guide portion.

6. 2. The optical system of claim 1 , wherein the second light guide portion and the third light guide portion are portions of a single light guide such that the pair of mutually parallel major surfaces extend continuously across the second and third light guide portions.

7. The optical system of claim 1 , wherein the coupling output arrangement comprises a second set of mutually parallel partially reflective internal surfaces deployed within the third light guide portion.

8. The optical system of claim 1 , wherein the coupling output arrangement comprises a diffractive optical element associated with the third light guide portion.

9. The optical system of claim 1 , wherein the PRLG is without an internal reflector.

10. The optical system of claim 1 , wherein a majority of the length of the PRLG does not include an internal reflector.

11. The optical system of claim 1 , wherein the PRLG includes at least one partially reflective interior surface that is parallel to the set of partially reflective surfaces of the second light guide portion.

12. 12. The optical system of claim 11 , wherein the at least one partially reflective internal surface comprises a surface having a reflectivity for at least one range of angles of incidence greater than 10%, the surface being located one-third of the length of the PRLG distal to the coupling input arrangement.

13. 12. The optical system of claim 11, wherein the at least one partially reflective internal surface comprises a surface having a reflectivity for at least one range of angles of incidence that is less than 10%, the surface being located one-third of the length of the PRLG proximate the coupling input arrangement.

14. The optical system of claim 1 , wherein the third major surface of the PRLG is coated with a reflective coating to support reflection at angles of incidence less than a critical angle of a transparent material in air.

15. 1. An optical system for directing a light beam corresponding to an image from an image projector towards a user for viewing, the optical system comprising: (a) a partially internally reflecting rectangular light guide (PRLG) formed from a transparent material and having first and second mutually parallel major surfaces, a third major surface perpendicular to the first and second major surfaces, the first, second, and third major surfaces supporting internal reflection for a range of angles of incidence, and a fourth major surface parallel to the third major surface, at least a portion of the fourth major surface comprising a partially reflective, non-diffractive coating; (b) a second light guide portion having a pair of mutually parallel major surfaces for transmitting the light beam by internal reflection, the second light guide portion optically coupled to at least a portion of an area of ​​the partially reflective coating, the second light guide portion comprising a diffractive optical element deployed to deflect a light beam propagating within the second light guide portion; (c) a third light guide portion formed as a continuation of or adjacent to the second light guide portion, the third light guide portion including a coupling out arrangement deployed to couple out the light beam propagating within the third light guide portion by internal reflection to direct the light beam towards the user; (d) a coupling input arrangement for coupling a light beam corresponding to a collimated image from the image projector into the PRLG for propagation within the PRLG by four internal reflections at the first, second, third, and fourth major surfaces; This results in an optical system in which the light beam from the image projector, propagating by four internal reflections within the PRLG, is progressively emitted from the PRLG through the partially reflective coating, enters the second light guide portion, is redirected by the diffractive optical element to propagate within the third light guide portion, and is coupled out of the third light guide portion towards the user by the coupling output configuration.

16. 1. An optical system for directing a light beam corresponding to an image from an image projector towards a user for viewing, the optical system comprising: (a) a partially internally reflecting rectangular light guide (PRLG) formed from a transparent material and having first and second mutually parallel major surfaces, a third major surface perpendicular to the first and second major surfaces, the first, second, and third major surfaces supporting internal reflection for angles of incidence greater than a critical angle, and at least the third major surface being transmissive for angles of incidence less than the critical angle, and a fourth major surface parallel to the third major surface, at least a portion of the fourth major surface comprising a partially reflective coating that is partially reflective for angles of incidence greater than the critical angle and transparent for angles of incidence less than the critical angle; (b) a second light guide having a pair of mutually parallel major surfaces for transmitting the light beam by internal reflection, the second light guide being optically coupled to at least a portion of the area of ​​the partially reflective coating of the fourth major surface, the second light guide including a set of planar, mutually parallel partially reflective surfaces located between and non-parallel to the pair of major surfaces; (c) a third light guide adjacent the third major surface, the third light guide including a coupling out configuration deployed to couple out the light beam propagating within the third light guide by internal reflection to direct the light beam towards the user; (d) a coupling input arrangement for coupling a light beam corresponding to a collimated image from the image projector into the PRLG for propagation within the PRLG by four internal reflections at the first, second, third, and fourth major surfaces; This results in an optical system in which the light beam from the image projector propagating by four internal reflections within the PRLG incident on the fourth major surface at an angle of incidence greater than the critical angle, progressively exits the PRLG through the partially reflective coating, incident on the second light guide, is redirected by reflection off the set of partially reflective surfaces to incident on the fourth major surface at an angle of incidence less than the critical angle, traverses the PRLG, passes through the third major surface to incident on the third light guide, propagates within the third light guide by internal reflection, and is coupled out of the third light guide towards the user by the coupling output configuration.

17. The optical system of claim 16 , wherein the pair of major surfaces of the second light guide are perpendicular to the fourth major surface of the PRLG.

18. 20. The optical system of claim 17, wherein the second light guide further comprises an additional major surface parallel to the fourth major surface, the additional major surface supporting internal reflection of the light beam at angles of incidence greater than the critical angle.

Images