Image light guide system with forward light mitigation

Abstract

An image light guide system including an image source configured to emit image-bearing light beams, and an image light guide. The image light guide including an in-coupling diffractive optic operable to diffract at least a portion of the image-bearing light beams into the image light guide, and an out-coupling diffractive optic operable to diffract a first sub-portion of the image-bearing light beams in a first direction toward an eyebox, and operable to diffract a second sub-portion of the image-bearing light beams in a second direction away from the eyebox. Wherein the second sub-portion of the image-bearing light beams is oriented at a forward output angle relative to a horizontal axis, and the magnitude of the forward output angle is at least twenty degrees (20˚) relative to the horizontal axis.

Description

IMAGE LIGHT GUIDE SYSTEM WITH FORWARD LIGHT MITIGATIONTECHNICAL FIELD

[0001] The present disclosure generally relates to electronic displays and more particularly relates to displays utilizing image light guides with diffractive optics to convey image-bearing light to a viewer.BACKGROUND

[0002] Head-Mounted Displays (HMDs) and virtual image near-eye displays are being developed for a range of diverse uses, including military, commercial, industrial, fire-fighting, and entertainment applications. For many of these applications, there is particular value in forming a virtual image that can be visually superimposed over the real-world image formed in the eye from within the field of view of the HMD user. Image light guides convey image-bearing light along a transmissive waveguide from a location outside the viewer’s field of view to a position in alignment with the viewer's pupil while preserving the viewer’s view of the environment through the waveguide.

[0003] In some image light guides, collimated, relatively angularly encoded light beams from an image source are coupled into a plate-shaped waveguide by an input coupling such as an incoupling diffractive optic, which can be mounted or formed on a surface of the plate-shaped waveguide or buried within the waveguide. Such diffractive optics can be formed as diffraction gratings, holographic optical elements, or in other known ways. After propagating along the waveguide, the diffracted light can be directed back out of the waveguide by a similar output coupling, which can be arranged to provide exit pupil expansion along one or two dimensions. In addition, an optional intermediate diffractive optic (also referred to as a turning diffractive optic) can be positioned along the waveguide between the input and output couplings to provide exit pupil expansion in at least one dimension. The two dimensions of exit pupil expansion define an expanded eyebox within which the viewer’s pupil can be positioned for viewing the virtual image conveyed by the image light guide.

[0004] Image light guides and diffractive optical elements may form a virtual image focused at optical infinity by conveying angularly encoded light beams of collimated light to the viewer eyebox. However, a virtual image may be focused at some closer distance, such as in the range from 1 m to 1.5 m, for example.

[0005] In conventional image light guide arrangements utilizing transmissive waveguide substrates, light is diffracted from the output coupling in more directions than the direction of the viewer eyebox. An image light guide system configured to control and / or eliminate light outputin undesired directions would benefit commercial and general consumers because the near-eye display may appear more like traditional eyewear (i.e., provide aesthetic improvement). Thus, there is a need for an image light guide system operable to produce the desired virtual image brightness and resolution while managing the output of light in undesired directions.SUMMARY

[0006] In a first exemplar}7embodiment, the present disclosure provides for an image light guide system including an image source configured to emit image-beanng light beams, and an image light guide. The image light guide including an in-coupling diffractive optic operable to diffract at least a portion of the image-bearing light beams into the image light guide, and an out-coupling diffractive optic operable to diffract a first sub-portion of the image-bearing light beams in a first direction toward an eyebox, and operable to diffract a second sub-portion of the image-bearing light beams in a second direction away from the eyebox. Wherein the second sub-portion of the image-bearing light beams is oriented at a forward output angle relative to a horizontal axis, and the magnitude of the forward output angle is at least twenty degrees (20°) relative to the horizontal axis.

[0007] In a second exemplary embodiment, the present disclosure provides for a method of aligning a virtual image, including aligning an image source at an initial angle relative to a first planar surface of an image light guide, angling the first planar surface of the image light guide at a tilt angle relative to a vertical axis, diffracting a portion of in-coupling image-bearing light into the image light guide via a first in-coupling optical element, diffracting a first sub-portion of the image-bearing light out of the image light guide in a first direction towards an eyebox, and diffracting a second sub-portion of the image-bearing light out of the image light guide in a second direction away from the eyebox, wherein the second sub-portion is diffracted out of the image light guide at a forward output angle, and wherein the forward output angle is at least twenty degrees (20 ) relative to the horizontal axis. In an example embodiment, the step of aligning the image source relative to the in-coupling optical element is accomplished by a physical rotation of the image source, by a digital movement of a virtual image within the image source, or by an optical coupler.BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0008] The accompanying drawings are incorporated herein as part of the specification. The drawings described herein illustrate embodiments of the presently disclosed subject matter and are illustrative of selected principles and teachings of the present disclosure. However, thedrawings do not illustrate all possible implementations of the presently disclosed subject matter and are not intended to limit the scope of the present disclosure in any way.

[0009] FIG. 1 is a top view of an image light guide with an exaggerated thickness for showing the propagation of light from an image source along the image light guide to an eyebox within which the virtual image can be viewed.

[0010] FIG. 2 is a perspective view of an image light guide including an in-coupling diffractive optic, a turning diffractive optic, and out-coupling diffractive optic for managing the propagation of image-bearing light beams.

[0011] FIGS. 3 and 4 are schematic top plan views of an image light guide showing the angular relationship between an in-coupled light ray vector and light rays emitted by the out-coupling diffractive optic according to an exemplary embodiment of the presently disclosed subject matter.

[0012] FIG. 5 is a side view of a head mounted near-eye display having an image light guide system with an image light guide oriented at a pantoscopic tilt according to an exemplary embodiment of the presently disclosed subject matter.

[0013] FIG. 6A is a side view of an image light guide system according to an exemplary embodiment of the presently disclosed subject matter.

[0014] FIG. 6B is a side view of an image light guide system according to an exemplary embodiment of the presently disclosed subject matter.

[0015] FIG. 6C is a side view of an image light guide system according to an exemplary embodiment of the presently disclosed subject matter.

[0016] FIG. 6D is a side view of an image light guide system according to an exemplary embodiment of the presently disclosed subject matter.

[0017] FIG. 7 is a flow chart of a method of aligning a virtual image according to an exemplary embodiment of the presently disclosed subject matter.

[0018] FIG. 8 is a top plan view of an image light guide system with an image light guide oriented at a chevron angle according to an exemplary embodiment of the presently disclosed subject matter.

[0019] FIG. 9 is a perspective view of an image light guide system taking the form of a headmounted display according to an exemplary embodiment of the presently disclosed subject matter.

[0020] FIG. 10 is a perspective view of an image light guide system taking the form of a monocular head-mounted display according to an exemplary' embodiment of the presently disclosed subject matter.DETAILED DESCRIPTION

[0021] It is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific assemblies and systems illustrated in the attached drawings and described in the following specification are simply exemplary embodiments of the inventive concepts defined herein. Hence, specific dimensions, directions, or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless expressly stated otherwise. Also, although they may not be, like elements in various embodiments described herein may be commonly referred to with like reference numerals within this section of the application.

[0022] One skilled in the relevant art will recognize that the elements and techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects of the present disclosure. Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. Thus, the appearance of the phrase “in one embodiment” or “in an embodiment” throughout the specification is not necessarily referring to the same embodiment. However, the particular features, structures, or characteristics described may be combined in any suitable manner in one or more embodiments.

[0023] Where used herein, the terms “first”, “second”, and so on, do not necessarily denote any ordinal, sequential, or priority relation, but are simply used to more clearly distinguish one element or set of elements from another, unless specified otherwise.

[0024] Where used herein, the terms “viewer”, “operator”, "observer", and “user” are considered to be equivalent and refer to the person, or machine, who wears and / or views images using a neareye display device.

[0025] Where used herein, the terms “coupled” or “coupler” (in the context of optics) refer to a connection by which light travels from one optical medium or device to another optical medium or device.

[0026] Where used herein, the term “about” when applied to a value is intended to mean within the tolerance range of the equipment used to produce the value, or, in some examples, is intendedto mean plus or minus 10%, or plus or minus 5%, or plus or minus 1%, unless otherw ise expressly specified.

[0027] Where used herein, the term “substantially” is intended to mean within the tolerance range of the equipment used to produce the value, or, in some examples, is intended to mean plus or minus 10%, or plus or minus 5%, or plus or minus 1%, unless otherwise expressly specified.

[0028] Where used herein, the terms “optical infinity” and “at infinity” correspond to conventional usage in the camera and imaging arts, indicating image formation using substantially collimated light, so that the focus distance exceeds at least about four meters (4 m).

[0029] Where used herein, the term “beam expansion” is intended to mean replication of a beam via multiple encounters with an optical element to provide exit pupil expansion in one or more directions. Similarly, as used herein, to “expand” a beam, or a portion of a beam, is intended to mean replication of a beam via multiple encounters with an optical element to provide exit pupil expansion in one or more directions.

[0030] An optical system, such as a HMD, can produce a virtual image. In contrast to methods for forming a real image, a virtual image is not formed on a display surface. That is, if a display surface were positioned at the perceived location of a virtual image, no image would be formed on that surface. Virtual images have a number of inherent advantages for augmented reality presentation. For example, the apparent size of a virtual image is not limited by the size or location of a display surface. Additionally, the source object for a virtual image may be small; for example, a magnifying glass provides a virtual image of an object. In comparison with systems that project a real image, a more realistic viewing experience can be provided by forming a virtual image that appears to be some distance away. Providing a virtual image also obviates the need to compensate for screen artifacts, as may be necessary when projecting a real image.

[0031] FIG. 1 is a schematic diagram showing a simplified cross-sectional view of one conventional configuration of an image light guide system 10. Image light guide system 10 includes a planar image light guide 12, an in-coupling diffractive optic IDO, and an out-coupling diffractive optic ODO. The image light guide 12 includes a transparent substrate S, which can be made of optical glass or plastic, with plane-parallel front and back surfaces 14 and 16. In this example, the in-coupling diffractive optic IDO is shown as a transmissive-type diffraction grating arranged on, in, or otherwise engaged with the front surface 14 of the image light guide 12. However, in-coupling diffractive optic IDO could alternately be a reflective-type diffraction grating or other type of diffractive optic, such as a volume hologram or other holographic diffraction element, that diffracts incoming image-bearing light beams WI into the image lightguide 12. The in-coupling diffractive optic IDO can be located on, in, or otherwise engaged with the front surface 14 or back surface 16 of the image light guide 12 and can be of a transmissive or reflective-type in a combination that depends upon the direction from which the image-bearing light beams WI approach the image light guide 12.

[0032] When used as a part of a near-eye or head-mounted display system, the in-coupling diffractive optic IDO of the conventional image light guide system 10 couples the image-bearing light beams WI from a real, virtual or hybrid image source 18 into the substrate S of the image light guide 12. Any real image or image dimension formed by the image source 18 is first converted into an array of overlapping, angularly related, collimated beams encoding the different positions within a virtual image for presentation to the in-coupling diffractive optic IDO. Typically, the rays within each bundle forming one of the angularly related beams extend in parallel, but the angularly related beams are relatively inclined to each other through angles that can be defined in tw o angular dimensions corresponding to linear dimensions of the image.

[0033] Once the angularly related beams engage with the in-coupling diffractive optic IDO, at least a portion of the image-bearing light beams WI are diffracted (generally through a first diffraction order) and thereby redirected by in-coupling diffractive optic IDO into the planar image light guide 12 as angularly encoded image-bearing light beams WG for further propagation along a length dimension x of the image light guide 12 by total internal reflection (TIR) between the plane-parallel front and back surfaces 14 and 16. Although diffracted into a different combination of angularly related beams in keeping with the boundaries set by TIR, the imagebearing light beams WG preserve the image information in an angularly encoded form that is derivable from the parameters of the in-coupling diffractive optic IDO. The out-coupling diffractive optic ODO receives the encoded image-bearing light beams WG and diffracts (also generally through a first diffraction order) at least a portion of the image-bearing light beams WG out of the image light guide 12, as image-bearing light beams WO, toward a nearby region of space referred to as an eyebox E, within which the transmitted virtual image can be seen by a viewer’s eye or other optical component. The out-coupling diffractive optic ODO can be designed symmetrically with respect to the in-coupling diffractive optic IDO to restore the original angular relationships of the image-bearing light beams WI among outputted angularly related beams of the image-bearing light beams WO. In addition, the out-coupling diffractive optic ODO can modify the original field points’ positional angular relationships producing an output virtual image at a finite focusing distance.

[0034] However, to increase one dimension of overlap among the angularly related beams populating the eyebox E (defining the size of the region within which the virtual image can beseen), the out-coupling diffractive optic ODO is arranged together with a limited thickness T of the image light guide 12 to encounter the image-bearing light beams WG multiple times and to diffract only a portion of the image-bearing light beams WG upon each encounter. The multiple encounters along the length (e.g., a first direction) of the out-coupling diffractive optic ODO have the effect of replicating the image-bearing light beams WG and enlarging or expanding at least one dimension of the eyebox E where the replicated beams overlap. The expanded eyebox E decreases sensitivity to the position of a viewer’s eye for viewing the virtual image.

[0035] The out-coupling diffractive optic ODO is shown as a transmissive-type diffraction grating arranged on or secured to the front surface 14 of the image light guide 12. However, like the incoupling diffractive optic IDO, the out-coupling diffractive optic ODO can be located on, in, or otherwise engaged with the front or back surface 14 or 16 of the image light guide 12 and can be of a transmissive or reflective-type in a combination that depends upon the direction through which the image-bearing light beams WG is intended to exit the image light guide 12. In addition, the out-coupling diffractive optic ODO could be formed as another type of diffractive optic, such as a volume hologram or other holographic diffraction element, that diffracts propagating imagebearing light beams WG from the image light guide 12 as the image-bearing light beams WO propagating toward the eyebox E.

[0036] FIG. 2 illustrates a perspective view of a conventional image light guide system 10 arranged for expanding the eyebox E in two dimensions, i.e., along both x- and y-axes of the intended image. To achieve a second dimension of eyebox expansion, the in-coupling diffractive optic IDO is oriented to diffract at least a portion of image-bearing light beams WG along a grating vector kl along the image light guide 12 toward an intermediate turning optic TO, whose grating vector k2 is oriented to diffract at least a portion of the image-bearing light beams WG in a reflective mode along the image light guide 12 toward the out-coupling diffractive optic ODO. It should be appreciated that only a portion of the image-bearing light beams WG are diffracted by each of the multiple encounters with intermediate turning optic TO, thereby laterally replicating each of the angularly related beams of the image-bearing light beams WG as they approach the out-coupling diffractive optic ODO. The intermediate turning optic TO redirects the imagebearing light beams WG toward the out-coupling diffractive optic ODO (having a grating vector k3) for longitudinally replicating the angularly related beams of the image-bearing light beams WG in a second direction before exiting the image light guide 12 as the image-bearing light beams WO. Grating vectors, such as the depicted grating vectors kl, k2, and k3, extend within a parallel plane of the image light guide 12 in respective directions that are normal to the diffractive features (e.g., grooves, lines, or rulings) of the diffractive optics and have respective magnitudes inverseto the period or pitch d (i.e., the on-center distance between the diffractive features) of the diffractive optics IDO, TO, and ODO.

[0037] As shown in FIG. 2, in-coupling diffractive optic IDO receives the incoming imagebearing light beams WI containing a set of angularly related beams corresponding to individual pixels or equivalent locations within an image generated by the image source 18, such as a projector. A full range of angularly encoded beams for producing a virtual image can be generated by a real display together with collimating optics or other optical components, by a beam scanner for more directly setting the angles of the beams, or by a combination such as a one-dimensional real display used with a scanner. In this configuration, the image light guide 12 outputs a replicated set of angularly related beams (replicated in two dimensions) by providing multiple encounters of the image-bearing light beams WG with both the intermediate turning optic TO and the out- coupling diffractive optic ODO in different orientations. In the depicted orientation of the image light guide 12, the intermediate turning optic TO provides eyebox expansion in a first dimension, e.g., the y-axis direction, and the out-coupling diffractive optic ODO provides a similar eyebox expansion in a second dimensions, e.g., the x-axis direction. The relative orientations and respective periods d of the diffractive features of the in-coupling optic IDO, intermediate turning optic TO, and out-coupling diffractive optic ODO provide for eyebox expansion in two dimensions while preserving the intended relationships among the angularly related beams of the image-bearing light beams WI that are output from the image light guide system 10 as the imagebearing light beams WO. It should be appreciated that the in-coupling diffractive optic IDO, the intermediate turning optic TO, and the out-coupling diffractive optic ODO, can each include diffractive features having a common pitch d (i.e., period), where the common pitch d of each optic can be different.

[0038] In the configuration shown, while the image-bearing light beams WI input into the image light guide 12 are encoded into a different set of angularly related beams by the in-coupling diffractive optic IDO, the information required to reconstruct the image is preserved by accounting for the systematic effects of the in-coupling diffractive optic IDO. The intermediate turning optic TO, located in an intermediate position between the in-coupling and out-coupling diffractive optics IDO and ODO, can be arranged so that it does not induce significant changes to the encoding of the image-bearing light beams WG. As such, the out-coupling diffractive optic ODO can be arranged in a symmetric fashion with respect to the in-coupling diffractive optic IDO, e.g., including diffractive features sharing the same period d. Similarly, the period of the intermediate turning optic TO can also match the common period of the in-coupling and out-coupling diffractive optics IDO and ODO. Although the grating vector k2 of the intermediate turning opticTO is shown oriented at 45 degrees with respect to the other grating vectors, which remains a possible orientation, the grating vector k2 of the intermediate turning optic TO can be oriented at 60 degrees to the grating vectors kl and k3 of the in-coupling and out-coupling diffractive optics IDO and ODO in such a way that the image-bearing light beams WG are turned 120 degrees. By orienting the grating vector k2 of the intermediate turning optic TO at 60 degrees with respect to the grating vectors kl and k3 of the in-coupling and out-coupling diffractive optics IDO and ODO, the grating vectors kl and k3 of the in-coupling and out-coupling diffractive optics IDO and ODO are also oriented at 60 degrees with respect to each other. By basing the grating vector magnitudes on the common pitch shared by the in-coupling, intermediate turning, and out-coupling diffractive optics IDO, TO, and ODO, the three grating vectors kl, k2, and k3 (as directed line segments) form an equilateral triangle and sum to a zero vector magnitude, which avoids asymmetric effects that could introduce unwanted aberrations including chromatic dispersion. Such asymmetric effects can also be avoided by grating vectors kl, k2, and k3 that have unequal magnitudes in relative orientations at which the three grating vectors kl, k2, and k3 sum to a zero vector magnitude.

[0039] In a broader sense, the image-bearing light beams WI that are directed into the image light guide 12 are effectively encoded by the in-coupling diffractive optic IDO, whether the in-coupling optic IDO uses gratings, holograms, prisms, mirrors, or some other mechanism. Any reflection, refraction, and / or diffraction of light that takes place at the input should be correspondingly decoded by the output to re-form the virtual image that is presented to the viewer. Whether any symmetries are maintained among the intermediate turning optic TO, the in-coupling optic IDO, and out-coupling diffractive optic ODO, or whether any change to the encoding of the angularly related beams of the image-bearing light beams WI takes place along the image light guide 12, the intermediate turning optic TO and the in-coupling and out-coupling diffractive optics IDO and ODO can be related so that the image-bearing light beams WO that are output from the image light guide 12 preserve or otherwise maintain the original or desired form of the image-bearing light beams WI for producing the intended virtual image.

[0040] As shown in FIG. 2, the letter “R” represents the orientation of the virtual image that is visible to the viewer whose eye is positioned within the eyebox E. As shown, the orientation of the letter “R” in the represented virtual image matches the orientation of the letter “R” as encoded by the image-bearing light beams WI. A change in the rotation about the z axis or angular orientation of incoming image-bearing light beams WI with respect to the x-y plane causes a corresponding symmetric change in rotation or angular orientation of outgoing light from out- coupling diffractive optic (ODO). From the aspect of image orientation, the intermediate turningoptic TO simply acts as a type of optical relay, providing one dimension of eyebox expansion through replication of the angularly encoded beams of the image-bearing light beams WG along one axis (e.g., along the y-axis) of the image. Out-coupling diffractive optic ODO further provides a second dimension of eyebox expansion through replication of the angularly encoded beams along another axis (e.g., along the x-axis) while maintaining the original orientation of the virtual image encoded by the image-bearing light beams WI. The intermediate turning optic TO is typically a slanted or square grating or, alternately, can be a blazed grating and is typically arranged on one of the plane-parallel front and back surfaces of the image light guide 12. It should be appreciated that the representation of the virtual image “R” as created by an image source is comprised of infinitely focused light that requires a lens (e.g., the lens in the human eye) to focus the image so that the orientations discussed above can be detected.

[0041] Together, the in-coupling, turning, and out-coupling diffractive optics IDO, TO, and ODO preferably preserve the angular relationships among beams of different wavelengths defining a virtual image upon conveyance by image light guide 12 from an offset position to a near-eye position of the viewer. While doing so, the in-coupling, turning, and out-coupling diffractive optics IDO, TO, and ODO can be relatively positioned and oriented in different ways to control the overall shape of the image light guide 12 as well as the overall orientations at which the angularly related beams can be directed into and out of the image light guide 12.

[0042] It should be appreciated that the intermediate turning optic TO is an optional element that may, or may not, be included in the image light guide systems described herein. In some example embodiments, the out-coupling diffractive optic includes compound diffractive features or crossed gratings configured to expand the eyebox in two dimensions without an intermediate turning optic TO.

[0043] Referring now to FIGS. 3 and 4, at least a portion of the image-bearing light WG incident upon the out-coupling diffractive optic ODO is directed out of the image light guide 12 as out- coupled image-bearing light beams WO1, WO2. As referred to herein, the image-bearing light beams WO1, WO2, the first portion of out-coupled image-bearing light beams WO1, the second portion of out-coupled image-bearing light beams WO2, the first sub-portion of the image-bearing light WO1, and the second sub-portion of the image-bearing light WO2 all refer to first order diffracted light out-coupled by a diffractive optic (e.g., out-coupling diffractive optic ODO), unless otherwise indicated. The first portion of out-coupled image-bearing light beams WO1 are emitted in a first direction toward the eyebox E where the viewer’s eye is operable to view a virtual image. The second portion of out-coupled image-bearing light beams WO2 are emitted in a second direction opposite the eyebox E.

[0044] In an example embodiment, a central ray of the image-bearing light beams WI is incident on the in-coupling diffractive optic IDO at an angle of incidence 30. The first portion of out- coupled image-bearing light beams WO1 are emitted toward the eyebox E at an angle of emission 32. The first portion of out-coupled image-bearing light beams WO1, which correspond to diffracted light of a selected diffraction order (e.g., first order diffraction), are diffracted and emitted toward the eyebox E, i.e., out of and away from the side 14 of the image light guide 12 on which the input image-bearing light beams WI are incident. The angle of incidence 30 and the angle of emission 32 are symmetric (i.e., have mirror symmetry with respect to planar surface 14). In other words, although emitted in an opposing direction to the image-bearing light WI, the magnitude of the angle of emission 32 of the out-coupled image-bearing light beams WO1 is equal to the magnitude of the angle of incidence 30 of the in-coupled image-bearing light WI.

[0045] The image-bearing light beams WO2 are emitted by the out-coupling diffractive optic ODO from the surface 16 in the same direction as the image-bearing beams WI are incident on the in-coupling diffractive optic IDO and at an exit angle 36 equal to the angle of incidence 30, rendering image-bearing light beams WI and image-bearing light beams WO2 parallel in angular space. FIG. 3 shows that image-bearing light beams WI incident on a surface 14 at an angle of incidence 30 of ninety degrees (90°) relative to the surface 14 produce image-bearing light beams WO2 emitted from surface 16 at an exit angle 36 of ninety degrees (90°) relative to the surface 16. As illustrated in FIG. 3, angle of incidence 30 and exit angle 36 are equal with respect to surface 16 and surface 14.

[0046] FIG. 4 illustrates image-bearing light beams WI having an angle of incidence 30 that is not ninety degrees (90°) relative to surface 14, and image-bearing light beams WO2 emitted through surface 16 at an exit angle 36 that is not ninety degrees (90°) relative to surface 16. The exit angle 36 of the outgoing image-bearing light beams WO2 is equal to the angle of incidence 30 and the outgoing image-bearing light beams WO2 are parallel to the image-bearing light beams WI. The angle of incidence 30 of the central ray of the image-bearing light beams WI emitted from image source 18 may be determined by a relative orientation of the image source 18 (e.g., orientation of a lens barrel) and / or an optical coupler (e.g., a prism or optical wedge) to the image light guide 12.

[0047] As the angle of incidence 30 of the image-bearing light beams WI is changed the exit angle 36 of the image-bearing light beams WO2 is correspondingly changed, such that the relationship of the angle of incidence 30 and the exit angle 36 is maintained. The angle of incidence 30 and the exit angle 36 are equal and are independent of the roll, pitch, and yaw of the image light guide 12. The relationship of the angle of incidence 30 and the exit angle 36 is a resultof the image-bearing light beam WI being incident upon the in-coupling diffractive optic IDO from the side of the image light guide 12 opposite of the side of emission of the second portion of the image-bearing light beams WO2. Conversely, the angle of incidence 30 and the angle of emission 32 of the first portion of the image-bearing light beams WO1 is not independent of the roll, pitch, and yaw of the image light guide 12. Instead, should the image light guide 12 move (change to pitch, yaw, and / or roll) relative to the image source, image-bearing light beams WO1 change in a corresponding or mirrored relationship to the angle of incidence 30.

[0048] The relationships of the angle of incidence 30 and the angle of emission 32 and exit angle 36 can be utilized to control the direction at which the image-beanng light beams WO2 are emitted to reduce the visibility of the image-bearing light beams WO2 to persons around the wearer of the image light guide system 10. Referring now to FIG. 5, in an example embodiment, the image light guide 12 is arranged at a tilt angle p to an imaginary vertical axis Al. In other words, the surfaces 14, 16 of the image light guide 12 are arranged generally parallel to a plane Pl which is oriented at the angle p to the vertical axis Al. For example, the vertical axis Al is arranged generally orthogonal to the floor or ground, and / or the vertical axis Al is arranged generally orthogonal to the horizontal axis A2 that is parallel to a forward gaze of the wearer. The arrangement of the image light guide 12 at the angle p may be referred to as the pantoscopic tilt of the image light guide 12.

[0049] Referring now to FIGS. 6A-6D, the angle of incidence 30 and the angle of emission 32 and exit angle 36 of the image-bearing light beams WO1, WO2 are referenced relative to a surface 14, 16 of the image light guide 12 generally between the in-coupling diffractive optic IDO and the out-coupling diffractive optic ODO for purpose of discussion. As illustrated in FIGS. 6A-6D, the image light guide 12 is oriented at atilt angle p which may be between 0° and 15°, between 5° and 12°, and / or between 10° and IF. The tilt angle p is selected from within the given ranges, as 7° - 12° appears to be an idealized range for what is a socially acceptable range of pantoscopic tilt for a pair of smart glasses. Nevertheless, it should be appreciated that other ranges of the tilt angle p are possible, e.g., ranges between 15° and 25° may be possible.

[0050] As illustrated in FIG. 6A, in an example embodiment, the incoming image-bearing light beams WI are oriented at an angle of incidence 30 such that the incoming image-bearing light beams WI are parallel to imaginary horizontal axis A2 oriented orthogonal to the vertical axis Al. The second portion of out-coupled image-bearing light beams WO2 are oriented at an exit angle 36 that is equal to the angle of incidence 30 such that the second portion of out-coupled imagebearing light beams WO2 are also parallel to the honzontal axis A2. The first portion of out- coupled image-bearing light beams WO1 are oriented at an angle of emission 32 in mirrorsymmetry to the angle of incidence 30 such that the virtual image conveyed to the wearer appears to originate at a position below the horizontal axis A2.

[0051] As illustrated in FIG. 6B, in an example embodiment, the incoming image-bearing light beams WI are oriented at an angle of incidence 30 such that the incoming image-bearing light beams WI are oriented at generally orthogonal to the surface 14 of the image light guide 12. The second portion of out-coupled image-bearing light beams WO2 are oriented at an exit angle 36 that is equal to the angle of incidence 30 such that the second portion of out-coupled image-bearing light beams WO2 are also oriented generally orthogonal to the surface 14 of the image light guide 12. The first portion of out-coupled image-bearing light beams WO1 are oriented at an angle of emission 32 in mirror symmetry to the angle of incidence 30 such that the virtual image conveyed to the wearer appears to originate at a position below the horizontal axis A2. Relative to the embodiment illustrated in FIG. 6A, the virtual image conveyed to the wearer in FIG. 6B is closer to the horizontal axis A2.

[0052] As illustrated in FIG. 6C, in an example embodiment, the incoming image-bearing light beams WI are oriented at an angle of incidence 30 such that the incoming image-bearing light beams WI are oriented at an obtuse angle to the surface 14 of the image light guide 12. The second portion of out-coupled image-bearing light beams WO2 are oriented at an exit angle 36 that is equal to the angle of incidence 30 such that the second portion of out-coupled image-bearing light beams WO2 are also oriented an obtuse angle to the surface 14 of the image light guide 12. The first portion of out-coupled image-bearing light beams WO1 are oriented at an angle of emission 32 in mirror symmetry to the angle of incidence 30 such that the virtual image conveyed to the wearer appears to originate at a position above the horizontal axis A2.

[0053] As illustrated in FIG. 6D, in an example embodiment, the incoming image-bearing light beams WI are oriented at an angle of incidence 30 such that the incoming image-bearing light beams WI are oriented at an obtuse angle to the surface 14 of the image light guide 12. The second portion of out-coupled image-bearing light beams WO2 are oriented at an exit angle 36 that is equal to the angle of incidence 30 such that the second portion of out-coupled image-bearing light beams WO2 are also oriented an obtuse angle to the surface 14 of the image light guide 12. The first portion of out-coupled image-bearing light beams WO1 are oriented at an angle of emission 32 in mirror symmetry to the angle of incidence 30 such that first portion of out-coupled imagebearing light beams WO1 are generally parallel with the horizontal axis A2 and the virtual image conveyed to the wearer appears to originate at a position on the horizontal axis A2 (e.g., on the horizon).

[0054] As illustrated in FIGS. 6A-6D, the angle of emission 32 and exit angle 36 of the first and second portions of the image-bearing light beams WO1 and WO2, respectively, are determined by the angle of incidence 30. From a different perspective, the image-bearing light beams WO1, WO2 leaving the image light guide 12 can be characterized by rearward and forward output angles al, a2 relative to the horizontal axis A2. As such, the rearward and forward output angles al, a2 are determined by the angle of incidence 30 and the tilt angle p of the image light guide 12. In other words, the magnitude of the forward output angle a2 of the second portion of the imagebearing light beams WO2 is equal to a magnitude of the tilt angle p plus the magnitude of the angle of incidence 30. In an example embodiment, the image light guide 12 is oriented at a tilt angle p of approximately 10° to 11° and the incident angle 30 of the incoming image-bearing light beams WI is selected to locate the virtual image conveyed to the wearer at or below the horizontal axis A2. In this example, the second portion of out-coupled image-bearing light beams WO2 are oriented at an exit angle 36 that arranges the forward output image-bearing light WO2 at a forward output angle a2 of at least 20° below the horizontal axis A2. In an example embodiment, the image source 18 has a vertical field-of-view (FOV), and the forward output angle a2 is equal to or greater than the image source 18 vertical FOV.

[0055] In an exemplary embodiment, the perception of the second portion of out-coupled imagebearing light beams WO2 may be further mitigated utilizing an out-coupling diffractive optic ODO having diffractive features optimized to maximize diffraction efficiency in a particular diffraction order to maximize the light intensity of the first portion of the image-bearing light beams WO1 guided to the eyebox E and reduce the light intensity of the second portion of out- coupled image-bearing light beams WO2. In an example embodiment, the diffractive features of the out-coupling diffractive optic ODO are blazed diffractive features (e.g., blazed gratings). In another example embodiment, the diffractive features of the out-coupling diffractive optic ODO are slanted diffractive features. In another example embodiment, the diffractive features of the out-coupling diffractive optic ODO are sawtooth diffractive features. In an exemplary embodiment, the diffractive features (e.g., blazed gratings) of the out-coupling diffractive optic ODO are configured by suitable parameter selection to concentrate a majority of the outcoupled image-bearing light in one diffraction order such that the ratio of the first portion of the imagebearing light beams WO1 to the second portion of out-coupled image-bearing light beams WO2 is at least 8: 1. In an example embodiment, the rearward output angle al of the first portion of the image-bearing light beams WO1 is within five degrees (5°) of the horizontal axis A2.

[0056] The in-coupling diffractive optic IDO is designed to efficiently diffract image-bearing light beams WI incident on the in-coupling diffractive optic IDO at a range of angles for a givenwavelength range of light. In an example embodiment, the forward output angle a2 is selected as a function of the vertical FOV of the image source 18 to locate all of the second portion of out- coupled image-bearing light beams WO2 below the horizontal axis A2. Knowing the forward output angle a2, the angle of incidence 30 and the tilt angle p are selected to achieve the forward output angle a2, and selected such that the rearward output angle al is within 5° of the horizontal axis A2. The in-coupling diffractive optic IDO is then designed such that the angular range of efficiently diffracted image-bearing light beams WI incident on the in-coupling diffractive optic IDO encompasses the entire angular range of the FOV of the image source 18, for example, by centering the acceptable angular range of the in-coupling diffractive optic IDO with a central ray of the image-bearing light beams WI at the angle of incidence 30.

[0057] As illustrated in FIG. 7, in an example embodiment, the relationships of the angle of incidence 30 and the angle of emission 32 and exit angle 36 can be utilized to control the direction at which the image-bearing light beams WO2 are emitted in a method of aligning a virtual image 100. The method of aligning a virtual image 100 includes a first step 102 of aligning the image source 18 at an initial angle relative to a first planar surface 14 of the image light guide 12. In a second step 104, the method 100 includes angling the first planar surface 14 of the image light guide 12 at a tilt angle p relative to the vertical axis Al. In a third step 106, the method 100 includes diffracting a portion of the image-bearing light WI into the image light guide 12 via the first in-coupling optical element IDO. In a fourth step 108, the method 100 includes diffracting a first sub-portion of the image-bearing light WO1 out of the image light guide 12 in a first direction towards the eyebox E. The method of aligning a virtual image 100 also includes a fifth step 110 of diffracting a second sub-portion of the image-bearing light WO2 out of the image light guide 12 in a second direction away from the eyebox E, wherein the second sub-portion of imagebearing light WO2 is diffracted out of the image light guide 12 at a forward output angle a2 of at least twenty degrees (20°) relative to the horizontal axis A2. In an example embodiment, the first step 102 of the method of aligning a virtual image 100 includes a step 102A comprising physically rotating the image source 18, digitally moving a virtual image within the image source 18, and / or angularly rotating the image-bearing light WI via an optical coupler. In an example embodiment, an optical coupler arranged optically between the image source 18 and the in-coupling optical element IDO is configured to orient the image-bearing light beams WI incident on the in-coupling optical element IDO at the selected angle of incidence 30.

[0058] As illustrated in FIG. 8, in an exemplary embodiment, the image light guide 12 is disposed at a “chevron” angle (|) with respect to an imaginary horizontal axis A3. The chevron angle (|) may also be referred to as the rake angle herein. The horizontal axis A3 is orientedorthogonal to both the vertical axis Al and the horizontal axis A2. While FIG. 8 illustrates a monocular system, it should be understood that the present disclosure also contemplates binocular systems. It should be understood that the horizontal angular relationship of the incident imagebearing light WI and the output image-bearing light WO1, WO2 shown in FIG. 8 behaves similarly to the vertical angular relationship of the angle of incidence 30 and the output imagebearing light WO1, WO2 shown in FIGS. 6A-6D. For example, a change in the chevron angle <|) will cause a mirrored angular change to the image-bearing light WO1 while not causing any angular change to the image-bearing light WO2. As such, the chevron angle (|) may be selected to orient the second portion of out-coupled image-bearing light beams WO2 to the side of the wearer (e.g., in the x-axis direction) such that persons standing in front of the wearer do not perceive the second portion of out-coupled image-bearing light beams WO2. In an example embodiment, the output angle a2 and the chevron angle <|) are both selected to mitigate perception of the second portion of out-coupled image-bearing light beams WO2.

[0059] The perspective view shown in FIG. 9 illustrates one example of image light guide system 10 in a display system 200 for augmented reality viewing of virtual images. In an example embodiment, the augmented reality display system 200 includes a frame 210 with a right-eye rim section 220, a left-eye rim section 222, and a nose bridge section 224 disposed between the righteye rim section 220 and the left-eye rim section 222. In an example embodiment, the frame 210 also includes a right temple section 230 connected with the right-eye rim section 220 and a left temple section 232 connected with the left-eye rim section 222. The augmented reality display system 200 may also include a hinge 240 connected with the right temple section 230 and the right-eye rim section 220, wherein the hinge 240 is configured to enable the right temple section 230 to pivot relative to the right-eye rim section 220. Similarly, the augmented reality display system 200 may also include a hinge 242 connected with the left temple section 232 and the lefteye rim section 222, wherein the hinge 242 is configured to enable the left temple section 232 to pivot relative to the left-eye rim section 222.

[0060] The image light guide system 10 uses one or more image light guides (e.g., image light guides 12). Image light guide system 10 is shown within the augmented reality display system 200 with a right-eye rim section 220 having an image light guide 12R proximate the user’s right eye. The image light guide system 10 includes image source 18, such as a pico-projector or similar device, energizable to generate one or more virtual images. The image light guide 12R is fixed within the right-eye rim section 220 such that the angle p (i.e., the pantoscopic tilt) of the image light guide 12R cannot be reconfigured once the augmented reality display system 200 is assembled without disassembly and recalibration of the image light guide system 10.

[0061] Referring now to FIG. 10, in an example embodiment, the augmented reality display system 200 includes a frame 210 which does not include the right-eye rim section 220 or the lefteye rim section 222. For example, the image light guide 12R may be fixedly connected with an optics module 250 housing the image source 18. In embodiments without eye rim sections 220, 222, the image light guide 12R is fixed with the optics module such that the angle p (i.e., the pantoscopic tilt) of the image light guide 12R cannot be reconfigured once the augmented reality display system 200 is assembled without disassembly and recahbration of the image light guide system 10.

[0062] Although not illustrated, in one example, image light guide system 10 includes a left-eye optical system including one or more image light guides and a second image source. In examples using both a right-eye rim section 220 and a left-eye nm section 222, the virtual images that are generated can be a stereoscopic pair of images for 3D viewing. During operation by a user or viewer, the virtual image or images formed by the image light guide system 10 can appear to be superimposed or overlaid onto the real-world scene content seen by the viewer through the right eye image light guide 12R and / or left eye image light guide. Additional components familiar to those skilled in the augmented reality visualization arts, such as one or more cameras mounted on the frame of the HMD for viewing scene content or viewer gaze tracking, can also be provided.

[0063] One or more features of the embodiments described herein may be combined to create additional embodiments which are not depicted. While various embodiments have been described in detail above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant arts that the disclosed subject matter may be embodied in other specific forms, variations, and modifications without departing from the scope, spirit, or essential characteristics thereof. The embodiments described above are therefore to be considered in all respects as illustrative, and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.

Claims

CLAIMSWhat is claimed is:

1. An image light guide system, comprising: an image source configured to emit image-bearing light beams; and an image light guide comprising: an in-coupling diffractive optic operable to diffract at least a portion of the imagebearing light beams into the image light guide; and an out-coupling diffractive optic operable to diffract a first sub-portion of the image-bearing light beams in a first direction toward an eyebox, and operable to diffract a second sub-portion of the image-bearing light beams in a second direction away from the eyebox; wherein the second sub-portion of the image-bearing light beams is oriented at a forward output angle relative to a horizontal axis, and wherein the magnitude of the forward output angle is at least twenty degrees (20°) relative to the horizontal axis.

2. The image light guide system of claim 1, wherein the image light guide comprises a first planar surface arranged at atilt angle between zero degrees and fifteen degrees (0° to 15°) relative to a vertical axis oriented orthogonal to the horizontal axis.

3. The image light guide system of claim 2, wherein the image-bearing light beams emitted by the image source are arranged at an angle of incidence relative to the first planar surface of the image light guide, wherein the angle of incidence is between zero degrees and fifteen degrees (0° to 15°) relative to the first planar surface of the image light guide.

4. The image light guide system of claim 3, wherein a magnitude of the forward output angle is equal to a magnitude of the tilt angle plus the magnitude of the angle of incidence.

5. The image light guide system of claim 3, wherein the image light guide is arranged at a chevron angle with respect to a second horizontal axis.

6. The image light guide system of claim 2, wherein a magnitude of the forward output angle is at least twice the magnitude of the tilt angle.

7. The image light guide system of claim 1, wherein the out-coupling diffractive optic comprises a set of diffractive features, wherein the set of diffractive features are selected from at least one of: surface relief gratings, volume holograms, and holographic optical elements.

8. The image light guide system of claim 7, wherein the out-coupling diffractive optic comprises a set of diffractive features, wherein the set of diffractive features are surface relief gratings, and wherein the surface relief gratings are selected from at least one of: blazed diffractive features, slanted diffractive features, and sawtooth diffractive features.

9. The image light guide system of claim 8, wherein the set of diffractive features are blazed diffractive features, such that the ratio of an intensity of the first sub-portion of image-bearing light to the intensity of the second sub-portion of the image-bearing light is at least 8: 1.

10. The image light guide system of claim 1, wherein the image source has a vertical field-of- view (FOV), and wherein the output angle is equal to or greater than the vertical FOV.

11. The image light guide system of claim 1, wherein the first sub-portion of image-bearing light is arranged at rearward output angle, wherein the rearward output angle is within five degrees (5°) of the horizontal axis.

12. The image light guide system according to any one of claims 1-11, further comprising a frame, wherein the image light guide is fixed relative to the frame.

13. The image light guide system or claim 12, wherein the frame comprises an optics module, and wherein the image light guide is secured to the optics module.

14. The image light guide system according to any one of claims 1-11, further comprising a frame having a right-eye rim section and a left-eye rim section, wherein the image light guide is secured at least partially within the right-eye rim section or the left-eye rime section.

15. The image light guide system of claim 1, wherein the first sub-portion and second subportion of image-bearing light are first order diffracted light.

16. A method of aligning a virtual image, comprising: aligning an image source at an initial angle relative to a first planar surface of an image light guide; angling the first planar surface of the image light guide at a tilt angle relative to a vertical axis; diffracting a portion of in-coupling image-bearing light into the image light guide via a first in-coupling optical element; diffracting a first sub-portion of the image-bearing light out of the image light guide in a first direction towards an eyebox; and diffracting a second sub-portion of the image-bearing light out of the image light guide in a second direction away from the eyebox, wherein the second sub-portion isdiffracted out of the image light guide at a forward output angle, and wherein the forward output angle is at least twenty degrees (20°) relative to the horizontal axis.

17. The method of claim 16, wherein the step of aligning the image source relative to the incoupling optical element is accomplished by a physical rotation of the image source, by a digital movement of a virtual image within the image source, or by an optical coupler.

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