Projector-combiner display with beam replication

A near-eye display (NED) includes an image replicator and an image combiner. The image replicator is configured for receiving a beam of image light from a source such as an image projector, and splitting the beam into a plurality of second beams of image light. The combiner is configured to relay the plurality of second beams to an eyebox of the NED such that the second beams at the eyebox are laterally offset from one another. The etendue of the NED may be increased by replicating and relaying the image beams.

TECHNICAL FIELD

The present disclosure relates to visual displays and display systems, and in particular to wearable displays.

BACKGROUND

Head mounted displays (HMDs) are used to provide virtual imagery to a user, or to augment real scenery with additional information or virtual objects. The virtual or augmented imagery can be three-dimensional (3D) to enhance the experience and to match virtual objects to the real 3D scenery observed by the user. In some HMD systems, a head and/or eye position and orientation of the user are tracked, and the displayed scenery is dynamically adjusted depending on the user's head orientation and gaze direction, to provide experience of immersion into a simulated or augmented 3D scenery.

One problem of head-mounted displays, and near-eye displays (NEDs) in particular, is a limited etendue of an optical system. The etendue can be defined as a product of an area of the display's eyebox, i.e. the exit pupil of the display, and the display's field of view solid angle. Existing displays, particularly those with compact form factors, having large fields of view tend to have small eyeboxes, and vice versa. Large fields of view is desirable for a greater degree of immersion into the virtual or augmented reality, while large eyeboxes provide the user with freedom and convenience of placing the display in front of the eyes and eye rotation. Larger eyeboxes provide a greater degree of accommodation of different users having individual size and shape of the head, and different distances between the eyes.

Although the size of the eyebox may depend on magnification of the optical imaging system, the etendue is invariant of the magnification. Because of the etendue invariance, providing larger eyebox results in a narrower field of view, and widening the field of view results in a smaller eyebox.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives and equivalents, as will be appreciated by those of skill in the art. All statements herein reciting principles, aspects, and embodiments of this disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.

As used herein, the terms “first”, “second”, and so forth are not intended to imply sequential ordering, but rather are intended to distinguish one element from another, unless explicitly stated. Similarly, sequential ordering of method steps does not imply a sequential order of their execution, unless explicitly stated.

In accordance with the present disclosure, the etendue of a near-eye display may be increased by replicating a beam of image light, and disposing a combiner element to redirect the replicated beams of image light to the eyebox in a grid-like pattern, such that at any position of the user's eye, at least one beam of image light impinges onto the eye's pupil, thereby expanding the eyebox of the display while preserving the field of view.

In accordance with the present disclosure, there is provided a near-eye display (NED) comprising an image replicator and an image combiner. The image replicator may be configured for receiving a first beam of image light and splitting the first beam into a plurality of second beams of image light propagating parallel to each other. The combiner may be configured for receiving the plurality of second beams and relaying the plurality of second beams to an eyebox of the NED. The combiner may be further configured to selectively redirect rays of the second beams depending on angle of incidence of the rays of the second beams on the combiner, such that rays of the second beams split from a corresponding ray of the first beam and redirected by the combiner are parallel to each other and laterally offset in a first direction at the eyebox.

In some embodiments, the first beam is diverging and comprises an image in angular domain, the second beams split by the image replicator are diverging, and the second beams relayed by the combiner to the eyebox are converging, parallel to each other, and comprise the image in angular domain. The combiner may be made angular- and wavelength-selective for relaying the plurality of second beams to the eyebox while transmitting external light to the eyebox substantially without modification. The NED may further include an image projector for providing the first diverging beam comprising the image in angular domain.

In some embodiments, the image replicator may include a first waveguide comprising first and second surfaces. The first surface may be partially reflective at a wavelength of the image light to split off second beams of the plurality of second beams at reflections from the first surface as the first beam propagates in the first waveguide between the first and second surfaces in a zigzag pattern. The first waveguide may include a coating at the first surface, e.g. a metallic coating or a dielectric coating. The second surface may be fully reflective at the wavelength(s) of the image light. The reflectivity of the first surface of the first waveguide may be spatially variant.

The first waveguide may contain a diffractive structure for in-coupling the first beam into the first waveguide, an out-coupling second beams of the plurality of second beams from the waveguide, or both. The diffractive structure may have a spatially varying diffraction efficiency for equating optical power of the second beams. A grating axis of the diffractive structure may be disposed at an acute angle to a plane of incidence of the first beam onto the diffractive structure, such that in operation, the second beams form a two-dimensional (2D) grid of beams at the eyebox.

In some embodiments, the first waveguide includes an internally embedded quarter-wave waveplate, and the first surface of the first waveguide includes a polarization-selective reflector. The polarization-selective reflector may be configured to transmit the first beam having a first polarization. The first waveguide may further include a side surface for receiving the first beam of image light, wherein the side surface is at a non-orthogonal angle to the first and second surfaces. In some embodiments, the first waveguide includes a first optical element comprising the first surface, and a second, distinct optical element comprising the second surface, such that in operation, the first beam propagates in an air gap between the first and second optical elements.

The image light may include a plurality of color channels. The first and second surfaces of the waveguide may be at least partially transmissive at wavelengths of visible light different from wavelengths of the plurality of color channels. The image replicator may include a second waveguide comprising third and fourth surfaces at an angle to the first and second surfaces of the first waveguide, for receiving each second beam from the first waveguide and splitting each second beam into a plurality of third beams of image light. The combiner may be configured for relaying each third beam at the eyebox of the NED such that the third beams at the eyebox are laterally offset in a second direction.

In some embodiments, the image replicator may include a first stack of reflectors in an optical path of the first beam. Each reflector of the first stack of reflectors may be configured for splitting a second beam of the plurality of second beams from the first beam. The image replicator may further include a second stack of reflectors disposed at an angle to the first stack of reflectors for receiving each second beam from the first stack of reflectors and splitting each second beam into a plurality of third beams of image light. The combiner may be configured for relaying each third beam at the eyebox of the NED, such that the third beams at the eyebox are laterally offset in a second direction. The first stack of reflectors may include at least one variable reflector.

An eye tracking system and a controller coupled to the eye tracking system may be provided in an NED of the present disclosure. The eye tracking system may determine at least one of position or orientation of a user's eye at the eyebox. The controller may be operably coupled to the at least one variable reflector and configured to vary reflectivity of the at least one variable reflector depending on the at least one of position or orientation of the user's eye determined by the eye tracking system.

The combiner may include an angularly multiplexed volume hologram comprising a succession of overlapping phase profiles for focusing the second beams at the eyebox. The succession of overlapping phase profiles may have a step corresponding to a lateral offset of the second beams at the eyebox, and each phase profile may be configured to selectively redirect rays of the second beams depending on angle of incidence of the rays of the second beams on the combiner. For example, an ellipsoidal phase profile may be provided. The combiner may also include a metasurface.

For embodiments where the NED includes an image projector for providing the first beam comprising an image in angular domain, the image projector may have an exit pupil smaller than the step; and/or an acceptance angle of a first phase profile of the succession of overlapping phase profiles may be no greater than an exit pupil size of the image projector divided by an optical distance between the volume hologram and the image projector. A holographic projector may be provided for generating the first beam of image light. The holographic projector may be configured to lessen optical aberrations of the combiner. The holographic projector may possess a variable focus.

In accordance with another aspect of the present disclosure, there is further provided a method for displaying an image by an NED. The method may include receiving a first beam of image light and splitting the first beam into a plurality of second beams of image light propagating parallel to each other, and receiving the plurality of second beams and relaying the plurality of second beams at an eyebox of the NED by selectively redirecting rays of the second beams depending on angles of the rays of the second beams, such that rays of the second beams split from a corresponding ray of the first beam are parallel to each other and offset in a first direction. In some embodiments, the first beam is diverging and comprises an image in angular domain, the second beams split by the image replicator are diverging, and the second beams relayed by the combiner at the eyebox are converging, parallel to each other, and comprise the image in angular domain.

Referring now toFIG. 1, a near-eye display (NED)100of the present disclosure includes an image replicator140and a combiner160. The image replicator140may include a waveguide having a translucent reflective surface141and blind, i.e. 100% reflective, surface142, which may be disposed parallel to the translucent reflective surface141. The image replicator140receives a diverging first beam101of image light, which can be provided by a projector108. The diverging first beam101of image light may include an image in angular domain, where different angles of the rays in the diverging first beam101correspond to different coordinates of a pixel in the image to be displayed.

The first beam101propagates in the waveguide of the image replicator140in a zigzag pattern, i.e. upwards inFIG. 1. The image replicator140splits the first beam101into a plurality of second beams102of image light, producing a plurality of virtual projectors108′ emitting virtual second beams102′ of image light carrying a copy of the image in angular domain. The virtual second beams102′ may be parallel to each other as shown. Herein, the term “parallel”, when applied to diverging or converging beams, means each pair of corresponding rays of the beams are parallel.

The second beams102propagate towards the combiner160. The combiner160may include a plurality of recorded holograms configured to receive the plurality of second beams102, to redirect the plurality of second beams102toward an eyebox112of the NED100, and to focus the images of projectors108′. The second beams102at the eyebox112are converging and laterally offset in a first direction113. In other words, the combiner160is configured to selectively redirect rays of the second beams102depending on angle of incidence of the rays of the second beams102on the combiner160, such that rays of the second beams102split from a corresponding ray of the first beam101and redirected by the combiner160are parallel to each other and laterally offset in a first direction at the eyebox112. A user's eye114can be placed anywhere at the eyebox112, while being able to receive at least one of the second beams102of image light to observe the image. Providing multiple second beams102increases the size of the eyebox112in the first direction113. The beams102can be made parallel to one another, such that the image observed by the eye114, does not undergo a shift when the eye114shifts from one second beam102to another. The eyebox112is an area where the observed image can be completely seen and has a required image quality. Different types of image replicators, combiners, and projectors with different degree of divergence of the image light may be used in the NED100. These will be described further below.

Non-limiting examples of the image replicator140will now be considered. Referring toFIG. 2A, a waveguide image replicator200A includes a planar waveguide, e.g. a glass plate240A having first241and second242parallel surfaces. At least a portion of the first surface241can be made partially reflective at wavelength(s) of the image light, e.g. by depositing a corresponding translucent metal or dielectric reflector coating at the first surface241. The first beam101impinges onto the glass plate240A at an input end211of the first surface241. In one embodiment, the input end211is antireflection (AR) coated to lessen optical losses at the first entry. The input end211may be left uncoated or, in yet another embodiment, the translucent metal or dielectric reflector extends to include the input end211. A partially or completely reflective coating may be deposited at the second surface242. The coating may include a metallic and/or dielectric coating. One second beam102of the plurality of second beams102is split off the first surface241at each reflection from the partially reflecting portion of the first surface241, as the first beam101propagates in the glass plate240A between the first241and second242surfaces in a zigzag pattern, i.e. upwards inFIG. 2A. The last second beam102exits the glass plate240A at an exit location212, which may also be AR coated. To equalize the optical powers carried by different second beams102, the reflectivity of the first surface241of the glass plate240A can be made spatially variant. For example, the reflectivity may decrease in going upwards inFIG. 2A, such that at each subsequent reflection from the first surface241, a larger portion of the remaining optical power of the second beam102is reflected, making the second beams102have nearly equal, or at least less different, optical power. In embodiments where the image light comprises a plurality of color channels, e.g. red (R) color channel, green (G) color channel, and blue (B) color channel, the first241and second242surfaces of the glass plate240A can be made at least partially transmissive at wavelengths of visible light other than wavelengths of the plurality of the color channels, to make the waveguide at least partially transparent at the other wavelengths. Narrowband, e.g. laser-line, color channels can reduce a residual coloring when looking through the glass plate240A. This may be convenient in applications where the waveguide is placed in a way of peripheral vision of a user wearing the near-eye display.

A waveguide image replicator200B ofFIG. 2Bis similar to the waveguide image replicator200A ofFIG. 2A. The waveguide image replicator200B ofFIG. 2Bincludes a glass plate240B having a side surface291for receiving the first beam101of image light. The side surface291can be slanted, i.e. disposed at a non-orthogonal angle to the first241and second242surfaces of the glass plate240B. The side surface291may be AR coated, and/or disposed at a Brewster angle to reduce reflection of the first beam101when the first beam101is linearly polarized in a plane of incidence onto the side surface291, i.e. in the plane ofFIG. 2B. The first beam101needs to be launched at such an angle that an incidence angle of the first beam101onto the first surface241is less than a TIR angle, such that the second beams102can exit the glass plate240A.

Referring toFIG. 2C, a waveguide image replicator200C is similar to the waveguide image replicator200A ofFIG. 2A. The waveguide image replicator200C ofFIG. 2Cincludes a first optical element251comprising a first, partially reflective surface254and a second, distinct optical element252comprising a second surface255which may be partially or fully reflective. In operation, the first beam101of image light is coupled to an air gap240C between the first251and second252optical elements. Thus, the air gap240C operates as a waveguide for the first beam101. The air gap240C waveguide reduces the travel distance of the first beam101in glass, which may avoid wavefront distortion of the first beam101due to non-uniformities of refractive index, inclusions, micro-bubbles, etc. in optical materials used.

Referring now toFIG. 2D, a waveguide image replicator200D is similar to the waveguide image replicator200A ofFIG. 2A. The waveguide image replicator200D ofFIG. 2Dincludes a waveguide e.g. a glass plate240D, a polarization-selective reflector261supported by the glass plate240D, and a stack of a full reflector262and a quarter-wave waveplate264, supported by the glass plate240D on the opposite side of the glass plate240D. The polarization-selective reflector261can be configured to transmit light at a first polarization, and partially reflect light at a second polarization orthogonal to the first polarization. The first and second polarizations may include horizontal and vertical linear polarizations, left- and right-circular polarizations, etc. In operation, the first beam101at the first polarization impinges onto the glass plate240D. Then, the first beam101propagates through the quarter-wave waveplate264, impinges on the100reflector262, and propagates back through the quarter-wave waveplate264. Double-pass propagation through the quarter-wave waveplate264is equivalent to propagation through a half-wave waveplate, which changes the polarization of the first beam101from the first polarization to the second polarization. This causes the first beam101to be partially reflected by the polarization-selective reflector261, and the reflections repeat in a zigzag pattern. The quarter-wave waveplate may be disposed on the opposite side of the glass plate240D. InFIG. 2D, only two generated second beams102are shown, for brevity. The polarization configuration ofFIG. 2Dmay also be used in the waveguide image replicator200D ofFIG. 2D, by replacing the partially reflective surface254with the polarization-selective reflector261and by placing a quarter-wave waveplate264inside the air gap240C, e.g. at the second surface255.

Turning toFIG. 2E, a waveguide image replicator200E is similar to the waveguide image replicator200A ofFIG. 2A. The waveguide image replicator200E ofFIG. 2Eincludes a waveguide e.g. a glass plate240E, an out-coupling diffraction grating271on one side of the glass plate240E, and an in-coupling diffraction grating274. The in-coupling grating274is configured to change the ray angles of the beam101so that light propagates through waveguide240E through total internal reflection from a rear surface272. The out-coupling grating271is designed to change the ray angles of the beam101so that light no longer propagates through total internal reflection (TIR) and exits the waveguide. The out-coupling271grating may have low diffraction efficiency to allow formation of multiple beams102. In operation, the first beam101is diffracted by the in-coupling diffraction grating274to propagate in the glass plate240E in a zigzag pattern, with a second beam102of the plurality of second beams102being diffracted out along the zigzag pattern, as shown. The diffraction efficiency of diffraction grating271may be varied spatially to improve the uniformity of the second beams102. The diffraction grating271may include any diffractive, holographic, polarization-based or resonant structures, e.g. surface relief gratings, volume holograms, metasurfaces, Pancharatnam-Berry phase (PBP) elements, or polarization volume holograms. The grating structures may have a spatially varying diffraction efficiency for equating optical power of the second beams102. A two-dimensional (2D) pupil replication may be obtained by orienting an axis of the diffraction grating271at an acute angle to the plane of incidence of the first beam101onto the in-coupling diffraction grating274. At such orientation, the light reflected back to propagate in the glass plate240E and light diffracted back to propagate in the glass plate240E will propagate in non-parallel planes, effectively producing a 2D grid of the multiple beams102.

Referring now toFIG. 2F, a 2D waveguide image replicator200F includes not one but two waveguides. In this example, a first waveguide281is similar to the waveguide image replicator200B ofFIG. 2B. A second waveguide282is disposed at an angle to the first waveguide281, as shown. The second waveguide282has third243and fourth244surfaces at an angle to the first241and second242surfaces of the first waveguide281. In operation, each second beam102is received from the first waveguide281at the third surface243of the second waveguide282(see “View A” inFIG. 2F), and is split into a plurality of third beams103of image light. The third beams103form a 2D array of beams of image light. The combiner, e.g. the combiner160ofFIG. 1, can be configured to relay and refocus each third beam103at the eyebox112of the NED such that the third beams103at the eyebox112are laterally offset in a second direction, i.e. perpendicular to the first direction113(horizontal direction inFIG. 1) and parallel to each other. The third beams103are disposed at the eyebox112in a 2D grid of beams.

Turning toFIG. 2G, a diffraction grating image replicator200G includes not one but two beam-expanding diffraction gratings. A first beam-expanding diffraction grating231spreads the first beam101in a horizontal direction, providing a plurality of the second beams102. A second beam-expanding diffraction grating232further spreads the plurality of second beams102in vertical direction, i.e. orthogonal to the horizontal direction, providing the plurality of third beams103, which form a 2D array of beams of image light.

Referring now toFIG. 3, an NED300is similar to the NED100ofFIG. 1. An image replicator340of the NED300ofFIG. 3includes a stack of reflectors (e.g. five reflectors)341,342,343,344, and345disposed in an optical path of the first beam101. Four first reflectors341,342,343, and344in the image replicator340are configured for splitting a second beam102of the plurality of second beams102from the first beam101, while the fifth reflector345may be a blind mirror reflecting the remaining optical power. The reflectors may be equally spaced apart. Such a configuration can provide the plurality of virtual projectors108′ emitting virtual second beams102′ of image light carrying the image in angular domain. The virtual second beams102′ can be parallel to each other.

The second beams102propagate towards the combiner160, which receives the plurality of second beams102and refocuses the plurality of second beams102at the eyebox112of the NED300. The second beams102at the eyebox112are converging, laterally offset in the first direction113, and parallel to one another. The user's eye114can receive at least one of the second beams102of image light to observe the image. Providing multiple second beams102increases the size of the eyebox112.

The image replicator340is shown in more detail inFIGS. 4A and 4B.FIG. 4Ashows the image replicator340in side view, which illustrates how the second beams102are split off the first beam101.FIG. 4Bis a top view showing corresponding points of reflection351,352,353,354, and355. The reflectivities of the individual reflectors341,342,343,344, and345may be identical or different. For example, the reflectivity of each subsequent reflector341,342,343,344, and345may increase, such that at each subsequent reflection, a larger portion of the remaining optical power of the second beam102is reflected, making the second beams102have nearly equal, or at least less different, optical power. The reflectors341,342,343,344, and345are parallel to each other. In some embodiments, the reflectors341,342,343,344, and345may also be disposed at an angle to one another. It is to be understood that the actual number of the second beams102may be much higher than shown inFIG. 4A, due to multiple reflections within the stack of reflectors341,342,343,344, and345.

Turning toFIG. 4C, a 2D image replicator440includes a first stack481of reflectors, which is basically the image replicator340ofFIGS. 4A and 4B, and further includes a second stack482of reflectors disposed at an angle to the first stack481of reflectors. The second stack482is disposed for receiving the second beams102from the first stack481of reflectors as shown inFIG. 4C, and splitting each second beam102into the plurality of third beams103of image light. The points of splitting are shown as dots450. In this embodiment, the combiner160ofFIG. 1can be configured to relay each third beam103at the eyebox112of the NED such that the third beams103at the eyebox112are laterally offset in a second direction, i.e. perpendicular to the first direction113(horizontal direction inFIG. 1) and parallel to each other. The third beams103are disposed at the eyebox112in a 2D grid of beams.

Referring now toFIG. 5, an NED500is similar to the NED300ofFIG. 3in that it also includes a mirror stack-based image replicator540. The image replicator540includes a stack of first to fourth variable reflectors541,542,543, and544, as well as a fifth mirror545, which can be a fully reflective mirror. Herein, the term “variable reflector” means that the reflector's coefficient of reflectivity can be varied in a controllable manner, e.g. by applying an external control signal. At least one variable reflector may be provided in the reflector stack of the image replicator540. The NED500further includes an eye tracking system580for determining at least one of position or orientation of the user's eye114in the eyebox112.

A controller590is operably coupled to the eye tracking system580and the variable reflectors541,542,543, and544of the image replicator540via respective control lines591,592,593, and594. The controller590can be configured to vary reflectivity of the variable reflectors541,542,543, and544of the image replicator540depending on the at least one of position or orientation of the user's eye114determined by the eye tracking system580. By way of example, when the eye tracking system580determines that the user's eye114is at a leftmost position denoted by “1” at the eyebox112, the controller590sends a control signal via the rightmost (first) control line591to set the reflectivity of the first reflector541to a maximum reflectivity, e.g. close to 100%. This increases a brightness of the perceived image, since otherwise the light propagated to the second542to fifth545reflectors would be wasted. When the eye tracking system580determines that the user's eye114is at a second position denoted by “2” at the eyebox112, as depicted inFIG. 5, the controller590can send a control signal via the second control line592to set the reflectivity of the second reflector542to maximum reflectivity, e.g. close to 100%, while setting the reflectivity of the first reflector541to minimum reflectivity and maximum throughput. When the user's eye114is at a third position “3”, the third mirror543is set to the max reflectivity; and when the user's eye114is at a fourth position “4”, the fourth mirror544is set to the max reflectivity. When the user's eye114is at the last (fifth) position “5”, all variable mirrors541-544can be set to minimum reflectivity and maximum transmission. When a single mirror is at maximum reflectivity, the image formed by the second beams102does not need to be at infinity, since the user's eye114observes a single image replica at a time. When at intermediate location between neighboring positions “1” to “5”, the two reflectors near those positions can be made to reflect more light. The gaze angle of the eye114may also be used to determine the set of optimal reflectivities of the mirrors541-544to maximize the overall brightness and clarity of the perceived image. The control lines591-595may be combined into a common control line or bus.

Turning toFIG. 6, a holographic combiner660may be used as the combiner160in the NEDs100,300, and500ofFIGS. 1, 3, and 5respectively. In this example, the holographic combiner660is a multiplexed volume hologram including a succession of superimposed phase profiles. These phase profiles can be configured for relaying the second beams102toward the eyebox112and focusing the image. For example, the phase profiles can be ellipsoidal, i.e. they can add an optical phase similar to an elliptical reflector, for refocusing a diverging second beam102at one focus to a converging second beam102at the other focus. Two such phase profiles, a first profile610and a second profile620, are illustrated schematically inFIG. 6. The first profile610receives a beam611of image light emitted by a first virtual source108′1, relays beam611as a beam612at the eyebox112, and focuses the beam612. The second profile620receives a beam621of image light emitted by a second virtual source108′2, relays beam621as a beam622at the eyebox112, and focuses the beam622. The first profile610and the beams611and612are shown in solid lines, and the second profile620and the corresponding beams621and622are shown with dashed lines. It is noted that only portions of the overlapping first610and second620phase profiles, reflecting the drawn rays of the beams611and621, are illustrated for brevity. The volume hologram comprising the combiner is configured so that the multiplexed holograms are angular-selective and wavelength-selective to the corresponding virtual sources108′1and108′2. Each phase profile610,620is configured to selectively redirect rays of the corresponding second beams611,621depending on angle of incidence of the rays of the second beams611,621on the combiner. That is, light received by the combiner from source108′1will affected substantially by phase profile610only, and light received by the combiner from source108′2will be affected substantially by phase profile620only; relaying the beams611and621to the eyebox112as the beams612and622, while transmitting external light to the eyebox112substantially without modification. The succession of overlapping phase profiles, i.e. the first610and second620profiles, may be identical profiles but with a translation of step K corresponding to a lateral offset of the second beams612,622at the eyebox112, as illustrated. The step K may be selected to lessen or minimize crosstalk between the holograms represented by the phase profiles610and620. It is to be understood that although only two virtual sources108′1and108′2are shown inFIG. 6, a linear or 2D array of virtual sources including many more virtual sources may be used. Any element with angular-selective properties, e.g. a metasurface, may be used instead of the holographic combiner660.

An example process of writing the multiplexed volume hologram of the holographic combiner660is briefly illustrated inFIG. 7A. A slab of photosensitive material700is illuminated with a pair of beams: an object beam702and a converging reference beam704. The object beam702may be placed at a position in the eye box, and the reference beam704may be placed at a position of a virtual source. The diverging object beam702can be formed e.g. by shining a laser beam through a microscope objective706, and the converging reference beam704can be formed e.g. by focusing a collimated wide laser beam with a lens708. Then, the slab of photosensitive material700is shifted by the step K, and the writing process is repeated, e.g. to cover multiple positions of the virtual sources and corresponding positions in the eyebox. The re-focusing of the diverging first beam101is in fact a playback of the hologram recorded by shining the first beam101emitted by the virtual projector108′ to obtain the second beam102, as illustrated inFIG. 7B.

Since multiple holograms are written into the recording medium700, a care must be taken to avoid, or at least reduce, crosstalk between different phase profiles. When a light source is imaged by an “incorrect” phase profile, a ghost image may be formed. To reduce the crosstalk effect, angular selectivities of the phase profiles need to be carefully controlled. Referring toFIG. 8, a hologram defined by the first profile610of phase should be able to accept and diffract a ray of light emitted from an exit pupil of the first virtual source108′1. The exit pupil of the first virtual source108′1has a linear dimension d and is shown inFIG. 8by a solid line with diamond ends. The corresponding acceptance angle is denoted as a. The acceptance angle α is a vertex angle of an acceptance cone840. Any rays within the acceptance cone840must be accepted, i.e. refocused by the first profile610, while any rays from the neighboring second virtual source108′2within a rejection cone850must be rejected to avoid crosstalk. In other words, the hologram defined by the first profile610should be recorded such that the acceptance cone840does not overlap with the rejection cone850. This condition will ensure that one does not have a spatial position on the holographic combiner660where two multiplexed holograms both have angular selectivity for a common ray direction. From this, one can determine the angular selectivity criterion of a hologram as
tan α≤d/L(1)

where L is a distance between the first virtual source108′1and the phase profile610. The distance L can be approximated by an optical distance between the image source and the volume hologram, i.e. the holographic combiner660. The criterion (1) above should hold for the first direction113for 1D beam replicators, or for each of the two directions for 2D beam replicators. When 1D replicators are used, image sources or projectors with an asymmetrical exit pupil may be used. For example, the exit pupil may remain small in a direction of the image beam replication, while in an orthogonal direction, where the image beams are not replicated, the exit pupil may be enlarged to provide a wide enough coverage in a corresponding dimension on the eyebox.

Referring back toFIGS. 1, 3, and 5, alternative embodiments of the combiner160may include, for example, a patterned metasurface comprising a stack of alternating metal and dielectric and/or metal/semiconductor layers. The projector108may include an image projector having an electronic display and beam collimating optics for converting the image displayed by the electronic display into a projected image in angular domain. Holographic projectors, e.g. those formed by a laser light source and a phase and/or amplitude spatial light modulator (SLM) may also be used. Holographic projectors have an advantage of having a variable focus and/or being able to at least partially compensate for optical aberrations of the combiner160. It may be preferable to create a display with exit pupils less than the step size K of repeating the phase profile in the holographic combiner160.

A method for displaying an image by an NED may include receiving a first beam of image light, e.g. the first beam101inFIGS. 1, 3, and 5. The first beam may be split into a plurality of second beams of image light, e.g. the second beams102inFIGS. 1, 3, and 5. The beam splitting may be performed e.g. by a waveguide image replicator200A,200B,200C,200D,200E, or200F ofFIGS. 2A, 2B, 2C, 2D, 2E, and 2F, respectively. The beam splitting may also be performed e.g. by a mirror stack based image replicator340ofFIGS. 3, 4A and 4B, 440ofFIG. 4C, or540ofFIG. 5. Then, the plurality of second beams may be relayed to an eyebox of the NED such that the second beams at the eyebox are laterally offset in a first direction. The relaying may be done e.g. by using the combiner160ofFIG. 1, the holographic combiner660ofFIG. 6, and/or a combiner including a metasurface. As explained above, the first beam may be diverging and comprise an image in angular domain; the second beams split by the image replicator may be diverging, and the second beams relayed by the combiner at the eyebox may be converging, parallel to each other, and comprise the image in angular domain.

Referring toFIGS. 9A and 9B, a near-eye AR/VR display900includes body or frame902of the near-eye coherent AR/VR display900has a form factor of a pair of eyeglasses, as shown. A display904includes a display assembly906(FIG. 14B) provides image light908to an eyebox910, i.e. a geometrical area where a good-quality image may be presented to a user's eye912. The display assembly906may include any one of the NEDs100,300, and500ofFIGS. 1, 3, and 5respectively. A separate AR/VR display module may be provided for each eye, or one AR/VR display module for both eyes. For the latter case, an optical switching device may be coupled to a single electronic display for directing images to the left and right eyes of the user in a time-sequential manner, one frame for left eye and one frame for right eye. The images are presented fast enough, i.e. with a fast enough frame rate, that the individual eyes do not notice the flicker and perceive smooth, steady images of surrounding virtual or augmented scenery.

An electronic display of the display assembly906may include, for example and without limitation, a liquid crystal display (LCD), an organic light emitting display (OLED), an inorganic light emitting display (ILED), an active-matrix organic light-emitting diode (AMOLED) display, a transparent organic light emitting diode (TOLED) display, a projector, a scanned laser beam display, a liquid crystal on silicon (LCOS) display, a phase spatial light modulator (SLM) or a combination thereof. The near-eye coherent AR/VR display900may also include an eye-tracking system914for determining, in real time, the gaze direction and/or the vergence angle of the user's eyes912. The determined gaze direction and vergence angle may be used for switching variable mirrors in a mirror stack of an image replicator, and may also be used for real-time compensation of visual artifacts dependent on the angle of view and eye position. Furthermore, the determined vergence and gaze angles may be used for interaction with the user, highlighting objects, bringing objects to the foreground, dynamically creating additional objects or pointers, etc. The near-eye coherent AR/VR display900may also include an audio system, such as small speakers or headphones.

Turning now toFIG. 10, an HMD1000is an example of an AR/VR near-eye wearable display system which encloses the user's face, for a greater degree of immersion into the AR/VR environment. The HMD1000can present content to a user as a part of an AR/VR system, which may further include a user position and orientation tracking system, an external camera, a gesture recognition system, control means for providing user input and controls to the system, and a central console for storing software programs and other data for interacting with the user for interacting with the AR/VR environment. The function of the HMD1000is to augment views of a physical, real-world environment with computer-generated imagery, and/or to generate the entirely virtual 3D imagery. The HMD1000may include a front body1002and a band1004. The front body1002is configured for placement in front of eyes of a user in a reliable and comfortable manner, and the band1004may be stretched and/or adjusted to secure the front body1002on the user's head. A display system1080may include the NEDs100,300, and500ofFIGS. 1, 3, and 5respectively. The display system1080may be disposed in the front body1002for presenting AR/VR imagery to the user. Sides1006of the front body1002may be opaque or transparent.

In some embodiments, the front body1002includes locators1008, an inertial measurement unit (IMU)1010for tracking acceleration of the HMD1000, and position sensor(s)1012for tracking position of the HMD1000. The locators1008are traced by an external imaging device of a virtual reality system, such that the virtual reality system can track the location and orientation of the entire HMD1000. Information generated by the IMU and the position sensors1012may be compared with the position and orientation obtained by tracking the locators1008, for improved tracking of position and orientation of the HMD1000. Accurate position and orientation is important for presenting appropriate virtual scenery to the user as the latter moves and turns in 3D space.

The HMD1000may further include an eye tracking system1014, which determines orientation and position of user's eyes in real time. The obtained position and orientation of the eyes allows the HMD1000to determine the gaze direction of the user and to adjust the image generated by the display system1080accordingly. In one embodiment, the vergence, that is, the convergence angle of the user's eyes gaze, is determined. The determined gaze direction and vergence angle may be used for switching variable mirrors in a mirror stack of an image replicator, and may also be used for real-time compensation of visual artifacts dependent on the angle of view and eye position. Furthermore, the determined vergence and gaze angles may be used for interaction with the user, highlighting objects, bringing objects to the foreground, creating additional objects or pointers, etc. An audio system may also be provided including e.g. a set of small speakers built into the front body1002.