Patent Publication Number: US-11048089-B2

Title: Augmented/virtual reality near-eye displays with edge imaging lens comprising a plurality of display devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 15/477,712 filed Apr. 3, 2017, which claims the benefit of U.S. Provisional Patent Application No. 62/318,468, filed Apr. 5, 2016, the entireties of which are incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to wearable, near-eye displays with particular emphasis on see-through near-eye displays having high optical performance that are easy to fabricate, aesthetically pleasing, compact and lightweight enough to wear comfortably. 
     BACKGROUND 
     Progress in miniaturization and in the capability of electronic image display devices is a key enabling technology for compact and high performance near-eye displays (NEDs) which have been extensively researched in recent years in view of increasingly popular augmented reality (AR) applications. 
     A challenge in the design of NEDs for AR applications has been to make such a system truly “wearable”. For this purpose, the NED system must present high quality virtual images, be comfortable to wear, and not look awkward in appearance. Further, the system must not impair the user&#39;s perception of real-world views and be sufficiently rugged for daily handling. All these requirements must be met under the constraints of low cost in mass production for wide adoption. 
     There are a number of prior art NEDs which attempt to meet the above requirements with varying degrees of success. See, for example, U.S. Pat. Nos. 5,696,521, 5,886,822 and 8,508,851B2 whose exemplary NEDs are shown in  FIGS. 1-3  respectively. These prior art NEDs take advantage of a refractive and/or reflective system with rotationally-symmetrical optical surfaces which are simple to fabricate and have average image performance but are undesirably bulky and/or have an awkward appearance. To better fit near-eye displays into a smaller volume on a user&#39;s face, some prior art NEDs employ an off-axis optical architecture which is symmetrical but only about the horizontal plane. See, for example, U.S. Pat. Nos. 6,353,503B1, 6,710,902B2, 6,147,807 and 9,239,453B2 whose exemplary NEDs are shown in  FIGS. 4-5 and 7-8 . In these prior art NEDs, a tilted off-axis segment of a partially-reflective concave surface in front of a users&#39; eye is used to image light from the display device and combine it with light from the real-world view. The “combiner” surface may be of a complex geometry such as toroidal or free-formed. It may also be a flat surface with micro-features. To balance optical aberrations that are introduced by such combiner elements, additional or other tilted/decentered and/or free-form optical surfaces are employed elsewhere in the system. 
     The resulting near-eye off-axis near-eye display system is undesirably difficult to fabricate and looks very different from ordinary eyeglasses and is often too bulky, thus not aesthetically pleasing from a consumer standpoint. For example, U.S. Pat. No. 9,134,535B2 discloses an exemplary NED as illustrated in  FIG. 6  in which the virtual image is combined with the real-world view through an ophthalmic lens-shaped combiner. The combiner has many minute facet features on the curved front and back surfaces. Since these minute facets are made parallel to each over the curved surfaces in the transversal zone, no additional aberration is introduced as imaging light is “Total Internal Reflection”- or “TIR”-bounced over these facets before being ejected toward the eye. As a result, other imaging surfaces anterior to the combiner can have rotational symmetry and fewer optical aberrations. Unfortunately, these imaging surfaces are large in diameter due to the required increased optical path to the eye pupil as the light must be guided through multiple (TIR) bounces within the combiner. This in turn detracts from the appearance experience of the NED. Further, the discontinuity between the minute facets can cause artifacts in the information light and the real-world view. Also, the parallelism between the minute facets must typically be held to very tight tolerances due to the necessary multiple TIR bounces off of the facets. 
     Another challenge in the design of prior art near-eye display systems has been the need to increase the system&#39;s field of view (FOV) so more information can be presented to the user for an improved user experience. 
     Optically, the FOV of a near-eye display can be increased merely by increasing the image display device size while keeping the system effective focal length (EFL) and numerical aperture (NA) unchanged. The product of NA and EFL defines the radius of the NED exit pupil within which an unobstructed virtual image can be viewed. This approach leads to a more complex optical system due to the associated size increase of optical surfaces/components and the need for aberration control for an increased FOV. In spite of the trade-off between system complexity and virtual image resolution at the corner fields, the resulting near-eye display will still grow in size due to the larger image display device and larger optical surfaces/components that must be employed to avoid vignetting. This in turn makes the near-eye display less wearable due to its excessive bulkiness. A larger display device is also undesirably less power efficient and is more costly. 
     To increase the FOV of a near-eye display, another approach has been to divide the FOV into multiple zones and to cover each zone with a channel which uses a smaller image display device. This angular tiling method has the advantage of each channel being relatively compact and the growth of system volume is proportional to the number of channels and the extent of the FOV. Also, the use of a number of smaller display devices is more cost-effective than the use of a single large device. See, for example, U.S. Pat. No. 6,529,331B2, and U.S. Pat. No. 9,244,277B2. 
       FIG. 9  shows a prior art NED incorporating multiple channels used to expand the FOV as disclosed in U.S. Pat. No. 6,529,331B2. Each channel comprises a display device and covers a limited FOV which is stacked or tiled to cover a larger FOV. The exit pupil planes of the various channels are overlapped at a single point which is coincident with the eye front point. The eye clearance (“ec”) distance from the exit pupil plane to the first optical surface of a channel is defined as the eye clearance of a channel while the ec&#39; distance from the eye front plane to the nearest optical surface among all channels is defined as the eye clearance of the NED.  FIG. 9  shows that ec must be greater than ec&#39; due to the arc arrangement of all channels around the eye front point. The associated increased ec requires that each channel grow in size and thus the entire system becomes bulkier. To fit multiple channels together, complex boundary conditions must be addressed and met which in turn increases NED fabrication challenges. To avoid a visual perception of gaps in the tiled-up image, the eye front must be finely aligned with the common point of the exit pupil planes of all channels. This requirement often makes the near-eye display difficult to wear. Further, the location of the display devices is in front along the vision line and is often not easy to use in connection with an AR application. 
       FIG. 10  shows a prior art NED with multiple channels to expand a FOV as disclosed in U.S. Pat. No. 9,244,277B2. A prism with a free-form optical surface is employed in each channel to deliver light from a display device image to the eye pupil. The exit pupil plane of each channel is made to be coincident with the eye pupil plane. This feature avoids the reduction of ec′ from that of ec for a single channel and makes the whole system generally more compact than that of U.S. Pat. No. 6,529,331 B2. The location of the display device is also moved away from the forward vision line for the convenience of an AR application. To undo the distortion of the real-world view caused by the imaging prism, a matched free-form prism is added anterior. To combine various channels, the imaging prism tip of each channel is trimmed and then butted together with glue or is bonded in a molding operation. Due to the required inclusion of the matched prism, this form of prior art NED typically becomes bulkier. Further, there is typically a seam line where the different channels adjoin. These seam lines often create distortion in the real-world view of the NED. Also, the trimming operation often leads to field-dependent pupil vignetting in each channel with more vignetting for the central FOV than for the peripheral FOV. The vignetted pupils of various channels complement each other at the eye pupil plane due to the symmetrical arrangement of various channels. Such a complement in pupil vignetting typically minimizes the appearance of image gaps in the tiled image as the eye rotates but to work well, the eye must be closely aligned to maintain the symmetrical arrangement of various channels. This again typically leads to a requirement of tight eye alignment with the NED. 
     To increase the FOV of a NED, yet another prior art approach has been to divide the FOV into a number of zones and to cover each zone in a time sequence with light from a single display device. A smaller display device can be used in this approach which in turn typically makes the NED smaller. The imaged light from the display device is switched among different zones using switchable optics components. See, for example, U.S. Pub. No. US2015/0125109A1 and U.S. Pub. No. US2014/0232651A1. 
       FIGS. 11A and 11B  show two switchable light paths in the prior art NED disclosed in U.S. Patent Pub. No. US2015/0125109A1 with the FOV tiled in time sequence based on a multi-layer planar waveguide with a switchable Bragg Diffraction Grating (SBDG). Light from the display device is collimated by optics disposed outside the layered waveguides before being coupled into the waveguide using an input diffraction grating. In the waveguide, the display light is coupled by TIR reflection and routed along various paths using grating pairs formed by non-output grating layers and output grating layers. In this prior art device, either the non-output or the output grating layer in each pair must be a SBDG. Each distinct path corresponds to a FOV tiling channel. The non-output grating layers may be input grating layers or light path folding grating layers. One of the advantages of such a system is the realization of a real-world view through a thin planar waveguide which acts as a flat eyeglass lens. Another advantage of this approach is the expansion of the eye box through multiple bounces off of the folding or output grating layers with a diffraction efficiency less than one (1). With the eye box expansion taken care of by the waveguide and embedded grating layers, the collimation optics anterior to the waveguide can have a relatively small exit pupil aperture and simpler architecture which helps the performance of the NED. The small exit pupil aperture of collimation optics make the collimation optics smaller, but it typically also matches the very small geometries of the waveguide. However, disadvantages of such a system often include: a) low optical efficiency due to the small aperture of the collimation optics anterior to the waveguide and the switching in time sequence; b) the high complexity of the SBDG system, its environmental susceptibility (temperature/pressure/stress) and endurance; c) grating-induced optical aberrations for which correction complicates the collimation optics architecture or lowers the system optical performance; d) restrictions in the achievable FOV imposed by the TIR and grating angular bandwidth; e) a large area on the planar waveguide is needed to accommodate various grating pairs to route the display light and to expand the eye box size; and; f) the near-eye display volume is increased by the required collimation optics attached to the waveguide. 
       FIG. 12  shows yet a further prior art NED with multiple channels used to expand the FOV as disclosed in U.S. Patent Pub. No. US2014/0232651A1. The system is based on time multiplexing with active components such as switchable mirrors or holographic optics. The display device and imaging optics are disposed in a straight line to simplify optics and reduce the aberration level. For the real-world view light path, the near-eye display acts like an optical plate and introduces no distortion. Since the FOV is stacked up with spatially-separated folding components, the optical path length for each channel varies greatly. This leads to the field-dependent vignetting at the eye pupil which undesirably varies from channel-to-channel. The near-eye display as a result has a relatively small eye box. 
     Another significant technical hurdle to the miniaturization of near-eye display systems is the availability of high brightness and compact display devices, as can be inferred from the above discussion. Common conventional display device technologies include Digital Micro-Mirror Display (DMD), Liquid Crystal Display (LCD), Liquid Crystal on Silicon (LCOS) and Organic Light Emitting Diode(OLED). Systems such as DMD and LCOS require an accompanying illumination optical system which adds system volume. LCD technology has associated low brightness and lower resolution. OLED technology is more compact than DMD and LCOS and has better brightness and resolution than LCD. OLED is also a promising display device format for near-eye displays, but OLED still needs to further improve its brightness and durability for wide adoption in NED applications. 
     A new class of emissive micro-scale pixel array imager devices has been introduced as disclosed in U.S. Pat. Nos. 7,623,560, 7,767,479, 7,829,902, 8,049,231, 8,243,770, 8,567,960, and 8,098,265, the contents of each of which is fully incorporated herein by reference. The disclosed light emitting structures and devices referred to herein may be based on the Quantum Photonic Imager or “QPI®” imager. QPI® is a registered trademark of Ostendo Technologies, Inc. These disclosed devices desirably feature high brightness, very fast multi-color light intensity and spatial modulation capabilities, all in a very small single device size that includes all necessary image processing drive circuitry. The solid-state light-(SSL) emitting pixels of the disclosed devices may be either a light emitting diode (LED) or laser diode (LD), or both, whose on-off state is controlled by drive circuitry contained within a CMOS chip (or device) upon which the emissive micro-scale pixel array of the imager is bonded and electronically coupled. The size of the pixels comprising the disclosed emissive arrays of such imager devices is typically in the range of approximately 5-20 microns with a typical emissive surface area being in the range of approximately 15-150 square millimeters. The pixels within the above emissive micro-scale pixel array devices are individually addressable spatially, chromatically and temporally, typically through the drive circuitry of its CMOS chip. The brightness of the light generated by such imager devices can reach multiple 100,000 cd/m2 at reasonably low power consumption. 
     The QPI imager referred to in the exemplary embodiments described below is well-suited for use in the wearable devices described herein. See U.S. Pat. Nos. 7,623,560, 7,767,479, 7,829,902, 8,049,231, 8,243,770, 8,567,960, and 8,098,265. However, it is to be understood that the QPI imagers are merely examples of the types of devices that may be used in the present disclosure. Thus, in the description to follow, references to the QPI imager, display, display device or imager are to be understood to be for purposes of specificity in the embodiments disclosed, and not for any limitation of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments herein are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and they mean at least one. 
       In the drawings: 
         FIG. 1  is a prior art see-through near-eye display system. 
         FIG. 2  is a further prior art see-through near-eye display system. 
         FIG. 3  is a yet further prior art see-through near-eye display system. 
         FIG. 4  is a yet further prior art see-through near-eye display system. 
         FIG. 5  is a yet further prior art see-through near-eye display system. 
         FIG. 6  is a yet further prior art see-through near-eye display system. 
         FIG. 7  is a yet further prior art see-through near-eye display system. 
         FIG. 8  is a yet further prior art see-through near-eye display system. 
         FIG. 9  is a prior art FOV tiling near-eye display system. 
         FIG. 10  is a yet further prior art FOV tiling see-through near-eye display system. 
         FIG. 11A  shows one path in a prior art time sequence FOV tiling see-through near-eye display system. 
         FIG. 11B  shows a different path in the same near-eye display system as in  FIG. 11A . 
         FIG. 12  is a yet further prior art time sequence FOV tiling see-through near-eye display system. 
         FIG. 13  illustrates an example for explaining a see-through near-eye display system according to an embodiment herein. 
         FIG. 14  illustrates an example for explaining a right eye channel of a see-through near-eye display system according to an embodiment herein. 
         FIG. 15  illustrates an example for explaining a horizontal view of a right eye channel of a see-through near-eye display system according to an embodiment herein. 
         FIG. 16A  illustrates an example for explaining a right eye FOV tiling see-through near-eye display system according to an embodiment herein. 
         FIG. 16B  illustrates a second example for explaining a right eye FOV tiling see-through near-eye display system according to an embodiment herein. 
         FIG. 17  illustrates an example for explaining a horizontal view of a right eye FOV tiling see-through near-eye display system according to an embodiment herein. 
         FIG. 18A  illustrates an example for explaining two overlapped Fresnel mirrors with optical power according to an embodiment herein. 
         FIG. 18B  illustrates an example for explaining spatially-multiplexed Fresnel mirrors with optical power according to an embodiment herein. 
         FIG. 18C  illustrates an example for explaining spatially-multiplexed Fresnel mirrors without optical power according to an embodiment herein. 
         FIG. 19  illustrates an example for explaining angular prismatic facet elements in a Fresnel mirror according to an embodiment herein, to implement a free-form optical surface. 
         FIG. 20A  illustrates an example for explaining a non-telecentric display element incorporated into a near-eye optical system having a flat lens according to an embodiment herein. 
         FIG. 20B  illustrates an example for explaining a non-telecentric display element incorporated into a near-eye optical system having a curved lens according to an embodiment herein. 
         FIG. 21  illustrates an example for explaining a non-telecentric display element incorporated into a near-eye optical system, the non-telecentric display element being comprised of refractive optical elements according to an embodiment herein. 
         FIG. 22  illustrates an example for explaining a non-telecentric display element incorporated into a near-eye optical system, the non-telecentric display element being comprised of tilted refractive optical elements according to an embodiment herein. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure and various of its embodiments are set forth in the following description of the embodiments which are presented as illustrated examples of the disclosure in the subsequent claims. It is expressly noted that the disclosure as defined by such claims may be broader than the illustrated embodiments described below. The word “exemplary” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other aspects or designs. 
     In one example embodiment, a near-eye display device is disclosed comprising a lens prism assembly comprising a viewer-facing surface, a scene-facing surface, a vertical dimension, a horizontal dimension and a lens thickness. The lens prism assembly may comprise an upper portion and a lower portion bonded together along a horizontal interface comprising a beam-splitting element. Two or more display devices may be disposed on a first edge surface and each configured to couple an optical image from the respective display devices through the lens thickness. 
     In one embodiment, a reflective element is disposed on a second edge surface that generally opposes the first surface. The reflective element, the beam-splitting element or both the reflective element and the beam-splitting element may comprise a region comprising a plurality of overlapping, prismatic facet elements that are configured to reflect or transmit the separate optical images (from the display devices) that overlap on the reflective element (i.e. areas where the two separate display device images overlap on the reflective element) in a predetermined interlaced optical pattern and back toward the beam-splitting element. The beam-splitting element may be further configured to couple the reflected optical images through the viewer-facing surface to an eye pupil of a user. 
     By virtue of the example embodiments herein, a compact NED is enabled that resembles the appearance of a pair of ordinary spectacle glasses (i.e., eyeglasses in the format of a typical consumer eyeglass frame), has good quality in both the displayed image and the “see-through” real-world view, is easy to fabricate in mass production and is comfortable to wear by accommodating large inter-pupil distance (IPD) variation among people. 
     The embodiments herein may be provided to take advantage of advances made in display device technologies, particularly self-emissive display devices with micro-pixels such as the Quantum Photonic Imager or “QPI®” imager. QPI® is a registered trademark of Ostendo Technologies, Inc. See U.S. Pat. Nos. 7,623,560, 7,767,479, 7,829,902, 8,049,231, 8,243,770, 8,567,960, and 8,098,265. Display devices such as the QPI imager offer high brightness and high resolution images in a very compact format and are particularly well-suited for the compact NED of present disclosure. 
     The embodiments herein may also take advantage of the ergonomic fact that, although humans&#39; IPD varies greatly among the general population and users tend to scan their eyes in a large latitude to their left or to their right, a human&#39;s vertical eye scan movement is much narrower and less frequent. Humans generally tend to scan their vertical field of view by tilting their head back or forward. The embodiments herein exploit this behavior so that the disclosed NED does not require a circular eye box but rather provides an elongated eye box or exit pupil having a horizontal dimension that is larger than its vertical dimension. The eye box can be a 2-D or 3-D viewing region within which the viewer&#39;s eye can move and still see the entire image. The NED system may be vertically aligned with a user&#39;s eyes by adjusting the system up or down along the wearer&#39;s nose bridge like an ordinary pair of eyeglasses are worn and adjusted. Since much of the bulkiness of prior art NEDs comes from the requirement of having a large circular eye box, the reduction of eye box size in the vertical direction beneficially reduces system bulkiness in the corresponding horizontal (eye sight) direction which is the dominant eye-scan direction. 
     A conventional eyeglass or spectacle lens generally has its largest dimension along its width (horizontal) and the smallest dimension through its thickness with the height (vertical) dimension somewhere between the width and thickness dimension. The shape of the front (scene-facing) and the back (viewer-facing) of a spectacle lens is typically decided by the ophthalmic function or fashion. Thus, the lens edge surfaces may be modified to implement the function of coupling and reflecting an electronic image that is displayed from a display device to the eye pupil of a viewer. By disposing one or more display devices on the upper (“first”) or the lower (“second”) edge surface of a lens and coupling the image outputs from the display device(s) using a reflective optic assembly at the opposing edge surface, the larger lens horizontal dimension can correspond to the larger horizontal eye box dimension and the smaller lens vertical dimension can correspond to the smaller vertical eye box dimension. 
     By virtue of embodiments described herein, it is possible to simplify the light path from the display device to eye pupil and minimize any break of symmetry to ensure maximum optical performance. A lens suitable for an embodiment of the NED of the present disclosure may comprise a beam-splitting bonding interface embedded along the horizontal dimension of the lens, about mid-way along the lens&#39; height dimension which divides the lens into two parts: an upper portion and a lower portion. The embedded interface can be provided with a partially-reflective coating whereby the interface functions as an optical beam-splitting element. In one embodiment, at least one, but preferably two or more, imaging source display devices such as QPI imagers are disposed on a first edge surface (“upper” in the described embodiment) facing the embedded beam-splitting interface and a novel reflective element surface, with or without optical power, is disposed on a second edge (“lower” in the described embodiment) facing the interface. It is expressly noted the above described embodiment is not limited to having the imager disposed on the first surface and the reflective element disposed on the second surface and that the terms “first” and “second” surfaces are used for convenience only and are considered interchangeable positions. For instance, disposing the imager on the second surface and the reflective element on the first surface are contemplated as being within the scope of the embodiments disclosed herein. 
     In one embodiment, a displayed image received from each of the display devices is coupled into the thickness of the lens body by optical transmission from the first edge. The information light transmits through the embedded partially-reflective interface and transverses the lens body along a straight path to the second edge upon which reflective element optics are disposed. Upon reflection from the reflective element disposed on the second edge, the information light travels back to the embedded partially-reflective interface. Upon reflection at the partially-reflective interface, information light transmits through the lens&#39; viewer-facing surface and enters the eye pupil where it is perceived as a virtual image superimposed on the real-world view along the vision line. 
     In this optical path layout, much of the imaging work is performed by the edge reflective element optics which may be generally centered relative to the opposing display devices. The folding reflection at the embedded plane interface introduces little to no aberration. Although the spectacle lens viewer-facing surface is generally curved and tilted relative to the vision line for fashion or vision correction reasons, the aberration introduced at the viewer-facing surface is manageable due to the low divergence of information light at the surface and can be corrected at the reflective element or by clocking the display device and reflective element around the vision line as a group. Although the layout of the optical path of the information light of the present disclosure may be provided as a 3D path in nature, the break of symmetry is gentle with good optical performance attainable. 
     Another advantageous aspect of the present disclosure is the increase in horizontal field of view through the use of two QPI imagers or suitable display devices that are disposed on the first edge of the lens. In one embodiment, the total FOV may be divided into two tiled-up zones: a first zone and second zone with each zone supported by a single QPI imager. To ensure the complete overlap of light paths from these two zones over the eye box of NED, micro-prismatic facet features similar to those found in a Fresnel lens can be employed on the reflective element surface at the second edge of a lens and on the partially-reflective interface embedded in the thickness of the lens. These micro-prismatic facet features are categorized into two types with each type working with a respective display device. These two types of micro-prismatic facet features are interspersed at a period comparable to the micro-prismatic facet feature size. A typical size of the micro-prismatic facet feature may be about 20 to 150 um at which scale the light is reflected/refracted rather than diffracted. Thus, the wavelength dependence can be much less than that of diffracted elements. As a result, the eye box of the NED is composed of interspersed zones for different display devices at a pitch of 20 to 150 um. Since this pitch is much smaller than the typical 4 mm eye pupil of a user, a user&#39;s eyes can move over the entire eye box without image gaps being observed in the FOV. An advantageous effect of this embodiment is the compactness of the near-eye device and large effective eye box for the total FOV. 
     In one embodiment, a Fresnel slope angle is contoured along a closed circle of a Fresnel optical prismatic facet element to implement free-form optical surfaces which are useful in optical aberration correction. 
     The description of the various embodiments of the NED of the disclosure is made with regard to one lens or one eye but it is expressly noted that the description is intended to include two lenses or both eyes which together provides stereoscopic or binocular vision. 
       FIG. 13  shows one embodiment of the present disclosure. In this embodiment, the lens prism assembly  1320 L and  1320 R for the left eye and right eye respectively are supported by a holding assembly  1300  which may be in the form of a conventional eyeglass frame. The holding device  1300  may be designed to resemble a conventional eyeglass frame but other styles are enabled. Display devices  1310 L and  1310 R can be disposed at the first upper edges  1311 R and  1311 L of  1320 L and  1320 R as shown in  FIG. 13 , but may be disposed at the second lower edges  1312 R and  1312 L of  1320 L and  1320 R as well, or any combination of first edge or second edge device positions may be employed. 
     Except for the embedded beam-splitting surfaces  1330 L and  1330 R, the lens prism assemblies  1320 L and  1320 R resemble and function in the transmission of light received from the real-world view as a pair of ophthalmic lenses and may be provided with or without vision correction. Other components like computers, sensors, antenna, control circuit boards and batteries may also be incorporated into the holding frame assembly  1300  or alternatively, the display devices  1310 L and  1310 R can be connected to an external computer through wireless means or cables coming out of the two ends of the temples of the glasses frame. Because of the mirror symmetry relationship between the systems for the left eye and the right eye, only the right eye system is described in the following but use in binocular applications is contemplated as within the scope of the disclosure and the claims. 
       FIG. 14  illustrates the lens prism assembly  1420 R formed by an upper portion  1425 R′ and a lower portion  1425 W′ with  1430 R noted as the beam-splitting element bonding interface of the two portions, according to one embodiment. The upper portion  1425 R′ on whose first edge  1426 R the display device  1410 R is disposed has a partially-reflective coating applied on its interface  1430 R with lower portion  1425 R″ to define a beam-splitting element. The lower portion  1425 R″ is provided with a reflective surface  1440 R disposed on its second edge  1427 R. The large plane feature of interface  1430 R can have a transmittance of about 80% and a reflectance of about 20% but any suitable user-defined transmittance/reflectance ratio may be selected. This can be achieved for instance by use of a metal coating or dielectric thin film coating as is known in the optical art. The scene-facing and viewer-facing surfaces of upper portion  1425 R′ and the lower portion  1425 R″ can be aligned with each other and bonded together with an index-matching optical cement. It is desirable that any misalignment between scene-facing and viewer-facing surfaces of the lens prism assembly  1420 R be smaller than 0.1 mm after bonding to minimize or avoid artifacts in the transmission of the real-world view to the user. 
       FIG. 14  illustrates a ray path from D, the center of display device  1410 R, through the lens prism assembly  1420 R, to E, the eye pupil plane  1450 R, according to one embodiment. The line EG  1460 R is the vision line when the right eye is looking straight ahead at infinity or at a plane located greater than two meters away. As the light ray leaves the display device center D, it travels down the lens thickness of upper portion  1425 R′ of the lens prism assembly  1420 R, transmits through the interface  1430 R, to lower portion  1425 R″ and incident at A on the reflective element  1440 R. Upon reflection from  1440 R, the ray travels back to intercept the interface  1430 R at B. The ray is then reflected toward G on the viewer-facing surface of the lens prism assembly  1420 R. After transmission through the viewer-facing surface, the ray enters the viewer&#39;s eye pupil at E along vision line direction  1460 R. The information in the form of an optical image received from display device  1410 R is then perceived as superimposed upon the real-world view along vision line  1460 R. 
       FIG. 14  also shows the elliptical beam footprint F at the eye pupil plane for light transmitted from a point on the display device. The horizontal dimension of the beam foot print F is several times larger than its vertical dimension. This is the result of the horizontal dimension of the lens prism assembly  1420 R being larger than its dimension along the vision line  1460 R direction. However, this may not reduce the performance of the NED of the embodiments of the disclosure. Although a relatively large horizontal beam footprint can accommodate the variation of IPD among the user population and the left and right sweeping of the eye pupil of a user, the smaller vertical beam footprint is compensated by adjusting the position of the eyeglass frame on user&#39;s nose ridge and its effect further minimized by the fact the general population of humans tend to tilt their head back and forth to view in the vertical direction instead of moving their eyes in vertical dimension. 
     Earlier attempts have been made at making the vertical dimension of the beam footprint as large as the horizontal dimension. The resulting systems can be bulky and/or complicated. On the other hand, the optical system of the present disclosure beneficially has most of its optical power contributed by the reflective element  1440 R which may be centered relative to the display device  1410 R. In one embodiment, reflective element  1440 R can be provided as a weak toroidal surface with small difference in curvature radius along two orthogonal directions. This departure from the rotational symmetry of reflective surface  1440 R accounts for the fact that the viewer-facing surface of the lens prism assembly  1420 R may be provided to have a different curvature radius along two orthogonal directions. Although the viewer-facing surface of lens prism assembly  1420 R may be generally of toroidal shape and tilted with respect to the vision line  1460 R, the information rays as focused by  1440 R have low divergence at the viewer-facing surface and form moderate angles with respect to the vision line  1460 R. In addition, the curvature radius on the scene-facing surface and viewer-facing surface of the lens prism assembly  1420 R is not strong for practical reasons and all of these features combine to provide an acceptable aberration level and good optical performance. 
       FIG. 15  shows the horizontal view of the ray path from D center of a display device  1510 R to the center E of eye pupil  1550 R for the lens prism assembly  1520 R, according to one embodiment. In this embodiment, the line GE and its extension defines the vision line  1560 R. For fashion, compactness, or the need of vision correction, the scene-facing surface and viewer-facing surface of lens prism system  1520 R is generally curved and tilted relative to vision line  1560 R as shown in  FIG. 15 . As a result, ray path DABGE does not lie within a single plane but lays in a 3D space. The NED imaging system of the disclosure is thus generally tilted without any symmetry plane. However, the imaging system can still be considered a quasi-axial one relative to the center ray as discussed above. In contrast to off-axis systems as disclosed in prior art patent literature, the quasi-axial system of the present disclosure has the advantage of easy fabrication and good optical performance. Also, in contrast to prior art on-axis systems as disclosed in prior art patent literature, the quasi-axial system of the present disclosure has the advantage of compactness and aesthetics. 
       FIGS. 16A and 16B  show example optics for the right eye of an embodiment of the present disclosure in which two display devices are disposed side-by-side to extend the horizontal FOV from that of a single display device NED. In  FIGS. 16A and 16B , the right eye optics assembly  1620 R are formed by an upper portion  1615 R′ and a lower portion  1615 R″ with  1630 R identified as the bonding interface of the two respective portions that defines a beam-splitting element. The upper portion  1615 R′ on whose first upper edge  1616 R the display devices  1610   t R and  1610   b R are disposed can be provided with a partially-reflective coating applied on its interface  1630 R to lower portion  1615 R″. The partially-reflective coating on  1630 R of the upper portion  1615 R′ can have a transmittance of about 80% and a reflectance of about 20% but other ratios may be implemented. This can be achieved for instance by use of a metal coating or dielectric thin film coating as is known in the optical arts. The scene-facing surface and viewer-facing-surface of upper portion  1615 R′ and the lower portion  1615 W′ can be aligned with each other and bonded using an index-matching optical cement applied on the bonding interface  1630 R. It is desirable for the misalignment between scene-facing surface and viewer-facing-surface of the lens prism assembly to be less than about 0.1 mm after bonding to minimize or avoid artifacts in the transmission of the real-world view to the user. 
       FIG. 16A  also shows chief ray paths from the center and four corners of display devices  1610   t R and  1610   b R, through the lens prism assembly  1620 R, all the way to the eye pupil plane, according to one embodiment. As the rays leave the display devices, they travel through the thickness of lens prism assembly  1620 R and straight down through upper portion  1615 R′ of the lens prism assembly  1620 R, partially transmit through the interface  1630 R to enter the lower portion  1615 R″ and are incident upon the reflective element  1640 R. Minute prismatic features are provided on  1640 R similar to those of a Fresnel mirror on a flat substrate. These prismatic features may be configured to impart optical power to the otherwise flat substrate surface and perform a role in imaging the output of the display devices to the eye. Upon reflection from  1640 R, the rays travel back to intercept interface  1630 R. They are then partially-reflected by  1630 R and exit the lens prism assembly  1620 R toward the eye through the viewer-facing surface. The optical information coupled from display devices  1610   t R and  1610   b R is perceived as superimposed upon the real-world view. There may also be prismatic features disposed on the  1630 R interface configured so that rays from either  1610   t R or  1610   b R are directed toward the eye pupil by an otherwise plane-like feature. As shown in  FIGS. 16A and 16B , there may be an overlap  1650  of light paths from display devices  1610   t R and  1610   b R which overlap is accommodated by the prismatic features. 
       FIG. 17  shows the horizontal view of chief ray paths from the center and corner points on display devices  1710   b R and  1710   t R to the center of eye pupil for right eye lens prism assembly  1720 R, according to one embodiment. The FOV contributed by each display device combines to form a large FOV with a minimal overlap. Due to the employment of micro-prismatic features on the embedded beam-splitter element interface  1630 R and reflective element  1640 R of  FIG. 16A , the interlacing of light from either display device over the eye pupil plane is on a micro-scale and image gaps or other directional artifacts are not observed during eye movement. For fashion, compactness, or the need of vision correction, the scene-facing surface and viewer-facing-surface of lens prism assembly  1720 R is generally curved, tilted and without a symmetry plane. However, the break of symmetry is gentle due to the major imaging path segment from each display device to its corresponding Fresnel prismatic features on reflective element  1640 R having rotational symmetry around the normal axis at the center of the respective display device. In contrast to off-axis systems as disclosed in prior art, the quasi-axial system of the present disclosure has the advantage of easy fabrication and good optical performance. Also, the optical interlacing of micro-prismatic features for different display devices permits a large overlap of light paths from different display devices, resulting in a reduction of system volume. 
       FIG. 18A  shows an example use of overlapped Fresnel mirrors  1850   b R and  1850   t R on the lower second edge  1840 R of the exemplar right eye NED optics, according to one embodiment. Each Fresnel mirror can be provided with concentric circular micro-prismatic features whose centers are approximately aligned with those of the respective display devices. Such alignment can ensure higher optical performance. 
       FIG. 18B  shows the interlacing of prismatic features from  1850   b R and  1850   t R by eliminating alternating selected concentric prismatic features of  1850   t R, according to one embodiment. Selected concentric prismatic features for  1850   t R may not be provided while all concentric features for  1850   b R are. In this embodiment, the display device for  1850   b R is perceived as less bright than the other device but the loss of brightness is compensated by the display device like the QPI imager. The advantage in system volume reduction resulting from this overlap approach is substantial. It is noted that, similarly, the alternative features on  1850   b R may be eliminated with all features on  1850   t R being on. 
       FIG. 18C  shows the micro-prismatic features on the embedded beam-splitting element interface  1830 R of the example right eye NED optics, according to one embodiment. Since there is no optical power on the interface  1830 R, the prismatic features may be generally linear in contour. The prismatic features are alternating for either of the display devices as designated by b or t as shown in  FIG. 18C . 
     For a given symmetrical optical surface, its equivalent Fresnel surface has concentric circular features with a constant slope angle. Optical power is imparted by providing a predetermined variation of slope angle across the various circular features. In a tilted optical system of the disclosure, a free-form optical surface may be used for aberration correction. A simple method to implement a free-form surface in the Fresnel type lens disclosed herein is to modulate the slope angle periodically at predetermined periods along the azimuth direction on a circle. An embodiment comprising the above approach is depicted in  FIG. 19 . Each curve represents a concentric circle of a Fresnel prism element at a certain radius. In this example, the slope angle and the contouring magnitude increase as the circle radius increases. This type of free-form Fresnel optics assists in balancing aberrations that may be introduced by slight symmetry break in NED for compactness and aesthetics. 
     Turning to  FIGS. 20A and 20B , embodiments involving reduction of “ghost” images are now discussed. By way of background, undesirable “ghost” images are typically generated in optical systems due to reflections off of the lens element that are visible in the eye-box of the system. Such reflections and the ghost images they cause are avoided in the flat and curved optical lens embodiments illustrated in  FIGS. 20A and 20B  by using “non-telecentric” emission display elements  2000 A and  2000 B. Examples of non-telecentric display elements suitable for use in these embodiments are disclosed in U.S. patent application Ser. No. 15/391,583, filed Dec. 27, 2016, entitled “Non-Telecentric Emissive Micro-Pixel Array Light Modulators and Methods of Fabrication Thereof”, the entirety of which is incorporated herein by reference. 
     In the non-telecentric embodiments disclosed herein, the respective pixels of each of display elements  2000 A and  2000 B are provided with an associated array of pixel-level micro optical elements (pixel-level micro lens array or “PMLA”) that is configured to collimate light emitted from each pixel to match a reflector optical aperture. Each pixel element is configured to direct the light emitted from the pixel toward the reflector optical aperture by means of the associated PMLA micro optical element. The individual light emission from each pixel is thus directionally modulated by the PMLA in a unique direction to enable a predetermined non-telecentric pixel light emission pattern from the pixel array of non-telecentric display element  2000 . 
     In one aspect, the PMLA layer of pixel-level micro optical elements is disposed above the pixels and is used to directionally modulate light coupled onto the PMLA micro optical elements from the corresponding pixels in a predetermined respective direction relative to an axis that is perpendicular to the surface of non-telecentric display element  2000 A or  2000 B. 
     Non-telecentric display element  2000 A or  2000 B may be incorporated into either a flat lens element design (as shown in  FIG. 20A ) or a curved lens element design (as shown in  FIG. 20B ) of a near-eye optical system. The flat lens element design may include a flat lens prism assembly  2020 A, embedded beam-splitting surface  2030 A, and reflective surface  2040 A. The curved lens element design may include a curved prism assembly  2020 B, embedded beam-splitting surface  2030 B, and reflective surface  2040 B. In these embodiments, the light emitted from each pixel is substantially coupled to the reflector&#39;s optical aperture with minimal light ray scattering toward the lens viewing surface and with minimal reflection toward the viewing eye-box that can result in undesired ghost images. It is therefore possible to reduce “ghost” images by virtue of the arrangements in  FIGS. 20A and 20B . 
       FIG. 21  illustrates an example non-telecentric display element  2000 C for use with a near-eye display system, in which individual non-telecentric micro optical elements making up PMLA  2000 C- 1  are realized as refractive optical elements (ROE) and are used to direct selected pixel light outputs at angles generally inclined relative to the surface of non-telecentric display element  2000 C. 
     In the embodiment of  FIG. 21 , directional modulation of the pixel-level refractive non-telecentric micro optical elements are realized using de-centered micro lenses formed by successive low and high refractive index layers of dielectric materials  2000 C- 2  and  2000 C- 3 , each having different indexes of refraction.  FIG. 21  is a schematic cross-section of non-telecentric display element  2000 C comprising an array  2000 C- 1  of non-telecentric refractive micro optical elements. In this embodiment, array  2000 C- 1  of pixel-level non-telecentric micro optical elements may be fabricated monolithically at the wafer level as multiple layers of semiconductor dielectric material, such as silicon oxide for low index layer  2000 C- 2  and silicon nitride for high index layer  2000 C- 3 , using known semiconductor lithography, etch and deposition techniques. As illustrated in  FIG. 21 , array  2000 C- 1  of pixel-level micro optical elements are realized using multiple layers; the dielectric materials  2000 C- 2  and  2000 C- 3  having different indexes of refraction are successively (sequentially) deposited to form the refractive surfaces of pixel-level micro optical elements, each of which progressively vary in refractive micro-lens element center position across array  2000 C- 1  as required to obtain the desired non-telecentric characteristics and image projection directions. 
       FIG. 22  illustrates an example of a non-telecentric display element  2000 D in which the array  2000 D- 1  of non-telecentric micro optical elements are realized as tilted refractive optical elements (ROE), again progressively varying across array  2000 D- 1  to obtain the desired non-telecentric characteristics and image projection directions, and may be used to direct selected pixel light outputs at angles generally inclined relative to the surface of non-telecentric display element  2000 D. In this embodiment, and as best illustrated in  FIG. 22 , directional modulation of the pixel-level refractive non-telecentric micro optical elements is realized using “tilted” micro lenses that are formed by successive layers of dielectric materials  2000 D- 2  and  2000 D- 3 , each having different indexes of refraction. 
       FIG. 22  is a side view of non-telecentric display element  2000 D comprising a plurality of tilted refractive micro optical elements. In this embodiment, array  2000 D-1 of pixel-level non-telecentric micro optical elements may be fabricated monolithically at the wafer level as multiple layers of semiconductor dielectric materials, such as silicon oxide for low index layer  2000 D-2 and silicon nitride for high index layer  2000 D-3, using known semiconductor lithography, etch and deposition techniques. As illustrated in  FIG. 22 , array  2000 D-1 may be realized using multiple layers dielectric materials  2000 D-2 and  2000 D-3 with different indexes of refraction successively (sequentially) deposited to form the refractive surfaces of the pixel-level non-telecentric micro optical elements that comprise array  2000 D-1. 
     Non-telecentric display elements  2000 C and  2000 D may each comprise refractive or diffractive micro optical elements which may be fabricated from a UV curable polymer. The diffractive micro optical elements of non-telecentric display elements  2000 C and  2000 D may each comprise blazed gratings or rail gratings. Blazed gratings used in the non-telecentric display element may be configured whereby the directional modulation of the pixel outputs is determined at least in part by a slant angle, a pitch or both a slant angle and a pitch, of the blazed grating elements. 
     Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the disclosure. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the disclosure as defined by any claims in any subsequent application claiming priority to this application. 
     For example, notwithstanding the fact that the elements of such a claim may be set forth in a certain combination, it must be expressly understood that the disclosure includes other combinations of fewer, more or different elements, which are disclosed in above even when not initially claimed in such combinations. 
     The words used in this specification to describe the disclosure and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus, if an element can be understood in the context of this specification as including more than one meaning, then its use in a subsequent claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. 
     The definitions of the words or elements of any claims in any subsequent application claiming priority to this application should be, therefore, defined to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense, it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in such claims below or that a single element may be substituted for two or more elements in such a claim. 
     Although elements may be described above as acting in certain combinations and even subsequently claimed as such, it is to be expressly understood that one or more elements from a claimed combination can in some cases be excised from the combination and that such claimed combination may be directed to a subcombination or variation of a subcombination. 
     Insubstantial changes from any subsequently claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of such claims. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
     Any claims in any subsequent application claiming priority to this application are thus to be understood to include what is specifically illustrated and described above, what is conceptually equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the disclosure.