This application claims priority to U.S. provisional patent application No. 62/373,336, filed on August 10, 2016, which is hereby incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

This disclosure relates generally to a holographic waveguide for use in Near-to-Eye (NTE) and Head Mounted (HMD) displays capable of projecting high definition, full color images, generated using broad bandwidth wavelength light sources, through an optically transmissive or "see-through" media to a viewer's eye. The holographic waveguide employs volume phase holographic (VPH) diffraction gratings in reflection. The holographic waveguide enables delivery of wavelength and angle of incidence (ΑΟΓ) selective information through a transparent material, allowing the user an unobstructed view of the real world environment with the addition of rich media content superimposed on the field of regard, hence facilitating the user experience often termed "augmented reality". The holographic waveguide is not limited to the use of VPH reflection diffraction structures, where transmission VPH diffraction gratings and surface relief diffraction gratings may also be used in similar embodiments.

BACKGROUND OF THE INVENTION

[0001] The amount of digital information available is ever increasing in today's world. Many display devices exist on the market today to provide a means to communicate information in visual format; including handheld smart phone displays, tablets, theater projection systems, heads up displays, various types of television sets, etc.

[0002] Augmented Reality (AR) See-through Near-to-Eye Displays (NEDs) or Head Mounted Displays (HMDs) are emerging technologies that attempt to enhance and/or increase the amount of visual information communicated to the viewer by over laying information in the user's field of view (FOV), thereby allowing the viewer to simultaneously experience the physical, surrounding environment and receive added content without negative impact to perception of the real world environment.

[0003] Holographic waveguide technology offers a viable solution in achieving see-through augmented reality displays. Prior art identifies advantages of holographic waveguides over other see-through display projection technologies such as free foml optics, louvered dichroic reflector based waveguides, or partially reflective elements in front of viewer's eyes to allow for superposition of a real world view with virtual imagery. The diffraction efficiency achievable using a holographic waveguide results in very high light throughput of the optical system. Additional advantages of holographic waveguides over comparable see- through display technologies include very high transmission of light from the surrounding or ambient environment, thin profile to allow for socially acceptable design embodiments, reduced weight, low material cost, and uncomplicated tooling and manufacturing design to achieve production volumes. However, prior art lacks in addressing issues such as color multiplexing, dispersion, chromatic aberrations, small field-of-view, small eye box, and complexity associated in designing and manufacturing full-color single layer, holographic waveguides. A volume phase holographic waveguide that enables a see-through, full- color, high resolution, wide FOV display without chromatic dispersion and optical aberrations in a compact and lightweight foml factor for use under varying ambient light conditions is highly desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 , a diagram of optical layout of a VPH waveguide based see-through display.

FIG. 2, a diagram showing a geometry of a VPH waveguide with continuous layer of photopolymer.

FIG. 3, a diagram showing a geometry of a VPH waveguide with discrete photopolymer sections at in-coupling and out-coupling hologram location only. FIG. 4, a diagram showing pupil replication and the resulting composite eye box in a VPH waveguide.

FIG. 5, a diagram showing variation in diffraction efficiency along the in-coupling and out- coupling holograms in one embodiment of a VPH waveguide.

FIG. 6, a raytrace diagram showing the finite/infinite imaging of the display and

projection into the in-coupling hologram.

FIG. 7, an illustration showing the characteristic angles and key features of a reflecti on VPH .

FIG. 8, an illustration showing the reconstruction setup to measure hologram diffraction efficiency.

FIG. 9, a plot showing comparison of empirical measurements of diffraction efficiency of a reflection VPH at different incident angles vs. analytical predictions generated via

coupled wave theory.

FIG. 10, a diagram showing compensation of chromatic dispersion caused by the in- coupling hologram via the out-coupling hologram.

FIG. 11, a plot showing a VPH wavelength dephasing curves for discrete AOIs and the enveloping superblaze curve.

FIG. 12, a diagram comparing characteristic angular and spectral response of transmission and reflection VPHs.

FIG. 13, images of a VPH HMD on a characteristic head form.

FIG. 14, images showing the working principles of photopolymer material and

microscopic structure of a recorded VPH. FIG. 15, an image showing the relationship of FOV, projection lens focal length, and display panel size.

FIG. 16, an image showing a wide FOV projected through a VPH.

FIG. 17, an image showing the collection and repositioning of extreme vertical field angles by the Y-waveguide.

FIG. 18 A, an image showing a VPH waveguide wit in-coupling and out-coupling holograms.

FIG. 18B, an image showing a VPH waveguide with in-coupling and out-coupling holograms and Y-direction hologram.

FIG. 19, a plot showing potential color coupling between discrete gratings within a multi-color hologram when color multiplexing.

FIG. 20, an image showing variation in color gamut for three discrete pixels across FOV FIG. 21, an image showing waveguide curvature. DETAILED DESCRIPTION

[0004] Embodiments of an apparatus, or system, employing volume phase holographic waveguide and methods to improve field of view and white balance are described herein. In the following description numerous specific details are set forth to provide a thorough

understanding of the embodiments. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects. Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0005] The optical architecture for one embodiment is shown in Figure 1. In this embodiment an LED illumination subsystem provides broadband optical input illumination to a display panel. In one embodiment the display may be a color sequential Ferro-electric Liquid Crystal on Silicon (FLCoS) panel. A polarizing beam splitter (PBS) allows only one polarization orientation to reach the display panel. The display panel reflects the incident light and rotates polarization for pixels that are "on" for a particular frame. The PBS then reflects polarization now orthogonal to the polarization initially incident on the display panel. A finite/mfinite conjugate lens images the display to an infinite image distance. Hence all field points at the object, the display panel, are projected as a focal field angles by the projection lens. The Y- waveguide diffracts the most extreme vertical field angles, from the top and bottom of the display panel, to preferentially adjust entrance location at the in-coupling hologram while maintaining propagation angle. Ultimately the Y- waveguide increases the vertical dimension of the eye box by shifting or offsetting extreme field angles spatially. The projected light then transmits through the thickness of the Main Waveguide and is reflected by a reflective coating or discrete mirror. The alignment mirror is used to steer the projection to the in-coupling hologram. The in-coupling hologram diffracts the light, steering all field angles down the length of the waveguide in Total Internal Reflection (TIR) A symmetric, balanced, out-coupling hologram then diffracts the light out of the Main Waveguide, conserving angular content of the infinite conjugate image across the full FOY. A full color, wide FOV, virtual image at infinity is presented to the viewer.

[0006] The holographic waveguide for a near-to-eye displ ay consists of an in-coupl ing hologram, an out-coupling hologram, an elongated see-through waveguide substrate material, and a see-through cover. The in-coupling and out-coupling holograms are each recorded in a single layer of photopolymer. Hence, the in-coupling and out-coupling holograms are "color multiplexed" holograms with red, green, and blue holographic diffraction gratings recorded in the same volume of the photopolymer The photopolymer is supported by a permanent carrier, or photopolymer substrate (not to be confused with the waveguide substrate). The

photopolymer substrate may be a polyimide film, acetate film, or other In one embodiment the single layer photopolymer may be continuous over the full substrate and the in-coupling and out-coupling holograms are recorded into specific spatial zones of the single layer

photopolymer. After the holograms have been recorded and the photopolymer has been photo- cured, the cover is adhered to the see-through substrate and photopolymer using a transparent adhesive such as an optically clear adhesive tape, UV cure epoxy, or other index of refraction matched adhesive. See Figure 2 for a potential cross section of the waveguide. Figure 3 then displays a cross section of another potential embodiment where smaller, discrete sections of photopolymer may be located at the in-coupling and out-coupling hologram sites on the see- through substrate and the photopolymer is no longer continuous over the full waveguide. With the attachment of the photopolymer to the substrate and subsequent bonding of the cover layer, the HOE based waveguide becomes a single component. The integral nature of the single component holographic waveguide with protective layers both reduces risk and increases ease of handling and overall environmental tolerance. Other advantages of photopolymer based single component holographic waveguide include high reliability, large range of thickness and shape, dry processing, reasonable shelf life, and low price.

[0007] The AOI of the l ight entering the waveguide and the grating vector of the in-coupl ing hologram are designed to generate reconstruction angles that guide the light down the length of waveguide. The light bounces along the waveguide experiencing total internal reflection (TIR) between the in-coupling and out-coupling holograms. The number of TIR bounces is dependent upon the separation distance between the in-coupling and out-coupling hologram, the thickness of the substrate, and the reconstruction angle caused by diffraction at the in-coupling hologram. The out-coupling hologram then redirects the light out of the waveguide in a ray bundle toward the eye of the viewer. By selecting waveguide geometry that results in numerous TIR bounces in the waveguide and an out-coupling hologram of sufficient width, the light will encounter the out- coupling hologram after multiple TIR. bounces. Further, by tail oring the diffraction efficiency of the out-coupling hologram to increase along the length of the hologram, multiple light bundles of equal brightness can be directed from out-coupling hologram. Where the ray bundle resulting from a single interaction with the out-coupling hologram will generate a relatively small exit pupil, the composite exit pupil generated by tiling individual exit pupils from each TIR out-coupling hologram interaction is much larger. This phenomena is referred to as "pupil replication". Pupil replication is used in this embodiment to create a large eye box that allows for variation in location of the viewer's eye with full field of view viewing as projected by the holographic NTE display. Figure 4 shows a cross section of the waveguide with the center field angle traveling down the waveguide in TIR with in-coupling and out-coupling ray bundles included. Note the waveguide cross section has been simplified for viewing clarity.

Likewise, only the center field point, or center field angle, of the projected image has been traced through waveguide.

[0008] As commented above, there is a desire to generate exit pupils of equal brightness. The NTE system described herein provides the viewer a constant brightness eye box and avoids brightness roll off across the FOV due to motion of the viewer's eye within the eye box. An example of tailoring the out-coupling hologram efficiency to achieve constant brightness is described below. In the case where the light intersects the out-coupling hologram three times due to TIR propagation, the near edge of the out-coupling hologram can be designed for a diffraction efficiency of ~33.3%. The intensity of the zero order diffracted light, or light that simply passes through the hologram at the AOI, would then be ~66.6% of the initial intensity. And, the diffraction efficiency of the hologram at the location of the second encounter should be ~66%to again diffract ~33.3% of the light (as compared with the initial intensity). Finally, the diffraction efficiency at the location of the final TIR encounter should then be 100% to diffract the remaining light out of the waveguide. Figure 5 presents an embodiment that allows for uniform eye box intensity distribution. In the recording optical layout a variable attenuator may be placed in the reference (or object) beam path to reduce laser optical intensity and preferentially modify the ratio of object beam intensity to reference beam intensity. Reduction in recording intensity at a given location, for a given exposure time, will reduce the amplitude of index of refraction modulation (Δη or nl) in the photopolymer and result in a reduction in diffraction efficiency. Both the reduction in total energy absorbed by the photopolymer and the difference in reference and object beam intensity can be used to tailor the index modulation of the photopolymer in localized regions. In other recording embodiments the variable attenuator can be designed with a linear attenuation profile, discrete attenuation regions, or any arbitrary attenuation geometry as desired by the holographer. The attenuation may be achieved via reflective, absorptive, polarization based optics, preferential selection of laser illumination profile, or other means.

[0009] A large eyebox is critical for a positive viewer experience. The results of a small eye box include a reduced observable FOV, difficulty locating the virtual image, and general difficulty aligning the viewing area of the waveguide to the user's eye, and effectively result in user discomfort. In this embodiment the field angles corresponding to field points at the display panel are afocal leaving the projection optics and incident upon the input hologram. Figure 6 shows a raytrace of the projection system coupling into the waveguide. All rays emanating from a single object field point are parallel after leaving the projections lens, and the angle of the parallel ray bundle corresponding to a field point is determined by the field point location at the object, in this case a display panel. Parallel rays may propagate through the waveguide between the in-coupling and out-coupling holograms through an arbitrary number of TIR bounces without experiencing image distortion or field angle mixing. Achieving multiple TIR bounces and pupil replication to increase the eye box without introducing image distortion from overlapping of light sources at a particular reflection requires parallel front and rear surfaces of the waveguide substrate. Issues arising from optical surfaces that are not parallel and of high optical quality include loss in resolution and contrast, reduced image quality, and scattered light. An eye box of at least 15mm horizontal at an eye relief distance of 25mm for a FOV of greater than 45 degrees diagonal is achieved for the current Near-to-Eye display configuration to accommodate for pupil size and motion.

[0010] Each volume phase hologram is able to couple in, or out, broadband illumination from a LED source in the visible spectrum. Multiple, distinct holographic gratings are generated during the hologram recording process via color multiplexing. The holograms can be tailored for desired performance by varying grating period "Λ", grating orientation vector "γ", index of refraction modulation "nl", and photopolymer thickness "d". These variables, along with incident angle "a" and diffraction angle "β' are displayed in Figure 7. In a "RGB", color multiplexed hologram, Red, Green, and Blue wavelengths are directed through the waveguide from a single layer of photopolymer that has been recorded using three discrete wavelengths. A full color image can then be directed to the viewer using with a single layer photopolymer. The substrate, cover layer, and adhesive materials are selected to match the index of refraction of the photopolymer to avoid undesired Fresnel reflections when the light propagates down to the waveguide and for the light entering from the ambient environment. Due to the angle and wavelength selectivity of the holograms and diffraction angles properly selected for TIR, light from the ambient environment in the viewer's normal field of view does not experience appreciable diffraction. Diffraction efficiency of light emanating from the surrounding

environment is low enough at angles defined by the viewer's FOY that the gratings do not upset, nor distort the view of the real world. Hence the waveguide does not adversely affect the viewer's perception of the ambient, surrounding environment, when the NTE display is on or off.

[0011] To achieve the wide FOY, the holographic waveguide utilizes the broadband spectral width of the illumination source (e.g. LEDs). For a gi ven volume phase holographic

diffraction grating, with determined fringe period and orientation relative to the substrate, Bragg's Law determines an angle with peak diffraction efficiency associated with every incident wavelength:

Bragg's Law;

where φ is the grating vector angle relative to the photopolymer surface normal, Θ is the Bragg angle, λ is the reconstruction wavelength, and Λ is the grating period.

The reverse relationship is also true, where there is an optimal efficiency diffraction wavelength associated with each angle in the FOV incident upon the in-coupling hologram. Figure 8 shows the reconstruction definitions used when measuring diffraction efficiency. Following equations are used for measurement of diffraction efficiency:

Where Idas-acted and Irefiected are the intensity of the diffracted and the first and second surface reflections of the reconstruction beam, respectively. To characterize the angular acceptance of the diffraction grating at a given wavelength, diffraction efficiency is measured as a function of reconstruction angles Θ. In the equation below, θο is the Bragg angle.

The plot of diffraction efficiency measured at varying incident angles results an angle dephasing curve. Figure 9 displays an empirically determined dephasing curve overlaid on a plot of simulated performance predicted via coupled wave theory.

The same may be executed for a wavelength dephasing curve by holding the AOI constant and varying the incident wavelength, λ.

For a given field angle incident upon the in-coupling hologram, with broadband illumination (e.g., LED), there will be a finite amount of optical power diffracted by the hologram as determined by the convolution of the dephasing efficiency curve and the broadband LED spectral power distribution. For any given incident field angle, the system takes advantage of the wavelength band and angular dispersion generated by the in-coupling hologram.

Chromatic dispersion causes the diffraction angle in the waveguide to vary with wavelength, though the Main Waveguide is designed such that the out-coupling hologram is symmetric to the in-coupling hologram about the mid plane of the waveguide. The symmetric out- coupling hologram compensates for the any chromatic dispersion introduced by the in- coupling hologram. In Figure 10 the dispersion resulting from diffraction at the in-coupling hologram is shown. It can be seen that the dispersion is compensated at the out-coupling hologram such that the spectral content associated with the input field angle is conserved and the exit angle is equal and opposite to that of the incident angle. Without compensation of the chromatic dispersion from the in-coupling hologram, an exit pupil including the full WFOV leaving the out-coupling hologram with any realistic waveguide dimensions would not be possible. Further, the symmetric nature of the in- and out-coupling holograms to correct chromatic dispersion in a single layer holographic waveguide with broadband illumination allows for a more power efficient optical system, capturing the LED power within each wavelength dephasing curve for a given field angle.

[0012] A "superblaze curve" envelopes all the dephasing curves for a given hologram and defines diffraction efficiency at the Bragg condition as a function of both wavelength and AOI, independent of illumination source. An example superblaze curve for an in-coupling hologram of one embodiment is shown in Figure 11. In Figure 11 the grating period is

186.2nm, the grating vector is oriented at 33 degrees relative to the surface normal, the modulation of index of fraction in the photopolymer is 0.025, the photopolymer thickness is 0.012mm, and the center field angle relative to the viewer's eye is 20 degree AOI.

[0013] While either reflection or transmission holograms may be used in one embodiment or another, one differentiation between reflection and transmission holograms is that the angular acceptance of a reflection volume phase hologram for a given wavelength is greater than that for a transmission volume phase hologram. The reverse is also true where, in general, the spectral acceptance of a transmission volume phase hologram for a given incident angle is greater in comparison with a reflection volume phase hologram.

Figure 12 depicts this comparison of angular and wavelength selectivity behavior for

transmission and reflection VPH gratings. It follows that a narrow band illumination source, e.g. laser, incident upon a volume phase hologram, reflection or transmission, has a

fundamental limitation in acceptance angle range for the hologram defined by the dephasing curve. Projection of a wide FOV through a single hologram using a narrow band source (e.g. laser) in this scenario is unfeasible. In the embodiment described herein, the system waveguide takes advantage of the superblaze characteristics of the reflection volume phase holograms. The composite diffraction efficiency of the hologram is increased by observing the Bragg response over the larger FOV with the broad wavelength illumination. In other embodiments, the illumination could be provided by any wide band illumination source, e.g. white light source with wide band filters, etc.

[0014] In other embodiments, additional gratings can be multiplexed into the single layer photopolymer if the recording configurations have sufficient separation in recording laser wavelength and/or angle from one another. In this respect, holographic gratings of increased numbers can be used to diffract different angular zones in the FOV and/or wavelength regions in the illumination spectra. Recording increased number of multiplexed gratings in a single photopolymer layer does however typically cause a reduction in modulation of index of refraction realizable in the photopolymer and results in peak diffraction efficiency loss per grating.

[0015] In one embodiment the in-coupling hologram is located on the side of the waveguide substrate, offset horizontally near the edge. The waveguide is constructed to facilitate packaging of coupling optics, a display device, and an illumination source to the side of the head near the temple. The out-coupling hologram is located directly in front of the line of sight of the viewer's eye with sufficient eye relief See Figure 13.

[0016] The angle of the waveguide relative to the viewer's temple and forward line of sight (LOS) may be designed for an ergonomic fit and aesthetic appeal The current embodiment employs a 20 degree slant angle of the waveguide to the head. The holographic grating period and orientation relative to the waveguide local surface normal will vary based on slant angle of the waveguide relative to the user. Any slant angle desirable for a reasonable NTE system is achievable by tailoring the grating period and orientation to diffract the projected image for TIR pupil replication and compact packaging design and integration of the projection module.

[0017] The optical design of the holographic waveguide locates the virtual image of the display in the far field to reduce or eliminate eye strain as experienced by the viewer. The virtual image is displayed at an image distance greater than the hypelfocal distance of the human eye (~6-7 meters for a standard human eye in indoor lighting). H ence the image of the object (or display) appears far away (at infinity) where the human eye is most relaxed.

[0018] VPH holograms can be recorded with different optical prescriptions to generate complex wavefronts. The complexity of optical prescription that can be generated in the hologram is defined by the recording optical set up. For complex phase functions desired in a hologram, the holographer may employ additional light manipulating devices in the recording set up, e.g. refractive optics, spatial light modulators, surface relief diffractive optics, additional VPH elements, etc. More specifically, through modification of the VPH recording set up the holographer can add optical power to the linear diff action grating of the out-coupling hologram to generate a spherical wavefront leaving the out-coupling hologram. As opposed to the afocal ray bundles leaving the out- coupling hologram which generate a virtual image at infinity, the spherical wavefront presented to the viewer will create a virtual image at a predetermined finite distance.

[0019] Figure 14 shows the working principle of the photopolymer and resulting structure of the hologram. This photopolymer is self-processing (requires no wet process) as the polymerization process initiated by the bright areas of the interference pattern continues until all the monomer is consumed or until the holographer determines based on desired efficiency less than the theoretical maximum. In order to increase transparency of the photopolymer, the photopolymer is photo-cured after the laser recording process. A combination of ul tra violet and visible l ight is used to photo-cure or bleach unexposed areas. In one embodiment of the waveguide a transparency of at least 95% is achieved. The diffraction efficiency achievable in the color multiplexed hologram can be very high which facilitates high light throughput of the entire optical system. Experimentally, peak diffraction efficiency can be more than 70% for RG & B in each hologram.

[0020] Low image brightness and image brightness uniformity, poor color contrast, substandard grey scale, among other degradations in optical performance, have plagued the daylight readability performance of see-through Near-to-Eye and Head-Mounted displays. Further, poor contrast due to a low quality display device, mediocre optical projection, suboptimal holograms or waveguide (in the case of holographic NTE systems), or other shortcoming, presents a blurry image to the viewer. To avoid such shortcomings, the LED illumination system is designed to best match the f/number of the display panel and projection optics and to achieve maximum color mixing prior to the display.

High brightness, high efficiency LEDs are employed in the systems. The projection optics are designed for sub-pixel resolution projection across the field of view for a viewer with

20/20vision or better. The NTE system is compatible with prescription lenses for those with vision worse than 20/20 vision. The projection optics are designed for an effective focal length corresponding to the panel size and desired FOY. In a 2D paraxial approximation, the focal length is determined by the relationship below (See Figure 15), though raytracing software has been used to design the lens and lens elements.

Where h is the display dimension, height or width, and FOVFULL is the full field of view, vertical or horizontal.

In one embodiment, an LED based illumination system with multiple RG and B LEDs is coupled to a high resolution Field Sequential Liquid Crystal over Silicon (FLCoS) display panel. The LED based NTE does not suffer from the same laser safety concerns associated with laser based projection systems. LED light is incoherent and hence the human eye may accept much higher values of luminous intensity. Unlikely, yet potential failures in an NTE system employing laser illumination could result in laser eye damage for the viewer. LED illumination mitigates this concern.

[0021] Historically, waveguides with multiple stacked holograms have suffered from inadequate combination (or overlay) of red, green, and blue images thereby exhibiting color bleeding and color blur In competing designs color spectra diffracted by the in-coupling holograms can become broader for each hologram overlaid in series due to color dispersions of the holographic diffractions. This leads to interference between diffracted spectra, causing color cross talk and ghost images. The current embodiment, on the other hand utilizes a single, monolithic, visibly transparent, single layer photopolymer waveguide. The RGB volume phase holographic diffraction gratings constituents are wavelength and angle of incidence selective. The design of the R.G-B gratings ensures that only the spectrum of the blue LED i s diffracted by the blue holograms. The same is true for the red and green holograms and their respective LED sources. The angle and wavelength selectivity of holograms, along with the high transmission of the waveguide substrate in the visible region, allows the waveguide to deliver the LED illuminated, wide FOV image through the waveguide without disrupting the user's view.

[0022] When operating with a small field of view, see-through Near-to-Eye or Head- Mounted display devices are subject to limitations in amount of content presented to the user, compromising the user experience. A small FOV relates to challenges in presenting a) full augmented reality experience b) media rich content, and c) fully immersive environment for the user Prior art identifies see-through waveguides in a Near-to-Eye configuration with a user FOV approaching 30°. The NTE system described herein can achieve an horizontal FOV approaching 50° in a single layer by tailoring the chief ray angle relative to the photopolymer surface normal, the LED illumination wavelength spectra, the hologram grating period, and the dimensions of the waveguide (e.g. thickness, in-coupling and out-coupling hologram size/location, and in-coupling to out-coupling hologram separation distance). As shown in Figure 16, the extreme object field angles are set at 12.5° and 32.5°, with chief ray angle relative to waveguide at 10°. The large FOV provided by the waveguide requires a high resolution display device (high resolution LCoS) such that resolution of the virtual image presented to the viewer is approaches the visual acuity of a healthy human eye (~1 arcmin).

[0023] The vertical field of view in the current embodiment is set to achieve a 16:9 wide screen aspect ratio common to HD fomlats. The current embodiment uses a vertically oriented waveguide, or Y-waveguide, which is also a single layer photopolymer that includes RGB chffraction gratings. The composition of the physical cross section is the same as that described for the main waveguide. From Figure 1, it can be seen that the Y-waveguide is located directly in front of the main waveguide coupling optics. In the current embodiment, the Y-waveguide diffracts the most extreme vertical field angles and transmits the field angles near to the chief ray of the system. The diffraction grating parameters are designed to direct the diffracted light in TIR toward the vertical mid-plane of the waveguide. With each 2nd TIR bounce the grating redirects the light out of the waveguide at a location closer to the mid-plane though conserving input or field angle. The diffraction efficiency of the RGB gratings are less than zero, so multiple bounces are out-coupled. In this manner the Y- waveguide collects extreme vertical field angles and injects them back into the projection system toward the optical mid-plane, allowing those field angles to propagate through the main waveguide and eye relief distance and to reach the eye box. In Figure 17 the center field point propagates through the diffraction gratings, generally unaffected. A section of the extreme field angle ray bundles, associated with the top and bottom rows of the display panel are diffracted into and out of the Y-waveguide with exit location closer to the midplane of the projection system. The Y-waveguide could be designed to collect one half of the extreme field angles and the display module could be offset vertically relative to the waveguide mid plane to achieve a similar viewer experience. It would also be possible to use the Y-waveguide to generate full pupil replication for the full vertical field of view prior to the main waveguide. Further, a Y-direction hologram, transmission or reflection, could be located between the in-coupling and out-coupling hologram of the main waveguide to perform similar function of directing extreme afocal field angles to the viewer's eye box. Figure 18A shows a Main Waveguide with the in-coupling and out- coupling holograms. Figure 18B then displays how a Y-direction hologram may be located between the in- coupling and out-coupling holograms, also in the single layer photopolymer of the Main

waveguide. The Y-direction hologram would perform the same function as the Y- Waveguide though would be able to continuously relocate extreme vertical field during horizontal propagation via TIR within the waveguide. In another embodiment the Y- waveguide may be omitted completely. In this embodiment optical apertures for the projection optics and the main waveguide optics could be selected to ensure the full vertical FOV is projected to the viewer's eye box.

[0024] One source of FOV limitations in the current system architecture is overlap of diffracted RGB spectra due to Bragg's Law. As the range of AOIs incident at the in- coupling hologram increase, the wavelength band that is diffracted also increases. At some point, the Green diffraction grating of the in-coupling RGB hologram will begin to diffract the extreme edge of the Blue illumination spectrum required to image the full field of view. Likewise, the required Red and Green spectra may intersect at opposite edges of the FOY. Figure 19 shows a graphical representation of how the Blue and Green spectra may overlap. The overlap in wavelength spectra will correspond to opposite field points, or field angles, and result in undesired blur and/or color mixing at the edges of the viewer's image. For example, the green grating in the in-coupling hologram may diffract the green reconstruction wavelength associated with a pixel at the far left of the display and the blue reconstruction wavelength for a pixel at the far right of the display. Color coupling due to the gratings at the edges of the display would be undesirable. A method to address the wavelength based FOV limitation and improve the optical efficiency of the system is to employ a means of wavelength selective illumination. The illumination profile could be tailored to provide a gradient in wavelength that matches the superblaze curve, resulting optimal diffraction efficiency across the field of view. In a system with external illumination (e.g. LCOS, DLP) the wavelength selective illumination could be achieved via addition of an optical prism or diffractive grating, multiple sources incremented in wavelength based on spatial location, inclusion of optical dichroic filters, or any combination of the aforementioned.

[0025] Due to Bragg's Law, the spectral content for any color RG or B will vary across the viewer's FOV A consequence of the spectral selection as a function of AO! at the grating is variation in the RG and B content for any given pixel at the display. In order to achieve image white balance for a wide FOV a software mask could be generated to independently control RGB intensity contribution and white balance at individual pixels. Hence each pixel would have slightly different RGB content, notated by RxyGxyBxy, different relative intensity values of R _X yGxyB _X y to achieve white balance, and a slightly different CIE XYZ color region. Figure 20 shows the CIE XYZ plot for three pixels, one at the far right of the display, one at the center of the display, and the last at the far right of the display. Likewise, with determined illumination spectra, projection optics, and diffraction characteristics, a physical, optical mask to attenuate RGB contributions locally could be used. The embodiment described herein is not limited to the two methods described above and any other reasonable approaches to locally defining white balance maybe employed.

[0026] Current embodiment uses waveguide substrate material with index of refraction greater than that of the surrounding media (e.g. n _air ~ 1.0003) for total internal reflection of the RGB light within the substrate. Both glass and plastic substrates can be employed. The embodiment described herein employs a glass substrate to take advantage of ease of manufacture of flat, parallel surfaces and low cost The substrate can be cut in any shape or form that facilities the optical architecture. The current embodiment utilizes a flat substrate and results in a flat waveguide. More complex holograms, with predetermined wavefront manipulation, can be recorded to correct for, or accommodate, curvature on the waveguide surface where total internal reflection occurs. See Figure 21.

[0027] In other embodiments of this system, the in-coupling hologram could be replaced with either an edge-lit hologram or in-coupling refractive prism. In either case, the out- coupling hologram would be designed to compensate for the dispersion generated at the input.

[0028] The see-through, full color, transmissive holographic waveguide described herein is a viable solution for a see-through Augmented Reality Near-to-Eye or Head : Mounted

Display. The waveguide described herein overcomes performance issues that have historically plagued holographic waveguides and establishes a new benchmark for this technology.

[0029] In accordance with an embodiment, a full color, transmissive holographic waveguide for use in see-through Near-to-Eye (NTE) and Head Mounted (HJVID) displays is provided that includes a single, monolithic, visibly transparent, elongated substrate, a single layer, thin film, photopolymer, a single, monolithic, visibly transparent, cover layer, an optically clear, visibly transparent adhesive layer attaching the cover layer to the photopolymer and substrate, an in- coupling volume phase hologram recorded in the photopolymer at a first location multiplexing three optical functions into the single layer, each function responsible for steering one specific spectral bandwidth range of visible wavelengths (R, G, & B) down the length of the waveguide at angles that achieve TIR, a waveguide stack configuration, of materials with matched index of refraction, capable of accepting multiple visible wavelengths (R, G, & B) from the in-coupling hologram located at a first location, and transmitting the input light along the length of the substrate through total internal reflection to a second location, an out-coupling volume phase hologram recorded in the photopolymer at a second location multiplexing three optical functions into the single layer, each function responsible for steering one specific spectral bandwidth range of visible wavelengths (R, G, & B) out of the waveguide toward the viewer's eye and correcting for the chromatic dispersion induced in the first, in-coupling volume phase hologram.

[0030] In accordance with another embodiment, said in-coupling hologram and out-coupling hologram are recorded to enable a field-of-view of at least 45° at an eye relief distance of 25mm.

[0031] In accordance with another embodiment, said in-coupling hologram and out-coupling hologram are recorded to cause TIR in the waveguide producing multiple exit pupil's and a large composite exit pupil with a horizontal eye box at least 15mm in size and a vertical eye box at least 10mm at an eye relief distance of 25mm.

[0032] In accordance with another embodiment said in-coupling hologram and out-coupling hologram are recorded and processed to be at least 90% transparent in the visible spectrum.

[0033] In accordance with another embodiment, the virtual image presented to the viewer is located beyond the hyperfocal distance of the human eye (e.g. virtual image at infinity).

[0034] In accordance with another embodiment, the virtual image presented to the viewer is located at a predetermined virtual image distance.

[0035] In accordance with another embodiment, the light directed to the viewer is composed of a complex wavefront.

[0036] In accordance with another embodiment, the substrate, photopolymer, and cover are curved along a single axis.

[0037] In accordance with another embodiment, the in-coupling and out-coupling holograms are color multiplexed to include more or (or less) than three optical functions corresponding to specific projection wavelength bands and field of view. [0038] In accordance with another embodiment, two or more holograms are stacked to diffract different zones of a larger field of view.

[0039] In accordance with another embodiment, the in-coupling hologram is located near the edge on waveguide, offset horizontally from the viewer's normal line of sight.

[0040] In accordance with another embodiment, which is compatible with prescription lenses.

[0041] In accordance with another embodiment, the photopolymer layer is replaced by

dichromated gelatin or other photorefractive material.

[0042] In accordance with another embodiment, reflection volume phase holograms are replaced by transmission volume phase holograms.

[0043] In accordance with another embodiment, the in-coupling hologram is replaced by an edge- lit hologram or a refractive prism.

[0044] In accordance with another embodiment, the substrate and cover material are either glass or plastic.

[0045] In accordance with an embodiment, a full color, transmissive holographic Y-waveguide for use in see-through Near-to-Eye (ΝΤΈ) and Head Mounted (HMD) displays is provided that includes a single, monolithic, visibly transparent, elongated substrate, a single layer, thin film, photopolymer, a single, monolithic, visibly transparent, cover layer, an optically clear, visibly transparent adhesive layer attaching the cover layer to the photopolymer and substrate, a waveguide stack configuration, of materials with matched index of refraction, capable of accepting multiple visible wavelengths (R, G, & B), a volume phase hologram recorded in the photopolymer multiplexing three optical functions into the single layer; each function responsible for steering one specific spectral bandwidth range of visible wavelengths (R, G, & B) down the length of the waveguide at angles that achieve TIR, a volume phase hologram that diffracts extreme field angles of the vertical field of view and directs those field angles toward the mid- plane of the waveguide, a volume phase hologram that diffracts TIR field angles and directs those field angles out of the waveguide at the input angle, though at a different location in the waveguide.

[0046] In accordance with another embodiment, the Y-waveguide function is executed by a Y- direction hologram located in the Main Waveguide between the in-coupling hologram and the out-coupling hologram.

[0047] In accordance with an embodiment, a method for increasing the field of view presented to the viewer beyond that listed in another embodiment by employing selective spectral illumination across the field of view.

[0048] In accordance with an embodiment, a method for achieving color uniformity and white balance across a display field of view by controlling R, G, and B brightness at an individual pixel level.

[0049] It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.