Patent Publication Number: US-2022214551-A1

Title: Environmentally Isolated Waveguide Display

Description:
PRIORITY CLAIMS 
     This application is a continuation of U.S. patent application Ser. No. 17/144,000 entitled “Environmentally Isolated Waveguide Display,” to Popovich et al., filed Jan. 7, 2021, which continuation of U.S. patent application Ser. No. 16/593,606 entitled “Environmentally Isolated Waveguide Display,” to Popovich et al., filed Oct. 4, 2019, which is a continuation of U.S. patent application Ser. No. 15/543,016, entitled “Environmentally Isolated Waveguide Display” to Popovich et al., filed Jul. 12, 2017, which is the U.S. National Phase of PCT Application No. PCT/GB2016/000005, entitled “Environmentally Isolated Waveguide Display” to Popovich et al., filed Jan. 12, 2016, which claims the benefit of U.S. Provisional Application No. 62/125,066, entitled “OPTICAL WAVEGUIDE DISPLAYS FOR INTEGRATION IN WINDOWS” to Waldern et al., filed Jan. 12, 2015, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a display device, and more particularly to a holographic optical waveguide display. 
     Optical waveguide devices are being developed for a range of display applications such as Head Mounted Displays (HMDs) and Heads Up Displays (HUDs). Another field of application of waveguides is in sensors such as eye trackers such as the ones disclosed in PCT Application No.: PCT/GB2014/000197 entitled HOLOGRAPHIC WAVEGUIDE EYE TRACKER by Popovich et al and finger print sensors such as the ones disclosed in PCT/GB2013/000005 entitled CONTACT IMAGE SENSOR USING SWITCHABLE BRAGG GRATINGS by Popovich et al. However, waveguide devices that use total internal reflection (TIR) to transmit image information suffer from the problem the beam propagation may be disturbed by damage to or contamination of the external waveguide surfaces by foreign materials. There is a requirement for a waveguide display in which image light propagated within the waveguide is isolated from the external environment. 
     SUMMARY OF THE INVENTION 
     It is a first object of the invention to provide a waveguide display in which image light propagated within the waveguide is isolated from the external environment. 
     The objects of the invention are achieved in one embodiment of the invention in which there is provided a waveguide display comprising: an input image generator providing image light projected over a field of view; a waveguide having first and second external surfaces; and at least one grating optically coupled to the waveguide for extracting light from the waveguide towards a viewer of the display. 
     The waveguide has a lateral refractive index variation between the external surfaces that prevents rays propagating within the waveguide from optically interacting with at least one of the external surfaces. 
     In one embodiment the waveguide contains a GRIN medium and the grating is disposed in proximity to one of the external surfaces. 
     In one embodiment the waveguide contains a GRIN medium, and the grating is disposed within the GRIN medium. 
     In one embodiment the waveguide contains a GRIN medium, and the grating is a surface relief structure formed on one of the external surfaces. 
     In one embodiment the waveguide comprises a first waveguide portion containing a GRIN medium abutting a second waveguide portion operating in TIR and containing at least one grating for extracting light from the second waveguide portion towards a viewer of the display. 
     In one embodiment the waveguide further comprises an input grating. 
     In one embodiment the waveguide is immersed in air. 
     In some embodiments the display provides a HUD, HMD or a light field display. 
     In some embodiments the waveguide is curved. 
     In some embodiments the waveguide comprises at least one GRIN waveguide portion optically coupled to at least one TIR waveguide portion, each the TIR waveguide portion containing at least one grating. 
     In one embodiment the waveguide comprises a stack of GRIN waveguides optically coupled to a stack of TIR waveguides, each the TIR waveguide containing at least one grating. 
     In one embodiment the waveguide is immersed in a low refractive index external medium and comprises a high refractive index core sandwiched by a low refractive index clad layer and at least one grating layer. TIR takes place between the interface of the core layer and the grating layer and the interface of the grating layer and the external medium. 
     In one embodiment the core and the grating layer have substantially same average refractive index. 
     In one embodiment the core has a refractive index greater than the grating layer average index. 
     In one embodiment the apparatus further comprises low refractive index layers overlaying at least one of the grating layer and the low refractive index clad layer. 
     In one embodiment the grating layer comprises a grating sandwiched by transparent substrates, the grating layer and the substrates having similar refractive indices. 
     In one embodiment the grating layer comprises an input grating and an extraction grating. 
     In some embodiments the display further comprises a beamsplitter layer. 
     In some embodiments the image light is collimated. 
     In some embodiments the grating is a Bragg Grating, a surface relief grating or a switchable Bragg grating recorded in a HPDLC material, a uniform modulation HPDLC material or a reverse mode HPDLC material. 
     A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, wherein like index numerals indicate like parts. For purposes of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a chart illustrates the spatial variation of refractive index in a GRIN device. 
         FIG. 2  is a schematic cross section view of a GRIN light guide operating at infinite conjugates and providing an intermediate focal plane in one embodiment. 
         FIG. 3  is a schematic cross section view of a GRIN light guide operating at infinite conjugates and providing two conjugate intermediate focal planes in one embodiment. 
         FIG. 4A  is a cross section view illustrating beam paths in a portion of a planar waveguide. 
         FIG. 4B  is a cross section view illustrating beam paths in a portion of a curved waveguide. 
         FIG. 5  is a schematic plan view of a curved GRIN light guide comprising abutting GRIN elements in one embodiment. 
         FIG. 6  is a schematic plan view of a near eye displays comprising curved GRIN light guide and a planar holographic waveguide in one embodiment. 
         FIG. 7  is a schematic view of a further embodiment of the invention. 
         FIG. 8  is a schematic view of a further embodiment of the invention. 
         FIG. 9  is a schematic view of a further embodiment of the invention. 
         FIG. 10  is a schematic view of a further embodiment of the invention. 
         FIG. 11  is a schematic view of a further embodiment of the invention. 
         FIG. 12  is a schematic view of a further embodiment of the invention. 
         FIG. 13  is a schematic view of a further embodiment of the invention. 
         FIG. 14A  is a schematic view of a further embodiment of the invention. 
         FIG. 14B  is a schematic view of a further embodiment of the invention. 
         FIG. 15  is a schematic side view of a GRIN waveguide in embodiment of the invention. 
         FIG. 16  is a schematic side view of a GRIN waveguide in embodiment of the invention. 
         FIG. 17  is a schematic side view of a GRIN waveguide in embodiment of the invention. 
         FIG. 18  is a schematic side view of a GRIN waveguide in embodiment of the invention. 
         FIG. 19  is a schematic side view of a GRIN waveguide in embodiment of the invention. 
         FIG. 20  is a schematic side view of a GRIN waveguide in embodiment of the invention. 
         FIG. 21  is a schematic side view of an aberration-correcting GRIN waveguide in embodiment of the invention. 
         FIG. 22  is a block diagram of an architecture for coupling GRIN waveguides to an image extraction grating waveguide in one embodiment. 
         FIG. 23  is a block diagram of an architecture for coupling GRIN waveguides to an image extraction grating waveguide in one embodiment. 
         FIG. 24  is a waveguide comprising a grating layer sandwiched by GRIN layers in one embodiment. 
         FIG. 25  is a waveguide architecture using GRINs recorded in HPDLC in one embodiment. 
         FIG. 26  is a GRIN device comprising concentric annular GRIN layers in one embodiment. 
         FIG. 27  is a GRIN device comprising concentric annular GRIN layers recorded in HPDLC in one embodiment. 
         FIG. 28  is a section of a TIR waveguide device with an external protective layer in one embodiment. 
         FIG. 29  is a section of a TIR waveguide device with an external protective layer in one embodiment. 
         FIG. 30  is a section of a TIR waveguide device with an external protective layer in one embodiment. 
         FIG. 31  is a section of a TIR waveguide device with an external protective layer in one embodiment. 
         FIG. 32  is a waveguide display using an external protective layer according to the principles of the invention in one embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be further described by way of example only with reference to the accompanying drawings. It will apparent to those skilled in the art that the present invention may be practiced with some or all of the present invention as disclosed in the following description. For the purposes of explaining the invention well-known features of optical technology known to those skilled in the art of optical design and visual displays have been omitted or simplified in order not to obscure the basic principles of the invention. Unless otherwise stated the term “on-axis” in relation to a ray or a beam direction refers to propagation parallel to an axis normal to the surfaces of the optical components described in relation to the invention. In the following description the terms light, ray, beam and direction may be used interchangeably and in association with each other to indicate the direction of propagation of light energy along rectilinear trajectories. Parts of the following description will be presented using terminology commonly employed by those skilled in the art of optical design. It should also be noted that in the following description of the invention repeated usage of the phrase “in one embodiment” does not necessarily refer to the same embodiment. 
     One known solution for conveying images down a waveguide without interference from surface contamination relies on Gradient Index (GRIN) optics. GRIN optics reproduces the optical properties of spherical lenses by gradual varying the refractive index of a material. In the most common application, GRIN fibers, the lens properties result from a radially varying index. Since the optical properties rely on the index distraction the input and output faces of a GRIN lens may be planar. GRIN lenses are manufactured by using various methods including neutron irradiation, chemical vapor deposition, ion exchange and partial polymerization, in which an organic monomer is partially polymerized using UV light at varying intensities. By precisely varying their refractive index, gradient index lenses are able to continuously bend light within the lens. This contrasts with conventional spherical lenses, which bend light only twice: when light meets the front surface of the lens and when it exits the back of the lens. Gradient index lenses can be positive (converging) or negative (diverging). GRIN lenses are capable of high quality imaging as demonstrated by their successful application in endoscopes. 
       FIG. 1  shows typical refractive index profile across a GRIN lens. The element has a thickness of 2 mm along the y-direction of the inset Cartesian coordinate frame (which will also apply to all of embodiments to be discussed below). The direction of beam propagation is in the z-direction. The refractive index of the GRIN varies from 1.46 to 1.5. 
       FIG. 2  shows a GRIN lightguide  10  divided into two regions  11  and  12 . The two regions may correspond to separate GRIN elements. The lightguide has an input surface  13  and intermediate focal plane  14  (which may correspond to the interface of two separate GRIN elements) and an output surface  15 . Input collimated light  1010  having a field of view (FOV) is incident on the input surface. The GRIN focuses the light onto the intermediate focal plane such that the three beams illustrated,  1011 - 1013 , form focal spots  1017 - 1019 . The beams are then re-expanded in the second portion of the light guide and subsequently exit the light guide as the near-collimated beams  1014 - 1016  through the exit pupil  1020 . From consideration of  FIG. 2  it should be apparent that by applying the same optical principle it should be possible to engineer longer light guides containing more than one intermediate focal plane.  FIG. 3 , for example, shows how the embodiment of  FIG. 2  may be extended to a light guide containing two intermediate focal planes. A beam collimator  20  provides collimated light  1024  over a FOV. The four GRIN regions  21 - 24  provide focal planes indicated by  26 , 28  with maximum beam expansion occurring at the planes  25 , 27 , 29 . Three separate beam paths through the light guide are indicated by  1021 - 1023  with the entrance and exit pupils being indicated by  1024 , 1025 . 
     To be of practical use in eye wear a GRIN lightguide should be curved in at least one plane of projection. In one embodiment the curvatures should match the profile of a spectacle lens. The inventors propose that the GRIN lightguide can be used to overcome the problem encountered when curved lightguide designs are attempted using total internal reflection (TIR) waveguides. The nature of the problem is illustrated in  FIG. 4  which compares beam propagation in small sections of a planar waveguide ( FIG. 4A ) and a curved waveguide ( FIG. 4B ). The beams in the planar waveguide are well separated as shown by the illustrated beam paths  1030 , 1031 . In the case of the curved waveguide element the beams paths are mixed leading to scrambling of the waveguide pupil. Unfortunately, the disorder cannot be corrected by modifying the curvatures of the waveguide surfaces. There may be some scope for compensation based in curving the input and output surfaces. However the number of optimisation degrees of freedom provided by the surface curvatures will be insufficient for most display applications. 
       FIG. 5  shows one embodiment of the invention in which a curved waveguide  40  comprises abutting or daisy-chaining GRIN elements. To achieve a smooth curve the input and output surfaces of the elements should have a small wedge angle (or may have parallel input and output surfaces but sandwich a wedge-shaped layer of index-matching material. Multiple focal surfaces are formed as in the embodiments of  FIG. 3 . Advantageously, the input is collimated and should fill the input pupil to fill the intermediate pupils within in and at the output of the waveguide. In some embodiments it may use non-collimated light. 
     In one embodiment shown in  FIG. 6  the invention provides a near eye display  50  comprising a GRIN lightguide  40 , identical to the one in  FIG. 5 , optically coupled to an image extraction waveguide  51  for directing the image light to the eye  53  of the viewer. In contrast to the GRIN light guide the image extraction waveguide transmits image light by total internal reflection (TIR) as indicated by the ray path  1051 - 1053 . The waveguide essentially comprises a holographic grating  52  sandwiched between transparent optical substrates. The grating is lossy, that is, it has diffraction efficiency (DE) varying from a low value at the end of the grating nearest the GRIN lightguide to a high value at its other end. The effect of the varying DE is to provide uniform light extraction along the length of the grating thereby expanding the exit pupil or eyebox of the display as indicated by the rays  1054 - 1056 . Coupling of the GRIN light guide and the waveguide may be accomplished in several different ways. In one embodiment the two devices are coupled end-to-end as indicated in  FIG. 6 . In this case it will be necessary to engineer a sharp bend to ensure that the entire angular image content  1050  emerging from the GRIN lightguide enters a TIR state in the waveguide. The lagging ray angle would need to be just above the TIR angle (&gt;41.5 deg in glass). In another embodiment the required beam steering may be provide by a coupling prism. The disadvantage of this approach is that it would entails a step from the lightguide to the holographic waveguide. In another embodiment the prism may be replaced by a grating. The precise details of the coupling interface will depend on the application. For example, ergonomic constraints on the maximum bend, waveguide thickness, field of view and other parameters will have an impact on the relative ease of implements of the various solutions. In one embodiment the apparatus of  FIG. 6  is embedded in one or both of the curved eyepieces of a pair of prescription glasses. In one embodiment the apparatus of  FIG. 6  is embedded within curved, emmetropic (that is, zero prescription), eyewear such as sunglasses). 
     The image extraction waveguide in  FIG. 6  may be based on a passive holographic technology. Alternatively, it may use switching grating technology such as Switchable Bragg Gratings (SBGs). The advantage of switching gratings is that they allow the field of view to be expanded using tiling. In the light of current waveguide fabrication limitations, it is likely the waveguide will be need to be a planar element. However, the invention is equally applicable to curved waveguides. The waveguide may be based on any of the holographic waveguide embodiments disclosed in U.S. Pat. No. 8,233,204 entitled OPTICAL DISPLAYS, U.S. patent application Ser. No. 13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY, PCT Application No.: GB2012/000677 entitled WEARABLE DATA DISPLAY, U.S. patent application Ser. No. 13/317,468 entitled COMPACT EDGE ILLUMINATED EYEGLASS DISPLAY, U.S. patent application Ser. No. 13/869,866 entitled HOLOGRAPHIC WIDE ANGLE DISPLAY, and U.S. patent application Ser. No. 13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY all of which are incorporated herein by reference in their entireties. 
     Holographic waveguides based on Switchable Bragg Gratings (SBGs). SBGs are fabricated by first placing a thin film of a mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the film. A volume phase grating is then recorded by illuminating the liquid material (often referred to as the syrup) with two mutually coherent laser beams, which interfere to form a slanted fringe grating structure. During the recording process, the monomers polymerize and the mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the film. When an electric field is applied to the grating via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range. The device exhibits near 100% efficiency with no voltage applied and essentially zero efficiency with a sufficiently high voltage applied. In certain types of HPDLC devices magnetic fields may be used to control the LC orientation. In certain types of HPDLC phase separation of the LC material from the polymer may be accomplished to such a degree that no discernible droplet structure results. SBGs may be used to provide transmission or reflection gratings for free space applications. In waveguide applications the parallel glass plates used to form the HPDLC cell provide a total internal reflection (TIR) light guiding structure. Light is “coupled” out of the SBG when the switchable grating diffracts the light at an angle beyond the TIR condition. Typically, the HPDLC used in SBGs comprise liquid crystal (LC), monomers, photoinitiator dyes, and coinitiators. The mixture frequently includes a surfactant. The patent and scientific literature contains many examples of material systems and processes that may be used to fabricate SBGs. Two fundamental patents are: U.S. Pat. No. 5,942,157 by Sutherland, and U.S. Pat. No. 5,751,452 by Tanaka et al. Both filings describe monomer and liquid crystal material combinations suitable for fabricating SBG devices. 
     One of the known attributes of transmission SBGs is that the LC molecules tend to align normal to the grating fringe planes. The effect of the LC molecule alignment is that transmission SBGs efficiently diffract P polarized light (ie light with the polarization vector in the plane of incidence) but have lower diffraction efficiency for S polarized light (ie light with the polarization vector normal to the plane of incidence. 
     The techniques for designing GRIN light guides should be well known to those skilled in the art and have been implemented in design software such as ZEMAX® (ZEMAX Development Corporation, Bellevue, Wash.). GRINs suitable for used with the invention are likely to require a polynomial profile which may also require additional odd-order polynomial terms to correct for the effects of lightguide curvature. Potential issues to be addressed in reducing the invention to practice include the limited number of degrees of freedom available for optimizing the design, the impact of curvature on aberrations, the tolerances of refractive index profile (and impact on relay functionality) and identifying the most efficient optical means for coupling the GRIN light guide to the image extraction waveguide 
     In many practical embodiments the GRIN lightguide will have unity magnification. However, the invention does not assume any particular magnification. In one embodiment at least one change in magnification is provided along the light guide. 
     In one embodiment illustrated in  FIG. 7  there is provided an afocal cylindrical GRIN waveguide element  60 suitable for use in a curved waveguide. The element comprises the back to back elements  61  and  62  which share a focal surface  63 . The input and output surfaces of the waveguide element are indicated by  63 ,  64 . In practice the input and output surfaces will be wedged to give the input and output surfaces  65 , 67  where  65  is the output surface of the refractive element  66 . In the particular embodiment illustrated the tilt angle will be equivalent to a field angle of ±15 degrees in air or ±9.48 degrees in glass. The tilt angle applied to each end of the waveguide matches the half field angles of the transmitted image, that is, the maximum design angles that can be transmitted by the waveguide. It should be apparent from consideration of the drawing that the relay therefore has a magnification of exactly unity. The positive field angles entering the relay exactly match the negative field angles at the re-imaged pupil. It should also be apparent that elements of the type illustrated can be daisy-chained to yield a progressing curve shape with the tilted surfaces  65 , 67  providing the interfaces between adjacent waveguide elements. Typically, a straight GRIN element would use a form such as n(x)=A 0 +A 2X   2 +A 4X   4 +A 6X   6 . Odd terms are added to this expression when tilt is introduced. At the above stated angles the wavefront errors are found to be negligible. Typically, the GRIN refractive index variation centre to edge is around 0.12. 
     In the embodiment of  FIG. 8  there is provided a waveguide based on the element shown in  FIG. 7 . Three such elements  70 - 72  are separated by intermediate, substantially afocal GRIN waveguides  74 , 75  of a second prescription. Collimated input  1070  and output  1071  light is shown. The approximate dimensions of the complete waveguide are  68  mm horizontal by 36.5 mm. vertical. The waveguide is approximately  3  mm. thick. The collimated output light emerging from the end of the waveguide may be coupled in a grating waveguide for image extraction. The waveguide is shown in side projection in  FIG. 9 . 
     In the embodiment shown in  FIG. 10  the GRIN prescription comprises an even polynomial up to sixth order coefficients, a radial offset term and a GRIN input tilt term. The latter is effectively an offset to the radial centre of rotation labelled by  1098 . It should be noted that this relay lens contains two internal focus points. The first of these focus points is formed as the light TIRs within the waveguide. The relay is symmetrical about the line bisection the extended input and output surfaces  91 , 92  in the plane of the drawing. In general, the detailed design of a GRIN waveguide must take into account skew rays, dispersion, number of GRIN index steps. Moreover, the GRIN must be manufactured to tight tolerances. Errors in the waveguide length would result in the output beams being imprecisely collimated. Tolerances of this nature could be mitigated either by focus adjustment on the input beam or by trimming the waveguide to the specific length required. 
     In the embodiment of  FIG. 11  a GRIN waveguide  100  provides finite conjugate 1:1 image imaging between the tilt surfaces  104 , 105 . The waveguide may be divided into the two symmetrical portions  101 , 102  abutting at the surface  103  where the beam is substantially collimated. The ray paths for one pair of conjugate points are indication by  1100 , 1101 . The waveguide is shown in plan view  110  on  FIG. 12 . Here a small ± 2 mm. pupil relay is shown. Note that the relayed surfaces  114 , 115  are non planar. This pupil aberration does not need to be corrected in the case of a  1 : 1  relay because it is arranged to be symmetrical about the stop at the surface  103 . Three beams  1110 - 1112  are illustrated. 
     In one embodiment there is provided a waveguide suitable This is illustrated in  FIG. 13  which shows a waveguide  120  for propagating beams from a multiplicity of points labelled by symbols A-C to a multiplicity of points labelled by the symbols A′-C′ Note that the GRIN profile is radial; there is no index variation and hence the image relay process is invariant along the axis of propagation. The GRIN prescription contains the series of tilt surfaces labelled by numerals  121 - 129 . In one embodiment the tilts are symmetrical about the surface  125 . Beam paths  1120 - 1122  from the points A-C to A′-C′ are illustrated. Only the upper half of the GRIN is used. Hence referring to the reference plane  1123  the points A-C could correspond to a first image plane and the points A′-C′ are light in proximity to a grating layer for extracting light from the waveguide. An example of grating coupling to an from a GRIN waveguide is provided by the embodiment of  FIG. 14  which shows a GRIN waveguide  130  immersed in air  131 , comprising a GRIN waveguide  132  similar in concept to the one of  FIG. 13 , an input grating  133  and an output grating  134 . The input and output gratings are design from 0 degrees in air coupling to 75 degrees in glass. Referring to  FIG. 14A , the input beam is indicated by  1130 , the GRIN guided light by the rays  1131  and the output light by  1132 .  FIG. 14B  is a plan view of the ray trace showing the GRIN-guided beam paths  1134 , 1135 . 
     We next consider a series of GRIN waveguide embodiments using gratings to extract collimate image light from the waveguide over a specified field of view. In the embodiment of  FIG. 15  a GRIN waveguide  140  comprises a planar GRIN substrate  141  having an output surface  142  and an input surface  143  which is also an intermediate focal surface. First and second ray paths through the GRIN medium are generally indicated by  1140  and  1141 . The collimated light beams refract out of the output surface as generally indicated by  1142 , 1143 . 
     In the embodiment of  FIG. 16  a GRIN waveguide  150  comprises a curved GRIN substrate  151  having a curved output surface  152  and a curved input surface  153  which is also an intermediate focal surface. First and second ray paths through the GRIN medium are generally indicated by  1150  and  1151 . The collimated light beams refract out of the output surface as generally indicated by  1152 , 1153 . 
     In the embodiment of  FIG. 17  a GRIN waveguide  160  comprises a curved GRIN substrate  161  having a curved output surface  162  which has the form of a surface relief grating, and a curved input surface  163  which is also an intermediate focal surface. First and second ray paths through the GRIN medium are generally indicated by  1160  and  1161 . The collimated light beams refract out of the output surface as generally indicated by  1162 , 1163 . 
     In the embodiment of  FIG. 18  a GRIN waveguide  170  comprises a curved GRIN substrate  171  having a curved output surface  172  which has the form of a surface relief grating, and a curved input surface  173 , which has the form of a surface relief grating and which is also an intermediate focal surface. First and second ray paths through the GRIN medium are generally indicated by  1170  and  1171 . The collimated light beams refract out of the output surface as generally indicated by  1172 , 1173 . 
     In the embodiment of  FIG. 19  a GRIN waveguide  180  comprises a curved GRIN substrate  181  having a curved output surface  182  and a curved input surface  183  which has the form of a surface relief grating and which is also an intermediate focal surface. A curved grating  184  is disposed inside the waveguide in proximity to the output surface. In one embodiment the grating is a volume or Bragg grating. In one embodiment the grating is a SBG. First and second ray paths through the GRIN medium are generally indicated by  1180  and  1181 . The collimated light beams refract out of the output surface as generally indicated by  1182 , 1183 . 
     In the embodiment of  FIG. 20  a GRIN waveguide  190  comprises a curved GRIN substrate  191  having a curved output surface  192  and a curved input surface  133  which is also an intermediate focal surface. A curved grating  194  is disposed inside the waveguide in proximity to the input surface. In one embodiment the grating is a volume or Bragg grating. In one embodiment the grating is a SBG. First and second ray paths through the GRIN medium are generally indicated by  1190  and  1191 . The collimated light beams refract out of the output surface as generally indicated by  1192 , 1193 . 
     It should be apparent from consideration of the preceding embodiments of  FIGS. 15-20  that other embodiments may be devised for the purposes of extracting collimate image light from the waveguide over a specified field of view by using different combinations of curved surfaces and gratings. In some cases multiple layers of gratings may be used for the purposes of increasing the field of view and propagating red green and blue image content according to the teaching of the above-cited related patent applications. 
     In one embodiment illustrated in  FIG. 21  a waveguide comprises two GRIN waveguide elements arrange in series in which the second element corrects the aberrations of the input image light. The input image light, which is represented by the curved wavefront  1200 , propagates down the first waveguide element  241  forming a wavefront  1201  in proximity to the boundary surface  244 . After refraction through the boundary surface the refracted wavefront  1202  propagates through the second waveguide element and is refracted out of the waveguides as the planar wave  1203 . 
     In one embodiment there is provided a general waveguide architecture which is schematically illustrated by the block diagram of  FIG. 22 . The GRIN waveguides  251 - 253  are coupled into a waveguide  254  containing an image extraction grating by coupling means symbolically indicated by  1210 - 1212 . The output from the image extraction waveguide comprises collimated light in the field of view regions or tiles labelled  1213 - 1215 . The coupling means are base on the teachings of the above described embodiments. 
     In one embodiment there is provided a general waveguide architecture which is schematically illustrated by the block diagram of  FIG. 23 . The GRIN waveguides  261 - 262  are coupled into waveguides  264 - 266  each containing an image extraction grating by coupling means symbolically indicated by  1220 - 1222 . The output from the image extraction waveguide comprises collimated light in the field of view regions or tiles labelled  1223 - 1225 . 
     In one embodiment illustrated in  FIG. 24  there is provided a waveguide  230  comprising at least one grating layer  231  sandwiched by first and second GRIN layers  231 , 232 . In one embodiment the grating is a volume or Bragg grating. In one embodiment the grating is a SBG. 
     In one embodiment illustrated in  FIG. 25  there is provided a waveguide  200  comprising GRIN layers formed in a HPDLC material sandwiched by transparent electrodes. In the example shown the electrodes layers  201 , 202  sandwich the stack of GRIN layers  201 - 203 . 
     In one embodiment illustrated in  FIG. 26  a GRIN structure  210  comprises cylindrical concentric GRIN layers  211 - 214 . The structure may be trimmed to form curved substrates or lens elements. 
     In one embodiment illustrated in  FIG. 27  a GRIN structure  220  comprises cylindrical concentric GRIN layers  222 - 224  sandwiched by transparent electrodes  221 , 225  The structure may be trimmed to form curved substrates or lens elements. 
     In one embodiment of the invention the refractive index of the GRIN varies radially and along the length of the waveguide. 
     GRIN waveguides several key advantages over TIR waveguides as used in the above-cited related patent applications. The first one is the GRIN eliminates the problem of banding. A major design challenge in waveguide optics is coupling the image content into the waveguide efficiently and in such a way the waveguide image is free from chromatic dispersion and brightness non uniformity. To overcome chromatic dispersion and to achieve the best possible collimation it is desirable to use lasers. However, lasers and other narrow band sources such as LEDs suffer from the problem of pupil banding artifacts which manifest themselves as output illumination non uniformity. Banding artifacts are formed when the collimated pupil is replicated (expanded) in a TIR waveguide. In very basic terms the light beams diffracted out of the waveguide each time the beam interacts with the grating have gaps or overlaps. This leads to an illumination ripple. The degree of ripple is a function of field angle, waveguide thickness, and aperture thickness. The effects are therefore most noticed in narrowband (e.g. laser) illumination sources. Banding can be smoothed by dispersion with broadband sources such as LEDs. However, current LEDs do not provide enough light output for waveguide displays. A second major benefit of GRIN waveguides is that the guided beams do not interact with the faces of the waveguide making the waveguides immune to external contaminants. Additional optical layers may be applied without interfering with the waveguiding. The third major benefit of GRIN is that curved waveguides can be engineered much more easily than with TIR waveguides. 
     Notwithstanding the above advantages of GRINs, in many applications a similar degree of protection may be provided by using a protective cladding applied to an exterior surface of the waveguide. Examples of such embodiments of the invention are shown in  FIGS. 28-31 . In each case the lower surface of the waveguide is nearest the viewer of the display and is in contact with air. In each case the uppermost layer isolates the TIR from the effects of windscreen damage or contamination and allows additional coatings to be applied to the exterior surface if required. In general, a waveguide must have a core of high index sandwiched by lower index (clad) layers, one of which may be air. However, if the TIR angles in the core are large enough low index refractive materials may be used. 
     In the embodiment shown in  FIG. 28  the display comprises the external clad layer  230 , providing a protective layer, a core layer  231  and a grating layer divided into an input grating  232  and a lossy output grating  233  which extracts light uniformly along the waveguide to provide an expanding exit pupil. The grating layer has an average refractive index substantially identical to that of the core. A typical ray path  1240 - 1242  is shown. An input ray is coupled into a TIR path inside the waveguide by the input grating and is eventually coupled out of the waveguide by the output grating. 
     The embodiment of  FIG. 29  is identical to the one of  FIG. 28  but with an additional outer clad layer. Note this can be done without penalty as the inner core satisfies the waveguiding index requirement. The display comprises an outer clad  240 , providing a protective layer, the clad layer  241  a core layer  242  and a grating layer divided into an input grating  243  and a lossy output grating  244 . The grating layer has an average refractive index substantially identical to that of the core. A typical ray path  1250 - 1252  is shown. 
     In the embodiment of  FIG. 30  the input and output grating layer media have a refractive index higher than that of the core. Since the clad layer has an index lower than that of the core it therefore allows TIR. The display comprises the external clad layer  250 , providing a protective layer, a core layer  251  and a grating layer divided into an input grating  252  and a lossy output grating  253 . A typical ray path  1260 - 1262  is shown. 
     Note that the grating layers in the above embodiments will in turn comprise a holographic material layer sandwiched by two substrates or alternatively a holographic material layer sandwiched by the core layer and a further substrate. The embodiments of  FIGS. 28-31  may be used to provide curved or planar displays. 
     In the embodiment of  FIG. 31  the display comprises an upper glass layer  260  a low index layer  261 , a layer  262  a grating layer  263  divided into an input grating  264  and a lossy output grating  265  which extracts light uniformly along the waveguide to provide an expanding exit pupil and a lower glass layer  266 . The grating layer has an average refractive index substantially identical to that of the core. In one embodiment the low index layer is an adhesive material of refractive index  1 . 315  in th 3 e visible band. A typical ray path  1240 - 1242  is shown. In one embodiment the upper and lower glass layers  260 , 266  and the layer  262  each have a refractive index of approximately 1.5. An input ray is coupled into a TIR path inside the waveguide by the input grating and is eventually coupled out of the waveguide by the output grating. The structure as illustrated comprises a section of a windscreen. Typically, the overall thickness is around 4.4 mm. with the upper glass layer being equal in thickness to the waveguide stack comprising layers  261 - 266 . A typical ray path is illustrated by  1270 - 1275 . Noted the extraction of light by the output grating only takes for downward propagating rays; the Bragg condition is not met by the upward propagating rays. 
       FIG. 32  illustrates an embodiment of the invention using the waveguide of  FIGS. 28-31 . The transparent display device  270 , which may form part of a HUD or HMD, comprises a waveguide  271  into which are recording three gratings: an input grating  273  a fold or turning grating  274  and an output grating  275 . The embodiments of  FIGS. 28-31  represent a cross section of the embodiment of  FIG. 32 . The waveguide also contains a thin beamsplitter homogenizer layer for vertical and horizontal homogenization. The input grating has high efficiency for maximum light in-coupling efficiency. The output grating is a lossy grating for extracting light uniformly out of the waveguide along the beam path in the waveguide. In contrast to the input and output gratings which deflect light in a plane orthogonal to the waveguide the fold grating deflects light substantially in the plane of the waveguide. The formation of the image viewed from the eye box takes place in four stages. In the first stage the input coupler couples image light  1280  from an input image generator  272  into a vertical TIR path generally indicated by  1281  within the waveguide. Typically, the image generator will further comprise an illumination source such as a laser or LED, a microdisplay panel and a collimating lens. Advantageously, the input coupler grating uses a vertically rolled K-vector. The K-vector is a vector aligned normal to the grating planes (or fringes) which determines the optical efficiency for a given range of input and diffracted angles. Rolling the K-vectors allows the angular bandwidth of the grating to be expanded without the need to increase the waveguide thickness. The input grating has a horizontal aperture width large enough to mitigate horizontal illumination non-uniformity, commonly referred to as banding (resulting from the gaps that appear between the TIR beam paths after several bounces within the waveguide). Laser-illuminated waveguide displays are particularly susceptible to banding. In the second stage the fold grating and homogenizing beamsplitter together provide first axis of pupil expansion directing light into a TIR path  1283  in the output grating. Note that a single fold grating is used to support the field of view. In the third stage the output grating, which is a lossy grating, provides uniform output coupling along the waveguide thereby providing the second axis of pupil expansion. Finally, in the fourth stage collimated light generally indicated by  1284 - 1287  is output towards the eye box  1288  from which a collimated image of the full field of view may be viewed. 
     In one embodiment the apparatus of  FIG. 32  is embedded in a vehicle windscreen. In such applications, the substrate indices used in the waveguide stack should be close the index currently used in windscreen glass, that is, around  1 . 5 . The inventors propose to use a modified PVB with a slightly higher index than the PVB material currently used in windshields (typically 1.48-1.50). A higher index is desirable to keep the diffraction angle at an acceptably low level during holographic recording. 
     In the above-described embodiments in which GRIN waveguides are combined with grating waveguides that are not embedded within a GRIN structures the embodiments of  FIGS. 28-31  may be used to environmentally isolate the grating waveguides. 
     The embodiments of  FIGS. 28-31  may be applied to any of the waveguide display devices disclosed in PCT Application No.: GB2012/000677 entitled WEARABLE DATA DISPLAY, United States Patent No.  8 , 233 , 204  entitled OPTICAL DISPLAYS, U.S. patent application Ser. No. 13/317,468 entitled COMPACT EDGE ILLUMINATED EYEGLASS DISPLAY, U.S. patent application Ser. No. 13/869,866 entitled HOLOGRAPHIC WIDE ANGLE DISPLAY, and U.S. patent application Ser. No. 13/844,456 entitled TRANSPARENT WAVEGUIDE DISPLAY and may benefit from the teachings provided therein. 
     Although the invention has been discussed in relation to a near eye display it should be apparent from consideration of the drawings that the invention may also be used in other displays such as Head Up Displays. The apparatus may also be used to provide an illumination system. By reversing the light paths the apparatus may also be used in an image sensing system. A further application of the invention is in an image delivery system for providing a secondary image source for use in microlens array light field display. The invention may also be applied to waveguide sensors such as eye trackers and fingerprint sensors. 
     A display according to the principles of the invention may include a waveguide despeckler based on principles disclosed in PCT Application No.: PCT/GB2013/000500 entitled WAVEGUIDE FOR HOMOGENIZING ILLUMINIATION, and U.S. Pat. No. 8,224,133 entitled LASER ILLUMINATION DEVICE both of which are incorporated herein by reference in their entireties. 
     It should be emphasized that the drawings are exemplary and that the dimensions have been exaggerated. 
     Any of the above-described embodiments may be implemented using plastic substrates using the materials and processes disclosed in PCT Application No.: PCT/GB2012/000680, entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES. Advantageously, the SBGs are recorded in a reverse mode HPDLC material in which the diffracting state of SBG occurs when an electric field is applied across the electrodes. An eye tracker based on any of the above-described embodiments may be implemented using reverse mode materials and processes disclosed in the above PCT application. 
     The method of fabricating the SBG pixel elements and the ITO electrodes used in any of the above-described embodiments of the invention may be based on the process disclosed in the PCT Application No. US2006/043938, entitled METHOD AND APPARATUS FOR PROVIDING A TRANSPARENT DISPLAY. 
     It should be understood by those skilled in the art that while the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. 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.