WO2014080155A1

WO2014080155A1 - Waveguide device for homogenizing illumination light

Info

Publication number: WO2014080155A1
Application number: PCT/GB2013/000500
Authority: WO
Inventors: Milan Momcilo Popovich; Jonathan David Waldern
Original assignee: Milan Momcilo Popovich; Jonathan David Waldern
Priority date: 2012-11-20
Filing date: 2013-11-18
Publication date: 2014-05-30

Abstract

An illumination device comprises: a laser; a waveguide comprising at least first and second transparent lamina; a first grating device for coupling light from the laser into a TIR path in the waveguide; a second grating device for coupling light from the TIR path out of the waveguide; and a third grating device for applying a variation of at least one of beam deflection, phase retardation or polarization rotation across the wavefronts of the TIR light. The first second and third grating devices are each sandwiched by transparent lamina.

Description

WAVEGUIDE DEVICE FOR HOMOGENIZING ILLUMINATION

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of United States Provisional Patent Application No.: 61/796,795 filed on 20 November 2012 entitled "COMPACT LASER ILLUMINATOR

INCORPORATING A DESPECKLER".

This application incorporates by reference in their entireties: United States Provisional Patent Application No.: 61/573,066, filed on 24 August 2011 entitled "HOLOGRAPHIC

POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES"; United States Patent No.: 8,224,133 with issue date 17 July 2012 entitled LASER ILLUMINATION DEVICE; PCT Application No.: PCT/GB2010/002023 filed on 2 November 2010 entitled APPARATUS FOR REDUCING LASER SPECKLE; and PCT Application No.: PCT/GB2012/000729 filed on 6 September 2012 entitled "METHOD AND APPARATUS FOR SWITCHING ELECTRO OPTICAL ARRAYS".

BACKGROUND OF THE INVENTION

The present invention relates to an illuminetion device, and more particularly to a laser illumination device based on electrically switchable Bragg gratings that homogenizes illuminination light.

Miniature solid-state lasers are finding their way into a range of display applications. The competitive advantage of lasers results from increased lifetime, lower cost, higher brightness and improved colour gamut. Although lasers offer much more compact illumination solutions than can be provided with conventional sources such as Light Emitting Diodes (LEDs) the demand

1

SUBSTITUTE SHEET RULE 26 for yet more compressed form factors continues unabated. Classical illumination designs using beam splitters and combiners fail to meet the requirements. It is known that delivering laser illumination via waveguide optics can result in 50% reduction in size compared with

conventional lens combiner splitter schemes.

Laser displays suffer from speckle, a sparkly or granular structure seen in uniformly illuminated rough surfaces. Speckle arises from the high spatial and temporal coherence of lasers. Speckle reduces image sharpness and is distracting to the viewer. Several approaches for reducing speckle contrast have been proposed based on spatial and temporal decorrelation of speckle patterns. More precisely, speckle reduction is based on averaging multiple sets of speckle patterns from a speckle surface resolution cell with the averaging taking place over the human eye integration time. Speckle can be characterized by the parameter speckle contrast which is defined as the ratio of the standard deviation of the speckle intensity to the mean speckle intensity. Temporally varying the phase pattern faster than the eye temporal resolution destroys the light spatial coherence, thereby reducing the speckle contrast. Traditionally, the simplest way to reduce speckle has been to use a rotating diffuser to direct incident light into randomly distributed ray directions. The effect is to produce a multiplicity of speckle patterns while maintaining a uniform a time-averaged intensity profile. This type of approach is often referred to as angle diversity. Another approach known as polarization diversity relies on averaging phase shifted speckle patterns. In practice neither approach succeeds in eliminating speckle entirely.

It is known that speckle may be reduced by using an electro optic device to generate variations in the refractive index profile of material such that the phase fronts of light incident on the device are modulated in phase and or amplitude. United States Patent No. 8,224,133 with issue date 17 July 2012 entitled LASER ILLUMINATION DEVICE, discloses a despeckler based on a new type of diffractive electro optical device known as an electrically Switchable Bragg Grating (SBG). Speckle is not the only source of illumination inhomogeneity.

Imperfections in the illumination optics and optical material nonuniformites may also contribute to inhomogeneity.

There is a need for a compact waveguide device for providing homogeneous

illumination. SUMMARY OF THE INVENTION

It is an object of the present invention to provide a compact waveguide device for providing homogeneous illumination.

The objects of the invention are achieved in a first embodiment comprising a first grating device for coupling light from an external source into a TIR path in the waveguide; a second grating device for coupling light from the TIR path out of the waveguide; and a third grating device for applying a variation of at least one of beam deflection or phase retardation across the wavefronts of the TIR light.

In one embodiment the waveguide comprises at least first and second transparent lamina and the first, second third grating devices are each sandwiched by the transparent lamina. In one embodiment the external source is a laser.

In one embodiment of the invention the third grating device is electrically switchable. Transparent interdigitated electrodes are applied to portions of a transparent lamina overlapping the grating device.

In one embodiment of the invention the optical prescription of the third grating device varies along the waveguide.

In one embodiment of the invention the first and second grating devices are grating lamina.

In one embodiment of the invention the third grating device comprises more than one grating adjacently disposed along the waveguide.

In one embodiment of the invention the third grating device comprises a two dimensional array of SBG elements. Transparent electrodes are applied to overlapping portions of transparent lamina sandwiching the SBG elements. At least one of the electrodes is pixelated into elements substantially overlapping the SBG elements.

In one embodiment of the invention the first and second grating devices are SBGs.

In one embodiment of the invention the third grating device is a SBG.

In one embodiment of the invention the third grating device comprises upper and lower overlapping gratings configured reciprocally for applying variation of at least one of beam deflection or phase retardation across the wavefronts of the TIR light. The third and fourth grating devices overlap. The third and fourth grating devices have identical prescriptions and are configured in a reciprocal sense.

In one embodiment of the invention the third grating device includes at least one grating that overlaps the first grating device.

In one embodiment of the invention the third grating device includes at least one grating that overlaps the second grating device.

In one embodiment of the invention the third grating device includes at least one grating disposed along the total internal reflection (TIR) path between the first and second grating devices.

In one embodiment of the invention the third grating device includes at least one grating that diffuses light into the direction of the TIR path.

In one embodiment of the invention at least one of the transparent lamina is wedged. In one embodiment of the invention at least one end of the waveguide is terminated by a reflector.

In one embodiment of the invention the illuminator further comprises a reflector disposed adjacent to an external surface of the waveguide. In one embodiment the reflector comprises a transmission grating and a mirror.

In one embodiment of the invention at least one end of the waveguide is terminated by a quarter wave plate and a mirror.

In one embodiment of the invention the second grating device comprises overlapping grating lamina separated by a transparent lamina. One grating lamina diffracts S-polarized light and the other grating lamina diffracts P-polarized light.

In one embodiment at least one of the birefringence of the SBG, the bulk PDLC scattering characteristics and waveguide surface roughness is varied along the length of said waveguide. 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.l is a schematic side elevation view of a laser illumination waveguide incorporating a despeckler for illuminating a transmissive display panel in one embodiment of the invention. FIG.2 is a schematic side elevation view of a laser illumination waveguide incorporating a despeckler for illuminating a reflective display panel in one embodiment of the invention.

FIG.3 is a schematic side elevation view of a laser illumination waveguide incorporating a despeckler in one embodiment of the invention in which the third grating device comprises two reciprocal overlapping SBG layers.

FIG.3 is a schematic side elevation view of a laser illumination waveguide incorporating a despeckler in one embodiment of the invention in which the third grating device has diffusing properties.

FIG.5 is a schematic side elevation view illustrating a light loss mechanism occurring in the embodiment of FIG.3.

FIG.6 is a table showing the light lost from the waveguide for each beam bounce as a function of diffraction efficiency in the embodiment of FIG.3.

FIG.7 is a schematic side elevation view of an embodiment similar to the one of FIG.3 in which the gratings in the third grating device encode diffusive properties. FIG. 8 is a schematic side elevation view of an embodiment of the invention in which the waveguide incorporates a reflector and a low refractive index layer overlapping the third grating device.

FIG.9 is a schematic side elevation view of an embodiment in which the third grating device comprises a single grating layer.

FIG.10 is a schematic side elevation view of an embodiment in which the third grating device comprises two overlapping reciprocal gratings.

FIG.11 is a schematic side elevation view of an embodiment in which a randomly scattering lamina is applied to an external surface of the waveguide.

FIG.12 is a schematic side elevation view of an embodiment in which a reflector is applied to an external surface of the waveguide overlapping the first grating device.

FIG.13 is a schematic side elevation view of an embodiment in which overlapping transmission grating and mirror layers are applied to an external surface of the waveguide overlapping the third grating device.

FIG.14 is a schematic side elevation view of an embodiment in which the third grating device comprises a two dimensional array of SBG elements.

FIG.15 is a schematic side elevation view of an embodiment in which the third grating device further comprises four layers overlapping the first grating device and each optimised for different angular bandwidth ranges to provide high efficiency diffraction over a large field angle.

FIG.16 is a schematic side elevation view of an embodiment in which the waveguide is wedged. FIG.17 is a schematic side elevation view of an embodiment in which the third grating device further comprises two gratings that overlap the second grating device. 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 important class of diffractive optical elements is 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 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. SBGs may be implemented as waveguide devices in which the HPDLC forms either the waveguide core or an evanescently coupled layer in proximity to the waveguide. In one particular configuration to be referred to here as Substrate Guided Optics (SGO) 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. SGOs are currently of interest in a range of display and sensor applications. Although much of the earlier work on HPDLC has been directed at reflection holograms transmission devices are proving to be much more versatile as optical system building blocks. 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: United States Patent No.5, 942,157 by Sutherland, and U. S Patent 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 nearly zero diffraction efficiency for S polarized light (ie light with the polarization vector normal to the plane of incidence. Transmission SBGs may not be used at near-grazing incidence as the diffraction efficiency of any grating for P polarization falls to zero when the included angle between the incident and reflected light is small. A glass light guide in air will propagate light by total internal reflection if the internal incidence angle is greater than about 42 degrees. Thus the invention may be implemented using transmission SBGs if the internal incidence angles are in the range of 42 to about 70 degrees, in which case the light extracted from the light guide by the gratings will be predominantly p-polarized.

The objects of the invention are achieved in a first embodiment illustrated in FIG.l comprising a laser 20 for illuminating a microdisplay 40; a waveguide 10 comprising transparent lamina 11-14; a first grating device 41 for coupling light from the laser into a TIR path in the waveguide; a second grating device 45 for coupling light from the TIR path out of the waveguide; and a third grating device 42 for applying a variation of at least one of beam deflection or phase retardation across the wavefronts of the TIR light. The third grating device essentially provides despeckling and beam homogenization according to the principles of the SBG array devices disclosed in United States Patent No. 8,224,133 by Popovich et al entitled LASER ILLUMINATION DEVICE. Input collimated light 100 from the laser is diffracted into a TIR pathlOl by the first grating device. TIR light 102 incident at the third grating device has at least one of its angle or phase varied across its wavefront to provide despeckled and

homogenized light 103.

The first grating is sandwiched by transparent lamina 11,12. The second grating device is sandwiched by the transparent lamina 12,13. The third grating device is sandwiched by the transparent lamina 13,14. Note that the thicknesses shown in FIG.l are greatly exaggerated. Typically, the lamina, which may be glass or optical plastics, are of thickness 500 micron but may be as thin as 100 micron or as thick as 500 micron. The layers may have different thicknesses. In contrast the grating devices are very thin, typically in the range 1.8 to 3 microns. The invention does not assume the grating devices lie in different layers of the waveguide structure as shown in FIG.l. The only requirement is that each grating device is sandwiched by two transparent lamina. Desirably, to achieve the thinnest waveguide architecture all three devices would be sandwiched between common transparent lamina. TIR proceeds up to the second grating device which diffracts TIR light indicated by 108 out of the waveguide and onto the microdisplay 40. The image modulated light from the microdisplay is then projected into the beam 108 by the projection lens 30.

In one embodiment of the invention shown in FIG.2 the microdisplay is a reflective device. In this case the second grating device 43 performs the dual functions of a beam deflector and a beam splitter according to the principles disclosed in United States Patent No. 6,115,152 by Popovich et al entitled HOLOGRAPHIC ILLUMIMATION SYSTEM. In the case of FIG.2 the second grating device is an SBG which preferentially diffracts P-polarised light. Despeckled, homogenized P-polarized light 104 is diffracted as light 105 towards the microdisplay 32 which is a liquid crystal microdisplay (typically, an LCoS device). The image modulated light 106 reflected from the LCoS has its polarization rotated from P to S and consequently is transmitted through the second grating device and without substantial deviation as image light 107 which leaves the waveguide and is projected by the lens 31 into the output beaml08.

To simplify the explanation of the invention the individual laminas will not be shown in the following drawings. It will also be assumed that the third grating device comprises SBG grating lamina (configured as SBG arrays) in various configurations to be described below. Transparent electrodes which are not shown in the drawings are applied to portions of transparent lamina sandwiching the grating device gratings. The electrodes substantially overlap the grating device providing electric fields at ninety degrees to the grating lamina.

In one embodiment of the invention the electrodes are transparent interdigitated electrodes which are applied to portions of a transparent lamina overlapping the grating device providing electric fields substantially parallel to the grating lamina.

Normally the first and second gratings are not required to switch and do not need to be SBGs. However, it may still be advantageous to use non-switching SBGs owing to the higher refractive index modulation and hence higher diffraction efficiency achievable with HPDLC.

The invention allows several different ways of configuring the third grating device. In one embodiment of the invention the optical prescription of the third grating device varies along said waveguide. The third grating device may comprise more than one grating lamina adjacently disposed along the waveguide. The third grating device may also compromise more than one layer. In other cases the third grating device may contain gratings that overlap one or both of the first and second grating devices. Some of these features are illustrated in the embodiment of the invention shown in FIG. 3 which shows a side elevation view of a waveguide containing the first and second grating devices 50,51 and a third grating device the third grating device uses two reciprocal overlapping SBG layers with each layer comprising two adjacent grating lamina 51 A, 52A and 51B,52B . By reciprocal we mean that the gratings have identical prescriptions so that by symmetry a ray input at a give angle leaves the second grating at the same angle after diffraction at each grating. This overcomes the problem of chromatic dispersion. In the case of FIG.3 the reciprocal gratings pairs are 51 A,51B and 52A,52B. If we consider the first pair we see that incident TIR light 111 is diffracted in the direction 112 by grating 51 A and is then diffracted into the direction 113 parallel to the ray direction 112 by the grating 5 IB. Note that some of the light incident at each grating is not diffracted and continues to propagate as zero order light. In the case of diffraction at the grating 51 A the 0-order light will continue along the TIR path. However, the zero order light at grating 5 IB, which will be substantially normal to the grating and consequently below the critical angle for TIR , will leave the waveguide. To minimise such losses it is desirable that the gratings have high diffraction efficiency.

In the embodiment of the invention shown in FIG.4 the third grating device comprises two adjacently disposed grating lamina 53,54 that each provide weak diffraction or scattering of TIR light. Hence incident TIR ray 115 is diffracted into the ray direction 117 and zero order ray direction 1 16.

FIG. 5 shows a detail of the embodiment of FIG.3. The diffracted light components of an incident TIR ray 130 after diffraction by the gratings 51A,51B are represented by the

raysl31,132. The 0 order light at grating 51 A follows the path labelled 133 and 0 order light at grating 5 IB is indicated by 134. FIG.6 is a table shows the light loss per TIR bounce for different SBG efficiencies where the transmission T at each bounce is given by the formula T= (1-DE) + DE² where DE is the diffraction efficiecny. Ignoring absorption, scatter and other losses the transmission loss at each bounce is then equal to 1 -T.

In one embodiment of the invention shown in FIG.7 which is similar to one shown in FIG.3 the gratings 55A,55B, which are reciprocal, encode diffusion in addition to their basic beam deflecting properties. The procedures for recording diffusing gratings are well known to those skilled in the art of holography The beam diffusion is indicated by the shaded regions 135,136. Each beam-grating interaction results in beam angle broadening, resulting in weaker diffraction. While this scheme will enhance despeckling and homogenisation it requires careful design of the grating prescriptions to avoid losses.

FIG.8 illustrates one strategy for overcoming 0-order losses in the two layer design. The illuminator further comprises a reflector overlapping the third grating device and further comprises a mesoporous layer 61 of near unity refractive index and a mirror 62. The third grating device comprises the reciprocal grating pair 51 A, 5 IB which provides the diffracted ray path 118,119. The 0 order light 120 is reflected by the mirror 62 in the ray path 121,122 which undergoes TIR back to the input end of the waveguide where is reflected at the mirror 73 as indicated by the ray pathesl24-126 resuming the original TIR path. The light diffracted out of the waveguide is bounced back into the waveguide off an external mirror and a mirror at the input end of the WG re-directs the light into the correct TIR direction. The mesoporous layer ensures that TIR is maintained for the non diffracted light. To counter the risk is that light may get diffracted out again by the input grating a quarter wave plate may be disposed in front of the external mirrors.

As shown in FIG.9 the third grating device may comprise the reciprocal grating pair 57A,57B disposed at the input end of the waveguide overlapping the first grating device indicated by 50 and a further despeckling and homogenising grating device 58 which extends over most of the waveguide length, By combining these gratings it is possible to achieve better control of speckle contrast and beam homogeneity. The grating 58 should be understood to represent any of the despeckling and homogenisation devices disclosed in the present application. That is it could comprise a single grating or and array of smaller gratings. The gratings could have diffusive properties. FIG.9 indicates that the grating 58 may be

implemented as single layer. However, a more typical implementation illustrated in FIG.10 would use two reciprocal gratings 58A,58B as discussed above. . FIG.11 shows an embodiment of the invention that increases angular diversity using a randomly scattering surface structure 59. The latter may be a slightly roughened surface.

Alternately the surface structure may comprise a weak blazed grating.

In the embodiment of the invention shown in FIG.12 the illuminator further comprises a reflector 63 disposed on an outer surface of the waveguide and overlapping the first grating device. The purpose of the reflector is to redirect zero order light 141 back into the TIR path. The reflector may comprise a reflection holographic grating with a diffraction angle equal to the waveguide TIR angle. Alternatively, the reflector may comprise a transmission holographic grating and a mirror coating. The grating steers the zero order light into the TIR path in the ray direction 142. Since this light will be off-Bragg after being reflected at the mirror coating it is not diffracted and re-enters the waveguide at the TIR angle FIG.13 illustrates an embodiment of the invention direction that overcomes the problem of leakage from the waveguide which occurs as a consequence of the increasing angular diversity along the TIR path. Again, the reflector could be a reflection hologram or transmission grating 64 and mirror 65 as illustrated. The ray 143 which is below the critical angle is diffracted into the TIR ray 145 by the hologram. A ray 144 which exceeds the critical angle lies outside the grating angular bandwidth is reflected into the ray 146 at the air interface and continues to undergo TIR.

In one embodiment of the invention shown in FIG.14 the third grating device comprises a two dimensional array of SBG elements 67 each element being switched at high speed.

Transparent electrodes 67,68 are applied to overlapping portions of transparent lamina sandwiching the SBG elements. At least one of the electrodes is pixelated into elements such as 67A substantially overlapping the SBG elements element 66A. The SBG elements may have varying grating vectors to provide angular diversity beam deflection. Alternatively, the grating elements may encode sub wavelength gratings to provide varying phase retardation. The diffracting properties of the grating elements may vary with position along the waveguide. In one embodiment the SBG array may comprise column shaped elements.

In one embodiment of the invention shown in FIG 15 the third grating device includes the stack of gratings 57A,57B 57C,5D overlapping the first grating device. The four gratings are each optimised for different angular bandwidth ranges to provide high efficiency diffraction over a large field angle.

In one embodiment of the invention shown in FIG.16 a thin wedge is applied to at least one of the waveguide substrates to create a wedged waveguide. The wedge angle helps to increase angular diversity.

In one embodiment of the invention the third grating device may include gratings disposed at the output end of the waveguide overlapping the second grating device. For example as shown in FIG.17 the third grating device may further comprise the reciprocal gratings

59A,59B. It should be apparent from consideration of the preceding description and the drawings that the invention allows many different combinations of gratings to be used to provide a waveguide despeckler and homogenizer. For example the third grating device may comprised grating disposed at the input and output ends and along the length of the waveguide as shown in FIG.17.

Further angular and phase diversity despeckling and homogenisation along the waveguide may be provided by the spatially varying at least one of the birefringence of the SBG, the bulk PDLC scattering characteristics and surface roughness.

In one embodiment of the invention the laser module comprises a laser source and a beam expander. Advantageously, the beam expander is comprises diffractive optical elements.

The transparent lamina used in the present invention may be implemented using plastic substrates using the materials and processes disclosed in United States Provisional Patent Application No. 61/573,066, filed on 24 August 2011 entitled "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. The reverse mode SBGs may be fabricated using the materials and processes disclosed in United States Provisional Patent Application No. 61/573,066, filed on 24 August 2011 entitled

"HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND

DEVICES ". However, the invention does not assume any particular type of SBG.

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 and PCT Application No.: PCT/GB2012/000729 filed on 6 September 2012 entitled "METHOD AND APPARATUS FOR SWITCHING ELECTRO OPTICAL ARRAYS".

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.

Claims

CLAIMS What is claimed is:

1. A waveguide illumination device comprising:

a first grating device for coupling light from an external source into a TIR path in said waveguide;

a second grating device for coupling light from said TIR path out of said waveguide; and a third grating device for applying a variation of at least one of beam deflection or phase retardation across the wavefronts of said TIR light.

2. The apparatus of claim 1 wherein said waveguide comprises at least first and second transparent lamina wherein said first, second third grating devices are each sandwiched by said transparent lamina.

3. The apparatus of claim 1 wherein at least one of said first second and third grating

devices is electrically switchable between a diffracting and a non diffracting state.

4. The apparatus of claim 1 wherein the optical prescription of said third grating device varies along said waveguide.

5. The apparatus of claim 1 wherein said third grating device includes at least one grating disposed along the TIR path between said first and second grating devices.

6. The apparatus of claim 1 wherein said third grating device comprises a two dimensional array of SBG elements, wherein electrodes are applied to overlapping portions of transparent lamina transparent lamina sandwiching said SBG elements, at least on of said electrodes being pixelated into elements substantially overlapping said SBG elements.

7. The apparatus of claim 1 wherein said third grating device comprises at least one pair of reciprocal upper and lower overlapping gratings.

8. The apparatus of claim 1 wherein said third grating device includes at least one grating that overlaps at least one of said first grating device of said second grating device.

9. The apparatus of claim 1 wherein said third grating device includes at least one pair of reciprocal upper and lower overlapping gratings that overlaps at least one of said first grating device of said second grating device.

10. The apparatus of claim 1 wherein at least one of said first, second and third grating

devices includes at least one grating that diffuses light into the direction of said TIR path.

11. The apparatus of claim 1 wherein at least one of said first, second and third grating

devices is a SBG.

12. The apparatus of claim 1 wherein at least one of said transparent lamina is wedged.

13. The apparatus of claim 1 further comprising a reflector disposed adjacent to an external surface of said waveguide, wherein said reflector comprises a reflection hologram or a transmission hologram and a mirror or a layer of material of index substantially equal to unity and a mirror.

14. The apparatus of claim 1 wherein at least one end of waveguide is terminated by a quarter wave plate and a mirror or by a mirror.

15. The apparatus of claim 1 wherein said source is a laser.

16. The apparatus of claim 1 wherein at least one of the birefringence of the SBG, the bulk PDLC scattering characteristics and waveguide surface roughness is varied along the length of said waveguide.