WEARABLE DATA DISPLAY

[0001] This application is a continuation of U.S. application Ser. No. 14/240,643 filed Feb. 24, 2014 which is U.S. national phase of PCT Application No. PCT/GB2012/000677 filed on Aug. 22, 2012, which claims the benefit of U.S. Provisional Patent Application No. 61/573,067 filed on Aug. 24, 2011, the disclosures of which are incorporated in their entirety by reference herein.

[0002] Each of the following applications is incorporated herein by reference in its entirety: PCT Application No. US2008/001909, with International Filing Date: 22 Jul. 2008, entitled LASER ILLUMINATION DEVICE; PCT Application No.: US2006/043938, entitled METHOD AND APPARATUS FOR PROVIDING A TRANSPARENT DISPLAY; PCT Application No.: PCT/GB2010/001982 entitled COMPACT EDGE ILLUMINATED EYEGLASS DISPLAY PCT Application No. PCT/GB2010/000835 with International Filing Date: 26 Apr. 2010 entitled COMPACT HOLOGRAPHIC EDGE ILLUMINATED EYEGLASS DISPLAY; and PCT Application No.: PCT/GB2010/002023 filed on 2 Nov. 2010 entitled APPARATUS FOR REDUCING LASER SPECKLE, U.S. patent application Ser. No. 10/555,661 filed 4 Nov. 2005 entitled SWITCHABLE VIEWFINDER DISPLAY; U.S. Provisional Patent Application No. 61/344,748 with filing date 28 Sep. 2010 entitled EYE TRACKED HOLOGRAPHIC EDGE ILLUMINATED EYEGLASS DISPLAY; No. 61/457,835 with filing date 16 Jun. 2011 entitled HOLOGRAPHIC BEAM STEERING DEVICE FOR AUTOSTEREOSCOPIC DISPLAYS; and U.S. Provisional Patent Application No. 61/573,066 with filing date 24 Aug. 2011 by the present inventors entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES.

[0003] This invention relates to a wearable display device, and more particularly to a wearable display using electrically switchable holographic optical elements.

[0004] There is a requirement for a compact see through data display capable of displaying image content ranging from symbols and alphanumeric characters to high-resolution pixelated images. The display should be highly transparent and the displayed image content should be clearly visible when superimposed over a bright background scene. The display should provide full colour with an enhanced colour gamut for optimal data visibility and impact. A prime requirement is that the display should be as easy to wear, natural and non-distracting as possible with a form factor similar to that of ski goggles or, more desirably, sunglasses. The eye relief and pupil should be big enough to avoid image loss during head movement even for demanding military and sports activities. The image generator should be compact, solid state and have low power consumption.

[0005] The above goals are not achieved by current technology. Current wearable displays only manage to deliver see through, adequate pupils, eye relief and field of view and high brightness simultaneously at the expense of cumbersome form factors. In many cases weight is distributed in the worst possible place for a wearable display, in front of the eye. The most common approach to providing see through relies on reflective or diffractive visors illuminated off axis. Microdisplays, which provide high-resolution image generators in tiny flat panels, do not necessarily help with miniaturizing wearable displays because the requirement for very high magnifications inevitably results in large diameter optics. Several ultra low form factor designs offering spectacle-like form factors are currently available but usually require aggressive trade-offs against field of view, eye relief and exit pupil.

[0006] The optical design benefits of diffractive optical elements (DOEs) are well known including unique and efficient form factors and the ability to encode complex optical functions such as optical power and diffusion into thin layers. Bragg gratings (also commonly termed volume phase gratings or holograms), which offer the highest diffraction efficiencies, have been widely used in devices such as Head Up Displays.

[0007] It is also known that diffractive optical elements can be used to provide virtual images for direct viewing or for viewing with the aid of optical systems. U.S. Pat. No. 6,052,540 by Koyama discloses a viewfinder device comprising a transmission hologram that can be located at a position other than in an image plane. The position of the virtual image formed by the transmission hologram is arranged to lie at the image plane of the optical system.

[0008] An important class of diffractive optical element known as an electrically Switchable Bragg Gratings (SBG) is based on recording Bragg gratings into a polymer dispersed liquid crystal (PDLC) mixture. Typically, SBG devices are fabricated by first placing a thin film of a mixture of photopolymerisable 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 PDLC layer. A Bragg grating is then recorded by illuminating the liquid material with two mutually coherent laser beams, which interfere to form the desired grating structure. During the recording process, the monomers polymerize and the PDLC 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 Bragg grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the PDLC layer, in the absence of an applied electric field the SBG remains in its diffracting state. When an electric field is applied to the hologram via the electrodes, the natural orientation of the LC droplets is changed thus reducing the refractive index modulation of the fringes and causing the hologram diffraction efficiency to drop to very low levels. The diffraction efficiency of the device can be adjusted, by means of the applied voltage, over a continuous range from essentially zero to near 100%. U.S. Pat. No. 5,942,157 by Sutherland et al. and U.S. Pat. No. 5,751,452 by Tanaka et al. describe monomer and liquid crystal material combinations suitable for fabricating SBG devices.

[0009] There is a requirement for a compact, lightweight wearable data display providing a high brightness, high contrast information display with a high degree of transparency to external light.

[0010] It is an object of the present invention to provide a compact, lightweight wearable data display providing high brightness and high contrast information visibility with a high degree of transparency to external light.

[0011] The objects of the invention are achieved in a first embodiment in which there is provided a transparent wearable data display comprising: a source; a means of collimating light from the source; a means for deflecting the collimated light into a scanned beam; a first array comprising one column containing N switchable grating elements sandwiched between first and second parallel transparent substrates, the substrates together functioning as a first light guide; a second array comprising integer M columns and integer N rows of switchable grating elements sandwiched between third and fourth parallel transparent substrates, the substrates together functioning as a second light guide. Transparent electrodes are applied to the first and second and the third and fourth substrates. Each switchable grating element has a diffracting state and a non diffracting state. The apparatus further comprises a first coupling means for directing the scanned beam into a first total internal reflection (TIR) light path between the outer surfaces of the first light guide along the first array column; and a second coupling means linking each element of the first array to the first element of a row of elements of the second array. Each element of the first array when in its diffracting state directing light via the second coupling means into a second TIR path along a row of the second array for directing the first TIR light into a second TIR path between the outer surfaces of the second light guide along a row of elements of the second array. At least one of said electrodes of the first array is patterned into 1×N independently switchable elements each element overlapping one of the, first array grating elements. At least one of the electrodes of said second array is patterned into M×N independently switchable elements, each element overlapping one of the second array grating elements. In one embodiment of the invention each element of the first array is disposed adjacent to a first element of a row of said second array.

[0012] In one embodiment of the invention each switchable grating element has a diffracting state when no electric field is applied across the electrodes sandwiching the grating element and a non diffracting state when a field is applied across the electrodes. Each element of the first array when in its diffracting state directs light from the first TIR path into the second TIR path starting at the first element of a row of elements of the second array and proceeding along said row. In one embodiment of the invention the elements of said first array are switched sequentially into their diffracting states. In one embodiment of the invention the elements of rows of the second array adjacent an element of the first array in its diffracting state are switched sequentially into their diffracting states. Each element of the second array when in its diffracting state deflects light through the fourth substrate.

[0013] In one embodiment of the invention each grating element of the second array encodes image information.

[0014] In one embodiment of the invention the outer surface of the fourth substrate faces a viewer of the display.

[0015] In one embodiment of the invention an element of the second array in its diffracting state forms an image of the information encoded within the grating element at a predefined viewing range and an angular bearing defined by the sweep angles of the scanned beam.

[0016] In one embodiment of the invention the substrates of the first array are parallel to the substrates of the second array.

[0017] In one embodiment of the invention the substrates of the first array are orthogonal to the substrates of the second array.

[0018] In one embodiment of the invention the first coupling means is a grating device.

[0019] In one embodiment of the invention the second coupling means is a grating device abutting each of the first and second arrays.

[0020] In one embodiment of the invention each switchable grating element of the output array is divided into independently switchable columns aligned orthogonally to the TIR path direction in the output array.

[0021] In one embodiment of the invention a switchable grating is a Switchable Bragg Grating (SBG).

[0022] In one embodiment of the invention the scanned beam is characterized by angular deflections in two orthogonal directions.

[0023] In one embodiment of the invention the intensity of the scanned beam is modulated by varying the refractive index modulation of at least one of the switchable grating elements traversed by the beam.

[0024] In one embodiment of the invention the source of collimated light provides first, second and third wavelength light.

[0025] In one embodiment of the invention the source of collimated light provides comprises first second and third wavelength light and each switchable grating element is a multiplexed SBG comprising a first grating for diffracting first wavelength light and a second grating for diffracting second and third wavelength light.

[0026] In one embodiment of the invention the source of collimated light provides comprises first second and third wavelength light and each switchable grating element is a multiplexed SBG comprising a first grating for diffracting first wavelength light, a second grating for diffracting second wavelength light and a third grating for diffracting third wavelength light.

[0027] In one embodiment of the invention a switchable grating element comprises a surface relief grating backfilled with an electrically variable refractive index medium.

[0028] In one embodiment of the invention each switchable grating element in at least one of the first array and second array is divided into independently switchable columns aligned orthogonally to the TIR paths. The refractive index modulation of each switchable column is dynamically controlled such that a predetermined amount of light is diffracted by the switchable column through the fourth substrate.

[0029] In one embodiment of the invention N is equal to 4 and M is equal to 4.

[0030] In one embodiment of the invention the data display is one of an identical pair of left and right eyepieces.

[0031] In one embodiment of the invention the means for providing a scanned beam comprises: a first transparent optical substrate with an input surface and an output surface; a second transparent optical substrate with an input surface and an output surface; transparent electrodes applied to the output surface of the first substrate and the input surface of the second substrate; an electrically variable refractive index layer having a planar surface and a second surface shaped to provide an array of prisms; and a fixed refractive index layer having a planar surface and a second surface shaped to provide an array of prismatic cavities. The prisms and prismatic cavities have identical and opposing geometries, each prism abutting one of said prismatic cavities. The planar surface of the variable refractive index layer abuts the output surface of the first substrate and the planar surface of the fixed refractive index layer abuts the input surface of the second substrate. The transparent electrodes are electrically coupled to a variable voltage generating means. At least one of the transparent electrodes is patterned into independently switchable electrode elements having substantially the same cross sectional area as the prisms such that said the refractive index prisms may be selectively switched in discrete steps from a fully diffracting to a non diffracting state by an electric field applied across the transparent electrodes.

[0032] 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.

[0050] The invention will now be further described by way of example only with reference to the accompanying drawings.

[0051] 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.

[0052] 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.

[0053] Parts of the following description will be presented using terminology commonly employed by those skilled in the art of optical design.

[0054] 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.

[0055] In a first embodiment of the invention illustrated in the schematic front elevation view of FIG. 1 there is provided a transparent wearable data display comprising: an illumination source 1, a first switchable grating array 2 and a second switchable grating array 3. The display provides an eyepiece that may be one of pair of identical elements used in a binocular display. Alternatively the display may simply provide a monocular eyepiece. The illumination source which will be discussed in more detail later in the description comprises a light source, a means for collimating the light; and a means for deflecting the collimated light into a scanned beam. Desirably, the source is a laser. The first array 2 comprises one column and integer number N switchable grating elements (1×N) sandwiched between first and second parallel transparent substrates 25, 26. The substrates 25, 26 together function as a first light guide. The second array comprises M columns and N rows of switchable grating elements sandwiched between third and fourth parallel transparent substrates 30, 31. The substrates 30, 31 together function as a second light guide. The substrates 30, 31 are in orthogonal planes to those of 25, 26. Transparent electrodes which are not illustrated are applied to the first and second and the third and fourth substrates. Advantageously the electrodes are applied to opposing faces of the substrates. The electrodes are configured such that the applied electric field will be perpendicular to the substrates. The electrodes would typically be fabricated from Indium Tin Oxide (ITO). In one embodiment of the invention the outer surface of the fourth substrate faces a viewer of the display.

[0056] In one embodiment of the invention the switchable grating is a Switchable Bragg Grating (SBG).

[0057] In the embodiment of FIG. 1 the integer M is equal to 4 and N is equal to 4 in other words the first array is a 1×4 array and the second array is a 4×4 array. The invention does not assume any particular value for M or N.

[0058] The illumination source further comprises a first coupling means for directing the scanned beam into a first TIR light path between the outer surfaces of the first light guide along the first array column. There is further provided a second coupling means 16 for directing the first TIR light into a second TIR path between the outer surfaces of the second light guide along a row of elements of the second array. In one embodiment of the invention the first coupling means is a grating device. In one embodiment of the invention the second coupling means is a grating device abutting each of the first and second arrays as indicated in FIG. 1.

[0059] At least one of said electrodes of the first array is patterned into 1×N independently switchable elements each element overlapping one of the first array grating elements. At least one of the electrodes of said second array is patterned into M×N independently switchable elements each element overlapping one of the second array grating elements. Again we will assume M=4 and N=4.

[0060] In one embodiment of the invention each element of the first array is disposed adjacent to a first element of a row of said second array. Each switchable grating element has a diffracting state when no electric field is applied across the electrodes sandwiching the grating element and a non diffracting state when a field is applied across the electrodes. Each element of the first array when in its diffracting state directs light from the first TIR path into the second TIR path starting at the first element of a row of elements of the second array and proceeding along said row.

[0061] In one embodiment of the invention the elements of said first array are switched sequentially into their diffracting states. The elements of rows of the second array adjacent an element of the first array in its diffracting state are switched sequentially into their diffracting states. Each element of the second array when in its diffracting state deflects light through the fourth substrate towards the eye of the user of the display. The rows of the second array are switched sequentially. For example, in FIG. 1 the switchable grating elements of the first array are indicated by 24A-24D with the element 24B being indicated as being in its diffracting state by a dashed line. The diffracted light 102R, 102G, 102B is diffracted into the row of elements 11A-11D of the second array starting at element 11A. Input colour sequential red, green blue light from the light source 40 is indicated by the rays 100R, 100G, 100B. It should be noted that the light is in collimated spaced throughout the optical process to be described. The rays are coupled into the first array light guide into the TIR paths 101R, 101G, 101B which are coupled the TIR paths indicated by the rays 102R, 102G, 102B along the row of elements 11A-11B by the grating element 24B which is in its active state.

[0062] FIG. 2 is a schematic cross-sectional view of the display showing the input array and the output array. The switchable grating element 24B of the first array and the row of switchable grating elements 11A-11B of the second array are illustrated. Only the red TIR path 102R is illustrated.

[0063] In one embodiment of the invention each grating element of the second array encodes image information. For the purpose of understanding the invention this image information may comprise a binary dot pattern or a symbol where the dots or symbols comprise regions of material into which gratings have been recorded surrounded by regions containing no gratings. In other words when illuminated by collimated light and in its diffracting state the grating element diffracts the light to form an image corresponding to said image information. In one embodiment of the invention an element of the second array in its diffracting state forms an image of the information encoded within the grating element at a predefined viewing range and an angular bearing defined by the instantaneous deflection angles of the scanned beam. The encoded information may comprise a numeric symbol or a portion of a numeric symbol. The information may be a gray level pixel. The information may be a binary pixel or symbol characterized solely by “on” and “off states. In other embodiments of the invention the information may provide a three dimensional or holographic image when the grating element is in its diffracting state. The invention does not assume any particular type of image information.

[0064] In one embodiment of the invention the source of collimated light provides color sequential red, green and blue illumination and each switchable grating element is a multiplexed Bragg grating comprising a first grating for diffracting red light and a second grating for diffracting blue and green light.

[0065] FIG. 3 illustrates the elements of the first array in more detail. FIG. 3A is a schematic plan view of a grating element of the first array. The grating contains two multiplexed gratings having slant angles in the YX plane. The fringes 22A, 22B from the first grating and the fringes as 23A, 23B in the second grating are indicated. The same fringes are shown in the orthogonal YZ plane in FIG. 3B.

[0066] FIG. 4 illustrates the elements of the second array in more detail. FIG. 3A is a schematic cross sectional view of a switchable grating element of the second array. The grating contains two multiplexed gratings having slant angles in the ZX plane. The fringes 32A, 32B from the first grating and the fringes as 33A, 33B in the second grating are indicated. The same fringes are shown in the orthogonal YX plane in FIG. 4B.

[0067] In a further embodiment of the invention based on the embodiment of FIGS. 3-4 the switchable grating multiplexes separate red, green and blue diffracting Bragg gratings.

[0068] It should be apparent from consideration of FIGS. 3-4 that the invention may provide a monochrome display by recording a single monochrome grating within each switchable grating element. Further, since the display is fundamentally transparent red green and blue diffracting arrays may be stacked to provide a colour display. However such an implementation of the invention would suffer from increased thickness.

[0069] FIG. 5 is a schematic plan view of an illumination source in one embodiment of the invention comprising a laser module emitting red, green and blue collimated light 110R, 110G, 110B, a scanner 42 providing the scanned beams 111R, 111G, 111B, and angular sweep expansion means 43 providing the beams 112R, 112G, 112B and a grating coupler 44 (essentially the first coupling means discussed above) for deflecting scanned beams 113R, 113G, 113B into a TIR path insider the light guide formed by the first array. The angular sweep expansion means may comprise a focal system of lenses or other equivalent means known to those skilled in the art of optical design. The invention does not assume any particular configuration of the grating coupler with respect to the first array and many alternative schemes should be apparent to those skilled in the art of optical design. The grating coupler may employ any known grating technology. In a typical eyeglass where the display provides left and right eyepieces it would be ergonomically advantageous to integrate the illumination source within the arms of the spectacles.

[0070] In one embodiment of the invention the scanned beams are characterized by angular deflections in two orthogonal directions which advantageously correspond to the Y and X coordinate directions indicated in FIG. 1. Techniques for scanning a beam in orthogonal direction are well documented in the prior art.

[0071] The invention does not assume any particular beam scanning method. Advantageously the scanner will be an electro optical device However, devices based on piezoelectric deflectors and micro electro mechanical systems (MEMS) may be also considered. Separate scanners may be provided for red, green and blue light. Alternatively, a single scanner operating on colour sequential light from separate red green and blue sources may be used. The relative merits of such technologies in terms of scanning speed, optical efficiency, physical robustness, size and cost should be apparent to those skilled in the art of optical design.

[0072] In one embodiment of the invention, the scanner is similar to the electro optical micro scanner disclosed in U.S. Provisional Patent Application No. 61/457,835 by the present inventors with filing date 16 Jun. 2011 entitled HOLOGRAPHIC BEAM STEERING DEVICE FOR AUTOSTEREOSCOPIC DISPLAYS. The micro scanner described in that reference comprises: a first transparent optical substrate with an input surface and an output surface; a second transparent optical substrate with an input surface and an output surface; transparent electrodes applied to the output surface of the first substrate and the input surface of the second substrate; an electrically variable refractive index layer having a planar surface and a second surface shaped to provide an array of prisms; and a fixed refractive index layer having a planar surface and a second surface shaped to provide an array of prismatic cavities. The prisms and prismatic cavities have identical and opposing geometries, each prism abutting one of the prismatic cavities. The planar surface of the variable refractive index layer abuts the output surface of the first substrate and the planar surface of the fixed refractive index layer abuts the input surface of the second substrate. The transparent electrodes are electrically coupled to a variable voltage generating means. At least one of the transparent electrodes is patterned into independently switchable electrode elements having substantially the same cross sectional area as the prisms such that the refractive index prisms may be selectively switched in discrete steps from a fully diffracting to a non diffracting state by an electric field applied across the transparent electrodes.

[0073] In one embodiment of the invention the scanner scans the light into discrete angular steps. In an alternative embodiment of the invention the scanner scans the light in continuous sweeps. In one embodiment of the invention the intensity of the scanned beam is modulated by varying the refractive index modulation of at least one of the switchable grating elements traversed by the beam. Advantageously the elements of the first array are used to modulate the beam. However, it will be apparent from consideration of the description and drawings that other modulation schemes based on varying the refractive index modulation of any of the grating elements along the beam path from the light source to the output surface of the display may be used.

[0074] The formation of a viewable image by the display is illustrated in more detail in FIGS. 6-7. In a typical application of the invention the viewable image is overlaid on the external scene in the manner of a Heads Up Display (HUD). FIG. 7 is a schematic cross-sectional view of a portion of the second array including the grating elements 11A-11C (see FIGS. 1-2). The element 11C is in its diffracting state. A voltage source for applying a voltage across each grating element is indicated by 5 and the circuit connection to the switching electrodes across the grating element is indicated by 51. Typically, an active matrix switching scheme would be used to control the voltages applied to the first and second arrays. The TIR path of the illumination light at one point in the beam angular sweep is indicated by the rays 114R, 114G, 114B. The light deflected out of the display at one extreme of the beam angular sweep is indicated by rays 115R, 115G, 115B and at the other extreme of the beam angular sweep by the rays 116R, 116G, 116B. The output light forms a virtual image 111 at infinity. It should be apparent from consideration of FIG. 6 that by scanning the beam in the X and Y directions and modulating the voltage applied across the active grating element a symbol image such as the one illustrated in FIG. 7 may be written. The symbol image comprises bright pixels 113 and dark pixels 114. In this case the voltage modulation as indicated by the chart 52 showing voltage V plotted against time t would have a binary waveform represented by the characteristic 53. The output light is viewed through the pupil 112. It should be noted that each element of the second array requires a unique prescription to that all light diffracted out of the eye glass passes through an exit pupil through which the eye may observe the entire displayed image. It should be apparent that by switching the voltage to provide grey levels and taking advantage of the colour gamut provided by the red, green blue illumination more complex images may be generated.

[0075] FIG. 8 is a schematic side elevation view of the display 15 in relation to the observer eye 17 in one embodiment of the invention, showing the angular extent of the display data in relation to the overall field of view defined by the physical aperture of the display. The limiting rays defining the overall field of view are illustrated by 115A, 115B. The rays 116A, 116B define the vertical extent of the displayed data. In typical applications such as data displays for sports it is desirable to project data into the lower portion of the field of view. The data may extend across the full horizontal field if necessary.

[0076] In one embodiment of the invention each switchable grating element in at least one of the input and output arrays is divided into independently switchable columns, aligned orthogonally to the TIR paths. FIG. 9 provides a front elevation of view of the elements 11A-11D of the second array. One column of the grating element 11A is indicated by the numeral 13. The invention does not place any restrictions on the width of and number of column elements in a column. The refractive index modulation of each switchable column is dynamically controlled by active matrix voltage control circuitry which is not illustrated. The refractive index modulation within a column can be set by the SBG recording conditions or can be varied dynamically by modulating each column in synchronization with the scanning of the input light. Alternatively, a combination of fixed and dynamic index modulation may be used.

[0077] The columns maximize the extraction of light from the light guide by diffracting a predetermined amount of light from an active column out of the display towards the eye. Non-diffracted or zero-order light which would otherwise be confined to the light guide by TIR is depleted in small steps each time the beam interacts with a column until all of the light has been extracted. In other applications of diffractive optical elements zero-order light is treated as a loss. However, in the present application the zero order light is recycled to allow uniform out-coupling of TIR light. The diffraction efficiency of individual column elements is controlled by adjusting the index modulation in synchronization with the beam scanning.

[0078] In the embodiment of the invention illustrated in FIG. 9 the grating elements are identical in size and contain equal numbers of columns. The use of columns elements as described above allows the grating element widths to vary across an array row as in the case of the grating elements indicated by 11E-11H in FIG. 10. The grating elements widths may be varied dynamically to match the extraction efficiency to the time varying beam angle. This overcomes the problem that TIR rays with incidence angles that do not meet the exact Bragg condition (off-Bragg rays) are diffracted with progressively diminishing efficiency as the angle increases up to the angular bandwidth limit, requiring more bounces before the beam or an acceptable portion of the beam is ejected from the light guide.

[0079] FIG. 11 is a schematic plan view of a portion of the grating element 11A illustrating the propagation of TIR light through the columns labelled by 13A-13C. The TIR path light inside the light guide is indicated by the ray 102R. The diffraction efficiencies of the column elements 13A, 13B, 13C for rays meeting the exact Bragg diffraction angle (referred to as on-Bragg rays) are k, k′, k″ respectively. If the TIR light is injected into the light guide with power P₀ the power diffracted at element 13A is kP₀ in to the ray direction 102RA. The power diffracted at the element 13B is k′(1−k)P₀ into the ray direction 102RB and so on until most of the beam power has been extracted and the output light is distributed over the ray directions generally indicated by 120R. The k factors are specified to give a fixed light output at each bounce of the TIR beam ensuring a uniform light distribution across the exit pupil of the display. Other light distributions maybe obtained by suitable specification of the k-factors.

[0080] In embodiments of the invention using the switchable column principle described above the grating element is no longer a fixed functional element of the display as discussed in relation to the embodiments of FIGS. 1-8. The term now describes the instantaneous extent of the set of columns over which extraction of the light corresponds to a defined image element (pixel) takes place. In addition to maximizing the extraction of light from the display the switchable columns principle also allow the output put light to be distributed uniformly over the exit pupil. Furthermore, the switchable column principle allow the size of the exit pupil to be expanded by using a sufficiently large subset of columns and matching the column prescriptions to the scanned beam ray directions. Switchable column designs for use with the present invention may be based on the embodiments and teachings disclosed in the U.S. Provisional Patent Application No. 61/457,835 with filing date 16 Jun. 2011 entitled HOLOGRAPHIC BEAM STEERING DEVICE FOR AUTOSTEREOSCOPIC DISPLAYS which is incorporated by reference herein in its entirety.

[0081] In the embodiment of FIG. 1 the first array is orthogonal the second array. In an alternative embodiment of the invention illustrated in FIGS. 12-15 the substrates of the first array are parallel to the substrates of the second array. The advantage of such a configuration which will now be discussed with reference to FIGS. 12-15 is that the first and second arrays may share common substrates and transparent electrode layers avoiding the fabrication problems of aligning the first and second arrays. Again the drawings are referred to the coordinate system defined by the axes labelled XYZ. FIG. 12 is a schematic front elevation view of the display showing the illumination source 1 the first array 6 which further comprises the elements 24A-24D and the second array 3. The illumination source and second array are unchanged from the embodiment of FIG. 1. FIG. 13 is a schematic cross-sectional view of the first and second arrays showing the propagation of red beam. FIG. 14 is a schematic cross sectional view of the first array 6 in the ZY plane. FIG. 15 is schematic cross sectional view of the first array in the ZX plane. The first and second arrays may abut as shown in FIG. 13. In alternative embodiments of the invention the first and second arrays may sandwich an air gap or a slab of transparent material. Turning now to FIG. 13 we see that the first and second arrays are sandwiched by the substrates 30, 31 to which transparent electrodes (not illustrated) are applied on opposing faces. The first array grating element 24B and the second array gratings elements 11A-11D are indicated. A passive grating device comprises a grating 29B sandwiched by substrates 28A, 28B abuts the substrate 30 overlapping the element 24A. As indicated in FIG. 14 the passive grating device extends over the entire length of the first array. Although the passive grating is illustrated as four distinct elements 29A-29D in FIG. 13 the grating will typically have a uniform prescription along its length. The illumination source injects colour-sequential TIR light 121R, 121G, 121B into the light guide formed by the first array substrates which is diffracted by the active element 24B into the ray directions 122R, 122G, 122B. The passive grating diffracts the light which is totally internally reflected at the outer surface of the substrate 28B as represented by the ray paths 123R, 124R lying in the plane ZY in FIG. 12 and FIG. 14. The light then proceeds to follow the TIR path 102R within the second array. At least one of the first or second arrays may use the column element scheme described earlier.

[0082] In one embodiment of the invention a switchable grating element according to the principles of the invention is a surface relief grating backfilled with an electrically variable refractive index medium based on the embodiments and teachings disclosed in the U.S. Provisional Patent Application No. 61/457,835 with filing date 16 Jun. 2011 entitled HOLOGRAPHIC BEAM STEERING DEVICE FOR AUTOSTEREOSCOPIC DISPLAYS which is incorporated by reference herein in its entirety.

[0083] In order to ensure high transparency to external light, high contrast of displayed data (i.e. high diffraction efficiency) and very low haze due to scatter the following material characteristics are desirable. A low index-modulation residual grating, with a modulation not greater than 0.007, is desirable. This will require a good match between the refractive index of the polymer region and the ordinary index of the liquid crystal. The material should have a high index modulation capability with a refractive index modulation not less than 0.06. The material should exhibit very low haze for HPDLC cell thicknesses in the range 2-6 micron. The HPDLC should have a good index match (to within +0.015) for glass or plastic at 630 nm. One option is 1 0.515 (for example, 1737F or BK7 glasses). An alternative option would be 1.472 (for example Borofloat or 7740 Pyrex glasses).

[0084] Desirably the light sources are solid-state lasers. The low etendue of lasers results in considerable simplification of the optics. LEDs may also be used with the invention. However, LEDs suffer from large etendue, inefficient light collection and complex illuminator and projection optics. A further disadvantage with regard to SBGs is that LEDs are fundamentally unpolarized.

[0085] Any display device using lasers will tend to suffer from speckle. The present invention may incorporate any type of despeckler. Advantageously, the despeckler would be based on electro-optical principles. A despeckler for use with the present invention may be based on the disclosed embodiments and teachings of PCT Application No. US2008/001909, with International Filing Date: 22 Jul. 2008, entitled LASER ILLUMINATION DEVICE, and PCT Application No.: PCT/GB2010/002023 filed on 2 Nov. 2010 by the present inventors entitled APPARATUS FOR REDUCING LASER SPECKLE each of which is incorporated by reference herein in its entirety. The need for a despeckler may be eliminated by using a miniature, broadband (4 nm) RGB lasers of the type supplied by Epicrystal Inc.

[0086] Speckle arising from laser sources can be reduced by applying decorrelation procedures based on combining multiple sets of speckle patterns or cells from a given speckle-generating surface during the spatio-temporal resolution of the human eye. Desirably the despeckler is an electro-optical device configured to generate set of unique speckle phase cells by operating on the angular or polarization characteristic of rays propagating through the device. Furthermore, the despeckler device may be configured in several different ways to operate on one of more of the phase, and ray angular characteristics of incoming light. The invention does not rely on any particular despeckler technology. Any method for generating and averaging speckle cells may be used with the invention. However, solid-state methods using SBGs offer more scope for miniaturization of the illuminator module.

[0087] The optical design of a wearable display according to the principles of the invention will be dictated by basic geometrical considerations well known to those skilled in the art of optical design. The goal is to maximize eye relief, exit pupil and field of view. Since these parameters will impact on geometrical aberrations, dispersion and other factors affecting image quality some performance versus form factor trade-offs are inevitable. The preferred light source is a laser. If broadband sources such as LEDs are used the design will require careful attention to the correction of chromatic dispersion and monochromatic geometrical aberrations. Dispersion is a problem for any DOE illuminated by a broadband source. The degree of defocus or image blur due to dispersion depends on the source spectral bandwidth and the distance from the DOE to the virtual image plane. Typically, the angular blur for a given wavelength and a source spectral bandwidth will be of the order of the bandwidth divided by the wavelength. The effect of monochromatic geometrical aberrations will depend on the field of view and pupil size.

[0088] A wearable display based on any of the above-described embodiments may be implemented using plastic substrates. Using sufficiently thin substrates such embodiments could be implemented as a long clear strip applique running from the nasal to ear ends of each eyeglass with a small illumination module continuing laser dies, light guides and display drive chip tucked into the sidewall of the eyeglass. Standard index matched glue would be used to fix the display to the surfaces of the eyeglasses. The plastic substrates may be fabricated from materials such as polycarbonate. The transparent electrodes may be fabricated from carbon nanotubes (CNTs) which may be more suitable than ITO for use with flexible substrates. The display may further comprise an environmental seal. A plastic SBG for use in the present invention may be based on the HPDLC material system and processes disclosed in a U.S. Provisional Patent Application No. 61/573,066 with filing date 24 Aug. 2011 by the present inventors entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES, which is incorporated by reference herein in its entirety.

[0089] Although a planar display element using flat substrates has been discussed in the above description an eyepiece according to the principles of the invention may be fabricated using curved surfaces. The invention the invention may be used to provide a facetted surface display. In one embodiment of the invention the switchable gratings are SBGs operated in reverse mode. In reverse mode the SBG has low diffraction efficiency when no electric field is applied and has high efficiency when a field is applied. A reverse mode SBG for use in the present invention may be based on the HPDLC material system and processes disclosed in U.S. Provisional Patent Application No. 61/573,066 with filing date 24 Aug. 2011 by the present inventors entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES which is incorporated by reference herein in its entirety.

[0090] A key feature of all of the embodiments described above is that they provide the benefit of see-through. The latter is of great importance in Head Up Displays for automobile, aviation and other transport applications; private see-through displays such for security sensitive applications; architectural interior signage and many other applications. With the addition of a holographic brightness enhancing film, or other narrow band reflector affixed to one side of the display, the purpose of which is to reflect the display illumination wavelength light only, the see-through display can be made invisible (and hence secure) in the opposite direction of view. Here the reflected display illumination is effectively mirrored and therefore blocked in one direction, making it ideal for transparent desktop display applications in customer or personal interview settings common in bank or financial services settings.

[0091] Although the present application addresses wearable displays it will be clear that in any of the above embodiments the eye lens and retina may be replaced by any type of imaging lens and a screen. Any of the above described embodiments of the invention may be used in either directly viewed or virtual image displays. Possible applications range from miniature displays such as those used in viewfinders to large area public information displays. The above described embodiments may be used in applications where a transparent display is required. For example the invention may be used in applications where the displayed imagery is superimposed on a background scene such as heads up displays and teleprompters. The invention may be used to provide a display device that is located at or near to an internal image plane of an optical system. For example any of the above described embodiments may be used to provide a symbolic data display for a camera viewfinder in which symbol data is projected at an intermediate image plane and then magnified by a viewfinder eyepiece. It will be clear the invention may be applied in biocular or monocular displays. The invention may also be used in a stereoscopic wearable display. Any of the above described embodiments of the invention may be used in a rear projection television. The invention may be applied in avionic, industrial and medical displays. There are applications in entertainment, simulation, virtual reality, training systems and sport.

[0092] SBG arrays may be fabricated using a diffractive optical mask formed on a transparent sapphire wafer. The SBG cell optical prescriptions are defined on a cell to cell basis. The process of fabricating the SBG array may start with the creation of a multiphase computer generated hologram encoding the desired optical functions which is then holographically recorded in the SBG.

[0093] It should be noted that the total internal reflection ray paths shown in the drawings are meant to be schematic only. The number of total internal reflections will depend on the scrolling scheme used and the overall geometry of the light guide formed by the display layers. Typically, in order to ensure that TIR occurs the incidence angles must lie in the range of about 42 to about 70 degrees. It should be emphasized that the drawings are exemplary and that the dimensions have been exaggerated.

[0094] 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, claiming priority to U.S. provisional patent application 60/789,595 filed on 6 Apr. 2006, entitled METHOD AND APPARATUS FOR PROVIDING A TRANSPARENT DISPLAY, which is incorporated by reference herein in its entirety.

[0095] The display devices disclosed in the present invention may employ features of the transparent edge lit display embodiments and teachings disclosed in U.S. patent application Ser. No. 10/555,661 filed 4 Nov. 2005 entitled SWITCHABLE VIEWFINDER DISPLAY which is incorporated by reference herein in its entirety.

[0096] The despeckler referred to in the above description may be based on the disclosed embodiments and teachings of PCT Application No. US2008/001909, with International Filing Date: 22 Jul. 2008, entitled LASER ILLUMINATION DEVICE, and PCT Application No.: PCT/GB2010/002023 filed on 2 Nov. 2010 by the present inventors entitled APPARATUS FOR REDUCING LASER SPECKLE each of which is incorporated by reference herein in its entirety.

[0097] The optical design of the display disclosed in the present application may be guided by the teachings of PCT Application No.: PCT/GB2010/001982 entitled COMPACT EDGE ILLUMINATED EYEGLASS DISPLAY by the present inventors (and also referenced by the Applicant's docket number SBG081PCT) which is incorporated by reference herein in its entirety.

[0098] The display disclosed in the present application may incorporate an eye tracker based on the embodiments and teachings disclosed in U.S. Provisional Patent Application No. 61/344,748 with filing date 28 Sep. 2010 entitled EYE TRACKED HOLOGRAPHIC EDGE ILLUMINATED EYEGLASS DISPLAY (and also referenced by the Applicant's docket number SBG092) which is incorporated by reference herein in its entirety.

[0099] The means for scanning collimated input light and the column array technique for improving the light extraction efficiency from switchable gratings discussed above may be based on the embodiments and teachings disclosed in the U.S. Provisional Patent Application No. 61/457,835 with filing date 16 Jun. 2011 entitled HOLOGRAPHIC BEAM STEERING DEVICE FOR AUTOSTEREOSCOPIC DISPLAYS which is incorporated by reference herein in its entirety.

[0100] The optical design of display disclosed in the present application may be guided by the teachings of PCT Application No.: PCT/GB2010/000835 with International Filing Date: 26 Apr. 2010 entitled COMPACT HOLOGRAPHIC EDGE ILLUMINATED EYEGLASS DISPLAY which is incorporated by reference herein in its entirety, which discloses eyeglass display architectures based on a light guiding eyepiece in which a two dimension array of SBG deflectors is combined with an input beam.

[0101] The display disclosed in the present application may fabricated using the HPDLC material system and processes disclosed in a U.S. Provisional Patent Application No. 61/573,066 with filing date 24 Aug. 2011 by the present inventors entitled IMPROVEMENTS TO HOLOGRAPHIC POLYMER DISPERSED LIQUID CRYSTAL MATERIALS AND DEVICES which is incorporated by reference herein in its entirety.

[0102] 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.