HOLOGRAPHIC WINDOWS

Abstract

We describe a window assembly comprising: a window pane comprising a glass or plastic sheet; and a layer of holographic recording medium attached to said glass or plastic sheet; wherein said layer of holographic recording medium has recorded within the medium a volume hologram configured to direct light incident onto said glass or plastic sheet to propagate within a thickness of said glass or plastic sheet. In embodiments the volume hologram is fabricated by recording a transmission hologram and shrinking the recorded hologram to convert the transmission hologram to an edge-directing hologram configured to direct light in a direction to be totally internally reflected within the window pane, for example at greater than 40, 50, 60, 70, 75 or 80 to a normal to the surface of the hologram.

Claims

1. A window assembly comprising: a window pane comprising a glass or plastic sheet; and a layer of holographic recording medium attached to said glass or plastic sheet; wherein said layer of holographic recording medium has recorded within the medium a volume hologram configured to direct light incident onto said glass or plastic sheet to propagate within a thickness of said glass or plastic sheet.

2. A window assembly as described in claim 1 wherein said volume hologram is configured to direct said incident light such that it propagates within said thickness of said sheet at an angle to a normal to said sheet equal to or greater than a critical angle of said glass or plastic sheet.

3. A window assembly as claimed in claim 1 wherein said volume hologram is configured to direct said incident light such that it propagates within said thickness of said sheet when said incident light has a wavelength longer than a threshold wavelength and to allow said incident light to pass through said thickness of said glass or plastic sheet when said incident light has a wavelength shorter than said threshold wavelength.

4. A window assembly as claimed in claim 1 wherein said volume hologram comprise fringes at a range of different angles such that light rays incident onto said glass or plastic sheet at a range of angles to a normal direction to said sheet are directed to propagate substantially parallel to one another.

5. A window assembly as claimed in claim 4 wherein said glass or plastic sheet defines two orthogonal axes each perpendicular to said normal direction, a first, vertical direction and a second, horizontal direction, and wherein said volume hologram comprises fringes at a range of different angles such that light rays incident onto said window and over a range of angles in each of said first and second directions are directed to propagate substantially parallel to one another

6. A window assembly as claimed in claim 4 wherein said volume hologram has a plurality of layers having fringes at a set of different angles, and wherein said volume hologram is indexed by wavelength such that at different angles of incidence of said light rays different wavelengths of said incident light are directed to propagate substantially parallel to one another.

7. A window assembly as claimed in claim 4 wherein said volume hologram has at least one layer having overlapping said fringes at said range of different angles.

8. A window assembly as claimed in claim 1 wherein said volume hologram is chirped such that a spacing of said fringes increases from a front to a rear surface of said hologram, or vice-versa.

9. A window assembly as claimed in claim 1 wherein said layer of holographic recording medium comprises a layer on a film substrate, and wherein said film substrate is glued to said glass or plastic sheet with said layer of holographic recording medium sandwiched between said sheet and said film substrate.

10. A window assembly as claimed in claim 9 wherein said film substrate bears an image separate to said volume hologram.

11. A window assembly as claimed in claim 1 wherein said volume hologram includes a hologram of an image of a spatial pattern such that said image is reproduced when said volume hologram or glass or plastic sheet is edge lit.

12. A window assembly as claimed in claim 1 further comprising a photovoltaic element mounted to receive light escaping from an edge of said glass or plastic sheet.

13. Holographic film for the window assembly of claim 1, comprising a film substrate bearing a layer of holographic recording medium, wherein said layer of holographic recording medium has recorded within the medium a volume hologram configured to direct light, incident onto the film or onto a glass or plastic sheet to which said film is attached, to propagate within a thickness of said film or said glass or plastic sheet, in particular wherein said volume hologram includes a hologram of an image of a spatial pattern such that said image in reproduced when said volume hologram or glass or plastic sheet is edge lit.

14. A method using the holographic film of claim 13 to convert a window pane comprising a glass or plastic sheet to a photovoltaic collector, the method comprising: applying the holographic film of claim 13 to said glass or plastic sheet said that light incident on said sheet is directed to propagate within a thickness of said glass or plastic sheet; and providing a photovoltaic element to receive light escaping from an edge of said glass or plastic sheet.

15-22. (canceled)

23. A method of providing solar power, the method comprising: mounting a layer of holographic recording medium on a window pane comprising a glass or plastic sheet; the method further comprising: recording a volume hologram in said holographic recording medium; directing sunlight falling on said window using said volume hologram to propagate within a thickness of said sheet; and illuminating one or more photovoltaic elements with sunlight escaping from a lateral edge of said window to provide said solar power.

24. A method as claimed in claim 23 wherein said directing comprises selecting an angle of said propagating light to be equal to or greater than a critical angle of said glass or plastic sheet.

25. A method as claimed in claim 23 further comprising using said volume hologram to selectively divert longer wavelengths of said sunlight to illuminate said photovoltaic elements and transmitting shorter wavelengths in a substantially unchanged direction through said window, the method further comprising varying a fringe rotation of said volume hologram from top to bottom of said window to compensate for changes in solar elevation.

26. (canceled)

27. A method as claimed in claim 23 further comprising providing a plurality of sets of fringes within said volume hologram, one for each of a plurality of different solar azimuth values, wherein said sets of fringes constitute a volume hologram of plurality of replayed holograms, one for each azimuth value.

28. (canceled)

29. A method as claimed in claim 23 further comprising providing a plurality of sets of fringes within said volume hologram, wherein said sets of fringes are located in different layers of said volume hologram and indexed by different respective wavelengths of said sunlight.

30. A method as claimed in claim 23 further comprising chirping fringes of said volume hologram from front to back.

31-38 (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0047] These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

[0048] FIG. 1a to 1e show a window assembly according to an embodiment of the invention;

[0049] FIG. 2 shows a first example of a hologram recording process for fabricating a volume hologram for use with the window assembly of FIG. 1, according to an aspect of the invention;

[0050] FIGS. 3a to 3d illustrate techniques for the fabrication of a volume hologram for the window assembly of FIG. 1 for handling a range of vertical and lateral (solar elevation and azimuth) angles of incident light;

[0051] FIGS. 4a to 4c illustrate a contact-copying based volume hologram fabrication process according to an aspect of the invention, and a drum-based volume hologram fabrication process according to an aspect of the invention;

[0052] FIGS. 5a to 5e show schematic illustrations of volume holograms for diffracting light at multiple different wavelengths, in embodiments of which fringe angle is indexed by wavelength;

[0053] FIG. 6 illustrates an example chirped volume hologram for use with embodiments of the invention;

[0054] FIGS. 7a to 7e illustrate, schematically, techniques for fringe rotation and for hologram fabrication for use in embodiments of aspects of the invention;

[0055] FIGS. 8a and 8b illustrate example target fringe angle and spacing requirements;

[0056] FIGS. 9a to 9e illustrate details of an example fringe rotation/spacing modification process according to embodiments of the invention;

[0057] FIGS. 10a and 10b illustrate incorporation of a replayed holographic image into a volume hologram for use in embodiments of the invention; and

[0058] FIG. 11 illustrates holographic recording film storing a volume hologram according to an embodiment of the invention and an additional image defined by the film substrate.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0059] Broadly speaking we will describe a system which employs a volume holographic grating mounted in contact with the surface of a glass plate, more particularly a window, using an index-matched adhesive bonding agent. The fringes of the volume holographic grating are arranged to diffract light incident upon the surface of the glass, preferably sunlight having a particular range of wavelengths, into the thickness of the plate. Preferably within the plate the direction of propagation of the diffracted light is such that a majority of the light exceeds the critical angle for a glass/air interface such that the diffracted light is totally internally reflected at one or both faces of the glass plate. Therefore the diffracted light continues within the thickness of the glass plate until it arrives at an edge of the plate where it is incident upon a linear array of photovoltaic cells, preferably arranged to capture substantially all of the light exiting the plate in this manner, for production of electrical energy.

[0060] FIG. 1a shows, schematically, a window assembly 100 of this type comprising a glass window an internal surface of which is provided with a layer of holographic film or, as illustrated, one or more volume hologram tiles 104 bearing one or more volume holograms, schematically illustrated as fringes within a layer of holographic recording medium 106. Incident sunlight 110 is transmitted through window 102 and the holographic film/tiles 104 to provide an exit beam 112 parallel to but slightly displaced from beam 110. A proportion of the incoming beam 110 is diffracted by the holographic layer 106 to provide a beam 116 which propagates within the thickness of the window 102 down towards one or more linear photovoltaic elements 120 such as a PV array. In some preferred embodiments the diffracted beam 116 comprises a relatively longer wavelength portion of incoming beam 110 as this is where both the majority of the solar energy is located and also where PV cells tend to be most efficient. Thus beam 112 exiting the interior surface of the window will tend to have a slightly cooler colour than beam 110.

[0061] Thus in broad terms providing a volume hologram with a suitable holographic grating can be fabricated red and infra-red components of sunlight can be exploited to generate significant PV power with very little effect on the overall function and appearance of the window. Thus, for example, the arrangement could be provided on a glass window of a south facing exterior wall of a building to receive sunlight which is at an angle of incidence which is a function of the latitude of the site. Red and infrared components of the sunlight are reflected from the holographic mirror grating at an obtuse angle down into the glass pane such that the light is totally internally reflected and arrives at the lower edge of the pane. At this lower edge light is incident upon the glass/air interface at a small angle (to the normal) and thus there is low internal reflection and around 95% of light can exit to fall upon the surface of the photovoltaic cell(s). In more detail sunlight incident at a range of angles around a central, design angle, and having a relatively narrow bandwidth around a particular wavelength, is directed along a first order diffraction direction of the volume hologram. This direction is the same direction as a ray would have if reflected by one of the fringes and they may therefore be considered as a reflection from the fringes of the volume hologram. Although we refer to a grating, in the embodiments we describe later the fringes do not form a simple grating and have a more complex structurebut it is helpful to consider this simplification for an initial understanding of the basic principles. The light propagating through the thickness of the glass pane is totally internally reflected at the front (exterior) surface of the window and at the interior surface to which the holographic film is attached with an index-matching UV-cured adhesive. The direction of diffracted (reflected) light is selected to achieve this total internal reflection, that is, so that if the does meet the interior surface of the glass pane it is reflected at the surface. The situation is slightly more complex for the red/infrared light grazing the interior surface of the window: in this case the volume hologram will not in general act as a mirror for such a ray (because of the angle of incidence) and instead the ray is totally internally reflected from the outer surface of the substrate of the volume hologram, for example at the film/air interface where holographic film is employed.

[0062] As will be appreciated from the forgoing discussion, the arrangement of FIG. 1a is a simplification of the system and we will describe further features of a practical arrangement later. However it is also useful for understanding the invention to describe some features of holographic recording media.

[0063] A typical hologram comprises a glass plate or film coated on the reverse side with a photosensitive recording layer. In the case of a volume hologram for embodiments of the invention the recording layer has a typical thickness of 4-20 microns (although it can be greater) and within this layer an interference pattern can be recorded which takes the form of a microstructure comprising modulation of the composition and hence refractive index of the layer. In a volume hologram the fringes defined by these modulations occupy the thickness of the layer rather than merely being defined as a surface pattern and thus volume (or thick) holograms typically have a thickness of at least 5 times, 7 times, or 10 times the wavelength, which may be the wavelength at which the hologram was recorded or a wavelength at which the hologram reflects. The presence of multiple fringes within the thickness of the hologram means that a volume hologram is relatively wavelength-specific; volume holograms can also provide a high diffraction efficiency, as previously described.

[0064] In the case of a silver halide recording medium such as Harman Holo FX from Harman Technologies Limited Mobberley, Cheshire UK, typically after exposing a high resolution recording plate or film to a standing wave of interference produced by coherent laser light the film is developed to create black silver metal. Typically this defines a network of ultrafine grains or filaments of silver defining granular planes of metallic silver resembling under a microscope the pages of a book. This provides an amplitude hologram which is inefficient as light is blocked and thus further chemical processing is employed to convert this to a phase hologram for example using a bleaching process. Thus a bleaching solution may be employed to convert the black silver metal in the (typically) gelatin emulsion layer back into silver bromide (refractive index 2.25 in red light). In general during this conversion process reagents may also be employed to encourage microscopic diffusion transfer of silver bromide into zones already rich in silver bromide. However, we control this process in moderation, since the existing of large crystals of silver bromide may be regarded as scattering centres, especially with respect to their interaction with light of shorter (blue/ultraviolet) wavelength. The overall chemical changes have the effect of both increasing the index modulation and rendering the film transparent to provide an efficient phase grating. Although very orderly planar fringes maybe be created, in embodiments of the invention the fringe patterns are more complex and are controlled to adapt to multiple angles of incidence to control the reflection to adapt the bandwidth and, potentially, even to include effects such as magnification. Apart from this flexibility one of the advantages of employing a volume hologram is that (for a narrow band of wavelengths) one can achieve very high diffraction efficiency corresponding, effectively to a reflectivity approaching 100%.

[0065] There are also processing techniques termed SHSG silver halide sensitized gelatin wherein the silver content is removed in its entirety and hardening techniques are used to preserve voids in the hardened gelatin, which provide sufficient index modulation in the layer to produce relatively high diffraction efficiency. The removal and recovery of the silver content has a cost saving and environmental advantages.

[0066] The skilled person will know that a volume hologram can also be fabricated in a photopolymer material, for example Bayfol HX, from Bayer Material Science, Chem Park, Leverkusen, Germany. Photopolymer volume recording materials are typically an order or magnitude (or more) less sensitive to light than silver halide recording materials but this can be compensated for by employing more powerful lasersfor example some embodiments of the invention described later employed a 660 nanometre diode pumped solid-state laser (a Flamenco laser from Colbolt Lasers, Sonia, Sweden). This wavelength broadly corresponds to the sensitive range of a silicon wafer photovoltaic cell, which is predominantly receptive to light in the longer wavelength part of the visible spectrum and is therefore convenient for embodiments of the invention. Photopolymers typically do not require chemical processing after exposure to laser light. Instead the holographic grating is formed in real time as a result of migration of species within the coated layer during the polymerisation process creating regions of relatively higher and lower density (refractive index); afterwards ultraviolet light is applied to cure the film and inhibit further monomer activity. Photopolymer material is also able to produce gratings with a diffraction efficiency close to 100% over a band of wavelengths. For both polymer and silver halide films the volume hologram itself is typically very low in colour content, scatter and optical density and thus in embodiments can appear almost invisible.

[0067] Referring now to FIG. 1b, this shows a more detailed version of FIG. 1a, in which like elements to those of FIG. 1a are indicated by like reference numerals. Thus an incident beam 110 from the sun 130 at angle to a normal to the window 102 is directed downwards through the thickness of window 102 at angle to a normal by hologram 106, as indicated by ray 116, towards PV element 120. The rays 110 from the sun are parallel, as are the diffracted rays 116 travelling within the thickness of the window. Depending upon how shallow an angle rays 116 make with a surface of window 102 (i.e. on how close angle is to 90 degrees), as well as on the distance of travel, a ray 116 may or may not totally internally reflect off a front (sun-facing) or rear surface of window 102. For a typical window height of order 1 metre it is useful but not essential that rays 116 are able to totally internally reflect off the internal front surface of window 102.

[0068] Hologram 106 is a volume hologram and may be considered to be a volume reflection hologram (although for reasons described later neither of the terms reflection hologram and transmission hologram is strictly appropriate). FIG. 1b illustrates the diffracted rays; preferably longer wavelengths are diffracted and shorter wavelengths are transmitted. Thus rays 116 may have a centre wavelength dependant on the fringe spacing of hologram 106, for example of around 600 nanometres. The position of sun 130 relative to the window movesthe sun moves in both elevation and azimuth. In a simple embodiment the diffraction of rays 116 at angle is optimised for a particular direction of the sun, for example the direction of the sun at noon. However in some preferred embodiments light is diffracted at substantially the same angle for a range of different solar elevation angles . Similarly in preferred embodiments rays 116 are directed in substantially the same direction, in particular vertically downwards, for a range of different solar azimuth angles , as schematically illustrated in FIG. 1c. If this were not done the solar energy would tend to accumulate in one or other lower corner of the window as schematically illustrated in FIG. 1d. The range of angles over which light is diffracted in substantially the same direction maybe a continuous range or a range of discrete angles as explained below.

[0069] In embodiments the volume hologram 106 on film or tile substrate at 104 is attached to window 102 by refractive index matching glue 118, as illustrated in FIG. 1e.

[0070] Referring now to FIG. 2, this shows an embodiment, of a first optical apparatus 200 which may be employed to record a volume hologram for the window assembly of FIG. 1. One difficulty with fabricating a volume hologram with fringes at the correct angles is that because the fringes lie at a relatively shallow angle to the surface of the hologram (they lie flat within the hologram) it is difficult to provide interfering laser beams at the correct angles because refraction at the boundary of the hologram limits the range of internal angles of propagation of the laser beams; even with a beam which has a grazing incidence on the front surface of the hologram the direction of travel of the beam within the hologram may not be sufficient to give a shallow enough angle to the fringes within the hologram. Thus in the arrangement of FIG. 2 the hologram is sandwiched between a pair of transparent, for example glass substrates or blocks 202, 204 optically coupled to the hologram with index matching fluid (not shown). One of the beams, for example beam 206, enters through the front face of one of the blocks/substrates; the other beam, for example beam 208, enters through the edge of the second glass block/substrate. This enables fringes to be formed at a very shallow angle to the surface of the hologram. In preferred embodiments the refractive indices of blocks 202, 204 are close that of the hologram 106 (which is generally provided on its substrate 104), for example matched to the holographic recording medium to within a refractive index value of better than 0.02. The particular angles of the laser beams are chosen so that after refraction by the glass blocks 202, 204 the beams are travelling in the right direction within the holographic recording medium 106 to generate fringes of the desired angle, in particular to direct rays 116 at the desired angle for the target solar elevation. The skilled person will appreciate that determining the angles of the rays within holographic recording medium 106 is a routine application of Snell's Law, and that the fringe direction in the mirror assembly of FIG. 1 is, in preferred embodiments, a direction in which the normal to the fringes bisects the angle between rays 110 and 116 (that is bisects +).

[0071] In one embodiment of apparatus for mass producing a volume hologram for the assembly of FIG. 1 an edge-illuminated glass block 204 is provided beneath a film transport system which receives illumination with coherent light from above, the system also including an exposure gate for the illumination. In embodiments a single laser with a split beam maybe employed, for example a 500 mw Flamenco laser operating at 659 nanometres as previously mentioned. Optionally a tuneable laser or multiple lasers of different wavelengths maybe employed to provide simultaneous or consecutive exposure to multiple different colours of interfering beams to increase the spectral bandwidth of the resulting holograph.

[0072] Referring to FIG. 3a, this shows an arrangement similar to that of FIG. 2 but in which one or both of beams 206 and 208 is slightly diverging rather than collimated. This results in fringes which are tilted differently at different lateral locations within the hologram. This enables the volume hologram/window assembly to operate effectively over a range of solar elevations. In the illustrated example the fringes 300 are tilted more towards the vertical (at a shallower angle to the surface of the hologram) at the bottom than at the top of the hologram (when installed) but this is not essential.

[0073] An alternative approach is shown, schematically, in FIG. 3b in which multiple exposures with collimated beams 206a, b at different angles are made to produce corresponding sets of fringes 302a, b at different angles within the hologram. FIG. 3c illustrates, schematically, how this can be achieved in a mass production system, in which a film or tile conveyer at 320 moves the holographic recording medium stepwise between positions 106a, b, c at which successive, spaciously overlapping exposures of the film are made. For example for a hologram with a repeat length of 1 metre (to match a target window size) exposures may be taken, say every 10 cm. This effectively angularly multiplexes the holograms stored within the film.

[0074] Such an approach may be employed to provide a volume hologram which is adapted to efficiently direct sunlight from a plurality of different (lateral azimuthal) angles onto a photovoltaic element. The skilled person will appreciate, however, that whether a range of azimuthal or elevation angles is covered is merely a matter of orientation of the fabricated volume hologram on the window.

[0075] Although we have described an example film publication system which employs a glass block to achieve the desired fringe angles within the hologram, we describe different approaches later, which employ a dimensional change of the hologram rather than a glass block to achieve the desired target fringe angles.

[0076] FIG. 3 illustrates an alternative approach which may be employed to fabricate a volume hologram with fringes at a range of different angles in order to deflect light from a range of vertical and/or lateral directions towards a PV element in the window assembly in FIG. 1. Thus in the approach of FIG. 3d a first master hologram, H0 is fabricated having a plurality of different regions 330a-e, for example a plurality of vertical stripes, so that within each region the fringes are substantially parallel to one or another but are at different angles from one region to another.

[0077] The H0 master may be fabricated as previously described. This H0 hologram is then illuminated by a further collimated light beam 332 to replay the stored holograms simultaneously to create a replayed wavefront 334 and a further beam 336 is then used to record the combination of holograms together in a second master hologram H1340. This second master hologram thus effectively comprises fringes suitable for directing light from a range of angles towards a PV element in the previously described system. Depending upon the direction from which light beam 336 impinges on hologram H1 the hologram may either be a transmission master (as illustrated) or a reflection master.

[0078] Referring to FIGS. 4a and 4b, these show examples of contact copying systems for 100a, b for copying the H1 master into a holographic recording medium 106 on film or a glass substrate. As illustrated, a transport mechanism, more particularly a hologram drive 402 may include a reservoir 404 of index matching fluid 406 to provide this to the interface between the copied master hologram and the holographic recording medium. (A similar approach may be employed with the previously described arrangement based on that illustrated in FIG. 2). The system of FIG. 4a shows a reflection master hologram 340a; out of FIG. 4b is suitable for a transmission master hologram 340b. In each case the master hologram is replayed to create a wave front which is copied by collimated beam 408 into the joining holographic recording medium 106, suitably index matched.

[0079] FIG. 4c illustrates an alternative drum-based hologram recording system 450 in which the holographic recording medium 106 on a film substrate is guided by a transport mechanism 452a,b around a rotating drum 454 where the hologram is recorded and embodiments the recorded film is then captured on a spall 456.

[0080] In one embodiment a pair of collimated laser beams 460a,b are overlapped in a region 462 of the recording medium 106 which is within a liquid bath 464 which serves the function of the glass block 204 in FIG. 2. In embodiments one of the beams, beam 460a, is projected into one end of the rotating drum 454, so that it is incident on the recording medium from within the drum. In embodiments this beam defines a plane within which the axis of rotation of the drum lies. Preferably this beam intersects the film at an acute or glancing angle. The second beam 460b may be arranged to intersect the holographic recording medium 106 at an appropriate angle within region 462 in order to achieve the target desired fringe orientation within the film layer.

[0081] In another approach the drum 454 may carry a reflection or transmission master hologram 340 as previously described which may be replayed and copied into the recording medium (in which case only a single laser beam is needed). In such an arrangement bath 464 may hold index matching fluid (which is preferable but not essential).

[0082] In still further embodiments, which may be combined with either of the previously described approaches, bath 464 may additionally or alternatively hold a liquid to swell or contract the holographic recording medium so that the spacing and rotation of the film fringes may afterwards be adjusted to a desired target angle by contracting/swelling the recorded hologram. This is described in more detail later.

[0083] Preferably in a drum-based hologram recording system as shown in FIG. 4c a relatively high powered laser such as a diode-pumped solid state laser is employed to facilitate rapid mass production of the recorded holographic material holographic. An approach which employs post-exposure fringe expansion is particularly advantageous for high speed mass production.

[0084] In a further mass production technique which is advantageous in embodiments for the production of simplistic single plane grating elements, FIG. 4d shows an alternative technique where direct exposure to the film is employed without index matching procedures. Here the laser beams of an appropriate wavelength are incident at equal angle in opposite directions upon either side of the film layer. The refraction at the film surface ensures that the grating produced has the correct orientation to produce the desired edge-directing hologram.

[0085] Referring now to FIG. 5a, this shows a further alternative approach which may be employed to fabricate a volume hologram able to direct incoming sunlight at a range of different solar elevation and/or azimuth angles in order to provide actinic light to the PV cell; a similar approach may be used for diffracting selected pairs or groups of wavelengths. In the approach of FIG. 5a a hologram 500 comprises a set of different layers 500a-d each of which is preferentially sensitive to a particular wavelength of light when recording the hologram. In this way different wavelengths of laser light used to record the hologram maybe employed to fabricate sets of fringes at different angles or frequencies within the thickness of the hologram. One advantage of such an approach is that fringes need not overlap within a single layer, which can result in improved diffraction efficiency. (This is particularly useful for a volume hologram as described earlier in which longer wavelengths are preferentially directed towards the PV element since the diffraction of longer wavelengths employs fringes with comparatively greater spacings than the diffraction of visible wavelengths, for example of order 500 nanometres) so that there are typically fewer fringes overall, which allows more precisely defined index modulation.

[0086] Suitable recording media are commercially available or maybe fabricated to order, for example by Harman Technologies Ltd. (Ilford Ltd); typically the different layers contain different spectral sensitizers. Additionally or alternatively such recording media may include one or more components in one or more of the layers which enable the layer thickness or density to be controlled in the chemical film processing subsequent to recording. The skilled person will recognise that photographic films are often coated in a plurality of layers, for example to achieve colour recording and we have previously described some particularly advantageous multilayer holographic recording media in US2011/0088050 (hereby incorporated by reference).

[0087] FIGS. 5b and 5c show, schematically, a first example multilayer volume hologram recording film 510 before and after recording of a hologram in the film. The film 510 comprises a substrate 512 and a pair of photosensitive layers 514, 516 both sensitive to red light (longer than the first threshold wavelength), but having different peak wavelength sensitivities within the red. Thus, for example, the surface layer 516 may be sensitive to wavelengths in the range 600 nanometres-700 nanometres, and the second layer 514 may be sensitive to wavelengths longer than 700 nanometres, for example comprising a dye or mixture of dyes of the type used in infrared photographic applications for the sensitisation of silver halide. Thus, for example, such a film may be exposed to a first standing wave (interference pattern) at a first wavelength, say 659 nanometres from a Cobolt Flamenco Laser, and a second standing wave (interference pattern) at, say, 1064 nanometres from a Cobolt Rumba Laser. As illustrated schematically in FIG. 5c the 2 layers record separate gratings of different spacing/angle which can thus separately diffract light. Such an approach may be used to increase the bandwidth over which the volume hologram operates and/or to direct light to waveguide within the window when incident upon the window at more than one angle of incidence.

[0088] FIGS. 5d and 5e illustrate an alternative approach using a multilayer holographic film which may be employed to achieve a similar result. Thus FIG. 5c shows holographic film 520 comprising a substrate 522 and two subsequent layers 524, 526 which, in this example, both contain the same spectral sensitizer (or correspondingly may both have the same peak spectral sensitivity). However one (or more) of the layers includes a material which may be employed to change a thickness of the layer when the film is developed, for example to reduce the thickness of in the layer in the developed and the dried film. Thus in one example one of layers 524, 526 may comprise gelatin, and the other gelatin in combination with a water soluble polymer (or other material which may dissolved during subsequent chemical processing). In the illustrated example layer 524 includes a soluble polymer so that as shown in FIG. 5e, after chemical processing and drying the thickness of layer 524 is reduced compared with that of layer 526 so that the microstructure of layer 524 has a relatively higher frequency of than of layer 526.

[0089] The skilled person will appreciate that the above described approach may readily be generalized to more than two layers.

[0090] Referring now to FIG. 6, this shows an example of a volume hologram 600 in which the fringes are chirped so that the hologram reflects light at an appropriate angle over a wider range of frequencies than would otherwise be the case (albeit at a slightly reduced level of efficiency). Thus hologram 600 comprises a substrate 602 baring a recorded hologram 604 in which the fringe microstructure shows a monotonic increase in the fringe spacing in moving from the front to the rear surface of the recording layer (as shown) or vice versa. This can be achieved by providing a gradual change in the thickness of the emulsion layer during chemical processing of the film; the end result is a chirped fringe frequency (by analogy with radar).

[0091] There are various techniques which can be employed to produce such chirping for example the film maybe processed prior to exposure or during or after the developing and bleaching so as to modulate the density of a gelatin layer so that this varies between the front and rear surfaces of the recording layer. For example, rapid processing with a relatively hot developer can act quickly on the surface without diffusing evenly into the depth of the layer as would normally be expected in typical processing technique for photography. This can result in a gradient of silver density in the layer which will then in turn produce a density/refractive index modulation within the layer during the bleaching stage, especially in the event that a solvent bleach is utilised for the purpose. In another approach a pre-swelling step with limited soaking time so as to affect the surface more than the depth of the material may also be employed. In general the forced removal of material(s) from the recording layer under non-equilibrium conditions (for example at excessive levels of activity) results in depth zones within the microstructure shrinking proportionately with respect to their proximity to the surface of the layer. The skilled person will recognise that there are other methods which may also be used to obtain, in effect, different degrees of shrinkage at different shrinkage at different depths within the emulsion layer.

[0092] The inventors have also recognised that related techniques may be employed to rotate fringes as well as to change fringe spacing for the volume hologram. This recognition is in part based on the observation that as a volume hologram dries in the laboratory there is a point at which the edge of the holographic plate frequently appears to light up. FIG. 7a illustrates what is believed to occur for certain fringe anglesinitially the volume hologram acts as a transmission hologram with fringes lying across the thickness of the film and, as the film dries, the holographic recording medium shrinks and the fringes rotate so that they eventually lie predominantly parallel to the surface of the hologram so that the hologram operates in reflection mode. Between the extremes the fringes pass through a rotation at which light is directed to propagate within the thickness of the film or plate substrate bearing the holographic recording medium. This principle is further illustrated in FIGS. 7b and 7c in which a film of thickness 2t at the time of recording shrinks to thickness t after recording, rotating a fringes so that incoming light is directed to propagate within the thickness of the holographic layer. This approach may be used to rotate the fringes in either direction (and to change our spacing)for example material may be added into the holographic recording medium and washed out (and hardened) after recording, or the holographic recording medium maybe subjected to a pre-swell treatment for example in a liquid bath, afterwards drying out; or material may be used to swell the recording medium after recording a hologram (subsequently hardening the swollen film). In one example material within the hologram recording layer is soluble in alkaline developer, thus allowing material to be removed from the recording layer so that the thickness of the recording layer is reduced upon drying after bleaching. Suitable film is available, for example Harman Technology Limited, UK. In another example a silver halide/gelatin emulsion layer is exposed whilst wet and hence substantially thicker than usual and post-drying shrinkage is reduced via for example, further post exposure expansion.

[0093] These techniques maybe applied in conjunction with or instead of any of those previously described. Broadly speaking they facilitate achieving fringe angles suitable for directing reflected light into the window glass at an angle in excess of the critical angle, to achieve total internal reflection within the window. The skilled person will recognise that expansion and/or contraction techniques to modify fringe spacing may be used in conjunction with various laser line wavelengths such as 514 nm, 532 nm, 561 nm, 594 nm, 639 nm, 659 nm, 694 nm, 1064 nm and so forth.

[0094] These techniques are also compatible with high speed mass production in particular, in embodiments a suitable volume hologram maybe fabricated as a transmission hologram with both interfering laser beams incident on the same side of the holographic recording medium. The transmission hologram may then be converted into a (window) edge-Illuminating hologram by shrinking the hologram post exposure. In embodiments such an approach provides further advantages in that the previously described index matching need not be employed. In embodiments, the recording medium need not necessarily be sensitive to infrared (increasing the available range of recording media and avoiding the difficulties of infra-red) and infrared lasers need not be employed to create the interference pattern (which reduces health and safety concerns).

[0095] An approach which writes a transmission hologram and then converts this to the desired window-edge Illuminating hologram can be employed with either a linear recording medium transport mechanism of the general type illustrated in FIGS. 4a and 4b or with a drum-based exposure system of the general type illustrated in FIG. 4c (but with the two beams incident present on the same, preferably outer surface of the drum. Again with such an approach there is no need for index matching fluid and the bath 464 is optional depending upon the approach used to shrink the film after exposure. FIG. 7d shows, in outline, a simplified hologram recording apparatus of this general type.

[0096] We now consider a geometrical approach to obtaining fringes at a desired target angle for the volume hologram in order to subject diffracted rays within a window on which a hologram is located to total internal reflection. The grating structure maybe positioned on either the outer surface of the window or the inner surface. In the former case the diffracted light passes through the grating before entering the window pane; in the latter case the diffracted light is reflected forwards into the glass. In both cases, however, the geometrical analysis is similar. Broadly speaking embodiments of a volume hologram to diffract light as desired provide an obtuse angle of diffraction, more particularly between 90 degrees and 135 degrees to a normal to the incident ray. Perhaps surprisingly, the configuration of the optical microstructure differs only slightly between these two apparent extremes.

[0097] FIG. 7e shows hologram recording apparatus for implementing a method in which dry film 106 is fed onto rotating drum 704. Index matching is facilitated by a carefully controlled capillary supply of, for example, a volatile solvent 700. This facilitates the entry of light into the recording medium at extremely acute (oblique) angles. Example volatile liquid which may be employed include, but are not limited to: ethanol, methanol, and iso-propyl alcohol (or other alcohol or polar solvent; or non-polar solvent). In embodiments the liquid may be introduced via a porous roller 701, which may be termed a doctor roller, preferably at a controlled rate. An optical element 702 is provided; this may be a lens, prism or the like, for example fabricated from glass. In embodiments liquid remains in the capillary space between the optical element 702 and surplus liquid is discharged by progress of the recording medium (film) through the apparatus; optionally it may be reclaimed after use. As the drum rotates the film is exposed in a region 462. In the illustrated example the laser beams 703 define planes which intersect and interfere along a line which runs generally parallel to the axis of rotation on the surface of the cylinder.

[0098] Referring now to FIG. 8a, this illustrates the determination of the angle of fringes 800 in a volume hologram 106 to achieve total internal reflection within a window pane for an example solar elevation. In London the sun's elevation at midday ranges between around 20 at the winter solstice and 60 at the summer solstice. For the sake of example we will consider a beam 802 from a solar elevation of 40 (the angle between ray 802 and the normal 804 to the surface of the hologram). It is desired, in this example to diffract ray 802 so that the rays 806 reflected from the fringes of the hologram are at an angle of 10 to the plane of the surface of the hologram as illustrated. Rays 806, comprise rays of a selected wavelength band or having a wavelength greater than a threshold wavelength; other light from beam 802 continues through the hologram and out as beam 808 to illuminate the far side of the window (in FIG. 8a the window pane is schematically illustrated by region 810).

[0099] Ray 802 is refracted to travel along an altered direction 802a within the hologram, in the illustrated example at an angle of 25.4 to normal 804. Line 812 defines a normal to fringe 800 and incoming ray 802a and reflected ray 806 make equal angles to this normal as illustrated each having an angle of 52.7 to normal 812. As can be seen from the figure, this in turn dictates that line 812, which defines a normal to the fringe, is at an angle of 27.3 to normal 814 to the surface of the hologram, and thus the fringes 800 themselves also have an angle of 27.3 to the plane of the hologram, that is to the film or tile surface. Thus when fabricating the volume hologram, for this example the fringes should be an angle of 27.3 with respect to the film surface. The skilled person will readily appreciate that the example given maybe modified for different solar elevations at different times/latitudes.

[0100] FIG. 8b illustrates an example target set of wavelengths for rays 806. Thus line 820 in FIG. 8b shows the solar spectrum and arrow 822 denotes a wavelength of 1100 nanometres which corresponds approximately to the 1.1 eV band gap of siliconthat is wavelengths shorter than 1100 nanometres can be converted to electricity by an inexpensive polycrystalline silicon solar photovoltaic cell. Line 824 notionally marks the start of the infrared region of the spectrum, here taken as light of a wavelength greater than 600 nanometres. Preferably, therefore, a volume hologram for the previously described window assembly has a fringe structure which is capable of Edge-Directing light of at least some wavelengths in the range 600 nanometres to 1100 nanometres although in this example there is no particular need to handle wavelengths greater than 1100 nanometres. The fringe structure described with reference to FIG. 8a could be fabricated using infrared film and interfering laser beams at appropriate angles, as previously described.

[0101] However in a preferred approach a transmission hologram is recorded using light of a shorter wavelength and then the fringes are rotated and to achieve an Edge-Directing fringe structure.

[0102] FIG. 9a shows the fabrication of a volume transmission hologram 900 comprising a layer of holographic recording medium 902 on a substrate 904. A pair of interfering laser beams at 906, 908, for example split from a single beam, are arranged to interfere over a region 910 of the recording medium 902, at an angle to one another to produce fringes with a spacing d. These are related to the wavelength by Bragg's Law:

/n=2d sin [0103] where [0104] =the wavelength of the laser light in air [0105] n=the average refractive index of the recording layer [0106] d=the fringe spacing [0107] =half the angle between the recording beams

[0108] For thin holograms the refractive index term is frequently overlooked since the interference occurs effectively in air where refractive index is unity. In this case, we specifically consider volume holograms, where index differential is significant, and which are produced in silver halide emulsions in either wet or dry condition. Bjelkhagen ISBN 3-540-58619-9 Silver Halide Recording materials estimates for Kodak and Agfa Holotest films, emulsion prior to exposure with refractive index of the order of 1.50-1.60 and aqueous-swollen emulsion of the order of 1.32.

[0109] In the final volume hologram the fringe spacing should be appropriate to reflect red and infrared lightfor example very roughly to reflect 800 nanometre light the fringe spacing should be approximately 0.4 m; for example two 659 nanometre laser beams with angle 2 between the beams ( is half the free space angle), incident onto film as shown in FIG. 9a, will produce fringes in silver halide film with spacings indicated by the table below for different angles :

TABLE-US-00001 10 20 30 d (nm) 1186 602 411
For two 1064 nanometre laser beams the corresponding table is:

TABLE-US-00002 10 20 30 d (nm) 1914 972 665
But for a laser of shorter wavelength such as 532 nm the fringe spacing is:

TABLE-US-00003 10 20 30 d (nm) 957 487 333

FIRST EXAMPLE

[0110] Consider, for the sake of example, using a 659 nm laser, selecting a relative angle (2) of 45 for the two beams, corresponding to a fringe spacing of 487 nanometres. Now, rather than locating the film plane normal to a line bisecting the angle between the interfering beams, the film is tilted with respect to the interfering beams as shown in FIG. 9b.

[0111] By way of example we will select a tilt angle of X degrees, which tilts the fringes shown in FIG. 9a away from the vertical direction 912 by the same X degrees (FIG. 9c). In the simple arrangement of FIG. 9a the angle of X degrees may be limited by Snell's Law, for example to 42 assuming a refractive index for the recording material of 1.50 (unexposed photopolymer may have a lower refractive index). After film shrinkage (as illustrated in FIG. 9d), the fringes are at a desired target angle for edge-directing use.

[0112] As can be seen from FIG. 9d, the effect of shrinkage of the thickness of the film is to rotate the fringes and to alter their spacing (although their frequency at the surface of the hologram does not change). The relationship between the tilt angle of X degrees and the target angle is thus given by straightforward trigonometryknowing distance I (FIG. 9c) and the final thickness of the filmthe tangent of the final fringe angle is the ratio of these two values.

[0113] In one illustrative example the film is tilted so that X=20 and the film shrinks from an original thickness of 8 m to 5.64 m (30% shrinkage is readily achievable in practice). Referring to FIG. 9d, the calculation is then as follows:

tan 20=I/8

Therefore

[0114]
I=2.91 m

[0115] In shrinking the frequency in the surface plane does not change (FIG. 9d) so the fringe angle and spacing (in a direction perpendicular to the fringes) will both change. Therefore a new fringe angle X is given by:

tan X=2.91/5.64

X=27.3

[0116] The original spacing of fringes with the example given above has d=487 nm

[0117] Therefore x.Math.cos 20=487 where x is the surface spacing (which stays constant)

x=518 nm

and

d.sub.new=518 cos 27.3

thus

d.sub.new=460 nm.

[0118] The ratio of the spacings, d/d.sub.new is given by the ratio of cos X/cos X. Thus in a similar manner an original fringe spacing of, say, 466 nm would be reduced to 439 nm. The 460 nm (or 439 nm) grating spacing could (with an appropriate angle of incidence) have a useful reflectivity for infrared light at 814 nm nanometres for total internal reflection in the window pane, well suited for generating electricity using a silicon PV cell.

[0119] In the example of FIG. 9d the fringes end up at an angle of 27.4 to the normal to the surface of the film. In this example the fringes are thus not tilted at a sufficiently shallow angle to the surface to the film to direct the light as shown in FIG. 8a, through the thickness of a relatively thin film. Nonetheless, depending upon the geometry of the application, the optical properties of the recording material, the thickness of the film/layer through which the light is directed, and upon how glancing an angle is needed for total internal reflection within the film/layer, this approach may be sufficient.

SECOND EXAMPLE

[0120] A second example is illustrated in FIG. 9e. In this example the fringes end up at an angle of around 27 to the surface of the film, as illustrated in FIG. 8a. In the example of FIG. 9e the beams are incident onto the film through a layer of liquid, as illustrated water, in contact with the film. The configuration of the tank which may be used to contain the liquid is arbitrary and may be designed to facilitate entry of light into the cell at a desired angle; or the water may be confined by capiliary action as previously described. In other approaches (for example as shown in FIG. 7e) a layer of solid (transparent) material such as glass may additionally or alternatively be employed, optionally with an index matching layer between the layer and the film. This allows the beams to enter the film at a shallower angle than would otherwise be the case; in the illustrated example one of the two beams enters from the normal position and the other enters from an angle of 50 in order to allow the resulting fringes to be formed at an angle which facilitates the ability for layer shrinkage to result in axial rotation of the microstructure.

[0121] This approach allows fringes to be formed with an initially shallower angle (to the surface of the film), and this can be further reduced by later shrinkage of the film. In the illustrated example the emulsion is initially swollen to 4 times its original thickness (4t), and afterwards shrunk back to its original thickness (t). This is readily achievable. Exposing the film through a liquid such as water facilitates such a procedure. This approach may be combined with that described previously with respect to FIGS. 4c and 7that is the film may be run over a drum located in a liquid bath to provide a substantially continuous recording process (with stepwise flash or continuous exposure to the laser beams). Preferably the film is given sufficient time in the liquid to reach an equilibrium swollen thickness; in the case of continuous process this may be achieved with sufficiently long previous swelling prior to the recording stage.

[0122] As previously described there are many ways in which an emulsion layer may be shrunk. For example water-soluble material may be added in to the emulsion layer when this is coated on to the substrate. Then significant quantities of this material will leave the layer during subsequent aqueous processing. Additionally or alternatively the use of a solvent bleach process can contribute to the reduction of the thickness of the layer of recording medium by removal of silver from the layer during processing. This latter approach has the additional advantage of reducing printout that is residual sensitivity of the processed film product to light in particular ultraviolet light.

[0123] FIG. 10a illustrates a solar voltaic system 1000 of the type previously described in combination with an energy storage system 1010 system such as a charger and rechargeable battery, charged by PV element 120 and providing electrical energy to an illumination source 1012 such as one or more light omitting diodes. As illustrated, sunlight is captured at the bottom of the window pane 102 and the light source 1012 Edge-Illuminates the hologram 106 from the top of the window assembly. In this way the system 1000 is able to collect light during the hours of daylight and to provide an illuminated holographic image at other times. Preferably hologram 106 is Edge-Illuminated by a substantially collimated light which, in embodiments, may be substantially monochromatic. One advantage of hologram 106 being configured to receive sunlight from a range of angles is that an additional hologram for display purposes encoded into volume hologram 106 is visible over a range of angles. The skilled person will appreciate that power for light source 1012 need not be provided by PV element 120, although this is convenient.

[0124] Commercial holograms may be produced by recording the interference between one specular laser beam, whose orderly component rays are predominantly parallel, together with a diffuse beam whose rays issue from a diffuse surface in randomised directions. In this case, the former beam may be referred to as the reference beam and one considers the holographic recording to result from its modulation. Such a diffuse hologram, which is capable of high diffraction efficiency, as well as being a useful medium for display technology is capable of acting, in its own right, as an efficient HOE, whose numerical aperture is helpful in the present system.

[0125] The system of FIG. 10a employs a volume hologram of the type previously described for directing light to propagate within the thickness of a window pane, but in addition there is an image recorded in the volume hologram, preferably a three dimensional image, for replay when the hologram is suitably illuminated. FIG. 10b illustrates one method for fabricating such a hologram: The arrangement of FIG. 3d may be adapted to include an image, for example a diffuser located on or adjacent the H0 hologram, which is then recorded into the H1 hologram.

[0126] The skilled person will recognise that there are many potential applications for such systems. Furthermore in embodiments the use of window pane 102 in the system 1000 of FIG. 10a is optionalfor example the hologram 106 (and its substrate) may itself direct sunlight towards PV element 120. Thus, for example, a film bearing the volume hologram could be used to provide signage, storing power from sunlight during the day and providing an illuminated screen at night. In one example application the rear or sides or windscreen of a container lorry could be provided with such signage. More generally one or more signals could be stored as images within the hologram, for example a red stop signal and/or orange turn signal which could then be lit by illuminating the hologram with light source 1012. In a little further application rather than reproducing an image such as a 3D image the hologram may instead be employed to produce specular or diffuse illumination of the interior or exterior region bounded by the window panel: in effect a window could be used as a source of light at night.

[0127] More generally, the sunlight itself may be employed to replay an image encoded in the volume hologram 106 even without Edge-Illumination 1012. This can be achieved by recording one or more images into the hologram rather than a simple grating structure; these one or more images maybe indexed by wavelength and/or angle. Further optionally where a plurality of different images is encoded dependent upon the innovation and/or azimuth angle of the sun, the position of the sun can be used to selectively display an image or image sequence. In this way a temporally animated image may be displayed, for example a display of local time based on the angular change in the direction of incident light on the surface of the hologram. This may be employed to provide an animated holographic image of a digital or analogue clock depicting the time based on the sun's position in the sky. Such an image may be a two dimensional or three dimensional image.

[0128] FIG. 11 illustrates a still further method encoding an image or other optical effect into the hologram: in this example the film or tile substrate 104 is modified to provide the image or optical effect without necessarily modifying the hologram 106. Thus, for example, a mirrored or frosted appearance may be provided on substrate 104, for example using a hard polyester substrate the surface of the substrate not bearing the hologram may be roughened to scatter light. More generally a toned, tinted, mirrored or frosted appearance may be provided by the substrate. This may be included as part of a window assembly either as a window pane or, for example, as part of a double glazing system.

[0129] It will be appreciated that there are many applications for this technology, including use in domestic, office or industrial buildings as well as, potentially, on vehicles. In principle embodiments of the techniques may also be employed on a window of a display, for example, of an electronic device. As previously described embodiments of the invention also have applications for signage and the like.

[0130] No doubt many other effective alternatives will occur to the skilled person and it will be understood that the invention is not limited to the described embodiments but encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.