GB2501754A

GB2501754A - Luminaire

Info

Publication number: GB2501754A
Application number: GB1207824.2A
Authority: GB
Inventors: Karen Nicola Harvey; Steven Lori Smith
Original assignee: De Montfort University; Harman Technology Ltd
Current assignee: De Montfort University; Harman Technology Ltd
Priority date: 2012-05-03
Filing date: 2012-05-03
Publication date: 2013-11-06
Also published as: GB201207824D0

Abstract

An edge lit lighting device is formed from a waveguide or sheet of transparent optical support 1 with one or more means of illumination 2 through the edge of the sheet 1, so the waveguide 1 is edge lit, and a multi layered hologram 3a,3b which is able to diffract one or more frequencies of illumination. The holographic element 3a, 3b may be reflective or transmissive. The holographic element 3a, 3b may be a volume hologram formed using silver halide. The edge of the waveguide may be chamfered or angled (fig 3),

Description

I

Luminaire

Field of the Invention

The present invention relates to lighting units comprising holographic elements, and in particular to edge lit lighting units comprising an LED light source and a holographic optical element.

Background of the Invention

Recent years have seen a considerable expansion and development in state-of-the-art LED (Light Emitting Diode) technology and its use for numerous lighting purposes.

White-light LED technology shows several advantages over conventional lighting systems 1 0 such as incandescent and fluorescent lamps, with the increased luminous efficiency (the amount of visible light produced per unit of electrical power input) being of particular importance. Further, LED lamps are compact, light in weight, mechanically robust, and offer long service life that is not affected by cycling the LED on arid off. Also, LED lamps contain no mercury.

On the other hand, the intense, directional, and concentrated characteristics of the light emission source show disadvantages for many general lighting applications, and in such instances it is common to use some means of diffusing or scattering the light. One means of achieving this is to provide a lighting device in the form of an edge-lit flat screen, in which the LED lamp or lamps are arranged to feed light into the edge of a flat substrate, and a means of causing the light to emerge over the flat surface of the screen is provided.

It is known that holographic optical elements may be used as a means of directing the light in displays. For instance, United States Patent 5,650,865 provides a holographic edge lit backlight for display panels comprising a cold cathode fluorescent tube for providing white light and a double layer reflection! transmission hologram. Similarly, PCT WO 01/46722 describes a holographic universal display that uses reflective lighting for a liquid-crystal panel and comprises at least two waveguide holograms. Further, PCT WO 2006/041278 relates to an edge-lit slab waveguide equipped with a slanted anisotropic holographic layer which couples out linearly polarised light.

LED lighting units comprising holographic optical elements are also known. For instance United States Patent 6,750,996 discloses an edge lit display device incorporating a waveguide and a volume hologram. The device is described for illumination of liquid crystal displays; the volume hologram may be an integral part of the substrate or may be laminated as a separate layer, and the light source may be a]ight emitting diode. Further, the waveguide may comprise two or more mutually separate holograms each laminated onto or formed integral with the waveguiding substrate.

In general, holographic optical elements comprising volume holograms for use in lighting devices have to be prepared by exposure of a photosensitive holographic material, such as for example a silver halide holographic material, using a laser. The inherent monochromatic characteristic of the laser emission during exposure means that the resulting optical element will replay at a single wavelength, and this is not of use for so-called white light lighting devices.

Silver halide holographic materials are known, and in general may be prepared using a very fine grain silver halide photographic emulsion in a protective colloid such as gelatin, coated on a suitable substrate. The material is then subjected to a holographic exposure using laser light to produce a series of interference fringes in the material that may be fixed in the material by a suitable photographic processing step. Silver halide holographic materials and mcthods for photographic processing of holograms are known and are described, for example, in the book Practical Holography by Graham Saxby, third edition published by Institute of Physics Publishing in 2004.

Colour silver halide holographic materials designed to provide holograms replaying at a plurality of wavelengths are known. For example, European Patent Application 0 428 332 A describes a holographic material which comprises coated on a photographic supporting base at least one silver halide emulsion which has a mean grain size of less than 0.1 iam and at least one emulsion of which has been optically sensitised with at least one speci fled green sensitising dye and at least one emulsion of which has been optically sensitised with at least one specified red sensitising dye. The material is designed for exposure by a plurality of lasers having different exposing wavelengths, in particular at least green and red lasers.

Further, colour silver halide holographic materials comprising more than one layer are also known. For instance, UK Patent Application GB 2474277 A discloses a material for recording a multicolour volume hologram, the material comprising: a carrier; a first photosensitive recording layer sensitive to one or both of red and green laser light; and a second photosensitive recording layer sensitive to blue laser light. 1-lowever use of this material in lighting devices is not disclosed.

Holographic elements for lighting devices may be produced from silver halide holographic materials by conventional holographic exposure with monochrome laser light and though such elements may realise high diffraction efficiency they will only replay at a single wavelength. However, it is found that when colour holographic materials such as those provided by EP 0 428 332 A are exposed with a plurality of lasers under similar conditions there is a detrimental effect on diffraction efficiency of the resulting optical elements. Therefore there is a requirement for an improved holographic material capable of providing holograms of good diffraction efficiency when exposed with a plurality of lasers.

Summary of the Invention

According to this invention there is provided an edge lit lighting device comprising a sheet of transparent optical support provided with one or more means of illumination through the edge of the sheet and a holographic element on at least one surface of the support capable of diffracting one or more frequencies of illumination wherein the holographic element comprises a plurality of layers.

Description of the Drawings

Figure 1 shows a schematic cross-section of one exemplary arrangement of the lighting device according to the invention operating in a reflective mode. In this figure, 1 represents the sheet of optical support, 2 represents the means of illumination through the edge of the sheet, 3a. 3b, etc. represent the holographic element comprising a plurality of layers provided on the surface of the support, 4a, 4b, etc. represent the path of light from the means of illumination, and S represents an optional element provided in the light path to direct the light rays into the sheet.

Figure 2 shows a schematic cross-section of an exemplary arrangement of the lighting device according to the invention operating in a transmissive mode. In this Figure, the same reference numbers as in Figure 1 are used to represent the same components. In Figures 1 and 2 the means of illumination is shown through only one edge of the sheet but may also be arranged through more than one edge of the sheet, Figure 3 shows a schematic cross-section of the exposing setup used for the example. In this Figure, 10 is an optical block, 11 is the holographic coating comprising a plurality of layers, 12 is the index-matching fluid holding the coating to the block, 13 is the reference exposing beam, 14 is the objective beam, iSa, 15b, etc. represent the interference fringes exposed in the emulsion, and 16 is the edge through which the reference beam enters the optical block. Separation between fringes 15a, 15b, etc. depends on the laser wavelength.

The objective beam and reference beam may be produced from a single laser by a beam splitting arrangement (not shown in the figure).

Figure 4 shows the results obtained from the example. In this Figure the measured light IS output is shown as a function of the wavelength: 20 is the plot of the light output emerging from a device according to the invention wherein the holographic element comprises two holographic layers, and 21 is the plot of the light output from the comparative device wherein the element comprises a single layer with mixed emulsions.

The greater peak heights from the inventive element show that the light output from this element is significantly better than that from the comparison.

Detailed Description of the Invention

The transparent optical support may be a sheet or panel of any suitable material such as glass, polycarbonate, polyacrylate, polyester, and the like. The sheet may be planar, but curved or shaped sheets may alternatively be used as long as they include at least one edge through which the panel can be internally illuminated.

The means of illumination may comprise any light source, but particularly comprises at least one Light Emitting Diode and especially comprises an array of Light Emitting Diodes arranged along the edge or edges being illuminated. The LED may emit coloured light or may be a so-called white LED emitting white light, but may also comprise an

S

array of separate LED elements emitting coloured light at different wavelengths, the number being arranged to give the overall visual impression of white light after passage through the device. Suitable LED lamps are known in the art. Alternatively, by choice of the appropriate LED and optical element the completed lighting device may also be arranged to provide coloured light. Further, in the case where the illuniination source is an array of LED elements emitting coloured light at different wavelengths, the individual elements of the array may be controlled separately to change the output colour of the device.

According to one arrangement of the lighting device as shown in Figure l,the light falling on the grating from within the transparent optical support is diffracted through the opposite side of the sheet to that carrying the grating as shown by rays 4a, 4b, etc. and the holographic element may be regarded as operating in a rcflective mode.

According to the alternative arrangement of the device shown in Figure 2, the light is diffracted out through the same side of the sheet as that carrying the holographic grating, and the holographic element may be regarded as operating in a transmissive mode.

The holographic element comprises a plurality of layers and may comprise any suitable holographic material known in the art, for example a silver halide holographic material, a photopolymer holographic material, or a diehromated gelatin holographic materiaL Typically the element comprises an appropriately constructed volume hologram providing a holographic grating such that internally incident light will replay at the appropriate angle. Preferably the holographic element comprising a plurality of layers is a volume hologram prepared using a silver halide holographic material.

For the purpose of the present invention, the silver halide holographic material is arranged to be sensitive to a variety of laser wavelengths, and in one especially preferred embodiment of the invention the different layers of the material are arranged to be sensitive to different wavelengths. The silver halide holographic material may be a two layer system comprising at least green and red sensitive layers, or may be a three layer system comprising blue, green, and red sensitive layers, or may also comprise further layers or layers of other sensitivity.

In general, the silver halide holographic material is inherently sensitive to blue light, that is to say light of wavelength less than about 500 nrn, and sensitivity to longer wavelengths may be achieved using one or more sensitising dyes as are known in the art. It is also possible to use blue sensitising dyes to increase sensitivity to blue light. In one specific embodiment, the silver halide of one of the layers is arranged to be sensitive to green light, that is to say light of wavelength in the region from about 500 nm to about 600 nm, and the silver halide of another of the layers is arranged to be sensitive to red light, that is to say light of wavelength greater than about 600 nm. by using different dyes in the two layers. The material may be arranged with either the green sensitive or the red sensitive layer uppermost.

In another specific embodiment comprising three layers, the silver halide of one of the layers is arraiged to be sensitive to blue light, that is to say light of wavelength less than about 500 nrn, the silver halide of another of the layers is arranged to be sensitive to green light, that is to say light of wavelength in the region from about 500 nm to about 600 nfl, and the silver halide of a third layer is arranged to be sensitive to red light, that is to say light of wavelength greater than about 600 nm, by using different dyes in the three layers.

The materiaL may be arranged with the three layers in any order; for example it may be coated with either the blue, green, or red sensitive layer uppermost.

Any suitable sensitising dyes known in the art may be used, but the green and red sensitising dyes disclosed in European Patent Application 0 428 332 A are particularly suitable for the green and red sensitive layers of a two or three layer system as hereinbefore described. A particularly suitable green sensitising dye is the dye shown as dye A in European Patent Application 0 428 332 A that has the formula: (cF-12)4So3Na This dye is hereinafter referred to as dye A. A particularly suitable red sensitising dye is the dye shown as dye B in European Patent Application 0428 332 A that has the formula: CH3 CH3 a 000H This dye is hereinafter referred to as dye B. Suitable silver halide holographic materials for the individual layers are known, and are generally made up of a holographic emulsion comprising silver halide particles in the size range below 1 pm suspended in gelatin. Further the material may comprise other components known in the art, for example sensitisisers, stabilisers, hardeners, surfactants.

and the like.

Means of exposing such holographic materials to a laser are known. In general, a silver halide holographic material may be exposed by taking the output from a suitable laser, separating it into two beams using a suitable beam splitting arrangement, and then arranging for the two beams to interfere in the holographic material to provide a diffraction grating. The attached Figure 3 shows one exemplary exposing arrangement for IS use when the holographic material is provided as a separate coating for subsequent attachment to the device, but others will be clear to those skilled in the art.

For the purpose of the present invention in which the silver halide holographic material is arranged to be sensitive to a variety of laser wavelengths, the material is exposed to a plurality of lasers having different wavelengths. In particular, for a material having one layer sensitive to green light and another layer sensitive to red light, the material is exposed to at least green laser light and red laser light.

Methods of processing the exposed silver halide holographic material to obtain the holographic element are known and are described, for example, in the book Practical Holography by Graham Saxby, third edition published by Institute of Physics Publishing in 2004. Any suitable processing method may be used in preparing the elements of the invention, but one specific method comprises a development stage using a photographic developing solution, and a bleaching stage using a rehalogenating bleach. Such processed holograms will replay at a similar wavelength to that of the original laser exposure.

The holographic grating formed in the element during exposure is composed of fringes of constructive and destructive interference between the object and reference laser beams.

Without wishing to be constrained by any particular theory, it is believed that during the development stage of this processing method the exposed silver halide particles in the constructive fringes are developed to tine particles of metallic silver, and that during the bleaching stage these particles are dissolved and rcprecipitated as silver halide on the nuclei of the remaining undeveloped silver halide emulsion particles in the unexposed fringes. The interference fringes forming the final processed holographic element arise from the refractive index difference between bleached fringes containing little or no residual silver, and rehalogenated fringes comprising silver halide particles.

The silver halide holographic material may be coated directly on the sheet of transparent optical support comprising the device of the invention, Alternatively and conveniently, the material may be coated on a suitable intermediate base, may be exposed and processed as hereinbefore described, and the intermediate base may then be fixed to the sheet of transparent optical support to form the device of the invention. Any base may be used, but a transparent flexible filmic base such as polyester is particularly suitable.

One advantage of the present invention is that the completed lighting device is almost clear, and therefore may be substituted for many applications wherein existing glazed panels are utilised such as, for example, window panes, mirrors, glass doors, a protective pane over a picture or photograph, or any other panel which is typically and conventionally made from a transparent material.

Further, the lighting device of the present invention may be arranged for illumination of displays incorporated in devices such as computer or television screens, mobile phones, laptops, tablets and the like. The display may be, for example, a liquid crystal display The device may also be configured as a conventional device for artificial illumination such as for example a wall or ceiling lighting panel, and may be used for any convenient use including residential, commercial, and industrial lighting applications. In this case the device may be backed by an opaque or diffusing surface, or by a reflective surface Alternatively, the device may be employed for vehicle lighting applications such as for example a car headlight or indicator light.

Further, the completed device may also carry a pattern or design. The paTtern may show information or data, such as for example a safety sign or exit sign, or may comprise an image, such as for example an advertisement or a display. The pattern may be arranged to appear only when the device is illuminated or also to be visible when the lighting device is not switched on. For certain signage applications, the background to the device may be darkened to give increased visibility and contrast to the image when illuminated.

The following example will serve to illustrate the invention.

EXAMPLE I

A gelatino silver iodobromide holographic emulsion having a mean particle size of approximately 20 -30 nm and comprising 96% silver bromide and 4% silver iodide was used. One portion of the emulsion was subjected to conventional sulphur and gotd sensitisation and stabilised using a 0.1% solution of 1-phenyl-5-mercapto tetrazole in dilute sodium carbonate. This portion of the emulsion was then sensitised to green light using a 1.8% solution of the green sensitising dye of formula A shown above to give a dye level of 1.08g/mole silver. A second portion of the emulsion was also sulphur and gold sensitised and stabilised as before, and was then sensitised to red light using a dispersion of the red sensitising dye of formula B shown above to give a dye level of 0,4gimole silver.

An experimental two-layer coating was prepared on a 63 ni thick potyester substrate base with both layers having the same silver coating weight; the lower layer comprised the green sensitised emulsion and the upper layer comprised the red sensitised emulsion and the total silver coating weight was measured at 4.63 g/m2. A second experimental two-layer coating was then prepared as before but wherein the lower layer comprised the red sensitised emulsion and the upper layer comprised the green sensitised ernulsion The total silver coating weight was measured at 4.63 g/m2. Finally equal proportions of the two emulsions were mixed and coated to give a comparison single-layer coating for which the total silver coating weight was measured at 4.62 g/m2.

The coatings were exposed using the optical setup shown in Figure 3, in which the coatings were reversibly attached to the surface of a glass optical block with an index-matching fluid. The optical block was fabricated from BK7 optical glass, annealed to remove any birefringence. The blocic was designed to have internal angle of incidence of the reference beam to the holographic material of 810, and the entrance edge shown as 16 in Figure 3 was cut and polished at an angle of 9° to provide a normal angle of incidence of the reference beam on entry to the block. The green laser used was a frequency-doubled Nd YAG continuous-wave laser emitting at 532 nm and the red laser was a continuous-wave 1-Ic-Ne laser emitting at 632 nm.

The exposed coatings were each processed at 21°C as follows: Distilled water pre-soak 1 minute Developer 30 seconds Dilute acetic acid stop bath 1 minute Bleach 5 minutes Water wash 5 minutes The developer made up by mixing equal volumes of the following solutions A and B immediately before use: Solution A Water 500 ml Metol 6 g Ascorbic Acid 40 g Water to make 1 litre Solution B Water 500 ml Sodium Carbonate anhyd 100 g Sodium Hydroxide 30 g Water to make 1 litre The bleach solution was made up as follows: Water 500 ml Ferric EDTA 40 g Potassium Bromide 60 g Acetic Acid 70 ml Water to make 1 litre The completed optical elements were then incorporated into a lighting device arranged as in Figure 1, illuminated with white light, and the output intensity at different wavelengths was then measured to determine the diffraction efficiency. Figure 4 shows the results plotted against wavelength, with line 20 a plot of the light output emerging from a device according to the invention wherein the holographic element comprises two holographic layers and line 21 representing the results from the comparative element. It will be seen that the light output from the inventive element is significantly better than that from the comparison, showing the improved diffraction cfficicricy of the inventive device and the usefulness of the invention.

Claims

C [aims 1. An edge lit lighting device comprising a sheet of transparent optical support provided with one or more means of illumination through the edge of the sheet and a holographic element capable of diffracting one or more frequencies of iHumination on at least one surface of the support wherein the holographic element comprises a plurality of layers.
2. A lighting device according to claim I wherein the holographic element comprises a volume hologram prepared using a silver halide holographic material.
3. A lighting device according to claim 2 wherein the silver halide holographic material comprises a plurality of layers having different spectral sensitivity.
4. A lighting device according to claim 3 wherein the silver halide holographic material is a two layer system comprising green and red sensitive layers.
5. A lighting device according to claim 3 wherein the silver halide holographic material is a three layer system comprising blue green and red sensitive layers.
6. A lighting device according to any of the preceding claims wherein the device operates in a reflective mode.
7. A lighting device according to any of claims 1 -5 wherein the device operates in a transinissive mode.
8. A lighting device according to any of the preceding claims wherein the means of illumination comprises a light emitting diode.
9. A method of making an edge lit lighting device comprising: * providing a light sensitive holographic material comprising a plurality of layers; * attaching the material to a optical block; * exposing and processing the material to provide a holographic element; * attaching the element to an optical support to provide a lighting device.
10. A method according to claim 9 wherein the holographic material comprises a silver halide holographic material,
11. A method according to claims 9 or 10 whcrein Ihe exposure is provided by interference between a reference laser beam and an objective laser beam.
12. A method according to any of claims 8 -10 wherein the entrance edge of the optical block is chamfered at an angle to provide a normal incidence of the exposing reference beam.12. Use of a holographic element comprising a plurality of layers in an edge lit lighting device.