WO2005121641A1

WO2005121641A1 - Illumination system

Info

Publication number: WO2005121641A1
Application number: PCT/IB2005/051803
Authority: WO
Inventors: Christoph G. A. Hoelen; Johannes P. M. Ansems; Lingli Wang; Rifat A. M. Hikmet
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-06-11
Filing date: 2005-06-02
Publication date: 2005-12-22
Also published as: TW200612157A

Abstract

An illumination system has a first transparent element (1) and a second transparent element (2). The first transparent element has a light-dispersing structure (7) for broadening an angular distribution of light propagating from the first to the second transparent element via the light-dispersing structure (7). The light-dispersing structure has a fixed refractive index n1. The second transparent element has a refractive index n2 being electrically adaptable. Preferably, the second transparent element is a liquid crystal layer. Preferably, the light-dispersing structure is in direct physical contact with the second transparent element. Preferably, the light-dispersing structure is a holographic diffuser, an array of micro-lenses or Fresnel-lenses.

Description

Illumination system

The invention relates to an illumination system comprising a first transparent element and a second transparent element, while, in operation, light propagates from the first transparent element towards the second transparent element. Such illumination systems are known per se. They are used, inter alia, as backlighting of (image) display devices, for example for television receivers and monitors. Such illumination systems can particularly suitably be used as a backlight for non-emissive displays, such as liquid crystal display devices, also referred to as LCD panels, which are used in (portable) computers or (cordless) telephones. In addition, such illumination systems are used for general lighting purposes, such as spot lights, flood lights and for large-area direct- view light emitting panels such as applied, for instance, in signage, contour lighting, and billboards. Generally, such illumination systems comprise a multiplicity of light sources, for instance light-emitting diodes (LEDs). LEDs can be light sources of distinct primary colors, such as, for example the well-known red (R), green (G), or blue (B) light emitters. In addition, the light emitter can have, for example, amber or cyan as primary color. These primary colors may be either generated directly by the light-emitting-diode chip, or may be generated by a phosphor upon irradiance with light from the light-emitting-diode chip. In the latter case, also mixed colors or white light is possible as one of the primary colors. Generally, the light emitted by the light sources is mixed in the transparent element(s) to obtain a uniform distribution of the light while eliminating the correlation of the light emitted by the illumination system to a specific light source. In addition, it is known to employ a controller with a sensor and some feedback algorithm in order to obtain high color accuracy.

US Patent Application US-A 2003/0 193 807 discloses a LED-based elevated omni-directional airfield light. The known illumination system comprises a LED light source, a light transformer, a hemispherical optical window, a circuit and a base. The light transformer includes a truncated hollow conical reflector, a curved reflective surface, and an optical element. A light shaping diffuser, particularly a holographic diffuser, may be used as dispersing optical element. The conical reflector has a truncated end facing the light source and a cone base opposite the truncated end. The conical reflector axis is coincident with a light source axis, and light passes through an opening on the truncated end. The curved reflective surface is between the truncated end and the cone base. The surface reflects light from the light source in a limited angle omni-directional pattern with a pre-determined intensity distribution. The optical element is adjacent the cone base in a plane perpendicular to the conical reflector axis, and disperses the light passing through the truncated hollow cone reflector. A drawback of the known illumination system is that the beam pattern emitted by the illumination system can not be changed.

The invention has for its object to eliminate the above disadvantage wholly or partly. According to the invention, this object is achieved by an illumination system comprising: a first transparent element and a second transparent element; the first transparent element comprising a light-dispersing structure for broadening an angular distribution of light propagating from the first transparent element to the second transparent element via the light-dispersing structure; the light-dispersing structure having a fixed refractive index n^ the second transparent element having a refractive index n₂ being electrically adaptable. A light beam traveling from the first transparent element towards the second transparent element experiences a change in refractive index at an interface between the first and the second transparent element. By electrically changing the refractive index n₂ of the second transparent element, the difference in refractive index between the light-dispersing structure of the first transparent element and the second transparent element is changed. The difference in refractive index between the first and the second transparent element influences the angular distribution of the light propagating from the first to the second transparent element. The width of the light beam emitted by the illumination system can be varied by changing the difference in refractive indices between the light-dispersing structure of the first transparent element and the second transparent element. By way of example, if a collimated light beam travels through the first transparent element via the light-dispersing structure towards the second transparent element, and the refractive index n₂ of the second transparent element would be electrically adapted to be the same as the refractive index ni of the light- dispersing structure of the first transparent element, i.e. n₂=nι, then the shape of the light beam would have the same shape in the second transparent element, i.e. a coUimated light beam would propagate through the first and the second transparent element. By way of a further example, if a coUimated light beam travels trough the first transparent element via the light-dispersing structure towards the second transparent element, and the refractive index n₂ of the second transparent element would be electrically adapted to be larger than the refractive index ni of the first transparent element, i.e. n₂>nι, then, according to SnelPs law, the shape of the light beam would have a divergent character in the second transparent element, i.e. a coUimated beam would propagate through the first transparent element with the light-dispersing structure and a widening beam would propagate through the second transparent element. The measure according to the invention allows the angular distribution of the light beam emitted by the illumination system to be influenced by electrically adapting the refractive index of the second transparent element. In particular, it is possible to switch electrically between various angles of the light beam emitted by the illumination system. By suitably adapting the difference between the refractive index of the light-dispersing structure and the second transparent element, the shape of a light beam emitted by the illumination system can be changed from, for instance, a "spot" light beam with a relatively narrow angular distribution to a "flood" light beam with a relatively broad angular distribution. A further advantage of the illumination system according to the invention is that the change in the beam pattern can be done (quasi) continuously. In addition, the shape of the light beam and/or the beam pattern of the illumination system can be adjusted dynamically. A preferred embodiment of the illumination system according to the invention is characterized in that the second transparent element comprises a liquid crystal layer. The orientation of a liquid crystal layer can be changed by applying a voltage across the liquid crystal layer resulting in a change in the effective refractive index experienced by a light beam traveling through such a layer. Preferably, two transparent electrodes are applied to enable the application of a voltage across the liquid crystal layer. Preferably, the electrodes are provided at opposite sites of a stack forming the liquid crystal layer. Preferably, the liquid crystal layer is in direct physical contact with the light-dispersing structure. Preferably, the electric field is applied across the stack of liquid crystal and the light dispersing structure. Preferably, the transparent electrodes are made of indium tin oxide (ITO). By changing the orientation of the liquid crystal layer under the influence of the applied electrical field, the effective difference in refractive index between the first and the second transparent elements is changed, by which the angles over which the distribution of light is scattered, are changed correspondingly. A very suitable liquid crystal layer is a so-called uniaxially oriented nematic liquid crystal layer. A liquid crystal layer for use in the illumination system according to the invention can be made as follows. Substrates are covered by the transparent electrodes. Preferably, the first transparent element with a light dispersing property is located on top of one of the electrodes. Various techniques can be used for producing the transparent element. Preferably, ultra-violet (UV) replication techniques are employed using reactive molecules. In this method a layer of reactive molecules in the liquid form are brought between a surface of the electrode and a substrate containing a structure to be replicated. Upon exposing the monomer to ultra-violet light, the material solidifies forming the first transparent element. In order to induce such configurations with a single domain preferably uniaxial orientation of the molecules on the interfaces is induced with a tilt with respect to the surface plane. Such a uniaxial planar tilted orientation in liquid crystals can be induced easily by first applying a thin layer of polymer and rubbing it uniaxially with for example a velvet cloth. In addition, a surface of the first transparent element is provided with a further orientation layer. A cell is constructed using a transparent electrode covered substrate with an orientation layer. Nematic. liquid crystal brought into such a cell becomes macroscopically oriented. Such a liquid crystal material can have uniaxial or twisted configurations as is well-known for persons skilled in the art of liquid crystals. In order to bring a twist into the liquid crystal, it is preferably doped with a suitable chiral dopant and the rubbing direction of the surfaces is adjusted accordingly. In such an orientation state, preferably, a liquid crystal is used with a positive dielectric anisotropy wherein application of electric field across such a layer change the orientation of the molecules from being perpendicular to the applied field to become oriented in the direction of the electric field. In a similar manner, it is possible to use a surface treatment such as surfactant molecules for orienting the liquid crystal molecules perpendicular to the surfaces. In that case, preferably, liquid crystal molecules are used with a negative dielectric anisotropy wherein under the influence of the applied field the liquid crystal molecules change their orientation from being aligned in the direction of the electric field to become oriented perpendicular to the applied field. It is known that a uniaxially oriented nematic liquid crystal layer has two refractive indices, indicated by n_o (ordinary) and n_e (extraordinary) reflecting the sensitivity with respect to the polarization direction of light. The extraordinary refractive index n_e of such a uniaxially oriented nematic layer is the refractive index experienced by light polarized in the direction parallel to the long axis of the liquid crystal molecules. The ordinary refractive index n_o is observed for lateral polarization directions. The sensitivity with respect to the polarization direction of light implies that upon applying an electric field in a direction perpendicular to the orientation direction of the liquid crystal layer with a positive dielectric anisotropy for light traveling in the direction of the applied field, light polarized in the orientation direction of the liquid crystal molecules the effective extraordinary refractive index shows a change while the effective ordinary refractive index for light polarized in the direction perpendicular to the orientation direction of the liquid crystal molecules remains unchanged. If such a uniaxially oriented liquid crystal layer is in direct contact with a transparent element with a light dispersing structure having an isotropic refractive index ni which is the same as the ordinary refractive index n₀ of the liquid crystal, only the light polarized in the orientation direction of the liquid crystal molecules becomes affected. The other polarization direction of light propagates without being influenced by the structure. There are various ways to compensate for this effect of polarization. A first manner is to use two cells each containing a light-dispersing structure and a liquid crystal layer. To this end, a first preferred embodiment of the illumination system according to the invention is characterized in that it contains two cells in optical contact with each other and each cell being provided with a second transparent element which is in optical contact the first transparent element provided with a light-dispersing structure. In this embodiment, the cells are positioned so that the liquid crystal layers are oriented such with respect to each other that the optical characteristics are perpendicular to each other. In this manner, upon electrically switching each of the liquid crystal layers, a change is effected in the refractive index for all polarization directions. A variation in this embodiment of the illumination system according to the invention is characterized in that between the first and second transparent elements with a light-dispersing structure an optical rotator is positioned causing a rotation of ninety degrees for plane polarized light. In the latter embodiment, the liquid crystal layers are oriented such with respect to each other that the optical characteristics are parallel to each other. In a very favorable, alternative way of compensating for the polarization effects of the liquid crystal layer two anisotropic layers with a light-dispersing structure are provided at opposite sites of the liquid crystal layer. To this end a first preferred embodiment of the illumination system according to the invention is characterized in that the light- dispersing structure of the first transparent element comprises a first anisotropic layer, and a surface of the second transparent element facing the first transparent element is provided with a further light-dispersing structure comprising a second anisotropic layer, the anisotropic characteristics of the first and second anisotropic layer being oriented perpendicular with respect to each other. In this embodiment the liquid crystal is in a configuration where the molecules rotate ninety degrees from one light-dispersing structure to the other. In addition, in this configuration, the refractive indices of the anisotropic light dispersing structure are the same as that of the liquid crystal. In this favorable embodiment in the field- off state, the refractive indices are perfectly matched and the cell functions as a so-called twisted nematic cell, therefore the cell is transparent. Upon applying an electric field, the twisted nematic structure disappears and the liquid crystal molecules become aligned in the direction of the electric field whereby one of the polarization directions is influenced at one of the anisotropic light-dispersing element while the other polarization direction is influenced at the other light-dispersing element. There are many embodiments to realize the light-dispersing structure.

According to a preferred embodiment of the illumination system the light-dispersing structure comprises an array of micro-lenses or Fresnel-lenses, or, more general, the surface texture is causing a change in the beam shape in response to a difference in the refractive index between the materials forming the textured interface. According to an alternative, preferred embodiment of the illumination system the light-dispersing structure comprises a holographic diffuser. Preferably, the holographic diffuser is a randomized holographic diffuser. The primary effect is a change in the beam shape. A secondary effect of the holographic diffuser is that a uniform spatial and angular color and light distribution is obtained. By the nature of the holographic diffuser, the dimensions of the holographic diffuser, or beam shaper, are so small that no details are projected on a target, thus resulting in a spatially and/or angularly smoothly varying, homogeneous beam pattern.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings: Figure 1A is a cross-sectional view of a first embodiment of the illumination system according to the invention; Figure IB is a cross-sectional view of a second embodiment of the illumination system according to the invention; Figures 1C and ID are two examples of the angular distribution of the light emitted by the illumination system according to the invention; Figure 2 is a cross-sectional view of a third embodiment of the illumination system according to the invention; Figure 3 is a cross-sectional view of a fourth embodiment of the illumination system according to the invention; Figure 4 is a cross-sectional view of a detail of an embodiment of the illumination system with two anisotropic layers and a light-dispersing structure; Figure 5 is a cross-sectional view of a detail of an embodiment of the illumination system with a replica isotropic layer, and Figure 6 is a plot showing intensity as a function of angle at various voltages applied across a cell. The Figures are purely diagrammatic and not drawn to scale. Notably, some dimensions are shown in a strongly exaggerated form for the sake of clarity. Similar components in the Figures are denoted as much as possible by the same reference numerals.

Figure 1 A very schematically shows a cross-sectional view of a first embodiment of the illumination system according to the invention. The illumination system comprises a plurality of light sources, for instance a plurality of light-emitting diodes (LEDs). LEDs can be light sources of distinct primary colors, such as in the example of Figure 1A, the well-known red R, green G, or blue B light emitters. Alternatively, the light emitter can have, for example, amber or cyan as primary color. The primary colors may be either generated directly by the light-emitting-diode chip, or may be generated by a phosphor upon irradiance with light from the light-emitting-diode chip. In the latter case, also mixed colors or white light is possible as one of the primary colors. In the example of Figure 1A, the LEDs R, G, B are mounted on a (metal-core) printed circuit board 4. In general, the LEDs have a relatively high source brightness. Preferably, each of the LEDs has a radiant power output of at least 100 mW when driven at nominal power. LEDs having such a high output are also referred to as LED power packages. The use of such high-efficiency, high-output LEDs has the specific advantage that, at a desired, comparatively high light output, the number of LEDs may be comparatively small. This has a positive effect on the compactness and the efficiency of the illumination system to be manufactured. If LED power packages are mounted on such a (metal-core) printed circuit board 4, the heat generated by the LEDs can be readily dissipated by heat conduction via the PCB. In a favorable embodiment of the illumination system, the (metal-core) printed circuit board 4 is in contact with the housing 3 of the illumination system via a heat-conducting connection. Preferably, so-called naked-power LED chips are mounted on a substrate, such as for instance an insulated metal substrate, a silicon substrate, a ceramic or a composite substrate. The substrate provides electrical connection to the chip as well as acts a good heat transfer to a heat exchanger. The embodiment of the illumination system as shown in Figure 1A comprises a first transparent element 1 functioning as a light mixing chamber filled with air with a refractive index of 1. Side walls of the first transparent element 1 are coated with a (diffuse) reflector 21 or are provided with a (diffuse) reflective coating. The light emitted by the LEDs R, G, B is mixed in the first transparent element 1 resulting in a substantially uniform distribution of the light while eliminating the correlation of the light emitted by the illumination system to a specific LED R, G, B. The first transparent element 1 is coupled to a second transparent element 2. The first transparent element 1 comprises a light-dispersing structure 7 for broadening an angular distribution of light (also see Figure 1C and ID) propagating from the first transparent element 1 towards the second transparent element 2 via the light-dispersing structure 7. In the example of Figure 1 A, the light-dispersing structure 7 is a holographic diffuser. Diffusers using holographic means are called holographic diffusers. A holographic diffuser can have a very specific light-shaping function. Holographic diffusers shape light by controlling the energy distribution along the horizontal and vertical axes. Holographic diffusers increase the brightness of any traditional light source and greatly enhance the brightness and contrast of optical images. In addition, holographic diffusers have light- mixing properties. The second transparent element 2 has a refractive index n₂ which is electrically adjustable. In the example of Figure 1A, the second transparent element 2 is a liquid crystal layer. The liquid crystal layer is powered by a applying a voltage V across the layer. By changing the LC orientation, the effective extraordinary refractive index n_e (extraordinary) of the second transparent element 2 is changed. Changing n_e causes a change in the effective refractive index difference at the interface between the light-dispersing structure on the surface of the first transparent element and the second transparent element and, accordingly, influences the angular distribution of the light propagating from the first to the second transparent element 1, 2. The effect of altering the effective refractive index difference between the light-dispersing structure 7 of the first transparent element 1 and the second transparent element 2 is used to vary the shape of the light beam emitted by the illumination system. By electrically adapting the angular distribution of the light beam emitted by the illumination system, it is possible to switch electrically between various angles of the light beam emitted by the illumination system (also see Figures 1C and ID). For instance, a "spot" light beam with an angular distribution of approximately 10° Full Width at Half Maximum (FWHM) can be converted into, for instance, a "flood" light beam with an angular distribution of approximately 30°. The change in the beam pattern can be done (quasi) continuously. In addition, the shape of the light beam and/or the beam pattern of the illumination system can be adjusted dynamically. Figure IB very schematically shows a cross-sectional view of a second embodiment of the illumination system according to the invention. In the embodiment of Figure IB, the first transparent element is not an empty cavity as in Figure 1 A, but comprises a body of a dielectric material. Suitable materials are e.g. PMMA, PC, glass, epoxy, silicon gel, ceramic or combinations thereof. In stead of three LEDs with the primary colors red, green and blue, in the embodiment of Figure IB, three white LEDs Wi, W , W₃ are mounted on the printed circuit board 4. In the example of Figure IB, the three white LEDs Wi, W₂, W₃ are embedded in a body of a dielectric material. The dielectric material has a collimating effect on the light emitted by the LEDs R, G, B. Depending on the height of the first transparent element 1 filled with dielectric material, a substantially coUimated beam arrives at the surface 11 of the first transparent element 1 facing the second transparent element 2. Due to the presence of the dielectric material, providing a reflector on the walls of the first transparent element 1 can be avoided. Preferably, the light-dispersing structure 7 of the first transparent element 1 and the second transparent element 2 are in optical contact coated with each other. Preferably, an index-matching material is employed promoting the optical contact. Figure 1C and ID schematically shows two examples of the angular distributions of the light. The example of Figure 1C shows a "spot" light beam with a relatively narrow angular distribution. This narrow angular distribution is achieved by starting with a substantially coUimated beam at the surface the surface 11 of the first transparent element 1 and having a relatively small gradient between the refractive index ni of the first transparent element 1 and the refractive index n₂ of the second transparent element 2. The example of Figure ID shows a "flood" light beam with a relatively broad angular distribution. This broad angular distribution is achieved by starting with a substantially coUimated beam at the surface the surface 11 of the first transparent element 1 and having a relatively large gradient between the refractive index ni of the light-dispersing structure of first transparent element 1 and the refractive index n₂ of the second transparent element 2. By suitably adapting the difference between the refractive index of the light-dispersing structures of the first transparent element 1 and the second transparent element 2, the shape of a light beam emitted by the illumination system can be changed dynamically and continuously. Figure 2 very schematically shows a cross-sectional view of a third embodiment of the illumination system according to the invention. Similar to Figure IB, the first transparent element 1 comprises a dielectric material. The light-dispersing structure 7 in Figure 2 comprises an array of micro-lenses or a Fresnel-lens. The effect of the array of micro-lenses or the Fresnel lens is that a uniform spatial and angular color and light distribution is obtained. Alternatively, an array of micro-prisms is employed. Figure 3 very schematically shows a cross-sectional view of a fourth embodiment of the illumination system according to the invention. In Figure 3, light emitted by the LEDs R, G, B is mixed in a dielectric optical collimator. The combination of the light- dispersing structure 7 and the second transparent element 2 is placed at a distance from the dielectric optical collimator with the LEDs R, G, B. The light-dispersing structure 7 can either be a holographic diffuser as shown in Figure 1A and Figure IB or an array of micro- lenses or a Fresnel-lens. Preferably, a wall 35 of the illumination system is coated with a (diffuse) reflector or is provided with a (diffuse) reflective coating. The illumination system of Figure 3 is provided with a sensor 25 for measuring one or more optical properties of the light which, in operation, is emitted by the LEDs R, G, B. Preferably, such a sensor 25 is coupled to control electronics (not shown in Figure 3) for suitably adapting the luminous flux of the LEDs R, G, B. By means of the sensor 25 and the control electronics, a feedback mechanism can be formed which is used to enhance the quality and the quantity of the light emitted by the illumination system. In an alternative embodiment, the illumination system may be provided with a temperature sensor (not shown in Figure 3) for measuring the temperature of the LEDs. By means of the temperature sensor and the control electronics, a feedback and/or feed forward mechanism can be formed to enhance the quality and the quantity of the light emitted by the illumination system. Figure 4 very schematically shows a cross-sectional view of a detail of an embodiment of the illumination system with a liquid crystal sandwiched between two anisotropic light-dispersing structures 13 and 23. In the example of Figure 4, two transparent electrodes 14, 24 for are shown applying a voltage across second transparent element 2. The horizontal lines in the second transparent element 2 indicate the orientation of the liquid crystal layer. Preferably, the transparent electrodes 14, 24 comprise indium tin oxide. In the example of Figure 4, the light-dispersing structure comprises a first anisotropic layer 13. In addition, a surface 12 of the second transparent element 2 in optical contact with the light- dispersing structure is provided with a further light-dispersing structure comprising a second anisotropic layer 23. The anisotropic characteristics of the first and second anisotropic layer 13, 23 are oriented perpendicular with respect to each other. Preferably, anisotropic light- dispersing structures are being made using so called liquid crystal gels. Such gels are produced using a liquid mixture of non reactive and reactive molecules. Such a liquid crystal mixture with a reactive component can be placed on top of a substrate with a transparent electrode and an orientation layer. As a next step, a mould with a light dispersing structure and orientation layer is placed on top of the mixture and polymerization is initiated using ultra-violet (UV) light. After removing the mould an anisotropic layer with a light dispersing structure on top of a glass substrate with a transparent electrode is obtained. Figure 5 very schematically shows a cross-sectional view of a detail of an embodiment of the illumination system comprising a switchable cell and a light-dispersing structure. In the example of Figure 5, two transparent electrodes 14, 24 for are shown applying a voltage across second transparent element 2. Broken lines in the second transparent element 2 indicate the orientation of the liquid crystal layer. Preferably, the transparent electrodes 14, 24 comprise indium tin oxide (ITO). In the example of Figure 5, the structure 7 has light-dispersing properties. Preferably, the light-dispersing structure 7 is made using acrylate replication technique. Acrylate monomer is placed on top of a substrate 42 with a transparent electrode 14. A mould with a light dispersing structure and is then placed on top of the monomer and polymerization is initiated using ultra-violet (UV) light. After the polymerization of the monomer, the mould is removed leaving behind a solid layer with a light-dispersing property. Subsequently an orientation layer 41 is provided by applying a layer of polyimide on top of the surface of the light-dispersing structure 7 and rubbing it uniaxially. A second substrate 43 provided with transparent electrodes and an orientation layer is used for producing a cell while using spacers to produce a gap between the two surfaces covered with the orientation layers. The sell is subsequently filled with a nematic liquid crystal which is oriented uniaxially under the influence of the orientation layers. Two of such cells are optically coupled whereby the orientations of the liquid crystal molecules in two different cells are oriented orthogonal with respect to each other. Preferably, the refractive index ni of the layer with a light-dispersing structure 7 is chosen to be the same as the ordinary refractive index n₀ of the liquid crystal. Preferably, the extraordinary refractive index n. of the liquid crystal has a higher value than n₀ and the cell showing light scattering in the field of state. Figure 6 shows the light intensity I (in cd/m²) as a function of angle θ (in °) for a coUimated beam of light at various voltages applied across a cell. It can be seen that with increasing voltage the intensity at half-width decreases. Not wishing to be held to any particular theory, it is believed that this effect is due to the fact that at high voltages molecules tend to become oriented in the direction of the applied field and the effective refractive index for the extraordinary ray tends to decrease to become the same as ordinary refractive index of the liquid crystal. In this manner, light travelling in the direction of the applied field at high voltages does not experience any change in refractive index at the interface. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:

1. An illumination system comprising: a first transparent element (1) and a second transparent element (2); the first transparent element (1) comprising a light-dispersing structure (7) for broadening an angular distribution of light propagating from the first transparent element (1) to the second transparent element (2) via the light-dispersing structure (7); the light-dispersing structure (7) having a fixed refractive index nι_; the second transparent element (2) having a refractive index n₂ being electrically adaptable.

2. An illumination system as claimed in claim 1, wherein the second transparent element (2) comprises a liquid crystal layer.

3. An illumination system as claimed in claim 1 or 2, wherein the light- dispersing structure (7) of the first transparent element (1) is in direct physical contact with the second transparent element (2).

4. An illumination system as claimed in claim 2, wherein the light-dispersing structure (7) of the first transparent element (1) comprises a first anisotropic layer (13), and a surface (12) of the second transparent element (2) facing the light-dispersing structure (7) of the first transparent element (1) is provided with a further light-dispersing structure comprising a second anisotropic layer (23); the anisotropic characteristics of the first and second anisotropic layer (13, 23) being oriented perpendicular with respect to each other.

5. An illumination system as claimed in claim 2, wherein the refractive index n₂ of the second transparent element (2) is changed by applying a voltage across the liquid crystal layer via two transparent electrodes (14, 24).

6. An illumination system as claimed in claim 4, wherein the transparent electrodes (14, 24) comprise indium tin oxide.

7. An illumination system as claimed in claim 2, wherein a surface (12) of the second transparent element (2) facing the first transparent element (1) is provided with a further light-dispersing structure and the second transparent element (2) comprises a further liquid crystal layer.

8. An illumination system as claimed in claim 1 or 2, wherein the light- dispersing structure (7) comprises an array of micro-lenses or Fresnel- lenses.

9. An illumination system as claimed in claim 1 or 2, wherein the light- dispersing structure (7) comprises a holographic diffuser.

10. An illumination system as claimed in claim 1 or 2, wherein the illumination system comprises a plurality of light-emitting diodes (R, G, B; W_l5 W₂, W₃) of distinct primary colors or of a single primary color.

11. An illumination system as claimed in claim 10, wherein each of the LEDs (R, G, B; Wi, W₂, W ) has a radiant power output of at least 100 mW when driven at nominal power.