Abstract:
The invention relates to a thin illumination device ( 1, 10, 20, 30, 40, 50, 60 ). The invention also relates to a display device and a luminary device comprising such a thin illumination device. The thin illumination device comprises a translucent plate ( 2, 23, 31, 41, 51, 66, 73 ) provided with an array of light-emitting diodes (LEDs) ( 3, 32, 47, 61, 75 ) connected by an electric conducting pattern, and a reflector ( 4, 20, 22, 43, 53, 62, 74 ) arranged on a first side of the plate at a distance from the plate. Such a construction can be made thinner and works more efficiently than a conventional LED thin-film illumination device.

Description:
This application is a national stage application under 35 U.S.C. §371 of International Application No. PCT/IB2007/054029 filed on Oct. 4, 2007, which claims priority to European Application No. 06122037.2, filed on Oct. 10, 2006, incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a thin illumination device. The invention also relates to a display device and a luminary device comprising such a thin illumination device. 
     BACKGROUND OF THE INVENTION 
     Light-emitting diodes (LEDs) can be used in thin illumination devices that provide backlighting, for instance, in liquid crystal displays (LCD) and luminaires. Thin illumination devices typically comprise an array of LEDs positioned behind objects to be backlit, such as liquid crystal plates, advertising screens or decorative tiles. A disadvantage is that LEDs emit light from a small surface area; hence the device must have a relatively thick structure in order to achieve an acceptable color and luminance uniformity. Conventional LED backlighting devices usually need more than 10 mm and additional diffuser layers in order to obtain a sufficiently uniform luminance. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved thin illumination device. 
     The invention provides a thin illumination device comprising a translucent plate provided with an array of light-emitting diodes (LEDs) connected by an electric conducting pattern, and a reflector arranged on a first side of the plate at a distance from the plate, wherein the array of LEDs is arranged to emit light which is substantially directed towards the reflector, and the reflector is arranged to reflect light from the LEDs through the translucent plate. As compared with a conventional LED backlight, wherein the LEDs are located on the reflector, such a thin illumination device results in a more diffuse light emitted through the translucent plate. Effectively, the shortest possible optical path of the light emitted by the LED array is doubled in comparison with a configuration in which the LEDs are located on the reflector. The LED may be any type of light-emitting diode or comparable light source, for instance, RGB LEDs or a mix of different LED types, in particular white LEDs and phosphor-converted LEDs. If differently colored LEDs are used, the device according to the invention also improves color mixing, leading to a uniform spectrum emitted by the device. Another advantageous effect is that the invention minimizes binning effects of the LEDs. Binning is the effect that leads to a decreased luminance uniformity due to slight differences in flux and/or color of individual LEDs within a batch of similar LEDs. The translucent plate is preferably made of a translucent glass or resin or polymer material. The LEDs may be arranged on either side of the translucent plate or may be integrated in the plate. The electric conducting pattern is arranged to power the LEDs and comprises connections to the LEDs and an external electric power source. The electric conducting pattern may be provided with an anti-reflective coating in order to improve the optical efficiency of the system. The reflector is arranged to reflect light from the LEDs through the translucent plate. The array of LEDs arranged to emit light which is substantially directed towards the reflector may comprise optical elements designed to control the direction of the light. The LEDs may be provided with a transparent package in order to minimize shadows on the side of the translucent panel opposite the reflector, in which, for instance, an LCD screen may be positioned. Depending on its application, the illumination device typically contains 100 to 1000 LEDs per m 2 , which is strongly dependent on the light output of the LEDs and the type of application. A LED array wherein the LEDs are positioned in an essentially square configuration is the easiest to produce. In a square configuration, four adjacent LEDs form a square. However, a better light uniformity is obtainable if a hexagonal configuration is used. In a hexagonal configuration, six adjacent LEDs form a hexagon. The reflector may be made of any light-reflecting material, depending on the preferred application. The invention is especially suitable for low-power LEDs (typically lower than 0.5 W per LED), dissipating only a small amount of power per LED. Placing the LEDs on glass or resin is therefore efficient enough without the use of bulky heatsinks. 
     The reflector is preferably substantially parallel to the translucent plate. A good luminance uniformity is easily obtainable with this configuration. Substantially parallel refers to the overall configuration of the device, whereas the reflector and/or the plate do not necessarily have to be perfectly flat or follow the same curvature at each position. 
     In a preferred embodiment, the reflector is a diffusing reflector. An improved uniformity of luminance is thus obtained. A diffusing reflector is typically provided with a light-diffusing surface. 
     In a more preferred embodiment, the diffusing reflector comprises diffusing elements. Such a diffuse reflector allows better fine-tuning of the uniformity of luminance. Useful diffusing elements are chosen in dependence on the application, and may have various forms such as pyramidal, conical and (hemi-)spherical forms protruding from the reflector surface. 
     It is advantageous if the electric conducting pattern comprises a translucent electric conducting material. Such translucent patterns improve the optical efficiency of the device. Examples of useful conducting materials comprise indium tin oxide (ITO) and fluor-doped tin oxide (SnO 2 :F). 
     Alternatively, it is advantageous if the electric conducting pattern comprises a metal wire. Metal wires may be loose wires or formed as patterns in thin layers, for instance, by depositing copper or aluminum, and generally have a better conductivity and less power dissipation than most translucent conducting materials. The form and size of the wires can be optimized to minimize interference with the luminance. The metal wire can be printed on the glass or made by means of standard lithography, but also a wire net can be used in which the wires connecting with the LED are only partially connected to the plate. 
     In a preferred embodiment, the distance between the reflector and the translucent plate is between 2 and 10 mm. Such a distance yields a compact device which has a sufficient luminance uniformity of the light emitted through the plate, in particular for LCD backlighting applications. 
     The translucent plate is preferably provided with a reflecting dot pattern. The reflecting dots further improve the luminance uniformity. The reflecting dots are typically made of a white reflecting material and may be applied to either side of the translucent plate, or on both sides. 
     In a preferred embodiment, the reflecting dots in the pattern have a relatively high density near a LED and a relatively low density remote from the LED. It is thus relatively easy to achieve an even more effective diffusion of light. This renders it possible to realize an even smaller distance between the plate and the reflector while still achieving a good luminance uniformity. 
     In a preferred embodiment, the array of LEDs comprises blue LEDs and/or UV LEDs and the reflector comprises a fluorescent material. This configuration allows fine-tuning of the color spectrum of light emitted by the device. The fluorescent material is typically a fluorescent phosphor, such as YAG:Ce, which may be integrated or coated onto the reflector, preferably in the form of a printed pattern. The preferred wavelengths emitted by the LEDs and the fluorescent material are preferably selected to complement each other. Blue LEDs emit light primarily at wavelengths which are shorter than 500 nm, while UV LEDs emit light at wavelengths which are shorter than 380 nm. The phosphor absorbs the blue and/or UV light, and emits light at longer wavelengths. 
     Mixtures of phosphors emitting at different wavelengths are preferably used to obtain an optimal spectrum for a certain application. Fine-tuning the spectrum of the illumination device can be done by balancing the amount of blue light scattered at the reflector and the amount of light generated by excitation of the phosphors on the reflector. Also a mixture of white and luminescent pigments can be used to tune the emitted color of the device, for instance, to obtain white light. 
     It is advantageous if at least one fluorescent light tube is arranged between the translucent plate and the reflector. As the light from the device is a mix of light from the LEDs and the fluorescent light tube, it is possible to optimize the spectrum emitted by the device. Typical conventional fluorescent light tubes used in backlighting are of the CCFL, EEFL and HCFL types. The light tubes and LEDs are preferably arranged in an alternating pattern with respect to each other. 
     It is preferred if the translucent plate is provided with an optically active front layer. This leads to an even better luminance uniformity. The optically active front layer may comprise, for instance, a diffuser plate, a diffuser film or a filter. Examples of effective optically active front layers are described in Philips&#39; patent application WO 2005/083317. 
     In a preferred embodiment, an optically active front layer is integrated with the translucent plate. A very compact device is thus made possible. 
     Alternatively, an optically active front layer is positioned at a distance from the translucent plate on a second side of the translucent plate opposite the reflector. The distance from the optically active front layer to the translucent plate is preferably smaller than the distance from the reflector to the translucent plate. 
     In a preferred embodiment, the optically active front layer and the plate enclose a space that is in open communication with the outer environment. The space thus forms an air channel through which heat can be dissipated away from the device during service, thereby lowering electric resistance and power consumption. 
     In a preferred embodiment, the translucent plate is a flexible polymer film. Great freedom of design is thus possible. The reflector may also be flexible or may be arranged as a support for the flexible polymer film. Such an embodiment is very useful in various applications as a backlighting device, in particular in luminary applications such as billboards, wall cladding and lighting in floor tiles. 
     The invention further provides a display device comprising a thin illumination device as defined in the claims, used as a backlight. A display device according to the invention is thinner and more efficient than known LED-based displays with a comparable light uniformity and color mixing. 
     The invention also provides a luminary device comprising a thin illumination device according to the invention. Such a luminary device according to the invention is thinner than a known LED-based display with a comparable light uniformity. Wall cladding and use under translucent floor tiles are typical luminary applications. 
     The invention will now be elucidated by way of non-limiting example with reference to the following embodiments. The technical measures shown in the embodiments may be combined to achieve cumulative effects, which may be advantageous in certain applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1   a ,  1   b  and  1   c  show a first preferred embodiment of the invention. 
         FIG. 2  shows a second preferred embodiment of the invention. 
         FIG. 3  shows a third preferred embodiment of the invention. 
         FIGS. 4   a  and  4   b  show a fourth preferred embodiment of the invention. 
         FIG. 5  shows a fifth preferred embodiment of the invention. 
         FIG. 6  shows a sixth preferred embodiment of the invention. 
         FIG. 7  shows a seventh preferred embodiment of the invention. 
         FIG. 8  shows an eighth preferred embodiment of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1   a  shows a thin illumination device  1  according to the invention. It comprises a thin glass plate  2  on which a LED array  3  is arranged. The LEDs  3  are directed towards a diffusing reflector  4  positioned parallel to the glass plate  2 , such that light  5  emitted by a LED  3  is reflected back as diffused light  6  through the glass plate  2 . A very effective diffusion of light is thus possible while using a relatively small distance h 1  between the glass plate  2  and the reflector  4 , resulting in a relatively thin illumination device  1  with a uniform luminance distribution. The LEDs, if used in known systems, would be attached on the reflector  4 ; in such a case, a larger thickness of the complete system would be needed to achieve a similar level of light diffusion at the position of the glass plate  2 . An additional optical stack  7 , comprising, for instance, a diffuser layer, a brightness-enhancing layer and optical filters can be placed in front of the glass plate  2 . The device  1  may be placed behind, for instance, an LCD screen as a backlight. Alternatively, the LED array  3  may be arranged at the side of the glass plate  2  facing the optical stack  7 , or it may be integrated in the glass plate, wherein the LEDs  3  can still be directed towards the reflector  4  through the glass plate. 
     Both the space between the glass plate  2  and the reflector  4  (thickness h 1 ) and the space between the optical stack  7  and the glass plate  2  (thickness h 2 ) may be in open air contact with the environment, thus allowing air cooling by convection for the LEDs  3  and the related electric circuitry during use. The distance between reflector  4  and glass plate  2  (h 1 ) will be preferably larger than the distance between optical stack  7  and glass plate  2  (h 2 ). It is even possible to integrate the optical stack with the glass plate, reducing the distance h 2  effectively to zero. For a given total thickness H (h 1 +h 2 ) of the system, the best configuration is a location of the translucent plate  2  with LEDs  3  at the minimal distance below the front optical stack  7 , such that no shadows in the reflected light from the reflector  4  due to the LEDs  3  or other light-blocking objects on the plate are formed on the optical stack  7 . 
       FIG. 1   b  shows a square pattern of the LED array  3 , wherein four adjacent LEDs form a square  8 . Such a pattern is relatively easy to produce. Electric power for the LED array is provided by a transparent conductor pattern (not shown), but metal patterns applied to the glass plate can also be applied. 
     As an alternative to the square pattern of  FIG. 1   b ,  FIG. 1   c  shows a hexagonal pattern of the LED array  3 , wherein adjacent LEDs are more tightly packed than in a square form and offer a better uniform luminance coverage of the area (circles  9 ). Such a hexagonal pattern  9  provides a relatively efficient distribution of light sources. Electric power for the LED array is provided by a transparent conductor pattern (not shown). 
       FIG. 2  shows a second preferred embodiment of a thin illumination device  10  according to the invention, wherein the reflector is provided with shaped reflecting elements  20 , allowing an even better diffusion of light. 
       FIG. 3  shows a third preferred embodiment of a thin illumination device  20  according to the invention, wherein the reflecting elements are orthogonal reflecting walls  21  directed from the reflector  22  towards the glass plate  23 , achieving a good luminance uniformity as well as a good color mixing when colored LEDs are used. The segments defined by the walls  21  can be illuminated separately, which is favorable in a scanning backlight to reduce motion blur in dynamic images. 
       FIG. 4   a  shows a fourth preferred embodiment of a thin illumination device  30  according the invention, wherein a pattern of reflecting dots  33  applied to the glass plate  31  is used to improve the light uniformity. The concentration of dots  33  is larger near the LEDs  32 , as is shown in a top view in  FIG. 4   b . The larger concentration of dots  33  near the LEDs  32  leads to a better luminance uniformity than an evenly divided dot pattern. The pattern of the embodiment shown in  FIG. 4   b  is an example, and alternative patterns may be used to a similar effect. 
       FIG. 5  shows a fifth preferred embodiment of a thin illumination device  40  according to the invention. The LEDs  47  positioned on the translucent plate  41  direct their LED light  42  towards the reflector  43 . The reflector is coated with a fluorescent phosphor dot pattern  46  that may have a configuration similar to the pattern of the reflective dots on the glass plate in  FIG. 4   b . Part of the light from the LEDs  47  is reflected and diffused by the reflector  43  through the glass plate  41 , denoted by the solid-line arrow  44 . Another part of the LED light  42  is absorbed by the phosphor dots  46  and re-emitted as light of a different wavelength, denoted by the broken-line arrows  45 . A mix of LED light and fluorescent light is thus obtained, allowing fine-tuning of the spectrum of light emitted by the device as a whole. LEDs  40  emitting mainly blue light (wavelength shorter than 500 nm) are preferably used for this application, wherein the phosphor is selected to emit mainly towards the red part of the spectrum (wavelength longer than 600 nm). For instance, YAG:Ce emits in the green-yellow range. In combination with blue light from the LEDs, this yields white as the emitted spectrum from the device as a whole. 
       FIG. 6  shows a sixth preferred embodiment of a thin illumination device  50  according to the invention. In this embodiment, a translucent decorative tile  54 , typically made of a ceramic material or a resin material, is positioned adjacent to the glass plate  51  provided with LEDs  52 , on the side of the glass plate opposite the reflector  53 . As the light from the LEDs  52  is emitted through the decorative tile  50  indirectly via the reflector  53 , a good uniformity of luminance is achieved while the device H (h 1 +h 2 ) can have a relatively small thickness. A device using side-emitting LEDs placed on the reflector  53  would have less uniformity of luminance at a comparable thickness. 
       FIG. 7  shows a seventh preferred embodiment of a thin illumination device  60  according to the invention, wherein a flexible polymer foil  66  with integrated LEDs  61  and circuitry (not shown) is bent in a desired form. The shape of the reflector  62  follows the curvature of the polymer foil  66 . An optical stack  64 , which, dependent on its application, further diffuses and filters the light  65  emitted by the device, is positioned on the viewer side of the polymer foil  66 . This embodiment allows great freedom of design, for instance, in wall cladding. 
       FIG. 8  shows an eighth preferred embodiment of a thin illumination device  70  according to the invention, wherein LEDs  75  and conventional tubular fluorescent lamps  71  are integrated in a single device  70 . The fluorescent lamps  71  are positioned between the glass plate  73  provided with LEDs  75  and the reflector  74 . The spectrum emitted by the LEDs  75  and the fluorescent lamps  71  are preferably selected to complement each other, dependent on their application. By combining LEDs  75  and fluorescent lamps  71 , it becomes relatively easy to obtain a desired spectrum, which can be further optimized by an optical stack  7 . Typical conventional fluorescent lamps  71  used, for instance, in backlighting are CCFL and EEFL lamps. 
     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. 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.