Abstract:
The present invention relates to a light emitting diode (LED) device ( 10 ) comprising (a plurality of LED chips ( 12 ) placed on a substrate, and a plurality of light guiding elements ( 16, 36 ) each having and an entrance window ( 24 ) and an exit window ( 26 ) wherein the elements are arranged for allowing light from at least one LED chip to be transferred from the entrance window to the exit window, and wherein the elements are further arranged so that said exit windows form several inner side surfaces ( 20 ) of a cavity ( 22 ), which cavity further has an exit aperture ( 30 ) for allowing extraction of light from the device. This configuration allows for a high brightness LED device.

Description:
The present invention relates to a light emitting diode (LED) device comprising a plurality of LED chips. 
     BACKGROUND OF THE INVENTION 
     LED chips are increasingly being used for illumination and lighting purposes. For example to create white light, blue or UV LEDs coated with phosphor are used, so-called phosphor converted LEDs. The phosphor converts at least some of the blue radiation to for example yellow light. Together, the non-converted blue light and the yellow light generate a white light. 
     However, individual LED chips do currently not provide sufficient brightness for many applications, like projections displays and car head lighting. Therefore, LED devices comprising arrays of LED chips are used, for example as disclosed in the document U.S. Pat. No. 6,325,524. In U.S. Pat. No. 6,325,524, as well as in many other known multi LED chip devices, several LED chips are positioned besides each other on a substrate. The light from the LED chips is then collected by an optical system covering the LED chips. However, due to the wide extension of the optical system covering all the LED chips, there is a low amount of light per unit of surface area, resulting in low brightness. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome this problem, and to provide an improved, high brightness LED device. 
     This and other objects that will be evident from the following description are achieved by means of an LED device comprising a plurality of LED chips placed on a substrate, and a plurality of light guiding elements each having an entrance window and an exit window, wherein the elements are arranged for allowing light from at least one LED chip to be transferred from the entrance window to the exit window, and wherein the elements are further arranged so that the exit windows form several inner side surfaces of a cavity, which cavity further has an exit aperture for allowing extraction of light from the device. 
     The invention is based on the understanding that by guiding light from a plurality of LED chips to a common cavity, the luminance of the exit aperture of the cavity can be higher than the luminance of a single LED chip. This without the size of the exit aperture significantly exceeding the size of one of the LED chips. Thus, a high brightness LED device can be achieved. 
     Preferably, at least one additional LED chip is placed on the substrate for emitting light at the bottom surface of the cavity opposite the exit aperture. This further increases the brightness of the LED device. The at least one additional LED chip can for example be positioned at the bottom surface of the cavity, in case the cavity is reasonably in level with the substrate. Alternatively, the at least one additional LED chip can be provided with an additional light guiding element for transferring light from the additional LED chip(s), from an entrance window to an exit window of the additional light guiding element, wherein the bottom surface of the cavity is essentially formed by the exit window. The additional light-guiding element is useful for example in case the cavity is not in level with the substrate. 
     Suppose that the cavity has four inner side surfaces, each inner side surface with one exit window transferring light from one LED chip. In addition, a fifth LED chip is placed at the bottom surface of the cavity. Further, the size of the bottom surface and the size of the exit aperture preferably correspond to the area of the bottom LED chip. In such a case, for a loss less system, the brightness could be five times larger than for a single LED chip, with maintained light emitting area. 
     In one embodiment of the present invention, each light guiding element is a triangular prism having a first, second and third side face, wherein the first and second side faces are arranged at an essentially right angle. That is, the area of the base of the prism is a right-angled triangle. Further, the entrance window can be formed on at least a portion of one of the right-angled side faces, and the exit window can formed on at least a portion of the other right-angled side face. Preferably, the size of the entrance and exit windows corresponds to the size of the LED chip. In a case where the entrance window and exit window cover essentially the entire right-angled side faces, respectively, the LED chips can be positioned closed together on the substrate, which allows a compact LED device. In a case where the entrance window and exit window only cover a portion of the right-angled side faces, respectively, the LED chips can be positioned further away from each other on the substrate, which facilitates manufacturing. 
     Preferably, the side face accommodating the exit window and the third side face are arranged at an angle of about 45°. However, the side face accommodating the exit window and the third side face could alternatively be arranged at an acute angle. This improves the total internal reflection inside the prism. 
     In another embodiment of the present invention, the light guiding elements further comprises a rectangular parallelepiped extending the triangular prism at one of the right-angled side faces of the prism, wherein the entrance window is formed on at least a portion of the other right-angled side face, and the exit window is formed on at least a portion of the face of the rectangular parallelepiped opposite the face towards the prism. This allows the LED chips to be positioned further away from each other on the substrate, which facilitates manufacturing. The triangular prism and the rectangular parallelepiped can be formed in one piece, i.e. they are created and formed integrally in one and the same piece or bit of material, as opposed to two separate parts joined together. 
     The LED device can further comprise phosphor for converting the wavelength of at least part of the light or radiation from the LED chips. The cavity can for example be provided internally with a phosphor coating. That is, the exit windows of the light guiding elements and the additional LED chip placed at the bottom of the cavity can be provided with a phosphor coating. As an alternative to the phosphor coatings, the entire cavity can be filled with a phosphor material. The phosphor, in combination with blue or UV LED chips, allows an LED device emitting for example white light. 
     Preferably, a filter is interposed between each LED chip and light guiding element, or between each light guiding element and the phosphor, which filter is adapted to transmit unconverted light from the LED chips and reflect converted light. The filters offer the advantage of preventing light losses and they direct all converted light forward towards the cavity. This results in efficient light extraction and increased brightness. Another filter can be placed on top of the phosphor, which filter is adapted to reflect unconverted light. Thus, any unconverted light from the LED chips is reflected back for conversion or mixing. This also results in efficient light extraction and increased brightness. 
     The cavity of the LED device can further be filled with a clear resin, such as silicone resin, to enhance the out coupling efficiency of the device. 
     The LED device can further comprise an optical element, such as a collimator, an out coupling lens or a compound parabolic concentrator (CPC), arranged to receive any light emitted from the exit aperture. The collimator offers the advantage that the emission angle can be reduced to any desired value, while the out coupling lens and CFC can enhance the out coupling of light from the device. Preferably, in case the cavity is filled with a clear resin, the optical element is in optical contact with the clear resin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiments of the invention. 
         FIG. 1   a  is a schematic cross-sectional side view of an LED device according to an embodiment of the invention; 
         FIG. 1   b  is a schematic perspective view of the LED device of  FIG. 1   a;    
         FIG. 2  is a schematic perspective view of a variant of the LED device of  FIGS. 1   a - 1   b;    
         FIGS. 3   a - 3   b  are schematic cross-sectional side views of other variants of the LED device of  FIGS. 1   a - 1   b.    
         FIG. 4  is a schematic cross-sectional side view of another variant of the LED device of  FIGS. 1   a - 1   b;    
         FIG. 5   a  is a schematic cross-sectional side view of an LED device according to another embodiment of the invention; 
         FIG. 5   b  is a schematic perspective view of the LED device of  FIG. 5   a;    
         FIG. 6   a  is a schematic cross-sectional side view of an LED device according to yet another embodiment of the invention; 
         FIG. 6   b  is a schematic perspective view of the LED device of  FIG. 6   a ; and 
         FIG. 7  is a schematic perspective view of an LED device having a rectangular shaped cavity. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1   a  and  1   b  show an LED device  10  according to an embodiment of the invention. The LED device  10  comprises five LED chips  12  placed on a flat substrate  14  in a cross formation. The LED chips  12  are positioned adjacent or almost adjacent to each other on the substrate  14 . 
     Each LED chip  12 , except for the centrally positioned LED chip  12   a , is provided with a light guiding triangular prism  16 . Each prism  16  has a first  18   a , second  18   b  and third  18   c  side face. The first  18   a  and the second  18   b  side faces are arranged at an essentially right angle. The first side face  18   a  faces the LED chip  12 , while the second side face  18   b  forms an inner side surface  20  of a cavity  22 . Further, an entrance window  24  is formed on the first side face  18   a , and an exit window  26  is formed on the second side face  18   b . All faces of the prisms  16 , except for the entrance and exit windows  24  and  26 , are covered with a reflective coating (not shown), for allowing transfer of light from the entrance window  24  to the exit window  26 . The reflective coating on the prisms is preferably in non-optical contact with the prism to preserve the loss less total internal reflections. 
     Thus, in  FIGS. 1   a  and  1   b , each of the four exit windows  22  forms one of the four inner side surfaces  20  of the cavity  22 . Preferably, the size of the entrance and exit windows  24  and  26  corresponds to the size of the square LED chips  12 , resulting in a cubic cavity  22 . The cavity  22  further has a bottom surface  28 , where the central LED chip  12   a  is positioned, and an exit aperture  30  opposite the bottom surface  28 . 
     Upon operation of the LED device  10 , light from the LED chips  12  provided with prisms  16  is coupled into the respective prism  16  through the entrance window  24 . The light is then guided through the prism  16  to the exit window  26 , and into the cavity  22 . At the same time, light from the LED chip  12   a  at the bottom surface is emitted into the cavity  22 . The light can finally exit the cavity  22  through the exit aperture  30 . Thus, even though the exit aperture&#39;s size corresponds to the size of a single LED chip, it emits light originating from five LED chips, resulting in a high-brightness LED device. 
     Preferably, the LED chips  12  are adapted to emit blue light or UV radiation. In that case, the exit windows  26  forming the inner surfaces of the cavity  22 , as well as the bottom LED chip  12   a , can be provided with a phosphor coating  32 . For example, in case of blue LED chips, the phosphor coating converts part of the blue light emitted from the LED chips  12  to for example yellow light, which yellow light together with unconverted blue light can generate white light. As an alternative to the phosphor coatings  32 , the cavity  22  can be completely filled with a phosphor material  34 , as illustrated in  FIG. 2 . In such a case, more of the blue light will be absorbed by the phosphor, thereby generating more yellow light. 
     A filter (not shown) which transmits blue light or UV radiation and reflects converted light can optionally be placed between each LED chip and prism or between the prisms and the phosphor or phosphor coating, as well as between the LED chip  12   a  and the phosphor or phosphor coating. In this way, converted light can be forwarded towards the cavity. The filter can for example be a dichroic mirror. Further, blue or UV reflecting filters (not shown) can be positioned on top of the phosphor coatings or on top of the phosphor material at the exit aperture, in order to reflect back any unconverted light and give it another chance to be converted or mixed. The above-mentioned filters contribute to improve the brightness of the LED device. 
     In order to enhance the out coupling efficiency of the LED device, the cavity  22  of the LED device can be filled with a clear resin  46 , such as silicone resin, as illustrated in  FIGS. 3   a - 3   b . In  FIG. 3   a , the clear resin is in optical contact (at the exit aperture) with an out coupling lens  48 , and in  FIG. 3   b  with a CPC  50 , to further enhance the out coupling. 
     Another variant of the LED device in  FIGS. 1   a - 1   b  is illustrated on  FIG. 4 . In  FIG. 4 , there is an acute angle between the second side face  18   b  accommodating the exit window  26  and the third side face  18   c , compared to the about 45° angle in  FIGS. 1   a - 1   b . The sharper angle improves the amount of total internal reflection inside the prisms  16 . 
     An LED device  10  according to another embodiment of the invention is illustrated in  FIGS. 5   a - 5   b . In  FIGS. 5   a - 5   b , each triangular prism  16  is extended, at the side face  18   b , by a rectangular parallelepiped  36 , forming a single light guiding structure. The entrance window  24  is formed on the side face  18   a  of the prism portion, and the exit window  26  is formed on the face  38   a  of the rectangular parallelepiped portion opposite the face  38   b  towards the prism. Each prism  16  and rectangular parallelepiped  36  is preferably formed in one piece. In this embodiment, the LED chips  12  can be positioned further apart compared to the LED device of  FIGS. 1-4 , which facilitates manufacturing. 
     An LED device  10  according yet another embodiment is illustrated in  FIGS. 6   a - 6   b . In  FIGS. 6   a - 6   b , the LED device comprises triangular prisms  16  as in for example  FIG. 1 , however the entrance window  24  is formed on only a portion of the first side face  18   a , and the exit window  26  is formed on only a portion the second side face  18   b . This results in an LED device where the LED chips can be placed apart from each other, and a cavity that is elevated above the surface of the substrate. In this embodiment, the central LED chip  12   a  preferably is provided with an additional light guiding element  40  having an entrance window  42  and an exit window  44 , wherein the exit window  44  forms the bottom surface of the cavity. Preferably, the prisms  16  and the additional light guiding element  40  are formed in one piece, which facilitates manufacturing of the LED device. 
     The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, even though an LED device having five LED chips is disclosed above, it will be appreciated that the invention is not limited in this respect, as a different number of LED chips may be used in the LED device. For example, an LED device having three LED chips and a cavity with a triangular bottom surface and exit aperture is envisaged. Further, several chips and light-guiding elements can be arranged side by side, an example of which is disclosed in  FIG. 7 . In  FIG. 7 , there is a rectangular shaped cavity with several LED chips arranged at the bottom surface, several LED chips  12  and prisms  16  arranged along the long side of the cavity, and a single LED chip  12  and prism  16  arranged at each short side of the cavity. Alternatively, a single elongated prism  16  can support several of the LED chips  12  placed at one of the long sides of the cavity. 
     Further, the various aspects described in relation to  FIGS. 1-4 , such as blue or UV LEDs, wavelength filters, phosphor filled cavity, etc., can be implemented also in the LED devices shown in  FIGS. 5-7 .