Abstract:
The present invention provides a lighting device for a liquid crystal display and the like. The device comprises: a light guide plate having a top surface serving as a light exiting surface, a bottom surface serving as a reflective surface and a side surface serving as a light entering surface; and a least one LED unit disposed adjacent to the side surface of the light guide plate. The LED unit comprises at least one LED chip, a transparent emission section having a front surface facing the side surface of the light guide plate, a back surface defined opposite to the front surface, and a circumferential surface extending between the front surface and the back surface. A substrate is disposed on and covering the back surface of the transparent or translucent member and electrically connected to the LED chip. The device further comprises a flat reflective member disposed over a top side of the emission section and extending in parallel with and partly overlapping the top surface of the light guide plate adjacent to the light entering surface of the light guide plate.

Description:
[0001]     This application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. JP2005-263081 filed Sep. 9, 2005, the entire content of which is hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a lighting device for a non-self-emitting display such as a small-size liquid crystal display used in a cellular phone, a PDA and so on.  
         [0004]     2. Description of the Related Arts  
         [0005]     In recent years, such a lighting device has employed a light emitting diode (hereinafter abbreviated as “LED”) for a light source thereof.  
         [0006]      FIG. 6   a  and  FIG. 6   b  show a typical type of the lighting device with LEDs. The lighting device comprises a plurality of LED units  64 - 1 ,  64 - 2 ,  64 - 3  and  64 - 4 , all of which are placed adjacent to a side edge surface  63  of a rectangular light guide plate so that light exiting from the LED unit  64 - 1 ,  64 - 2 ,  64 - 3  and  64 - 4  enters the light guide plate  10  through the side edge surface  63 . The light guide plate  10  includes reflective prisms  62  formed on a bottom surface thereof which are successively arranged from the side edge surface  63  to the opposite side edge surface  65 .  
         [0007]     As shown in  FIG. 6   b,  there are provided a reflecting sheet  16  below the light guide plate  10  and a diffusion sheet  70  and prism sheets  68  and  66  above the same and all of those elements are accommodated in a housing  60 . A non-self-emitting display such as a liquid crystal display is placed on top of the housing.  
         [0008]      FIG. 7   a  illustrates a behavior of the light for illumination. A light  15  from the LED unit  64  that has entered the light guide plate  10  advances in the light guide plate  10  while repeatedly bouncing between reflective prisms  62  arranged in the lower surface of the light guide plate and an upper surface of the light guide plate. During the repeated bouncing, an angle of incidence of the light impinging on the upper surface of the light guide plate is gradually made small and finally become smaller than a critical angle, thereby allowing the light to exit upward through the upper surface of the light guide plate. In addition, the light that may exit from the lower surface of the light guide plate is to be reflected by the reflecting sheet  16  and returned to the light guide plate  10 . The light exited from the light guide plate  10  may be diffused by the diffusion sheet  70  and then directed toward a non-self-emitting display  74  by the prism sheets  68  and  66  so as to illuminate the display.  
         [0009]     It is to be noted that reference numeral  20  designates a flexible printed circuit board serving for establishing an electric connection between the LED unit and an external device.  
         [0010]     Referring now to  FIGS. 8   a  and  8   b,  coordinate axes used in the present specification are illustrated. Z-axis extends from an LED unit  64 , which comprises an emission section  76  and a substrate section  12 , toward the light guide plate  10  and perpendicularly to the side edge surface  63  of the light guide plate  10 , X-axis is orthogonal to the Z-axis and extending in parallel to the side edge surface  63  of the light guide plate  10 , and Y-axis is orthogonal to the Z-X plane.  
         [0011]     Further, referring to  FIG. 8   b,  with regard to the light emitted from the LED unit  64 , an angle between a component X′ of the light projected onto the X-Z plane and the Z-axis and an angle between a component Y′ of the light projected onto the Y-Z plane and the Z-axis are both denoted by θ which will be referred to as “light emission angle” hereinunder.  
         [0012]      FIG. 7   b  is a diagram representing the directivity of the light emitted from the LED unit as shown in  FIG. 7   a.  In the diagram, the lateral axis represents the light emission angle θ, and the vertical axis represents ratio of the light intensity of the components of the light projected on the X-Z plane and the Y-Z plane wherein the ratio of the component of the light emission angle θ equal to zero is defined as 1. As can be seen from the diagram, the profiles of the ratios of the light intensity of the components are substantially the same and therefore represented by a single curve.  
         [0013]      FIG. 9   a  is a perspective view of the LED unit  64 ,  FIG. 9   b  shows the LED unit  64  viewed from the right side along the Z-axis,  FIG. 9   c  is a sectional view of the LED unit  64  in an approximately central region taken along a plane (horizontal plane) parallel to the X-Z plane, and  FIG. 9   d  shows the LED unit  64  viewed from the left side along the Z-axis.  
         [0014]     The substrate section  12  comprises a substrate  26  having internal terminals  32  and external terminals  34  which are electrically connected by vias  36  formed in the substrate  26 . The emission section  76  comprises a light shielding wall  78  in the shape of a rectangular cylinder and a resin block  80  filling up the interior space in the light shielding wall  78 . Within the resin block  80  are embedded a LED chip  28  mounted on the substrate  26  and gold wires  30  connecting the internal terminals  32  with the LED chip  28 . The resin block  80  has a front surface  77 , which faces to the light entering surface of the light guide plate, and thus, the front surface  77  serves as an emission surface for the light emitted from the LED chip  28  toward the light guide plate.  
         [0015]     With reference to  FIG. 10   a,  a diffused light emitted from the LED unit  64  is as indicated by a shaded triangle  22  enters the light guide plate  10  and contributes as the light for illumination. However, other light diffused outside the shaded triangle  22  will not enter the light guide plate  10 , thereby not being used for illumination.  FIG. 10   b  shows such a matter by using the same diagram as that of  FIG. 7   b,  i.e., the light indicated by shaded portions  78 ,  80  is not be used for illumination.  
         [0016]     The quantity of the light that cannot be used for illumination may increase or decrease in dependence on a distance L 1 , between the LED unit  64  and the light guide plate  10  and a thickness T, of the light guide plate as shown in  FIG. 10   a.    
         [0017]     If the distance L 1  is made zero, all of the light could be taken into the light guide plate. However, in actuality, it is impossible to make the distance L 1  zero in the lighting device.  
         [0018]     In addition, if the thickness T of the light guide plate is made thicker, more light could be taken into the light guide plate to be used for illumination. However, the thickness T is not allowed to be increased for the recent trend of a lower profile of the lighting device, which has been desired in association with the demand for a low-profile, lighter portable device. Actually, it is said that in order to reduce the thickness of the lighting device by 20% to 30%, the thickness of the light guide plate is needed to be reduced by around half.  
         [0019]     Unfortunately, as described with reference to  FIG. 10 , if the thickness of the light guide plate is decreased, less light is taken into the light guide plate.  
         [0020]     Accordingly, although there is the trend to make the lighting device thinner, it will deteriorate the emission efficiency of the light source, and, therefore, it is impossible to establish the coexistence of “making thinner” and “making brighter” in the lighting device.  
         [0021]     In connection with the lighting device with LEDs, there has been a proposal for solving a problem inherent to the lighting device that there appear dark regions or low brightness regions on the light exiting surface of the light guide plate at locations between the LEDs and adjacent to the side edge surface of the light guide plate along which the LED units are arranged. According to the proposal, the upper and lower surfaces of the emission section of the LED unit are mirror-finished or provided with light shielding layers made from material having a high reflectance without subjecting the left and right side surfaces to such treatment (see, for example, Japanese Patent Laid-open Publication No. 2004-127604). The treatments as stated above are not easy but expensive.  
       SUMMARY OF THE INVENTION  
       [0022]     An object of the present invention is to provide a lighting device which is of low-profile and inexpensive and has a higher intensity of illumination.  
         [0023]     According to the present invention, a lighting device comprises: 
        a light guide plate having a top surface serving as a light exiting surface, a bottom surface serving as a reflective surface and a side surface serving as a light entering surface;     at least one LED unit disposed adjacent to the light entering surface of the light guide plate, comprising:     an emission section comprising a transparent or translucent member having a front surface facing the side surface of the light guide plate, a back surface opposite to the front surface, and a circumferential surface extending between the front surface and the back surface;     an LED chip embedded within the emission section; and,     a substrate disposed on and covering the back surface of the transparent or translucent member and electrically connected to the LED chip; and     a flat reflective member provided separately from the LED unit and disposed over a top side of the circumferential surface and extending in parallel with the top surface of the light guide plate so as to cover a region of the top surface of the light guide plate adjacent to the light entering surface of the light guide plate.        
 
         [0030]     Compared to a conventional LED unit with the reflective member formed on an overall circumferential surface of the emission section of the LED unit, an LED unit according to the present invention can be inexpensive. In addition, since the leak light upward from the emission section can be reflected and returned to the light guide plate, light can be effectively supplied to the light guide plate. Further, since light can be emitted from a left side and a right side of the circumferential surface of the emission section of the LED unit according to the present invention, dark regions or low brightness regions emerging on the light exiting surface of the light guide plate between the LEDs at adjacent area to the side edge surface of the light guide plate can be prevented. Specifically, the reflective member may have a width at least as wide as that of the emission section.  
         [0031]     Preferably, the lighting device comprises a reflective plate disposed beneath and adjacent to the bottom surface of the light guide plate and extending toward the substrate of the LED unit under the emission section of the LED unit, thereby utilizing the light from the LED unit more effectively.  
         [0032]     The reflective member may be made of a reflective sheet. This may reduce the cost to form the reflective layer.  
         [0033]     In addition, the reflective member may have a surface on which an array of reflective prisms are formed so that the reflective prisms facing the emission section deflect light exiting from the circumferential surface of the emission section toward the front surface side of the emission section. Thus, this can reduce a number of bouncing of the light in the emission section, thereby enabling the light to be more effectively supplied to the light guide plate.  
         [0034]     Further, the lighting device may comprise a flexible printed circuit board that is attached to and the electrically connected to the LED chip through the substrate, the flexible printed circuit board extending toward and in parallel with the top surface of the light guide plate, wherein the reflective sheet is disposed between and in contact with the flexible printed circuit board and the LED unit.  
         [0035]     Still further, the reflective member facing the LED unit may be formed on a surface of the flexible printed circuit board.  
         [0036]     Thus, according to the present invention, even if the light guide plate is made thinner, the light from the LED unit can be still effectively supplied to the light guide plate for illumination, and the light can be additionally emitted from the left and the right of the emission section of the LED unit, so that it can prevent the dark regions or low brightness regions from emerging between the LED unit, in an area of the light exiting surface of the light guide plate adjacent to the LED unit. In addition, since the reflective layer is provided separately from the LED unit, it can be inexpensive. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0037]      FIG. 1   a  is a partial sectional view illustrating a lighting device of a first embodiment of the present invention;  
         [0038]      FIG. 1   b  is a partial sectional view illustrating a variation of the device of  FIG. 1   a;    
         [0039]      FIG. 2   a  shows a lighting device of a second embodiment of the present invention;  
         [0040]      FIG. 2   b  shows a lighting device of a third embodiment of the present invention;  
         [0041]      FIG. 3  is a perspective view illustrating main components of a lighting device according to the present invention;  
         [0042]      FIG. 4   a  is a perspective view illustrating an LED unit used in the present invention;  
         [0043]      FIG. 4   b  shows a light exiting surface of the LED unit of  FIG. 4   a;    
         [0044]      FIG. 4   c  is a sectional view taken along the X-Z plane of the LED unit of  FIG. 4   a;    
         [0045]      FIG. 4   d  is a back view of the LED unit of  FIG. 4   a;    
         [0046]      FIG. 5   a  is a schematic view for illustrating how a light is emitted in case an LED unit, used in the present invention, is applied to a conventional lighting device;  
         [0047]      FIG. 5   b  is a diagram illustrating an emission angle characteristic of a LED unit as shown in  FIG. 5   a;    
         [0048]      FIG. 6   a  is a plan view of a conventional lighting device;  
         [0049]      FIG. 6   b  is a side view of the lighting device of  FIG. 6   a;    
         [0050]      FIG. 7   a  is a schematic view for illustrating a behavior of an illumination light in the lighting device as shown in  FIG. 6   a;    
         [0051]      FIG. 7   b  is a diagram illustrating a directivity of a light from an LED unit of the lighting device of  FIG. 6   a;    
         [0052]      FIG. 8   a  is a schematic diagram for illustrating coordinate axes used in the present specification;  
         [0053]      FIG. 8   b  is a diagram for illustrating an emission angle “θ” of a light emitted from an LED unit and directed toward an light guide plate, with respect to the coordinate axes shown in  FIG. 8   a;    
         [0054]      FIG. 9   a  is a perspective view illustrating an LED unit with a shading wall provided on an overall circumferential surface of the emission section of the LED unit;  
         [0055]      FIG. 9   b  shows a light emitting surface of the LED unit of  FIG. 9   a;    
         [0056]      FIG. 9   c  is a sectional view taken along the X-Z plane of the LED unit of  FIG. 9   a;    
         [0057]      FIG. 9   d  is a back view of the LED unit of  FIG. 9   a;    
         [0058]      FIG. 10   a  is a schematic side view of the lighting device for explaining a problem associated with the device; and  
         [0059]      FIG. 10   b  is a diagram for illustrating the problem associated with the lighting device of  FIG. 10   a.   
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0060]     Embodiments of the present invention will be explained below with reference to the accompanying drawings.  
         [0061]     LED unit shown in  FIG. 4   a  is a perspective view of the LED unit  13 ;  FIG. 4   b  shows the LED unit  13  viewed from the right side along the Z-axis;  FIG. 4   c  is a sectional view of the LED unit  13  taken along a plane parallel to the X-Z plane; and  FIG. 4   d  shows the LED unit  13  viewed from the left side along the Z-axis.  
         [0062]     As illustrated, the LED unit  13  comprises a substrate section  12  and an emission section  14 . The substrate section  12  comprises a substrate  26  on which a LED chip  28  is mounted, internal terminals  32  disposed on an inner surface of the substrate  26 , and external terminals  34  on an outer surface of the substrate  26 . The internal terminals  32  and the corresponding external terminals  34  are interconnected by vias  36 , respectively. At least one LED chip  28  is mounted on the substrate, and gold wires  30  for connecting the internal terminals  32  with terminals of the LED chip  28 .  
         [0063]     The emission section  14  comprises a transparent resin block  80 , and, the LED chip  28 , and gold wires  30  connecting the internal terminals  32  with the LED chip  28  which are embedded in the transparent or translucent resin block  80 , which protects all the elements embedded therein and controls the illumination color. The emission section  14  is configured such that in operation, the light can exit not only from a front surface of the resin block  80  facing to light guide plate but also from a top side  14 - 4 , a bottom side  14 - 5 , a left side  14 - 3  and a right side  14 - 2  of the same, while the back surface  14 - 6  is covered by the substrate  26  fixedly attached thereto. As shown in  FIG. 4   d,  the external electrode  34  of the substrate  26  and the via  36  for connecting the external electrode  34  with the internal electrode  32  are seen in the back surface of the LED unit  13 .  
         [0064]     As apparent from the comparison of  FIGS. 4   a - 4   d  to  FIGS. 9   a - 9   d,  the LED unit  13  used in the present invention as shown in  FIGS. 4   a - 4   d  has advantageously employed five of the six surfaces thereof as the emission surfaces by removing the light shielding surface  78  in association with the conventional LED unit  64  as shown in  FIGS. 9   a - 9   d.    
         [0065]     As a result of measurement, it has been found that this configuration of the present invention can increase a total volume of light emitted from the emission section by a multiple of 1.3 to 1.4 over the conventional LED unit  64  that has employed one of the six surfaces of the emission section. Incidentally, it has also been found that when the LED unit of  FIG. 9   a  is modified by removing the left and right side shading surfaces retaining the upper and lower shading surfaces, a total volume of light exiting therefrom increases by a multiple of 1.1.  
         [0066]     If the LED unit  13  is applied to a lighting device having the conventional configuration as stated above, however, the quantity of light entering the light guide plate could decrease as explained below.  
         [0067]     Similarly to  FIG. 10   b,    FIG. 5   b  is a diagram for illustrating the directivity of the light directed toward the light guide plate  10  by taking the front surface  14 - 1  facing to the light guide plate  10  as a reference, and it can be seen from the diagram that the quantity of light spreading in the left and the right sides with respect to the direction of θ=0 in the Y-Z plane increases. Specifically, the characteristic on the Y-Z plane exhibits peaks, i.e., 1 of the relative intensity at + and −35 degrees and around 0.4 of the relative intensity even at + and −90 degrees as indicated by a solid line, while the characteristic on the X-Z plane exhibits substantially the same characteristic as in the conventional device as indicated by a dotted line. Reference numerals  22 ,  38  and  40  in  FIG. 5   a  symbolically represent the manner in which the light exits from the LED unit  13 . Namely, when the LED unit  13  is placed in relation to the light guide plate  10  as shown in  FIG. 5   a,  the light denoted by  38 ,  40  does not enter the light guide plate  10 , thereby not being used for illumination.  
         [0068]     The present invention intends to achieve a low-profile lighting device with LED units  13  as shown in  FIG. 4  which device is capable of highly intensive illumination.  
         [0069]      FIG. 1   a  is a partial sectional view of a lighting device according to a first embodiment of the present invention.  
         [0070]     In this lighting device, a light guide plate  10  having reflective prisms formed on a bottom surface thereof is provided, in this illustration, on the right side with respect to LED units  13  arranged as in  FIG. 6   a  (only one of which is shown in the figure). The LED unit  13  comprises a substrate section  12  and an emission section  14 . A flexible printed circuit  20  is attached on the top of the substrate section  12  of the LED unit  13  and electrically connected with terminals of at least one LED chip embedded in the emission section. A reflective plate  16  is disposed beneath the LED unit  13  and the light guide plate  10 . A reflective sheet  18  is further disposed between and in contact with the emission section  14  of the LED unit  13  and the flexible printed circuit  20 .  
         [0071]     It should be noted that the reflective plate  16  lies below the entire bottom surface of the light guide plate and extends to the underside of the substrate section  12 . It is important that the reflective plate  16  lies below the bottom surface of the emission section  14 .  
         [0072]     Further, a length, L 3 , of the reflective sheet  18  is set longer than a length, L 2 , of the emission section  14  in the longitudinal direction of the light guide plate. Specifically, the length L 3  is at least two times as long as the length L 2 . This allows larger quantity of light to be collected, which would otherwise leak without entering or out of the light guide plate  10 .  
         [0073]     With this configuration, the light  38  as shown in  FIG. 5   a  that is not to be used for illumination can be reflected on the reflective sheet  18  back to the light guide plate  10  as denoted by  24  in  FIG. 1   a,  while the light  40  as shown in  FIG. 5   a  that is not to be used for illumination can be also reflected on the reflective plate  16  back to the light guide plate  10 , as denoted by  26  in  FIG. 1   a,  so that the both portions of light can be used for illumination. Thus, the light exiting from the top and the bottom surfaces of the LED unit are reflected on the reflective members disposed in vicinity of the top and the bottom surfaces, so that a larger quantity of light can be supplied to the light guide plate. In this manner, the lighting device of the present invention can improve the efficiency in using the light from the light source significantly. The reflective sheet  18  may be made by cutting a preformed large-size reflective sheet which may be economically prepared by using various kinds of reflective materials, so that the reflective sheet  18  can be advantageous as compared with a case where such a reflective member as the reflective sheet  18  is formed through the mirror-surface finishing as is in  FIG. 9 . The preformed large-size reflective sheet may be a sheet made of PET film with a silver film thereon and a coating layer formed thereon or a reflective sheet referred to as “ESR” available from Sumitomo 3M Limited. The ESR, because of its non-conductivity, has an advantage that the LED electrode cannot be electrically shorted.  
         [0074]     Accordingly, the lighting device according to the present invention can accomplish significant superiority in the efficiency in using the light, or the brightness, and the cost performance as well.  
         [0075]     Since the lighting device of the present invention can increase the efficiency in using the light in this manner, even if the thickness of the light guide plate is reduced by around half, the lighting device can still illuminate at substantially the same level as that achieved by the conventional device and thus can realize the low-profile, bright lighting device with low production cost.  
         [0076]     In  FIG. 1   a,  there is provided a gap between a flat upper surface of the emission section  14  of the LED unit  13  and the flexible printed circuit  20 , while in  FIG. 1   b,  there is provided no gap between the emission section  14  and the flexible printed circuit  20  but instead the reflective sheet  18  is inserted therebetween. The reflective sheet is so thin that it can be inserted as the above manner.  
         [0077]      FIG. 2   a  shows another aspect of a lighting device according to the present invention. This lighting device is different from that shown in  FIG. 1   a  in the reflective layer  19 , wherein reflective prisms are arranged on its surface facing the top side of the emission section of the LED unit, as illustrated. This is provided for the purpose of reflecting the light exiting from the top side of the emission section on the reflective layer  19 , and this configuration can prevent light from leaking without entering the light guide plate, as the light reflected on the reflective layer  19  and returned to the light guide plate is reflected repeatedly within the light guide plate until the light can finally exit for illumination.  
         [0078]      FIG. 2   b  shows a lighting device according to another embodiment of the present invention, which comprises terminals  46  and  48  on an bottom surface of a flexible printed circuit  21 , wherein the terminal  46  is adapted to be connected with the LED chip and the terminal  48  is adapted not to be connected with the LED chip but has a reflective layer  50  formed thereon. This can reduce a number of parts required for assembling the device. The terminals  46  and  48  may be constructed as a single unit, and the reflective layer  50  may be formed over the both terminals  46  and  48 .  
         [0079]      FIG. 3  is a perspective view of main components of a lighting device according to the above described embodiments of the present invention. As shown in  FIG. 3 , the lighting device includes the reflective sheet  18  and the reflective plate  16  over and below the LED unit, respectively, so that the light from the LED unit can be directed toward an end surface of the light guide plate  10  effectively. Films for collecting the light are disposed above the light guide plate  10 , including, for example, a diffusion plate  70 , and prism films  68 ,  66  which are disposed in respective orientations perpendicular to each other, so that the light exited from the LED unit  13  along the Z-axis can be deflected upward in the light guide plate and thereby turn to be the exiting light along the Y-axis with the aid of the light guide plate  10  and the films  70 ,  68  and  66  so as to illuminate a non-self-emitting display (not shown) disposed on the films. Thus the light from the LED unit can illuminate the non-self-emitting display in an efficient manner, and so according to the present invention it becomes possible to provide the low-profile and inexpensive lighting device with a high illumination intensity.  
         [0080]     Although the present invention has been described in terms of specific embodiments, it is anticipated that alternations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alternations and modifications as fall within the true spirit and scope of the invention.