Patent Publication Number: US-2010110727-A1

Title: Planar lighting device

Description:
The entire contents of the documents cited in this specification are incorporated herein by reference. 
     BACKGROUND 
     The present invention relates to a planar lighting device used for a liquid crystal display device and the like. 
     Liquid crystal display devices use a backlight unit (planar lighting device) for radiating light from behind the liquid crystal display panel to illuminate the liquid crystal display panel. A backlight unit is configured using a light guide plate for diffusing light emitted by an illuminating light source to irradiate the liquid crystal display panel and optical parts such as a prism sheet and a diffusion sheet for rendering the light emitted from the light guide plate uniform. 
     Currently, large liquid crystal televisions predominantly use a so-called direct illumination type backlight unit comprising a light guide plate disposed immediately above the illuminating light source. This type of backlight unit comprises a plurality of cold cathode tubes serving as a light source provided behind the liquid crystal display panel whereas the inside of the backlight unit provides white reflection surfaces to ensure uniform light amount distribution and necessary brightness. 
     To achieve a uniform light amount distribution with a direct illumination type backlight unit, however, a thickness of about 30 mm in a direction perpendicular to the liquid crystal display panel is required, making further reduction of thickness difficult with the direct illumination type backlight unit. 
     Among backlight units that allow reduction of thickness thereof is a backlight unit using a light guide plate in which light emitted by an illumination light source and entering the light guide plate is guided in given directions and emitted through a light exit plane that is different from the plane through which light entered. 
     There has been proposed a backlight unit of a type using a light guide plate formed by mixing scattering particles for diffusing light into a transparent resin, wherein light is admitted through one or more end faces of the plate adapted to be light entrance planes and one of the largest planes is adapted to be the light exit plane. 
     JP 07-36037 A, for example, discloses a light diffusion light guide light source device comprising a light diffusion light guide member having at least one light entrance plane region and at least one light exit plane region and light source means for admitting light through the light entrance plane region, the light diffusion light guide member having a region that has a tendency to decrease in thickness with the increasing distance from the light entrance plane. 
     JP 08-248233 A discloses a planar light source device comprising a light diffusion light guide member, a prism sheet provided on the side of the light diffusion light guide member closer to a light exit plane, and a reflector provided on the rear side of the light diffusion light guide member. JP 08-271739 A discloses a liquid crystal display comprising a light emission direction correcting element formed of sheet optical materials provided with a light entrance plane having a repeated undulate pattern of prism arrays and a light exit plane given a light diffusing property. JP 11-153963 A discloses a light source device comprising a light diffusion light guide member having a scattering power therein and light supply means for supplying light through an end face of the light diffusion light guide member. 
     SUMMARY OF THE INVENTION 
     Some of the backlight units using a light guide plate as described above may also use such films (sheets) as a prism sheet, a light diffusion sheet, and the like having various functions located in front of the light exit plane in order to render uniform the light emitted from the light exit plane of the light guide plate. 
     These films used for the backlight are generally fixed to the light guide plate or secured with a fixing means to a given position. 
     The light guide plate and these films are liable to expand and contract when heated or moistened. Thus, in the conventional backlight unit, the film prevents the light guide plate from expanding or contracting, or the light guide plate is affected by the expansion and contraction of the film, causing the light guide plate to warp or distort and making it impossible to emit appropriate illumination light. Even when only the film expands and contracts without affecting the light guide plate, the film cannot produce intended effects if the film warps or distorts, making it impossible to emit intended illumination light. 
     Further, when the light exit plane of the light guide plate touches the film, an interference fringe may be produced, which also makes it impossible to admit appropriate illumination light into the liquid crystal display panel. 
     While the light guide plate and the film are preferably disposed with a slight gap between them, the light guide plate and the film in the conventional backlight units as described above can come into contact with each other if the light guide plate and/or the film warps or distorts. 
     An object of the present invention is to solve the problems associated with the prior art and provide a planar lighting device used in, for example, the backlight unit of a liquid crystal display panel comprising a light source unit, a light guide plate, and a sheet or sheets of film disposed in front of the light exit plane of the light guide plate, wherein the sheet or sheets of film do not affect the expansion and contraction of the light guide plate due to moisture absorption and heating, expansion and contraction of the light guide plate due to moisture absorption and heating do not affect the light guide plate, distortion and warping of the light guide plate and the film can be prevented in an optimum manner, and the contact between the light guide plate and the film that might otherwise be caused by this can be prevented. 
     To solve the above problems, the invention provides a planar lighting device comprising a light guide plate comprising a rectangular light exit plane and a light entrance plane existing on one side of the light exit plate, the one side being located above the opposite side; a light source unit disposed opposite the light entrance plane; a film support means provided with the light guide plate; and at least one sheet of film suspended from and supported by the film support means and disposed in front of the light exit plane. 
     In such a planar lighting device, the at least one sheet of film is preferably suspended from the film support means using long holes formed in one of the at least one sheet of film and the film support means and oriented in a normal direction normal to an up-and-down direction of the light guide plate and pins formed on the other and inserted into the long holes, respectively. 
     Preferably, the planar lighting device further comprises a film securing means on a center line of the at least one sheet of film in a normal direction normal to an up-and-down direction for preventing displacement of the at least one sheet of film in the normal direction. 
     Preferably, the planar lighting device further comprises a light source support means supporting the light source unit, wherein the light source unit is suspended from the light source support means. 
     Preferably, the light source unit is suspended from the light source support means using long holes formed in one of the light source unit and the light source support means and oriented in a normal direction normal to an up-and-down direction and pins formed on the other and inserted into the long holes, respectively. 
     Preferably, the planar lighting comprises a light source securing means on a center line of the light source unit in a normal direction normal to an up-and-down direction for preventing displacement of the light source unit in the normal direction. 
     Preferably, the gap between the light exit plane and one of the at least one sheet of film closest to the light exit plane is in a range of 0.05 mm to 0.2 mm. 
     Preferably, the at least one sheet of film has sheets of film, and a gap between adjacent two sheets of film is in a range of 0.05 mm to 0.2 mm. 
     Preferably, the planar lighting device further comprises another light source unit, wherein the light guide plate comprises another light entrance plane existing on the opposite side of the light exit plate, and the another light source unit is disposed opposite the another light entrance plane. 
     Preferably, the thickness of the light guide plate gradually increases with an increasing distance from the light entrance plane. 
     Preferably, the light source unit comprises a plate-like portion disposed so as to cover a plane located opposite from the light exit plane of the light guide plate. 
     Preferably, the length of the plate-like portion in a direction away from the light source unit is set to a maximum possible length with the plate-like portion located closest possible to the light exit plane depending upon a shape of the light source unit instead of away from the rear side opposite from the light exit plane of the light guide plate. 
     According to the invention providing the above configuration, the prism sheet, the diffusion sheet, etc., although held by the film support means provided on the light guide plate, are so held that they are suspended (hang) from the film support means. Thus, the light guide plate and the film are disposed in the same manner as when they are disposed totally independently of each other. 
     Thus, even when the light guide plate and the film expand and contract when heated or moistened, they do not affect each other. In addition, since the film is supported by hanging, the warping and distortion of the light guide plate and the film caused by the expansion and contraction can be prevented. Further, since the warping and distortion of the film can be prevented, the contact between the light exit plane of the light guide plate and the film that might otherwise be caused by such warping and distortion can be prevented. In addition, the film can be supported with the gaps between the film and the light guide plate and between the films maintained to an appropriate distance by a simple configuration where the film is merely suspended from the film support means provided on the light guide plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view illustrating an example of a liquid crystal display device using the planar lighting device of the invention. 
         FIG. 2A  is a schematic front view of the backlight unit of the liquid crystal display device illustrated in  FIG. 1 ;  FIG. 2B  is a cross section taken along line I-I of  FIG. 2A ;  FIG. 2C  is a cross section taken along line II-II of  FIG. 2A . 
         FIG. 3A  is a schematic perspective of the light guide plate of the backlight unit illustrated in  FIG. 2 ;  FIG. 2B  is schematic side view. 
         FIG. 4A  is a partial perspective of a light source unit of the backlight unit illustrated in  FIG. 2 ;  FIG. 4B  is a partial perspective of an LED chip; and  FIG. 4C  is a partial schematic front view of the light source unit. 
         FIGS. 5A and 5B  are views representing a concept of a film used for the backlight unit illustrated in  FIG. 2 . 
         FIG. 6  is a schematic front view of another example of the backlight unit according to the planar lighting device of the invention. 
         FIG. 7A  is a schematic side view of the light source of the backlight unit illustrated in  FIG. 6 ;  FIG. 7B  is a schematic front view of a guide member used in the backlight unit;  FIG. 7C  is a schematic side view thereof. 
         FIG. 8  is a partial schematic side view of another example of the backlight unit according to the planar lighting device of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Now, the planar lighting device of the invention will be described in detail referring to preferred embodiments illustrated in the accompanying drawings. 
     While the planar lighting device described below as a representative examples is a two-light entrance plane type whereby light from two light source units is admitted through two light entrance planes existing two sides of the light guide plate, respectively, the example is not limitative of the scope of the present invention. 
       FIG. 1  is a schematic perspective view illustrating an example of a liquid crystal display device using the planar lighting device of the invention. 
     A liquid crystal display device  10  illustrated in  FIG. 1  is a display device such as a liquid crystal television using a so-called liquid crystal display panel (LCD)  12  and basically comprises a liquid crystal display panel  12 , a backlight unit  20  according to the planar lighting device of the invention, and a drive unit  14  for driving the liquid crystal display panel  12 . 
     The liquid crystal display device  10  is basically installed so that the longer sides of the liquid crystal display panel  12  having a rectangular image display plane (viewing surface) are positioned one above the other vertically. The backlight unit  20  is disposed behind the liquid crystal display panel  12  (on the opposite side from the image display plane), with the light exit plane directed toward the liquid crystal display panel  12 . In the description to follow, the vertical direction in which the shorter sides of the liquid crystal display panel  12  (light guide plate  30  described later) extend, i.e., the vertical direction of the liquid crystal display panel  12  as correctly installed, will be referred to also as up-and-down (UD) direction, and the direction in which the longer sides of the liquid crystal display panel  12  extend, i.e., the horizontal direction normal to the up-and-down (vertical) direction, will be referred to also as left-and-right (LR) direction for the purpose of the invention. 
     In  FIG. 1 , a part of the liquid crystal display panel  12  is not shown to better illustrate the configuration of the backlight unit  20 . 
     In the liquid crystal display panel  12 , an electric field is partially applied to liquid crystal molecules, previously arranged in a given direction, to change the orientation of the molecules. The resultant changes in refractive index in the liquid crystal cells are used to display characters, figures, images, etc., on the liquid crystal display panel  12 . 
     The drive unit  14  applies a voltage to transparent electrodes in the liquid crystal display panel  12  to change the orientation of the liquid crystal molecules, thereby controlling the transmittance of the light transmitted through the liquid crystal display panel  12 . 
     The backlight unit  20  is a lighting device for illuminating the whole surface of the liquid crystal display panel  12  from behind the liquid crystal display panel  12  and comprises a light exit opening  24  having substantially a same shape as the image display plane of the liquid crystal display panel  12 . 
     As illustrated in  FIGS. 1 and 2 , the illustrated example of the backlight unit  20  comprises two light source units  28  ( 28   a  and  28   b ), a light guide plate  30 , three sheets of films  32  ( 32   a ,  32   b , and  32   c ), a reflection plate  34 , an upper housing  38 , and a lower housing  40 . 
       FIG. 2A  is a view representing the concept of the front side (the display side of the liquid crystal display panel  12 ) of the backlight unit  20  with the upper housing  38  removed;  FIG. 2B  is a cross section taken along line I-I of the backlight unit  20  of  FIG. 2A ;  FIG. 2C  is a cross section taken along line II-II of  FIG. 2A . 
     The lower housing  40  is a housing in the form of a rectangular solid opening on one side (the largest plane) and accommodates/holds the two light source units  28 , the light guide plate  30  comprising a rectangular light exit plane, the three sheets of films  32 , and the reflection plate  34  in their given positions. The light guide plate  30  is encased in the lower housing  40 , with the light exit plane  30   a  described later directed toward the opening side. A power unit casing  49  is provided on the rear side of the lower housing  40  to house power supply units that supply the light source units  28  with electrical power. 
     The upper housing  38  has the same shape as the lower housing  40  so that the lower housing  40  may be inserted into it as into a lid. The upper housing  38  is formed in its plane facing the opening with the light exit opening  24  for irradiating the rear side of the liquid crystal display panel  12  with light emitted from the light guide plate  30 . 
       FIG. 3  represents a concept of the light guide plate  30 .  FIG. 3A  is a perspective;  FIG. 3B  is a schematic side view (as seen from the direction normal to the shorter sides). 
     As illustrated in  FIG. 3 , the light guide plate  30  is a plate member having a rectangular light exit plane  30   a  that can contain the whole surface of the light exit opening  24  and be accommodated in the lower housing. The light guide plate  30  comprises the light exit plane  30   a , inclined planes  30   b  and  30   c  located on the opposite side from the light exit plane  30   a , and light entrance planes  30   d  and  30   e  being the end faces on the longer sides of the light exit plane. The light entrance planes  30   d  and  30  are rectangles having exactly the same shape and have the same thickness (dimension in the direction normal to the light exit plane). 
     As described above, the liquid crystal display device  10  is installed with one longer side located above the other or, in other words, in such a posture that the longer sides extend in the left-and-right direction. Accordingly, the light guide plate is installed so that the light entrance planes  30   d  and  30   e , the longer sides, are also installed with one located above the other. In the illustrated example, the light entrance plane  30   d  is located in the upper position by way of example. 
     In the illustrated example representing a preferred embodiment, the light guide plate  30  is thickest on the rear side in a position corresponding to a ridge line  30   f , which is parallel to a center line α, a bisector of the rectangular light exit plane  30   a  in the up-and-down direction (i.e., a bisector of the shorter sides), that is, thickest in a plane normal to the light exit plane  30   a  and passing through the center line α. Accordingly, the inclined plane  30   b  is a rear plane, which is a flat plane inclined with respect to the light exit plane  30   a  and connects the ridge line  30   f  and the side of the light entrance plane  30   d  closer to the rear side; the inclined plane  30   c  is a rear plane which is a flat plane inclined with respect to the light exit plane  30   a  and connects the ridge line  30   f  and the side of the light entrance plane  30   e  closer to the rear side. 
     The light entrance planes  30   d  and  30   e , having the same thickness, are symmetrical with respect to the inclined planes  30   b  and  30   c , the center line α, and the ridge line  30   f . Accordingly, as illustrated in  FIG. 3B , the light guide plate  30  has a configuration such that it gradually grows thicker with the increasing distance from the light entrance planes  30   d  and  30   e , becoming the thickest at the ridge line  30   f  corresponding to the center line α (the configuration being referred to as “wedge shape” below for the purpose of the invention). 
     The joint at the ridge line  30   f  between the inclined planes  30   b  and  30   c  is not limited to a joint where two flat planes meet; the otherwise flat planes may be curved close to the ridge line  30   f.    
     In the illustrated example, light is admitted through the two light entrance planes  30   d  and  30   e , propagates in the light guide plate  30  in the direction parallel to the shorter sides, and passes through the light exit plane  30   a  to illuminate the rear side of the liquid crystal display panel  12 . 
     There is no specific limitation to the thickness of the light entrance planes. If the light entrance planes are too thin, a sufficient amount of light cannot be admitted, and thus the light having a sufficient brightness cannot be emitted. Conversely, too thick light entrance planes add to the whole weight of the light guide plate, making the light guide plate unfit as an optical member of the liquid crystal display device and the like while reducing the light use efficiency as the light is allowed to easily pass through the light guide plate. Taking the above into consideration, the thickness of the light entrance planes  30   d  and  30   e  is preferably in a range of 0.5 mm to 3.0 mm. 
     The thickness at the ridge line  30   f  (thickness at the center of the light guide plate  30  in the propagating direction) is also not limited. If the thickness at the ridge line  30   f  is too great, the weight increases, making the light guide plate unfit as an optical member of the liquid crystal display device and the like, while reducing the light use efficiency as the light is allowed to easily pass through the light guide plate. Conversely, if the thickness there is too small, not only is shaping difficult, the effects to be produced by the wedge shape cannot be obtained sufficiently. Taking the above into consideration, the thickness of the light guide plate at the ridge line  30   f  is preferably in a range of 1.0 mm to 6.0 mm. 
     As in the illustrated example, the light guide plate  30 , shaped such that its thickness increases with the increasing distance from the light entrance planes makes it easier for the incoming light to travel still deeper into the light guide plate (in the direction away from the light entrance planes), thus improving the in-plane uniformity while maintaining the light use efficiency and further achieving a high-in-the-middle, bell-curve brightness distribution. In other words, such a shape achieves a uniform or a high-in-the-middle, bell-curve distribution where the conventional, flat light guide plate could only provide a distribution that is dark in the middle. 
     The light guide plate  30  preferably has fine scattering particles for scattering light therein and may be formed, for example, by extrusion molding or injection molding using a transparent resin into which scattering particles are kneaded and dispersed. 
     Transparent resin materials that may be used to form the light guide plate  30  include optically transparent resins such as PET (polyethylene terephthalate), PP (polypropylene), PC (polycarbonate), PMMA (polymethyl methacrylate), benzyl methacrylate, MS resins, and COP (cycloolefin polymer). 
     The scattering particles kneaded and dispersed into the light guide plate  30  are exemplified by TOSPEARL (trademark), silicone, silica, zirconia, and a derivative polymer. The light guide plate  30  containing such scattering particles is capable of emitting uniform illumination light through the light exit plane  30   a  with a greatly reduced level of brightness unevenness. 
     The diameter of the scattering particles dispersed in the light guide plate  30  used in the planar lighting device of the invention preferably is in a range of 4.0 μm to 12.0 μm both inclusive. The diameter of the scattering particles within that range is preferable as it permits achieving a high scattering efficiency, a great forward scattering property, and a small wavelength dependency, and optimally reduces uneven color representation. 
     The scattering particles used may have a single particle diameter, or scattering particles having a plurality of diameters may be mixed and used. 
     In the light guide plate  30 , the higher the light use efficiency is, the more preferable. This is because given a low light use efficiency, a light source capable of generating a greater output is needed to obtain a required brightness, and a light source capable of generating a greater output not only assumes a higher temperature and consumes a greater amount of electricity but also causes the light guide plate  30  to warp or expand considerably, making it impossible to achieve a required brightness distribution, i.e., a high-in-the-middle or bell-curve brightness distribution. Taking the above into consideration, the light use efficiency of the light guide plate is preferably 55% or more. 
     Further, thin, large-screen liquid crystal televisions require a high-in-the-middle (bell-curve) brightness distribution, wherein the area closer to the center of the screen is brighter than the periphery. Therefore, the light guide plate  30  preferably yields a brightness distribution in the light exit plane  30   a  such that the middle-high ratio, which represents a ratio of the brightness of the light emitted through the central area to the brightness of the light emitted through the areas closer to the light entrance planes, is in a range of 0% to 25% inclusive. 
     In the light guide plate  30 , at least one plane of the light entrance planes  30   d  and  30   e , the light exit plane  30   a , and the inclined planes  30   b  and  30   c , which act as light reflection planes, preferably has a surface roughness Ra of less than 380 nm. 
     When the light entrance planes have a surface roughness Ra of less than 380 nm, diffuse reflection on the surfaces of the light guide plate can be ignored or, in other words, diffuse reflection on the surfaces of the light entrance planes can be prevented and, thus, light admission efficiency can be improved. Further, when the light exit plane  30   a  is given a surface roughness Ra of less than 380 nm, transmission by diffuse reflection through the surface of the light guide plate can be ignored or, in other words, transmission by diffuse reflection on the surface of the light guide plate can be prevented and, therefore, light is allowed to travel further deep into the light guide plate by total reflection. Further, when the reflection planes are given a surface roughness Ra of less than 380 nm, diffuse reflection can be ignored or, in other words, diffuse reflection on these reflection planes can be prevented and, therefore, all the reflected light components are allowed to travel further deep into the light guide plate. 
     In the backlight unit  20  of the invention, a reflection plate for reflecting light toward the light guide plate  30  is preferably provided where necessary such as in the inclined planes  30   b  and  30   c  acting as light reflection planes, as in known light guide plates. Provision of a reflection plate improves the light use efficiency. 
     The reflection plate may be formed of any material as appropriate used for light guide plates including, for example, a resin sheet produced by kneading, for example, PET or PP (polypropylene) with a filler and then drawing the resultant mixture to form voids therein for increased reflectance; a sheet with a specular surface formed by, for example, depositing aluminum vapor on the surface of a transparent or white resin sheet; a metal foil such as an aluminum foil or a resin sheet carrying a metal foil; and a thin sheet metal having a sufficient reflective property on the surface. 
     As will be described, support members  84   a  to  84   e  are secured to the four corners and the center in the left-and-right direction of the upper side of the light exit plane  30   a  of the light guide plate  30  to suspend the light guide plate  30  and the films  32  described later from the light source units  28  (the light source supports  52 ) described later.  FIG. 3A  does not show the support member  84   c  provided at the center in the left-and-right direction of the upper side.  FIG. 3B  does not show the support members  84   a  to  84   e.    
     This will be described later in detail. 
     The support members  84   a  to  84   e  may be secured to the light guide plate  30  by any method, and any of the known methods may be used according to the material and shape of the light guide plate  30  and the support members. A method using an adhesive is preferable, however, in that no holes need be made and hence breaking or otherwise damaging the light guide plate  30  can be avoided. 
       FIG. 4A  is a schematic perspective illustrating a part (near an end portion in the left-and-right direction) of the light source unit (hereinafter also referred to as “light source”)  28 . 
     As illustrated in  FIG. 4A , the light source  28  comprises a plurality of LED chips (light emitting diodes)  50  and a light source mount  52  such that the LED chips  50  are arranged on the light source mount  52 . 
     Each LED chip  50  is a chip of a light emitting diode emitting blue light the surface of which has a fluorescent substance applied thereon. It has a given area of light emission face  58  for emitting white light. 
     Specifically, the LED chip  50  has a property of generating fluorescence as blue light emitted by the light emitting diode is transmitted through the fluorescent substance applied to the surface of the light emitting diode. Thus, when blue light is emitted from the light emitting diode, the fluorescent substance through which the blue light is transmitted also generates light so that the blue light emitted by the light emitting diode and transmitted through the fluorescent substance and the light emitted as the fluorescent substance generates fluorescence blend to produce and emit white light. 
     The LED chip  50  may for example be formed by applying a YAG (yttrium aluminum garnet) base fluorescent substance to the surface of a GaN base light emitting diode, an InGaN base light emitting diode, and the like. 
     Besides the LED chips  50 , the light source  28  of the planar lighting device of the invention may use various light emitting devices that can be applied to the planar lighting device. 
     For example, in the case of the liquid crystal display device  10  as illustrated by way of example, an LED unit formed using three kinds of LEDs, i.e., a red LED, a green LED, and a blue LED, may be used. Alternatively, a semiconductor laser (LD) may be used instead of an LED. 
     A light source support  52  is a long plate member having an L-shaped side elevation (cross section) formed by joining two long plates at right angles on longitudinal sides thereof. 
     In the illustrated example, the LED chips  50  are arranged at given intervals on the surface of the shorter side of the L-shaped configuration of the light source support  52 . The light source support  52  comprises a circuit board (not shown) for driving the LED chips  50 . 
     The light source  28   a  is disposed with the L-shaped configuration inverted upside down so that the surface bearing the LED chips  50  faces the light entrance plane  30   d  of the light guide plate  30  whereas the longer side of the L-shaped configuration faces the inclined plane  30   b  of the light guide plate  30  and that the light source  28   a  extends in the left-and-right direction of the light guide plate  30  to have the LED chips  50  face the light entrance plane  30   d.    
     The light source  28   b  is likewise disposed so that the surface bearing the LED chips  50  faces the light entrance plane  30   e  of the light guide plate  30  whereas the longer side of the L-shaped configuration faces the inclined plane  30   c  of the light guide plate  30 . Thus, light is admitted through the light entrance plane  30   d  or  30   e  and propagates in the light guide plate  30  to be emitted through the light exit plane  30   a.    
     As illustrated in  FIG. 4B , the illustrated example of the LED chips  50  have a rectangular shape such that their sides perpendicular to the direction in which the LED chips  50  are arrayed are shorter than their sides lying in the direction in which the LED chips  40  are arrayed or, in other words, their sides lying in the direction of thickness of the light guide plate  30  are the shorter sides. Expressed otherwise, the LED chips  50  each have a shape defined by b&gt;a where “a” denotes the length of the side perpendicular to the light exit plane  30   a  of the light guide plate  30  and “b” denotes the length of the side in the array direction. 
     Now, given “q” as the distance by which the arrayed LED chips  50  are spaced apart from each other, then q&gt;b holds. Thus, the length “a” of the side of the LED chips  50  perpendicular to the light exit plane  30   a  of the light guide plate  30 , the length “b” of the side in the array direction, and the distance “q” by which the arrayed LED chips  50  are spaced apart from each other preferably have a relationship satisfying q&gt;b&gt;a. 
     Providing the LED chips  50  each having the shape of a rectangle allows a thinner design of the light source to be achieved while producing a large amount of light. A thinner light source, in turn, enables a thinner design of the planar lighting device to be achieved. Further, the number of LED chips that need to be arranged may be reduced. 
     While the LED chips  50  each preferably have a rectangular shape with the shorter sides lying in the direction of the thickness of the light guide plate  30  for a thinner design of the light source, the present invention is not limited thereto, allowing the LED chips to have any shape as appropriate such as a square, a circle, a polygon, and an ellipse. 
     In the illustrated example, the LED chips  50  are arrayed in a single row but the invention is not limited this way; the LED chips  50  may be arrayed in a plurality of rows. 
     As schematically illustrated in  FIG. 4C , the light source support  52  of the upper light source  28   a  is provided through its plate bearing the LED chips  50  arrayed thereon with long holes  70   a  and  70   b  through which pins  60   a  and  60   b  of the lower housing  40  described later are passed and a through-hole  72  through which a support pin  62  of the lower housing  40  is passed. The light source support  52  further has support pins  74   a  and  74   b  secured thereto close to both ends thereof in the left-and-right direction for supporting support members  84   a  and  84   b  of the light guide plate  30  described later. 
     As illustrated in  FIG. 2A , the lower light source  28   b  has secured thereto support pins  74   d  and  74   e , which engage with the support members  84   d  and  84   e  secured to the light guide plate  30 . 
     This will be described later in detail. 
     As illustrated in  FIGS. 2B and 2C , three sheets of films  32  ( 32   a ,  32   b , and  32   c ) are disposed in front of the light guide plate  30  (the light exit plane  30   a ). 
     In the illustrated example, the film  32   a  is a diffusion sheet to diffuse the illumination light emitted through the light guide plate  30  (the light exit plane  30   a ) to reduce brightness unevenness. The film  32   b  is a prism sheet having micro prism arrays formed thereon. The film  32   c  is a diffusion sheet to diffuse the illumination light emitted through the film  32   b  (prism sheet) to reduce brightness unevenness. The prism sheet being the film  32   b  is disposed so that the ridge lines of the micro prism arrays lie parallel to the center line α (in the left-and-right direction). 
     The films  32  have long holes  90   a  and  90   b  and a through-hole  90   c  through which support pins  86   a ,  86   b  and  86   c  secured to the support members  84   a ,  84   b , and  84   c  are passed (see  FIG. 5A ). 
     The diffusion sheets and the prism sheet may be any known diffusion sheets and a known prism sheet. The sheets described in paragraphs [0028] through [0033] of JP 2005-234397 A by the Applicant of the present application may be cited as preferred examples. 
     While in the illustrated example, the backlight unit  20  uses the films  32  composed of two diffusion sheets ( 30   a  and  30   c ) and a prism sheet ( 30   b ) disposed between them, there is no limitation according to the invention to the order in which the films are disposed and the number in which they are provided. 
     These films are not limited to a prism sheet and diffusion sheets and various optical films may be used provided that they can reduce brightness unevenness of the illumination light emitted through the light exit plane  30   a  of the light guide plate  30 . For example, one may use optical films formed of transmittance adjusting films each comprising a number of transmittance adjusters consisting of diffusion reflectors distributed according to the brightness unevenness in addition to or in place of the diffusion sheets and the prism sheet described above. Besides these, one may use various films having a variety of functions that are used for backlight units (planar lighting devices). 
     In the backlight unit  20  according to the illustrated example, the upper light source  28   a  is suspended from the lower housing  40 , the light guide plate  30  is suspended from the light source  28   a , and the films  32  and the lower light source  28   b  are suspended from the light guide plate  30 . 
     Now, their configuration will be described referring to  FIGS. 2 ,  3 , and  5 A. 
     A support pin  62  is erected on the inner wall surface on the rear side of the lower housing  40  at the center in the left-and-right direction (LR). A support pin  60   a  is erected in the same position in the up-and-down (UD) direction as the support pin  62  but spaced a given distance toward one side in the left-and-right direction from the support pin  62 , and a support pin  60   b  is likewise erected in a position spaced the same distance toward the other side. 
     As described earlier, the light source support  52  of the upper light source  28   a  has a through-hole  72  located at the center in the left-and-right direction of the plate member bearing the LED chips  50  and formed through the plate member in the direction of thickness of the light guide plate  30 . Further, the light source support  52  has a long hole  70   a , also a through-hole, oriented in the left-and-right direction and spaced a given distance toward one side in the left-and-right direction from the through-hole  72  and a similar long hole  70   b  oriented in the left-and-right direction spaced the same distance toward the other side. Both long holes  70   a  and  70   b  are formed such that the distance from the center of the through-hole  72  to the center in the left-and-right direction of these holes is equal to the distance between the support pin  62  and the support pins  60   a  and  60   b.    
     In the backlight unit  20  according to the illustrated example, the support pin  62  of the lower housing  40  is inserted into the through-hole  72  of the light source  28   a , the support pin  60   a  of the lower housing  40  is inserted into the long hole  70   a  of the light source  28   a , and the support pin  60   b  of the lower housing  40  is inserted into the long hole  70   b  of the light source  28   a  so that the lower housing  40  supports the light source  28   a  in a given position. 
     The backlight unit  20  is disposed so that the shorter sides of the light guide plate  30  lie in the up-and-down direction (the longer sides being aligned one above the other). Thus, the light source  28   a , located above the light guide plate  30 , is supported so as to hang from the lower housing  40 . 
     The light source support  52  of the light source  28   a  has a support pin  74   a  erected on the end face on the front side of the plate member bearing the LED chips  50  arrayed thereon close to one end in the left-and-right direction and a support pin  74   b  erected at the same position in the up-and-down direction but close to the other end. A support pin  74   c  is erected in the same position in the up-and-down direction as the support pin  74 , etc. but at the center in the left-and-right direction. 
     As described above, the support members  84   a ,  84   b ,  84   d , and  84   e  are secured to the four corners of the light exit plane  30   a  of the light guide plate  30 . The support members  84   a ,  84   b  are secured to the upper side; the support members  84   d ,  84   e  are secured to the lower side. The support member  84   c  is secured to the upper side at the center thereof in the left-and-right direction. 
     The support members  84   a  and  84   b  secured to the end portions in the left-and-right direction of the upper side of the light guide plate  30  each have long holes  88   a  and  88   b , respectively, which, both a through-hole, are formed at the same position in the up-and-down direction thereof and oriented in the left-and-right direction. Both long holes  88   a  and  88   b  are formed such that the distance from the center of the light guide plate  30  to the center in the left-and-right direction of these holes (long holes) is equal to the distance between the through-hole  72  and the support pins  74   a ,  74   b  formed in the light source support  52 . 
     The support member  84   c  secured to the upper side at the center thereof has a through-hole  88   c  at the same position in the up-and-down direction as the above long hole  88   a , etc. but at the center in the left-and-right direction of the light guide plate  30 . 
     In the backlight unit  20  according to the illustrated example, the support pin  74   c  of the light source support  52  of the light source  28   a  is inserted into the through-hole  88   c , the support pin  74   a  of the light source  28   a  is inserted into the long hole  88   a  of the support member  84   a , and the support pin  74   b  of the light source  28   a  is inserted into the long hole  88   b  of the support member  84   b.    
     Thus, the light guide plate  30  is so supported by the light source  28   a  (the light source support  52  thereof), with the former suspended from the latter, that the light entrance plane  30   d  of the light guide plate  30  faces the LED chips  50 . Needless to say, the relative positions in the up-and-down direction of the support pins and the respective support members are set so that the distance between the light source  28   a  (the light emission face of the LED chips  50 ) and the light entrance plane  30   d  is appropriate. 
     The support member  84   a  secured to the light guide plate  30  has the support pin  86   a  erected thereon, the support member  84   b  has the support pin  86   b  erected thereon, and the support member  84   c  located at the center in the left-and-right direction has the support pin  86   c  erected thereon. 
     As illustrated in  FIG. 5A , a reinforcing member  92   a  is secured to the upper side of the films  32  at one end thereof in the left-and-right direction, a reinforcing member  92   b  is secured at the other end, and a reinforcing member  92   c  is secured at the center in the left-and-right direction. 
     The reinforcing members  92   a  and  92   b  each have long through-holes  90   a  and  90   b , respectively, which are formed at the same position in the up-and-down direction and oriented in the left-and-right direction. (The through-holes  90   a  and  90   b  are formed also through the film  32 .). Both long holes  90   a  and  90   b  are formed such that the distance from the center of the light guide plate  30  to the center in the left-and-right direction of these holes (long holes) is equal to the distance between the support pin  86   c  of the support member  84   c  of the light guide plate  30  and the through-hole  72  formed in the light source support  52  on the one hand and the support pin  86   a  of the support member  84   a  and the support pin  86   b  of the support member  84   b  on the other hand. The reinforcing member  92   c  located at the center in the left-and-right direction has a through-hole  90   c  (same as above) at the same position in the up-and-down direction as the support pins  86   a , etc. but at the center in the left-and-right direction. 
     In the backlight unit  20  according to the illustrated example, the support pin  86   c  of the support member  84   c  secured to the light guide plate  30  is inserted into the through-hole  90   c  of the reinforcing member  92   c , the support pin  86   a  of the support member  84   a  is inserted into the through-hole  90   a  of the reinforcing member  92   a , and the support pin  86   b  of the support member  84   b  is inserted into the through-hole  90   b  of the reinforcing member  92   b.    
     Thus, the films  32  are supported by the light guide plate  30  with the former suspended from the latter. 
     As described above, the support members  84   d  and  84   e  are secured to the lower side of the light guide plate  30  at both ends thereof in the left-and-right direction. The support member  84   d  has a through-hole  88   d  and the support member  84   e  has a long through-hole  88   e  that is formed at the same position in the up-and-down direction as the through-hole  88   d  and oriented in the left-and-right direction. 
     In the light source support  52  of the lower light source  28   b , support pins  74   d  and  74   e  are secured to the end face on the front side of the plate member bearing the LED chips  50  arrayed thereon. The support pin  74   d  is secured at a position spaced the same distance from the center in the left-and-right direction as the through-hole  88   d  of the support member  84   d  and also spaced the same distance from the center in the left-and-right direction as the center in the left-and-right direction of the long hole  88   e  of the support member  84   e.    
     In the backlight unit  20  according to the illustrated example, the support pin  74   d  of the lower light source  28   b  is inserted into the through-hole  88   d  of the support member  84   d  secured to the light guide plate  30 , and the support pin  74   d  of the lower light source  28   b  is inserted into the long hole  88   e  of the support member  84   e.    
     Thus, the light source  28   a  is so supported by the light guide plate  30 , with the former suspended from the latter, that the light entrance plane  30   e  of the light guide plate  30  faces the LED chips  50  of the light source  28   b . Needless to say, the relative positions in the up-and-down direction of the support pins and the respective support members are set so that the distance between the light source  28   b  (the light emission face of the LED chips  50 ) and the light entrance plane  30   e  is appropriate. 
     As will be apparent from the foregoing, in the backlight unit  20 , the upper light source  28   a  is supported by the lower housing  40  with the former suspended from the latter, the light guide plate  30  is supported by the light source  28   a  with the former suspended from the latter, and the films  32  and the lower light source  28   b  are supported by the light guide plate  30  with the former suspended from the latter, thereby disposing these components in their given positions inside the lower housing  40 . 
     In the backlight unit  20 , the heat of the light sources  28 , the moisture produced in the environments in which the backlight unit  20  is installed, and the like cause the light guide plate  30  and the films  32  to expand and contract. 
     In the conventional backlight units (planar lighting devices), the film is secured to the light guide plate and the support members. Accordingly, the film affects the expansion and contraction of the light guide plate, and the expansion and contraction of the film itself affect the light guide plate, leading to the expansion and contraction of the light guide plate caused by the film whereas the distortion of the film brings it into contact with the light guide plate. 
     According to the backlight unit  20  of the invention, however, the light guide plate  30  and the films  32  can be disposed with respect to each other in the same manner as if they were totally independent of each other in the up-and-down direction. This is achieved by disposing the films  32  suspended in a given position by a film support means secured to the light guide plate  30 . 
     Thus, even when the light guide plate  30  and the films  32  expand and contract, they do not affect each other. In addition, because of the support by suspension, the warping or distortion of the films  32  caused by the expansion and contraction in particular in the up-and-down direction can be prevented. Further, since the warping and distortion of the films  32  can be prevented, the contact between the light guide plate  30  and the films  32  can be prevented. In addition, the film can be supported with the gaps between the films  32  and the light guide plate  30  and between the films themselves appropriately maintained by a simple configuration where the films are solely suspended from the film support means provided on the light guide plate  30 . 
     Further, the films  32  disposed in front of the light guide plate  30  act as a moisture prevention member for the light guide plate  30  because the films  32  obstruct the opening of the upper housing  38  for the light guide plate  30 . Thus, according to the invention, the expansion and contraction of the light guide plate  30  due to moisture absorption can be greatly curbed, and the warping and distortion of the light guide plate  30  due to moisture absorption can also be greatly curbed. 
     Further, in the illustrated example, also the light guide plate  30  is disposed so that it is suspended from the upper light source  28   a  whereas the lower light source  28   b  is disposed so that it is suspended from the light guide plate  30  as a preferred embodiment. Thus, the light guide plate  30  itself can be prevented from warping and distorting. 
     The present invention is not limited to such a configuration, however. The light guide plate  30  and/or the lower light source  28   b  may be fixedly disposed using a known fixing means. 
     Although, in the illustrated example, the independent reinforcing members  92   a ,  92   b , and  92   c  are provided at both ends and at the center in the left-and-right direction of the upper side of the films  32  as illustrated in  FIG. 5A , the invention is not limited to such a configuration. 
     For example, as illustrated in  FIG. 5B , one may provide a single reinforcing member  94  extending in the left-and-right direction of the upper side of the films  32  having long holes  90   a  and  90   b  and a through-hole  90   c  formed at both ends and at the center in the left-and-right direction of this reinforcing member. 
     In the backlight unit  20  of the invention, there is no specific limitation to the gap between the light exit plane  30   a  of the light guide plate  30  and the sheet of the films  32  closest to the light guide plate  30  but the gap is preferably in a range of 0.05 mm to 0.2 mm. When the distance between them is in that range, the films  32  can optimally produce their optical properties, and the moisture prevention effect for the light guide plate  30  can be produced in a more preferable manner. 
     There is also no specific limitation to the gaps between the films  32  themselves but the gaps are preferably in a range of 0.05 mm to 0.2 mm. When the distances between the films  32  are in that range, the films  32  can optimally produce their optical properties, and the moisture prevention effect for the light guide plate  30  can be produced in a more preferable manner. 
     In the present invention, the gap between the light exit plane  30   a  and the films  32  and the gap between the films  32  themselves may be maintained each to an appropriate distance by, for example, forming a groove in the support pins, providing spacers such as washers between them, and the like. 
     In the backlight unit  20  according to the illustrated example, the engagement of the light guide plate  30  and the films  32  is achieved by means of the long holes that are oriented in the left-and-right direction and the support pins. This configuration permits absorbing the expansion and contraction of the light guide plate  30  and the films  32  in not only the up-and-down direction but also the left-and-right direction and preventing distortion and warping of the light guide plate  30  and the film  32  caused by the expansion and contraction in the left-and-right direction. 
     This also applies to the relationship between the lower housing  40  and the upper light source  28   a , the upper light source  28   a  and the light guide plate  30 , and the light guide plate  30  and the lower light source  28   b.    
     In the illustrated example, the support pin  86   c  is erected at the center in the left-and-right direction of the light guide plate  30 , and the through-hole  90   c  having about the same size as the support pin  86   c  (the through-hole  90   c  is larger only by a dimension for allowing the support pin  86   c  to pass therethrough) is formed at the center in the left-and-right direction of the films  32  whereby the support pin  86   c  is inserted into the through-hole  90   c  as a preferred embodiment. 
     Fixedly positioning the center in the left-and-right direction with respect to the left-and-right direction permits distribution of the left-and-right expansion and contraction onto both sides and, hence, halving the movement of the end portions caused by the expansion and contraction. In addition, provision of the support pin  86   c  and the through-hole  90   c  permits positioning the films  32  using these as a reference position. 
     This also applies to the relationship between the lower housing  40  and the upper light source  28   a  and the relationship between the upper light source  28   a  and the light guide plate  30 . 
     According to the invention, fixing the center in the left-and-right direction is not essential; the component parts may be suspended using only the long holes and the support pins provided at both ends in the left-and-right direction (or at positions spaced a given distance leftwards and rightwards from the center, respectively). 
     Although the support pins are erected on the support members of the light guide plate  30  and the through-holes are formed through the films  32  to suspend the films  32  from the light guide plate  30  in the illustrated example, the present invention is not limited to such a configuration; conversely, the pins may be erected on the films  32  and the through-holes may be formed in the support members of the light guide plate (or in the light guide plate). Alternatively, the support pins may be erected directly on the light guide plate  30  without providing the support members. 
     This also applies to the relationship between the lower housing  40  and the upper light source  28   a , the upper light source  28   a  and the light guide plate  30 , and the light guide plate  30  and the lower light source  28   b.    
     The liquid crystal display device  10  is basically configured as described above. 
     In the backlight unit  20 , light emitted by the light sources  28  provided opposite the upper and lower sides of the light guide plate  30  strikes the light entrance planes  30   d  and  30   e  of the light guide plate  30 . Then, the light admitted through the respective planes is scattered by scatterers contained inside the light guide plate  30  as will be described later in detail as the light travels through the inside of the light guide plate  30  and, directly or after being reflected by the inclined plane  30   b  or the inclined plane  30   c , is emitted through the light exit plane  30   a.    
     Thus, light emitted through the light exit plane  30   a  of the light guide plate  30  is transmitted through the films  32  and emitted through the light exit opening  24  to illuminate the liquid crystal display panel  12 . 
     The liquid crystal display panel  12  uses the drive unit  14  to control the transmittance for the light according to the position so as to display characters, figures, images, etc. on its surface. 
       FIG. 6  illustrates the concept of another example of the backlight unit according to the planar lighting device of the invention with the upper housing removed (as in  FIG. 2A ). 
     A backlight unit  100  illustrated in  FIG. 6  basically has the same configuration as the backlight unit  20  described earlier except for guide members  102  ( 102   a  to  102   d ). In the following, like components will be given like characters, and the description will be focused on the components different between these backlight units. 
     In the backlight unit  100  illustrated in  FIG. 6 , a guide member  102   a  is provided at a position corresponding to one end of the end face in the left-and-right direction of the upper light source  28   a , and a guide member  102   b  is provided at a position corresponding to the other end. Further, a guide member  102   c  is provided at a position corresponding to one end of the end face in the left-and-right direction of the lower light source  28   b , and a guide member  102   d  is provided at a position corresponding to the other end. 
     The guide members  102   a  and  102   b  for the upper light source  28   a  may be dispensed with in this embodiment. 
     As illustrated in  FIG. 7A , a groove  52   a  is formed so as to extend in the up-and-down direction in each outer end face of the longer side of the L-shaped configuration (i.e., that part extending in the up-and-down direction behind the light guide plate  30 ) of the light source support  52  forming a part of each light source  28 . 
       FIG. 7B  illustrates the concept of the front view of the guide member  102  (as seen from the left-and-right direction);  FIG. 7C  illustrates the concept of the side view thereof (as seen from the direction of the thickness of the light guide plate  30 ). 
     The guide member  102  comprises a base  104  in the form of a rectangular solid and a spring member  108 , which is formed by bending a central portion of a long flat plate, with the flat portions thereof secured to the base  104 . 
     The guide member  102  has the spring member  108  extending in the up-and-down direction and is secured to the inner wall of each of the lateral sides of the lower housing  40  with the bent portion of the spring member  108  inserted into the groove  52   a  of the respective light sources  28 . The groove  52   a  and the spring member  108  are disposed and configured so that the light sources  28  and the guide members  102  can slide in the up-and-down direction relative to each other. 
     In the backlight unit  100  provided with such guide members  102  according to the illustrated example, the engagement of the groove  52   a  and the spring member  108  prevents the light guide plate  30  from moving in the thickness direction. 
     Since the groove  52   a  and the spring member  108  both extend in the up-and-down direction, the expansion and the contraction of the light guide plate  30  in the up-and-down direction are not hindered. Further, since the spring member  108  has an elasticity in the left-and-right direction as is apparent from its shape, the spring member  108  does not hinder but properly follows the expansion and the contraction of the light guide plate  30  in the left-and-right direction. 
     As described earlier, the light source support  52  forming a part of each of the light sources  28  has an L-shaped structure having the LED chips  50  arrayed on the shorter side (lower side) of the L-shaped configuration and is disposed so that the surface bearing the LED chips  50  faces the light entrance planes ( 30   d ,  30   e ) of the light guide plate  30  and the longer side of the L-shaped configuration faces the rear side (inclined planes  30   b ,  30   c ) of the light guide plate. 
     The plate portion, the longer side of the L-shaped configuration of the light source support  52 , also acts as heat dissipation member for the light sources  28  for releasing heat of the LED chips  50 , etc. Therefore, the light source support  52  is preferably formed of a material having a good thermal conductivity. 
     Accordingly, the length of the longer side of the L-shaped configuration lying in the up-and-down direction preferably extends a maximum possible length without touching the rear side of the light guide plate  30  to obtain a good heat dissipation effect as schematically illustrated in  FIG. 8 . 
     Such a configuration permits obtaining a maximum heat dissipation effect and greatly reducing the quantity of or eliminating the need to use grease generally used for heat dissipation in backlight units. 
     The effect of such a configuration is significantly great where, in particular, the lower light source  28   b  is also disposed so that it is suspended from the light guide plate  30  as in the illustrated example of the backlight unit  20 , the obtained heat dissipation efficiency is poor because the lower light source  28   b  is not in contact with the housing, etc. However, the configuration wherein the light source support  52  is given the maximum possible length enables an excellent heat dissipation effect to be obtained. 
     In all the backlight units described above, the two opposite end faces (lateral planes) of the plate-like light guide plate  30  are adapted to be light entrance planes but the invention is not limited to such a configuration; all the four end faces of the plate-like light guide plate  30  may be light entrance planes or only one of the end faces may be a light entrance plane. 
     The light entrance planes need not necessarily be provided on the longer sides of the light guide plate  30  and may be provided on the shorter sides; the light entrance planes need not necessarily be the upper and lower end faces of the light entrance plane  30  but may be the end faces in the left-and-right direction. 
     While the light guide plate of the backlight unit according to the illustrated examples has the above-described wedge shape where the thickness gradually increases with the increasing distance from the two light entrance planes, the invention is not limited to such a configuration and permits the use of various light guide plates used in a variety of planar lighting devices. 
     For example, the light guide plate may be in the form of a flat plate or may have a configuration where the rear side gradually decreases in thickness with the increasing distance from the two light entrance planes. Alternatively, one may use a light guide plate having a configuration having a ridge line located away from the center line α and closer to one of the end faces so that the one end face close to the ridge line is adapted to be a light entrance plane. 
     While the planar lighting device of the invention has been described above in detail, the present invention is not limited in any manner to the above embodiments and various improvements and modifications may be made without departing from the spirit of the invention.