Patent Publication Number: US-2005141242-A1

Title: Optical member and lighting apparatus

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to an optical member and a lighting apparatus.  
      A lighting apparatus for emitting light through an area has been known. Such a lighting apparatus includes a light emitting element such as a light emitting diode (LED), cold cathode tube or electroluminescent (EL) element as a light source. One lighting apparatus is an organic EL panel comprising an organic EL element provided on a transparent substrate. The organic EL panel can be used as backlight for a liquid crystal display apparatus.  
      The area of the light exit surface of the organic EL panel equals the projection area of an organic EL layer in the organic EL element. A further increase in size of the light exit surface of the organic EL panel is currently required, but it is not easy to meet the requirement by increasing the projection area of the organic EL layer. This is because the thickness of the organic EL layer is normally very small, i.e., about several tens to hundreds of nanometers, and it is thus difficult to uniformly form an organic EL layer having a large projection area with a good yield. Thus, it has been proposed that a plurality of cells each including one organic EL element be arranged on a plane to meet the requirement for increase in size of the display surface of the display apparatus and the light exit surface of the organic EL panel (e.g. see Japanese Patent Laid-Open No. 2001-52858).  
      However, a terminal and a wire for connecting an anode and a cathode of each organic EL element to an external power supply are usually placed on a periphery of the transparent substrate of the organic EL element. Accordingly, light from the organic EL element is not emitted from a section of the transparent substrate between adjacent organic EL elements. Thus, a lighting apparatus emitting light uniformly can not be obtained if a plurality of organic EL elements are simply arranged on a plane.  
      Japanese Laid-Open Patent Publication No. 2002-214411 discloses a lighting apparatus comprising an optical sheet or optical member, wherein the optical sheet or optical member has a plurality of raised portions for performing the function of scattering light. As shown in  FIGS. 6 and 7 , the optical sheet comprises a base sheet section  52  and a diffusion section  53  composed of a plurality of raised portions  54 . The raised portions  54  are mutually spaced. The base sheet section  52  and the raised portion  54  each include a scattering material  51 . A trough  55  is provided between adjacent raised portions  54 . An optical waveguide (not shown) is placed below the optical sheet, and light from the optical waveguide enters the base sheet section  52 . A cross section of the raised portion  54  is a tetragon of 113 μm square, the height of the raised portion  54  is 60 μm, and the pitch between raised portions  54  is 195 μm. The optical member has a configuration the same as that of the optical sheet except that its thickness is larger than that of the optical sheet. The optical sheet and the optical member are used for reducing unevenness of luminance of light from the optical waveguide. In a lighting apparatus comprising the optical sheet or optical member, it is required to arrange the optical sheet or optical member and the light emitting element more satisfactorily.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is an objective of the present invention to provide a lighting apparatus capable of emitting light in a suitable manner and to provide an optical member suitable for such a lighting apparatus.  
      To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an optical member is provided. The optical member includes a substrate section that permits light transmission. The optical member has a first surface and a second surface. The first surface and the second surface are on opposite sides of the substrate section. The first surface functions as a light exit surface. A guide section is provided on the second surface of the substrate section. A plurality of housing sections each houses a light emitting element. The housing sections are defined by the guide section. Light emitted from a side face of the light emitting element housed in each housing section is guided to the substrate section by the guide section.  
      The present invention also provides a lighting apparatus. The lighting apparatus includes the above optical member and a plurality of light emitting elements. The light emitting elements are each housed in its corresponding housing section of the optical member.  
      Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
       FIG. 1  is a schematic sectional view of a lighting apparatus according to a first embodiment of the present invention;  
       FIG. 2  is a schematic plan view of the lighting apparatus shown in  FIG. 1 ;  
       FIG. 3  is a schematic sectional view of the lighting apparatus according to a second embodiment of the present invention;  
       FIG. 4  is a schematic sectional view of the lighting apparatus according to another embodiment of the present invention;  
       FIG. 5  is a schematic plan view of an optical member provided in the lighting apparatus shown in  FIG. 4 ;  
       FIG. 6  is a partial schematic view of an optical sheet according to prior art; and  
       FIG. 7  is a partial schematic view showing the action of the optical sheet shown in  FIG. 6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      An optical member and a lighting apparatus according to the first embodiment of the present invention will now be described with reference to  FIGS. 1 and 2 .  
      As shown in  FIG. 1 , a lighting apparatus  11  comprises an optical member  12  and a plurality of organic EL elements  14 . Each organic EL element  14  is housed in a corresponding one of a plurality of housing sections  13  provided in the optical member  12  and functions as a light emitting element.  
      The optical member  12  comprises a substrate section  15  that permits light transmission and a guide section  16 . The substrate section  15  comprises a light exit surface  15   a , and the guide section  16  is provided on a surface of the substrate section  15  that faces away from the light exit surface  15   a . In other words, the light exit surface  15   a  corresponds to a first surface which is one of first and second surfaces being on opposite sides of the substrate section  15 , and the guide section  16  is provided on the second surface of the substrate section  15 . The guide section  16  guides light, emitted from a side face of the organic EL element  14  housed in each housing section  13 , to the substrate section  15 .  
      The guide section  16  is integral with the substrate section  15 . The substrate section  15  and the guide section  16  may be made of, for example, a transparent acrylic resin. In this specification, the term “transparent” means that at least visible light is allowed to pass.  
      The guide section  16  protrudes from the second surface of the substrate section  15 . As shown in  FIG. 2 , the guide section  16  has a planar tetragonal frame shape corresponding to the planar tetragonal shape of the organic EL element  14 . The housing sections  13  are defined by the substrate section  15  and the guide section  16 . As shown in  FIG. 1 , the depth of each housing section  13  is almost equal to the thickness of the corresponding organic EL element  14 . Openings of the housing sections  13  are closed with a bottom plate  18 .  
      As shown in  FIG. 1 , each organic EL element  14  comprises a transparent substrate  19 , a first electrode  20 , an organic EL layer  21  and a second electrode  22 . The first electrode  20 , the organic EL layer  21  and the second electrode  22  are provided on the transparent substrate  19  in this order. Each organic EL element  14  is covered with a protective film  23  so that the organic EL layer  21  does not contact outside air. The protective film  23  functions to prevent permeation of at least moisture and oxygen. The protective film  23  may be made of, for example, silicon nitride. In each organic EL element  14 , light emitted from the organic EL layer  21  is made to pass through the transparent substrate  19  to the outside. That is, each organic EL element  14  has a bottom emission structure. Organic EL elements  14  are electrically connected in series.  
      Each transparent substrate  19  is made of glass. Each transparent substrate  19  comprises an incident surface  19   a  and a light exit surface  19   b  being on opposite sides of the transparent substrate  19 . The incident surface  19   a  of each transparent substrate  19  contacts the first electrode  20 . Each organic EL element  14  is housed in the corresponding housing section  13  with the light exit surface  19   b  of the transparent substrate  19  facing the substrate section  15  and a side face  19   c  of the transparent substrate  19  contacting the guide section  16 . The side face  19   c  of the transparent substrate  19  is at least part of the side face of the organic EL element  14 . The light exit surface  15   a  of the substrate section  15  is orthogonal to the side face  19   c  of the transparent substrate  19 .  
      Each first electrode  20  is made of a transparent conductive material such as indium tin oxide (ITO). The first electrode  20  functions as an anode.  
      Each organic EL layer  21  has a well known configuration. Each organic EL layer  21  may be composed of a hole injection layer, a light emitting layer and an electron injection layer arranged in this order from the first electrode  20  side, or may be composed of a hole injection layer, a hole transportation layer, a light emitting layer and an electron transportation layer arranged in this order from the first electrode  20  side. Light emitted from each organic EL layer  21  is white light.  
      Each second electrode  22  is made of a metal such as aluminum, and has light reflectivity. The second electrode  22  functions as a cathode.  
      The guide section  16  includes light scattering bodies  17 . Each light scattering body  17  comprises an interface for scattering light emitted from the side face of the organic EL element  14  (more specifically, light exiting from the side face  19   c  of the transparent substrate  19  and entering the guide section  16 ). In this specification, “scattering” includes reflection and refraction. Each light scattering body  17  may be a scar or mark caused by application of laser light, or may be a portion of the guide section  16  different in refraction index from other portions of the guide section  16 . The shape of each light scattering body  17  is not specifically limited, but may be appropriately designed according to the thickness and the width of the guide section  16 , and may be, for example, spherical or circular in cross section. However, it is desirable that the light scattering bodies  17  should be capable of guiding light entering the guide section  16  toward the substrate section  15  efficiently.  
      The scattering bodies  17  may be formed by application of a laser marking method as disclosed in Japanese Laid-Open Patent Publication No. 2001-276985. According to the laser marking method, high-output laser light is converged into a transparent material, whereby degeneration of the transparent material associated with damage, a change in index of refraction and a change in density occurs only near a focal point. Thus, if the optical member  12  is placed on a stage capable of being moved three-dimensionally, and the optical member  12  is three-dimensionally moved while laser light is converged into the guide section  16  of the optical member  12  using a lens, the light scattering bodies  17  are formed at predetermined positions in the guide section  16 .  
      For a laser light source, for example, an Nd-YAG laser is used. The laser light source is preferably a pulse laser because of easy control. As the pulse width of the laser decreases, it becomes easier to make the marking depth uniform. In this respect, a laser light source having a pulse width equal to or less than a subnano second (e.g. femtosecond laser having a pulse width in the order of 10 to 15 femtoseconds) is useful.  
      A light reflection film  24  functioning as a light reflection portion is provided at the end of guide section  16  located away from the substrate section  15 , i.e., at the leading end of guide section  16 . The light reflection film  24  is made of, for example, a metal such as aluminum.  
      The actual thickness of the transparent substrate  19  is, for example, about 0.5 to 1 mm, and the actual thicknesses of the first electrode  20 , the organic EL layer  21  and the second electrode  22  are, for example, about several tens to 1000 nm. Thus, ratios of the thicknesses of the first electrode  20 , the organic EL layer  21  and the second electrode  22  to the thickness of the transparent substrate  19  in  FIG. 1  are different from those in actuality. The actual thickness of the protective film  23  is equal to or greater than the actual thicknesses of the first electrode  20 , the organic EL layer  21  and the second electrode  22  but in  FIG. 1 , the thickness of the protective film  23  is smaller than the thicknesses of the first electrode  20 , the organic EL layer  21  and the second electrode  22  for the sake of illustrative convenience. Relative sizes of the organic EL element  14 , the substrate section  15 , the guide section  16  and the transparent substrate  19  in  FIG. 1  are also different from those in actuality. In this way, the relative size of each component of the lighting apparatus  11  shown in  FIG. 1  is different from those in actuality.  
      Not only the light scattering bodies  17  of the guide section  16 , but also the substrate section  15 , performs the function for scattering light. However, the haze level of the substrate section  15  is set to be small so that the substrate section  15  does not scatter light more intensively than the light scattering bodies  17 .  
      Operation of the lighting apparatus  11  shown in  FIG. 1  will now be described.  
      The lighting apparatus  11  is placed on, for example, a back surface (surface opposite to the display surface) of a transmissive liquid crystal panel (not shown) and used as a backlight. When the lighting apparatus  11  is ON, a voltage is applied between the first electrode  20  and the second electrode  22  and as a result, the organic EL layer  21  emits light. Light emitted from the organic EL layer  21  enters the transparent substrate  19  through the first electrode  20 . Among light entering the transparent substrate  19 , light with the angle of incident to the light exit surface  19   b  smaller than the critical angle exits from the transparent substrate  19 , and then enters the substrate section  15  of the optical member  12 . Light with the angle of incidence to the light exit surface  19   b  larger than the critical angle is totally reflected on the light exit surface  19   b , and therefore it does not exit from the transparent substrate  19  through the light exit surface  19   b . The totally reflected light travels through the transparent substrate  19  toward the side face  19   c , and exits from the transparent substrate  19  through the side face  19   c . Thus, the organic EL element  14  emits light from not only the top face but also the side face.  
      The light exiting from the organic EL element  14  through the side face  19   c  enters the guide section  16  of the optical member  12 . The light entering the guide section  16  is reflected or refracted at the light scattering bodies  17 . Part of the light entering the guide section  16  travels toward the substrate section  15  as a result of reflection or refraction at the light scattering bodies  17 , and then exits from the substrate section  15  through the light exit surface  15   a . The light exiting from the substrate section  15  through the light exit surface  15   a  in this way is applied to a liquid crystal panel, and an image is displayed on the display surface of the liquid crystal panel.  
      In this way, in the lighting apparatus  11  shown in  FIG. 1 , not only light exiting from the top face of the organic EL element  14 , but also light exiting from the side face of the organic EL element  14  are effectively used for display of images.  
      The first embodiment provides the following advantages.  
      (1) According to the lighting apparatus  11  shown in  FIG. 1 , light emitted from the side face of the organic EL element  14  is guided to the substrate section  15  by action of the guide section  16  to exit from the substrate section  15  through the light exit surface  15   a . Thus, a larger amount of light exits from the substrate section  15  through the light exit surface  15   a  compared to the case where the guide section  16  is absent. The light guided to the substrate section  15  by the action of the guide section  16  exits from a portion of the substrate section  15  located between adjacent organic EL elements  14 , and therefore unevenness of luminance between a portion of the substrate section  15  corresponding to each organic EL element  14  and a portion of the substrate section  15  located between adjacent organic EL elements  14  is reduced. According to the lighting apparatus  11 , light can be made to exit uniformly from the substrate section  15  through the light exit surface  15   a.    
      (2) The guide section  16  is integral with the substrate section  15 . Thus, the optical member  12  is produced more easily compared to a configuration in which the guide section  16  is independent of the substrate section  15 .  
      (3) The guide section  16  includes the light scattering bodies  17 . Light emitted from the side face of the organic EL element  14  is guided to the substrate section  15  while being scattered and diffused by the light scattering bodies  17 . Thus, the light guided to the substrate section  15  exits uniformly from the light exit surface  15   a  without the necessity to strongly scatter the light at the substrate section  15 . On the other hand, if light entering the guide section  16  is simply guided to the substrate section  15  by a mirror surface, it is required to scatter light strongly at the substrate section  15  in order for the light to exit uniformly from the light exit surface  15   a.    
      (4) The light reflection film  24  is provided at the leading end of the guide section  16 . The light reflection film  24  reflects, toward the substrate section  15 , at least part of the light reflected or refracted so as to travel away from the substrate section  15  at the light scattering bodies  17 . Thus, a larger amount of light exits from the substrate section  15  through the light exit surface  15   a  compared to the case where the light reflection film  24  is absent and the surface of the leading end of the guide section  16  is optically absorptive or optically transparent.  
      (5) The organic EL element  14  is used as a light emitting element. Thus, reduction in thickness of the lighting apparatus  11  is more easily achieved compared to the case where a light emitting diode (LED) or cold cathode tube is used as a light emitting element.  
      (6) The organic EL element  14  has a bottom emission structure. Thus, light emitted from the side face of the organic EL element  14  is more efficiently guided to the substrate section  15  through the guide section  16  compared to an organic EL element having a top emission structure in which light emitted from the organic EL layer exits to the outside from a side opposite to the substrate.  
      (7) Organic EL elements  14  are electrically connected in series. Thus, the amounts of current passing through the organic EL elements  14  are the same, and the amounts of light emitted from the organic EL elements  14  are the same. Therefore, uniform light is easily made to exit from the light exit surface  15   a.    
      (8) Light emitted from each organic EL layer  21  is white light. Thus, if the lighting apparatus  11  is used as a backlight for a liquid crystal panel, a full color display can be provided by using a color filter.  
      (9) The light scattering bodies  17  are formed by application of a laser marking method. Thus, the light scattering bodies  17  can be formed at predetermined positions in the guide section  16  without scratching or marking the surface of the guide section  16 . Light scattering bodies  17  having various shapes can easily be formed at desired positions.  
      (10) Not only the light scattering bodies  17  of the guide section  16 , but also the substrate section  15 , performs the function for scattering light. Accordingly, light exiting from a portion of the light exit surface  15   a  corresponding to the guide section  16  is almost equivalent to light exiting from a portion of the light exit surface  15   a  not corresponding to the guide section  16 .  
      (11) The second electrode  22  located away from the transparent substrate  19  compared to the first electrode  20  is optically reflective. Thus, the amount of light exiting from the light exit surface  19   b  of the transparent substrate  19  increases compared to the case where the second electrode  22  is not optically reflective.  
      The second embodiment of the present invention will now be described with reference to  FIG. 3 . The same reference numerals are given to those components that are similar or the same as the corresponding components of the first embodiment, and detailed explanations are omitted.  
      In the lighting apparatus  11  according to the second embodiment shown in  FIG. 3 , prisms  25  and  26  functioning as light reflection portions are provided at the end of the guide section  16 . The prisms  25  and  26  are formed by notching the leading end of the guide section  16 . The prism  25  provided in a portion of the guide section  16  located in the outer periphery of the optical member  12  is larger in size than the prism  26  provided in a portion of the guide section  16  located between housing sections  13 . Specifically, the length of an inclined face  25   a  of the prism  25  is almost twice the length of the inclined face  26   a  of the prism  26 . The angle between the inclined face  25   a  of the prism  25  and the side face of the guide section  16  is set so that the angle of incidence of light vertically entering the side face of the guide section  16  relative to the inclined face  25   a  of the prism  25  is larger than the critical angle. The angle between the inclined face  26   a  of the prism  26  and the side face of the guide section  16  is set so that the angle of incidence of light vertically entering the side face of the guide section  16  relative to the inclined face  26   a  of the prism  26  is larger than the critical angle.  
      In  FIG. 3 , the thickness of a portion of the organic EL element  14  excluding the transparent substrate  19  is equal to the thickness of the transparent substrate  19  for the sake of convenience, and therefore the inclined faces  25   a  and  26   a  of the prisms  25  and  26 , respectively, do not match the side face  19   c  of the transparent substrate  19  in location. However, in actuality, the thickness of a portion of the organic EL element  14  excluding the transparent substrate  19  is smaller than {fraction (1/100)} of the thickness of the transparent substrate  19 , and therefore the inclined faces  25   a  and  26   a  of the prisms  25  and  26 , respectively, match the side face  19   c  of the transparent substrate  19  in location.  
      In the lighting apparatus  11  according to the second embodiment, the guide section  16  does not include the light scattering bodies  17 , but instead, a portion of the substrate section  15  corresponding to the guide section  16  includes the light scattering bodies  17 .  
      Light emitted from the transparent substrate  19  through the side face  19   c  enters the guide section  16 , and then travels through the guide section  16  toward the inclined face  25   a  or  26   a . The light arriving at the inclined face  25   a  or  26   a  is totally reflected toward the substrate section  15  at almost a right angle to the light exit surface  15   a  of the substrate section  15 . Thereafter, the light is scattered by the light scattering bodies  17  of the substrate section  15  to diffuse almost uniformly, and then exits from the light exit surface  15   a.    
      The second embodiment provides the following advantages in addition to advantages (1), (2), (5) to (8), (10) and (11).  
      (12) Light emitted from the side face of organic EL element  14  enters the guide section  16  and is then guided to the substrate section  15  by the action of the prisms  25  and  26 . Thus, the light entering the guide section  16  is efficiently guided to the substrate section  15  even if the light reflection film  24  shown in  FIG. 1  is absent.  
      (13) A portion of the substrate section  15  corresponding to the guide section  16  includes the light scattering bodies  17 . Thus, light guided to the substrate section  15  is scattered by the light scattering bodies  17  to exit uniformly from the light exit surface  15   a.    
      It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms.  
      The substrate section  15  and the guide section  16  may be formed independently and then integrated. For example, as shown in  FIGS. 4 and 5 , the optical member  12  may be composed of a lattice frame-shaped member  27  including light scattering bodies  17  and transparent members  28  fitted in meshes of lattices of the frame-shaped member  27 . The top face of each transparent member  28  is flush with the top face of the frame-shaped member  27 . In this case, the substrate section  15  is composed of a portion of the frame-shaped member  27  located in the periphery of the transparent member  28  and the transparent member  28 , and the guide section  16  is composed of other portion of the frame-shaped member  27 . The substrate section  15  has boundaries between the frame-shaped member  27  and the transparent members  28 , but the boundaries are hardly seen when light exits from the light exit surface  15   a  of the substrate section  15 . This is because the boundaries are hidden from view by light scattered by the light scattering bodies  17  included in a portion of the substrate section  15  corresponding to the guide section  16 .  
      In the lighting apparatus  11  shown in  FIG. 1 , a light reflection film may be provided at least in a portion of the bottom plate  18  corresponding to the guide section  16 , or the top face of the bottom plate  18  may be formed into a mirror face instead of providing the light reflection film  24  at the leading end of the guide section  16 .  
      The optical member  12  may be formed by bonding a lattice frame-shaped member to a planar substrate. In this case, the substrate and the frame-shaped member may be bonded together with an adhesive or by welding.  
      The optical member  12  may be housed in a bottomed box-shaped housing instead of using the bottom plate  18 . As a result, stability is improved compared to the case where the bottom plate  18  is used even if prisms  25  and  26  are provided at the leading end of the guide section  16 .  
      The light reflection film  24  may be omitted.  
      The light reflection film may be provided on the outer face of the substrate section  15 . The light reflection film preferably reflects light irregularly. In this case, at least part of the light exiting from the outer face of the substrate section  15  can be made to exit from the light exit surface  15   a.    
      Each light scattering body  17  may have any shape as long as it has an interface for scattering light entering the guide section  16 . For example, it may have a long and narrow shape extending along the thickness of the substrate section  15 .  
      The scattering bodies  17  may not be necessarily formed by application of the laser marking method. For example, the light scattering bodies  17  may be formed by dispersing into the guide section  16  beads having an index of refraction different from that of the guide section  16 .  
      Each organic EL element  14  may have a top emission structure instead of a bottom emission structure.  
      Light emitted from each organic EL element  14  is not limited to white light, but may be monochromatic light, or two or more colors of monochromatic light may be emitted. For example, each organic EL element  14  may emit red, blue, green or yellow monochromatic light. Alternatively, all organic EL elements  14  do not emit light of the same color, but some of the organic EL elements  14  may emit light different in color from light of other organic EL elements  14 .  
      The planar shapes for the optical member  12  and the organic EL element  14  may be a square, rectangle, triangle, pentagon, other polygon, circle or sector form.  
      The shape of the optical member  12  is not limited to an analog of the organic EL element  14 . For example, the optical member  12  may be tetragonal and the organic EL element  14  may be circular or triangle.  
      The optical member  12  may be made of a transparent resin other than a transparent acrylic resin, or glass.  
      A light emitting element other than the organic EL element  14 , such as an inorganic EL element, LED or cold cathode tube, may be used. However, use of the organic EL element  14  or inorganic EL element can contribute to a reduction in thickness of the lighting apparatus  11  compared to use of an LED, cold cathode tube or the like.  
      A diffusion sheet (scattering sheet) may be provided on the light exit surface  15   a  of the lighting apparatus  11 .  
      The first electrode  20  may be made of a transparent conductive material other than ITO, such as zinc oxide.  
      The first electrode  20  may be made transparent with a very thin metal foil. In this case, the thickness of the metal foil is preferably 50 nm or smaller, more preferably 0.5 to 20 nm.  
      The first electrode  20  may function as a cathode and the second electrode  22  may function as an anode. In this case, the configuration of the organic EL layer  21  is changed accordingly. Specifically, for example, the organic EL layer  21  is changed to be composed of an electron injection layer, a light emitting layer and a hole injection layer arranged in this order from the first electrode  20  side, or changed to be comprised of an electron injection layer, an electron transportation layer, a light emitting layer, a hole transportation layer and a hole injection layer arranged in this order from the first electrode  20  side.  
      The organic EL layer  21  may be composed only of a light emitting layer, or may be composed of at least one selected from the group consisting of a hole injection layer, a hole transportation layer, a hole injection and transportation layer, a hole blocking layer, an electron injection layer, an electron transportation layer, an electron injection and transportation layer and an electron blocking layer, and a light emitting layer.  
      The transparent substrate  19  may be made of a resin instead of glass. The transparent substrate  19  may be flexible if it is made of a resin. A transparent substrate  19  made of a resin is lighter than a transparent substrate  19  made of glass.  
      The lighting apparatus  11  may be used in applications other than for backlighting.