Patent Publication Number: US-6986600-B2

Title: Lighting device and display device using the lighting device

Description:
This is a Division of application Ser. No. 08/927,498 filed Sep. 11, 1997, now U.S. Pat. No. 6,612,709 which in turn is a Divisional Application of application Ser. No. 08/816,489 filed Mar. 13, 1997, now U.S. Pat. No. 5,704,703, which is a continuation of Ser. No. 08/204,686 filed Mar. 2, 1994, now abandoned. The disclosure of the prior applications is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention generally relates to lighting devices, and more particularly to a lighting device applied to a back-lighting device of a transmitted type liquid crystal display device. 
     Recently, a display unit has widely been employed in wordprocessors, personal computers or the like together with improvements in the display capacity and lighting performance. Further, it has been required to provide thin and light display units applicable to notebook-type devices and workstations using a large-size display. Particularly, there has been considerable activity in the development of high-luminance color display units. Under these situations, it has been necessary to provide high-luminance, high-efficiency lighting devices. 
     An edge light type back-lighting device has been applied to a liquid crystal display device. In such a type of back-lighting device, light is incident to a side surface of a transparent light conducting plate. The incident light is propagated through the light conducting plate so that the light is totally reflected at an interface of the light conducting place. The light conducting plate has a slope at its center portion, and is provided with white ink portions. Hence, the condition for total reflection is broken and the light is emitted via a light-emitting surface. 
       FIG. 1  is a diagram of a conventional edge-light type lighting device  90 , which includes two fluorescent tubes  81 , a light conducting plate  82  made of a transparent resin, incident surfaces  82   a  of the light conducting plate  82 , a back surface  82   b  of the light conducting plate  82  on which a diffusion reflection pattern is printed, a light-emitting surface  82   c  of the light conducting plate  82 , a reflection sheet  83 , an emission surface  84 , reflection mirrors  85  respectively enclosing the fluorescent tubes  81 , a linear prism  86 , a transparent diffusion sheet  87  and an inner propagation light  88  and an emitted light  89 . 
     As shown in  FIG. 1 , the fluorescent tubes  81  covered by the reflection mirrors  85  are arranged so that lights emitted therefrom enter into the incident surfaces  82   a  of the light conducting plate  82 . The light conducting plate  82  has slopes from the incident surfaces  82   a  so that the light conducting plate  82  becomes thinner toward the center thereof from the incident surfaces  82   a . These slopes of the light conducting plate  82  form the light-emitting surface  82   c . The diffusion reflection pattern which has a weighted white-ink pattern is formed on the back surface  82   b  so that the printed area of the pattern is increased as the distances from the fluorescent tubes  81  increase. The reflection sheet  83  provided on the surface opposite to the back surfaces  82   b  functions to efficiently and effectively emit light scattered by the diffusion reflection pattern. The linear prism  86  is provided on the emission surfaces  84  of the light conducting plate  82  in order to effectively collect the emitted light  89  in the normal line direction. Further, the diffusion sheet  87  which prevents the diffusion reflection pattern from being seen from the outside of the lighting device is disposed on the emission side of the linear prism  86 . 
     The diffused lights emitted from the fluorescent tubes  81  enters into the incident surfaces  82   a  of the light conducting plate  82  and are propagated through the light conducting plate  82  while the condition for total reflection is satisfied. The angle of the inner propagation light  88  becomes sharp by an angle θ of the light-emitting surface  82   c  each time the inner propagation light  88  is totally reflected. When the angle of the inner propagation light  88  becomes greater than the critical angle, the light is emitted via the light-emitting surface  82   c  as the emitted light  89 . The inner propagation light  88  totally reflected by the light-emitting surface  82   c  reaches the back surface  82   b  is emitted via the emission surface  84  because the diffusion reflection pattern breaks the condition for total reflection. 
     However, the prior art lighting device has the following disadvantages. All lights emitted from fluorescent tubes  81  are not emitted via the emission surface  84 . Some light emitted from one of the fluorescent tubes  81  is propagated through the light conducting plate  82  and is returned to the associated reflection mirror  85 . At this time, the returned light hits the reflection mirror  85  and loss of light occurs. Hence, the efficiency in use of light is not good. This problem may be reduced by means of the sloped light-emitting surface  82   c  of the light conducting plate  82 . However, it has not been possible to obtain sufficient and satisfactory lighting performance. 
     The linear prism plate  86  disposed between the light conducting plate  82  and the diffusion sheet  87  contributes to a reduction in the above loss of light to enhance luminance in the normal line direction. However, an interference will occur unless the pitch of linear prisms of the linear prism plate  86  is optimally selected with respect to the pitch between electrodes arranged in rows and columns of a display panel. Hence, in practice, the diffusion sheet  87  having a high degree of diffusion is used or optimal pitches of the linear prisms are selected with respect to the respective electrode pitches. However, the luminance in the normal line direction is decreased as the degree of diffusion is increased. Further, dies for the respective pitches of the linear prisms must be prepared, which leads to an increase in the production cost. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a lighting device in which the above disadvantages are eliminated. 
     A more specific object of the present invention is to provide a thin, light and efficient lighting device having a high uniform luminance distribution. 
     The above objects of the present invention are achieved by a lighting device comprising: a light source; a light conducting plate having an incident surface receiving light emitted from the light source, a back surface and a light-emitting surface; and a reflection member having a first portion facing the back surface, and a second portion located on a side opposite to a side of the light conducting plate on which the light source is located, the second portion being spaced apart from the light conducting plate so that light emitted from the light conducting plate is oriented toward an emission surface of the lighting device. 
     The above objects of the present invention are also achieved by a lighting device comprising: a light source; a light conducting plate having an incident surface receiving light emitted from the light source, a back surface and a light-emitting surface; and a reflection member having a first portion facing the back surface, and a second portion located on a side opposite to a side of the light conducting plate which the light source is located, the light conducting plate comprising a plurality of concave portions which are formed on the back surface and orient light emitted from the light conducting plate via the back surface toward an emission surface of the lighting device, a parameter related to the plurality of concave portions being weighted so that an even luminance distribution can be obtained on the emission surface. 
     The above objects of the present invention are also achieved by a lighting device comprising: a plurality of units located on a plane; the plurality of units respectively comprising: light sources; light conducting plates, each having an incident surface receiving light emitted from the light sources, a back surface, a light-emitting surface and an edge opposite to the incident surface; and a reflection member facing the back surfaces of the light conducting plate. 
     The above objects of the present invention are also achieved by a lighting device comprising; a plurality of units located; the plurality of units respectively comprising: light sources; light conducting plates, each having an incident surface receiving light emitted from the light sources, a back surface, a light-emitting surface and an edge opposite to the incident surface; a reflection member facing the back surfaces of the light conducting plate; light diffusing means, formed on the back surface of each of the light conducting plates, for diffusing light traveling in each of the light conducting plates in the vicinity of the incident surface more greatly than in other portions of each of the light conducting plates; a first linear prism plate partially allowing the lights emitted from the light conducting plates to pass through the first linear prism plate and partially reflecting the lights toward a space between the light conducting plates and the first linear prism plate; and a second linear prism plate collecting the lights from the first linear prims plate in a normal-line direction on the emission surface, wherein: the light conducting plates are arranged so that a space is defined by edges of the light conducting plates; and the light-emitting surface of each of the light conducting plates comprises an inclined surface which goes down toward the second portion of the reflection member. 
     Another object of the present invention is to provide a display device comprising any of the above-mentioned lighting device. 
     This object of the present invention is achieved by a display device comprising a display panel, a lighting device configured as mentioned above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram of a conventional edge-light type lighting device, 
         FIG. 2  is a cross-sectional view of a lighting device according to a first embodiment of the present invention; 
         FIG. 3  is a cross-sectional view showing the operation of the lighting device shown in  FIG. 2 ; 
         FIG. 4  is a graph showing an effect of a space provided in the lighting device shown in  FIG. 2 ; 
         FIG. 5A  is a cross-sectional view of a lighting device according to a second embodiment of the present invention; 
         FIG. 5B  is a graph of a luminance characteristic of the lighting device shown in  FIG. 5A ; 
         FIG. 6A  is a cross-sectional view of a lighting device according to a third embodiment of the present invention; 
         FIG. 6B  is a graph of a luminance characteristic of the lighting device shown in  FIG. 6A ; 
         FIG. 7  is a cross-sectional view of a lighting device according to a fourth embodiment of the present invention; 
         FIG. 8A  is a cross-sectional view of a lighting device according to a fifth embodiment of the present invention; 
         FIG. 8B  is a plan view showing the positional relationship between a linear prism plate shown in  FIG. 8A  and electrodes of a liquid crystal display device; 
         FIG. 9A  is a perspective view for explaining the directivity of a first lighting device; 
         FIG. 9B  is a graph of the directivity of the first lighting device shown in  FIG. 9A ; 
         FIG. 10A  is a perspective view for explaining the directivity of a second lighting device; 
         FIG. 10B  is a graph of the directivity of the second lighting device shown in  FIG. 10A ; 
         FIG. 11A  is a perspective view for explaining the directivity of a third lighting device; 
         FIG. 11B  is a graph of the directivity of the third lighting device shown in  FIG. 11A ; 
         FIG. 12A  is a perspective view for explaining the directivity of a fourth lighting device; 
         FIG. 12B  is a graph of the directivity of the fourth lighting device shown in  FIG. 12A ; 
         FIG. 13A  is a perspective view for explaining the directivity of a fifth lighting device; 
         FIG. 13B  is a graph of the directivity of the fifth lighting device shown in  FIG. 13A ; 
         FIG. 14A  is a plan view of a lighting device according to a sixth embodiment of the present invention; 
         FIG. 14B  is a cross-sectional view of the lighting device shown in  FIG. 14A  taken along line A–A′; 
         FIG. 14C  is a cross-sectional view taken along line B–B 7 ; 
         FIG. 15  is a cross-sectional view of a lighting device according to a seventh embodiment of the present invention; 
         FIG. 16  is a cross-sectional view of a lighting device according to an eighth embodiment of the present invention; 
         FIG. 17  is a cross-sectional view of a lighting device according to a ninth embodiment of the present invention; 
         FIG. 18  is a cross-sectional view of a lighting device according to a tenth embodiment of the present invention; 
         FIG. 19  is an exploded perspective view of the lighting device shown in  FIG. 18 ; 
         FIG. 20  is a cross-sectional view of a normal linear prism plate used in the lighting device shown in  FIG. 18 ; 
         FIG. 21  is a diagram for explaining a diffusion pattern formed on the back surface of a light conducting plate used in the lighting device shown in  FIG. 18 ; 
         FIG. 22  is a cross-sectional view of a structure of a special linear prism plate used in the lighting device shown in  FIG. 18 ; 
         FIG. 23  is a cross-sectional view showing the operation of the lighting device shown in  FIG. 18 ; 
         FIG. 24  is a cross-sectional view of a variation of the special linear prism plate; 
         FIG. 25  is a cross-sectional view of a special lenticular plate; 
         FIG. 26  is a diagram showing a lighting device according to an eleventh embodiment of the present invention; 
         FIG. 27  is a cross-sectional view showing a groove formed in a light conducting plate used in the lighting device shown in  FIG. 26 ; 
         FIG. 28  is a diagram showing a lighting device according to a twelfth embodiment of the present invention; 
         FIG. 29  is a diagram showing a lighting device according to a thirteenth embodiment of the present invention; 
         FIG. 30  is a diagram of a lighting device according to a fourteenth embodiment of the present invention; 
         FIG. 31  is a diagram of a lighting device according to a fifteenth embodiment of the present invention; 
         FIG. 32  is a perspective view showing an arrangement of a group of pits formed on the back surface of a light conducting plate used in the lighting device shown in  FIG. 31 ; 
         FIG. 33  is a diagram of a lighting device according to a sixteenth embodiment of the present invention; 
         FIG. 34  is a bottom view of the back surface of the light conducting plate shown in  FIG. 33 ; 
         FIG. 35  is a perspective view of the back surface of the light conducting plate shown in  FIG. 34 ; 
         FIG. 36  is a diagram of a lighting device according to a seventeenth embodiment of the present invention; 
         FIG. 37  is a diagram of a lighting device according to an eighteenth embodiment of the present invention; 
         FIG. 38  is a bottom view of the back surface of a light conducting plate shown in  FIG. 37 ; 
         FIG. 39  is a cross-sectional view of a lighting device according to a nineteenth embodiment of the present invention; 
         FIG. 40  is a cross-sectional view of a lighting device according to a twentieth embodiment of the present invention; 
         FIG. 41  is a diagram of a lighting device according to a twenty-first embodiment of the present invention; 
         FIG. 42  is a cross-sectional view of one of possible combinations of the above embodiments of the present invention; 
         FIG. 43  is a graph showing a variation in the pitch with which grooves or pits are arranged; and 
         FIG. 44  is a cross-sectional view of a variation of the structure shown in  FIG. 30 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  is a diagram of a lighting device  10  according to a first embodiment of the present invention. The lighting device  10  shown in  FIG. 2  includes a light source  1  formed with, for example, a fluorescent tube, a transparent light conducting plate  2  having an incident surface  2   a , a back surface  2   b  and a light-emitting surface  2   c , a reflection member  3  having a reflection surface, a sidewall surface  3   a  on a side of the light conducting plate  2  opposite to the side thereof on which the light source  1  is provided, an emission surface  4 , a space  5 , and a reflection mirror  6 . 
     The light-emitting surface  2   c  of the light conducting plate  2  is formed so that the thickness of the light conducting plate  2  becomes smaller as the distance from the light source  1  increases. The cross section of the light conducting plate  2  is of an approximately right-angled triangle and of a wedge shape. The space  5  is defined by the light-emitting surface  2   c , the sidewall surface  3   a  and the reflection member  3 . The most important feature of the first embodiment of the present invention is that the space  5  is provided as described above. 
       FIG. 3  shows the operation of the lighting device shown in  FIG. 2 . The light emitted from the light source  1  is reflected by the reflection mirror  6  having a cross section of a half circle intended to function to collect light. The light directly coming from the light source  1  and the light reflected by the reflection mirror  6  are combined, and the combined light enters into the light conducting plate  2  via the incident surface  2   a.    
     The light incident to the light conducting plate  2  enters at an angle of approximately ±42° with respect to the normal line of the incident surface according to the Snell&#39;s law. If the surfaces of the light conducting plate  2  are vertical to the incident surface  2   a , the angle of the light reaching the surfaces of the light conducting plate  2  will be approximately ±48° or greater with respect to the normal line of the surfaces of the light conducting plate  2 . Hence, the light will be totally reflected by the surfaces of the light conducting plate  2 , and will be propagated through the light conducting plate  2 . In the above manner, the propagation light will be emitted via the end surface of the light conducting plate  2  opposite to the incident surface  2   a  thereof facing the light source  1  if the surfaces of the light conducting plate  2  are vertical to the incident surface  2   a.    
     However, according to the first embodiment of the present invention, the light conducting plate  2  has the sloped light-emitting surface  2   c  forming an angle e with respect to the back surface  2   b  so that the thickness of the plate  2  becomes thinner as the horizontal distance from the light source  1  increases. Hence, the angle of the propagation light  7  entering into the light conducting plate  2  via the incident light  2   a  with respect to the light-emitting surface  2   c  becomes shape (increased) by the angle θ each time the propagation light  7  is totally reflected by the light-emitting surface  2   c . Then, some of the propagation light  7  is emitted, as the emitted light  8 , from the light-emitting surface  2   c  while the total reflection of the propagation light  7  is repeatedly performed, and goes toward the emission surface  4 . The remaining of the propagation light  7  is totally reflected repeatedly. Further, some of the propagation light  7  is emitted from the back surface  2   b  and is then reflected by the reflection surface  3  facing the back surface  2   b.    
     The emitted light  8  includes a component which directly reaches the emission surface  4  and another component which is reflected by the sidewall surface  3   a  of the reflection member  3  and is then emitted from the emission surface  4 . 
       FIG. 4  is a graph of the results of an experiment, and more particularly shows the amount of luminance vs. the size D of the space  5  provided in the first embodiment of the present invention. It will be noted that the size D of the space  5  corresponds to the distance between the edge of the light conducting plate  2  and the sidewall surface  3   a . The light conducting plate used in the experiment is 5 mm thick and 130 mm long. In the experiment, a linear prism plate and a transparent diffusion sheet were provided as shown in  FIG. 4 . A solid line curve indicates a characteristic obtained in a case where the reflection member has a mirror surface. A broken line curve indicates a characteristic obtained in a case where the reflection member has a diffusion pattern formed on the surface thereof. It can be seen from the graph of  FIG. 4  that the amount of luminance is increased as the size D of the space  5  is increased. It will be noted that a minimum amount of luminance was obtained when the size D of the space  5  is zero, that is, the sidewall surface  3   a  is in contact with the edge of the light conducting plate  2 . Further, a maximum amount of luminance was obtained when the sidewall surface  3   a  is not provided. In practice, it is preferable to set the distance D to 2 mm or more. 
     When the sidewall surface  3   a  is in contact with the edge of the light conducting plate  2 , a large amount of light is reflected thereby and loss of light becomes great. Further, in this case, light outgoing from the light conducting plate  2  and directly reaching the emission surface and light reflected by the sidewall surface and reaching the emission surface are superposed in an inner portion of the light conducting plate close to the edge thereof. Hence, an even luminance distribution cannot be obtained. 
     The space  5  located between the light conducting plate  2  and the sidewall surface  3   a  and sandwiched between the reflection member  3  and the emission surface  4  functions to introduce most of the emitted light  8  emitted from the light-emitting surface  2   c  toward the emission surface  4 . Hence, an even luminance distribution can be obtained and loss of light can be reduced. 
       FIG. 5A  is a diagram of a lighting device  20  according to a second embodiment of the present invention. The lighting device  20  shown in  FIG. 5A  includes a light source  11 , a transparent light conducting plate  12  having an incident surface  12   a , a back surface  12   b , a first light-emitting surface  12   c ′ and a second light-emitting surface  12   c ″, a reflection member  13  having a reflection surface, a sidewall surface  13   a  on a side of the light conducting plate  12  opposite to the side thereof on which the light source  11  is provided, an emission surface  14 , a space  15 , and a reflection mirror  16 . 
     The light conducting plate  12  is located so that the incident light  12   a  faces the light source of the fluorescent tube covered by the reflection mirror  16  having a cross-section of a half-circle shape functioning to collect light emitted by the light source  11 . The light conducting plate  12  has a light-emitting surface which is sloped so that the thickness of the light conducting plate  12  becomes smaller as the distance from the light source  11  increases. The above sloped light-emitting surface includes the first light-emitting surface  12   c ′ and the second light-emitting surface  12   c ″. The first light-emitting surface  12   c ′ is inclined at an angle θ 1  with respect to the back surface  12   b . The second light-emitting surface  12   c ″ is inclined at an angle θ 2 , which is greater than the angle θ 1 . Further, an interface between the first light-emitting surface  12   c ′ and the second light-emitting surface  12   c ″ is rounded so that the interface is curved. The reflection surface  13  faces the back surface  12   b  of the light conducting plate  12 . The sidewall surface  13   a  is provided on the side of the light conducting plate  12  opposite to the side thereof on which the light source  11  is provided. The emission surface  14  is provided on the upper side of the light conducting plate  12  opposite to the lower side thereof facing the reflection surface  13 . The sidewall surface  13   a  is spaced apart from the edge of the light conducting plate  12 , and the space  15  is defined by the light-emitting surfaces  12   c ′ and  12   c ″, the sidewall surface  13   a  and the emission surface  14 . 
     With the structure in which the slant angle θ 2  is greater than the slant angle θ 1 , it becomes possible to prevent the emitted light from being collected at the edge portion of the light conducting plate  12  and to appropriately distribute emission of light. Hence, it becomes possible to more efficiently utilize the light and make the even luminance distribution. Further, the rounded end of the light conducting plate  12  forming the second light-emitting surface  12   c ″ can be formed more easily than a sharp end thereof. Furthermore, the rounded end is not likely to be broken. For the above reasons, the yield can be improved. 
       FIG. 5B  is a graph showing a luminance distribution as a function of the angle θ 1  with the angle θ 2  kept constant (θ 2 &gt;θ 1 ). A solid line  401  relates to a case where the angle θ 1  is relatively small, and a solid line  402  relates to a case where the angle θ 2  is relatively large. As shown in  FIG. 5B , a small amount of light is emitted from the first light-emitting surface  12   c ′ and is propagated toward the edge of the light conducting plate  12  when the angle θ 1  is relatively small. Hence, a large mount of light is emitted from the edge portion of the light conducting plate  12 , and hence a large amount of luminance is obtained around the edge portion. When the angle θ 1  is relative large, the amount of light emitted from the first light-emitting surface  12   c ′ is increased, while the amount of light emitted from the second light-emitting surface  12   c ″ is decreased. Hence, the even luminance distribution can be obtained. However, the luminance distribution on the tip-end side is increased or decreased in proportion to the angle θ 1 . 
     The reflection surface  13  can be a diffusion surface or a mirror surface. When the reflection surface  13  is a diffusion surface, the light emitted from the back surface  13   b  of the light conducting plate  12  is diffused, whereby the amount of light emitted from the vicinity of the light source  11  is increased and the luminance distribution is further leveled. 
       FIG. 6A  is a diagram of a lighting device  30  according to a third embodiment of the present invention. The lighting device  30  shown in  FIG. 6A  includes a light source  21  formed with, for example, a fluorescent tube, a transparent light conducting plate  22  having an incident surface  22   a , a back surface  22   b , a first light-emitting surface  22   c ′ and a second light-emitting surface  22   c ″, a reflection member  23  having a reflection surface, a sidewall surface  23   a  on a side of the light conducting plate  22  opposite to the side thereof on which the light source  21  is provided, an emission surface  24 , a space  25 , and a reflection mirror  26 . the light conducting plate  22  shown in  FIG. 6A  differs from the light conducting plate  12  shown in  FIG. 5A  in that the first light-emitting surface  22   c ′ is a plane with shaped distribution portions and a diffusion reflection pattern is printed on the back surface  22   b . The shaped distribution portions are convex and concave portions formed on the surface of the first light-emitting surface  22   c ′. The diffusion reflection pattern is painted with white paint. The shaped distribution portions and the diffusion reflection pattern contribute to make the even luminance distribution. 
     Alternatively, it is possible to form a diffusion reflection pattern on the first light-emitting and provide the back surface  22   b  with shaped distribution portions. Further, it is possible to weight the density of diffusion reflection pattern taking into account the luminance distribution. For example, the sizes of the patterned areas are weighted. However, such weighting may be not needed when the lighting device is of a relatively small size and the slant angle θ 1  is relatively great. 
       FIG. 6B  is a graph of a luminance distribution obtained when a weighted diffusion reflection pattern is printed on the back surface  22   b  of the light conducting plate  22 . A broken line  403  indicates the luminance distribution made by light emitted from the first light-emitting surface  22   c ′, and a one-dot chained line  404  indicates the luminance distribution made by light emitted from the second light-emitting surface  22   c ″. Further, a solid line  405  indicates the whole luminance distribution. It can be seen from  FIG. 6B  that weighted diffusion reflection pattern formed on the back surface  22   b  contributes to making the luminance distribution even. Hence, it can be said that the lighting device shown in  FIG. 6A  efficiently utilizes light and has an even luminance distribution. 
       FIG. 7  is a diagram of a lighting device  40  according to a fourth embodiment of the present invention. The lighting device  40  shown in  FIG. 7  includes a light source  31  formed with, for example, a fluorescent tube, a transparent light conducting plate  32  having an incident surface  32   a , a back surface  32   b , a first light-emitting surface  32   c ′ and a second light-emitting surface  32   c ″, a reflection member  33  having a reflection surface, a sidewall surface  33   a  on a side of the light conducting plate  32  opposite to the side thereof on which the light source  31  is provided, an emission surface  34 , a space  35 , and a reflection mirror  36 . 
     The shape of the light conducting plate  32  used in the fourth embodiment of the present invention differs from the previously described light conducting plates  12  and  22 . The incident surface  32   a  includes a wavy surface having convex and concave portions. The axis of the wavy surface is parallel to the light source  31 . Further, a transparent diffusion reflection sheet  37  on which a diffusion reflection pattern is printed is provided between the back surface  32   b  of the light conducting plate  32  and the reflection surface  33 . 
     The fourth embodiment of the present invention is suitable for a relatively compact back-lighting device. The wavy surface which includes concave and convex portions formed on the incident light  32   a  and is parallel to the light source  31 . Hence, the direction in which the light entering into the light conducting plate  32  via the incident surface  32   a  is propagated can be set to be equal to or greater than ±42° with respect to the normal line on the incident surface  32   a  perpendicular to the back surface  32   b . Hence, it becomes possible to increase the amount of light obtained in the vicinity of the light source  31 . Further, the diffusion reflection sheet  37  provided between the back surface  32   b  and the reflection surface  33  functions to provide an even luminance distribution. 
       FIG. 8A  is a diagram of a lighting device  50  according to a fifth embodiment of the present invention. The lighting device  50  shown in  FIG. 8A  includes a light source  41  formed with, for example, a fluorescent tube, a transparent light conducting plate  42  having an incident surface  42   a , a back surface  42   b , a first light-emitting surface  42   c ′ and a second light-emitting surface  42   c ″, a reflection member  43  having a reflection surface, a sidewall surface  43   a  on a side of the light conducting plate  42  opposite to the side thereof on which the light source  41  is provided, an emission surface  44 , a space  45 , a reflection mirror  46 , a linear prism plate  47  and a diffusion sheet  48 . 
     The shape of the sidewall surface  43   a  differs from those of the sidewall surfaces  13   a ,  23   a  and  33   a  in that the sidewall surface  43  is a continuous curved surface extending from the reflection surface  43 . A weighted deflection pattern is printed on the back surface  32   b  of the light conducting plate  32 . The linear prism plate  47  is provided on the emission surface  44  in order to provide a directional emission in the normal line direction. Further, as shown in  FIG. 8B , the linear prism plate  47  is disposed so that the axis  47   a  thereof and the axes of a matrix arrangement of electrodes  49   a  and  49   b  of a display device placed over the linear prism plate  47  are neither parallel to nor orthogonal to each other. The diffusion sheet used to prevent the diffusion sheet  48  printed on the back surface  42   b  from being seen from the outside of the lighting device is provided above the linear prism plate  47 . 
     The curved sidewall surface  43   a  contributes to reducing loss of light emitted from the light conducting plate  42  whereby light can be efficiently utilized and a uniform luminance distribution can be obtained. Further, as shown in  FIG. 8B , the linear prism plate  47  is arranged at an angle φ (0°&lt;φ&lt;90°) with respect to the electrodes  49   a  and  49   b . With this arrangement, it is possible to prevent degradation of the display quantity due to interference. 
       FIG. 9A  is a perspective view of a first lighting device, and  FIG. 9B  is a graph of the directivity thereof. 
     The lighting device shown in  FIG. 9A  includes a light source  51  formed with, for example, a fluorescent tube, a transparent light conducting plate  52  having an incident surface  52   a , a back surface  52   b  and a light-emitting surface  52   c , and a reflection mirror  54 . The light-emitting surface  52   c  is inclined at an angle θ of 10° with respect to the back surface  52   b . The reflection mirror  54  is provided so as to cover the light source  51 . A light emitted from the light source  51  is propagated through the light conducting plate  52 , as indicated by reference number  58 . A reference number  59  indicates light emitted from the light-emitting surface  59   c . It will be noted that the structure shown in  FIG. 9A  does not employ a reflection surface or the like. 
       FIG. 9B  shows the directivity distribution characteristics around the edge of the light conducting plate  52  and the light source  51 . The vertical axis denotes a relative luminance, and the horizontal axis denotes the angle α of emission of light. A solid line  406  indicates the directivity characteristic around the tip-end, and a broken line  407  indicates the directivity characteristic around the light source. As shown in  FIG. 9B , there is little difference between the width of the directivity peak of the directivity characteristic curve related to the edge and that related to the light source  54  when the reflection surface is not provided. However, the amount of luminance obtained on the light source side is less than that obtained on the edge side, and the directivity on the light source side has an angle less than that on the edge side. It will be noted that the luminance peak shown in  FIG. 9B  is the reference of the relative luminance in graphs shown in  FIGS. 10B ,  11 B,  12 B and  13 B. 
       FIG. 10A  is a perspective view of a second lighting device, and  FIG. 10B  is a graph of the directivity thereof. In  FIG. 10A , parts that are the same as those shown in  FIG. 9A  are given the same reference numbers. A reflection member  53  having a reflection surface is provided below the light conducting plate  52  so as to face the back surface  52   b . The angle θ is set to 10°. The lighting device shown in  FIG. 10A  is made up of the fluorescent tube  51 , the reflection mirror  54 , the light conducting plate  52  and the reflection surface  53 . 
       FIG. 10B  shows two directivity distribution characteristics respectively obtained when the reflection surface  53  is a mirror surface and a diffusion reflection surface. A solid line  408  indicates the directivity characteristic obtained when the reflection surface  53  is a mirror surface, and a broken line  409  indicates the directivity characteristic obtained when the reflection surface  53  is a diffusion reflection surface. As shown in  FIG. 10B , the directivity peak of the directivity distribution curve  409  is almost the same as that obtained when no reflection surface is provided. However, the width of the directivity peaks are quite different. This is because light is emitted from the back surface  52   b  of the light conducting plate  52  at an angle sharper than the angle θ formed between the light-emitting surface  52   c  and the back surface  52   b . When the reflection surface  53  is a diffusion reflection surface, as shown in the curve  409 , the peak of luminance becomes lower, while the width of the peak becomes greater. This is because the emitted light  59  is distributed at portions on the light-emitting surface  52   c , and the luminance distribution obtained on the emission surface can be improved. 
       FIG. 11A  is a perspective view of a third lighting device, and  FIG. 11B  is a graph of the directivity thereof. In  FIG. 11A , parts that are the same as those shown in  FIG. 10A  are given the same reference numbers. A transparent diffusion reflection sheet  55  is provided between the back surface  52   b  of the light conducting plate  52  and the reflection surface  53 . A weighted diffusion reflection pattern is formed on the diffusion reflection sheet  55 . The reflection surface  53  is a mirror surface. The angle θ is set to 10°. The lighting device shown in  FIG. 11A  is made up of the fluorescent tube  51 , the reflection mirror  54 , the light conducting plate  52 , the reflection member  53 , and the diffusion reflection sheet  55 . 
       FIG. 11B  shows the directivity distribution characteristic of the lighting device shown in  FIG. 11A  equipped with the diffusion reflection sheet  55 . A solid line  410  indicates the directivity characteristic of the lighting device shown in  FIG. 11A . As shown in  FIG. 11B , the peak luminance level is decreased as in the case of the curve  409  shown in  FIG. 10B  in which the reflection surface  53  is a diffusion reflection surface. Instead, the width of the directivity peak is increased, because the emitted light  59  is obtained so that the light is distributed at portions on the light-emitting surface  52   c . Hence, the luminance distribution can be improved. 
       FIG. 12A  is a perspective view of a fourth lighting device, and  FIG. 11B  is a graph of the directivity thereof. In  FIG. 12A , parts that are the same as those shown in  FIG. 11A  are given the same reference numbers. A linear prism plate  56  is used instead of the transparent diffusion reflection sheet  55  and is provided between the back surface  52   b  of the light conducting plate  52  and the reflection surface  53 . The surface of the linear prims plate  56  on which concave and convex portions are formed faces the light conducting plate  52  so that the axis of the prism is parallel to the longitudinal direction of the lighting device. The angle θ is set to 10°. The lighting device shown in  FIG. 12A  is made up of the fluorescent tube  51 , the reflection mirror  54 , the light conducting plate  52 , the reflection member  53 , and the linear prism plate  56 . 
       FIG. 12B  shows the directivity distribution characteristic of the lighting device shown in  FIG. 12A  equipped with the linear prism plate  56 . A solid line  411  indicates the directivity characteristic of the lighting device shown in  FIG. 12A . As shown in  FIG. 12B , a high peak luminance level is obtained and the amount of emission of light around the light source  51  can be increased so that the luminance distribution can be improved. 
       FIG. 13A  is a perspective view of a fifth lighting device, and  FIG. 13B  is a graph of the directivity thereof. In  FIG. 13A , parts that are the same as those shown in  FIG. 10A  are given the same reference numbers. A linear prism plate  57  is attached to the incident surface  52   a  so as to face the light source  51  in such a manner that the prism axis of the plate  57  is perpendicular to the longitudinal direction of the light source  51 . The angle θ is set to 10°. The lighting device shown in  FIG. 13A  is made up of the fluorescent tube  51 , the reflection mirror  54 , the light conducting plate  52 , the reflection member  53 , and the linear prism plate  57 . 
       FIG. 13B  shows the directivity distribution characteristic of the lighting device shown in  FIG. 13A  equipped with the linear prism plate  57 . A solid line  412  indicates the directivity characteristic of the lighting device shown in  FIG. 13A  in the up and down direction (angle α) in which the linear prism plate  57  is provided. A broken line  413  indicates the directivity characteristic of the lighting device in the up and down direction (angle α) in which the linear prism plate  57  has been removed. A solid line  414  indicates the directivity characteristic of the light device shown in  FIG. 13A  in the left and right directions (angle β) in which the linear prism plate  57  is provided. A broken line  415  indicates the directivity characteristic of the light device in the left and right directions (angle β) in which the linear prism plate  57  has been removed. 
     As shown in  FIG. 13B , the linear prism plate  57  functions to sharpen the directivity in the left and right directions, as indicated by the characteristic curves  413  and  415 , and to improve the luminance characteristic of the light device. 
     A description will now be given, with reference to  FIGS. 14A ,  14 B and  14 C, of a lighting device  70  according to a sixth embodiment of the present invention.  FIG. 14A  is a plan view of the lighting device  70  according to the sixth embodiment of the present invention,  FIG. 14B  is a cross-sectional view taken along line A–A′ shown in  FIG. 14A , and  FIG. 14C  is a cross-sectional view taken along line B–B′ shown in  FIG. 14A . The lighting device  70  includes four units, each of which includes one light source and one light conducting plate. 
     The lighting device  70  shown in  FIGS. 14A through 14C  includes four light sources  61  formed with, fluorescent tubes, four transparent light conducting plates  62 , a reflection member  63  having a reflection surface, an emission surface  64 , a space  65 , a reflection mirror  66 , a linear prism plate  67 , a diffusion sheet  68 , and a light interrupting portion  69 . 
     The four units are arranged on the same plane as shown in  FIGS. 14A through 14C . The units may be any of the lighting devices according to the first through fifth embodiments of the present invention. The light sources  66  are located on the four sides of the lighting device and are covered by the reflection mirror  66 . The four light conducting plates  62  which are separately provided are arranged so that these plates face the corresponding light sources  61 . The shape of each of the light conducting plates  62  has a slope  62  inclined toward the edge thereof at an angle θ with respect to a back surface  62   b . Further, the width of each of the light conducting plates  62  becomes narrow toward the edge. A diffusion reflection pattern weighted taking into account the luminance distributions of the light conducting plates  62  is formed on the back surface  62   b  of each of the light conducting plates  62 . 
     The reflection member  63  has a convex reflection surface  63   a  of a quadrangular pyramid shape. The convex reflection surface  63   a  is located in the space  65  defined by the edges of the four light conducting plates  62 . The linear prism plate  67  is provided on the emission surface  64  so that the prism axis is neither parallel to nor orthogonal to the axes of the matrix arrangement of electrodes of a display device located above the linear prism plate  67 . The diffusion sheet  68  is used to prevent the diffusion reflection pattern printed on the back surface  62   b  from being seen from the outside of the lighting device. 
     As shown in  FIG. 14C , the light conducting plates  62  are separated from and spaced apart from each other in order to prevent rays propagated through the plates  62  from returning toward the light sources  61 . The space formed between the adjacent the plates  62  functions as the light interrupting space. A reflection side surface  62   d  is formed in each of the light conducting plates  62  and prevents light from being emitted from the side surfaces of the light conducting plates  62 . The reflection side surfaces  62  are inclined as shown in  FIG. 14C  so that light reflected thereby is oriented toward the emission surface  64 . 
     The lighting device  70  according to the sixth embodiment of the present invention is suitable for a large-scale lighting device required to provide a large amount of luminance. By arranging a plurality of units (which can be any of the lighting devices according to the first through fifth embodiments of the present invention), it is possible to efficiently utilize light and provide an even high-luminance distribution. 
     It may be possible to use a single light conducting plate having portions corresponding to the light conducting plates  62 . However, a lighting device having such a single light conducting plate will not provide a luminance as high as that of the lighting device shown in  FIGS. 14A through 14C . 
     Further, it is possible to use an arbitrary number of units rather than four units. For example, two units which are located on the same plane and face each other can be used. Further, instead of the diffusion reflection patterns printed on the back surfaces of the light conducting plates  62 , it is possible to employ other means for leveling the luminance distribution, as shown in  FIGS. 5A  through  FIG. 13 . 
       FIG. 15  is a partially cross-sectional side view of a lighting device  80  according to a seventh embodiment of the present invention. The lighting device  80  includes a plurality of light sources  71  formed with, for example, fluorescent tubes, a plurality of light conducting plates  72  having incident surfaces  72   a  and light-emitting surfaces  72   b , a plurality of reflection members  73  having reflection surfaces, a plurality of reflection mirrors  74 , and a diffusion sheet  75 . 
     The lighting device  80  includes a plurality of units, each having one light source  71 , one light conducting plate  72  and one reflection member  73 . Each of the units may be any of the lighting devices used in the first through fifth embodiments of the present invention. The units are arranged side by side so that the light-emitting surfaces  72   b  of the light conducting plates  72  face upward. The side surface of each of the light conducting plates  72  are adjacent to the reflection members  73 . The angle θ formed by each light-emitting surface  72   b  and each reflection member  73  is, for example, 30°. The light conducting plates  72  are made of a transparent member such as an acrylic resin. The diffusion sheet  75  functions to provide a uniform luminance distribution and prevent the reflection members  73  from being seen from the outside of the lighting device  80 . Light emitted from the light-emitting surface  72   b  of one unit except for light directly reaching the diffusion sheet  75  is reflected by the reflection surface  73  of the adjacent unit, and is oriented to the diffusion sheet  75 . 
     The lighting device  80  according to the seventh embodiment of the present invention is suitable for a large-scale lighting device required to provide high luminance. The arrangement of units shown in  FIG. 15  makes it possible to provide an even luminance characteristic, a sharp directivity and high efficiently in use of light. 
       FIG. 16  is a partially cross-sectional side view of a lighting device  80 ′ according to an eighth embodiment of the present invention. The lighting device  80 ′ includes a plurality of light sources  71 ′ formed with, for example, fluorescent tubes, a plurality of light conducting plates  721  having incident surfaces  72   a ′ and light-emitting surfaces  72   b ′, a plurality of reflection members  73 ′ having reflection surfaces, a plurality of reflection mirrors  74 ′, and a diffusion sheet  75 ′. The shape of the light conducting plates  72 ′ differs from that of the light conducting plates  72  shown in  FIG. 15  in which the light conducting plates  72 ′ have a cross section of an approximately equilateral triangle. The light conducting plates  72 ′ are arranged side by side via the reflection members  73 ′ so that the apexes thereof face upward. Two surfaces forming the apex of each of the light conducting plates  72 ′ function as light-emitting surfaces  72   b ′. The angle of each apex is, for example, 30°. 
     The lighting device  80 ′ has a directivity sharper than that of the lighting device  80 , and utilizes light more efficiently. 
       FIG. 17  is a partially cross-sectional side view of a lighting device  80 ″ according to a ninth embodiment of the present invention. The lighting device  80 ″ includes a plurality of light sources  71 ″ formed with, for example, fluorescent tubes, a plurality of light conducting plates  72 ″, a plurality of reflection members  73 ″ having reflection surfaces, a plurality of reflection mirrors  74 ″, and a diffusion sheet  75 ″. The shape of the light conducting plates  72 ″ is almost the same as that of the light conducting plates  72 . However, the arrangement of the plates  72 ″ is different from that of the plates  72 . The light conducting plates  72 ″ having incident surfaces  72   a ″ are obliquely arranged side by side and stacked so that the side surfaces  72   b ″ thereof are adjacent to the reflection surfaces  73 ″ associated with the neighboring light conducting plates  72 ″. The side surfaces  72   b ″ of the light conducting plates  72 ″ function as light-emitting surfaces. The angle of each apex is, for example, 30°. The lighting device shown in  FIG. 17  can be made to be thinner than that of the lighting devices shown in  FIGS. 15 and 16 . 
     A description will now be given, with reference to  FIGS. 18 and 19 , of a lighting device  100  according to a tenth embodiment of the present invention. The lighting device  100  corresponds to an improvement in the lighting device  70  shown in  FIGS. 14A ,  14 B and  14 C. The lighting device  100  differs from the lighting device  70  in the structure of a diffusion pattern  101  and use of a special linear prism plate  102 . 
     More particularly, the lighting device  100  includes two light sources  105  formed with, for example, fluorescent tubes, two light conducting plates  106 , a reflection member  107  having a reflection surface, the special linear prism plate  102 , reflection mirrors  108 , a normal linear prism plate  110 , and a diffusion sheet  116 . 
     Each of the two light conducting plates  106  has an incident surface  106   a , a back surface  106   b  a light-emitting surface  106   c , and an edge  106   d . The thickness t of each of the light conducting plates  106  becomes smaller as the distance from the associated light source increases. The light-emitting surface  106   c  is inclined at an angle θ with respect to the back surface  106   b . Each of the two light conducting plates  106  has a wedge shape and has a cross section of an approximately right-angled triangle. The reflection member  107  is provided below the two light conducting plates  106 . The reflection mirrors  108  cover the associated light sources  105 , as shown in  FIG. 18 . The cross section of the reflection mirror may be of a half-cycle shape instead of the cross section shown in  FIG. 18 . 
     The two light conducting plates  106  are separated apart from each other so that the light-emitting surfaces  106   c  thereof face each other. With this arrangement of the plates  106 , a space  109  is defined between and above the light-emitting surfaces  106   c . The normal linear prism plate  110  is provided above the light conducting plates  106  so that a space  109  is formed between the linear prism plate  110  and the plates  106 . 
     As shown in  FIG. 19 , the normal linear prism plate  110  has a plurality of linear prisms  111 , each having a vertical angle of 90°. The linear prisms are arranged side by side and inclined at an angle a with respect to the direction (indicated by a reference number  112 ) orthogonal to the longitudinal direction of the light sources  105 . As shown in  FIG. 20 , the linear prism plate  100  functions to collect incident light beams  113  entering within a wide angle range toward a normal line  114  of the linear prism plate  110 . The degree of collection is equal to, for example,  400  as shown in  FIG. 20 . 
     It will be noted that the two lighting units shown in  FIG. 19  can be replaced by any of the embodiments described in the specification. 
     Turning again to  FIG. 10 , the diffusion sheet  116  is located above the normal linear prism plate  110  and diffuses light from the normal linear prism plate  110 . The upper surface of the diffusion sheet  116  serves as an emission surface  117  of the lighting device  100 . 
     A description will now be given of the above-mentioned two essential features of the lighting device  100 . First of all, the diffusion pattern  101  will be described below. 
     The diffusion pattern  101  includes white ink parts  120  ( FIG. 18 ) formed on the back surface  106   b  of each of the light conducting plates  106 . The white ink parts  120  are arranged so as to form a predetermined pattern. Light entering into the white ink parts  102  is diffused. As indicated by a curve I shown in  FIG. 21 , the white ink parts  102  are arranged on the back surface  106   b  with a high density in the vicinity of the incident surface  106   a . That is, each of the white ink parts  102  located in the vicinity of the incident surface  106   a  is weighted so as to have a relatively large area. The diffusion pattern  101  having the above white ink parts  102  functions to diffuse much light particularly, in the vicinity of the incident surface  106   a.    
     The special linear prism plate  102  will be described below.  FIG. 22  is a cross-sectional view of the special linear prism plate  102 . As shown in  FIG. 22 , the special linear prism plate  102  includes first linear prisms  121  having a vertical angle of 140° and second linear prisms  122  having a vertical angle of 70°. Each of the first linear prisms  121  has a cross section of an approximately equilateral triangle shape, and each of the second linear prisms  122  also has a cross section of an approximately equilateral triangle shape. The ratio of the number of first prisms  121  to the number of second prisms  122  is 3:1. 
     As shown in  FIGS. 18 and 19 , the special linear prism plate  102  is provided so that the linear prisms  121  and  122  face downward and the longitudinal direction thereof is orthogonal to the aforementioned line  112 , that is, parallel to the longitudinal direction of the light sources  105 . Further, the special linear prism plate  102  is located above the light conducting plates  106  and below the normal linear prism plate  110 . There is an air layer between the normal linear prism plate  110  and the special linear prism plate  102 . 
     As shown in  FIG. 22 , each of the linear prisms  122  having a vertical angle (the angle of each apex) of 70° totally reflects an incident light  123  to thereby introduce the light  123  upward. An incident light  125  enters into each of the linear prisms  121  having a vertical angle of 140° so that a light  126  propagated through the prism plate  102  reaches at an angle equal to or greater than the critical angle. The light  126  is totally reflected by an upper surface  102   a  of the special linear prism plate  102 , as indicated by the reference number  127 . Then, the totally reflected light  127  goes out of the special linear prism plate  102  and is obliquely emitted downward, as indicated by the reference number  128 , into the space  109 . The first linear prisms  121  are parallel to the longitudinal direction of the light sources  105 . Hence, the light  128  is efficiently oriented toward the center of the lighting device  100 . That is, the linear prisms  121  and  122  of the special linear prism plate  102  are arranged in parallel with the longitudinal direction of the light sources  105  in order to facilitate propagation of light toward the center of the lighting device  100 . It is preferable that the vertical angle of the first prisms  121  is equal to or greater than 110° and the vertical angle of the second prisms  122  is equal to or less than 110°. 
     A description will now be given of the operation of the lighting device  100  with reference to  FIG. 23 . In  FIG. 23 , there are illustrated various arrows indicating rays of light. The direction of each arrow indicates the direction of propagation of the light. The thickness of each arrow indicates the amount of light. As the amount of light becomes larger, the corresponding arrow becomes thicker. 
     A light  130  emitted from the fluorescent tube  105  enters into the light conducting plate  106  via the incident surface  106   a , and goes toward the edge  106   d  thereof. Most of the light  106  entering into the light conducting plate  106  is diffused in the vicinity of the incident surface  106  due to the diffusion pattern  101 . Hence, a large amount of light  131  is emitted from a portion of the light-emitting surface  106   b  located in the vicinity of the incident light  106   a . The reset  132  of light is propagated through the light conducting plate  106  toward the edge  106   d  thereof. 
     The light  132  originally has a small amount, and is diffused and emitted, as lights  133  and  134 , from the light-emitting surface  106   b  during the propagation through the light conducting plate  106 . Hence, the rays of light-toward the edge  106   d  becomes smaller, as indicated by reference numbers  135  and  136 , and a small amount of light  137  is emitted from the edge  106   d . As a result of the above-mentioned mechanism, it becomes possible to eliminate a disadvantage in that the amount of luminance in a portion close to the edge  106   d  becomes greater than that in other portions. 
     The light  131  enters into the special linear prism plate  102 . Some of the light which has entered into the special linear prism plate  102  is passed therethrough and emitted upward, as indicated by a reference number  140 . The reset of the above light is reflected by the special linear prism plate  102  is returned to the space  109 , as indicated by a reference number  141 . The lights  133  and  134  partially pass through the special linear prism plate  102  and are emitted upward, as indicated by reference numbers  142  and  143 . The rest of the lights  133  and  134  is reflected by the special linear prism plate  102  and are returned to the space  109 , as indicated by reference numbers  144  and  145 . The lights  141 ,  144  and  145  are reflected by the light-emitting surface  106   c , and enter into the special linear prism plate  102  again. These lights partially pass through the special linear prism plate  102  and the rest thereof is reflected thereby. 
     The above operation is repeatedly performed, and an approximately constant amount of light is emitted from the whole upper surface  102   a  of the special linear prism plate  102 . Hence, as will be described later, the whole emission surface  117  has a uniform brightness. 
     The light emitted upward from the special linear prism plate  102  enters into the normal linear prism plate  110 , and is collected in the normal direction and emitted, as indicated by a reference number  145 . Then, the light  145  is reflected by the diffusion sheet  116 , and-is emitted from the emission surface  117 , as indicated by a reference number  146 . 
     With the above-mentioned mechanism, there is no part having a larger amount of luminance than that of other parts in the emission surface  117  of the lighting device  100 . That is, the part in the emission surface  117  corresponding to the edge  106   d  of the light conducting plate  106  has the same amount of luminance as the other parts. Further, the amount of luminance in the part of the emission surface  117  close to the fluorescent tube  105  is almost the same as that in the other parts. As a result, it becomes possible to obtain a uniform luminance distribution over the whole emission surface  117 . 
     A description will now be given of characteristics of a liquid crystal display device equipped with the lighting device  100  as a back-lighting source for a liquid crystal panel. In  FIG. 19 , there is illustrated a liquid crystal panel  150  located above the lighting device  100 . The liquid crystal panel  150  includes electrodes  151  for display in the X direction extending in the X direction, and electrodes  152  for display in the Y direction extending in the Y direction. A line  112  orthogonal to the fluorescent tube  105  extends in the X direction. 
     A description will now be given of the positions of the linear prisms  111 ,  121  and  122  ( FIG. 19 ) with respect to the display electrodes  151  and  152 . The linear prisms  111  and the linear prisms  121  and  122  cross at an angle (90−α)°. Hence, it is difficult for the prisms  111 ,  121  and  122  to interfere with each other, so that Moire interference fringes cannot be generated. The linear prisms  111  cross the display electrodes  151  and  152  at an angle of approximately 45°. Hence, it is difficult for the linear prisms  111  and the display electrodes  151  and  152  to interfere with each other, so that Moire interference fringes cannot be generated. 
     A description will now be given of variations of the tenth embodiment of the present invention. 
       FIG. 24  shows a variation  102 A of the special linear prism plate  102  in which the ratio of the number of linear prisms  121  to the number of linear prisms  122  is 4:1. The linear prism plate  102 A shown in  FIG. 24  is capable of orienting a larger amount of light along the surface thereof than the linear prism plate  102  shown in  FIG. 22 . 
       FIG. 25  is an enlarged view of a special lenticular plate  160  used instead of the special linear prism plate  102 . The lenticular plate  160  is located so that lenses face downward. More particularly, the lenticular plate  160  includes first lenses  161 , each having a radius r 1  and a height h 1 , and second lenses  162 , each having a radius r 2  and a height h 2 , in which h 2 &gt;h 1  and r 2 &gt;r 1 . The ratio of the number of first lenses  161  to that of second lenses  162  is, for example, 3:1. As indicated by a reference number  163 , the second lenses  162  function to orient light upward. Further, as indicated by a reference number  164 , the first lenses  161  function to orient light downward and propagate it along the surface of the lenticular plate  160 . 
     A description will now be given, with reference to  FIG. 26 , of a lighting device  200  according to an eleventh embodiment of the present invention. The lighting device  200  includes a linear light source  201  formed with, for example, a fluorescent tube. A transparent light conducting plate  202  includes a back surface  202   a , an emission surface  202   b , an incident surface  202   c  and an end surface  202   d . The incident surface  202   c  and the end surface  202   d  are perpendicular to the back surface  202   a  and the emission surface  202   b . A reflection plate  203  having a reflection surface  204  is provided so as to face the back surface  202   a  of the light conducting plate  202 . A reflection mirror  205  covers the light source  201 . The lighting device  200  has an emission surface  206 . 
     A plurality of grooves (concave portions)  207  are formed on the back surface  202   a  of the light conducting plate  202 , and extend in the direction parallel to the incident surface  202   c  (vertical to the drawing sheet). The grooves  207  are arranged with a pitch P 1  in a center portion  202   a   −1  of the back surface  202   a . The pitch of the grooves  207  becomes larger as the distance from the center portion  202   a   −1  increases. The grooves  207  located in a portion  202   a   −2  close to the incident surface  202   c  and a portion  202   a   −3  close to the end surface  202   d  are arranged with a pitch P 2  less (narrower) than the pitch P 1  in the center portion  202   a   −1 . 
     As shown in  FIG. 27 , each groove  207  has a cross section of a triangular shape, and includes two flat slant surfaces  208  and  209 , which are inclined at an angle θ with respect to the horizontal surface. The angle θ is selected so that the incident light is prevented from being returned to the fluorescent tube  201 , and is set to, for example, 30°. 
     The grooves  207  function as follows. Some of the light that is emitted by the fluorescent tube  201  and enters into the light conducting plate  202  via the incident light  202   c  goes to one groove  207  shown in  FIG. 27 . The above light going to the groove  207  is classified into one of three rays of light  210 ,  211  or  212  due to the angle at which the light is projected on the slant surface  208 . The light  210  is totally reflected by the slant surface  208 , and travels to the emission surface  202   b  as indicated by a reference number  210   a . The light  211  goes in the groove  207 , and is reflected by the reflection surface  204  of the reflection plate  203 , and enters into the light conducting plate  202  again via the slant surface  209 . Then, the light  211  goes toward the emission surface  202   b  as light  211   a . The light  212  goes in the groove  207 , and passes therethrough. Then, the light  212  enters into the light conducting plate  202  again via the slant surface  209 , and travels toward the end surface  202   d , as light  212   a . As described above, the grooves  207  function to efficiently orient the light that is propagated through the light conducting plate  202  and goes toward the back surface  202   a  to the light emitting surface  202   b.    
     Conventionally, as disclosed in Japanese Laid-Open Patent Application 2-165504, grooves related to the above-mentioned grooves  207  are arranged with an equal pitch, and the surfaces forming the grooves are inclined so that all rays of light incident to the grooves are totally reflected. Hence, the luminance distribution on the emission surface obtained with the structure disclosed in the above Japanese document is as indicated by a curve II shown in  FIG. 26 , in which the luminance obtained in the vicinity of the incident surface  202   c  and in the vicinity of the end surface  202   d  is less than the luminance obtained in other portions. 
     On the other hand, according to the eleventh embodiment of the present invention, the arrangement of the grooves  207  is weighted as has been described previously. The weighted-pitch arrangement of the grooves  207  functions to increase the amounts of luminance obtained in the vicinity of the incident surface  202   c  and in the vicinity of the end surface  202   d . Hence, an even luminance distribution as indicated by a curve III shown in  FIG. 26  can be obtained in the entire emission surface. 
       FIG. 28  shows a lighting device  220  according to a twelfth embodiment of the present invention. In  FIG. 28 , parts that are the same as those shown in  FIG. 26  are given the same reference numerals as previously. The grooves  207  formed on a back surface  202 Aa of a transparent light conducting plate  202 A are arranged with an identical pitch P 3  in an area close to the fluorescent tube  201 . The pitch in the other areas may be the same as or different from the pitch P 3 . Reference numerals  207   −1 ,  207   −2 , . . . are given to the grooves  207  from the side close to the fluorescent tube  201 . A reflection mirror  205 A having an approximately U-shaped cross section covers not only the fluorescent tube  201  but also a portion  202 Ab- 1  corresponding to the groove  207   −1 . The portion  202 Ab −1  is a part of a light-emitting surface  202   a B of the light conducting plate  202 A. A reference number  205 Ab is a part of the and covers the groove  207   −1  formed on a back surface  202 Aa of the light conducting plate  202 A. A portion  202 A −1  which is a part of the light conducting plate  202 A functions as an emission surface (area). A reference numeral  202 A −2  indicates a light accumulating area, which accumulates light as will be described later. 
     A light  221  incident to the light conducting plate  202 A from the fluorescent tube  201  via an incident surface  202 Ac is reflected by one of the surfaces defining the groove  207   −1 , and is oriented toward the surface  202 Ab −1 , as indicated by a reference numeral  221   a . Then, the light goes out of the surface  202 Ab −1 , and is reflected by the upper cover portion  205 Aa. Then, the reflected light enters into the light conducting plate  202 A. The light goes toward the back surface  202 Aa, and goes out of the back surface  202 Aa. Then, the light is reflected by the lower cover portion  205 Ab, and enters into the light conducting plate  202 A again. Then, the light goes upwards. The above operation is repeatedly performed, so that the light from the fluorescent tube  201  goes toward the emission area  202 A −1.    
     The surface area of the light-emitting surface  202 Ab except for the part covered by the upper cover portion  205 Aa is an effective light-emitting surface  202 Ab −2 . The grooves  207 - 2  through  207   −1 , located on the effective light-emitting surface function in the same manner as those used in the eleventh embodiment of the present invention, whereby the light propagated through the light conducting plate  202 A is oriented upward. In a portion in the effective light-emitting surface  202 Ab −2  close to the fluorescent tube  201 , a light  221   b  leaked from the light accumulating area  202 A −2  and going upward is superposed onto a light  222  refracted by the groove  207   −2 . Hence, the amount of light is increased in the above portion. Hence, the luminance on the emission surface  206  of the lighting device  220  is even in the vicinity of the fluorescent tube  201 , as indicated by a curve IV shown in  FIG. 28 . 
       FIG. 29  shows a lighting device  230  according to a thirteenth embodiment of the present invention. The lighting device  230  corresponds to a combination of the lighting device  200  shown in  FIG. 26  and the lighting device  220  shown in  FIG. 28 . In  FIG. 29 , parts that are the same as those shown in the previously described figures are given the same reference numerals as previously. 
     A light accumulating area  202 A −2  is formed by the groove  207   −1  and the upper cover portion  205 Aa of the reflection mirror  205 A. In a portion in the effective light-emitting surface  202 Ab −2  close to the fluorescent tube  201 , the light  221   b  leaked from the light accumulating area  202 A −2  and going upward is superposed onto the light  222  refracted by the groove  207   −2  The pitch P 2  with which the grooves  207   −1  and  207   −2  are arranged is less than the pitch P 1  with which the grooves are arranged in the center portion of the back surface. Hence, the amount of the light  222  is increased. Hence, as indicated by a line V shown in  FIG. 29 , a constant luminance distribution can be obtained over the whole emission surface  206 . 
       FIG. 30  shows a lighting device  240  according to a fourteenth embodiment of the present invention. The lighting device  240  corresponds to a modification of the lighting device  200  shown in  FIG. 26  in which the light conducting plate  202  is modified. In  FIG. 30 , parts that are the same as those shown in  FIG. 26  are given the same reference numerals as previously. 
     As shown in  FIG. 30 , a transparent light conducting plate  241  used in the lighting device  240  shown in  FIG. 30  has an approximately wedge-shaped cross section, and has a slant light-emitting surface  241   b  and a curved end surface  241   c . A back surface  241   d  of the light conducting plate  241  is a flat surface on which the grooves  207  are formed. An incident surface  241   a  of the light conducting plate  241  is perpendicular to the back surface  241   d  thereof. As compared with the vertical end surface  202   d  shown in  FIG. 29 , the curved end surface  241   c  functions to make it difficult for the light propagated through the light conducting plate  241  from being reflected by the surface  241  and to cause the light to be emitted upward. Hence, the light is efficiently emitted from the end portion of the light conducting plate  241 , so that the luminance obtained at the end portion thereof can be enhanced. As indicated by a line VI shown in  FIG. 30 , an even luminance distribution can be obtained over the whole emission surface  243 . 
     The length L 1  of the light conducting plate  243  is approximately equal to, for example, 210 mm, and the thickness T 1  thereof is approximately equal to, for example, 5 mm. Further, the width of the light conducting plate  243  is approximately equal to, for example, 160 mm. The distance D 1  (the size of the space) is approximately equal to, for example, 9 mm. The angle φ of the grooves  207  is equal to, for example, 120°, and the depth of the grooves is equal to, for example, 0.025 mm. In practice, it is preferable to set the distance D 1  to 9 mm or more and set the thickness T 1  to 5 mm or more. 
       FIG. 31  shows a lighting device  250  according to a fifteenth embodiment of the present invention. In  FIG. 31 , parts that are the same as those shown in  FIG. 25  are given the same reference numerals as previously. The lighting device  250  includes a transparent light conducting plate  251 , which includes a group  252  of pits (concave portions) formed on a back surface  251   a  as shown in  FIGS. 31 and 32 . The group of pits  252  includes a large number of pits  253 . Each of the pits  253  has an approximately triangle-shaped cross section, and functions to orient propagated light reaching the back surface  251   a  toward a light-emitting surface  251   b  in the same manner as the grooves  207  shown in  FIG. 25 . 
     The pit group  252  includes a pit alignment including pits  253   −1  through  253   −4 , a pit alignment  254   −1  including pits  253   −5  through  253   −7 , and a pit alignment  254   −3  including pits  253   −8  through  253   −11 . The pit alignments  254   −1 ,  254   −2  and  254   −3  are parallel to each other and are arranged in a zigzag form. With the above arrangement, it becomes possible to make it possible to further equalize the amount of light oriented from the back surface  251   a  to the light-emitting surface  251   b  in the whole back surface  251   a , a compared with use of the grooves  207 . Hence, a luminance fluctuation caused on an emission surface  255  of the lighting device  250  due to the presence of pits can be suppressed, as compared with the lighting device  200  shown in  FIG. 26 , as indicated by a line VII shown in  FIG. 31 . 
       FIG. 33  shows a lighting device  260  according to a sixteenth embodiment of the present invention. In  FIG. 33 , parts that are the same as those shown in  FIG. 26  are given the same reference numerals as previously. The lighting device  260  includes a transparent light conducting plate  261 , which includes a group  262  of grooves formed on a back surface  261   a , as shown in  FIGS. 34 and 35 . The group  262  includes first grooves  262   a  and second grooves  262   b . The first grooves  262   a  are obliquely arranged at an acute angle al with respect to a line  264  orthogonal to an axial line  263  of the fluorescent tube  201 . The second grooves  262   b  are obliquely arranged at an angle α 2  with respect to the above line  264 . The grooves  262   a  and  262   b  cross at a large number of points. Hence, it becomes difficult for the grooves to be seen from the outside of the lighting device  260 , as compared with the lighting device  200  shown in  FIG. 26 . Hence, the lighting device  260  has an even luminance distribution over the whole emission surface, as indicated by a line VIII shown in  FIG. 33 . 
       FIG. 36  shows a lighting device  270  according to a seventeenth embodiment of the present invention. In  FIG. 36 , parts that are the same as those shown in  FIG. 26  are given the same reference numbers as previously. The lighting device  270  includes a transparent light conducting plate  271 , which includes a group  271  of grooves formed on a back surface  271   a  thereof. The group  271  includes grooves  272  through  277 . The sizes of the grooves  272  through  277  become larger as the distance from the fluorescent tube  201  becomes smaller. The grooves  272  through  277  are arranged with a constant pitch P 4 , which is less than the pitch P 1  shown in  FIG. 26 . Hence, it becomes difficult for the grooves to be seen from the outside of the lighting device  270 , as compared with the lighting device  200  shown in  FIG. 26 . Hence, the lighting device  270  has an even luminance distribution over the whole emission surface, as indicated by a line IX shown in  FIG. 36 . 
       FIG. 37  shows a lighting device  280  according to an eighteenth embodiment of the present invention. In  FIG. 37 , parts that are the same as those shown in  FIG. 26  are given the same reference numerals as previously. The lighting device  280  includes a transparent light conducting plate  281 . As shown in  FIGS. 37 and 38 , a group  282  of pits are formed on a back surface  281   a  of the light conducting plate  281 . The pit group  282  includes a large number of pies  283 . Each of the pits  283  has a triangular cross section, and functions to orient light propagated through the light conducting plate  281  and reaching the back surface  281   a  toward a light-emitting surface  281   b . The pit group  282  includes first pit alignments  284  inclined on the left-hand side, and second pit alignments  284  inclined on the right-hand side. The pit alignments  284  and  285  cross. Hence, it becomes possible to orient an increased amount of light toward the light-emitting surface  281   b , as compared with use of the grooves  207 . As a result, the lighting device  28  has a suppressed luminance fluctuation and has an even luminance distribution over the whole emission surface, as indicated by a line X shown in  FIG. 37 . 
       FIG. 39  shows a lighting device  290  according to a nineteenth embodiment of the present invention. In  FIG. 41 , parts that are the same as those shown in  FIG. 26  are given the same reference numerals. The lighting device  290  includes a transparent light conducting plate  291 , which includes grooves  292  formed on a back surface  291   a  thereof. Each of the grooves  292  has a triangular cross section. The angle θ 10  of the slope of each groove  292  with respect to the flat portions of the back surface  291   a  is considerably less than the angle θ 11  shown in  FIG. 26 . When the angle θ 10  is relatively small, each groove  292  has a small capability of orienting light toward a light-emitting surface  291   b . In the above manner, it becomes possible to vary the amount of light emitted from the light-emitting surface  291   b  by-changing the angle θ 10 . 
       FIG. 40  shows a lighting device  300  according to a twentieth embodiment of the present invention, which includes a transparent light conducting plate  310 . As shown in  FIG. 40 , U-shaped grooves  302  are formed on a back surface  301   a  of the light conducting plate  310 . 
       FIG. 41  shows a lighting device  310  according to a twenty-first embodiment of the present invention, which includes a light conducting plate  311  and a reflection plate  312 . Grooves  318  are formed on a back surface  311   a  of the light conducting plate  311  with a constant pitch. The reflection plate  312  includes a large number of projections  313 , which face the back surface  311   a . The projections  313  reflect light leaked from the grooves  318  formed on the back surface  311   a , and cause the leaked light to enter into the light conducting plate  311  again, as indicated by reference number  314   a . The projections  313  are arranged with a pitch P 10  in a center portion  314 . The pitch of projections  313  becomes smaller as the distance from the center portion  314  increases. The projections  313  are arranged in areas  315  and  316  respectively close to the light source  201  and an end surface  311   d  with a pitch P 11  less than the pitch P 10 . Rays of light leaked from the area  315  in the vicinity of the light source  201  and from the area  316  in the vicinity of the end surface  311   d  can be returned to the light conducting plate  311  and emitted from an emission surface  317  more efficiently than the rays of light leaked from the center portion  314 . Hence, as indicated by a line XI shown in  FIG. 41 , an even luminance distribution can be obtained over the emission surface  317 . 
     It will be noted that various combinations of the aforementioned embodiments can be-made with ease. 
       FIG. 42  shows a combination of the structures shown in  FIGS. 18 ,  29  and  30 . In  FIG. 42 , those parts that are the same as those shown in  FIGS. 18 ,  29  and  30  are given the same reference numerals as previously. A lighting device  320  shown in  FIG. 42  includes two transparent light conducting plates  241 , two light sources  201 , and two reflection mirrors  205 . Each of the light conducting plates  241 , which are arranged so that the edges  241   c  face each other via the space  109 , has the same structure as shown in  FIG. 30 . Each of the reflection mirrors  205  includes the upper cover portion  205 Aa, which covers one or more grooves close to the associated light source  201 . The grooves  207  of the light conducting plates  241  face the reflection plate  203  having the reflection surface  204 . The special linear prism plate  102 , the normal linear prism plate  110  and the diffusion sheet  116  are provided in the same manner as shown in  FIG. 18 . The vertical angle of each of the grooves  207  is, for example, 120°. The angle of the inclined surface  241   c  with respect to the reflection plate  203  is, for example, 30°. 
     The length L 2  of the two identical light conducting plates  243  is equal to, for example, 210 mm, and the thickness T 2  thereof is approximately equal to, for example, 9.3 mm. Further, the width of the light conducting plate  243  is approximately equal to; for example, 83 mm. The distance D 2  (the size of the space  109 ) is equal to 4 mm or more. The angle φ of the grooves  207  is equal to, for example, 120°, and the depth of the grooves is equal to, for example, 0.025 mm. 
       FIG. 43  is a graph a variation in the pitch as a function of the distance on the light conducting plate  241  from the incident surface  241   a . As shown in  FIG. 43 , the pitch P 1  for the grooves  207  located in the center portion has a relatively large value, and the pitch P 2  for the grooves located in the vicinity of the incident surface  241   a  and the end surface  241   c  has a relative small value. As shown in  FIG. 43 , it is possible to gradually vary the pitch. The graph of  FIG. 43  can be applied to the embodiments having the grooves. In general, the plurality of concave portions are arranged at pitches expressed by a multiple-order curve as a function of a distance on the bottom surface of the light conducting plate from the side thereof facing the light source. 
     The curve of the  FIG. 43  can be by a seventh-order approximate expression having the following coefficients X^7, X^6, X^5, X^4, X^3, X^2, X^1, X^0:
         X^7=−8.74289E-15   X^6=3.86566E-12   X^5=−5.315E-10   X^4=1.20258E-08   X^3=2.67616E-06   X^2=−2.86593E-04   X^1=0.014992   X^0=0.664993       

       FIG. 44  shows a variation of the light conducting plate  241  shown in  FIG. 30 . In  FIG. 44 , parts that are the same as those shown in  FIG. 30  are given the same reference numerals as previously. A projection  241   e  functioning as a reflection surface is formed at the end opposite to the end thereof facing the light source  201 . The projection  241   e  is integrally formed with the other parts of the light conducting plate  241 . The light conducting plate shown in  FIG. 44  provides almost the same advantages as those of the light conducting plate shown in  FIG. 30 . 
     The length L 3  of the two identical light conducting plates  243  is equal to, for example, 210 mm, and the thickness T 3  thereof is approximately equal to, for example, 5.0 mm. Further, the width of the light conducting plate  243  is approximately equal to, for example, 160 mm. The angle φ of the grooves  207  is equal to, for example, 120°, and the depth of the grooves is equal to, for example, 0.025 mm. 
     Further, it is possible to apply the grooves  207  to the structure shown in  FIG. 6A  as a diffusion pattern. It is also possible to apply the reflection plate shown in  FIG. 41  to the other embodiments, for example, the structure shown in  FIG. 2 . The linear special linear prism plate  102  shown in  FIGS. 18 and 42  can be applied to the other embodiments of the present invention. 
     The present invention are-not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.