Abstract:
In a method of forming a thin light guide plate which includes a compression step in an injection molding step, an optical pattern is liable to be adhered to a mold. Further, in removing the light guide plate from the mold using an ejector pin, a stress is concentrated on a local area of the light guide plate thus generating warping, deformation or irregularities in size of the light guide plate. To overcome such drawbacks, a liquid crystal display device is configured such that an optical pattern portion is compressed, and the light guide plate is removed by making use of a peripheral portion of a mold thus preventing the generation of stress in a local area of the light guide plate due to an ejector pin.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light source of a non-self-luminous display device, and more particularly to a liquid crystal display device having a backlight which includes a light guide plate and uses an LED as a light source. 
     2. Background Art 
     Recently, the liquid crystal display device has been popularly used as a display device. Particularly, the liquid crystal display device is used as a display part of portable equipment because the liquid crystal display device is thin and light-weighted, and consumes small electric power. 
     However, the liquid crystal display device is not self-luminous and hence, the liquid crystal display device requires a lighting means. In general, as a lighting device which is used for the liquid crystal display device, a planar lighting device referred to as a backlight is popularly used. Conventionally, although a cold cathode discharge tube has been used as a light emitting element (also referred to as a light source) of the backlight, an LED (light emitting diode) has been also recently used as the light emitting element. 
     As a thin backlight, there has been known a side-light-type backlight which includes a light emitting element on a side surface thereof. The side-light-type backlight includes a plate-shaped light guide plate. A material of the light guide plate is a light transmitting resin or the like, and light incident on the light guide plate from the light emitting element propagates in the inside of the light guide plate. A reflection/scattering member such as grooves, projections or a printed material is formed on the light guide plate, and the light which propagates in the inside of the light guide plate due to such a reflection/scattering member is directed and radiated toward a liquid-crystal-display-device side. 
     When LEDs are used as a light emitting element, there arises a drawback that a thickness of the LED becomes larger than a thickness of the light guide plate. Accordingly, in JP-A-2004-12747 (patent document 1), for example, there has been proposed a light guide plate having the constitution in which the light guide plate has a large thickness at a light incident surface on which light is incident from a light source and sets a thickness thereof at a light radiating surface smaller than the thickness of the light guide plate at the light incident surface. Further, in patent document 1, there has been also proposed a light guide plate having the constitution in which a thickness of a light guide plate is set in two stages by forming an inclined surface which extends toward a light radiating surface from a light incident surface. However, patent document 1 neither discloses nor suggests a manufacturing method of the light guide plate whose thickness is further reduced at the light radiating surface. 
     On the other hand, JP-A-2001-341177 (patent document 2) discloses a technique which forms a light guide plate by compressing a resin at the time of forming the light guide plate by injection molding. However, in the technique disclosed in patent document 2, the whole light guide plate is compressed so that a portion to be compressed is not limited to the light incident portion. Further, patent document 2 neither discloses nor suggests a method of taking out the light guide plate which is suitable for the mass production. 
     SUMMARY OF THE INVENTION 
     The further reduction of the thickness of the light guide plate makes the manufacture of the light guide plate by injection molding difficult. Particularly, it is difficult to form the light guide plate by injection molding using the resin within a manufacturing time suitable for mass production and hence, it has been impossible to form a thin light guide plate having stable quality by molding and to take out the light guide plate from a mold. 
     To overcome the above-mentioned drawbacks, according to one aspect of the present invention, there is provided a liquid crystal display device which includes a display panel, and a backlight which radiates light to the display panel, wherein the backlight includes a light emitting element and a light guide plate on which light from the light emitting element is incident, and the light emitting element is mounted on a side surface of the light guide plate. The light guide plate is formed using a mold. A resin is injected and filled in the mold such that the resin is filled in spaced defined in the mold for forming a light incident portion and a light radiating portion of the light guide plate. Thereafter, the resin filled in the space for forming the light incident portion is compressed by the mold. 
     A constraining portion is formed on the mold for facilitating the removal of the light guide plate from the mold. A stepped portion which is formed by the constraining portion is formed on a periphery of the light guide plate. 
     Even when the light radiating portion of the light guide plate is made thin, by injecting and filling a resin into the space formed in the mold corresponding to the light incident portion and, thereafter, by compressing the resin in the space, it is possible to manufacture a thin light guide plate having stable quality in a short time. 
     The light guide plate is pressed by the constraining portion at the time of removing the light guide plate from the mold and hence, the light guide plate can be easily removed from the mold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the schematic constitution of a liquid crystal display device of an embodiment according to the present invention; 
         FIG. 2A  and  FIG. 2B  are schematic views showing a light emitting diode of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 3A  and  FIG. 3B  are schematic views showing a light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 4A  and  FIG. 4B  are schematic cross-sectional views showing the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 5  is a schematic cross-sectional view showing an area in the vicinity of a light incident surface of the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 6  is a schematic cross-sectional view showing a mold for forming the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 7  is a schematic cross-sectional view showing the mold for compression-forming the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 8  is a schematic cross-sectional view showing the mold for compression-forming the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 9  is a schematic cross-sectional view showing the mold for compression-forming the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 10  is a schematic cross-sectional view showing the mold for compression-forming the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 11  is a schematic cross-sectional view showing the mold for compression-forming the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 12  is a schematic cross-sectional view showing the mold for compression-forming the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 13  is a schematic cross-sectional view showing the mold for compression-forming the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 14  is a schematic perspective view showing an area in the vicinity of a light incident portion of the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 15  is a schematic perspective view showing an area in the vicinity of a light incident portion of the light guide plate of the liquid crystal display device of the embodiment according to the present invention; 
         FIG. 16  is a schematic perspective view showing an area in the vicinity of a light incident portion of the light guide plate of the liquid crystal display device of the embodiment according to the present invention; and 
         FIG. 17  is a schematic perspective view showing an area in the vicinity of a light incident portion of the light guide plate of the liquid crystal display device of the embodiment according to the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 1  is a plan view showing a liquid crystal display device  100  according to the present invention. The liquid crystal display device  100  is constituted of a liquid crystal panel  1 , a backlight  110  and a control circuit  80 . Signals and power source voltages necessary for a display of the liquid crystal display device  100  are supplied from the control circuit  80 . The control circuit  80  is mounted on a flexible printed circuit board  70 , and signals are transmitted to the liquid crystal panel  1  via lines  71  and terminals  75 . 
     The backlight  110  is constituted of a light guide plate  120 , LEDs  150  and a housing casing  180 . The backlight  110  is provided for radiating light to the liquid crystal panel  1 . The liquid crystal panel  1  performs a display by controlling a transmission quantity or a reflection quantity of light radiated from the backlight  110 . Here, the backlight  110  is mounted on a back-surface side or a front-surface side of the liquid crystal panel  1  in an overlapping manner as viewed from a viewer. However, in  FIG. 1 , to facilitate the understanding of the constitution of the liquid crystal display device, the backlight  110  is shown in a state that the backlight  110  is arranged parallel to the liquid crystal panel  1 . 
     The light guide plate  120  has a substantially rectangular shape, and the LEDs  150  are arranged on the side surface thereof. Numeral  160  indicates a flexible printed circuit board which electrically connects the plurality of LEDs  150  with each other. The flexible printed circuit board  160  and the control circuit  80  are electrically connected with each other by lines  161 . 
     A side surface  125  on which the LEDs  150  are arranged is referred to as a light incident surface or a light entering surface, and the light is incident on the light guide plate  120  from the light incident surface  125 . The light incident on the light guide plate  120  from the light incident surface  125  is radiated from a light radiating portion  121 . An inclined portion  128  is formed between the light incident surface  125  and the light radiating portion  121  for guiding the light from the light incident surface  125  to the light radiating portion  121 . Further, a light incident portion  124  is formed of the light incident surface  125  and the inclined portion  128 , and the light incident portion  124  is provided for efficiently transmitting the light emitted from the LED  150  to the light radiating portion. Here, the light incident portion  124  is explained in detail later. 
     Next, the liquid crystal panel  1  is explained. The liquid crystal panel  1  includes two substrates consisting of a TFT substrate  2  and a color filter substrate  3  which overlap with each other, and the liquid crystal composition sandwiched between these two substrates. Pixel portions  8  are arranged on the TFT substrate  2 , and each pixel portion  8  includes a pixel electrode  12 . Here, although the liquid crystal panel  1  includes a large number of pixel portions  8  arranged in a matrix array, to prevent the drawing from becoming complicated, only one pixel portion  8  is shown in  FIG. 1 . The pixel portions arranged in a matrix array form a display region  9 , each pixel portion  8  plays a role of a pixel of a displayed image, and an image is displayed in the display region  9 . 
     In  FIG. 1 , gate signal lines (also referred to as scanning lines)  21  which extend in the x direction and are arranged parallel to each other in the y direction in the drawing, and drain signal lines (also referred to as video signal lines)  22  which extend in the y direction and are arranged parallel to each other in the x direction in the drawing are provided, wherein the gate signal lines  21  and the drain signal lines  22  intersect with each other. Further, each pixel portion  8  is formed in a region surrounded by the gate signal lines  21  and the drain signal lines  22 . 
     A switching element  10  is provided to the pixel portion  8 . A control signal is supplied to the switching element  10  via the gate signal line  21  so as to control an ON/OFF state of the switching element  10 . When the switching element  10  is turned on, a video signal transmitted via the drain signal line  22  is supplied to the pixel electrode  12 . 
     The drain signal lines  22  are connected to a drive circuit  5 , and the video signals are outputted to the drain signal lines  22  from the drive circuit  5 . The gate signal lines  21  are connected to a drive circuit  6 , and the control signals are outputted to the gate signal lines  21  from the drive circuit  6 . Here, the gate signal lines  21 , the drain signal lines  22 , the drive circuit  5  and the drive circuit  6  are formed on the same TFT substrate  2 . Further, in addition to the drive circuit  5  and the drive circuit  6 , the control circuit  80  can be formed on one semiconductor chip. 
     Next,  FIG. 2A  and  FIG. 2B  are schematic views showing the LED  150  which constitutes a light emitting element, wherein  FIG. 2A  is a schematic cross-sectional view of the LED  150 , and  FIG. 2B  is a front view of the LED  150  as viewed from a light-emission side. 
     The LED  150  is configured such that an LED chip  151  which constitutes a light emission portion is mounted on a chip substrate  154 . The LED chip  151  has a pn junction and, when a voltage is applied to the pn junction, the LED chip  151  emits light at a specified wavelength. A p electrode (anode)  158  is formed on a p-type semiconductor layer which forms the pn junction, and an n electrode (cathode)  159  is formed on an n-type semiconductor layer which forms the pn junction. 
     Wires  152  are respectively connected to the p electrode and the n electrode  159 . Chip terminals  153  are provided for connecting the LED  150  to an external portion, and the chip terminals  153  are electrically connected with the p electrode and the n electrode  159  using the wires  152 . 
     A fluorescent light emission part  156  may be arranged on a light radiating portion side of the LED chip  151 . The fluorescent light emission part  156  has a function of converting a wavelength of light emitted from the LED chip  151 . Numeral  155  indicates a reflection portion, and the reflection portion  155  reflects light toward a front side. A light emission surface from which light is emitted is formed on a front surface side of the LED  150 . 
     Next,  FIG. 3A  is a schematic plan view of the light guide plate  120 , and  FIG. 3B  is a schematic side view of the light guide plate  120 . As shown in  FIG. 3A , the light guide plate is formed into an approximately rectangular shape and, as shown in  FIG. 3B , the light guide plate  120  includes an upper surface (also referred to as a light radiating portion)  121  and a lower surface  122 . The light guide plate  120  is made of a material such as an acrylic resin or a polycarbonate which allows light to pass therethrough. The light guide plate  120  is formed into a plate shape, and a thickness of the light guide plate  120  is set to 0.1 mm to 11.0 mm. 
     In  FIG. 3B , although a cross section of the light guide plate  120  has an approximately rectangular shape, the inclined portion  128  is formed toward the light radiating portion  121  from the light incident surface  125 . The inclined portion  128  is effective when a thickness of the LED  150  is larger than a thickness of the light radiating portion  121  of the light guide plate  120 . 
     In  FIG. 3A  and  FIG. 3B , the positional relationship between the light guide plate  120 , the LED  150  and the flexible printed circuit board  160  is shown. On at least one side of the light guide plate  120 , the light incident surface  125  is arranged and, in the vicinity of the light incident surface  125 , a plurality of LEDs  150  is arranged. The LEDs  150  are arranged below the flexible printed circuit board  160  and along the light incident surface  125 . 
     An adhesive sheet (not shown in the drawing) is arranged on a light-guide-plate- 120  side of the flexible printed circuit board  160 . By adhering and fixing the flexible printed circuit board  160  to the light guide plate  120 , a position of the LED is adjusted with respect to the light incident surface  125 . Projecting portions  220  are formed on the light guide plate  120  for ensuring a large adhesive area between the flexible printed circuit board  160  and the light guide plate  120  by way of the adhesive sheet. The projecting portions  220  are formed on a light-incident-surface- 125  side of the light guide plate in a state that each LED  150  is sandwiched between the projecting portions  220 . By adhering the projecting portions  220  and the flexible printed circuit board  160  to each other, the relative position of the LEDs  150  with respect to the light guide plate  120  can be adjusted with high accuracy. 
     Next, light  131  emitted from the LED  150  is explained in conjunction with  FIG. 3B . The light  131  emitted from the LED  150  is incident on the light guide plate  120  from the light incident surface  125 . A refractive index of the light guide plate  120  is larger than a refractive index of air and hence, light which reaches the light incident surface  125  at an angle larger than a specified angle with respect to the direction perpendicular to the light incident surface  125  is reflected on the light incident surface  125 , while light which reaches the light incident surface  125  at an angle smaller than the specified angle with respect to the direction perpendicular to the light incident surface  125  enters the inside of the light guide plate  120 . 
     The upper surface  121  and the lower surface  122  of the light guide plate  120  are arranged to be substantially orthogonal to the light incident surface  125 , and the light which enters the inside of the light guide plate  120  advances in the inside of the light guide plate  120  while repeating the total reflection between the upper surface  121  and the lower surface  122  of the light guide plate  120 . Grooves  126  having a V-shaped cross section are formed in the lower surface  122  as reflection portions. A part of the light which advances through the light guide plate  120  is reflected toward the upper-surface- 121  side on the grooves  126  formed in the lower surface  122 , and is radiated from the upper surface  121 . Here, the explanation is made with respect to a case in which the reflection portion is formed of the grooves  126  having a V-shaped cross section as one example. However, any reflection portion may be used provided that the reflection portion has a function of directing the light which advances in the inside of the light guide plate toward the upper-surface- 121  side. For example, white dots formed by printing or the like may be used as the reflection portions. 
     Next, the light which is reflected on the grooves  126  is explained in conjunction with  FIG. 4A  and  FIG. 4B .  FIG. 4A  shows a case in which the grooves  126  are recessed inwardly, while  FIG. 4B  shows a case in which the grooves  126  project outwardly. Each groove  126  includes a reflection surface (also referred to as an inclined surface)  127 , wherein the reflection surface  127  makes an angle of 1 to 35 degrees with respect to the lower surface  122 . The light which is reflected on the reflection surface  127  is reflected toward the upper surface of the light guide plate  120 . By reflecting the light on the reflection surface  127 , it is possible to set an angle of light with respect to the upper surface  121  to an angle which allows the light to be radiated from the upper surface  121 . That is, although the light repeats the total reflection in the inside of the light guide plate  120  as described above, due to the formation of the reflection surfaces  127 , the angle of the light with respect to the upper surface  121  becomes an angle which allows the radiation of light from the light guide plate  120 , and the light is radiated from the light guide plate  120 . 
     As shown in  FIG. 4A , prism sheets  112 ,  113  are arranged on the upper surface  121  of the light guide plate  120  so as to control the direction of light radiated from the light guide plate  120 . Here, in  FIG. 4A , the prism sheets  112 ,  113  are arranged in a state that ridges of triangular columns of the prism sheet  112  and ridges of triangular columns of the prism sheet  113  intersect with each other. Accordingly, the prism sheet  113  can refract the advancing direction of light which is radiated from the light guide plate  120  in the lateral direction thus directing the light toward the inside (liquid-crystal-panel side). Here, numeral  114  indicates a diffusion plate, and numeral  115  indicates a reflection sheet. 
     Next,  FIG. 4B  shows a case in which one sheet of asymmetric prism sheet is used. The light which is reflected on the reflection surface  127  makes an obtuse angle with respect to the perpendicular direction of the upper surface  121 , and the light is radiated from the upper surface  121  such that the light expands outwardly (toward a right side in the drawing). On the light guide plate  120 , an asymmetrical prism sheet  116  is formed so as to refract the outgoing light toward a liquid-crystal-panel (not shown in the drawing) side. 
       FIG. 5  is a perspective view showing the vicinity of the light incident surface  125  of the light guide plate  120 . Lenses  123  are formed on the light incident surface  125  of the light guide plate  120 . The lenses  123  have a function of scattering light incident from the light incident surface  125 . The light incident from the light incident surface  125  is guided to the light radiating portion  121  by way of the inclined portion  128 . A projecting portion  220  is formed between the neighboring lenses  123  in a state that the projecting portion  220  projects from the light incident surface  125 . The light incident portion  124  is formed of the projecting portions  220 , the light incident surface  125 , the lenses  123 , the inclined portion  128  and the like. 
     When the light guide plate  120  is made thin, a thickness of the LED  150  becomes larger than a thickness of the light guide plate  120  defined between the upper surface  121  and the lower surface  122 . Accordingly, a thickness of the light guide plate  120  at the light incident surface  125  is set large so as to form the inclined portion  128  thus guiding the light toward the upper-surface- 121  side. 
     The light is radiated toward a liquid-crystal-panel side from the upper surface  121 . A portion of the light guide plate  120  from which the light is radiated toward the liquid crystal panel is referred to as a light radiating portion  129 . To satisfy a demand for further reduction of a thickness of the light guide plate  120 , a thickness of the light guide plate  120  at the light radiating portion  129  is steadily reduced. However, when a distance between the upper surface  121  and the lower surface becomes 1 mm or less, it becomes difficult to manufacture the light guide plate  120  by injection molding. 
     For realizing the reduction of thickness of the light guide plate  120 , a means which realizes thin-plate molding by forming a resin plate having a thickness equal to or more than a thickness of the light guide plate  120  by molding and by compressing the resin plate in a mold may be effective. However, when a plate thickness of the thin wall portion becomes 0.4 mm or less, a drawback that a resin is cooled and solidified rapidly arises conspicuously. In this case, even when the resin is compressed after being filled in the mold, the light guide plate having a desired shape cannot be formed by molding and hence, the transfer of a shape of the mold for forming the light incident portion  124  to the light incident portion  124  of the formed light guide plate  120  becomes unstable. Accordingly, it is necessary to cope with such a drawback. 
     On the other hand, when the compression is not used, there arises a following drawback. That is, the light incident portion  124  has fine portions and hence, it is necessary to accurately transfer a shape of the mold to the light incident portion  124 . When the thickness of the light radiating portion is small, with respect to a resin which reaches the light incident portion  124  after passing the light radiating portion at the time of performing injection molding, since a thickness of the light incident portion  124  is larger than a thickness of the light radiating portion  129 , a pressure which pushes the resin is decreased in the light incident portion so that the resin is not sufficiently filled in the mold under pressure. 
     In view of the above, according to the present invention, a resin is injected and filled in spaces defined in the mold for forming the light radiating portion  129  and the light incident portion  124  and, thereafter, the resin filled in the space in the mold corresponding to the light incident portion is compressed thus allowing a shape of the mold to be transferred to the light incident portion  124  sufficiently. 
       FIG. 6  is a schematic cross-sectional view of the mold. The mold  300  is constituted of a first side-surface portion  311 , an upper surface portion  312  and a lower surface portion  331 . The light guide plate is formed by injecting resin into a gap surrounded by the first side-surface portion  311 , the upper surface portion  312  and the lower surface portion  331 . Here,  FIG. 6  to  FIG. 8  and  FIG. 12  are cross-sectional views of the mold taken along the long-side direction (x direction in  FIG. 3 ) of the light guide plate  120 . 
     A resin is injected into the mold  300  in the direction indicated by an arrow  410  from an opening portion which is referred to as a sprue or a gate  400 . Due to the pressure which is applied from the outside, the resin is filled in the inside of the mold  300 . First of all, the resin which enters the mold from the gate  400  is firstly filled in a light-radiating-portion forming space  322  and, thereafter, reaches a light-incident-portion forming space  324 . 
     In the light-incident-portion forming space  324 , a compression portion  351  is formed. As shown in  FIG. 7 , after the light-incident-portion forming space  324  is filled with the resin, the compression portion  351  is lowered in the direction indicated by an arrow  361  thus compressing the resin. By compressing the resin by the compression portion  351 , the configuration of the lenses  123  or the like is accurately transferred to the light incident portion  124 . Here, a plug  420  is arranged in the gate  400  so as to prevent the leaking of the resin from the gate  400 . 
     Next,  FIG. 8  shows a case in which not only the light incident portion  124  but also the light radiating portion  129  are compressed. In a light-radiating-portion forming space  322 , a second compression portion  352  is formed, and the second compression portion  352  can compress the resin in the direction indicated by an arrow  362  after the resin is filled in the light-radiating-portion forming space  322 . 
     A second side-surface portion  313  is formed so as to surround the second compression portion  352 , and the second compression portion  352  is movably held by a second side-surface portion  313  and a third side-surface portion  332  shown in  FIG. 9 . 
     After compressing the light incident portion  124  and the light radiating portion  129 , as shown in  FIG. 9 , the second compression portion  352  is moved in the direction indicated by an arrow  363 , and the compression operation is finished. Here, there arises a drawback that grooves  126  formed on the light radiating portion  129  are adhered to the lower surface portion  331  of the mold thus making the removal of the light guide plate  120  from the mold difficult.  FIG. 9  to  FIG. 11  and  FIG. 13  are cross-sectional views of the mold taken along the short-side direction (y direction in  FIG. 3 ) of the light guide plate  120 . 
     When a thickness of the light radiating portion  129  is set to 0.4 mm or less, such a thickness is close to a resin-made-product forming thickness limit and hence, the light guide plate  120  cannot be surely formed. Accordingly, by filling the resin in the mold  300  such that a thickness of the resin is set to a desired thickness or more and, thereafter, by compressing the resin, the light guide plate  120  having a small thickness can be realized. In this case, however, there arises a drawback that the resin is adhered to an optical pattern such as the grooves  126  and hence, it is difficult to remove the light guide plate  120  from the mold  300 . 
     In an attempt to remove the light guide plate  120  from the mold  300  forcibly, the manufactured light guide plate  120  having a thickness of 0.4 mm or less exhibits deformation or irregularities in size thus lowering manufacturing efficiency. That is, although the method which compresses the resin is adopted for forming the thin light guide plate  120  having a thickness of 0.4 mm or less, such a compression method exhibits the drawback that it is difficult to remove the light guide plate  120  from the mold  300  due to a small thickness of the light guide plate  120 . 
     In view of the above, as shown in  FIG. 10 , a constraining portion  352  is formed around the optical pattern such as grooves  126 . In  FIG. 10 , the constraining portion  352  is formed on the third side-surface portion  332 . As shown in  FIG. 11 , the constraining portion  352  prevents, when the lower surface portion  331  of the mold is moved in the direction indicated by an arrow  364 , the light guide plate  120  from moving in the direction indicated by an arrow  364  thus allowing the lower surface portion  331  of the mold to be easily removed from the light guide plate  120 . 
     Next,  FIG. 12  and  FIG. 13  show a case in which the light guide plate  120  is compressed by the compression portion  351  from above and is also compressed by the compression portion  353  from below. In the light-incident-portion forming space  324 , the compression portion  351  is moved in the direction indicated by an arrow  361  thus compressing the light incident portion  124 , and the light guide plate  120  is also compressed in the direction indicated by an arrow  365  by the compression portion  353  from below. 
     With the provision of the compression portion  353 , the compression portion  353  is moved in the direction indicated by an arrow  364  in  FIG. 13  after compressing the light guide plate  120  so that the compression portion  353  is peeled off from the light guide plate  120 . Also in  FIG. 13 , the constraining portion  352  is formed on the third side-surface portion  332 . 
       FIG. 14  is a partially enlarged view of the light incident portion  124 . On the first side-surface portion  311 , concaves and convexes for transferring shapes of the lenses  123  and the projecting portions  220  to the light guide plate  120  are formed.  FIG. 14  is also a view showing an area of the light guide plate in the vicinity of the light incident portion  124  before the light incident portion  124  is compressed by the compression portion  351 . 
     Before the light incident portion  124  is compressed, the inclined surface  128  and the upper surface  121  are connected with each other without a stepped portion therebetween. Next,  FIG. 15  shows an area of the light guide plate  120  in the vicinity of the light incident portion  124  after the light incident portion  124  is compressed. After the light incident portion is compressed, the inclined surface  128  is pressed by the compression portion  351  and the upper surface  121  is not pressed and hence, the inclined surface  128  is downwardly pressed to a level below the upper surface  121  thus forming a stepped portion  353 . For minimizing a loss of optical performance which the light guide plate  120  suffers, it is desirable to set a size of the stepped portion  353  to approximately 0.05 mm. 
       FIG. 16  is a partial perspective view of the light incident portion  124  as viewed from a light incident surface  125  side. The inclined surface  128  is downwardly pressed by compression, and the stepped portion  353  is formed between the inclined surface  128  and the upper surface  121 . Further, on a light-incident-surface- 125 -side edge of the inclined surface  128 , an upper end  354  of the inclined surface  128  is downwardly pressed and is formed so as to be arranged close to upper ends of the lenses  123  and the projecting portions  220 . However, by forming the upper end  354  of the inclined surface  128  such that the upper end  354  of the inclined surface  128  is arranged above the upper ends of the lenses  123  and the projecting portions  220 , it is possible to prevent leaking of light from the lenses  123 . 
     That is, the upper end of the inclined surface  128  constitutes an upper end of the light incident surface  125  and hence, when the lenses  123  project from the light incident surface  125 , light which is not incident on the light incident surface  125  from the lenses  123  is generated. To prevent the generation of such light, the upper end of the inclined surface is arranged above the upper ends of the lenses  123 . 
       FIG. 17  is a perspective view of a lower-surface- 122  side of the light guide plate  120 . On the lower surface  122 , a mark which is formed by the constraining portion by pressing remains in a form of a stepped portion  356 , inclination or the like.