Abstract:
A liquid-crystal display device is provided which includes: a display panel; a backlight that applies light onto the display panel; a light emitting device of the backlight; a light guide into which the light from the light emitting device comes; and a mold around the light guide, wherein the mold reflects the light from the light emitting device.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to the light source of nonluminous display devices, and more particularly, to a liquid-crystal display device having a backlight that uses an LED as a light source with a light guide. 
         [0003]    2. Background Art 
         [0004]    Liquid-crystal display devices have been frequently used as display devices in recent years. Particularly, liquid-crystal display devices have been used for the displays of portable equipment because of their thin, lightweight, and energy saving features. 
         [0005]    However, liquid-crystal display devices need illuminating means because they are not of emissive type. 
         [0006]    Popular lighting units used in liquid-crystal display devices include planar lighting units called backlights. Cold-cathode Fluorescent Lamp tubes have been generally used as the light-emitting devices (also referred to as light sources) of the backlights, while LEDs (light-emitting diodes) are also used as the light-emitting devices. 
         [0007]    The backlights have a planar light guide. The light guide is made of light-transmissive resin, so that light incident on the light guide from the light-emitting devices transmits through the light guide. The light guide has reflecting or dispersing members such as grooves, protrusions, or prints. The reflecting or dispersing members cause the light that transmits through the light guide to advance toward the liquid-crystal display device. 
         [0008]    The use of LEDs as light-emitting devices poses the problem of difficulty in letting out uniform light from the light guide because they are point light sources. To cope with such a problem, for example, JP-A-9-092886 proposes a technique of dispersing the light around the LEDs evenly. 
         [0009]    Another arrangement is also known in which the light-exiting portions of LEDs have lenses so that uniform light can exit from the LEDs. 
       SUMMARY OF THE INVENTION 
       [0010]    Backlights that use a plurality of LEDs as light-emitting devices so as to emit high-luminance light have separate light-emitting points. This makes it difficult to emit uniform light in the incident surface of the light guide. Even when lenses are particularly disposed at the light-exiting portions of LEDs in consideration of the dispersion of the light from the LEDs, it is difficult to eliminate darkness to be caused between the LEDs. 
         [0011]    According to a first aspect of the invention, there is provided a liquid-crystal display device including: a display panel; a backlight that applies light onto the display panel; a light emitting device of the backlight; a light guide into which the light from the light emitting device comes; and a mold around the light guide. The mold has slopes inclined with respect to the light incident surface of the light guide. The light from the light emitting device is advanced also along the light incident surface of the light guide. The light traveling along the light incident surface of the light guide is reflected by the slopes toward the light incident surface of the light guide. 
         [0012]    Thus, the light can be introduced to the light guide through between the LEDs, so that dark portions to be generated between the LEDs can be reduced. In other words, the light that is emitted from the LEDs and travels in parallel to the light incident surface of the light guide is directed forward of the slopes using the slopes provided to the mold, so that the light can be introduced to the light guide ahead of the slopes. This increases the light that enters perpendicularly on the light incident surface of the light guide even from between LEDs. 
         [0013]    According to a second aspect of the invention, there is provided a liquid-crystal display device including a liquid crystal panel; and a planar lighting unit that applies light onto the liquid crystal panel. The planar lighting unit includes: a light guide having a light exiting surface and a bottom surface facing the light exiting surface. The light guide has side faces crossing the light exiting surface or the bottom surface. A plurality of LEDs is disposed along a first side of the light guide. The light of the LEDs is entered from the first side of the light guide to make the first side as the light incident surface. A mold is disposed around the light guide and the LEDs. The mold has slopes facing the light incident surface of the light guide. 
         [0014]    A part of the light emitted from the LEDs is emitted along the light incident surface of the light guide the light traveling along the light incident surface of the light guide by the slopes is reflected toward the light incident surface of the light guide. 
         [0015]    The above-described structure can thus increase the amount of light incident on the light guide from between adjacent two LEDs. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a plan view of a liquid-crystal display device according to an embodiment of the invention; 
           [0017]      FIG. 2A  is a schematic cross sectional view of a light emitting diode of the liquid-crystal display device according to an embodiment of the invention; 
           [0018]      FIG. 2B  is a light-exiting-side front view of the light emitting diode according to an embodiment of the invention. 
           [0019]      FIG. 3A  is a schematic perspective view of the light emitting diode of the liquid-crystal display device according to an embodiment of the invention; 
           [0020]      FIG. 3B  is a schematic cross sectional view of the same; 
           [0021]      FIG. 4A  is a schematic plan view of the light guide of the liquid-crystal display device according to an embodiment of the invention; 
           [0022]      FIG. 4B  is a schematic side view of the same; 
           [0023]      FIG. 5A  shows an optical path in the case where the grooves of the light guide project outward, according to an embodiment of the invention; 
           [0024]      FIG. 5B  shows an optical path in the case where the grooves are recessed inward, according to an embodiment of the invention; 
           [0025]      FIG. 6  is a schematic plan view of the backlight according to an embodiment of the invention, showing a problem of the liquid-crystal display device having a plurality of light emitting devices; 
           [0026]      FIG. 7  is a schematic plan view of the backlight according to an embodiment of the invention, in which the light incidence surface has curves; 
           [0027]      FIG. 8  is a schematic plan view of the flexible board having white or high-reflectance members according to an embodiment of the invention; 
           [0028]      FIG. 9  is a schematic plan view of the light guide and a mold that houses the light guide according to an embodiment of the invention; 
           [0029]      FIG. 10  is a schematic plan view of the light guide and a mold that houses the light guide according to an embodiment of the invention; and 
           [0030]      FIG. 11  is a schematic plan view of the light guide and the mold according to an embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]      FIG. 1  is a plan view of a liquid-crystal display device  100  according to an embodiment of the invention. The liquid-crystal display device  100  includes a liquid crystal panel  1 , a backlight  110 , and a control circuit  80 . Signals and supply voltage necessary for display on the liquid crystal panel  1  are fed from the control circuit  80 . The control circuit  80  is mounted on a flexible board  70 , from which signals are sent to the liquid crystal panel  1  through lines  71  and terminals  75 . 
         [0032]    The backlight  110  includes a light guide  120 , LEDs  150 , and a mold  180 . The backlight  110  is disposed to illuminate the liquid crystal panel  1  with light. The light from the backlight  110  whose amount of transmission or reflection is controlled is used for the display on the liquid crystal panel  1 . While the backlight  110  is placed on the back or front surface of the liquid crystal panel  1 , as viewed from the viewer,  FIG. 1  shows the backlight  110  below the liquid crystal panel  1  for the convenience of description. 
         [0033]    The light guide  120  is substantially rectangular in shape and has the LEDs  150  on one side. Reference numeral  160  denotes a flexible board that electrically connects the LEDs  150 . The flexible board  160  and the control circuit  80  are electrically connected by a line  161 . For the interests of simplicity, a mold provided between the light guide  120  and the LEDs  150  is omitted. The details of the mold and the backlight  110  will be described later. 
         [0034]    The liquid crystal panel  1  will now be described. A pixel section  8  of the liquid crystal panel  1  has a pixel electrode  12 . While the liquid crystal panel  1  has a large number of the pixel sections  8  in matrix form, only one pixel section  8  is shown in  FIG. 1  for the sake of simplicity. The matrix pixel sections  8  constitute a display region  9 . The pixel sections  8  serve as the pixels of a display image to provide an image on the display region  9 . 
         [0035]    Referring to  FIG. 1 , the liquid crystal panel  1  has gate signal lines (also referred to as scanning lines)  21  which extend in the X direction and arranged in lines in the Y direction and drain signal lines (also referred to as video signal lines)  22  which extend in the Y direction and arranged in lines in the X direction. The gate signal lines  21  and the drain signal lines  22  intersect each other. The pixel sections  8  are each formed in the region surrounded by the gate signal lines  21  and the drain signal lines  22 . 
         [0036]    Each pixel section  8  has a switching element  10 . A control signal is supplied through the gate signal line  21  to control the on-off action of the switching element  10 . When the switching element  10  is turned on, a video signal sent to the pixel electrode  12  through the drain signal line  22 . 
         [0037]    The drain signal lines  22  are connected to a driving circuit  5 . The driving circuit  5  outputs video signals. The gate signal lines  21  are connected to a driving circuit  6 . The driving circuit  6  outputs control signals. The gate signal lines  21 , the drain signal lines  22 , the driving circuit  5 , and the driving circuit  6  are formed on the same TFT substrate  2 . 
         [0038]      FIGS. 2A and 2B  show the schematic structure of the LED  150  that is a light-emitting device, wherein  FIG. 2A  is a schematic cross sectional view of the LED  150 , and  FIG. 2B  is a light-exiting-side front view of the same. 
         [0039]    The LED  150  has a structure in which an LED chip  151  serving as a light emitter is mounted on a chip board  154 . The LED chip  151  has a PN junction. When voltage is applied, the PN junction emits light of a specific wavelength. A P-type semiconductor layer of the PN junction has a P electrode (anode)  158 , while an N-type semiconductor layer has an N electrode (cathode)  159 . 
         [0040]    The P electrode  158  and the N electrode  159  each connect to a wire  152 . The wires  152  electrically connect the P electrode  158  and the N electrode  159  to chip terminals  153  for connecting the LED  150  externally, respectively. 
         [0041]    The LED chip  151  may have a fluorescent emission section on the light exiting surface. The fluorescent emission section has the function of converting the wavelength of the light emitted from the LED chip  151 . A lens portion  157  disperses the exiting light. A transparent resin portion  155  allows the light emitted from the LED chip  151  to pass therethrough to the front and the sides. 
         [0042]    Referring then to  FIGS. 3A and 3B , light emitted from the LED  150  will be described.  FIG. 3A  is a schematic perspective view of the LED  150 , and  FIG. 3B  is a schematic cross sectional view of the same, which illustrate the exiting light. 
         [0043]    The LED chip  151  emits light mainly along an arrow  131 . The arrow  131  is perpendicular to the light-emitting surface of the LED chip  151  and directed to the front. The direction indicated by the arrow  131  is referred to as a main exiting direction. 
         [0044]    In this embodiment, light exits also in directions crossing the main exiting direction  131 . The directions crossing the main exiting direction  131  and along the chip board  154  are designated by arrows  132 . The directions indicated by the arrows  132  are hereinafter also referred to as lateral directions. 
         [0045]    The LED  150  shown in  FIGS. 3A and 3B  has the lens portion  157 , through which the exiting light is dispersed evenly in the main exiting direction  131 . The light exiting surface of the lens portion  157  is orthogonal to the exiting light so as to minimize the reflection and refraction of the light on the light exiting surface. 
         [0046]    Light of angles lager than a predetermined angle is reflected at the boundary between the light guide  120  and air on the incident surface of the light guide  120  because of the refractivity of the light guide  120 . Accordingly, the light exiting in the lateral directions  132  will not enter the light guide  120  or reflected by the light guide  120  because it is substantially parallel to the incident surface of the light guide  120 . 
         [0047]    This embodiment is constructed such that the light from the LEDs  150  travels also in the directions parallel to the incident surface of the light guide  120 , and is reflected by the mold, to be described later, to the light guide  120 , so that the amount of light that enters between the LEDs  150  is increased. 
         [0048]      FIG. 4A  shows a schematic plan view of the light guide  120 , and  FIG. 4B  shows a schematic side view of the same. The light guide  120  is rectangular in shape as shown in  FIG. 4A , and has a top face  121  and a bottom face  122 , as shown in  FIG. 4B . The light guide  120  is made of a light transmissive material such as acrylic resin, and has the shape of a plate with a thickness from 1.0 mm to 0.2 mm. Although the light guide  120  in FIG.  4 B has a rectangular cross section, it may have a wedge shape whose thickness decreases from a light incident surface  125 . 
         [0049]      FIGS. 4A and 4B  show the positional relationship between the light guide  120  and the LEDs  150 . The LEDs  150  are disposed in the vicinity of the light incident surface  125  at least one side of the light guide  120 . The LEDs  150  are disposed on the flexible board  160  and along the light incident surface  125 . 
         [0050]    The light that has exited from the LEDs  150  enters the light incident surface  125 . Since the refractivity of the light guide  120  is higher than that of air, as described above, light incident on the light incident surface  125  at angles larger than a specified angle with respect to the normal to the light incident surface is reflected, while light incident at angles lower than that enters the light guide  120 . 
         [0051]    The top face  121  and the bottom face  122  of the light guide  120  are substantially perpendicular to the light incident surface  125 . The bottom face  122  has V-grooves  126  serving as a reflector. The light that has come into the light guide  120  repeats total reflection between the top face  121  and the bottom face  122  to advance in the light guide  120 . The light traveling in the light guide  120  is reflected by the grooves  126  provided on the bottom face  122  to the top face  121  and exits from the top face  121 . 
         [0052]    Referring to  FIGS. 5A and 5B , the light reflected by the grooves  126  will be described.  FIG. 5A  shows a case in which the grooves  126  protrude outward, while  FIG. 5B  shows a case in which the grooves  126  are recessed inward. The grooves  126  each have a reflecting surface (also referred to as a slope)  127 . The reflecting surface  127  forms an angle from 2 to 35 degrees with the bottom face  122 . The light reflected by the reflecting surface  127  exits such that it expands externally at a large angle with respect to the line perpendicular to the top face  121  of the light guide  120  (at an obtuse angle with respect to the top face  121 ). Therefore, prism sheets  113  and  112  are disposed above the light guide  120  to reflect the outward light toward the liquid crystal panel (not shown). Numeral  114  denotes a diffuser, and numeral  115  designates a reflecting sheet. 
         [0053]    Referring to  FIG. 6 , the light in the neighborhood of the LEDs  150  will be described. The LEDs  150  are disposed to face the light incident surface  125  of the light guide  120 . The main light exiting direction  131  (the Y direction in  FIG. 6 ) of the LEDs  150  is substantially perpendicular to the light incident surface  125 , so that most of the light exiting from the LEDs  150  enters the light guide  120 . 
         [0054]    Since light of angles above a predetermined angle with respect to the vertical direction to the light incident surface  125  is reflected by the light incident surface  125 , as described above, extremely little light reaches the regions of the light incident surface  125  beyond the predetermined angle to form the dark regions  210 . 
         [0055]      FIG. 7  shows the light incident surface  125  having curves  129  similar to the lens portions  157  of the LEDs  150  to decrease the dark region  210 . The curves  129  are formed on the light incident surface  125  so as to face the lens portions  157 . Since the light exiting from the lens portions  157  enters the curves  129  at right angles, thus enhancing the light utilization of the light guide  120 . 
         [0056]    Moreover, the light traveling in the X direction can also enter the light guide  120  because the curves  129  have surfaces substantially perpendicular to the X direction. This reduces the dark regions  210  to be formed between LEDs  150 . 
         [0057]    In  FIG. 7 , the distance between the lens portions  157  and the curves  129  is set small so as to achieve high light utilization. To that end, a portion  161  of the flexible board  160  on which the LEDs  150  are mounted overlaps with the light guide  120 . 
         [0058]    Specifically, since the lens portions  157  of the LEDs  150  are embedded in the concave portions of the curves  129 , the portion  161  of the flexible board  160  located on the straight line connecting the LEDs  150  overlaps with the light guide  120 . 
         [0059]      FIG. 8  shows the flexible board  160  having white or high-reflectance members  211 . The flexible board  160  thus reflects light to the light guide  120 , thereby enhancing the light utilization. 
         [0060]    The flexible board  160  shown in  FIG. 8  has the white or high-reflectance members  211  corresponding to the dark regions  210 , and further has light-absorbing members  212  on the surface adjacent to the light incident surface  125  of LED  150 . The light-absorbing members  212  have alternate black or gray dots of different sizes so as to adjust the amount of light reflected in the neighborhood of the LEDs  150 . 
         [0061]    Referring next to  FIG. 9 , the mold  180  housing the light guide  120  will be described. The mold  180  has a shape that surrounds the light guide  120 . The light guide  120  and the mold  180  are arranged close to each other on the side of the light guide  120  except the light incident surface  125 , while they are illustrated separately for the convenience of illustration. The proximity of the light guide  120  and the mold  180  reduces the leakage of light therebetween, and allows the light exiting from the side to be reflected into the light guide  120  again. 
         [0062]    In  FIG. 9 , part of the mold  180  adjacent to the back of the LEDs  150  is apart from the light incident surface  125 , below in the drawing, because the LEDs  150  are present between the mold  180  and the light incident surface  125 . However, part of the mold  180  between the LEDs  150  forms protrusions  181  close to the light incident surface  125 . 
         [0063]    The LEDs  150  emits light also in the direction along the light incident surface  125  (in the lateral direction  132 ). For that, a clearance is provided between the protrusions  181  and the light incident surface  125 . The presence of the protrusions  181  allows the light from the LEDs  150  traveling in the lateral direction  132  to be reflected to the light guide  120 , and allows the light that has exited from the light incident surface  125  to the mold  180  to be reflected to the light guide  120  again. 
         [0064]    The flexible board  160  may have the high-reflectance members  211  and the light-absorbing members  212  shown in  FIG. 8  (not shown in  FIG. 9 ). Similarly, the other flexible boards of this embodiment may have the high-reflectance members  211  and the light-absorbing members  212  as necessary. 
         [0065]      FIG. 10  shows the protrusions  181  having slopes  182 . The light that has exited from the LEDs  150  in the lateral direction  132  along the light incident surface  125  is reflected by the slopes  182  to become light  133  directed to the light incident surface  125 . 
         [0066]    The light  133  reflected by the slopes  182  enters the light guide  120  between the LEDs  150 , so that the dark regions  210  can be remarkably reduced. By controlling the angle of the slopes  182 , the light  133  reflected by the slopes  182  can enter the light incident surface  125  of the light guide  120  substantially vertically. 
         [0067]    Thus, the presence of the slopes  182  allows the light to enter through the part of the light incident surface  125  between the LEDs  150 . This allows even the LEDs  150 , or point light sources, to emit light substantially vertically with respect to the light incident surface  125 . 
         [0068]      FIG. 11  shows a case in which the light guide  120  is disposed close to the protrusions  181 . The light guide  120  has protrusions  128  such that they fill the clearance between the LEDs  150 . The protrusions  128  of the light guide  120  and the protrusions  181  of the mold  180  form spaces  183 , in which the LEDs  150  are housed. 
         [0069]    The protrusions  128  of the light guide  120  have slopes  184 . The slopes  184  reflect the light traveling along the light incident surface  125  into the light  133  that is directed to the light guide  120 . The slopes  184  increase the amount of vertical light that enters the light incident surface  125  between the LEDs  150 . 
         [0070]    The light emitted from the LEDs  150  is also reflected in the inner surfaces of the spaces  183 . Of the light reflected by the inner surfaces of the spaces  183 , the light that has reached the light incident surface  125  at angles permitting entrance can enter the light guide  120 , thus enhancing light utilization.