Abstract:
Provided is a backlight device which can reduce the size and weight of light-guiding members while also improving light use efficiency. The backlight device ( 10 ) is provided with a light-guiding plate ( 11 ) and a plurality of LEDs ( 12 ) which irradiate the light-guiding plate with light. The light-guiding plate includes a plurality of side surfaces ( 11   a ) and a plurality of light incident surfaces ( 11   b ) which are formed in a plurality of corners and are arranged opposite the LEDs. The side surfaces are arranged between adjacent light incident surfaces and are formed in convex shapes.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a backlight device and a display device, and in particular, the present invention relates to a backlight device and a display device that have a point light source disposed at a corner of a light guide member. 
       BACKGROUND ART 
       [0002]    Conventionally, there has been known a backlight device including a point light source such as an LED (Light Emitting Diode) and a light guide member which light from the point light source enters. In a case of using point light sources in a backlight device, it is possible to reduce the number of point light sources by arranging the point light sources at corners of the light guide member. 
         [0003]      FIG. 21  is a top view showing a configuration of a backlight device according to a conventional example where LEDs (point light sources) are disposed at the corners of a light guide plate (a light guide member). As shown in  FIG. 21 , a backlight device  501  according a conventional example includes a rectangular light guide plate (a light guide member)  510  and LEDs (point light sources)  520  which are disposed at the four corners (four corner portions) of the light guide plate  510  to face the four corners. 
         [0004]    In the backlight device  501  according to the conventional example shown in  FIG. 21 , a substantial entirety of the light guide plate  510  is an effective light emitting region corresponding to a display region of a display panel (not shown). Besides, in the backlight device  501 , in each of the four corners of the light guide plate  510 , a portion that is located to face an LED  520  is formed inclined with respect to a light emitting surface  520   a  of the LED  520 . That is, in each of the four corners of the light guide plate  510 , the surface on which light is incident is not formed perpendicular to the optical axis  501  of the LED  520 . As a result, light emitted from the LED  520  is prone to be reflected on a surface of the light guide plate  510 , and this is inconvenient. This makes it disadvantageously difficult to improve light use efficiency. 
         [0005]    A possible method of dealing with such inconveniences is to form a surface of the light guide plate on which light is incident to be perpendicular with respect to an LED optical axis. 
         [0006]      FIG. 22  is a top view showing a configuration of a backlight device that is capable of reducing reflection of LED light on a surface of a light guide plate, an aspect ratio (length of a long side:length of a short side) of the light guide plate being approximately 1:1.  FIG. 23  is a top view of the light guide plate of the backlight device shown in  FIG. 22 , showing a light emitting region which emits light by being irradiated with light from an LED.  FIG. 24  is a top view of the light guide plate of the backlight device shown in  FIG. 22 , showing light emitting regions which emit light by being irradiated with light from four LEDs.  FIG. 25  is a top view of the light guide plate of the backlight device shown in  FIG. 22 , showing an effective light emitting region of the light guide plate. 
         [0007]    As shown in  FIG. 22 , a backlight device  601  includes a light guide plate (a light guide member)  610  which is substantially square shaped and LEDs (point light sources)  620  which are disposed at four corners of the light guide plate  610  to face the corners. In the backlight device  601 , the LEDs  620  are placed such that a center of the light guide plate  610  (a center O 601  of a later-described effective light emitting region S 601  (see  FIG. 25 )) is located on optical axes L 601  of the LEDs  620 . In addition, in the four corners of the light guide plate  610 , light incident surfaces  610   a  are formed to be parallel to light emitting surfaces  620   a  of the LEDs  620 ) (that is, to be perpendicular to the optical axes L 601  of the LEDs  620 ). With this configuration, it is possible to reduce reflection of LED light emitted from the LEDs  620  on surfaces (the light incident surfaces  610   a ) of the light guide plate  610 , to thereby improve light use efficiency. 
         [0008]    The light guide plate  610  is typically formed of an acrylic resin, a polycarbonate resin, or the like. Thus, in the backlight device  601  shown in  FIG. 22 , a spread angle with which the light emitted from the LEDs  620  spreads on entering the light guide plate  610  is smaller than 90°. 
         [0009]    Specifically, a refraction index of an acrylic resin is approximately 1.49. Thus, in a case in which the light guide plate  610  is formed by using an acrylic resin, even if a spread angle of light at the time of being emitted from the LEDs  620  is set to, for example, 178°, the spread angle of the light at which it spreads on entering the light guide plate  610  is approximately 84° according to Snell&#39;s law. A refraction index of a polycarbonate resin is approximately 1.59. Thus, in a case in which the light guide plate  610  is formed by using a polycarbonate resin, even if the spread angle of the light at the time of being emitted from the LEDs  620  is set to, for example, 178°, a spread angle of the light at which it spreads on entering the light guide plate  610  is approximately 78° according to Snell&#39;s law. 
         [0010]    As a result, a light emitting region R 601  of the light guide plate  610  which emits light by being irradiated with light from one of the LEDs  620  (the one at the top left in  FIG. 23 ) is as shown as a hatching region in  FIG. 23 . And, in the backlight device  601 , in which the LEDs  620  are disposed at the four corners of the light guide plate  610  as shown in  FIG. 24 , the entire light guide plate  610  is a light emitting region. Incidentally, in  FIG. 24 , boundary surfaces of the light emitting regions that emit light by being irradiated with light from the LEDs  620  are indicated by lines of as many different types to help distinguish which of the light emitting regions corresponds to which of the LEDs  620 . 
         [0011]    In addition, with the backlight device  601 , in a case in which, as shown in  FIG. 25 , the effective light emitting region S 601  of the light guide plate  610  corresponding to a display region of a display panel (not shown) is a square-shaped region (a hatching region enclosed in a bold line frame in  FIG. 25 ) defined by lines connecting centers of the light incident surfaces  610   a  of the light guide plate  610 , the effective light emitting region S 601  of the light guide plate  610  is maximum. 
         [0012]      FIG. 27  is a top view showing a configuration of a backlight device that is capable of reducing reflection of LED light on a surface of a light guide plate, an aspect ratio (length of a long side:length of a short side) of the light guide plate being approximately 4:3.  FIG. 28  is a top view of the light guide plate of the backlight device shown in  FIG. 27 , showing a light emitting region that emits light by being irradiated with light from an LED.  FIG. 29  is a top view showing an effective light emitting region of the light guide plate of the backlight device shown in  FIG. 27 . 
         [0013]    As shown in  FIG. 27 , a backlight device  701  includes a light guide plate (a light guide member)  710  which is substantially rectangular and LEDs (point light sources)  720  which are disposed at four corners of the light guide plate  710  to face the corners. In the backlight device  701 , the LEDs  720  are placed such that a center of the light guide plate  710  (a center O 701  of a later-described effective light emitting region S 701  (see  FIG. 29 )) is located on optical axes L 701  of the LEDs  720 . In addition, in the four corners of the light guide plate  710 , light incident surfaces  710   a  are formed to be parallel to light emitting surfaces  720   a  of the LEDs  720 ) (that is, to be perpendicular to the optical axes L 701  of the LEDs  720 ). With this configuration, it is possible to reduce reflection of LED light emitted from the LEDs  720  on the surfaces (the light incident surfaces  710   a ) of the light guide plate  710 , to thereby improve light use efficiency. 
         [0014]    In the backlight device  701 , a light emitting region R 701  of the light guide plate  710  which emits light by being irradiated with light from one of the LEDs  720  (the one at the top left in  FIG. 28 ) is as shown as a hatching region in  FIG. 28 . And, in the backlight device  701 , as shown in  FIG. 27 , as in the above-described backlight device  601 , the entire light guide plate  710  is a light emitting region. Incidentally, in  FIG. 27 , the light emitting regions that emit light by being irradiated with light from the LEDs  720  are indicated by lines of as many different types to help distinguish which of the light emitting regions corresponds to which of the LEDs  720 . 
         [0015]    In addition, with the backlight device  701 , in a case in which, as shown in  FIG. 29 , the effective light emitting region S 701  of the light guide plate  710  corresponding to a display region of a display panel (not shown) is a rectangular region (a hatching region enclosed in a bold line frame in  FIG. 29 ) that does not include the light incident surfaces  710   a,  the effective light emitting region S 701  of the light guide plate  710  is maximum. 
         [0016]      FIG. 32  is a top view showing a configuration of a backlight device that is capable of reducing reflection of LED light on a surface of a light guide plate, the backlight device having LEDs disposed at two adjacent corners of the light guide plate such that the LEDs face the two adjacent corners.  FIG. 33  is a top view of the light guide plate of the backlight device shown in  FIG. 32 , showing a light emitting region that emits light by being irradiated with light from an LED.  FIG. 34  is a top view showing a non-light-emitting region of the light guide plate of the backlight device shown in  FIG. 32 .  FIG. 35  is a top view showing an effective light emitting region of the light guide plate of the backlight device shown in  FIG. 32 . 
         [0017]    As shown in  FIG. 32 , a backlight device  801  includes a light guide plate (a light guide member)  810  which is substantially rectangular and LEDs (point light sources)  820  which are disposed at two adjacent corners of the light guide plate  810  such that the LEDs  820  face the two adjacent corners. In the backlight device  801 , the LEDs  820  are placed such that a center O 801  of a later-described effective light emitting region S 801  (see  FIG. 35 ) is located on optical axes L 801  of the LEDs  820 . In addition, in the two corners of the light guide plate  810 , light incident surfaces  810   a  are formed to be parallel to light emitting surfaces  820   a  of the LEDs  820  (that is, to be perpendicular to the optical axes L 801  of the LEDs  820 ). With this configuration, it is possible to reduce reflection of light emitted from the LEDs  820  on the surfaces (the light incident surfaces  810   a ) of the light guide plate  810 , to thereby improve light use efficiency. 
         [0018]    In the backlight device  801 , a light emitting region R 801  of the light guide plate  810  which emits light by being irradiated with light from one of the LEDs  820  (the one at the top left in  FIG. 33 ) is as shown as a hatching region in  FIG. 33 . And, with the backlight device  801 , where the LEDs  820  are disposed only at two corners of the light guide plate  810 , a region excluding a hatching region (a non-light-emitting region R 802 ) in  FIG. 34  is the light emitting region. Incidentally, in  FIG. 34 , boundary surfaces of the light emitting regions that emit light by being irradiated with light from the LEDs  820  are indicated by lines of as many different types to help distinguish which of the light emitting regions corresponds to which of the LEDs  820 . 
         [0019]    In addition, with the backlight device  801 , in a case in which, as shown in  FIG. 35 , the effective light emitting region S 801  of the light guide plate  810  corresponding to a display region of a display panel (not shown) is a rectangular region (a hatching region enclosed in a bold line frame in  FIG. 35 ) that does not include the light incident surfaces  810   a,  the effective light emitting region S 801  of the light guide plate  810  is maximum. 
         [0020]    Examples of backlight devices capable of reducing reflection of light from point light sources on a surface of a light guide member include the backlight devices disclosed in Patent Literatures 1 and 2. 
       CITATION LIST 
     Patent Literature 
       [0021]    Patent Literature 1: JP-A-H11-133425 
         [0022]    Patent Literature 2: JP-A-2001-215338 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0023]    With the backlight device  601  shown in  FIG. 22 , however, since a region R 602  (a hatching region in  FIG. 26 ) which is a region other than the effective light emitting region S 601  of the light guide plate  610  also emits light, it is difficult to improve light use efficiency. 
         [0024]    In addition, the backlight device  601  suffers a problem where the existence of the region R 602  (the hatching region in  FIG. 26 ) which is a region other than the effective light emitting region S 601  of the light guide plate  610  makes it difficult to make the light guide plate  610  compact (resource saving) and lightweight. 
         [0025]    With the backlight device  701  shown in  FIG. 27 , where a region R 702  (a hatching region in  FIG. 30 ) which is a region other than the effective light emitting region S 701  of the light guide plate  710  also emits light as in the case with the backlight device  601 , it is difficult to further improve light use efficiency. 
         [0026]    In addition, as in the case with the backlight device  601 , the backlight device  701  suffers a problem where the existence of the region R 702  (the hatching region in  FIG. 30 ) which is a region other than the effective light emitting region S 701  of the light guide plate  710  makes it difficult to make the light guide plate  710  compact and lightweight. 
         [0027]    Furthermore, in the backlight device  701 , a region R 703  (the hatching region in  FIG. 31 ) which is not irradiated with light from either of two adjacent LEDs  720  and the effective light emitting region S 701  (see  FIG. 29 ) overlap each other. The region R 703  (the hatching region in  FIG. 31 ), which is not irradiated with light from either of two adjacent LEDs  720  and thus is not as bright as the other regions, is liable to be recognized as uneven brightness by a viewer. Note that it is more difficult to make the light guide plate  710  compact and lightweight if the light guide plate  710  is formed such that the region R 703  which is not irradiated with light from either of two adjacent LEDs  720  and the effective light emitting region S 701  do not overlap each other. 
         [0028]    In addition, as in the cases with the backlight devices  601  and  701 , the backlight device  801  shown in  FIG. 32  suffers a problem where the existence of a region R 803  (the hatching region in  FIG. 36 ) which is a region other than the effective light emitting region S 801  of the light guide plate  810  makes it difficult to make the light guide plate  810  compact and lightweight. 
         [0029]    Furthermore, in the backlight device  801 , as in the case with the backlight device  701 , a region R 802  (the hatching region in  FIG. 34 ) which is not irradiated with light from either of two adjacent LEDs  820  and the effective light emitting region S 801  (see  FIG. 35 ) overlap each other. Thus, the region R 802  (the hatching region in  FIG. 34 ), which is not irradiated with light from either of the two adjacent LEDs  820 , is liable to be recognized as uneven brightness by a viewer. 
         [0030]    As described hereinabove, the backlight devices  601 ,  701 , and  801  all suffer the problem where it is difficult to make the light guide plate compact and lightweight. 
         [0031]    The present invention has been made to solve the above problems, and an object of the present invention is to provide a backlight device and a display device that are capable of both improving light use efficiency and achieving a compact and lightweight light guide member. 
       Solution to Problem 
       [0032]    To achieve the above object, according to a first aspect of the present invention, a backlight device includes a light guide member and a plurality of point light sources which irradiate the light guide member with light. Here, the light guide member includes a plurality of light incident surfaces, the light incident surfaces being formed in at least two adjacent corners thereof such that the point light sources are disposed to face the light incident surfaces, and a plurality of side surfaces. And, at least one of the side surfaces between adjacent ones of the light incident surfaces is formed in a depressed shape. 
         [0033]    In the backlight device according to the first aspect, as described above, the light guide member includes the plurality of light incident surfaces, and the point light sources are disposed to face the light incident surfaces. With this configuration, it is possible to reduce reflection of light emitted from the point light sources on surfaces (the light incident surfaces) of the light guide member, and this makes it possible to improve light use efficiency. 
         [0034]    Furthermore, with the backlight device according to the first aspect, by forming, as described above, at least one of the side surfaces between adjacent ones of the light incident surfaces in a depressed shape, it is possible to achieve a compact (resource saving) and lightweight light guide member. 
         [0035]    Note that, in a case in which no side surface of the light guide member is formed in a depressed shape, if a light incident surface is formed in a corner of the light guide member, there appears, in the light guide member, a region other than the effective light emitting region (that is, a region that does not contribute to illumination of the display panel). Furthermore, if the effective light emitting region is maximized, there appears a region other than the effective light emitting region (that is, a region that does not contribute to illumination of the display panel) around at least one side surface between adjacent ones of the light emitting surfaces. Thus, even if at least one of the side surfaces between adjacent ones of the light incident surfaces is formed in a depressed shape, the effective light emitting region of the light guide member is not reduced. That is, an effective light emitting region of the same size can be obtained without making the light guide member larger. 
         [0036]    Moreover, with the backlight device according to the first aspect, since at least one of the side surfaces between adjacent ones of the light incident surfaces is formed in a depressed shape, it is possible to reduce emission of light from a region other than the effective light emitting region of the light guide member. This makes it possible to further improve light use efficiency. 
         [0037]    In the backlight device according to the first aspect described above, it is preferable that the plurality of side surfaces include a pair of first side surfaces which face each other and a pair of second side surfaces which are disposed along a direction that crosses the first side surfaces, that a length of the first side surfaces be not longer than a length of the second side surfaces, and that at least one of the pair of first side surfaces be located between the plurality of light incident surfaces and formed in a depressed shape. With this configuration, in a case, for example, in which the light guide member is formed in a substantially rectangular shape, a side surface (a first side surface) along a short side of the light guide member is formed in a depressed shape. Here, in order to maximize the effective light emitting region in the case in which the light guide member is formed substantially rectangular, it is necessary to form a side surface along a short side of the light guide member in a depressed shape, without forming a side surface (a second side surface) on a long side of the light guide member in a depressed shape. Thus, by forming, as described above, at least one of the pair of first side surfaces in a depressed shape, it is possible to maximize the effective light emitting region. That is, an effective light emitting region of the same size can be obtained with a smaller light guide member. As a result, a compact and lightweight light guide member can be obtained. 
         [0038]    In the backlight device in which the plurality of side surfaces include a pair of first side surfaces and a pair of second side surfaces, it is preferable that the length of the first side surfaces be shorter than the length of the second side surfaces, that the light guide member include an effective light emitting region corresponding to a display region of a display panel, and that a center of the effective light emitting region of the light guide member be disposed a predetermined distance away from optical axes of the point light sources. With this configuration, in contrast to a case in which the center of the effective light emitting region of the light guide member is located on the optical axes of the point light sources, it is possible to reduce overlap of a region that is not irradiated with light from either of two adjacent point light sources and the effective light emitting region, and thus it is possible to reduce occurrence of uneven brightness in the effective light emitting region. Also, since there is no need of making the light guide member larger in order to prevent the region that is not irradiated with light from either of two adjacent point light sources and the effective light emitting region from overlapping each other, it is possible to reduce increase in size and weight of the light guide member. 
         [0039]    In the backlight device in which the plurality of side surfaces include the pair of first side surfaces and the pair of second light sources, it is preferable that the light incident surfaces be formed in four corners of the light guide member, and that the pair of first side surfaces be both formed in a depressed shape. With this configuration, in a case in which the point light sources are disposed at the four corners of the light guide member, it is possible to both maximize the effective light emitting region and make the light guide member compact and lightweight. 
         [0040]    In the backlight device in which the plurality of side surfaces include the pair of first side surfaces and the pair of second light sources, it is preferable that the light incident surfaces be formed in two adjacent corners of the light guide member, and that only one of the pair of first side surfaces be formed in a depressed shape. With this configuration, in a case in which the point light sources are disposed at two adjacent corners of the light guide member, it is possible to both maximize the effective light emitting region and make the light guide member compact and lightweight. In addition, since the number of point light sources can be reduced in comparison with the case in which the point light sources are disposed at the four corners of the light guide member, it is possible to achieve a compact and lightweight backlight device. 
         [0041]    In the backlight device according to the first aspect, it is preferable that an angle formed by two of the side surfaces that are adjacent to the light incident surfaces be substantially as large as a spread angle at which light from the point light sources spreads on entering the light guide member. With this configuration, it is possible to reduce appearance of a useless region (that does not contribute to illumination of the display panel) through which light emitted from the point light sources does not pass, and thus it is easy to achieve a compact and lightweight light guide member. 
         [0042]    In the backlight device according to the first aspect, it is preferable that the light guide member include an effective light emitting region corresponding to a display region of a display panel, and that a side surface among the side surfaces that is formed in a depressed shape have a function of reflecting light that enters the light guide member to the effective light emitting region side. With this configuration, it is possible to reflect, to the side of the effective light emitting region of the light guide member, light that travels toward the side surface that is formed in a depressed shape after entering the light guide member, and thus to reduce loss of light. That is, it is possible to reduce degradation of light use efficiency. 
         [0043]    In the backlight device in which a side surface among the side surfaces that is formed in a depressed shape has a function of reflecting light that enters the light guide member to the effective light emitting region side, it is preferable that a prism be formed on the side surface that is formed in the depressed shape. With this configuration, in comparison with a case in which no prism is formed on the side surface that is formed in a depressed shape, it is easier to reflect the light to an inside (a side surface opposite to the side surface formed in a depressed shape) of the light guide member. This makes it possible to achieve uniform emission of light from the light guide member and to achieve more uniform brightness of the display panel as well. 
         [0044]    In the backlight device according to the first aspect, it is preferable that the light incident surfaces be formed as a flat surface or a curved surface. With this configuration, it is possible to easily reduce reflection of light emitted from the point light sources on the surfaces (the light incident surfaces) of the light guide member, and this makes it easy to improve light use efficiency. 
         [0045]    In the backlight device according to the first aspect, it is preferable that a side surface among the side surfaces that is formed in a depressed shape include a plurality of flat surface portions or at least one curved surface portion. With this configuration, it is easy to form a side surface in a depressed shape. 
         [0046]    According to a second aspect of the present invention, a display device includes any one of the above-configured backlight devices and a display panel which is illuminated by the backlight device. With this configuration, it is possible to obtain a display device that is capable of both improving light use efficiency and making the light guide member compact and lightweight. 
       Advantageous Effects of Invention 
       [0047]    As described hereinabove, according to the present invention, it is possible to obtain a display device capable of both improving light use efficiency and making the light guide member compact and lightweight. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0048]      FIG. 1  is a side view schematically showing a configuration of a liquid crystal display device according to a first embodiment of the present invention; 
           [0049]      FIG. 2  is a top view showing a configuration of a backlight device according to the first embodiment of the present invention shown in  FIG. 1 ; 
           [0050]      FIG. 3  is a top view of a light guide plate of the backlight device according to the first embodiment of the present invention shown in  FIG. 1 , showing a light emitting region that emits light by being irradiated with light from an LED; 
           [0051]      FIG. 4  is a top view of the light guide plate of the backlight device according to the first embodiment of the present invention shown in  FIG. 1 , showing a region that is not irradiated with light from either of two adjacent LEDs; 
           [0052]      FIG. 5  is a top view of the light guide plate of the backlight device according to the first embodiment of the present invention shown in  FIG. 1 , showing light emitting regions which emit light by being irradiated with light from four LEDs; 
           [0053]      FIG. 6  is a top view of the light guide plate of the backlight device according to the first embodiment of the present invention shown in  FIG. 1 , showing an effective light emitting region of the light guide plate; 
           [0054]      FIG. 7  is a top view showing a configuration of a backlight device according to a second embodiment of the present invention; 
           [0055]      FIG. 8  is a top view of a light guide plate of the backlight device according to the second embodiment of the present invention shown in  FIG. 7 , showing a light emitting region that emits light by being irradiated with light from an LED; 
           [0056]      FIG. 9  is a top view of the light guide plate of the backlight device according to the second embodiment of the present invention shown in  FIG. 7 , showing a region that is not irradiated with light from either of two adjacent LEDs; 
           [0057]      FIG. 10  is a top view of the light guide plate of the backlight device according to the second embodiment of the present invention shown in  FIG. 7 , showing light emitting regions which emit light by being irradiated with light from four LEDs; 
           [0058]      FIG. 11  is a top view of the light guide plate of the backlight device according to the second embodiment of the present invention shown in  FIG. 7 , showing an effective light emitting region of the light guide plate; 
           [0059]      FIG. 12  is a top view showing a configuration of a backlight device according to a third embodiment of the present invention; 
           [0060]      FIG. 13  is a top view of a light guide plate of the backlight device according to the third embodiment of the present invention shown in  FIG. 12 , showing an effective light emitting region of the light guide plate; 
           [0061]      FIG. 14  is a top view of the light guide plate of the backlight device according to the third embodiment of the present invention shown in  FIG. 12 , showing a light emitting region of the light guide plate that emits light by being irradiated with light from an LED, on the assumption that the light guide plate is not formed in a depressed shape; 
           [0062]      FIG. 15  is a top view of the light guide plate of the backlight device according to the third embodiment of the present invention shown in  FIG. 12 , showing a light emitting region that emits light by being irradiated with light from an LED; 
           [0063]      FIG. 16  is a top view of the light guide plate of the backlight device according to the third embodiment of the present invention shown in  FIG. 12 , showing light emitting regions which emit light by being irradiated with light from two LEDs; 
           [0064]      FIG. 17  is a top view showing a configuration of a backlight device according to a fourth embodiment of the present invention; 
           [0065]      FIG. 18  is an enlarged top view of a light guide plate of the backlight device according to the fourth embodiment of the present invention shown in  FIG. 17  for illustrating a prism of the light guide plate; 
           [0066]      FIG. 19  is a top view showing a configuration of a light guide plate of a backlight device according to a first modified example of the present invention; 
           [0067]      FIG. 20  is an enlarged top view showing a configuration of a light guide plate of a backlight device according to a second modified example of the present invention; 
           [0068]      FIG. 21  is a top view showing a configuration of a backlight device according to a conventional example where LEDs are disposed at the corners of a light guide plate; 
           [0069]      FIG. 22  is a top view showing a configuration of a backlight device that is capable of reducing reflection of LED light on surfaces of a light guide plate, an aspect ratio (length of a long side:length of a short side) of the light guide plate being approximately 1:1; 
           [0070]      FIG. 23  is a top view of the light guide plate of the backlight device shown in  FIG. 22 , showing a light emitting region that emits light by being irradiated with light from an LED; 
           [0071]      FIG. 24  is a top view of the light guide plate of the backlight device shown in  FIG. 22 , showing light emitting regions which emit light by being irradiated with light from four LEDs; 
           [0072]      FIG. 25  is a top view of the light guide plate of the backlight device shown in  FIG. 22 , showing an effective light emitting region of the light guide plate; 
           [0073]      FIG. 26  is a top view of the light guide plate of the backlight device shown in  FIG. 22 , showing a region other than the effective light emitting region of the light guide plate; 
           [0074]      FIG. 27  is a top view showing a configuration of a backlight device that is capable of reducing reflection of LED light on surfaces of a light guide plate, an aspect ratio (length of a long side:length of a short side) of the light guide plate being approximately 4:3; 
           [0075]      FIG. 28  is a top view of the light guide plate of the backlight device shown in  FIG. 27 , showing a light emitting region that emits light by being irradiated with light from an LED; 
           [0076]      FIG. 29  is a top view of the light guide plate of the backlight device shown in  FIG. 27 , showing an effective light emitting region of the light guide plate; 
           [0077]      FIG. 30  is a view of the light guide plate of the backlight device shown in  FIG. 27 , showing a region other than the effective light emitting region of the light guide plate; 
           [0078]      FIG. 31  is a top view of the light guide plate of the backlight device shown in  FIG. 27 , showing a region that is not irradiated with light from either of two adjacent LEDs; 
           [0079]      FIG. 32  is a top view showing a configuration of a backlight device that is capable of reducing reflection of LED light on surfaces of a light guide plate, the backlight device having LEDs disposed at two adjacent corners of the light guide plate; 
           [0080]      FIG. 33  is a top view of the light guide plate of the backlight device shown in  FIG. 32 , showing a light emitting region that emits light by being irradiated with light from an LED; 
           [0081]      FIG. 34  is a top view of the light guide plate of the backlight device shown in  FIG. 32 , showing a non-light-emitting region of the light guide plate; 
           [0082]      FIG. 35  is a top view of the light guide plate of the backlight device shown in  FIG. 32 , showing an effective light emitting region of the light guide plate; and 
           [0083]      FIG. 36  is a top view of the light guide plate of the backlight device shown in  FIG. 32 , showing a region other than the effective light emitting region of the light guide plate. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0084]    Embodiments of the present invention will be described below with reference to the accompanying drawings. In some of the top views, some portions are indicated by hatching for ease of understanding. 
       First Embodiment 
       [0085]    With reference to  FIGS. 1 to 6 , a description will be given of a liquid crystal display device  1  provided with a backlight device  10  according to a first embodiment of the present invention. 
         [0086]    The liquid crystal display device  1  according to the first embodiment of the present invention is applicable to portable devices such as mobile phones, for example, and as shown in  FIG. 1 , the liquid crystal display device  1  is provided with a liquid crystal display panel  2  and a backlight device  10  which is disposed on a rear surface side of the liquid crystal display panel  2 . With respect to the first embodiment, descriptions will deal with a case where the aspect ratio (length of a long side:length of a short side) of a display region (not shown) of the liquid crystal display panel  2  is approximately 1:1. The liquid crystal display device  1  is an example of the “display device” of the present invention, and the liquid crystal display panel  2  is an example of the “display panel” of the present invention. 
         [0087]    The liquid crystal display panel  2  is formed of two glass substrates which hold a liquid crystal layer (not shown) in between. The liquid crystal display panel  2  functions as a display panel by being illuminated by the backlight device  10 . 
         [0088]    As shown in  FIG. 2 , a backlight device  10  is provided with a light guide plate  11 , which is substantially square shaped, and four LEDs  12  which are disposed at four corners of the light guide plate  11  to face the corners. The backlight device  10  may be provided with components such as an optical sheet, a reflection sheet, and a circuit board, which are not illustrated. The light guide plate  11  is an example of the “light guide member” of the present invention, and the LEDs  12  are an example of the “point light source” of the present invention. 
         [0089]    In the backlight device  10 , the LEDs  12  are each disposed such that a center of the light guide plate  11  (a center O 1  of a later-described effective light emitting region S 1  (see  FIG. 6 ) of the light guide plate  11 ) is located on optical axes (center lines) L 1  of the LEDs  12 . 
         [0090]    The light guide plate  11  is formed of, for example, an acrylic resin having a refraction index of approximately 1.49. The light guide plate  11  includes four side surfaces  11   a,  and four light incident surfaces  11   b  formed in four corners thereof (where adjacent ones of the side surfaces  11   a  are connected to each other). The side surfaces  11   a  are an example of a “side surface located between adjacent light incident surfaces,” a “first side surface,” and a “second side surface.” 
         [0091]    The light incident surfaces  11   b  are formed to be inclined with respect to the side surfaces  11   a  and parallel to light emitting surfaces  12   a  of (that is, perpendicular to optical axes L 1  of) the LEDs  12 . And, the light emitting surfaces  12   a  are disposed to face the light incident surfaces  11   b.    
         [0092]    The side surfaces  11   a  are located between two adjacent ones of the light incident surfaces  11   b.  The four side surfaces  11   a  include a pair of side surfaces  11   a  that are disposed to face each other in an A-direction and another pair of side surfaces  11   a  that are disposed to face each other in a B-direction which is perpendicular to (which crosses) the A-direction. Note that, in the first embodiment, the pair of side surfaces  11   a  which are disposed to face each other in the A-direction and the pair of side surfaces  11   a  which are disposed to face each other in the B-direction are all formed to have a same length and a same shape. 
         [0093]    Here, in the first embodiment, the side surfaces  11   a  of the light guide plate  11  are each formed in a depressed shape that is depressed toward a center of the light guide plate  11  (a center O 1  of a later-described effective light emitting region of the light guide plate  11 ). The side surfaces  11   a  are formed of a plurality of (two) flat surface portions  11   c.    
         [0094]    The light guide plate  11  can be formed by, for example, injection molding which is performed by using an injection mold, and the depressed shape (the flat surface portions  11   c ) of the side surfaces  11   a  can also be formed simultaneously in the injection molding. Incidentally, the depressed shape (the flat surface portions  11   c ) of the side surfaces  11   a  may be formed by cutting. 
         [0095]    Furthermore, in the first embodiment, the flat surface portions  11   c  are formed such that an angle θ 1  formed by one of the flat surface portions  11   c  (the side surfaces  11   a ) and an optical axis L 1  of an adjacent one of the LEDs  12  is approximately 42°. 
         [0096]    Here, a spread angle of light from the LEDs  12  at which the light spreads on entering the light guide plate  11  is smaller than 90°. Specifically, since the refraction index of the light guide plate  11  (made of an acrylic resin) is approximately 1.49, even if a spread angle of light at the time of being emitted from the LEDs  12  is set to, for example, 178°, a spread angle of the light at which the light spreads on entering the light guide plate  11  is approximately 84° according to Snell&#39;s law. 
         [0097]    That is, in the first embodiment, an angle θ 2  (=θ 1 ×2) formed by any two of the flat surface portions  11   c  (the side surfaces  11   a ) that are connected to a same one of the light incident surfaces  11   b  is substantially as large as the spread angle (approximately 84°) at which light from the LEDs  12  spreads on entering the light guide plate  11 . Note that, in the light guide plate  11 , a light emitting region R 1  which emits light by being irradiated with light from one of the LEDs  12  (the one at the top left in  FIG. 3 ) is as shown as a hatching region in  FIG. 3 . 
         [0098]    Furthermore, as shown in  FIG. 3 , the flat surface portions  11   c  (the side surfaces  11   a ) are each formed to coincide with part of a boundary surface of a light emitting region R 1  that emits light by being irradiated with light from an adjacent one of the LEDs  12 . In other words, the side surfaces  11   a  are each formed in a depressed shape so as to coincide with a boundary surface of a region R 2  (a hatching region in  FIG. 4 ) which is a region that is not irradiated with light from either of two adjacent ones of the LEDs  12  (a region through which light from the LEDs  12  does not pass). The hatching regions (regions R 2 ) in  FIG. 4  are regions that would not be irradiated with light from either of two adjacent ones of the LEDs  12  even if the side surfaces  11   a  were not formed in a depressed shape, the regions R 2  not contributing to the illumination of the liquid crystal display panel  2 . 
         [0099]    Moreover, in the first embodiment, since the LEDs  12  are disposed at the four corners of the light guide plate  11  as shown in  FIG. 5 , the entire light guide plate  11  is a light emitting region. Incidentally, in  FIG. 5 , the boundary surfaces of regions that emit light by being irradiated with light from the LEDs  12  are indicated by lines of as many different types to help distinguish which of the light emitting regions corresponds to which of the LEDs  12 . 
         [0100]    In addition, in the first embodiment, since the effective light emitting region S 1  of the light guide plate  11  corresponding to the display region (not shown) of the liquid crystal display panel  2  is a square-shaped region (a hatching region enclosed in a bold line flame in  FIG. 6 ) that is defined by lines connecting centers of the light incident surfaces  11   b  of the light guide plate  11 , and in this case, the effective light emitting region S 1  is maximum. Besides, an aspect ratio (length of a long side:length of a short side) of the effective light emitting region S 1  of the light guide plate  11  is approximately 1:1. 
         [0101]    In the first embodiment, as described above, the light guide plate  11  is provided with a plurality of light incident surfaces  11   b,  and the LEDs  12  are disposed to face the light incident surfaces  11   b.  With this configuration, it is possible to reduce reflection of light emitted from the LEDs  12  on surfaces (the light incident surfaces  11   b ) of the light guide plate  11 , and this makes it possible to improve light use efficiency. 
         [0102]    Furthermore, in the first embodiment, it is possible to make the light guide plate  11  compact (resource saving) and lightweight by forming the side surfaces  11   a,  which are located between two adjacent ones of the light incident surfaces  11   b,  in a depressed shape as described above. 
         [0103]    Note that, in a case in which the light incident surfaces  11   b  are formed in the corners of the light guide plate  11  without forming the side surfaces  11   a  of the light guide plate  11  in a depressed shape, the regions R 2  (which do not contribute to the illumination of the liquid crystal display panel  2 ), which are regions that are not irradiated with light from either of two adjacent ones of the LEDs  12 , appear in the light guide plate  11 . Furthermore, when the effective light emitting region S 1  is maximum, the regions R 2  (which do not contribute to the illumination of the liquid crystal display panel  2 ), which are regions that are not irradiated with light from either of two adjacent ones of the LEDs  12 , appear around the side surfaces  11   a  which are located between two adjacent ones of the light incident surfaces  11   b.  Thus, even if the side surfaces  11   a  of the light guide plate  11  are formed in depressed shapes by omitting the regions which are not irradiated with light from either of two adjacent ones of the LEDs  12 , the effective light emitting region S 1  of the light guide plate  11  is not reduced in size. That is, the effective light emitting region S 1  of the same size can be obtained without increasing the size of the light guide plate  11 . 
         [0104]    Furthermore, in the first embodiment, since the side surfaces  11   a  of the light guide plate  11  are formed in a depressed shape, it is possible to reduce emission of light from regions other than the effective light emitting region S 1 . This makes it possible to further improve light use efficiency. 
         [0105]    Moreover, in the first embodiment, by forming all of the side surfaces  11   a  in a depressed shape as described above, it is possible to make the light guide plate  11  more compact and lightweight while maximizing the effective light emitting region S 1 . 
         [0106]    Besides, in the first embodiment, as described above, the angle θ 2  formed by any two of the side surfaces  11   a  that are connected to a same one of the light incident surfaces  11   b  is substantially as large as the spread angle (approximately 84°) at which light from the LEDs  12  spreads on entering the light guide plate  11 . With this configuration, it is possible to reduce appearance of the useless regions R 2  (which do not contribute to illumination of the display panel  2 ) through which light emitted from the LEDs  12  does not pass, and this makes it easy to make the light guide plate  11  compact and lightweight. 
         [0107]    In addition, in the first embodiment, as described above, the light incident surfaces  11   b  are formed as flat surfaces, and this makes it easy to reduce reflection of light emitted from the LEDs  12  on the surfaces (the light incident surfaces  11   b ) of the light guide plate  11 . Thus, it is possible to easily improve light use efficiency. 
         [0108]    Also, in the first embodiment, the side surfaces  11   a  are formed of two flat surface portions  11   c  as described above, and this makes it easy to form the side surfaces  11   a  in a depressed shape. 
       Second Embodiment 
       [0109]    In the second embodiment, with reference to  FIGS. 7 to 11 , a description will be given of a case in which a light guide plate  111  is formed in a substantially rectangular shape in contrast to the first embodiment. 
         [0110]    As shown in  FIG. 7 , a backlight device  110  according to the second embodiment is provided with the light guide plate  111 , which is substantially rectangular, and four LEDs  12  which are disposed at four corners of the light guide plate  111  to face the corners. Here, in the second embodiment, the description will deal with a case where the aspect ratio (length of a long side:length of a short side) of a display region (not shown) of a display panel is 4:3. The light guide plate  111  is an example of the “light guide member” of the present invention. 
         [0111]    In the second embodiment, the LEDs  12  are each disposed such that a center of the light guide plate  111  (a center O 101  of a later-described effective light emitting region S 101  (see  FIG. 11 ) of the light guide plate  111 ) is not located on optical axes L 1  of the LEDs  12 . In other words, the center O 101  of the later-described effective light emitting region S 101  is located a predetermined distance away from the optical axes L 1  of the LEDs  12 . 
         [0112]    The light guide plate  111  includes a pair of side surfaces  111   a  that are located to face each other in an A-direction (a long-side direction), a pair of side surfaces  111   b  that are located to face each other in a B-direction (a short-side direction) which is perpendicular to the A-direction, and light incident surfaces  111   c  formed in four corners (at each of which a side surface  111   a  and a side surface  111   b  which are adjacent to each other are connected to each other). The side surfaces  111   a  are an example of the “side surface located between adjacent light incident surfaces” and the “first side surface.” Also, the side surfaces  111   b  are an example of the “second side surface” of the present invention. 
         [0113]    The length of the side surfaces  111   a  is shorter than that of the side surfaces  111   b.  The side surfaces  111   a  form a short-side portion of the light guide plate  111  while the side surfaces  111   b  form a long-side portion of the light guide plate  111 . 
         [0114]    Furthermore, in the second embodiment, both of the pair of side surfaces  111   a  of the light guide plate  111  are formed in a depressed shape which is depressed toward the center of the light guide plate  111  (the center O 101  of the later-described effective light emitting region S 101  of the light guide plate  111 ). The side surfaces  111   a  are formed of a plurality of (two) flat surface portions  111   d.    
         [0115]    On the other hand, in the second embodiment, the pair of side surfaces  111   b  of the light guide plate  111  are not formed in a depressed shape. 
         [0116]    Moreover, the flat surface portions  111   d  (the side surfaces  111   a ) are formed such that an angle θ 101  formed by one of the flat surface portions  111   d  and the optical axis L 1  of an adjacent one of the LEDs  12  is approximately 42°. Likewise, the side surfaces  111   b  are formed such that an angle θ 102  that is formed by each of the side surfaces  111   b  and the optical axis L 1  of an adjacent one of the LEDs  12  is approximately 42°. 
         [0117]    Thus, like in the first embodiment, an angle θ 103  (=θ 101 +θ 102 ) formed by a flat surface portion  111   d  (a side surface  111   a ) and a side surface  111   b  which are connected to a same one of the light incident surfaces  111   c  is substantially as large as a spread angle (approximately 84°) at which light emitted from the LEDs  12  spreads on entering the light guide plate  111 . Note that, in the light guide plate  111 , a light emitting region R 101  which emits light by being irradiated with light from one of the LEDs  12  (the one at the top left in  FIG. 8 ) is as shown as a hatching region in  FIG. 8 . 
         [0118]    Besides, as shown in  FIG. 8 , the flat surface portions  111   d  (the side surfaces  111   a ) are each formed to coincide with part of a boundary surface of a light emitting region R 101  that emits light by being irradiated with light from an adjacent one of the LEDs  12 . In other words, the side surfaces  111   a  are each formed in a depressed shape so as to coincide with a boundary surface of a region R 102  (a hatching region in  FIG. 9 ) which is not irradiated with light from either of two adjacent ones of the LEDs  12 . Note that the hatching regions (regions R 102 ) in  FIG. 9  are regions that are not irradiated with light from either of two adjacent ones of the LEDs  12  even if the side surfaces  111   a  are not formed in a depressed shape, and the regions R 102  do not contribute to illumination of a liquid crystal display panel. 
         [0119]    The side surfaces  111   b  are each formed to coincide with a boundary surface of one of the light emitting regions R 101  that emits light by being irradiated with light from an adjacent one of the LEDs  12 . 
         [0120]    Furthermore, in the second embodiment, since the LEDs  12  are disposed at the four corners of the light guide plate  111  as shown in  FIG. 10 , the entire light guide plate  111  is a light emitting region. Incidentally, in  FIG. 10 , the boundary surfaces of the light emitting regions that emit light by being irradiated with light from the LEDs  12  are indicated by lines of as many different types to help distinguish which of the light emitting regions corresponds to which of the LEDs  12 . 
         [0121]    Moreover, in the second embodiment, as shown in  FIG. 11 , since the effective light emitting region S 101  of the light guide plate  111  corresponding to a display region (not shown) of a liquid crystal display panel is a rectangular region (a hatching region enclosed in a bold line flame in  FIG. 11 ) that does not include the light incident surfaces  111   c,  and in this case, the effective light emitting region S 101  is maximum. An aspect ratio (length of a long side:length of a short side) of the effective light emitting region S 101  of the light guide plate  111  is approximately 4:3. 
         [0122]    The other features of the second embodiment are similar to those of the first embodiment. 
         [0123]    In the second embodiment, as described above, a length of the side surfaces  111   a  is formed to be shorter than a length of the side surfaces  111   b  (that is, the light guide plate  111  is formed in a substantially rectangular shape), and the side surfaces  111   a  are formed in a depressed shape. Here, in order to maximize the effective light emitting region S 101  in the case where the light guide member  111  is formed in a substantially rectangular shape, it is necessary to form the side surfaces  111   a  along the short sides of the light guide member  111  in a depressed shape, without forming the side surfaces  111   b  along the long sides of the light guide member  111  in a depressed shape. Thus, by forming the side surfaces  111   a  in a depressed shape without forming the side surfaces  111   b  in a depressed shape as described above, it is possible to maximize the effective light emitting region S 101 . That is, the effective light emitting region S 101  of the same size can be obtained with a smaller light guide plate  111 . As a result, it is possible to make the light guide plate  111  more compact and lightweight. 
         [0124]    Besides, in the second embodiment, as described above, the center O 101  of the effective light emitting region S 101  of the light guide plate  111  is located a predetermined distance away from the optical axes L 1  of the LEDs  12 . With this configuration, in contrast to a case in which the center O 101  of the effective light emitting region S 101  of the light guide plate  111  is located on the optical axes L 1  of the LEDs  12 , it is possible to reduce overlap of the regions (the regions R 102 ) which are not irradiated with light from either of two adjacent ones of the LEDs  12  and the effective light emitting region S 101 , and this makes it possible to reduce occurrence of uneven brightness in the effective light emitting region S 101 . Also, since there is no need of making the light guide plate  111  larger in order to prevent the regions (the regions R 102 ) which are not irradiated with light from either of two adjacent ones of the LEDs  12  and the effective light emitting region from overlapping each other, it is possible to reduce increase in size and weight of the light guide plate  111 . 
         [0125]    The other advantages of the second embodiment are similar to the advantages of the first embodiment. 
       Third Embodiment 
       [0126]    In a third embodiment, with reference to  FIGS. 12 to 16 , a description will be given of a case in which LEDs  12  are disposed at two corners of a light guide plate  211 , in contrast to the first and second embodiments described above. 
         [0127]    As shown in  FIG. 12 , a backlight device  210  according to the third embodiment of the present invention is provided with the light guide plate  211 , which is substantially rectangular, and two LEDs  12  which are disposed at two corners of the light guide plate  211  to face the corners. Here, in the third embodiment, the description will deal with a case where the aspect ratio (length of a long side:length of a short side) of a display region (not shown) of a display panel is 16:9. The light guide plate  211  is an example of the “light guide member” of the present invention. 
         [0128]    In the third embodiment, the LEDs  12  are each disposed such that a center O 201  of a later-described effective light emitting region S 201  (see  FIG. 13 ) of the light guide plate  211  is not located on optical axes L 1  of the LEDs  12 . That is, the center O 201  of the later-described effective light emitting region S 201  of the light guide plate  211  is located a predetermined distance away from the optical axes L 1  of the LEDs  12 . 
         [0129]    The light guide plate  211  includes a pair of side surfaces  211   a  and  211   b  that are located to face each other in an A-direction (a long-side direction), a pair of side surfaces  211   c  that are located to face each other in a B-direction (a short-side direction) which is perpendicular to the A-direction, and light incident surfaces  211   d  formed in two adjacent corners of the light guide plate  211 . The side surface  211   a  is an example of the “side surface located between adjacent light incident surfaces” and the “first side surface” of the present invention. The side surface  211   b  is an example of the “first side surface,” and the side surfaces  211   c  are an example of the “second side surface” of the present invention. 
         [0130]    A length of the side surface  211   a  and a length of the side surface  211   b  are shorter than a length of the side surfaces  211   c.  And, the side surfaces  211   a  and  211   b  form a short-side portion of the light guide member  211  while the two side surfaces  211   c  form a long-side portion of the light guide plate  211 . 
         [0131]    Here, in the third embodiment, among the side surfaces  211   a  to  211   c,  only the side surface  211   a,  which is located between the two LEDs  12 , is formed in a depressed shape that is depressed toward a center of the light guide plate  211  (the center O 201  of the later-described effective light emitting region S 201  of the light guide plate  211 ). Furthermore, the side surface  211   a  is formed of a plurality of (two) flat surface portions  211   e.    
         [0132]    On the other hand, in the third embodiment, the side surface  211   b  and the pair of side surfaces  211   c  of the light guide plate  211  are not formed in a depressed shape. 
         [0133]    Moreover, in the third embodiment, as shown in  FIG. 13 , the effective light emitting region S 201  of the light guide plate  211  corresponding to a display region (not shown) of a liquid crystal display panel is a rectangular region (a hatching region enclosed in a bold line frame in  FIG. 13 ) that does not include the light incident surfaces  211   d,  and in this case, the effective light emitting region S 201  is maximum. Besides, an aspect ratio (length of a long side:length of a short side) of the effective light emitting region S 201  of the light guide plate  211  is approximately 16:9. 
         [0134]    In addition, in the third embodiment, a portion  211   f  where the two flat surface portions  211   e  of the side surface  211   a  cross (are connected to) each other is located in the vicinity of the effective light emitting region S 201  or on a boundary surface of the effective light emitting region S 201 . That is, no part of the side surface  211   a  formed in a depressed shape exists inside the effective light emitting region S 201 . 
         [0135]    Here, in the third embodiment, as shown in  FIG. 12 , the LEDs  12  are disposed such that an angle θ 201  formed between an optical axis L 1  of one of the LEDs  12  and an adjacent one of the side surface  211   c  is approximately 48°. That is, the LEDs  12  are disposed such that, as shown in  FIG. 14 , an angle θ 202  that is formed by an optical axis L 1  of one of the LEDs  12  and a side surface F 211   a,  which is a substitute of the side surface  211   a  in a case in which the side surface  211   a  is assumed not to be formed in the depressed shape, is approximately 42°. 
         [0136]    Thus, part of light that is emitted from the LEDs  12  to enter the light guide plate  211  travels parallel to the B-direction, and thus, in a case in which it is assumed that the side surface  211   a  is not formed in a depressed shape, a light emitting region R 201  that emits light by being irradiated with light from one of the LEDs  12  (the one at the top left in  FIG. 14 ) is as shown as a hatching region in  FIG. 14 . 
         [0137]    In the third embodiment, since the side surface  211   a  is formed in the depressed shape, a light emitting region R 202  that emits light by being irradiated with light from one of the LEDs  12  (the one at the top left in  FIG. 15 ) is as shown as a hatching region in  FIG. 15 . 
         [0138]    The side surface  211   a  (the flat surface portions  211   e ) has a function of reflecting light that travels toward the side surface  211   a  after entering the light guide plate  211  to the effective light emitting region S 201  side. 
         [0139]    Furthermore, in the third embodiment, since the LEDs  12  are disposed at two corners of the light guide plate  211  as shown in  FIG. 16 , the entire light guide plate  211  is a light emitting region. Incidentally, in  FIG. 16 , boundary surfaces of regions that emit light by being irradiated with light from the LEDs  12  are indicated by lines of as many different types to help distinguish which of the light emitting regions corresponds to which of the LEDs  12 . 
         [0140]    In the other respects, the configuration of the third embodiment is similar to those of the above-described first and second embodiments. 
         [0141]    In the third embodiment, as described above, the LEDs  12  are disposed at two adjacent corners of the light guide plate  211 , and only the side surface  211   a  is formed in a depressed shape. With this configuration, in a case in which the LEDs  12  are disposed at two adjacent corners of the light guide member, it is possible to both maximize the effective light emitting region S 201  and make the light guide plate  211  compact and lightweight. In addition, since the number of LEDs  12  can be reduced in comparison with the case in which the LEDs  12  are disposed at the four corners of the light guide member, it is possible to make the backlight device  210  still more compact and lightweight. 
         [0142]    Moreover, in the third embodiment, the side surface  211   a  is formed to reflect light that travels thereto after entering the light guide plate  211  to the effective light emitting region S 201  side. This makes it possible to reduce loss of light that enters the light guide plate  211 . That is, it is possible to reduce degradation of light use efficiency. 
         [0143]    The other advantages of the third embodiment are similar to those of the above-described first and second embodiments. 
       Fourth Embodiment 
       [0144]    In a fourth embodiment, with reference to  FIGS. 17 and 18 , a description will be given of a case in which, in contrast to the third embodiment, a prism  311   g  is formed on a side surface  311   a  of a light guide plate  311 . 
         [0145]    As shown in  FIG. 17 , a backlight device  310  according to the fourth embodiment of the present invention is provided with the light guide plate  311 , which is formed in a substantially rectangular shape, and two LEDs  12  which are disposed at two corners of the light guide plate  311  to face the corners. The light guide plate  311  is an example of the “light guide member” of the present invention. 
         [0146]    The light guide plate  311  includes a pair of side surfaces  311   a  and  211   b  that are located to face each other in an A-direction (a long-side direction), a pair of side surfaces  211   c  that are located to face each other in a B-direction (a short-side direction) which is perpendicular to the A-direction, and light incident surfaces  211   d  formed in two adjacent corners of the light guide plate  311 . The side surface  311   a  is an example of the “side surface located between adjacent ones of the light incident surfaces” and the “first side surface.” 
         [0147]    Furthermore, the side surface  311   a  is formed of a plurality of (two) flat surface portions  311   e.    
         [0148]    Here, in the fourth embodiment, a plurality of prisms  311   g  are formed on the flat surface portions  311   e  (the side surface  311   a ). The prisms  311   g  include a reflection surface  311   h  (see  FIG. 18 ) which reflects light traveling toward the side surface  311   a  to an inside of the light guide plate  311  (the side surface  211   b  side). The reflection surface  311   h  is formed, as shown in  FIG. 18 , such that an angle θ 301  formed by the reflection surface  311   h  and the side surface  211   c  is approximately 45°. 
         [0149]    The prisms  311   g  can be formed simultaneously when the light guide plate  311  is formed by injection molding. Incidentally, the prisms  311   g  may be formed by cutting. 
         [0150]    In the other respects, the configuration of the fourth embodiment is similar to that of the above-described third embodiment. 
         [0151]    In the fourth embodiment, the prisms  311   g  are made, as described above, on the side surface  311   a  (the flat surface portions  311   e ). With this configuration, it is possible to reflect light travelling toward the side surface  311   a  to an inside of the light guide plate  311  (the side surface  211   b  side), and this makes it possible to achieve uniform emission of light from the light guide plate  311  and to achieve more uniform brightness of a liquid crystal display panel as well. 
         [0152]    The other advantages of the fourth embodiment are similar to those of the first to third embodiments. 
         [0153]    The embodiments disclosed herein are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is set out in the appended claims and not in the description of the embodiments hereinabove, and includes any variations and modifications within the sense and scope equivalent to those of the claims. 
         [0154]    For example, the above-described embodiments have dealt with cases where a display panel and a display device are applied to a liquid crystal display panel and a liquid crystal display device, respectively, but this is not meant to limit the present invention, and the display panel and the display device may be applied to those of any other type. 
         [0155]    Furthermore, in the above-described embodiments deal with examples where liquid crystal display devices are used in portable devices such as mobile phones, but this is not meant to limit the present invention, and liquid crystal display devices may be used in portable devices other than mobile phones, and electronic devices other than portable devices. Note that, however, portable devices are particularly required to be compact, lightweight, and power saving (improved light use efficiency), and thus, it is particularly effective to use the liquid crystal display device of the present invention in portable devices. 
         [0156]    Moreover, although the above-described embodiments deal with examples where LEDs are used as point light sources, this is not meant to limit the present invention, and point light sources other than LEDs may be used. 
         [0157]    Besides, although the above-described embodiments deal with examples where a side surface that is formed in a depressed shape is formed of two flat surface portions, but this is not meant to limit the present invention, and a side surface that is formed in a depressed shape may be formed of three or more flat surface portions. In addition, like, for example, a light guide plate  411  of a backlight device according to a first modified example of the present invention shown in  FIG. 19 , a side surface  411   a  which is formed in a depressed shape, may be formed of one or more curved surface portions  411   b.    
         [0158]    Furthermore, although the above-described embodiments deal with examples where a light incident surface is formed in a flat surface shape, but this is not meant to limit the present invention, and a light incident surface may be formed of a plurality of flat surface portions. Moreover, like, for example, a light guide plate  421  of a backlight device according to a second modified example of the present invention shown in  FIG. 20 , a light incident surface  421   a  may be formed in a curved surface shape. 
         [0159]    Besides, although the above-described embodiments deal with examples where a light guide plate is formed of a acrylic resin, this is not meant to limit the present invention, and a light guide plate may be formed of, for example, a resin other than acrylic resins, such as a polycarbonate resin having a refraction index of approximately 1.59. A light guide plate may be formed of a material other than resins. In the case where a light guide plate is formed of a material other than acrylic resins, an angle formed by a side surface and an optical axis of an LED may be set to an angle that is suitable to the refraction index of the material. 
       LIST OF REFERENCE SYMBOLS 
       [0160]      1  liquid crystal display device (display device) 
         [0161]      2  liquid crystal display panel (display panel) 
         [0162]      10 ,  110 ,  210 ,  310  backlight device 
         [0163]      11 ,  111 ,  211 ,  311 ,  411 ,  421  light guide plate (light guide member) 
         [0164]      11   a  side surface (side surface located between adjacent light incident surfaces, first side surface, second side surface) 
         [0165]      11   b,    111   c,    211   d,    421   a  light incident surface 
         [0166]      11   c,    111   d,    211   e,    311   e  flat surface portion 
         [0167]      12  LED (point light source) 
         [0168]      111   a,    211   a,    311   a,    411   a  side surface (side surface located between adjacent light incident surfaces, first side surface) 
         [0169]      111   b,    211   c  side surface (second side surface) 
         [0170]      211   b  side surface (first side surface) 
         [0171]      311   g  prism 
         [0172]      411   b  curved surface portion 
         [0173]    L 1  optical axis 
         [0174]    O 101 , O 201  center 
         [0175]    S 101 , S 201  effective light emitting region 
         [0176]    θ 2 , θ 103  angle