Patent Publication Number: US-2007121341-A1

Title: Backlight device and liquid crystal display using the same

Description:
This patent document claims the benefit of Japanese Patent Application No. 2005-342015 filed on Nov. 28, 2005, which is hereby incorporated by reference.  
     BACKGROUND  
      1. Field  
      The present embodiments relate to a backlight device and a liquid crystal display using the same.  
      2. Related Art  
      In a related art, a liquid crystal display generally includes a backlight structure composed of a light guiding plate which introduces light of a light source from an end portion onto a main surface, a diffusing plate formed on the main surface of the light guiding plate, and a reflecting plate formed on the main surface opposite to the light guiding plate (for example, see JP-A-10-161119). The liquid crystal display has a structure preventing a bright line or a hot spot from occurring on a display surface in the vicinity of a light source, such as a cathode tube or an LED (Light Emitting Diode).  
      As shown in  FIG. 4 , a band-shaped light absorbing layer  106  is formed on a surface facing a light source of at least one of a light guiding plate  102  in the vicinity of a light source  101 , a diffusing plate  103 , a prism plate  104  or a reflecting plate  105  A band-shaped light reflecting layer  107  is formed on at least one light absorbing layer  106 .  
      The light reflecting layer  107  reflects light in the vicinity of the light source  101 , which enters the light guiding plate  102  and is output toward the light absorbing layer  106 , or toward the light guiding plate  102 , thus deterring loss due to absorbing of light on the light absorbing layer  106 . The light absorbing layer  106  absorbs light traveling from an end portion of the light guiding plate  102  to the liquid crystal display surface. The light reflecting layer  107  reflects the light.  
      When a liquid crystal display panel is relatively small in size, for example, a display panel in a cellular phone, it is impossible to sufficiently prevent occurrence of the bright line or the hot spot by the above-described method.  
     SUMMARY  
      The present embodiments may obviate one or more of drawbacks inherent in the related art. For example, in one embodiment, a backlight device can reliably prevent occurrence of a bright line and a hot spot even in a relatively small liquid crystal display panel.  
      In one embodiment, a backlight device includes a light guiding member which guides light from a light source, and an optical element disposed on the light guiding member. An end portion of the optical element facing the light source is farther from the light source than an end portion of the light guiding member facing the light source.  
      In one embodiment, the end portion of the optical element facing the light source is distanced from the end portion of the light guiding member facing the light source. In this embodiment, it is possible to block light traveling from the light guiding plate toward the liquid crystal display panel in a region where the optical element does not exist. In this exemplary embodiment, it is also possible to reliably prevent occurrence of a bright line and a hot spot in a liquid crystal display using a relatively small liquid crystal display panel.  
      In one embodiment, the optical element may be composed of a plurality of optical elements, and an end portion of an upper optical element facing the light source may be farther from the light source than an end portion of a lower optical element facing the light source.  
      In one embodiment, the plurality of optical elements include a diffusing plate formed on the light guiding member. An end portion of the diffusing plate facing the light source is disposed substantially at the same position as the end portion of the light guiding plate facing the light source.  
      In one embodiment, the backlight device includes a frame body accommodating the light guiding member and the optical element. The optical element has a first positioning portion, and the frame body has a second positioning portion corresponding to the first positioning portion. In this embodiment, it is possible to easily mount the optical elements on the frame body.  
      In one embodiment, a liquid crystal display includes the above-described backlight device. The liquid crystal display panel is operatively disposed on the backlight device and introduces light from the backlight device and outputs the light to the outside. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a cross-sectional view showing one embodiment of a backlight structure of a liquid crystal display;  
       FIGS. 2A  to  2 F are views showing exemplary embodiment of optical elements mounted on a frame material of a liquid crystal display;  
       FIG. 3A  is a view showing a display state of the liquid crystal display using a backlight device according to one embodiment;  
       FIG. 3B  is a view showing a display state of the liquid crystal display using a backlight device according to the related art; and  
       FIG. 4  is a cross-sectional view showing the backlight structure of the liquid crystal display according to the related art. 
    
    
     DETAILED DESCRIPTION  
       FIG. 1  is a cross-sectional view showing a liquid crystal display having a backlight device according to one embodiment. In one embodiment, as shown in  FIG. 1 , a backlight structure of the liquid crystal display includes a light source LED  11 . A light guiding plate  12  is disposed adjacent to and in the vicinity of the LED  11 . The light guiding plate  12  has a flat plate shape, and a side surface  12   a  thereof is made as a light incident surface and disposed to face the LED  11 . On one main surface  12   b  of the light guiding plate  12 , a diffusing plate  13  is disposed to diffuse light output from the light guiding plate  12 . On the diffusing plate  13 , a pair of prism plates  14  and  15 , which diffuse light in different directions, is disposed. On the other main surface  12   c  of the light guiding plate  12 , the reflecting plate  16  is disposed to reflect light output from the light guiding plate  12  back to the light guiding plate  12 . A liquid crystal display panel  17  is disposed in the backlight device having the above-described construction. As shown in  FIG. 1 , the liquid crystal display panel  17  is disposed on the pair of prism plates  14  and  15 .  
      In one embodiment, end portions  13   a  to  15   a  of the optical elements (here, the diffusing plate  13  and the prism plate  14 ), facing the LED  14 , disposed on the light guiding plate  12  are farther from the LED  11  than an end portion of the light guiding plate  12  facing the LED  11 .  
      In one embodiment, the end portion  13   a  of the diffusing plate  13  is disposed substantially at the same position as the end portion  12   a  of the light guiding plate  12 . Therefore, the end portion  14   a  of the prism plate  14 , for example, a lower layer (a layer that is closer to the light guiding plate) is d 1  farther (receded) from the LED  11  than the end portion of the light guiding plate  12  facing the LED  11 . The end portion  15   a  of the prism plate  15 , for example, an upper layer (a layer that is farther from the light guiding plate) is d 1 +d 2  farther from the LED  11  than the end portion of the light guiding plate  12  facing the LED  11 . For example, the prism plate  15  is d 2  farther from the LED  11  than the end portion of the lower prism plate  14  facing LED  11 .  
      In one embodiment, the optical elements laminated on the light guiding plate  12  constitute a structure in which the end portion facing the LED  11  goes farther from (recedes from) the LED  11 , as it goes to an upper layer. The d 1  and d 2  are preferably set in the range of about 0.5 mm to 2.0 mm, taking into consideration the occurrence of the bright line and the hot spot.  
      In one embodiment, the end portion facing the LED  11  of the optical elements disposed on the light guiding plate  12  is disposed so as to be farther from the LED  11  than the end portion (side surface  12   a ) facing the LED  11  of the light guiding plate  12 . In this embodiment, it is possible to block light traveling from the light guiding plate  12  toward the liquid crystal display panel  17  in a region where the optical element does not exist. Accordingly, it is possible to reliably prevent occurrence of the bright line and the hot spot in a liquid crystal display using a relatively small liquid crystal display panel.  
       FIGS. 2A  to  2 F are views showing a state in which optical elements, for example, a diffusing plate  13  or prism plates  14  and  15 , are mounted on a frame body  18  of a liquid crystal display.  
      With respect to the length (light in a vertical direction in the drawing) of the diffusing plate  13  shown in  FIG. 2A , the prism plates  14  and  15  have a smaller length. For example, a distance L 2  from the end portion  14   a  of the prism plate  14  (intermediate layer of the optical elements) disposed on the diffusing plate  13  on the light guiding plate  12  to a closest positioning protrusion  14   b  is smaller than a distance L 1  from the end portion  13   a  of the diffusing plate  13  (lower layer of the optical elements) disposed on the light guiding plate  12  to a closest positioning protrusion  13   b.    
      A distance L 3  from the end portion  15   a  of the prism plate  15  (upper layer of the optical elements) disposed on the prism plate  14  on the light guiding plate  12  to a closest positioning protrusion  15   b  is smaller than the distance L 2  from the end portion  14   a  of the prism plate  14  (intermediate layer of the optical elements) disposed on the diffusing plate  13  on the light guiding plate  12  to the closest positioning protrusion  14   b.  For example, the distance relationship may be indicated by L 1 &gt;L 2 &gt;L 3 . In the respective optical elements, distances between the protrusions are substantially equal.  
      In one embodiment, the liquid crystal display includes a frame body  18 . In the frame body  18 , positioning concave portions  18   a  are formed to correspond to the positioning protrusions  13   b,    14   b,  and  15   b,  respectively. In the case of mounting the diffusing plate  13  on the frame body  18 , as shown in  FIG. 2D , the protrusion  13   b  is inserted into the concave portion  18   a  of the frame body  18  while the end portion  13   a  faces the LED  11 .  
      In one embodiment, as shown in  FIG. 2E , the prism plate  14  is mounted on the frame body  18 . The protrusion  14   b  is inserted into the concave portion  18   a  of the frame body  18  while the end portion  14   a  faces the LED  11 .  
      In one embodiment, as shown in  FIG. 2F , the prism plate  15  is mounted on the frame body  18 . The protrusion  15   b  is inserted into the concave portion  18   a  of the frame body  18  while the end portion  15   a  faces the LED  11 . As positioning portions are formed in the optical elements and the frame body, respectively, the optical elements are mounted on the frame body using the positioning elements. In this embodiment, it becomes possible to easily mount the optical elements on the frame body  18  in an accurately positioned state.  
      In one embodiment, the distance from the end portion of the optical elements to the closest positioning protrusion differs according to the optical elements. In this embodiment, it is possible to prevent occurrence of an error of the optical element. In another embodiment, the direction of the optical elements can be recognized. In this embodiment, it is possible to prevent the optical elements from being mounted on the frame body  18  in a reversed direction.  
      In one embodiment, light from the LED  11  enters the side surface  12   a  of the light guiding plate  12  and propagates inside the light guiding plate  12 . In this embodiment, the light output from the main surface  12   c  of the light guiding plate  12  is reflected by the reflecting plate  16  and returns to the light guiding plate  12 . Light output from the main surface  12   b  of the light guiding plate  12  is diffused by the diffusing plate  13  serving as an optical element, polarized by the prism plates  14  and  15 , and sent to the liquid crystal display panel  17 .  
      The light sent to the liquid crystal display panel  17  is optically modulated and output to the outside. A user recognizes the light output to the outside.  
      In this embodiment, in the vicinity of the side surface  12   a  of the light guiding plate  12 , the end portion of the optical element facing the LED is disposed father from the LED  11  than the end portion of the light guiding plate  12  facing the LED, so that light traveling from the end portion of the light guiding plate  12  toward the liquid crystal display panel  17  in a region where the optical element does not exist. Accordingly, it is possible to reliably prevent a bright line and a hot spot from occurring in the liquid crystal display using a relatively small liquid crystal display panel.  
      In the liquid crystal display having the construction of  FIG. 1  when light of the backlight device is irradiated on the liquid crystal display panel the liquid crystal display produces results similar to those shown in  FIG. 3A .  
      In the liquid crystal display having the construction of the  FIG. 4 , when light of the backlight device is irradiated on the liquid crystal display panel the liquid crystal display produces results similar to those shown in  FIG. 3B . For example, the size of the liquid crystal display panel is about 1.3 inches.  
      As shown in  FIG. 3A , in the liquid crystal display using the backlight device, the hot spot is deterred in the vicinity of the LED, and a region with high luminance spreads over a wide area (portion A in the drawing).  
      As shown in  FIG. 3B , in the liquid crystal display using the backlight device according to the related art, the hot spot is partially found in the LED (portions B in the drawing).  
      On exemplary embodiment makes it possible to reliably prevent the bright line and the hot spot from occurring as well in the relatively small liquid crystal display panel.  
      The application of the invention is not limited to the above-described embodiment, and it can be modified in various forms. For example, according to one exemplary embodiment, the optical elements include the diffusing plate and the prism plate, but the invention is not limited to this exemplary embodiment. For example, an additional optical element may be included in the construction in which the end portion of the optical element facing the light source is farther from the light source than the end portion of the light guiding plate facing the light source. In addition, the number of layers of the optical elements are not limited.  
      According to one embodiment, two protrusions of the optical elements are formed on the side surfaces facing each other; however, the invention is not limited thereto, and the location and number of protrusions are not limited as long as the location and number of concave portions of the frame body correspond thereto.  
      According to another embodiment, the positioning portion of the optical element is the protrusion, and the positioning portion of the frame body is the concave portion. However, if the optical element is positioned with respect to the frame body, the positioning portion of the frame body is the protrusion, and the positioning portion of the optical element is the concave portion. The numerical value described in the embodiment is not limited to a specific value. In addition, it can be modified in various forms.  
      In one embodiment, the backlight device according to the invention can be used in electronic equipment having a relatively small liquid crystal display panel, for example, a mobile phone or a note book computer.  
      According to one embodiment, the backlight device includes a light guiding member which guides light from a light source, and an optical element disposed on the light guiding member. An end portion of the optical element facing the light source is farther from the light source than an end portion of the light guiding member facing the light source. Therefore, it is possible to reliably prevent the bright line and the hot spot from occurring in the liquid crystal display using the relatively small liquid crystal display panel.  
      Various embodiments described herein can be used alone or in combination with one another. For example, the invention can be one embodiment or a combination of embodiments. The forgoing detailed description has described only a few of the many possible implementations of the present invention. For this reason, this detailed description is intended by way of illustration, and not by way of limitation. It is only the following claims, including all equivalents that are intended to define the scope of this invention.