Abstract:
A linear light source includes a plurality of LED chips that are linearly arranged along a first direction, and a reflector including a first tapered sidewall sandwiching the LED chips in the first direction and a second tapered sidewall sandwiching the LED chips in a second direction intersecting with the first direction. The LED chips are surrounded by first tapered sidewall and second tapered sidewall, and the second tapered sidewall is lower than the first tapered sidewall.

Description:
The present application is based on Japanese patent application No. 2012-214139 filed on Sep. 27, 2012, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a linear light source and a planar light source. 
     2. Description of the Related Art 
     A conventional linear light source is known that is provided with LED chips linearly arranged in the longitudinal direction of the linear light source and a reflector for upwardly reflecting light horizontally emitted from the LED chips (see, e.g., JP-A-2004-235139 and JP-A-2010-15709). 
     The linear light source disclosed in JP-A-2004-235139 is constructed such that the reflector sandwiches each LED chip in the longitudinal direction of the linear light source and a reflection sheet is provided on sides along the longitudinal direction of the linear light source. Accordingly, of light horizontally emitted from the LED chip, light emitted in the longitudinal direction of the linear light source is reflected by the reflector and light emitted in the lateral direction of the linear light source is reflected by the reflection sheet. 
     The linear light source disclosed in JP-A-2010-15709 is constructed such that the reflector surrounds the LED chip. Accordingly, of the light horizontally emitted from the LED chip, both light emitted in the longitudinal direction of the linear light source and light emitted in the lateral direction of the linear light source are reflected by the reflector. 
     SUMMARY OF THE INVENTION 
     However, in the linear light source disclosed in JP-A-2004-235139, since the reflection sheet is disposed vertically to a surface to mount the LED chip, light horizontally emitted from the LED chip can be inputted back to the LED chip after it is reflected by the reflection sheet and fail to be extracted to the outside. 
     On the other hand, since the linear light source disclosed in JP-A-2010-15709 is provided with the reflector having a tapered portion to cause an increased thickness, it may prevent a decrease in thickness of the linear light source and a planar light source using thereof, etc. In general, the light reflectivity of the reflector is lower than that of the reflection sheet. 
     It is an object of the invention to provide a linear light source and a planar light source that are excellent in light extraction efficiency and suited to decrease in thickness. 
     (1) According to one embodiment of the invention, a linear light source comprises: 
     a plurality of LED chips that are linearly arranged along a first direction; and 
     a reflector comprising a first tapered sidewall sandwiching the LED chips in the first direction and a second tapered sidewall sandwiching the LED chips in a second direction intersecting with the first direction, 
     wherein the LED chips are surrounded by first tapered sidewall and second tapered sidewall, and 
     wherein the second tapered sidewall is lower than the first tapered sidewall. 
     In the above embodiment (1) of the invention, the following modifications and changes can be made. 
     (i) A height of the second tapered sidewall is equal to or greater than a height of an upper edge of a light-emitting layer of the LED chips. 
     (ii) A height of the second tapered sidewall is equal to a height of an upper edge of the LED chips. 
     (2) According to another embodiment of the invention, a planar light source comprises: 
     the linear light source according to the above embodiment (1); 
     a light guide plate comprising a side to cover a light extraction surface of the linear light source and to input a light emitted from the linear light source therethrough to the plate and a top surface to output the light therethrough to an outside of the plate; and 
     a reflection sheet covering at least a light passing section in a side along the first direction of the linear light source, the light passing section being defined between the second tapered sidewall and the light guide plate. 
     In the above embodiment (2) of the invention, the following modifications and changes can be made. 
     (iii) A light reflectivity of the reflection sheet is higher than that of the reflector. 
     (iv) The reflection sheet comprises a specular reflection-type reflection sheet. 
     Points of the Invention 
     According to one embodiment of the invention, a linear light source is constructed such that a second tapered sidewall of a reflector along the longitudinal direction (D1) of the linear light source is lower than a first tapered sidewall of the reflector along the lateral direction (D2) of the linear light source. Since the second tapered sidewall has a tapered portion, the thickness thereof increases with an increase in height so as to increase the thickness of the linear light source. The same goes for the planar light source using the linear light source. On the other hand, the light reflectivity of a reflection sheet is higher than that of the second tapered sidewall (reflector) and the light extraction efficiency thereof is higher than the case of reflecting by the second tapered sidewall. In a planar light source using the linear light source, the second tapered sidewall is covered by the reflection sheet up to the first tapered sidewall, light horizontally emitted in the lateral direction (D2) from an LED chip is reflected by the reflector and light emitted obliquely upward in the lateral direction (D2) from the LED chip is reflected by the reflection sheet. Due to this configuration, the planar light source is excellent in light extraction efficiency and suited to decrease in thickness. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Next, the present invention will be explained in more detail in conjunction with appended drawings, wherein: 
         FIGS. 1A and 1B  are perspective views showing planar light sources in an embodiment; 
         FIG. 2  is a perspective view showing a linear light source in the embodiment; 
         FIGS. 3A and 3B  are vertical cross sectional views showing the planar light source, respectively taking along a first direction and a second direction; 
         FIG. 4  is a vertical cross sectional view schematically showing a trajectory of light emitted from an LED chip of the planar light source in the embodiment; and 
         FIGS. 5A and 5B  are vertical cross sectional views schematically showing trajectories of lights emitted from LED chips of planar light sources in Comparative Examples. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 
       FIG. 1A  is a perspective view showing a planar light source in the embodiment. A planar light source  1  has a linear light source  10 , a light guide plate  20  provided so that a side surface thereof covers a light extraction surface of the linear light source  10 , and reflection sheets  30  which cover one side of the light guide plate  20  and both sides of the linear light source  10 . 
       FIG. 2  is a perspective view showing the linear light source  10 . The linear light source  10  has plural LED chips  11  linearly arranged along a first direction D1, a reflector  12  provided to surround each LED chip  11  and a sealing material  14  for sealing the LED chip  11 . The reflector  12  is provided so that each LED chip  11  is sandwiched by first tapered sidewalls  12   a  in the first direction D1 and by second tapered sidewalls  12   b  in a second direction D2 which intersects with the first direction D1, and the height of the second tapered sidewall  12   b  is lower than that of the first tapered sidewall  12   a.    
       FIGS. 3A and 3B  are vertical cross sectional views showing the planar light source  1 , respectively taking along the first direction D1 and the second direction D2. Note that, a typical structure of the linear light source  10  is shown in  FIGS. 2 ,  3 A and  3 B, where the first direction D1 coincides with a long axis direction and the second direction D2 is orthogonal to the first direction D1 and also coincides with a short axis direction of the linear light source  10  as viewed in a light-extracting direction. 
     The LED chip  11  is, e.g., a flip-chip or face-up LED chip and is connected to a non-illustrated conductive portion, such as a lead frame, of the linear light source  10 . The LED chip  11  has a chip substrate and a crystal layer thereon which are not illustrated in the drawings. The crystal layer has a light-emitting layer which is sandwiched between an n-type semiconductor layer and a p-type semiconductor layer. 
     The reflector  12  has a function of upwardly reflecting light laterally emitted from the LED chip  11  (reflecting in a light-extracting direction), thereby improving light extraction efficiency of the linear light source  10 . The reflector  12  is formed of, e.g., a resin material which contains light-reflecting particles such as titanium dioxide. For example, thermoplastic resins such as polyphthalamide resin, LCP (Liquid Crystal Polymer) or PCT (Polycyclohexylene Dimethylene Terephalate), etc., or thermosetting resins such as silicone resin, modified silicone resin, epoxy resin or modified epoxy resin, etc., are used as the resin material. 
     The height of the second tapered sidewall  12   b  is set to be equal to or higher than the upper edge of the light-emitting layer of the LED chip  11  so that light horizontally emitted from the LED chip  11  is reflected by the second tapered sidewalls  12   b  of the reflector  12 . Alternatively, the height of the second tapered sidewall  12   b  may be set to be equal to or higher than the upper edge of the LED chip  11 . In such a case, it is also possible to reflect the light horizontally emitted from the LED chip  11  and, in addition, it is easier to confirm the upper edge of the LED chip  11  than the upper edge of the light-emitting layer and it is thus easy to set height of the second tapered sidewall  12   b.    
     The sealing material  14  is formed of, e.g., a resin material such as silicone-based resin or epoxy-based resin, etc., or glass. In addition, the sealing material  14  may contain phosphor particles. For example, when emission color of the LED chip  11  is blue and that of the phosphor contained in the sealing material  14  is yellow, emission color of the linear light source  10  and the planar light source  1  is white. 
     The light guide plate  20  is a rectangular plate formed of a translucent material such as acrylic resin, which takes light emitted from the linear light source  10  therein and emits the light in a top surface thereof. The light guide plate  20  may be a wedge-shaped plate having a cross section in which a surface-emitting portion is thinner than a portion for taking light from the linear light source  10  therein, as shown in  FIG. 1B . 
     The reflection sheet  30  is, e.g., a multi-layer sheet formed of polyester. The reflection sheet  30  covers at least a light passing section  13  of a side surface in the first direction D1 of the linear light source  10  between the second tapered sidewall  12   b  and the light guide plate  20  so that light emitted from the LED chip  11  toward above the second tapered sidewall  12   b  of the reflector  12  is reflected. 
     Light reflectivity of the reflection sheet  30  is higher than that of the reflector  12 . The reflection sheet  30  is preferably a specular reflection-type reflection sheet, not a diffuse reflection-type. In addition, the reflection sheet  30  is more susceptible to heat or light than the reflector  12  and is more likely to deteriorate due to light absorption. 
       FIG. 4  is a vertical cross sectional view schematically showing a trajectory of light emitted from the LED chip  11  of the planar light source  1 . The cross section shown in  FIG. 4  corresponds to the cross section shown in  FIG. 3B . 
     As shown in  FIG. 4 , light horizontally emitted in the second direction D2 from the LED chip  11  is upwardly reflected by the second tapered sidewall  12   b  of the reflector  12  and does not return into the LED chip  11 . In addition, due to reflection by the second tapered sidewall  12   b , such light travelling at an angle nearly vertical to the reflection sheet  30  is not absorbed by the reflection sheet  30  and thus does not cause deterioration of the reflection sheet  30 . 
     In addition, as shown in  FIG. 4 , light emitted obliquely upward in the second direction D2 from the LED chip  11  is upwardly reflected by the reflection sheet  30 . Such light having a large incident angle with respect to the reflection sheet  30  is hardly absorbed by the reflection sheet  30  and does not cause deterioration of the reflection sheet  30 . In addition, such light does not travel toward the LED chip  11  after being reflected by the reflection sheet  30 . Meanwhile, since light reflectivity of the reflection sheet  30  is higher than that of the reflector  12 , light extraction efficiency is larger than the case of reflecting by the reflector  12 . 
       FIG. 5A  schematically shows a trajectory of light in Comparative Example in which the reflector  12  does not have the second tapered sidewall  12   b . The cross section shown in  FIG. 5A  corresponds to the cross section shown in  FIG. 4 . In this case, the light horizontally emitted in the second direction D2 from the LED chip  11  may return into the LED chip  11  by being reflected by the reflection sheet  30 , as shown in  FIG. 5A . In addition, such light is incident on the reflection sheet  30  at an angle close to vertical and is thus likely to be absorbed by the reflection sheet  30 . In general, the reflection sheet is more likely to deteriorate due to light absorption and to be subjected to a decrease in reflectance as compared to the reflector. Therefore, light incident on the reflection sheet  30  at an angle close to vertical is not preferable. 
     Furthermore,  FIG. 5B  schematically shows a trajectory of light in another Comparative Example in which the height of the second tapered sidewall  12   b  is equal to that of the first tapered sidewall  12   a . The cross section shown in  FIG. 5B  corresponds to the cross section shown in  FIG. 4 . In this case, the light emitted obliquely upward in the second direction D2 from the LED chip  11  is upwardly reflected by the second tapered sidewall  12   b , as shown in  FIG. 5B . However, the light reflectivity of the second tapered sidewall  12   b  is lower than that of the reflection sheet  30  and light extraction efficiency is therefore lower than the case of reflecting by the reflection sheet  30 . In addition, since the second tapered sidewall  12   b  has a tapered portion, the thickness thereof increases with an increase in height, which increases the thickness of the planar light source  1 . 
     Effects of the Embodiment 
     In the embodiment, by providing the second tapered sidewall  12   b  of which height is lower than the first tapered sidewall  12   a , the light horizontally emitted in the second direction D2 from the LED chip  11  is reflected by the reflector  12  and the light emitted obliquely upward in the second direction D2 from the LED chip  11  is reflected by the reflection sheet  30 . Due to such a configuration, a linear light source and a planar light source using thereof are excellent in light extraction efficiency and are suitable for thinning. In addition, it is possible to suppress deterioration of the reflection sheet  30 . 
     The present invention is not intended to be limited to the above-mentioned embodiment, and the various kinds of modifications can be implemented without departing from the gist of the invention. 
     In addition, the invention according to claims is not to be limited to the above-mentioned embodiment. Further, please note that all combinations of the features described in the embodiment are not necessary to solve the problem of the invention.