Abstract:
A method of producing an edge-light type light guide plate having a light exiting surface and a light reflecting surface having light reflective dots formed thereon in a scattered fashion. The method includes (a) producing a first prototype of the light guide plate having primary dots formed on the light reflecting surface, (b) examining optical characteristics of the first prototype relating to light exiting from the light exiting surface thereof, and (c) producing a second prototype of the lightguide plate when the examined first prototype exhibits unsatisfactory optical characteristics. The second prototype has primary dots that are the same as the primary dots of the first prototype in size and distribution, and secondary dots arranged between the primary dots in at least part of the light reflecting surface. The steps (b) and (c) are repeated until a prototype is produced which exhibits satisfactory optical characteristics.

Description:
[0001]    This application claims priority under 35 U.S.C. §119 to Japanese Patent application No. JP2007-158073 filed on Jun. 14, 2007, the entire contents of which are hereby incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to light guide plates for use in display apparatus and the like. 
       RELATED ART 
       [0003]    Liquid crystal display apparatus have been widely used in mobile phones, personal digital assistants (PDAs), automatic teller machines (ATMs), displays of personal computers, etc. These liquid crystal display apparatus employ a backlight unit that applies light to a liquid crystal display panel from the back thereof to enhance a luminance of the display screen. 
         [0004]    The backlight unit uses a light guide plate that receives light from a light source such as LEDs (light-emitting diodes) and that applies the light toward the entire back surface of the liquid crystal display panel. This type of light guide plate is demanded to apply light to the liquid crystal display panel uniformly, and various improvements have been made thereto. 
         [0005]    Japanese Patent Application Publication No. 2005-026223 and Japanese Patent No. 3136189 disclose an edge-light type light guide plate having a light exiting surface that emits light and a light reflecting surface opposite to the light exiting surface, in which the light reflecting surface has light diffusing reflective dots formed thereon in a desired distribution pattern to uniformly disperse light while the light is transmitted through the light guide plate. 
         [0006]    Japanese Patent No. 3136189 also proposes a method of making a desired dot distribution pattern by a computer simulation using a preset relational expression. 
         [0007]    The above-described conventional techniques, however, still have the following problems to be solved. 
         [0008]    In the manufacture of a light guide plate, a mold having a dot pattern determined on the basis of a simulation or the like is made, and a resin is injected into the mold to produce a prototype of the light guide plate. Examination of luminance uniformity at the light exiting surface of the prototype, however, often reveals a failure to achieve a target luminance uniformity based on a simulation or the like. This may be due to various factors such as the flow of resin during the injection molding process. Accordingly, it is often necessary to remake the mold a plurality of times, which leads to an increased cost. 
       SUMMARY OF THE INVENTION 
       [0009]    An object of the present invention is to improve the above-described disadvantages. 
         [0010]    The present invention provides a method of producing a light guide plate having a light exiting surface and a light reflecting surface opposite to the light exiting surface. The light reflecting surface has a plurality of light reflective dots formed thereon in a scattered fashion. The method includes (a) producing a first prototype of the light guide plate in which primary dots are formed on a light reflecting surface of the first prototype, (b) examining optical characteristics of the first prototype relating to light exiting from the light exiting surface of the first prototype, (c) producing a second prototype of a light guide plate when the examined first prototype exhibits optical characteristics which are not satisfactory, the second prototype having, on the light reflecting surface thereof, primary dots that are the same as the primary dots of the first prototype in size and distribution, and secondary dots arranged between the primary dots in at least part of the light reflecting surface to improve the optical characteristics of the second prototype, and (d) repeating steps (b) and (c) until a prototype is produced which exhibits satisfactory optical characteristics, wherein the second prototype is examined in step (b). 
         [0011]    According to this method, one mold is made, and a prototype of the light guide plate is produced by using the mold. Based on the prototype, the mold is subjected to additional processing. By repeating the steps, the mold can be surely finished to produce a light guide plate having satisfactory optical characteristics. Accordingly, the production cost of the mold can be reduced to a considerable extent as compared to the above-described conventional method. If a prototype of the light guide plate produced in the first prototype producing step (a) exhibits satisfactory optical characteristics, the prototype producing process is completed. 
         [0012]    Specifically, the secondary dots formed in step (c) may be smaller in size than the previously formed dots. 
         [0013]    More specifically, the primary dots and the secondary dots may be hemispherical or columnar projections projecting from the light reflecting surface. 
         [0014]    In this case, the hemispherical or columnar secondary dots formed in step (c) may be smaller in diameter and height than the previously formed dots. 
         [0015]    The secondary dots provided in a boundary area that borders the surrounding part of the at least part of the light reflecting surface may be distributed at a lower density than the secondary dots provided in an other area of the at least part of the light reflecting surface. 
         [0016]    With the above-described arrangement, optical characteristics can be improved without a sharp change at the boundary area. 
         [0017]    In addition, the present invention provides a light guide plate having a light exiting surface and a light reflecting surface opposite to the light exiting surface. The light reflecting surface has a plurality of light reflective dots formed thereon in a scattered fashion. The dots include primary dots and secondary dots arranged between the primary dots in at least part of the light reflecting surface to improve optical characteristics of the light guide plate relating to light exiting from the light exiting surface. The secondary dots have a lower reflectivity than the primary dots surrounding them. 
         [0018]    The light guide plate can be produced at a reduced cost by the above-described method. 
         [0019]    Specifically, the primary dots and the secondary dots may be hemispherical or columnar projections. The secondary dots may be smaller in diameter and height than the primary dots. More specifically, the primary dots may be hemispherical projections, and the secondary dots, which are additionally provided, may be columnar projections. In this case, the secondary dots may be smaller in diameter and height than the primary dots. Hemispherical dots are superior to columnar dots in light diffusing and reflecting characteristics. On the other hand, columnar dots can be produced at a lower cost. Therefore, hemispherical dots are used as primary dots, and columnar dots are used as supplementary secondary dots. 
         [0020]    The secondary dots provided in a boundary area that borders the surrounding part of the at least part of the light reflecting surface may be distributed at a lower density than the secondary dots provided in an other area of the at least part of the light reflecting surface. 
         [0021]    In addition, the present invention provides a backlight unit including the above-described light guide plate and a light source disposed adjacent to a light entrance surface of the light guide plate to emit light into the light guide plate through the light entrance surface. 
         [0022]    In addition, the present invention provides a display apparatus including an image display panel and the above-described backlight unit disposed at the back of the image display panel. 
         [0023]    Embodiments of the present invention will be explained below with reference to the accompanying drawings. It should be noted that the scale of constituent members shown in the figures used in the following description is properly changed for the sake of explanation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a plan view of a first prototype formed in a first prototype producing step of a light guide plate producing method according to an embodiment of the present invention. 
           [0025]      FIG. 2  is a plan view of a second prototype formed in a second prototype producing step of the light guide plate producing method. 
           [0026]      FIG. 3  is a plan view of a third prototype formed in the second prototype producing step of the light guide plate producing method. 
           [0027]      FIG. 4  is a schematic perspective view showing a backlight unit using a light guide plate formed by using the light guide plate producing method. 
           [0028]      FIG. 5  is a schematic sectional view showing a display apparatus having the backlight unit shown in  FIG. 4 . 
           [0029]      FIG. 6  is a plan view showing a different prototype formed by the light guide plate producing method. 
           [0030]      FIG. 7  is a perspective view showing hemispherical dots as one form of dots on the light guide plate according to the present invention. 
           [0031]      FIG. 8  is a perspective view showing columnar dots as another form of dots on the light guide plate according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0032]    A light guide plate producing method according to an embodiment of the present invention is to produce a light guide plate  4 , as shown in  FIG. 5 , which is used in a backlight unit  5  of a liquid crystal display apparatus  10 . The light guide plate  4  has a light exiting surface  1   b , which is an upper surface as viewed in  FIG. 5 , a light reflecting surface  1   a , which is a lower surface, and a light entrance surface that receives light from a light source  2 . The light reflecting surface  1   a  has a multiplicity of light reflective dots formed thereon to diffuse and reflect light entering the light guide plate  4  so as to allow the light to exit from the light exiting surface  1   b  uniformly. In the following embodiment, the light reflective dots are formed as short hemispherical or columnar projections projecting from the light reflecting surface  1   a  (see  FIGS. 7 and 8 ). The light reflective dots, however, may be recesses and may be formed by printing. 
         [0033]    In the light guide plate producing method, firstly, as shown in  FIG. 1 , a distribution pattern of primary dots D 1  to be formed on the light reflecting surface  1   a  is determined on the basis of a computer simulation or the like, and a first prototype  4 A is produced which has the primary dots D 1  formed on the light reflecting surface  1   a  (first prototype producing step). More specifically, a mold is made that has a plurality of recesses for forming the primary dots D 1 , and a first prototype  4 A is produced by injection-molding a transparent polycarbonate or acrylic resin using the mold. 
         [0034]    Although in the illustrated example the primary dots D 1  are distributed in a polka-dot pattern, they may be arranged in any of various two-dimensional patterns in which the primary dots D 1  are spaced from each other, such as a lattice pattern. As shown in  FIG. 1 , the primary dots D 1  are, in general, formed so that the diameter thereof is small near the light sources  2  and becomes larger as the distance from the light sources  2  increases. The height of the primary dots D 1  also becomes higher as the distance from the light sources  2  increases. 
         [0035]    Next, optical characteristics of the first prototype  4 A relating to light exiting from the light exiting surface  1   b , such as luminance at the light exiting surface  1   b , are measured by making light enter the first prototype  4 A through the light entrance surface, locating a part X where optical characteristics are not satisfactory. 
         [0036]    After the part X has been located, secondary dots D 2  are formed between the primary dots D 1  in the part X (second prototype producing step). More specifically, the above-described mold is subjected to additional processing to form secondary dots D 2 , thereby producing a second prototype  4 B. If the luminance at a part X of the light exiting surface  1   b  that is closer to the light entrance surface is lower than at the other part, for example, secondary dots D 2  are formed in the part X as shown in  FIG. 2 . 
         [0037]    The secondary dots D 2  are each provided at the midpoint between a pair of adjacent primary dots D 1  and formed with a smaller diameter and a lower height than the surrounding primary dots D 1 . 
         [0038]    In a boundary area X 1  of the part X that borders a surrounding part, secondary dots D 2  are formed at a lower density and with a smaller diameter and a lower height than the dots D 2  formed in the remaining area X 2  of the part X. 
         [0039]    The above-described measurement of optical characteristics relating to exiting light is performed, for example, by using a luminance and color shading analyzer (e.g. EYE-Scale, available from I System Corporation). With the luminance and color shading analyzer, the light exiting surface is divided into a predetermined number of areas, and the luminance at each divided area is measured, allowing discrimination between bright and dark areas. Accordingly, secondary dots D 2  are formed in a dark area located by this inspection to obtain a predetermined luminance. For example, the luminance at an area immediately in front of each light source  2  and the luminance at an area between areas immediately in front of mutually adjacent light sources  2  are measured with the above-described analyzer, and secondary dots D 2  are additionally formed in a dark area in front of and between the light sources  2 . Thus, the luminance at the dark area can be increased. 
         [0040]    The above-described measurement is also performed on the second prototype  4 B. If the second prototype  4 B does not exhibit satisfactory optical characteristics described above, a third prototype producing step similar to the above is carried out.  FIG. 3  shows a third prototype  4 C with tertiary dots D 3  produced through the additional prototype producing step. The dimensional relationship between the tertiary dots D 3  and the secondary dots D 2  is the same as that between the secondary dots D 2  and the primary dots D 1 . The relationship between tertiary dots D 3  formed in the areas X 1  and X 2  is also the same as in the case of the second prototype  4 B. 
         [0041]    Thus, the optical characteristics of a prototype are measured each time additional dots are formed, and the same one mold is additionally processed to form additional dots until a prototype is produced which exhibits satisfactory optical characteristics. 
         [0042]      FIG. 6  shows a second prototype  4 B having secondary dots D 2  formed thereon in the same way as the above when the luminance at a part X of the first prototype  4 A remote from the light sources  2  is low, in reverse relation to the case shown in  FIG. 1 . 
         [0043]    The backlight unit  5  in this embodiment has, as shown in  FIGS. 4 and 5 , the above-described light guide plate  4 , light sources  2 , a diffusing sheet  6 , a first prism sheet  8 A, a second prism sheet  8 B, and a reflecting sheet  9 . The diffusing sheet  6  is disposed over the light guide plate  4  to diffuse light exiting from the light guide plate  4 . The first and second prism sheets  8 A and  8 B direct light exiting from the diffusing sheet  6  toward a liquid crystal display panel  7  as illuminating light. The reflecting sheet  9  is disposed underneath the light guide plate  4 . 
         [0044]    The first and second prism sheets  8 A and  8 B each have a plurality of mutually parallel elongated prisms on their respective upper surfaces, and their respective prisms of the first and second prism sheets  8 A and  8 B are disposed to substantially perpendicularly intersect each other in plan view. 
         [0045]    The reflecting sheet  9  is a metal plate, film, foil or the like having light-reflecting characteristics. In this embodiment, the reflecting sheet  9  is a film provided with an evaporated silver layer. It should be noted, however, that an evaporated aluminum layer or the like may be used in place of the evaporated silver layer. 
         [0046]    The light sources  2  are a plurality of white light-emitting diodes (LEDs) disposed along the light entrance surface of the light guide plate  4 . The white LEDs are, for example, each formed by sealing at least one semiconductor light-emitting element on a substrate with a resin material. The semiconductor light-emitting element is, for example, a blue (wavelength λ: 470 to 490 nm) LED element or an ultraviolet (wavelength λ: less than 470 mn) LED element, which is formed, for example, by stacking a plurality of semiconductor layers of a gallium nitride compound semiconductor (e.g. InGaN compound semiconductor) on an insulating substrate, e.g. a sapphire substrate. 
         [0047]    The resin material used to seal the semiconductor light-emitting element is formed by adding, for example, a YAG fluorescent substance into a silicone resin as a main component. The YAG fluorescent substance converts blue or ultraviolet light from the semiconductor light-emitting element into yellow light, and white light is produced by color mixing effect. It should be noted that various LED elements in addition to those described above can be used as the white LEDs. 
         [0048]    The display apparatus  10  in this embodiment is a liquid crystal display apparatus applicable, for example, to a liquid crystal display for use in a mobile phone, a personal digital assistant (PDA) or an automatic teller machine (ATM). The display apparatus  10  has a liquid crystal display panel  7  and the above-described backlight unit  5  disposed at the back of the liquid crystal display panel  7 . 
         [0049]    The liquid crystal display panel  7  is a transmissive or semitransmissive type liquid crystal display panel. For example, the liquid crystal display panel  7  in this embodiment is of the semitransmissive type and has a panel body  25  having a liquid crystal material L sealed with a sealant  24  in a gap between an upper substrate  22  and a lower substrate  23 . A semitransmitting-reflecting sheet  26  having both light-transmitting and reflecting characteristics is disposed underneath the panel body  25 . As the liquid crystal material L, for example, TN liquid crystal or STN liquid crystal may be used. 
         [0050]    The upper substrate  22  comprises an upper transparent substrate  22   a  made of glass, for example. An upper transparent electrode layer  22   b  made of an ITO (Indium Tin Oxide) film is disposed on the lower surface of the upper transparent substrate  22   a . An upper alignment film  22   c  is disposed on the lower surface of the upper transparent electrode layer  22   b.  The upper alignment film  22   c  is formed by applying an alignment treatment to a transparent polyimide resin film or the like. In addition, an upper polarizer  22   d  is disposed on the upper surface of the upper transparent substrate  22   a.    
         [0051]    The lower substrate  23  comprises a lower transparent substrate  23   a  made of glass, for example. A lower transparent electrode layer  23   b  made of an ITO film is disposed on the upper surface of the lower transparent substrate  23   a . A lower alignment film  23   c  is disposed on the upper surface of the lower transparent electrode layer  23   b . The lower alignment film  23   c  is formed by applying an alignment treatment to a transparent polyimide resin film or the like. In addition, a lower polarizer  23   d  is disposed on the lower surface of the lower transparent substrate  23   a.    
         [0052]    The semitransmitting-reflecting sheet  26  may be an aluminum-evaporated sheet having light-transmitting properties, or a reflective polarizer, for example. It should be noted that a spacer (not shown) comprising silica balls or plastic balls, for example, is dispersedly disposed in the gap between the upper and lower substrates  22  and  23 , ensuring a desired amount of gap. 
         [0053]    It should be noted that the present invention is not necessarily limited to the foregoing embodiments but can be modified in a variety of ways without departing from the scope of the present invention. 
         [0054]    Although it is preferable to use white LEDs as the light sources  2 , as has been stated above, a linear fluorescent tube light source may be used as the light sources  2 . 
         [0055]    Although in the foregoing embodiments the liquid crystal display panel  7  is used as an image display panel, other types of image display panels may be used, for example, an electronic paper.