Patent Publication Number: US-2007115660-A1

Title: Backlight unit and liquid crystal display comprising the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority under 35 U.S.C. §119 (a) Korean Patent Application No. 2005-0111041, filed on Nov. 19, 2005, in the Korean Intellectual Property Office, which is hereby incorporated in its entirety by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present general inventive concept relates to a backlight unit and a liquid crystal display comprising the same, and more particularly, to a backlight unit having a point light source and a liquid crystal display comprising the same.  
      2. Description of the Related Art  
      Recently, a flat panel display apparatus, such as a liquid crystal display (LCD), a plasma display panel (PDP), and an organic light emitting diode (OLED), has been developed to substitute for a conventional display such as a cathode ray tube (CRT).  
      The liquid crystal display (LCD) includes an LCD panel having a thin film transistor (TFT), a color filter substrate and a liquid crystal disposed therebetween. Since the LCD panel does not emit light by itself, the LCD includes a backlight unit at a rear side of the TFT substrate as a light source for providing light. The transmittance of the light generated from the backlight unit is adjusted according to an alignment of the liquid crystal. The LCD panel and the backlight unit are accommodated in a chassis of the flat panel display apparatus.  
      Depending on a location of the light source, the backlight unit may be classified as an edge type or direct type backlight unit. The edge type backlight unit is provided with the light source at a lateral side of a light guiding plate and is typically used for relatively small sized LCDs, such as those used in laptops and desktop computers. The edge type backlight unit provides high light uniformity and good endurance and is suitable for use in thin profile LCDs. However, its light efficiency is decreased because the emitted light is lost while getting through the light guiding plate. Also, the light guiding plate cannot be manufactured by using one mold in a case of a large sized LCD panel.  
      As the size of the LCD panel is increased, development of the direct type backlight unit has been emphasized. The direct type backlight unit provides light to the entire surface of the LCD panel by disposing a plurality of light sources a rear side of the LCD panel. The direct type backlight unit provides a high level of brightness by using a plurality of light sources, as compared with the edge type backlight unit, but the brightness is not sufficiently uniform due to a blur of color.  
     SUMMARY OF THE INVENTION  
      The present invention provides a backlight unit having an improved color uniformity and a good light efficiency and a liquid crystal display comprising the same.  
      Additional aspects and advantages of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept.  
      The foregoing to and/or other aspects of the present invention may be achieved by providing a backlight unit comprising a point light source circuit board, a plurality of point light sources seated on the point light source circuit board, and a diffusion lens provided on each point light source and comprising a depressed point and a curved surface radially protruding from the depressed point.  
      The curved surface may have a section shaped like a symmetrical non-circular curves.  
      The depressed point may be spaced apart from the point light sources by a predetermined distance.  
      The curved surface may be formed by the following equation, and A 1  is not equal to zero.  
         z   (           ⁢   r   )     =           ⁢         c   ·     r   2         1   +       1   -       (     1   -   k     )     ⁢     c   2     ⁢     r   2               +           ⁢       A   1     ⁢           ⁢   r     +           ⁢       A   2     ⁢           ⁢     r   2       +           ⁢       A   3     ⁢           ⁢     r   3       +           ⁢       A   4     ⁢           ⁢     r   4       +           ⁢       A   5     ⁢           ⁢     r   5       +           ⁢   ⋯   +           ⁢       A   n     ⁢           ⁢     r   n             
 
 where c=curvature of the diffusion lens, and k=Conic constant. 
 
      The diffusion lens may comprise a first lens surface adjacent to the point light sources and a second lens surface as the curved surface with the depressed point, the c may be a negative number, and the A 1  may be a positive number.  
      At least a portion of a section of the curved surface may be shaped like at least two or more different circular curves.  
      The backlight unit may further comprise a prominence and depression part formed on a surface of the diffusion lens.  
      The foregoing to and/or other aspects of the present invention may also be achieved by providing a liquid crystal display comprising a liquid crystal display panel, point light sources provided on an entire rear surface of the liquid crystal display panel, and a diffusion lens provided between the liquid crystal display panel and each point light source and comprising a depressed point and a curved surface radially protruding from the depressed point.  
      The curved surface may have a section shaped like a symmetrical non-circular curves.  
      The curved surface may be formed by the following equation, and A 1  is not equal to zero.  
         z   (           ⁢   r   )     =           ⁢         c   ·     r   2         1   +       1   -       (     1   -   k     )     ⁢     c   2     ⁢     r   2               +           ⁢       A   1     ⁢           ⁢   r     +           ⁢       A   2     ⁢           ⁢     r   2       +           ⁢       A   3     ⁢           ⁢     r   3       +           ⁢       A   4     ⁢           ⁢     r   4       +           ⁢       A   5     ⁢           ⁢     r   5       +           ⁢   ⋯   +           ⁢       A   n     ⁢           ⁢     r   n             
 
 where c=curvature of the diffusion lens, and k=Conic constant. 
 
      At least a portion of a section in the curved surface may be shaped like at least two or more different circular curves.  
      The liquid crystal display may further comprise a prominence and depression part formed on a surface of the diffusion lens.  
      The foregoing to and/or other aspects of the present invention may also be achieved by providing a backlight unit usable in a flat panel display, the backlight unit including a circuit board, a plurality of light sources disposed on the circuit board to generate light, and a diffusion lens having a first surface to receive the light from the corresponding light source, and having a second surface to emit the received light and having a depressed point having a depressed distance with the first surface and a surface extended from the depressed point and having a distance with the first distance, the distance varying from the depressed distance to a highest distance and a lowest distance according to a distance from the depressed point.  
      The distance may increase from the depressed distance to the highest distance and then decreases from the highest distance to the lowest distance according to the radius from a center of the diffusion lens. The light source may include an LED having a plastic mold disposed on the circuit board, a lead disposed in the plastic mold, and a chip disposed on the lead to be electrically connected to the light source through the lead to generate the light; and the first surface is disposed on the plastic mold and spaced apart from the chip.  
      The depressed point and the chip may be disposed on a line perpendicular to the circuit board, and the first surface of the diffusion lens may be extended from the plastic mold in a direction parallel to the circuit board. The first surface may include a first portion contacting a top surface of the plastic molding and a second portion extended from the first position in a direction perpendicular to a line connecting the chip and the depressed point, and an edge of the second portion may meet the second surface.  
      The light source may be disposed on a line perpendicular to the circuit board, and the depressed point is disposed on the line. The second surface may include a first spherical surface having a first radius and a second spherical surface having a second radius, the first spherical surface has the distance varying from the depressed distance to the highest distance, and the second spherical surface has the distance varying from the highest distance to the lowest distance.  
      The first surface may be a flat surface and the second surface is a curved surface, and the second surface may include a curved surface and a non-curved surface. The non-curved surface may include a flat surface formed in a circumferential direction of the depressed point.  
      The second surface may include a curved surface linearly varying with respect to a line passing though the depressed point, and may include a first portion having the distance which varies linearly and a second portion having the distance which varies non-linearly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and/or other aspects and advantages of the present general inventive concept will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings of which:  
       FIG. 1  is an exploded perspective view illustrating an LCD according to an embodiment of the present general inventive concept;  
       FIGS. 2A and 2B  are a sectional view and an exploded perspective view illustrating a diffusion lens of the LCD of  FIG. 1 , respectively;  
       FIG. 3  is a sectional view illustrating a diffusion lens according to an embodiment of the present general inventive concept;  
       FIG. 4  is a sectional view illustrating a diffusion lens according to an embodiment of the present general inventive concept; and  
       FIG. 5  is a view illustrating brightness of a diffusion lens according to the embodiment of the present general inventive concept and brightness of a conventional diffusion lens. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The embodiments are described below so as to explain the present general inventive concept by referring to the drawings.  
      A liquid crystal display (LCD) according to an embodiment of the present general inventive concept will be described with reference to the  FIGS. 1, 2A ,  2 B and  5 .  FIG. 1  is an exploded perspective view of an LCD  1  according to the embodiment of the present general inventive concept,  FIGS. 2A and 2B  are a sectional view and an exploded perspective view of the LCD of  FIG. 1 , and  FIG. 5  is a view illustrating brightness of a diffusion lens according to the embodiment of the present general inventive concept and brightness of a conventional diffusion lens.  
      The LCD  1  comprises an LCD panel  20 , a light regulating member  30 , a reflecting plate  40 , a light emitting diode (LED) circuit board  51  disposed in back of the LCD panel  20  in order; and an LED  60  seated on the LED circuit board  51  and disposed corresponding to an LED aperture  41  of the reflecting plate  40 . A plurality of LEDs is disposed on the LED circuit boards  51  as the LED  60 . The LED  60  may be used as an example of the LEDs.  
      The LCD panel  20 , the light regulating member  30 , and the LED circuit board  51  are accommodated between an upper chassis  10  and a lower chassis  100 .  
      The LCD panel  20  comprises a TFT substrate  21  on which TFTs are formed, a color filter substrate  22  facing the TFT substrate  21 , a sealant  23  attached to the two substrates  21  and  22  to form a cell gap, and a liquid crystal layer  24  surrounded by the two substrates  21  and  22  and the sealant  23  and disposed in the cell gap. The LCD panel  20  according to the present embodiment is provided as a rectangular shape having a long side and a short side.  
      The LCD panel  20  controls molecular alignment of liquid crystal of the liquid crystal layer  24 , thereby forming an image thereon. However, the LCD panel  20  must be supplied with light from the LED  60  disposed at its rear, because the LCD panel  20  does not emit light by itself. On a side of the TFT substrate  21  is disposed a driving part  25  to apply driving signals to the LCD panel  20 . The driving part  25  comprises a flexible printed circuit (FPC)  26  connected to the LCD panel  20 , a driving chip  27  mounted on the FPC  26  to drive the LCD panel  20 , and a printed circuit board (PCB)  28  connected on a side of the FPC  26  to control the driving chip  27 . Here, the driving part  25  shown in  FIG. 1  is a COF (chip on film) type. However, other types of driving parts may be used, such as TCP (tape carrier package) or COG (chip on glass) type. Alternatively, the driving part  25  may be formed on the TFT substrate  21  where wirings are formed.  
      The light regulating member  30  disposed at a rear side of the LCD panel  20  may comprise a diffusion plate  31 , a prism film  32 , and a protection film  33 .  
      The diffusion plate  31  comprises a base plate and a coating layer having beads formed on the base plate. The diffusion plate  31  diffuses light from the LED  60 , thereby improving a uniformity of the brightness.  
      Triangular prisms are placed on the prism film  32  in a predetermined arrangement. The prism film  32  concentrates the light diffused from the diffusion plate  31  in a direction perpendicular to a surface of the LCD panel  20 . When two prism films  32  are used, the micro prisms formed on the prism film  32  form a predetermined angle each other. The light passing through the prism film  32  progresses vertically, thereby forming uniform brightness distribution. A reflective polarizing film may be used along with the prism film  32  as necessary, or only the reflective polarizing film may be used without the prism film  32 .  
      The protection film  33 , positioned at the top of the light regulating member  30 , protects the prism film  32 , which is vulnerable to scratching.  
      On the LED circuit board  51  on which the LEDs  60  are not seated is placed the reflecting plate  40 . One or more LED apertures  41  are disposed in the reflecting plate  40  corresponding to the arrangement of LEDs  60 . A plurality of sets of LED apertures  41  comprise one or more lines in parallel to each other, and each line includes a plurality of LED apertures  41  disposed at a regular interval. The LED apertures  41  between the adjacent lines are in staggered positions relative to each other. In each LED aperture  41  is disposed a white colored light providing unit  61  of the LED  60 . The LED aperture  41  may be formed slightly larger than the white colored light providing unit  61 .  
      Most part in addition to a chip  62  ( FIG. 2A ) generating light of the LED  60  is disposed over the reflecting plate  40 . For example, when the white colored light providing unit  61  is disposed in the corresponding LED aperture  41 , the most part and the chip  62  may protrude from the reflecting plate  40  such that the reflecting plate  40  reflects the light delivered downward and directs the reflected light to the diffusion plate  31 . The reflecting plate  40  may be made of, e.g., polyethylene terephthalate (PET) or polycarbonate (PC), and/or be coated with silver (Ag) or aluminum (Al). In addition, the reflecting plate  40  may be formed with a sufficient thickness so as to prevent distortion or shrinkage due to heat generated from the LED  60 .  
      Because the LED  60  may generate a significant amount of heat, the LED circuit board  51  may be primarily made of aluminum (Al) having an excellent thermal conductivity. Although not shown in drawings, the LCD  1  may further comprise a heat pipe, a heat radiating fin, a cooling fan, or other cooling mechanisms for removing the heat generated by the LED  60 .  
      The LEDs  60 , seated on the LED circuit board  51 , are disposed across the entire rear surface of the LCD panel  20 . A predetermined number of LEDs  60  are included in each of the plurality of white colored light providing units  61  to provide white colored light. The predetermined number of the LEDs  60  may be disposed in the corresponding LED aperture  41 . In the present embodiment, the white colored light providing unit  61  comprises a red LED, a blue LED and a pair of green LEDs which respectively generate red, blue and green lights to be combined into the white color light. The white colored light providing units  61  are disposed on the LED circuit board  51  at a regular interval.  
      Referring to  FIG. 2A , the LED  60  comprises the chip  62  to generate light, a lead  63  to connect the chip  62  with the LED circuit board  51 , a plastic mold  64  to accommodate the lead  63  and surrounding the chip  62 , a filling material  65  comprising silicon and disposed on an upper part of the chip  62 , and a diffusion lens  70 . A pattern of the light generated from the LED  60  is mainly influenced by a shape of the diffusion lens  70 . The diffusion lens  70  according to the present embodiment will be described in detail hereinafter.  
       FIG. 2A  is a sectional view of the LED  60  and  FIG. 2B  is a perspective view illustrating the diffusion lens  70  in three-dimensional. As illustrated in  FIGS. 2A and 2B , the diffusion lens  70  according to the present embodiment comprises a surface  73  radially protruding with respect to a depressed point  71  having a shape similar to an upper shape of an apple. The surface  73  may be a curved surface having the depressed point  71  at its center. The depressed point  71  and the chip  62  are spaced apart from each other by a predetermined distance.  
      The depressed point  71  may be disposed on a line corresponding to a z axis of  FIG. 2A . The diffusion lens  70  may include a bottom surface  74  disposed on top surfaces of the plastic mold  60  and/or the filing material  65 , and a distance d between the surface  73  and the bottom surface  74  in a direction parallel to a z axis of  FIG. 2A  may vary according to a radius r from the z axis and/or the depressed point  71 . The distance d (d 1 ) may increase within a first radius ra to a highest distance dh, and the distance d (d 2 ) may become decrease according to a second radius rb longer than the first radius ra. A distance dz (the predetermined distance) between the depressed point  71  and the bottom surface  74  is less than the highest distance dh.  
      The diffusion lens  70  may be made of polymethylmetharcylate (PMMA) or polycarbonate (PC). It is preferred that the filling material  65  contacted with the diffusion lens  70  on an upper part of the chip  62  may have a refractive index similar to that of the diffusion lens  70 . It is preferred that a ratio of the refractive index in the filling material  65  to that of the diffusion lens  70  is between 0.8 and 1.2. Also, it is preferred that bonding material such as epoxy connecting the diffusion lens  70  with the chip  62  has the refractive index similar to that of the diffusion lens  70 .  
      The curved surface  73  as an aspheric surface made of the diffusion lens  70  is a shape in which a non-circular curved line rotates with respect to a z-axis at an angle of 360 degrees as shown in the sectional view. In other words, a pair of the non-circular curved lines symmetrically shapes along a section of the curved surface  73 . An aspheric surface equation made of the curved surface  73  is the same as the following mathematical equation 1 and A 1  is not equal to zero.  
      [Mathematical Equation 1] 
         z   (           ⁢   r   )     =           ⁢         c   ·     r   2         1   +       1   -       (     1   -   k     )     ⁢     c   2     ⁢     r   2               +           ⁢       A   1     ⁢           ⁢   r     +           ⁢       A   2     ⁢           ⁢     r   2       +           ⁢       A   3     ⁢           ⁢     r   3       +           ⁢       A   4     ⁢           ⁢     r   4       +           ⁢       A   5     ⁢           ⁢     r   5       +           ⁢   ⋯   +           ⁢       A   n     ⁢           ⁢     r   n             
 
 where r=x 2 +y 2 , c=curvature of the diffusion lens, and k=Conic constant. 
 
      The variable r corresponds to an x-y planar distance from a center illustrated in  FIG. 2B  in three-dimensional. Accordingly, the mathematical equation 1 is an equation of the non-circular curved line in  FIG. 2A .  
      In case of a general lens, the aspheric surface equation does not comprise an odd order term such as a first term coefficient A 1  because the aspheric surface of the lens is asymmetry with respect to the z-axis, when the aspheric surface equation comprises the odd order term. However, in the diffusion lens  70  of the present embodiment, r may be positive numbers in the aspheric surface equation. Also, the non-circular curved line rotates with respect to the z-axis and then the diffusion lens  70  is formed. Because the aspheric surface is formed, the diffusion lens  70  may have a rotationally symmetric shape. The diffusion lens  70  may be formed in various modifications using a coefficient of the odd order term as well as that of an even order term to form the aspheric surface. Also, when the diffusion lens  70  is formed, degree of freedom in a design is increased due to various combinations of the coefficients. The Conic constant and the curvature of the diffusion lens  70  are regulated in various values.  
      It is possible that c is a negative number and a first term coefficient A 1  is a positive number opposite to a value of c to form a spherical surface protruding to a direction of the z-axis from the x-y plane.  
      If the aspheric surface equation comprises the first term coefficient A 1  among odd order terms, a discontinuous portion like the depressed point  71  according to the present embodiment is formed. Because a concave shape toward a direction of the chip  62  like the ingression point  71  disperses light which is concentratedly radiated to a very upper portion of the chip  62 , with a large light emitting angle, a hot spot is decreased and thereby a uniformity of light brightness distribution and a color uniformity are increased.  
      The aspheric surface according to the present embodiment may comprise a plane surface not a curved surface. On the other words, two or more different curved surfaces may be formed as the surface  73 , and the aspheric surface may comprise a two-dimensional plane surface. In this case, the curve of the section in the diffusion lens  70  partially comprises a straight line.  
       FIG. 5  is a graph illustrating an improved brightness of the diffusion lens  70  according to the present embodiment and brightness of a conventional diffusion lens. The white colored light providing unit  61  used in the present embodiment comprises the red LED, the blue LED and a pair of the green LEDs. The diffusion lens  70  in which the depressed point  71  is formed thereon is used so that a range in which light emitted therefrom is extended, that is, light is dispersed into more broaden area. Accordingly, the brightness of the diffusion lens  70  is higher than the conventional brightness of the conventional diffusion lens when the diffusion lens  70  which is in a position separated at a predetermined distance from a center in which an LED  60  is disposed as well as in the center in which the LED  60  is disposed. A fine shape adjustment of a lens surface using a polynomial expression in the aspheric surface equation expressing the curved surface controls the light emitting angle of the lens in effective. Also, after emitted from the lens surface, light toward a bottom is decreased and light toward the LCD panel  20  is increased. Accordingly, the brightness of the center is increased about 40% as compared with the conventional brightness and power consumption is decreased according to the increasing brightness.  
      Also, a prominence and depression part may be formed on the surface of the diffusion lens  70  according to another embodiment of the present invention. A surface roughness of the diffusion lens  70  is enhanced by the prominence and depression part and a diffusion of light is induced. Accordingly, the brightness uniformity and the color uniformity of the light provided with the LCD panel  20  is enhanced. Size and shape of the prominence and depression part is not limited and may be formed by scratching the surface in the diffusion lens  70 , for example.  
       FIG. 3  is a view illustrating a diffusion lens  80  according to an embodiment of the present general inventive concept. Referring to  FIG. 3 , a section of the diffusion lens  80  has a shape in which a pair of circular curved lines are symmetrically connected to form a surface  83 . The diffusion lens  80  according to the present embodiment is constituted of a spherical surface in which a portion of a semicircle is rotated with respect to the z-axis different from the embodiment illustrated in  FIG. 2B .  
      A radius of a circle is R 1  and a distance from the z-axis to a center of the circle is R 2 . Here, it is possible that R 2  is shorter than R 1 . If R 2  is the same as R 1 , a function of the diffusion may be remarkably decreased because the diffusion lens  80  becomes a hemisphere type. Also, if R 2  is longer than R 1 , a hollow space is formed on the center of the diffusion lens  80  and light emitted from the chip  62  does not pass the diffusion lens  80 .  
      If a distance in which the depressed point is spaced from a bottom surface  84  of the diffusion lens  80  is d 1 , the distance d 1  is equal to (R 1   2 −R 2   2 ) 1/2  because the center of the circle is located on an r-axis in the present embodiment. If a z-axis coordinate of the center in the circle is a positive number, that is, the distance d 1  is longer than (R 1   2 −R 2   2 ) 1/2 , a process for producing the diffusion lens  80  may be troublesome. More particularly, an injection of the diffusion lens  80  is difficult in a molding process because the center of the diffusion lens  80  is depressed downwardly. Accordingly, if the distance d 1  is longer than (R 1   2 −R 2   2 ) 1/2 , it is possible that an edge of the diffusion lens  80  is formed as a straight line.  
       FIG. 4  is a view illustrating a diffusion lens  90  according to an embodiment of the present general inventive concept. Referring to  FIG. 4 , a section of the diffusion lens  90  has a shape in which a pair of circular curved lines are symmetrically connected. A curved surface  93  of the diffusion lens  90  according to the present embodiment is constituted of a shape in which two different circles are composed.  
      A radius of an inner circle is R 4  and a radius of an outer circle is R 3 , and R 3  is longer than R 4 . Alternatively, each radius of the two circles and the relative relation of their lengths are is not limitable. However, each slope of tangent lines has the same value in a node in which the two circles are united so that the two circles are smoothly connected without an infection.  
      Also, if a z-axis coordinate of begins with a positive number, it is possible that an edge of the diffusion lens  90  is formed as a straight line.  
      Light generated from the LED  60  may have higher uniformity through the diffusion lenses  70 ,  80  and  90  to provide it to the LCD panel  20 . The shapes of the diffusion lenses  70 ,  80  and  90  are not limited the aforementioned embodiments and may have various modifications.  
      Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.