Abstract:
A liquid crystal display with low manufacturing cost, small dark area and compact exterior is provided by utilizing a plurality of differently radiating light emitting diodes at different angles. The liquid crystal of present invention includes a thin film transistor panel for displaying image, a backlight assembly for providing light to the thin film transistor panel with a plurality of light emitting diodes with different light radiation angles and a light guiding plate, and a frame unit for encompassing the thin film transistor panel and the backlight assembly. The light emitting diodes are placed either at the mid-point of light incident surface or the corner portion of the light guiding plate.

Description:
This application claims priority to Korean Patent Application No. 2007-59720, filed on Jun. 19, 2007 and all the benefits accruing therefrom under 35 U.S.C. §119, and the contents of which in its entirety are herein incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid crystal display (LCD) backlight assembly, and more particularly, the present invention relates to a backlight assembly using different radiation angles of different light emitting diodes for low manufacturing cost, compact exterior and luminance efficiency. 
     2. Description of the Related Art 
     As display devices are used in everyday life, the liquid crystal display (LCD) has been gaining popularity. The LCD has a thin film transistor (TFT) panel which includes a pair of opposing substrates with a liquid crystal layer therebetween. The LCD also has a backlight assembly since the liquid crystal layer is not self-emissive. The backlight assembly is located behind the TFT panel to provide light to the liquid crystal layer. With light from the backlight assembly, transmittance of the TFT panel is controlled by arranging the liquid crystal molecules for each pixel. 
     The backlight assembly is classified into two groups according to the location of a light source, a direct light backlight assembly and an edge light backlight assembly. In the edge light backlight assembly, the light source is located at a lateral side of a light guiding plate (LGP) which is placed between the TFT panel and the light source. A linear fluorescent lamp has been mainly used as the light source, but point light sources such as a light emitting diode (LED) are also popular for a small LCD. 
     Being a semiconductor device, the merit of an LED lies in long lifetime, low power consumption and compact exterior. However, on the other hand, a LED has the demerit of limited light emitting angle which is called a light radiation angle. In an edge light backlight assembly, the LGP has a dark area close to a light incident area due to a limited light radiation angle of the LED and has low space efficiency for illuminating. 
     To improve the low efficiency, an increased number of LEDs or a widened radiation angle for each LED may be used. However, in order to widen the radiation angle for each LED, each of the LED packages must be altered raising the LED package price. Accordingly, an increased number of LEDs or widening radiation angle of each LED may lead to high manufacturing cost, high power consumption and a bulky exterior. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, it is an aspect of the present invention to provide a backlight assembly with less of a dark area and a compact exterior with a low manufacturing cost by using different radiation angles of different LEDs. 
     According to an embodiment of present invention, an LCD includes a TFT panel, a backlight assembly and a frame unit. The TFT panel has a pair of transparent glasses, a liquid crystal layer inbetween the glasses and a pair of polarizers attached to each glass to selectively pass light in response to the electric charge of each pixel. 
     The backlight assembly has a plurality of light emitting diodes (LEDs), a light guiding plate (LGP) next to the LEDs and at least one optical sheet above the LGP for providing uniform luminance to the TFT panel. Finally, the frame unit has an upper frame and a lower frame to enclose the TFT panel, and the backlight assembly unit to make LCD into one module. 
     Here, in the backlight assembly, a first LED has a first light radiation angle and a second LED has a second radiation angle while the LGP has a light incident surface, a light reflection surface, a first lateral surface and a second lateral surface. 
     In detail, the first LED faces the mid-point of the light incident surface; the second LEDs are paired and may be separated by the same distance from the mid point of the light incident surface and on opposite sides of the first LED. There, the second LEDs may face the corner portions at edges of the light incident surface where the light incident surface meets either the first lateral surface or the second lateral surface. 
     At the corner portion, the second LED may be slanted and, thus, may distribute its emitted light to the LGP more effectively. Namely, the light radiated to the outer area of the LGP can be minimized by placing the second LED at the corner portion of the LGP. With respect to above configuration, accordingly, the dark area of the LGP can be reduced and the whole backlight assembly can be compact. 
     According to another embodiment of the present invention, the first LED is placed out of the mid-point of the light incident surface while the second LED faces the corner portion at the edge of the light incident surface of the LGP. 
     According to yet another embodiment of the present invention, the corner portion has an additional surface located between the light incident surface and either of the first or second lateral surfaces of the LGP. 
     According to still another embodiment of the present invention, the second LED, at the corner portion of the LGP, has first and second radiation edge lines with respect to the light radiation angle. Here, either of the first or the second radiation edge lines extends into the gap between the lateral surface and active light emitting area of the LGP. 
     According to an alternate embodiment of the present invention, the LED has a package of flat light emitting faces whose thickness is less than the thickness of either light incident surface or corner portion of the LGP and is closely located near the LGP. Also, the LED package&#39;s outermost portion can exceed either the first or second lateral surface of the LGP. 
     According to yet another alternate embodiment of the present invention, the dark area of the LGP in the vicinity of the light incident surface has an inactive light emitting distance. The inactive light emitting distance may be the same or within the distance from the light of the incident surface to the active light emitting area of the LGP. 
     According to still another alternate embodiment of the present invention, the TFT panel has a first transparent substrate and a second transparent substrate which is larger than the first substrate with an area of circuitry unit. The circuitry unit overlaps with the inactive light emitting area of the LGP and/or LED packages. In addition, the top frame of the frame unit fully covers both the inactive light emitting area and LEDs while the top frame overlaps the edge portion of the first substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and/or other aspects and advantages of the present invention will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a plan view of a backlight assembly having a light guide plate (LGP) and a plurality of light emitting diodes (LEDs) with the same light radiation angle; 
         FIG. 2  is a perspective view of an exemplary embodiment of a liquid crystal display (LCD) having a plurality of LEDs whose light radiation angles are different from each other according to the present invention; 
         FIG. 3  is a plan view of an exemplary embodiment of a backlight assembly having LEDs in different angles according to the present invention; 
         FIG. 4  is a plan view of an exemplary embodiment of a backlight assembly of  FIG. 3  which shows a package of the LED and its relevance to a corner portion of an LGP according to the present invention; 
         FIG. 5  is a plan view of an exemplary embodiment of a backlight assembly of  FIG. 3  which shows the relevance of a light radiation angle of the LED and an active light emitting area of the LGP according to the present invention; 
         FIG. 6  is a plan view of an exemplary embodiment of a backlight assembly with the LED&#39;s location deviated from the mid-point of a light incident surface of the LGP according to the present invention; 
         FIG. 7  is a cross-sectional view of an exemplary embodiment of the LCD, the LCD having a backlight assembly of  FIG. 2  through  FIG. 6 , and a TFT panel overlapping the backlight assembly according to the present invention; 
         FIG. 8  is another cross-sectional view of an exemplary embodiment of the LCD, the LCD having a backlight assembly of  FIG. 2  through  FIG. 6 , and a TFT panel overlapping the backlight assembly according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. The embodiments are described below in order to explain the present invention by referring to the figures. 
       FIG. 1  is a plan view illustrating a dark area of a backlight assembly caused by a plurality of LEDs with the same light radiation angle. According to  FIG. 1 , the backlight assembly comprises an LGP  10  and a plurality of LEDs  20 . Each LED  20  has a light emitting semiconductor chip enclosed by a package and accordingly has a limited light emitting range called light radiation angle  25 . In detail, the light emitted from the LED  20  is spread within two edge lines which define the light radiation angle  25 . 
     In  FIG. 1 , the backlight assembly includes three LEDs  20  whose light radiation angel  25  is 72 degrees. Due to the LEDs&#39; linear arrangement and light radiation angle  25  of each LED  20 , the backlight assembly has dark areas  30  between neighboring LEDs  20  in the vicinity of light incident surface  23 . As a result, the dark area  30  and the region between the dark areas are useless for the display area of an LCD and is collectively called an inactive light emitting area. In other words, the inactive light emitting area has an inactive light emitting distance  40 . 
     For example, the inactive light emitting distance is 7 mm when the length of the light incident surface is 33.9 mm and the light radiation angle of each LED is 72 degrees. Meanwhile, as the LCD should have a more effective display area, the inactive light emitting distance  40  should be minimized. 
       FIG. 2  is a perspective view of LCD  100  with different LEDs  122  facing the light incident surface  123  of the LGP  124 . Here, each LED  122  has at least two different light radiation angles. In  FIG. 2 , LCD  100  includes a TFT panel  110 , a backlight assembly  120  and a frame unit  132 . The TFT panel  110  includes a liquid crystal layer and a pair of transparent glasses. Among the glasses, one glass is larger than the other to have circuitry unit  112 . On the circuitry unit  112 , a drive IC or film bonding area for connection with other driving circuits may be provided. 
     The backlight assembly  120  includes a light source  122  such as an LED, an LGP  124  for uniformly illuminating with the incident light from the light source  122 , and at least one optical sheet  128  for accommodating luminance or uniformity enhancement. 
     The light source  122  may include a plurality of LEDs with at least two different light radiation angles. Further, each LED faces either the light incident surface  123  of the LGP  124  or the corner portion  125  at the edge of the light incident surface  123  of the LGP  124 . Here, the LED  122  placed near the corner portion  125  of the LGP  124  can be slanted from the light incident surface  123  to effectively distribute its emitted light to the LGP  124 . 
     The LGP  124  has an active light emitting area (not shown) which is substantially coincident with an active display area (not shown) of the TFT panel  110 . Within the light emitting surface of the LGP  124 , the inactive light emitting area may overlap with the circuitry unit  112  of the TFT panel  110 . According to the present invention, the light source  122  and inactive light emitting area are collectively covered by the circuitry unit  112  of the TFT panel  110 . 
     Finally, a frame unit  130  encompasses both the TFT panel  110  and backlight assembly  120  for completely assembling the LCD. The frame unit  130  includes a bottom frame  132  for accommodating the LGP  124  and top frame (not shown) for surrounding the edge of the TFT panel  110 . Next, by conjoining the top frame and the bottom frame  132 , the overall LCD is completed. 
       FIG. 3  is a plan view of a backlight assembly having LEDs of at least two different light radiation angles. The backlight assembly of  FIG. 3  includes a LGP  250 , a first LED  220  and a plurality of second LEDs  225  and a series of optical sheets (not shown). The first LED  220  has a first light radiation angle  260  while facing the light incident surface  230  of the LGP  250 . Meanwhile, the second LED  225  has a second light radiation angle  265  while facing the corner portion  270  of the LGP  250  where the edge of the light incident surface  230  and one of the lateral surfaces  255  of the LGP meet. 
     At the corner portion, the second LED  225  is slanted and, thus, may distribute its emitted light to the LGP  290  more effectively. Namely, the light radiated to outer area of LGP  290  can be minimized by placing the second LED  225  at the corner portion of the LGP  290 . With respect to the above configuration, accordingly, a dark area of the LGP can be reduced and the whole backlight assembly can be compact. 
     According to the present invention of  FIG. 3 , the light radiation angles  260 ,  265  of the first LED  220  and the second LED  225  are different. For example, the first light radiation angle  260  is wider than 90 degrees; the second light radiation angle  265  is narrower than 90 degrees. As an exemplary embodiment,  FIG. 3  has a first light radiation angle  260  of 160 degrees at the mid-point of light incident surface  230  and accordingly the first LED  220  has a first inactive light emitting distance  240 . In the case of a 1.9 inch diagonal LCD, the light incident surface  230  is 33.9 mm long and results in an inactive light emitting distance  240  of 2.8 mm. 
     On the other side, the second LED  225  faces the corner portion  270  of the LGP  250 , has a second light radiation angle  265  of 72 degrees and has a second inactive light emitting distance  243 . Here, the corner portion  270  is a cutaway surface  275  meeting both the light incident surface  230  and the lateral surface  255  at opposing edges of the light incident surface  230 . Specifically, the cutaway surface  275  is slanted to the light incident surface  230  at an angle of 54 degrees. 
     Assuming the same 1.9 inch diagonal LCD above, the second inactive light emitting distance  243  is 10.3 mm which is longer than the first inactive light emitting distance  240  of 2.8 mm. 
     However, applying both the first and second LED  220 ,  225  within the same LCD makes the final inactive light emitting distance  246  of the inactive light emitting area shorter than either the first or second inactive light emitting distance  240 ,  243  as can be seen in  FIG. 3 . The final inactive light emitting distance  246  is calculated to be 2.0 mm. 
     On the contrary, by assuming that the second LED  225  of  FIG. 3  is moved to the same position of  FIG. 1 , the final inactive light emitting distance is changed to 7.0 mm. As a result, the elongated inactive light emitting distance has bad influence on the active light emitting area ratio within the light emitting surface of the LGP. 
       FIG. 4  is another plan view of a backlight assembly showing a package of LEDs and its relevance to a corner portion of an LGP. For a brief explanation, the same elements shown in  FIG. 3  are expressed with the same reference numeral and corresponding explanations will be omitted. 
     Referring to  FIG. 4 , the second LED  225  is a package  226  having a self emissive semiconductor (not shown) enclosed by a housing  227 . Specifically, the housing  227  of the LED package  226  has a square shape with long sides and short sides. One of the long sides of the housing  227  is a flat LGP facing surface  228  which includes a light emitting opening  229 . 
     Meanwhile, the LGP  290  of  FIG. 4  has a slanted corner surface  275  at the edge of the light incident surface. The slanted corner surface  275  faces the LGP facing surface  228  of the LED  225  with a larger surface area than the light emitting opening  229  of the LED  225 . 
     The long side of the LED package  226  is slanted to the light incident surface  230  and parallel with the corner surface  275  of the LGP; thus, the LED package  226  has an outermost portion  280 . For designing the overall backlight, the outermost portion  280  should be within the region of extensive area from the inactive light emitting area of the LGP because outermost portion  280  within the extensive area of active light emitting area  290  of LGP may shrink the active light emitting area in turn. Therefore, the outermost portion  280  should be within the range extended from the inactive light emitting area. That is to say, the distance from the extensive line from the light incident surface  230  to the outermost portion  280  should be the same or shorter than the inactive light emitting distance  246 . 
       FIG. 5  is a plan view of a backlight assembly of  FIG. 3  showing the relevance between a light radiation angle of the LED and the active light emitting area of the LGP. For a brief explanation, the same elements shown in  FIG. 3  and  FIG. 4  are expressed with the same reference numeral and corresponding explanations will be omitted. 
     With respect to  FIG. 5 , the LGP has the light incident surface  230 , the corner surface  275  and the lateral surface  255  in a series. Further, the LGP has an active light emitting area  290  whose edges are spaced apart from the light incident surface  230  and lateral surface  255  of the LGP by distances d 1  and d 2  respectively. 
     The second LED  225  faces the corner surface  275  of the LGP and has a light radiation angle  265  defined by a first radiation edge line  310  and a second radiation edge line  315 . Here, the first radiation edge line  310  is close to the light incident surface  230  while the second radiation edge line  315  is close to the lateral surface  255  of the LGP. 
     In this instance, the second radiation edge line  315  is located between the lateral surface  255  and the active light emitting area  290  to secure a maximized active light emitting area. In other words, if the second radiation edge line  315  exceeds the lateral surface  255 , the first radiation edge line  310  will move toward the inner side of the active light emitting area  290 . Then, radiation edge  310  prolongs the inactive light emitting distance  243 . Consequently, the active light emitting area will be limited. 
     On the contrary, if the second radiation edge line  315  moves toward the inner space of light emitting surface  290 , a dark area will be shown in close proximity to the second lateral surface  255  and prohibits the formation of an evenly illuminated light emitting area. As a result, the second radiation edge line  315  standing inbetween the second lateral surface  255  and the edge of active light emitting area  290  will promote the maximum ratio of active light emitting area on the light emitting surface of the LGP. 
       FIG. 6  is a plan view of a backlight assembly wherein the LED is located at the point deviated from the mid-point of a light incident surface of the LGP. For a brief explanation, the same elements shown in  FIGS. 3 through 5  are expressed with the same reference numeral and corresponding explanations will be omitted. 
     Referring to  FIG. 6 , the mid-point of the light incident surface  230  is denoted as m and the middle line extended from the mid-point is denoted as l. Here, the first LED  220  has a wider radiation angle  260  than the second LED  225  and is spaced apart from m by distance d 5  to the first lateral surface  253 . Thus, the first dark area  236  to the first lateral side  253  is narrower than the second dark area  237  to the second lateral side  255 . 
     To solve the discrepancy, the second LED  225  may be positioned near the second lateral side  255  by a distance d 6  from the mid-point d 6 . By applying both the first LED  220  and the second LED  225 , the backlight assembly of  FIG. 6  has three dark areas  236 ,  238 ,  239 . 
     Specifically, the first dark area  236  is between a fourth light radiation edge  260   a  and the first lateral surface  253  whose shortest point to the active light emitting area is P 1 . The third dark area  238  is placed between a third light radiation edge  260   b  and the second light radiation edge  316  whose shortest point to the active light emitting area is P 3 . The fourth dark area  239  is disposed between a first light radiation edge  317  and the second lateral surface  255  whose contact point to the active light emitting area is P 4 . 
     Here, for making maximized light emitting area  290 , the shortest points to the active light emitting area P 1 , P 2 , P 3 , P 4  should be close to the light incident surface  230 . Also, either the distance between the mid-point m and the first LED d 5  or the distance between the mid-point m and the second LED d 6  can be controlled to maximize the active light emitting area  290 . In other words, to acquire the maximum active light emitting area  290  and minimize the inactive light emitting distance  246 , the closest point among the dark areas to the edge of the active light emitting area in the vicinity of the light incident surface should be close to the light incident surface  230 . 
     Hence, minimized fourth dark area  239  makes the shortest inactive light emitting distance  246  in  FIG. 6 . Furthermore, the asymmetric arrangement of LEDs&#39; of  FIG. 6  can be accomplished with fewer LEDs than the symmetric arrangement of LEDs&#39; since the symmetric arrangement needs additional LEDs for symmetry. However, in considering the asymmetric arrangement of LEDs, the overall backlight assembly should have enough luminance for display. 
     Alternatively, to acquire the minimized fourth dark area  239 , the edge line  317  of the second light radiation angle  265  meets the corner of the active light emitting area  290 . In this respect, the narrower second light radiation angle  265  accommodates the second LED&#39;s location farther than the first LED&#39;s location from the mid-point m of the light incident surface  230 . 
     In  FIG. 6 , even though the second LED  225  is positioned to face the light incident surface like the first LED  220  faces the light incident surface, the second LED  225  may be placed at the corner portion as long as one of the first and second light radiation edges  316 ,  317  of the second LED  225  exceeds the active light emitting area  290 . When the second LED  225  is located at the corner portion at the cross region between the second lateral surface  255  and the incident surface  230 , the first LED  220  can move farther from the mid-point m toward the first lateral surface  253  as long as adequate luminance uniformity in the active light emitting area  290  is acquired. 
     For example, when the second LED  225  has a radiation angle of less than 90 degrees and the first light radiation edge  317  is placed between the second lateral surface  255  and the active light emitting area  290 , the second light radiation edge  316  still remains within the LGP; then, a third light radiation edge  260   b  of the first LED  220  can meet the second light radiation edge  316  at point P 3 . Here, as the first LED  220  moves toward the first lateral surface  253 , the point P 3  moves toward the light incident surface  230  which result in the expansion of active light emitting area  290 . 
       FIG. 7  is a cross-sectional view of the LCD having a backlight assembly of  FIGS. 2 through 6  in overlapping relationship with the TFT panel. For a brief explanation, the same elements shown in  FIGS. 2 through 6  are expressed with the same reference numeral and corresponding explanations will be omitted. 
     Now, referring to  FIG. 7 , the LCD comprises a TFT panel  110 , a backlight assembly  120  and a frame unit  130 . The backlight assembly  120  consists of an LGP  124 , a plurality of LEDs  122  of different light radiation angle beside the LGP, and a series of sheets  128 . Here, the LGP has the inactive light emitting distance  246  as explained in  FIGS. 3 to 6 . The optical sheets  128  are disposed over the LGP  124  and may overlap the inactive light emitting area  246 . 
     The TFT panel  110  has a first transparent substrate  114 , a second transparent substrate  116  which overlaps the first substrate  114  in one part and extends from the first substrate in another part. In detail, the extended area of the second substrate  116  is a circuitry unit  112  which is served as mounting area for drive ICs  119  or a film bonding area for electrical connection with other external circuits. 
     The circuitry unit  112  may cover the inactive area  246  of the LGP  124  for acquiring a compact LCD. In  FIG. 7 , the drive IC  119  is mounted at the circuitry unit  112 , does not face the LEDs  122  and is covered by the top frame  134 . The top frame  134  extends to the inner space of the TFT panel  110  from the circuitry unit  112  while the top frame  134  partially overlaps the TFT panel  110 . 
     Because the first LED is thinner than the light incident surface of the LGP, light from the first LED does not propagate to the optical sheets or the TFT panel directly. On the other hand, the second LED may be placed to face the corner portion of the LGP and may not be placed within the active light emitting area of the LGP. Pursuant to the above explanation, the light emitting opening of each LED is covered by the circuitry unit of the TFT panel to direct light to the LGP. 
       FIG. 8  is another cross-sectional view of the LCD having a backlight assembly of  FIGS. 2 through 6  in overlapping relationship with the TFT panel. For a brief explanation, the same elements shown in  FIGS. 2 through 7  are expressed with the same reference numeral and corresponding explanations will be omitted. 
     In  FIG. 8 , the drive IC  119  on the circuitry unit  112  is mounted differently from  FIG. 7  to face the LEDs  122 . Here, the circuitry unit  112  may serve as the film bonding area for connection with the external circuit. With the structure of  FIG. 8 , because the drive IC is mounted in an opposite orientation than as shown in  FIG. 7 , space between the TFT panel  110  and the top frame  134  may be saved resulting in a thinner LCD. 
     Even though, the present invention is explained with an LED as a point light source, other light generating sources can be used for accomplishing the spirit of the invention. For example, the point light source may be at least one of organic light emitting diode, plasma display panel, and field emission device. 
     The above-described embodiments of the present invention are merely meant to be illustrative and not limiting. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects. Therefore, the appended claims encompass all such changes and modifications as fall within the true spirit and scope of this invention.