Patent Publication Number: US-2016238895-A1

Title: Backlight unit and liquid crystal display including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2015-0021679 filed in the Korean Intellectual Property Office on Feb. 12, 2015, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     (a) Technical Field 
     The present disclosure relates to a backlight unit and a liquid crystal display (LCD) including the same. 
     (b) Description of the Related Art 
     A liquid crystal display (LCD) is a widely used type of display device at present. Generally, the LCD includes a liquid crystal material filled between an upper substrate formed with common electrodes, color filters, and the like, and a lower substrate formed with thin film transistors, pixel electrodes, and the like. By applying different voltages to the pixel and common electrodes to generate an electric field, thereby changing the arrangement of liquid crystal molecules, the LCD is able to adjust the transmittance of light to display an image. 
     An LCD panel of the LCD is a non-emissive type of light receiving element, so the LCD generally includes a backlight unit for supplying light to the LCD panel at a rear side thereof. 
     As a light source for the backlight unit, a cold cathode fluorescent lamp (CCFL) and a light emitting diode (LED) are generally used. Conventionally, the CCFL has been widely used as a light source for the backlight unit since it consumes less power and provides bright white light. However, the LED has recently been gaining popularity since it provides superior color reproducibility, a longer lifespan, and less power consumption. 
     The backlight unit may be classified as an edge type or a direct type, depending on where the light source is positioned in relation to the LCD panel. In the edge type, the light source is positioned at sides of the LCD panel to provide light through a light guide, whereas in the direct type, the light source is positioned at a rear side of the LCD panel to provide light thereto. Among them, the direct type of backlight unit has merits, such as, high light utilization, easy handling, no limitation in size, and a relatively cheaper price. 
     When a point light source, such as the LED, is used as the light source of the direct type of backlight unit, an optical lens may be provided on a light emitting surface of an LED package, such that the LED light&#39;s property of a straight-line propagation is not concentrated on an upper part of a light emitting surface, but is uniformly distributed across the entire LCD panel. Typically, a reflective sheet is positioned below the optical lens to reflect light, and a diffuser is positioned above the optical lens to uniformly distribute light. 
     The above information disclosed in this Background section is only to enhance the understanding of the background of the disclosure and therefore may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY 
     An objective of the present disclosure is to provide a backlight unit that uniformly supplies light to an LCD panel by improving local luminance increases or decreases and a display device including the same. 
     A backlight unit according to an exemplary embodiment of the present disclosure includes: a bottom chassis; a light source disposed on the bottom chassis; a reflective sheet disposed on the bottom chassis; at least one optical sheet disposed above the light source and the reflective sheet; and a supporter fixed to the bottom chassis and supporting the optical sheet. The supporter includes a locking portion fastened to the bottom chassis and a supporting portion extending from the locking portion, and the locking portion is positioned lower than the reflective sheet. 
     The locking portion may include a supporting plate positioned between the bottom chassis and the reflective sheet, and a hook penetrating the bottom chassis to be at least partially positioned below the bottom chassis. 
     The bottom chassis may include a substantially flat base portion and a recess downwardly depressed further than the base portion, in which recess the supporting plate is positioned, and a depth of the recess may be the same as or greater than a thickness of the supporting plate. 
     A top surface of the supporting plate may be positioned at the same height as or lower than a top surface of the base portion. 
     The reflective sheet may include a hole through which the support portion passes. 
     The backlight unit may further include a substrate on which h the light source is mounted, wherein the substrate may be fixed to the bottom chassis and positioned lower than the reflective sheet. 
     The bottom chassis may include a substantially flat base portion and a groove downwardly depressed further than the base portion, in which groove the substrate is positioned, and a depth of the groove is the same as or greater than a thickness of the substrate. 
     A top surface of the substrate may be positioned at the same height as or lower than a top surface of the base portion. 
     The reflective sheet may be attached to the substrate through an adhesive. 
     The backlight unit may further include a side emitting lens attached to the substrate and positioned to cover the light source. 
     The optical sheet may include a diffuser. 
     A manufacturing method of a backlight unit according to an exemplary embodiment of the present disclosure includes: preparing a bottom chassis; fixing a light source unit including a light source to the bottom chassis; fixing a supporter to the bottom chassis such that the supporter supports at least one optical sheet; and placing a reflective sheet on the bottom chassis by using at least a portion of the supporter as a guide after fixing the supporter. 
     The supporter may include a locking portion fastened to the bottom chassis and a supporting portion extending from the locking portion, the reflective sheet may include a hole that is formed at a corresponding position of the support portion, and the placing of the reflective sheet on the bottom chassis may include fitting the hole into the support portion. 
     The light source unit may include a substrate and a light source mounted on the substrate, and the placing of the reflective sheet on the bottom chassis may include attaching the reflective sheet to the substrate. 
     A display device according to an exemplary embodiment of the present disclosure includes: a display panel; and a backlight unit for supplying light to the display panel. The backlight unit includes: a bottom chassis; a light source disposed on the bottom chassis; a reflective sheet disposed on the bottom chassis; at least one optical sheet disposed above the light source and the reflective sheet; and a supporter fixed to the bottom chassis and supporting the optical sheet. The supporter includes a locking portion fastened to the bottom chassis and a supporting portion extending from the locking portion, and the locking portion is disposed lower than the reflective sheet. 
     The locking portion may include a supporting plate positioned between the bottom chassis and the reflective sheet, and a hook penetrating the bottom chassis to be at least partially positioned below the bottom chassis. 
     The bottom chassis may include a substantially flat base portion and a recess downwardly depressed further than the base portion, in which recess the supporting plate is positioned, and a depth of the recess is the same as or greater than a thickness of the supporting plate. 
     A top surface of the supporting plate may be positioned at the same height as or lower than a top surface of the base portion. 
     The reflective sheet may include a hole through which the support portion passes. 
     The backlight unit may further include a substrate on which the light source is mounted, and the substrate may be fixed to the bottom chassis and positioned lower than the reflective sheet. 
     The bottom chassis may include a substantially flat base portion and a groove downwardly depressed further than the base portion, in which groove the substrate is positioned, and a depth of the groove may be the same as or greater than a thickness of the substrate. 
     A top surface of the substrate may be positioned at the same height as or lower than a top surface of the base portion. 
     The reflective sheet may be attached to the substrate through an adhesive. 
     The backlight unit may further include a side emitting lens attached to the substrate and positioned to cover the light source. 
     The optical sheet may include a diffuser. 
     The backlight unit according to the present disclosure minimizes exposure of the supporter in an optical space, so unnecessary light scattering due to the supporter is minimized, which improves a profile of the light emitted from the backlight unit. 
     In addition, the supporter can be used as the guide for assembling the reflective sheet, so an additional means is not required to assemble the reflective sheet at a proper position, and local luminance deterioration does not occur due to such an additional means. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a liquid crystal display (LCD) including a backlight unit according to an exemplary embodiment of the present disclosure. 
         FIG. 2  is a cross-sectional view of  FIG. 1  taken along the line A-A. 
         FIG. 3  is an enlarged partial cross-sectional view of  FIG. 2 . 
         FIGS. 4A and 4B  are perspective views of supporters according to exemplary embodiments of the present disclosure. 
         FIGS. 5, 6, 7, 8 and 9  are schematic diagrams for illustrating an assembly process of a backlight unit according to an exemplary embodiment of the present disclosure. 
         FIGS. 10 and 11  are drawings that illustrate light reflectance and a luminance profile of the backlight unit according to an exemplary embodiment of the present disclosure. 
         FIGS. 12 and 13  are drawings that illustrate light reflectance and a luminance profile of a backlight unit according to a comparative example. 
         FIG. 14  is a drawing that illustrates a luminance distribution of light emitted from the backlight unit according to an exemplary embodiment of the disclosure. 
         FIG. 15  is a graph illustrating a luminance profile of a B-B region of  FIG. 14  together with that of a comparative example. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present system and method are described more fully hereinafter with reference to the accompanying drawings in which exemplary embodiments of the present system and method are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present system and method. 
     In the drawings, the thickness of layers, films, panels, regions, etc. are enlarged or exaggerated for clarity. It will be understood that when an element, such as, a layer, film, region, or substrate is referred to as being “on” another element, it may be directly on the other element, or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, there are no intervening elements present. 
     A liquid crystal display (LCD) including a backlight unit according to an exemplary embodiment of the present disclosure is described in detail with reference to  FIGS. 1 to 4 . 
       FIG. 1  is an exploded perspective view of a liquid crystal display (LCD) including a backlight unit according to an exemplary embodiment of the present disclosure.  FIG. 2  is a cross-sectional view of  FIG. 1  taken along the line A-A.  FIG. 3  is an enlarged cross-sectional view of a part of  FIG. 2 .  FIGS. 4A and 4B  are perspective views of supporters according to exemplary embodiments of the present disclosure. 
     Referring to  FIGS. 1 and 2 , the LCD includes an LCD panel  100  and backlight unit  200   
     The backlight unit  200  supplies light to the LCD panel, and the LCD panel  100  displays an image by controlling the supplied light. The LCD further includes a mold frame  300  that is positioned between the LCD panel  100  and the backlight unit  200 . The LCD further includes a top chassis  400  that protects the LCD panel  100  while enclosing a rim thereof and prevents the LCD panel  100  from being separated from the backlight unit  200 . In some exemplary embodiments, either one or both of the mold frame  300  and the top chassis  400  may be omitted. 
     The LCD panel  100  includes a lower display substrate  110 , an upper display substrate  120 , and a liquid crystal layer (not shown). The lower display substrate  110  and the upper display substrate  120  are attached to each other while maintaining a predetermined interval therebetween where the liquid crystal layer is formed. 
     The lower display substrate  110  includes a transparent insulation substrate, such as glass, and a plurality of thin film transistors, data lines, gate lines, pixel electrodes, etc. that are formed on the insulation substrate. A data line is connected to a source terminal of a thin film transistor, and a gate line is connected to a gate terminal thereof. A pixel electrode formed of a transparent conductive material, such as indium tin oxide (ITO), is connected to a drain terminal of the thin film transistor. 
     The upper display substrate  120  is positioned to face the lower display substrate  110  and includes a transparent insulation substrate on which color filters, common electrodes, etc. are formed. Each of the color filters may represent a primary color, such as red, green, and blue. A common electrode is formed of a transparent conductive material, such as indium tin oxide (ITO) or indium zinc oxide (IZO). In some embodiments, at least one of the color filter and the common electrode may be positioned in the lower display substrate  110  instead. 
     Polarizers may be attached to the lower display substrate  110  and the upper display substrate  120 , respectively. The polarizers may polarize incident light on the LCD panel  100  to transmit only light vibrating in one direction. In the LCD panel  100 , when the thin film transistor is turned on by a signal applied to the gate line, a signal applied to the data line is applied to the pixel electrode, which causes an electric field to be generated between the pixel electrode and the common electrode. The intensity of the electric field is based on the voltage level of the signal applied to the pixel electrode. By controlling the voltage level, and thereby the intensity of the electric field, the alignments of liquid crystal molecules of the liquid crystal layer may be controlled to change the polarization direction of incident light by the liquid crystal layer. The combined polarization effects of the liquid crystal layer and the polarizers determine the transmittance thereof. Accordingly, transmittance of light passing through the liquid crystal layer and the polarizers may be controlled to display an image. 
     The LCD includes at least one driving device (not shown), such as a driver and a controller, that controls signals applied to the LCD panel  100 . As an IC chip, the driving device may be mounted on the LCD panel  100  or on a printed circuit board (PCB) and a flexible printed circuit board (FPCB) to be electrically connected to the LCD panel  100 . Some driving devices may be integrated into the LCD panel  100 . 
     The backlight unit  200  is positioned below the LCD panel  100  to supply light to the LCD panel  100 . 
     The backlight unit  200  includes a bottom chassis  210 , a light source unit  220  that is accommodated in and fixed to or supported by the bottom chassis  210 , an optical lens  230 , a supporter  240 , a reflective sheet  250 , and an optical sheet  260 . 
     The bottom chassis  210  is a kind of container with its upper surface open and an accommodating space having a predetermined depth. For example, the bottom chassis  210  may have an overall quadrangular tray-like shape. The bottom chassis  210  includes a bottom  210   a , a wing  210   b  extending from edges of the bottom  210   a  in a substantially slanted and upward direction, and a rim  210   c  extending from an upper end of the wing  210   b  in a substantially parallel and outward direction (e.g., parallel to the bottom  210   a ). The bottom  210   a  includes a substantially flat base portion  211 , and a groove  212  and a recess  214  downwardly depressed further than the base portion  211  (in a negative z direction). The bottom chassis  210  may further include a wall  210   d  that downwardly extends from the rim  210   c . A height from the bottom  210   a  to the rim  210   c  corresponds to a depth of the bottom chassis  210 . The accommodating space defined by the bottom  210   a  and the wing  210   b  of the bottom chassis  210  may have a cross-section of an overall reverse trapezoid. 
     The bottom chassis  210  may be formed of a metallic material such as an aluminum plate, an aluminum alloy plate, or zinc-plated steel. In some exemplary embodiments, the bottom chassis  210  may be formed of a plastic material such as polycarbonate (PC). 
     The light source unit  220  is accommodated in the bottom chassis  210 . The light source unit  220  includes a substrate  221 , and a light source  222  mounted thereon. The light source  222 , which may be a light emitting diode (LED) package, is mounted on the circuit board  221  such that a light emitting surface of the light source  222  faces the LCD panel  100 . A white LED package for emitting white light may be used as the LED package, or a mixed arrangement of red, green, and blue LED packages may also be used. In addition to the LED packages, other point or line light sources may be used as the light source  222 . 
     The substrate  221  may have a narrow elongated bar shape. The substrate  221  supplies power to and supports the light source  222 . The substrate  221  may be a circuit board, for example, a metal core printed circuit board (MCPCB) that quickly discharges heat generated from the light source  222 . The light source  222  is electrically connected to wires of the substrate  221  to receive power, and converts electrical energy into light energy to emit light. One or more light sources  222  may be disposed on one substrate  221 . The total number of the light sources  222  and their arrangement may be modified in various ways depending on the size of the LCD panel, the output of the light source, etc. 
     The optical lens  230  is also mounted on the substrate  221 . The optical lens  230  is positioned to substantially cover the light source  222 , so light emitted from the light source  222  is refracted and diffused through the optical lens  230 . Since the optical lens  230  diffuses the light directed upward of the light source  222 , such that it is not concentrated, the optical lens  230  may be used to reduce the number of the light sources  222  and to apply a high power light source. 
     The optical lens  230  may be a side emitting lens that refracts and diffuses the light directed upward of the light source  222  mostly in side directions. The optical lens  230  may be a top emitting lens that refracts and diffuses the light of the light source  222  mostly in an upward direction. Since the side emitting lens may reduce an optical path further than the top emitting lens, the optical sheet  260 , as described later, may be positioned closer to the light source  222 , so an overall thickness of the backlight unit  200  may be reduced to realize a slimmer LCD. 
     The substrate  221 , the light source  222 , and the optical lens  230  together are referred to as a light source assembly in the present specification. A plurality of light source assemblies may be disposed at the bottom  210   a  of the bottom chassis  210  at predetermined intervals, and the number of the light source assemblies is the same as that of the substrates  221 . As illustrated in  FIG. 3 , the light source assembly is mounted on the groove  212  of the bottom  210   a . A depth d 1  of the groove  212  may be greater than a thickness of the substrate  221  such that the substrate  221  of the light source assembly is positioned at the same height as or lower than the base portion  211  that forms an overall flat surface of the bottom  210   a . Accordingly, when viewed in a cross-section, a top surface of the substrate  221  is positioned at the same height as or lower than the top surface of the base portion  211 . As a result, the reflective sheet  250 , as described later, is prevented from protruding or being lifted near the light source assembly. That is, when the reflective sheet  250  is attached to the substrate  221 , the top surfaces of the substrate  221  and the base portion  211  may be positioned nearly at the same height to prevent the reflective sheet  250  from being lifted. If there is a height difference between them, an attached portion of the reflective sheet  250  may not be smooth. 
     When the plurality of light source assemblies are disposed as illustrated, the light source unit  220  may further include one or a plurality of connecting boards  223  for supplying power to each of the light source assemblies. The plurality of light source assemblies may be connected to the connecting board  223 . For example, an end portion of the substrate  221  may be inserted into an insertion hole (not shown) of the connecting board  223  to be connected thereto, like a plug. The bottom  210   a  may include a groove (not shown) for mounting the connecting board  223  such that the connecting board  223  is positioned at the same height as or lower than the base portion  211  of the bottom  210   a . In some exemplary embodiments, instead of the connecting board  223 , electric wires may be used to supply power to the light source assembly. 
     The supporter  240  for supporting the optical sheet  260  is disposed on the bottom chassis  210 . A plurality of supporters  240  may be disposed at the bottom  210   a  of the bottom chassis  210  at predetermined intervals. The supporter  240  includes a locking portion ( 242 ,  243 ) that is locked to the bottom chassis  210 , and a supporting portion  241  that extends upwardly therefrom. Accordingly, the supporter  240  is secured to the bottom chassis  210 , and the supporting portion  241  protrudes upwardly from the bottom chassis  210  with a predetermined height. 
     The supporter  240  may be formed of a plastic, such as a polycarbonate or a metal, and may be formed as a single component. The supporter  240  may be coated with an optical material, such as a light absorptive material, to minimize light scattering or reflection by the supporter  240 . 
     As a kind of pillar for substantially supporting the optical sheet  260 , the supporting portion  241  has a smaller cross-section than the supporting plate  242  and extends straight therefrom. An upper end of the supporting portion  241  may contact the optical sheet  260  to support the optical sheet  260  and prevent the optical sheet  260  from drooping and to maintain the optical path between the light source  222  and the optical sheet  260 . 
     The supporting portion  241  may have a thin, elongated shape (e.g., circular cylindrical or polygonal shape) to minimize the light scattering and reflection due to the supporting portion  241 , and may have a shape in which its cross-sectional area gradually decreases towards its upper end. The supporting portion  241  may have, for example, a conical shape, as illustrated in  FIG. 4A , or a polypyramidal shape, such as, a triangular pyramid, a quadrangular pyramid (refer to  FIG. 4B ), etc. However, the end portion of the supporting portion  241  where it directly contacts the optical sheet  260  may be formed to be round or flat. 
     The locking portion serves to stably fix the supporting portion  241  to the bottom chassis  210 , so that the supporting portion  241  supports the optical sheet  260  at a proper position. The locking portion may include the supporting plate  242  contacting the top surface of the bottom chassis  210 , and the hook  243  contacting the bottom surface of the bottom chassis  210 . In order for, The bottom chassis  210  includes a penetration hole  215  through which the hook  243  may pass through to contact the bottom surface of the bottom chassis  210  to be fixed thereto. The hook  243  may be elastically restored after passing through the penetration hole  215 . Thus, the supporter  240  is fastened to the bottom chassis  210  by sandwiching the bottom chassis  210  between the supporting plate  242  and the hook  243 . The shape of the hook  243  illustrated in the drawings is only exemplarily provided and may be variously modified. 
     In some embodiments, the supporting plate  242  secures a wide contact area with the bottom chassis  210  such that the supporter  240  does not shake and is stably fastened. Accordingly, the supporting plate  242  may have a flat shape, and may be a circular plate or a polygonal plate, such as, for example, a quadrangular plate, a pentagonal plate, or the like. 
     The supporter  240  is fastened to position the supporting plate  242  in the recess  214  of the bottom  210   a  of the bottom chassis  210 . The penetration hole  215  is formed at a center of the recess  214 , and the hook  243  of the supporter  240  is inserted into the penetration hole  215  to be closely attached to the bottom surface of the recess  214 . A depth d 2  of the recess  214  may be greater than a thickness of the supporting plate  242 , such that the supporting plate  242  of the supporter  240  is positioned at the same height as or lower than the base portion  211  that forms the overall flat surface of the bottom  210   a . Accordingly, when viewed in a cross-section, a top surface of the supporting plate  242  is disposed at the same height as or lower than the top surface of the base portion  211 . As a result, the supporting plate  242  is flatly covered by the reflective sheet  250 , undesirable scattering and reflection of light due to the supporting plate  242  is prevented. In addition, the reflective sheet  250  is prevented from protruding and being lifted near the supporting plate  242 . 
     The reflective sheet  250  is positioned on the bottom chassis  210 . The reflective sheet  250  allows light emitted from the optical lens  230  and light reflected by other structures, such as the diffuser  261 , to be reflected and directed toward the LCD panel  100 , thereby improving optical efficiency. 
     The entire reflective sheet  250  may be mounted on an inner surface of the bottom chassis  210 . For example, the reflective sheet  250  may include a flat portion  250   a , a wing  250   b  extending from an edge of the flat portion  250   a  in a substantially slanted and upward direction, and a rim  250   c  extending substantially horizontally from the wing  250   b , such that they respectively correspond to the bottom  210   a , the wing  210   b , and the rim  210   c  of the bottom chassis  210 . The reflective sheet  250  may be formed of a plastic material such as polyethylene terephthalate (PET), polycarbonate (PC), or polystyrene (PS). The reflective sheet  250  may include a light reflective material, such as titanium dioxide TiO 2 , to increase light reflectance. 
     The flat portion  250   a  of the reflective sheet  250  is formed with a first hole  253  through which the optical lens  230  can pass and a second hole  254  through which the supporting portion  241  of the supporter  240  can pass. Accordingly, when the reflective sheet  250  is mounted onto the bottom chassis  210 , as shown in  FIG. 3 , the optical lens  230  is exposed above the reflective sheet  250  through the first hole  253 , and the supporting portion  241  is exposed above the reflective sheet  250  through the second hole  254 . However, a part of the substrate  221  where the optical lens  230  is not positioned is covered by the reflective sheet  250 . The supporting plate  242  of the supporter  240  is also covered by the reflective sheet  250 . Accordingly, an entire inner surface of the bottom chassis  210  where the optical lens  230  and the supporting portion  241  of the supporter do not protrude above the reflective sheet  250  is covered by the reflective sheet  250 . A space defined by the reflective sheet  250  and the diffuser  261 , as described later, is herein referred to as an optical space of the backlight unit  200 . 
     The first hole  253  may have a shape that corresponds to a horizontal cross-sectional shape of the optical lens  230 , and may have, for example, a circular shape. The first hole  253  may be of a size through which a thickest part (i.e., part having the largest cross-sectional area) of the optical lens  230  can pass. 
     The second hole  254  may have a shape that corresponds to a horizontal cross-sectional shape of the supporting portion  241 . For example, the second hole  254  may have a circular shape if the supporting portion  241  has a circular horizontal cross-sectional shape, and the second hole  254  may have a quadrangular shape if the supporting portion  241  has a quadrangular horizontal cross-sectional shape. The second hole  254  may be of a size through which the thickest part of the supporting portion  241  can pass. For example, when the second hole  254  and the supporting portion  241  have a circular horizontal cross-sectional shape, an inner diameter of the second hole  254  is the same as or nearly the same as an outer diameter of the thickest part of the supporting portion  241 . 
     A vicinity of the first hole  253  through which the optical lens  230  passes may be lifted up in the flat portion  250   a  of the reflective sheet  250 . In this case, the lifted part has an adverse effect on a profile of the light transmitted through the optical sheet  260  to be provided to the LCD panel  100  because it reflects light differently than the rest of the flat portion  250   a . For example, the light may be displayed as being locally brighter or darker on the entire light emitting surface. Accordingly, to prevent the reflective sheet  250  from being lifted, the vicinity of the first hole  253  may be attached to the substrate  221  or the base portion  211  of the bottom chassis  210  by a double-sided adhesive tape (not shown) or an adhesive. 
     The optical sheet  260  is positioned above the bottom chassis  210  and the reflective sheet  250 . Accordingly, the optical sheet  260  is positioned above the light source  222 , the optical lens  230 , and the supporter  240 . An edge of the optical sheet  260  may be placed on the rim  210   c  of the bottom chassis  210 . In this case, the rim  250   c  of the reflective sheet  250  may be positioned between the edge of the optical sheet  260  and the rim  210   c  of the bottom chassis  210 . The optical sheet  260  is supported by the supporting portion  241  of the supporter  240  protruding through the second hole  254  of the reflective sheet  250  and therefore does not droop while maintaining a predetermined optical path. 
     The optical sheet  260  may include a diffuser  261 , a prism sheet  262 , a protecting sheet  263 , and the like. The diffuser  261  is used to scatter light to produce a surface light source of uniform brightness. The prism sheet  262  is used to control the traveling direction of the light such that the light is concentrated, thereby improving luminance. The protecting sheet  263  is used to protect a prism of the prism sheet  262  from being scratched and the like. Further, the protecting sheet  263  may widen a viewing angle that is narrowed by the prism sheet  262 . 
     The optical sheet  260  may exclude one of the prism sheet  262  and the protecting sheet  263 , while including a plurality of the others. The optical sheet  260  may further include an optical sheet having characteristics other than those described above. For example, the optical sheet  260  may include a reflective polarizer sheet that improves luminance efficiency by separating, transmitting, and reflecting polarization components of the light. 
     Though not illustrated, an inverter board and/or a printed circuit board (PCB) for signal conversion and supplying power may be mounted as a printed circuit board (PCB) on a lower surface of the bottom chassis  210 . The inverter board converts an external power supply into a constant voltage level and supplies it to the light source  222 . The printed circuit board (PCB) for signal conversion may convert an analog data signal into a digital data signal and transmit it to the LCD panel  100  through the flexible printed circuit board attached to the LCD panel  100 . 
     The LCD may include a mold frame  300  in which the LCD panel  100  is stably fixed onto the backlight unit  200  with a predetermined height therebetween. The mold frame  300  may be a square frame of a substantial cuboidal shape with top and bottom sides open. The mold frame  300  may be combined with the bottom chassis  210 . For example, the mold frame  300  may be hooked and fixed to a hook (not shown) and the like that are positioned in the wall  210   d  of the bottom chassis  210  and enclose the rim  210   c  of the bottom chassis  210 . In this case, a part of the mold frame  300  presses the edge of the optical sheet  260  that is placed on the rim  210   c  of the bottom chassis  210 , so movement of the optical sheet  260  may be limited. The LCD panel  100  is fixed onto the mold frame  300 . The LCD panel  100  may be attached to the flat surface of the mold frame  300  through an adhesion member (not shown), which may be a double-sided cushion tape with impact-absorbing capability for cushioning an impact applied to the LCD panel  100 . 
     In a backlight unit  200  having the aforementioned structure, a process of supplying light to an LCD panel  100  is now briefly described. First, when power is supplied to a light source  222  through a connecting board  223  and a substrate  221 , the light source  222  emits light. The emitted light is refracted and diffused to lateral sides while passing through an optical lens  230  and is reflected by a reflective sheet  250  toward an optical sheet  260 . Some of the light emitted from the optical lens  230  may be directly directed toward the optical sheet  260 . Then, the light is diffused while being transmitted through the optical sheet  260 , and its direction of travel is controlled, thereby supplying the light to an entire surface of the LCD panel  100 . 
     An assembling process of a backlight unit according to an exemplary embodiment of the present disclosure is now described with reference to  FIGS. 5 to 9 . 
       FIGS. 5 to 9  are schematic diagrams for illustrating an assembly process of a backlight unit according to an exemplary embodiment of the present disclosure. 
     In  FIGS. 5 to 9 , two light source assemblies of a bottom chassis  210  and one supporter  240  installed therebetween are illustrated, and they are relatively exaggerated, but it should be understood that this is only to simplify description of the assembling sequence and the relation of respective components, as well as to avoid an overly complicated drawing. 
     Referring to  FIG. 5 , a bottom chassis  210  is prepared. In the bottom chassis  210 , a groove  212  for mounting a light source assembly and a recess  214  for mounting a supporter  240  are formed. The groove  212  and the recess  214  may have corresponding shapes and depths to accommodate the substrate  221  of the light source assembly. The recess  214  may have a corresponding shape to accommodate a supporting plate  242  of a supporter  240 . A penetration hole  215  through which a hook  243  of the supporter  240  may be inserted is formed in the recess  214 . 
     Referring to  FIG. 6 , the light source assembly is mounted on the groove  212  of the bottom chassis  210 . The light source assembly includes a substrate  221 , and a light source  222  and an optical lens  230  that are mounted on the substrate  221 . Since the groove  212  of the bottom chassis  210  is formed to have a depth greater than a thickness of the substrate  221 , the substrate  221  does not protrude beyond a base portion  211  of the bottom chassis  210  after the light source assembly is mounted on the groove  212 . 
     Referring to  FIG. 7 , the supporter  240  is mounted on the recess  214  of the bottom chassis  210 . Mounting of the supporter  240  may be performed by pushing the hook  243  into the penetration hole  215  that is provided in the recess  214 . The hook  243  is stopped by a bottom surface of the recess  214  after passing through the penetration hole  215 , and in this case, the supporting plate  242  is closely attached to a top surface of the recess  214 . Accordingly, as a locking portion of the supporter  240 , the supporting plate  242  and the hook  243  are interlocked with the top and bottom surfaces of the recess  214  to fix the supporter  240  to the bottom chassis  210 . In some exemplary embodiments, the hook  243  may be inserted into the penetration hole  215  such that it is tightly fitted into the penetration hole  215  and is fixed thereto. Since the recess  214  of the bottom chassis  210  is formed to have a depth greater than a thickness of the supporting plate  242 , the supporting plate  242  does not protrude beyond the base portion  211  of the bottom chassis  210  after the supporter  240  is mounted on the recess  214 . 
     When the locking portion has a structure of the supporting plate  242  and the hook  243 , the supporter  240  may be locked with the bottom chassis  210  by pushing the hook  243  into the penetration hole  215  that is provided in the bottom chassis  210 . Accordingly, compared with other locking means, such as, a bolt and nut, a screw, an adhesive, etc., assembly time may be reduced, and the supporter  240  can be easily separated when being disassembled for repair. 
     In some exemplary embodiments, the supporter  240  is described to be mounted after mounting the light source assembly, but the light source assembly may be mounted after mounting the supporter  240 . 
     Referring to  FIG. 8 , a reflective sheet  250  is placed on the bottom chassis  210 . A first hole  253  through which the optical lens  250  can pass and a second hole  254  through which the supporting portion  241  of the supporter  240  can pass are formed in reflective sheet  250 . When placing the reflective sheet  250  on the bottom chassis  210 , it should be correctly positioned. Since the second hole  254  of the reflective sheet  250  is formed to correspond to the supporting portion  241 , and the supporting portion  241  protrudes from the bottom  210   a  of the bottom chassis  210 , the reflective sheet  250  may be correctly positioned by simply fitting the second hole  254  into the supporting portion  241 . When a plurality of supporters  240  are provided in the bottom chassis  210 , the plurality of supporting portions  241  can be used as a guide so that the reflective sheet  250  is correctly placed at an exact location. 
     Since the supporting portion  241  of the supporter  240  serves as an installation guide of the reflective sheet  250 , there is no need to form an additional guide protrusion and an additional hole in the reflective sheet  250  to correctly position the bottom chassis  210 . If the guide protrusion and the additional hole are formed, the guide protrusion is not covered by the reflective sheet  250 , but is exposed by the additional hole, so the guide protrusion may look relatively darker due to a difference in reflectance between the guide protrusion and the reflective sheet  250 . According to the exemplary embodiment of the present disclosure, the guide protrusion and the additional hole are not formed, so the aforementioned problems do not occur. 
     As described above, after mounting the supporter  240 , the reflective sheet  250  is placed on the bottom chassis  210  using the supporting portion  241  of the supporter  240  as the guide. After the reflective sheet  250  is placed thereon, the optical lens  230  is placed above the reflective sheet  250  through the first hole  253 , and the supporting portion  241  is placed above the reflective sheet  250  through the second hole  254 . As such, except for exposed portions above the reflective sheet  250  through the first and second holes  253  and  254 , an inner surface of the bottom chassis  210  and the components mounted thereon (e.g., the substrate  221  of the light source assembly and the supporting plate  242  of the supporter) may be completely covered by the reflective sheet  250 . In addition, since the substrate  221  mounted with the optical lens  230  and the supporting plate  242  of the supporter  240  are respectively accommodated in the groove  212  and the recess  214 , the reflective sheet  250  may have an overall flat surface with no protruding or lifted parts. Accordingly, the exposed portions above the reflective sheet  250  can be minimized to reduce unnecessary light scattering, so the reflective sheet  250  reflects the light uniformly. 
     Prior to the installation of the reflective sheet  250 , an adhesion member, such as a double-sided adhesive tape, may be applied around the first hole  253 . In this case, since the reflective sheet  250  is attached to the bottom chassis  210  or the substrate  221  after the installation of the reflective sheet  250 , lifting of the reflective sheet  250  is prevented. 
     Referring to  FIG. 9 , an edge of the optical sheet  260  is placed on the rim  210   c  of the bottom chassis  210  to be supported thereby. In this case, an inner part of the optical sheet  260  may be supported by the supporting portion  241  of the supporter  240 . Accordingly, the optical sheet  260  does not droop and uniformly maintains a predetermined distance with the light source  222 . 
     A mold frame  300  may be mounted on the backlight unit  200  in which the bottom chassis  210  are assembled with the light source assembly, the supporter  240 , the reflective sheet  250 , and the optical sheet  260 . In addition, an LCD may be assembled by placing an LCD panel  100  on the mold frame  300  and then installing a top chassis  400  to enclose an edge of the LCD panel  100 . 
     Optical characteristics of cases in which the supporting plate  242  of the supporter  240  is positioned below and above the reflective sheet  250  inside the recess  214  according to exemplary embodiments of the present disclosure are now described with reference to  FIGS. 10 to 15 . 
       FIGS. 10 and 11  are drawings that illustrate light reflectance and a luminance profile of the backlight unit according to an exemplary embodiment of the present disclosure.  FIGS. 12 and 13  are drawings that illustrate light reflectance and a luminance profile of a backlight unit according to a comparative example.  FIG. 14  is a drawing that illustrates luminance distribution of light emitted from a backlight unit according to an exemplary embodiment of the disclosure.  FIG. 15  is a graph illustrating a luminance profile of a B-B region of  FIG. 14  together with that of a comparative example. 
     Referring to  FIGS. 10 and 11 , when light emitted from the light source  222  is transmitted through the optical lens  230 , which in this case is a side emitting lens, it is refracted toward sides of the optical lens  230 . The light refracted toward the sides of the optical lens  230  is partially directed toward the supporter  240 . Since the supporting portion  241  of the supporter  240  protrudes above the reflective sheet  250 , the supporting portion  241  may scatter the light. However, since the supporting plate  242  of the supporter  240  is positioned lower than the reflective sheet  250 , light scattering is not caused by the supporting plate  242 . As a result, luminance does not significantly increase near the supporter  240 , and a relatively uniform luminance profile is obtained. 
     Unlike the aforementioned exemplary embodiment, in comparative examples illustrated in  FIGS. 12 and 13 , both the supporting portion  241  and the supporting plate  242  of the supporter  240  are exposed above the reflective sheet  250 . In this case, the supporting plate  242  may be useful to prevent the reflective sheet  250  from being lifted because it is operated to press the reflective sheet  250 , but uniformity of the luminance profile deteriorates due to light scattering that is caused by the exposed supporting plate  242 . The light emitted from the sides of the optical lens  230  to be directed toward the supporter  240  may be scattered upward by the supporting plate  242 , and therefore the luminance where the supporter  240  is positioned increases, as shown in  FIG. 13 . 
       FIG. 14  is a drawing that illustrates a luminance distribution of light emitted from the backlight unit  200  according to an exemplary embodiment of the present disclosure. In the luminance distribution, the light emitted above the optical sheet  260  of backlight unit  200  is photographed by a camera, and parts with relatively high luminance are represented by red, and parts with relatively low luminance are represented by blue. In other words, the luminance is higher closer to red and is lower closer to blue. In this case, the backlight unit  200  has a total of 45 light sources  222  arranged in a matrix form and 8 supporters  240  arranged at predetermined intervals. There are virtually no differences observed in luminance between centers of circled portions and vicinities thereof in the drawing. 
       FIG. 15  illustrates a luminance profiles corresponding to the solid B-B line shown in  FIG. 14  for an exemplary embodiment and a comparative example. In the drawing, dotted lines represent positions of the light sources  222 , while a digit on a horizontal axis represents a relative location of the light source  222  from a center column of the backlight unit  200 . As described above, in the exemplary embodiment, the supporting plate  242  of the supporter  240  is positioned lower than the reflective sheet  250 , and is covered by the reflective sheet  250 . As shown in the graph of  FIG. 15 , a point where the supporter  240  is positioned shows slightly higher luminance than a vicinity thereof. Presumably, this is because the light scattering due to the supporting portion  241  of the supporter  240  has caused local luminance to increase to a certain degree. 
     Further, in  FIG. 15 , a luminance profile of the comparative example in which the supporting plate  242  of the supporter  240  is positioned above the reflective sheet  250  is shown, while the other conditions that are the same as in the exemplary embodiment of  FIG. 14  are represented by an alternated long and short dash line. In this case, the luminance of points where the supporting bodies  240  are positioned are relatively higher than the vicinities thereof, and an overall luminance profile is not as uniform as the exemplary embodiment of  FIG. 14 . In other words, when the supporting plate  242  of the supporter  240  is positioned in the recess  214  of the bottom chassis  210 , and the supporting plate  242  is positioned to cover the reflective sheet  250 , in accordance with an exemplary embodiment of the present disclosure, the light scattering due to the supporting plate  242  does not occur, thereby minimizing the unnecessary light scattering by the supporter  240 . Accordingly, the luminance increase generated near the supporter  240  is minimized. 
     While the present system and method have been described in connection with exemplary embodiments, it is to be understood that the present system and method are not limited to the disclosed embodiments. On the contrary, the present system and method cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.