Patent Publication Number: US-11662624-B2

Title: Backlight unit and display device using the same

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 17/122,851, filed on Dec. 15, 2020, which claims the benefit of Korean Patent Application No. 10-2019-0177565, filed on Dec. 30, 2019, which is hereby incorporated by reference for all purposes as if fully set forth herein. 
    
    
     BACKGROUND 
     Technical Field 
     Embodiments of the present disclosure relate to a backlight unit and a display device including the backlight unit. 
     Description of the Related Art 
     As the information society develops, the demand for display devices for displaying images is increasing in various forms. Various types of display devices such as a liquid crystal display device (LCD), and an organic light emitting display device (OLED) have been used for this purpose. 
     The display device may include the backlight unit and may display the image in response to light emitted from the backlight unit. The display device may utilize a light emitting diode (LED), a cold cathode fluorescent lamp (CCFL), and a hot cathode fluorescent lamp (HCLF) as a light source of the backlight unit. Recently, the light emitting diodes having excellent light efficiency and high color reproducibility are widely used as light sources of the backlight units. 
     The backlight unit may be classified into an edge-type or a direct-type according to the arrangement of the light source and the transmission mode of light. In the direct-type backlight unit, the light source such as the LED may be disposed on the rear surface of the display device. The light source device used in the direct-type backlight unit may include the light emitting diode, a substrate including the light emitting diode and circuit elements for driving the light emitting diode. 
     Recently, for display devices employed in smartphones, tablet PCs, etc., there is an increasing demand for the light weight, low power consumption and high light efficiency. 
     BRIEF SUMMARY 
     Various embodiments of the present disclosure provide a backlight unit capable of improving image quality and a display device including such a backlight unit. 
     Various embodiments of the present disclosure provide a backlight unit having a high light efficiency and a display device including the same. 
     In accordance with one or more embodiments of the present disclosure, there may be provided with a backlight unit comprising: a plurality of light emitting elements disposed on a substrate and each having a flip chip structure; a reflector disposed between the plurality of light emitting elements and having a plurality of grooves each having a predetermined size on an upper surface of the reflector; and a transparent sheet disposed on the reflector and the plurality of light emitting elements and including a plurality of optical path changing patterns disposed at positions overlapping the plurality of light emitting elements, respectively, on an opposite side of a surface of the transparent sheet adjacent to the plurality of light emitting elements and each having a central region thicker than the outer region. 
     In accordance with one or more embodiments of the present disclosure, there may be provided with a display device comprising: a display panel; and a backlight unit disposed under the display panel and emitting light to the display panel, wherein the backlight unit includes, a plurality of light emitting elements disposed on a substrate and each having a flip chip structure; a reflector disposed between the plurality of light emitting elements and having a plurality of grooves each having a predetermined size on an upper surface of the reflector; and a transparent sheet disposed on the reflector and the plurality of light emitting elements and including a plurality of optical path changing patterns disposed at positions overlapping the plurality of light emitting elements, respectively, on an opposite side of a surface of the transparent sheet adjacent to the plurality of light emitting elements and each having a central region thicker than the outer region. 
     According to embodiments of the present disclosure, it is possible to provided with the backlight unit capable of improving image quality and the display device including the backlight unit. 
     According to embodiments of the present disclosure, it is possible to provided with the backlight unit having a high light efficiency and the display device including the backlight unit. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating a schematic configuration of a display device according to embodiments of the present invention; 
         FIG.  2    is a partial cross-sectional view illustrating a part of the display device according to embodiments of the present invention; 
         FIG.  3    is a partial cross-sectional view illustrating a substrate on which a plurality of light emitting elements and a reflector are disposed in a backlight unit according to an embodiment of the present invention; 
         FIGS.  4  and  5    are partial cross-sectional views illustrating backlight units according to embodiments of the present invention; 
         FIG.  6    is a partial plan view illustrating a part of the backlight unit; 
         FIGS.  7  and  8    are partial cross-sectional views illustrating backlight units according to embodiments of the present invention; and 
         FIGS.  9 A to  9 C  are diagrams illustrating a process of forming the backlight unit shown in  FIG.  2   . 
     
    
    
     DETAILED DESCRIPTION 
     In the following description of examples or embodiments of the present disclosure, reference will be made to the accompanying drawings in which it is shown by way of illustration specific examples or embodiments that can be implemented, and in which the same reference numerals and signs can be used to designate the same or like components even when they are shown in different accompanying drawings from one another. Further, in the following description of examples or embodiments of the present disclosure, detailed descriptions of well-known functions and components incorporated herein will be omitted when it is determined that the description may make the subject matter in some embodiments of the present disclosure rather unclear. The terms such as “including”, “having”, “containing”, “constituting” “make up of”, and “formed of” used herein are generally intended to allow other components to be added unless the terms are used with the term “only”. As used herein, singular forms are intended to include plural forms unless the context clearly indicates otherwise. 
     Terms, such as “first”, “second”, “A”, “B”, “(A)”, or “(B)” may be used herein to describe elements of the present invention. Each of these terms is not used to define essence, order, sequence, or number of elements, etc., but is used merely to distinguish the corresponding element from other elements. 
     When it is mentioned that a first element “is connected or coupled to”, “contacts or overlaps”, etc., a second element, it should be interpreted that, not only can the first element “be directly connected or coupled to” or “directly contact or overlap” the second element, but a third element can also be “interposed” between the first and second elements, or the first and second elements can “be connected or coupled to”, “contact or overlap”, etc., each other via a fourth element. Here, the second element may be included in at least one of two or more elements that “are connected or coupled to”, “contact or overlap”, etc., each other. 
     When time relative terms, such as “after,” “subsequent to,” “next,” “before,” and the like, are used to describe processes or operations of elements or configurations, or flows or steps in operating, processing, manufacturing methods, these terms may be used to describe non-consecutive or non-sequential processes or operations unless the term “directly” or “immediately” is used together. 
     In addition, when any dimensions, relative sizes, etc., are mentioned, it should be considered that numerical values for an elements or features, or corresponding information (e.g., level, range, etc.) include a tolerance or error range that may be caused by various factors (e.g., process factors, internal or external impact, noise, etc.) even when a relevant description is not specified. 
       FIG.  1    is a diagram illustrating a schematic configuration of a display device according to embodiments of the present disclosure. 
     Referring to  FIG.  1   , the display device  10  according to embodiments of the present disclosure may include a display panel  100  including an active area A/A and a non-active area N/A, a gate driving circuit  120  and a data driving circuit  130  for driving the display panel  100 , a controller  140  and the like. 
     In the display panel  100 , a plurality of gate lines GL and a plurality of data lines DL may be disposed, and multiple subpixels SP may be located in areas where the gate lines GL and the data lines DL intersect. Also, the display panel  100  may be a liquid crystal panel. The liquid crystal panel may include a pixel electrode, a common electrode, and a liquid crystal layer disposed between the pixel electrode and the common electrode. The liquid crystal layer may display an image by blocking or transmitting light by changing its molecular arrangement in response to a voltage applied to the pixel electrode and the common electrode. 
     The gate driving circuit  120  may be controlled by the controller  140 , and may sequentially output a scan signal to the plurality of gate lines GL arranged on the display panel  100 , thereby can control the driving timing of the multiple subpixels SP. The gate driving circuit  120  may include one or more gate driver integrated circuits (GDICs), and may be located on one side of the display panel  100  or on both sides according to a driving method. Each gate driver integrated circuit (GDIC) may be connected to a bonding pad of the display panel  100  by a tape-automated bonding (TAB) method or a chip-on-glass (COG) method, or may be implemented as a gate-in-panel (GIP) type and directly disposed on the display panel  100 . Alternatively, each gate driver integrated circuit (GDIC) may be integrated and disposed in the display panel  100 . Further, each gate driver integrated circuit (GDIC) may be implemented in a chip on film (COF) method in which each gate driver integrated circuit (GDIC) is mounted on a film connected to the display panel  100  and electrically connected to the display panel  100  through lines on the film. 
     The data driving circuit  130  may receive image data DATA from the controller  140  and convert the image data DATA to an analog data voltage. The data driving circuit  130  may output the analog data voltage to each data line DL according to the timing at which the scan signal is applied through the gate line GL, so that each subpixel SP can express brightness according to the image data DATA. The data driving circuit  130  may include one or more source driver integrated circuits (SDICs). Each source driver integrated circuit (SDIC) may include a shift register, a latch circuit, a digital-to-analog converter, and an output buffer, however, is not limited thereto. 
     Each source driver integrated circuit (SDIC) may be connected to the bonding pad of the display panel  100  by the tape automated bonding (TAB) method or the chip-on-glass (COG) method, or directly disposed on the display panel  100 , or, in some cases, may be integrated and directly disposed in the display panel  100 . In addition, each source driver integrated circuit (SDIC) may be implemented in a chip-on-film (COF) method in which each source driver integrated circuit (SDIC) is mounted on a film connected to the display panel  100  and electrically connected to the display panel  100  through lines on the film. 
     The controller  140  may supply various control signals to the gate driving circuit  120  and the data driving circuit  130 , and can control the operation of the gate driving circuit  120  and the data driving circuit  130 . The controller  140  may be mounted on a printed circuit board, a flexible printed circuit, or the like, and may be electrically connected to the gate driving circuit  120  and the data driving circuit  130  through a printed circuit board, a flexible printed circuit, or the like. The controller  140  may control the gate driving circuit  120  to output the scan signal according to the timing implemented in each frame, and may output the converted image data by converting the externally received image data according to the data signal format used by the data driving circuit  130  to the data driving circuit  130 . The controller  140  may receive various timing signals including vertical synchronizing signal (VSYNC), horizontal synchronizing signal (HSYNC), input data enable signal (DE), clock signal (CLK), and the like along with image data from the outside (e.g., a host system). 
     The controller  140  may generate various control signals using various timing signals received from the outside and output the control signals to the gate driving circuit  120  and the data driving circuit  130 . For example, in order to control the gate driving circuit  120 , the controller  140  may output various gate control signals GCS including a gate start pulse (GSP), a gate shift clock (GSC), and a gate output enable signal (GOE), etc. Here, the gate start pulse GSP may control the operation start timing of one or more gate driver integrated circuits (GDICs) constituting the gate driving circuit  120 . The gate shift clock (GSC) is a clock signal commonly input to one or more gate driver integrated circuits (GDICs), and can control the shift timing of the scan signal. The gate output enable signal (GOE) may specify timing information of one or more gate driver integrated circuits (GDICs). 
     In addition, in order to control the data driving circuit  130 , the controller  140  may output various data control signals DCS including a source start pulse (SSP), a source sampling clock (SSC), and a source output enable signal (SOE), etc. Here, the source start pulse (SSP) may control the data sampling start timing of one or more source driver integrated circuits (SDICs) constituting the data driving circuit  130 . The source sampling clock (SSC) may be a clock signal that controls sampling timing of data in each of the source driver integrated circuits (SDICs). The source output enable signal (SOE) may control the output timing of the data driving circuit  130 . 
     The display device  10  may further include a power management integrated circuit which supplies various voltages or currents to the display panel  100 , the gate driving circuit  120 , and the data driving circuit  130 , or controls various voltages or currents to be supplied. 
       FIG.  2    is a partial cross-sectional view illustrating a part of the display device according to embodiments of the present disclosure, and  FIG.  3    is a partial cross-sectional view illustrating a substrate on which a plurality of light emitting elements and a reflector are disposed in a backlight unit according to an embodiment of the present invention. 
     Referring to  FIGS.  2  and  3   , the display device  10  according to embodiments of the present disclosure may include the display panel  100  and the backlight unit  200  disposed below the display panel  100  and supplying light to the display panel  100 . 
     A plurality of structures may be disposed between the backlight unit  200  and the display panel  100 . As an example, a guide panel  400  and a foam pad  500  may be included. The display panel  100  may be disposed on the backlight unit  200  by the guide panel  400  and the foam pad  500 . 
     The display device  10  may include a cover bottom  300  that accommodates the backlight unit  200 . The substrate  210  may be disposed on the cover bottom  300 , and a plurality of light emitting elements  211  may be disposed on the substrate  210 . 
     The substrate  210  may be a printed circuit board. The substrate  210  may be coated with a reflective film. The reflective film may include a white pigment. However, it is not limited thereto. The reflective film may reflect light irradiated to the substrate  210  in the direction of the display panel  100  or a transparent sheet  213 , which will be described later, to further increase the light efficiency of the backlight unit  200 . 
     The light emitting element  211  may be, for example, a light emitting diode (LED), or may be a small mini light emitting diode (Mini LED) or a small micro light emitting diode (μLED). In addition, the light emitting element  211  may have a flip chip structure. 
     The light emitting element  211  of the flip chip structure may be disposed in a form in which the chip type light emitting element  211  is mounted on the substrate  210 , thereby reducing the thickness of the backlight unit  200  and providing a light source having a wide emission angle and a high light efficiency. 
     The light emitting element  211  may emit light in a white wavelength band, or in some cases, emit light in a specific wavelength band (e.g., blue wavelength band). 
     In addition, a reflector  212  may be disposed on the substrate  210 . The reflector  212  may reflect light emitted from the light emitting elements  211  to increase light efficiency. The reflector  212  may include a plurality of grooves  212   g  each having a predetermined size formed on the upper surface of the reflector as shown in  FIG.  3   . The depth of the groove  212   g  may be at least 50 however, is not limited thereto. The reflector  212  may be white polyethylene terephthalate (PET). 
     The reflector  212  may include a plurality of protrusions protruding in the direction of the transparent sheet  213  from portions of the reflector adjacent to the plurality of light emitting elements  211 , and the plurality of grooves  212   g  may be located between the plurality of protrusions. 
     In addition, the transparent sheet  213  may be disposed on the substrate  210  on which the plurality of light emitting elements  211  are disposed. The transparent sheet  213  may protect the plurality of light emitting elements  211  and may provide a function of diffusing light emitted from the light emitting elements  211 . The transparent sheet  213  may include polymethyl methacrylate (PMMA), polycarbonate (PC), or glass, but is not limited thereto. 
     In addition, a plurality of optical path changing patterns  213 R may be disposed on the upper surface of the transparent sheet  213 . The optical path changing pattern  213 R may be disposed at a position overlapping the light emitting element  211 . The optical path changing pattern  213 R may reflect a part of light emitted from the light emitting element  211 . The other part of the light emitted from the light emitting element  211  may be absorbed by the optical path changing pattern  213 R or transmitted through the optical path changing pattern  213 R. 
     The light emitted from the light emitting element  211  may be reflected by the optical path changing pattern  213 R, so that it is possible to suppress the occurrence of hot spots in the backlight unit  200  due to the light emitted vertically. 
     In addition, the occurrence of hot spots of the backlight unit  200  is suppressed, so that, in the backlight unit  200 , it is possible to suppress the occurrence of mura, which is not uniform in the characteristics of the screen and is stained in the peripheral area of the light emitting element  211 . Accordingly, the luminance of light emitted from the backlight unit  200  may be uniform. 
     In addition, the light emitted from the light emitting element  211  may be reflected by the optical path changing pattern  213 R, the light path is converted in the direction of the substrate  210 , and then may be reflected back by the reflector  212  to proceed in the direction of the display panel  100 . At this time, since the light reflected from the optical path changing pattern  213 R is incident on the reflector  212  in a diagonal direction, the light irradiated to the reflector  212  by the optical path changing pattern  213 R and reflected back may pass through the transparent sheet  213  at a position remote from the light emitting element  211 . 
     Accordingly, even when the distance between the adjacent light emitting elements  211  is large due to the small number of light emitting elements  211  disposed on the substrate  210  of the backlight unit  200 , the amount of light irradiated to the region between the adjacent light emitting elements  211  may be increased by the optical path changing pattern  213 R. Therefore, the luminance uniformity of the backlight unit  200  may be constant. 
     As a result, the number of light emitting elements  211  disposed in the backlight unit  200  can be reduced, so that the manufacturing cost of the backlight unit  200  can be reduced. 
     A diffusion plate  214  for diffusing the light incident from the bottom may be disposed on the transparent sheet  213 . Then, one or more optical sheets  215  may be disposed on the diffusion plate  214 . The structure disposed on the transparent sheet  213  is not limited thereto. 
     In addition, an adhesive film may be disposed on the transparent sheet  213 . The adhesive film may be an optical clear adhesive (OCA) film. The diffusion plate  214  may be fixed on the transparent sheet  213  by the adhesive film. 
       FIGS.  4  and  5    are partial cross-sectional views illustrating backlight units according to embodiments of the present disclosure, and  FIG.  6    is a partial plan view illustrating a part of the backlight unit. 
     Referring to  FIGS.  4  and  5   , the backlight unit  200  may include the plurality of light emitting elements  211  disposed on the substrate  210  and each having a flip chip structure, and the reflector  212  disposed between the plurality of light emitting elements  211  and having the plurality of grooves  212   g  each having a predetermined size on the upper surface of the reflector. In addition, the backlight unit  200  may include the transparent sheet  213  disposed on the reflector  212  and the plurality of light emitting elements  211 , and the transparent sheet  213  may include the plurality of optical path changing patterns  213 R each of which is disposed at a position overlapping each of the light emitting elements on an opposite side of a surface of the transparent sheet adjacent to the plurality of light emitting elements  211  and has a central region thicker than the outer region. 
     Alternatively, as illustrated in  FIG.  4   , the optical path changing patterns  213 R may be formed on a surface of the transparent sheet adjacent to the diffusion plate  214  rather than a surface of the transparent sheet adjacent to the plurality of light emitting elements  211 , but is not limited thereto. 
     The reflector  212  may include a plurality of reflector reinforcements  212 R disposed in the plurality of grooves  212   g  disposed on the upper surface of the reflector. Also, the reflector reinforcement  212 R may include resin. The height of the reflector  212  may be uniform by the plurality of reflector reinforcements  212 R. The depth D 1  of the groove  212   g  may be at least 50 μm. In addition, the reflector reinforcement  212 R may include a material having a refractive index in the range of 1.4 to 1.6, however, is not limited thereto. In addition, the air layer may be disposed in the groove  212   g  disposed on the upper surface of the reflector  212  instead of the reflector reinforcement  212 R formed of resin. 
     The transparent sheet  213  may include polymethyl methacrylate (PMMA), polycarbonate (PC), or glass. However, it is not limited thereto. 
     The light (i) having the first path emitted from the light emitting element  211  passes through the transparent sheet  213  and is reflected in the direction of the substrate  211  from the optical path changing pattern  213 R to become the light (ii) having the second path. The optical path changing pattern  213 R has a shape in which the thickness of the central region is greater than the thickness of the outer region, so that the light (ii) having the second path reflected from the optical path changing pattern  213 R can be entered diagonally to the reflector  212 . 
     The light (ii) having the second path may become the light (iii) having the third path reflected from the lower surface of the transparent sheet  213 , and the light (iv) having the fourth path reflected from the boundary between the reflector  212  and the reflector reinforcement  212 R by using the reflector reinforcement  212 R disposed in the groove  212   g  of the reflector  212 . 
     The light (iv) having the fourth path may pass through the reflector reinforcement  212 R disposed on the reflector  212  and may be reflected at the boundary between the reflector  212  and the reflector reinforcement  212 R of the reflector  212 . Accordingly, the reflection point P 2  at which the light (iv) having the fourth path is reflected may be farther away from the light emitting element  211  than the reflection point P 1  at which the light (iii) having the third path is reflected. Therefore, light can travel to a position far from the light emitting element  211 . 
     Therefore, even if the distance between the adjacent light emitting elements  211  is large, the amount of light irradiated to the region between the adjacent light emitting elements  211  may be increased by the optical path changing pattern  213 R, so that the luminance uniformity of the backlight unit  200  may be constant. 
     In addition, a spacer  250  may be disposed between the transparent sheet  213  and the reflector  212  so that the distance between the transparent sheet  213  and the reflector  212  can be maintained over a predetermined distance. 
     As illustrated in  FIG.  4   , the optical path changing pattern  213 R may be formed by forming a semi-spherical engraved groove in the transparent sheet  213 , and filling the groove with a highly reflective material having high reflectance. Therefore, the optical path changing pattern  213 R may have a constant curvature along which the thickness of the optical path changing pattern  213 R is changed from the central region to the outer region. The highly reflective material may be a white-based UV curing type ink containing titanium oxide (TiO2) or a thermal curing type ink. However, the highly reflective material is not limited thereto. In addition, the reflectance of the highly reflective material may be 95% or more. 
     Alternatively, as shown in  FIG.  5   , in order to form the optical path changing pattern, the groove having a certain inclination may be formed in the transparent sheet  213 . Accordingly, the optical path changing pattern  213 R formed by filling the groove formed in the transparent sheet  213  with the highly reflective material having high reflectance may have a constant change in thickness from the central region to the outer region. 
     The maximum depth D 2  of the engraved groove formed in the transparent sheet  213  may be between 1.5 and 1 mm, and the radius R of the groove may be determined by multiplying the distance between the light emitting element and the transparent sheet  213  by tan 60°. However, it is not limited thereto. 
     In addition, the backlight unit  200  may further include a color resin  240  disposed to overlap the light emitting elements  211  and surrounded by the reflector  212 . The color resin  240  may have a circular shape as illustrated in  FIG.  6   . The light emitted from the light emitting element  211  may pass through the color resin  240  disposed at a position overlapping the light emitting element  211 . 
     The color resin  240  may include a resin and a phosphor. The resin included in the color resin  240  may be a UV curing resin or a thermosetting resin. The phosphor may excite the incident light so as to emit the light in a specific wavelength band. Accordingly, the light passing through the color resin  240  may be a specific color included in the color resin  240  or a color mixed with a specific color. For example, when the light emitting element  211  emits light in the first wavelength band (e.g., blue light), the color resin  240  may emit the light in the second wavelength band (e.g., green light) and the light in the third wavelength band light (e.g., red light) in response to the incident light. Accordingly, in the case that the light emitting element  211  emits blue light, light emitted from the light emitting element  211  may pass through the color resin  240 , and may be converted into white light and emitted. 
     In addition, the phosphor included in the color resin  240  may be yellow or green-red or yellow-red phosphor. 
     In addition, the diffusion plate  214  for diffusing the light incident from the bottom may be disposed on the transparent sheet  213 . 
     As described above, embodiments of the present disclosure include the transparent sheet  213  including the optical path changing pattern  213 R disposed at a position corresponding to the light emitting element  211 , and various optical elements, thereby improving the image quality represented by the backlight unit  200  while reducing the thickness of the backlight unit  200 . 
       FIGS.  7  and  8    are partial cross-sectional views illustrating backlight units according to embodiments of the present invention. 
     Referring to  FIGS.  7  and  8   , the backlight unit  200  may include the plurality of light emitting elements  211  disposed on the substrate  210  and each having a flip chip structure, and the reflector  212  disposed between the plurality of light emitting elements  211  and having the plurality of predetermined grooves formed on the upper surface of the reflector. In addition, the backlight unit  200  may include the transparent sheet  213  disposed on the plurality of light emitting elements  211 , and the transparent sheet  213  may include the plurality of optical path changing patterns  213 R each of which is disposed at a position overlapping each of the light emitting elements on an opposite side of a surface of the transparent sheet adjacent to the plurality of light emitting elements  211  and has the thickness of the central region greater than the thickness of the outer region. 
     The reflector  212  may include the plurality of reflector reinforcements  212 R disposed in the plurality of grooves  212   g  disposed on the upper surface of the reflector. Also, the reflector reinforcement  212 R may include resin. The height of the reflector  212  may be uniform by the plurality of reflector reinforcements  212 R. The depth D 1  of the groove  212   g  may be at least 50 μm. In addition, the reflector reinforcement  212 R may include a material having a refractive index in the range of 1.4 to 1.6, however, is not limited thereto. In addition, the air layer may be disposed in the groove  212   g  disposed on the upper surface of the reflector  212  instead of the reflector reinforcement  212 R formed of resin. 
     The transparent sheet  213  may include polymethyl methacrylate (PMMA), polycarbonate (PC), or glass. However, it is not limited thereto. 
     The light (i) having the first path emitted from the light emitting element  211  passes through the transparent sheet  213  and is reflected in the direction of the substrate  210  from the optical path changing pattern  213 R to become the light (ii) having the second path. The optical path changing pattern  213 R has a shape in which the thickness of the central region is greater than the thickness of the outer region. The light (ii) having the second path reflected from the optical path changing pattern  213 R can be entered diagonally to the reflector  212 . 
     The light (ii) having the second path may become the light (iii) having the third path reflected from the lower surface of the transparent sheet  213 , and the light (iv) having the fourth path reflected from the boundary between the reflector  212  and the reflector reinforcement  212 R by using the reflector reinforcement  212 R disposed in the groove  212   g  of the reflector  212 . 
     The light (iv) having the fourth path may pass through the reflector reinforcement  212 R disposed on the reflector  212  and may be reflected at the boundary between the reflector  212  and the reflector reinforcement  212 R of the reflector  212 . Accordingly, the reflection point P 2  at which the light (iv) having the fourth path is reflected may be farther away from the light emitting element  211  than the reflection point P 1  at which the light (iii) having the third path is reflected. Therefore, light can travel to a position far from the light emitting element  211 . 
     Therefore, even if the distance between the adjacent light emitting elements  211  is large, the amount of light irradiated to the region between the adjacent light emitting elements  211  may be increased by the optical path changing pattern  213 R, so that the luminance uniformity of the backlight unit  200  may be constant. 
     In addition, the spacer  250  may be disposed between the transparent sheet  213  and the reflector  212  so that the distance between the transparent sheet  213  and the reflector  212  can be maintained over a predetermined distance. 
     As illustrated in  FIG.  7   , the optical path changing pattern  213 R may be formed by forming a semi-spherical engraved groove in the transparent sheet  213  and disposing the reflection filter reflecting light having the first wavelength emitted from the light emitting element  211  in the groove. Therefore, the reflection filter may have a constant curvature along which the thickness of the reflection filter is changed from the central region to the outer region. The reflection filter may be formed by stacking the highly reflective material reflecting light having the first wavelength in the semi-spherical engraved groove. The reflection filter may be formed by stacking a plurality of reflection layers including the highly reflective material. The highly reflective material may include polyethylene terephthalate (PET). In addition, the reflection layer may include PET, and the reflection filter may be formed by stacking reflection layers containing PET. 
     In addition, a part of the light having the first wavelength emitted from the light emitting element  211  may be reflected in the first reflection layer and the other part may pass through the first reflection layer. Light passing through the first reflection layer may be partially reflected by the second reflection layer and the other part may pass through the second reflection layer. Finally, the reflection filter may cause light having the first wavelength emitted from the light emitting element  211  to be reflected in the direction of the reflector  212 . 
     In this case, the thickness and refractive index of the plurality of reflection layers stacked and disposed in the reflection filter may be the same or different from each other. In addition, the highly reflective material may include PC (Poly carbonate), PMMA (Poly methyl methacrylate), however, is not limited thereto. 
     In addition, in the optical path changing pattern  213 R as shown in  FIG.  8   , each of the grooves formed in the transparent sheet  213  may be formed to have a certain slope. Therefore, the optical path changing pattern  213 R formed by stacking the highly reflective material reflecting light having the first wavelength in the groove formed in the transparent sheet  213  may have a constant change in thickness from the central region to the outer region. 
     The maximum depth D 2  of the groove formed in the transparent sheet  213  may be between 0.5 and 1 mm, and the radius R of the groove may be determined by multiplying the distance between the light emitting element and the transparent sheet  213  by tan 60°. However, it is not limited thereto. 
     In addition, the phosphor sheet  241  may be disposed on the transparent sheet  213 . The phosphor sheet  241  may include a phosphor. Further, the phosphor may be excited by light emitted from the light emitting element  211  to emit light. The light emitted from the light emitting element  211  and the light emitted from the phosphor of the phosphor sheet  241  may be mixed to become white light. For example, when the light emitting element  211  emits light in the first wavelength band (e.g., blue light), the phosphor sheet  241  may react to the incident light so as to emit the light in the second wavelength band (e.g., green light) and the light in the third wavelength band (e.g., red light). Accordingly, in the case that the light emitting element  211  emits blue light, when light emitted from the light emitting element  211  passes through the phosphor sheet  241 , it may be converted into white light and emitted. 
     In addition, the phosphor included in the phosphor sheet  241  may be yellow, green-red or yellow-red phosphor. 
     In addition, the phosphor sheet  241  may be disposed in some areas on the diffusion plate  214 . 
     As described above, embodiments of the present disclosure include the transparent sheet  213  including the optical path changing pattern  213 R disposed at a position corresponding to the light emitting element  211 , and various optical elements, thereby improving the image quality represented by the backlight unit  200  while reducing the thickness of the backlight unit  200 . 
       FIGS.  9 A to  9 C  are diagrams illustrating a process of forming the backlight unit shown in  FIG.  2   . 
     Referring to  FIGS.  9 A to  9 C , the plurality of light emitting elements  211  may be disposed on the substrate  210 . The reflective film may be coated and disposed on the substrate  210 . The coated reflective film may include the white pigment. That is, the surface of the substrate  210  may be white. 
     In addition, the reflector  212  may be disposed on at least some of the regions except the region in which the light emitting element  211  is disposed on the substrate  210 . 
     The reflector  212  may be formed in an area in which a subarea corresponding to the light emitting element  211  is opened and may be disposed on the substrate  210 . In addition, the reflector  212  may reflect light emitted from the light emitting element  211  to the front surface of the backlight unit  200  to increase the light efficiency of the backlight unit  200 . 
     In the case that the light emitting element  211  is disposed in the form of the chip, the light emitting element  211  may be implemented small, so that the height of the reflector  212  may be greater than the height of the light emitting element  211 . 
     Accordingly, light emitted in the lateral direction of the light emitting element  211  may be reflected from the side of the reflector  212  and may be emitted to the front surface of the backlight unit  200 , thereby further improving the light efficiency of the backlight unit  200 . 
     The reflector  212  may include the plurality of grooves formed in the upper surface thereof. Further, the resin  212 R may be disposed in the groove  212   g  formed on the upper surface of the reflector  212 . 
     In addition, the transparent sheet  213  may be disposed on the plurality of light emitting elements  211  and the reflector  212 . The transparent sheet  213  may include polymethyl methacrylate (PMMA), polycarbonate (PC), or glass, but is not limited thereto. 
     The transparent sheet  213  may include the plurality of optical path changing patterns  213 R each disposed at a position overlapping with the light emitting element  211 . The hot spots may be prevented from being generated in the backlight unit  200  by the optical path changing patterns  213 R. 
     The transparent sheet  213  may serve to protect the plurality of light emitting elements  211  disposed on the substrate  210 , and may also provide the function of the light guide plate by diffusing light emitted from the light emitting elements  211 . Therefore, the light emitted from the light emitting element  211  can be spread evenly to the upper surface of the transparent sheet  213  by the transparent sheet  213 . At this case, even if the reflector  212  adjusts the traveling direction of light by the transparent sheet  213  or the like, the intensity of light emitted in the vertical direction of the light emitting element  211  may be large, and accordingly, the uniformity of luminance may be reduced. 
     In embodiments of the present disclosure, the optical path changing pattern  213 R may be disposed at a position corresponding to the light emitting element  211  on the upper surface of the transparent sheet  213 , thereby reducing the thickness of the backlight unit  200  while improving the uniformity of luminance of the display panel  100 . 
     The plurality of optical path changing patterns  213 R may be disposed on the upper surface of the transparent sheet  213 . However, the present disclosure is not limited thereto, and the plurality of optical path changing patterns  213 R may be disposed on the lower surface of the transparent sheet  213 . Also, the spacer  250  may be disposed between the transparent sheet  213  and the reflector  212 . 
     Each of the plurality of optical path changing patterns  213 R disposed on the lower surface or the upper surface of the transparent sheet  213  may be located to correspond to each of the plurality of light emitting elements  211  disposed on the substrate  210 . For example, the optical path changing pattern  213 R may be arranged such that at least a portion of the optical path changing pattern  213 R overlaps with the light emitting element  211 , and when considering diffusion characteristics of light, the optical path changing pattern  213 R may be disposed to overlap the region including the region where the light emitting element  211  is disposed. 
     The optical path changing pattern  213 R may reflect light emitted from the light emitting element  211 . In addition, the optical path changing pattern  213 R may reflect light emitted in the vertical direction from the light emitting element  211  and may make it be reflected again by the reflector  212  so that may make the light emit to the region between the adjacent light emitting elements  211 . 
     As described above, the light emitted from the light emitting element  211  is reflected by the optical path changing pattern  213 R and the reflector  212 , thereby improving the image quality of the backlight unit  200 . 
     In addition, the color resin  212 R may be disposed and surrounded by the reflector  212  at a position overlapping with the light emitting element  211 . The color resin  212 R may make light emitted from the light emitting element  211  become white light. 
     The diffusion plate  214  may be disposed on the transparent sheet  213 . In addition, the phosphor sheet  241  may be disposed between the transparent sheet  213  and the diffusion plate  214 . However, the present disclosure is not limited thereto, and the phosphor sheet  241  may be disposed on the diffusion plate  214 . 
     The phosphor sheet  241  may include the phosphor having a specific color, and may excite incident light to emit light in a specific wavelength band. Therefore, the light passing through the phosphor sheet  241  may be a specific color included in the phosphor sheet  241  or a color mixed with a specific color. For example, when the light emitting element  211  emits light in the first wavelength band (e.g., blue light), the phosphor sheet  241  may react to incident light and emit the light in the second wavelength band (e.g., green light) and the light in the third wavelength band light (e.g., red light). Accordingly, in the case that the light emitting element  211  emits blue light, when light emitted from the light emitting element  211  passes through the phosphor sheet  241 , it may be converted into white light and be emitted. 
     In addition, one or more optical sheets  215  may be disposed on the phosphor sheet  241 . 
     The above description and the accompanying drawings provide an example of the technical idea of the present disclosure for illustrative purposes only. Those having ordinary knowledge in the technical field, to which the present disclosure pertains, will appreciate that various modifications and changes in form, such as combination, separation, substitution, and change of a configuration, are possible without departing from the essential features of the present disclosure. Therefore, the embodiments disclosed in the present disclosure are intended to illustrate the scope of the technical idea of the present disclosure, and the scope of the present disclosure is not limited by the embodiment. The scope of the present disclosure shall be construed on the basis of the accompanying claims in such a manner that all of the technical ideas included within the scope equivalent to the claims belong to the present disclosure. 
     The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.