Patent Publication Number: US-2010118222-A1

Title: Backlight unit

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2008-0111873, filed on Nov. 11, 2008, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a backlight unit, and more particularly, to a backlight unit having improved luminous efficiency. 
     2. Description of the Related Art 
     In general, electron emission devices may be classified as either a type which uses a hot cathode as an electron emission source or a type which uses a cold cathode as an electron emission source. Field Emission Devices (FEDs), Surface Conduction Emitters (SCEs), Metal Insulator Metals (MIMs), Metal Insulator Semiconductors (MISs), and Ballistic electron Surface Emitting types (BSEs), among others, are known as cold cathode type electron emission devices. 
     FED type electron emission devices are based on the principle that when a material having a low work function or a high beta function is used as an electron emission source, electrons are easily emitted in a vacuum due to an electric field difference. The following types of FED type electron emission devices have been developed: ones which employ a tapered tip structure formed of, for example, molybdenum (Mo) or silicon (Si) as a main component, ones which employ a carbonaceous material such as graphite or Diamond-Like Carbon (DLC), and ones which employ a nano material such as nanotubes or nanowires, as electron emission sources. 
     FED type electron emission devices may be largely classified as either top gate type or under gate type, according to an arrangement of a cathode electrode and a gate electrode. FED type electron emission devices may also be classified as diode type, triode type, or tetrode type according to the total number of electrodes used. In such a conventional electron emission device, electrons are emitted from an electron emission source due to an electric field formed between a cathode electrode and a gate electrode. That is, electrons are emitted from the electron emission source disposed around one of a cathode electrode and a gate electrode, which operates as a cathode. The emitted electrons travel toward the other of the cathode electrode and the gate electrode, which operates as an anode, in a beginning stage, and are then accelerated toward a phosphor layer due to a strong electric field of an anode electrode. 
     In general, a backlight unit, which is a type of electron emission device, includes a polarizer. The polarizer is a type of device which obtains linear polarized light based on the principle that the color of transmitted polarized light varies according to the location of an optical isomer. In general, the transmissivity of non-polarized light that is primarily incident on a polarizer is about 30 to 40 percent (%) and the remaining roughly 60 percent (%) of the non-polarized light is absorbed or reflected by the polarizer. Absorption or reflection of light by the polarizer is a main factor of degradation of luminous efficiency of a backlight. 
     In order to solve such a problem, various methods of using a wire grid polarizer have been introduced. The wire grid polarizer is formed by performing line patterning on a conductor in order to divide the conductor into regions having dimensions similar to a wavelength of light of a region of interest, so that electric-field oscillating components that intersect a grid may pass through the grid, but the energy of electric-field oscillating components that are not correctly aligned with respect to the grid is partially absorbed or reflected by the grid. 
     The wire grid polarizer has an advantage in that an optical reproducing efficiency is high since internal absorption is less than in a conventional polarizer, and the transmissivity of selectively transmitted polarized light and the rate of back reflection of non-selected polarized light are higher than in a conventional polarizer. In this case, conversion of polarized light, such as oval polarized light, should be performed in order to reuse light reflected in a rear surface. 
     In a conventional backlight unit, a diffusion sheet is disposed between an optical source and a wire grid polarizer, and thus, light reflected on a rear surface in an optical path passes through the diffusion sheet two or more times, thereby causing an optical loss. 
     Even if the wire grid polarizer is disposed between the optical source and the diffusion sheet, the wire grid polarizer and the optical source are installed to be separated from each other. Thus an optical loss occurs in a space between the wire grid polarizer and the optical source. 
     Also, an increase in the optical diffusion performance of the diffusion sheet results in a reduction in the transmissivity of light, and therefore, the diffusion sheet itself causes an optical loss. 
     Furthermore, in an electron emission type backlight unit, since voltage is applied to an anode surface, static electricity is induced onto an external surface of a substrate. The induced static electricity is one of a number of factors that cause a device to operate unstably, and may cause unnecessary particles, such as dust, to stick to an external surface of the backlight unit. 
     SUMMARY OF THE INVENTION 
     Exemplary embodiments of the present invention provide a backlight unit having improved luminous efficiency. 
     According to an aspect of an exemplary embodiment according to the present invention, there is provided a backlight unit which functions as a surface light source of a non-emissive display device, the backlight unit including a wire grid polarizer being formed in a single body. 
     The non-emissive display device may include a liquid crystal display (LCD) panel, and the wire grid polarizer may be on a surface of the backlight unit and may be configured to face the LCD panel. 
     A static electricity prevention layer may be on the surface of the backlight unit and may be configured to face the LCD panel. 
     The static electricity prevention layer may be grounded. 
     The wire grid polarizer may be for scattering light emitted from the backlight unit. 
     The wire grid polarizer may be grounded. 
     According to another aspect of an exemplary embodiment according to the present invention, there is provided a backlight unit including a first substrate and a second substrate facing each other; a plurality of first electrodes on the first substrate; a plurality of second electrodes on the first substrate and electrically insulated from the first electrodes; an electron emission source on the first substrate and electrically connected to the first electrodes; a phosphor layer on the second substrate for emitting light corresponding to the electron emission source; a third electrode on the second substrate for accelerating electrons emitted from the electron emission source toward the phosphor layer; and a wire grid polarizer on the second substrate for polarizing light emitted from the phosphor layer. 
     A liquid crystal display (LCD) panel may be at one side of the backlight unit, and the wire grid polarizer may be on a surface of the second substrate facing the LCD panel. 
     The wire grid polarizer may be for scattering light emitted from the phosphor layer. 
     A side of the wire grid polarizer opposite a side of the wire grid polarizer facing the second substrate may be configured to face a diffusion sheet for scattering light emitted from the phosphor layer. 
     The wire grid polarizer may be grounded. 
     The phosphor layer may be on the third electrode, and the wire grid polarizer may be on the phosphor layer. 
     The phosphor layer and the wire grid polarizer may be adhered to each other. 
     The third electrode and the wire grid polarizer may be grounded. 
     A static electricity prevention layer may be on the second substrate. 
     The static electricity prevention layer may be grounded. 
     The third electrode may reflect light reflected from the wire grid polarizer. 
     The light reflected from the wire grid polarizer may be reflected again from the third electrode to change into oval polarized light. 
     According to yet another aspect of an exemplary embodiment according to the present invention, there is provided a liquid crystal display (LCD) including an LCD panel and a backlight unit for supplying light to the LCD panel, the backlight unit including: a first substrate and a second substrate facing each other, the second substrate positioned between the first substrate and the LCD panel; a plurality of first electrodes on the first substrate; a plurality of second electrodes on the first substrate and electrically insulated from the first electrodes; an electron emission source on the first substrate and electrically connected to the first electrodes; a phosphor layer on the second substrate for emitting light corresponding to the electron emission source; a third electrode on the second substrate for accelerating electrons emitted from the electron emission source toward the phosphor layer; and a wire grid polarizer on the second substrate for polarizing light emitted from the phosphor layer toward the LCD panel. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and aspects of the present invention will become more apparent by describing in detail exemplary embodiments thereof, with reference to the attached drawings, in which: 
         FIG. 1  is a partial perspective view of an electron emission type backlight unit according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of  FIG. 1  taken along the line II-II; 
         FIG. 3  is a cross-sectional view of an electron emission type backlight unit according to another embodiment of the present invention; 
         FIG. 4  is a cross-sectional view of an electron emission type backlight unit according to another embodiment of the present invention; and 
         FIG. 5  is a cross-sectional view of an electron emission type backlight unit according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a partial perspective view of an electron emission type backlight unit  100  including an electron emission device  101  according to an embodiment of the present invention.  FIG. 2  is a cross-sectional view of  FIG. 1  taken along the line II-II. A diffusion sheet  190  and a liquid crystal display (LCD) panel  200  illustrated in  FIG. 2  are not illustrated in  FIG. 1 . 
     As illustrated in  FIGS. 1 and 2 , the backlight unit  100  includes the electron emission device  101  and a front panel  102  that are aligned in parallel to form a light emitting space  103 , which is a vacuum space, and spacers  160  disposed to maintain a distance between the electron emission device  101  and the front panel  102 . 
     The electron emission device  101  includes a first substrate  110 , a plurality of first electrodes  120 , an insulating layer  130 , a plurality of second electrodes  140 , and a plurality of electron emission sources  150 . 
     Here, the first electrodes  120  and the second electrodes  140  are arranged to cross one another on the first substrate  110 . The insulating layer  130  is formed between the first and second electrodes  120  and  140  in order to electrically insulate the first and second electrodes  120  and  140  from each other. A plurality of electron emission holes  131  are respectively formed on locations where the first and second electrodes  120  and  140  cross each other. The electron emission sources  150  are respectively included in the electron emission holes  131 . 
     The first substrate  110  is a plate having a thickness (e.g., a predetermined thickness), and may be formed of quartz glass, glass containing an amount of impurities (e.g., Na), plate glass, a glass substrate coated with SiO 2 , or an aluminum oxide or ceramic substrate. In embodiments having a flexible display apparatus, the first substrate  110  may be formed of a flexible material. 
     The first and second electrodes  120  and  140  may be formed of an electrically conductive material that is well known in the art. For example, the first and second electrodes  120  and  140  may be formed of metals such as Al, Ti, Cr, Ni, Au, Ag, Mo, W, Pt, Cu, or Pd, or an alloy thereof. The first and second electrodes  120  and  140  may also be formed of a printed conductor including glass and a metal such as Pd, Ag, RuO 2 , or Pd—Ag, or a metal oxide thereof. The first and second electrodes  120  and  140  may also be formed of a transparent conductor such as In 2 O 3  or SnO 2 , or a semiconductor material such as polysilicon. 
     The insulating layer  130  insulates the first and second electrodes  120  and  140 . The insulating layer  130  may be formed of a general insulating layer. For example, the insulating material may be a silicon oxide, a silicon nitride, frit, or a similar material. Examples of the frit include, but are not limited to, PbO—SiO 2 -based frit, PbO—B 2 O 3 —SiO 2 -based frit, ZnO—SiO 2 -based frit, ZnO—B 2 O 3 —SiO 2 -based frit, Bi 2 O 3 —SiO 2 -based frit, and Bi 2 O 3 —B 2 O 3 —SiO 2 -based frit. 
     The electron emission sources  150  contain an electron emission material. The electron emission material may be a carbon nanotube (CNT) that has a low work function and a high beta function. In particular, the carbon nanotube has good electron emission characteristics, thus enabling a low voltage operation. Thus, an apparatus using a carbon nanotube as an electron emission source may be easily manufactured on a large scale. However, the electron emission material is not limited to a carbon nanotube, and may be, for example, a carbonaceous material such as graphite, diamond, or diamond-like carbon, or a nano material such as nanotube, nanowire, or nanorod. The electron emission material may also include a carbide-derived carbon. 
     The front panel  102  includes a second substrate  171  which may be transparent, a phosphor layer  175  which is disposed on the second substrate  171  and which is excited by electrons emitted from the electron emission device  101  for emitting visible light, a third electrode  173  which accelerates the electrons emitted from the electron emission device  101  toward the phosphor layer  175 , and a wire grid polarizer  180 . 
     The second substrate  171  may be formed of the same material as the first substrate  110  and may be transparent. 
     The third electrode  173  may be formed of the same material as the first or second electrodes  120  or  140 . That is, the third electrode  173  may be formed of a metal such as Al, Ti, Cr, Ni, Au, Ag, Mo, W, Pt, Cu, or Pd, or an alloy thereof; a printed conductor including glass and a metal such as Pd, Ag, RuO 2 , or Pd—Ag or a metal oxide thereof; a transparent conductor such as In 2 O 3  or SnO 2 ; or a semiconductor material such as polysilicon. Preferably, the third electrode  173  may be formed of an Al thin film. If the third electrode  173  is formed of an Al thin film, accelerated electrons may pass through the third electrode  173  to reach the phosphor layer  175 . The third electrode  173  may also function as a reflection plate. That is, the third electrode  173  may function as a reflection plate that reflects light reflected from a rear surface of the wire grid polarizer  180 . The relationship between the third electrode  173  as a reflection plate and the wire grid polarizer  180  is described in detail later. 
     The phosphor layer  175  may be formed of a cathode luminescence (CL) type phosphor which is excited by accelerated electrons to emit visible light. A phosphor that may be used in the phosphor layer  175  may be a red-emitting phosphor such as SrTiO 3 :Pr, Y 2 O 3 :Eu, or Y 2 O 3 S:Eu, a green-emitting phosphor such as Zn(Ga, Al) 2 O 4 :Mn, Y 3 (Al, Ga) 5 O 12 :Tb, Y 2 SiO 5 :Tb, or ZnS:Cu,Al, or a blue-emitting phosphor such as Y 2 SiO 5 :Ce, ZnGa 2 O 4 , or ZnS:Ag,Cl. Of course, the present invention is not limited to the above phosphors. 
     The wire grid polarizer  180  is disposed on the second substrate  171 , i.e., on, or attached to, a surface of the second substrate  171  which faces the diffusion sheet  190  and the LCD panel  200 . The wire grid polarizer  180  will be described in detail later. 
     In order to normally operate the backlight unit  100 , a space between the phosphor layer  175  and the electron emission device  101  must be maintained in a vacuum state. For this, the spacers  160  maintaining a distance between the phosphor layer  175  and the electron emission device  101 , and glass frit (not shown) for sealing the vacuum space between the phosphor layer  175  and the electron emission device  101 , may be further used. The glass frit is provided around the vacuum space to seal the vacuum space. 
     The diffusion sheet  190  is disposed on the backlight unit  100 , and particularly, on the wire grid polarizer  180 , and the LCD panel  200  is disposed on the diffusion sheet  190 . 
     The diffusion sheet  190  is disposed on the backlight unit  100  to scatter light emitted from the backlight unit  100 . 
     The LCD panel  200  includes an upper substrate  205  and a lower substrate  201  that face each other, a thin film transistor (TFT) array  202 , liquid crystal  203  and a color filter  204  which are disposed between the upper and lower substrates  205  and  201 , and a polarizer  206  disposed on the upper substrate  205 . The LCD panel  200  is well known in the art and thus will not be described in detail here. 
     The backlight unit  100  operates as follows. When a negative (−) voltage and a positive (+) voltage are respectively applied to the first electrodes  120  and the second electrodes  140  of the electron emission device  101 , electrons are emitted from the electron emission sources  150  toward the second electrode  140  due to an electric field formed between the first and second electrodes  120  and  140 . At this time, when a positive (+) voltage which is much higher than the positive (+) voltage applied to the second electrodes  140  is applied to the third electrode  173 , the electrons emitted from the electron emission sources  150  are accelerated toward the third electrode  173 . The electrons excite the phosphor layer  175  adjacent to the third electrode  173  to emit visible light. The emission of the electrons may be controlled by the voltage applied to the second electrodes  140 . 
     In some embodiments, a negative (−) voltage is not necessarily applied to the first electrodes  120  provided that an appropriate electric potential necessary for electron emission is formed between the first and second electrodes  120  and  140 . 
     The backlight unit illustrated in  FIGS. 1 and 2  is a surface light source and may be used as a backlight unit of a non-emissive display device such as a TFT-LCD. Furthermore, in order to display images instead of simply emitting a visible ray from a surface light source, or in order to use a backlight unit having a dimming function, the first and second electrodes  120  and  140  of the electron emission device  101  may be arranged to cross each other. For this, one of the first and second electrodes  120  and  140  may be formed to have a main electrode part and a branch electrode part. In some embodiments, the main electrode part intersects the other electrode of the first and second electrodes  120  and  140 , and the branch electrode part protrudes from the main electrode part to face the other electrode. The electron emission sources  150  may be formed on the branch electrode part or a part facing the branch electrode part. 
     The wire grid polarizer  180  of the backlight unit  100  according to the current embodiment will now be described in greater detail. 
     As described above, the wire grid polarizer  180  is formed by performing line patterning on a conductor in order to divide the conductor into regions having dimensions similar to a wavelength of light of a region of interest, so that electric-field oscillating components that intersect the grid may pass through the grid, but the energy of electric-field oscillating components that are not correctly aligned with respect to the grid is partially absorbed or reflected by the grid. 
     More specifically, when non-polarized light is incident on the wire grid polarizer  180 , all electric field vectors in a certain direction are reflected by a medium and other electric field oscillating components penetrate the medium. That is, when non-polarized light is incident on the wire grid polarizer  180 , all polarized light having electric-field components parallel to a metal wire is reflected, and polarized light of electric-field components perpendicular to the grid passes through the medium. Thus, current induced by the electric-field components parallel to the metal wire flows through the metal wire to scatter polarized energy that is in the same direction. Accordingly, linear polarized light may be generated by reflecting the polarized light in a direction parallel to the metal wire. 
     Here, the wire grid polarizer  180  reflects beams polarized in a direction parallel to the wire grid polarizer  180  and allows beams polarized in a direction perpendicular to the wire grid polarizer  180  to pass through from among diffused-polarized beams emitted from the phosphor layer  175 . The wire grid polarizer  180  is formed of devices or materials that may be divided into an S-polarizer and a P-polarizer to transmit or reflect light. The wire grid polarizer  180  may be manufactured using a metal material according to a combination of a lift-off method and one of a hologram lithography method and an e-beam lithography method. Also, a cycle of the wire grid polarizer  180  is not limited and may range from several nm to several μm. The efficiency and characteristics of the wire grid polarizer  180  may vary according to a line-width, cycle and thickness thereof. In one embodiment, a pattern dimension of the wire grid polarizer  180  of the backlight unit  100  is less than or equal to 1 μm, and the distance between patterns of the wire grid polarizer  180  is also less than or equal to 1 μm. 
     In the backlight unit  100  according to the current embodiment, the wire grid polarizer  180  is formed on the second substrate  171 . 
     More specifically, conventionally, since a diffusion sheet is installed between an optical source and a wire grid polarizer, reflected light may pass through the diffusion sheet two or more times, thereby causing an optical loss. Even if the wire grid polarizer is installed between the optical source and the diffusion sheet, the wire grid polarizer is separated from the optical source and thus an optical loss occurs in a space between the wire grid polarizer and the optical source. An optical loss also occurs due to the diffusion sheet itself. 
     In the backlight unit  100  according to the current embodiment, the wire grid polarizer  180  is combined with the second substrate  171 . Thus the backlight unit  100  and the wire grid polarizer  180  are combined in a single body. In this case, light emitted from the phosphor layer  175  of the backlight unit  100  is first polarized by the wire grid polarizer  180  and then passes through the diffusion sheet  190 . Light reflected from a rear surface of the wire grid polarizer  180  is substantially reflected by the third electrode  173  functioning as a reflection plate, thereby changing a polarized state of the light. In the backlight unit  100  according to the current embodiment, a diffusion sheet is not included between the phosphor layer  175  and the wire grid polarizer  180 , thereby improving the reproduction efficiency of light. Also, the wire grid polarizer  180  and the phosphor layer  175  are respectively disposed on both surfaces of the second substrate  171 . Accordingly, an optical loss occurring in a space between the wire grid polarizer  180  and the phosphor layer  175  is reduced. 
     Although  FIG. 2  illustrates that the diffusion sheet  190  is disposed between the backlight unit  100  and the LCD panel  200 , the present invention is not limited thereto. In detail, an arrangement pitch between wire grids and the width of an aperture of each of the wire grids are designed to be equal to or less than the wavelength of a visible light region, i.e., 200 to 800 nm, in order to increase selectivity of polarized light. Light passing through the wire grid with the above dimensions contains polarized light, and is also influenced by diffraction due to multiple slits, thereby spreading out light reaching the LCD panel  200 . In general, the optical transmissivity of the diffusion sheet  190  is inversely proportional to a diffusion degree thereof. Accordingly, in the backlight unit  100  according to an embodiment of the present invention, the wire grid polarizer  180  may perform a part of the function of the diffusion sheet  190  and thus is capable of increasing the optical transmissivity of the diffusion sheet  190  by reducing haze in the diffusion sheet  190 . Furthermore, the wire grid polarizer  180  may also function as a diffusion sheet without having to use an additional diffusion sheet. Accordingly, a backlight unit may not include a diffusion sheet, and thus, an optical loss from a diffusion sheet may be reduced, and luminous efficiency may be increased. Also, in this case, the total number of constitutional elements in the backlight unit decreases, and a manufacturing process is simplified 
       FIG. 3  is a schematic cross-sectional view of an electron emission type backlight unit  100  according to another embodiment of the present invention. Referring to  FIG. 3 , the backlight unit  100  includes an electron emission device  101  and a front panel  102  that form a light emitting space  103  therebetween, and spacers  160  disposed to maintain a distance between the electron emission device  101  and the front panel  102 . The electron emission device  101  includes a first substrate  110 , a plurality of first electrodes  120 , an insulating layer  130 , a plurality of second electrodes  140 , and a plurality of electron emission sources  150 . The front panel  102  includes a second substrate  171 , a phosphor layer  175 , a third electrode  173 , and a wire grid polarizer  180 . A diffusion sheet  190  is disposed on the backlight unit  100  and an LCD panel  200  is disposed on the diffusion sheet  190 . 
     The current embodiment differs from the previous embodiment in that one side of the wire grid polarizer  180  is grounded. In detail, in the backlight unit  100 , a voltage is applied to a surface of the third electrode  173  which is an anode, and thus, static electricity may be generated on an external surface of the second substrate  171 . Static electricity is one of a number of factors that cause a device to unstably operate, and may cause unnecessary particles, such as dust, to stick to an external surface of the backlight unit  100 . To reduce static electricity, one side of the wire grid polarizer  180  of the backlight unit  100  according to the current embodiment is grounded (see reference numeral G 1  of  FIG. 3 ). That is, the wire grid polarizer  180  includes a conductor, and thus, one side of the wire grid polarizer  180  is grounded to apply an appropriate voltage thereto, thereby reducing occurence of static electricity. 
       FIG. 4  is a schematic cross-sectional view of an electron emission type backlight unit  100  according to another embodiment of the present invention. Referring to  FIG. 4 , the backlight unit  100  includes an electron emission device  101  and a front panel  102  that form a light emitting space  103  therebetween, and spacers  160  disposed to maintain a distance between the electron emission device  101  and the front panel  102 . The electron emission device  101  includes a first substrate  110 , a plurality of first electrodes  120 , an insulating layer  130 , a plurality of second electrodes  140 , and a plurality of electron emission sources  150 . The front panel  102  includes a second substrate  371 , a third electrode  373 , a phosphor layer  375 , and a wire grid polarizer  380 . A diffusion sheet  190  is disposed on the backlight unit  100  and an LCD panel  200  is disposed on the diffusion sheet  190 . 
     The current embodiment differs from the previous embodiments in that the wire grid polarizer  380  is disposed on the second substrate  371  facing the first substrate  110 . More specifically, the wire grid polarizer  380 , the phosphor layer  375 , and the third electrode  373  are sequentially formed on the second substrate  371  facing the first substrate  110 , and the wire grid polarizer  380  and the phosphor layer  375  are closely adhered to each other. In the current embodiment, since the second substrate  371  is not disposed between a rear surface of the wire grid polarizer  380  and the third electrode  373 , an optical loss due to reflected light passing through the second substrate  371  may be reduced. Also, the phosphor layer  375  is surrounded by the third electrode  373  and the wire grid polarizer  380  which may both include conductive materials. Thus, electrons colliding against the phosphor layer  375  are not discharged to the outside, preventing a voltage drop. 
     Also, in the backlight unit  100  according to the current embodiment, one side of each of the wire grid polarizer  380  and the third electrode  373  may be grounded (see reference numeral G 2  of  FIG. 4 ). That is, the wire grid polarizer  380  and the third electrode  373  include conductive materials, and thus, one side of each of the wire grid polarizer  380  and the third electrode  373  is grounded to apply an appropriate voltage thereto, thereby reducing occurrence of static electricity. 
       FIG. 5  is a schematic cross-sectional view of an electron emission type backlight unit  100  according to another embodiment of the present invention. Referring to  FIG. 5 , the backlight unit  100  includes an electron emission device  101  and a front panel  102  that form a light emitting space  103 , and spacers  160  disposed to maintain a predetermined space between the electron emission device  101  and the front panel  102 . The electron emission device  101  includes a first substrate  110 , a plurality of first electrodes  120 , an insulating layer  130 , a plurality of second electrodes  140 , and a plurality of electron emission sources  150 . The front panel  102  includes a second substrate  471 , a third electrode  473 , a phosphor layer  375 , and a wire grid polarizer  480 . A diffusion sheet  190  is disposed on the backlight unit  100  and an LCD panel  200  is disposed on the diffusion sheet  190 . 
     The current embodiment differs from the embodiment of  FIG. 4  in that a static electricity prevention layer  485  is further disposed on the second substrate  471 . In detail, the static electricity prevention layer  485  may be further formed on the second substrate  471  facing the diffusion sheet  190  and the LCD panel  200 , and one side of the static electricity prevention layer  485  may further be grounded (see reference numeral G 3  of  FIG. 5 ). That is, one side of each of the static electricity prevention layer  485 , the wire grid polarizer  480 , and the third electrode  473  is grounded to apply an appropriate voltage thereto, thereby reducing occurrence of static electricity. 
     The above exemplary embodiments of a backlight unit according to the present invention provide improved luminous efficiency. 
     While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present invention as defined by the following claims.