Abstract:
Provided is a plasma flat lamp. The provided lamp includes a discharge gas filled in a discharge area of a discharge container, at least two electrodes generating a gas discharge in the discharge area, a low work function material layer located in a discharge path between the electrodes and collided against gas ions that are generated by the gas discharge, and a fluorescent layer generating visible rays by ultraviolet rays that are generated by the gas discharge in the discharge container. The provided plasma flat lamp reduces a driving voltage due to the low work function material layer against which ions are collided, and increases luminescent efficiency by reducing the absorption of ultraviolet rays of the low work function material layer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This application claims the priority of Korean Patent Application No. 2003-84958, filed on Nov. 27, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
         [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a plasma flat lamp, and more particularly, to a plasma flat lamp with high luminance and luminescent efficiency and a uniform luminance distribution.  
         [0004]     2. Description of the Related Art  
         [0005]     A flat lamp used as a back-light of a liquid crystal display (LCD) has been developed from an edge-light type or a direct-light type using a cold cathode fluorescent lamp to a surface discharge type or a facing surfaces discharge type using a lower portion of a luminescent surface as a discharge area to improve luminescent efficiency and luminance uniformity.  
         [0006]     The U.S. Published Patent Application No. US-2003-0098643-A1 discloses problems of various discharge types and a method of solving the problems. It is important to improve the luminescent efficiency of a plasma lamp and to develop a technology of driving a plasma lamp at a low power in order to improve the performance of a plasma lamp and to reduce the cost of a plasma lamp. In general, a surface discharge type plasma lamp has a merit of a stable discharge characteristic compared to a facing surfaces discharge type plasma lamp; however, the luminance of the surface discharge type plasma lamp is lower than that of the facing surfaces discharge type plasma lamp. In order to improve the luminescent efficiency, a discharge gap is increased. Here, the increase of a discharge gap is limited by the size of a discharge area. Another method of improving the luminescent efficiency is increasing the total gas pressure of a discharge gas, for example, Ne—Xe, or increasing the partial pressure of Xe. However, when the total gas pressure or the partial pressure of Xe is increased, a high discharge voltage is required. When the discharge voltage is increased, the lifespan of a lamp is reduced and a manufacturing cost of a driver, which drives the lamp, is increased.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention provides a plasma flat lamp with high luminescent efficiency and a low operation voltage to increase a lifespan and to decrease a manufacturing cost.  
         [0008]     According to an aspect of the present invention, there is provided a plasma flat lamp comprising a discharge container including a first plate and a second plate that maintain a predetermined distance to form a discharge area in which a discharge gas is filled, the discharge gas filled in the discharge area of the discharge container, at least two electrodes formed on the discharge container and generating a gas discharge in the discharge area, a fluorescent layer generating visible rays by ultraviolet rays that are generated by the gas discharge in the discharge container, and a low work function material layer located in a discharge path between the electrodes and collided against gas ions that are generated by the gas discharge.  
         [0009]     According to another aspect of the present invention, there is provided a plasma flat lamp comprising a first plate and a second plate maintaining a predetermined distance to form a discharge area in which a discharge gas is filled, the discharge gas filled in the discharge area, at least two electrodes formed on a surface of the first plate facing the second plate, a dielectric layer formed on a surface of the first plate facing the second plate and covering the electrodes, a low work function material layer formed on the dielectric layer to correspond to the electrodes, and a fluorescent layer formed on portions of the dielectric layer where the low work function material layer is not formed to expose the low work function material layer to the discharge area.  
         [0010]     In this case, the fluorescent layer may extend to areas between the dielectric layer and the low work function material layer, or the low work function material layer may be formed on an entire surface of the dielectric layer.  
         [0011]     According to still another aspect of the present invention, there is provided a plasma flat lamp comprising a first plate and a second plate maintaining a predetermined distance to form a discharge area in which a discharge gas is filled, the discharge gas filled in the discharge area, at least two electrodes formed on a surface of the first plate facing the second plate, a dielectric layer formed on a surface of the first plate facing the second plate and covering the electrodes, a fluorescent layer formed on the dielectric layer, and a low work function material layer formed on the dielectric layer to a thickness of 80 to 200 Å.  
         [0012]     Here, the electrodes are formed on an inner surface of an outer surface of the discharge container, more specifically, on an inner surface or an outer surface of at least one of the first plate and the second plate.  
         [0013]     In addition, the low work function material layer is formed in a lower portion or an upper portion of the fluorescent layer. The fluorescent layer may be formed at a portion deviated from the discharge path. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0015]      FIG. 1  is a sectional view illustrating a plasma flat lamp according to a first embodiment of the present invention;  
         [0016]      FIG. 2  is a sectional view illustrating a plasma flat lamp according to a second embodiment of the present invention;  
         [0017]      FIG. 3  is a sectional view illustrating a plasma flat lamp according to a third embodiment of the present invention;  
         [0018]      FIG. 4  is a sectional view illustrating a plasma flat lamp according to a fourth embodiment of the present invention;  
         [0019]      FIG. 5  is a sectional view illustrating a plasma flat lamp in which a substrate is used as a dielectric layer according to a fifth embodiment of the present invention;  
         [0020]      FIG. 6  is a sectional view illustrating a plasma flat lamp in which a substrate is used as a dielectric layer according to a sixth embodiment of the present invention;  
         [0021]      FIG. 7  is a sectional view illustrating a plasma flat lamp in which a substrate is used as a dielectric layer according to a seventh embodiment of the present invention;  
         [0022]      FIGS. 8A and 8B  are a perspective view and a sectional view illustrating a first example of the plasma flat lamp according to the seventh embodiment of the present invention shown in  FIG. 7 , respectively;  
         [0023]      FIGS. 9A and 9B  are a perspective view and a sectional view illustrating a plasma flat lamp in which symmetrical electrodes are formed on substrates according to a second example of the seventh embodiment of the present invention shown in  FIG. 7 , respectively; and  
         [0024]      FIG. 10  is a sectional view illustrating a plasma flat lamp in which symmetrical electrodes are formed on substrates according to an eighth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown.  
         [0026]     Referring to  FIG. 1 , electrodes  11  and  12  connected to a driving power  16  are formed on a first plate  10   a , and a dielectric layer  13  is formed on the electrodes  11  and  12 . In addition, a fluorescent layer  14  and low work function material layers  15  are formed on the dielectric layer. Here, the low work function material layers  15 , such as MgO, are arranged on a discharge path between the electrodes  11  and  12  to generate secondary electrons due to the collision of ions during a discharge. In addition, the fluorescent layer  14  is arranged to prevent the reduction of the incidence amount of ultraviolet rays to the fluorescent layer  14  by preventing the low work function material layers  15  from absorbing the ultraviolet rays. Thus, the low work function material layers  15  are formed on the discharge electrodes  11  and  12 , and the fluorescent layer  14  is formed on the other portions. On the other hand, another fluorescent layer  14  is formed on a surface of a second plate  10   b  facing the first plate  10   a.    
         [0027]     According to the above-described structure, the ions collide against the low work function material layers  15  on the electrodes  1  and  12  when a gas discharge occurs between the electrodes  11  and  12 , thus the secondary electrons are generated to reduce a discharge voltage. On the other hand, the ions do not collide against the fluorescent layer  14 , thus the fluorescent layer  14  is protected from the ion collision.  
         [0028]     Referring to  FIG. 2 , electrodes  11  and  12  connected to a driving power  16  are formed on a first plate  10   a , and a dielectric layer  13  is formed on the electrodes  11  and  12 . In addition, a fluorescent layer  14  is formed on an entire surface of the dielectric layer, and low work function material layers  15  are partially formed on the fluorescent layer  14 . Here, the low work function material layers  15 , such as MgO, are arranged on the portions above the electrodes  11  and  12  to generate secondary electrons due to the collision of ions during a discharge. In addition, the low work function material layers  15  are not formed on the portions deviated from the electrodes  11  and  12  to prevent the reduction of the incidence amount of ultraviolet rays to the fluorescent layer  14  by preventing the low work function material layers  15  from absorbing the ultraviolet rays. On the other hand, another fluorescent layer  14  is formed on a surface of a second plate  10   b  facing the first plate  10   a.    
         [0029]     According to the above-described structure, the ions collide against the low work function material layers  15  on the electrodes  11  and  12  when a gas discharge occurs between the electrodes  11  and  12 , thus the secondary electrons are generated to reduce a discharge voltage. On the other hand, the ions do not collide against the fluorescent layer  14 , thus the fluorescent layer  14  is protected from the ion collision.  
         [0030]     Referring to  FIG. 3 , electrodes  11  and  12  connected to a driving power  16  are formed on a first plate  10   a , and a dielectric layer  13  and a low work function material layer  15  are sequentially formed on the electrodes  11  and  12 . In addition, a fluorescent layer  14  is formed on the low work function material layers  15  except the portions above the electrodes  11  and  12  in order to generate secondary electrons by colliding ions against the low work function material layer  15  when a discharge occurs on a discharge path between the electrodes  11  and  12 . On the other hand, another fluorescent layer  14  is formed on a surface of a second plate  10   b  facing the first plate  10   a.    
         [0031]     According to the above-described structure, the ions collide against the low work function material layer  15  on the electrodes  11  and  12  when the gas discharge occurs between the electrodes  11  and  12 , thus the secondary electrons are generated to reduce a discharge voltage. On the other hand, the ions do not collide against the fluorescent layer  14 , thus the fluorescent layer  14  is protected from the ion collision.  
         [0032]     Referring to  FIG. 4 , electrodes  11  and  12  connected to a driving power  16  are formed on a first plate  10   a , and a dielectric layer  13  is formed on the electrodes  11  and  12 . In addition, a fluorescent layer  14  is formed on the dielectric layer  13 , and a low work function material layer  15  is formed on the fluorescent layer  14 . Here, the low work function material layer  15  is formed on the entire surface of the fluorescent layer  14  as well as on the portions above the electrodes  11  and  11  against with ions collide when a discharge occurs between the electrodes  11  and  12 . When the low work function material layer  15  is formed on the entire surface of the fluorescent layer  14 , a driving voltage may be lowered due to the generation of secondary electrons; however, the incidence amount of ultraviolet rays to the fluorescent layer  14  may be reduced because the low work function material layer  15  absorbs the ultraviolet rays. In order to minimize the absorption of the ultraviolet rays by the low work function material layer  15 , the low work function material layer  15  is formed to a thickness of from 80 to 200 Å.  
         [0033]     On the other hand, the function of the dielectric layer  13  may be performed by the first plate  10   a  by forming the electrodes  11  and  12  on one surface of the first plate  10   a  and forming the fluorescent layer  14  and the low work function material layer  15  on the other surface of the first plate  10   a.    
         [0034]      FIGS. 5 through 7  are sectional views illustrating plasma flat lamps in which a first plate operate as a dielectric for AC driving as well as an element of the plasma flat lamp.  
         [0035]     Referring to  FIG. 5 , fluorescent layers  14  and low work function layers  15  are formed on a surface of a first plate  10   a  facing a second plate  10   b , and discharge electrodes  11  and  12  are formed on the other surface of the first plate  10   a . The low work function material layers  15  are formed to correspond to the electrodes  11  and  12 , and the fluorescent layers  14  are formed on the other portions.  
         [0036]     Referring to  FIG. 6 , a low work function layer  15  is formed on an entire surface of a first plate  10   a  facing a second plate  10   b , and discharge electrodes  11  and  12  are formed on the other surface of the first plate  10   a . In addition, fluorescent layers  14  are formed on the low work function material layer  15  on the portions except for the portions corresponding to the electrodes  11  and  12 . Thus, portions of the low work function material layer  15  corresponding to the electrodes  11  and  12  are exposed.  
         [0037]     Referring to  FIG. 7 , a fluorescent layer  14  is formed on an entire surface of a first plate  10   a  facing a second plate  10   b , and discharge electrodes  11  and  12  are formed on the other surface of the first plate  10   a . In addition, low work function material layers  15  are formed on portions of the fluorescent layer  14  corresponding to the electrodes  11  and  12 . Thus, the portions of the fluorescent layer  14  except for the portions corresponding to the electrodes  11  and  12  are exposed.  
         [0038]      FIGS. 8A and 8B  illustrate a first example of the plasma flat lamp according to the seventh embodiment of the present invention shown in  FIG. 7 . Referring to  FIGS. 8A and 8B , a first plate  10   a  and a second plate  10   b  are separated to a predetermined distance by walls  10   c  to form a discharge area  10   d  in which a discharge gas is filled.  
         [0039]     A low wok function material layer  15  is formed on a surface of the first plate  10   a  facing the second plate  10   b , and electrodes  11   a  and  11   b  are formed on the other surface of the first plate  10   a . On the other hand, fluorescent layers  14  are formed on surfaces of the first plate  10   a  and the second plate  10   b  facing each other. Here, the fluorescent layer  14  is not formed on portions of the first plate  10   a  corresponding to the electrodes  11   a  and  11   b . Thus, when a discharge occurs between the electrodes  11   a  and  11   b , ions collide against the portions of the low work function material layer  15  corresponding to the electrodes  11   a  and  11   b  and exposed to the discharge area  10   d.    
         [0040]      FIGS. 9A and 9B  illustrate a second example of the plasma flat lamp according to the seventh embodiment of the present invention shown in  FIG. 7 . Referring to  FIGS. 9A and 9B , a first plate  10   a  and a second plate  10   b  are separated to a predetermined distance by walls  10   c  to form a discharge area  10   d  in which a discharge gas is filled.  
         [0041]     A low wok function material layer  15  is formed on a surface of the first plate  10   a  facing the second plate  10   b , and electrodes  11   a  and  11   b  are formed on the other surface of the first plate  10   a  and a surface of the second plate  10   b  not facing the first plate  10   a . On the other hand, fluorescent layers  14  are formed on surfaces of the first plate  10   a  and the second plate  10   b  facing each other. Here, the fluorescent layer  14  is not formed on portions of the first plate  10   a  corresponding to the electrodes  11   a  and  11   b.    
         [0042]     The couples of the discharge electrodes  11   a  and  11   b  formed on the first plate  10   a  and the second plate  10   b  face each other with the discharge area  10   d  therebetween, and the electrodes  11   a  on the first plate  10   a  and the second plate  10   b  are electrically connected to maintain the same potential, thus a discharge does not occur between the electrodes  11   a . In the same manner, the electrodes  11   b  on the first plate  10   a  and the second plate  10   b  maintain the same potential, thus a discharge does not occur between the electrodes  11   b.    
         [0043]      FIG. 10  is a sectional view illustrating a plasma flat lamp in which symmetrical electrodes are formed on substrates as in  FIGS. 9A and 9B , according to an eighth embodiment of the present invention. In this case, fluorescent layers  14  are formed on surfaces of a first plate  10   a  and a second plate  10   b  facing each other, and low work function material layers  15  corresponding to electrodes  11   a  and  11   b  are formed on the fluorescent layers  14 .  
         [0044]     An experiment was performed to examine the performance of a plasma flat lamp according to the present invention. Here, Ne—Xe was used as a discharge gas at a gas pressure of 152 mbar. A driving frequency was controlled to 20 KHz with a duty of 20%. The experiment was performed on a first specimen in which a MgO layer is formed on a fluorescent layer to a thickness of 5,000 Å, and a second specimen in which a MgO layer is not formed on a fluorescent layer. A breakdown voltage of the second specimen with the MgO layer was 2.76 KV; however, a breakdown voltage of the first specimen according to the present invention was 2.12 KV. As a result, when the MgO layer was formed on the fluorescent layer, the breakdown voltage was reduced by about 640 V. In addition, when the MgO layer was formed on the fluorescent layer, a discharge maintain voltage is reduced by about 620 V, from 1.72 KV to 1.10 KV.  
         [0045]     As described above, when the low work function material layer  15  is formed on the portions deviated from the discharge path, the ultraviolet rays may be absorbed by the low work function material layer  15 . Thus, it is preferable that the low work function material layer  15  is formed on or exposed to the discharge path and the fluorescent layer  14  is exposed to other portions, as shown in  FIGS. 1 through 3 . On the other hand, it is preferable that the low work function material layer  15  is formed to a thickness of from 80 to 200 Å to minimize the absorption of the ultraviolet rays as shown in  FIG. 4 . Here, the thickness of the low work function material layer  15  is determined when the transmission rate of ultraviolet rays, for example, vacuum ultraviolet rays (VUV) with a wavelength of 147 nm, to a MgO layer with an extinction coefficient of 0.3, to 80%.  
         [0046]     The low work function material layer  15  is formed of MgO. Such a low work function material layer  15  may be formed of any one selected from MgF 2 , CaF 2 , LiF, Al 2 O 3 , ZnO, CaO, SrO, SiO 2 , and La 2 O 3 .  
         [0047]     A plasma flat lamp according to the present invention has a low driving voltage compared to a conventional flat lamp. In order to prevent or repress the absorption of ultraviolet rays, for example, VUV by a low work function material layer to reduce a discharge voltage, the low work function material layer is formed not to cover a fluorescent layer at portions deviated from a discharge path. Thus, the ultraviolet rays are directly input to the fluorescent layer. In addition, when the low work function material layer is formed on the fluorescent layer, the thickness of the low work function material layer is controlled to minimize the loss of the ultraviolet rays.  
         [0048]     According to the present invention, a plasma flat lamp with a low driving voltage and high luminescent efficiency is obtained. Such a plasma flat lamp may be used as a light source, for example, a back-light of a liquid crystal display (LCD).  
         [0049]     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 therein without departing from the spirit and scope of the present invention as defined by the following claims.