Patent Publication Number: US-2006006805-A1

Title: Flat lamp

Description:
BACKGROUND OF THE INVENTION  
      Priority is claimed to Korean Patent Application No. 10-2004-0052986, filed on Jul. 8, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.  
      1. Field of the Invention  
      The present invention relates to a flat lamp, and more particularly, to a flat lamp that can lower a discharge voltage and improve luminance efficiency.  
      2. Description of the Related Art  
      Flat lamps used as backlights for LCDs have been developed from an edge-light type or direct-light type flat lamp using a cold cathode fluorescent lamp to a surface discharge type or facing discharge type flat lamp in which the entire lower portion of a light emitting surface is used as a discharge space, in consideration of a luminance efficiency and uniformity in brightness.  
      Although the surface charge type flat lamp is advantageous in that it exhibits a stable discharge property compared to the facing discharge type flat lamp, the overall brightness of the surface charge type flat lamp is lower than that of the facing discharge type.  
       FIG. 1  is a perspective view showing part of a conventional surface discharge type flat lamp. Referring to  FIG. 1 , a lower substrate  10  and an upper substrate  20  are arranged to face each other by being separated at a predetermined distance by spacers  14 . A discharge space where plasma discharge is generated is formed between the lower substrate  10  and the upper substrate  20 . The discharge space is filled with a discharge gas that is a mixture of neon (Ne) gas and xenon (Xe) gas.  
      A fluorescent layer  30  which is excited by ultraviolet rays generated during discharge and generates visible light is formed on interior surfaces of the lower substrate  10  and the upper substrate  20  and both side surfaces of the spacers  14 . A plurality of discharge electrodes for generating a plasma discharge is formed on the lower substrate  10  and the upper substrate  20 . In detail, a plurality of first and second lower electrodes  12   a  and  12   b  and first and second upper electrodes  22   a  and  22   b  are formed in pairs on exterior surfaces of the lower substrate  10  and the upper substrate  20 , respectively. The same voltage is applied to the first lower electrode  12   a  and the first upper electrode  22   a  so that discharge is not induced therebetween. Also, the same voltage is applied to the second lower electrode  12   b  and the second upper electrode  22   b  so that discharge is not induced therebetween. Meanwhile, a predetermined difference in electric potential exists between the first lower electrode  12   a  and the second lower electrode  12   b  and between the first upper electrode  22   a  and the second upper electrode  22   b , so that a surface discharge is induced in a direction parallel to the lower substrate  10  or the upper substrate  20 .  
      In the flat lamp configured as above, although the luminance efficiency may be improved by increasing a partial pressure of the xenon gas or an absolute pressure of the discharge gas, a discharge voltage increases accordingly. Also, although the luminance efficiency may be improved by increasing a width between the electrodes to extend a discharge path, the discharge voltage increases as well in this case.  
     SUMMARY OF THE INVENTION  
      To solve the above and/or other problems, the present invention provides a flat lamp that can lower a discharge voltage and improve a luminance efficiency.  
      According to an aspect of the present invention, a flat lamp comprises a lower panel and an upper panel arranged to face each other and forming a discharge space therebetween, a plurality of discharge electrodes located on at least one of the lower and upper panels, and a plurality of auxiliary electrodes located on a panel where the discharge electrodes are located and positioned such that a start discharge is generated by a voltage induced in the auxiliary electrodes by a voltage applied to the discharge electrodes.  
      A dielectric layer can be located between the discharge electrodes and the auxiliary electrodes.  
      The discharge electrodes can be located in pairs parallel to each other and the auxiliary electrodes can be located in pairs parallel to each other and corresponding to the discharge electrodes. The auxiliary electrodes are located in a direction parallel to the discharge electrodes.  
      A distance between the auxiliary electrodes can be less than a distance between the discharge electrodes.  
      A plurality of spacers can be located between the lower and upper panels to maintain a uniform distance therebetween.  
      A fluorescent layer can be located on an interior wall of the discharge space. The discharge space is filled with a discharge gas including xenon (Xe) gas.  
      According to another aspect of the present invention, a flat lamp comprises a lower substrate and an upper substrate arranged to face each other and forming a discharge space therebetween, a dielectric layer located on an outer surface of at least one of the lower and upper substrates, a plurality of discharge electrodes located on a surface of the dielectric layer, and a plurality of auxiliary electrodes located on the outer surface of a substrate where the discharge electrodes are located and embedded in the dielectric layer, wherein the auxiliary electrodes are positioned such that a start discharge can be generated by a voltage induced in the auxiliary electrodes by a voltage applied to the discharge electrodes.  
      The lower and upper substrates can be glass substrates.  
      The auxiliary electrodes can be formed of ITO or SnO 2 . The auxiliary electrodes can be formed of a material selected from a group consisting of RuO 2 , Ag, Cu, and Cr.  
      The dielectric layer can be formed of a ferroelectric.  
      According to another aspect of the present invention, a flat lamp comprises a lower substrate and an upper substrate arranged to face each other and forming a discharge space therebetween, a plurality of discharge electrodes located on an outer surface of at least one of the lower and upper substrates, and a plurality of auxiliary electrodes locaed on an inner surface of a substrate where the discharge electrodes are located, and positioned such that a start discharge is generated by a voltage induced in the auxiliary electrodes by a voltage applied to the discharge electrodes.  
      A dielectric layer, in which the auxiliary electrodes are embedded, can be located on an inner surface of a substrate where the auxiliary electrodes are formed.  
      A trench can be located in the dielectric layer between the auxiliary electrodes.  
      The trench can be parallel to the auxiliary electrodes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:  
       FIG. 1  is a perspective view illustrating part of a conventional flat lamp;  
       FIG. 2  is a sectional view illustrating part of a flat lamp according to an embodiment of the present invention;  
       FIG. 3  is a sectional view illustrating a modified example of the flat lamp of  FIG. 2 ;  
       FIG. 4  is a sectional view illustrating another modified example of the flat lamp of  FIG. 2 ;  
       FIG. 5  is a sectional view illustrating part of a flat lamp according to another embodiment of the present invention;  
       FIG. 6  is a sectional view illustrating part of a flat lamp according to yet another embodiment of the present invention;  
       FIG. 7  is a sectional view illustrating a modified example of the flat lamp of  FIG. 6 ;  
       FIGS. 8A through 8C  are views illustrating flat lamps used to compare the discharge voltage and luminance efficiency between the conventional flat lamp and the flat lamp according to the present invention;  
       FIG. 9  is a graph showing the results of comparison in the discharge voltage between the conventional flat lamp and the flat lamp according to the present invention; and  
       FIG. 10  is a graph showing the results of comparison in the luminance efficiency between the conventional flat lamp and the flat lamp according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In the accompanying drawings, the same reference numerals indicate the same constituent elements.  
       FIG. 2  is a sectional view illustrating part of a flat lamp according to an embodiment of the present invention. Referring to  FIG. 2 , a flat lamp according to an embodiment of the present invention includes a lower panel and an upper panel arranged to be separated from each other. A discharge space  130  where a plasma discharge is generated is formed between the lower panel and the upper panel. The discharge space  130  is filled with a discharge gas that is a mixture of neon (Ne) gas and xenon (Xe) gas.  
      The lower panel includes a lower substrate  110  and a dielectric layer  115  formed on a lower surface of the lower substrate  110 . A glass substrate is generally used as the lower substrate  110 . At least one pair of first and second electrodes  112   a  and  112   b  are formed on a lower surface of the dielectric layer  115 , parallel to each other. The first and second electrodes  112   a  and  112   b  are discharge electrodes, to which a voltage in the form of pulses from a power source is applied, and formed of a conductive material.  
      In this embodiment, at least one pair of first and second auxiliary electrodes  111   a  and  111   b  are formed on a lower surface of the lower substrate  110 , parallel to each other. The dielectric layer  115  is formed on the lower surface of the lower substrate  110  such that the first and second auxiliary electrodes  111   a  and  111   b  can be buried therein. The first and second auxiliary electrodes  111   a  and  111   b  correspond to the first and second electrodes  112   a  and  112   b , respectively, and are formed in a direction parallel to the first and second electrodes  112   a  and  112   b . The distance between the first and second auxiliary electrodes  111   a  and  111   b  is less than that between the first and second electrodes  112   a  and  112   b . The first and second auxiliary electrodes  111   a  and  111   b  are floating electrodes, to which a voltage is induced via the dielectric layer  115  as a predetermined voltage is applied to the first and second electrodes  112   a  and  112   b . The first and second auxiliary electrodes  111   a  and  111   b  may be formed of a transparent conductive material such as ITO (indium tin oxide) or SnO 2 , or a conductive material such as RuO 2 , Ag, Cu, or Cr. The same is true of first and second electrodes  122   a  and  122   b . To reduce a voltage drop by the dielectric layer  115 , the dielectric layer  115  may be formed of a material having a high dielectric constant. The dielectric layer  115  may be formed of a ferroelectric exhibiting a hysteresis property.  
      The upper panel includes an upper substrate  120  that is separated a predetermined distance from the lower substrate  110 . A glass substrate is generally used as the upper substrate  120  like the lower substrate  110 . A plurality of spacers  114  is provided between the lower substrate  110  and the upper substrate  120  to maintain a uniform distance therebetween. A fluorescent layer  113  for generating visible light by being exited by ultraviolet rays generated from the discharge gas by a plasma discharge is formed on portions constituting an interior wall of the discharge space  130 , that is, inner surfaces of the lower substrate  110  and the upper substrate  120  and side surfaces of the spacers  114 .  
      In the operation of the flat lamp configured as above, a voltage in the form of pulses is applied from the power source to the first and second electrodes  112   a  and  112   b . When the pulse type voltage is applied to the first and second electrodes  112   a  and  112   b , the voltage between the first and second electrodes  112   a  and  112   b  changes to reach a predetermined value. As the voltage between the first and second electrodes  112   a  and  112   b  changes, a voltage corresponding to the voltage between the first and second electrodes  112   a  and  112   b  is induced between the first and second auxiliary electrodes  111   a  and  111   b  via the dielectric layer  115 . When the dielectric layer  115  is formed of a material having a high dielectric constant, since a voltage drop due to the dielectric layer  115  can be greatly reduced, the voltage that is substantially the same as that between the first and second electrodes  112   a  and  112   b  can be induced between the first and second auxiliary electrodes  111   a  and  111   b . A start discharge  150   a  is primarily generated between the first and second auxiliary electrodes by the induced voltage. This is because the distance between the first and second auxiliary electrodes  111   a  and  111   b  is less than that between the first and second electrodes  112   a  and  112   b . In the flat lamp according to the present embodiment, due to the first and second auxiliary electrodes  111   a  and  111   b , the start discharge  150   a  is generated at a voltage lower than that of a conventional flat lamp.  
      Next, the voltage between the first and second electrodes  112   a  and  112   b  is maintained constantly after reaching a predetermined value. In this step, since the voltage between the first and second electrodes  112   a  and  112   b  does not change, the voltage is not induced in the first and second auxiliary electrodes  111   a  and  111   b  and a sustain discharge  150   b  is generated between the first and second electrodes  112   a  and  112   b . Luminance efficiency can be improved by extending a discharge path by increasing the distance between the first and second electrodes  112   a  and  112   b . Then, the start discharge  150   a  and the sustain discharge  150   b  are repeatedly generated in order in the discharge space  130 .  
       FIG. 3  is a sectional view illustrating a modified example of the flat lamp of  FIG. 2 . Referring to  FIG. 3 , the upper panel includes the upper substrate  120  and a dielectric layer  125  formed on an upper surface of the upper substrate  120 . The lower panel includes the lower substrate  110 , arranged to be separated by a predetermined distance from the upper substrate  120 .  
      At least one pair of first and second electrodes  122   a  and  122   b  are formed on an upper surface of the dielectric layer  125 , parallel to each other. The first and second electrodes  122   a  and  122   b  are discharge electrodes, to which a voltage in the form of pulses is applied from the power source. At least one pair of first and second auxiliary electrodes  121   a  and  121   b  are formed on the upper surface of the upper substrate  120 , parallel to each other. The dielectric layer  125  is formed on the upper surface of the upper substrate  120  such that the first and second auxiliary electrodes  121   a  and  121   b  can be buried therein. The first and second auxiliary electrodes  121   a  and  121   b  correspond to the first and second electrodes  122   a  and  122   b , respectively, and are formed in a direction parallel to the first and second electrodes  122   a  and  122   b . The first and second auxiliary electrodes  121   a  and  121   b  are formed such that the distance therebetween is less than that between the first and second electrodes  122   a  and  122   b . The first and second auxiliary electrodes  121   a  and  121   b  are floating electrodes in which a voltage is induced via the dielectric layer  125  as a predetermined voltage is applied to the first and second electrodes  122   a  and  122   b . The first and second auxiliary electrodes  121   a  and  121   b  may be formed of a transparent conductive material such as ITO and SnO 2  to transmit visible light. Alternatively, the first and second auxiliary electrodes  121   a  and  121   b  may be formed of a conductive material such as RuO 2 , Ag, Cu, and Cr. The same is true of first and second electrodes  122   a  and  122   b . The dielectric layer  125  may be formed of a material having a high dielectric constant or a ferroelectric having a hysterisis property.  
      Since the operation of the flat lamp having the above structure is the same as that described above, a detailed description thereof is omitted.  
       FIG. 4  is a sectional view illustrating another modified example of the flat lamp of  FIG. 2 . Referring to  FIG. 4 , the lower panel includes the lower substrate  110  and a first dielectric layer  215  formed on the lower surface of the lower substrate  110 . The upper panel includes the upper substrate  120 , arranged to be separated a predetermined distance from the lower substrate  110 , and a second dielectric layer  225  formed on the upper surface of the upper substrate  120 .  
      At least one pair of first and second lower electrodes  212   a  and  212   b  is formed on a lower surface of the first dielectric layer  215 , parallel to each other. The first and second lower electrodes  212   a  and  212   b  are discharge electrodes, to which a voltage in the form of pulses is applied from the power source. At least one pair of first and second lower auxiliary electrodes  211   a  and  211   b  are formed on a lower surface of the lower substrate  110 , parallel to each other. The first dielectric layer  215  is formed on the lower surface of the lower substrate  110  such that the first and second lower auxiliary electrodes  211   a  and  211   b  can be buried therein. The first and second lower auxiliary electrodes  211   a  and  211   b  correspond to the first and second lower electrodes  212   a  and  212   b , respectively, and are formed in a direction parallel to the first and second lower electrodes  212   a  and  212   b . The distance between the first and second lower auxiliary electrodes  211   a  and  211   b  is less than that between the first and second lower electrodes  212   a  and  212   b . The first and second lower auxiliary electrodes  211   a  and  211   b  are floating electrodes, to which a voltage is induced via the first dielectric layer  215  as a predetermined voltage is applied to the first and second lower electrodes  212   a  and  212   b . The first and second lower auxiliary electrodes  211   a  and  211   b  may be formed of a transparent conductive material such as ITO or SnO 2 , or a conductive material such as RuO 2 , Ag, Cu, or Cr. The same is true of first and second electrodes  122   a  and  122   b . The first dielectric layer  215  may be formed of a material having a high dielectric constant, or a ferroelectric exhibiting a hysterisis property.  
      At least one pair of first and second upper electrodes  222   a  and  222   b  are formed on an upper surface of the second dielectric layer  225 , parallel to each other. The first and second upper electrodes  222   a  and  222   b  are formed parallel to the first and second lower electrodes  212   a  and  212   b . The first and second upper electrodes  222   a  and  222   b  are discharge electrodes, to which a voltage in the form of pulses is applied from the power source. At least one pair of first and second upper auxiliary electrodes  221   a  and  221   b  are formed on the upper surface of the upper substrate  120 , parallel to each other. The second dielectric layer  225  is formed on the upper surface of the upper substrate  120  such that the first and second upper auxiliary electrodes  221   a  and  221   b  can be buried therein. The first and second upper auxiliary electrodes  221   a  and  221   b  correspond to the first and second electrodes  122   a  and  122   b , respectively, and are formed in a direction parallel to the first and second upper electrodes  222   a  and  222   b . The first and second upper auxiliary electrodes  221   a  and  221   b  are formed such that the distance therebetween is less than that between the first and second upper electrodes  222   a  and  222   b . The first and second upper auxiliary electrodes  221   a  and  221   b  are floating electrodes in which a voltage is induced via the second dielectric layer  225  as a predetermined voltage is applied to the first and second upper electrodes  222   a  and  222   b . The first and second upper auxiliary electrodes  221   a  and  221   b  may be formed of a transparent conductive material such as ITO and SnO 2  to transmit visible light. Alternatively, the first and second upper auxiliary electrodes  221   a  and  221   b  may be formed of a conductive material such as RuO 2 , Ag, Cu, and Cr. The same is true of first and second electrodes  122   a  and  122   b . The second dielectric layer  225  may be formed of a material having a high dielectric constant or a ferroelectric having a hysterisis property.  
      In the flat lamp configured as above, since the discharge electrodes, which are the first and second lower and upper electrodes  212   a  and  212   b , and  222   a  and  222   b , and the auxiliary electrodes, which are the first and second lower and upper auxiliary electrodes  211   a  and  211   b , and  221   a  and  221   b , are formed on both the lower and upper panels, the brightness and the luminance efficiency are further improved.  
       FIG. 5  is a sectional view illustrating part of a flat lamp according to another embodiment of the present invention. In the following description, only the differences from the above-described embodiments are described below.  
      Referring to  FIG. 5 , first and second auxiliary electrodes  111 ′ a  and  111 ′ b  generating a start discharge are formed on the lower surface of the lower substrate  110 , parallel to each other. A dielectric layer  115 ′ is formed on the lower surface of the lower substrate  110  such that the first and second auxiliary electrodes  111 ′ a  and  111 ′ b  can be buried therein. The dielectric layer  115 ′ is formed thinner than in the above-described embodiments and formed of a material having a high dielectric constant. A pair of first and second electrodes  112 ′ a  and  112 ′ b  generating a sustain discharge are formed on the lower surface of the dielectric layer  115 ′ a , parallel to each other. The distance between the first and second electrodes  112 ′ a  and  112 ′ b  is greater than that between the first and second auxiliary electrodes  111 ′ a  and  111 ′ b . The areas where the first electrode  112 ′ a  overlaps the first auxiliary electrode  111 ′ b  and the second electrode  112 ′ b  overlaps the second auxiliary electrode  111 ′ b  are greater than those in the above-described embodiments.  
      When a material that is thin and has a high dielectric constant is used for the dielectric layer  115 ′ and the areas where the discharge electrodes, which are the first and second electrodes  112 ′ a  and  112 ′ b , overlap the auxiliary electrodes, which are the first and second auxiliary electrodes  111 ′ a  and  111 ′ b , increase, capacitance increases so that a voltage drop is further reduced compared to the above-described embodiments.  
       FIG. 6  is a sectional view illustrating part of a flat lamp according to yet another embodiment of the present invention. Referring to  FIG. 6 , a flat lamp according to the present embodiment includes a lower panel and an upper panel, which are arranged to be separated from each other. A discharge space  330  where a plasma discharge is generated is formed between the lower and upper panels. The discharge space  330  is filled with a discharge gas that is a mixture of neon (Ne) gas and xenon (Xe) gas.  
      The lower panel includes a lower substrate  310  and a dielectric layer  315  formed on a lower surface of the lower substrate  310 . A glass substrate is generally used as the lower substrate  310 . At least one pair of first and second electrodes  312   a  and  312   b  are formed on a lower surface of the lower substrate  310 , parallel to each other. The first and second electrodes  312   a  and  312   b  are discharge electrodes, to which a voltage in the form of pulses from the power source is applied, and formed of a conductive material.  
      At least one pair of first and second auxiliary electrodes  311   a  and  311   b  are formed on an upper surface of the lower substrate  310 , parallel to each other. The first and second auxiliary electrodes  311   a  and  311   b  correspond to the first and second electrodes  312   a  and  312   b , respectively, and are formed in a direction parallel to the first and second electrodes  312   a  and  312   b . The distance between the first and second auxiliary electrodes  311   a  and  311   b  is less than that between the first and second electrodes  312   a  and  312   b . Unlike the auxiliary electrodes  111   a  and  111   b  of the embodiment shown in  FIG. 2 , outer edges of the auxiliary electrodes  311   a  and  311   b  of the embodiment of  FIG. 6  may be substantially co-extensive with the outer edges of the discharge electrodes  312   a  and  312   b , but are wider than the discharge electrodes  312   a  and  312   b , such that the inner edges are closer together. Also, a dielectric layer  315  may be formed on the upper surface of the lower substrate  310  such that the first and second auxiliary electrodes  311   a  and  311   b  can be buried therein.  
      The first and second auxiliary electrodes  311   a  and  311   b  are floating electrodes, to which a voltage is applied via the lower substrate  310  that is a dielectric material as a predetermined voltage is induced to the first and second electrodes  312   a  and  312   b . The first and second auxiliary electrodes  311   a  and  311   b  may be formed of a transparent conductive material such as ITO or SnO 2 , or a conductive material such as RuO 2 , Ag, Cu, or Cr. The same is true of first and second electrodes  122   a  and  122   b.    
      The upper panel includes an upper substrate  320 , which is separated a predetermined distance from the lower substrate  310 . A glass substrate is generally used as the upper substrate  320  like the lower substrate  310 . A plurality of spacers  314  is provided between the lower substrate  310  and the upper substrate  320  to maintain a uniform distance therebetween. A fluorescent layer  313  for generating visible light by being exited by ultraviolet rays generated from the discharge gas by a plasma discharge is formed on portions constituting an interior wall of the discharge space  330 , that is, inner surfaces of the lower substrate  310  and the upper substrate  320  and side surfaces of the spacers  314 .  
      Since the operation of the flat lamp configured as above is the same as that of the above-described embodiments, a detailed description thereof is omitted.  
       FIG. 7  is a sectional view illustrating a modified example of the flat lamp of  FIG. 6 . Referring to  FIG. 7 , a dielectric layer  315 ′ is formed on the upper surface of the lower substrate  310  such that the first and second auxiliary electrodes  311   a  and  311   b  can be buried therein. A trench  315 ′ a  having a predetermined shape to expose the lower substrate  310  is formed in the dielectric layer  315 ′ between the first and second auxiliary electrodes  311   a  and  311   b . The trench  315 ′ a  is formed in a direction parallel to the first and second auxiliary electrodes  311   a  and  311   b . Since not only a surface discharge but also a facing discharge can be generated by the trench  315 ′ a  when a discharge is generated between the first and second auxiliary electrodes  311   a  and  311   b , a luminance efficiency is improved.  
      Although in the present embodiment the discharge electrodes and the auxiliary electrodes are described as being formed in the lower panel only, they can be formed on the upper panel or both the upper and lower panels.  
       FIGS. 8A through 8C  are views illustrating flat lamps used to compare the discharge voltage and luminance efficiency between the conventional flat lamp and the flat lamp according to the present invention.  FIG. 8A  shows a conventional flat lamp in which the distance between discharge electrodes  412   a  and  412   b  is 8 mm.  FIG. 8B  shows a conventional flat lamp in which the distance between discharge electrodes  412 ′ a  and  412 ′ b  is 16 mm.  FIG. 8C  shows a flat lamp according to an embodiment of the present invention in which the distances between discharge electrodes  512   a  and  512   b  and between auxiliary electrodes  511   a  and  511   b , are 16 mm and 8 mm, respectively. In  FIGS. 8A through 8C , copper tapes are used for the discharge electrodes and auxiliary electrodes. In  FIG. 8C , an acetate tape having a dielectric constant of about 2-3 is used as a dielectric layer  415  formed between the discharge electrodes  512   a  and  512   b  and auxiliary electrodes  511   a  and  511   b . In  FIGS. 8A through 8C , reference numerals  410 ,  413 ,  414 , and  420  denote a lower substrate, a fluorescent layer, a spacer, and an upper substrate.  
       FIGS. 9 and 10  are graphs showing the results of the discharge voltage and the luminance efficiency of the flat lamps shown in  FIGS. 8 through 8 C.  FIGS. 9 and 10  show the results measured when a voltage in the form of pulses having a frequency of 20 KHz and a duty ratio of 20% is applied to the discharge electrodes. Here, A and B denote the flat lamp shown in  FIGS. 8A and 8B , respectively, and C and D indicate cases in which the thickness of the dielectric layer of the flat lamp shown in  FIG. 8C  is 40 μm and 120 μm, respectively.  
       FIG. 9  shows a discharge start voltage Vf and a discharge sustain voltage Vs. Referring to  FIG. 9 , the discharge start voltage Vf is 2.48 KV in the conventional flat lamp (case B) in which the distance between the discharge electrodes  412 ′ a  and  412 ′ b  is large. The discharge start voltage Vf is 2.03 kV for the flat lamp (case C) according to the present invention. Thus, it can be seen that the discharge start voltage Vf of the flat lamp (case C) according to the present invention is lowered by about 18% compared to the conventional flat lamp (case B). While the discharge sustain voltage Vs is 1.90 kV in the conventional flat lamp (case B) in which the distance between the discharge electrodes  412 ′ a  and  412 ′ b  is large, the discharge sustain voltage Vs of the flat lamp (case C) according to the present invention is 1.46 kV. Thus, it can be seen that the discharge sustain voltage Vs of the flat lamp (case C) according to the present invention is lowered by about 23% compared to the conventional flat lamp (case B).  
       FIG. 10  shows the results of comparison in the luminance efficiency between the conventional flat lamp and the flat lamp according to the present invention. Referring to  FIG. 10 , while the luminance efficiency is 14.21 lm/W in the conventional flat lamp (case B) in which the distance between the discharge electrodes  412 ′ a  and  412 ′ b  is large, the luminance efficiency of the flat lamp (case C) according to the present invention is 17.9 lm/W. Thus, it can be seen that the luminance efficiency of the flat lamp (case C) according to the present invention is improved by about 26% compared to the conventional flat lamp (case B).  
      As described above, in the flat lamp according to the present invention, since the auxiliary electrodes in which the voltage is induced as the voltage is applied to the discharge electrodes is formed at least one of the upper and lower substrates, the discharge voltage is lowered and the luminance efficiency is improved, compared to the conventional flat lamp.  
      Also, when the range of the discharge voltage applied to the flat lamp according to the present invention and the conventional flat lamp is the same, since a more amount of xenon (Xe) gas can be applied in the flat lamp according to the present invention than in the conventional flat lamp, the luminance efficiency can be further improved.  
      While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. For instance, the auxiliary electrodes and discharge electrodes are shown as being layered on a surface of the substrates, but it should be recognized that the phrase “on the substrates” includes embodiments where the electrodes are embedded in the substrates.