Patent Publication Number: US-6981903-B2

Title: External electrode driven discharge lamp

Description:
This is a divisional application claiming the benefit of U.S. application, Ser. No. 09/647,078, filed Sep. 22, 2000, entitled EXTERNAL ELECTRODE DRIVEN DISCHARGE LAMP, and filed by Jackson P. Trentelman, now U.S. Pat. No. 6,603,248 which is a 371 of PCT/US98/23722 filed Nov. 9, 1998, which claims the benefit of Provisional Application No. 60/079,198, filed Mar. 24, 1998. 

   BACKGROUND OF INVENTION 
   1. Field of Invention 
   The present invention relates to a low-pressure discharge lamp in which external electrodes are employed to drive an electrical gas discharge confined within a laminated envelope. More particularly the present invention relates to such a discharge lamp which could be utilized for the purpose of automotive rear lighting applications. 
   2. Description of Related Art 
   In the neon signage industry, the standard type of electrode employed in low-pressure discharge lamps is the internal electrode. Internal electrodes, as the name provides, are located within the glass tubing and typically consist of a metal shell coated with an emissive coating. A connection to an external power source is made via a wire which is glass-to-metal sealed in the tubing see generally W. Strattman,  Neon Techniques, Handbook of Neon Sign and Cold Cathode Lighting , ST Publications, Inc., Cincinnati, Ohio (1997). 
   A significant problem associated with low-pressure discharge lamps comprising internal electrodes is a reduction in lifetime due to electrode failure resulting from bombardment of the electrode by gas ions, and sputtering away of material from the electrode. Further, failure in these discharge lamps is also associated with leakage at the glass-to-metal seal i.e., at the seal between the glass envelope and the electrode. This mode of failure is particularly true in discharge lamps having borosilicate-to-tungsten wire seals. 
   In contrast to internal electrodes, the activation of an ionizable gas by external electrodes eliminates the aforementioned destruction of electrodes, resulting in longer lamp life, i.e., external electrodes are on the outside of the glass tubing and therefore are not subject to bombardment by gas ions. The term “external electrodes” is meant to refer to electrodes that are not internal to a glass article containing an ionizable gas. 
   An additional feature of driving a discharge through external electrodes is that multiple separate channels can be driven in parallel, unlike driving a discharge through internal electrodes, which will only follow the path of least resistance. 
   Capacitive coupling to a low-pressure discharge, i.e., driving a discharge through external electrodes has been disclosed in U.S. Pat. No. 4,266,166 Proud et al.) and U.S. Pat. No. 4,266,167 (Proud et al.). U.S. Pat. No. 4,266,166 discloses a fluorescent lamp comprising a pear-shaped glass envelope with a reentrant cavity in the lamp envelope. An outer and inner conductor, typically a conductive mesh, is disposed on the outer surface of the envelope and on the reentrant cavity surface, respectively. Similarly, U.S. Pat. No. 4,266,167 discloses a fluorescent lamp comprising a pear-shaped glass envelope with a reentrant cavity. An outer conductor, typically a conductive mesh, is disposed on the outer surface of the lamp envelope, and an inner conductor, typically a solid conductive device, fills the reentrant cavity. Both patents disclose the use of a high frequency of operation, in the range of 10 MHz to 10 GHz. 
   A fluorescent lamp wherein a twin-tube lamp envelope comprises electrodes at or near the ends thereof for capacitive coupling to a low pressure discharge lamp is disclosed in U.S. Pat. No. 5,289,085 (Godyak et al.). Externally located electrodes comprising metal layers or bands at or near the ends of the tube envelope are disclosed. Frequencies in the range of 3 MHz to 300 MHz are suggested. 
   U.S. Pat. No. 5,041,762 (Hartai) discloses a luminous panel comprising a flat glass envelope formed from two plates of glass, the flat glass envelope comprising a gas discharge channel formed by machining a groove on the surface of the plates. Although the preferred embodiment discloses internal electrodes, electrodes of the capacitive type are also suggested. 
   OBJECTS AND ADVANTAGES 
   An object of the present invention is to provide a discharge lamp for use in automotive rear lighting applications having packaging simplicity, long life, energy and cost efficiency by employing external electrodes to drive an electrical gas discharge confined within a laminated envelope. 
   Another object of the present invention is to optimize the capacitive reactance the external electrode site by manipulating the electrode&#39;s geometry with the laminated envelope forming process. 
   SUMMARY OF THE INVENTION 
   According to the present invention, these and other objects and advantages are achieved in a discharge lamp comprising a laminated envelope and external electrodes for inducing an electrical gas discharge. The laminated envelope comprises at least one gas-discharge channel and an ionizable gas confined within the gas discharge channel. The ionizable gas is activated by external electrodes which are in communication with the gas-discharge channel. The external electrodes comprise an electrode surface and a conductive medium on the electrode surface. The electrode surface is integral with the body of the laminated envelope. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention, with reference to the accompanying drawings, in which: 
       FIG. 1  is a plan view of a discharge lamp comprising a laminated envelope, the laminated envelope containing a gas-discharge channel and a pair of external electrodes in communication with the gas-discharge channel. 
       FIG. 1A  is a cross section on line X—X of  FIG. 1 . 
       FIG. 2  is an equivalent, parallel-plate circuit of the discharge lamp shown in  FIG. 1 . 
       FIG. 3  is a plan view of a discharge lamp comprising a laminated envelope, the laminated envelope containing a gas-discharge channel and a pair of external electrodes of a different geometry than the external electrodes of  FIG. 1 . 
       FIG. 3A  is a cross-section on line Y—Y of  FIG. 3 . 
       FIG. 4  is a perspective view of a discharge lamp comprising a laminated envelope, the laminated envelope including four separate gas-discharge channels, in a horizontal parallel arrangement, and external electrodes in communication with and located at opposite ends of each gas-discharge channel. 
       FIG. 5  is a perspective view of a discharge lamp comprising a laminated envelope, the laminated envelope including a continues gas-discharge channel in a serpentine configuration and external electrodes in communication with and located on each of the parallel sections of the gas-discharge channel. 
       FIG. 6  is a cross-sectional view of a laminated envelope suitable for the discharge lamp of the present invention, the laminated envelope including a gas-discharge channel and external electrodes located on the outer top surface, at opposite ends of the gas-discharge channel. 
       FIG. 6A  is a cross-sectional view of a laminated envelope suitable for the discharge lamp of the present invention, the laminated envelope including a gas-discharge channel and external electrodes located on the outer top surface, at opposite ends of the gas-discharge channel. 
       FIG. 6B  is a cross-sectional view of a laminated envelope suitable for the discharge lamp of the present invention, the laminated envelope including a gas-discharge channel and external electrodes located on the outer top and bottom surfaces, at opposite ends of the gas-discharge channel. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is based on a discharge lamp containing a laminated envelope with at least one gas-discharge channel, wherein the discharge is driven by external electrodes, the electrodes comprising a electrode surface integral with the laminated envelope and a conductive medium disposed on the electrode surface. 
   The laminated envelope of the present invention is made according to the methods disclosed in U.S. patent application Ser. No. 08/634,485 (Allen et al.), and in U.S. Pat. No. 5,834,888 (Allen et al.) and Co.-Pending U.S. Provisional Pat. Appln. Ser. No. 60/076,968 having the title “Channeled Glass Article and Method Thereof” and having Stephen R. Allen as sole inventor; co-assigned to the instant assignee and herein incorporated by reference. 
   In U.S. patent application Ser. No. 08/634,485 (Allen et al.), and in U.S. Pat. No. 5,834,888 (Allen et al.) the method of forming glass envelopes containing internally enclosed channels or laminated envelopes comprises the following steps: (a) delivering a first or channel-forming ribbon of molten glass to a surface of a mold assembly having a mold cavity possessing at least one channel-forming groove formed therewithin and a peripheral surface, wherein the channel-forming ribbon overlies the mold cavity and the peripheral surface of the mold assembly; (b) causing the channel-forming ribbon of molten glass to substantially conform to the contour of the mold cavity resulting in the formation of at least one channel in the ribbon of the molten glass; (c) delivering and depositing a second or sealing ribbon of molten glass to the outer surface of the channel-forming ribbon of molten glass wherein the viscosity of the sealing ribbon is such that the sealing ribbon bridges but does not sag into contact with the surface of the channel of the channel-forming ribbon but is still molten enough to form a hermetic seal wherever the sealing ribbon contacts the channel-forming ribbon, thereby resulting in a glass article possessing at least one enclosed channel; and, (d) removing the glass article from the mold. Conformance of the channel-forming molten glass ribbon to the mold cavity is attained by gravity forces, vacuum actuation or a combination of both. The glass envelope formed by the above described method comprises a front surface and a back surface laminated and integrated together to form a unitary envelope body essentially free of any sealing materials and having at least one gas discharge channel. The laminated glass envelope exhibits a weight to area ratio of ≦1.0 g/cm 2 . 
   In Co.-Pending U.S. Provisional Pat. Appl. Ser. No. 60/076,968 the method of forming glass envelopes or laminated envelopes comprises the following steps: (a) delivering and depositing a first or channel-forming ribbon of molten glass to a surface of a mold assembly having a mold cavity possessing at least one channel-forming groove formed therewith and a peripheral surface, wherein the channel-forming ribbon overlies the mold cavity and the peripheral surface of the mold assembly; (b) causing the channel-forming ribbon of molten glass to substantially conform to the contour of the mold cavity resulting in the information of at least one channel in the ribbon of the molten glass; (c) delivering and depositing a second or sealing ribbon of molten glass to the outer surface of the channel-forming ribbon of molten glass wherein the viscosity of the sealing ribbon is such that the sealing ribbon (i) bridges but does not sag into complete contact with the surface of at least one channel of the channel-forming ribbon and (ii) forms a hermetic seal wherever the seal ribbon contacts the channel-forming ribbon to form a glass article with at least one enclosed channel; (d) causing the sealing ribbon to stretch so that the sealing ribbon has a thin cross-section and so that the hermetic seal between the sealing ribbon and the channel ribbon has a thin cross-section; and, (e) removing the glass article from the mold. The glass envelope formed by the above described method comprises a front surface and a back surface laminated and integrated together to form a unitary envelope body essentially free of any sealing materials and having at least one gas discharge channel, wherein the gas-discharge channel has a front surface having a thin cross-section and wherein the laminated glass envelope has a thin cross-section. The laminated glass envelope exhibits a weight to area ratio of ≦1.0 g/cm 2 . 
     FIGS. 1 and 1A  present a typical embodiment of the discharge lamp of the present invention. 
   Discharge lamp  20  comprises a laminated envelope  24  having a front surface  28  and a back surface  32  laminated and integrated together to form a unitary body essentially free of any sealing materials. Laminated envelope  24  preferably exhibits a weight to area ratio of ≦1.0 g/cm 2 . Laminated envelope  24  includes gas-discharge channel  36 . Tubulation port  40  is in communication with the external environment and gas-discharge channel  36 . At tubulation port  40 , gas-discharge channel  36  is evacuated and backfilled with an ionizable gas. After evacuation and backfilling, tubulation port  40  is sealed, whereby communication with the external environment is discontinued. 
   Any of the noble gases or mixtures thereof may be used for the ionizable gas, including but not limited to neon, xenon, krypton, argon, helium and mixtures thereof with mercury. In a preferred embodiment discharge lamp  20  is a neon lamp. A pressure preferably of 5–6 torr is used for neon. 
   Laminated envelope  24  disclosed hereinabove is preferably comprised of a transparent material such as glass selected from the group consisting of soda-lime silicate, borosilicate, aluminosilicate, boro-aluminosilicate and the like. 
   External electrodes  44  are in communication with, and located at each end of gas-discharge channel  36 . Communication between external electrodes  44  and gas-discharge channel  36  is achieved via passageways  48 . It is to be understood, however, that passageway  48  is present only for styling or process related reasons. Alternatively, passageway  48  may be removed, whereby the gas-discharge channel is contiguous with the external electrodes. It may also be contemplated to apply a conductive medium to the passageways, whereby the passageways effectively become part of the external electrode structure. 
   A ballast or a high voltage source  100  is connected to the external electrodes via connector leads  98  to drive the discharge. Suitable ballasts and connector leads are well known in the art. 
   Referring now to  FIG. 1A , external electrode  44  comprises electrode surface  52  and conductive medium  60  disposed on said electrode surface  52 . Electrode surface  52  forms an elongated receptacle. A key aspect of the present invention is that the electrode surface is integral with the laminated envelope structure. As such, the envelope forming process herein above described requires modification to allow for simultaneous formation of at least one electrode surface integral with the laminated envelope. This can be achieved by modifying the mold cavity to include an electrode surface-forming groove, whereby there is formation of a laminated envelope comprising a gas-discharge channel and an electrode surface. 
   As used herein “electrode surface” refers to that section of the laminated envelope which if coated with a conductive medium forms an external electrode capable of coupling to a power source. It is to be understood that the described method of electrode surface formation is a preferred embodiment and that other methods of formation can be utilized to achieve a similar envelope structure, one such being separate formation of an electrode surface receptacle and attachment thereof to the discharge channel via a sealant such as a glass frit. 
   The discharge lamp shown in  FIGS. 1 and 1A  comprises a laminated envelope with two external electrodes. Alternatively, a laminated envelope comprising one electrode surface integral with the body of the laminated envelope and a conductive medium disposed on the electrode surface is suitable for the present invention. A discharge lamp comprising a laminated envelope with one external electrode and one gas-discharge channel is capable of illumination since, as it is well known, the surrounding environment is a conductive medium and hence effectively becomes a second external electrode. Nonetheless, to achieve optimum operating conditions in a discharge lamp comprising the above described laminated envelope a second external electrode should be provided, i.e., application of conductive tape or a separate, external electrode glass structure to the laminated envelope whereby the second electrode is in communication with the gas-discharge channel. 
   In the present invention it has been found that the ability to couple effectively is a direct result of the envelope forming process herein above described. More specifically, the forming process is particularly suitable for producing external electrodes having a maximum electrode area and a minimum electrode thickness. The terms “electrode area” and “electrode thickness” refer to the area of the conductive medium disposed on the electrode surface, and to the thickness of the glass at the electrode surface, respectively. 
   The importance of electrode area and electrode thickness in the present invention becomes apparent after an investigation of  FIG. 2 . This figure presents a simple, parallel-plate RC circuit of discharge lamp  20 , herein illustrated in  FIGS. 1 and 1A . The RC circuit is connected to a ballast  68 . The schematic shows in series, two parallel-plate capacitors C 1  and C 2 , each having a dielectric D, and a resistance R L . The two parallel-plate capacitors represent external electrodes  44  and the ionizable gas in gas-discharge channel  36 , which effectively form the conductors of capacitors C 1  and C 2 . The ionizable gas in gas-discharge  36  is a conductive medium and has an effective resistance represented by R L . The glass of gas-discharge channel  36  effectively acts as dielectric D between the conductors of capacitors C 1  and C 2 . 
   It is well known that the capacitance (C) of filled capacitors C 1  and C 2 , in a parallel-plate capacitor, is given by the formula:
 
 C =κ(∈ 0   A/d )
 
where
 
κ=dielectric constant
 
∈ 0 =permitivity of space ( C   2   /N·m   2 )
 
A=electrode area
 
d=electrode thickness.
 
   The capacitive reactance (C R ) associated with capacitors C 1  and C 2  is given by the formula:
 
 C   R =1/(2 πƒC )
 
where
 
ƒ=frequency of ballast  68 
 
C=capacitance.
 
   A preferred situation is attained when C R  is small. At low values of C R , excess voltage across the electrode is small thereby reducing the maximum voltage requirement of the ballast. The light output of the discharge lamp is optimized by tuning the drive circuit to the load impedance. This is most easily achieved when C R  is small compared to R L , i.e., when C R  is a fraction of R L . 
   Low values of C R  are obtained by increasing C or by using high frequencies of operation, i.e., 10 MHz to 1 GHz or more. High frequencies of operation, however, are expensive and lead to other problems such as high electromagnetic interference. In order to meet customer requirements of low cost and energy efficiency, an objective of the present invention is to use low operating frequencies, preferably in the range of 100 kHz to 1000 kHz, and most preferably about 250 kHz. 
   Therefore, in order to operate at low frequencies and to have low values of C R , C must be large. C for a filled capacitor is inversely proportional to the thickness of the dielectric, and proportional to the surface area of the conductors. In the present invention, a large C is obtained by decreasing the electrode thickness and increasing the electrode area. 
   As described herein above a small electrode area and thickness are achieved via the envelope forming process. Briefly and more specifically, the stretching of the glass during the forming process to the contour of a preformed mold cavity by gravity, vacuum actuation or a combination of both, renders a structure of maximum area and minimum thickness at the electrode site. Therefore, in the present invention C R  is a function of the envelope forming process. 
   For effective coupling at 250 kHz, the electrode surface area is in the range of 6.54-25.81 cm 2 , and the electrode thickness is in the range from 0.5 mm to 1.5 mm, preferably about 0.75 mm. 
   The present invention allows for discharge lamp designs incorporating equivalent light output by decreasing the gas-discharge channel length and increasing the current correspondingly. Increasing the current and hence sputtering does not have an effect on the external electrodes since their location is on the outside of the envelope and not in direct contact with the ionizable gas ions. 
   The present invention is illustrated by the nonlimiting examples given in the following Table. Neon discharge lamps comprising laminated envelopes were driven with both internal and external electrodes. Example 1 is a discharge lamp comprising a laminated envelope having a gas-discharge channel of 210 cm, the channel having a non-circular inner diameter of approximately 8 mm. Example 2 is a discharge lamp comprising a laminated envelope having a gas-discharge channel of 37 cm, the channel having a non-circular inner diameter of approximately 5 mm. Example 3 is a discharge lamp comprising a laminated envelope having a gas-discharge channel of 140 cm, the channel having a non-circular diameter of approximately 5 mm. Example 4 is a discharge lamp comprising a laminated envelope having a gas-discharge channel of 55 cm, the channel having alternating wide and narrow sections and an inner diameter in the narrow sections of 3 mm. 
   Examples 1, 2, and 3 have an electrode thickness of 0.75 mm, and Example 4 has an electrode thickness of 0.50 mm. 
   The power source for the internal electrodes was a 30 mA DC driven ballast. The operating point was chosen as the point at which the light emitting efficiency was the greatest, i.e., at a lamp resistance of 50 kohm. An equal light output condition was maintained for the internal and external electrode configurations. The power source for the external electrodes was a variable frequency plasma generator. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE 
             
           
          
             
                 
                 
             
             
                 
               1 
               2 
               3 
               4 
             
          
         
         
             
             
             
             
             
             
             
             
             
          
             
                 
               Internal 
               External 
               Internal 
               External 
               Internal 
               External 
               Internal 
               External 
             
             
                 
               Electrode 
               Electrode 
               Electrode 
               Electrode 
               Electrode 
               Electrode 
               Electrode 
               Electrode 
             
             
                 
               Coupling 
               Coupling 
               Coupling 
               Coupling 
               Coupling 
               Coupling 
               Coupling 
               Coupling 
             
             
                 
                 
             
          
         
         
             
             
             
             
             
             
             
             
             
          
             
               Frequency 
               28 
               292 
               29 
               278 
               28 
               285 
               28 
               290 
             
             
               (kHz) 
             
             
               R L  (kohms) 
               50 
               50 
               50 
               50 
               50 
               50 
               50 
               50 
             
             
               C R  (kohms) 
               — 
               9 
               — 
               50 
               — 
               8 
               — 
               6 
             
             
               Light 
               350 
               350 
               60 
               60 
               244 
               244 
               73 
               73 
             
             
               Output 
             
             
               (lux) 
             
             
               Power 
               45.8 
               45.8 
               9.4 
               9 
               36.8 
               34.5 
               12.2 
               12.5 
             
             
               (watts) 
             
             
               Light 
               7.64 
               7.95 
               6.38 
               6.67 
               6.63 
               7.07 
               5.98 
               5.84 
             
             
               Emitting 
             
             
               Efficiency 
             
             
               (lux/watt) 
             
             
                 
             
          
         
       
     
   
   It has been observed that there is no fundamental difference in how power is applied to the discharge lamps of the following Table, i.e., whether the discharge is driven by internal or external electrode configurations, as long as the circuit is tuned to the proper operating frequency when driving through external electrodes, i.e., the frequency at which the greatest light emitting efficiency is achieved. In the laboratory experiment examples tuning was achieved with a variable frequency plasma generator. In a non-laboratory environment tuning may be achieved either through a self-tuning ballast or a ballast that is tuned to the circuit of each discharge lamp. 
   In each example, the light emitting efficiency is the same for both internal and external electrode configurations, within experimental error. Hence, in a discharge lamp of the present invention external electrodes provide the same or better light emitting efficiency as an internal electrodes, with the added advantage of no sputtering or leakage failure mechanisms at the electrode site. 
     FIGS. 3 and 3A  illustrates another embodiment of a discharge lamp according to the present invention. The embodiment has a preferred external electrode geometry. The discharge lamp  80  includes laminated envelope  82 , which has a first or front surface  102  and a second or back surface  104 . External electrodes located at or near opposite ends of gas-discharge channel  84  are in communication with the gas-discharge channel through passageways  90 . Tubulation port  86  is separate from the electrode. The external electrodes  88  comprise a conductive medium  94  disposed on electrode surface  92 . The electrode surface forms a plurality of contiguous round receptacles, each with a rounded shape. Electrical leads  164 , such as wires or other conduits, conduct a power source  160  with the external electrodes  88  to activate the lamp. 
   The conductive medium  94  is either applied as a coating or a film and includes but is not limited to conductive coatings, conductive epoxies, conductive inks, frit with conductive filler, and the like or mixtures thereof. An example of a conductive coating suitable as a conductive medium is indium tin oxide. A coating of indium tin oxide is formed by, but is not limited to sputtering, evaporation, chemical deposition and ion implantation. 
   In a further embodiment a discharge lamp comprises a laminated envelope, where the laminated envelope comprises a plurality of separate gas-discharge channels and external electrodes in communication with said channels, whereby a discharge is driven in parallel, as illustrated in  FIG. 4 . Discharge lamp  50  comprises laminated envelope  54 , wherein said laminated envelope comprises four separate gas-discharge channels  56 , in a parallel arrangement. External electrodes  58  are in communication with and located at opposite ends of each gas-discharge channel  56 . Connection to ballast  62  is made with connector leads  60 . 
     FIG. 5  illustrates another embodiment of discharge lamp  70 , which comprises a laminated envelope  72 , with a continuous gas-discharge channel  74  in a serpentine configuration. External electrodes  76  are located on parallel sections of the gas-discharge channel  74 . As shown, two external electrodes, one at each end, are in capacitative communication with each section of the gas-discharge channel. Connection to ballast  80  is made with connector leads  78 . 
   Referring now to  FIGS. 6 ,  6 A, and  6 B illustrated therein are cross-sectional views of further embodiments of laminated sheet envelopes suitable for the present invention. Laminated envelope  90  comprises gas-discharge channel  94  and external electrodes  98 . In the embodiments illustrated in  FIGS. 6 and 6A , the external electrodes are applied as a coating or film directly to the top outer surface of gas-discharge channel  94 , and are located at each end of the channel. In the embodiment illustrated in  FIG. 6B , the external electrodes are applied as a coating or film directly to the top and bottom outer surfaces of gas-discharge channel  94 . 
   Although the now preferred embodiments of the invention have been set forth, it will be apparent to those skilled in the art that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as set forth in the following claims.