Gas discharge tube, lighting device using such a gas discharge tube and a method of operating such a lighting device

Abstract: The present invention is to provide a gas discharge tube which is properly lighted to improve the reproducibility of a satisfactory light emission state. The gas discharge tube includes an envelop (11) for accommodating a hot cathode (25) for emitting thermoelectrons, an anode (24) for receiving the thermoelectrons, a focusing electrode (26) having a focusing opening for converging paths of the thermoelectrons, and a discharge shielding member (21) supporting the focusing electrode and the anode, the discharge shielding member further supporting a cathode box for surrounding the hot cathode while electrically insulated from the focusing electrode. And since a lighting device for the gas discharge tube sets the focusing electrode a zero or negative potential while an electric field generated between the hot cathode and the anode, the lighting device will be certainly turn on.

Description: The invention relates to a gas discharge tube, a lighting device therefor, and a method of operating the lighting device. Embodiments of the invention may be used as an ultraviolet light source for a spectrophotometer, liquid chromatrography, or the like. A gas discharge tube is a discharge light source using positive column light emission by arc discharge of a gas sealed in a tube. As a typical gas discharge tube, a deuterium discharge tube in which ultraviolet light is emitted by discharge of sealed deuterium is well known. The deuterium discharge tube is mainly used as an ultraviolet continuous spectrum source for a spectrophotometer or the like. Note that a deuterium discharge tube having a light-emitting portion is described in, e.g., Japanese Utility Model Laid-Open JP-U-60-29400. In EP-A-473378 there is described a gas discharge tube having an outer envelope in which deuterium gas is enclosed. In the envelope, an anode, a cathode and a first shield cover for surrounding these electrodes are disposed. A second shield cover is disposed within the first shield cover and at a position adjacent the anode to divide an internal space defined by the first shield cover into a first chamber in which the anode is positioned and a second chamber in which a cathode is positioned. A plasma arc generating portion is positioned adjacent the second shield cover. A plasma arc generated on the plasma arc generating portion provides an optical axis extending linearly toward the outer envelope through an opening in the first shield cover. The cathode is disposed at a position offset from the optical axis for providing a flow line of electrons from the cathode to the anode in an oblique direction with respect to the optical axis. A shield member is further provided at a position immediately adjacent the plasma arc generating portion for largely bending the flow line of the electrons around the tip end portion of the shield member and for directing the flow line substantially coincident with the optical axis. It is desirable to provide a gas discharge tube which is properly lighted to improve the reproducibility of a satisfactory light emission state. It is also desirable to provide a method of operating a lighting device for a gas discharge tube and a lighting device for a gas discharge tube, which maintain stable discharge and increases a light emission intensity. According to the invention there is provided a gas discharge tube comprising: an envelope for sealing a gas therein; lead pins introduced into an inner space of said envelope from externally of said envelope; and a light-emitting section in said envelope, positioned at distal ends of said lead pins and supported by said lead pins while spaced from an inner side wall of said envelope, said light-emitting section including: a hot cathode for emitting thermoelectrons; an anode for receiving the thermoelectrons emitted from said hot cathode; a focusing electrode for focusing said thermoelectrons on to said anode, the focusing electrode having a focusing opening through which the thermoelectrons pass to be received by said anode; and a cathode box surrounding said hot cathode, for partially shielding discharge of said hot cathode, said cathode box having a cathode slit electrode located between said hot cathode and said focusing electrode, said cathode slit electrode having a slit through which the thermoelectrons emitted from said hot cathode pass, and a front electrode facing to said focusing electrode so as to accommodate said hot cathode and said cathode slit electrode in a space defined by said front electrode and said focusing electrode, said front electrode having a window located at a position which faces to the focusing opening of said focusing electrode, for outputting light caused by discharge, characterized in that: said cathode slit electrode and said front electrode are arranged so as to be spaced from and insulated from said focusing electrode. According to the invention there is also provided a lighting device for use with such a gas discharge tube. According to the invention there is also provided a method of operating such a lighting device. In Fig. 1, there is shown a gas discharge tube 10, which comprises: a glass envelope 11 for sealing a gas therein; lead pins 13 to 16 introduced into an inner space of the envelope 11 from external of the envelope 11; and a light-emitting section 20 in the glass envelope 11, positioned at distal ends of the lead pins 13 to 16 and supported by these lead pins while spaced from an inner side wall of the glass envelope 11. The anode 24 is a metal plate mounted on and supported by the distal end of the lead pin 14. In particular, as shown in Figs. 2, 6 and 7, the light-emitting section 20 includes: a discharge shielding member 21 of an insulator, such as a ceramic, having a front surface 21A and a rear surface 21B which is opposite to the front surface 21A; a hot cathode 25 for emitting thermoelectrons, positioned at the front surface side of the discharge shielding member 21; an anode 24 for receiving the thermoelectrons emitted from said hot cathode 25, mounted on and supported by the rear surface 21B of the discharge shielding member 21; a focusing electrode 26 having a focusing opening 262 for converging paths of the thermoelectrons and for passing through the thermoelectrons which is emitted from the hot cathode 25 and which is received by the anode 24; and a cathode box surrounding the hot cathode 25, for partially shielding discharge of the hot cathode 25. The cathode box is mounted on and supported by the front surface 21A of the discharge shielding member 21, and is electrically insulated from the focusing electrode 26. The cathode box is constituted by: a cathode slit electrode 27 located between the hot cathode 25 and the focusing electrode 26, having a slit 270 for passing through the thermoelectrons emitted from the hot cathode 25, and a front electrode 23 facing to the focusing electrode 26 so as to accommodate the hot cathode 25 and the cathode slit electrode 27 in a space defined by the front electrode 23 and the focusing electrode 26, the front electrode 23 having an opening window 230 located at a position which faces to the focusing opening 262 of the focusing electrode 26, for outputting ultraviolet light (U.V) caused by discharge. And, the cathode slit electrode 27 and the front electrode 23 of the cathode box are in directly contact with each other, thereby each electrode has a same potential. Furthermore, as shown in Fig. 3, the discharge shielding member 21 has a recessed portion 211 at the rear surface 21B of the discharge shielding member 21, for accommodating the anode plate 24. As shown in Fig. 8, the recessed portion 211 of the discharge shielding member 21 has a depth W21 matching a sum of a height W225 of the projections 225 of the support member 21 and a thickness W24 of the anode 24, thereby the front surface 22B of the support member 22 is in partially contact with the rear surface 21B of the discharge shielding member 21. The discharge shielding member 21 is in contact with parts C2 (see Fig.4) of the anode plate 24 at parts C1 of the rear surface 21B of said discharge shielding member 21. Therefore, the anode plate 24 covers an opening of the through hole 210 at the rear surface side of the discharge shielding member 21. The focusing electrode 26 is also supported by the discharge shielding member 21 and in contact with the front surface 21A of the discharge shielding member 21, thereby a distance between the focusing electrode 26 and the anode plate 24 is kept at a constant distance. The light-emitting section, as shown in Figs. 2 and 4, further comprises a support member 22 of an insulator, such as a ceramic, being in the glass envelope 11 and located on an opposite side to the discharge shielding member 21 through the anode plate 24, the support member 22 having projections 225 for pushing parts C4 of the anode plate 24 onto parts C1 of the rear surface 21B of the discharge shielding member 21. Therefore, the parts C1 of the discharge shielding member 21 are brought into contact with the parts C2 of the anode plate 24, and the parts C3 of the support member 22 are brought into contact with the parts C4 of the anode plate 24. Particularly, a top surface of each projection 225 contains the part C3 contacting the associated part C4 of the anode plate 24, but other part of the top surface of each projection 225 faces the rear surface 21B of the discharge shielding member 21 without through the anode plate 24. As a result, the anode plate 24 is gripped at a plurality of parts thereof, and obtains a structure that is effective to increase the heat radiation efficiency of the anode plate. In other words, this structure decreases area contacting insulator having a lower heat conductivity. The support member 22 further has a depression 224 at a front surface 22B which faces to the rear surface 24B of the anode 24, thereby the depression 224 of the support member 22 provides heat dissipation space for the anode 24. Next, as shown in Figs.9, 11, and 12, a lighting device for a gas discharge tube which has at least an anode, a focusing electrode, and a hot cathode, comprises: an anode terminal 142 adapted to be electrically connected to the anode of the gas discharge tube; a focusing electrode terminal 132 adapted to be electrically connected to the focusing electrode of the gas discharge tube; a first and second cathode terminals 152 and 162 adapted to be electrically connected to both ends of the hot cathode of the gas discharge tube; a cathode heating voltage source V F for heating the hot cathode, disposed between the first and second cathode terminals 152 and 162; an electric field generating voltage source V 0 for generating an electric field from the anode to the hot cathode of the gas discharge tube, disposed between the anode terminal 142 and the first and second cathode terminals 152 and 162; a discharge start circuit for supplying a trigger voltage to the focusing electrode in order to start discharge between the hot cathode and the focusing electrode; and a luminance adjustment circuit for adjusting a potential to be supplied to the focusing electrode of the gas discharge tube. A first terminal V10 of the electric field generating voltage source V 0 is electrically connected to the anode terminal 142, and a second terminal V20 of the electric field generating voltage source V 0 is electrically connected to the first and second cathode terminal 152 and 162. A first terminal V30 of the cathode heating voltage source V F is electrically connected to the first cathode terminal 152, and a second terminal V40 of the cathode heating voltage source V F is electrically connected to the second cathode terminal 162. Particularly, the luminance adjustment circuit is constituted by the luminance adjustment voltage source V A and the variable resistor R V arranged in series so that a second terminal V60 of the luminance adjustment voltage source V A and a first terminal R10 of the variable resistor R V are connected to each other. A first terminal V50 of the luminance adjustment voltage source V A is electrically connected to the first and second cathode terminal 152 and 162, and a second terminal R20 of the variable resistor R V will be electrically connected to the focusing electrode terminal 132 through a luminance adjustment switch S A . The discharge start circuit as shown in Fig. 9, comprises an RC parallel circuit disposed between the anode terminal 142 and the focusing electrode terminal 132. A first terminal RC10 of the RC parallel circuit will be electrically connected to the focusing electrode terminal 132 through the luminance adjustment switch S A , and a second terminal RC 20 of the RC parallel circuit will be electrically connected to the anode terminal 142 through a trigger switch S T . The discharge start circuit shown in Fig.11, is constituted by a fixed resistor R C and a capacitor C. A first terminal R30 of the fixed resistor R C is electrically connected to the anode terminal 142, and a second terminal R40 of the fixed resistor R C will be connected to a second terminal C20 of the capacitor C. A first terminal C10 of the capacitor C is electrically connected to the first and second cathode terminals 152 and 162, and the second terminal C20 of the capacitor C will be electrically connected to one of the second terminal R40 of the fixed resistor R C and the focusing electrode terminal 132 through the trigger switch S T and the luminance adjustment switch S A . The discharge start circuit shown in Fig. 12, comprises a discharge start voltage source V T and an RC series circuit arranged in parallel. A first terminal V70 of the discharge start voltage source V T will be electrically connected to the second terminal RC40 of the RC series circuit through the trigger switch S T , and a second terminal V80 of the discharge start voltage source V T is electrically connected to the first and second cathode terminals 152 and 162. A first terminal RC30 of the RC series circuit is electrically connected to the first and second cathode terminals 152 and 162, and the second terminal RC40 will be electrically connected to one of the anode terminal 142 and the first terminal V70 of the discharge start voltage source V T through the trigger switch S T . The operation for the above described lighting device is carried out as follows. First, the cathode heating voltage source V F supplies a predetermined voltage to the hot cathode for heating the hot cathode, sequentially (after the hot cathode has been heated) the discharge start voltage source V 0 supplies a predetermined voltage between the anode and the hot cathode. After an electric field is generated from the anode to the hot cathode, the discharge start circuit supplies a predetermined voltage sufficient to generate discharge between the hot cathode and one of the focusing electrode and the anode, to one of the focusing electrode and the anode. From now on, the luminance adjustment circuit adjusts a potential of the focusing electrode by supplying a predetermined voltage so as to set the potential of the focusing electrode zero or negative. In the gas discharge tube embodying the invention, the focusing electrode 26 is electrically insulated from the cathode slit electrode 27 and the front electrode 23. With this structure, after the hot cathode is sufficiently heated, and a predetermined electric field is supplied to a space between the hot cathode and the anode, a positive potential higher than that of the front electrode and the cathode slit electrode will be supplied to the focusing electrode in generation of trigger discharge between the hot cathode and the focusing electrode. For this reason, the trigger discharge area extending from the cathode extends from a space surrounded by the front electrode and the cathode slit electrode, i.e., a space in the cathode box to reach the focusing electrode. Therefore, arc discharge will be easily generated between the hot cathode and the anode. In addition, in the lighting device for use with the gas discharge, the luminance adjustment circuit constituted by the luminance adjustment voltage source, the variable resistor, and the luminance adjustment switch, all of which are sequentially arranged in series, is arranged between the hot cathode and the focusing electrode. With this arrangement, when the luminance adjustment switch is set in an ON state with respect to the luminance adjustment circuit upon occurrence of arc discharge, a zero potential or a negative potential will be supplied from the luminance adjustment voltage source to the focusing electrode through the variable resistor. Since the focusing electrode repels electrons and attracts cations, an ion sheath is generated in a space in front of or near the focusing electrode. Therefore, the arc ball can be compressed or focused by this ion sheath. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention will become apparent to those skilled in the art form this detailed description. IN THE DRAWINGS Fig. 1 is a perspective view showing a gas discharge tube. Fig. 2 is an exploded perspective view of a light-emitting portion 20 of the gas discharge tube in Fig. 1. Fig. 3 is a perspective view showing a discharge shielding member and an anode in the light-emitting portion in Fig. 2 before assembly. Fig. 4 is a perspective view showing a support member and the anode in the light-emitting portion in Fig. 2 before assembly. Fig. 5 is a plan view showing a positional relationship among the discharge shielding member, the anode, and the support member in the light-emitting portion in Fig. 2. Fig. 6 is a sectional view taken along a line A - A in Fig. 5. Fig. 7 is a sectional view taken along a line B - B in Fig. 5. Fig.8 is a sectional view for explaining a structure of the recessed portion of the discharge shielding member in Fig. 3. Fig. 9 is a circuit diagram showing a lighting device for the gas discharge tube in Fig. 1. Fig. 10 is a graph showing a relationship between the potential of a focusing electrode of the gas discharge tube in Fig. 1 and an optical output. Fig. 11 is a circuit diagram showing the lighting device for the gas discharge tube in Fig. 1. Fig. 12 is a circuit diagram showing the lighting device for the gas discharge tube in Fig. 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The arrangement and function of a gas discharge tube will be described below in detail with reference to Figs. 1 to 12. Note that this gas discharge tube is a side-on type deuterium discharge tube which extracts light (U.V) from the side portion or the tube. In a gas discharge tube 10 shown in Fig. 1, a light-emitting section 20 is accommodated in a glass envelope 11 while being supported by lead pins 13 to 16. Deuterium gas in the glass envelope 11 is sealed at about several Torr. The envelop 11 with its head portion sealed has a cylindrical shape, and the bottom portion of the envelope 11 is hermetically sealed by a glass stem 12. The envelop 11 is formed of ultraviolet light transmitting glass or quartz glass having a high permeability to ultraviolet light. The four lead pins 13 to 16 which are in parallel aligned in line extend through the glass stem 12 from the bottom portion of the light-emitting section 20, and are covered by insulating members 130, 140, 150, and 160, respectively. Terminals 131, 141, 151, and 161 will be connected to terminals 132, 142, 152, and 162 of an external power supply. The light-emitting section 20 has a shielding box structure in which a ceramic support member 22 and a metal front electrode 23 are bonded each other while a ceramic discharge shielding member (spacer) 21 is sandwiched therebetween. The anode plate 24 is sandwiched by the discharge shielding member 21 and the support member 22, and is accommodated in a space defined by these members 21 and 22. The embodiments of this invention relate to for example US Patent Application No. 08/305,972, published as US-A-5,552,669, Japanese Patent Laid-Open JP-A-4-255662, and the like. Particularly, a basic structure of the gas discharge tube and materials of members such as anode, etc. are described in these references. For example, the discharge shielding member 21 and the support member 22 are made of ceramic as insulator, and the anode plate 24 is made of molybdenum as high melting point metal. In this specification, the high melting metal means material group having a higher melting point than sillicon (1414°C). The structure of the light-emitting section 20 will be described below in detail with reference to Figs. 2 to 8. As shown in Figs. 2 and 3, the flat plate-like discharge shielding member 21 has a swell section thinner and wider than the support member 22. The discharge shielding member 21 has a through hole 210, a recessed portion 211, a vertical through hole 212, four horizontal through holes 213, two horizontal through holes 214 and four horizontal through holes 215. The through hole 210 extends through the discharge shielding member 21 almost at its central portion to oppose the anode 24. The recessed portion 211 is recessed from the rear surface 21B of the discharge shielding member 21. The recessed portion 211 also includes an opening edge portion located on the rear surface side of the through hole 210. The vertical through hole 212 extends in the vertical direction with respect to the bottom surface 11A of the envelope 11, and passes through the discharge shielding member 21. The four horizontal through holes 213 respectively extend in the horizontal direction with respect to the bottom surface 11A of the envelope 11. The two horizontal through holes 214 of the discharge shielding member 21 are formed at positions to insert lock pawls 271 of a cathode slit electrode 27 (to be described later). The four horizontal through holes 215 are formed at positions to insert lock pawls 231 of the front electrode 23 (to be described later). One side of a substantially L-shaped electrode rod 216 is inserted in the vertical through hole 212, and the lower end of the electrode rod 216 is exposed from the discharge shielding member 21 so as to be welded to the distal end of the lead pin 15. Therefore, the discharge shielding member 21 will be held by the glass stem 12 through the electrode rod 216. Electrode rods 250 and 251 are welded to the two ends of a hot cathode (filament) 25. The electrode rod 250 is welded to the distal end of the electrode rod 216, and the electrode rod 251 is welded to the distal end of the lead pin 16. With this structure, the hot cathode 25 will be held by the stem 12. As shown in Figs. 2 and 4, the support member 22 is a prism having a swell section 22A. A vertical through hole 220, concave grooves 221 to 223, a depression 224, four protrusions portions (projections 225), and four horizontal through holes 226 are formed in the support member 22. The vertical through hole 220 vertically extends in the vertical direction with respect to the bottom surface 11A of the envelope 11, and passes through the section 22A of the support member 22. The concave grooves 221 to 223 and the depression 224 are recessed on the front surface 22B of the support member 22 and sequentially extend toward the bottom surface 11A of the envelope 11, thereby appropriately accommodating the lead pin 14 and an insulating member 141. The four projections 225 are formed close to the opening edge portions of the concave grooves 221 and 222 and project from the front surface 22B of the support member 22 so as to oppose the corners of an anode plate 24. The four horizontal through holes 226 extend in the horizontal direction to oppose four the horizontal through holes 213 of the discharge shielding member 21. The support member 22 is held by the glass stem 12 through the lead pin 13 inserted in the vertical through hole 220 and the lead pin 14 accommodated in the concave grooves 221 to 223. The rectangular flat plate-like anode 24 is welded and fixed to the distal end of the lead pin 14 and pushed onto the rear surface side of the discharge shielding member 21 by the four projections 225 of the support member 22. A heat dissipation space is ensured at the back of the anode 24 by providing the depression 224 having an opening substantially matching the surface area of the anode 24. As shown in Figs. 5, 7, and 8, the rectangular anode 24 indicated by a broken line is accommodated in the recessed portion 211 of the discharge shielding member 21. The corner portions of the anode 24 are sandwiched by cooperation of the bottom surface 211A of the recessed portion 211 of the discharge shielding member 21 and the four projections 225 of the support member 22. The four sides of the anode 24 almost match the slightly round through hole 210 having a substantially rectangular shape. The remaining portion of the opening edge portion of the through hole 210 is joined to the four corner portions of the anode 24. The four projections 225 each having a circular surface are joined to the four corner portions of the anode 24, thereby pressing the anode 24. In particular, as shown in Fig. 8, the rectangular recessed portion 211 has a depth W21 matching the sum of the height W225 of the four projection 225 and the thickness W24 of the anode 24. As a result, the peripheral portion formed on the front surface 22B of the support member 22 will be brought into contact with the rear surface 21B of the discharge shielding member 21. On the other hand, as shown in Figs. 3 and 4, the parts C1 of the rear surface 21B of the discharge shielding member 21 are brought into contact with the parts C2 of the anode plate 24, and the parts C3 of the top surface of each projection 225 are brought into contact with the parts C4 of the anode plate 24. Thereby, the anode plate 24 is gripped by the projections 225 and the rear surface 21B of the discharge shielding member 21. The top surface of each projection 225 is defined by the part C3 contacting the associated part C4 of the anode plate 24 and other part facing the rear surface 21B of the discharge shielding member 21 without through the anode plate 24. That is, this structure increase the heat ratio efficiency of the anode plate 24 because a specific area of the anode plate 24 decreases, the area being directly in contact with insulator (the discharge shielding member 21, the support member 22, etc.) having a lower heat conductivity decreases. As shown in Figs. 2 and 6, a focusing electrode 26 constituted by bending a metal plate into a substantially L-shape and an opening limit plate 261, and has an opening 260 and four horizontal through holes 263. The opening 260 is arranged coaxial with the through hole 210 of the discharge shielding plate 21. The opening limit plate 261 for limiting the diameter of the opening is welded to the peripheral region of the opening 260. The opening limit plate 261 has a focusing opening 262 bent toward the anode 24 to pass through the opening 260. The four horizontal through holes 263 extend in the direction of thickness of the focusing electrode 26 and oppose the four horizontal through holes of the discharge shielding member 21. The focusing electrode 26 is arranged so as to be in contact with the front surface 21A of the discharge shielding member 21. Since a distal end portion 26A bent backward is welded to the distal end of the lead pin 13 projecting from the support member 22, the focusing electrode 26 is fixed to the discharge shielding member 21 and the support member 22. Note that the distance between the opening limit plate 261 and the anode 24 is smaller than the thickness of the discharge shielding member 21. The horizontal through holes 226 of the support member 22, the horizontal through holes 213 of the discharge shielding member 21, and the horizontal through holes 263 of the focusing electrode 26 are aligned in line. Therefore, when the discharge shielding member 21, the support member 22, and the focusing electrode 26 are bonded each other, and four metal rivets 28 are inserted into the through holes, all these components will be integrally fixed to the stem 12. As shown in Figs. 2, 6, and 7, the cathode slit electrode 27 is bent at three portions in correspondence with the shape of the stepped portion 217 of the discharge shielding member 21 and has an opening (slit 270) and two lock pawls 271. With respect to the bottom surface 11A of the envelope 11, the opening 270 having a vertically elongated rectangular shape extends through the cathode slit electrode 27. The two lock pawls 271 formed at the upper and lower ends of the cathode slit electrode 27 are bent backward. The cathode slit electrode 27 opposes the hot cathode 25 and is positioned in front surface side of the discharge shielding member 21 on its one side. When the two lock pawls 271 are inserted into the two horizontal through holes 214 of the discharge shielding member 21, the cathode slit electrode 27 is engaged with the discharge shielding member 21. Note that the slit 270 is arranged between the hot cathode 25 and the opening limit plate 261. The metal front electrode 23 is bent at four portions to have a substantially U-shaped section and has an opening window 230 and four lock pawls 231. The rectangular opening window 230 is arranged coaxial with the focusing opening 262 of the focusing electrode 26. The four lock pawls 231 formed at the corners of the front electrode 23 project backward. Note that the opening window 230 is arranged at a position to project ultraviolet light from the space in front of or near the focusing opening 262. The front electrode 23 is arranged at the front surface 21A of the discharge shielding member 21 on its both sides. When the four lock pawls 231 are inserted into the four horizontal through holes 215 of the discharge shielding member 21, the front electrode 23 is engaged with the discharge shielding member 21. The front end of the cathode slit electrode 27 is brought into contact with the inner wall of the front electrode 23, thereby separating a space where the hot cathode 25 is arranged from a light-emitting space where arc discharge occurs. The operation of this embodiment will be described below. When a power of about 10 W is supplied from the external power supply in 20 seconds before discharge, a predetermined rated voltage is applied to the hot cathode 25, and the hot cathode 25 is preheated. After the hot cathode 25 is sufficiently heated, a DC open voltage of about 150 V is applied between the hot cathode 25 and the anode 24. Upon completion of preparation for arc discharge, a trigger voltage of 350 to 500 V is applied between the hot cathode 25 and the anode 24, thereby starting arc discharge among the hot cathode 25, the focusing electrode 26, and the anode 24. At this time, the path of thermoelectrons moving in a direction indicated by an arrow 30 is limited to only a discharge path 31 because of the convergent effect of the opening limit plate 261 of the focusing electrode 26 and the shielding effect of the cathode slit electrode 27 and the front electrode 23. More specifically, the thermoelectrons emitted from the hot cathode 25 pass through the focusing opening 262 of the opening limit plate 261 from the slit 270 of the cathode slit electrode 27 and through the through hole 210 of the discharge shielding member 21 and are received by the anode 24. An arc ball 32 as a high-density discharge area by arc discharge is generated in a space in front of or near the opening limit plate 261 on the opposite side to the anode 24 through the discharge shielding member 21. Ultraviolet light (U.V) extracted from the arc ball 32 as positive column light emission is emitted toward the front of the anode 24, i.e., in a direction indicated by an arrow 33 and is projected through the opening window 230 of the front electrode 23. The anode 24 and the focusing electrode 26 are arranged so as to be in contact with the two opening edge portions of the through hole 210 through the discharge shielding member 21. For this reason, the insulated state between the two electrodes 24 and 26 is maintained by the insulating properties of the material of the discharge shielding member 21. Upon occurrence of arc discharge, the anode 24 receives thermoelectrons to generate heat, and the focusing electrode 26 also generates heat upon bombardment of cations. However, the through hole 210 of the discharge shielding member 21 hardly covers the front surface 24A of the anode 24, and the front surface 24A of the anode 24 is exposed at maximum. The projections 225 of the support plate 22 are hardly in contact with the rear surface 24B of the anode 24, and the rear surface 24B of the anode 24 is exposed at maximum. The depression portion 224 of the support member 22 enlarges the back space of the anode 24. For this reason, heat dissipation from the anode 24 and the focusing electrode 26 is promoted. Although the material constituting the discharge shielding member 21 and the support member 22 and having electrical insulating properties generally has a low thermal conductivity, the temperature in the space near the anode 24 and the focusing electrode 25 can be maintained relatively low. As a result, for the anode 24 and the focusing electrode 26 deformation of the material due to a residual stress in a high temperature state, or a loss of the material due to an evaporation or sputtering phenomenon can be minimized. More specifically, a variation in distance between the two electrodes or energization hardly makes the arc discharge state unstable. Additionally, a loss of the material of the two electrodes hardly makes it difficult to generate arc discharge. Therefore, for long-time continuous light emission, the positions of the anode 24 and the focusing electrode 26 are precisely held, thereby realizing a continuously stable operation for a long time. In particular, in the focusing electrode 26, the cathode slit electrode 27, and the front electrode 23 having the above arrangement, the focusing electrode 26 is electrically insulated from the cathode slit electrode 27 and the front electrode 23 through the discharge shielding member 21. On the other hand, the cathode slit electrode 27 and the front electrode 23 are in contact with each other and set at the same potential, for example a zero potential. The arrangement of a lighting circuit connected to the gas discharge tube will be described below with reference to Fig. 9. As shown in Fig. 9, this lighting device is constituted as an auxiliary electrode system. The lighting device has an anode terminal 142 to be connected to the terminal 141 fixed at the distal end of lead pin 14, a focusing electrode terminal 132 to be connected to the terminal 131 fixed at the distal end of the lead pin 13, a first cathode terminal 152 to be connected to the terminal 151 fixed at the distal end of the lead pin 15, and a second cathode terminal 162 to be connected to the terminal 161 fixed at the distal end of the lead pin 16. Between ends of the hot cathode (E C ), a cathode heating voltage source (V F ) is disposed, and the first terminal V30 and the second terminal V40 of the cathode heating voltage source V F are respectively connected to the first and second cathode terminals 152 and 162. An electric field generating voltage source V 0 is disposed between the hot cathode (E C ) 25 and the anode (E A ) 24, and the first terminal V10 and the second terminal V20 of the electric field generating voltage source V 0 are respectively connected to the anode terminal 142 and the first and second cathode terminals 152 and 162. Note that the cathode 25 is set at a voltage of about 2.5 V as a cathode filament rated value. The electric field generating voltage source V 0 supplies a DC open voltage of about 150 V and a constant current of about 300 mA. As a discharge start circuit, an RC parallel circuit constituted by a capacitor C and a fixed resistor R C which are connected in parallel to each other and a trigger switch S T connected in series with the above circuit are disposed between the anode (E A ) 24 and the focusing electrode (E R ) 26. The first terminal RC10 of the RC parallel circuit will be connected to the focusing electrode terminal 132 through a luminance adjustment switch S A , and the second terminal RC20 of the RC parallel circuit will be connected to the anode terminal 142 through the trigger switch S T . Therefore, the trigger switch S T functions to electrically connect the second terminal RC20 of the RC parallel circuit to the anode terminal 142, and the luminance adjustment switch S A functions to electrically connect the first terminal RC10 of the RC parallel circuit to the focusing electrode terminal 132 at a predetermined timing. As a luminance adjustment circuit, a luminance adjustment voltage source V A and a variable resistor R V are disposed in series between the hot cathode (E C ) 25 and the focusing electrode (E R ) 26. The first terminal V50 of the luminance adjustment voltage source V A is connected to the first and second terminals 152 and 162, and the second terminal R20 of the variable resistor R V will be connected to the focusing electrode terminal 132 through the luminance adjustment switch S A . Therefore, the luminance adjustment switch S A also functions to connect the second terminal R20 of the variable resistor R V to the focusing electrode terminal 132 at a predetermined timing. The operation of this embodiment will be described below. First of all, the trigger switch S T is set in an OFF state, and the luminance adjustment switch S A is set in an ON state with respect to the discharge start circuit. With this operation, a voltage of about 2.5 V is applied from the cathode heating voltage source V F to ends of the hot cathode (E C ) 25 for about 20 seconds before discharge, and the hot cathode (E C ) 25 is preheated. After the hot cathode (E C ) 25 is sufficiently heated to a temperature of about 1,100°C, a voltage of about 150 V is applied between the hot cathode (E C ) 25 and the anode (E A ) 24 from the electric field generating voltage source V 0 , thereby generating an electric field from the anode (E A ) 24 to the hot cathode (E C ) 25. Upon completion of preparation of trigger discharge, the trigger switch S T is set in an ON state. With this operation, a potential of about 150 V is supplied from the electric field generating voltage source V 0 to the focusing electrode (E R ) 26 through the capacitor C and the fixed resistor R C (RC parallel circuit), which are connected in parallel to each other, thereby generating trigger discharge between the hot cathode (E C ) 25 and the focusing electrode (E R ) 26. Since the focusing electrode (E R ) 26 is electrically insulated from the cathode slit electrode 27 and the front electrode 23, a positive potential higher than that of the cathode slit electrode 27 and the front electrode 23, both of which are set at about 0 V, can be supplied to the focusing electrode (E R ) 26. For this reason, as shown in Fig. 6, a trigger discharge area 30 extending from the hot cathode (E C ) 25 extends from a space surrounded by the front electrode 23 and the cathode slit electrode 27, i.e, a space in the cathode box to reach the focusing electrode (E R ) 26. Therefore, trigger discharge is generated between the hot cathode (E C ) 25 and the focusing electrode (E R ) 26, so that the gas discharge tube 10 is properly lighted. By generating trigger discharge in this manner, arc discharge is generated between the hot cathode (E C ) 25 and the anode (E A ) 24. The arc discharge stably continues on the basis of a current of about 300 mA, which is supplied between the hot cathode (E C ) 25 and the anode (E A ) 24 from the electric field generating voltage source V 0 . At this time, the paths of thermoelectrons is limited to only one discharge path because of the convergent effect of the opening limit plate 261 of the focusing electrode (E R ) 26 and the shielding effect of the cathode slit electrode 27 and the front window electrode 23. More specifically, the thermoelectrons emitted from the hot cathode (E C ) 25 pass through the focusing opening 262 of the opening limit plate 261 from the focusing opening 260 of the cathode slit electrode 27 and through the through hole 210 of the discharge shielding member 21 and are received by the anode (E A ) 24. An arc ball as a high-density discharge area by arc discharge is generated in a space in front of the opening limit plate 261 on the opposite side to the anode (E A ) 24. Ultraviolet light (U.V) extracted from the arc ball as positive column light emission is projected toward the front of the anode 24 through the opening window 230 of the front electrode 23. Upon occurrence of arc discharge, the luminance adjustment switch S A is set in an ON state with respect to the luminance adjustment circuit. With this operation, a potential of 0 V or a negative potential is supplied from the luminance adjustment voltage source V A to the focusing electrode (E R ) 26 through the variable resistor R V . The focusing electrode (E R ) 26 repels electrons and attracts cations. At this time, since an ion sheath is generated in a space in front of the focusing electrode (E R ) 26. the arc ball can be compressed or focused by this ion sheath. Therefore, when the resistance value of the variable resistor R V is appropriately set, the luminance of the gas discharge tube 10 can be highly increased. Experiments for this embodiment will be described below. For the gas discharge tube of this embodiment and a conventional gas discharge tube, trigger voltages required to be applied to the focusing electrodes for trigger discharge, i.e., discharge start voltages were measured. As a result, the discharge start voltage for the gas discharge tube of this embodiment was 90 to 100 V while that for the conventional gas discharge tube was 130 to 170 V. In the gas discharge tube of this embodiment, the discharge start voltage is included within a small variation range, and it is found that lighting is highly facilitated. In addition, for the gas discharge tube of this embodiment, pulse lighting was repeatedly performed. As a result, no lighting failure was observed. Therefore, it is found that the reliability of lighting of the gas discharge tube of this embodiment is highly improved. Furthermore, for the gas discharge tube of this embodiment, the potential of the focusing electrode was changed on the basis of the resistance value of the variable resistor after lighting, and an optical output extracted from the positive column light emission was measured. As a result, as shown in Fig. 10, when the potential of the focusing electrode increased in the negative direction, the optical output increased. In this case, however, a positive potential of about 34 V is supplied to the focusing electrode from the luminance adjustment circuit. In Fig. 10, an optical output value corresponding to the potential of about 34 V is defined as 100%, and an optical output value corresponding to each potential of the focusing electrode is normalized and plotted. As shown in Fig. 10, the amount of an increase in optical output is almost 10% when the potential of the focusing electrode is 0 V. The amount of an increase in optical output reaches almost 40% when the potential of the focusing electrode is -120 V. Therefore, it is found that, in the gas discharge tube of this embodiment, the light emission intensity will be easily adjusted and increased. The arrangement and function of a lighting device for the gas discharge tube according to the present invention, will be described below with reference to Fig. 11. This lighting device is different from that illustrated in Fig. 9 only in the discharge start circuit. As shown in Fig. 11, this lighting device is constituted as an auxiliary electrode system. The discharge start circuit is also disposed between the anode (E A ) 24 and the hot cathode (E C ) 25, and constituted by the fixed resistor R C and the capacitor C. The first terminal R30 of the fixed resistor R C is connected to the anode terminal 142, the second terminal R40 will be connected to the capacitor C through the trigger switch S T . The first terminal C10 is connected to the first and second cathode terminals 152 and 162, and the second terminal C20 will be connected to one of the fixed resistor R C and the focusing electrode terminal 132 through the trigger switch S T and the luminance adjustment switch S A . Therefore, the trigger switch S T functions to connect the capacitor C to one of the fixed resistor R C and the luminance adjustment switch S A , and the luminance adjustment switch S A functions to connect the focusing electrode terminal 132 to one of the trigger switch S T and the luminance adjustment circuit. According to this arrangement, first of all, the trigger switch S T and the luminance adjustment switch S A are set in an ON state with respect to the discharge start circuit, and thereafter, preparation for trigger discharge is made in the same manner as in the above embodiment. At this time, a voltage of about 150 V is applied from the electric field generating voltage source V 0 to the capacitor C through the fixed resistor R C , and the capacitor C is sufficiently charged. Subsequently, the trigger switch S T is set in an ON state with respect to the focusing electrode (E R ) 26, thereby supplying a potential of about 150 V to the focusing electrode (E R ) 26 by a pulse voltage based on discharge of the capacitor C. With this operation, trigger discharge occurs between the hot cathode (E C ) 25 and the focusing electrode (E R ) 26. After the trigger switch S T is set in an OFF state, generation of arc discharge and adjustment of the luminance are performed in the same manner as described above. The arrangement and function of a lighting device for a gas discharge tube will be described below with reference to Fig. 12. This lighting device is different from that illustrated in Fig. 9 only in the discharge start circuit. As shown in Fig. 12, this lighting device is not constituted as a normal auxiliary electrode system. Between the anode (E A ) 24 and the hot cathode (E C ) 25, the discharge start circuit, and the discharge start circuit is constituted by a discharge start voltage source V T and an RC series circuit. A first terminal V70 of the discharge start voltage source V T will be connected to the RC series circuit through the trigger switch S T , and a second terminal V80 is connected to the first and the second cathode terminals 152 and 162. A first terminal RC30 of the RC series circuit will be connected to one of the discharge start voltage source V T and the anode terminal 142 through the trigger switch S T . Therefore, the trigger switch S T functions to connect the second terminal RC40 of the RC series circuit one of the first terminal V70 and the anode terminal 142 at a predetermined timing. The discharge start voltage source V T supplies a DC open voltage of about 500 to 650 V. The luminance adjustment switch S A functions to connect the second terminal R20 of the luminance adjustment circuit to the focusing electrode terminal 132 at a predetermined timing. According to this arrangement, first of all, the trigger switch S T is set in an ON state with respect to the discharge start circuit, and at the same time, the luminance adjustment switch S A is set in an OFF state. Thereafter, preparation for trigger discharge is made in the same manner as described above. At this time, a voltage of 500 to 550 V is applied from the discharge start voltage source V T to the capacitor C through the fixed resistor R C , and the capacitor C is sufficiently charged. Subsequently, the trigger switch S T is set in an ON state with respect to the anode (E A ) 24, thereby supplying a potential of about 500 to 650 V to the anode (E A ) 24 by a high pulse voltage based on discharge of the capacitor C. With this operation, trigger discharge occurs between the hot cathode (E C ) 25 and the anode (E A ) 24 on the basis of a reduced impedance. Although the discharge start voltage is about 350 at the start of lighting, the discharge start voltage increases according to continuation of lighting in some cases. For this reason, the trigger voltage to be applied between the anode (E A ) 24 and the hot cathode (E C ) 25 is set to about 500 to 650 V in consideration of the reliability. After the trigger switch S T is set in an OFF state, generation of arc discharge and adjustment of the luminance are performed in the same manner as described above. Note that the present invention is not limited to the above embodiments, and various changes and modifications can also be made. In the above embodiment as to the gas discharge tube, the light-emitting section is constituted as a side-on type which extracts light from the side portion of the tube. However, with a head-on type light-emitting portion which extracts light from the head portion of the tube, the same effect as in the above embodiment can be obtained. In the above embodiment, as the material of the discharge shielding member and the support member, a normal ceramic is used. On the other hand, when a so-called conductive ceramic such as beryllium oxide or aluminum nitride having a high thermal conductivity is used as the material of the discharge shielding member and the support member, a sink tank for the anode which is heated to a high temperature due to self heat generation is formed. For this reason, dissipation of the heat accumulated in che light-emitting section is promoted, and the operation of the gas discharge tube can be stabilized. As has been described above in detail, in the gas discharge tube embodying the invention, the focusing electrode is electrically insulated from the cathode slit electrode and the front electrode. With this structure, after the hot cathode is sufficiently heated, and a predetermined electric field is supplied to a space between the hot cathode and the anode, a positive potential higher than that of the front electrode and the cathode slit electrode will be supplied to the focusing electrode in generation of trigger discharge between the hot cathode and the focusing electrode. For this reason, the trigger discharge area extending from the cathode extends from a space surrounded by the front electrode and the cathode slit electrode, i.e., a space in the cathode box to reach the focusing electrode. Therefore, arc discharge will be easily generated between the hot cathode and the anode. Since this gas discharge tube will be certainly turned on, a gas discharge tube which improves the reproducibility of a satisfactory light emission state will be provided. In addition, in the lighting device for use with gas discharge tubes embodying the invention, the luminance adjustment circuit constituted by the luminance adjustment voltage source, the variable resistor, and the luminance adjustment switch, all of which are sequentially connected in series, is arranged between the hot cathode and the focusing electrode. With this arrangement, when the luminance adjustment switch is set in an ON state with respect to the luminance adjustment circuit upon occurrence of arc discharge, a zero potential or a negative potential will be supplied from the luminance adjustment voltage source to the focusing electrode through the variable resistor. Since the focusing electrode repels electrons and attracts cations, an ion sheath is generated in a space in front of the focusing electrode. Therefore, the arc ball can be compressed or focused by this ion sheath. When the resistance value of the variable resistor is appropriately set, the luminance of the gas discharge tube will be increased. Therefore, a lighting device for a gas discharge tube, which highly increases the light emission intensity while maintaining stable discharge, will be provided. From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims: A gas discharge tube comprising: an envelope (11) for sealing a gas therein; lead pins (13-16) introduced into an inner space of said envelope (11) from externally of said envelope (11); and a light-emitting section (20) in said envelope (11), positioned at distal ends of said lead pins (13-16) and supported by said lead pins (13-16) while spaced from an inner side wall of said envelope (11), said light-emitting section (20) including: a hot cathode (25) for emitting thermoelectrons; an anode (24) for receiving the thermoelectrons emitted from said hot cathode (25); a focusing electrode (26) for focusing said thermoelectrons on to said anode, the focusing electrode having a focusing opening (262) through which the thermoelectrons pass to be received by said anode (24); and a cathode box surrounding said hot cathode (25), for partially shielding discharge of said hot cathode (25), said cathode box having a cathode slit electrode (27) located between said hot cathode (25) and said focusing electrode (26), said cathode slit electrode (27) having a slit (270) through which the thermoelectrons emitted from said hot cathode (25) pass, and a front electrode (23) facing to said focusing electrode so as to accommodate said hot cathode (25) and said cathode slit electrode (27) in a space defined by said front electrode (23) and said focusing electrode (26), said front electrode (23) having a window (230) located at a position which faces to the focusing opening (262) of said focusing electrode (26), for outputting light caused by discharge, characterized in that: said cathode slit electrode (27) and said front electrode (23) are arranged so as to be spaced from and insulated from said focusing electrode (26). A gas discharge tube according to claim 1, wherein said light-emitting section (20) further comprises a discharge shielding member (21) of an insulator positioned between said focusing electrode (26) and said anode (24), and having a front surface (21A) and a rear surface (21B) which is opposite to the front surface (21A), wherein said focusing electrode (26) is mounted on and supported by the front surface (21A) of said discharge shielding member (21), and wherein said cathode slit electrode (27) and said front electrode (23) of said cathode box are respectively mounted on and supported by the front surface (21A) of said discharge shielding member (21). A gas discharge tube according to claim 1 or 2, wherein said cathode slit electrode (27) and said front electrode (23) of said cathode box are in direct contact with each other so as to have a same potential. A gas discharge tube according to claim 1, wherein said light-emitting section (20) further comprises a discharge shielding member (21) of an insulator positioned between said focusing electrode (26) and said anode (24), and having a front surface (21A) and a rear surface (21B) which is opposite to the front surface (21A), wherein said focusing electrode (26) is mounted on and in contact with the front surface (21A) of said discharge shielding member (21), and said anode (24) is mounted on and in contact with the rear surface (21B) of said discharge shielding member (21). A gas discharge tube according to claim 4, wherein said light-emitting section (20) further comprises a support member (22) of an insulator being in said envelope (11) and located on an opposite side to said discharge shielding member (21) through said anode (24), said support member (22) having a projection (225) for pushing said anode (24) onto the rear surface (21B) of said discharge shielding member (21). A gas discharge tube according to claim 5, wherein said support member (22) further has a depression (224) at a surface (22B) which faces to said anode (24), for providing a heat dissipation space for said anode (24). A gas discharge tube according to claim 5 or 6, wherein said discharge shielding member (21) and said support member (22) are formed of a ceramic. A gas discharge tube according to any of claims 4 to 7, wherein said discharge shielding member (21) has a recessed portion (211) at the rear surface (21B) of said discharge shielding member (21), for accommodating said anode (24). A gas discharge tube according to any of claims 4 to 8, wherein said discharge shielding member (21) is disposed in the inner space of said envelope (11) while being supported by lead pins (13-16) penetrating said envelope (11). A gas discharge tube according to claim 9, wherein said discharge shielding member (21) is spaced from an inner side wall of said envelope (11) through a space containing the gas. A gas discharge tube according to claim 9, wherein the recessed portion (211) of said discharge shielding member (21) has a depth matching a sum of a height of said projection (225) of said support member (22) and a thickness of said anode (24), whereby a front surface (22B) of said support member (22) is in partially contact with the rear surface (21B) of said discharge shielding member (21). A gas discharge tube according to any preceding claim, wherein said anode (24) is a metal plate mounted on and supported by a distal end of a lead pin (14) penetrating said envelope (11). A lighting device for use with a gas discharge tube as claimed in any preceding claim, said lighting device comprising: an anode terminal (142) adapted to be electrically connected to the anode (24) of the gas discharge tube; a focusing electrode terminal (132) adapted to be electrically connected to the focusing electrode (26) of the gas discharge tube; a first and a second cathode terminals (152; 162) adapted to be electrically connected to both ends (251; 252) of the hot cathode (25) of the gas discharge tube; an electric field generating voltage source (Vo) for generating an electric field from the anode (24) to the hot cathode (25) of the gas discharge tube, disposed between said anode terminal (142) and said first and second cathode terminals (152; 162), having a first terminal (V10) being connected to said anode terminal (242) and a second terminal (V20) being connected to said first and second cathode terminals (152; 162); and a luminance adjustment circuit for adjusting a potential to be supplied to the focusing electrode (26) of the gas discharge tube, disposed between said focusing electrode terminal (132) and said first and second cathode terminals (152; 162), said luminance adjustment circuit including: a luminance adjustment voltage source (V A ) having a first terminal (V50) being connected said first and second cathode terminals (152; 162) and a second terminal (V50); a variable resistor (R V ) having a first terminal (R10) being connected to said second terminal (V60) of said luminance adjustment voltage source (V A ) and a second terminal (R20) to be connected to said focusing electrode terminal (132) through a luminance adjustment switch (S A ). A lighting device according to claim 13, further comprising a cathode heating voltage source (V F ) for heating the hot cathode (25), disposed between said first and second cathode terminals (152; 162), having a first terminal (V30) being connected to said first cathode terminal (152) and a second terminal (V40) being connected to the second cathode terminal (162). A lighting device according to claim 13 or 14, further comprising a discharge start circuit including: an RC parallel circuit constituted by a capacitor (C) and a fixed resistor (R C ) arranged in parallel, having a first terminal (RC10) to be connected to said focusing electrode terminal (132) through said luminance adjustment switch (S A ) and a second terminal (RC20) to be connected to said anode terminal (142) through a trigger switch (S T ). A lighting device according to claim 13, further comprising a discharge start circuit including: a fixed resistor (R C ) having a first terminal (R30) being connected to said anode terminal (142); and a capacitor (C) having a first terminal (C10) being connected to said first and second cathode terminals (152; 162) and a second terminal (C20) to be connected to one of a second terminal (R40) of said variable resistor (R C ) and said focusing electrode terminal (132) through a trigger switch (S T ) and said luminance adjustment switch (S A ). A lighting device according to claim 13, further comprising a discharge start circuit including: a discharge start voltage source (V T ) for generating an electric field from said anode (24) to said hot cathode (25), having a first terminal (V80) being connected to said first and second cathode terminals (152; 162); and an RC series circuit constituted by a fixed resistor (R C ) and a capacitor (C) arranged in series, having a capacitor side terminal (RC30) being connected to said first and second cathode terminals (152; 162) and a capacitor side terminal (RC40) to be connected to one of said anode terminal (142) and a second terminal (V70) of said discharge start voltage source (V T ) through a trigger switch (S T ). A method of operating a lighting device according to any of claims 13 to 17, said method comprising: a first step of heating the hot cathode (25) by supplying a predetermined voltage to ends (251; 252) of the hot cathode (25); a second step of supplying a predetermined voltage between the heated hot cathode (25) and the anode (24) after the first step so as to generate an electric field from the anode (24) to the heated hot cathode (25); a third step of supplying a predetermined voltage sufficient to generate discharge between the hot cathode (25) and one of the focusing electrode (26) and the anode (24), to one of the focusing electrode (26) and the anode (24) after the second step; and a fourth step of adjustment a potential of the focusing electrode (26) by supplying a predetermined voltage so that a potential of the focusing electrode (26) is lower than or equal to that of the focusing electrode (26) in the second step after the third step. A method according to claim 18, wherein the potential of the focusing electrode (26) in the fourth step is set at a zero potential or a negative potential.

Inventors: IKEDO TOMOYUKI, ITO YOSHINOBU, MATUI RYOTARO, IKEDO, TOMOYUKI, ITO, YOSHINOBU, MATUI, RYOTARO, Ikedo, Tomoyuki, c/o Hamamatsu Photonics K.K., Ito, Yoshinobu, c/o Hamamatsu Photonics K.K., Matui, Ryotaro, c/o Hamamatsu Photonics K.K.
Applicants: HAMAMATSU PHOTONICS KK, HAMAMATSU PHOTONICS K.K.
Application year: 1995
Main classification: H01J61
IPC codes: H01J61, G01J3