High-pressure discharge lamp and lighting method

Abstract: The invention provides a high-pressure discharge lamp incorporating an outer envelope (1) storing a pair of electric terminals (13) a plurality of arc tubes (5) which are respectively stored in this outer envelope (1) and electrically connected in parallel, and a plurality of ignition aids (17) available for assisting operating of these arc tubes (5), where these ignition aids (17) are provided for each of these arc tubes (5) and contain potentials different from each other. In addition, the invention also provides a lighting system for operating the high-pressure discharge lamp. The lighting system comprises the first and second power supply lines which respectively provides mutual connection between a pair of terminals of the high-pressure discharge lamp, a ballast (21) which is at least provided for either of the first and second power supply lines, a power switch (71) which is provided for either the first or the second power supply line, a pulse generating means (22) which generates either the positive or the negative ignition pulses to be superimposed on AC power voltage output from an AC power supply source (20), and a control circuit (52) which alters polarity of the ignition pulses output from the pulse generator (22).

Description: The present invention relates to a high-pressure discharge lamp incorporating a plurality of arc tubes in the outer envelope and the method of operating. While operation of a high-pressure metal vapor discharge lamp like a high-pressure sodium lamp or a metal halide lamp for example, normally, internal pressure of arc tube rises beyond 1 atmospheric pressure. As a result, after once turning the above mentioned discharge lamp off, in order to again light up the discharge lamp, controller needs to spend a certain period of time to initiate discharge until the arc tube is cooled off to some extent to condense mercury and luminous metal and decrease pressure in the arc tube. For example, in order to re-start a high-pressure sodium lamp with an external ignitor, normally, it takes about one minute. On the other hand, in order to reactivate a metal halide lamp, normally, it takes more than 10 minutes, and yet, even after being reactivated, at least several minutes are required until the luminous output is fully stabilized. As a result, when power service is momentarily interrupted, unlike an incandescent lamp or a fluorescent lamp capable of quickly reaching full luminous condition, at least more than 10 minutes of interval is required for any conventional high-pressure metal vapor discharge lamps before it can again recover full luminous power. To solve this problem, as is typically described in the publication of USP4,287,454, a high-pressure sodium lamp is provided, which comprises a pair of arc tubes in the outer envelope by way of electrically connecting them in parallel with each other. When the proposed high-pressure sodium lamp normally lights up, either of the pair of arc tubes remains being lit. When power service is resumed after a momentary interruption, the other until arc tube containing low pressure lights up. In this case, while the former arc tube remains being lit, the latter arc tube slightly raises internal pressure due to preliminarily applied heating effect. As a result, this arc tube can start and reach its full output in a few minutes. In other words, the high-pressure sodium lamp cited above fully restarts in a very short period, thus offering much convenience for constantly illuminating highways and tunnels. Furthermore, even when either of these arc tubes cannot light up itself, the other arc tube lights up. Thus in turn significantly extends life of the high-pressure sodium lamp cited above. Theoretically, the service life of this high-pressure sodium lamp is twice as long as that of a conventional high-pressure sodium lamp merely housing a single arc tube. On the other hand, when operating such a high-pressure sodium lamp incorporating a pair of parallel-connected arc tubes in the outer envelop, either of these arc tubes whichever has a lower starting voltage lights up. In other words, due to unexpected irregularity incidental to a manufacturing process, starting voltages may slightly be uneven between these two arc tubes. As a result, either of these arc tubes remaining lower starting voltage lights up first. Hence, when activating the high-pressure sodium lamp cited above, either of these arc tubes whichever has a lower starting voltage always lights up first. In other words, whenever operating the high-pressure sodium lamp with a pair of arc tube cited above, the one of lower starting voltage tends to light up first. It means that the arc tube with a lower starting voltage often light up itself, thus resulting in dissipation of sodium filled in this arc tube. If this symptom even occurs, then a lamp voltage of the arc tube rises to cause the luminous characteristic of the lamp itself to quickly degrade. When either of these arc tube is no longer available, the other one can be operated. Since one of a pair of the arc tube cannot instantaneously light up again, the high-pressure sodium lamp can no longer maintain own objective function. Mostly, rise of voltage in the arc tube accounts for the generation of disabled arc tube caused by unilateral operating. Consequently, during the latter half of the service life of the high-pressure sodium lamp, either one of those arc tubes that tends to initially light up frequently extinguishes. When this arc tube is extinct, the other arc tube starts lighting up in turn. After the former arc tube subjects to repeated extinction, leakage is eventually occurred in it, thus causing rare gas in it to leak into the outer envelope. As a result, leaked rare gas absorbs the ignition pulse to prevent the other arc tube from being activated. For example, even when the arc tube is operated, as a result of heat dissipation by rare gas, neither temperature nor voltage rises in the arc tube, thus lowering luminous efficiency. In the same way, the high pressure discharge lamp incorporating a plurality of arc tubes is adversely affected by those symptoms described above. Document US-A-4 513 227 discloses an apparatus for starting and operating high intensity discharge lamps. This apparatus includes a nonlinear dielectric element and time delay means for performing the desired operation. Document EP-A-0 240 066 relates to a high intensity discharge lamp with a plurality of discharge devices and including a starting aid for starting one of the discharge devices. With this apparatus, upon initial application of power to the lamp the discharge device with the starting aid will start first, and the other discharge devices will not start. The object of the invention is to provide a novel high-pressure discharge lamp which can securely maintain for a long time its own functional capability of instantaneously irradiating light by alternately activating a plurality of arc tubes on halves to prevent either half of the plural arc tubes from constantly being subject to operating, and at the same time, the invention also provides the method of lighting up the high-pressure discharge lamp in the invention. According to the invention, a high-pressure discharge lamp as defined by the features of claim 1 and an apparatus for operating this discharge lamp as defined by the features of claim 4 are respectively provided. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 schematically designates the front view of the whole structure of the high-pressure discharge lamp according to the first embodiment of the invention; Fig. 2 schematically designates the block diagram of the operation circuit according to the first embodiment of the invention; Fig. 3 designates the chart of ignition pulse waveforms generated from the first embodiment of the invention; Fig. 4 schematically designates the circuit block diagram of the high-pressure discharge lamp incorporating a starter according to the second embodiment of the invention; Fig. 5 schematically designates the operating circuit of the high-pressure discharge lamp according to the third embodiment of the invention; Fig. 6 concretely designates the block diagram of the lamp operating control circuit according to the third embodiment of the invention; Fig. 7A designates the timer circuit introduced to the third embodiment of the invention; Fig. 7B designates waveforms output from the timer circuit introduced to the third embodiment of the invention; Fig. 8 designates the timing chart explanatory of lighting operation executed by the third embodiment of the invention; Fig. 9A designates the waveform of AC power supply source available for the third embodiment of the invention; Fig. 9B designates the positive ignition pulse voltage (+) available for the third embodiment of the invention; Fig. 9C designates the negative ignition pulse voltage (-) available for the third embodiment of the invention; Fig. 10A designates the waveform of AC power supply source available for the third embodiment of the invention; Fig. 10B designates the waveform of counter swing voltage available for the third embodiment of the invention; Fig. 10C designates the magnitude of the counter swing voltage available for the third embodiment of the invention; Fig. 11 concretely designates the block diagram of the operation circuit introduced to the high-pressure discharge lamp according to the fourth embodiment of the invention; Fig. 12 concretely designates the block diagram of the operation circuit introduced to the high-pressure discharge lamp according to the firth embodiment of the invention; Fig. 13 designates the timing charge explanatory of lighting operation according to the fifth embodiment of the invention; Fig. 14 more concretely designates the operation circuit shown in Fig. 12; Fig. 15 designates a variation of the operation circuit of the fifth embodiment; Fig. 16 designates another variation of the operation circuit of the fifth embodiment; Fig. 17 designates a still further variation of the operation circuit of the fifth embodiment; Fig. 18 schematically designates the block diagram of the operation circuit of the high-pressure discharge lamp according to the sixth embodiment of the invention; Fig. 19 schematically designates the high-pressure discharge lamp according to the seventh embodiment in conjunction with the operation circuit; Fig. 20A designates waveform output from a pulse-generating circuit PG1 introduced to the seventh embodiment of the invention; Fig. 20B designates waveform output from a pulse-generating circuit PG2 introduced to the seventh embodiment of the invention; Fig. 21 schematically designates the block diagram of the operation circuit of the high-pressure discharge lamp according to the eighth embodiment of the invention; Fig. 22A schematically designates the front view of the high-pressure discharge lamp according to the ninth embodiment of the invention; Fig. 22B schematically designates the lateral view of the high-pressure discharge lamp according to the ninth embodiment of the invention; Fig. 23 designates the dimensions of arc tubes according to the ninth embodiment of the invention; Fig. 24 designates the chart showing uneven light distribution when a plurality of arc tubes are disposed by way of intersecting each other and in parallel with each other; Fig. 25 designates the sectional view of an illumination instrument introduced to the tenth embodiment of the invention; and Fig. 26 designates the lateral view of the illumination instrument showing the method of installing it according to the tenth embodiment of the invention. Referring now more particularly to Figures 1 through 3, the high-pressure discharge lamp according to the first embodiment of the invention is described below. Fig. 1 designates the front view of the high-pressure discharge lamp according to the first embodiment. Fig. 2 schematically designates the block diagram of the operation circuit according to the first embodiment. Refer now to Fig. 1. The reference numeral 1 designates an outer envelope incorporating a unit of high-pressure sodium lamp. The outer envelope 1 is composed of glass which is of BT shape, where a screw base 2 is supported to an end of the outer envelope 1. The screw base 2 is of the Edison base which is provided with a shell 3 and an eye-let terminal 4. The outer envelope 1 incorporates a pair of arc tubes 5a and 5b and is internally filled with N2 gas which prevents arc discharge from being generated in the outer envelope 1 otherwise caused by accidental leakage of gas from these arc tubes 5a and 5b. These arc tubes 5a and 5b are respectively composed of the following: See Fig. 1. An end disk made from alumina ceramic serving as shielding wall is airtightly sealed to an end of tubular light-transmitting envelope made from polycrystalline or monocrystalline alumina. Main discharge terminals 6 shown in Fig. 2 are respectively supported to a pair of conductive members 7 which are respectively installed by way of penetrating through the end disk. Each of these arc tubes 5a and 5b is filled with sodium, mercury, and Xenon gas. Those conductive members 7 shown at the upper position of Fig. 1 are respectively connected to a pair of bulb holders 8a and 8b made from heat-resistant metal like niobium or tantalum by way of electrical and mechanical connection. Those two arc tubes 5a and 5b are installed in parallel with each other inside of the outer envelope 1. Those bulb holders 8a and 8b are respectively connected to supporting wires 9a and 9b via both edges. The bottom-side conductive members 7 and 7 are respectively held by an insulated holder 10 whose both ends are supported to the supporting wires 9a and 9b. Those supporting wires 9a and 9b are respectively conductive. The upper edges are connected to each other via an insulated bridge 11, and yet, these upper edges of the supporting wires 9a and 9b are respectively engaged with the tip region of the outer envelope 1 via a pair of elastic plates 12a and 12b. On the other hand, the bottom edges of these supporting wires 9a and 9b are respectively supported to lead-in conductors 13a and 13b by means of welding, whereas these lead-in conductors 13a and 13b are respectively supported to a stem 14 of the outer envelope 1. These shield-supporting lines 13a and 13b are respectively connected to the shell 3 and the eye-let terminal 4 of the screw base 2 via external conductive lines 15a and 15b. The conductive member 7 below the arc tube 5a is connected to the supporting wire 9b via a lead wire 16a, whereas the other conductive member 7 below the other arc tube 5b is connected to the other support wire 9a via the other lead wire 16b. A pair of ignition aid 17a and 17b available for assisting lighting-ignition operation are respectively provided in the axial direction outside of those arc tubes 5a and 5b. These ignition aid 17a and 17b are respectively provided on the external surfaces of those arc tubes 5a and 5b. The upper ends of these ignition aid 17a and 17b are rotatably held by those bulb holders 8a and 8b, whereas the bottom ends are respectively connected to bimetallic element of bimetal switch 18a and 18b by means of welding. Likewise, these bimetallic element of bimetal switch 18a and 18b are respectively supported to the supporting wires 9a and 9b by means of welding. The reference numeral 19 designates a getter. The inner space of the outer envelope 1 is constantly maintained at 1,3·10 -2 Pa (10-4 torr) of vacuum condition. The high-pressure discharge lamp composed of the above structure is made available by way of connection to the operation circuit shown in Fig. 2. Since the lighting control circuit is conventionally known in conjunction with a choke-coil type ballast 21 connected to the AC power supply source 20, description of the lighting control circuit shown in Fig. 2 is deleted. On the other hand, the lighting control system is provided with a ignition pulse generator 22 which is installed in association with the ballast 21. Concretely, according to the embodiments of the invention, the ignition pulse generator 22 generates a specific ignition pulse voltage on both ends of the ballast 21. An independent pulse transformer may also be provided for the ignition pulse generator 22 in order to feed pulses from the transformer to the arc tubes. This method is well known by those who are skilled in the art. The high-pressure discharge lamp embodied by the invention externally uses the choke-coil type ballast 21 and the ignition pulse generator 22 to provide their own functional effect. When activating the high-pressure discharge lamp, as shown in Fig. 3, the lighting control circuit respectively superimposes pulses P generated on both ends of the ballast 21 by the ignition pulse generator 22 onto AC voltage V delivered from the AC power supply source 20 before feeding the superimposed pulses to the arc tubes 5a and 5b. Before activating these arc tubes 5a and 5b, since these lamps are still cold, those bimetal pieces 18a and 18b respectively bring those ignition aids 17a and 17b to positions close to those arc tubes 5a and 5b. Next, as shown in Fig. 3, before activating those arc tubes 5a and 5b, the lighting control circuit superimposes ignition pulses P onto the AC voltage V in order to feed the superimposed pulses to those arc tubes 5a and 5b. Concretely, high-voltage pulses are added to the positive and negative components of the AC voltage V. This in turn means that the ignition pulses are generated every half cycle. When a negative pulse is delivered to either of those ignition aids 17a and 17b, the arc tube close to the pulse-added ignition aid is easily activated. For example, when the positive pulse is delivered to the eye-let terminal 4 of the screw base 2, the ignition aid 17a turns into the negative, and as a result, this activates the arc tube 5a close to the ignition aid 17a. Conversely, when the positive pulse is delivered to the shell 3 of the screw base 2, the other ignition aid 17b turns into the negative, thus activating the other arc tubes 5b close to the ignition aid 17b. In this way, those ignition aid 17a and 17b respectively receive positive pulses based on 50% of probability. In consequence, those arc tubes 5a and 5b can respectively be operated based on 50% of probability as well. However, relative to the increase of the lighting rounds, the rate of operating between both arc tubes is evenly levelled off, and as a result, neither of these arc tubes can unilaterally and intensively be operated. This in turn prevents either of those arc tubes from unilaterally being activated so often. As a result, the high-pressure discharge lamp embodied by the invention effectively prevents voltage in either of these arc tubes from sharply rising itself as a result of promoted dissipation of sodium in either of these arc tubes and also prevents either of these arc tubes from quickly degrading own luminous characteristic. Substantially, these arc tubes can extend own service life that doubles any conventional high-pressure discharge lamp merely incorporating a single arc tube. Furthermore, in the event that either of these arc tubes under illumination is turned off as a result of momentary power interruption and then the power service is resumed, the other arc tube containing low pressure thus far remained off lights up itself. Since the latter arc tube contains heat preliminarily provided by the former arc tube while being lit, the internal pressure of the latter arc tube is slightly raised in advance, and as a result, luminous condition of the latter tube is stabilized in a very short period of time. As a result, the high-pressure discharge lamp embodied by the invention can securely restart in a very short period of time. Since the predetermined luminosity can quickly be restored, application of the high-pressure discharge lamp embodied by the invention to the illumination of highways and tunnels securely promotes traffic safety. When either of these arc tubes 5a and 5b lights up, those bimetallic element of bimetal switch 18a and 18b are thermally deformed to cause those ignition aids 17a and 17b to leave those arc tubes 17a and 17b. This in turn minimizes interception of radiant light emitted from either of those arc tube 5a and 5b caused by presence of those ignition aids 17a and 17b. The above description has referred to such a case in which the first embodiment solely provides the ignition pulse generator outside of the high-pressure discharge lamp. However, the scope of the invention is not merely confined to this, but the invention also provides such structure like the second embodiment shown in Fig. 4. Concretely, Fig. 4 designates the structure which solely stores the ignition pulse generator inside of the outer envelope 1. This ignition pulse generator is composed of a thermosensitive switch like a bimetal switch 40 and a heater 41 which are connected to each other in series. This serial circuit is connected to a pair of arc tubes 5a and 5b in parallel. When the high-pressure discharge lamp is activated, the bimetal switch 40 remains closed to feed power to the heater 41, which then thermally opens the bimetal switch 40 to cause the ballast 21 to generate kick voltage pulses. These pulses are then superimposed on the power voltage. Likewise, the ignition pulse generator composed of the bimetal switch 40 and the heater 41 generates positive and negative high-voltage pulses every half cycle of the AC voltage. In consequence, those arc tubes 5a and 5b respectively feed the positive pulses to the ignition aids 17a and 17b based on 50% of probability, and therefore, these arc tubes 5a and 5b are respectively operated at 50% of probability. Referring now to Figures 5 through 9, the high-pressure discharge lamp according to the third embodiment of the invention is described below. Fig. 5 schematically designates the circuit block diagram of the high-pressure discharge lamp having the structure identical to that is shown in Fig. 1 by way of combination with the lighting control circuit. Fig. 6 schematically designates further detail of the lighting control circuit shown in Fig. 4. Fig. 7 designates a timer circuit and output waveform shown in conjunction with another timer circuit. Fig. 8 designates a timing chart which is explanatory of functional operation of the lighting control circuit. Fig. 9A through 9C respectively designate waveforms of the AC power-supplier and the waveforms of the AC voltage superimposed with ignition pulses output from the ignition pulse generator. Note that those components shown in Fig. 5 identical to those which shown in Fig. 2 are respectively designated by the identical reference numerals, and thus, description of these is deleted here. In the third embodiment of the invention, the ignition pulse generator 22 activates operation of the ballast 21 to selectively output the positive and negative ignition pulses based on the control operation performed by the lighting controller. Concretely, the lighting controller selectively outputs the positive and negative ignition pulses in correspondence with the polarity of the AC power at the moment of turning the power switch ON. Concretely, the ignition pulse generator 22 incorporates a pair of pulse generators 22a and 22b. The pulse generator 22a outputs the positive pulse P1 shown in Fig. 9A, whereas the other pulse generator 22b outputs the negative pulse P2 shown in Fig. 9B, respectively. One ends of these pulse generators 22a and 22b are respectively connected to an intermediate point of the ballast 21, whereas the other ends are respectively grounded via contact "a" of relay Ryl and another contact "a" of relay Ry2. As shown in Fig. 5, a lighting control circuit 52 is connected to both ends of the AC power-supply source 20 via the power switch 51. Referring now to Fig. 6, detailed structure of the lighting control circuit 52 is described below. An end of the AC power supply source is connected to an end of the primary coil of a transformer 62 via the power switch 51 and a zero-cross circuit 61. The other end of the primary coil is connected to the other end of the AC power supply source 20. Furthermore, both ends of the primary coil of the transformer are serially connected to a parallel circuit composed of a pair of photocouplers pc1 and pc2 which are respectively connected to a resistor r1 by inverting polarity from each other. Both ends of the secondary coil of the transformer 62 are respectively connected to the input terminal of a diode bridge DB1. Another resistor r2 and a Zener diode D1 are respectively connected to the output terminal of the diode bridge DB1 in series. A capacitor C1 is connected to both ends of the Zener diode D1. An end of the capacitor C1 is connected to the collector of transistor Q1, whereas the emitter of this transistor Q1 is grounded via a serial circuit composed of another resistor r3 and another capacitor C2. Another resistor r4 is connected between the collector and the base of the transistor Q1. Furthermore, as shown in Fig. 7, collector and emitter of another transistor Q2 are respectively connected to terminal A connected to the base of the transistor Q1 and the grounded terminal B. As shown in Fig. 7B, the base of the transistor Q2 is connected to the output terminal of a timer 63 which outputs HIGH signal for a predetermined period T2 after the power switch 51 is turned ON. As a result, those terminals A and B are connected to each other only for the predetermined period T2 after the power switch 51 is ON. The timer 63 provides the preset period T2 which is longer than the time actually needed to fully light up either of those arc tubes 5a and 5b, for example, for a period of 3 minutes. The emitter of the transistor Q1 is grounded via the coil of the relay Ry1, photocoupler pc3, and a thyristor SCR1. A non-grounded terminal of the resistor r6 is connected to the gate of the thyristor SCR1. On the other hand, emitter of the transistor Q1 is grounded via resistor r7, photocoupler PC2 and a parallel circuit consisting of photocoupler PC3, resistor r8, and capacitor C4. The emitter of the transistor Q1 is also grounded via the coil of the relay Ry2, photocoupler PC4, and thyristor SCR2. The non-grounded terminal of the resistor r8 is connected to the gate of the thyristor SCR2. The contact between the resistor r3 and the capacitor C2 is grounded via the zener diode D2 and another resistor r9. A non-grounded terminal of the resistor r9 is connected to the base of another transistor Q3, whereas the emitter is grounded. Diodes D3 and D4 are connected to each other in the forward direction on the contact lines between the photocoupler PC4 and the thyristor SCR1 and between the photocoupler PC3 and the thyristor SCR1 extended from the collector of the transistor Q3. Next, functional operation of the control circuit according to the third embodiment of the invention is described below. First, when the AC power switch 51 is ON, as shown in Fig. 8, either the photocoupler PC1 or photocoupler PC2 alternately turns ON in correspondence with the actual polarity of the AC power supply source 20. After the AC power switch 51 is ON, terminals A and B shown in Fig. 7A are shorted for the predetermined period T2. In the event that the photocoupler PC1 initially turns itself ON simultaneous with the operating of the power switch 51, the control circuit according to the third embodiment of the invention performs those operations described below. When the photocoupler PC1 turns itself ON, simultaneously the photocoupler PC1 also turns itself ON. As a result, trigger signal is generated by the non-grounded terminal of the resistor r6 to cause the thyristor SCR1 to also turn itself ON. This excites the coil of the relay Ry1 to close contact "a" of the relay Ry1 shown in Fig. 5. This permits the positive ignition pulse voltage P (+) to be delivered to both ends of the high-pressure discharge lamp 1. The positive ignition pulse voltage P (+) is generated as a result of the superimposition of the positive pulse P1 output from the pulse generator 22a onto the AC voltage V. This permits the arc tube 5b to light up. Incidentally, when the coil of the relay Ry1 is excited, the photo-PC3 turns itself ON. As a result, both ends of the resistor r8 are shorted to cause the trigger signal to be delivered to the gate of the thyristor SCR2 to inhibit this thyristor SCR2 to turn itself ON. In other words, after exciting the coil of the relay Ry1, in the course of feeding the positive ignition pulse voltage P (+) to both ends of the high-pressure discharge lamp 1, excitation of the coil of the relay Ry2 is inhibited to prevent the negative ignition pulse voltage P (-) from being delivered to both ends of the high-pressure discharge lamp 1. On the other hand, in the event that the photocoupler PC2 initially turns itself ON immediately after activating the power switch 51, the control circuit according to the third embodiment executes those functional operations described below. When the photocoupler PC2 turns itself ON, the photocoupler PC2 also turns itself ON. As a result, trigger signal is generated by the non-grounded terminal of the resistor r8 to turn the thyristor SCR2 ON. This excites the coil of the relay Ry2 to close contact "a" of the relay Ry2. This in turn permits the negative ignition pulse voltage P (-) to be delivered to both ends of the high-pressure discharge lamp 1. The negative ignition pulse voltage P (-) is generated as a result of the superimposition of the negative pulse P2 output from the ballast 21 via the function of the pulse generator 22b onto the AC voltage V. This permits the arc tube 5a to light up. Incidentally, when the coil of the relay Ry2 is excited, the photocoupler PC4 turns itself ON. As a result, both ends of the resistor r6 are shorted to permit the trigger signal to be delivered to the gate of the thyristor SCR1 to inhibit the thyristor SCR1 to turn itself ON. In other words, after exciting the coil of the relay Ry2, in the course of feeding the negative ignition pulse voltage P (-) to both ends of the high-pressure discharge lamp 1, excitation of the coil of the relay Ry1 is inhibited to prevent the positive ignition pulse voltage P (+) form being delivered to both ends of the high-pressure discharge lamp 1. The preset period T1 shown in Fig. 7B is determined by the time constant of the resistor r3 and the capacitor C2. When the preset period T1 is past after turning the transistor Q1 ON, the transistor Q3 turns itself ON. As a result, coils of those relays Ry1 and Ry2 are respectively excited to close the contacts "a" of those relays Ry1 and Ry2. Owing to this functional mechanism, even when the arc tube 5b cannot light up itself as a result of the initial excitation of the coil of the relay Ry1 resulting in the delivery of the positive ignition pulse voltage P (+) to both ends of the high-pressure discharge lamp 1, the negative ignition pulse voltage P (-) is securely delivered to both ends of the high-pressure discharge lamp 1 to light up the other arc tube 5a. In the event that both the arc tubes 5a and 5b cannot be lit even when feeding the positive and negative pulse voltages P (+) and P (-) to the high-pressure discharge lamp 1, those terminals A and B are opened after passing the preset period T2 from the moment at which the power switch 51 is activated. As a result, the transistor Q1 turns itself OFF to free the coils of those relays Ry1 and Ry2 from the excited condition. This in turn prevents the terminals 6 from incurring unwanted damage otherwise caused by unnecessarily feeding both the positive and negative ignition pulse voltages P (+) and P (-) to the high-pressure discharge lamp 1. In other words, according to the third embodiment of the invention, whenever the power switch 51 is ON, either the photocoupler PC1 or the photocoupler PC2 can be activated at 50% of probability. Based on this reason, if the polarity of those ignition aids provided for each arc tube were preliminarily arranged to be inverse from each other, since the available ignition pulses respectively contain 50% of probability, these two arc tubes 5a and 5b can pro-portionally be activated based on 50% of probability. This in turn securely prevents either of these two arc tubes from unilaterally being lit up all the time, but instead, rounds for operation both tubes can evenly be levelled off. In other words, this securely prevents either of these arc tubes from quickly degrading own luminous characteristic, but instead, service life can securely be promoted double. As a result, these two arc tubes can securely and fully exert own function to instantaneously light up themselves after a momentary power interruption until the expected service life fully expires. Furthermore, the control circuit according to the third embodiment of the invention securely feeds both the positive and negative ignition pulse voltages P (+) and P (-) to the high-pressure discharge lamp 1 after passing the preset period T1 from the moment at which the power switch 51 is activated. This in turn permits one of those two arc tubes to securely light up itself even when the other arc tube cannot be lit up. According to the control circuit provided for the third embodiment, either the positive ignition pulse voltage P (+) or the negative ignition pulse voltage P (-) shown in Fig. 9B or 9C is delivered to both ends of the high-pressure discharge lamp 1 in correspondence with the actual polarity of the AC power-supply source 20 simultaneous with the operating of the power switch 51 so that either the arc tube 5a or the other arc tube 5b can securely light up itself. The control circuit of the third embodiment then feeds both the positive and negative ignition pulse voltages P (+) and P (-) to the high-pressure discharge lamp 1 after passing the preset period T1 from the moment at which the power switch 51 is activated so that either of these two arc tubes can securely light up itself even when one of these arc tubes cannot be lit up. A consideration is hereby given on such a specific case in which, after activating the power switch 51, the positive ignition pulse voltage P (+) is delivered to both ends of the high-pressure discharge lamp in correspondence with the polarity of the AC power supply source 20, and then, after confirming that one of these two arc tubes does not light up, and then once turns the power switch 51 OFF before the preset period T1 is past, and again turns the power switch ON. It is probable that, as a result of reactivating the power switch 51, the positive ignition pulse voltage p (+) may be delivered to the high-pressure discharge lamp 1 in a rare case. Likewise, as a result of reactivating the power switch 51 over again, the positive ignition pulse voltage P (+) may incidentally be delivered to both ends of the high-pressure discharge lamp 1. If the identical pulse voltage were repeatedly delivered to a arc tube that does not light up, terminals 6 will soon incur unwanted damage. To prevent this, the control circuit of the third embodiment internally provides the positive ignition pulse voltage P (+) with counter swing voltage waveform undershooting itself into the negative region like the one shown in Fig. 10B. This securely lights up either of these arc tubes even when one of these does not light up. As shown in Fig. 10C, the counter swing voltage has an absolute value above the voltage B capable of illuminating the extinct arc tube independent of polarity in the course of lighting, extinction, and lighting cycle. The absolute value of the counter swing voltage can be set below the voltage A which is capable of securely lighting up one of those arc tubes with any polarity when either of these arc tubes should be lit from the extinct condition. The case shown in Fig. 10B adds the counter swing voltage to the positive ignition pulse voltage P (+). However, it is also possible for the third embodiment of the invention to add the counter swing voltage to the negative ignition pulse voltage P (-). Although not shown in the accompanying drawings, the counter swing voltage value can be set by initially determining the constants of the capacitor of the ignition pulse generator 22 and the inductance of the ballast 21 before eventually determining the resonant frequency. In this way, since the third embodiment adds the counter swing voltage to the positive ignition pulse voltage P (+), and yet, in the event that one of those arc tubes does not light up itself, the other arc tube can securely be lit by means of the counter swing voltage. In consequence, this system prevents the positive ignition pulse voltage P (+) from repeatedly being delivered to the extinct arc tube, thus eventually preventing those terminals 6 from incurring unwanted damage. By virtue of this arrangement, even when one of those two arc tubes cannot light up itself, the other arc tube can securely and instantaneously be reactivated for illumination, thus securely extending service life of the high-pressure contained electric discharge lamp itself. Referring now to Fig. 11, the fourth embodiment of the invention is described below. Note that those components shown in Fig. 5 identical to those shown in Figures 4 and 5 are respectively designated by the identical reference numerals, and thus, description of these is deleted here. See Fig. 11. Both ends of the AC power supply source 20 are respectively connected to input terminals of a diode bridge DB2 via a power switch 51. A serial circuit consisting of a coil of a latching relay Ry-S and a thyristor SCR3 and the other serial circuit consisting of a coil of the other latching relay Ry-R and a thyristor SCR4 are respectively connected to the output terminal of the diode bridge DB2 in parallel. A resistor r10, transistor Q3, and an emitter resistor r11, are respectively connected to the output terminal of the diode bridge DB2 in series. The emitter of the transistor Q3 is connected to the gate of the thyristor SCR3. In addition, a resistor r12, transistor Q4, and an emitter r13, are respectively connected to the output terminal of the diode bridge DB2 in series. The emitter of the transistor Q4 is connected to the gate of the thyristor SCR4. An end of the pulse generator 22a externally delivering the positive pulse P1 is connected to the intermediate point of the ballast 21, whereas the other end is connected to contact S of the latching relay Ry-S. An end of the other pulse generator 22b externally delivering the negative pulse P2 is also connected to the intermediate point of the ballast 21, whereas the other end is connected to contact R of the latching relay Ry-R. The movable contact of relay switch 71 of the latching relay Ry-R is connected to the AC power supply source 20 and the grounding terminal of the diode bridge DB2. Normally, the relay switch 71 remains closed at contact R. Those main terminals 6 on the part of the arc tube 5a are connected to the AC power supply source 20 and the grounding terminal of the diode bridge DB2 via the primary coil of transformer 72. The secondary coil of this transformer 72 is connected to the output terminal of another diode bridge DB3. A resistor r14 and a capacitor C5 are connected to the output terminal of the diode bridge DB3 in series. The contact between the resistor 14 and the capacitor C5 is connected to the base of the transistor Q3 via a Zener diode D5 and a resistor r15. Likewise, those main terminals 6 on the part of the arc tube 5b are connected to the AC power supply source 20 and the grounding terminal of the diode bridge DB3 via the primary coil of another transformer 73. The secondary coil of the transformer 73 is connected to the input terminal of another diode bridge DB4. A resistor r16 and a capacitor C6 are connected to the output terminal of the diode bridge DB4 in series. The contact between the resistor r16 and the capacitor C6 is connected to the base of the transistor Q4 via a Zener diode D6 and a resistor r17. Next, functional operation of the control circuit according to the fourth embodiment of the invention is described below. When the power switch 51 is ON, the negative pulse P2 is superimposed on the AC power voltage output from the AC power supply source 20, and then delivered to both ends of the high-pressure discharge lamp 1. This cause the arc tube 5a to light up. When the arc tube 5a lights up, the lamp current flows through the primary coil of the transformer 72, whereas the AC voltage generated by the secondary coil is delivered to the diode bridge DB3, which then rectifies full waveforms of the received AC voltage. The wave-rectified voltage is then smoothed by the resistor r14 and then capacitor C5. Smoothed voltage at the contact between the resistor 14 and the capacitor C5 is delivered to the base of the transistor Q3 via the zener diode D5 and the resistor r15. As a result, the transistor Q3 is turned ON, and then trigger signal is output to the gate of the thyristor SCR3. The in turn activates the thyristor SCR3 to excite the coil of the latch relay Ry-S. In response to this the relay switch 71 is closed by way of switching itself to contact S, and then this condition is held on. When the power switch 51 is opened after lighting up the arc tube 5a, the arc tube 5a turns itself OFF. Next, when the power switch 51 is again activated, since the relay switch 71 remains closed on the part of contact S, the AC voltage output from the AC power supply source 20 is superimposed with the positive pulse P1, and then the positive-pulse added AC voltage is delivered to both ends of the high-pressure discharge lamp 1 to light the arc tube 5b. When the arc tube 5b lights up, the lamp current flows through the primary coil of the transformer 73, whereas the AC voltage generated by the secondary coil of this transformer 73 is delivered to the diode bridge DB4, which then fully rectifies waveform of the input AC voltage. Next, the rectified waveforms of the AC voltage are smoothed by the resistor r16 and the capacitor C6. The smoothed AC voltage at the contact between the resistor r16 and the capacitor C6 is delivered to the base of the transistor Q4 via the Zener diode D6 and the resistor r17. As a result, the transistor Q4 turns itself 0N, and then trigger signal is output to the gate of the thyristor SCR4. This in turn activates the thyristor SCR4 to excite the coil of the latching relay Ry-R. In response to this, the relay switch 71 is closed by way of switching itself to contact R, and then this condition is held on. The fourth embodiment of the invention causes the control system to detect lamp current via the transformers 72 and 73 in order to detect the lit-up arc tube. Instead of using these transformers, the fourth embodiment may also provide a plurality of photoelectric conversion elements in specific position close to these arc tubes in order to convert light beam emitted from the lit-up arc tube into electric signals. In addition, the fourth embodiment may also provide a thermosensor adjacent to each arc tube in order to detect the actually lit-up arc tube. According to the fourth embodiment of the invention, whenever the power is ON, those arc tubes 5a and 5b alternately lights up, and thus, the lighting probability of these arc tubes 5a and 5b can evenly be levelled off at 50%. This in turn significantly extends the service life of the electric discharge lamp itself. Theoretically, the service life of the electric discharge lamp embodied by the invention doubles the service life of any conventional electric discharge lamp merely incorporating a single arc tube. Referring now to Figures 12 and 13, the fifth embodiment of the invention is described below. Note that those components shown in Fig. 12 identical to those which are shown in Figures 4 and 5 are designated by the identical reference numerals, and thus the description of these is deleted here. Refer to Fig. 12. An input terminal of a diode bridge DB5 is connected to both ends of the AC power supply source 20 via the power switch 51. A serial circuit consisting of a resistor r17 and a Zener diode D7 is connected to the output terminal of the diode bridge DB5. Another serial circuit consisting of a diode D8 and a capacitor C7 is connected to both ends of the Zener diode D7. Another serial circuit consisting of a resistor r18 and a capacitor C8 is connected to both ends of the capacitor C7. The non-grounded terminal of the capacitor C8 is grounded via a Zener diode D9 and a pair of photocouplers PC5 and PC8. The anode of the Zener diode D9 is grounded via a pair of photocouplers PC6 and PC7. Furthermore, a serial circuit consisting of the coil of a latching relay Ry-S, the photocoupler PC7, and a thyristor SCR5 and another serial circuit consisting of the coil of a latching relay Ry-R, the photocoupler PC8, and a thyristor SCR6, are respectively connected to both ends of the capacitor C7. The non-grounded terminal of the capacitor C7 is connected to the movable contact of a relay switch 81 via a resistor r19. Contact S of this relay switch 81 is grounded via the photocouplers PC5 and PC6. The contact between the photocouplers PC5 and PC8 is grounded via a resistor 20, whereas the non-grounded terminal of the resistor 20 is connected to the gate of the thyristor SCR5. The contact between those photocouplers PC6 and PC7 is grounded via a resistor r21, whereas the non-grounded terminal of the resistor r21 is connected to the gate of the thyristor SCR6. Next, functional operation of the control circuit for lighting the high-pressure discharge lamp according to the fifth embodiment of the invention is described below. First, when the power switch 51 is ON, AC voltage output from the AC power supply source 20 is superimposed with the negative pulse P2, and then the negative-pulse superimposed AC voltage is delivered to both ends of the high-pressure discharge lamp 1 to light up the arc tube 5a. When the power switch 51 is ON, AC voltage output from the AC power supply source 20 is delivered to the diode bridge DB5, which then fully rectifies waveforms, and then the rectified waveforms are smoothed by a smoothing circuit composed of the resistor r17 and the capacitor C7. After passing a predetermined period of time determined by the constant of a circuit consisting of resistor r18, capacitor C8, and the Zener diode D9, trigger signal is output to the gate of the thyristor SCR5, and as a result, the thyristor SCR5 turns itself ON. This in turn excites the coil of the latching relay Ry-S to switch the relay switches 71 and 81 to come into contact with terminal S, and then this condition is held on. Since the photocoupler PC7 turns itself ON simultaneous with the excitation of the coil of the latching relay Ry-S, the gate potential of the thyristor SCR6 is grounded to inhibit the thyristor SCR6 from turning itself ON. As a result, the coil of the latching relay Ry-R is prevented from being excited simultaneous with the excitation of the coil of the latching relay Ry-S. After one lighting up the arc tube 5a, when the power switch 51 is opened, the arc tube 5a again turns itself OFF. When the power switch 51 is again activated, since the relay switch is closed on the part of the contact S, the positive pulse P2 is superimposed on the AC voltage output from the AC power supply source 20, and then the positive-pulse superimposed AC voltage is delivered to both ends of the high-pressure discharge lamp 1 to light up the arc tube 5b. When the power switch 51 is turned on, AC voltage output from the AC power supply source 20 is delivered to the diode bridge DB5, which then fully rectifies waveforms of the received AC voltage. The wave-rectified and positive-pulse superimposed AC voltage is then smoothed by a smoothing circuit composing of the resistor r17 and the capacitor C7. Then, after passing a predetermined period of time determined by constant of a circuit consisting of the resistor r18, capacitor C8, and the Zener diode D9, trigger signal is output to the gate of the thyristor SCR6 so that the thyristor SCR6 can be activated. As a result, the coil of the latching relay Ry-R is excited to switch the relay switches 71 and 81 over to the terminal R, and then this condition is held on. In this way, whenever the power switch 51 is activated, those arc tubes 5a and 5b alternately light up. As a result, the lighting probability of these arc tubes 5a and 5b can evenly be levelled off at 50%. This in turn significantly extends the service of the high-pressure discharge lamp itself. Theoretically, the service life of the electric discharge lamp embodied by the invention doubles the service life of any conventional high-pressure discharge lamp. Referring now to Fig. 14, the first variation of the fifth embodiment of the invention is described below. Note that those components shown in Fig. 14 identical to those which are shown in Fig. 12 are respectively designated by the identical reference numerals, and thus, description of these is deleted here. Using the relay switch 71, the control circuit shown in Fig. 12 switches the pulse generators 22a and 22b as required. The control circuit according to the first variation of the first embodiment switches the positive and negative pulses by applying the photocouplers PC10 and PC11. In Fig. 14, a power-factor adjusting diskacitor C10 is provided between power supply lines "a" and "b" connected to both ends of the AC power supply source 20. A serial circuit composed of the ballast 21 and those resistors r31 and r32 is connected to the power-factor adjusting diskacitor C10. The outer envelope 1 housing a pair of arc tubes 5a and 5b is connected to both ends of a serial circuit composed of those resistors r31 and r32. The intermediate point of the ballast 21 is connected to the power supply line "b" via diskacitors C11 and C12 and a constantly closed triode AC switch T1. An inductance coil L1 and a two-way-two-pin thyristor D are respectively connected to an end of the ballast 21 and the other end of the diskacitor C11. A resistor r33 is connected to both ends of the diskacitor C12. In addition, a resistor r34, the photocoupler PC11 and a diode D11 (which is connected in the forward direction) are respectively connected to both ends of a serially connected circuit composed of the inductance coil L1 and the thyristor D. The diode D11 (which is connected in the direction inverse from the photocoupler PC11) is connected to a serially connected circuit composed of the photocoupler PC10 and the diode D10. A capacitor C13 is connected to both ends of the resistor r32. The contact between the resistors r31 and r32 is connected to the triode AC switch T1 via the photocoupler PC12 and another triode AC switch T2. Both ends (points C and D) of the capacitor C7 are connected to a timer 91. The output terminal of the timer 91 is connected to the base of a transistor Q10 via a resistor r35. An inversely connected diode D12 and a photocoupler PC12 are respectively connected between the collector of the transistor Q10 and the point C. A capacitor C14 and a resistor r36 are connected in parallel with each other between the base and the emitter of the transistor Q10. After passing a predetermined period of time from the operating of the power switch 51, the transistor Q10 is activated by the timer 91. The photocoupler PC10 is connected between contact S of a relay switch 81 and the photocoupler PC6. The photocoupler PC11 is connected between contact R of the relay switch 81 and the photocoupler PC5. When the power switch 51 is activated while the relay switch 81 is closed on the part of the contact S, then, the photocoupler PC10 turns itself ON. As a result, the capacitor C11 is charged with specific voltage during a period in which the voltage flowing through the line "b" is higher than that flows through the other line "a". As soon as the charged voltage rises beyond the breakdown voltage of the thyristor D, the capacitor C11 discharges it to cause the negative pulse to superimpose on the AC power voltage. On the other hand, when the power switch 51 is activated while the relay switch 81 is closed on the part of the contact R, then, the photocoupler PC11 turns itself ON. As a result, the capacitor C11 is charged with specific voltage during a period in which the voltage flowing through the line "a" is higher than that flows through the other line "b". As soon as the charged voltage rises beyond the breakdown voltage of the thyristor D, the capacitor C11 discharges it to cause the positive pulse to superimpose on the AC power voltage. After a predetermined period of time is past from the operating of the power switch 51, the transistor Q10 turns itself ON to cause the photocoupler PC12 to also turn itself ON, and as a result, the triode AC switch T1 is no longer conductive. As a result, after a predetermined period of time is past from the operating of the power switch 51, the ignition pulse cannot superimpose on the AC power voltage output from the AC power supply source 20 at all. Referring now to Fig. 15, the second variation of the fifth embodiment of the invention is described below. Note that those components shown in Fig. 15 identical to those which are shown in Figures 12 and 14 are respectively designated by the identical numerals, and thus, the description of these is deleted here. To implement the second variation, the line "a" connected to an end of the AC power supply source 20 is provided with a pair of ballasts including the main ballast 21a and an auxiliary ballast 21b by dividing the stabilizer 21 into two parts. The line "a" is connected to the intermediate point of the auxiliary ballast 21b. A parallel connected circuit composed of a capacitor C11 and a resistor r41, another parallel connected circuit composed of a capacitor C12 and a resistor r33, and a triode AC switch T1, are respectively connected between the intermediate point of the auxiliary ballast 21b and the line "b". A resistor r42, a photocoupler PC12, and a resistor r43, are respectively connected to both ends of the triode AC switch T1, and in addition, contact between the photocoupler PC12 and the resistor r43 is also connected to the gate of the triode AC switch T1. By virtue of the above arrangement, conductivity of the triode AC switch T1 is restrained for a predetermined period of time after turning the power switch 51 ON. As shown in Fig. 15, if the relay switch 81 were closed on the part of contact S when the power switch 51 is turned ON, as was done for the first variation of the fifth embodiment, the negative pulse superimposes on the AC voltage output from the AC power supply source 20. On the other hand, if the relay switch 81 were closed on the part of the other contact R when the power switch 51 is turned ON, then, as was done for the first variation described above, the positive pulse superimpose on the AC voltage output form the AC power supply source 20. Furthermore, since the second variation of the fifth embodiment discretely provides the main ballast 21a and the auxiliary ballast 21b, these ballasts can effectively minimize attenuation of the positive or negative pulse. Referring now to Fig. 16, the third variation of the fifth embodiment of the invention is described below. Note that those components shown in Fig. 16 identical to those which are shown in Figures 12, 14 and 15, are respectively designated by the identical reference numerals, and thus, description of these is deleted here. See Fig. 16. A power-factor adjusting capacitor C10 is connected between lines "a" and "b" connected to both ends of the AC power supply source 20. A stabilizer 21, the secondary coil of a pulse transformer 92, a resistor r51, a pair of capacitors C14 and C15, and a constantly open triode AC switch T1, are respectively connected to both ends of the capacitor C10 in series. Furthermore, the primary coil of the pulse transformer 92, a pair of capacitors C16 and C17, are respectively connected to both ends of the triode AC switch T1. Furthermore, owing to switching operation of a relay switch 93, either of those diodes D11 and D12 (which are respectively connected in the direction inverse from each other) can selectively be connected to both ends of the triode AC switch T1 in parallel. Furthermore, a resistor r52, a photocoupler PC12, and a resistor r53, are respectively connected between an end of the resistor r51 and the other end of the triode AC switch T1. A capacitor C18 is connected to both ends of the resistor r53. Contact between the photocoupler PC12 and the resistor r53 is connected to the triode AC switch T1 via a resistor r54 and another triode AC switch T2. By virtue of the above structure, if the relay switch 93 were at the switched position shown to the right of Fig. 16 after activating the power switch 51, if the voltage flowing through the line "a" rises beyond the voltage flowing through the other line "b", the voltage of the line "a" flows into those capacitors C14 through C17 via the arrowed route X. Then, when the photocoupler PC12 turns ON after passing a predetermined period of time from the operating of the power switch 51, the triode AC switch T1 also turns itself ON. As a result, a discharge circuit like the one shown with broken line is formed to cause the positive pulse to superimpose on the AC voltage output from the AC power supply source 20. On the other hand, while the relay switch 93 is closed in the direction opposite from the position shown in Fig. 16, if the voltage flowing through the line "b" rises beyond the voltage flowing through the other line "a", it flows in the direction opposite from the arrowed direction X to effect charge. When the photocoupler PC12 turns ON after a predetermined period of time is past from the operating of the power switch 51, the triode AC switch T1 also turns ON, thus forming a discharge circuit in the direction opposite from broken line Y to cause the negative pulses to superimpose themselves on the AC voltage delivered from the AC power supply source 20. Although the control circuit shown in Fig. 16 solely provides the ballast 21 for the line "a", as shown in Fig. 17, another ballast 21' having performance characteristic identical to that of the ballast 21 may be provided for the line "b". This permits the control circuit to more securely switch those arc tubes 5a and 5b to light up. Referring now to Fig. 18, the sixth embodiment of the invention is described below. Note that those components shown in Fig. 18 identical to those which are shown in Fig. 4 are respectively designated by the identical reference numerals, and thus, the description of these is deleted here. The reference numeral 20 designates the AC power supply source. Both ends of this AC power supply source 20 are connected to the high-pressure discharge lamp 1 via the power switch 51, control switch 91, and the ballast 21. An end of a control circuit 52 is connected to the contact between the control switch 91 and the ballast 21, whereas the other end is grounded. An end of a pulse circuit 22 is connected to the intermediate point of the ballast 21, whereas the other end is grounded. In addition to those functional features described in relation to those structures shown in Figures 4 and 5, the control circuit 52 incorporates such function to open and close the control switch 91 at predetermined intervals. Owing to the structure mentioned above, the control circuit can effectively prevent the arc tube 5a from continuously being lit otherwise caused by the closed state of the power switch 51 after being turned ON. More particularly, when detecting that a predetermined period of time is past after the power switch 51 is ON, the control circuit 52 once opens the control switch 91, and then closes it. As a result, this simulates the opening and closing operation of the power switch 51 to cause the other arc tube 5b to light up so that the lighting probability can evenly be levelled off at 50%. When applying a large number of high-pressure discharge lamps each incorporating a plurality of arc tubes to the illumination of highways in tunnels without turning the power switch 51 OFF at all, the control system according to the sixth embodiment of the invention can securely maintain the lighting probability of each arc tube substantially at 50%. Since the simultaneous extinction of a plurality of high-pressure discharge lamps endangers the traffic safety, it is desired that those high-pressure discharge lamps illuminating highways in tunnels sequentially be switched on the individual or group basis with the delayed timing. Using a remote-controlled monitoring system, operations of the control switch 91 can properly be managed. Referring now to Figures 19 and 20, the high-pressure discharge lamp according to the seventh embodiment of the invention is described below. Note that those components shown in Fig. 19 identical to those which are shown in Fig. 1 are respectively designated by the identical reference numerals, and thus, description of these is deleted here. See Fig. 19. A conductive member 7 below the arc tube 5a is connected to a shield-supporting line 13a via a lead wire 16a, whereas the other conductive member 7 below the other arc tube 5b is connected to the other shield-supporting line 13b via a lead wire 16b. A connecting member connecting the bottom end of a pair of supporting wires 9a and 9b is bonded to a shield-supporting line 13c by means of welding. These shield-supporting lines 13a through 13c are airtightly connected to a stem 14 of the outer envelope 1. The shield-supporting line 13a is connected to one side of a screw base 2 by an external conductor 15a, where the screw base 2 has a pair of metallic members which are electrically insulated across a screw base insulator 2a. The shield-supporting line 13b is connected to the other side of the screw base 2 by an external conductor 15b, whereas the other shield-supporting line 13c is connected to an eye-let terminal 4 by an external conductor 15c. A socket insulator 3a is provided on the internal circumferential surface of a socket 3 which is engaged with the screw base 2 and faces the screw base insulator 2a. The socket 3 itself has a pair of metallic members 3a and 3b which are electrically insulated from each other across the socket insulator 3a. These metallic members 3a and 3b are respectively connected to an end of the AC power supply source 20 via ballasts 21a and 21b. Contact 3c coming into contact with the eye-let terminal 4 is connected to the other end of the AC power supply source 20. Pulse generators PG1 and PG2 are respectively connected between the other end of the AC power supply source 20 and the lines connected to those ballasts 21a and 21b. These pulse generators PG1 and PG2 respectively output ignition pulses in response to the phase of received voltage. These pulse generators PG1 and PG2 respectively start to generate the ignition pulses when the phase of the input voltage inverts. For example, as shown in Fig. 20A, the pulse generator PG1 starts to generate the ignition pulses when the positive-phase voltage is received. On the other hand, the pulse generator PG2 starts to generate the ignition pulses when the negative-phase voltage is received. For example, when the power switch 51 is activated, if the positive-phase voltage were delivered to the pulse generator PG1 as shown in Fig. 20A, then, the pulse generator PG1 initially starts to generate the ignition pulses to cause the arc tube 5a to light up first. On the other hand, if the negative-phase voltage were delivered to the pulse generator PG2 when the power switch 51 is activated, then, the pulse generator PG2 initially starts to generate the ignition voltage, and as a result, the arc tube 5b lights up first. Since there is 50% of probability to feed either the positive-phase voltage or the negative-phase voltage to these pulse generators PG1 and PG2, these arc tubes 5a and 5b can respectively be operated at 50% of probability. The lighting probability is levelled off relative to the increased lighting rounds, and thus, neither of these arc tubes 5a and 5b can unilaterally and intensively be lit up. This in turn prevents either of these arc tubes from being lit up more frequently than the other, and thus, the voltage in the lamp can be prevented from rising itself as a result of the promoted dissipation of sodium in either of these arc tubes 5a and 5b. As a result, neither of these arc tubes quickly degrades own arc characteristic. Substantially, service life of the high-pressure discharge lamp embodied by the invention doubles that of any conventional high-pressure discharge lamp merely incorporating a single arc tube. Furthermore, when either of these arc tubes goes OFF while being lit as a result of a momentary power interruption, and then when the power service is restored, then the other arc tube containing low pressure and thus far remained OFF lights up. In this case, the latter arc tube were preliminarily heated while the former arc tube remained lit, and yet, since the pressure inside of the latter arc tube is slightly up, the lighting condition can quickly be stabilized. In other words, the high-pressure discharge lamp embodied by the invention can be reactivated in an extremely short period of time to restore the predetermined luminosity, and thus, if a plurality of the high-pressure discharge lamps embodied by the invention were made available for the illumination of highways and tunnels, traffic safety can significantly be promoted. Furthermore, the control circuit according to the seventh embodiment can simultaneously light up those arc tubes 5a and 5b by effectively controlling those pulse generator PG1 and PG2, thus making the luminosity double. The above description on the seventh embodiment has solely referred to the system for controlling the illumination of the high-pressure discharge lamp 1 incorporating a pair of arc tubes 5a and 5b. If more than two of arc tubes 5a, 5b, ... 5n were stored in the high-pressure discharge lamp 1, operation for lighting these arc tubes is subject to control by applying the control system of the eighth embodiment shown in Fig. 21. See Fig. 21. The AC power supply source 20 is connected to a lighting control device 100. A control circuit 101 is connected to this lighting control device 100. On the other hand, the lighting control device 100 is connected to one ends of terminals 6 of these arc tubes 5a through 5n via power transmission lines "a" through "n", and yet, the lighting control device 100 is also connected to the other ends of terminals 6 of those arc tubes 5a through 5n via another line 102. In response to the control signal from the control circuit 101, the lighting control device 100 superimposes either the positive pulse P1 or the negative pulse P2 on the AC voltage from the AC power supply source 20, and then delivers the pulse-superimposed AC voltage to a specific power transmission line selected from those lines "a" through "n" before selectively lighting up any of those arc tubes 5a through 5n. For example, the light control device 100 divides those arc tubes 5a through 5n into two groups in order to evenly level off the lighting probability of both-group arc tubes substantially at 50% whenever activating the power switch (not shown). Next, referring to Figures 22A, 22B, 23 and 24, the ninth embodiment of the invention is described below. Although not shown in Figures 22A and 22B, the outer envelope stores a pair of the frosted arc tubes 5a and 5b by way of inclining themselves by about 10 degrees part from the vertical line so that the tubular axes can intersect themselves at this angle. These frosted arc tubes 5a and 5n are respectively of the structure comprising a pair of end disks made from niobium, which are provided by way of shielding both ends of each tubular bulb composed of a ceramic tube made from either polycrystalline or monocrystalline alumina and airtightly bonded to both ends of the tubular bulb. Each of these end disks internally secures main terminals 6 at the upper and lower regions. These terminals 6 are respectively connected to the corresponding conductive members 7 projecting themselves from the end disks. These arc tubes 5a and 5b airtightly contain sodium, mercury, and Xenon gas, respectively. Those upper conductive members 7 shown in Fig. 22A are electrically and mechanically connected to a pair of bulb holders 8a and 8b made from thermally resistant metal like niobium or tantalum. Both ends of these bulb holders 8a and 8b are respectively coupled with supporting wires 9a and 9b. Those lower conductive members 7 shown in Fig. 22A are respectively held by insulated holders 10a and 10b, where both ends of these insulated holders 10a and 10b are respectively supported to the supporting wires 9a and 9b. Substantially, these supporting wires 9a and 9b are conductive. The upper ends of these supporting wires 9a and 9b are interconnected via an insulated bridge 11. The upper ends of these supporting wires 9a and 9b are respectively engaged with the tip region of the outer envelope via a pair of elastic plates 12a and 12b. The bottom ends of these supporting wires 9a and 9b are bonded to an inner lead wire 120a by means of welding. Those conductive members 7 below those arc tubes 5a and 5b are respectively connected to an inner lead wire 120b via a pair of silver lead wires 121a and 121b for example. These inner lead wires 120a and 120b are supported to the steam 122 of the outer envelope. According to the ninth embodiment, since a pair of arc tubes 5a and 5b stored in the outer envelope are respectively frosted, and yet, since the axes of these arc tubes 5a and 5b intersect with each other by about 10 degrees, the system can minimize the unwanted rate of obliging light from a radiant tube to be shielded by the other extinct tube. Fig. 24 designates unevenness of light distribution when those arc tubes 5a and 5b cross each other and align themselves in parallel with each other. Table shown in Fig. 24 designates the comparative unevenness of light distribution right below the illuminator when the extinct arc tube is at a position 180 degrees apart from the illuminated arc tube which is at the 0 degree position. As shown in Fig. 23, assume that the external diameter of each arc tube is D and the length L, and yet, interval between the arc tube 5a and 5b is less than 3D/2 and the intersecting angle . If the intersecting angle  were set in a range D/3L ≦ sin  < 2D/L, then these arc tubes 5a and 5b can maintain satisfactory proportion of light distribution. Next, referring to Figures 25 and 26, the tenth embodiment of the invention is described below. Fig. 25 designates the sectional view of an illuminator housing a high-pressure discharge lamp incorporating a pair of arc tubes like the one shown in Fig. 1. Fig. 26 designates a perspective view of an illuminator installed on road. The reference numeral 130 shown in Figures 25 and 26 designates a pole erected on a side of rod. An illuminator 131 is supported to the top of the pole 130. A socket 134 is supported to a flange 133 inside of the back plate 132 of the illuminator 131. The socket 134 accommodates a high-pressure discharge lamp 135. The illuminator 131 incorporates a lighting control device 136 which lights up a pair of arc tubes stored in the high-pressure discharge lamp based on substantially even probability. The lighting control device 136 incorporates those electrical circuits described earlier in relation to those preceding embodiments. The lighting control device 136 may discretely be provided outside of the illuminator 131. In this way, application of the high-pressure discharge lamp housing a pair of arc tubes as per the embodiments of the invention to the illumination of roads and tunnels is quite useful. Since the high-pressure discharge lamp embodied by the invention securely prevents either of arc tubes from unilaterally and more frequently being activated, and thus, actual service life doubles that of any conventional high-pressure discharge lamp merely housing a single arc tube. The above description has solely referred to the high-pressure discharge lamp incorporating a pair of arc tubes. However, the scope of the invention is not merely confined to the use of a pair of arc tubes, but the high-pressure discharge lamp of the invention may also store more than two of arc tubes, and yet, the lighting probability of these arc tubes can evenly be levelled off substantially at 50%. Furthermore, those ignition aids 17a and 17b may not necessarily be provided outside of those arc tubes 5a and 5b. Furthermore, when implementing any of those embodiments described above, a pair of ballasts may be provided like the embodiment shown in Fig. 21.

Claims: A high-pressure discharge lamp comprising: an outer envelope (1) formed from light-translucent material; a plurality of arc tubes (5a, 5b) which are respectively enclosed in said cuter envelope (1) and electrically connected in parallel with each other; and a plurality of ignition aids (17a, 17b) available for assisting operating of said arc tubes (5a, 5b), characterized in that said ignition aids (17a, 17b) are respectively provided for each of said arc tubes (5a, 5b) and have electric potentials different from each other. A high-pressure discharge lamp according to claim 1, characterised by further comprising: a high-voltage pulse generating means (40, 41) housed in said outer envelope (1) and substantially consisting of a thermal reaction switch (40) and a heater (41) for heating said thermal switch, both said switch (40) and said heater (41) being connected in series. A high-pressure discharge lamp according to claim 1, characterized in that said plural arc tubes (5a, 5b) are respectively disposed by way of intersecting each other. An apparatus for operating a high-pressure discharge lamp according to any one of claims 1 to 3, comprising: an AC power supply source (20); first and second power-supply lines which respectively connect both terminals of said AC power supply source (20) to a pair of terminals of said high-pressure discharge lamp (1); a ballast (21) which is at least connected on either of said first and second power supply lines; a power switch (51) which is connected on either the first or the second power supply line; a pulse generating means (22) which generates either positive or negative ignition pulses to be superimposed on AC power voltage delivered from said AC power supply source (20); a control means (52) which alters polarity of said ignition pulses output from said pulse generating means (22). An apparatus for operating a high-pressure discharge lamp according to claim 4, characterized in that said control means (52) alters polarity of ignition pulses output from said pulse generating means (22) in correspondence with polarity of AC voltage output from said AC power supply source (20) at the moment of activating said power switch (51). An apparatus for operating a high-pressure discharge lamp according to claim 4, characterized in that said control means (52) alters polarity of ignition pulses output from said pulse generating means (22) in correspondence with polarity of AC voltage output from said AC power supply source (20) at the moment of activating said power switch (51), and then, after a predetermined period from the operating of said power switch (51), said control means (52) controls switching operation in order that said pulse generating means (22) can output those pulses containing polarity inverse from that of those ignition pulses output from said pulse generating means (22) when activating said power switch (51). An apparatus for operating a high-pressure discharge lamp according to claim 4, characterized by further comprising: a means (DB4) for detecting an arc tube actually being lit; and wherein said control means (52) controls switching operation in order that said pulse generating means (22) can output ignition pulses to operate an arc tube opposite from another which is detected by said detection means (DB4) and actually being lit. An apparatus for operating high-pressure discharge lamp according to claim 4, characterized in that said control means alters pulse polarity from said pulse generating means (22) after a predetermined period from the operating of said power switch (51). An apparatus for operating a high-pressure discharge lamp according to claim 4, characterized by further comprising: a normally closed control switch (71, 81) which is connected to said power switch (51) in series; and wherein said control means (52) alter pulse polarity from said pulse generating means (22) after a predetermined period from the operating of said power switch (51), and then after another predetermined period from the operating of said power switch (51), said control means (52) executes a control operation to open and close said control switch (71, 81). An apparatus for operating a high-pressure discharge lamp according to claim 4, characterized in that counter swing voltage of said ignition pulses output from said pulse generating means is lower than the lamp voltage while said arc tubes (5a, 5b) remain cold, and conversely, said counter swing voltage is higher than the voltage capable of operation an extinct art tube while said arc tube (5a, 5b) remain hot. An apparatus for operating a high pressure discharge lamp according to claim 4, characterized by a further ballast (21') connected on said second power supply lines.

Inventors: ITOH AKIRA, KOTABE MITSUHO, MORI YASUKI, OKAMURA KAZUYOSHI, SATO KIMIHITO, SHIOHAMA HIROCHIKA, UCHIDA KAZUIKI, UCHINO KATSUSUKE, ITOH, AKIRA, KOTABE, MITSUHO, MORI, YASUKI, OKAMURA, KAZUYOSHI, SATO, KIMIHITO, SHIOHAMA, HIROCHIKA, UCHIDA, KAZUIKI, UCHINO, KATSUSUKE
Applicants: TOSHIBA LIGHTING & TECHNOLOGY, TOSHIBA LIGHTING & TECHNOLOGY CORPORATION
Application year: 1991
Main classification: H01J61
IPC codes: H05B41, H01J61