CA1256154A - Low pressure mercury discharge lamp for dc operation and method of operating the lamp - Google Patents
Low pressure mercury discharge lamp for dc operation and method of operating the lampInfo
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- CA1256154A CA1256154A CA000488058A CA488058A CA1256154A CA 1256154 A CA1256154 A CA 1256154A CA 000488058 A CA000488058 A CA 000488058A CA 488058 A CA488058 A CA 488058A CA 1256154 A CA1256154 A CA 1256154A
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Abstract
Abstract of the Disclosure A low pressure mercury discharge lamp for DC
operation has a discharge tube sealing therein an ionizable medium containing a predetermined amount of mercury, a cathode, arranged at one end of the discharge tube, for emitting electrons, an anode, arranged at the other end of the discharge tube, for collecting the electrons, and an amalgam, arranged at the anode side of the discharge tube, for adsorbing mercury in the discharge tube when the lamp is turned off and for liberating mercury into the discharge tube when the lamp is turned on.
operation has a discharge tube sealing therein an ionizable medium containing a predetermined amount of mercury, a cathode, arranged at one end of the discharge tube, for emitting electrons, an anode, arranged at the other end of the discharge tube, for collecting the electrons, and an amalgam, arranged at the anode side of the discharge tube, for adsorbing mercury in the discharge tube when the lamp is turned off and for liberating mercury into the discharge tube when the lamp is turned on.
Description
256~54 The present invention relates to a low pressure mercury discharge lamp for DC operation and a method of operating the lamp.
When a low pressure mercury discharge lamp exemplified by a fluorescent lamp is DC-operated, electrode loss is reduced as compared to the case of AC operation and luminous efficacy with respect to lamp input is increased. Flickering of the lamp is also decreased.
However, DC operation of such lamps is also liable to the problem of the cataholesis phenomenon as de-scribed in S. Yabashi et al, ~DC operation of vertically placed fluorescent lamp~, preprint of Japan Illuminating Engineering Soc., No. 18, 1984, p.18. When a lamp is DC-operated, mercury atoms in the tube are converted into positive ions so that most of the mercury vapor filled within the tube is attracted to the cathode side.
As a result, the mercury pressure at the anode side is greatly decreased, and the discharge tube becomes dark at the anode side as lamp ON time elapses.
It is an object of the present invention to provide a low pressure mercury discharge lamp for DC operation which is restrained from non-uniform brightness due to the cataholesis phenomenon.
~ 25 In order to achieve the above object, a low ; pressure mercury discharge lamp for DC operation ~ comprises: a discharge tube filled with an ionizable ' ~
. .
L256~5~
medium containing a predetermined amount of mercury;
a cathode, arranged at one end of the discharge tube, for emitting electrons; an anode, arranged at the other end of the discharge tube, for collecting the electrons; and an amalgam, arranged at the anode side of the discharge tube, for adsorbing mercury in the discharge tube when the lamp is turned off and for liberating mercury into the discharge tube when the lamp is turned on.
In the lamp of the above construction according to the present invention, when the mercury vapor pressure at the anode side begins to decrease during the operating time of the lamp, a corresponding amount of mercury is liberated from the amalgam.
Since mercury is replenished near the anode, apparently, mercury is uniformly distributed in the interior of the tube. For this reason, the cataholesis phenomenon in which the anode side is darkened according to ;~ ~ the operating time of the lamp is suppressed, and light of uniform brightness can be emitted from the overall discharge tube over a long period of time.
Other objects and advantages will be apparent from / :
the following description taken in conjunction with the accompanying drawings, in which:
Fig. 1 is a partial sectional view of a low pressure mercury discharge lamp for DC operation 12S6~54 according to a first embodiment of the present invention;
Fig. 2 is a circuit diagram of a circuit for operating the discharge lamp shown in Fig. l;
Fig. 3 is a partial sectional view of a low pressure mercury discharge lamp for DC operation according to a second embodiment of the present invention;
Fig. 4 shows a partial sectional view of a low pressure mercury discharge lamp for DC operation and a circuit diagram thereof according to a third embodiment of the present invention;
Fig. 5 is a partial sectional view of a low pressure mercury discharge lamp for DC operation with an amalgam arranged at a different position;
Fig. 6 is a partial sectional view of a low pres-sure mercury discharge lamp for DC operation according to a fourth embodiment of the present invention;
Fig. 7 is a front view of a display screen in which a low pressure mercury discharge lamp for DC operation according to the present invention is arranged;
Fig. 8 is a sectional view of a low pressure mercury discharge lamp for DC operation according to a fifth embodiment of the present invention:
Fig. 9 is a sectional view of an amalgam assembly used in the embodiment shown in Fig. 8;
Fig. 10 is a graph showing light output as ~Z5~1S4 a function of mercury vapor pressure;
Fig. 11 is a graph showing mercury vapor pressure distribution within a discharge tube;
Fig. 12 is a sectional view of a low pressure mercury discharge lamp for DC operation according to a sixth embodiment of the present invention;
Fig. 13 is a graph showing light output from a central portion of the tube as a function of temperature of coldest point on the cathode side;
Fig. 14 is a circuit diagram of a circuit for explaining a first embodiment of an operating method of a low pressure mercury discharge lamp for DC operation according to the present invention;
Fig. 15 is a sectional view of a discharge lamp for explaining a modification of the operating method according to the first embodiment of the present invention;
Fig. 16 is a circuit diagram of a circuit for explaining a second embodiment of an operating method of a low pressure mercury discharge lamp for DC operation according to the present invention;
Fig. 17 is a graph showing time T required to develop the cataholesis phenomenon as 8 function of a turn off period t2;
Fig. 18 is a graph showing time T required to develop the cataholesis phenomenon as a function of an inverse of lamp current IL;
~S6~5~
Fig. 19 is a graph showing time T required to develop the cataholesis phenomenon as a function of an inverse of a distance ~ between a cathode and an anode;
Fig. 20 is a graph showing time T required to develop the cataholesis phenomenon as a function of an inverse of a cross-sectional area S of the discharge tube; and Fig. 21 is a graph showing coefficient A as a function of mercury vapor pressure Po at the coldest point near the cathode in the lamp OFF state.
A low pressure mercury discharge lamp for DC
operation according to a first embodiment of the present invention will be described with reference to Figs. 1 and 2. A phosphor film 12 is coated on the inner surface of a straight discharge tube 10 made of soft glass. Mounts 14a and 14b are sealed to the two ends of the tube 10. Internal lead-in wires 18a and 18b are sealed in a stem tube 16a of the mount 14a. A coil filament type anode 20 is arranged between one end of internal lead-in wire 18a and one end of internal lead-in wire 18b. Internal lead-in wires 18c and 18d are sealed in a stem tube 16b of the mount 14b. A coil filament type preheat cathode 22 is arranged between one end of the internal lead-in wire 18c and one end of the internal lead-in wire 18d. The cathode 22 emits electrons, and the anode 20 collects electrons. After ' ~
~2S6154 the interior of the tube 10 is evacuated through one of exhaust tubes 24a and 24b extending externally from the stem tubes 16a and 16b, an inert gas as an ionizable medium is introduced therein through one of the exhaust tubes 24a and 24b. The exhaust tubes 24a and 24b are hermetically sealed. Amalgam 26 for controlling the mercury vapor pressure at the anode side of the tube 10 during normal operating time is contained in the tube 24a. The amalgam 26 is, e.g., an alloy obtained by adding mercury to an alloy of bismuth (Bi) and indium (In). In the lamp ON time, the amalgam 26 liberates mercury, corresponding to the lack of the mercury vapor pressure at the anode side, into the tube 10 due to heat from the anode 20 and, in the lamp OFF time, it adsorbs free mercury in the tube 10. Instead of placing the amalgam, an amalgam-forming metal and mercury can be respectively introduced in the tube 24a and in the tube 10, and an amalgam can be formed within the tube 24a.
In this embodiment, small amounts of auxiliary amalgàms 28a and 28b are arranged near the anode 20 and the cathode 22. The amalgams 28a and 28b are obtained by forming thin films of an amalgam forming metal, e.g., indium, on the surfaces of metal bases (e.g., stainless ;~ steel bases), and are respectively supported by the internal lead-in wires 18b and 18c. When the lamp is ignited, the amalgams 28a and 28b temporarily liberate compensational mercury into the tube 10 before ., ~256~54 substantial mercury is liberated within the tube 10 from the amalgam 26. In the lamp OFF state, the amalgams 28a and 28b adsorb free mercury in the tube 10, which is necessary for liberating into the tube 10 in reignition.
The low pressure mercury discharge lamp of the above construction is assembled in a circuit 30 having the two ends connected to an AC power supply 32, as shown in Fig. 2. One end of the anode 20 is connected to one end of the AC power supply 32 through a resist-ance ballast 34 as a current limiting element and a rectifying bridge circuit 36. One end of the cathode 22 is connected to the other end of the AC power supply 32 through the rectifying bridge circuit 36. A capacitor 38 is connected to the output terminal of the circuit 36. The other end of the anode 20 is connected to the other end of the cathode 22 through a glow switch starter 40 and a primary winding 44 of a pulse transformer 42. A secondary winding 46 of the pulse transformer 42 is connected to an outside auxiliary conductor 48 arranged near the wall of the tube 10.
An output voltage from the AC power supply 32 is converted into a DC voltage by the rectifying bridge circuit 36. The DC voltage is applied to the anode 20 and the cathode 22 through the capacitor 38 and the resistance ballast 34. A high voltage pulse is gen-erated by the pulse transformer 42 in response to an action of the starter 40 and the high voltage pulse is .~
-` ~256~54 applied to the wall of the tube 10. As a result, a glow discharge is generated in the tube 10 to cause a glow arc transition, and the discharge lamp is DC-operated.
When the lamp operates in this manner, the auxiliary amalgams 28a and 28b near the electrodes 20 and 22 are influenced by heat from these electrodes 20 and 22 and by ion bombardment due to discharge. The amalgams 28a and 28b are quickly heated and actively liberate mercury into the tube 10. Therefore, a sufficient amount of mercury is supplied into the tube 10 within a short period of time, and rising characteristics of light output are excellent.
When the wall temperature of the tube 10 begins to increase by discharge, the temperature of the amalgam 26 in the exhaust tube 24a is gradually increased. Then, mercury is liberated into the tube 10 through an opening of the tube 24a in an amount corresponding to the temperature increase. The internal mercury vapor pressure after the light output reaches normal operation state is controlled to be a vapor pressure which is determined by the temperature at the position of the amalgam 26.
When the low pressure mercury discharge lamp is ; DC-operated in this manner, mercury liberated in the tube 10 is converted into positive ions which are gradually attracted toward the cathode 22 having a negative potential. As a result, the amount of mercury - ~256~54 at the anode 20 side is reduced considerably, and the mercury vapor pressure at this side begins to decrease. As noted earlier, the amalgam 26 is contained in the exhaust tube 24a at the side of the anode 20.
Therefore, when the mercury vapor pressure begins to decrease, mercury is liberated from the amalgam 26 in an amount corresponding to the decreased amount, and is replenished near the anode 20. Apparently, mercury is uniformly distributed within the tube 10, and a generation of the cataholesis phenomenon in which the anode 20 side is darkened is suppressed. Decrease and non-uniformity of brightness during a long time is suppressed, and the overall tube 10 uniformly emits light.
According to an operation test performed by the present inventors, with a conventional low pressure mercury discharge lamp, the cataholesis phenomenon occurred within about 30 minutes after an ignition of the lamp. However, in a low pressure mercury discharge lamp having an amalgam 26 near an anode 20 according to the present invention, the cataholesis phenomenon did not occur even after 10 hours from the ignition of the lamp. These results demonstrate that non-uniform distribution of mercury in the tub~ 10 is prevented by liberation of mercury from the amalgam 26.
When the discharge lamp operating in the state `:
~:~ as described above is turned off, a larger amount of ~256~54 mercury is distributed at the side of the cathode 22.
As the temperatures of the tube 10 and the portion near the amalgam are decreased, free mercury in the tube 10 begins to be adsorbed by the auxiliary amalgams 28a and 28b. Since the adsorbing capacities of the auxiliary amalgams 28 and 28b are small, most of the free mercury in the tube 10 is adsorbed by the amalgam 26 at the anode 20 side. Thus, the state before lamp operation is restored, and the same operation is repeated when the lamp is turned on again.
A low pressure mercury discharge lamp for DC
operation according to a second embodiment of the present invention will be described with reference to Fig. 3. A discharge tube 110 bent in a substantially double-U shape is housed in an envelope 152 with a base 150 for an incandescent lamp. A phosphor 112 is coated on the inner surface of the tube 110. The envelope 152 houses therein a resistance ballast 34, a rectifying bridge circuit 36, a glow switch starter 40, a pulse transformer 42, and a capacitor (not shown) which together constitute a circuit 30 as shown in Fig. 2.
A small incandescent lamp is used as the resistance ballast 34. An amalgam 126 obtained by adding mercury to an alloy of bismuth and indium is housed in an exhaust tube 124a at the anode side. This discharge lamp is a compact fluorescent lamp which can replace a normal incandescent lamp.
5615~
According to this embodiment, as in the case of the first embodiment, the cataholesis phenomenon upon DC
operation can be prevented. When the compact fluores-cent lamp is DC-operated, the circuit 3û is rendered small and the overall device can be rendered light in weight. Since flickering is eliminated, the discharge lamp can be conveniently used in place of a normal incandescent lamp.
A low pressure mercury discharge lamp for DC
operation according to a third embodiment of the present invention will be described with reference to Fig. 4.
In this embodiment, a cylindrical nickel anode 120 is mounted between internal lead-in wires 18a and 18b.
When the anode 120 of this shape is used, sputtering of the anode material by electrons is eliminated, leading to a long life of the discharge lamp. Since other parts remain the same as in the first embodiment, the same reference numerals denote the same parts and a detailed description thereof will be omitted.
In order to operate the discharge lamp, the inter-nal lead-in wires 18a and 18b connected to the anode 120 are coupled to one end of an AC power supply 32 through a rectifying bridge circuit 36, and an internal lead-in wire 18d connected to a cathode 22 is coupled to the other end of the AC power supply 32 through the circuit 36 and an inductor 60. A glow switch starter 40 is inserted between an internal lead-in wire 18c and the lead-in wires 18a and 18b.
In the above embodiments, an amalgam 26 is put in an exhaust tube 24a or 124a. However, a mesh member 126 to which an amalgam is adhered can be winded on a wall of a stem tube 16a, as shown in Fig. 5.
The present invention is similarly applicable to a low pressure mercury discharge lamp of separated inner tube type disclosed in U.S.P. No. 4,199,7û8. In this lamp, an inner tube which regulates a discharge path is lû arranged in an outer tube filled with mercury and an inert gas. The present invention is also applicable to a DC discharge lamp disclosed in Japanese Patent Laid-open Application No. 58-204468. In this lamp, an inner tube which regulates a discharge path to form a plurality of discharge paths is housed in an outer tube, a single cathode is arranged in the inner tube, and a plurality of anodes are arranged in the outer tube but outside the inner tube. In other words, the structure of a low pressure mercury discharge lamp of the present 2û invention is not particularly limited, as long as it is DC-operated.
A low pressure mercury discharge lamp for DC
operation according to a fourth embodiment of the present invention will be described with reference to Fig. 6. In this embodiment, an amalgam 26a is put in an exhaust tube 24a at an anode 20 side, and an amalgam 26b is also put in an exhaust tube 24b at a cathode 22 side.
':
~256~54 The amalgams 26a and 26b are obtained by adding mercuryto an alloy of bismuth and indium. Since the remaining parts are the same as in the first embodiment, the same reference numerals denote the same parts and a detailed description thereof will be omitted. A circuit for operating the discharge lamp of this construction can be similar to that as shown in Fig. 2.
When a low pressure mercury discharge lamp is DC-operated, mercury emitted in the tube 10 is turned into positive ions which are gradually attracted toward the cathode 22 having a negative potential. As a result, the mercury at the anode 20 side is decreased in amount, and the mercury vapor pressure at the anode 20 side begins to decrease.
However, with the lamp of the above-described construction, the amalgam 26a is put inside the exhaust tube 24a at the anode 20 side. When the mercury vapor pressure begins to decrease, mercury is liberated from the amalgam 26a in the amount corresponding to the pressure decrease, and mercury is replenished near the anode 20 side. Apparently, mercury is uniformly distributed inside the tube lO, and the generation of the cataholesis phenomenon in which the anode 20 side is darkened is suppressed. Non-uniform brightness is suppressed for a long period of time, and the overall tube 10 can uniformly emit light.
In the ON state of the lamp, mercury in the tube 10 12S6~5~
_ 14 -is attracted to the cathode 22 side. When mercury continues to be supplied to the anode 20 side, the amount of mercury at the cathode 22 side increases.
Especially when the lamp is turned on for a long period of timej an excessive amount of mercury is supplied to the cathode side, and the mercury vapor pressure at the cathode 22 side exceeds the optimal pressure.
In this case, in the lamp of the above-mentioned construction, the amalgam 26b is also arranged in the exhaust tube 24b at the cathode 22 side. The amalgam 26b adsorbs excess mercury at the cathode 22 side.
Thus, an excessively high pressure of mercury vapor pressure at the cathode 22 side is suppressed, and a decrease in brightness at the cathode 22 side is suppressed.
As a result, the mercury amount is kept optimum at both the anode 20 and cathode 22 sides, so that the overall tube 10 can emit uniform light over a long period of time.
In the lamp OFF time, the mercury vapor pressure at the cathode 22 side is slightly higher than that at the anode 20 side. Therefore, the amalgam 26a at the anode 20 side slowly adsorbs mercury, and mercury is shifted from the cathode 22 side to the anode 2û side. The supply of mercury at the anode 20 side will not be deficient.
According to the above-mentioned construction, ::
125~;154 since the amalgams 26a and 26b are arranged at both the anode 20 and cathode 22 sides of the tube 10, polarity designation is not necessary when the lamp is operated in DC voltage in a luminaire, and handling is easy.
A low pressure mercury discharge lamp according to a fifth e~bodiment of the present invention will be described with reference to Figs. 7 to 9. The discharge lamp of this embodiment is a discharge lamp used as a display element of, e.g., a color video display apparatus.
Fig. 7 shows a large display screen which is used in a scoreboard at a ballpark or a sports stadium or on ~; ~ a wall of a building. Several thousands or ten times that of discharge lamps 212-1 to 212-n are densely arranged on a rectangular screen board 210. Any picture can be displayed on the screen board 210 by selectively switching or light controlling a plurality of discharge lamps 212-1 to 212-n arranged in the screen board 210.
Fig. 8 illustrates details of the discharge lamp 212-1.
One open end of a straight glass tube 214 is sealed by a convex Iens 216, and the outer circumferential surface ; of the lens 216 serves as a display surface 216a. The other open end of the glass tube 214 is sealed by a ::
glass stem 218. Lead-in wires 220a and 220b are sealed in the glass stem 218. A preheat cathode 222 comprising a filament coil is connected between the distal ends of the lead-in wires 220a and 220b which are guided into ~Z5615 the glass tube 214. A support wire 224 serving also as a power supply line is sealed in the glass stem 218.
The support wire 224 extends along the axis of the tube 214 and the distal end of the wire 224 is located near the lens 216. In order not to cause short-circuit discharge with the cathode 222, the support wire 224 is inserted into a glass insulating tube 226. An annular anode 228 is coaxially supported at the distal end of the wire 224 and is electrically connected thereto. A
light reflecting film 230 of an alumina coating film or the like is coated on the inner circumferential wall surface of the glass tube 214. A phosphor film 232 is formed on the light reflecting film 230. The phosphor film 232 extends to one end of the glass tube 214 in which the lens 216 exists, and covers the anode 228.
A base member 234 is mounted at the other end of the glass tube 214. The lead-in wires 220a and 220b and the support wire 224 are connected to base pins 236a, 236b and 236c projecting from the base member 234. The glass tube 214 is filled with predetermined amounts of mercury and an inert gas.
An amalgam assembly 238 for controlling the internal mercury vapor pressure during steady lamp ON
time is arranged at the anode side of the insulating tube 226. The amalgam assembly 238 of this embodiment consists of a plate-like metal base 240 of stainless steel or the like, and a thin film of an amalgam $256~54 forming metal 242 such as indium (In) formed on the circumferential surface of the base 240, as shown in Fig. 9. The amalgam assembly 238 bonds with mercury in the glass tube 214 and forms an amalgam. During the lamp ON time, the amalgam assembly 238 liberates mercury into the glass tube 214 by heat from the anode 228.
However, when the temperature within the glass tube 214 is lowered as in lamp OFF time, the amalgam assembly 238 adsorbs free mercury in the tube 214. The discharge lamps 212-1 to 212-n of this construction are densely arrayed on the screen board 210 with its display surface 216a facing the display direction. The base pins 236a, 236b and 236c are connected to a controller unit 246 connected to a DC power supply 244.
In order to operate the discharge lamp 212-1, a ; preheating current is supplied to the preheat cathode 222 so as to allow it to emit electrons into the glass tube 214. The emitted electrons are attracted toward the anode 228 by a voltage applied to the electrodes 222 and 228, and migrate within the glass tube 214, resulting in starting of discharge between the two electrodes. When discharge is started in this manner, the temperature of the amalgam assembly 238 is gradually increased, and mercury is liberated into the tube 214 in an amount corresponding to the temperature increase. A
positive column is formed between the preheat cathode 222 and the anode 228 in the tube 214. The phosphor '~
:
~256~54 film 232 is excited by ultraviolet rays emitted from the positive column and emits visible light. The visible light is radiated outward through the lens 216. When the light output reaches the steady state, the subse-quent mercury vapor pressure is controlled to be a vaporpressure determined by the temperature of the amalgam assembly 238.
When the discharge lamp 212-1 is DC-operated in this manner, as described above, mercury liberated in the glass tube 214 is turned into positive ions which are attracted toward the preheat cathode 222 having a negative potential. As a result, the mercury amount at the anode side 228 is decreased considerably, and the mercury vapor pressure at the anode side 228 begins to decrease.
In the above configuration, the amalgam assembly 238 is arranged at the anode 228 side in the glass tube 214. When the mercury vapor pressure begins to decrease at the anode 228 side as described above, mercury is liberated from the amalgam assembly 238 in an amount corresponding to the pressure decrease, and mercury is replenished near the anode 228. Therefore, apparently, mercury ions are uniformly distributed within the tube 214, and the generation of the cataholesis phenomenon in which the anode 228 side is darkened is suppressed. As a result, the phosphor near the display surface 216a which is most closely associated with the clearness of ~25615~
the image is actively excited. Therefore, the overall display sur~ace 216a can emit bright light over a long period of time, and a clear image of excellent quality can be displayed.
When the discharge lamp 212-1 in this state is turned off, a larger amount of mercury is present at the preheat cathode 222 side. However, as the temperature of the glass tube 214 and the amalgam assembly 238 decreases, free mercury in the glass tube 214 is gradually adsorbed by the amalgam assembly 238. Thus, the state before starting the operation is restored, and the same operation is repeated when the lamp is turned on again.
A low pressure mercury discharge lamp for DC
operation according to a sixth embodiment of the present invention will be described with reference to Figs. 10 to 13. The discharge lamp of this embodiment is a discharge lamp used as a display element as in the case of the fifth embodiment.
In a fluorescent lamp, a maximum output is known , to be obtainable if the mercury vapor pressure during ON time is kept at about 6 x 10-3 Torr. As shown in Fig. 10, light output decreases if the mercury vapor pressure deviates from 6 x 10-3 Torr in either direc-tion. Therefore, the mercury vapor pressure during lamp ON time is preferably kept at about 6 x 10 3 Torr.
A straight fluorescent lamp for AC operation has lZ56~5
When a low pressure mercury discharge lamp exemplified by a fluorescent lamp is DC-operated, electrode loss is reduced as compared to the case of AC operation and luminous efficacy with respect to lamp input is increased. Flickering of the lamp is also decreased.
However, DC operation of such lamps is also liable to the problem of the cataholesis phenomenon as de-scribed in S. Yabashi et al, ~DC operation of vertically placed fluorescent lamp~, preprint of Japan Illuminating Engineering Soc., No. 18, 1984, p.18. When a lamp is DC-operated, mercury atoms in the tube are converted into positive ions so that most of the mercury vapor filled within the tube is attracted to the cathode side.
As a result, the mercury pressure at the anode side is greatly decreased, and the discharge tube becomes dark at the anode side as lamp ON time elapses.
It is an object of the present invention to provide a low pressure mercury discharge lamp for DC operation which is restrained from non-uniform brightness due to the cataholesis phenomenon.
~ 25 In order to achieve the above object, a low ; pressure mercury discharge lamp for DC operation ~ comprises: a discharge tube filled with an ionizable ' ~
. .
L256~5~
medium containing a predetermined amount of mercury;
a cathode, arranged at one end of the discharge tube, for emitting electrons; an anode, arranged at the other end of the discharge tube, for collecting the electrons; and an amalgam, arranged at the anode side of the discharge tube, for adsorbing mercury in the discharge tube when the lamp is turned off and for liberating mercury into the discharge tube when the lamp is turned on.
In the lamp of the above construction according to the present invention, when the mercury vapor pressure at the anode side begins to decrease during the operating time of the lamp, a corresponding amount of mercury is liberated from the amalgam.
Since mercury is replenished near the anode, apparently, mercury is uniformly distributed in the interior of the tube. For this reason, the cataholesis phenomenon in which the anode side is darkened according to ;~ ~ the operating time of the lamp is suppressed, and light of uniform brightness can be emitted from the overall discharge tube over a long period of time.
Other objects and advantages will be apparent from / :
the following description taken in conjunction with the accompanying drawings, in which:
Fig. 1 is a partial sectional view of a low pressure mercury discharge lamp for DC operation 12S6~54 according to a first embodiment of the present invention;
Fig. 2 is a circuit diagram of a circuit for operating the discharge lamp shown in Fig. l;
Fig. 3 is a partial sectional view of a low pressure mercury discharge lamp for DC operation according to a second embodiment of the present invention;
Fig. 4 shows a partial sectional view of a low pressure mercury discharge lamp for DC operation and a circuit diagram thereof according to a third embodiment of the present invention;
Fig. 5 is a partial sectional view of a low pressure mercury discharge lamp for DC operation with an amalgam arranged at a different position;
Fig. 6 is a partial sectional view of a low pres-sure mercury discharge lamp for DC operation according to a fourth embodiment of the present invention;
Fig. 7 is a front view of a display screen in which a low pressure mercury discharge lamp for DC operation according to the present invention is arranged;
Fig. 8 is a sectional view of a low pressure mercury discharge lamp for DC operation according to a fifth embodiment of the present invention:
Fig. 9 is a sectional view of an amalgam assembly used in the embodiment shown in Fig. 8;
Fig. 10 is a graph showing light output as ~Z5~1S4 a function of mercury vapor pressure;
Fig. 11 is a graph showing mercury vapor pressure distribution within a discharge tube;
Fig. 12 is a sectional view of a low pressure mercury discharge lamp for DC operation according to a sixth embodiment of the present invention;
Fig. 13 is a graph showing light output from a central portion of the tube as a function of temperature of coldest point on the cathode side;
Fig. 14 is a circuit diagram of a circuit for explaining a first embodiment of an operating method of a low pressure mercury discharge lamp for DC operation according to the present invention;
Fig. 15 is a sectional view of a discharge lamp for explaining a modification of the operating method according to the first embodiment of the present invention;
Fig. 16 is a circuit diagram of a circuit for explaining a second embodiment of an operating method of a low pressure mercury discharge lamp for DC operation according to the present invention;
Fig. 17 is a graph showing time T required to develop the cataholesis phenomenon as 8 function of a turn off period t2;
Fig. 18 is a graph showing time T required to develop the cataholesis phenomenon as a function of an inverse of lamp current IL;
~S6~5~
Fig. 19 is a graph showing time T required to develop the cataholesis phenomenon as a function of an inverse of a distance ~ between a cathode and an anode;
Fig. 20 is a graph showing time T required to develop the cataholesis phenomenon as a function of an inverse of a cross-sectional area S of the discharge tube; and Fig. 21 is a graph showing coefficient A as a function of mercury vapor pressure Po at the coldest point near the cathode in the lamp OFF state.
A low pressure mercury discharge lamp for DC
operation according to a first embodiment of the present invention will be described with reference to Figs. 1 and 2. A phosphor film 12 is coated on the inner surface of a straight discharge tube 10 made of soft glass. Mounts 14a and 14b are sealed to the two ends of the tube 10. Internal lead-in wires 18a and 18b are sealed in a stem tube 16a of the mount 14a. A coil filament type anode 20 is arranged between one end of internal lead-in wire 18a and one end of internal lead-in wire 18b. Internal lead-in wires 18c and 18d are sealed in a stem tube 16b of the mount 14b. A coil filament type preheat cathode 22 is arranged between one end of the internal lead-in wire 18c and one end of the internal lead-in wire 18d. The cathode 22 emits electrons, and the anode 20 collects electrons. After ' ~
~2S6154 the interior of the tube 10 is evacuated through one of exhaust tubes 24a and 24b extending externally from the stem tubes 16a and 16b, an inert gas as an ionizable medium is introduced therein through one of the exhaust tubes 24a and 24b. The exhaust tubes 24a and 24b are hermetically sealed. Amalgam 26 for controlling the mercury vapor pressure at the anode side of the tube 10 during normal operating time is contained in the tube 24a. The amalgam 26 is, e.g., an alloy obtained by adding mercury to an alloy of bismuth (Bi) and indium (In). In the lamp ON time, the amalgam 26 liberates mercury, corresponding to the lack of the mercury vapor pressure at the anode side, into the tube 10 due to heat from the anode 20 and, in the lamp OFF time, it adsorbs free mercury in the tube 10. Instead of placing the amalgam, an amalgam-forming metal and mercury can be respectively introduced in the tube 24a and in the tube 10, and an amalgam can be formed within the tube 24a.
In this embodiment, small amounts of auxiliary amalgàms 28a and 28b are arranged near the anode 20 and the cathode 22. The amalgams 28a and 28b are obtained by forming thin films of an amalgam forming metal, e.g., indium, on the surfaces of metal bases (e.g., stainless ;~ steel bases), and are respectively supported by the internal lead-in wires 18b and 18c. When the lamp is ignited, the amalgams 28a and 28b temporarily liberate compensational mercury into the tube 10 before ., ~256~54 substantial mercury is liberated within the tube 10 from the amalgam 26. In the lamp OFF state, the amalgams 28a and 28b adsorb free mercury in the tube 10, which is necessary for liberating into the tube 10 in reignition.
The low pressure mercury discharge lamp of the above construction is assembled in a circuit 30 having the two ends connected to an AC power supply 32, as shown in Fig. 2. One end of the anode 20 is connected to one end of the AC power supply 32 through a resist-ance ballast 34 as a current limiting element and a rectifying bridge circuit 36. One end of the cathode 22 is connected to the other end of the AC power supply 32 through the rectifying bridge circuit 36. A capacitor 38 is connected to the output terminal of the circuit 36. The other end of the anode 20 is connected to the other end of the cathode 22 through a glow switch starter 40 and a primary winding 44 of a pulse transformer 42. A secondary winding 46 of the pulse transformer 42 is connected to an outside auxiliary conductor 48 arranged near the wall of the tube 10.
An output voltage from the AC power supply 32 is converted into a DC voltage by the rectifying bridge circuit 36. The DC voltage is applied to the anode 20 and the cathode 22 through the capacitor 38 and the resistance ballast 34. A high voltage pulse is gen-erated by the pulse transformer 42 in response to an action of the starter 40 and the high voltage pulse is .~
-` ~256~54 applied to the wall of the tube 10. As a result, a glow discharge is generated in the tube 10 to cause a glow arc transition, and the discharge lamp is DC-operated.
When the lamp operates in this manner, the auxiliary amalgams 28a and 28b near the electrodes 20 and 22 are influenced by heat from these electrodes 20 and 22 and by ion bombardment due to discharge. The amalgams 28a and 28b are quickly heated and actively liberate mercury into the tube 10. Therefore, a sufficient amount of mercury is supplied into the tube 10 within a short period of time, and rising characteristics of light output are excellent.
When the wall temperature of the tube 10 begins to increase by discharge, the temperature of the amalgam 26 in the exhaust tube 24a is gradually increased. Then, mercury is liberated into the tube 10 through an opening of the tube 24a in an amount corresponding to the temperature increase. The internal mercury vapor pressure after the light output reaches normal operation state is controlled to be a vapor pressure which is determined by the temperature at the position of the amalgam 26.
When the low pressure mercury discharge lamp is ; DC-operated in this manner, mercury liberated in the tube 10 is converted into positive ions which are gradually attracted toward the cathode 22 having a negative potential. As a result, the amount of mercury - ~256~54 at the anode 20 side is reduced considerably, and the mercury vapor pressure at this side begins to decrease. As noted earlier, the amalgam 26 is contained in the exhaust tube 24a at the side of the anode 20.
Therefore, when the mercury vapor pressure begins to decrease, mercury is liberated from the amalgam 26 in an amount corresponding to the decreased amount, and is replenished near the anode 20. Apparently, mercury is uniformly distributed within the tube 10, and a generation of the cataholesis phenomenon in which the anode 20 side is darkened is suppressed. Decrease and non-uniformity of brightness during a long time is suppressed, and the overall tube 10 uniformly emits light.
According to an operation test performed by the present inventors, with a conventional low pressure mercury discharge lamp, the cataholesis phenomenon occurred within about 30 minutes after an ignition of the lamp. However, in a low pressure mercury discharge lamp having an amalgam 26 near an anode 20 according to the present invention, the cataholesis phenomenon did not occur even after 10 hours from the ignition of the lamp. These results demonstrate that non-uniform distribution of mercury in the tub~ 10 is prevented by liberation of mercury from the amalgam 26.
When the discharge lamp operating in the state `:
~:~ as described above is turned off, a larger amount of ~256~54 mercury is distributed at the side of the cathode 22.
As the temperatures of the tube 10 and the portion near the amalgam are decreased, free mercury in the tube 10 begins to be adsorbed by the auxiliary amalgams 28a and 28b. Since the adsorbing capacities of the auxiliary amalgams 28 and 28b are small, most of the free mercury in the tube 10 is adsorbed by the amalgam 26 at the anode 20 side. Thus, the state before lamp operation is restored, and the same operation is repeated when the lamp is turned on again.
A low pressure mercury discharge lamp for DC
operation according to a second embodiment of the present invention will be described with reference to Fig. 3. A discharge tube 110 bent in a substantially double-U shape is housed in an envelope 152 with a base 150 for an incandescent lamp. A phosphor 112 is coated on the inner surface of the tube 110. The envelope 152 houses therein a resistance ballast 34, a rectifying bridge circuit 36, a glow switch starter 40, a pulse transformer 42, and a capacitor (not shown) which together constitute a circuit 30 as shown in Fig. 2.
A small incandescent lamp is used as the resistance ballast 34. An amalgam 126 obtained by adding mercury to an alloy of bismuth and indium is housed in an exhaust tube 124a at the anode side. This discharge lamp is a compact fluorescent lamp which can replace a normal incandescent lamp.
5615~
According to this embodiment, as in the case of the first embodiment, the cataholesis phenomenon upon DC
operation can be prevented. When the compact fluores-cent lamp is DC-operated, the circuit 3û is rendered small and the overall device can be rendered light in weight. Since flickering is eliminated, the discharge lamp can be conveniently used in place of a normal incandescent lamp.
A low pressure mercury discharge lamp for DC
operation according to a third embodiment of the present invention will be described with reference to Fig. 4.
In this embodiment, a cylindrical nickel anode 120 is mounted between internal lead-in wires 18a and 18b.
When the anode 120 of this shape is used, sputtering of the anode material by electrons is eliminated, leading to a long life of the discharge lamp. Since other parts remain the same as in the first embodiment, the same reference numerals denote the same parts and a detailed description thereof will be omitted.
In order to operate the discharge lamp, the inter-nal lead-in wires 18a and 18b connected to the anode 120 are coupled to one end of an AC power supply 32 through a rectifying bridge circuit 36, and an internal lead-in wire 18d connected to a cathode 22 is coupled to the other end of the AC power supply 32 through the circuit 36 and an inductor 60. A glow switch starter 40 is inserted between an internal lead-in wire 18c and the lead-in wires 18a and 18b.
In the above embodiments, an amalgam 26 is put in an exhaust tube 24a or 124a. However, a mesh member 126 to which an amalgam is adhered can be winded on a wall of a stem tube 16a, as shown in Fig. 5.
The present invention is similarly applicable to a low pressure mercury discharge lamp of separated inner tube type disclosed in U.S.P. No. 4,199,7û8. In this lamp, an inner tube which regulates a discharge path is lû arranged in an outer tube filled with mercury and an inert gas. The present invention is also applicable to a DC discharge lamp disclosed in Japanese Patent Laid-open Application No. 58-204468. In this lamp, an inner tube which regulates a discharge path to form a plurality of discharge paths is housed in an outer tube, a single cathode is arranged in the inner tube, and a plurality of anodes are arranged in the outer tube but outside the inner tube. In other words, the structure of a low pressure mercury discharge lamp of the present 2û invention is not particularly limited, as long as it is DC-operated.
A low pressure mercury discharge lamp for DC
operation according to a fourth embodiment of the present invention will be described with reference to Fig. 6. In this embodiment, an amalgam 26a is put in an exhaust tube 24a at an anode 20 side, and an amalgam 26b is also put in an exhaust tube 24b at a cathode 22 side.
':
~256~54 The amalgams 26a and 26b are obtained by adding mercuryto an alloy of bismuth and indium. Since the remaining parts are the same as in the first embodiment, the same reference numerals denote the same parts and a detailed description thereof will be omitted. A circuit for operating the discharge lamp of this construction can be similar to that as shown in Fig. 2.
When a low pressure mercury discharge lamp is DC-operated, mercury emitted in the tube 10 is turned into positive ions which are gradually attracted toward the cathode 22 having a negative potential. As a result, the mercury at the anode 20 side is decreased in amount, and the mercury vapor pressure at the anode 20 side begins to decrease.
However, with the lamp of the above-described construction, the amalgam 26a is put inside the exhaust tube 24a at the anode 20 side. When the mercury vapor pressure begins to decrease, mercury is liberated from the amalgam 26a in the amount corresponding to the pressure decrease, and mercury is replenished near the anode 20 side. Apparently, mercury is uniformly distributed inside the tube lO, and the generation of the cataholesis phenomenon in which the anode 20 side is darkened is suppressed. Non-uniform brightness is suppressed for a long period of time, and the overall tube 10 can uniformly emit light.
In the ON state of the lamp, mercury in the tube 10 12S6~5~
_ 14 -is attracted to the cathode 22 side. When mercury continues to be supplied to the anode 20 side, the amount of mercury at the cathode 22 side increases.
Especially when the lamp is turned on for a long period of timej an excessive amount of mercury is supplied to the cathode side, and the mercury vapor pressure at the cathode 22 side exceeds the optimal pressure.
In this case, in the lamp of the above-mentioned construction, the amalgam 26b is also arranged in the exhaust tube 24b at the cathode 22 side. The amalgam 26b adsorbs excess mercury at the cathode 22 side.
Thus, an excessively high pressure of mercury vapor pressure at the cathode 22 side is suppressed, and a decrease in brightness at the cathode 22 side is suppressed.
As a result, the mercury amount is kept optimum at both the anode 20 and cathode 22 sides, so that the overall tube 10 can emit uniform light over a long period of time.
In the lamp OFF time, the mercury vapor pressure at the cathode 22 side is slightly higher than that at the anode 20 side. Therefore, the amalgam 26a at the anode 20 side slowly adsorbs mercury, and mercury is shifted from the cathode 22 side to the anode 2û side. The supply of mercury at the anode 20 side will not be deficient.
According to the above-mentioned construction, ::
125~;154 since the amalgams 26a and 26b are arranged at both the anode 20 and cathode 22 sides of the tube 10, polarity designation is not necessary when the lamp is operated in DC voltage in a luminaire, and handling is easy.
A low pressure mercury discharge lamp according to a fifth e~bodiment of the present invention will be described with reference to Figs. 7 to 9. The discharge lamp of this embodiment is a discharge lamp used as a display element of, e.g., a color video display apparatus.
Fig. 7 shows a large display screen which is used in a scoreboard at a ballpark or a sports stadium or on ~; ~ a wall of a building. Several thousands or ten times that of discharge lamps 212-1 to 212-n are densely arranged on a rectangular screen board 210. Any picture can be displayed on the screen board 210 by selectively switching or light controlling a plurality of discharge lamps 212-1 to 212-n arranged in the screen board 210.
Fig. 8 illustrates details of the discharge lamp 212-1.
One open end of a straight glass tube 214 is sealed by a convex Iens 216, and the outer circumferential surface ; of the lens 216 serves as a display surface 216a. The other open end of the glass tube 214 is sealed by a ::
glass stem 218. Lead-in wires 220a and 220b are sealed in the glass stem 218. A preheat cathode 222 comprising a filament coil is connected between the distal ends of the lead-in wires 220a and 220b which are guided into ~Z5615 the glass tube 214. A support wire 224 serving also as a power supply line is sealed in the glass stem 218.
The support wire 224 extends along the axis of the tube 214 and the distal end of the wire 224 is located near the lens 216. In order not to cause short-circuit discharge with the cathode 222, the support wire 224 is inserted into a glass insulating tube 226. An annular anode 228 is coaxially supported at the distal end of the wire 224 and is electrically connected thereto. A
light reflecting film 230 of an alumina coating film or the like is coated on the inner circumferential wall surface of the glass tube 214. A phosphor film 232 is formed on the light reflecting film 230. The phosphor film 232 extends to one end of the glass tube 214 in which the lens 216 exists, and covers the anode 228.
A base member 234 is mounted at the other end of the glass tube 214. The lead-in wires 220a and 220b and the support wire 224 are connected to base pins 236a, 236b and 236c projecting from the base member 234. The glass tube 214 is filled with predetermined amounts of mercury and an inert gas.
An amalgam assembly 238 for controlling the internal mercury vapor pressure during steady lamp ON
time is arranged at the anode side of the insulating tube 226. The amalgam assembly 238 of this embodiment consists of a plate-like metal base 240 of stainless steel or the like, and a thin film of an amalgam $256~54 forming metal 242 such as indium (In) formed on the circumferential surface of the base 240, as shown in Fig. 9. The amalgam assembly 238 bonds with mercury in the glass tube 214 and forms an amalgam. During the lamp ON time, the amalgam assembly 238 liberates mercury into the glass tube 214 by heat from the anode 228.
However, when the temperature within the glass tube 214 is lowered as in lamp OFF time, the amalgam assembly 238 adsorbs free mercury in the tube 214. The discharge lamps 212-1 to 212-n of this construction are densely arrayed on the screen board 210 with its display surface 216a facing the display direction. The base pins 236a, 236b and 236c are connected to a controller unit 246 connected to a DC power supply 244.
In order to operate the discharge lamp 212-1, a ; preheating current is supplied to the preheat cathode 222 so as to allow it to emit electrons into the glass tube 214. The emitted electrons are attracted toward the anode 228 by a voltage applied to the electrodes 222 and 228, and migrate within the glass tube 214, resulting in starting of discharge between the two electrodes. When discharge is started in this manner, the temperature of the amalgam assembly 238 is gradually increased, and mercury is liberated into the tube 214 in an amount corresponding to the temperature increase. A
positive column is formed between the preheat cathode 222 and the anode 228 in the tube 214. The phosphor '~
:
~256~54 film 232 is excited by ultraviolet rays emitted from the positive column and emits visible light. The visible light is radiated outward through the lens 216. When the light output reaches the steady state, the subse-quent mercury vapor pressure is controlled to be a vaporpressure determined by the temperature of the amalgam assembly 238.
When the discharge lamp 212-1 is DC-operated in this manner, as described above, mercury liberated in the glass tube 214 is turned into positive ions which are attracted toward the preheat cathode 222 having a negative potential. As a result, the mercury amount at the anode side 228 is decreased considerably, and the mercury vapor pressure at the anode side 228 begins to decrease.
In the above configuration, the amalgam assembly 238 is arranged at the anode 228 side in the glass tube 214. When the mercury vapor pressure begins to decrease at the anode 228 side as described above, mercury is liberated from the amalgam assembly 238 in an amount corresponding to the pressure decrease, and mercury is replenished near the anode 228. Therefore, apparently, mercury ions are uniformly distributed within the tube 214, and the generation of the cataholesis phenomenon in which the anode 228 side is darkened is suppressed. As a result, the phosphor near the display surface 216a which is most closely associated with the clearness of ~25615~
the image is actively excited. Therefore, the overall display sur~ace 216a can emit bright light over a long period of time, and a clear image of excellent quality can be displayed.
When the discharge lamp 212-1 in this state is turned off, a larger amount of mercury is present at the preheat cathode 222 side. However, as the temperature of the glass tube 214 and the amalgam assembly 238 decreases, free mercury in the glass tube 214 is gradually adsorbed by the amalgam assembly 238. Thus, the state before starting the operation is restored, and the same operation is repeated when the lamp is turned on again.
A low pressure mercury discharge lamp for DC
operation according to a sixth embodiment of the present invention will be described with reference to Figs. 10 to 13. The discharge lamp of this embodiment is a discharge lamp used as a display element as in the case of the fifth embodiment.
In a fluorescent lamp, a maximum output is known , to be obtainable if the mercury vapor pressure during ON time is kept at about 6 x 10-3 Torr. As shown in Fig. 10, light output decreases if the mercury vapor pressure deviates from 6 x 10-3 Torr in either direc-tion. Therefore, the mercury vapor pressure during lamp ON time is preferably kept at about 6 x 10 3 Torr.
A straight fluorescent lamp for AC operation has lZ56~5
- 2~ --a mercury vapor pressure distribution substantially uniform along the axial direction of the tube, as indicated by characteristic curve ~ shown in Fig. 11.
Therefore, the mercury vapor pressure can be kept at a level of 6 x 10-3 Torr along the entire tube. However, in a straight fluorescent lamp for DC operation subject to the cataholesis phenomenon, since mercury vapor is attracted toward the cathode side, the mercury vapor pressure at the anode side is considerably decreased, and the mercury vapor pressure at the cathode side is slightly increased. As a result, the mercury vapor pressure distribution within the tube is as indicated by characteristic curve ~ in Fig. 11.
With the mercury vapor pressure as indicated by characteristic curve ~, the maximum output portion is shifted toward the cathode side (point P), resulting in a large decrease in light output at the anode side.
In order to prevent the cataholesis phenomenon, an amalgam-forming metal can be arranged at the anode side in the light emitting tube so as to adsorb mercury in the tube in the lamp OFF time and to emit mercury in the lamp ON time, as in the first to fifth embodiments.
~; However, the amount of mercury which can be liberated from an amalgam is limited. When the lamp is turned on over an extended period of time, no more mercury can be liberated from the amalgam, and the cataholesis phenomenon later occurs. The sixth ::~;
~ ~ `f~' ~2~i6~54 embodiment is to resolve this problem.
A discharge lamp 310 shown in Fig. 12 is a dis-charge lamp as a display element arranged in a screen board as shown in Fig. 7. The inner surface of a cylindrical reflecting cylinder 312 has a light reflecting surface 316 coated with a visible light reflecting substance 314 such as an alumina deposition film. A base member 318 is connected to one open end of the reflecting cylinder 312. A U-shaped fluorescent lamp 320 as a light source of the display element is arranged inside the reflecting cylinder 312. A
discharge tube 322 is obtained by bending a straight soft glass tube having an inner diameter of 10 mm, a length of 100 mm and a wall thickness of 1.0 mm into a substantially U shape at the central portion between two ends 324a and 324b. The tube 322 has a substantially U-shaped portion 326 between the two ends 324a and 324b.
An anode 328 and a cathode 330 are sealed to the ends 324a and 324b of the tube 322. The tube 322 is filled with predetermined amounts of mercury and an inert gas.
A phosphor 332 for emitting one color light of red, green or blue is coated on the inner surface of the discharge tube 322. The ends 324a and 324b of the lamp 320 are supported by the base member 318. The anode 328 and the cathode 330 are connected to four base pins 334a, 334b and so on (remaining two pins not shown) of the base member 318. The portion 326 is located inside ' :
` ` 125~154 the other open end of the reflecting cylinder 312. A
lens 336 for projecting light from the fluorescent lamp 320 is fitted in the other open end of the reflecting cylinder 312.
An amaigam 338 is put an exhaust tube 334a which is at the anode 328 side. The amalgam 338 is put in the exhaust tube 334a in the form of an alloy obtained by adding mercury (Hg) to at least one of bismuth (Bi), indium (In) and tin (Sn).
The base member 318 covers the ends 324a and 324b of the discharge tube 322. At least one air hole (two holes 340a and 340b in this case) is formed in the portion around the end 324a of the base member 318. The air holes 340a and 340b communicate the interior and exterior of the reflecting cylinder 312. An opening 342 ; is formed in the reflecting cylinder 312 at a position near the linear portion at the anode side. An air flow along a direction indicated by arrow D or a direction opposite thereto is generated by the air holes 340a and 340b and the opening 342. Thus, the anode side end 324a is cooled with air, and the anode side end 324a is effectively cooled. ~_¦
~;~ In contrast, no air hole or opening is formed inthe cathode side end 324b. Rather, the cathode side end 324b is covered with the base member 318 and is insulated thereby with respect to heat. A heat insu-:;~
l~; lating film can be formed on the cathode side end 324b :
~:256~54 so as to increase the temperature at this end.
The discharge lamp 310 described above is operated by an operating circuit as shown in Fig. 2 or 4. In the ON state of the discharge lamp 310, mercury vaporized in the tube 322 is turned into positive ions which are gradually attracted toward the cathode 330 side which is at a negative potential. For this reason, the mercury vapor pressure at the anode 328 side begins to decrease. However, since the amalgam 338 is arraged in the exhaust tube 344a, the amalgam 338 liberates mercury in an amount corresponding to the decrease amount, thus replenishing the required amount of mercury. As a result, the cataholesis phenomen.on in an early ON period of the lamp is prevented, and the time required to develop the cataholesis phenomenon is delayed.
In this embodiment, since the cathode side end 324b is covered with the base member 318 or a heat insulating film, the temperature at the coldest point at the cathode side is increased above that in a conventional case. Thus, the mercury vapor pressure at the cathode side end 324b is higher than that in the case wherein the end 324b is not insulated with respect to heat. It is difficult for the mercury vapor at the anode side end 324a to move to the cathode side end 324b which is at a higher pressure. Mercury at the cathode side is return-ed to the anode side end 324a due to Brown Motion. The ~;25~;154 generation of the cataholesis phenomenon is suppressed more effectively.
The mercury vapor pressure distribution in the lamp of this embodiment can be represented by characteristic curve y in Fig. 11. The point corresponding to the pressure of 6 x lû 3 Torr at which a maximum luminous efficacy is obtained is at the central position of the tube. With this arrangement, a decrease in the luminous flux at the anode side can be decreased, and a light output decrease of the overall lamp can be reduced to a minimum. In the U-shaped discharge tube 322 shown in Fig. 12, a position of maximum light output corresponds to the U-shaped bent portion 326. In the case of the light source 310 as the display element, light from the lamp 320 is projected toward the front through the lens 336. Therefore, it is most preferable that a position of maximum light output correspond to the U-shaped bent portion 326 which faces the lens 336. With this arrangement, the light output of the overall discharge lamp 310 is improved.
As can be seen from the characteristic curve r in Fig. 11, in order to obtain a mercury vapor pressure of 6 x 10 3 Torr at the central portion of the tube, the mercury vapor pressure at the coldest point at the cathode side must be doubled, i.e., 2 x 6 x 10 3 Torr.
The present inventors experimentally confirmed the temperature of the coldest point at the cathode side to 25~5d, obtain the mercury vapor pressure of 6 x 10-3 Torr at the central portion of the tube.
The results obtained are shown in Fig. 13. It is seen from these results that in order to obtain a maximum light output at the central portion of the discharge tube, the temperature of the coldest point must be set at about 6ûC~ However, in order to obtain a light output of 90 % or more in consideration of lamp manufacturing allowance and pressure distribution allowance, the coldest point temperature at the cathode side may be kept within the range of 50 to 80C.
The application of a fluorescent lamp is not limited to a light source of the display element 310 but can be extended to a U-shaped lamp for normal illumination or a double U-shaped fluorescent lamp obtained by bending the U-shaped lamp into a U shape having a smaller radius of curvature. This embodiment is also applicable to a straight fluorescent lamp. The amalgam may be arranged on a wall of the stem tube or at the other position at the anode side.
A method of operating a low pressure mercury discharge lamp for DC operation according to a first embodiment of the present invention will be described with reference to Fig. 14.
A phosphor film 412 is formed on the inner surface of a straight discharge tube 410 of soft glass. An anode 414 and a cathode 416 are arranged at the two ends ~256~54 of the tube 410. The tube 410 is filled with predeter-mined amounts of mercury and a starting rare gas. An amalgam 418 is put in an exhaust tube 428a at the anode 414 side of the tube 410. The amalgam 418 consists of an alloy obtained by adding mercury (Hg) to at least one of bismuth (Bi), indium (In), and tin (Sn).
The discharge tube 410 is mounted in a luminaire (not shown) such that the anode 414 is below the cathode 416. The tube 410 is used in a vertical orientation with the anode 414 is located below the cathode 416.
One end of the anode 414 is series-connected to one end of the cathode 416 through a glow switch starter 420. The other end of the anode 414 is connected to the other end of the cathode 416 through a rectifying bridge circuit 422. The rectifying bridge circuit 422 is connected to an AC power supply 426 through a choke coil ballast 424. An output voltage from the AC power ~; supply 426 is converted into a DC voltage by the circuit 422. The DC voltage is applied to the anode 414 and the cathode 416. Thus, the tube 410 is DC-operated.
In this ON state, mercury vaporized in the tube 410 is turned into positive ions which are gradually attracted toward the cathode side 416. Then, the mercury vapor pressure at the anode 414 side begins to decrease. However, since the amalgam 418 is arranged in the exhaust tube 428a at the anode 414 side, it liberates mercury in the amount corresponding to the ~256~54 decrease amount at the anode 414 side. The cataholesis phenomenon will not occur during an early ON period of the lamp.
In addition to the above-mentioned function of the amalgam 418, mercury in the tube 410 and mercury liberates from the amalgam 418 is gradually attracted downward by gravity and tends to collect at the lower end of the tube 410, since the tube is arranged such that the anode 414 is located below the cathode 416.
The gravity acting on mercury ions balances with the attractive force acting on mercury ions toward the cathode 416 side, thereby balancing the mercury vapor pressure inside the tube 410.
In this manner, the generation of the cataholesis phenomenon in an early ON period of the lamp is delayed ; by a combination of the mercury liberating function from the amalgam 418 and the gravity action of mercury.
In the OFF period of the lamp, extra mercury vapor in the tube 410 is attracted toward the amalgam 418.
Mercury in the tube collects near the amalgam 418 by gravity to stimulate adsorption of mercury by the amalgam 418. Mercury which is not adsorbed by the amalgam 418 collects near the anode 414 located below.
As a result, when the lamp is turned on again, it takes a long period of time for mercury to collect at the cathode 416 side, thereby suppressing the generation of the cataholesis phenomenon.
.~ ~
:
2S~54 When a DC operation of the lamp of the above embodiment was performed, the following results were obtained. With a straight fluorescent lamp having an inner diameter of 15 mm, a distance between the anode and cathode of 300 mm and a lamp current of 0.215 A and using no amalgam, when the lamp was oriented vertically with the anode 414 located above, the total luminous ; flux after a 10 hour operation was 410 ~m. On the other hand, when the identical lamp was operated in a vertical orientation with the anode 414 located below, the total luminous flux after a 10 hour operation was 820 lm.
When an amalgam 418 was put in an identical lamp and the anode 414 was located above, the total luminous flux after a 10 hour operation was 650 Qm. However, when the amalgam 418 was used and the anode 414 was located above, the total luminous flux after a 10 hour operation was 860 ~m.
~; The tube 410 need not be operated in a vertical orientation. The tube 410 can be operated in an inclined orientation as long as the anode 414 is located below the cathode 416. The amalgam 418 may be arranged on a wall of the stem tube or at the other position at the anode side. Thus, in the case of the U-shaped ; fluorescent lamp shown in Fig. 15, a similar effect can be obtained if the anode 414 is located below the cathode 416.
~:
A method of operating a low pressure mercury discharge lamp according to a second embodiment of the present invention will be described with reference to Figs. 16 to 21.
Fig. 16 shows the construction of a U-shaped fluorescent lamp and an operating circuit therefor. A
phosphor film 512 is coated on the inner surface of a discharge tube 510 obtained by bending a soft glass tube into a U shape. An anode 514 and a cathode 516 are mounted at the two ends of the tube 510. The tube 510 is filled with predetermined amounts of mercury and a starting rare gas. An amalgam 518 is put in an exhaust tube 528a at the anode 514 side of the tube 510. The amalgam 518 consists of an alloy obtained by adding i~ mercury (Hg) to one of bismuth (Bi), indium (In) and tin (Sn).
One end of the anode 514 is series-connected to one end of the cathode 516 through a glow switch starter 520. The other end of the anode 514 is connected to the other end of the cathode 516 through a rectifying bridge circuit 522. The circuit 522 is connected to an AC
power supply 526 through a choke coil ballast 524.
~; Therefore, an output voltage from the AC power supply 526 is converted into a DC voltage by the circuit 522.
The DC voltage is applied between the anode 514 and the cathode 516 to DC operate the tube 510.
In the ON state, mercury vaporized in the tube ~ 510 is turned into positive ions which are gradually `:::
~2~ 5~
attracted toward the cathode 516 side. Therefore, themercury vapor pressure at the anode 514 side begins to decrease. However, since the amalgam 518 is arranged in the exhaust tube 528a at the anode 514 side, the amalgam 518 liberates mercury in an amount corresponding to the decrease amount, so that apparently the mercury vapor pressure distribution within the tube is maintained to be uniform. As a result, the cataholesis phenomenon in an early ON period of the lamp is prevented.
Now, the present inventors found that the time required to develop the cataholesis phenomenon differed in accordance with turn OFF period t2 even if the same lamp was used in the various experiments. Based on this, the present inventors surmised that a better ; 15 effect in suppressing the cataholesis phenomenon could be obtained if the turn OFF period t2 were controlled properly.
; Based on this assumption, the present inventors ;~ performed various experiments, and found that if the following relation is satisfied:
tl/t2 < 780(S/l) (Po/Pg)-(l/IL) ... (1) where tl: turn ON period t2: turn OFF period S: discharge tube cross-sectional area (cm2) l: distance between the anode and the cathode telectrode distance) ~25615~
Po: mercury vapor pressure (Torr) near the cathode during lamp turn ON period Pg: filling gas pressure (Torr) IL: lamp current (A) the cataholesis phenomenon can be further suppressed.
The experiments performed will be described below.
Experiment 1 The present inventors knew prior to the experiments that when the turn OFF period t2 is long, the time T
required to develop the cataholesis phenomenon is different even if an identical lamp is used. In order to confirm this experimentally, the present inventors performed the following experiment.
A fluorescent lamp used had a constructian as ~-~ 15 shown in Fig. 16 and an inner diameter of 15 mm (cross-;~ sectional area S = 1.77cm2?, a distance between the anode and cathode (I) of 29 cm, a lamp current (IL) of 0.215 A, and a filling gas pressure tPg) of 5 Torr. The relationship between the turn OFF period t2 and the time I required to develop the cataholesis phenomenon was examined. The obtained characteristic data is as shown ~:
in Fig. 17. The characteristics shown in Fig. 17 are average values obtained with three lamps of identical ~ specifications (the same applies to Figs. 18 to 21). In - ~ 25 these lamps, the coldest point at the cathode 516 side was located at the tube end and was 40C.
The characteristics shown in Fig. 17 satisfy:
~ ~ .
~256~5~
T - . O. 28t2 ..(2) Therefore, the time ~ to develop the cataholesis phenom-enon upon lamp ON operation is proportional to the turn OFF period t2 before an ignition of the lamp. That is, T ~ t2 - (3) This is attributable to the fact that the longer the turn OFF period t2, the larger the amount of mercury adsorbed in the amalgam 518 at the anode 514 side and the more difficult for mercury to move toward the lû cathode 516 side.
Experiment 2 In the course of investigation, the present inventors realized that the larger the lamp current IL, the shorter the time T required to develop the cataholesis phenomenon. Ir, view of this, the relationship between the lamp current IL and the cataholesis phenomenon was examined in detail.
In this experiment, a discharge tube used had an inner diameter of 15 mm (cross-sectional area S =
1.77 cm ), and a distance between the anode and cathode (~) of 29 cm. The coldest point at the cathode side was forcibly cooled at 40C. The filling gas pressure was kept at 5 (Torr), and the turn OFF period t2 was 2 hours. Under these conditions, the relationship between the lamp current IL and the time T was examined.
The obtained results are shown in Fig. 18. From the results shown in Fig. 18, the relation:
--` 1256~54 T (l/IL) ...(4) is satisfied.
The above phenomenon is considered to be caused by the following. The attraction force of the mercury toward the cathode 516 side is influenced by the lamp current IL. That is, when the lamp current is large, the number of mercury ions is increased, and the amount of mercury ions attracted to the cathode 516 side is increased. As a result, the mercury vapor pressure lû distribution within the tube has a gradient, resulting in the cataholesis phenomenon.
Experiment 3 Meanwhile, the gradient of the mercury vapor pressure due to electric potential gradient should be cancelled in response to turning off the lamp.
Cancellation of the pressure gradient is attributed to diffusion of mercury atoms. One of the factors influ-encing the time required for mercury vapor attracted to the cathode 516 side to be recovered to the anode 514 side by diffusion in the lamp OFF period is presumed to be the distance ~ between the anode and cathode. In other words, the longer the distance ~, the longer the time required to recover the mercury.
A discharge tube used had an inner diameter of 15 mm (cross-sectional area S = 1.77 cm2). Forcible cooling was performed to keep the coldest point at the cathode side at 40C. The filling gas pressure (Pg) was .
5 Torr, the lamp current (IL) was 0.215 A, and the turn OFF period t2 was 2 hours. Under these conditions, the relationship between the distance Q and the time T
required to develop the cataholesis phenomenon was examined.
The obtained results are shown in Fig. 19. It is seen from Fig. 19 that, the relation:
T ~ (1/Q) - (5) is established.
Experiment 4 A factor preventing recovery of the mercury vapor attracted to the cathode 516 side to the anode 514 side by diffusion during the lamp OFF period is the cross-sectional area of the tube 510, i.e., the cross-sectional area of the light emitting space. The smaller the cross-sectional area, the longer the time required for recovering the mercury due to diffusion resistance.
A discharge tube used had a distance (Q) of 29 cm.
The coldest point at the cathode 516 side was kept at 40C, the filling gas pressure (Pg) was kept at 5 Torr, the lamp current ~IL) was set at 0.215 A, and the lamp turn OFF period (t2) was 2 hours. Under these condi-tions, the relationship between the cross-sectional area S of the tube and the time T was examined.
The results obtained are shown in Fig. 20. It is seen from the results shown in Fig. 20, that the following relation:
::
~256~S4 T S (6) is established.
From experiments 1 to 4, the following relation is established:
T (S/Q)-(l/IL)-t2 ....................... (7) It is known that when the filling pressure of a starting rare gas is high, the amount of mercury evaporation is regulated and movement of the mercury vapor is restricted. When the relationship between the time T and the filling gas pressure Pg was examined, the following relation was obtained:
T (l/Pg) ...(8) Substitution of relation (8) in relation (7) yields:
T (S/Q) (l/Pg) (l/IL) t2 ... (9) Relation (9) can be rewritten as:
T = A (S/Q)-(l/Pg)-(l/IL) t2 ... (10) where A is a coefficient.
Experiment 5 The mercury vapor pressure in the discharge tube 510 varies in accordance with the coldest point temperature. When the mercury vapor pressure at the cathode side changes, the amount of mercury shifted to the cathode 516 side also changes, resulting in a change in the time T.
In view of this, the present inventors examined the relationship between the cataholesis phenomenon and the mercury vapor pressure at the cathode 516 side affected ~2S6~5~
by the coldest point temperature at the same side.
A lamp used had an inner diameter of 15 mm (cross-sectional area S = 1.77 cm2), a distance (~) of 29 cm, a filling gas pressure (Pg) of 5 Torr, and a lamp current (IL) of 0.215 A. When these conditions are substituted in relation (10) and t2 = T/O. 28 obtained from T -, 0.28t2 of relation (2) is substituted in relation (10), A '_ 5 is obtained. The tube end defining the coldest point at the cathode 516 side was at 40C
and the mercury vapor pressure Po at this point was 6.34 x 10 3 Torr.
A similar experiment was performed by variously changing the temperature of the coldest point. The mercury vapor pressure Po and the coefficient A corre-sponding to each different temperature was examined.
The results as shown in Tabie 1 below were obtained:
Table 1 point Mercury vapor Coefficient tempera- P0 (Torr) A
0 2.00 x 10-4 0.16 1.27 x lû-3 0.99 6.34 x 10-3 5.0 2.6 x lo-2 20 9.10 x 10-2 71 100 2.77 x 10~1 220 - -~256~54 When the relationship between the mercury vapor pressure PO and the coefficient A in Table 1 was represented in a graph, the relationship was confirmed to be linear as shown in Fig. 21. As a result, we have:
A Po ... (11) Relation (11) is equivalent to A = BPo ... (12) and relation (10) can be rewritten as:
T = B-(S/Q)-(Po/Pg) (l/IL)-t2 ... (13) Since A = 780Po in the characteristics shown in Fig. 21, substitution of B = 780 in relation (13) yields:
T = 780(5/Q)-(Po/Pg)-(l/IL)-t2 ............ (14) Since T is time required to develop the cataholesis phenomenon, the lamp can be turned on within the time T
after the turn OFF period t2 in order not to cause the cataholesis phenomenon. That is, when tl < T, the cataholesis phenomenon is not caused. Therefore, when the following relation:
,20 tl < 780(S/Q) (Po/Pg)-(l/IL)~ ~ ............... (15) i.e., tl/t2 < 78û(S/)-(Po/Pg)-(l/IL) _ ...(16) is established, the cataholesis phenomenon is not caused.
An experiment was performed under the above conditions. A general fluorescent lamp is used which has conditions of a filling gas pressure of 1 Torr to 12.~6:~5~
142Torr and a lamp current of 60 mA to 1,000 mA. The fluorescent lamp has an inner diameter of 38 mm and a tube length of 7 to 100 cm. It was confirmed that no cataholesis phenomenon occurred.
In the lamp used in Experiment 5 and having the coldest point at 40C, tl/t2 < 0.28. Therefore, if the turn ûFF period t2 exceeds 18.75 hours, and the lamp is turned on for a time interval not exceeding 5.25 hours, no cataholesis phenomenon occurs.
The above embodiments are described with reference to a U-shaped fluorescent lamp. However, the present invention is similarly applicable to a straight fluorescent lamp, an ultraviolet lamp or the other low pressure mercury discharge lamp. The location of the amalgam is not limitted to within the exhaust tube. The amalgam may be arranged on a wall of the stem tube or at the other portion at the anode side.
Therefore, the mercury vapor pressure can be kept at a level of 6 x 10-3 Torr along the entire tube. However, in a straight fluorescent lamp for DC operation subject to the cataholesis phenomenon, since mercury vapor is attracted toward the cathode side, the mercury vapor pressure at the anode side is considerably decreased, and the mercury vapor pressure at the cathode side is slightly increased. As a result, the mercury vapor pressure distribution within the tube is as indicated by characteristic curve ~ in Fig. 11.
With the mercury vapor pressure as indicated by characteristic curve ~, the maximum output portion is shifted toward the cathode side (point P), resulting in a large decrease in light output at the anode side.
In order to prevent the cataholesis phenomenon, an amalgam-forming metal can be arranged at the anode side in the light emitting tube so as to adsorb mercury in the tube in the lamp OFF time and to emit mercury in the lamp ON time, as in the first to fifth embodiments.
~; However, the amount of mercury which can be liberated from an amalgam is limited. When the lamp is turned on over an extended period of time, no more mercury can be liberated from the amalgam, and the cataholesis phenomenon later occurs. The sixth ::~;
~ ~ `f~' ~2~i6~54 embodiment is to resolve this problem.
A discharge lamp 310 shown in Fig. 12 is a dis-charge lamp as a display element arranged in a screen board as shown in Fig. 7. The inner surface of a cylindrical reflecting cylinder 312 has a light reflecting surface 316 coated with a visible light reflecting substance 314 such as an alumina deposition film. A base member 318 is connected to one open end of the reflecting cylinder 312. A U-shaped fluorescent lamp 320 as a light source of the display element is arranged inside the reflecting cylinder 312. A
discharge tube 322 is obtained by bending a straight soft glass tube having an inner diameter of 10 mm, a length of 100 mm and a wall thickness of 1.0 mm into a substantially U shape at the central portion between two ends 324a and 324b. The tube 322 has a substantially U-shaped portion 326 between the two ends 324a and 324b.
An anode 328 and a cathode 330 are sealed to the ends 324a and 324b of the tube 322. The tube 322 is filled with predetermined amounts of mercury and an inert gas.
A phosphor 332 for emitting one color light of red, green or blue is coated on the inner surface of the discharge tube 322. The ends 324a and 324b of the lamp 320 are supported by the base member 318. The anode 328 and the cathode 330 are connected to four base pins 334a, 334b and so on (remaining two pins not shown) of the base member 318. The portion 326 is located inside ' :
` ` 125~154 the other open end of the reflecting cylinder 312. A
lens 336 for projecting light from the fluorescent lamp 320 is fitted in the other open end of the reflecting cylinder 312.
An amaigam 338 is put an exhaust tube 334a which is at the anode 328 side. The amalgam 338 is put in the exhaust tube 334a in the form of an alloy obtained by adding mercury (Hg) to at least one of bismuth (Bi), indium (In) and tin (Sn).
The base member 318 covers the ends 324a and 324b of the discharge tube 322. At least one air hole (two holes 340a and 340b in this case) is formed in the portion around the end 324a of the base member 318. The air holes 340a and 340b communicate the interior and exterior of the reflecting cylinder 312. An opening 342 ; is formed in the reflecting cylinder 312 at a position near the linear portion at the anode side. An air flow along a direction indicated by arrow D or a direction opposite thereto is generated by the air holes 340a and 340b and the opening 342. Thus, the anode side end 324a is cooled with air, and the anode side end 324a is effectively cooled. ~_¦
~;~ In contrast, no air hole or opening is formed inthe cathode side end 324b. Rather, the cathode side end 324b is covered with the base member 318 and is insulated thereby with respect to heat. A heat insu-:;~
l~; lating film can be formed on the cathode side end 324b :
~:256~54 so as to increase the temperature at this end.
The discharge lamp 310 described above is operated by an operating circuit as shown in Fig. 2 or 4. In the ON state of the discharge lamp 310, mercury vaporized in the tube 322 is turned into positive ions which are gradually attracted toward the cathode 330 side which is at a negative potential. For this reason, the mercury vapor pressure at the anode 328 side begins to decrease. However, since the amalgam 338 is arraged in the exhaust tube 344a, the amalgam 338 liberates mercury in an amount corresponding to the decrease amount, thus replenishing the required amount of mercury. As a result, the cataholesis phenomen.on in an early ON period of the lamp is prevented, and the time required to develop the cataholesis phenomenon is delayed.
In this embodiment, since the cathode side end 324b is covered with the base member 318 or a heat insulating film, the temperature at the coldest point at the cathode side is increased above that in a conventional case. Thus, the mercury vapor pressure at the cathode side end 324b is higher than that in the case wherein the end 324b is not insulated with respect to heat. It is difficult for the mercury vapor at the anode side end 324a to move to the cathode side end 324b which is at a higher pressure. Mercury at the cathode side is return-ed to the anode side end 324a due to Brown Motion. The ~;25~;154 generation of the cataholesis phenomenon is suppressed more effectively.
The mercury vapor pressure distribution in the lamp of this embodiment can be represented by characteristic curve y in Fig. 11. The point corresponding to the pressure of 6 x lû 3 Torr at which a maximum luminous efficacy is obtained is at the central position of the tube. With this arrangement, a decrease in the luminous flux at the anode side can be decreased, and a light output decrease of the overall lamp can be reduced to a minimum. In the U-shaped discharge tube 322 shown in Fig. 12, a position of maximum light output corresponds to the U-shaped bent portion 326. In the case of the light source 310 as the display element, light from the lamp 320 is projected toward the front through the lens 336. Therefore, it is most preferable that a position of maximum light output correspond to the U-shaped bent portion 326 which faces the lens 336. With this arrangement, the light output of the overall discharge lamp 310 is improved.
As can be seen from the characteristic curve r in Fig. 11, in order to obtain a mercury vapor pressure of 6 x 10 3 Torr at the central portion of the tube, the mercury vapor pressure at the coldest point at the cathode side must be doubled, i.e., 2 x 6 x 10 3 Torr.
The present inventors experimentally confirmed the temperature of the coldest point at the cathode side to 25~5d, obtain the mercury vapor pressure of 6 x 10-3 Torr at the central portion of the tube.
The results obtained are shown in Fig. 13. It is seen from these results that in order to obtain a maximum light output at the central portion of the discharge tube, the temperature of the coldest point must be set at about 6ûC~ However, in order to obtain a light output of 90 % or more in consideration of lamp manufacturing allowance and pressure distribution allowance, the coldest point temperature at the cathode side may be kept within the range of 50 to 80C.
The application of a fluorescent lamp is not limited to a light source of the display element 310 but can be extended to a U-shaped lamp for normal illumination or a double U-shaped fluorescent lamp obtained by bending the U-shaped lamp into a U shape having a smaller radius of curvature. This embodiment is also applicable to a straight fluorescent lamp. The amalgam may be arranged on a wall of the stem tube or at the other position at the anode side.
A method of operating a low pressure mercury discharge lamp for DC operation according to a first embodiment of the present invention will be described with reference to Fig. 14.
A phosphor film 412 is formed on the inner surface of a straight discharge tube 410 of soft glass. An anode 414 and a cathode 416 are arranged at the two ends ~256~54 of the tube 410. The tube 410 is filled with predeter-mined amounts of mercury and a starting rare gas. An amalgam 418 is put in an exhaust tube 428a at the anode 414 side of the tube 410. The amalgam 418 consists of an alloy obtained by adding mercury (Hg) to at least one of bismuth (Bi), indium (In), and tin (Sn).
The discharge tube 410 is mounted in a luminaire (not shown) such that the anode 414 is below the cathode 416. The tube 410 is used in a vertical orientation with the anode 414 is located below the cathode 416.
One end of the anode 414 is series-connected to one end of the cathode 416 through a glow switch starter 420. The other end of the anode 414 is connected to the other end of the cathode 416 through a rectifying bridge circuit 422. The rectifying bridge circuit 422 is connected to an AC power supply 426 through a choke coil ballast 424. An output voltage from the AC power ~; supply 426 is converted into a DC voltage by the circuit 422. The DC voltage is applied to the anode 414 and the cathode 416. Thus, the tube 410 is DC-operated.
In this ON state, mercury vaporized in the tube 410 is turned into positive ions which are gradually attracted toward the cathode side 416. Then, the mercury vapor pressure at the anode 414 side begins to decrease. However, since the amalgam 418 is arranged in the exhaust tube 428a at the anode 414 side, it liberates mercury in the amount corresponding to the ~256~54 decrease amount at the anode 414 side. The cataholesis phenomenon will not occur during an early ON period of the lamp.
In addition to the above-mentioned function of the amalgam 418, mercury in the tube 410 and mercury liberates from the amalgam 418 is gradually attracted downward by gravity and tends to collect at the lower end of the tube 410, since the tube is arranged such that the anode 414 is located below the cathode 416.
The gravity acting on mercury ions balances with the attractive force acting on mercury ions toward the cathode 416 side, thereby balancing the mercury vapor pressure inside the tube 410.
In this manner, the generation of the cataholesis phenomenon in an early ON period of the lamp is delayed ; by a combination of the mercury liberating function from the amalgam 418 and the gravity action of mercury.
In the OFF period of the lamp, extra mercury vapor in the tube 410 is attracted toward the amalgam 418.
Mercury in the tube collects near the amalgam 418 by gravity to stimulate adsorption of mercury by the amalgam 418. Mercury which is not adsorbed by the amalgam 418 collects near the anode 414 located below.
As a result, when the lamp is turned on again, it takes a long period of time for mercury to collect at the cathode 416 side, thereby suppressing the generation of the cataholesis phenomenon.
.~ ~
:
2S~54 When a DC operation of the lamp of the above embodiment was performed, the following results were obtained. With a straight fluorescent lamp having an inner diameter of 15 mm, a distance between the anode and cathode of 300 mm and a lamp current of 0.215 A and using no amalgam, when the lamp was oriented vertically with the anode 414 located above, the total luminous ; flux after a 10 hour operation was 410 ~m. On the other hand, when the identical lamp was operated in a vertical orientation with the anode 414 located below, the total luminous flux after a 10 hour operation was 820 lm.
When an amalgam 418 was put in an identical lamp and the anode 414 was located above, the total luminous flux after a 10 hour operation was 650 Qm. However, when the amalgam 418 was used and the anode 414 was located above, the total luminous flux after a 10 hour operation was 860 ~m.
~; The tube 410 need not be operated in a vertical orientation. The tube 410 can be operated in an inclined orientation as long as the anode 414 is located below the cathode 416. The amalgam 418 may be arranged on a wall of the stem tube or at the other position at the anode side. Thus, in the case of the U-shaped ; fluorescent lamp shown in Fig. 15, a similar effect can be obtained if the anode 414 is located below the cathode 416.
~:
A method of operating a low pressure mercury discharge lamp according to a second embodiment of the present invention will be described with reference to Figs. 16 to 21.
Fig. 16 shows the construction of a U-shaped fluorescent lamp and an operating circuit therefor. A
phosphor film 512 is coated on the inner surface of a discharge tube 510 obtained by bending a soft glass tube into a U shape. An anode 514 and a cathode 516 are mounted at the two ends of the tube 510. The tube 510 is filled with predetermined amounts of mercury and a starting rare gas. An amalgam 518 is put in an exhaust tube 528a at the anode 514 side of the tube 510. The amalgam 518 consists of an alloy obtained by adding i~ mercury (Hg) to one of bismuth (Bi), indium (In) and tin (Sn).
One end of the anode 514 is series-connected to one end of the cathode 516 through a glow switch starter 520. The other end of the anode 514 is connected to the other end of the cathode 516 through a rectifying bridge circuit 522. The circuit 522 is connected to an AC
power supply 526 through a choke coil ballast 524.
~; Therefore, an output voltage from the AC power supply 526 is converted into a DC voltage by the circuit 522.
The DC voltage is applied between the anode 514 and the cathode 516 to DC operate the tube 510.
In the ON state, mercury vaporized in the tube ~ 510 is turned into positive ions which are gradually `:::
~2~ 5~
attracted toward the cathode 516 side. Therefore, themercury vapor pressure at the anode 514 side begins to decrease. However, since the amalgam 518 is arranged in the exhaust tube 528a at the anode 514 side, the amalgam 518 liberates mercury in an amount corresponding to the decrease amount, so that apparently the mercury vapor pressure distribution within the tube is maintained to be uniform. As a result, the cataholesis phenomenon in an early ON period of the lamp is prevented.
Now, the present inventors found that the time required to develop the cataholesis phenomenon differed in accordance with turn OFF period t2 even if the same lamp was used in the various experiments. Based on this, the present inventors surmised that a better ; 15 effect in suppressing the cataholesis phenomenon could be obtained if the turn OFF period t2 were controlled properly.
; Based on this assumption, the present inventors ;~ performed various experiments, and found that if the following relation is satisfied:
tl/t2 < 780(S/l) (Po/Pg)-(l/IL) ... (1) where tl: turn ON period t2: turn OFF period S: discharge tube cross-sectional area (cm2) l: distance between the anode and the cathode telectrode distance) ~25615~
Po: mercury vapor pressure (Torr) near the cathode during lamp turn ON period Pg: filling gas pressure (Torr) IL: lamp current (A) the cataholesis phenomenon can be further suppressed.
The experiments performed will be described below.
Experiment 1 The present inventors knew prior to the experiments that when the turn OFF period t2 is long, the time T
required to develop the cataholesis phenomenon is different even if an identical lamp is used. In order to confirm this experimentally, the present inventors performed the following experiment.
A fluorescent lamp used had a constructian as ~-~ 15 shown in Fig. 16 and an inner diameter of 15 mm (cross-;~ sectional area S = 1.77cm2?, a distance between the anode and cathode (I) of 29 cm, a lamp current (IL) of 0.215 A, and a filling gas pressure tPg) of 5 Torr. The relationship between the turn OFF period t2 and the time I required to develop the cataholesis phenomenon was examined. The obtained characteristic data is as shown ~:
in Fig. 17. The characteristics shown in Fig. 17 are average values obtained with three lamps of identical ~ specifications (the same applies to Figs. 18 to 21). In - ~ 25 these lamps, the coldest point at the cathode 516 side was located at the tube end and was 40C.
The characteristics shown in Fig. 17 satisfy:
~ ~ .
~256~5~
T - . O. 28t2 ..(2) Therefore, the time ~ to develop the cataholesis phenom-enon upon lamp ON operation is proportional to the turn OFF period t2 before an ignition of the lamp. That is, T ~ t2 - (3) This is attributable to the fact that the longer the turn OFF period t2, the larger the amount of mercury adsorbed in the amalgam 518 at the anode 514 side and the more difficult for mercury to move toward the lû cathode 516 side.
Experiment 2 In the course of investigation, the present inventors realized that the larger the lamp current IL, the shorter the time T required to develop the cataholesis phenomenon. Ir, view of this, the relationship between the lamp current IL and the cataholesis phenomenon was examined in detail.
In this experiment, a discharge tube used had an inner diameter of 15 mm (cross-sectional area S =
1.77 cm ), and a distance between the anode and cathode (~) of 29 cm. The coldest point at the cathode side was forcibly cooled at 40C. The filling gas pressure was kept at 5 (Torr), and the turn OFF period t2 was 2 hours. Under these conditions, the relationship between the lamp current IL and the time T was examined.
The obtained results are shown in Fig. 18. From the results shown in Fig. 18, the relation:
--` 1256~54 T (l/IL) ...(4) is satisfied.
The above phenomenon is considered to be caused by the following. The attraction force of the mercury toward the cathode 516 side is influenced by the lamp current IL. That is, when the lamp current is large, the number of mercury ions is increased, and the amount of mercury ions attracted to the cathode 516 side is increased. As a result, the mercury vapor pressure lû distribution within the tube has a gradient, resulting in the cataholesis phenomenon.
Experiment 3 Meanwhile, the gradient of the mercury vapor pressure due to electric potential gradient should be cancelled in response to turning off the lamp.
Cancellation of the pressure gradient is attributed to diffusion of mercury atoms. One of the factors influ-encing the time required for mercury vapor attracted to the cathode 516 side to be recovered to the anode 514 side by diffusion in the lamp OFF period is presumed to be the distance ~ between the anode and cathode. In other words, the longer the distance ~, the longer the time required to recover the mercury.
A discharge tube used had an inner diameter of 15 mm (cross-sectional area S = 1.77 cm2). Forcible cooling was performed to keep the coldest point at the cathode side at 40C. The filling gas pressure (Pg) was .
5 Torr, the lamp current (IL) was 0.215 A, and the turn OFF period t2 was 2 hours. Under these conditions, the relationship between the distance Q and the time T
required to develop the cataholesis phenomenon was examined.
The obtained results are shown in Fig. 19. It is seen from Fig. 19 that, the relation:
T ~ (1/Q) - (5) is established.
Experiment 4 A factor preventing recovery of the mercury vapor attracted to the cathode 516 side to the anode 514 side by diffusion during the lamp OFF period is the cross-sectional area of the tube 510, i.e., the cross-sectional area of the light emitting space. The smaller the cross-sectional area, the longer the time required for recovering the mercury due to diffusion resistance.
A discharge tube used had a distance (Q) of 29 cm.
The coldest point at the cathode 516 side was kept at 40C, the filling gas pressure (Pg) was kept at 5 Torr, the lamp current ~IL) was set at 0.215 A, and the lamp turn OFF period (t2) was 2 hours. Under these condi-tions, the relationship between the cross-sectional area S of the tube and the time T was examined.
The results obtained are shown in Fig. 20. It is seen from the results shown in Fig. 20, that the following relation:
::
~256~S4 T S (6) is established.
From experiments 1 to 4, the following relation is established:
T (S/Q)-(l/IL)-t2 ....................... (7) It is known that when the filling pressure of a starting rare gas is high, the amount of mercury evaporation is regulated and movement of the mercury vapor is restricted. When the relationship between the time T and the filling gas pressure Pg was examined, the following relation was obtained:
T (l/Pg) ...(8) Substitution of relation (8) in relation (7) yields:
T (S/Q) (l/Pg) (l/IL) t2 ... (9) Relation (9) can be rewritten as:
T = A (S/Q)-(l/Pg)-(l/IL) t2 ... (10) where A is a coefficient.
Experiment 5 The mercury vapor pressure in the discharge tube 510 varies in accordance with the coldest point temperature. When the mercury vapor pressure at the cathode side changes, the amount of mercury shifted to the cathode 516 side also changes, resulting in a change in the time T.
In view of this, the present inventors examined the relationship between the cataholesis phenomenon and the mercury vapor pressure at the cathode 516 side affected ~2S6~5~
by the coldest point temperature at the same side.
A lamp used had an inner diameter of 15 mm (cross-sectional area S = 1.77 cm2), a distance (~) of 29 cm, a filling gas pressure (Pg) of 5 Torr, and a lamp current (IL) of 0.215 A. When these conditions are substituted in relation (10) and t2 = T/O. 28 obtained from T -, 0.28t2 of relation (2) is substituted in relation (10), A '_ 5 is obtained. The tube end defining the coldest point at the cathode 516 side was at 40C
and the mercury vapor pressure Po at this point was 6.34 x 10 3 Torr.
A similar experiment was performed by variously changing the temperature of the coldest point. The mercury vapor pressure Po and the coefficient A corre-sponding to each different temperature was examined.
The results as shown in Tabie 1 below were obtained:
Table 1 point Mercury vapor Coefficient tempera- P0 (Torr) A
0 2.00 x 10-4 0.16 1.27 x lû-3 0.99 6.34 x 10-3 5.0 2.6 x lo-2 20 9.10 x 10-2 71 100 2.77 x 10~1 220 - -~256~54 When the relationship between the mercury vapor pressure PO and the coefficient A in Table 1 was represented in a graph, the relationship was confirmed to be linear as shown in Fig. 21. As a result, we have:
A Po ... (11) Relation (11) is equivalent to A = BPo ... (12) and relation (10) can be rewritten as:
T = B-(S/Q)-(Po/Pg) (l/IL)-t2 ... (13) Since A = 780Po in the characteristics shown in Fig. 21, substitution of B = 780 in relation (13) yields:
T = 780(5/Q)-(Po/Pg)-(l/IL)-t2 ............ (14) Since T is time required to develop the cataholesis phenomenon, the lamp can be turned on within the time T
after the turn OFF period t2 in order not to cause the cataholesis phenomenon. That is, when tl < T, the cataholesis phenomenon is not caused. Therefore, when the following relation:
,20 tl < 780(S/Q) (Po/Pg)-(l/IL)~ ~ ............... (15) i.e., tl/t2 < 78û(S/)-(Po/Pg)-(l/IL) _ ...(16) is established, the cataholesis phenomenon is not caused.
An experiment was performed under the above conditions. A general fluorescent lamp is used which has conditions of a filling gas pressure of 1 Torr to 12.~6:~5~
142Torr and a lamp current of 60 mA to 1,000 mA. The fluorescent lamp has an inner diameter of 38 mm and a tube length of 7 to 100 cm. It was confirmed that no cataholesis phenomenon occurred.
In the lamp used in Experiment 5 and having the coldest point at 40C, tl/t2 < 0.28. Therefore, if the turn ûFF period t2 exceeds 18.75 hours, and the lamp is turned on for a time interval not exceeding 5.25 hours, no cataholesis phenomenon occurs.
The above embodiments are described with reference to a U-shaped fluorescent lamp. However, the present invention is similarly applicable to a straight fluorescent lamp, an ultraviolet lamp or the other low pressure mercury discharge lamp. The location of the amalgam is not limitted to within the exhaust tube. The amalgam may be arranged on a wall of the stem tube or at the other portion at the anode side.
Claims (13)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A low pressure mercury discharge lamp for DC
operation comprising:
a discharge tube filled with an ionizable medium containing a predetermined amount of mercury;
a cathode, arranged at one end of said discharge tube, for emitting electrons;
an anode, arranged at the other end of said discharge tube, for collecting the electrons; and an amalgam, arranged at the anode end of said discharge tube, for adsorbing mercury in said discharge tube when said lamp is off and for liberating mercury into said discharge tube throughout the operation of the discharge lamp while said lamp is on.
operation comprising:
a discharge tube filled with an ionizable medium containing a predetermined amount of mercury;
a cathode, arranged at one end of said discharge tube, for emitting electrons;
an anode, arranged at the other end of said discharge tube, for collecting the electrons; and an amalgam, arranged at the anode end of said discharge tube, for adsorbing mercury in said discharge tube when said lamp is off and for liberating mercury into said discharge tube throughout the operation of the discharge lamp while said lamp is on.
2. The discharge lamp according to claim 1, further comprising an exhaust tube provided at the anode end of the discharge tube, said amalgam being arranged in said exhaust tube.
3. The discharge lamp according to claim 1, further comprising:
stem tubes for sealing both ends of said discharge tube, said amalgam being arranged in contact with said stem tube at the anode end.
stem tubes for sealing both ends of said discharge tube, said amalgam being arranged in contact with said stem tube at the anode end.
4. The discharge lamp according to claim 1, wherein said discharge tube is a curved tube having a central portion bent in a U shape.
5. The discharge lamp according to claim 1, wherein a phosphor is coated on an inner surface of said discharge tube.
6. The discharge lamp according to claim 1, further comprising maintaining means for maintaining a coldest point, located at the cathode end with respect to a central portion of said discharge tube, at not lower than 50°C.
7. The discharge lamp according to claim 6, wherein said maintaining means maintains the coldest point at not higher than 80°C.
8. The discharge lamp according to claim 6, wherein said discharge tube is a curved tube having a central portion bent in a U shape.
9. The discharge lamp according to claim 1, further comprising another amalgam, arranged at the cathode end of said discharge tube, for adsorbing extra mercury to present an excessive mercury vapor pressure at the cathode.
10. The discharge lamp according to claim 1, further comprising:
projector means, arranged at the anode end of said discharge tube, for externally guiding light emitted from said discharge tube; and means, arranged at the cathode end of said discharge tube, for supplying a DC voltage to said cathode and said anode.
projector means, arranged at the anode end of said discharge tube, for externally guiding light emitted from said discharge tube; and means, arranged at the cathode end of said discharge tube, for supplying a DC voltage to said cathode and said anode.
11. The discharge lamp according to claim 10, wherein a phosphor is coated on an inner circumferential surface of at least the anode end of said discharge tube.
12. An operating method of a low pressure mercury discharge lamp for DC operation, comprising the steps of:
(1) preparing a low pressure mercury discharge lamp, said mercury discharge lamp including:
a discharge tube sealing therein an ionizable medium containing a predetermined amount of mercury;
a cathode, arranged at one end of said discharge tube, for emitting electrons;
an anode, arranged at the other end of said discharge tube, for collecting the electrons; and an amalgam, arranged at the anode end of said discharge tube, for adsorbing mercury in said discharge tube when said lamp is turned off and for liberating mercury into said discharge tube when said lamp is turned on; and (2) operating said discharge lamp with said anode end of said lamp located below said cathode end thereof.
(1) preparing a low pressure mercury discharge lamp, said mercury discharge lamp including:
a discharge tube sealing therein an ionizable medium containing a predetermined amount of mercury;
a cathode, arranged at one end of said discharge tube, for emitting electrons;
an anode, arranged at the other end of said discharge tube, for collecting the electrons; and an amalgam, arranged at the anode end of said discharge tube, for adsorbing mercury in said discharge tube when said lamp is turned off and for liberating mercury into said discharge tube when said lamp is turned on; and (2) operating said discharge lamp with said anode end of said lamp located below said cathode end thereof.
13. A method of operating a low pressure mercury discharge lamp using direct current, comprising the steps of:
(1) preparing a low pressure mercury discharge lamp, said mercury discharge lamp including:
a discharge tube sealing therein an ionizable medium containing a predetermined amount of mercury;
a cathode, arranged at one end of said discharge tube, for emitting electrons;
an anode, arranged at the other end of said discharge tube, for collecting the electrons; and an amalgam, arranged at the anode end of said discharge tube, for adsorbing mercury in said discharge tube when said lamp is turned off and for liberating mercury into said discharge tube when said lamp is turned on; and (2) repeatedly turning said lamp successively on and off, when a turned on period of said discharge lamp is represented by t1 and a turned off period thereof is represented by t2, such that:
t1/t2 ? 780(S/?l)?(Po/Pg)? (1/IL) where S is the cross-sectional area in square centimetres of said discharge tube;
Q is the distance between the anode and cathode in centimetres;
Po is the mercury vapor pressure in Torr adjacent the cathode during the turned on periods of the lamp;
Pg is the filling gas pressure in Torr; and IL is the lamp current in Amperes.
(1) preparing a low pressure mercury discharge lamp, said mercury discharge lamp including:
a discharge tube sealing therein an ionizable medium containing a predetermined amount of mercury;
a cathode, arranged at one end of said discharge tube, for emitting electrons;
an anode, arranged at the other end of said discharge tube, for collecting the electrons; and an amalgam, arranged at the anode end of said discharge tube, for adsorbing mercury in said discharge tube when said lamp is turned off and for liberating mercury into said discharge tube when said lamp is turned on; and (2) repeatedly turning said lamp successively on and off, when a turned on period of said discharge lamp is represented by t1 and a turned off period thereof is represented by t2, such that:
t1/t2 ? 780(S/?l)?(Po/Pg)? (1/IL) where S is the cross-sectional area in square centimetres of said discharge tube;
Q is the distance between the anode and cathode in centimetres;
Po is the mercury vapor pressure in Torr adjacent the cathode during the turned on periods of the lamp;
Pg is the filling gas pressure in Torr; and IL is the lamp current in Amperes.
Applications Claiming Priority (12)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP59168973A JPS6147057A (en) | 1984-08-13 | 1984-08-13 | Low pressure mercury vapor discharge lamp |
| JP168973/84 | 1984-08-13 | ||
| JP142750/84 | 1984-09-20 | ||
| JP14275084U JPS6157465U (en) | 1984-09-20 | 1984-09-20 | |
| JP197230/84 | 1984-09-20 | ||
| JP19723084A JPS6177248A (en) | 1984-09-20 | 1984-09-20 | Electric-discharge lamp for display element |
| JP3051185A JPS61193355A (en) | 1985-02-20 | 1985-02-20 | Low pressure mercury vapor discharge lamp |
| JP3051085A JPS61193357A (en) | 1985-02-20 | 1985-02-20 | Low pressure mercury vapor discharge lamp |
| JP3050985A JPS61193356A (en) | 1985-02-20 | 1985-02-20 | Lighting of low pressure mercury vapor discharge lamp |
| JP30511/85 | 1985-02-20 | ||
| JP30510/85 | 1985-02-20 | ||
| JP30509/85 | 1985-02-20 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1256154A true CA1256154A (en) | 1989-06-20 |
Family
ID=27549536
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000488058A Expired CA1256154A (en) | 1984-08-13 | 1985-08-02 | Low pressure mercury discharge lamp for dc operation and method of operating the lamp |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1256154A (en) |
-
1985
- 1985-08-02 CA CA000488058A patent/CA1256154A/en not_active Expired
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| Date | Code | Title | Description |
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| MKEX | Expiry |