STABILIZED HIGH INTENSITY SODIUM VAPOR LAMP
United States Patent 3851207
A high intensity sodium vapor arc lamp utilizing a slender tubular envelope of alumina ceramic having self-heating electrodes mounted at opposite ends and containing an excess pool of liquid sodium-mercury amalgam at the cold spot in the lower end. The electrode at the lower end includes a resistive heater portion and lamp current flowing therethrough provides supplementary heat to the amalgam pool varying according to lamp current. In conjunction with the inductive ballast characteristic this stabilizes the cold spot temperature and amalgam vapor pressure, and counters the tendency to arc voltage rise with life. It also reduces the sensitivity of the lamp to radiant energy reflected back from the fixture.
US Patent References:
Electric lamp
Mey - July 1940 - 2208998
Electrode for high-pressure mercury-vapor discharge device
Francis - October 1940 - 2217438
Discharge lamp with magnetic stabilization
Schlegel - December 1963 - 3113234
High-pressure mercury vapor halogen lamp having an electrode thermally insulated from lead-in conductor
Jacobs et al. - October 1967 - 3349276
High pressure saturation vapor sodium lamp containing mercury
Schmidt - May 1968 - 3384798
Electric lamp
Mey - July 1940 - 2208998
Electrode for high-pressure mercury-vapor discharge device
Francis - October 1940 - 2217438
Discharge lamp with magnetic stabilization
Schlegel - December 1963 - 3113234
High-pressure mercury vapor halogen lamp having an electrode thermally insulated from lead-in conductor
Jacobs et al. - October 1967 - 3349276
High pressure saturation vapor sodium lamp containing mercury
Schmidt - May 1968 - 3384798
Application Number:
05/277095
Publication Date:
11/26/1974
Filing Date:
08/01/1972
View Patent Images:
Export Citation:
Assignee:
General Electric Company (Schenectady, NY)
Primary Class:
Other Classes:
313/628, 313/550
International Classes:
H01J61/073; H01J61/28; H01J61/06; H01J61/24; H01J17/34; H01J61/06
Field of Search:
313/184,217,213,229 315/49
Primary Examiner:
Demeo, Palmer C.
Attorney, Agent or Firm:
Legree, Ernest Truesdell Henry Neuhauser Frank W. P. L.
Claims:
What I claim as new and desire to secure by Letters Patent of the United States is
1. A high intensity sodium vapor stabilized discharge lamp comprising a slender tubular ceramic envelope, closures sealing the ends of said envelope, electrodes in opposite ends of said envelope, a filling of sodium, mercury, and inert gas within said envelope, said lamp containing in operation an excess pool of sodium-mercury amalgam at the lower end, the lower electrode comprising an electron-emissive portion and a resistive heater portion interposed between the emissive portion and the lower closure and through which lamp current flows to reach the emissive portion, said resistive heater portion being supported by the lower closure and supporting the emissive portion.
2. A lamp as in claim 1 wherein the emissive portion of the lower electrode is tungsten activated with electron-emissive material, and the resistive heater portion is tungsten wire.
3. A lamp as in claim 1 wherein the emissive portion of the lower electrode comprises close wound turns of tungsten wire activated with electron-emissive material, and the resistive heater portion comprises spaced turns of tungsten wire.
4. A lamp as in claim 1 wherein the emissive portion of the lower electrode comprises axially wound close turns of tungsten wire activated with electron-emissive metal oxides coated thereon and lodged in the interstices between turns, and the resistive heater portion comprises axially wound spaced turns of tungsten wire.
5. A high intensity sodium vapor stabilized discharge lamp comprising a slender tubular ceramic envelope, closures sealing the ends of said envelope, at least one of said closures including a metal exhaust tube sealed at the outer end and projecting inwardly into said envelope, electrodes supported from said closures at opposite ends of said envelope, a filling of sodium, mercury, and inert gas within said envelope, the closure at the lower end of said envelope containing in operation an excess pool of sodium mercury amalgam, the lower electrode comprising an electron-emissive portion and a resistive heater portion interposed between the emissive portion and the lower closure, said resistive heater portion supporting the emissive portion and being attached to and supported by the inwardly projecting metal tube of the lower closure so that lamp current must flow through the resistive portion to reach the emissive portion.
6. A lamp as in claim 5 wherein the emissive portion of the lower electrode comprises close wound turns of tungsten wire activated with electron-emissive material, and the resistive heater portion comprises spaced turns of tungsten wire.
7. A lamp as in claim 5 wherein the emissive portion of the lower electrode comprises axially wound close turns of tungsten wire activated with electron-emissive metal oxides coated thereon and lodged in the interstices between turns, and the resistive heater portion comprises axially wound spaced turns of tungsten wire.
8. A lamp as in claim 6 wherein the turns of the emissive portion are wound around a tungsten stud.
9. A lamp as in claim 7 wherein the turns of the emissive portion are wound around a tungsten stud.
1. A high intensity sodium vapor stabilized discharge lamp comprising a slender tubular ceramic envelope, closures sealing the ends of said envelope, electrodes in opposite ends of said envelope, a filling of sodium, mercury, and inert gas within said envelope, said lamp containing in operation an excess pool of sodium-mercury amalgam at the lower end, the lower electrode comprising an electron-emissive portion and a resistive heater portion interposed between the emissive portion and the lower closure and through which lamp current flows to reach the emissive portion, said resistive heater portion being supported by the lower closure and supporting the emissive portion.
2. A lamp as in claim 1 wherein the emissive portion of the lower electrode is tungsten activated with electron-emissive material, and the resistive heater portion is tungsten wire.
3. A lamp as in claim 1 wherein the emissive portion of the lower electrode comprises close wound turns of tungsten wire activated with electron-emissive material, and the resistive heater portion comprises spaced turns of tungsten wire.
4. A lamp as in claim 1 wherein the emissive portion of the lower electrode comprises axially wound close turns of tungsten wire activated with electron-emissive metal oxides coated thereon and lodged in the interstices between turns, and the resistive heater portion comprises axially wound spaced turns of tungsten wire.
5. A high intensity sodium vapor stabilized discharge lamp comprising a slender tubular ceramic envelope, closures sealing the ends of said envelope, at least one of said closures including a metal exhaust tube sealed at the outer end and projecting inwardly into said envelope, electrodes supported from said closures at opposite ends of said envelope, a filling of sodium, mercury, and inert gas within said envelope, the closure at the lower end of said envelope containing in operation an excess pool of sodium mercury amalgam, the lower electrode comprising an electron-emissive portion and a resistive heater portion interposed between the emissive portion and the lower closure, said resistive heater portion supporting the emissive portion and being attached to and supported by the inwardly projecting metal tube of the lower closure so that lamp current must flow through the resistive portion to reach the emissive portion.
6. A lamp as in claim 5 wherein the emissive portion of the lower electrode comprises close wound turns of tungsten wire activated with electron-emissive material, and the resistive heater portion comprises spaced turns of tungsten wire.
7. A lamp as in claim 5 wherein the emissive portion of the lower electrode comprises axially wound close turns of tungsten wire activated with electron-emissive metal oxides coated thereon and lodged in the interstices between turns, and the resistive heater portion comprises axially wound spaced turns of tungsten wire.
8. A lamp as in claim 6 wherein the turns of the emissive portion are wound around a tungsten stud.
9. A lamp as in claim 7 wherein the turns of the emissive portion are wound around a tungsten stud.
Description:
BACKGROUND OF THE INVENTION
The invention relates to high intensity metal vapor arc lamps utilizing a slender tubular ceramic envelope, and more particularly to high intensity sodium vapor lamps of the kind described in U.S. Pat. No. 3,248,590 to Schmidt, entitled "High Pressure Sodium Vapor Lamp."
Such lamps utilize an elongated inner envelope of ceramic material resistant to the attack of sodium at high temperatures, such as a high density polycrystalline alumina tube, for containing the arc discharge. The filling comprises an amalgam of sodium and mercury and a rare gas, preferably xenon, to facilitate starting. The ends of the alumina tube are sealed by suitable closure members such as caps or plugs which carry the electrodes between which the discharge passes in operation. One construction utilizes a refractory metal closure member comprising an end cap of niobium whose coefficient of expansion is close to that of alumina and which serves as an electrical inlead and supports an electrode internally. The ceramic arc tube is supported within an outer vitreous envelope or jacket having at one end a screw base providing terminals to which the end caps of the arc tube are connected. The jacket is evacuated in order to conserve heat and maintain the arc tube at a sufficiently high temperature.
The high pressure sodium vapor lamp has a relatively high starting voltage due in part to the small diameter of the arc tube and in part to the use of xenon as a starting gas. Also the metal end caps used for the arc tube closures do not lend themselves conveniently to the incorporation of an auxiliary starting electrode. To start and operate the lamp, a ballast circuit is generally used which supplies high voltage pulses near the peak of the open circuit waveform until the lamp ignites. When the lamp starts, the reduction in voltage at the lamp terminals from the open circuit value to the lamp running value disables the pulsing circuit until the next time the lamp is started.
High intensity sodium vapor lamps of the present kind containing an excess of sodium-mercury amalgam typically have an arc voltage drop which increases as the lamp ages. The voltage rise with life which is characteristic of this lamp is due to the conjunction of several factors. As the electrodes deteriorate they must run hotter to sustain the same current. At the same time, deposits of emission material and tungsten from the electrodes on the arc tube walls of relatively small diameter entail rapid darkening in the vicinity of the electrodes. The darkened walls absorb more energy causing the ends of the lamp to run even hotter. The lamp contains an excess of sodium mercury amalgam and the vapor pressure in the arc tube is determined primarily by the temperature at the lower end where the excess amalgam collects as a liquid pool. The rise in vapor pressure is extremely rapid in relation to the rise in temperature of the liquid pool. The black deposits on the walls also tend to hold amalgam by surface tension effect and this further accentuates the rise in vapor pressure with life.
The end result of the foregoing is that high intensity sodium vapor lamps produced for commerce have generally had an operating voltage rising in a measured and predictable fashion with life. In the first lamps manufactured starting about 1966, the voltage rise with life characteristic was excessive, for instance from 100 to 160 volts in about 6000 hours. Various improvements over the intervening years have made possible a large reduction in the voltage rise characteristic. The most important improvement has been in the activating material applied to the electrodes and consisting of dibarium calcium tungstate Ba 2 CaWO 6 described and claimed in copending application Ser. No. 97,907 filed Dec. 14, 1970 by Willian E. Smyser et al., entitled "Discharge Lamp Thermionic Cathode Containing Emission Material," and assigned like this invention. At the present time (1972), the voltage rise characteristic in production lamps is about 20 volts in 15,000 burning hours, from nominally 100 to 120 volts.
As a high intensity sodium vapor lamp ages, its operating voltage climbs and eventually reaches the ballast sustaining voltage; this determines the end of life for that particular lamp. However the life of that lamp could be extended by reducing the voltage rise characteristic and this would also improve the lamp's reliability. A reduced voltage rise characteristic makes the lamp less sensitive to transients such as lightning strokes, less likely to drop out on a line voltage dip. It also reduces variations in light output and color with line voltage.
The high intensity sodium vapor lamp, by reason of the slenderness of its arc tube, is particularly suitable for use in compact fixtures or luminaires designed to provide a precise lighting pattern. However, the sensitivity of the lamp, in respect of the arc voltage drop, to the temperature of the liquid amalgam pool has been a decided drawback in such applications. For instance it has been necessary to avoid passing reflected radiation back through the arc tube and to avoid directing radiation at the lower closure in which the liquid amalgam pool collects.
SUMMARY OF THE INVENTION
The object of the invention is a high intensity sodium vapor lamp which is improved with respect to the voltage rise characteristic.
In accordance with my invention, the voltage rise characteristic of the high intensity sodium vapor lamp is greatly reduced and the tolerance of the lamp to external conditions is improved by incorporating in the electrode at the lower end of the arc tube a resistive heater portion through which lamp current is caused to flow. Lamp current flowing through the heater portion provides supplementary heat to the amalgam pool which, at constant heater resistance, is proportional to the square of the lamp current. The rise in resistance of the heater with temperature due to the positive temperature coefficient of resistance of tungsten, is also in a direction to increase the heating. This stabilizes the cold spot temperature and amalgam vapor pressure and counters the tendency to arc voltage rise with life. It also reduces the sensitivity of the lamp to radiant energy reflected back from the fixture.
DESCRIPTION OF DRAWING
In the drawing:
FIG. 1 illustrates a jacketed high pressure sodium vapor lamp embodying the invention.
FIG. 2 is a sectioned view of the lower end of the arc tube to a larger scale showing the construction of the electrode with integral heater portion.
FIG. 3 is a graph showing the lamp wattage versus lamp voltage characteristic for a typical ballast-lamp combination.
DESCRIPTION OF PREFERRED EMBODIMENT
A high intensity sodium vapor discharge lamp 1 in which the invention is embodied is illustrated in FIG. 1 and comprises an outer vitreous envelope or jacket 2 of elongated ovoid shape. The neck 3 of the jacket is closed by re-entrant stem 4 terminated in a press 5 through which extend stiff inleads or current conductors 6, 7 which are connected to the threaded shell 8 and insulated center contact 9 of a conventional mogul screw base.
The inner envelope or arc tube 11 is made of sintered high density polycrystalline alumina ceramic per U.S. Pat. No. 3,026,210 to Coble, "Transparent Alumina and Method of Preparation," or of other light-transmitting ceramic capable of withstanding the attack of sodium vapor at high temperatures. The ends of the tube are closed by thimble-like niobium metal end caps 12, 13 hermetically sealed to the alumina by means of a sealing composition comprising a major proportion of aluminum oxide and calcium oxide and a minor proportion of magnesium oxide and barium oxide.
Thermionic electrodes are mounted in the ends of the arc tubes and supported from the end caps. Upper electrode 15 is conventional and comprises for instance a double layer tungsten wire coil or helix 16 wound around a tungsten shank or core 17 extending from a niobium tube 18 welded through the end cap. The electrode is activated by metal oxides retained in the interstices between turns of the coil, the preferred material being dibarium calcium tungstate Ba 2 CaWO 6 described and claimed in the previously referred to copending application of William E. Smyser et al. Upper niobium tube 18 has an opening into the arc tube and may contain a small quantity of yttrium metal which serves as a brazing material to assure a hermetic seal where the tungsten shank 17 penetrates the niobium tube. By so doing, the pinch 19 at the upper end of the niobium tube need not be hermetic.
Electrode 20 at the lower end of the arc tube is best seen in FIG. 2. It comprises the electrode portion proper wherein tungsten wire is wound around a tungsten stud 21 in two layers of close wound turns 22 and continued in spread or spaced turns 23. The bottom turn 24 of the spaced turns is seized in the outspread end of the niobium exhaust tube 25 which is folded over the turn to provide a mechanical lock. Since niobium had a greater expansion coefficient than tungsten, the joint tightens when the lamp heats up and also some diffusion bonding takes place. The electrode portion proper consists of the tungsten stud 21 and the two close wound layers of tungsten wire turns which form a compact body. The electrode is activated by metal oxides retained in the interstices between turns in the same fashion as for the upper electrode, preferably utilizing dibarium calcium tungstate Ba 2 CaWO 6 .
Lower niobium tube 25 is pierced through at 26 to allow its use as an exhaust tube during manufacture. After the sodium-mercury amalgam and the gas filling consisting of xenon have been introduced into the arc tube, exhaust tube 25 is hermetically pinched off by a cold weld indicated at 27 and serves thereafter as a reservoir for the excess of condensed sodium-mercury amalgam which forms a liquid pool during operation. The illustrated lamp is intended for baseup operation wherein the longer niobium tube 25 which must be the coolest portion of the arc tube for the excess amalgam to condense therein, is located lowermost. In a base-down version of the same lamp, the arc tube is reversed relative to the outer jacket.
The arc tube is supported within the outer envelope by means of a mount comprising a single side rod 29 which extends the length of the envelope from inlead 6 at the stem end to a dimple 30 at the dome end to which it is anchored by a resilient clamp 31. End cap 13 of the arc tube is connected to the frame by band 32 welded to expansion strap 33 while end cap 12 is connected to inlead 6 through band 34 and connecting rod 35. Prior to sealing off the outer jacket, the interenvelope space is desirably evacuated in order to reduce the heat loss from the arc tube during operation. A getter, for instance barium aluminum alloy powder pressed into channeled rings 36 is flashed after sealing in order to assure a high vacuum.
In accordance with my invention, the heater portion of the lower electrode consisting of the spaced turns 23 serves as a supplementary heat source to the liquid amalgam pool regulating its temperature, and thereby its vapor pressure and the lamp operating voltage. The common sizes of high intensity sodium vapor lamps presently available are 250 watts, 400 watts and 1000 watts. The heater portion of the electrode is designed to have an ohmic dissipation in the range from 0.5 to 10.0 watts depending upon the size of lamp for which it is intended.
In order for the compensating characteristic to function most effectively in preventing arc drop voltage rise, the lamp should preferably be designed to operate somewhat near the peak in the lamp wattage versus lamp voltage curve of the ballast and lamp combinations. A typical curve is shown in FIG. 3 wherein maximum power transfer occurs when the voltage drop across the lamp load is about 60 percent of line voltage, as is conventional with a generator having internal reactance and supplying energy to a substantially resistive load. When the lamp is operated on a typical inductive ballast as represented by the curve, an increase in lamp voltage, regardless of the cause, will result in a decrease in lamp current. Lower lamp current means reduced electrode ohmic dissipation and reduced supplemental heat to the amalgam pool. The latter results in lower amalgam vapor pressure and reduced arc drop voltage thereby tending to stabilize lamp operation. Since the ohmic dissipation of the electrode is proportional to the square of the lamp current at constant electrode resistance, the control mechanism is quite sensitive and effectively stabilizes the amalgam pool temperature and the vapor pressure in the lamp.
By way of example, of the invention, in the illustrated lamp which is a 400 watt size, the arc tube is 7.24 millimeters in internal diameter and 112 millimeters long. The dose consists of about 54 milligrams of sodium mercury amalgam made up of 14 mg. of sodium and 40 mg. of mercury, and results in about 80 torr P Na and 290 torr P Hg for an appendix control temperature of about 675°C. The lamp operates at about 400 watts with an efficacy of about 120 lumens per watt. In lower electrode 20, tungsten stud 21 is 0.057 inch in diameter and 0.240 inch long. The coiling consists of 0.020 inch tungsten 218 wire whereof two layers of 10 turns each are close wound 0.200 inch high for portion 22, and 10 turns are space wound 0.300 inch high at 0.135 inch internal diameter for spread turns 23.
In operation of the foregoing lamp at 400 watts, heater portion 23 of the electrode produced about 4 watts of ohmic dissipation. The regulatory effect of this supplemental heat was demonstrated by a comparison test with a conventional high pressure sodium production lamp of 400 watt size. The production lamp was operated with a simple inductive reactor ballast in an open socket without reflector at 102 volts arc drop. An aluminum foil reflector was then placed about the production lamp to simulate a compact fixture which would reflect radiation on the arc tube and on the lower closure, and the arc drop voltage rose to 180 volts. The test was then repeated with the lamp of this invention which operated in the open socket at an arc drop voltage of 95 volts at 400 watts input. The addition of the same aluminum foil reflector caused an increase in arc drop voltage to only 130 volts at the same wattage.
In another comparison test, a 400 watt production lamp and the present lamp embodying the invention were operated in turn on an unmodified commerically available ballast for 400 watt high pressure sodium lamps (LU-400) having a maximum sustaining voltage of only 140 volts. The test was conducted with and without the aluminum foil reflector on both lamps operated successively. With the foil reflector, the reflected energy raised the voltage of the production lamp beyond the maximum sustaining voltage of the ballast and the lamp extinguished. The stabilized lamp of the invention having ohmic regulation could not be extinguished through the use of the foil reflector on the same ballast. Both lamps were photometered at 400 watts and each produced about 120 lumens per watt. As indicated previously, approximately 4 watts of ohmic dissipation occurred, resulting in a decrease in energy available to the arc of about 1 percent and a reduction in light output of about 1 percent.
The invention relates to high intensity metal vapor arc lamps utilizing a slender tubular ceramic envelope, and more particularly to high intensity sodium vapor lamps of the kind described in U.S. Pat. No. 3,248,590 to Schmidt, entitled "High Pressure Sodium Vapor Lamp."
Such lamps utilize an elongated inner envelope of ceramic material resistant to the attack of sodium at high temperatures, such as a high density polycrystalline alumina tube, for containing the arc discharge. The filling comprises an amalgam of sodium and mercury and a rare gas, preferably xenon, to facilitate starting. The ends of the alumina tube are sealed by suitable closure members such as caps or plugs which carry the electrodes between which the discharge passes in operation. One construction utilizes a refractory metal closure member comprising an end cap of niobium whose coefficient of expansion is close to that of alumina and which serves as an electrical inlead and supports an electrode internally. The ceramic arc tube is supported within an outer vitreous envelope or jacket having at one end a screw base providing terminals to which the end caps of the arc tube are connected. The jacket is evacuated in order to conserve heat and maintain the arc tube at a sufficiently high temperature.
The high pressure sodium vapor lamp has a relatively high starting voltage due in part to the small diameter of the arc tube and in part to the use of xenon as a starting gas. Also the metal end caps used for the arc tube closures do not lend themselves conveniently to the incorporation of an auxiliary starting electrode. To start and operate the lamp, a ballast circuit is generally used which supplies high voltage pulses near the peak of the open circuit waveform until the lamp ignites. When the lamp starts, the reduction in voltage at the lamp terminals from the open circuit value to the lamp running value disables the pulsing circuit until the next time the lamp is started.
High intensity sodium vapor lamps of the present kind containing an excess of sodium-mercury amalgam typically have an arc voltage drop which increases as the lamp ages. The voltage rise with life which is characteristic of this lamp is due to the conjunction of several factors. As the electrodes deteriorate they must run hotter to sustain the same current. At the same time, deposits of emission material and tungsten from the electrodes on the arc tube walls of relatively small diameter entail rapid darkening in the vicinity of the electrodes. The darkened walls absorb more energy causing the ends of the lamp to run even hotter. The lamp contains an excess of sodium mercury amalgam and the vapor pressure in the arc tube is determined primarily by the temperature at the lower end where the excess amalgam collects as a liquid pool. The rise in vapor pressure is extremely rapid in relation to the rise in temperature of the liquid pool. The black deposits on the walls also tend to hold amalgam by surface tension effect and this further accentuates the rise in vapor pressure with life.
The end result of the foregoing is that high intensity sodium vapor lamps produced for commerce have generally had an operating voltage rising in a measured and predictable fashion with life. In the first lamps manufactured starting about 1966, the voltage rise with life characteristic was excessive, for instance from 100 to 160 volts in about 6000 hours. Various improvements over the intervening years have made possible a large reduction in the voltage rise characteristic. The most important improvement has been in the activating material applied to the electrodes and consisting of dibarium calcium tungstate Ba 2 CaWO 6 described and claimed in copending application Ser. No. 97,907 filed Dec. 14, 1970 by Willian E. Smyser et al., entitled "Discharge Lamp Thermionic Cathode Containing Emission Material," and assigned like this invention. At the present time (1972), the voltage rise characteristic in production lamps is about 20 volts in 15,000 burning hours, from nominally 100 to 120 volts.
As a high intensity sodium vapor lamp ages, its operating voltage climbs and eventually reaches the ballast sustaining voltage; this determines the end of life for that particular lamp. However the life of that lamp could be extended by reducing the voltage rise characteristic and this would also improve the lamp's reliability. A reduced voltage rise characteristic makes the lamp less sensitive to transients such as lightning strokes, less likely to drop out on a line voltage dip. It also reduces variations in light output and color with line voltage.
The high intensity sodium vapor lamp, by reason of the slenderness of its arc tube, is particularly suitable for use in compact fixtures or luminaires designed to provide a precise lighting pattern. However, the sensitivity of the lamp, in respect of the arc voltage drop, to the temperature of the liquid amalgam pool has been a decided drawback in such applications. For instance it has been necessary to avoid passing reflected radiation back through the arc tube and to avoid directing radiation at the lower closure in which the liquid amalgam pool collects.
SUMMARY OF THE INVENTION
The object of the invention is a high intensity sodium vapor lamp which is improved with respect to the voltage rise characteristic.
In accordance with my invention, the voltage rise characteristic of the high intensity sodium vapor lamp is greatly reduced and the tolerance of the lamp to external conditions is improved by incorporating in the electrode at the lower end of the arc tube a resistive heater portion through which lamp current is caused to flow. Lamp current flowing through the heater portion provides supplementary heat to the amalgam pool which, at constant heater resistance, is proportional to the square of the lamp current. The rise in resistance of the heater with temperature due to the positive temperature coefficient of resistance of tungsten, is also in a direction to increase the heating. This stabilizes the cold spot temperature and amalgam vapor pressure and counters the tendency to arc voltage rise with life. It also reduces the sensitivity of the lamp to radiant energy reflected back from the fixture.
DESCRIPTION OF DRAWING
In the drawing:
FIG. 1 illustrates a jacketed high pressure sodium vapor lamp embodying the invention.
FIG. 2 is a sectioned view of the lower end of the arc tube to a larger scale showing the construction of the electrode with integral heater portion.
FIG. 3 is a graph showing the lamp wattage versus lamp voltage characteristic for a typical ballast-lamp combination.
DESCRIPTION OF PREFERRED EMBODIMENT
A high intensity sodium vapor discharge lamp 1 in which the invention is embodied is illustrated in FIG. 1 and comprises an outer vitreous envelope or jacket 2 of elongated ovoid shape. The neck 3 of the jacket is closed by re-entrant stem 4 terminated in a press 5 through which extend stiff inleads or current conductors 6, 7 which are connected to the threaded shell 8 and insulated center contact 9 of a conventional mogul screw base.
The inner envelope or arc tube 11 is made of sintered high density polycrystalline alumina ceramic per U.S. Pat. No. 3,026,210 to Coble, "Transparent Alumina and Method of Preparation," or of other light-transmitting ceramic capable of withstanding the attack of sodium vapor at high temperatures. The ends of the tube are closed by thimble-like niobium metal end caps 12, 13 hermetically sealed to the alumina by means of a sealing composition comprising a major proportion of aluminum oxide and calcium oxide and a minor proportion of magnesium oxide and barium oxide.
Thermionic electrodes are mounted in the ends of the arc tubes and supported from the end caps. Upper electrode 15 is conventional and comprises for instance a double layer tungsten wire coil or helix 16 wound around a tungsten shank or core 17 extending from a niobium tube 18 welded through the end cap. The electrode is activated by metal oxides retained in the interstices between turns of the coil, the preferred material being dibarium calcium tungstate Ba 2 CaWO 6 described and claimed in the previously referred to copending application of William E. Smyser et al. Upper niobium tube 18 has an opening into the arc tube and may contain a small quantity of yttrium metal which serves as a brazing material to assure a hermetic seal where the tungsten shank 17 penetrates the niobium tube. By so doing, the pinch 19 at the upper end of the niobium tube need not be hermetic.
Electrode 20 at the lower end of the arc tube is best seen in FIG. 2. It comprises the electrode portion proper wherein tungsten wire is wound around a tungsten stud 21 in two layers of close wound turns 22 and continued in spread or spaced turns 23. The bottom turn 24 of the spaced turns is seized in the outspread end of the niobium exhaust tube 25 which is folded over the turn to provide a mechanical lock. Since niobium had a greater expansion coefficient than tungsten, the joint tightens when the lamp heats up and also some diffusion bonding takes place. The electrode portion proper consists of the tungsten stud 21 and the two close wound layers of tungsten wire turns which form a compact body. The electrode is activated by metal oxides retained in the interstices between turns in the same fashion as for the upper electrode, preferably utilizing dibarium calcium tungstate Ba 2 CaWO 6 .
Lower niobium tube 25 is pierced through at 26 to allow its use as an exhaust tube during manufacture. After the sodium-mercury amalgam and the gas filling consisting of xenon have been introduced into the arc tube, exhaust tube 25 is hermetically pinched off by a cold weld indicated at 27 and serves thereafter as a reservoir for the excess of condensed sodium-mercury amalgam which forms a liquid pool during operation. The illustrated lamp is intended for baseup operation wherein the longer niobium tube 25 which must be the coolest portion of the arc tube for the excess amalgam to condense therein, is located lowermost. In a base-down version of the same lamp, the arc tube is reversed relative to the outer jacket.
The arc tube is supported within the outer envelope by means of a mount comprising a single side rod 29 which extends the length of the envelope from inlead 6 at the stem end to a dimple 30 at the dome end to which it is anchored by a resilient clamp 31. End cap 13 of the arc tube is connected to the frame by band 32 welded to expansion strap 33 while end cap 12 is connected to inlead 6 through band 34 and connecting rod 35. Prior to sealing off the outer jacket, the interenvelope space is desirably evacuated in order to reduce the heat loss from the arc tube during operation. A getter, for instance barium aluminum alloy powder pressed into channeled rings 36 is flashed after sealing in order to assure a high vacuum.
In accordance with my invention, the heater portion of the lower electrode consisting of the spaced turns 23 serves as a supplementary heat source to the liquid amalgam pool regulating its temperature, and thereby its vapor pressure and the lamp operating voltage. The common sizes of high intensity sodium vapor lamps presently available are 250 watts, 400 watts and 1000 watts. The heater portion of the electrode is designed to have an ohmic dissipation in the range from 0.5 to 10.0 watts depending upon the size of lamp for which it is intended.
In order for the compensating characteristic to function most effectively in preventing arc drop voltage rise, the lamp should preferably be designed to operate somewhat near the peak in the lamp wattage versus lamp voltage curve of the ballast and lamp combinations. A typical curve is shown in FIG. 3 wherein maximum power transfer occurs when the voltage drop across the lamp load is about 60 percent of line voltage, as is conventional with a generator having internal reactance and supplying energy to a substantially resistive load. When the lamp is operated on a typical inductive ballast as represented by the curve, an increase in lamp voltage, regardless of the cause, will result in a decrease in lamp current. Lower lamp current means reduced electrode ohmic dissipation and reduced supplemental heat to the amalgam pool. The latter results in lower amalgam vapor pressure and reduced arc drop voltage thereby tending to stabilize lamp operation. Since the ohmic dissipation of the electrode is proportional to the square of the lamp current at constant electrode resistance, the control mechanism is quite sensitive and effectively stabilizes the amalgam pool temperature and the vapor pressure in the lamp.
By way of example, of the invention, in the illustrated lamp which is a 400 watt size, the arc tube is 7.24 millimeters in internal diameter and 112 millimeters long. The dose consists of about 54 milligrams of sodium mercury amalgam made up of 14 mg. of sodium and 40 mg. of mercury, and results in about 80 torr P Na and 290 torr P Hg for an appendix control temperature of about 675°C. The lamp operates at about 400 watts with an efficacy of about 120 lumens per watt. In lower electrode 20, tungsten stud 21 is 0.057 inch in diameter and 0.240 inch long. The coiling consists of 0.020 inch tungsten 218 wire whereof two layers of 10 turns each are close wound 0.200 inch high for portion 22, and 10 turns are space wound 0.300 inch high at 0.135 inch internal diameter for spread turns 23.
In operation of the foregoing lamp at 400 watts, heater portion 23 of the electrode produced about 4 watts of ohmic dissipation. The regulatory effect of this supplemental heat was demonstrated by a comparison test with a conventional high pressure sodium production lamp of 400 watt size. The production lamp was operated with a simple inductive reactor ballast in an open socket without reflector at 102 volts arc drop. An aluminum foil reflector was then placed about the production lamp to simulate a compact fixture which would reflect radiation on the arc tube and on the lower closure, and the arc drop voltage rose to 180 volts. The test was then repeated with the lamp of this invention which operated in the open socket at an arc drop voltage of 95 volts at 400 watts input. The addition of the same aluminum foil reflector caused an increase in arc drop voltage to only 130 volts at the same wattage.
In another comparison test, a 400 watt production lamp and the present lamp embodying the invention were operated in turn on an unmodified commerically available ballast for 400 watt high pressure sodium lamps (LU-400) having a maximum sustaining voltage of only 140 volts. The test was conducted with and without the aluminum foil reflector on both lamps operated successively. With the foil reflector, the reflected energy raised the voltage of the production lamp beyond the maximum sustaining voltage of the ballast and the lamp extinguished. The stabilized lamp of the invention having ohmic regulation could not be extinguished through the use of the foil reflector on the same ballast. Both lamps were photometered at 400 watts and each produced about 120 lumens per watt. As indicated previously, approximately 4 watts of ohmic dissipation occurred, resulting in a decrease in energy available to the arc of about 1 percent and a reduction in light output of about 1 percent.