US3069581A - Low pressure discharge lamp - Google Patents

Description

Dec. 18, 1962 E. LEMMERS LOW PRESSURE DISCl-iARGE LAMP Filed Feb. 9, 1961 Inven tor: Eugene Le mew-s 9 His ATTOT'HGS ilnit 3,969,531 LGW PRESSURE DEQHARGE LAMP Eugene Lemrncrs, Cleveland Heights, Ohio, assignor to General Electric Company, a corporation of New Yorlr Filed Feb. 9, 1961, Ser. No. 88,228 Claims. (Cl. 313185) This invention relates to low pressure electric discharge lamps such as fluorescent lamps comprising a pair of thermionic electrodes sealed into opposite ends of an elongated tube containing mercury vapor and an inert gas for the ionizable medium. The present application is a continuation-in-part of my copending application Serial No. 812,235, filed May 11, 1959, of the same title and assignee and now abandoned.

In the fluorescent lamp, the electric discharge through a mixture of mercury vapor at a few microns pressure and an inert gas or mixture of inert gases at a few millimeters pressure produces ultraviolet radiation principally at 2537 A. The ultraviolet radiation is converted by the phosphor coated internally on the walls of the envelope into visible light which is transmitted through the glass walls. The source of ultraviolet radiation resides in the excited mercury atoms and there is an optimum mercury vapor pressure, usually about six microns, at which the efficiency of conversion of electrical energy into ultraviolet energy is a maximum. The inert starting gas, which in the past has most commonly been argon at a pressure generally not less than approximately 3 millimeters, is necessary principally at starting. The starting gas lowers the voltage required to initiate the arc discharge between the electrodes and also serves to protect the electrodes from destructive ion bombardment during the starting interval.

It has been appreciated for a long time that it is possible to increase the luminous efilciency, that is the ratio of luments output to watts input, in conventional fluorescent lamps such as the common 40-watt size by lowering the pressure of the inert starting gas. However, fluorescent lamps having argon gas filling at pressures below approximately three millimeters yield progressively shorter life, greater end darkening and poorer maintenance of lumen output as the pressure is reduced. These effects are caused principally by more destructive positive ion bombardment of the cathodes especially at starting. In the rapid-start type of lamp where the electrodes are heated by circulating current therethrough, cathode life can readily be brought back to normal by increasing the cathode area and raising the electrode temperature to a higher value whereby to achieve increased thermionic emission, in accord with the teachings of Hull Patent 1,790,153. The drawback which has up to now defeated such a scheme for improving the efiiciency of the fluorescent lamp has been that the increase in cathode heating watts required to maintain cathode life augments the overall losses to the point where the gains in efiiciency achieved by lowering the argon gas pressure are substantially completely ofiset.

The severe end darkening associated with lowered filling gas pressure can be prevented by the use of shields about the cathodes which trap the sputtered or vaporized electrode material before it can reach the phosphor on the bulb wall. However it is now generally accepted that shields, at least as used in the past, effectively shorten lamp life. This may be rather surprising in view of the fact that the principal purpose of the shield has been to prevent end darkening. However I have determined that shields as ordinarily used to reduce end darkening actually shortened the life of the cathode; in other words, the shields while hiding the damage have accelerated it.

The principal object of the invention is to provide new and improved low pressure electric discharge lamp such 3,6@,58l Patented Dec. 18, 1952 as fluorescent lamps having higher efficiencies than heretofore achieved.

A more specific object of the invention is to provide improved fluorescent lamps operating at substantially the same ratings and under substantially the same conditions as the common sizes of fluorescent lamps which have been commercially available but having higher overall efliciency and producing greater lumen output for the same wattage input.

Another object of the invention is to provide improved shielded electrode structures suitable for use in high efficiency lamps according to the invention.

In accordance with the invention, I have discovered that shields or anode plates about the thermionic electrodes in a low pressure discharge lamp such as a fluorescent lamp may be correlated in design to the filamentary electrodes and particularly to the thermal inertia and operating temperature thereof so as to achieve the desired shielding eltect and prevention of end darkening of the lamp without any acceleration of damage to the electrode.

Preferably lamps in accordance with the invention use an inert starting gas (including mixtures of starting gases) at a lower filling pressure than has heretofore been commonly used and which, in the absence of suitable counter measures, would ordinarily result in shortened cathode life resulting in greater end darkening and poorer maintainance of lumen output. However in accordance with the invention, such greater end darkening or poorer maintenance of lumen output is substantially prevented through the use of shields about the filamentary electrodes to which are correlated the electrode thermal inertia and normal operating temperature to prevent damage to the electrodes.

Ordinarily putting shields about a cathode as has been done in the past (in the absence of a polarized structure where the anode shield current is caused to pass through the filament) causes damage to the cathode by several mechanisms. Usually the shields have been placed close to the cathode in order to trap sputtered and evaporated material more effectively and the general practice has been to place them so close as to produce substantial ion trapping. Such ion trapping is of course equivalent to increase-d wall losses and alters the degree of ionization in the sheath about the cathode. The cathode automatically makes up this loss by adjusting to a higher cathode fall whereby to increase the degree of ionization. A higher cathode fall results in greater ion bombardment with resulting shorter life. In addition, if the shield is connected to the electrode inleads as it normally is, it will carry a substantial proportion of the anode current during the anode half-cycle, the proportion depending upon the area of the shield and its position relative to the cathode. Current carried by the shield during the anode halfcycle is of course current which must be subtracted from what the cathode would otherwise be carrying, with the result that the reduced current flow through the filamentary cathode causes a reduction in cathode temperature. Reduced cathode temperature normally entails a reduction in electron emission and again the cathode must make up therefor by readjusting to a higher cathode fall in order to increase the electrode emission to the degree necessary to support the discharge current.

In accordance with the invention, these drawbacks to the addition of a shield are circumvented by reducing the size or thermal mass of the cathode so as to achieve the optimum temperature despite the presence of the shields and their tendency to rob ions from the sheath and to subtract current from the cathode. I have discovered that by using a smaller filamentary cathode designed to operate in the usual manner without shields at 50% to and preferably at 50 to 62.5% of the normal 3 discharge current, proper operating conditions can be remtablished at normal current, that is at 100% current, with the use of shields. In effect, for the preferred range, the cathode is designed as regards resistance and thermal mass including its disposition to lose heat by radiation and otherwise, to achieve an emission spot temperature in the range of 1050 to 1200 C. while providing 100% of the electron emission on the cathode half cycle and only to 25% of the electron collection on the anode half cycle. The balance of electron collection on the anode half cycle is made up for the most part by the shields with possibly a minor fraction by the inleads. The shields or anode plates are spaced far enough away from the cathode, a minimum of approximately 3 millimeters, that ion trapping has a negligible effect on cathode fall.

In the usual rapid start lamp, the lamp starting voltage which sustains the discharge current is applied at the same time as the lamp filament voltage which produces the current circulating through the filaments to heat them. One result of this situation is that a demand is set up for electrons from the cathode immediately and before it has reached a temperature where a copious supply can be emitted. As a result, the discharge which normally occurs at starting and which is frequently referred to as a glow discharge occurs with a high cathode fall, for instance as high as 100 volts and this heats the oathode and quickly raises its temperature to the necessary electron emitting level. However the high cathode fall at starting also causes positive ion bombardment and sputtering of the conductors including the lead-in Wires and shields about the cathode. The material thus sputtered or vaporized deposits on the cathode and reduces its electron emissivity, in effect poisoning the cathode. Existing rapid start lamps have been so designed that the damage due to this high cathode fall or glow current at starting is insufiicient to cause less than published or rated life. However when shields of large area are attached to the cathode mount structure they may take a portion of the glow current at starting by emitting electrons and thus reduce the proportion emitted by the oathode. This tends to increase the time taken by the cathode to come up to the necessary emitting temperature. This phenomenon of course entails more sputtered material produced to poison the cathodes. Thus shields of large area which can operate as anodes further tend to reduce cathode life due to the phenomena occurring at starting. In accordance with yet another feature of the invention, these drawbacks are prevented by making the thermal inertia of the cathode such that even with large area shields connected to the mount structure, the combined effects of cathode heating resulting from filament heating voltage, that is, from ballast cathode voltage and glow current, will bring the filament to full emission temperature at least as quickly as in the standard lamp. Preferably the filament is brought up and stabilized at a temperature in the range of 850 to 1050 C. in a time interval less than 1 second after the application of filament heating voltage. Thus loss of life due to the conditions at starting is substantially avoided and cathode life again restored to normal.

Another feature of the invention is the use of shields or anode plates of relatively large area and consisting of a metal such as nickel preferably, or tantalum, tungsten or molybdenum having the characteristic of forming a suitable base member for a barium oxide cathode. Nickel or nickel plated iron is preferred for effectiveness and low cost. This permits a lower anode voltage drop during the anode half-cycle and thereby raises the elliciency.

In a preferred lamp construction embodying the invention, the foregoing features of the invention are combined with a lower inert starting gas filling pressure, for instance in the range of l to 2 millimeters, and the starting gas consists of a mixture of argon with a minor proportion its normal current rating,

diameter of the circular section envelope from a nominal diameter of 1%. to 1% inches. In addition the preferred lamp construction makes use of improved phosphors which have been treated to reduce the rate of depreciation, such treatment being particularly desirable in view of the higher rate of flow of ions to the wall in the instant lamp with resulting tendency towards more rapid depreciation of the phosphor.

The invention itself, together with further objects and advantages thereof, may best be understood with reference to the following description, taken in conjunction with the accompanying drawing. The novel features believed to be characteristic of the invention are set forth in the a pended claims.

in the drawings:

PEG. 1 is a partially cutaway perspective view of a fluorescent lamp representative of the invention.

FIG. 2 illustrates the mount and electrode structure of the lamp in side elevation.

FIG. 3 illustrates the same mount and electrode struc-- ture in end view.

FIGS. 4 and 6 illustrate successive stages in making the cathode filament. I

Referring to FIG. 1, the low pressure discharge lamp 1 embodying the invention may correspond, in regards to its" size and general configuration, to the ordinary 40-watt rapid start fluorescent lamp of 48" nominal length and 1 /2 nominal diameter The lamp comprises an elongated cylindrical envelope 2 having shouldered ends to which are secured bases 3 each provided with a pair of insulated contact terminals or pins 4, 5. As shown at the end of the lamp where a fragment of the envelope Wall has been broken out, electrode mount 6 comprises a relatively short stem tube 7 having its flared outer end 8 sealed peripherally into the shouldered tube end and having a press at its inner end through which are sealed current inlead wires 11, 12. The inward projections of the inlead wires support the filamentary cathode 13 whereas the outward projections are connected to the terminal pins 4, 5'. In addition the transverse extensions of the inleads support the cathode shields 14, 15. The other end of the lamp is provided with a similar cathode and one of the stem flares is provided with an exhaust tube which is sealed or tipped off in the usual fashion.

The lamp contains a quantity of mercury indicated by droplet 16 exceeding in amount the quantity vaporized during operation of the lamp. The lamp contains in addition a filling of an inert starting gas, for instance argon and neon in a proportion and at a total pressure to be more particularly specified hereinafter. A phosphor coating indicated at 17 on the inside of the envelope wall converts the resonance radiation of the discharge through the mercury vapor into visible light. The lamp may be coated externally with a water repellent substance to facilitate starting under adverse atmospheric conditions as when the humidity is high.

The shields 14, 15 are eifective in reducing and substantially preventing the end darkening associated with lowered inert starting gas pressure. They do this by trapping: the vaporized or sputtered darkening material before it. can reach the phosphor on the bulb wall. Each shield comprises a main portion 18 which is disposed generally' parallel to the filament i3 and transversely to the longi-- tudinal axis of the lamp, and an auxiliary portion 19 which extends approximately at right angles. This shield configuration serves to more or less box in the cathode where by to trap the darkening material more effectively. The shields are located about at the boundary of the cathode glow and in any case at a distance greater than approximately 3 millimeters from the emitting surface of the cathode. For example, in a preferred construction suit;

able for a 40-watt fluorescent lamp, the distance d is 5 to 7 millimeters. With this spacing, the ion trapping effect of the shields has negligible effect on the cathode fall.

As previously mentioned, the tendency'of the shields to reduce the operating temperature of the cathode by subtracting from it a substantial portion of the discharge current during the anode half-cycle, is ofiset by using a smaller cathode which heats up more quickly and which will be maintained at the required electron emitting temperature, for instance in the range of approximately 900 to 1000 C., under the conditions encountered. I have found that in general a cathode which is designed to operate in the usual manner, without shields, at a current which is 50 to 62.5% of the normal lamp current, will operate in the desired fashion at normal current with shields.

As an example of a cathode in accordance with the invention suitable for a 40-watt fluorescent lamp and utilizing an overwind in accordance with the teachings of US. Fatent 2,306,925, Aicher, the following construction may be used. Referring to KG. 4, the first coiling provides the overwind convolutions consisting of 0.4 mil tungsten wire 21 wrapped at 290 turns per inch around a composite mandrel formed by a 1.9 mil tungsten Wire 22 and a 1.5 mil molybdenum wire 23 laid alongside each other to provide a composite wire 24-. In the second coiling illustrated in PEG. 5, the product 24 of the first coiling is wrapped at 13% turns per inch around a mandrel consisting of a 3.5 mil molybdenum wire 25 to provide a composite wire 2-5. In the illustration of FIG. 5, molybdenum mandrel wire 23 has been omitted for the sake of clarity; also in FIG. 6, both molybdenum wires 23 and 25 are omitted for the same reason. In third coiling illustrated in FIG. 6, the product as of the second coiling is Wrapped around a mandrel ccnsistirn of a 13.5 mil steel pin, the cathode coil being thereafter removed from it by slipping it off. The cathode coil or filament 13 may consist of approximately 18 turns of the final coiling with straight extensions or legs which are clamped between the folded inner ends of the inlead wires 11, 12. Previous to clamping between the inlead wires, the cathode convolutions are set by suitable heat treatment and the molybdenum mandrel wires 23 and 25 are removed by dissolving in acid. Thus the filament 13 is a hollow or skeletal structure consisting of the triple-coiled overwind wire 21 fitting loosely around the turns of the double-coiled mandrel wire 22. This particular filament is merely an example of a cathode meeting the requirements of the invention in regards to correlation with shield design so that the cathode carries 50 to 62.5% of the total discharge current. Other cathode structures may readily be proportioned to the sar e criteria. including if desired simpler cathode structures not using an overwind or triple coiling and using for instance only double coiling.

The shield structure comprises a pair of thin nickel strips 14, 15, each having a width W of approximately 8 mm. and a total length L of approximately 21 mm. The short leg 1? of each stri welded to the inlead is approximately 6 mm. long. and the long leg 18 approximately mm. long. The long legs are preferably positioned parallel to the filament as illustrated in FIG. 3. The distance D between strips is approximately 15 mm. and the overt'll shield structure surrounds the filament in a plane transverse to the longitudinal axis of the lamp and is generally box-like around the filament. The distance d from the emitting surface of the filament to the shield strip is ap proximately 6 millimeters, a distance substantially greater than the minimum of 3 millimeters required to reduce ion trapping to the point where it has negligible efiect on cathode fall. The total inside surface of the shield members 14, 15 surrounding the cathode is approximately /2 in. or 3.3 cm. However this is not critical and may be varied within the principles stated, and in particular may be inc eased substantially. With the given dimensions, the cathode or filament carries from 12 to 18% of the 6 current on the anode half-cycle so that the cathode supports, on a full cycle average, 56 to 59% of the total discharge current.

The usual procedures are followed in making a lamp using the cathodes of the present invention. Previous to mounting the lamp envelope, the cathode mounts are processed to coat the filamentary cathodes with activating material such as barium, strontium, and cal ium carbonates or mixtures thereof. The mounts are then sealed to the ends of the glass envelope and the electrodes are activated during lamp manufacture by passing a heating current through them to reduce the carbonates to oxides.

The shields are preferably made of a metal which forms a good base member for activation by barium or barium oxide. Suitable metals are tungsten, molybdenum and nickel; nickel-plated iron is satisfactory and is preferred because it is cheapest. The shields of nickel-plated iron become sotospeak activated during operation, and this reduces the anode fall and permits an increase of 3 to 5% in lamp efiiciency. The activation of the shields comes about as a result of the deposition thereon of evaporated emission material, principally barium, sputtered or vaporiZed from the cathodes. A surface which is a good electron emitter is also a good electron collector and as a result when the shields operate as anodes during the anode half-cycle, the anode voltage drop and of course the lamp voltage is decreased by several volts. If desired, the shields may be made of screening or else of perforated strip in order to reduce trapping of 2537 A. radiation originating from the cathode glow. The shield plates need not necessarily be L-shaped as described and illustrated; another convenient form consists in C-shaped shield seg ments whereby the shield encircles the filament in a circu lar or ring-like configuration.

With the shields placed in the preferred position about the cathodes, namely the outer fringes of the expanded cathode glow, and with the inert filling gas pressure lowered into the range of l to 2 millimeters, a 48" long 1 /2" diameter lamp is found to consume only about 37 watts in order to produce the same lumens output as the standard 40-watt lamp while at the same time realizing an efficiency approximately 8% higher.

In the foregoing lamp which requires an input of 37 watts only to produce the same lumens output as the prior art 40-watt lamps, the wattage input may be restored to the 40-watt level and the total lumen output raised proportionately by decreasing the bulb diameter or by changing the inert starting gas composition. The latter may be done in accordance with the teachings of copending application Serial No. 812,236, filed May 11, 1959, of Kurt Schmidt, entitled Fluorescent Lamp Gas Filling, and assigned to the same assignee as the present invention. In accordance therewith, at relatively low loadings in the range of 5 to 15 watts per foot, an increase in efiiciency may be achieved by replacing up to approximately 50% of the usual argon filling gas by neon in the pressure range from 1.5 to 3 millimeters. At a pressure of approximately 2 millimeters, the preferred proportions of neon is in the range from 25 to 40%, the latter being preferred where the emphasis is on maximum output, and the former being preferred where the emphasis is on maximum life. This allows an improvement in efficiency of approximately 2% over the maximum efficiency which may be achieved with pure argon at the same total pressure. For the instant 40-watt lamp, the preferred filling gas mixture consists of 65% argon and 35% neon at a total pressure of approximately 2 millimeters of mercury.

It is known that reducing the filling pressure of the inert starting gas or using a starting gas of low atomic weight such as neon in lieu of part of the argon, entails a greater rate of depreciation of the phosphor coating. The foregoing measure increases the rate of flow of ions to the wall which appears to be responsible for a reaction occurring between the constituents of the bulb glass and the phosphor. Among the end products of this reaction are a phosphor surface which is mercurophilic, that is one to which mercury is more attracted and becomes more readily attached. The attachment of mercury to the phosphor surface causes a reduction in lumen output because the mercury layer is opaque to both the exciting ultraviolet radiation and to the emitted visible light. To reduce this deleterious effect as much as possible in the instant lamp, the phosphor may be treated to remove the surface metallic ions such as antimony, manganese, etc. Also the envelope wall may be treated to remove alkali materials therefrom whereby to make it a better insulator and also to leave less material for reaction with the phosphor surface or to serve as attachment points for mercury. Another procedure which may be used is to apply a potential to the bulb wall with such polarity that all the electro-positives such as sodium are moved to the outer surface.

Comparative tests of the foregoing improved 40-watt lamp in accordance with the invention and of regular production 40-watt rapid start fluorescent lamps produced the following results. The cathode (filament) temperature of the regular lamp at nominal cathode heating voltage (3.65 volts) stabilized at approximately 930 C. in 1.6 seconds after inital application of voltage, and the temperature of the emission spot on the cathode with normal discharge current (430 milliamperes) was approximately 1130 C. In the improved lamp using the rapid heating lament and the shields and a filling of 65% argon and- 35% neon at a total pressure of 2 millimeters of mercury, the cathode (filament) temperature stabilized at approximately 985 C. in 0.6 second after initial application of voltage, and the temperature of the emission spot with normal discharge current was approximately 1125" C. The foregoing temperatures were measured with an optical pyrorneter focused on the oxide coated surface of the electrode and include no correction for departure from black body radiation. Thus the improved lamp stabilizes as to cathode temperature in one half to one third the time and has substantially the same emission spot temperature despite the large area shields which operate as anodes. By comparison, When the shields are merely added about the cathodes of the regular lamp, the emission spot temperature drops from 1130 C. to 970 C., a value too low for satisfactory life and maintenance.

The improved 40-watt lamp in accordance with the invention realizes an improvement in efficiency of 8% over prior lamps, and for the first time makes possible an output in excess of the 3000 lumens level for an input of 40 watts. By using in this lamp more eflicient phosphors of controlled particle size for maximum brightness, an output of 3100 lumens at an efliciency of 77.5 lumens per watt is achieved. Such high efficiency has never belore now been rcaiized in a 40-watt lamp.

While a certain specific embodiment of the invention has been illustrated and described in detail, various modifications will readily occur to those skilled in the art inasmuch as the underlying principles may readily be applied to different sizes of lamps. The elongated discharge channel may be otherwise than straight and tubular; for instance a channel of re-entrant cross-section may be used, or a curved channel as in circline lamps, or a sinuous channel as in labyrinthine panel lamps. The appended claims are therefore intended to cover any such modifications coming Within the true spirit and scrpe of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A low pressure electric discharge lamp comprising a vitreous envelope defining an elongated discharge channel having a pair of electrode structures at opposite ends and containing an ionizable medium comprising mercury vapor and an inert starting gas, each electrode structure comprising a coiled tungsten filament coated with activating material and supported at opposite ends by inlead wires sealed through said envelope, said filament being proportioned in resistance and thermal mass to achieve an electron emission spot temperature in the range of 1050 to 1200" C. with emission of d.'s:harge current on the cathode half cycle and 0 to 25% eclectic-n on the anode half cycle, and conductive shields fastened to said inleads encompassing the cathode glow region of said electrode and collecting substantially the balance of discharge current.

2. A low pressure electric discharge lamp comprising a vitreous envelope defining an elongated discharge channel having a pair of electrode structures at opposite ends and containing an ionizable medium comprising mercury vapor and an inert starting gas, each electrode structure comprising a coiled tungsten filament coated with activating material and supported at opposite ends by inlead wires sealed through said envelope, said filament being proportioned in resistance and thermal mass to achieve an electron emission spot temperature in the range of 1050 to 1200 C. with 100% emission on the cathode half cycle and 0 to 25% collection on the anode half cycle, and conductive shields fastened to said inleads generally surrounding said filament and encompassing its cathode glow region, said conductive shields consisting of metal subject to activation by deposition thereon of activating material from said filament and being substantial in surface area whereby to collect substantially the balance of discharge current on the anode half cycle with a low anode voltage drop.

3. A low pressure electric discharge lamp comprising a vitreous envelope defining an elongated discharge channel having a pair of electrode structures at opposite ends and containing an ionizable medium comprising mercury vapor and an inert starting gas, each electrode structure comprising a coiled tungsten filament coated with activating material and supported at opposite ends by inlead wires sealed through said envelope, said filament being proportioned in resistance and thermal mass to stabilize at a temperature in the range of 850 to 1050 C. in a time interval less than 1 second after the application of filament heating voltage and to achieve in operation an electron emission spot temperature in the range of 1050 to 1200 C. with 100% emission of discharge current on the cathode half cycle and 0 to 25% collection on the anode half cycle, and conductive shields fastened to said inleads encompassing the cathode glow region of said electrode and collecting substantially the balance of discharge current.

4. A low pressure electric discharge lamp comprising a vitreous envelope defining an elongated discharge channel having a pair of electrode structures at opposite ends and containing an ionizable medium comprising mercury vapor and an inert starting gas, each electrode structure comprising a coiled tungsten filament coated with activating material and supported at opposite ends by inlead wires sealed through said envelope, said filament being proportioned in resistance and thermal mass to stabilize at a temperature in the range of 850 to 1050 C. in a time interval less than 1 second after the application of filament heating voltage and to achieve in operation an electron emission spot temperature in the range of 1050 to 1200 C. with 100% emission on the cathode half cycle and 0 to 25% collection on the anode half cycle, and conductive shields fastened to said inleads generally surrounding said filament and encompassing its cathode glow region, said conductive shields consisting of metal subject to activation by deposition thereon of activating material from said filament and collecting substantially the balance of discharge current on the anode half cycle.

5. A low pressure electric discharge lamp comprising a vitreous envelope defining an elongated discharge channel having a pair of electrode structures at opposite ends and containing an ionizable medium comprising mercury vapor and an inert starting gas, each electrode structure comprising a coiled tungsten filament coated with activating material and supported at opposite ends by inlead wires sealed through said envelope, said filament being proportioned in resistance and thermal mass to stabilize at a temperature in the range of 850 to 1050 C. in a time interval less than 1 second after the application of filament heating voltage and to achieve in operation an electron emission spot temperature in the range of 1050 to 1200 C. with 100% emission on the cathode half cycle and to 25% collection on the anode half cycle, and a pair of conductive shields fastened to said inleads generally surrounding said filament and located at a distance from the emission spot surface of said filament not less than approximately 3 millimeters and disposed at about the limits of the cathode glow region, said conductive shields being substantial in surface area and consisting of metal subject to activation by deposition thereon of activating material from said filament whereby to collect substantially the balance of discharge current on the anode half cycle with a low anode voltage drop.

6. A low pressure electric discharge lamp comprising a vitreous envelope defining an elongated discharge channel having a pair of electrode structures at opposite ends and containing an ionizable medium comprising mercury vapor and an inert starting gas, each electrode structure comprising a multiple coiled tungsten filament coated with alkaline earth oxide activating material and supported at opposite ends by inlead wires sealed through said envelope, said filament being proportioned in resistance and thermal mass to stabilize at a temperature in the range of 850 to 1050 C. in a time interval less than 1 second after the application of filament heating voltage and to achieve under operating conditions an emission spot temperature in the range of 1050 to 1200 C. with 100% emission on the cathode half cycle and 0 to 25 collection on the anode half cycle, and a pair of conductive shields fastened to said inleads generally surrounding said filament in a plane transverse to the longitudinal axis of said channel thereat and located at a distance from the center of said filament not less than approximately 3 millimeters and disposed at about the limits of the cathode glow region, said conductive shields being substantial in surface area and consisting of metal subject to activation by deposition thereon of activating material from said filament whereby to collect substantially the balance of discharge current on the anode half cycle with a low anode voltage drop.

7. A low pressure electric discharge lamp comprising an elongated vitreous envelope having a pair of electrode structures at opposite ends and containing gas consisting of an ionizable medium comprising mercury vapor and an inert starting mixture of argon and not over 50% neon at a total pressure less than 3 millimeters of mercury, each electrode structure comprising a coiled tungsten filament coated with activating material and supported at opposite ends by inlead wires sealed through said envelope, said filament being proportioned in resistance and thermal mass to achieve an electron emission spot temperature in the range of 1050 to 1200 C. with 100% emission of discharge current on the cathode half cycle and 0 to 25% collection on the anode half cycle, and conductive shields fastened to said inleads encompassing the cathode glow region of said electrode and collecting substantially the balance of discharge current.

8. A low pressure electric discharge lamp comprising an elongated vitreous envelope having a pair of electrode structures at opposite ends and containing and ionizable medium comprising mercury vapor and an inert starting easm gas consisting of a mixture of argon and not over 50% neon at a total pressure less than 3 millimeters of mercury, each electrode structure comprising a coiled tungsten filament coated with activating material and supported at opposite ends by inlead wires sealed through said envelope, said filament being proportioned in resistance and thermal mass to stabilize at a temperature in the range of 850 to 1050 C. in a time interval less than 1 second after the application of filament heating voltage and to achieve an electron emission spot temperature in the range of 1050 to 1200 C. with emission of discharge current on the cathode half cycle and 0 to 25% collection on the anode half cycle, and conductive shields fastened to said inleads encompassing the cathode glow region of said electrode and collecting substantially the balance of discharge current.

9. A low pressure electric discharge lamp comprising an elongated vitreous envelope having a pair of electrode structures at opposite ends and containing an ionizable medium comprising mercury vapor and an inert starting gas consisting of a mixture of argon with appoximately 25 to 40% neon at a total pressure of approximately 2 millimeters of mercury, each electrode structure comprising a coiled tungsten filament coated With activating material and supported at opposite ends by inlead wires sealed through said envelope, said filament being proportioned in resistance and thermal mass to stabilize at a temperature in the range of 850 to 1050 C. in a time interval less than 1 second after the application of filament heating voltage and to achieve an electron emission spot temperature in the range of 1050 to 1200 C. with 100% emission of discharge current on the cathode half cycle and 0 to 25% collection on the anode half cycle, and conductive shields fastened to said inleads encompassing the cathode glow region of said electrode and collecting substantially the balance of discharge current.

10. A low pressure fluorescent lamp comprising an elongated vitreous envelope coated internally with a phosphor and having a pair of electrode structures at opposite ends and containing an ionizable medium comprising mercury vapor and an inert starting gas consisting of a mixture of argon and 25 to 40% neon at a total pressure of approximately 2 millimeters of mercury, each electrode structure comprising a multiple coiled tungsten filament coated with alkaline earth oxide activating material and supported at opposite ends by inlead wires sealed through said envelope, said filament being proportioned in resistance and thermal mass to stabilize at a temperature in the range of 850 to 1050 C. in a time interval less than 1 second after the applicationof filament heating voltage and to achieve in operation an electron emission spot temperature in the range of 1050 to 1200 C. with 100% emission of discharge current on the cathode half cycle and 0 to 25% collection on the anode half cycle, and a pair of conductive shields fastened to said inlead generally surrounding said filament in a plane transverse to the longitudinal axis of said envelope and located at a distance from the center of said filament not less than approximately 3 millimeters and disposed at about the limits of the cathode glow region, said conductive shields being substantial in surface area and having a nickel surface subject to activation by deposition thereon of activating material from said filament whereby to collect substantially the balance of discharge current on the anode half cycle with a low anode voltage drop.

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