US3445719A

US3445719A - Metal vapor lamp with metal additive for improved color rendition and internal self-ballasting filament used to heat arc tube

Info

Publication number: US3445719A
Application number: US655969A
Authority: US
Inventors: Wolfgang E Thouret; Herbert S Strauss
Original assignee: Duro Test Corp
Current assignee: Duro Test Corp
Priority date: 1967-05-31
Filing date: 1967-05-31
Publication date: 1969-05-20
Anticipated expiration: 1986-05-20

Description

W. E. THOURET ETAL May 20, 1969 7 3,445,719

METAL'VAPOR LAMP WITH METAL ADDITIVE FOR IMPROVED COLOR RENDITION AND INTERNAL SELF-BALLASTING FILAMENT USED TO BEAT ARC TUBE Sheet Filed May 31, 1967 ill ' VENTORS HERBERT S.'STRAUSS WOLFGANG THOURET ATTORNEYS y 1969 w. E. THOURET ETAL 3,445,719

METAL VAPOR LAMP WITH METAL ADDITIVE FOR IMPROVED COLOR RENDITION AND INTERNAL SELF-BALLASTING FILAMENT USED TO BEAT ARC TUBE Filed May 51, 1967 Sheet 2 of 5 YNVENTORS HERBERT S. STRAUSS WOLFGANG E. THOURET I j ATTORNEYS May 20, 1969 w. E. THOURET ETAL 3,445,719

METAL VAPOR LAMP WITH METAL ADDITIVE FOR IMPROVED COLOR RENDITION AND INTERNAL SELF-BALLASTING FILAMENT USED TO HEAT ARC TUBE Filed May 31, 1967 Sheet 3 of 3 INVENTORS HERBERT s. STRAUSS WOLFGANG E THOURET ATTORNEYS United States Patent Int. Cl. H01j 7/44, 13/46, 17/34, 29/96 Us. 01. s15-49 13 Claims ABSTRACT OF THE DISCLOSURE An electric arc discharge lamp in which metal elements are placed in the arc discharge tube to contribute to the arc discharge and improve its color rendition. The are tube is heated by the ballast filament to produce a sufliciently high vapor pressure for the metal elements which are preferably used in halide form. In one embodiment, rare earth metal halides are used in the arc tube which is heated by an exposed ballast filament. In another embodiment, alkali metal halides are used and the ballast filaments are enclosed in separate envelopes to prevent loss of the alkali from the arc tube.

This application is a continuation-in-part of our copending application Ser. No. 394,255, filed Sept. 3, 1964, which is assigned to the same assignee.

Self-ballasted high pressure vapor lamps, such as those of the mercury vapor type, are well known in the art. These lamps usually include an arc discharge tube of quartz or other similar material which contains the metal for the vapor and the electrodes, and a series ballast resistance to limit and control the current in the lamp to stabilize its operation to a design wattage for a given applied line voltage. In lamps of this general type the ballast resistance is usually a filament wire, of tungsten or other suitable refractory metal, which may be either singly coiled or of the coiled-coil configuration. The arc discharge tube and the separate ballasting filament are mounted within an outer envelope of suitable vitreous material, such as glass, which has a fill gas therein. The fill gas is usually any of the relatively inert gases such as argon or nitrogen, or a mixture of gases such as argon and nitrogen for example, at pressures which meet the requirements of proper arc tube warm-up and lowest possible rate of ballast filament evaporation.

As is known, self-ballasted high pressure mercury vapor lamps using a tungsten ballast filament have less lumen per watt efficiency than lamps of the type using a reactive ballast, such as an inductor or a capacitor. This is because of the power consumed in heating the filament. Thus, it becomes desirable to try to increase the efliciency of self-ballasted lamps.

When mercury is used as the metal to be vaporized in a discharge lamp, the lamps are commonly called mercury lamps. As is known, the color rendition of high pressure mercury lamps without ballasting filaments, is known to be poor in the red or warm color regions. This is due to the absence of radiated energy in the red portion of the color spectrum. To correct this, some color correcting phosphor is usually applied to the inner wall of the outer envelope which improves the color rendition by absorbing the arc tube ultra-violet mercury radiation and re-emitting energy in response thereto in the red region.

The color rendition of self-ballasted mercury lamps, i.e., those with ballast filaments, in the warm or red region is better than that of mercury lamps without such filaments. In such self-ballasted lamps a phosphor coated envelope is often used to improve the spectral content of the light output. In this type of lamp an improvement in color rendition is obtained due to the use of the ballast filament which also emits energy in the red region. All in all, a self-ballasted mercury lamp with a phosphor coating has a higher red color out-put rendition than a reactive ballast type mercury lamp with a phosphor coating. However, these coated lamps also have manufacturing disadvantages such as cost of the phosphor, coating, the additional coating process, etc.

From the foregoing it should be apparent that it would be desirable to construct an arc discharge lamp with improved efficiency and better color rendition and to accomplish both of these results in as simple and economical a manner as possible. One attempt to construct such a lamp is disclosed in United States Patent No. 3,234,421 to G. H. Reiling, issued on Feb. 8, 1966. In the are discharge lamps of that patent, metal halides are added to the arc tube of a lamp which is externally ballasted, that is, no ballasting filaments are used. The metal halides are selected from various alkali halide and metal halide compounds and an attempt is made to raise the temperature of the arc tube to a point high enough to vaporize the metal additives primarily by reducing the size of arc tube and using relatively high voltages and currents.

The lamps of the aforesaid patent have several disadvantages. First of all, the proposed arrangements for temperature control of the arc tube are, in some cases, not adequate. Under some conditions of operation the required temperature for vaporization of the metal halides is not reached and/ or cannot be maintained. Further, the use of higher voltages and currents to attempt to maintain the required temperature is undesirable. In addition, the use of most alkali metal halides, does not greatly improve the color rendition of the lamp.

The advantages possible by using a metallic element additive in an electric arc discharge lamp are secured in the present invention in a self-ballasted lamp. Here, the filament performs a function in addition to its normal ballasting function of heating the arc tube. This heating ensures that the metal additives are properly heated so that they can be vaporized and contribute to the arc stream discharge. The novel heating arrangement permits rare earth metals, preferably in halide form, to be utilized. This is desirable since such metals have better color properties than most alkali earth metals. In addition, an arrangement is provided wherein alkali metals can be used in a self-ballasted lamp without the danger of loss of the alkali material brought about by the presence of an exposed ballasting filament.

In accordance with the present invention a self-ballasted vapor lamp is provided which has an increased lumen per watt efiicacy and increased spectral color rendition in a desired region such as the red and warm color regions. This is accomplished by the addition to the arc discharge tube of a suitable metallic element such as an alkali metal, rare earth metal, or combinations of various metals. The use of these metallic elements increases the lumen per watt output of the lamp as well as improve its color rendition in the warm color region characteristic of the additive. In a preferred embodiment of the invention the metallic elements used are rare earth metals and alkali metals, preferably introduced in metallic halide compound form, such as an iodine compound for example, to obtain a vapor pressure of the metallic element additive sufiiciently high to influence the dis charge and have the active halogen element chemically bound to the metallic element in the cooler (Wall) regions of the discharge chamber, so as not to attack the chamber vapor discharge producing material and the electrode material. In the arc discharge path the metallic halide compound is dissociated by the heat of the arc and the metallic element is freed to contribute to the discharge.

It is, therefore, an object of the present invention to provide self-ballasted vapor lamps with improved lumen output and color rendition.

A further object is to provide a self-ballasted vapor lamp using metal halide compounds to increase the efficacy of the lamp output and its color rendition.

An additional object is to provide self-ballasted mercury vapor lamps in which alkali metals are used and the ballast filament is housed in a separate envelope to prevent loss of the alkali material.

Another object is to provide improved self-ballasted mercury vapor lamps in which the surface loading of the high pressure mercury vapor arc discharge tube is controlled to control the vapor pressure of a metal halide compound within the tube.

Other objects and advantages of the present invention will become more apparent upon reference to the follow ing specification and annexed drawings, in which:

FIGURE 1 is an elevational view, taken partly in section of one embodiment of the lamp made in accordance with the present invention;

FIGURE 2 is a plan elevational view, taken partly in section, of a portion of the lamp of FIGURE 1 rotated by 90;

FIGURE 3 is a plan elevational view, taken partly in section, of another embodiment of the lamp;

FIGURE 4 is a view, taken partly in section, of the lamp of FIGURE 3 rotated by 90;

FIGURE 5 is a plan elevational view, taken partly in section, of another embodiment of the lamp made in accordance with the invention; and

FIGURE 6 is a plan elevational view of the lamp of FIGURE 5 rotated by 90.

FIGURES 1 and 2 show one embodiment of the selfballasted mercury arc lamp made in accordance with the invention. The lamp 10 includes an arc tube chamber 12 of quartz or other suitable material which is filled with a quantity of a vapor producing material such as mercury 14. The arc tube also has a metal additive therein to improve the lamps color rendition. This is described below.

The are discharge tube 12 is supported on a frame rod 16 having support braces 17 and 17a connected at each end thereof which are fastened around a boss or

press

19 and 19a at each end of the tube 12. Throughout the specification the latter a refers to the elements at the upper end of the tube as shown in the drawings.

A-

main electrode

21 and 21a and an auxiliary starting electrode 22 and 22m is sealed into each end of the tube. The end of each

main electrode

21 and 21a and one end of an auxiliary electrode 22 and 22a are connected to one end of a molybdenum ribbon 23 and 23a sealed in a

respective press

19 and 19a and a respective connecting

lead

25 and 25a is brought out from the other side of the ribbon. The other end of the lower auxiliary electrode 22 is connected in the press 19 to another molybdenum ribbon 27 which has a lead 29 connected thereto.

The ballasting filament 30, which is illustratively of tungsten, has a number of support rods 31 connected thereto at spaced points. These rods 31 are inserted into an insulating button member 33 of glass or other suitable insulating material. One end of the botton is held within a portion of the lower support brace 17.

The are chamber 12 and

filament assembly

30, 31 and 33 are mounted on a stem 40 at the base of the envelope by a pair of stem leads 42 and 43 which are electrically connected (not shown) to portions of the base 45 in the conventional manner, i.e., one of the stem leads is brought out to the screw threaded member and the other brought out to the insulated bottom contact 47 at the bottom of the base.

Stem lead 42 is directly connected to frame rod 16 while stem lead 43 is connected to another frame rod 16a. Rod 16a has one end inserted into the button 33. The points of contact of the two stem leads 42 and 43 with

rods

16 and 16a act as the fulcrum points on the rods and support springs 50 are attached at the bottom of each rod to provide a restoring force against mount distortion. An arrangement of this type is described in US. Patent No. 2,966,600 entitled, Electric Lamp Mount, by Herbert S. Strauss, which is assigned to the same assignee.

A tungsten rod 52 (FIG. 2) of suitable diameter is sealed into the lower press 19 to serve as one contact of a bimetallic switch whose other contact 5-3 is welded to an auxiliary insulated stem lead 55 pressed into the stem 40. The auxiliary stem lead 55 serves as a switch support and has its free end electrically connected through lead 29 to the end of the lower auxiliary starting electrode 22 which is remote from the main electrode. A rigid wire lead 57 connected to rod 52, which has the other switch contact 53 thereon, runs the length of the arc tube and connected to the end of the auxiliary electrode 22a which is remote from the main electrode 21a.

The electrical circuit for the lamp of FIGS. 1 and 2 is as follows: from the stern lead 43 connection is made through rod 16a and a connecting lead 60 attached thereto to the bottom of one of the supporting rods 31 connected to one end of the filament 30. The other end of the filament is connected by a support rod 31, a connecting lead 61 (FIG. 2) and lead 25 to the ribbon 23 for the bottom main electrode 21 and one end of the auxiliary starting electrode 22. Lead 25a of the upper main electrode 21a and auxiliary electrode 22a are connected to the rod 16 and back to the other stern lead 43 to complete the circuit for the arc discharge tube.

The auxiliary electrode circuit is as follows: From the stem lead 42 through the frame rod 16 and lead 25a, connection is made to the end of the auxiliary electrode 22a connected to the main electrode 21a. The circuit continues from the other end of electrode 22a through the connecting wire 57, through contact rod 52 and contact 53 of the switch and lead 29 to the free end of the auxiliary electrode 22. The circuit is completed through the auxiliary electrode 22, lead 25, and connecting lead 61 back to the filament.

The are discharge tube and filament assembly are located within an outer envelope which is filled with a suitable fill gas, such as nitrogen or argon or a combination thereof. The inner wall of the envelope may be coated, if desired, with a suitable phosphor for emitting colors in the red range, such as magnesium fluorogermanate, for example.

The operation of the lamp of FIG. 1 is described below. When current is applied the normally closed bimetallic switch formed by

elements

52 and 53 permits preheating current to flow through the auxiliary electrodes 22 and 22a. The time the preheating current flows is controlled by the design of the bimetallic switch which opens after a given time interval due to heat generated by the auxiliary starting electrodes and the ballast filament. When the switch opens the full line voltage is applied across the tube 12 causing the arc tube to ignite.

As indicated above, a quantity of a metal element additive 15 is present in the arc tube to improve the color rendition of the lamp. In the embodiment of the lamp of FIGS. 1 and 2 this additive is selected from the rare earth group of metals such as, for example, lanthanum, dysprosium, scandium and cerium. Other suitable rare earth metals can also be utilized but the foregoing, and particularly lanthanum, dysprosium and thorium are generally considered to have the most desirable characteristics for enhancing color rendition in the visible spectrum. Satisfactory results also have been obtained with a radioactive rare earth metal such as thorium.

It is preferred that the metal additive be introduced into the arc tube in the form of a metal halide, e.g., dysprosium halide, cerium halide, scandiurn halide or lanthanum halide or thorium halide, rather than as the metal alone. The reason for this is to obtain a vapor pressure of the additives sufficiently high to influence the discharge and have the active elements chemically bound in the cooler wall regions, so as not to attack the cham ber material. In the discharge column within the envelope 12, the additive compound is dissociated and the metallic element is freed to contribute to the discharge and increase its lumen output and improve its color rendition. The metal reacts in the arc stream to achieve the desired results. The halogen is freed in the arc stream and the two elements recombine at the cooler areas, such as the walls, of the tube into the metal halide.

The rare earth metals referred to above, when dissociated from the halogen in the discharge, emit a dense line spectrum which lies predominantly in visible region, thus producing good source color. H wever, sufficiently high rare earth halide vapor pressure must be obtained in order to achieve such emission. It has been found that the temperature of the coolest part of the arc tube must be maintained at 900 C. to 1200 C. to allow the pr per vapor pressures to be obtained.

Heretofore, the use of such rare earth additives was not considered in an arc discharge lamp because of the problem of maintaining the arc tube at the high temperature required to keep the vapor pressure needed for the vaporization of the additive. This is accomplished in the subject invention by the filament 30 which is disposed adjacent the arc tube 12, preferably in relatively close proximity thereto, to surface heat the tube. This heating is used to raise the temperature of the tube contents to a point where the rare earth halide dissociates and contributes to the arc discharge color. Of course, the surface temperature of the tube 12 must not be permitted to exceed the allowable value for either its own material or the material within the chamber. It should be understood that without the heating provided by filament 30 that it would be difficult, if not impossible, for the rare earth halide to be properly dissociated under normal lamp operating conditions at a relatively low voltage and current. Of course, the filament 30 in addition to its function as a ballast, permits the arc tube to be maintained at almost any desired temperature such as by changing the filament size, its spacing with respect to the arc tube, having several filament coils wrapped around the arc tube, etc.

FIGS. 3 and 4 show another embodiment of the lamp useful for rare earth metal additives. Similar reference numerals have been use where appropriate. Here, the auxiliary starting electrodes 22 and 22a have been replaced by a pair of

probe electrodes

74 and 74a. Electrode 74 is connected to one side of the line through lead 75, starting resistor 72 and frame rod 16. Electrode 74a is connected to the end 61 of the ballast filament 30 and the other side of the line by a resistor 72a and a lead 76 through filament 30, filament support 31, lead 60 and support rod 16a. In this case the starting bimetallic switch of FIGS. 1 and 2 is eliminated. This configuration of lamp is designed for higher starting voltage than the lamps of FIGS. 1 and 2.

It should be noted that a slightly different filament configuration is used from that of FIGS. 1 and 2 to provide more dispersed heating of the tube 12. Also, re-

flectors and 80a, located either inside or outside of the tube or both, are placed behind the

respective electrodes

21 and 21a to create more uniform wall temperatures by heating the usually cool portions of the tube behind these electrodes. The reflectors may be of any suitable metallic or non-conductive material. The lamp of FIGS. 3 and 4 operates in the same manner as the lamp of FIGS. 1 and 2 except that added and more uniform surface loading of the arc tube 12 is obtained by the filament configuration and the use of the reflectors.

Lamps of the type shown in FIGS. 1-4 have the ballast filament exposed to the fill gas of the outer envelope 70. Normally, the fill gas within the outer envelope is selected to minimize evaporation of the filament and to develop the highest possible filament efficacy for its designed life.

The lamps of FIGS. 1-4 described above, all use rare earth metal. While such metals, when properly dissociated so as to be able to contribute to the arc stream discharge do enhance the color rendition of the lamp they do not produce as efiicient a lamp as could be obtained if an alkali metal is used. It has heretofore been known to use alkali metals in the arc tube of an electric discharge lamp, see for example Railing, United States Patent 3,234,421. Generally, such prior art lamps have been unsuccessful for several reasons. One of these is that no efiicient way was provided, such as by a separate heating source, to heat the arc tube to obtain a sufficiently high vapor pressure.

While the embodiments of lamps shown in FIGS. 1-4 could produce the temperatures required to properly heat the arc tube when an alkali metal is used, such an arrangement, in which the ballasting and heating filament is exposed gives rise to another critical problem. That is, loss of alkali material from the arc tube, which is usually of quartz, by electrolysis through the hot wall of the arc tube.

The alkali loss mechanism is believed to operate in approximately the following manner. An incandescent tungsten ballast filament operating at high temperature in close proximity to an arc tube emits a considerable amount of thermionic electrons. Some of these reach the arc-tube outer wall and produce a negative charge on that wall. The electric field produced by this negative charge causes positive alkali ions to migrate from the inner arc-tube chamber, through the hot quartz wall, to the arc-tube outer surface. There the alkali ions are neutralized by the caption of thermionic electrons from the ballast filament. The alkali atoms thus formed evaporate off the arc-tube outer surface and eventually settle as a deposit on the inner surface of the lamps outer envelope. Such progressive loss of alkali from the quartz arc tube alters the optical output and the electrical characteristics of the lamp. It has been found that loss of one half of the original amount of alkali from the discharge will lead to lamp failure.

When the alkali metal is introduced as a metal halide, for the same reasons as described with respect to the use of rare earth metal halides, another problem arises. Here the free halogen, created by the loss of alkali atoms, will readily combine with the arc tube mercury, forming mercuric halide. The presence of this compound increases the starting voltage of the lamp. As the alkali loss continues, the starting voltage increases more and more until it climbs beyond the limit of the available supply voltage.

The problems engendered by the use of an alkali metal additive are overcome by the lamp shown in FIGS. 5 and 6. Here the ballast filament 30 is split into two sections 30a and 30b which are housed and held by coil springs 120 within the separate chambers a and 90b.

The chambers 90a and 90b run substantially the length of tube 12 and are spaced on opposite sides thereof so that good surface loading of the arc chamber surface may be achieved as Well as a more uniform radiation of the light from the filaments. The two chambers 90a and 90b are held to tube 12 by two supports 117 and 117a which are connected to frame rods 116a and 1161). Rod 116a is connected to one of the stem leads (not shown) which goes to the line while rod 116!) is connected to another supporting stem lead (not shown).

The electrical circuit of the lamp is from one side of the line via rod 116a through lead 25a to electrode 21a. The auxiliary probe electrode 74 is connected to rod 116a by starting resistor 72 and lead 130. A lead 116 is connected from the other side of the line by a stem lead (not shown) to the lower end of filament 30b which is connected in series to filament 30a by a connecting lead 135. The lower end of filament 30a is connected by lead 25 to the main electrode 21. A connection is made from the main electrode 21 to the starting probe 74a through the lead 76 and resistor 72a.

The arc tube 12 of the lamp of FIGS. -6 contains a quantity of an alkali metal preferably in an alkali halide form. Suitable compounds which can be used are, for example, sodium iodide, lithium iodide and cesium iodide. Other alkali metals, whose color properties are not as good but which can be used are potassium and rubidium, preferably in the halide form. In operation, the arc tube is heated by the ballasting filament to vaporize the metal additive which then contributes both to the color and efiiciency of the arc stream.

As should be apparent, the use of the ballasting filaments in the enclosed chambers not only provides the necessary heating for the arc tube but also prevents the loss of the alkali from the arc tube through the loss mechanism previously described. This is so because the electrons emitted from the ballast filaments cannot pass through the walls of the filament envelopes 90a and 90b. Therefore, there are no electrons available at the outer wall of the arc tube to attract and combine with the alkali ions.

An additional advantage is made possible using the embodiment of lamp shown in FIGS. 5 and 6. As is known, when a tungsten filament is used, the tungsten material evaporates and blackens the wall of any adjacent envelope. Thus, in the lamps of FIGS. 1-4, the tungsten could blacken both the outer wall of arc tube 12 and the inner wall of the envelope 70. In the embodiment of FIGS. 5 and 6, a halogen scavenger, such as iodine, can be used in the filament envelopes 90a and 90b to produce a halogen cycle which prevents the inner walls of these envelopes from becoming blackened. Of course, the tungsten material cannot reach either tube 12 or envelope 70.

In all of the embodiments of the lamp described, socalled filler materials also can be added. For example, indium iodide and thallium iodide can be added to either the alkali or rare earth metals. These materials aid in filling out the color spectrum.

It should also be understood that the features of the various embodiments of the lamps described above may be combined. Thus, for example, the lamp of FIGS. 1 and 2 may be provided with the filament configuration and reflectors of the lamp of FIGS. 3 and 4 and/or the separate filament ballast envelopes of the lamps of FIGS. 5 and 6. Similarly, the reflectors may be provided for the lamp of FIGS. 5 and 6. While the rare earth halides can be used in the lamp of FIGS. 5 and 6, the separate filament envelopes are not necessary since the rare earth halides do not migrate through a quartz wall.

By using the principles of the present invention a self ballasted mercury arc discharge lamp has been constructed with a higher lumen per watt output as compared to conventional mercury arc discharge lamps of the same type. In one case, an increase of from 30 to 42-55 lumens per watt was achieved with the partial pressure of the additive in the order of tens of millimeters and mercury pressure in the order of several atmospheres.

While iodide compounds for the alkali and rare earth metals have been described, it should be understood that chloride and bromide compounds also can be utilized. In

general, it has been found that fluoride compounds are too highly reactive with the walls of the tube and outer envelope.

While preferred embodiments of the invention have been described above, it will be understood that these are illustrative only, and the invention is limited solely by the appended claims.

What is claimed is:

1. A self-ballasted discharge lamp comprising:

an outer bulb,

a sealed envelope within said bulb having a pair of electrodes,

a quantity of a material within said envelope for producing an arc discharge therein between said electrodes,

a quantity of a metal additive within said envelope which is vaporizable to contribute to the are discharge and improve its color rendition,

and a thermionic filament located within said outer bulb and external to said envelope, means electrically connecting said filament to at least one of said electrodes to serve as a self-ballast for the operating current of the lamp, and means locating said filament adjacent the envelope to direct a suflicient amount of heat onto said envelope to raise and maintain the temperature of the inner wall of the sealed envelope and the vapor pressure of the metal additive so that the metal additive is vaporized and contributes to the color of the arc discharge.

2. A self-ballasted lamp as set forth in claim 1 wherein said metal additive comprises a metal halide.

3. A self-ballasted lamp as set forth in claim 2 wherein said metal halide additive comprises a halide of a rare earth metal.

4. A self-ballasted lamp as set forth in claim 3 wherein said filament maintains a temperature in said envelope in the range between about 900 C. to 1200 C.

5. A self-ballasted lamp as set forth in claim 3 wherein said metal halide comprises a halide of a rare earth metal selected from a group of rare earth metals consisting of halides of lanthanum, dysprosium, scandium and cerium.

6. A self-ballasted lamp as set forth in claim 1 wherein said metal additive comprises a halide of thorium.

7. A self-ballasted lamp as set forth in claim 3 wherein said filament is exposed within said bulb.

8. A self-ballasted lamp as set forth in claim 1 wherein said filament is housed within a second sealed envelope and said second sealed envelope is located adjacent the first-mentioned envelope to direct heat onto the first mentioned envelope to raise the vapor pressure of the metal additive.

9. A self-ballasted discharge lamp comprising:

an outer bulb,

a sealed envelope within said bulb having a pair of electrodes,

a quantity of an alkali metal additive within said envelope which is vaporizable to contribute to the arc discharge and improve its color rendition,

a thermionic filament, means electrically connecting said filament to at least one of said electrodes to serve as a self-ballast for the operating current of the lamp,

a sealed tube in which said filament is mounted to prevent the electrons emitted by the filament from reaching the outer wall of the sealed envelope to thereby prevent interaction of the electrons with the alkali ions at the outer wall of said envelope,

and means for mounting said filament tube within said outer bulb external to said envelope and at a position located to direct a sufiicient quantity of heat onto said envelope to raise the temperature of the inner wall of the sealed envelope and the vapor pres- 9 10 sure of the alkali metal additive so that the alkali References Cited metal additive is vaporized and contributes to the UNITED STATES PATENTS color of the arc dlscharge. 10. A self-ballasted lamp as set forth in claim 9 where- 2,966,600 12/1960 IW 315-49 XR in said alkali metal comprises an alkali metal halide. 5 3,234,421 2/1966 Ralhng 313205 X 11. A self-ballasted lamp as set forth in claim 10 3334361 8/1967 Butler et --313229 wherein said alkali metal halide is selected from the 3,351,798 11/1967 Bauer 313 229 X 3,048,741 8/1962 Thouret 315-49 group consisting of sodium, lithium, and cesium halide.

12. A self-ballasted lamp as set forth in claim 8 Wherein said second sealed envelope has a quantity of material 10 JAMES LAWRENCE Prlmary Exammer' therein to prevent darkening of the second envelope wall C. R. CAMPBELL, Assistant Examiner. by evaporative filament material.

13. A self-ballasted lamp as set forth in claim 12 further comprising at least one reflector means adjacent said 11 184 225 315 115 179 envelope containing the arc discharge electrodes for di- 15 recting the heat therein.