US3521108A - Metallic vapor arc-lamp having high intensity sun-like emission - Google Patents

Description

United States Patent 3,521,108 METALLIC VAPOR ARC-LAMP HAVING HIGH INTENSITY SUN -LIKE EMISSION Rodney E. Hanneman, Burnt Hills, N.Y., assignor to General Electric Company, a corporation of New York Filed July 17, 1968, Ser. No. 745,502

Int. Cl. H01j 17/20, 61/22 U.S. Cl. 313-184 14 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to metal vapor arc-lamps utilizing a plurality of metallic vapors as the dischargesupporting and light-emitting media and which produce light of a spectral distribution so as to achieve substantially sun-like or white emission at extremely high efficacies.

In the past, the standard arc-lamp light sources for the production of high-intensity light were the low pressure sodium lamp and the mercury vapor lamp. The low pres sure sodium vapor lamp had a distinct disadvantage, in that its light emission was substantially that of the sodium D-line, a couplet having wavelengths of 5,896 and 5,890 A.U., respectively. This emission is chromatically' unbalanced and generally not acceptable.

Emission of the prior art mercury vapor lamps likewise suffered from chromatic deficiencies, in that the emission thereof had a substantially bluish tint under which reds did not appear in their true color, thus rendering such light acceptable only for large area street lighting and, even in such instances, such lamps were only competitive with incandescent lamps, and exhibited eflicacies of only about 60 lumens per watt, at best.

In recent years, significant and substantial advances have been made in the art to improve metallic vapor arc-lamps, particularly with respect to the efficacy and chromatic response thereof. More specifically, the mercury-metallic-halide electric discharge lamp as disclosed and claimed in Pat. 3,234,421-Reiling, issued Feb. 8, 1966, brought to the lighting industry a lamp which had higher efiicacies than that of the mercury vapor lamp such that, in commercial operation, efiicacies of 80 to 90 lumens per watt are readily available. Additionally, the chromatic characteristic of these lamps was such as to approximate closely that of White light. Although the lamps of the general class exemplified by the aboveidentified Reiling patent, and further developments thereof, are a substantial improvement over mercury vapor lamps, the efficacies are still somewhat short of the desired and theoretically achievable eflicacies from metallic vapor arc-lamps. Some problems in lumen maintainance have also been encountered in such lamps.

In yet another and highly significant advance in the lighting art, the high pressure sodium vapor arc-lamp, such as is disclosed and claimed in Pat. 3,248,590 Schmitt, issued Apr. 26, 1966, brought to the lighting art a new lamp utilizing a light-transmissive, high temperature arc-tube with a pair of electrodes therein and fillings including light-emitting sodium vapor at greatly increased pressures as compared with the prior art sodium vapor lamps, generally in the order of 30 to 1,000 torr of sodium. Such lamps, utilizing line-broadening and the increased efiicacy of high-temperature operation, are capable of the attainment of high-lumen efficacies, in excess of lumens per watt with a chromaticallyacceptable emission.

Although such lamps represent a very great advance in the lighting arts search for the ultimate in metallic vapor arc-lamps, the spectral distribution of the high pressure sodium vapor lamps is still less than desired in some applications, in that the lamp has a golden spectral emission, weak in green radiation. Although such lamps render substantially improved chromatic response over mercury and low pressure sodium vapor lamps and permit the identification of reds and other similar colors, not identifiable under prior art lamps, the emission thereof is not fully acceptable in some lighting applications in which a more nearly white spectrum is required.

Accordingly, it is an object of the present invention to provide metal vapor arc-lamps having very high lumen efiicacies and substantially sun-like, White chromatic emission which is completely acceptable for general illumination purposes.

Yet another object of the present invention is to provide metal vapor arc-lamps having a plurality of ionizable metallic species therein which are compatable with one another to create thermodynamic conditions within the arcing volume of a vapor arc-lamp whereby the presence of all species are compatible with one another and consistent operation over long periods of time is achievable.

Still another object of the present invention is to provide metal vapor arc-lamps having a plurality of metal species in the charge added thereto which charge is partially vaporized to provide a light-emitting metallic vapor arc which exhibits the characteristics of good chromatic, high-efficacy operation, and long-life operation at a highlumen efiicacy.

Briefly stated, in accord with one embodiment of the present invention, I provide a metallic vapor arc-lamp including an arc-tube composed of a high-temperatureresistant, light-transmissive ceramic material, which is also resistant to chemical attack, containing therein a pair of oppositely-disposed solid or nonmelting arc-electrodes which define therebetween an arc path. Within the lamp envelope, I provide a charge comprising sodium and at least two or more additional atomic species from the group including cadium, thallium, and mercury. The foregoing constituents are used under such conditions that the presence of some of each of the chosen constituents in a non-volatilized reservoir within the lamp is assured at the operating temperatures and further chosen in such proportions as to provide thermodynamic conditions which insure white or near-white spectral emission, together with the characteristic of long maintainance at high efficacy.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof, may best be understood by reference to the attached drawing in which:

FIG. 1 illustrates, in schematic vertical cross section, a typical lamp constructed in accord with the present invention,

FIG. 2' illustrates, in graphic form, the nonlinear activity characteristic of a typical constituent binary system of charge metals useful in understanding some characteristics of lamps made in accord with the present invention.

As shown in FIG. 1, high-pressure metallic vapor arclamp 1 comprises an outer, transparent vitreous envelope or jacket 2 of elongated ovoid shape. Neck 3 of envelope 3 2 is closed by a reentrant stem 4, having a press 5 through which extend stilf wire inleads 6 and 7, connected at their outer ends to threaded shell 8 and center contact 9 of a conventional screw base.

The inner envelope, or arc-tube, 12 which forms the discharge lamp tube proper is, for example, made of sintercd, high-density, polycrystalline alumina ceramic such as is disclosed and claimed in US. Pat. No. 3,026,210 Coble, Transparent Alumina and Method of Preparation." Alternatively, any other similar light-transmissive, high-temperature ceramic which is resistant to chemical reaction with sodium may be used. Tungsten electrodes 16 and 17 are supported in position at the upper and lower ends of tube 12 by dummy end cap 18 and exhaust end cap 19, respectively, hermetically sealed to the arc-tube. The shanks of electrodes 16 and 17 are supported from niobium end caps through niobium tubes 20 and 21, respectively, which project hermetically through the end caps. Each electrode may, for example, comprise a double-wound tungsten wire coil, with interstices filled with electron-emitting material, suitably alkaline earth oxides including barium oxide, for example. Tube 21 is pierced through at 22 and is used as an exhaust tube during manufacture and to introduce an inert gas filling such as xenon and the ionizable dissociable light-emitting charge into the arc-tube. The lower end of tube 21 is thereafter hermetically pinched off by a cold weld at 23. A quantity of the charge, somewhat exaggerated for ease of illustration, is shown at 24 in the lower end cap 19. Excess charge may also collect in the projecting portion of exhaust tube 21, which tends to be at a cooler temperature in operation of the lamp.

The upper end of arc-tube 12 is supported within en velope 2 by means of band and rod frame 25 which extends from inlead 6 at the stem end to a dimple '27 at the dome end to which it is anchored by a resilient clamp 28. The lower end of the arc-tube is connected in inlead 7 by band 29 and a short length of rod 30. A strap 31 mechanically interconnects rods and 30 to stiffen the assembly, while insulator 32 prevents a short-circuit. The interenvelope space is evacuated in order to conserve heat and this is done prior to sealing off the outer envelope. Thereafter, a conventional getter, suitably barium metal powder pressed into channeled rings 33, is flashed after sealing in order to assure a good vacuum. Alternatively, a high molecular weight noble 0r inert gas may be used.

In order to prevent the presence of oxygen within arctube 12 and the consequent formation of sodium aluminate or similar compounds with the alumina of the arctube 12 or with constituents of the seals, an oxygen getter may be incorporated within the arc-tube or within the inter-bulb space, as for example as a deposit of a reactive metal getter 43 on tab 44 suspended by rod 46 from upper support 25. Such getters are meals having, over a given temperature range, a free energy of reaction in the formation of the oxide thereof per mole of oxygen more negative than the free energy of formation of sodium aluminate per mole of oxygen within the alumina arctube. Such getters utilized in high-pressure vapor arclamps including sodium in the vapor state are not my sole invention, but are disclosed and claimed in the copending application of Hanneman, Jorgensen, and Charles entitled High Pressure Sodium Vapor Lamp, Ser. No. 616,538 now abandoned, filed Feb. 2, 1967, of which application Ser. No. 753,143, filed Aug. 16, 1968, is a continuation-in-part and both of which are assigned to the present assignee.

The inert gases which may be used for starting purposes in the arc-tubes of electric discharge lamps in accord with the invention, in increasing order of atomic weight, are helium, neon, argon, krypton, and xenon and preferably xenon. Since xenon is expensive, for reasons of economy, krypton or argon may be used alone or adimtted with xenon, but generally at some sacrifice of efficacy. The specific Xenon pressure for maximum efiicacy in a given lamp depends upon many factors. It has been determined, however, that pressures of xenon from 5 to torr are acceptable, however, lamps in accord with the present invention are preferably constructed using approximately 20 torr OI xenon.

Lamps in accord with the present invention, the structure of which is described hereinbefore, are substantially that as disclosed in the above-identified Schmitt patent. The inventive concept involved, in accord with the present invention, relates to changes in charge constituents and parameters of operation which provide for the achievement of markedly improved spectral emission characteristics, and, not only a retention of the high efiicacies of the characteristic of the high-pressure sodium vapor lamp, but in many instances, an increase in the efiicacv thereof,

due to additional radiating constituents to the metalliccharge contained within the lamp envelope.

Basically, in accord with the present invention, the fill charge of the lamp contains predetermined quantities of sodium and at least two, and preferably all, of the additional materials of the group including thallium, cadmium, and mercury.

Prior art as far back as 1916, and the writings and patents of C. P. Steinmetz, for example US. Pat. No. 1,025,932, have recognized that, in theory, it might be possible to correct the spectrally undesirable emission of a given metallic vapor arc-lamp by the addition of constituents which supply, when operated alone, a spectral emission characteristic which is lacking in that of the lamp desired to be improved. Thus, for example, one may readily suggest that the high-pressure sodium vapor lamp of the Schmitt patent, being somewhat deficient in the green area of the visible spectrum at its operating temperatures, could be improved by the addition of a green-emitting material, such as thallium to remedy this deficiency. As a practical matter, however, one may not simply add an amount of thallium, predetermined from its own vapor pressure and temperature characteristics, to such a lamp and achieve the desired objectives. Such additions not only fail to correct the spectral emission, but may deleteriously affect the lamp efficacy and voltage.

From my work I have determined that the thermodynamics of the liquid-vapor equilibrium conditions existing within a lamp of the metallic vapor arc discharge type contains a number of parameters which are mutually dependent, and that to change one parameter, greatly affects other parameters. Thus, for example, the addition of thallium to a sodium vapor lamp, greatly affects the characteristics of the sodium already present within the lamp as contributing to the spectral emission.

Due to the foregoing, and to the number of the variables in such lamps, including the number and quantity of additives, the lamp operating temperature, as well as current and voltage therethrough, it is a virtual impossibility to determine the amount and identity of the constituents of a charge in a metal vapor arc-lamp which will achieve white light at efiicacies in excess of 100 lumens per watt by trial and error methods.

In general, I have discovered that the activity of a material, which is defined as the ratio of a partial pressure within an enclosed space over an alloy, containing the given material, to the partial pressure of the same material in the absence of any other constituent in equilib rium with its own liquid or solid phase, may be greatly affected by the presence of other constituents within the system.

FIG. 2 of the drawing represents a typical activity diagram for a simple two-constituent system of sodium and thallium. In FIG. 2, the axis of abscissas runs from Zero percent thallium and 100 percent sodium, at one end, to 100 percent thallium and zero percent sodium at the other end. The left axis of ordinates runs from an activity for sodium from 0 to 1.0 and right hand axis of ordinates runs from an activity of from 0 to 1.0 for thallium, the

activities being normalized to take into account the differing vapor pressures of the pure constituents. In FIG. 2, the diagonal lines represent the ideal linear variation which might be expected of the activities, while the curved lines represent the actual activities.

As may be seen from the graph of FIG. 2, the addition of low percentages of thallium to a charge originally consisting of sodium, produces a very low activity of thallium and hence the corresponding vapor pressure of thallium over the liquid will be much less than over pure thallium and also substantially less than over an ideal linear type solution. Similarly, at very low thallium concentrations, the concentration and, hence pressure, of the sodium in the vapor phase is not seriously affected. (The same is true for thallium pressures for small additions of sodium at the high thallium end of the diagram.) Of great interest, however, is the fact that by the time a quantity of thallium has been added which is sufficient to cause a significant presence of thallium in the vapor phase, the amount of sodium in the vapor phase has, by the addition of thallium, been greatly decreased, so that the activity of both the sodium and thallium is very much less than would be present by a similar addition of either one to a lamp in the absence of the other. This foregoing brief description of a simplified two-constituent system illustrates the type of complexities due to the presence of three or more simultaneous constituents in the charge of a metal vapor arc-lamp. For this reason, it is obvious that one cannot simply add a predetermined quantity of a given material to the lamp charge, based upon its own vapor pressure characteristics, to achieve its radiation from the lamp without drastically affecting the contribution of the other constituents to such an extent that efficacy and and color rendition may be seriously and detrimentally affected.

I have further found, quite unexpectedly, that an additional governing criterion, in the achievement of improved spectral emission characteristics and the maintenance of high-lumen efiicacy in metal vapor arc-lamps utilizing two or more constituents is that, in order that the desired proportion, as determined analytically, of a given constituent in the vapor phase within the lamp envelop be present, some of that constituent must exist in excess in a liquid phase in equilibrium with the vapor phase. I have further determined that, if one constituent of a charge of a plurality of metals in a metal vapor arc-lamp is to be present in excess in the liquid phase within the lamp envelope, then it is required that some of each of the other constituents also be present in excess in the liquid phase. When these criteria are met, and only then, is it possible to control properly the desired amount of partial pressure of each of the given constituents within the charge of a metallic vapor arc-lamp.

I have further determined that the operating temperature of the metallic vapor arc-lamp must especially take into consideration, and be consistent with, the vapor pressure characteristics of the lowest vapor pressure metal constituent of the charge. As used herein, the phrase operating temperature of the lamp is meant to denote the temperature of the liquid-metal alloy of the charge which is in equilibrium with the vapor phase within the lamp envelope. In general, this is also the temperature of the coldest portion of the bulb envelope wall, although by extreme, often unnecessary, and unusual seal configurations, it is conceivable that the end seal portions of the,

envelope could be so designed as to cause a thermal gradient through the liquid phase which would cause the coldest portion of the bulb wall to be somewhat, although not significantly, different from that of the liquid surface in equilibrium with the vapor phase, which is that portion referred to herein as defining the operating temperature of the lamp. As an example of the foregoing temperature relationship, it is to be noted that, should thallium be a constituent utilized in the charge of lamps in accord with the present invention, due to its relatively low vapor pressure as compared with the other materials utilized herein, operating temperatures must be raised to approximately 750 C. while, on the other hand, should thallium not be one of the materials utilized, it is possible to operate the lamp at temperatures of less than 700 C.

Another parameter which is of great importance, in the operation of the lamps in accord with the present invention, is the operating temperature of the lamp. Although a given range of metallic constituents of the charge added to the lamp envelope may be sufficient to achieve the desired spectral emission at a particular temperature, such a range of constituents is not necessarily correct for the same lamp operated at a substantially different temperature. Once this is appreciated, however, it does not present an insurmountable problem, because, once the useful range of constituents for a given ternary or quaternary system has been determined for a given temperature, and the direction of the variation of the quantity of any constituent necessary to produce the desired spectral output and efiicacy of operation is known, the correct range of percentages for operation at different temperatures may be determined. In general, the criteria of the lamps of the invention are as follows:

(1) Irrespective of the number of constituents in the charge, at least some of each of the constituents must remain in the unvaporized state at the lamp operating temperature.

(2) The lamp operating temperature and composition of the added charge must be chosen so as to cause the desired proportion of each of the constituents to be in both the liquid and vapor phases individually within the lamp envelope during lamp operation.

(3) The efficacy of the radiation from the lamp under operating conditions should be of the order of lumens per watt or higher.

(4) The spectral emission of the lamp must have one or more components with peak intensities in the range of 5,050 to 5,500 AU, and having a corresponding peak intensity which is at least 10 percent of the peak intensity of the total radiation.

In achieving the foregoing criteria, I cause the charge within the lamp envelope to be chosen from the quaternary system including sodium and two or more of thallium, cadmium, and mercury. Although sodium and two other constituents may be present. I prefer that all constituents be present. When all constituents are present, a useful operating temperature range of approximately 750 C. to 1,000 C. may be utilized to achieve highly efficacious, substantially-white radiation. In the case of a quaternary system in which all constituents are present, the operating range for all temperatures is obtained when the partial pressures of the constituents within the lamp are within the following ranges and satisfy the following criteria.

TABLE I Element: Partial pressure, atm. Sodium 1 10 -0.6 Thallium 0-10 Cadmium 0-1.5 Mercury 03.0

Since thallium, cadmium, or mercury may be absent or present in a very small proportion, the following additional conditions must be met.

.(1) The sum of the partial pressures of mercury and cadmium are in the range of from approximately 0.05 atm. to 4 atm.

(2) Should the partial pressure of thallium be less than approximately 1X10 atm., the pressure of cadmium is euqal to or greater than approximateliy 0.1 atm.

In the quaternary system, as is indicated hereinbefore, I prefer that all four constituents be present, and it is preferred that they be present in the charge in such quantities as to cause the vapor in equilibrium with the nonvolatilized portions thereof have the following partial pressures:

TABLE 11 Element: Partial pressure, atm. Sodium 3 X 10- -03 Thallium 10 -3 X 10 Cadmium 0.1-0.6 Mercury 0.05-1

Since all of the constituents are present in the ranges set forth in Table II and since the pressure of thallium does not fall below 10* atm., no further conditions need be added, except that the lamp operating temperature be such as to cause the partial pressures within the envelope to fall Within the above ranges and that the spectral emission have the peaks within the specified ranges as indicated hereinbefore. Such a temperature range is approximately 750 C. to l,000 C.

A further added requirement is placed upon the amount of sodium added to lamps in accord with the present invention. This is because sodium, being a reactive metal, tends to react with the high-density alumina comprising the lamp envelope wall, in a preferred embodiment, to form a sodium aluminate (NaAIO which causes thermal problems with the envelope wall and further reduces the light-transmissive characteristic thereof. In order to avoid the formation of sodium aluminate or the formation of related complex aluminates from seal materials, the sodium partial pressure within the arc-tube must be maintained below specified values which are a function of temperature. The basic chemical reaction for this reaction is 3Na+2Al O +3NaAlO +Ai the maxi-mum tolerable sodium pressure is governed by the relationship g1o Nn 5-61 X where P is in atmospheres and T is in degrees K. Some expletive maxima are as follows:

TABLE III Maximum partial Temperatures, C.: pressures of sodium, atm. 680 0.05 780 0.30 900 0.90 1,000 2.40

This, together with the foregoing, constitutes the limits of constraint upon the addition of metallic materials to the charge of the lamps of the present invention.

In lamps of the invention, the materials other than sodium, which is a necessary constituent, which are chosen to comprise the charge from which the light-emissive arc receives the arc-sustaining vapor, are added for the following reasons.

Mercury is utilized because the sodium-mercury molecular complex radiates a valuable constituent in the red portion of the visible spectrum. Additionally, in the presence of mercury, the current-voltage-time starting characteristics are such as to facilitate rapid start of the main lightemitting arc. Mercury, being chemically inert, is useful in the lamp, in that the mercury atoms tend to buffer thermally the lamp envelope wall from the highly-corrosive sodium vapors and makes it possible to substantially constrict the light-emitting arc to approximately two thirds to three quarters of the inner diameter of the arctube. Thus, although arc temperatures may raise to the order of 4,000 C. and higher, it is readily possible to maintain the hottest portion of the envelope wall no hotter than 1,300" C. at the absolute and 1,200 C. to l,250C. in a preferred embodiment. A very useful and unobvious result of the presence of mercury in a lamp envelope containing both sodium and thallium under the conditions of the present invention, wherein some of each constituent of the charge remains non-volatilized, is that the mercury uniquely controls and makes possible the optimization of the thermodynamics of the sodium-thallium chemistry at ,the temperatures and pressures utilized in the lamps of the present invention.

Thallium and cadmium are found to be useful in the lamps of the invention for the following reasons. Neither thallium or cadmium contribute in any way to the adverse attack of the vapors upon the alumina envelonpe wall. Similarly, they do not contribute to any deleterious action upon the seals utilized to pass current-conducting electrodes into the arc-tube. Additionally, the melting point and ionization efficiencies of thallium and cadmium are within the ranges such as to permit their use in a quaternary system and achieve the desired chemical and luminescent output results. Finally, thallium and cadmium are useful in the lamps of the present invention in that no additional getters are necessary, other than those normally provided in a high-pressure sodium vapor lamp, to remove deleterious constituents such as hydrogen, oxygen, or water vapor While the foregoing general criteria have been set forth as a framework within which the constituents of the charge and operating temperatures of lamps in accord with the present invention may be varied, more specific criteria are hereinafter set forth with regard to specific systems in specific volumes and configurations of lamp envelopes. It will be appreciated, however, that for a given temperature of operation, partial pressures and atom fractions of constituents in the excess are substantially independent of lamp volume. As indicated hereinbefore, the lamps of the present invention, generally involve an outer envelope containing an inner envelope or arc-tube in which the arc-electrodes and metallic charge are contained. Structural support for the inner envelope is also contained within the outer envelope, as may be a suitable gaseous atmosphere to help dissipate intense heat of the inner envelope and maintain optimum operating conditions. The inner envelope is generally constructed of a high-density alumina ceramic or similar light-transmissive polycrystalline ceramic having high resistance to active metals and metal vapors such as sodium and sodium vapor at elevated temperatures of the order of l,200 C., or higher, and being at least translucent and light transmissive to radiation within the visible spectrum.

A 400 watt lamp, constructed in accord with the present invention, utilizes a high-density alumina polycrystalline cylindrical arc-tube having a 7 mm. inside diameter a length of approximately 11 cm., and an interelectrode gap of approximately 8 cm.; and defining a volume of approximately 4 cubic centimeters. A quaternary system of sodium, thallium, mercury, and cadmium utilized at a lamp operating temperature of approximately 780 C. may conveniently have the following ranges of partial pressure within the lamp envelope.

TABLE IV Element: Partial pressure, atm. Sodium l 10' 0.3 Thallium 0-4 X 10 Cadmium 0-0.8 Mercury 04.0

Due to the possibilities of zero partial pressure of either of thallium, cadmium, or mercury, the following conditions must also be observed.

(1) Should the partial pressure of thallium be less than approximately 1 l0- atm., the partial pressure of cadmium is at least approximately 0.1 atm. As a corollary to this requirement, should the partial pressure of cadmium be less than approximately 0.1 atm., the partial pressure of thallium is at least as high as approximately 1x 10- atm.

(2) The sum of the partial pressures of cadmium and mercury is at least approximately 0.1 atm., but does not exceed approximately 4.0 atm.

Within the framework of the foregoing suitable ranges of partial pressures of the foregoing materials utilized at a lamp operating temperature of approximately 780 C., a preferred operating range is as follows.

TABLE V Element: Partial pressures, atm. Sodium 3.2 10" -0.25 Thallium l 3.0 Cadmium 0-0.6 Mercury 00.8

Since either cadmium or mercury may have zero partial pressure in the foregoing ranges, the following additional criterion must be met.

The sum of the partial pressures of mercury and cadmium is within the range of approximately 0.1 to 0.8 atm.

A specific example of one embodiment of a 400 watt lamp in accord with the present invention falling Within the framework of the foregoing generalized criteria, contains the following partial pressures.

TABLE VI Element: Partial pressures, atm. Sodium 6 X 10* Thallium 2.2)(10 Cadmium 0.23 Mercury 0.12

The foregoing partial pressures may be achieved within the lamp having the dimensions given above with the following proportions of the constituents Within the charge.

A 400 watt, 4 cc. arc-tube lamp having this charge utilized a standard commercial ballast such as that obtainable with Power Door, Cat. No. 962490D3 from General Electric Company, Outdoor Lighting Department, Hendersonville, N.C., for high pressure sodium vapor lamps, and sustained an arc voltage of 101 volts, resulting in a lamp operating temperature of 780 C. The efficacy of this lamp was 110 lumens per watt with near-white spectral emission.

As yet another set of governing criteria, the following data set forth the ranges of partial pressures in the broader aspects and also in the preferred embodiments for a quaternary system involving sodium, cadmium, thallium, and mercury at a lamp operating temperature of approximately 900 C. At this temperature, the broad suitable ranges of the constituents and conditions, in terms of the partial pressures of the constituents produced within the lamp envelope at the operating temperature, are

TABLE VIII Element: Partial pressure range, atm. Sodium 0.0010.6 Mercury 03 Cadmium 0-1 Thallium 0-3 1() Since it is possible that any one of the above-mentioned constituents in the group, mercury, cadmium, and thallium, can be absent, the following additional conditions apply.

(1) The sum of the partial pressures of cadmium and mercury is from approximately 0.05 to 3 atm.

(2) Should the thallium partial pressure be less than approximately 3X 10 atm., the partial pressure of cadmium is greater than approximately 0.1 atm.

At approximately 900 C., in a specific embodiment having superior, white spectral emission, it is desired that the ranges of the added constituents be such as to provide the following partial pressures of each of the constituents.

TABLE IX Element: Partial pressure, atm. Sodium 0.0050.35 Cadmium 0. 10.6 Mercury 0.05-l .0 Thallium 1 10 3 x 10- Within the general framework of the aforementioned criteria, the following represents a specific example of partial pressures, quantities of elements in the remaining reservoir, and the dosage thereof necessary to attain the required partial pressure, at a lamp operating tempera- For purposes of simplicity, in accord with the present invention, I find that certain ternary systems which are subspecies of the above-mentioned quaternary system, are suitable to provide lamps of high efiiciency and pleasing spectral response having white or sun-like spectral emission.

One such ternary system comprises the vapors of sodium, thallium, and mercury. One particular advantage of this system is that fewer metallic elements are required for the fabrication thereof.

For the range of operating temperatures of approximately 750 C. to 1,000 O, the values of partial pressures and corresponding atom fractions of the constituents in the reservoir of each of the constituents of this ternary system are as follows.

TABLE XI Atom fraction in reservoir Partial ressure Element; at 750 C. at 1,000 O (atm.) p

Sodium 13-. 3 .02-. 52 1 x 10- to 06. Mercury. 015-. 25 015-.15 0.05 to 4. Thallium... 58-. 33-. 98 1 x 10" to 1 x 10*.

For temperatures intermediate 750 C. and 1,000 C., the ranges of permissible atom fractions are intermediate those listed above. For any given charge the sum of the atom fractions is unity.

Preferably, however, the lamps of the present invention utilizing this ternary system are operated within the following range of partial pressures and atom fractions A specific example of a filling utilizing only sodium, mercury, and thallium and producing high-efliciency, near-white light at a lamp operating temperature of approximately 900 C. is as follows.

TABLE XIII Atom fraction of Weight of Con- Partral pressure excess element stituent added Element (atm.) in reservoir to charge (mg) Sodium 0. 01 147 1. 0 Thallium. 0. 0022 835 50. 6 Mercury 0. 3 018 3. 0

In yet another embodiment of the present invention, the ternary system of mercury, sodium, and cadmium may be utilized. This particular system is advantageous in that, containing no thallium, the minimum operating temperature may be substantially lowered, as for example, to approximately 650 C. while maintaining sufficiently high vapor pressures of sodium, cadmium and mercury to achieve acceptable operating characteristics. The general useful ranges of partial pressures of consituents of the ternary system herein is set forth in the following table for temperatures within the range of approximately 650 C. to 900 C.

TABLE XIV Element: Partial pressure, atm. Sodium 0.01-0.2 Cadmium 0.l-1.0 Mercury 0-4.0

As a special sub-category of the above system, is the sodium-cadmium binary which can also produce operative lamps. In this case the cadmium pressure should lie in the range of 0.1 to 1.0 atm. The total pressure of mercury and cadmium must not exceed 4 atm. in any case.

Within the framework of the foregoing range of useful combinations, the following table sets forth the preferred range of constituents for the ternary system involving sodium, cadmium, and mercury, which range has been found to produce high-efiicient, near-white-light-emitting,

metallic vapor arc-lamps.

TABLE XV Element: Partial pressure, atm. Sodium 0.015-005 Mercury 0l.00 Cadmium 0.1-0.25

The foregoing ranges of pressures of the ternary system involving mercury, cadmium, and sodium are useful at a lamp operating temperature of as low as 680 C. to produce the desired result.

One specific embodiment of a lamp in accord with this embodiment of the invention, involves a lamp with an operating temperature of approximately 680 C. having the same geometrical configuration and dimensions as set forth above, namely, a 7 millimeter ID wall and a volume of approximately 4 cubic centimeters, is constructed so as to have the following partial pressures of each of the constituents relating to the partial pressures of each of the constituents relating to the listed atom fraction of each of the constituents in excess of the liquid reservoir in the envelope and may be achieved by the addition of the listed weights of each of the constituents to the lamp envelope.

TABLE XVI From the foregoing, it is apparent that other ternary systems including sodium and two other elements of the group including thallium, cadmium, and mercury may be devised and, with appropriate charge compositions and lamp operating temperatures, utilized to emit highefficiency, chromatically-pleasing light.

Although thallium and cadmium have been selected as the ideal green component source elements in accord with the present invention, and have been described in a quaternary system, it is possible to follow the teachings of the present invention to add certain other metallic vapor-radiating species, and to use more than four species simultaneously, providing that the thermodynamic criteria set forth herein are followed.

Other specie radiating in the green spectral range with strong lines are magnesium, barium, strontium, copper, silver, scandium, lanthanium, cerium, praseodymium, and samarium. Thallium and cadmium, as green emitters, have been chosen from the others above, as well as other green emitters, for combination with mercury and sodium, under the conditions and constraints set forth hereinbefore, as contributing to an ideal system for the prodnotion of white-light-emitting vapor arc lamps having the ideal combination of good chromatic characteristics, high efliciency, and good maintainance. Their selection is due to their unusual and superior compliance with the thermodynamic criteria set forth herein.

From the foregoing general criteria, detailed description and specific examples of operative lamps in accord with the present invention, it is self-evident that I have provided a new and improved type of metal vapor arclamp containing sodium and other chemically compatible metal vapors present in thermodynamically consistent porportions, as to produce sun-like, white light of very high efficacy, in excess of lumens per watt.

The lamps of the invention are characterized by the identity and amount of the metals present in the vapor phase, by the lamp operating temperature, and by the all-important requirement that an excess of each metal in the charge be present in the liquid reservoir in equilibrium with the vapor phase.

Thus, the lamps of the present invention may be characterized as excess limited pressure lamps as contrasted to many prior art metallic vapor arc-lamps which may be characterized as amount limited pressure lamps. This difference has at least two distinct features which contribute to the superiority of lamps of this invention over prior art amount limit pressure lamps.

In amount limited pressure lamps, any partial removal of any constituent from the vapor phase, as by clean-up or chemical activity, reduces the partial pressure of that constituent, changes the thermodynamics of the lamp operation, and deleteriously afiects spectral emission and efficacy.

Additionally, it may be noted that the partial pressures utilized are, in many instances quite low. If an amount sufficient to yield only such a partial pressure is added to a 400 watt or a 1,000 watt lamp, for example, additions of less than one milligram of at least sodium must be made. Obviously, in addition to the difficulty of such a task, accurate control is extremely difficult to achieve.

The foregoing difiiculties are of great importance, for, as disclosed hereinbefore, if even one metallic charge constituent is amount limited, then so must be the others.

While the invention has been set forth hereinbefore with respect to certain preferred embodiments and specific examples, many modifications and changes will readily occur to those skilled in the art. Accordingly, by the appended claims, I intend to cover all such modifications and changes as fall within the true spirit and scope of the invention.

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

1. An electric arc discharge lamp comprising:

(a) an hermetically-sealed, light transmissive ceramic envelope;

(b) a pair of solid arc-electrodes disposed in spaced relationship with one another and defining therebetween a discharge path for a high-current light-emissive are;

(c) a filling within said envelope of a starting gas having a relatively low ionization potential;

(d) a thermally ionizable light-emissive charge within said envelope and including sodium and at least two of the materials selected from the group consisting of mercury, thallium, and cadmium,

(d said materials being present in concentrations such that a portion of each constituent of said 13 charge remains unvaporized at lamp operating temperatures,

(d such materials being present in said charge in quantities sufficient to provide at lamp operating temperatures of approximately 650 C. to 1,000 C. the following partial pressures of said materials within said envelope,

(d sodiumfrom approximately 1X10- to less than 0.6 atm. (d mercury-from to approximately 3 atm.

(d2c) thalliumfrom 0 to approximately atm.

(d cadmiumfrom 0 to approximately 1.5 atm.

(d said constituents being present within said envelop subject to the following conditions,

((1 the sum of the partial pressures of cadmium and mercury must be from approximately 0.1 to 4 atm.,

(d should the partial pressure of thallium within the said envelope be less than approximately 1x10 atm., the partial pressure of cadmium must be at least approximately 0.1 atm.,

(d should the pressure of cadmium be less than approximately 0.1 atm., the partial pressure of thallium must be greater than approximately 1 10- atm.;

(e) the total spectral emission of said lamp containing at least one distinct component having a peak intensity within the wavelength range from 5,050 A.U. to 5,500 A.U. and having a peak intensity of at least 10 percent that of the peak intensity of said total spectral emission.

2. The lamp of claim 1 wherein said charge materials are present within said arc-tube in quantities to produce therein at lamp operating temperatures of approximately 750 C. to 1,000 C. the following partial pressures:

(a) sodium-approximately 3X10 atm. to 0.3 atm.

(b) thalliumapproximately 1X10 atm. to 3X10 atm.

(c) cadmiumapproximately 0.1 atm. to 0.6 atm.

(d) mercury-approximately 0.05 atm. to 1.0 atm.

3. The lamp of claim 1 wherein said charge materials are present within said arc-tube in quantities sufficient to produce therein at a lamp operating temperature of approximately 780 C. the following partial pressures:

(a) (a sodium-approximately l l0 atm. to 0.3

atm.

(a thallium -approximately 0 to 4X10" atm.

(a cadmiumapproximately 0 to 0.8 atm.

(a mercury-approximately 0 to 4.0 atm.

(b) said charges being present in said arc-tube subject to the following conditions:

(b should the partial pressure of thallium be below approximately 1 10- atm., the partial pressure of cadmium is at least as high as approximately 0.l atm.,

(b should the partial pressure of cadmium be below approximately 0.1 atm., the partial pressure of thallium is at least as high as approximately 1 l0 atm.,

(b the sum of the partial pressures of mercury and cadmium are within the range of approximately 0.1 atm. to 4.0 atm.

4. The lamp of claim 3 wherein the charge materials are present within said arc-tube in quantities sufiicient to produce therein at a lamp operating temperature of approximately 780 C. the following partial pressures:

(a)(a sodiumapproximately 3.2 10* atm. to

0.25 atm.

(a thallium-approximately 1X 10- atm. to 3 10* atm.

(a cadmiumapproximately 0 to 0.6 atm.

(a mercuryapproximately 0 to 0.8 atm.

(b) said materials are present in quantities such as to satisfy the following condition:

(b the sum of the partial pressures of mercury and cadmium is within the range of approximately 0.1 to 0. 8 atm.

5. The lamp of claim 4 wherein said charge materials are present within said arc-tube in quantities to produce therein the following partial pressures at said lamp operating temperature:

(a) sodiumapproximately 6X 10- atm.

(b) thalliumapproximately 2.2 10- atm.

(c) cadmiumapproximately 0.23 atm.

(d) mercuryapproximately 0.12 atm.

6. The lamp of claim 1 wherein said charge materials are present within said arc-tube in quantities sufficient to produce therein at a lamp operating temperature of approximately 900 C. the following partial pressures:

(a) (a sodiumapproximately 0.001 atm. to 0.6 atm.

*(a mercury-approximately 0 to 3 atm.

(a cadmiumapproximately 0 to 1 atm.

(a thalliumapproximately 0 to 3 10- atm.

(b) said materials being present in quantities suflicient to yield partial pressures which also satisfy the following conditions:

(b the sum of the partial pressures of cadmium and mercury is from approximately 0.05 to 3 atm.,

(b should the partial pressure of thallium be below approximately 3x10- atm., the partial pressure of cadmium is at least approximately 0.1 atm.,

(b should the partial pressure of cadmium be less than approximately 0.1 atm., the partial pressure of thallium is at least approximately 3 10- atm.

7. The lamp of claim 6 wherein said charge materials are present within said arc-tube in quantities sufficient to produce therein at said lamp operating temperature the following partial pressures:

(a) sodium-approximately 0.005 atm. to 0.35 atm.

'(b) cadmiumapproximately 0.1 atm. to 0. 6 atm.

(c) mercury-approximately 0.05 atm. to 1.0 atm.

(d) thalliumapproximately 1 1=0- atm. to 3X10 atm.

8. The lamp of claim 7 wherein said charge materials are present within said arc-tube in quantities sufiicient to produce therein at said lamp operating temperature the following partial pressures:

(at) sodium-approximately 0.3 atm.

(b) thalliurnapproximately 4.7)(10 atm.

(c) mercury-approximately 0.3 atm.

(d) cadmiumapproximately 0.12 atm.

'9. An electric arc discharge lamp comprising (a) an hermetically-sealed, light-transmissive ceramic envelope;

(b) a pair of solid arc-electrodes disposed in spaced relationship with one another and defining therebetween a discharge path for a high-current light-emissive arc;

(c) a filling within said envelope of a starting gas having a relatively low ionization potential;

(d) a thermally ionizable light-emissive charge within said envelope and including sodium, mercury, and thallium,

(d said materials being present in concentrations such that a portion of each constituent of said charge remains unvaporized at lamp operating temperatures,

(d such materials being present in said charge in quantities sufficient to provide at lamp operating temperatures of approximately 750 C. to 1,000 C. the following partial pressures of said materials within said envelope,

15 (d sodium-approximately 1 10 atm.

to 0.6 atm. (d mercury-approximately 0.05 atm. to

4 atm. (d thallium-approximately 1X10 atm.

to l atm.

'(e) the total spectral emission of said lamp containing at least one distinct component having a peak intensity within the wavelength range from 5,050 A.U. to 5,500 AU. and having a peak intensity of at least 10 percent that of the peak intensity of said total spectral emission.

10. The lamp of claim 9 wherein said charge materials are present within said arc-tube in quantities sufiicient at said lamp operating temperatures to provide therein the following partial pressures:

(a) sodium-approximately 2 l0- atm. to 0.3 atm.

(-b) mercury-approximately 0.08 atm. to 0.8 atm.

(c) thallium-approximately 2 10- atm. to

2.5 10- atm.

11. The lamp of claim 10 wherein said charge materials are present within said arc-tube in quantities sufficient at a lamp operating temperature of approximately 900 C. to provide therein the following partial pressures:

(a) sodium-approximately 0.01 atm.

(b) thallium-approximately 0.0022 atm.

(c) mercury-approximately 0.3 atm.

12. An electric arc discharge lamp comprising:

(a) an hermetically-sealed, light-transmissive ceramic envelope;

(b) a pair of solid arc-electrodes disposed in spaced relationship with one another and defining therebetween a discharge path for a high-current lightemissive arc;

(c) a filling within aid envelope of a starting gas having a relatively low ionization potential;

(d) a thermally ionizable light-emissive charge within said envelope and including sodium, cadmium, and mercury;

(d said materials being present in concentrations such that a portion of each constituent of said charge remains unvaporized at lamp operating temperatures,

(d such materials being present in said charge in quantities suflicient to provide at lamp operating temperatures the following partial pressures of said materials within said envelope,

(() mercury approximately 0 to 4.0 atm.

(e) said charge materials further being present in quantities sufficient to produce a sum of the partial pressures of cadmium and mercury of approximately 0.1 atm. to 4 atm.,

(f) the total spectral emission of said lamp containing at least one distinct component having a peak intensity within the wavelentgh range from 5,050 A.U. to 5,500 AU. and having a peak intensity of at least 10 percent that of the peak intensity of said total spectral emission.

133. The lamp of claim 12 wherein the charge materials are present within said arc-tube in quantities suflicient to provide therein at said lamp operating temperatures the following partial pressures:

(a) sodiumapproximately 0.015 atm. to 0.05 atm.

(b) mercury-approximately 0 to 1.0 atm.

(c) cadmiumapproximately 0.1 atm. to 0.25 atm.

14. The lamp of claim 13 wherein said charge materials are present within said arc-tube in quantities sufficient to produce therein at lamp operating temperatures of approximately 680 C. the following partial pressures:

(a) sodium-approximately 0.04 atm.

(b) cadmium-approximately 0.14 atm.

(c) mercury-approximately 0.10 atm.

References Cited UNITED STATES PATENTS 3,248,590 4/1966 Schmidt 313-184 3,384,798 5/1968 Schmidt 313-184 3,453,477 7/1969 Hanneman et al 313-184 JAMES W. LAWRENCE, Primary Examiner P. C. DEMEO, Assistant Examiner US. Cl. X.R. 313-225, 229

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