METHOD OF DOSING THE ARC TUBE OF AN UNSATURATED HIGH PRESSURE SODIUM LAMP

Abstract: An unsaturated vapor type high pressure sodium lamp and fabrication process includes an arc tube within an evacu­ ated envelope and a dosage of amalgam, selected from a plurality of doses, located within the arc tube whereby the lamp dosage has a deviation less than the deviation obtain­ able with single doses. The lamp is fabricated by selecting and depositing multiple doses having a standard deviation into an arc tube to provide a dosage of reduced deviation with respect to the standard deviation of single doses.

Description: "UNSATURATED VAPOR HIGH PRESSURE SODIUM LAMP FABRICATION PROCESS AND STRUCTURE CROSS REFERENCE TO OTHER APPLICATIONS Concurrently filed Applications entitled "Unsaturated Vapor Pressure Type High Pressure Sodium Lamp", bearing Attorney's Docket No. 24,340 corresponding to U.S. Serial No. 473895, "Unsaturated Vapor High Pressure Sodium Lamp Getter Mounting", bearing Attorney's Docket No. 83-1-021 corresponding to U.S. Serial No. 473897 and "Arc Tube Fabrication Process" bearing Attorney's Docket No. D-24,833, corresponding to U.S. Serial No. 473896, relate to an arc tube and an arc tube fabricating process for unsaturated vapor high pressure sodium lamps. Also, concurrently filed Applications entitled "Unsaturated Vapor High Pressure Sodium Lamp Arc Tube Fabrication Process", bearing Attorney's Docket No. 83-1-036 corresponding to U.S. Serial No. 473894, "Arc Tube Dosing Process For Unsaturated High Pressure Sodium Lamps", bearing Attorney's Docket No. 24,517 corresponding to U.S. Serial No. 473892, relate to tube fabrication and arc tube dosing of unsaturated vapor type high pressure sodium lamps. This invention relates to the manufacture of high pressure sodium lamps of the unsaturated vapor type and more particularly to a process for dosing arc tubes for use in high pressure sodium lamps of the unsaturated vapor types. In the manufacture of high pressure sodium lamps, it is a normal practice to provide a tubular ceramic arc tube which is supported within an evacuated envelope which is, in turn, affixed to an ordinary screw-in type base member. Usually, the arc tube is filled with an excess amount of sodium in an attempt to compensate for undesired losses thereof during operation of the lamp. Thus, available high pressure sodium lamps are, for the most part, of the so-called saturated vapor type and are known to provide undesired variations in lamp voltage and color rendition because of this saturated condition. In an effort to eliminate or at least reduce such undesired effects, it has long been known that a lamp wherein the amount of sodium and mercury employed is only that which will become totally vaporized would provide the desired result. In other words, a high pressure sodium lamp of the unsaturated vapor type wherein sodium and mercury are introduced in only such an amount as to become totally vaporized is a highly desirable structure insofar as efficiency, cost of manufacture and enhanced lighting capability are concerned. However, one of the major problems encountered in the fabrication of unsaturated vapor high pressure sodium lamps is the introduction therein of the proper amounts of sodium and mercury. Since the sodium content is of a relatively small amount and sodium is such a chemically active material, it has been found most difficult to dose or introduce the proper amount thereof into the arc tube of an unsaturated vapor high pressure sodium lamp. One known suggestion for dosing an arc tube for a high pressure sodium lamp with the proper amount of sodium and mercury is set forth in U.S. Patent 4,156,550, issued to Furukubo et al on May 29, 1979. Therein, sodium azide (NaN3) was dissolved in a solvent, placed in a container and the solvent evaporated. Also, a mercury dispenser in the form of an Al-Zr-Ti-Hg alloy was placed in the container. Thereafter, the container was positioned within one exhaust pipe affixed to the arc tube, and this one exhaust pipe was closed or pinched off. Another exhaust pipe or tube was affixed to the other end of the arc tube and to an exhaust system. The exhaust tube having the container therein was heated to decompose the sodium and mercury-containing compounds and provide the desired sodium and mercury within the arc tube. Also, the arc tube was evacuated and re-filled with a starting gas. Another technique for dosing the arc tube of an unsaturated vapor type high pressure sodium lamp includes the employment of an amalgam of sodium-mercury within the arc tube of the lamp. Normally, the amalgam is in the form of a single pellet introduced into the arc tube to provide sodium and mercury in an amount which becomes totally vaporized. However, it has been found that such arc tube dosing process leaves something to be desired. More specifically, it has been found that a dosage achieved with a single pellet tends to have a relatively large deviation from the mean. In other words, an arc tube having a dosage effected by means of a single pellet has an undesirably large deviation from the mean whereupon undesired relatively large deviations in lamp voltages result. An object of the present invention is to provide an enhanced unsaturated vapor type high pressure sodium lamp and fabrication process. Another object of the invention is to improve the fabrication of an arc tube for an unsaturated vapor type high pressure sodium lamp. A further object of the invention is to improve the dosing of an arc tube in-an unsaturated vapor type high pressure sodium lamp. A still further object of the invention is to provide an improved structure and process for dosing an arc tube of an unsaturated vapor type high pressure sodium lamp. These and other objects, advantages and capabilities are achieved in one aspect of the invention by a process for fabricating an unsaturated vapor type high pressure sodium lamp wherein an arc tube supported within an evacuated glass envelope is dosed by selecting a plurality of doses of amalgam having a sum substantially equal to a given dosage for the lamp with the average value of dosages located within the arc tube having a standard deviation less than the standard deviation from the mean of dosages in lamps where said dosage was effected by a single pellet of sodium-mercury amalgam. The invention is illustrated by way of example in the accompanying drawings, in which: FIG. 1 is an elevational view of a preferred form of unsaturated vapor type high pressure sodium lamp of the invention and FIG. 2 is a comparison chart showing the change in deviation from the mean of a dosage effected by employing a plurality of doses. For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the accompanying drawings. Referring to the drawings, FIG. 1 illustrates an unsaturated vapor high pressure sodium lamp having a hermetically sealed and evacuated glass envelope 5 formed to fit into an ordinary screw-type base member 7. A glass stem member 9 is sealed to the envelope 5 and projects therein. Electrical conductors, 11 and 13 respectively, are sealed into and pass through the stem member 9 to provide electrical connections from the interior to the exterior of the glass envelope 5. An electrically conductive support member 15 is affixed to one of the electrical conductors 11 and has a pair of crossbars 17 and 19 affixed thereto at either end. Also, a plurality of spring-like members 21 are affixed to the support member 15 and formed for contact with the glass envelope 5. Moreover, a pair of getters 23 and 25 are attached to the support member 15 and serve to insure the integrity of the evacuated envelope 5. Disposed within the glass envelope 5 and supported by the crossbars 17 and 19 is an arc tube 27. This arc tube 27, preferably of a material such as polycrystalline alumina for example, includes an electrode 27 and 31 at either end thereof. One electrode 29 is affixed to and supported by the crossbar 17 while the other electrode 31 is insulatingly supported by the other crossbar 19, but electrically connected to the electrical conductor 13 passing through the stem member 9. Heat conserving elements 33 may be wrapped about the arc tube 27 at each end thereof in the vicinity of the electrodes 29 and 31 in order to reduce the heat differential thereat from the center of the arc tube 27. As to fabrication of the above-mentioned unsaturated vapor type high pressure sodium lamp for the arc tube 27, a dosage of sodium and mercury is selected which will become totally vaporized to provide the desired unsaturated condition. Actually, multiple doses of a sodium-mercury amalgam are selected in an amount to provide the desired and necessary dosage of the lamp. These multiple doses of amalgam are introduced into the arc tube 27 and provide an average dosage therein which has a deviation from the mean which is less than the deviation of the dosages of lamps dosed with single doses. Moreover, this reduced lamp dosage deviation is substantially equal to the standard deviation of single doses divided by the square root of the number of doses employed. In other words: EMI5.1 where x = standard deviation of lamp dosages employing multiple doses s = standard deviation of dosages employing single doses n - Number of doses employed. Referring to the comparative chart of FIG. 2, let it be assumed that a desired given dosage for an unsaturated vapor type high pressure sodium lamp is about 3-milligrams of a sodium-mercury amalgam having about 3.4% by weight of sodium. If It is assumed that the required given dosage is effected by a single 3-mg pellet, the present standard deviation of available materials is about 0.6 mgs (curve A). However, selecting a plurality of doses, three (3) In this example, each having a weight of about 1.0 mg and a standard deviation of about 0.6 mgs to provide the desired given dosage- results in a dosage having a standard deviation which is less than the standard deviation of dosage obtained by singly dosing lamps with 3 mg pellets. In other words, providing a 3-mg dosage selected from (3) pellets of l-mg each provides a dosage having a deviation of about 0.35 mg (Curve B). Thus, the dosage remains unchanged but the deviation from the mean thereof is enhanced by a plurality of doses in an amount where the sum of multiple doses is equal to the given dosage. Preferably, the doses are selected from a sodium-mercury amalgam having pellets which have weight in the range of about 0.05 to 20.0 mgs. Also, it is to be noted that pellets of a size smaller than the previously mentioned weights or powdered material tend to undesirably absorb moisture which is deleterious to the resultant lamp structure. On the other hand, pellets larger than the above-mentioned range seriously limit the number of pellets suitable to unsaturated high pressure sodium lamps and seriously limit the desired reduction in deviation from a mean of the lamp dosage. As a specific example, 400-watt unsaturated vapor type high pressure sodium lamps having an arc tube with a volume of about 4.3 cubic centimeters were fabricated. The arc tube was provided with a dosage which includes 3-pellets of about 0.62 mgs of sodium-mercury amalgam having therein about 3.4X sodium by weight. Also, xenon in an amount of about 15 Torr was dispensed into the arc tube which was thereafter sealed and located within an evacuated envelope to provide an unsaturated vapor type high pressure sodium lamp. These lamps exhibited a reduction in lamp voltage range from a value greater than about 30 volts for lamps dosed with a single pellet to a value less than about 8 volts for lamps dosed with multiple pellets. While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined by the appended claims.

Claims: CLAIMS 1. In a process for fabricating unsaturated vapor type high pressure sodium lamps having an arc tube positioned within an evacuated glass envelope and requiring a given amalgam dosage within the arc tube, the improvement comprising the steps of: selecting from a collection of amalgam pellets to provide multiple doses In an amount sufficient to provide said given amalgam dosage; and locating said multiple doses within said arc tube to provide said given dosage whereby the average dosage of said lamps will have a standard deviation less than the standard deviation of lamps dosed with a single pellet. 2. The unsaturated vapor type high pressure sodium lamp fabricating process of Claim 1 wherein said deviation of said dosage within said arc tube is substantially equal to the standard deviation of single pellet doses divided by the square root of the number of doses. 3. The unsaturated vapor type high pressure sodium lamp fabricating process of Claim I wherein said amalgam is in the form of sodium-mercury pellets. 4. The unsaturated vapor type high pressure sodium lamp fabricating process of Claim 1 wherein each of said multiple doses of amalgam has a weight in the range of about 0.05 to 20.0 mgs. 5. A process for dosing an arc tube of an unsaturated vapor type high pressure sodium lamp comprising the steps of: selecting a multiple of amalgam pellets in an amount sufficient to provide a required given lamp dosage, said pellets having a standarad deviation from a mean value; and depositing said multiple amalgam pellets within said arc tube to provide said required given lamp dosage with said dosage having a deviation from a mean value less than the standard deviation of dosages obtained by single doses of amalgam pellets. 6. The process for dosing an arc tube of an unsaturated vapor type high pressure sodium lamp of Claim 5 wherein said deviation (x) of said lamp dosage is substantially equal to the standard deviation (s) of said single doses of amalgam divided by the square root of the number of single doses (n). 7. The process for dosing an arc tube of an unsaturated vapor type high pressure sodium lamp of Claim 5 wherein said amalgam pellets are in the range of about 0.05 to 20.0 mgs 8. An unsaturated vapor type high pressure sodium lamp having an arc tube located within an evacuated envelope characterized by the improvement wherein an amalgam dosage of a plurality of amalgam pellets having a sum equal to a given dosage value has a standard deviation from a mean less than the deviation of lamps having a dosage of said given dosage value in the form of single amalgam pellets. 9. The unsaturated vapor type high pressure sodium lamp of Claim 8 wherein said plurality of doses are in each in the range of about 0.05 to 20.0 mgs. 10. The unsaturated vapor type high pressure sodium lamp of Claim 8 wherein said dosage of said lamp has a deviation from a mean value less than the deviation of single doses.

Inventors: WHITE, PHILIP J.
Applicants: GTE PRODUCTS CORPORATION
Application year: 1984
Main classification: H01J9
IPC codes: H01J9, H01J61