HIGH PRESSURE SODIUM LAMP HAVING IMPROVED COLORING RENDITION

Abstract: A high pressure sodium lamp operated at increased pressure and having an improved color rendition is dis­ closed. The improved high pressure sodium lamp has an arc tube which reduces the sodium losses typically ex­ perienced by high pressure operation.

Description: BIGH PRESSURE SODIUM LAMP BAVING IMPROVED COLORING RENDITION CROSS REFERENCE TO RELATED APPLICATION U.S. Patent Application Serial No. 444,777 having Attorney Docket No. LD 9000, filed concurrently herewith, for An Improved Efficacy High Pressure Sodium Lamp' of C. I. Mcvey, assigned to the same assignee of the present invention, is related to the present invention. BACKGROUND OF THE INVENTION This invention relates to a high pressure sodium vapor lamp utilizing alumina ceramic inner envelope and is particularly concerned with a high pressure sodium lamp having an inner arc tube of alumina ceramic operated at a relatively high pressure and having a desired color rendition which is maintained during the life of the high pressure sodium lamp. High pressure sodium vapor lamps have found widespread use during the past decade for commercial lighting applications, especially outdoor lightings. Such lamps are described in U.S. Patent No. 3,248,590 Schmidt, High Pressure Sodium Vapor Lamps. High pressure sodium lamps typically utilize a slender tubular inner envelope of transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or electron emissive material. The ceramic arc tube is generally supported within an outer vitreous envelope or jacket provided at one end with the usual screw base. The electrodes of the inner envelope are connected to terminals of the base, that is to shell and center contacts. The space between the inner and outer envelope is typically evacuated in order to conserve heat. The color rendition of standard high pressure sodium (EPS) lamps may be improved by increasing the internal sodium partial pressure within the arc chamber of the inner envelope formed of polycrystalline alumina. An improved color rendition of a standard EPS lamp may desire that the polycrystalline alumina arc chamber be operated with two to three times the internal sodium partial pressure relative to a conventional standard EPS lamp. In addition, for such increased sodium pressure the wall temperature between electrode tips within the arc chamber may be increased by IOOOC relative to a standard RPS lamp in order to offset the reduction of luminous efficacy which normally accompanies operation at increased sodium pressures. Bigher internal sodium pressures as well as higher arc chamber wall temperature typically shortens the life of improved color HPS lamps relative to their standard color, high efficacy EPS lamp counterparts. For example, manufacturers of improved color high pressure sodium lamps typically classify their products at 8,000-10,000 life hours as compared to 20,000-30,000 life hours for standard color products. The life reduction of improved color HPS lamps typically occurs by the major factors of sodium losses from the arc chamber due to reactions with the inner surface of the arc chamber and through migration of the sodium through the walls of the arc chamber. Sodium losses result in HPS lamp voltage rise, decrease of correlated color temperature of the HPS lamp, decrease of color rendition index and a color shift from a desired warm white color to an undesired pink color. In addition, the sodium which migrates through the arc chamber deposits on the inside wall of the evacuated outer chamber where reaction with the glass and evaporation products from the alumina arc chamber cause a brownish stain on the outer chamber, which, in turn, reduces the light output of the HPS lamp. The brownish stain further aggravates undesirable lamp voltage rise of the HPS lamp. Accordingly objects of the present invention are to provide means for reducing the loss of sodium from an improved color BPS lamp operated at relatively high pressures contributed by the sodium within the arc chamber of the BPS lamp, and to provide sta hign pressure sodium lamp having a color rendition index which remains substantially constant over the life of the improved color BPS lamp. These are the objects of the present invention and will become more apparent upon consideration of the following description of the invention. SUMMARY OF THE INVENTION This invention is directed to an improved color rendition high pressure sodium lamp operated at increased sodium pressures having means for reducing the sodium losses typically created by the increased sodium high pressure operation. Accordingly, in one embodiment of the present invention, a high pressure petal vapor lamp having an outer vitreous envelope enclosing an inner arc tube having thermionic electrodes sealed in its ends and containing a charge of vaporable metal having a sodium partial pressure in the range of 100 to 300 torr and xenon having a pressure in the range of 10 to 30 torr is disclosed. The inner arc tube has a chamber comprised of an optically translucent polycrystalline alumina ceramic chamber formed of the composition containing MgO and having an additive selected from the group consisting of ZrO2 and EfO2 and mixtures thereof. The high pressure vapor lamp according to the present inventon has a general color rendition index in the range of 50-85. BRIEF DESCRIPTION OF TEE DRAWINGS The only drawing, FIG. 1, is a front elevational view of > high pressure sodium vapor discharge lamp according to the present invention. DETAILED DESCRIPTION A high pressure sodium vapor lamp 10 embodying the invention and corresponding to a conventional 400 watt size is illustrated in FIG. 1. The high pressure sodium (BOPS) vapor lamp 10 comprises a vitreous outer envelope 12 with a standard mogul screw base 13 attached to the stem end which is shown uppermost in Fig. 1. A re-entrant stem press 14 has a pair of relatively heavy lead-in conductors 15 and 16 extending through the stem 14 and having outer ends of conductors 15 and 16 connected to the screw shell 17 and eyelet 18 of the base. The BPS lamp 10 has inner envelope or arc tube 19 centrally located within the outer envelope 12. The arc tube 19 is comprised of a length of light transmitting ceramic formed of a polycrystalline alumina ceramic which is translucent. The upper end of the arc tube 19 is closed by an alumina ceramic plug 20 trough which extends hermetically a niobium inlead 21 which supports an upper electrode (not shown) within the arc tube 19. The lower end of arc tube 19 has a closure which comprises a ceramic plug 22 through which extends a thin-walled niobium tube 23. The niobium tdbe 23 serves both as an inlead for arc tube 19 and a reservoir for excess alkali metal and mercury of the arc tube 19. The shank of the lower electrode (not shown) of arc tube 19 projects into tube 23 and is locked in place by crimping the tube 23 about the lower electrode at location 24 as shown in Fig. 1. The crimping leaves restricted channels which allow passage cf tbe alkali metal and mercury in vapor form but prevents movement of a liquid amalgam whereby the lamp may be burned not only 2se-up, as shown in FIG. 1, but also burned as base-down. The ceramiC seals are described in greater detail in U.S. Patent 4,065,651 Mcvey, Ceramic Lamp Having Electrode Support by Crimp Tubular Inleads. The arc tube 19 is of primary interest to the present invention and is illustrated in Fig. 1 as partially broken-away so as to partially show the arc chamber 40 and the walls 42 both of arc tube 19. The walls 42 have a thickness in the range 0.5 to 1.5 mm. The arc tube 19 comprises an optically translucent polycrystalline alumina ceramic formed of a composition containing magnesium oxide (MgO) and having an additive selected from the group consisting of zirconium oxide (ZrO2) and hafnium oxide (HfO2) and mixtures thereof. The inner tube 19 is formed of 633 ppm by weight of MgO and 400 ppm of the additive ZrO2 or HfO2 or mixtures thereof. The BPS vapor lamp 10 of FIG. 1 having the are tube 19 has a general color rendition index in the range of 50-85. The arc tube 19 formed of alumina and containing MgO, along with the additives ZrO2 and HfO2 is described in U.S. Patent 4,285,732 - Aluminum Ceramic, assigned to the same assignee of the present invention and herein incorporated by reference. U.S. Patent 4,285,732 describes a method for producing an optically translucent polycrystalline sintered body through which light or radiation in the visible wavelengths is able to pass through sufficiently to make such body useful for optical applications such as enclosures for arc tubes such as arc tube 19. I have determined that an arc tube such as my arc tube 19, having the compositions described in U.S. Patent 4,285,732, has an unexpected and surprising superior characteristic of reducing the undesirable sodium losses within the arc tube 19 described in the 'Background' section, which, in turn, provides a BPS lamp operable at increased pressures so as to improve the color rendition and also the color rendition of the improved lamp is maintained over increased life. Reference is now made to Table 1 showing typical BPS parameters relative to a standard HPS lamp and my improved color BPS lamp 10. TABLE 1 Standard Color BPS Improved BPS Parameters Lamp BPS Lamp 10 Bore 7.25 mm 7.25 mm Arc Gap 87 mm 44 mm Wall Loading 18.0 w/cm2 22.0 w/cm2 Amalgam 25% Na 25% Na Xenon Fill 17 Torr 17 Torr Cold Spot 6800 7200 Max Wall Temperature 11400 11050 Sodium Pressure 60 Torr 120 Torr Efficacy 125 LPW 90 LPW X, Y .512, .505, Chromaticity .420 .410 Cor. Color Temp. 21000K 22500K Color Rendering 20 65 Index The standard BPS lamp of Table 1 may be of the commercially available LU400 400 watt type available from the Lighting Business Group of the General Electric Company, whereas, the color improved lamp 10 of Table 1 is of a 250 watt type. A comparison between the standard BPS lamp and my color improved BPS lamp both of Table 1 reveals, (1) my improved color HPS lamp 10 has a Color Rendering Index of 65, whereas, the standard BPS lamp has a Color Rendering Index of 20 (2) my improved color BPS lamp 10 has a correlated color temperature (CCT) of 22500ski whereas, the standard HPS lamp has a CCT of 21000K, and (3) my improved color BPS lamp 10 has a sodium pressure of 120 Torr which is twice that of the standard BPS lamp having a typical sodium pressure of 60 Torr. In order that the invention may be more fully appreciated, references is now made to comparative test data obtained between a standard HPS lamp and my improved color EPS lamp in accordance with the practice of this invention. The standard HPS lamp with which the comparative test data was obtained was a 400 watt rating having a typical operating sodium partial pressure of 60 torr and having a general color rendition of 20. A total of 10 standard HPS lamps were subjected to comparative testing. The improved color HPS lamp 10 was similar to the standard HPS 400 watt lamp except that the HPS lamp 10 had the arc tube 19, previously described, having a selective additive of ZrO2 and operated at a pressure in the range of 120 to 160 torr. A total of 10 improved EPS lamps were subjected to the comparative testing. The groups of the standard and improved HPS lamps were subjected to operation at 800 watts for a 100 hour duration. During the testing at 800 watts, both groups were determined to have an increased, relative to the 400 watt operation, general color index of approximately 65 and both groups had a maximum ceramic temperature approximately midway between the associated electrode tips of approximately 1,35ooh. After the lamps were burned for approximately 100 hours, an inspection of the lamps was accomplished. The inspection revealed that subjecting of the standard HPS lamp to 800 watt operation created a dark brown staining of the outer envelope. However, the improved color EPS lamps 10 of the present invention only manifested a slight staining on their outer envelope. In addition, the arc tube chambers of the standard EPS lamps showed white spots concentrations of sodium beta-alumina, which extended into the arc chamber wall. This sodium beta-alumina is detrimental to the operation of the arc tube in that it chemically binds sodium making it unavailable for participation in the discharge condition of the arc tube and also the sodium beta-aluminum serves as a conductor path of sodium through the arc tube thereby further aggravating sodium losses and promoting arc tube jacket staining where it deposits. The arc tube 19 of the improved BPS lamps of the invention showed no white spots, A Scanning Electron Microscope (SEM) analysis on the inner surface of the arc tube 19 showed some scattered 'whiskers' of sodium t-alumina but no heavy concentrations or evidence of penetrion of sodium reaction through the wall of the arc tube 19. It should now be appreciated that the practice of the present invention provided an improved color BPS lamp 10 operable at at a pressure in the range of 100 to 300 Torr and having a general color index of 65. ft should be noted that although the comparative testing of the improved color BPS lamp 10 discussed bereinbefore determined the general color index of 65, the teachings of my invention along with my previous experience in the BPS lamp art contemplate a HPS lamp 10 having a general color index of approximately 85. Still further, although the hereinbefore given comparative testing description was related to a 400 watt BPS lamp 10 and the parameters of Table 1 was related to a 250 watt HPS lamp 10, it should be recognized that the practice of this invention contemplates lower wattage rating of HPS lamps such as 35 along with higher wattage ratings such as 1000.

Claims: CLAIMS What I claim as new and desire to secure by United States Letters Patent of the United States is: 1. A high pressure metal vapor lamp having an outer vitreous envelope enclosing an inner arc tube having thermionic electrodes sealed in its ends and containing a charge of vaporable metals having a sodium partial pressure in the range of 100 to 300 torr and xenon gas in the range of 10 to 30 torr, said inner arc tuke comprising; an optfclly translucent polycrystalline alumina ceramic formed of a cossposition containing MgO and having an additive selected from the 9up consisting of Zro2 and EfO2 and mixtures thereof. 2. A high pressure metal vapor lamp according to claim 1 having a general color rendition index in the range of 50-85. 3. A high pressure metal vapor lamp according to claim 1 wherein said inner arc tube is formed of 633 ppm MgO by weight and the additives ZrO2 or EfO2 or mixtures thereof are 400 ppm by weight. 4. A high pressure sodium lamp according to claim I wherein said inner arc tube has a wall thickness in the range of .5 to 1.5 mm.

Inventors: STROK, JACK MACK
Applicants: GENERAL ELECTRIC COMPANY
Application year: 1983
Main classification: H01J61
IPC codes: H01J61