LOW WATTAGE METAL HALIDE DISCHARGE LAMP

Abstract: A low wattage metal halide discharge lamp (5) includes an evacuated envelope (7) wherein is disposed a heat reducing member (25) having an arc tube (33) therein. The heat redu­ cing member and the arc tube have a metal band (31) and out­ er strap member (39) adjacent one another and adjacent an electrode (35) with the metal band, strap member and elec­ trode all electrically connected to an electrical lead (13) of one polarity whereby sodium losses from the arc tube are reduced.

Description: This invention relates to low wattage metal halide discharge lamps according to the generic clause of claim 1 Generally, metal halide discharge lamps are of the intermediate or relatively high wattage variety such as about 175 to 1500 watts for example. Also, it is known that the efficacy of the lumen output to input power decreases as the wattage of the lamp decreases. Thus, it has been generally presupposed that at lower wattages, wattages of 100 watts or less, metal halide discharge lamps would be entirely unsatisfactory in so far as efficacy is concerned. Also, it has been a common practice in the intermediate and relatively high wattage lamps to provide an inert fill gas in the outer envelope in order to prevent oxidation of metal parts of the arc tube mount. Another advantage of an inert gas fill in an outer envelope is a high breakdown voltage which prevents arcing between metal parts of the arc tube mount. However, an undesired heat loss due to convection currents of the inert gas in the outer envelope reduces the lamp efficacy significantly. One known attempt to reduce these undesired heat losses due to convection currents is disclosed in an application filed August 18, 1982 bearing U.S. Serial No. 409,280 and assigned to the Assignee of the present application. Therein, a quartz envelope is disposed within the gas filled outer envelope of a metal halide discharge lamp in an effort to reduce heat losses due to convection currents. Another attempt to reduce undesired heat loss due to convection currents is set forth in U.S. Patent No. 4,281,274. Therein, a glass cylinder surrounds a fuse tube with an outer glass envelope. The outer glass envelope includes one or more lamp filaments and is filled with a gas under pressure. Thus, a glass cylinder and a gas filled outer envelope are employed to reduce the heat loss due to convection currents. However, structures having gas filled envelopes and accompanying convection currents leave something to be desired in reduction of heat loss in so far as relatively high pressure lamps are concerned. EP-A 0 101 519 discloses a metal halide discharge lamp comprising an arc tube having a chemical fill including a sodium halide, an electrode therein at each end thereof, a covering member in the form of a bulb having a domed end and an open end with an outer metal band affixed thereto, said covering member surrounding a part of the arc tube, an outer envelope filled with an inert gas, surrounding said covering member, said envelope having a pair of electrical conductors sealed therein and passing therethrough, and a base member externally affixed to one end of the envelope. The object of the present invention is to overcome the difficulties of the prior art and to provide an improved low wattage metal halide discharge lamp having reduced heat and sodium losses and reduced thermal differences. This object is solved by the features of the characterizing part of claim 1. Embodiments of the invention are described in the following Figures, in which: Fig. 1 is a cross-sectional view of one embodiment of a low wattage metal halide discharge lamp of the invention; and FIG. 2 is a chart comparing lumen and color temperature maintenance during operational use of a metal halide lamp prior to and including the present invention. BEST MODE FOR CARRYING OUT THE INVENTION 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 conjunction with the accompanying drawings. Referring to the drawings, FIG. 1 illustrates a low wattage metal halide discharge lamp 5 which, importantly, includes an evacuated outer envelope 7. This evacuated outer envelope 7 is hermetically sealed to a glass stem member 9. An external base 11, formed for easy connection to an electrical source, is affixed to the hermetically sealed stem member 9 and outer envelope 7. A pair of electrical conductors 13 and 15 are sealed into and pass through the stem member 9 and electrically connected to the base 11 external of the outer envelope 7 to provide access for energization of the discharge lamp 5. Within the evacuated outer envelope 7 and affixed to one of the electrical conductors 13 is an electrically conductive support member 17. This electrically conductive support member 17 extends along an axis substantially parallel to the longitudinal axis of the discharge lamp 5 and includes a circular configuration 19 at or near the upper most portion 20 of the evacuated outer envelope 7. This circular configuration 19 in conjunction with the uppermost portion 20 of the outer envelope 7 serves to maintain the support member 17 in proper alignment and resistant to deformation due to external shock to the discharge lamp 5. Also disposed within the evacuated envelope 7 and affixed to the electrical conductors 13 and 15 therein are a pair of barium getters 21 and 23 respectively. These barium getters 21 and 23 are positioned at one end of the outer envelope 7 and adjacent to the glass stem member 9 and external base 11. As is well known, these barium getters 21 and 23 are important in any structure wherein an evacuated or vacuum is desired such as the above-described evacuated outer envelope 7. Disposed within the evacuated envelope 7 is a heat reducing covering member 25 in the form of a quartz sleeve. This heat reducing member 25 includes a domed portion 27, which is positioned closest to the getters 21 and 23 and base 11, and an open-ended portion 29 which is furthest from and faces away from the getters 21 and 23 and base 11. A metal band 31 surrounds and is affixed to the heat reducing member 25 and is electrically and mechanically connected to the support member 17. Within the heat reducing means 25 is an arc tube 33. This arc tube 33 has a chemical fill including a sodium halide and in a preferred embodiment includes iodides of sodium and scandium of a ratio in the range of about 20:1 to 28:1. The arc tube 33 also includes an electrode, 35 and 37 at each end thereof with a metal strap member 39 affixed to the outer surface thereof and electrically and mechanically connected to the support member 17. Moreover, the electrode 35 is mechanically and electrically connected to the support member 17 while the other electrode 37 is affixed to an electrical conductor 41 which passes through the dome portion 27 of the heat reducing member 25 and is electrically and mechanically connected to the other electrical connector 15. Importantly, the metal strap member 39 is immediately adjacent the metal band 31 affixed to the heat reducing member 25 and the one electrode 35 of the arc tube 33. Moreover, the strap member 39, metal band 31 and one electrode 35 are all electrically connected to the support member 17 and to the one electrical conductor 13. Although the low wattage metal halide discharge lamps are preferably in the range of about 40 to 150, the comparison chart of FIG. 2 illustrates the variations in maintenance of color temperature, voltage and lumen output of a 100 watt metal halide lamp. As can readily be seen, lumen maintenance for a 100 watt metal halide discharge lamp wherein the strap member 39 is adjacent the metal band 31 surrounding an electrode 35 and all are connected to one electrical conductor 13, Curve A, is at least about 80% and in this example about 81% after 2000 hours of operational use as compared with about 69%, Curve B, when the electrical connection and positional locations differ from the above-mentioned configuration. Similarly, it can readily be seen that the color temperature rise after 2000 hours of operational use of a discharge lamp having the strap member 39, metal band 31 and electrode 35 adjacent one another and electrically connected to the same electrical conductor 13 is not more than about 500 K and specifically only about 411 K (Curve C). In contrast, a discharge lamp operated for 2000-hours without having the above-mentioned polarity and positional location of parts had an undesired color temperature rise (Curve D) in the range of about 1027 K. Moreover, the tests provide a voltage rise of not more than about 8.0 volts and an actual test result of 5.8 volts for the above-described positionally located and electrically connected lamp as compared with an undesired rise of about 24.0 volts when the strap member 39, metal band 31 and electrode 35 were not connected and positioned as previously described. Accordingly, it has been found that a metal halide discharge lamp employing a heat reducing means with an affixed metal strap connected to an electrical lead of a given polarity, an arc tube with a metal band affixed and positioned adjacent the metal strap and connected to the same electrical lead and an electrode of the arc tube adjacent the metal strap and metal band and connected to the same electrical conductor provide an enhanced capability in a discharge lamp. Moreover, it is believed that the enhanced electrical characteristics are clear indications of a reduction in loss of sodium from the arc tube during operational life of the discharge lamp.

Claims: A low wattage metal halide discharge lamp of a wattage less than about 175 W comprising an arc tube (33) having a chemical fill including a sodium halide, an electrode (35,37) therein at each end thereof, a covering member (25) in the form of a bulb having a domed end (27) and an open end (29) with an outer metal band (31) affixed thereto, said covering member (25) being made of quartz and surrounding completely the arc tube (33), an outer envelope (7) enclosing said arc tube (33) and covering member (25) and having at one end thereof a pair of electrical conductors (13,15) sealed therein and passing therethrough, and a base member (11) externally affixed to said one end of the envelope (7), wherein the outer envelope (7) is evacuated and has, at said one end thereof, at least one getter (21,23) connected to the electrical conductors (13,15) respectively, an electrically conductive support member (17) is connected to one electrical conductor (13), the support member (17) is connected to one electrode (35) of the arc tube (33) at the end of the envelope (7) opposite to said one end of the envelope (7) to which the base member (11) is affixed, an outer strap member (39) is arranged on the arc tube (33), said outer strap member (39) of said arc tube (33) is adjacent said one electrode (35) at one end thereof, said outer metal band (31) of the covering member (25) is immediately adjacent said outer strap member (39), said outer strap member (39), adjacent electrode (35) and adjacent outer metal band (31) are electrically and mechanically connected to said one (13) of said pair of electrical conductors (13,15) through said support member (17), the other electrode (37) at the other end of said arc tube (33) is electrically connected to the other one (15) of said pair of said electrical conductors (13,15) of said evacuated envelope (7). A discharge lamp according to Claim 1, wherein the domed end of the covering member (25) is positioned toward the getters (21,23), thereby interrupting the line-of-sight between said getters (21,23) and the inside of the covering member (25) and the open ended portion (29) is furthest from and faces away from the getters (21,23) and base (11). A discharge lamp according to Claim 1 or 2, the electrically conductive support member (17) extends along an axis substantially parallel to the longitudinal axis of the discharge lamp (5) and includes a circular configuration (19) at or near the portion (20) of the outer envelope away from the base member (11), this circular configuration (19) in conjunction with said portion (20) of the outer envelope (7) serving to maintain the support member (17) in proper alignment and resistant to deformation due to external shock to the discharge lamp (5). A discharge lamp according to any of the preceding claims, wherein the arc tube (33) has a chemical fill including iodides of sodium and scandium of ratio in the range of about 20:1 to 28:1. A discharge lamp according to any of the preceding claims, wherein the electrode (37), which is not adjacent said strap member (39), is electrically connected through said domed end (27) of said covering member (25) to the other one (15) of the electrical conductors by a linear electrical conductor (41) extending parallel to the support member (17) and perpendicularly to the domed end of the covering member (25). A discharge lamp according to any of the preceding claims, wherein said lamp is a 100 watt lamp and said lamp has a lumen maintenance of at least 80% after 2000 operational hours. A discharge lamp according to any of the preceding claims, wherein said lamp is a 100 watt lamp and has a rise in color temperature of less than about 500 K after 2000 operational hours. A discharge lamp according to any of the preceding claims, wherein said lamp is a 100 watt lamp and experiences a voltage rise of not more than about 8.0 volts after 2000 operational hours. A discharge lamp according to Claim 1, wherein said lamp is of a wattage in the range of about 40 to 150 W.

Inventors: KARLOTSKI, ROBERT J., KEEFFE, WILLIAM M., KRASKO, ZEYA K.
Applicants: GTE PRODUCTS CORPORATION
Application year: 1985
Main classification: H01J61
IPC codes: H01J61