US3325662A - Metal vapor lamp having a heat reflecting coating of calcium pyrophosphate - Google Patents

Description

3EQ-M1 5R {W mmuHRoom June 13, 1967 c. G. COOK 3,325,662

METAL VAPOR LAMP HAVING A HEAT REFLECTINO COATING OF CALCIUM PYROPHOSPHATE sUBSTH-UTE FQR M\SS1NG XR Filed Sept. 19, 1965 Invefvtor: Clarence .Cook

His A FL vneg I V I i i in 4 X/WXU/IL' United States Patent Q 3,325,662 METAL VAPOR LAMP HAVING A HEAT REFLECT- ING COATlNG OF CALCIUM PYROI'HOSPHATE Clarence G. Cook, Mayfield Heights, Ohio, assignor to General Electric Company, a corporation of New York Filed Sept. 19, 1963, Ser. No. 309,965 2 Claims. (Cl. 31317) This invention relates to metallic vapor lamps using an arc discharge in mercury and metal halide vapors to produce visible light, and is more particularly concerned with reflective coatings on the ends of the are tube for controlling its temperature distribution.

The mercury arc lamp has achieved commercial acceptance by virtue of its long life and reasonably good efficiency but suffers from relatively poor color rendition due to the bluish-green quality of its light. Also its efliciency in the range of 50 to 60 lumens per watt is appreciably below the 70 to 80 lumcns per arc watt range of the ordinary fluorescent lamp. A radical improvement in both color rendition and efliciency may be achieved by adding to the mercury one or more vaporizable metal halides under proper control of loading, temperature and pressure, the preferred metal halide additive being sodium iodide, optionally with thallium iodide. Such improved lamps are described and claimed in copcnding application Ser. No. 84,068 of Gilbert l-l. Rolling, filed Jan. 23, 1961, entitled Gaseous Electric Discharge Lamps" and assigned to the same assignec as the present invention, now Patent 3,234,421. For convenience, such lamps will henceforth be referred to herein as mercury metal halide lamps.

in its general construction and appearance, the mercury metal halide lamp resembles the conventional highpressure mercury vapor lamp comprising a quartz arc tube mounted within a glass outer jacket having a screw base at one end. Thermionic main electrodes are provided at the ends of the arc tube which contains a quantity of mercury and metal halide such as sodium iodide along with an inert gas such as argon for starting purposes.

The mercury vapor lamp is usually designed to operate with its charge of mercury entirely vaporized, the vapor then being unsaturated. This requires that all parts of the p the electrodes, that is the ends of the arc tube, are the coolest regions in normal operation of thelamp. In the absence of special measures to raise the temperature of the ends, it is found that in a mercury metal halide lamp, the metal halide such as sodium iodide rapidly condenses on the envelope wall behind the electrodes, making the lamp ineffective.

In certain sires of mercury vapor lamps where the ends of the are tube tend to operate at too low a temperature, various kinds of reflective coatings have been applied to the tube ends n order to raise their temperature. However, I have found that metallic reflective coatings, such as coatings of platinum, nickel, stainless steel, gold or silver which can be used with mercury vapor lamps where .he interenvelope space is filled with an inert or inactive gas, are not suitable for use with mercury metal halide lamps where the intercnvelope space is evacuated as a heat conservation measure. in evacuated jackets, the material of such reflective coatings vaporizes away from the arc tube, resulting in poor heat shields and simultaneously causing severe blackening and discoloration of the jacket.

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Such materials may also give off gases which destroy the effectiveness of the vacuum in the jacket.

The object of the invention is to provide rcflectively coated ends on the arc tube of a mercury metal halide lamp which are effective for raising the temperature of the ends and which are suitable for use in a high vacuum jacket.

Another object is to provide a coating of the recited kind which does not release gas, adheres well to the quartz and which is inexpensive and easy to apply.

Aecording'to my invention, I have found that calcium pyrophosphate, c r o, may be used as a coating on the ends of the arc tube and meets all the necessary requirements for a heat shield. It is free from gases, adheres well to the quartz tube, has excellent reflectivity and low cost, and is easy to apply. It is preferably applied as a suspension in water with a fractional percentage of fine silica to promote adherence.

For further objects and advantages and for a better appreciation of the invention, attention is now directed to the following detailed description of a preferred embodiment to be read in conjunction with the accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

The single figure of the drawing is a side view of a mercury metal halide arc lamp embodying the invention.

Referring to the drawing there is shown a high-pressure mercury vapor lamp 1 comprising an outer vitreous envelope or jacket 2 of generally tubular form modified by a central bulbous portion 3. It is provided at its outer end with a re-entrant stem 4 having a press 5 through which extend relatively stifl' inlead wires 6, 7 connected at their outer ends to the contacts of the usual screw-type base 8, namely the threaded shell 9 and the insulated center contact 10.

The inner arc tube 12 is made of quart or fused silica and has sealed therein-at opposite ends a pair of main arcing electrodes 13, 14. The electrodes have inlcads 15, 16, respectively, each including an intermediate thin foil section 17 hermetically sealed through full diameter pinch seals 18, 19 at the ends of the arc tube. Each electrode comprises a tungsten wire helix wrapped around a tungsten core wire and may be activated by providing a small elongated piece or sliver of thorium metal (not shown in the drawing) inserted between the core and the helix. Alternatively, the electrodes may be activated by a very thin layer of thorium metal vacuum'deposited thereon. The are tube contains a quantity of mercury which is entirely vaporized during operation of the lamp and which at such time exerts a pressure in the range of 1 to 15 atmospheres. A rare inert gas such as argon is provided at a low pressure, for instance at approximately 25 millimeters of mercury, within the arc tube to facilitate starting and warm-up. In addition a quantity of a metal halide, suitably sodium iodide, is provided in excess of that vaporized at the operating temperature of the arc tube which should be not less than 500 C. at any place.

The are tube is supported within the outer jacket by a frame or harp comprising a single side rod 21 and rod portions 22 on the open side. The frame is welded at its base end to inlead wire 7 and has a pair of transverse spring members 23, 24 which against the outer tubular portion of the jacket to provide lateral support. The are tube is fastened to the frame by a pair of metal straps 25, 26 which extend between rod 21 and rod portions 22 and encompass and clamp the pinch seals. The straps are spaced :1 distance away from the ends of the arc chamber in order. to limit the cooling effect and also to prevent possible devitrification of the quartz about the electrode inleads. Electrode 13 is connected by conductor 27 a, r a j e threaded through insulating glass sleeve 28 to inlead wire 6 of the outer jacket and thereby to base shell 9. Electrode 14 is connected by conductor 29 to rod 21 which in turn is connected by inlcad wire 7 to center contact 10 of the base.

For maximum efliciency, it is desirable to reduce the heat losses from the are tube. Therefore, as a heat conservation measure, the interenvelope space is evacuated prior to scaling off the outer jacket. Getter material is provided in the channelled rings 30'and flashed after sealing of the jacket in order to assure high vacuum; a suitable getter is barium metal powder pressed into the rings. To prevent condensation of sodium iodide at the ends of the arc tube behind the electrodes the temperature in these regions during operation should be not less than- 500 C. This minimum temperature is assured by applying a heat reflective coating, indicated by the speckling, to the ends of the are tube and to the adjacent portions of the pinch seals. Desirably, the reflective coating should extend along the walls of the are tube up to where it is approximately flush with the tip of the electrode. There is thus formed a concave reflector which is highly effective in preserving heat while obstructing the visible light gencratcd in the interelectrode path to a minimum extent. In the direction of the outer end, the reflective coating should extend over the pinch at least up to the beginning of the foil section of thc inlead. However the extent is not critical and suitably the reflective coating may be extended up to the straps which clamp the arc tube to the frame.

My invention is more particularly concerned with the nature and manner of application of the reflective coating pyrophosphate. CilzPgOq is a superior material for a heat at the ends of the arc tube. I have found that calcium reflective gttm inasmuch as it meets all the necessary ie qtriren'tents. It is relatively easy to apply, it adheres well to the quartz tube, has excellent reflectivity and is low in cost. Also it does not release any gases which would dcleteriously affect the vacuum in the intcrcnvclopc space.

A suitable method of applying the reflective coating is as follows. A suspension of Cit-W in a water soluble binder is prepared by mixing the material in an ammoniacal aqueous solution of the copolymer of polyvinyl methyl ether and maleic anhydride. Suitable proportions are 0.3% to 0.8% by weight of the copolymcr relative to weight of Ca P O To this mixture is added from 0.1% to 1% of fine silica, suitably /2% by weight relative to Ca P O This combination of ingredients is milled for approximately 5 hours before application to the arc tube. Prior to application, the quartz surface of the are tube must be freed of any grease, dust or other foreign parthe exposed ends are heated by directing the flames of gas 'burners against them. After the quartz has been preheated, the Ca P O suspension is applied as a spray using a high pressure air gun. The flames are then again directed on the ends in order to bake out the binder and any other residue which may have been captured within the Ca P O suspension. The preheating of the quartz is important and results in much better adherence of the coating.

Life tests of Ca P O coatings applied in the manner described show no evidence of a decrease in reflectance of the coating with time and temperature. The coating on the ends results in an increase in light output due to better hcat conservation and less light absorption. The metal halide such as sodium iodide is prevented from condensing behind the electrodes whereby higher efliciency and better color rendition are achieved.

While the invention has been described by reference to a specific preferred embodiment, the details of construction described are intended as exemplary and not in order ticles which might interfere with proper adhesion. Scrubhing with alcohol followed by distilled water has proven sat sfactory. A mask is then placed over the are tube leaving only the ends exposed to which the reflective coating is to be applied. The tube is held at both ends in a suitable fixture and rotated about its axis at a relatively low speed, for instance revolutions per minute. At the same time,

to limit the invention thereto except insofar as any may be included in the accompanying claims.

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

1. An electric discharge lamp comprising a vitreous outer jacket enclosing a quartz arc tube having a pair of thermionic electrodes sealed therein at opposite ends and containing an ionizable filling comprising mercury, a metal halide, and an inert starting gas, the interenvelope space between said jacket and said are tube being evacuated, and a heat reflective coating on the ends of said are tube extending approximately flush with the tips of said electrodes and consisting essentially of calcium pyrophosphate Ca P O 2. An electric discharge lamp comprising a vitreous outer jacket enclosing a quartz arc tube having a pair of thermionic electrodes sealed therein at opposite ends and containing an ionizablc filling comprising mercury, sodium iodide, and an inert starting gas, frame means supporting said are tube within said jacket, the intcrcnvclope space between said jacket and said are tube being evacuated, and a heat reflective coating on the ends of saidare tube extending approximately flush with the tips of said electrodes and consisting essentially of calcium yrophosphate CHZPZO'].

References Cited UNITED STATES PATENTS 2,149,658 3/1939 Armstrong 313-178 2,556,254' 12/1951 Garne 313178 2,706,691 4/1955 Schaefer 117-33 2,748,303 5/1956 Thorington 313-25 2,982,668 5/1961 Gunther 117-33 3,023,337 2/1962 Repsher 117124 3,094,640 6/1963 Gustin 313-25 JAMES W. LAWRENCE, Primary Exrmiiner.

S. SCHLOSSER, R. SEGAL, Assistant Examiners.

Claims (1)

1. AN ELECTRIC DISCHARGE LAMP COMPRISING A VITREOUS OUTER JACKET ENCLOSING A QUARTZ ARC TUBE HAVING A PAIR OF THERMIONIC ELECTRODES SEALED THEREIN AT OPPOSITE ENDS AND CONTAINING AN IONIZABLE FILLING COMPRISING MERCURY, A METAL HALIDE, AND AN INERT STARTING GAS, THE INTERENVELOPE SPACE BETWEEN SAID JACKET AND SAID ARC TUBE BEING EVACUATED, AND A HEAT REFLECTIVE COATING ON THE ENDS OF SAID ARC TUBE EXTENDING APPROXIMATELY FLUSH WITH THE TIPS OF SAID ELECTRODES AND CONSISTING ESSENTIALLY OF CALCIUM PYROPHOSPHATE CA2P2O7.

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