US2707247A - Vapor electric discharge lamp - Google Patents

Description

April 26, 1955 W. T. ANDERSON, JR

VAPOR ELECTRIC DISCHARGE LAMP Filed June 5, 1951 INVENTOR. l/V/L 4 MW ZA'A/omm/a; we. BY

ATTOAMA'Y vAPoR ELECTRIC DISCHARGE LAMP William T. Anderson, Jr., Maplewood, N. 3., assignor to Hanovia Chemical and Mfg. Company, Newark, N. 1., a corporation of New Jersey Application June 5, 1951, Serial No. 229,971

3 Claims. (Cl. 313-184) The present invention deals with vapor electric discharge lamps and particularly with vapor electric discharge lamps of the high pressure type.

A light source such as a tungsten filament incandescent lamp produces full illumination almost immediately because the time required to heat the filament to full brilliancy is less than one second.

Commercial fluorescent lamps operate with mercury vapor at very low pressures, e. g. 0.015 millimeter, and require less than one-half minute to reach full brightness.

Commercial high pressure lamps and superhigh pressure lamps require the attainment of mercury Vapor pressures from about 300 to about 60,000 millimeters, depending on the type of lamp, before full brilliancy is reached. A time elapse of several minutes to about one-half hour may be required for these lamps to reach full light output. Lamps of this type employ metals such as mercury, cadmium, sodium, caesium and the like for providing the metal vapors in which during operation, the electric discharge occurs. At room temperatures (25 centigrade) these metals are mostly either in a liquid or solid condition and have in the vapor phase pressures of less than about 0.002 millimeter on the mercury scale. As a result, when such lamps are started there is a very appreciable time period during which the metals must be evaporated and raised to high pressure vapor conditions.

During the beginning of this period the lamps produce very little useful illumination.

The employment of mercury and a rare gas in the same lamp is not new. Rare atmospheric gases such as xenon, neon, argon and krypton at a few to about 150 milliliters pressure have been used in mercury lamps for purposes of starting the lamps and of providing the initial evaporation of the mercury. In these lamps the contribution of the rare gas to the light ouput of the lamp is entirely negligible. During the first few minutes of operation such lamps produce less than one percent of their ultimate average brightness. For some applications this behavior is not objectionable, e. g. for street lighting and the like, but for other applications the initially low light output and brightness of these lamps makes them impractical.

Metal vapor lamps operate with an efficiency which cannot be approached by filament type lamps and have a unit brightness greatly in excess of other light sources. Hence, they are desired for very many applications. Their slowness in starting and their slow attainment of a usable intensity have limited their employment. Thus, these lamps might be excellent for the lighting of airport runways, but the necessity to wait for several minutes after the lamp has been started-for sufiicient usable illumination to be producedis most undesirable for such usage.

It is an object of the present invention to provide a high pressure metal vapor discharge lamp capable of producing brilliant light immediately upon starting. It is another object of the present invention to provide a high pressure or super high pressure metal vapor discharge lamp characterized by a comparatively short warm-up period. It is a further object of the present invention to provide a high pressure metal vapor discharge lamp capable of application for many illumination purposes where existing high pressure type vapor lamps are not practical. Other objects and advantages of this invention Will become apparent from the description hereinafter following and the drawing forming part hereof and which represents an elevational view of a lamp embodying the subject matter of the present invention.

2,707,247 Patented Apr. 26, 1955 The present invention applies in particular to lamps in which the final or operating vapor pressure exceeds about 10,000 millimeters and which have short discharge paths of the order of one centimeter and less. The ionizable filling of the lamp according to this invention consists of a rare gas at a pressure between 2000 and 11,000 millimeters at about 25 centigrade and a vaporizable metal preferably in an amount sufiicient to become completely vaporized during the operation of the lamp. The lamp, therefore, is from the moment of start always a high pressure discharge lamp.

Although I may employ the rare gases argon, neon and krypton with a vaporizable metal in accordance with the above and to certain advantage, I prefer to provide an electric arc in xenon at a pressure between 2,000 and 11,000 millimeters, and to have a content of mercury evaporate completely during lamp operation as this will increase the arc stability. Instead of the highly diffuse discharge obtained With a lamp are started in gas pressures less than 300 millimeters, I obtain immediately a very concentrated and constricted arc of high brightness. I have found that if I provide a lamp with a high pressure gas filling of xenon in addition to sufiicient mercury to provide the required mercury vapor pressure during operation of the device, I can obtain a very marked improvement in light production immediately upon the starting of the lamp at 25 C. approximate ambient temperature and can achieve much faster warm-up of the lamp to the optimum mercury pressure. It is at the latter that full brilliancy of the arc is attained.

The xenon arc produces a brilliant light immediately and rapidly warms the lamp so that the mercury evaporates and is raised to high pressure, since xenon has an ionization potential of 12.08 volts and mercury an ionization potential of 10.30 volts, the mercury is more rapidly ionized than the xenon. Consequently, as the mercury evaporates the characteristics of the lamp change. The white discharge of the xenon arc becomes more bluish, the arc becomes more constricted with higher unit brightness and the lamp efficiency is improved.

In order to obtain the high pressures of xenon and mercury required in my invention, I estimate the capacity of the lamp envelope, e. g. in milliliters, and for each milliliter of available space I provide a certain number of xenon molecules between about 5.4 10 and about 30 l0 molecules per milliliter, and sufiicient mercury to provide in the vapor phase preferably about an equal number of mercury molecules, e. g. from about 4.3 l0 to 30 10 molecules per milliliter.

Xenon and mercury are monatomic gases and obey the gas laws of Boyle so that the total pressure in the lamp envelope is the sum of the partial pressures of xenon and mercury.

A preferred type lamp to which my invention is applicable is illustrated.

Referring to the illustration, the lamp is a compact type lamp comprising either a substantially spherical or elliptical envelope 1 to provide the required strength and made of fused quartz or a high temperature glass, since it must withstand operating temperatures of the order of 1200 Kelvin. The envelope 1 is provided with tubular extensions, e. g. tubes 2 and 3 projecting outwardly therefrom preferably oppositely of each other, which support a pair of spacer electrodes 4 and 5, which are preferably tungsten electrodes in solid or coiled form and spaced from each other. The other structures such as the reflecting members 6 and 7, the vacuum-tight seals 8 and 9, and the terminals 10 and 11, are merely illustratory for purposes of the present invention and are intended to show a type of lamp unit to which the essential embodiments of the invention are applicable.

The electrodes 4 and 5 are provided to supply electric power to the gas are which forms between them and are spaced from each other at a distance of the order of one centimeter and less. As illustrated the electrodes are made of tungsten in the form of coils and are of substantially equal size or mass which indicates that the lamp is designed for alternating current operation in which case the anode electrode would be about five-thirds the mass of the cathode electrode in order to withstand the intense negative ion bombardment.

The inner diameter normal to the arc of the envelope is approximately five times the distance between the main electrodes 4 and 5 (arc-length) since this proportioning provides the greatest stability for the high pressure xenon are.

For example, the arc-length of a lamp according to this invention is 0.6 centimeter and the inside diameter of the envelope normal to the arc is about 3.0 centimeters. The envelope with electrodes assembled is degased and evacuated. It is then provided with a filling consisting of 352 milligrams of Xenon by weight, equivalent to l3.5 molecules of xenon per milliliter, and 300 milligrams of mercury by weight, equivalent to 8.2 10 molecules of mercury per milliliter. (The effective capacity of the envelope was 12 milliliters.) At an average ambient temperature of centigrade, the xenon has a pressure of about 4,000 millimeters, slightly in excess of 5 atmospheres. The mercury has a vapor pressure of only 0.0018 millimeter, being mostly on the walls of the envelope in minute droplets.

The lamp is operated on 110 volt alternating current electric supply with a ballast to limit the current to about 24 amperes. The discharge can be initiated by a high voltage shock to ionize the gas. It can be assisted if de sired by radium emanations. At the instant of start a highly concentrated are forms in the xenon. The concentration is greatest if the lamp is supported so that the arc is mostly in the vertical position. The gas is heated rapidly and increases in pressure so that about 3 seconds after start the xenon gas pressure has changed from 4,000 millimeters to about 16,000 millimeters, corresponding to an average gas temperature of about 1300 Kelvin. At this time the average brightness of the arc is about 26,000 lamberts.

A lamp made in accordance with my invention is brighter after 3 seconds start than any obtainable tungsten filament projection lamp. A 1,000 Watt tungsten projection lamp 26 lumens per watt has an average brightness of 7,500 larnberts. Thus a lamp made in accordance with my invention can successfully compete with incandescent lamps for many illumination applications Where existing mercury lamps are initially too low in brightness to be even considered.

I have described a lamp for operation at one kilowatt electric input. My invention applies equally well to lamps of other capacities. I have made lamps for 150, 500, and 700 watts to cite a few examples. I have used xenon pressures to 11,000 millimeters at 25 C. as well as those in the vicinity of 3,000 millimeters pressure. I have operated lamps at combined pressures in excess of 100 atmospheres. In every case my invention has enabled me to operate lamps at an initially useful brightness.

Krypton gas can be used in place of xenon gas, but is not as effective. It has an ionization potential of 13.93 volts compared to 12.08 for xenon and 10.30 for mercury so that the ionization of krypton in the presence of mercury is rapidly quenched by the latter. It krypton is used, the initial brightness is only about one-third of that obtained by xenon and there is subsequent decrease in brightness While the mercury is evaporating due, possibly, to a quenching action. With xenon and mercury there is a negligible quenching action.

My invention is not limited to mercury. Other metals evaporate even more slowly than mercury. However,

because of the quenching action, it seems desirable to use mercury in conjunction with the xenon and the second metal. Thus cadmium has an ionization potential of 8.96 volts. At high pressures of several atmospheres cadmium will completely quench the xenon and substantially quench the mercury. Unless mercury is used in combination, the initially high brightness obtained by the high pressure xenon arc will decrease as the cadmium evaporates and the final brightness will be less than the initial. The combination of cadmium and mercury can result in a lamp of higher brightness than the xenon alone with the additional advantage that the mercury light emission is color-corrected by the addition of the cadmium green and red. In making such a lamp I provide the xenon gas pressure in the manner described and provide a combination of mercury and cadmium such that the final operating pressure is about 50 per cent the partial pres sure of xenon, about 40 per cent the partial pressure of mercury, and about 10 per cent the partial pressure of cadmium.

What I claim is:

l. A vapor electric discharge lamp comprising an envelope of light transmissive high temperature glass having a substantially spherical shape, a pair of electrodes spaced apart approximately one-fifth the inner diameter of the envelope and less than one centimeter, said envelope containing said electrodes and a filling of xenon gas at a pressure in excess of 2000 millimeters and less than 11,000 millimeters at 25 C. and mercury between 4.3 10 and 30x10 molecules per milliliter of envelope volume. V

2. A vapor electric discharge lamp comprising a substantially spherical fused quartz envelope containing a pair of electrodes spaced apart approximately one-fifth the inner diameter of the envelope and less than one centimeter, said diameter being normal to the lamp arc, and a gas filling of xenon between about 5.4 1 0 and 30 10 molecules per milliliter of envelope volume, and mercury between 4.3 l0 and 30 10 molecules per milliliter of envelope volume.

3. A vapor electric discharge lamp comprising a substantially spherical fused quartz envelope containing a pair of electrodes spaced apart about one-fifth the inner diameter of the envelope and less than one centimeter, one of said electrodes being an anode and one a cathode, said anode electrode having a mass approximately five-thirds that of said cathode, a gas filling of xenon between 5.4 10 and 30 10 molecules per milliliter of envelope volume and mercury between 4.3 10 and 30 10 molecules per milliliter of envelope volume.

References Cited in the file of this patent UNITED STATES PATENTS Noel Jan. 18,

Claims (1)

1. A VAPOR ELECTRIC DISCHARGE LAMP COMPRISING AN ENVELOPE OF LIGHT TRANSMISSIVE HIGH TEMPERATURE GLASS HAVING A SUBSTANTIALLY SPHERICAL SHAPE, A PAIR OF ELECTRODES SPACED APART APPROXIMATELY ONE-FIFTH THE INNER DIAMETER OF THE ENVELOPE AND LESS THAN ONE CENTIMETER, SAID ENVELOPE CONTAINING SAID ELECTRODES AND A FILLING OF XENON GAS AT A PRESSURE IN EXCESS OF 2000 MILLIMETERS AND LESS THAN 11,000 MILLIMETERS AT 25* C. AND MERCURY BETWEEN 4.3X1019 AND 30X1019 MOLECULES PER MILLILITER OF ENVELOPE VOLUME.

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