US2451043A - Gas discharge lamp - Google Patents

Description

UCL 12, l948 M. PENNYBACKER GAs DISCHARGE LAMP Filed Dec. 23, 1944 INVENTUR Miles/Ue A//LW TTORNEY lto they negative glow discharge.l

'Patented ooi. 12, 194s UNITED 'sTATEsf ,PATENT Gas DISCHARGE Larra Milqes Pennybaolrer, Westport, (linnn.l Application December 23, 1944, Serial No. 56.9.4818

1 This invention relates to electrical lamps of the I 'gaseous discharge type, more especially to lamps scloims. (Cl. 11e-122i o .o tubo current. `'11' the material of the triphas ofthis type employing o positive commons discharge and producing light having wave lengths either within or without the visible spectrum.

4'Ihis invention employs a novel starting conductor between electrodes, similar in some respects to the starting conductor described in United States Patent #1,961,617 but employing onthe conductor a coating or surface material of relatively high electrical resistance and permeable Positive columngas discharge lamps are usu- -ally constructed in the form of a relatively long and narrow cylinder of glass, quartz or other suitable-transparent material, provided with suitable electrodes at both ends thereof. In one widely used form. the internal surface of the transparent envelope is coated with a phosphor, which acts to transform luminous energy from one portion of the spectrum, such as ultra violet, to another portion of the spectrum better suited for general illuminatng purposes.

.For most purposes, it is desiree that the naht from a llamp be emitted over an are of not exceeding 180 degrees. Accordingly, when'a cylindrica'lly4shaped la'mp is employed, it is usually found necessary.:or atleast highly desirable, to

i employ a'. reflector which will Arender available a portion of the light otherwise 'wasted by reason.

/of the fact thatit is emitted in an undesired'diwhen the surface is en -unobstructedconductor, o

it is liable to carry 'enough current, as the vol-tage approaches that necessary to start the main positive discharge, to cause objectionable localized heating at least at one end of the starting conductor. This is trueparticularly if the positive column path is long. or of relatively high resistan ce, as in a fluorescent lamp used at low ambient temperature, before the mercury has vaporized, or during the processing of the tube as a manufacturing step. Furthermore. especially if the conductor is ofgreater surface area per 'unit of 1ensth. it yis noble to carry too muon or the normal l tube is ready -to be sealed. When positive column gas discharge.tubesare v very high work functlonfin order to minimize this starting current. then thevoltage necessa' tolnltiateflow of current to the conductor is/ an undesirable value. q o

- Qne object of this invention the con-struction, over a wide range of dimensi and of various different materials, of astarting Y I conductor or strip, by providing a coatingwhich will cause the negative glow to spread substantlglly along the length of the starting'strip atvery f small currents and thus' will facilitate starting the positive column discharge at lower currents and voltages than would be required with un coated starting strips of the prior art. 1 l

Another object is to provide starting conductors of much greater cross-sectional area, thereby improving their durability, facilitating their handling and mounting inthe tube, and permiti.

ting theiraddition-al use as reflectors or as fluores-V cent surfaces for transformation of the wave lengths of theluminous output. whereby theover all effectiveness of the tube as 'a llght'source is improved. in addition to the above-described improvements in starting characteristics.

Another object of this invention is to provide a novel formof elongated cylindrical lamp from" which the externally available light is emitted substantially wholly alongone longitudinal half of the cylinder, thus causing the luminous output of the lamp to be completely available without the need of employing a reflector.

Another purpose ofv this invention is toprovide a tubular lamp with an internal reflector,l in the form of an element not closely connected. either mechanically or chemically, with the internal wall of the tube. in distinction from reflecting elements which have hitherto been forpled as metallic deposits, or the like, upon such internal wall, and whichalso acts as a starting strip. and optionally also as a support for a fluorescent coating, al though this invention mayalso be practiced with theV use of -a starting strip of the deposited type.

A still fur-ther purpose of this invention is to provide a tubular lamp in which the reflector or starting strip can be manufactured independently of the tube and only inserted therein when the to be started. it has been found necessary to apply to such tubes a starting potential in excess of the potentiall existing between electrodes whenI the positive column discharge has once been estab- "lished.

In order to. reduce the starting potential re- 3 quired in a cold cathode tube, it has been proposed -to employ a starting strip. Such strip usually is in the form of a conductor extending from 'the region near one electrode to the region near the opposite electrode. Such starting strip operetes by the establishment of a-cathode glow dlscharge at one or both ends of such strip. and at .the portion of one or both of the electrodes adiacent to the strip. 'I'he initiation of a cathode glow discharge tends toward increased ionization, which latter eifect promotes the establishment of a positive column discharge at a lower volt-age. Insome proposed forms of starting strip, the cathode glow spreads along the strip, with in- .creasing rise 4in potential, until the current eventually selects the path of a positive column discharge between the electrodes. as constituting the path of least resistance. 'I'his process is described in detail in United States Patent #1,961,617, previously referred to. After the establishment of the positive dolumn discharge, it is highly desirable that all discharge through the path including the starting strip cease. vThis is desirable for reasons of luminous efficiency. and because such starting strip is not designed to constitute an eilicient locus for primary emission of light.

Wherever the term positive column discharge; is used in this specification, there is meant such discharge of normal current value and producing y a useful amount of radiation, and this term does not apply to any minor positive column discharges taking place at substantially lower voltages and at relatively low current intensities.

With the gas discharge tube of this invention, an improved form of starting strip is employed. overcoming diillculties hitherto found in constructing such starting strip and locating it at the desired position within the tube. Further-f;- more this invention provides an arrangement wherein the effective resistance of substantially the entire surface of the starting strip to the establishment of excessive discharge therefrom is greatly increased, so that there will be available a greater potential for establishing the desired column discharge.

Another object of this invention is tov provide, in-a gaseous discharge tube, a starting device including a relatively wide strip of metal or other conducting substance, which can be fabricated as a discrete unit and which can be placed in the desired position within a tube, when the tube is assembled, such discrete construction being adapted to economical large scale production, in contrast to certain hitherto employed methods of constructing a starting strip, in situ, within the tube itself.

Another object of this invention is to provide in a4 positive column discharge lamp, a starting strip bearing upon at least a portion of the surface thereof a coating of insulating material whereby such strip tends to oii'er a. higher effective resistance, over a given length, then does the positive to be diverted from the desired positive column discharge path.

A yet further purpose of this invention is to prcvide a positive column discharge tube, in which the difference between starting and operating voltages is greatly reduced from that hitherto found necessary. V

Still another object of this invention is to provide a cold cathode positive 'column discharge tube having suilicient length so that its efllciency will be relatively high and yet possessing starting characteristics such that the voltage applied column discharge, so that current will not tend 4 thereto may be kept within an upper limit, below which special insulation of the so-called high voltage type is not needed. Such an upper linut isoften determined in practice by codes or safety regulations. a figure of 600 volts being usually placed as such limit.

A certain advantage of one form of this invention is that it provides a positive column discharge tube in which a plurality of conductive starting strips are so arranged as to present, as a whole, a relatively high resistance to gas discharge therefrom. Such effectively high resistance is due to the provision of a; number of gaps between individual starting strips and between such strips and theI tube electrodes, since this will result in virtually placing in series with, one another the cathode fallwoccurring at each gap. This iorm of the'invention allows the construction of a tube in which the positive column dls charge may be made of greater length, and consequently having greater drop of potential thereacross than would otherwise be possible without the discharge being shunted by thestarting strip, to a greater or lesser degree.

Other purposes and advantages of this invention will be apparent to those skilled in the art from the following description of several embodiments of this invention.

Referenceis now made to lthe drawings hereunto attached, where Y Fig. 1 is a perspective view of a tube embodying one form of this invention,

Fig. 2A is a cross-sectional view of the Fig. l,

Fig. 2B is a cross-sectional view of a tube embodying another form of this invention,

Fig. 3 is a single-end connected tube showing another form of this invention, and- Flg. 4 shows, partly schematically, an alternatube of 40 tive form ofthis invention, using a divided startchosen to' be transparent to radiations having such wave lengths as are to be utilized externally. For example, the tube may be constructed of ultra violet transmittingglass or of quartz, if the radiations lie in the ultra violet portion of the spectram.. suitame electrodes or the com cathode that this strip extend for approximately 18o degrees around the internal periphery of the tube. However, strips extending over a greater or lesser angle may be employed without departure from this invention. Strip I may be formed of any suitable conducting material which will withstand the temperature and treatment incidental to the manufacture and operation of the tube. Such material should be capable of ready de-gasification and should not evolve gases during operation of the tube. For'reasons hereinafter to be explained in detail, I prefer to makethis strip of comparatively thin metal or alloy, possessing some N degree of,elasticity. Suitable\materials include iron, nickel-plated iron, magnesium and alu- `rnlnium, but this invention is not confined to the employment ofthese particular metals. The strip may be sprungA into a semi-cylindrical form, as it is being inserted into the tube, or prior` to such insertion therein. The elasticity of the trlp will then cause it to bepressed more or less against the portion of the tube wall covered tube.

As known in the art, all portions of a startingstrip do not act uniformly, and by the active portions of the strip are meant those upon which cathode glow discharges tend to be established.

In Fig. 2B there is shown a variant form I' I which the strip Just described may assume. In

this particular form, the apex of the V. and extremities of the two arms of the V constitute three points of contact extending longitudinally along the tube wall. The tendency of the V to expand laterally may be used to hold the strip in place.

When a strip having 'an interior reflecting surface is employed. -it is evident that radiations produced by the positive column discharge in the tube will pass in part directly through the rela` my use of a separate strip bearing the phosphor, 4such -type of lamp may bev manufactured very v reflecting strip is madeto extend more than 180 degrees, the light will be emitted throughout an arc correspondingly less than 180 degrees. This last described arrangement will in many cases eliminatefthe need for external reflectors. vBi

' simply, whereas it has been found very dimcult proximately 180 degrees, and covered'with a phostube.

tively unobstructed portion of the tube wall opposite the reflecting strip. Radiations impinglng upon the strip will be reflected therefrom and will also pass through the same transmitting portion of the tube wall.`

Evidently the surface of the reflecting strip should be chosen of a nature to act as an efficient reect'or for the particular wave lengths which are to be emitted from the tube.-

For confining the positive column discharge to the proper path thereof, it has been found desirable that the surface of the lreflecting strip which faces this path within the tube be covered with a layer of insulating material 4. If the material constituting the suface of the starting strip be metallic in nature, it may be desirable to place over it an outside layer of some material which Lfis relatively translucent or reflecting, but which is also hon-conducting. Many white materials such as aluminum oxide, magnesium oxide, or other metallic oxides, and the like', function both as a suitable reflecting surface and as a suitable insulator. A material such as aluminum .oxide also functions well in cases where reflection of ultra violet radiations, as well as visible, is desired.

These materials are applied in the form of discrete particles, as indicated in Figs. 2A, 2B and 3, each particle, being impervious and insulative to the discharge, whilethe interstices between particles allow current flow to and from the strip. Any organic binder, initially used as a vehicle for the coating, is burned away prior to or during the tube exhaust, leaving the material in the desired form of discrete particles.

In the case of gas discharge tubes of the fluorescent-type, the visible light proceeds from a material which is usuallynon-conducting and only somewhat transparent. Such phosphors may be used to cover the surface of the reecting strip, in which case the material probably acts,

. not only as a light emitter. but also toa certain extent as a light reflector.

In certain embodiments of this invention, the longitudinal wall ofv the tube opposite such a jstrip coated with a phosphor is relatively transparent. andflnversely related tothe arc coveredl by the relatively opaque reflecting strip. As the to coat the internal wall of a tube overv only a portion of the periphery thereof.

Measurements made upon a fluorescent type tube employing a reflecting strip covering apphor, showed a luminous output over approximately degrees, of 33 units in comparison with 23 units obtained from a similar tube, drawing 4the same amount of electrical energy, but employing a phosphor distributed over the entire inner surface of the tube, according to the hitherto prevailingl practice inthe art.

It will be evident that for ease in replacement. it will be desirable that the starting strip be placed within the tube at a predetermined angular relationship to the contact making pi'ns usually provided in pairs at both/ends of the This will. ensure that, when a fixture or other device mounting the two sockets in which thetube is to be placed, has once been put in position, the reflecting strip will be found in the desired position relative to the direction in which the useful light output is to be radiated. In the case of direct lighting, this direction usually will be downward and no external reflector -will be needed. In the case of indirect lighting, the direction usually will be upward or sidewise. but no shield between the light source and the eye will be needed because of the opaque strip in the reached `by dust and dirt.

It is thought unnecessary to describe iny detail the connection of a gas discharge tube to th power supply, via a transformer or similar inductive device. The following points, however, are of importance in discussing the problems solved 4by this invention. Such 'power supply transformer is preferably designed to exhibit poor regulation. Such transformers are well known in the gas tube art and may utilize a relatively high magnetic leakage arrangement, or other methods of obtaining such poor regulation may -be employed. Since the transformer must afford, upon an open circuit, va potential sufficiently high to initiate the gas discharge, but also must be so arranged that, when the normal discharge c'urrent has once been established, the potential supplied to the tube electrodes will not exceed that necessary to maintain the normal current, it is evident that thediflerence between the starting and operating voltages of the transformer will be determined, to a great degrees by the characteristics ofthe tube. It is likewise apparent that, the lower the' value at which this difference of potential may be established, the better will be the regulation of the transformer, and conselquentiy the higher willbe bothI power factor and tional type of screw base.

discharges are established by the starting conductors in exactly the same .manner in either case.

Referring now to Fig. 8,there is here shown a bulb I0, provided at one end with a conven- The leads from electrodes Ii and i2 pass through the press i8 and connect to the external screw-base. A partition or strip lil extends `from the press upwardiy to a point near the other end of the bulb. This partition is shaped and sized to meet the internal walls of the bulb, so that the bulb is divided into two discrete compartments, open to one another only at the upper end of the bulb, and may be formed either of metal, as shown. or of insulating material such as glass or mica, sultably coated as later described. The sides oi this partition are coated with a suitable insulating material I, such as previously described, so that the partition will function as the improvedA starting strip of this invention. The discharge extends upwardly in one chamber, crosses the upper end of the bulb, and descends in the other chamber. If the partition be made of an insulating substance, then it may be first coated with a conducting material, and then with one of the insulating and preferably reflecting or translucent types of material previously described. Likewise, the phosphor, in the case of a wavelength conversion type -oi lamp, may be placed upon this partition, as also previously described.

In Fig. 4 there is shown a tube of the same general type as that shown in Fig. 1.. However, in this case the starting strip is divided, at or near the center. so that there are really two starting strips, discrete from one another, and effectlvelyvoperating in series. The general method of operation of this bulb is similar to that already described in connection with Fig. 1. However,. the two starting strips, 31 and 3:, present, together with the two electrodes, a total of three gaps, over which the cathode glow discharge must be initiated, before the eventual establishment of the positive column discharge.l This would seem at first sight Ito be a disadvantage, in that it increases'the` needed starting voltage, since each gap will require at least approximately 100 volts in order to establish a discharge thereacross.J

8 With the prior art tubes, not employing a starting strip, the following values may be taken as illustrative. For a' cold cathode tube -having a bore of about 22 mm. and a positive column length of about 90 inches and an operating current of about 150 milliamperes, the terminal potential when operating will be found to be in the region n of 450 volts. n the other hand, in order to cause volts to be constructed with special insulation of However this form 'of construction of the tube has Y the advantage 'that the maximum length of the positive column discharge may be increased over the length possible when employing a single starting strip. This arises from the fact that, with increase in length oi' a single starting strip. a point is reached where the voltage required to initiate the positive column discharge will be` come so great that the strip will permit the full value of the 'normal discharge current to ilow as a negative glow discharge from its surface without reaching thefvoltage required to initiate the positive column discharge. I have found improved results by making the, two starting strips of unequal length, for example in a ratio of lengths of 39 to 46.

With the multiple starting strip employedin ,this particular form of structure of this inventhe initiation of the positive column discharge, a potential in excess of 600 volts is usually required. Since the underwriters regulations require electrical apparatus employing more than 600 similar considerations apply to other inductiveballast devices.

When a tube having the same physical dimensions and the same operating current as that just described. was provided with a starting strip having two sections according to the embodiment of this invention shown in Fig. 4, actual measurements showed that a cathode glow discharge would appear at the tube electrodes with a terminal potential of 380 volts. Ata potential in the neighborhood oi 535 volts, the positive column discharge was initiated, upon which the terminal voltage immediately fell to the normal operating value of 450 volts. 'Ihe respective values just given illustrate the advantage of voltage reduction secured by the use of this invention.

Among the novel advantages zof, that form of this invention which embodies a separable strip is the fact that the actual application of the insulating or fluorescent material does not have to take place within the relatively narrow confines of the glass tube. The strip is preferably coated before insertion in the tube, thusl making possible 'the use of the spray gun, dipping,dust ing, or other high-speed production methods not so readily applicable to coating the inside of glass tubes. .These methods permit the use of fluorescent powder or pigments which do not have to be subjected to the prolonged grinding or ballmilling now customary in making suspensions for coating the inner surfaces oi uorescent lamps. The less the grinding of the fluorescent particles, the more emcient they are. Likewise, less care will be needed in applying this coating than is needed in applying it to an ordinaryy yinch in thickness and still be sumciently 4permeable to the discharge to permit the negative glow to spread along the strip as desired to facilitate starting. Also the iluorescent materials may readily be appliedA in trade mark and decorative designs of different fluorescent colors,

l by means of silk screen reproduction methods for example.

It has been found that the coated starting conductors of this invention are less limited in tuhe suitable range of gas pressures than are those strips having highly conducting surfaces. It is preferred to nil the bulbs of this invention at, or

. scope of this invention is limited 9 slightly above, the gas pressure corresponding to the minimum positive column drop under n-ormal conditions of operation for that particular transverse cross-sectional area of positive column. This optimum gas pressure is'well known for conventional-bulbs, and is not materially altered by the addition of the coated conductor of this invention. In a bulb having a mm.l inside diameter, for example, it has been found that 3 mm. of argon. or argonl and neon, gas pressure operates satisfactorily. The coated conductor also functions according to the previous description if the pressure is varied from approximately 1 mm. to 5 mm. 'of such gases with the vcoating of insulating material inthe form of discrete particles, each particle being relatively impermeable and insulative, said insulating coating covering substantially the entire active interior surface of said strip and being porous, to

allow a cathode glow discharge to pass through the interstices between said particles, and yet offering a material electrical resistance, whereby current iiow via said starting strip is minimized both before and after establishment of the positive column discharge, and whereby the oute'v surface of said strip reduces light emission from a portion-of said tube.

2. A device according to claim 1, wherein said metallic strip is constituted by a plurality of dis-l crete sections, separated electrically from one. another by gaps.

3. A tube according to claim 2, in which said starting strips are of unequal length, whereby starting is facilitated.

4. A tube according to claim 3, in which said starting strips are two in number and .the length of one strip is to the length of` the other strip substantially in the ratio of 39 to 46.

5. A device according to claim 1, wherein one side of said strip has an area not substantially less than the longtitudinal cross-sectional area of said positive column discharge.

6. A positive column gas discharge tube, including an elongated envelope containing an ionizable gas, a pair of spaced electrodes sealed within said envelope at the respective ends thereof, a metallic trough-shaped strip formed of' conductive material, extending within said tube for a substantial portion of the path of the positive column discharge between said electrodes and partly surrounding said path, said strip being spaced apart from both electrodes and'bearing at least upon the side thereof facing said discharge a substantially complete covering of a porous material formed of discrete particles, each `particle being relatively impermeable and insulative. said coating acting by its presenceto give said strip a relatively rapidly rising voltage characteristic when said strip functions as a cathode. thereby permitting establishment of a starting'cathode glow discharge but minimizing the current thereof, said cathode glow discharge taking place through the interstices between said particles, and whereby said strip reduces light emission from said tube at a portion thereof back of said strip.

7. A fluorescent lamp including an elongated glass tube having therein a filling of at least one inert ionizable gas and mercury vapor, two main discharge electrodes spaced apart within and at the endsvof said tube, so that there is formed therebetween, upon excitationV of the lamp, a

luminous positivecolumn discharge circumferentially defined by the wall of the tube, a metallic -trough-shaped starting strip partly surrounding the dmcharge space and extending longitudinally a substantial distance along the discharge 'path'. e

and having upon the active side thereof a coating of a porous fluorescent insulating material .formed of discrete particles, each particle being relatively impermeable and insulative, the length and area of said strip exceeding that suiicient to shunt current from the positive column during continued operation o f the lamp in the absence of said coating, whereby said strip reduces light emission vfrom the tube in a rearward direction and passes threthrough substantially no current during the time when said positive column discharge exists, due to the presence upon said strip of said coating, said coating being suiiicientlypermeable to allow the establishment therethrough of a starting cathode glow discharge, said discharge taking place through the interstlcesbetween said particles.

8, Device according to claim 1, wherein said tube is formed as an elongated cylinder and said starting strip has a semi-cylindrical configuration whereby said strip can be separately formed and inserted into said tube as a discrete element lying closely adjacent to a portion of the internal longitudinal wall of said tube.

i MILES PENNYBACKER.

REFERENCES CITED The following references are yof record in the file of this patent:

UNITED STATES PATENTS

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