US3780331A

US3780331A - Apparatus and method for eliminating microcracks in alumina ceramic discharge devices

Info

Publication number: US3780331A
Application number: US00291489A
Authority: US
Inventors: W Knochel; L Werner; F Lin
Original assignee: Westinghouse Electric Corp
Current assignee: Philips North America LLC
Priority date: 1972-09-22
Filing date: 1972-09-22
Publication date: 1973-12-18
Anticipated expiration: 1990-12-18
Also published as: JPS5245431B2; ES418972A1; BE805060A; GB1439045A; DE2346132A1; JPS4976359A; CA984441A; FR2200617B1; NL7313047A; IT995463B; FR2200617A1

Abstract

A ceramic discharge device including an outer vitreous envelope sealed to a metallic base, a pair of lead-in conductors connected to the base and extending into the envelope with one of the leadin conductors connected to one end of an alumina arc tube mounted within the outer envelope and the other lead-in conductor forming a portion of a support frame for the arc tube and being connected to the other end of the arc tube. The lead-in conductor forming a portion of the support frame being constructed of stainless steel and having a chrome oxide coating formed on its surface. Additionally, a non-photoelectron emissive element forming a photoelectron collector electrically connected to the first leadin conductor may be interposed between the arc tube and the second lead-in conductor forming a portion of the frame and, if desired, the lower portion of the frame may be surrounded by a quartz or ceramic sleeve.

Description

United States Patent [191 Knochel et a].

[ Dec. 18, 1973 APPARATUS AND METHOD FOR ELIMINATING MICROCRACKS IN ALUMINA CERAMIC DISCHARGE DEVICES Inventors: William J. Knochel, West Orange;

Leo C. Werner, Cedar Grove, both of N.J.; Francis C. M. Lin, Flushing, NY.

Westinghouse Electric Corporation, Pittsburgh, Pa.

Filed: Sept. 22, 1972 Appl. No.: 291,489

Assignee:

US. Cl 313/219, 313/227, 313/229 Int. Cl. H01j 61/06 Field of Search 313/219, 220, 227,

Primary Examiner-Roy Lake Assistant Examiner-Darwi n R. Hostetter Attorney-A. T. Stratton et a1.

[ 5 7] ABSTRACT A ceramic discharge device including an outer vitreous envelope sealed to a metallic base, a pair of leadin conductors connected to the base and extending into the envelope with one of the lead-in conductors connected to one end of an alumina arc tube mounted within the outer envelope and the other lead-in conductor forming a portion of a support frame for the arc tube and being connected to the other end of the arc tube. The lead-in conductor forming a portion of the support framebeing constructed of stainless steel and having a chrome oxide coating formed on its surface. Additionally, a non-photoelectron emissive element forming a photoelectron collector electrically,

connected to the first lead-in conductor may be interposed between the arc tube and the second lead-in conductor forming'a portion of the frame 'and,if desired, the lower portion of the frame may be surrounded by a quartz or ceramic sleeve.

10 Claims, 4 Drawing Figures APPARATUS AND METHOD FOR ELIMINATING MICROCRACKS IN ALUMINA CERAMIC DISCHARGE DEVICES BACKGROUND OF THE INVENTION This invention relates to gas discharge lamps employing an alumina ceramic arc tube containing a discharge sustaining fill which includes sodium and more particularly to the structure and method for preventing lamp failure from the formation of dendritic cracks in the are tube wall. i i

The phenomenon of photoelectrons being emitted from the arc tube supporting frame in a gas discharge lamp due to the irradiation of that frame by light from the lamp was known with respect to gas discharge lamps employing glass or quartz arc tubes. These negative electrons which pass out of adjacent metal parts such as the supporting frame pass to the arc tube wall and charge its outer wall negatively. This negatively charged outer wall would cause positively charged sodium ions to migrate through the quartz arc tube wall eventually causing a depletion of sodium ions in the discharge medium and a corresponding excess of iodine ions which results in unstable burning behavior as well as voltage rise in he lamp. Several methods were devised to inhibit this phenomenon. US. Pat. No. 3,484,637 to Van Boort et al. discloses the use of a ceramic tube to surround the nickel lead wire to prevent ultraviolet radiations from the quartz arc tube from reaching the lead wire and hence prevented the release of photoelectrons from the lead wire. British Pat. No. 1,223,955 to Amlong et al suggested intercepting the negative electrons emitted from the lead-in conductor by interposing an oppositely charged strut to intercept the electrons passing out of the lead-in conductor so that they could no longer reach the burner wall and hence draw portions of the additives through the quartz wall. The problems solved by the foregoing prior art was considered to be of no consequence with respect to discharge lamps employing alumina ceramic arc tubes since it was well known to those skilled in the art that sodium and similar metallic ions could not migrate through the ceramic arc tube body as they could a quartz arc tube. One of the principle reasons for chaging to the alumina ceramic arc tube as opposed to the traditional quartz tube was the ability of the ceramic arc tube to withstand such migration and retain within its confines all of the elements originally provided to support a continuing stable discharge.

RECENT OBSERVATIONS Efforts by the lamp industry to develop 1,000 watt ceramic discharge lamps has been characterized by a substantial increase in lamp failures due to leaks developing in the arc tube wall in a pattern of crazed or treelike sports developing on the inside surface of the ceramic arc tube, which tree-like spots, as a result of onoff cycling of the lamp eventually penetrate, as a crack, through the arc tube wall. This phenomenon although not unknown with respect to lesser wattage lamps has been particularly magnified to the 1,000 watt lamp.

In conventional ceramic discharge lamps, nickel plated iron side rods of the frame carry the current to the electrode located at the end of the lamp opposite from the base and a barium getter flash is employed in the base end of the lamp to getter impurities from within the outer envelope and retain a hard vacuum therein. During gettering, since the ceramic discharge lamp operates on AC current, the side rods of the frame are alternately negative and positive with respect to the electrode-tubulation assembly at the lower end of the lamp. These side rods are illuminated with visible light from the arc tube which has much lower quantum energy than does ultraviolet radiation with the result that photoelectrons are emitted from the side rod when the side rod is at a negative potential. These photoelectrons are attracted to the positive surface of the arc tube and on the other half cycle there is no return of the photoelectrons because the polycrystalline alumina is a very poor photo-emitter for the radiation of wavelengths particularly in the visible range. Furthermore, the alumina is a poor conductor of electrons and therefor the charge can not be dissipated through conductor. Accordingly, the electrons that reach the arc tube cannot return. On the next half cycle, when the side rods are negative again more electrons are added to the outside surface of the arc tube until the ceramic surface acquires a DC potential almost equal to the potential of the negative side rods, i.e., approximately 250 volts in the case of a 1,000 watt lamp. This surface of the are tube cannot be discharged since there is no ionic current through the wall thickness of the ceramic body and there is now a DC voltage stress existing across the wall thickness of the ceramic body with the negative charge on the outside and the positivesodium ions on the inside. I

It has been known for many years that the use of a sealing frit to seal the end caps to the tubular ceramic arc tube, which sealing frit generally includes calcia, alumina, silica and magnesia, results in deposits of the frit materials on the inside wall of the ceramic arc tube presumably through evaporation from the sealing frit. It is postulated that the failure from dendritic cracks in the ceramic arc tube is a result of the negative DC potential established on the arc tube wall acting as a driving force to cause the sodium to react with the alumina, calcia, silica, etc., in the presence of oxygen to form crystals on the inside of the arc tube especially at the grain boundaries. These compounds of sodium silicate, sodium aluminate, magnesium sodium aluminate and magnesium aluminate which are formed have different thermal expansion rates than the bulk alumina surface and therefore, during on/off cycling of the lamp creates thermal cracking at locations near surface irregularity defects or at grain boundaries and these microcracks further propagate along the grain. boundaries and flnally crack through the tube wall thus allowing sodium to leak through the crack area and deposit on the wall of the outer glass bulb and hence cause lamp failure.

SUMMARY OF THE INVENTION Three mechanisms have been discovered which inhibit the formation of dendritic cracks in alumina ceramic arc tubes adjacent the base electrode. Each of these mechanisms inhibit the formation of dendritic cracks in the arc tube wall individually with varying degrees of success. In combinations of two or three of the mechanisms, the inhibiting effect is cumulative.

The elimination of conventional nickel plated iron side rods in the current carrying frame which supports the arc tube within the outer envelope and the substitution therefore of non-photoelectroln emissive materials such as stainless steel, chrome iron, tantalum clad nickel or iron, titanium, or platinum or gold plated or coated metal are effective. The second mechanism is the interposition of a photo-electron collector between the lead-in conductor frame and the arc tube which is preferably stainless steel and electrically connected to the other lead wire so that when the frame is negative, the photo-electron collector is at a positive potential. 'Ihirdly, a ceramic or fused quartz insulator, as for example, a thin walled alumina fused quartz tube can be placed over the support frame thus preventing the barium from depositing on the support wire and hence impeding photoelectron emission.

The foregoing is accomplished in a ceramic discharge lamp which comprises an elongated outer vitreous envelope sealed to a metallic base and having a pair of lead-in conductors connected to said base and extending into said envelope; and an arc tube mounting frame connected to one of said lead-in conductors and extending within the envelope for a substantial portion of the length thereof; an alumina ceramic arc tube containing a discharge sustaining fill including sodium mounted axially within the outer envelope between the other of the lead-in conductors and the frame; by providing a frame of stainless steel having a chrome oxide coating thereon and additionally a photoelectron intercept'or means mounted on said second lead-in conductor and extending between one end of the arc tube and the frame. Alternatively, or in addition a tubular ceramic or fused quartz shield may be provided surrounding a substantial portion of the frame particularly in the area of the lower electrode.

BRIEF DESCRIPTION OF THE DRAWING Many of the attendant advantages of this invention will become more readily apparent and better understood as the following detailed description is considered in connection with the accompanying drawing, in which:

FIG. 1 is a front elevational view of a ceramic discharge lamp incorporating the principles of this invention;

FIG. 2 is a side elevational view of the discharge lamp of FIG. 1;

FIG. 3 is a front elevational view of an alternative embodiment of a ceramic discharge lamp incorporating the features of this invention; and

FIG.'4 is a side elevational view of the ceramic'discharge lamp of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in detail to the drawings wherein like reference characters represent like parts throughout the several views, there is illustrated in FIG. 1 a gas discharge lamp generally designated 10 which illustrates a typical construction for a thousand watt ceramic discharge lamp. The lamp includes a conventional threaded mogal base 12 having sealed thereto a conventional glass outer envelope 14. The glass bulb is sealed to a conventional stem press 16 which has sealed therethrough first and second lead-in

conductors

18 and 20 respectively. The lead-in conductor 20 is connected to the base eyelet 22 while the lead-in conductor 18 is connected to the side wall of the base shell. The are tube supporting frame 24 is mounted to the lead-in conductor 18 and extends longitudinally to the top of the bulb 12. A conventional alumina ceramic arc tube 26 including the alumina ceramic body 28 closed off at each end by niobium end caps 30, and having mounted therein at each end tungsten electrodes 32 is electrically connected to the lead-in conductor adjacent the base of the lamp and to the lead-in conductor 18 through the mounting frame 24 at the upper end of the lamp. The alumina ceramic arc tube generally encloses a discharge sustaining fill including an inert gas, mercury and sodium. The are tube supporting frame 24 is centrally retained within the outer envelope [4 by means of upper and lower pairs of conventional arc tube retaining springs 34, which are secured to the supporting frame 24 and coact with the internal walls of the bulb 14. Stainless steel getter mounting frames 36 are mounted to each of the lead-in conductors l8 and 20 and carry the barium getter at 38.

In accordance with this invention, the lead-in

conductors

18 and 20 as well as the current carrying upright members 24 of the support frame and the upper cross piece 24a of the support frame 24 are all constructed of stainless steel, which stainless steel has been wet hydrogen baked to form a chrome oxide coating on the surface thereof. The upper end of the arc tube 26 is physically connected to the support frame 24 by a tantalum, niobium or stainless steel slider 40 which at its ends loosely surrounds the upright members 24 to compensate for linear expansion of the arc tube during operation of the lamp. Electrical connection of the arc tube upper electrode 32 to the current carrying

support frame

24, 24a is by means of a flexible tantalum, niobium or stainless steel loop strap 42 which is connected between the upper end cap 30 and the support frame crossbar 24a. Although the arc

tube support frame

24, 24a is preferably constructed of stainless having a chrome oxide coating thereon because of the nonphotoelectron emissive characteristics of that material it could also be constructed of chrome iron, tantalum clad nickel or iron, titanium, or platinum or gold plated or coated metal since each of these is more or less non photoelectron emissive under irradiation by visible light and particularly visible light in the yellow orange area of the spectrum.

Since in conventional lamps, the deposition of barium on the nickel surface of conventional arc lamp support frames caused by the gettering of barium which during the gettering operation will substantially coat the lower end of the glass bulb as illustrated at 44 as well as the lower portions of the supporting frame, deposition of the barium on the support frame of this invention as well as the inhibiting of the radiations reaching the arc tube support frame can be accomplished by sheathing all, as on the right side of the FIG. 1 embodiment or the lower portion of the supporting frame, as

illustrated on the left side of the FIG. 1 embodiment, with a ceramic or fused quartz sleeve as illustrated at 46 and 48 respectively. The use of this tubular ceramic or fused quartz shield on the arc tube support frame is one method by which the buildup of a negative charge on the arc tube wall can be accomplished.

An even more effective mechanism by which the migration of negative ions from the support frame to the arc tube wall can be prevented is illustrated in FIG. 1 wherein a U-shaped photoelectron interceptor 50 is electrically connected to the lead-in conductor 20 and is interposed between the arc tube support frame 24 and the ceramic envelope 28. Since the U-shaped photoelectron interceptor 50 is electrically connected to the lead-in conductor 20 and the lamp is operated from a source of AC potential, when the frame is at a negative potential and susceptible to emitting photoelectrons when irradiated by the visible light from the arc tube, the U-shaped photo-electron interceptor will be at a positive potential as is the lower electrode 32 and will cause photoelectrons which would normally migrate to the arc tube wall to be intercepted by the positively charged photoelectron interceptor 50. Since the photoelectron interceptor 50 is also constructed from stainless steel having a chrome oxide coating on the surface, it is not particularly photoelectron emissive and therefore will not release photoelectrons to the frame members 24 when, due to the character of the AC operation, the photoelectron interceptor is at a negative potential and the arc tube support frame 24 is at a positive potential.

The embodiment of FIGS. 3 and 4 illustrates an alumina ceramic discharge lamp of the 400 watt type wherein a shorter arc tube is employed and therefore can be suitably support with a single arc tube support frame member. In accordance with the present invention, the embodiment of FIGS. 3 and 4 includes a con ventional outer envelope 52 sealed by means of a stem press 54 to a standard mogul base 56. A pair of lead-in

conductors

58 and 60 are secured through the stem press to the side wall and eyelet respectively of the mogul base 56. The single armed arc tube support frame 62 is electrically connected to the lead-in conductor 58 and extends longitudinally of the outer enve lope to the upper end of the lamp. An alumina ceramic arc tube generally designated 64 including an alumina ceramic tubular envelope 66 closed at its end by niobium end caps 68 and carrying on the interior surface of the end caps tungsten, are supporting electrodes 70 is electrically connected at its lower end to lead-in conductor 60 through electrical connecting straps 72 and electrically connected at its upper end to the lead-in conductor 58 through the are tube support frame 62. The arc tube is mounted at its upper end to the 7 shaped arc tube support frame 62 through a slider mechanism 72 similar to that of the FIG. 1 embodiment and the upper end of the arc tube support frame 62 is physically retained centrally disposed within the outer envelope by means of conventional spring retainers 74 which coact with the interior surface of the outer envelope wall. A tantalum, niobium or stainless steel loop strap 76 is utilized to electrically connect the upper cross member of the arc tube support frame 62 to the upper electrode of the arc tube 70. Similar to the FIG. 1 embodiment a barium getter carrying stainless steel member 78 is secured to the lead-in conductor 60 in the lower portion of the outer envelope.

This conventionally constructed alumina ceramic discharge lamp illustrated in FIGS. 3 and 4 for a400 watt discharge lamp is to a lesser extent subject to early lamp failure because of te irradiation of the barium coated nickel iron support frame by light from the arc tube which causes photoelectrons to be emitted and build up on the arc wall creating, during on/off cycling of the lamp, the dendritic cracking earlier described. To inhibit the migration of photoelectrons from the support frame 62 to the are tube wall where they tend to build up and cause the dendritic cracking, the support frame 62 of this invention is again preferably constructed from chrome oxide coated stainless steel and a photoelectron interceptor 80 is electrically connected to the lead-in conductor 60, and therefor always of an opposite potential from that carried by the support frame, and is interposed between the lower portion of the arc tube support frame 62 and the arc tube body 66 to intercept any photoelectrons which may be emitted from the arc tube support frame when the arc tube support frame is negatively charged and thus prevent those negative ions from building up on the outer wall of the arc tube body 66.

In accordance with the concepts of FIG. 1 embodiment, the FIGS. 3 and 4 embodiment may also-include a ceramic or fused quartz sleeve positioned over the en tire length, or at least the lower portion, of the arc tube support frame 62 as illustrated in phantom at 82.

As will be apparent from the foregoing, the alumina ceramic discharge lamp of this invention may employ any one or all of three photoelectron emission inhibitors either singly or in combination to thereby inhibit the migration of photo-electrons from the arc tube support frame to the wall of the alumina ceramic arc tube. Each of these mechanisms inhibit photo-electron emission to a greater or lesser extent and may be employed singly or in combination to the extent that it is necessary to inhibit such emission.

What we claim is:

1. A ceramic discharge lamp comprising an elongated outer vitreous envelope sealed to a metallic base, a pair of lead-in conductors connected to said base and extending into said envelope;

an arc tube mounting frame connected to one of said lead-in conductors and extending within said envelope for a substantial portion of the length thereof, an alumina ceramic arc tube containing a discharge sustaining fill including sodium mounted axially within the said outer envelope between the other of said lead-in conductors and said frame, said frame being stainless steel and having a chrome oxide coating formed on the surface thereof.

2. A ceramic discharge lamp according to claim 1 wherein photoelectron interceptor means is mounted on said second lead-in conductor and extends between one end of said are tube and said frame.

3. A ceramic discharge lamp according to claim 1 wherein a tubular ceramic shield surrounds a substantial portion of said frame.

4. The method of improving the performance of a high intensity discharge device adapted to be operated from an AC potential source and comprising an elongated alumina arc tube having electrodes operatively sealed therein proximate the ends: thereof and enclosing a discharge sustaining filling comprising inert gas, sodium and mercury, said are tube being supported within an outer envelope by supporting frame means comprising at least one elongated lead-in conductor spaced from said are tube and longitudinally extending substantially the entire length of said are tube, and a short lead-in conductor positioned substantially at an end of said outer enveloped, said lead-in conductors being connected to said electrodes of said device, and said outer envelope enclosing a vacuum which method comprises:

a. making all components of said supporting frame means which are exposed to the intense radiation as generated within said are tube, when operated, substantially non-photo-electron emissive under bombardment of such radiations, and electrically connecting to said short lead-in conductor an addi tional conductor which is non-photoelectron emissive when exposed to said intense radiations, and supporting said additional conductor in receptive proximity to said are tube to intercept and neutralize any photo-electrons which might be generated from the radiation of said elongated lead-in conductor and any supporting members associated therewith when said elongated lead-in conductor carries a negative half cycle for the AC potential use to energize said device.

5. The method of improving the performance of a high intensity discharge device adapted to be operated from an AC potential source, which discharge device includes an elongated alumina arc tube having electrodes operatively sealed therein proximate the ends thereof and enclosing a discharge sustaining fill including sodium, said are tube being supported within an outer envelope by a supporting frame, said supporting frame being spaced from said are tube and extending longitudinally substantially the entire length of said outer envelope, and a short lead-in conductor, said lead-in conductors connected to said electrodes of said device, and said outer envelope enclosing a vacuum, which method comprises:

a. interposing a non-photo-electron emissive metallic member between said are tube and said elongated lead-in conductor and b. electrically connecting said non-photo-electron emissive metallic member to said short lead-in conductor to thereby intercept and neutralize any photoelectrons which might be generated from the irradiation of said elongated lead-in conductor and associated supporting members with said nonphoto-electron emissive metallic member being, during operation, always of the same electrical potential as said short lead-in conductor and of the opposite electrical potential of said elongated leadin conductor.

6. A ceramic discharge lamp comprising an elongated outer vitreous envelope sealed to a metallic base,

a pair of lead-in conductors connected to said base and extending into said envelope,

an arc tube mounting frame connected to one of said lead-in conductors and extending within said envelope for a substantial portion of the length thereof;

an alumina ceramic arc tube containing a discharge sustaining fill which includes sodium mounted axially within said outer envelope between the other of said lead-in conductors and said frame, said frame being constructed of an electrically conducting non-photo-electron emissive material.

7. A discharge lamp according to claim 6 wherein photoelectron interceptor means is electrically con- 7 nected to said other lead-in conductor and extending between one end of said are tube and said frame, said photoelectron interceptor means at all times during operation of said lamp being of the same electrical potential as said other lead-in conductor and at the opposite potential of said frame.

8. A discharge lamp according to claim 6 wherein said lead-in conductors and said frame are constructed from chronic oxide coated stainless steel.

9. A ceramic discharge lamp according to claim 6 wherein a tubular insulating shield surrounds a portion of said frame.

10. A ceramic discharge lamp according to claim 9 wherein said insulating shield surrounds a substantial portion of said frame and is fused quartz.

Claims

1. A ceramic discharge lamp comprising an elongated outer vitreous envelope sealed to a metallic base, a pair of lead-in conductors connected to said base and extending into said envelope; an arc tube mounting frame connected to onE of said lead-in conductors and extending within said envelope for a substantial portion of the length thereof, an alumina ceramic arc tube containing a discharge sustaining fill including sodium mounted axially within the said outer envelope between the other of said lead-in conductors and said frame, said frame being stainless steel and having a chrome oxide coating formed on the surface thereof.

2. A ceramic discharge lamp according to claim 1 wherein photoelectron interceptor means is mounted on said second lead-in conductor and extends between one end of said arc tube and said frame.

4. The method of improving the performance of a high intensity discharge device adapted to be operated from an AC potential source and comprising an elongated alumina arc tube having electrodes operatively sealed therein proximate the ends thereof and enclosing a discharge sustaining filling comprising inert gas, sodium and mercury, said arc tube being supported within an outer envelope by supporting frame means comprising at least one elongated lead-in conductor spaced from said arc tube and longitudinally extending substantially the entire length of said arc tube, and a short lead-in conductor positioned substantially at an end of said outer enveloped, said lead-in conductors being connected to said electrodes of said device, and said outer envelope enclosing a vacuum which method comprises: a. making all components of said supporting frame means which are exposed to the intense radiation as generated within said arc tube, when operated, substantially non-photo-electron emissive under bombardment of such radiations, and electrically connecting to said short lead-in conductor an additional conductor which is non-photo-electron emissive when exposed to said intense radiations, and supporting said additional conductor in receptive proximity to said arc tube to intercept and neutralize any photo-electrons which might be generated from the radiation of said elongated lead-in conductor and any supporting members associated therewith when said elongated lead-in conductor carries a negative half cycle for the AC potential use to energize said device.

5. The method of improving the performance of a high intensity discharge device adapted to be operated from an AC potential source, which discharge device includes an elongated alumina arc tube having electrodes operatively sealed therein proximate the ends thereof and enclosing a discharge sustaining fill including sodium, said arc tube being supported within an outer envelope by a supporting frame, said supporting frame being spaced from said arc tube and extending longitudinally substantially the entire length of said outer envelope, and a short lead-in conductor, said lead-in conductors connected to said electrodes of said device, and said outer envelope enclosing a vacuum, which method comprises: a. interposing a non-photo-electron emissive metallic member between said arc tube and said elongated lead-in conductor and b. electrically connecting said non-photo-electron emissive metallic member to said short lead-in conductor to thereby intercept and neutralize any photoelectrons which might be generated from the irradiation of said elongated lead-in conductor and associated supporting members with said non-photo-electron emissive metallic member being, during operation, always of the same electrical potential as said short lead-in conductor and of the opposite electrical potential of said elongated lead-in conductor.

6. A ceramic discharge lamp comprising an elongated outer vitreous envelope sealed to a metallic base, a pair of lead-in conductors connected to said base and extending into said envelope, an arc tube mounting frame connected to one of said lead-in conductors and extending within said envelope for a substantial portion of the length thereof; an alumina ceramic aRc tube containing a discharge sustaining fill which includes sodium mounted axially within said outer envelope between the other of said lead-in conductors and said frame, said frame being constructed of an electrically conducting non-photo-electron emissive material.

7. A discharge lamp according to claim 6 wherein photoelectron interceptor means is electrically connected to said other lead-in conductor and extending between one end of said arc tube and said frame, said photoelectron interceptor means at all times during operation of said lamp being of the same electrical potential as said other lead-in conductor and at the opposite potential of said frame.

8. A discharge lamp according to claim 6 wherein said lead-in conductors and said frame are constructed from chrome oxide coated stainless steel.