CA1243721A - Dischage lamp arc tube having opposite hemispherical ends and an intermediate conical region - Google Patents

Abstract

ABSTRACT
This invention provides a specially shaped arc tube for a high-intensity arc discharge lamp, particularly a lamp for vertical operation. The arc tube has a hemispherical top with radius R1, a hemispherical bottom with radius R2, and a middle section being a frustum of a right circular cone which on one end mates with the top and on the other end mates with the bottom so than the arc tube has a smooth and continuous surface. R2 is greater than or equal to two millimeters. The ratio R1/R2 is always greater than one, and in preferred embodiments R1/R2 is within the range of 1.5 to 3, inclusive. This arc tube virtually eliminates species segregation in metal halide lamps even though the arc is vertical.
The invention also includes an arc discharge lamp employing an arc tube as described herein together with an optimum choice of design parameters, such as wall loading and nitrogen pressure within the outer envelope. The virtual elimination of species segregation of the additives along the arc axis provides this lamp with higher luminous output, lower color temperature, improved luminous efficacy, extended life, an no perceptible flicker with electrical current at fifty hertz compared with counterparts in the existing art.

Description

372~
D-24,816 -1-DISCHARGE LAMP ARC TUBE HAYING OPPOSITE HEMISPHERICAL ENDS
AND AN INTERMEDIATE CONICAL REGION

CROSS REFERENCES TQ RELATED APPLICATTQNS
U.S. Appl~cation (S.N. 645,659), containing related subject matter ~iled concurrently herewith and assigned to the assignee of this application, issued September 9, 1985 as U.S. Patent No. 4,540,373.
TECHNICAL FIELD
The present invention relates to the field of high~intensity arc discharge lamps and more particularly to such lamps employ7ng an arc tube having t~o opposed hemispherical regions with an intermediate conical region.
BACKGRQUND ART
High-intenslty arc discharge lamps are those in which the light-producing arc ls stabilized by wall temperature, and the arc tube has a wall loading ln excess of three watts per square sentlmeter. These lamps include groups of lamps known as mercury, metal halide, and high-pressure sodium.
Within the arc tube of a high-intensity discharge lamp, operating temperatures generally range between 500 and 1000C and operating pressures may range bet~een one and ten atmospheres. The fill gas within the arc tube may comprise, for example, mercury, an inert gas, and one more metal-halide additives. Thus, the chemical reactions extant within an operating arc tube are quite complex.
Moreover, the arc discharge may be affected by convection currents within the arc tube.
The chemistry ~ithin the arc tube is affected by the shape of the arc tube. Within the existing art, various arc-tube shapes are employed; certain benefits and detriments are associated with each shape.
~..

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- - D-24,816 ~ any co~mer ial ~etal halide ~rc d~scharge l~mp6 or ~eneral illu~ination have ~ sub~t~atially tubul~r ~rc tube with ~ unlfor~
dl~meter. These l~mp~ have the ~dvnnt~ge that t~ey ~ay be operated either horizontally or vertlcally. Some l~mps, desl~ned for horlzontal operatlon only, have sn arched arc tube. ~he ~rcbed arc tube confcr~s ~ith the stead~ ~ate fihape of the arc dlschar~e ~which i5 bowed up~ard because of convectlon currents) thereb~
improvinK l&mp performence. ~or an e~ample of thi~ type of are tube, see U.S. Patent No. 3,858,078, by Roury.
In U.S. Pstent 3,883,766, Pohl te~ches that an arc tube having a non-uniform di~meter, i.e., havin~ an e~panded fiecti3n a~ or near the center of the ~rc tube, ~ncreases efficacy in vertically operated lamps. The e~psnded central section reduces the shear between the upward and downward convective flow ~ithin the arc tube during operation of the lamp.
See also U.S. P~tent No. 3,B96,326, by Pohl, where an arc tube with ~n expanded section has an additional benefit of reducing species segreeation in metal-halide l~p5 for vertical op~ratlon.
Some types of compact source arc discharge lamps haYe arc tubes wlth non-uniform diameter6, such as short arc lamps and he~vily loaded capillary lsmps. Short arc lamps ~enerslly cont~in a spherical arc tube and have an arc di~char~e that is electrode stabili~ed; in these l~mps, the urc length is small compared to the arc-tube diameter, the shape of the ~rc discharge is independent of the shape of the arc tube, ~nd the arc discharge is not sffected by convectlon currents within the arc tube.
Capillsry arc discharge lamps have been ~ade ~ith a slisht bulse at tbe hottest portion of the arc t~be ln order to prevent ~elting of the glass wall. In these lamps, the arc discharge extends to tbe walls of the arc tube and is confined thereby. The arc discharge is not significantly ~ffected by convection currents within the arc 72~1L
D-24,816 tube. Capillar~ ps ar~ unerall~ so ~eavil~ loAded (lu~p wattn~e per square centi~eter of l~t~rnal ~ur~ce areA o~ tbe ~rc tube~ thst they ~st be ~rtlficially ~oled i~ order to prevent the ~rc tu~e ~ro~ nelt~n~.
lu U.S. Patent No. 2,190,6S7, by Ger~er, tblsre l~ disclosed ~n arc tube for vertlcal operatio~ ~herei~ there i~ ~ bul~e in tbe upper end ~ the tube. Germer teache6 that the bul~e is required to prevent meltin~ of the upper end o~ the tube ~llch Is heated ~ore than the remainder of the tube b~ upri~i~p heat. Gsrmer also te~cbes th~t e~ceedlngl~ thlck ~alls of gu~rtz material are required f or the arc tube in order to resist the hl~h temperstures a~d pressures.
ID Canadian Patent No. 508,525, isgued December 28, 1954, by Frsnci~ et al., there ls disclosed nn oval-sh~ped arc tube havi~ a wider diameter qt the upper end than at the lower end in order thnt the operstin~ wall temperature will not vary ~reatly over the surfsce of the arc tube.
Despite the contribut~ons to the art made by the sbove-mentioned patentees, ~lgniflcant challen~es still remain in the fiold.
persi~tent proble~ is that of species se~regation in vertically opersted metal halide lsmps. When tbe src tube is operated vertically (or other than horizontally~, the smount of metal h~lide additive in vapor phase Nay vary substantiall~ alon~ the arc di~charge. Nonunifor~ species concentr~tion results in non-un~orm spectral emission alon~ the arc discharge which adversely affects the color te~perature sad efficacy of these ls~ps.
Lamps desi~ners have sou~ht to minimize specles se~reg~tion in vertic~lly oper~ted l~mps with only modest degrees of success.
Furthermore, there i5 ~n ongoin~ quest to impsove l~p oper~tin~
characteristics, such as luminous eEficacy, eolor temperature, l~mp l~fe, ~nd flicker. It ~ould be sn Advancement ln the art if a~
opti~um desi~n could be provided for vertic~lly operated hl~h-intensity ~rc dischar~e lamps which substsntially eli~inates species se~reeation and provides improved operatin~ ch~r~cteristlcs.

~2~72~

D-24,816 It i~, therefore, an object of the l~vent~on to obviate the dcficlenc~es io the prlor art.
It i~ ~nother ob~ect o the inventio~ to pro~lde an ~rc tube for a high-lntensity arc dlscharge lamp which substantially ell~inates ~peries se~re~tlon along the n~iu of the ~rc ~i~schar~e during operation of the lamp iQ a Yertical (or other thQn hori~ontal) po~itioD.
It is a further object of the in~entlon to provide an ~rc tube Por a hi~h-intensity arc discharge lamp which provldes improved luminous efflcacy ~ith no increase l~ ~al~ loadin~.
It i6 8till another object of this invention to provide ~n nrc tube for a hi~h-intensity discharge la~p whicb provldes light with n lower color temperature thsn that of counterpcrts in the e~istlng art with minl~al sacrlfice in the eeneral color rendering inde~.
Another ob~ect of the inventloll is to provida a hi~h-intensity dlschar~e lamp with e~tended life compared to counterparts in the e~lstlng art.
A further obJcct of the inventlon is to provide ~n src tube for a hi~h-intensity dischar~e lamp ~hich produces light that has virtually no flicker perceptible to the human eye when the lamp ls operated with alternating electricsl current of fifty hert~.
These ob~ects are accomplished, in one aspect of the invention, by the provi~ion of an arc tube for a high-intensity arc discharse lamp, as well as a hi~h-intensity arc discharge lamp for vertical oper~tion employing same, such ~rc tube comprisin~ an elon~ated body with first and second opposed ends. The body of the src tube her~etically encloses an interior. The body comprises a first re~io~ ad~oinin~ the first end, a second re~ion ad~oinin~ the second end, and a third re~ion intermedlate the first a~d second re~ion~.

'7~
D-24,~16 The flr~t re~ion ~5 subst~ntlally hemispheriea'L in ~hape ~lt~ r~d~us ~1 The second re~ion is ~ubstantially hemi~pberical in shape ~ith ~dlus a2. The th~rd r2giDn is ~ub~tantially ~ frustum of a r~ht clrcular cone. R2 is ~reater tha~ or equ&l to two 5 millimeters, and the rstio ~l/R2 ls ~reater than one.
Tbe arc tube h~s two electrodes, one bein~ ~ounted in the firæt end and protruding into the interior sf the ~c tube, the other elect~de being ~ounted in the cecond end and psotrud~ng ~nto the interinr of the arc tube. A fill ;ncludln~ mercury and at lesst one l0 ~et&l halide addit~ve i6 contained within the interior of the arc tube. The fill is capable of eeneratlng and sustalning an electrical arc therethrou~ eans are provided for supplyin~
electrical ener~y to the electrodes.
Vertically operating hl~h-intensity l~mps havln~ arc tubes as 15 hereln described will h~ve substaDtially improved oparatln~
characteristlcs eompared with their counterparts ln the existing art.
B _ F DESCRIPTION OF THE DRAWINGS
FIG. 1 i0 an enlar~ed elevntlonal view of an embodimellt of the lnventlon;
FIG. 2 is an elevational Yiew of snother e~bodiment of tbe invention;
FIG. 3 is a ~raph of luminous efficacy in lumens per watt w;th respect to ~all loading in watts per square centimeter for the embodiment of the invention shown i~ FIG. 2;
FIG. 4 is a graph of l~minous efficacy in lumens per wstt with respect to ~as pressure within the outer envelope measured in Torr for the embodiment of the invention shown in FIG. 2;
FIG. 5 is an elevational view oP the arc tube of FIG. l showing observed temperatures on the wall of the arc tube durin~ vertical 30 operation thereof;
FIG. 6 contains plots of the arc plasma temperature in ~elvin as n function of distance ln millimeters from the top electrode for the embodiment of the invention contained in ~IG. 2 and for a lamp representative o$ the e~isting art; and 37~
D-24,~16 PIC. 7 is ~n elev~tional ~iew of the ~mbodi~ent o~ the ~ent~on shown in FIG. 1 wher~ the contours of th~ co~ve~tlon current ~ith~ a~ opesatln~ arc tube are ~llu3tr~ted.
~ST ~ODE ~0~ C~RRYING ~UT THE INVENTION
For a better understanding of the present lnventlon, to~ether ~ith other and further ob~ects, adv~ntage~, and capsbilities thereof, reference I~ ~ade to the following disclo~ure ~nd appended c181~s taken in conjunction w~th the ~bo~e-described drawlngs.
~I~. 1 shows arc tube 10 having an elon~ated body 12 and central a~is ~-~. Body 12 comprlses regio~ 18 adjolning end 14, reglon 20 ~djoining end 16, and re~ion 22 being intermedlate re~ions 18 ~nd 20. ~egion 18 i5 ~ubstantislly bemispherical in shape w~th radius a1. Line B-8 shows the ~ppro~imate bound~r~ between re~ions 18 and 22. ~eeion 20 i8 sub6tsntlal}y hemlspherical in shape ~ith radius ~2. Line C-C showa the sppro~imate boundary between regions 20 ~nd 22. Region 22 is substantlally 8 frustum of a rlght circular cone wherein the rad~i of the two parsllel plsnes ~-o~ the frustum are appro~imately al and R2 BO that region 22 will join regions 18 and 20, respectlvely, to form a smooth snd continuouS surfcce.
In the e~bodiment shown in FIG. 1, arc tube 10 ~s designed to be operated vertically wlth end 16 beiog the l~wer end, i.e., end 16 is closer to the earth when arc tube 10 is operationally posltioned, and end 14 bein8 the upper end.
Arc tube 10 eDcloses a hermetically sesled Interior 28.
Interior 28 contalns a fill (not shown in the drawing) whlch is capable of ~enerating and sustaining an electrical ~rc therethrough. Electrodes 24 and 26 are ~ounted into ends 14 end 16, re~pectively, of arc tube 10; these ele~trodes protrude into lnterior 28. St~rting electrote 30, ~hl~h is optional, is mounted in e~d 14.

~ ~ ~ 37 D 24j816 -7-Ends 14 and 16 are formed by a novel ~ethod of press ~ealing. For a detailed description of this method, see the U.SO Patent No. 4,54G,373.

As 111ustrated ~n the dra~ing, electrical-current ~s provlded fr4m the lead-1n ~ires to elsctrodes 24 and 26 by means of conventlonal fo~l strlps, e.g., ~olybdenum, imbedded in the press seals.
In the embodiment of FIG. 1, Rl ~ 8 mm, R2 ~ 5 mm, and the ratio Rl/R2 . 1.6. The distance L, between the centers of regions 18 and 20 ~s 20.5 mm. The ~nsertion depth dl of electrode 24 is 4 m~; and the insertion depth d2 of electrode 26 is 4 mm.
With these parameters defined, distance G bet~een the tnterior extremities of electrodes 24 and 26 is 25.5 mm. Body 12 has wall thickness W a~ 1.5 mm. The valu@s of the parameters provided herein are approx~mate and not critical to the 1nvention.
Within intertor 28 in the embod1ment of FIG. 1, the fill is a mixture of mercury, an inert gas (to a~ lamp starting), and metal halide add~tives, e.g.~ sodium iodide (NaI) and scandium iodide ~ScI3~. A conventional heat-reflecting coating 32, e.g., zirconium oxide (ZrO2), covers port~ons of bottom region 20 and end 16 ~n ord~r to ref1ect ~nfrared radiation back into the lower portion of arc tube 10.
FIG. 2 sho~s a high-intensity arc dischar~e lamp 40 intended for vertical operation .~ith lamp base 42 up. Arc tube 10 is operationally mounted within light-transmissive outer envelope 44 with end 16 down. In the embodiment o~ FIG. 2, environment 46 surrounds arc tube 10 and 1s hermetically sealed t~ithin outer envelope 44.

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D-24,816 Sever~l ~xample~ of limp 40 were eon~tructled and ~es~od. ~IC. 3 sho~s a plot of luminou~ output of lamp ~0 in lumens per watt a~
functlon of wall losdi~ of ~rc tube 10 ln ~tt~ per sgu~re centl~eter. As ~a~ be ~een ro~ the plot, the wall loading v~r~ed fro~ ~ppro~lmately 12 to 16.7 ~atts per square centlmeter. This ran~e corresponds to ~atta~e applie~ to l~p 40 r~n~ng from 150 to 212 ~atts. The data in ~IG. 3 perta~ns to en~lron~ent 46 bein~ a vacuu~. The plot 6hows that the l~minous output lncre~ses only slightly with increasln~ watta~e indlcating thQt the arc tube ~eometry is nearly optlmu~.
Further tests ~ere conducted witb lsmp 40 wherein environment 46 wns changed from a vacuum to bein~ a saseous ~111, e.~., nitroge~, ~t v~r~ln~ pressures. FIG. 4 is a plot of luminous output in lu~ens per watt for l~mp 40 operatin~ ~ith 175 watts pow~r ~ith respect to nitro~en pressure withiD outer envelDpe 44 measured in Torr. Tbe plot ~hows thst an optimum heat tr~nsfer is obtained w;th 200 Torr of nitrogen. The dashed coordinate lines in FIG. 4 indlcate the optlmum point on the curve correspondin~ to approximntely 113 lumens per watt w;th 200 Torr of nltrogen. ~hen environment 46 l~ not ~
vacuu~, ~ddl~ional cooling of the outer arc tube wsll6 occurs due to convection currents within the outer e~velope.
The curve in FIG. 4 comprises two parts, Pl and P2, as shown ln the drawing. P1 corresponds to outer pressures of n;trogen less than 200 Torr; P2 corresponds to outer pressures o~ nitrogen e~ceeding 200 Torr. The position of the ~rc dischar~e ~as unstable during oper~tlon of lamp 40 in the lower Pl-region. The ~rc tended to bo~ to one side thereby reduci~g the effective plasma volume producing light. By ~djustlng the outer pressure o~ nltrogen up to appro~imately 200 Torr, the ~rc stably assumed a central pnslt;on within the arc tube whereby ~a~imum utilizatlon of the arc tube volume was realized. In the P2 region, further increasing the outer pressure of nit~ogen h~s llttl effect, ~ any, on luminou~ output.

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D-24,B16 _9_ Once ~he ærc di~charge hag been centrally stabili~ed by conYectl~e cooli~ of the outerwall~ Df tha arc tube, no ~ddltional beneflt~
sre apparent rom urther increa~ing the outer pre~sure.
Tbe 175 ~att power ratin~ used in obtaininls the dat~ of ~IG. 4 corresponds to a wall loadlng o~ 3pproI~mately 13.R ~attg per sguare centimeter ~or ~he arc t~e of l~mp 40. ~eferlr~ to ~IG. 3, tbe dashed coordinate lines indlcate ~ lumlnous output of appro~imately 96 lu~ens per w~tt for l~p 40 oper~ted ~t 175 watts with a ~acuum withio the outer envelope. Thus, Qn iDcrease ln luminous output ~ro~ g6 to 113 lumens per watt 518%), Qpprosimatel~, has been realized by sn optimal choice of pressure wlthin the outer envelope.
The wall of arc tube 10 ~8 very nearly isothermal durin~
operstion ln the embodlment o~ FIG. 2. FIG. S shows the temperature di6tribution over the arc tube ~all durinK vertical operation of arc tube 10 with end 16 down. The temperatures are shown ~t 3is locatlons, labelled Ll, L2,..., L6, in the drawin~. Note that the temperAture variatlon is only approximately 25 over the body of arc tube 10. Thls reBult i8 ~ttrlbutable to the speclal geometry of arc tube 10.
It is believed that lower color temperature is the result of redured species se~reg~tion. A ~qjor function of tbe metal h~lide additives is to improve the output spectrum of the la~p over that o~
~ercury lamps. The metal halide additives, partlcularly ~etal iodides, emit considerable ener~y in the red and other ~igible parts of the spectrum which results in vsstly improY0d color rendi~lon.
On the other b~nd, to the e~tent the metal halides are not present within the arc plasma, or a portion thereof, the benefits of the additi~es are lost. Accordin~ly, the de~ree of species se~re~ation csn be estimated by measurements of corollated color te~perature.
~he Syl~ania ~175 high-intensity arc discharge lamp was selected as A representati~e o~ the existing art. This l~p iG intended for vertical operation; it was 8 strAight tubular ~rc tube; the power-rating is 175 watts. The ~175 ~s a corollated color 3'~
1)-24 ,Blb temper~ture oP 4500~ ~ ~nd A co1Or rendarl~ in~e~ of 65. L~p 40, whlc~ Ig essent~s11~ the ~ame 1~mp ~ the ~175 e~cept for the ~pecial1y ~haped ~rc tube ~ ~a~ been de~cr~bed herei~, hhs a coro11ated color tempernture of 3400~ d ~ color rendin~ inde~
of ~0. ~hus, 1amp 40 exhiblts ~ 6ub~tAntia1 i~provement ln ~o1Or rendering abl1ity (more th~n 1000 ~ decresse 1~ coro11nted eo10r temper~ture). The two 1amps have a s11ght dlffer~nce i~ their co10r render~n~ Ind~ces; however ~ comparl~o~ of the two ~ndlces i5 not meaningful because the chromaticity or co10r temperature~ of the two 1amps dlffer by 60 ~reat ~ de~ree.
Add~tiona1 observatlon~ were made on the 6a~e two 1a~ps wh~rh further 6upport the conc1usion that lamp 40 has ne~llg~ble specie~
~e~re~atlon even thou~h the nrc dischar~e 15 vertiea1. The p1as~a temper~tures of the ~rc~ of both 1nmps were spectro~coplca11y observed nt different polnts nlon~ the arc. PIG. 6 contnlns le~st-sguares ~itted graphs o~ these tamperatures in ~elvln a~ a function of the distance in millimeters Erom tbe top electrode.
In the case of the Sylv~nia ~175 l~mp, labelled as "E~isting ~tt" in the din~rh~, tbe arr plajma temperature is considerably hi~her nesr the top electrode. The ~rc temperature ~ncre~ses appro~imstely 7.5 I per ~llimeter ~rom the bottom electrode towsrd the top electrode. Since the ~ercur~ in the fill burns hotter thsn do the metal halide additlves, thls lndicate6 an ab~ence or undersupply of metal halide additives ~Q the upper arc portlon. On the other hand, lamp 40 has ~ore un~for~ plasma temperatures theou~hout the entire nrc ~ith the upper arc be~ng slightly cooler than the lower arc. The arc temperature decreases approximately

2.2~ ~ per milll~eter from the bottom electrode toward the top electrode. The temperatu~e varlation over the entire arc len~th has bee~ reduced, by spproximately 70~, with lamp 40. ~he ~act that lamp ~0 operstes with an arc di~charge having a sub~tantially uniform plasma temperature over the entire arc di~charge ~37'~
D-24,8l6 corroborates the conclus~on th~t t~e ~et~l h~llde add~tive~ ~r~ in plentiful supply at all points ln t~e arc ~nd tb~t t~e problem of species ~egre~tlon hss been cverco~e by the ~pec~ rc tube geometry employed by lamp 40.
As ~as been explained in the fore~oin~, the speclQl geometr~ oE
~rc tube 10 provldes the improved cper~tln~ ch~r~cterl~tlc~ of la~p 40. Referring to ~IG. 1~ point ~ is the l~;dpoint between the internal e~tremlties of electrodes 24 snd 26. Since ~rc tube lO ~s desi~ned for subst~ntially verticsl operationS arrow U pcin~s upward (~a~ from the earth) and ~rro~ D points downwsrd (toward the earth) when arc tube 10 i~ operationally positioned.
~ IG. 1 shows thst arc tube 10 has greater sur~ace ~rea nnd interior volume in the upper portion of the ~rc tube. Thi~ deslgn fe~ture provides 8reater surface coolln~ of the upper portion of the ~rc tube. Wlthln arc tube 10, heat wlll ri~e into the upper portlon because of gr~vity and inte~ior convectlon currents. The greater cooling abllit~ of the upper surface BreQ is necessary to obtRln a substantially isothermal temperature distribut}on over the wslls of nrc tube 10.
~IG. 7 illustrates contours of coDvection currents within ~n operating ar~ tube 10 positioned with end 16 down. Ihese contours nre 5~0wn as dashed lines in the drswing. The arc di~ch~r~e ~
po~itioned on central a~is A-A. The direction of the convectlon current i5 upward surrounding the ~rc di~chnrge ~nd downward adjacent tv ~he ~nlls of arc tube 10. ~nintenance of continuous convective flow within arc tube 10 i5 esSentiA1 for replenlshin~ the arc discharge with the ~etal halide sdditives so th~t species segre~ation does not occur. Hemi~pher~csl top region 1~ and hemisphe~ical bottom region 20 are critlcal re~ion6 because e~ch re~ion chan~es the direction of convective ~low. Such redirection ~hould occur wlth B ~inlmu~ of turbulence and without causin~ a vorte~ within nrc tube 10. The hemispherical sh~pes of regions 18 and 20 hsve experimentally been determined to be optlmum.

7;~:~
D-24,816 ~ eglon 20 enclo6es the lowest p~etlon of l~terior 28 whlch ~s potentially the cQol~st portion of the interlor. A be~t-reflectlng coatl~, such as ~irconl~ o~ide, i6 frequentl~ employed o~ the e~terior o~ region 20 to assist in ~QintainlD~ the ~et~l halides in 5 vapor state. ~ continuous convective flo~ ~lon~ the walls of re~ion 20 ls l~portant for the purpose~ of supplyl~g ~le~t to the ee~lon (vla ~ot ~ases from the upper re~lon) ~nd b~ s~reepin6 the ~etal halide addltives out of re~ion 20 upward to arc dischar~e.
Experi~entatlon has shown that ~2 should be greater thao or equal 10 to t~o millimeters 80 th~t the ~etal balides ~ddltives will not collect in condensate for~ in the ~pes portion of region 20.
Conlcal re~lon 22 provides an optimum inter~ediate re~ion bet~een hemisphericQl top 18 and hemispherical bottom 20. The increasing r~dlus o~ a conic~l cross-se~tlon as the cross-~ection is 15 advanced toward top 18 providss ~reater sur~ace area snd interlor volume in the upper portlon Oe arc tube 10 ~hich is essentlal to the isothermal property oE the arc tube. Re~ion 22 also provides essentially strul~ht convectlve contours between the opposed hemispherical ends of differin~ rsdil.
~he ratio Rl/a~ is an important desi~n parQmeter in the overall ~eat balance eguation for the arc tube. ~ince the relative surface areas of the upper and lower portions of the arc tube are directly related to the respective radii, the coolin~ abllity o~
each portion ls like~ise related. An opti~um choice of Rl~R2 is 25 dependent on ~any factors, such as the len~th of the arc diLcharge Qnd the electrode insertion depths; the chemical propertles of the fill and ~etal halide additives as well as the internal operatin~
pressure; the electrical volta~e across the electrodes; etc. An optimum ran~e for this ratio for hi~h intensity src dischar~e lamps 30 generally of lO00 wstts or less has been co~puted theoretic~lly ~nd eYperimentslly veri~ied to be approxi~ately between 1.5 a~d 3, preferably wlthin 1.5 and 2.5.

D-24,816 ~24372~

Initial data fsom llfe te~t of twenty e~ple~ of lsmp 4~
indicate that lu~inou~ effic~cy and la~p life w~ll be ~mproYed ~ue to the optimu~ ~hapln~ o~ the ~rc tube. ~t lOO hours, the ~vera~
luminous efficacy for the e~Qmple l~ps I~ ~ppro~imately 90 lu~e~s per wstt. This vslue i6 more than tec percent bl~her than th~t of the st~ndard vertically ~urninS l~p with B tubular arc tube and equ~alent loadin~. The substantisll~ ~mproved efficac~ together ~ith the lower wall loadln~ cu~ests ~hat the avers~e life of la~p 40 will be appreclably e~tended.
La~p 40 provides light which has virtually no fllcker when t~e lamp ~s operated with alternatin~ electrlcal current of approsimQtely fifty hert~. ~or purposes herein, ~llcker is defined as half-frequency l~ht-intensity differences ~hlch ~re parceptlble to the hunan eye.
Photometric measurements of the li~ht output made on lamps In the esisting art in lamp ~O indicAte that the perceptlon of flicker results from mercury radi~tion modulatIon whlch i5 not concealed by radIation produced by metal-halide additlves. The 6rester the de~ree of a~ial mixin~ of the met~l h~lida additives along the arc dischdrge, the less pronounced i5 the eye's perception of flicker.
Flic~er is not a proble~ in arc dischsr~e lamps operated horizontally, because there i5 no aY~al se~re~ation when the arc i~
horizontal. Nor is flickes a problem when the alternatin~ current is si~ty hertz (or higher) because the frequency ~ust exceeds t~e eye's detection ability.
~ licker i~ a serious proble~ in vertically operated ~etal halide lamps of the e~isti~ art operated at appro~imately ~ifty hertz.
Because of the deficiency of additives In the upper arc, mercury radiation is prevalent and flicker occurs. Since lamp 4~ has the feature that a~i~l mi~in~ of the additives i8 unlform alon~ the vertical arc, flicker i6 virtually eliminated 8t fifty hertz~
Radiation emitted from metals in the additives, e.~.~ 60dium and scsnd;um, when ~i~ed with the mersury radiation, is sufficiently intense to overcome the eye' 6 ~ensation of flic~er.

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While there ha~/e been chown wh~t ~re ~t pre~cnt consl~ered to be preferred embsdlmerlt~ of the ~nYesitlon, ~t ~ill be appE~rent to those ~l~llled in tbe ~rt th~t Yarlou~ chcn~5e6 ~nd 3liodif~c~tlon~ c~n be ~ade herein wi~hout departin~ from the scope of the inventlon a~
S def ined by the appended cl~ims .

Claims (7)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A high-intensity arc discharge lamp for vertical operation comprising:
(a) a light-transmissive outer envelope;
(b) an arc tube operationally mounted within said outer envelope, said arc tube having an elongated body with a longitudinal axis, said body having upper and lower opposed ends wherein said lower end is closer to the earth than said upper end when said lamp is operationally positioned, said body hermetically enclosing an interior, said body comprising an upper region adjoining said upper end, a lower region adjoining said lower end, and a third region intermediate said upper and lower regions, said upper region being substantially hemispheri-cal in shape with radius R1, said lower region being substanti-ally hemispherical in shape with radius R2, said third region being substantially a frustum of a right circular cone, wherein R2 is greater than or equal to two millimeters and the ratio R1/R2 is greater than one;
(c) two electrodes, one electrode being mounted in said first end along said longitudinal axis and protruding into said interior, the other electrode being mounted along said longitudinal axis in said second end and protruding into said interior;
(d) a fill contained within said interior, said fill including mercury and at least one metal halide additive, said fill being capable of generating and sustaining an elec-trical arc therethrough;
(e) an arc discharge extending between said elec-trodes during operation of said lamp, the distribution of said one or more metal halide additives being substantially uniform along said arc discharge;
(f) a gaseous fill hermetically enclosed within said outer envelope, said gaseous fill having a steady state operational pressure such that said arc discharge stably assumes a central position within said arc tube; and (g) means for structurally and electrically com-pleting said lamp.

2. A lamp as described in claim 1 wherein said steady state operational pressure of said gaseous fill is ap-proximately the lowest pressure for which said arc discharge stably assumes a central position within said arc tube.

3. A lamp as described in claim 1 wherein the ratio R1/R2 is approximately within the range of 1.5 to 3.

4. A lamp as described in claim 1 wherein the dis-tance between the centers of the upper hemispherical region and the lower hemispherical region is approximately 80 milli-meters or less.

5. A lamp as described in claim 1 wherein said fill within said interior of said arc tube includes sodium iodide (NaI) and scandium iodide (ScI3).

6. A lamp as described in claim 1 wherein said gaseous fill within said outer envelope is nitrogen.

7. A lamp as described in claim 1 wherein said lamp has no perceptible flicker when operated with alternating electrical current of approximately fifty hertz.