Energy Balance of a High- Pressure Sodium Arc Tube

Energy balance of a high- pressure sodium arc tube

H idezoh AKUTSU* and Naoki SAITO**

This paper describes in detail the dependence of the energy batance of a high･pressure sodium arc tube on discharge parameters. The total radiation and the thermal conduction loss of the arc tube were estimated by calculating the thermal dissipation loss from the measured wall temperature of the tube envelope. The results thus estimated showed a reasonable consistency with experimental data on the absolute visible radiant power and the published results by previous authors as well. in the low sodium vapor pressure region, the total radiation Pr (W/cm) is influenced sharply by various discharge parameters, while in the high sodium vapor pressure region (above about 70 to 100 Torr), it is determined mainly by the power input Pin (W/cm) through the linear equation Pr;~~0.80 (Pin-6.7) for a sapphire arc tube. It has been concluded that the high efficacy of a high-pressure sodium lamp stems from the low thermal conduotion loss of its arc tube, as well as its high luminous efficacy of visible radiation.

properties of the arc tube. Furthermore, a gen- 1. Introduction eral guidance fcr improving the luminous ef ficacy Knowledge about the energy balance in an arc of a high-pressure sodium lamp is shown. tube of a lamp will offer a foothold not only for understanding the processes occurring in the arc tube also fcr improving its luminous efficacy. The 2. Experimental procedures energy balance of a high-pressure sodium arc tube Fig. I illustrates the construction of arc tubes has been studied by several authors ;i)~) theoreti- employed in our experiments. The tube envelope cally, through calculating the radiant power and was made of sapphire, with inner diameter 8.0 mm the thermal conduction loss on the basis of the and wall thickness 0.8 mm. Electrodes were sealed temperature distribution in the arc tube, and ex- with a gap of around 80 mm between the tips. The perimentally, through zneasurement of the absolute sodium amalgam having 66% sodium mole rate radiant power from the arc tube. The obtained was introduced in the arc tube at a fixed weight results have not shown the whole aspect of de- of 20 mg. Xenon or neon-argon Penning mixture pendence of the energy balance on various dis- was also put in the arc tube as a starting gas at charge parameters, partly because of relatively 2 different pressure levels, 4 and 30 Torr. The limited ranges of the discharge parameters chosen thermal protective layer of tantalum foil was ap- in those studies. In this paper another method for analyzing the energy balance is discussed, in whieh the radiant -80 9 l~ power and the thermal conduction loss over a wide o range of each diseharge parameter are estimated through calculating the thermal dissipation loss 3 2 984 56 7 from the measured wall temperature of the tube J SQpphire Tube 2. envelope. The estimated results are examined in Alumino End CGP comparison with experimental data on the radiant s- Nb - Tube 4. W - Coil E ectrode 5. Sodium ArnolgGm 6, Xe-Gas or Ne-0.5eleAr Mixture * Research Laboratory 7. Thern~GI ProtecNve LO yer ** Light Division 8, Coldest Spot 9. pt - pt, 13alcRh Thermocoup!e Matsushita Electronics Corporation, Takatsuki, Osaka, Ja pan Fig. 1 Colestructio,e of the are tube.

J. Light & Vis. Env. Vo!. 3 No. 2 1979 ll plied at both ends of the arc tube to adjust the I 300 o Na -ernolgem 20mg ( Na 66mole Qlo) cold-spot temperature, viz. the temperature of the coldest spot which exists inside either one of the ~ ~~~~t~)~~/ Pewer Input Pifl (W/cm) ends of the arc tube. It should be noted that, in 200 our experiments, the sodium and mercury vapor o 5~(~e~ ¥:~',~ ~~ pressures inside the arc tube were determined only ~ by the cold-spot temperature, as the sodium amal- ~t~~~1~ ¥~Q~¥ tS~ 'b:0~1" gam composition was kept the same. When iumi- i OO nous output and spectral distribution were meas- ¥~~~ ured the arc tube was mounted in an evacuated ~ E ¥ ~"'e~" outer-bulb. ~ Arc tubes were operated horizontally on 60 Hz l OOO ¥.,. ,..~ ac supply with choke ballast in series. Each tube ~ Xe 50 Tor~ was measured for its wall temperature, its lumi- 'e-o" Xe 4 Torr nous output and spectral distribution against two 900 variables ; power input and cold-spot temperature. 650 700 These 2 variables were independently adjusted by Cold600 - Spo? Temperature 750 ('C ) 800 changing the dimensions of the thermal protective layer. For measuring the wall temperature the arc Fig. 2 Wall teemperature as a fut~ction of tube was set up in an evacuated demountable ap- eold-spot tev~rperature at the ce?~tral paratus, with a Pt-Pt, 13%Rh thermocouple (di- poi,et of the (~rc tubes filled with ameter 200 ,~) cemented at the measuring spot of xenon. the tube surface. The wall temperature T~,* at the spot confronting the coldest spot inside the arc I 300 tube was measured with the assumption that it oNe-emeleem 20mg (NQ66melee/*) represented the nearest value of the cold-spot tem- O perature, as shown in Fig. 1. All the measured ~S¥ values were reproducible within a maximum :200 - o ¥ ¥~:~~ ~~' _~~__e--j~~~~ spread of :t:5'C. ~~ h ¥ 1~q.¥ ~~c~_~~ /~~~.~ 3. Results oa b¥ ¥~e¥.eL. 11,~~~ The experimental data in this paper were ana- ¥e ¥¥ ¥ lyzed mainly as a function of the cold-spot tem- ~ ~~_ l; 8 oo ~:s Power ~~p:Jt Pin (W'er~~ perature T,~... Based on experimental results of ~ ':4~ Ozaki5) on the relationship between the sodium vapor pressure and the self-reversal width of ~ 4~- Ne-o.5S<,Ar 30Torr -o*e- Ne-0.50/.Ar 4Torr broadened sodium D-lines, the cold-spot temper- 90 o ature T~,, of 650 to 750'C in our experiments seems to correspond with sodium vapor pressure of 20 to 600 6 so 700 7 50 800 80 Torr. coid -Spot Tempe~a tu~e ('C ) Fig. 3 Wall te,?rper(~ture as a function of cold-spot temperature at the central 3.1 Luminous efficacy and wall tem- poi,et of the arc tubes filled with perature ,eeo,e-argo,~ mixture. Fig. 2 shows the wall temperature T~(O) at the central point of the arc tubes filled with xenon, as difi:erence between the arc tubes filled with xenon a function of the cold-spot temperature T~,.. The and neon-argon was such that the wall temperature power input per unit length Pi~ which will be de- T~(O) went higher with the increase of neon-argon fined later on was chosen as a parameter. Within filling pressure and, even in the region of high the region below the cold-spot temperature T~,, of cold-spot temperature, it still continued dropping 700'C, the wall temperature T,.(O) at a fixed power in different modes depending on neon-argon filling input dropped rather sharply with the rising of the pressure as the cold-spot temperature was raised. cold-spot temperature, and took a slightly lower Then these tubes were fabricated to make lamps. locus to the increase of xenon filling pressure from The overall luminous efficacy eyL and the luminous 4 to 30 Torr. Then, as the cold-spot temperature efficacy per watt of visible radiation K of each lamp was raised higher still, the wall ternperature T~ (O) were obtained from the measurements of its lumi- approached a given level for each power input, nous output and spectral distribution, respectively. irrespective of the xenon filling pressure. Fig. 3 Its visible radiant efficiency ~.~ was calculated by : shows the wall temperature T~ (O) of the arc tubes filled with neon-argon mixture, which intrinsically ~･~= vi/K ･････････････ (1) gives a higher wall temperature than xenon. The Figs. 4 and 5 show the obtained radiant proper-

12 J. Illum. Engng. inst. Jpn. 舛o oNo－omotgom20mg〔封G66mole％l o至thew呂ll七e瓢perature．掛hisindic我tes捻att盗e oXe50Torr wall temperature is useful as a 搬eas縫re for ana－シ蝸、κ＼ 1yzing t負e energy b＆1ance in the arc t厭be． E 130 考＼

E 3、2 ↑o看a箋rad韮a重韮on and漁erm…蝸 J ω 120 co轟d“cl韮on望OSS η之‡マrv・K Poweゴ1鵬ut PinlWlc鮪 →←r〆一；～58．5 T難e eiectyical power iBp就P伽supPlied per unit 一悼：～46．5 lengt難 of the arc t駁be まs 最rst traゑsferred to the o l lG 瞬一1～34．5 駐rc plasma並the＆xiai core portion of the tube、 O，36 Almost all o妻the tr＆nsfeyred powerまs co強sidered タ＾420 旦一K ／／F一 to be delivered to the tube w段ll，except in the vicinity of t虹e electrodes，by various processes such ⊂400 0，34 ＆s radi＆tioR，the℃m＆1 con（luction and di登usion o£ ≧ 石！ ch＆rged or excited pεtどt重cles． T｝1e energy b＆lance ① α：380 ㌦ 0．32 equεしtion on 七簸e tube envelope for unit lengtL of ＼ t｝注e翫rc t摂be t五us becomes：む 3560－暑＼・o．30 ω ｝ P搬＝Pr＋P麟｝側……一………・一…………（2｝ in which PT a豊d p翫，加（ienote t蓋e tot31r＆di鼠tioB per 028 E 340 u麟lengthandt鼓ether撚aHossd重ssまpatedfro加

650 700 ア50 施et騒beenvelopeperunitlength，respectively，and Pt1、，船isgivenby： Cold－S轟ot TefnperGtロre ｛。C l P傭，御＝Pc季切十P7，盟 Fig．4 Rα識α？鴬斜¢0ηθゲ撹θSαSα∫％冗0一一3・え（κ）d2釜婆）＋πφ・σ・ε（」じ）・姻4…（3｝玩0π0／ooJ4－Sρ0言むθ？πpε？ヤα重％ゲε 0∫ 古んε αゲ0 む％bθ ガ泥θd ω茗亡1乞 where P。，，．is the thermal cond縫ction loss to the end の6？30？30 direction through the tubing w呂11（sapphire）per unit｝ength，P．，，，is塩e thermal radねtion loss from oNo－omαgom20mg｛NG66mo1豊％｝ the tube e簸velope pey駿niも1ength，x is t蓋e distance 120 o鯉6一〇・5％Ar 30Torr x＿ン x1x’xイ £rom the central poiηt to施e end direction of the ε uO ／ tube，Tω（x）is the wall temper我撮re我t distaRce」‘， 8 重s t難e cross－sectional 翫rea of the t級bing wailン iOO ノノ入（x） and ε（x） are the thermal conductまvity εm（i

E… 田 the emissivity， yespectively， of the t罎be envelope go J at （iis七＆nce κンσ is Ste賃an－Boltzmann consta盤t and φo is t溢e o級ter di我狐eもer of the tube． 80 o 艸：～33，0 丁簸etot段1r呂di駐tioゑper臓nitlengt簸P．c3nbe obt我童ne（i fro磁Eq雛εしtion（2） by giving both values， 440 o，35 》 pi札我ndP漁，”丁烈ev＆lueP｛帆三nW／c瓢wasesもimated ／！ f罫om the re1＆tion：鼠！x ／x 420 P珈＝＝伊珈（γ五一78）／yLL…・……一…・……・・（4） 03⊂）礁き、望＞ぐ一編 w簸ere W｛．まs甑e tot＆！power三np貸t to施e arc tube 』 0400① α二 i強watt，Listhegapbetweent強etipsoftheelec一 o．25コ廿odes沁cm，yL is the arc tube volt段ge in volt 皮毒＞一 ’F580 aηdγ。is the total eiectyode－f＆11voltageンw短ch w呂s ヒ」恥 £ ass登med to be g V、G）In c＆1c慧1ating the塩ermal o．20、豊 diss量patio灘loss Pθ、、，。，we de翫lt with the cen加al part 〉・一 560E… of the 貧rc tube w鼓ere the w＆11－tempey3tu薫e distri－」 650 1GO 750 buもi・nw呂s至airlyunifor磁．1，難evalueP．，甜was Ca1Cu玉段もed U簸der ＆n ＆SS登τnptiOn t烈at bOt｝生 ValueS， Cold－Spot Tempero㌻ure 1。C｝「加（x）andε（x），areconstantovertheuniもlength F圭9．5 ∫～α（滉α？覚ヌ）ザo夢θ¢擁θ8αsα∫％乾o一 o£ t墾e tube envelope；T，。（x） is yepyesente（i by the あ0％0／00菖d－8pO孟重θ伽pθ？U哲呪ゲθ walltempeヱ厩ureT，。（0）at捷eceηtr我1point（x－0） 0∫ オんθ α？・0 オ％～）θ 五π8d 側¢舌1乳 of the t疑be as alrea（iy given i益 t猿e p夢ecediRg sec一箆θ0？Z一α？轟90π ？π茗ρ5カ％？・θ．伽nandε（x）takesaconsもantvalαeO．25wh重c難 will be disc群ssed玉＆もer o捻．For esti狐＆ti簸g抜e value ties ＆s a f疑netion o£ the cold－spot te通per3t蓑i・e． P。，甜，t盈etぬermalconduetiv三tyofsaP晦reλ（x）in Fro孤 comparison with experime鍛t＆1res級1七s in atemper＆turera強gebetweeR1，20Gand1，500Kwas Eigs，2＆n（i3，重t beca搬e dear t強a七七難e increase and assumedtobe＆PProximatelyt簸es磁eas協atof decre＆seo£thevisibleraδi我n七e銀ciencyη，。closely 簸ormal opaque ai磁in＆l vi名．0．G63W／cm・K，7）be－ related to the droPPing ＆n（i risi貧9， respective玉y， c3use the value of sapPhire ls not㎞own．Wi撫

J、Ljgh圭＆Vis、εnv、 Vd、3No．2 工979 13 60 exceeds ～100甲orrンa s重milar linear relation holds：

Xe3。T。r「ムロ65。｛75。 Pγ応0．78（P乞π一6．7＞一一・一一一一一一・”一一・・一一一一・｛6）諭・ If suc蓋a1三near relation were valid董or the pres－ 40 ent ＆rc tube oper＆ted ＆t e3c蓋 cold－spoも tenlper段一！轍 t被妻e，七熟e t蓋er灘al conduction loss per unit Iength P。and the t笠ansτ嶽ittanceτcould be easily obt＆ined ε from t鼓e hoyizontal axis abscissa and the slope，起 20 ≧ re8pect重veiy，of each straまght li簸e plo七ted in Fig．6・

／江 However，this lea（is to a contrad三ctio豊 in t簸翫t the 6・7 ／ 1 ／癒ermal cond殺ction loss並cl■eases when惚e cold－ f ／o O spot七empe1・ぬre重sr我重sed（短f呂ctthewalltem－韓e－O、5％Ar5GTorr ／ per抗七ure drops as show蒸重n Figs．2and3）、More－／る／ overアもhe transmitt＆nce becomes highe翌＆9aまnst the ／ 750。Cぼ環．／ inαease of sodium vapor press根re．There量ore，it may be more reasonable to considαin the fo110w一重ng way：The塩ermal coRduction loss varies with 施e power玉np就，wh三le the tr踊smi枕翫nce stays aL most constant ＆9＆inst the sodium vapor pressure since t五e probability t五at absorbed photons＆re re－ er註itted ＆s ya（iiation is hig蓋．／ In order to est重lnate the thermal conduction loss o 20 40 60 80 per 縦nit length P，by apPlying Equ我tion（5），the Power 工n puち Pi自（W／om〕 tr＆nsmitt＆nceアw＆s ass登med to be＆co簸st翫且t value O．8．This was supported by the alre＆dy魚entio簸ed Fig．6銃θ？・8」罐o箆s1匹勿bθ伽εθ冗重o翅負nding that the val縦e τbeca搬e apProximεttely O．8 ゲα｛滉α寵0冗 Pヅ αり㍑！得0初θTを箆P祝古 i簸h主⑳coid－spot temperature regio難．Based o盤 P額． th重s ass殺mpt重on and both of the （3alcul＆ted valuesナ P｛．andP．，伽ev段luesP。asafunctionofthecold－ t蓋is ＆ssu磁ption and the meas殺red va1根e of spo七tempey我ture were esも重mated，＆s s蓋own in Figs． 20K／cm2for｛4町，．（x）ノαx2｝／津淵。，t漉v麟eP。，マ，w鵠 7我nd8．頚the region below the cold－spot tem－ estimated to be around O．3W／cm． perature o£ 700。C，t｝隻e thermal conduction loss of F呈9．6s姦ows the total radiation per嚢nit iength xenon一且11edtubes decreasedw三th the increas沁9 P．，caicu13ted foy the aγc t職be負11ed with xenon or cold・spottemperatureandxe捻on負n重ngPressare，簸eon－argo鍛m重xture of30Torr呂nd plotted3gainst whiie itまnc王℃ased wi捷愉e increasing Power input．俵e powey input per unit lengt難P｛，、．？he cold－s夢ot In a higher teml）eratu1’e reg重on t簸a盤this，it became te狐peraturewast＆ken＆s＆P撚獄eteLThe虻e1＆一 iess dependen七〇n t五〇se p＆ra搬eters and converge（i tion曲ip betweeね施e tot＆I radiat圭on我nd施e power to a ce王・taill value between6＆nd7W／cm．The iBput took str＆ight lines pert重nent to eac狼 cold－旗ermal conduction Ioss o£伽e＆rc tubes丘lied with spottemper＆ture；asthecold－spottemper3ture neon一段rgOnm重xtureS紮oWedSimi1＆ytrendSinde－ was raised，those slopes first bec我me steeper and Pende簸ce oη 七烈e col（i－spot te1捻perat嚢re and the then，in the region above the cold－spot temperature power input．丁熟e diffαence was s疑ch that，con－ of700。C （xe葺on盗11e（1） or750。C （neon一鼠rgon漁ix－ tr＆ry to xenon一且旦ed t犠bes，it greatly圭Rcre象sed with t疑re 登lle（i），apPro＆ched t蓋e consもant vεし1縦e O．8． Elenba＆s8）has shown that the following達inear

rel＆tion between 施e toもal radi3tio難 Pヂ＆nd 撒e 16 power 重nput P雀砂 is val圭d £o支’＆｝1igh－P1■essure mer－ ONG－omolgom20mgtNG66獅o乱eシ』） ε 14 cury arc tube：ミ、誌 we「Inpu†Pln｛W／cm｝ ≧ P7需τ（P伽一Pc）一……・・…一…………・一…（5） 12 衛0．72（P茗π一10）一・一一一＋・一一・一一一一一・一一・一一｛5｝ノ lO Jo whe王・e P。is the撚ermal conduction loss per unit C＝ o 8 leng旗at a periphery o至the arc plasm呂po瑚on oコ where the electric21 cond慧ct重vity （irops to zero， T コ 6 o仁 is t豆e transmitt段nce of t｝三e radiation escaping o δ 4 伽o域ght難ecooleroUterpoi・tio葺of七hearcColamn E Xe30Torr お 2 Xe4 Torr and七hen the tube envelope，3｝and P。an（iτεぼe al－ £ ← o most const段nt valaes10W／cm and O．72，respecもive－ 600 650 700 750 800 ly，i買espective of施e power三np級t＆nd the mercury v＆por pressare重n t五e arc加be．We蓋＆ve31so re－ Cold－Spo㌻Temperoturel。C） poyted in the previous paper9｝ that，for a h19五一 F圭9．7Tんθ臓Zoo箆伽痂o鴨Zo8s斜θゲ％翻 pressure sodium arc tube whic蓋is made ofεむtra強s－」ε？Z9カん Pc α8 α ∫％冗0擁0％ 0／ 00♂α一 1uce盤も polycrystεtlli難e alumina cera瓶ic tube a箪d spO哲重θ？πPθゲα古％？’θ ∫0ゲオんε αゲ0 operated in a regio簸where sodium vapor press蟹e オ脱δθS／i泥εdω覚ん0σε冗0りZ．

14 」．mum．獲Rgng．lns七．Jpn． 20 E o Na-amalge~1 20mg ( N066*/･Fnole'/.) o Nc-GmQigam 20mg ( NQ6emole'/.) ~~ o Xe 30Torr ~P,~~et~~~- Power input Pin(Wlcm) ~: 25 E ･~s 8. 5 ¥o J:: ~~P[~~ ~r(~ " ~ 6 c~ ~~l J i4 ~ ~~~5t,~L 20 ¥V~y ":i~Yv~'¥ ~46.5 ~~~~ Z ¥(~ ¥ :~ J ¥ ¥~~)¥7.¥~ ¥¥ ¥:¥ ¥ ¥ ~h ¥ ¥ g O ¥¥ ~~¥ ¥ ~ ¥ ¥ ¥¥¥¥ ¥¥¥ 5 (,, ¥ ¥ 8 ¥ ¥ ¥ .. ~34.5 l~~ ¥¥ ¥ c: ¥ 1' f o 6 o:: Power l~~u$ PiR tWlcm) IO- c, Ne-0.5*/.Ar 30To~r 4 Je PEV $, e 565 pr ~~ - : {¥je-0,5"/eAr 4Tarr ~ 2 > h O 5 650 750 eOO 650 700750 800 Co[d600 - Spot TemperGture700 ( C 800) Cald- Spot Temperoture ('C) e vi5ible redie,on Ptv ebtGined from the experimerttal data Fig. 8 Therm(el eo?2;duction loss per u?~it of the visible radient efflcier)cy ~rv length Pe as a fuLnctiolh the cold- o tctal radie~ion Pt multiplied by constan~ Crv of 0.5e5 spot temperature for the (~rc tubes Fig. 9 Colr~paf'iso'~ of the total radiatiolt filled with ~~eon-a4'gon ,?eixtu~'e. (Pr) e8ti??~ated from the tube wall te,mperature with the visible ,'a- the increasing neon-al'gon filling pressure and con- diatio't (Prv) obt(~i,ted from the verged at a higher cold-spot temperature than that ~'isible r(~dionLt effi(~iency of the of xenon-filled tubes. (~l'c tube filled udth xe'zon. From the above, it is confirmed that, in the high cold-spot temperature region, the thermal conduction loss per unit length P. takes a certain value e Na-ama~ge~120r~g(~aeerTlo le 'x' ) iE pertinent to the high-pressure sodium arc tube ; so ¥v e ~a-0'5'/eA r30To~r !v55te the total radiation per unit length P. can be to ~ 20- first approximation represented by the same linear equation of the power input per unit length Pi,, as ~ Equation (6) in the previous paper: 5

P.~'0.80( Pin ~ 6.7) - :D where the reason why the transmittance 7 difEers '~'33 o o / from the previous value 0.78 in Equation (6) may pewer ~1?Pur pin~wlern' be that the tube used here was sapphire instead of 1:: polycrystalline alumina ceramic. o: 5 prv s~ 0'55 pr J:? > 4. Discussion eoo 650 700750 80a We shall now consider the validity of the as- Co!~ - Spot Temperature ('e ) sumptions adopted in the preceding section. e visib:e radiation prv The value of the emissivity e, which was assumed o toto I rediation pF multipli'ed by constant crv of 0.55 to be 0.25,' infiuences the estimated value of the total radiation P,, as is clear from Equation (3). Fig. 10 Co?7upa;riso?~ of the total r(~dia- For comparison with the value P., the visible radiatio,e (P.) estim(~ted from the tion per unit length P~ was calculated from the tube wall temperature with the measured value of the visible radiant e~ciency ~.. ~,isible radiatio?e (P*v) obt(:~it~ed frotn the 1;isible radiant effi- Fig. 2 and 3 : ciency Of the (erc tube filled with P.*=1.02 ~.v Wi*/L ･･････････(8) leeon-argon mixture. where constant 1.02 is the correction factor for the glass outer-bulb. As shown in Figs. 9 and 10, the typical high-pressure sodium lamp with xenon start- valucs P of the arc tubes filled with xenon and ing gas. Besides this, the emissivity of sapphire neon-argon mixture were in good agreement with can not be higher than that of opaque alumina the P. values multiplied by constants C.. of 0.565 ceramic (known to have constant emissivity be- and 0.550, respectively. These constants represent tvveen 0.22 and 0.25 in a temperature range be- the ratio of the visible component against total tween 1,100 and 1,500 Klo)), since the effect of the radiation. It should be noted that the constant C.~ radiant inter-refiection in the former is less than for xenon-filled tube is almost consistent with the that in the latter. These indicate that the assump- published visible c.omponent ratio (O.593)) of a tion on the value e is i'easonable. Although the

J. Light & Vis. Env. Vol. 3 No. 2 1979 15 thermal conductivity of sapphire was assumed to The dependence of P. on starting gas is explained be the same as that of opaque alumina ceramic, by the following fact : The thermal conductivity actually it must be higher t.han that. As to total of the mixed gas in the arc tube varies depending electrode-fall voltage, another value 4 Vn) has been on the ratio of each starting gas pressure to the reported besides 9 V, assumed in this paper. How- sodium-mercury vapor pressure. It decreases with ever, the difference brought about no substantial the increasing xenon gas pressure, while it in- difEerence in the estimated values P. and P.. creases in the case of neon-argon mixture gas, and As already shown in the preceding section, it is such influence by the starting gas becomes in- clear that the estimated value of the thermal con- significant as the sodium-mercury vapor pressure duction loss per unit length P. corresponds closely is increased. Recently a 360W high-pressure sodi- to the characteristics of the visible radiant effi- um lamp having as high an efficacy as 130 Im/Wi6} ciency ~... Now, the dependence of P. on discharge was developed through not only increasing the xenon parameters can be physically interpreted in relation filling pressure from 20 to 350 Torr but also de- with the gas temperature in the arc plasma column signing a new arc tube that can operate at a rela- as follows : In a high-pressure sodium arc tube, tively low sodium vapor pressure. From the above- the axial gas temperature Tg" drops with the in- mentioned reason, it is obvious that such a high creasing sodium vapor pressure and also the de- efficacy is attributed to the reduced thermal con- creasing power input.i~)i3) This reduces the gas duction loss. On the other hand, the efficacy of temperature gr

Table I Co,mpa;rison between the d(~ta, ilt this pa per a?td those alre(~dy Published ol~ the total energy bal(e?tce of a high-pt'essut'e sodiul,~ la,,rp.

Present cvthor LinZ Jack et ci 3) Woymouth 4) ( ex periment/calcv I ot ion) ( ex pe~iment } ( experiment) ( experiment/cGicniction) Tube Deslgn Tube Fncterial ( A!a03) Sopphlre Poi ycrystalline Po I ycr ystc I I i n e Poiycrysta I I ine Inner diameter 8.0 mm 7.5mm 7.0 mm Electrede spacing 78 mm 8 a mm 9 Omm Amalgam eomposition Ne 66fnole'/. Na 69 mol e'l. Xenon fi!ling pressure S･OTorr i 5 Tot~ I 4Torr

Tu b e Ope rat i o n Cold-spot temperature 720'C 685･C Sodiurn vepor pressure t 60- i50Torr) - / 102Torr Mercury vapo~ pressure (400-800Torr) = / 1049Tcrr TotGI power input 400 W 400 W 400 W 400 / S99.9W Arc current tr,m.s.) 4.01 A 5.2 A 4.4 A 5.27 / 5.27 A A~c drop ( r, m,s. ) 120 V !OO V I05 V 84/ 847V

Lamp F?adiGr~t Prope~ties l 1 7.5lm/w 120 Imlw im Overali luminous efflcacy, ~L i29lrn~W/ - ll5.9 /I15.3 Iw Total radiQn$ efficiency, n. - I O.el8 O.500 0.535 /0.599 Visible redient efficiency, nrv O. 3 5 / - 0.287 0.295 Luminous efficacy of r( 36dm/w/ - 4 1 O Imlw 407lrnlw vlsible ~adi(1?ioR,

E:1e~gy Bala~ce E I ectrode I oss - / 30W 24 w - / 26.4W Radiant power : visibie i40W / - l i4.7 W I 18W i nv is i ble 8a w to tal - / 247.2W 200 w 214 / 239,4W Nen - radiative I osses x - / 122.8W 7ew - / 34.1 W

X con sist ing mGin!y of the the~rnGi conduc?ion Qnd radient trensrnission iosses

16 J. Illum. Eng~g. Inst. Jpn. 5．Conclusion References The energy balance of a high－pressure sodium （1）」．」．Lowke：Joumal of Quantitative Spectroscopy and Radiat圭ve Transfer，9（1969）839． a1℃tube has been analyzed by estimating its total （2） F．C．Lin：111um．Engng，65Apri1（1970）250． radiation and thermal con（iuction loss from the （3） A．C．Jack and M．Koedam：J．111um，Engng Soc、3July energy balance equation on the tube envelope． The （1974）323． estimated results are fairly consistent with the （4） J．F．Waymouth：」．111um．Engn Soc．6April（1977）131．（5） N．Ozaki：J．App1．phys．42July（1971）317L me乱sured （iata on its absolute visible radiation．（6） N．Ozaki＝ Journal of Quantitative SPectroscoPy and It has been shown that the kigh eHi（！acy of a Radiative Transfer，11（1971）1111． high－pressure sodium lamp is based not only on （71Y．S．Touloukian＝Thermophysical Properties of High high luminous e伍cacy per watt of visible radia－ Temperature Materials，4，The Macmillan Co．，New York， 1967． tion also on its low thermal conduction loss per一 18〕W．Elenbaas：The High Pressure Mercury Vapcur Disch－七inent to high－pressu1・e sodium vapor discharge． arge，Nortk Holland Publishing Co．，Amsterdam，（1951）20． For improvement of the lamp e伍cacy within the 〔91H．Akutsu，H．Yamazaki，T．Okamoto and Y．Watarai： limited power input，the combination of sodium J．111um・Engng Inst．Jpn．58（1974）658．乱nd st乱rting gas pressures plays an important role． llol J．C．Richmond，G．J．Kneissel，D．L．Kelley and F．J、Kelley： Techhical Report of Air Force Materials Laboratory AF・ MI－TR－66・302，0h王o，1966． 1111P．L、Denbigh and D，O．Wharmby；Light．Res．TechnoL Acknowledgemen量s 8（1976）141 The author would like to acknowledge the con－ 02｝J．J．deGroot and J．A．J．M．van Vliet；Joumal of Physics D： Appl．Physics，8（1975）651． tinuing guid＆nce and encouragement of Dr．H． H．Akutsu：J．111um．Engng Inst，Jpn．59（1975）498．⑬ Mizuno．He is also deeply indebted to Dr．Y．Wata－〔14｝ K、Schmidt：Proceedings of7th Intemational Conference rai for his helpful discussion，＆nd to Mr．H．Yama－ on Ionization Phenomena in Gases，Beograd，1（1965）654． zaki and Mr．T．Matsuba for their considerable 肛51T・S．Jen，M．F．Hoyaux and L．S・Frost：Journal of Quan・ titative SPectroscopy and Radiative Transfer，9（1969）487． assist＆nce in the performance of the experiments．〔1⑤1．Iwai，M．Ochi and M．Masui：J．Light＆Vis・Env・1 （1977）7

Received4，0ct．1979；Revision Received15，Feb．1980．

J．Light＆Vis．Env． VoL3No．21979 17