Physics and Chemistry of High Pressure Sodium Lamps - ACS

28

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on May 29, 2018 | https://pubs.acs.org Publication Date: March 8, 1982 | doi: 10.1021/bk-1982-0179.ch028

Physics and Chemistry of High Pressure Sodium Lamps R. J. Z O L L W E G Westinghouse Research and Development Center, Pittsburgh, P A 15235

In this review of the high pressure sodium lamp, emphasis i s placed on evidence concerning the interaction of resonantly excited sodium atoms with sodium, mercury or xenon atoms, which modifies the spectral radiance of lamps to improve the color or efficacy. The influence of mercury and xenon buffer gases on the thermal and e l e c t r i c a l conduc tivities and hence on lamp efficacy are also i n d i cated. The a t o m i c s p e c t r a o f s o d i u m b e a r s a u n i q u e r e l a t i o n s h i p t o t h e s e n s i t i v i t y o f t h e human e y e s i n c e t h e s t r o n g r e s o n a n c e l i n e s a p p e a r i n a s p e c t r a l r e g i o n v e r y n e a r t h e maximum e y e r e s p o n s e . T h i s i s i l l u s t r a t e d i n F i g u r e 1. W h i l e t h e maximum v i s u a l r e sponse i s 683 lumens p e r w a t t o f e n e r g y r a d i a t e d a t 555 nm, s o dium " D " l i n e r a d i a t i o n y i e l d s a b o u t 500 lumens p e r w a t t o f r a d i a n t energy. Thus s o d i u m i s c u r r e n t l y used i n t h r e e q u i t e d i f f e r e n t types o f commerical e l e c t r i c d i s c h a r g e lamps. Table I summarizes a few o f t h e c o m p a r a t i v e p a r a m e t e r s o f t h e s e t h r e e t y p e s o f l a m p s , t h e l o w p r e s s u r e sodium d i s c h a r g e , t h e m e t a l h a l i d e , and t h e h i g h p r e s s u r e sodium d i s c h a r g e l a m p s . Note t h e p a r t i c u l a r l y w i d e r a n g e o f s o d i u m p r e s s u r e s used f r o m m i l l i t o r r t o hundreds o f t o r r . That p r e s s u r e d i f f e r e n c e p l u s t h e d i f f e r e n c e s i n power l o a d i n g and t e m p e r a t u r e s n e c e s s i t a t e s d i f f e r e n c e s in discharge containing envelopes. Low p r e s s u r e sodium lamps ( L P S ) have t h e h i g h e s t lamp e f f i cacy. However, s i n c e t h e s p e c t r u m i s e s s e n t i a l l y m o n o c h r o m a t i c , s u c h l i g h t s o u r c e s g i v e v i r t u a l l y no c o l o r d i s c r i m i n a t i o n o f c o l o r e d , n o n - l u m i n o u s o b j e c t s i l l u m i n a t e d by them. Such lamps a r e used m o s t l y f o r s u c h u t i l i t a r i a n p u r p o s e s a s h i g h w a y , t u n n e l and s e c u r i t y l i g h t i n g , e s p e c i a l l y i n Europe. B o t h m e t a l h a l i d e (MH) and h i g h p r e s s u r e s o d i u m (HPS) lamps do c o n t a i n a v a r i e t y o f wavelengths i n t h e i r s p e c t r a which permits c o l o r d i s c r i m i n a t i o n i n t h e i l l u m i n a t e d scene. The c o l o r r e n d e r i n g i n d e x ( C R I ) i s an a p p r o x i m a t e measure o f how s u c c e s s f u l l y t h e s e lamps compare t o a

0097-6156/82/0179-0407$05.00/0 ©

1982 A m e r i c a n Chemical Society

Gole and Stwalley,; Metal Bonding and Interactions in High Temperature Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

METAL BONDING AND INTERACTIONS


408

b l a c k body s o u r c e o f t h e same c h r o m a t i c i t y . The s o u r c e t h e n has a c o r r e l a t e d c o l o r t e m p e r a t u r e (CCT) c o r r e s p o n d i n g t o t h e b l a c k body s o u r c e . In T a b l e I, I have l i s t e d o n l y two o f t h e most w i d e l y used m e t a l h a l i d e lamps and t h e " s t a n d a r d " h i g h p r e s s u r e s o d i u m lamps t o g e t h e r w i t h t y p i c a l v a l u e s o f t h e i r CRI and CCT. Other formu l a t i o n s o f m e t a l h a l i d e a d d i t i v e s a r e sometimes used y i e l d i n g s i g n i f i c a n t l y h i g h e r CRIs t o 8 5 - 9 0 and c o r r e l a t e d c o l o r t e m p e r a t u r e s o f 5 0 0 0 - 6 0 0 0 K, m a t c h i n g d a y l i g h t . High p r e s s u r e sodium lamps w i t h s u b s t a n t i a l l y h i g h e r CRI a n d / o r h i g h e r c o l o r t e m p e r a t u r e w i l l be d i s c u s s e d b r i e f l y b e l o w . A t p r e s e n t , t h e u s u a l me t a l h a l i d e lamp t y p i c a l l y has b e t t e r c o l o r r e n d e r i n g a b i l i t y and a somewhat h i g h e r c o l o r t e m p e r a t u r e t h a n t h e s t a n d a r d HPS l a m p . However, t h e HPS lamp has an edge i n lamp e f f i c a c y and i n i t s m a i n t e n a n c e o f lamp e f f i c a c y t h r o u g h o u t t h e l i f e o f t h e l a m p . The r e s t o f t h i s r e v i e w w i l l be l i m i t e d t o t h e h i g h p r e s s u r e s o d i u m lamp. R e c e n t r e v i e w s by d e G r o o t et_ aj_. ( 1 ) , Wharmby ( 2 j and by McVey ( 3 j summarize t h e s c i e n t i f i c and t e c h n i c a l a s p e c t s o f t h e high p r e s s u r e sodium lamp. Thus t h e p r e s e n t r e p o r t has l e s s need t o be c o m p r e h e n s i v e and w i l l t e n d i n s t e a d t o e m p h a s i z e a s p e c t s o f s o d i u m and s o d i u m - b u f f e r gas i n t e r a c t i o n s o f i n t e r e s t t o t h i s symposium. We s h a l l a t t e m p t t o i n d i c a t e a r e a s where b a s i c m a t e r i a l d a t a a r e e i t h e r unknown o r a r e i n a d e q u a t e . The

Spectrum

F i g u r e 1 shows c l e a r l y how t h e b r o a d e n e d w i n g s o f t h e s o d i u m "D" l i n e s dominate the v i s i b l e spectrum o f the h i g h p r e s s u r e s o dium l a m p s . S e l f - a b s o r p t i o n i s so s t r o n g t h a t r a d i a t i o n a t t h e undeviated l i n e center is n e g l i g i b l e . The b r o a d e n i n g i n c r e a s e s s t r o n g l y and a p p r o x i m a t e l y s y m m e t r i c a l l y w i t h t h e s o d i u m p r e s sure (4). A l t h o u g h t h e s p e c t r u m may a p p e a r t o be a c c o u n t e d f o r by r e s o n a n c e b r o a d e n i n g , most o f t h e e m i t t e d r a d i a t i o n a p p e a r s o u t s i d e of the r e g i o n of v a l i d i t y of the impact a p p r o x i m a t i o n . The l a t t e r i s v a l i d o n l y w i t h i n -1 nm o f l i n e c e n t e r f o r t h e c o n d i t i o n s o f T a b l e I. I n d e e d , bumps a p p e a r on b o t h t h e b l u e and r e d t a i l s i n F i g u r e 1 w h i c h can be a s s o c i a t e d w i t h Na-Na and N a Hg m o l e c u l e s . M i z u n o ejt al_. (_5) showed t h a t t h e bump on t h e r e d i n c r e a s e s w i t h Na and Hg p a r t i a l p r e s s u r e s and s h i f t s t o l o n g e r w a v e l e n g t h i f Cd i s s u b s t i t u t e d f o r Hg. F i g u r e 2 shows i n more d e t a i l t h a t t h e r e a r e 2-3 components t o t h e s e r e d bumps a r i s i n g f r o m t h e N a Hg and Na-Cd i n t e r a c t i o n s . B e c a u s e t h e g r o u n d s t a t e s o f t h e NaHg and NaCd m o l e c u l e s a r e w e a k l y - b o u n d van d e r Waals s t a t e s ( 6 ) , t h e red s h i f t i n d i c a t e s t h a t the e x c i t e d m o l e c u l a r s t a t e s formed by s o d i u m r e s o n a n c e atoms and g r o u n d s t a t e Hg o r Cd have h i g h e r d i s s o c i a t i o n e n e r g i e s than the ground s t a t e s . I t s h o u l d be men t i o n e d t h a t t h e s p e c t r a o f F i g u r e 2 were o b t a i n e d u n d e r somewhat s p e c i a l i z e d c o n d i t i o n s t o enhance t h e f e a t u r e s . A sapphire arc



28.

ZOLLWEG

T a b l e I.

High

Pressure Sodium

409

Types o f C o m m e r c i a l D i s c h a r g e Lamps C o n t a i n i n g Sodium Low P r e s s u r e Sodium (LPS)

Gases

Lamps

Na/Ne, 1% A r

Metal

Halide

(MH)

High Pressure Sodium (HPS)

NaI,ScI /Hg/Ar (NaI,TlI,InI /Hg/Ar)

Na/Hg/Xe

-1

50-100

Hg 3-5 a t m .

Hg - 3 0 0 Xe - 1 4 0

Quartz

Al 0 2 3

3

x

P

N a

(torr)

0.003

Buffer

Ne/1% A r torr

Envelope

Na-resistant glass

Efficacy (lumens/ watt)

X and t h e b l u e - g r e e n N a B •> X ( 4 7 6 . 7 - 4 9 2 . 5 nm) r e g i o n s , t h e r e i s net e m i s s i o n from h i g h p r e s s u r e sodium lamps. A paper by Woerdman and d e G r o o t (1_4) a t t h i s symposium w i l l d i s c u s s t h e N a c o n t r i b u t i o n t o t h e p r o m i n e n t bump a t 551.5 nm. We have a l s o o b s e r v e d a b r o a d bump a t 4 3 6 . 3 nm a t h i g h e r r e s e r v o i r t e m p e r a t u r e s t h a t , l i k e t h e 4 5 2 . 5 nm b a n d , has been i d e n t i f i e d by W o e r d man (8) as a b o u n d - f r e e t r a n s i t i o n o f N a e x c e p t t h a t t h e i n t e n s i t y o f t h e 4 3 6 . 3 nm band i s i n s e n s i t i v e t o b u f f e r gas p r e s s u r e . In h i s c a r e f u l , h i g h r e s o l u t i o n s t u d i e s o f t h e HPS l a m p , W h i t t a k e r (JK3) o b s e r v e d most o f t h e e x p e c t e d a t o m i c l i n e s o f N a , e . g . , f r o m t h e p r i n c i p a l , s h a r p , d i f f u s e and f u n d a m e n t a l s e r i e s as c l a s s i c a l l y d e s i g n a t e d . A l s o o b s e r v e d were f o r b i d d e n l i n e s o f t h e L e n a r d s e r i e s f r o m P -> P and F -> P t r a n s i t i o n s . Two f o r b i d d e n l i n e s a t 552.7 and 5 5 3 . 2 nm a r e p r o m i n e n t on t h e " k n e e " o f the b l u e wing r e v e r s a l of the resonance l i n e s . Many o f t h e s e a t o m i c l i n e s a r e a l s o b r o a d e n e d a t h i g h e r lamp r e s e r v o i r t e m p e r a t u r e s and b u f f e r gas p r e s s u r e s . In a d d i t i o n a t o m i c l i n e s o f Hg and i m p u r i t y l i n e s o f A l , B a , C a , K, Mg and S r were o b s e r v e d . The xenon s t a r t i n g gas a l s o c o n t r i b u t e s t o t h e Na r e s o n a n c e l i n e r e v e r s a l and much h i g h e r t h a n n o r m a l xenon p r e s s u r e s have been f o u n d t o i m p r o v e lamp e f f i c a c y and lumen m a i n t e n a n c e . Fig u r e 3 shows s p e c t r a o b t a i n e d f o r two lamps c o n t a i n i n g xenon a t d i f f e r e n t p r e s s u r e s w i t h s o d i u m b u t no m e r c u r y . Rather high r e s e r v o i r t e m p e r a t u r e s o f 812 C (1085 K) were used s u c h t h a t t h e Na p a r t i a l p r e s s u r e s a p p r o a c h 400 T o r r . The xenon p r e s s u r e s d u r i n g lamp o p e r a t i o n were a p p r o x i m a t e l y 140 and 2100 T o r r r e s p e c tively. A t t h e h i g h e r xenon p r e s s u r e i t i s e v i d e n t t h a t : (1) t h e s o d i u m r e v e r s a l i s g r e a t e r , (2) e m i s s i o n i n t h e f a r r e d w i n g i s much h i g h e r , (3) t h e r e i s a p p r e c i a b l y more r a d i a t i o n i n t h e 5 5 2 - 5 6 6 nm b l u e w i n g r e g i o n , (4) t h e r e i s a d d i t i o n a l s t r u c t u r e a t a b o u t 5 5 7 . 5 and 5 6 2 . 5 nm, and (5) t h e peaks o f t h e 5 6 8 . 6 and 615.9 nm s e l f - a b s o r b e d d o u b l e t s a r e a p p r e c i a b l y l o w e r . It is r e a d i l y e v i d e n t t h a t t h e Na-Xe i n t e r a c t i o n b r o a d e n s t h e e n t i r e sodium r e s o n a n c e l i n e r e g i o n a t s u c h h i g h s o d i u m and xenon p r e s -


2

1

1

2

2

2

2

2

2

2

2

2



412


or a> rq a>

or

540

565

590 615 Wavelength, nm

640

665

Figure 3. Dependence on Xe pressure of Na reversal is shown at higher sodium pressure. At Xe pressure of ~2100 ( ) vs. ~140 ( ) torr, separation between reversal maxima is increased, radiance in both wings increases, and additional features appear at 557.5 and 562.5 nm.


28.

ZOLLWEG

High

Pressure Sodium

413

Lamps

sures. The two a d d i t i o n a l bumps a t 557.5 and 562.5 nm depend upon b o t h h i g h s o d i u m and h i g h xenon p r e s s u r e s and m i g h t a r i s e from t h e s e l f r e v e r s e d s a t e l l i t e o f NaXe* ( E + E) o r from a t r i a t o m i c m o l e c u l e such as N a X e * . The r e d u c e d h e i g h t s o f t h e 568.6 and 615.9 nm d o u b l e t s m i g h t r e s u l t p r i m a r i l y f r o m a l o w e r a x i a l arc temperature. A l t h o u g h we s t a r t e d t h i n k i n g a b o u t t h e s o d i u m r a d i a t i o n i n t e r m s o f r e s o n a n c e l i n e b r o a d e n i n g , we now see t h a t t h e r e a r e components o f N a , NaHg, NaXe and p e r h a p s t r i a t o m i c m o l e c u l e s as w e l l . Much o f t h e r a d i a t i o n e m i t t e d i s a b s o r b e d , e s p e c i a l l y by t h e c o l d e r g a s n e a r t h e w a l l , s o t h e r a d i a t i o n t r a n s f e r p r o b lem h a s a c o n s i d e r a b l e i n f l u e n c e on t h e s p e c t r u m e m a n a t i n g f r o m the lamp. The t e m p e r a t u r e p r o f i l e o f t h e a r c and t h e a r c t u b e w a l l t e m p e r a t u r e m o d i f y t h e l a y e r o f a b s o r b i n g g a s and hence t h e sodium r e v e r s a l o b s e r v e d . Waymouth and Wyner (15J have empha s i z e d t h a t h i g h e r lamp e f f i c a c y may r e s u l t f r o m r a i s i n g t h e a r c t u b e w a l l t e m p e r a t u r e a t c o n s t a n t d i a m e t e r and power l o a d i n g . The o p t i c a l p r o p e r t i e s o f t h e w a l l a l s o c o n t r i b u t e t o t h e e m i t ted s p e c t r a . The u s u a l a r c t u b e w a l l m a t e r i a l , p o l y c r y s t a l l i n e a l u m i n a ( P C A ) , has r a t h e r l o w " i n - l i n e " t r a n s m i s s i o n and s c a t t e r s most o f t h e i n c i d e n t r a d i a t i o n . T h a t p o r t i o n w h i c h i s s c a t t e r e d back i n t o t h e d i s c h a r g e medium and t h a t i s a l s o i n t h e s p e c t r a l r e g i o n i n s i d e o r near the peaks o f the s e l f - r e v e r s e d r e s o n a n c e l i n e s , i s l i k e l y t o be a b s o r b e d . Thus we have f o u n d g r e a t e r Na r e v e r s a l f o r PCA a r c t u b e lamps t h a n f o r s a p p h i r e lamps w i t h t h e same d i a m e t e r s , Na-Hg a l l o y c o m p o s i t i o n and r e s e r v o i r temperatures. I n d e e d , v a r i o u s a u t h o r s have r e p o r t e d 3-8% h i g h e r e f f i c a c i e s f o r s a p p h i r e v s PCA lamps d e p e n d i n g upon t h e q u a l i t y o f t h e PCA and i t s s u r f a c e t r e a t m e n t {3). 2

2

2


2

W i t h a l l o f t h e c o m p l i c a t i o n s e n u m e r a t e d a b o v e , how c a n one a c c o u n t f o r t h e o b s e r v e d HPS lamp s p e c t r a ? Lamp d e v e l o p m e n t e n g i n e e r s have t y p i c a l l y t a k e n t h e s i m p l e s t a p p r o x i m a t i o n s and e x tended the L o r e n t z i a n , i m p a c t - a p p r o x i m a t i o n p r o f i l e f o r the e n t i r e l i n e - f a r beyond i t s r e g i o n o f s t r i c t v a l i d i t y - and com bined t h i s w i t h r a d i a t i o n t r a n s f e r models. Such p r o c e d u r e s y i e l d c a l c u l a t e d r e s u l t s s u r p r i s i n g l y c l o s e t o e x p e r i m e n t (1) p r o v i d e d d e t a i l s o f the s p e c t r a are not too c l o s e l y s c r u t i n i z e d " . A g r e a t d e a l o f g u i d a n c e on how t o i m p r o v e t h e s e c a l c u l a t i o n s have come f r o m t h e work o f G a l l a g h e r and h i s c o w o r k e r s (16) who have used l a s e r - i n d u c e d f l u o r e s c e n c e t o measure d e t a i l e d l i n e s h a p e s f o r a l k a l i m e t a l s p e r t u r b e d by r a r e g a s e s and d e t e r mined i n t e r a t o m i c p o t e n t i a l s . F u r t h e r a i d i s a v a i l a b l e from the c a l c u l a t e d p o t e n t i a l s o f B a y l i s (17), P a s c a l e {]8) and Duren (19). However, t h e a c c u r a c y o f t h e t h e o r e t i c a l c a l c u l a t i o n s i s o f t e n i n s u f f i c i e n t f o r our needs. C o n d i t i o n s i n our discharges are o f t e n a t h i g h e r k i n e t i c temperatures than those o f the f l u o r e s c e n c e measurements s u c h t h a t t h e r e p u l s i v e p o t e n t i a l c u r v e s p l a y a more i m p o r t a n t r o l e . We a l s o have t h e c o m p l i c a t i o n t h a t s e v e r a l broadening processes occur t o g e t h e r , e . g . , Na-Na, Na-Xe, Na-Hg, i n a d d i t i o n t o the s t r o n g s e l f - a b s o r p t i o n . To s o l v e t h e


414


r a d i a t i o n t r a n s f e r problem the a r c temperature p r o f i l e i s needed. T h i s can be m e a s u r e d e x p e r i m e n t a l l y o r c a l c u l a t e d by s o l v i n g t h e energy balance e q u a t i o n .


The E n e r g y

Balance

C a l c u l a t i o n s o f t h e HPS lamp s p e c t r u m and o f t h e e n e r g y b a l ance o f t h e a r c h e l p us t o u n d e r s t a n d t h e i m p o r t a n c e o f v a r i o u s c o n t r o l l a b l e a r c p a r a m e t e r s and t h u s a i d i n lamp d e v e l o p m e n t . The a s s u m p t i o n o f l o c a l t h e r m o d y n a m i c e q u i l i b r i u m (LTE) ( e x c e p t f o r r a d i a t i o n ) has been f o u n d t o be a good a p p r o x i m a t i o n , even f o r p u l s e d a r c s , a l t h o u g h non-LTE e f f e c t s have been d e m o n s t r a t e d i n some d e t a i l s , e . g . , i n t h e a b s o l u t e r a d i a n c e a t t h e s o d i u m Dl i n e r e v e r s a l m i n i m a and i n h i g h f r e q u e n c y o p e r a t i o n . F o r t h e ac o p e r a t e d a r c i n L T E , we s o l v e n u m e r i c a l l y t h e a c e n e r g y b a l a n c e equation _ p C | | = aE - d i v ( - K g r a d T) - u (1) 2

p

Ohm's Law

R = 2TTE / a r d r (2) o and t h e c i r c u i t e q u a t i o n w h i c h c o n n e c t s t h e power s o u r c e and b a l l a s t w i t h the a r c column. In e q u a t i o n s (1) and (2) t h e a r c c u r r e n t I and a x i a l e l e c t r i c f i e l d E a r e f u n c t i o n s o f t i m e t . The bulk m a t e r i a l p r o p e r t i e s i n c l u d e the e l e c t r i c a l c o n d u c t i v i t y a , t h e r m a l c o n d u c t i v i t y K , d e n s i t y p , s p e c i f i c h e a t C p , and n e t r a d i a t i o n e m i s s i o n u and a r e a l l f u n c t i o n s o f t e m p e r a t u r e T. These a r e i n p u t p a r a m e t e r s f o r t h e c a l c u l a t i o n as a r e n e c e s s a r y b o u n dary c o n d i t i o n s . F i g u r e s 4 and 5 show how t h e t h e r m a l and e l e c t r i c a l c o n d u c t i v i t i e s o f HPS lamps can be d e c r e a s e d w i t h h i g h e r Hg o r Xe b u f f e r gas p r e s s u r e s . F o r t e m p e r a t u r e s b e l o w an a x i a l a r c t e m p e r a t u r e o f - 4 0 0 0 K, l o w e r t h e r m a l c o n d u c t i v i t y i s d e s i r a b l e t o r e d u c e t h e c o n d u c t i o n power l o s s w h i c h r e d u c e s lamp e f f i cacy. A lower e l e c t r i c a l c o n d u c t i v i t y r e s u l t s i n a h i g h e r impe dance lamp w i t h l o w e r c u r r e n t and t h e r e f o r e l o w e r e l e c t r o d e l o s ses. However, b o t h Hg and Xe b u f f e r g a s e s change t h e e m i t t e d s p e c t r u m , and t o o h i g h p r e s s u r e s r e d u c e lamp e f f i c a c y . The q u a n t i t y p Cp i s a l s o i m p o r t a n t b e c a u s e i t has a m a j o r i n f l u e n c e on t h e t e m p e r a t u r e change o f t h e a r c d u r i n g t h e ac c y c l e . The n e t r a d i a t i o n e m i s s i o n o f a volume e l e m e n t o f t h e a r c i s a p a r t i c u l a r l y d i f f i c u l t parameter to determine. Because of the s t r o n g s e l f - a b s o r p t i o n of the arc i t i s not only a f u n c t i o n of t h e p a r t i a l p r e s s u r e s o f t h e r a d i a t i n g s p e c i e s and t h e t e m p e r a t u r e , b u t a l s o depends upon t h e s u r r o u n d i n g r a d i a t i n g r e g i o n a n d , i n t h i s c a s e , t h e a b s o r p t i o n w i l l a l s o be t i m e - d e p e n d e n t . We have found t h a t a p p l i c a t i o n o f a m o d i f i e d r a d i a t i o n imprisonment f o r m u l a t i o n o r i g i n a l l y d e v e l o p e d by H o l s t e i n g i v e s us n e t e m i s s i o n c o e f f i c i e n t s i n good a g r e e m e n t w i t h t h o s e we d e r i v e f r o m d e G r o o t ' s a r c t e m p e r a t u r e p r o f i l e measurements o f HPS a r c s ( 2 0 ) . d e G r o o t j e t aj_. (]_) show e x a m p l e s o f t h e changes i n a x i a l a r c I



ZOLLWEG

High

Pressure Sodium

Lamps

Figure 4. Calculated thermal conductivity of Na vapor and mixtures of Na vapor with Hg or Xe. Thermal conductivity is reduced significantly by the addition of Hg or Xe buffer gases.


415



6

2500

3000

3500

4000

4500

5000

5500

6000

Temperature, °K Figure 5. Calculated electrical conductivity of Na vapor between 3000 and 4500 K can be reduced more by the addition of Hg than by Xe as the buffer gas. A higher impedance arc tends to reduce electrode losses.



28.

ZOLLWEG

High

Pressure Sodium

All

Lamps

temperature w i t h time during the ac c y c l e . They a l s o show how t h e r e i g n i t i o n v o l t a g e s p i k e s o b s e r v e d f o r HPS lamps a r e m o d i f i e d a t h i g h e r b u f f e r gas p r e s s u r e s . In o r d e r t o c a l c u l a t e t h e m a t e r i a l t r a n s p o r t p r o p e r t i e s a s g i v e n i n F i g u r e s 4 and 5 we f i r s t must c a l c u l a t e t h e c h e m i c a l e q u i l i b r i u m m a k i n g use o f t h e a v a i l a b l e t h e r m o c h e m i c a l d a t a b a s e . T a b l e I I shows, f o r example, t h e c a l c u l a t e d e q u i l i b r i u m d e n s i t i e s o f n e u t r a l and c h a r g e d s p e c i e s n e a r t h e a x i s (-4000 K) o f a t y p i c a l c o m m e r c i a l HPS a r c l a m p . We t h e n use t h e b e s t a v a i l a b l e i n t e r a c t i o n data f o r cross s e c t i o n s t o c a l c u l a t e the t r a n s p o r t p r o perties. To o p t i m i z e t h e h i g h p r e s s u r e s o d i u m lamp i s s t i l l a v e r y f o r m i d a b l e t a s k (2J b e c a u s e o f t h e m u l t i p l i c i t y o f p a r a m e t e r s u n d e r t h e d e s i g n e r ' s c o n t r o l and a number o f m a t e r i a l s l i m i t a t i o n s w h i c h must be t a k e n i n t o a c c o u n t f o r a v e r y l o n g l i f e lamp (>20,000 h o u r s ) . Waymouth (21_) h a s d e v e l o p e d a s i m p l i f i e d a n a l y s i s t h a t r e l a t e s lamp o u t p u t q u a n t i t i e s , e . g . , e f f i c a c y , r a d i a t i o n i n v a r i o u s s p e c t r a l r e g i o n s , h e a t c o n d u c t i o n l o s s e s and a r c tube w a l l t e m p e r a t u r e , t o v a r i o u s parameters under t h e d e s i g n e r ' s control. A p p l i c a t i o n o f t h e model showed t h a t h i g h e r lamp e f f i c a c y c o u l d be a t t a i n e d by i n c r e a s i n g t h e PCA w a l l t e m p e r a t u r e (now l i m i t e d by A l and 0 v a p o r i z a t i o n ) a t t h e same a r c t u b e d i a m e t e r o r by i n c r e a s i n g t h e d i a m e t e r b u t h o l d i n g t h e w a l l t e m p e r ature constant. Such g o a l s c o u l d be p a r t i a l l y a c h i e v e d b y s u b s t i t u t i o n o f an a r c t u b e o f l o w e r t o t a l e m i s s i v i t y , e . g . , p o l y c r y s t a l l i n e y t t r i a , f o r PCA. A l i m i t a t i o n o f t h e model i s t h e s u b s t i t u t i o n o f e m p i r i c a l parameters f o r important p h y s i c a l c h a r a c t e r i s t i c s o f t h e a r c and t h e u n c e r t a i n t y a s t o how t h e s e a p proximations are r e f l e c t e d i n the numerical r e s u l t s . However, t h e d e v e l o p m e n t o f a more r i g o r o u s model i s a v e r y f o r m i d a b l e t a s k w h i c h i s p r e s e n t l y l i m i t e d by u n c e r t a i n t i e s o f t h e m a t e r i a l p a r a m e t e r s , s u c h a s t h e s p e c t r a l r a d i a n c e i n v a r i o u s bands i n c l u d i n g t h e i n f r a r e d c o n t i n u u m , a s w e l l a s t h e t h e r m a l and e l e c trical conductivities. High Pressure

Sodium Lamp

Problems

B e c a u s e o f t h e r e a c t i v i t y o f s o d i u m w i t h q u a r t z and o t h e r g l a s s e s an e n t i r e l y new s y s t e m o f a r c t u b e e n v e l o p e s and s e a l s had t o be d e v e l o p e d f o r HPS lamps {3). The l o s s o f s o d i u m d u r i n g lamp l i f e h a s n o t been c o m p l e t e l y e l i m i n a t e d . T h e r e f o r e , an e x c e s s q u a n t i t y o f s o d i u m i s added t o t h e a r c t u b e t o g e t h e r w i t h m e r c u r y and t h e s t a r t i n g g a s x e n o n . The a c t i v i t y o f m e r c u r y i n t h e c o n d e n s e d amalgam i s v e r y l o w (-10" t o 10" ) a n d , m o r e o v e r , i s s t r o n g l y t e m p e r a t u r e d e p e n d e n t ; hence t h e m e r c u r y v a p o r p r e s sure i s g r e a t l y reduced. A l t h o u g h t h e a c t i v i t i e s and t h e r m o d y n a m i c p r o p e r t i e s o f s o d i u m amalgams a t l o w t e m p e r a t u r e s have been known f o r o v e r a d e c a d e , q u a n t i t a t i v e d a t a a t lamp o p e r a t i n g t e m p e r a t u r e s and c o m p o s i t i o n s have o n l y r e c e n t l y been p u b l i s h e d (22). Thus t h e s o d i u m and m e r c u r y p a r t i a l p r e s s u r e s i n o p e r a t i n g 2

3



418

Table II. C a l c u l a t e d S p e c i e s D e n s i t i e s i n LTE a t 4000 K f o r a Gas M i x t u r e T y p i c a l o f a HPS Lamp (Na 75 t o r r , Hg 300 t o r r , Xe 140 t o r r ) . Neutral Species


Density 6.2,

+15

Na"

4.1,

+12

+17

Na

6.2,

+15

1.5,

+14

Na"

1.7,

+12

HgXe

2.2,

+14

Hg

+

2.2,

+10

Xe

3.4,

+17

6.9,

+7

Xe

+

1.9,

+8

Na Na

2

Hg Hg

*

Charged Species

Density

2

1.7,

+17*

e~

9.1,

+12

7.2,

1.7, +17

= 1.7

x 10

1 7

+

cm

- 3

HPS lamps depend upon t h e c o n d e n s e d phase amalgam c o m p o s i t i o n and the t e m p e r a t u r e o f t h i s r e s e r v o i r . These p a r t i a l p r e s s u r e s t o g e t h e r w i t h t h e b a l l a s t ( d e s i g n e d p r i m a r i l y t o l i m i t lamp c u r r e n t ) d e t e r m i n e t h e lamp v o l t a g e and power. B o t h t h e amalgam c o m p o s i t i o n and t h e r e s e r v o i r t e m p e r a t u r e may change d u r i n g lamp life. The u s u a l r e s u l t i s t h a t t h e lamp v o l t a g e i n c r e a s e s w i t h lamp l i f e u n t i l t h e power l i n e - b a l l a s t s y s t e m i s u n a b l e t o s u p p l y s u f f i c i e n t v o l t a g e and t h e lamp e x t i n g u i s h e s . Thus HPS lamp b a l l a s t s a r e d e s i g n e d t o o p e r a t e lamps o v e r a r a t h e r w i d e r a n g e o f v o l t a g e s , e . g . , 8 0 - 1 6 0 V. Changes i n t h e amalgam r e s e r v o i r t e m p e r a t u r e may r e s u l t f r o m a number o f c a u s e s , s u c h as changes i n e m i s s i v i t y o f (1) t h e PCA n e a r t h e end o f t h e lamp ( f r o m e l e c t r o d e s p u t t e r i n g ) o r (2) o f t h e n i o b i u m end a s s e m b l y o r m e t a l h e a t s h i e l d , o r changes i n t h e h e a t r e t u r n e d f r o m t h e o u t e r e n v e l o p e when i t r e c e i v e s a d i s c o l oring or m e t a l l i c deposit. Such d e p o s i t s r e s u l t f r o m (1) a l u mina v a p o r i z a t i o n as A l and 0 , (2) e v a p o r a t i o n f r o m s u p p o r t w i r e s o r w e l d s , (3) sodium l e a k a g e f r o m t h e a r c t u b e , e t c . Sodium can be l o s t by l e a k i n g t h r o u g h t h e s e a l as w e l l as by l o s s i n t o t h e a l u m i n a as o b s e r v e d by M i z u n o e_t aj_. ( 5 j . H i n g ( 2 3 j has s u g g e s ted t h a t the l o s s i s a c c e l e r a t e d i f c a l c i u m i s present. Wyner (24) has shown t h a t s u c h Na l o s s can be a c c e l e r a t e d by a p p l y i n g an e l e c t r i c f i e l d a c r o s s 'the PCA s u r f a c e . Other authors (25) have f o u n d most o f t h e l o s t s o d i u m i n t h e s e a l r e g i o n . However, J a c o b s ejt aj_. (26) have r e l a t e d s o d i u m l o s s t o t h e e l e c t r o d e emission m a t e r i a l . Thus a m u l t i p l i c i t y o f mechanisms e x i s t f o r c a u s i n g an i n c r e a s e i n lamp v o l t a g e , a l l o f w h i c h must be c o n t r o l l e d and a v o i d e d w h i l e m a n u f a c t u r i n g lamps a t r a p i d r a t e s t o s e l l at competitive prices.


28.

ZOLLWEG


Improved

High Pressure Sodium

High P r e s s u r e

Sodium

Lamps

419

Lamps

A l t h o u g h s t a n d a r d HPS lamps have h i g h e f f i c a c y , good lumen m a i n t e n a n c e , and t h e v o l t a g e r i s e can be c o n t r o l l e d , t h e y have two d e f i c i e n c i e s i n c o l o r q u a l i t y . The c o l o r r e n d i t i o n as mea s u r e d by t h e CRI i s low and t h e s o u r c e c o l o r as i n d i c a t e d by t h e CCT i s l o w . McVey (2J has amply r e v i e w e d p r o g r e s s i n t h i s a r e a . I t has been known s i n c e 1973 (27) t h a t h i g h e r s o d i u m p r e s s u r e s c o u l d be u s e d t o i n c r e a s e t h e CRI t o 85 and t h e CCT t o 2400 K w i t h a modest d r o p i n t h e e f f i c a c y t o 92 lumens p e r w a t t . Lamps a p p r o a c h i n g t h i s g o a l have r e c e n t l y become c o m m e r c i a l l y p r a c t i c a l through improvements i n c e r a m i c - s e a l i n g t e c h n i q u e s ( 2 8 ) . H i g h e r CCT can a l s o be a c h i e v e d by lamp o p e r a t i o n i n a low d u t y c y c l e , p u l s e d mode a t r e p e t i t i o n r a t e s o f 500 t o 2000 Hz. The a r c t e m p e r a t u r e p r o f i l e i s a p p r e c i a b l y m o d i f i e d i n t h i s mode, becoming n a r r o w e r w i t h a p p r e c i a b l y h i g h e r peak a x i s t e m p e r a t u r e s . More r a d i a t i o n f r o m s o d i u m l i n e s a r i s i n g f r o m h i g h e r e x c i t e d s t a t e s and f r o m t h e c o n t i n u u m r e s u l t s . Disadvantages of t h i s a p p r o a c h a r e t h e need f o r s p e c i a l p u l s i n g c i r c u i t r y t o o p e r a t e t h e lamps and s p e c i a l l y c o n s t r u c t e d a r c t u b e s t o w i t h s t a n d f r a c t u r e f r o m t h e a c o u s t i c s h o c k waves g e n e r a t e d by t h e p u l s e s . Lamps w i t h h i g h e r t h a n normal xenon f i l l p r e s s u r e s have a l s o r e c e n t l y been i n t r o d u c e d (_3)>to i m p r o v e lamp e f f i c a c y and m a i n t e nance r a t h e r t h a n f o r c o l o r i m p r o v e m e n t . Reliable s t a r t i n g of s u c h lamps i s a p r o b l e m w h i c h may r e q u i r e s p e c i a l b a l l a s t s and lamp s o c k e t s . Conclusions The HPS lamp i s a c o m p l e x d e v i c e t h a t has r e a c h e d a r a t h e r a d v a n c e d s t a g e o f d e v e l o p m e n t by l a r g e l y e m p i r i c a l means. There remain u n c e r t a i n t i e s about the s p e c i e s i n t e r a c t i o n s i n the d i s c h a r g e medium w h i c h l i m i t t h e a c c u r a c y o f o u r m o d e l i n g and q u a n t i t a t i v e understanding of the d i s c h a r g e . A number o f s p e c t r a l f e a t u r e s , e s p e c i a l l y a t h i g h m e r c u r y and xenon p r e s s u r e s a r e n o t well understood. B e t t e r v a l u e s f o r such m a t e r i a l p a r a m e t e r s w o u l d p e r m i t more r i g o r o u s lamp m o d e l i n g and b e t t e r a s s e s s m e n t o f t h e c o n s e q u e n c e s o f changes o f p a r a m e t e r s n o t a c c e s s i b l e t o convenient empirical t e s t .

Literature Cited 1. 2. 3. 4.

deGroot, J . J.; van V l i e t , J . A. J . M . ; Waszink, J . H. P h i l ips Tech. Rev. 1975, 35, 334. Wharmby, D. O. IEE Proc. 1980, 127A, 165. McVey, C. I. IEE Proc. 1980, 127A, 158. Schmidt, K. Proc. 6th Int. Conf. on Ion. Phen. in Gases, 1963, Paris, p. 323.


420

5. 6. 7. 8. 9.


10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.


Mizuno, H.; Akutsu, H.; Watarai, Y. Proc. CIE Conf. (Barce lona) 1971, Paper P-71. 14. Buck, U.; Pauly, H. J . Chem. Phys. 1971, 54, 1929. Barratt, S. Trans. Faraday Soc. 1929, 25, 758. Woerdman, J . P. Opt. Commun. 1978, 26, 216. Singh, R. B.; Rai, D. K. Ind. J. Pure Appl. Phys. 1965, 3, 475. Verma, Κ. K.; Vu, T. H.; Stwalley, W. C. J . Molec. Spectrosc. 1981, 85, 131. Zemke, W. T.; Verma, K. K.; Vu, T. H.; Stwalley, W. C. J. Molec. Spectrosc. 1981, 85, 150. Konowalow, D. D.; Rosenkrantz, M. E . ; Olsen, M. L. J . Chem. Phys. 1980, 72, 2612. Whittaker, F. L. Thorn Lighting Ltd. Tech. Rept. LRD 2005, 1972, (unpublished). See also Reference 2. Woerdman, J . P.; deGroot, J . J., ACS Symposium Series 1981, XX, xxxx [this volume]. Waymouth, J . F.; Wyner, E. F . , Paper presented at Annual Tech. Conf. of Ill. Eng. Soc., Dallas, Texas, August 1980. York, G.; Scheps, R.; Gallagher, A. J . Chem. Phys. 1975, 63, 1052. Baylis, W. E. J . Chem. Phys. 1969, 51, 2665. Pascale, J.; Vandeplanque, J . J . Chem. Phys. 1974, 60, 2278. Düren, R. J . Phys. Β 1977, 10, 3467. Zollweg, R. J.; Liebermann, R. W.; McLain, D. K. J. Appl. Phys., 1981, 52, 3293. Waymouth, J . F. J . Ill. Eng. Soc. 1977, 6, 131. Hirayama, C.; Andrew, K. F . ; Kleinosky, R. L. Thermochemica Acta, 1981, 45, 23. Hing, P., Paper presented at Annual Tech. Conf. of Ill. Eng. Soc., Dallas, Texas, August 1980. Wyner, E. F. J . Ill. Eng. Soc. 1979, 8, 166. Inouve, Α.; Higashi, T.; Ishigami, T.; Nagano, S.; Shimojima, H. J . Light and Vis. Env. 1979, 3, 1. Jacobs, C. A. J.; Sprengers, L . ; DeVaan, R. L. C. J . Ill. Eng. Soc. 1978, 7, 125. Kühl, B., paper presented at Conf. Assoc. of Public Lighting Engineers, Scarborough, 1973. Bhalla, R. S.; Larson, D. Α.; Unglert, M. C. J . Ill. Eng. Soc. 1979, 8, 202.

RECEIVED August 26,

1981.


Physics and Chemistry of High Pressure Sodium Lamps - ACS

Recommend Documents