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Prog, Energy Combust. Sci .1991, Vo l. 17, pp. 211-262 0360-1285191 $0.00 + .50Printed in Gre at Britain.All rights eserved, 1991 PergamonPress plc

O P T I C L I M G I N G O F S P R Y S

N O R M A N C H I G I E R

Dep artme nt of Mechanica l Engineering, Carnegie-M ellon University, Pi t tsburgh, PA 15213, U .S.A.

Received 13 November 1990

CONTENTS

1. Imaging1 . 1 . P h o t o g r a p h y1 .2 . Imaging1 .3 . S ti ll pho togra phy

1 . 4 . C a m e r a s1 .5 . F lash un i t s1 .6 . L igh t ing1 . 7 . M i c r o p h o t o g r a p h y1 .8 . Opt ica l sys tems1 . 9 . H i g h s pe e d m i c r o p h o t o g r a p h y s y s te m

1.9.1. S park l ight source1 .10 . Doub le f l ash sys tem for ve loc i ty measurem ent1 .11 . Pho tograph ic and op t ica l sys tem1.12 . Photo graph ic mate r ia l s

1.12.1. Emu lsion1 .12 .2 . La ten t im age form at ion1.12.3. Developing agentsI. 12.4. Fix ing

1.13. Sensi tometry

1.14. Processing1 .15 . Dens i tomet ry1.16. Reciproci ty1.17. Image character is t ics

1.17.1. Hala t ion1.17.2. Image relief1.17.3. Gra in1.17.4. Noise1.17.5. Reso lut ionI . 17,6. Late ral dim ensio nal s tabi l i ty

2 . H i g h S p e ed C i n e m a t o g r a p h y2 .1 . Fun dam enta l fac tors2 .2 . In te rm i t ten t p in reg i s te red cameras2 .3 . Rota t ing pr i sm cameras2 .4 . S t reak came ras

2 .5 . Ul t rah igh-speed cameras2.6. Resolut ion2 .7 . Fram ing ra te2.8. Exp osure t ime2.9. Lenses2.10. Lighting2.11. Picture qual i ty2 .12 . S te reoscopic c inem atograp hy

3 . H o l o g r a p h y3 .1 . Bas ic p r inc ip les o f op t ica l ho logra phy3 .2 . Types o f ho log ram3.3 . Holocameras3.4. Part icle f ield holo grap hy3.5. Dr ople t s izing3 .6 . Four ie r t rans form s

4 . A u t o m a t e d A n a l y s is o f H o l o g r a m s5 . D a t a R e d u c t i o n b y F o u r ie r Tr a n s f o r m A n a l y s is5.1. Pulsed fuel inject ion5 .2 . F low v isua l iza t ion ho logra phy5 .3 . On- l ine par t i c le and f low holog raphy

6 . Au tom at ic Image Analys i s6 .1 . Image com puters fo r ana lys i s o f pho tograp hs

211

212212213215

215216216216219222222224224225225226226227227

228228229229229229229230230230231231231231232

232232232232233233233233234234235236238238239

240240242242243244244


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212 N. CHIGIER

6.2 . Video spra y analyzer 2476.3. Optical imaging for drop size measurement 2476.4 . Dro plet detection and focus measurement 2496.5. Shading correction 2496.6. Au tom ated imaging spray analysis 2506.7 . Da ta analysis 2516.8 . Image processing for size and velocity measurement 252

6.9. Au tom atic image analysis 2536.10. Im age processing 2536.11. Preprocessing, feature extra ction and classification 2546.12. Image processing for nonspherical particles 2566.13. Outline deter min ation and focus selection 2576.14. Shape char acteriz ation 2576.15. Laser imaging system for electronic fuel injection 2586.16. Analysis of holograms 2596.17. Co mp arison of measurements ma de by imaging and scattering instruments 260

References 261

l. IMAGING

1.1. Photography

P h o t o g r a p h y i s t h e p r o c e s s o f f o r m i n g v i s i b l ei m a g e s d i r e c t ly o r i n d i r e c t ly b y t h e a c t i o n o f l i g h t o ro the r fo rms o f r ad ia t ion on sens i t i ve su r faces .Tr a d i t i o n a l l y p h o t o g r a p h y u t il i ze s t h e a c t i o n o f l ig h tt o b r i n g a b o u t c h a n g e s i n s i l v er h al i d es . T h e c h a n g e sm a y b e i n v i s ib l e a n d r e q u i r e d e v e l o p i n g t o r e v e a l th eimag e by conv er t ing the ex posed s i lve r ha l ide to s i lve r.T h e b r i g h t p a r t s o f t h e s u b j e c t g i v e m o r e e x p o s u r e

than the da rk pa r t s so tha t a nega t ive r e su l t s . Ap o s i ti v e , i n w h i c h t h e r e l a t i o n b e t w e e n d a r k a n d l i g h ta r e a s c o r r e s p o n d s t o t h a t o f t h e s u b je c t , is o b t a i n e dw h e n a n e g a t i v e is p r i n t e d o n t o a s h e e t o f s i m i l a rma te r i a l so tha t t he nega t ive tones a re r eve r sed . In ther e v e r s a l p r o c e s s , d i r e c t p r o d u c t i o n o f t h e p o s i t i v eo c c u r s i f t h e d e v e l o p e d n e g a t i v e s i l v e r i s r e m o v e dc h e m i c a l l y a n d t h e r e m a i n i n g s i lv e r h a l i d e i s t h e nredeve loped .

T h e c o m m o n m a t e r i a ls o f p h o t o g r a p h y c o n si st o fan emuls ion o f f ine ly d i spe r sed s i lve r ha l ide c rys t a l s( c h l o r id e , b r o m i d e , o r i o d i d e d e p e n d i n g o n t h e

purp ose ) in ge la t in , coa ted in a th in l a ye r (usua l ly le s st h a n 2 5 # m ) o n g l a ss , f l ex i b le t r a n s p a r e n t f i l m o rpape r. The m os t s ens i t i ve ma te r i a l s , u sed fo r nega t ive -m a k i n g , c o n s i s t o f s i lv e r b r o m i d e c o n t a i n i n g s o m es i lv e r i o d id e . M a t e r i a l s o f i n t e r m e d i a t e s e n s i ti v i ty a r eo f s i l v e r b r o m i d e o r s i lv e r b r o m i d e c h l o r i d e .

A f t e r e x p o s u r e o f t h e e m u l s i o n - c o a t e d m a t e r i a l i n acamera , t he shee t i s deve loped , f ixed in a so lu t ionw h i c h d i s s o lv e s t h e u n d e v e l o p e d s i lv e r h a li d e , w a s h e dt o r e m o v e t h e s o l u b l e s al t s, a n d d r i e d . P r i n t i n g f r o mt h e n e g a t i v e i s d o n e b y c o n t a c t w i t h o r o p t i c a l p r o j ec -t i o n o n t o a n e m u l s i o n - c o a t e d f il m o r p a p e r a n d t h esame sequence o f s t eps i s fo l lowe d as fo r t he nega t ive .G l a s s i s u s e d a s a s u p p o r t w h e n f l a tn e s s a n d r i g i d i t ya re r equ i red .

H i g h s p e e d p h o t o g r a p h y d e a l s w i t h p h o t o g r a p h y a te x p o s u r e t i m e s s h o r t e r t h a n t h o s e g i v e n b y n o r m a ls h u t te r s . N o r m a l s t i ll - c a m e r a m e c h a n i c a l s h u t t e r s

g ive exposu res no t mu ch sh or t e r t ha n 1 /1000 sec . Verys h o r t e x p o s u r e s a r e g i v e n b y m a g n e t o - o p t i c a l s h u t t er su s i n g t h e F a r a d a y e f fe c t, e l e c t r o - o p t i c a l s h u t t e r s u s i n gt h e K e r r e f f e c t a n d p u l s e d e l e c t r o n i m a g e t u b e s .Pu l sed l igh t sou rces a re used to g ive ve ry in t ensei l l u m i n a t i o n o f s h o r t d u r a t i o n . T h e y i n c l u d e x e n o ngaseou s -d i scha rge tubes , an e l ec t r i c spa r k in a i r,e x p l o d i n g w i re s , th e a rg o n f la s h b o m b , h i g h v o l t a g ef l y i n g - sp o t c a t h o d e r a y t u b e s , a n d l a se r s. H i g h s p e e ds in g l e X - r a y p i c t u r e s h a v e b e e n t a k e n b y d i s c h a rg i n ga s h o r t - d u r a t i o n h i g h p o t e n t i a l t h r o u g h t h e X - r a yt u b e . H i g h i n t e n s i ty r e p e t i ti v e fl a sh e s c a n b e o b t a i n e d

w i t h x e n o n f l as h t u b e s f o r s t r o b o s c o p i c p h o t o g r a p h ywi th f r equenc ie s up to 1 05per sec . S ing le expo suresc a n b e u s ed f o r s h a d o w p h o t o g r a p h y, f o r r e f le c te d o rt r a n s m i t t e d l i g h t p h o t o g r a p h y a n d f o r s e lf l u m i n o u ss u b je c t s. F l a s h d u r a t i o n a s s h o r t a s a f e w n a n o s e c h a sbeen a t t a ined .

P h o t o g r a p h y o f f e rs s e v e r al i m p o r t a n t a d v a n t a g e so v e r a l t e r n a t i v e p a r t i c l e s i z i n g t e c h n i q u e s . A m o n gt h e se a d v a n t a g e s a r e t h e s i m p l i c i t y a n d r e l a ti v ei n e x p e n s i v e n e s s o f p h o t o g r a p h i c a p p a r a t u s , f l e x i -b i l i t y, c a p a b i l i t y o f d e t e c t i n g a n d a n a l y z i n g n o n -s p h e r i c a l p a r t i c l e s . F u r t h e r a d v a n t a g e s i n c l u d e : t h e

re l a t ive in sens i t iv i ty to o p t i ca l p ro pe r t i e s o f pa r t i c l e s ,d e r i v a t i o n o f p a r t i c l e v e lo c i ti e s b y u s i n g d o u b l e o rm u l t i p l e e x p o su r e s , a n d t h e v i s u a l e v id e n c e t h a t m a yi n v a l i d a t e m e a s u r e m e n t s b y o t h e r t e c h n i q u e s e .g . d u eto mul t ip l e sca t t e r ing .

I m a g i n g t e c h n i q u e s c a n b e u s e d f o r d e t e c t i n g s o l id ,l i qu id o r s lu r ry pa r t i c l e s ove r a wide r ange o f pa r t i c l ed i a m e t e r s , f r o m l es s t h a n 1 0 # m t o s e v e r al m m w i t hve loc i t i e s up to seve ra l hun dred m/sec . Op t i ca lp r o p e r t i e s c a n v a r y f r o m t r a n s p a r e n t t o o p a q u e .M e a s u r e m e n t s c a n r e a d i l y b e m a d e i n h i g h l yluminous f l ames by us ing appropr i a t e l i gh t f i l t e r s .S p a r k l i g h t s o u r c e s o f t h e o r d e r o f 1 # s e c i n d u r a t i o nwi l l f r eeze pa r t i c l e s wi th v e loc i t i e s up to 50m /secw h i l e p u l s e d l a s e r s w i t h p u l s e d u r a t i o n s o f s e v e r a lnsec can f r eeze imag es o f pa r t i e l e s mo v ing a t ve loc i t i e sm u c h g r e a t e r t h a n 1 00 m / s e c . B e c a u se o f t h e c o h e r e n tn a t u r e o f i l l u m i n a t i n g l i g h t, p h o t o g r a p h s t e n d t o h a v e


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Optical imaging of sprays 213

bject

9 f =

U

moge

FIG. 1. Single lens imaging sy stem.

very p ron oun ced d i ff rac t ion r ings a round images anda l so ' speck le ' no i se which compl ica te au tomatedanalysis.

Work ing range (magni f i ed image)

ob jec t space 2 f > u > f

1.2. Imaging

Image fo rm ing is a d irec t me thod o f r ecord ingin format ion on pa r t i c l es a s opposed to the ind i rec tmethods ut i l iz ing diff ract ion, scat ter ing, resonantsca t t e r ing and in te r fe romet ry. Di rec t pho tographyal lows one plane a t a t ime to be imaged with expo suret imes do wn to a psec wi th shor t dura t ion f la sh l ampsor a nanose c with pulsed lasers. Fi lms have the ma joradvan tage o f p rov id ing h igh reso lu t ion an d ve ry l a rges to rage capac i ty. The overa l l pe r fo rmance o f an

imaging system is in i t ia l ly determined by the opt icalsubsys tem whi le the remainder o f the sys tem can on lydegrade tha t pe r fo rmance . Overa l l pe r fo rmanceincludes such parameters as spat ia l and temporal re-solut ion, d epth of focus, dep th of field, f ield of viewetc.

A s ingle lens imagin g system is show n in Fig. 1 fora diff ract ion- l imited system operat ing with in-coheren t ly t r ans i l lumina ted o r f ron t i l lumina tedob ject s . N ar ro w b and l igh t i s a s sumed . Po ly chrom at icl ight and other aberrat ions wil l degrade the system.The fo l lowing pa ramete r s app ly :

Res olut ion in image space R~1.22v2

2 d

image space 2 f < v < oo.

The ob jec t and image space pa ramete r s a re s implyre lated by the magni f i ca tion . F igure 2 show s a p lo t o fthe resolut ion as a funct ion of effect ive f # for e i therthe object or the image space for three different waveleng ths 600, 500 and 40 0nm . Sm al l f # ' s can beob ta ined by us ing h igh pow er m ic roscope ob jec tives .I t can be seen tha t d rop le t s o f one o r two p m can bedetected, but they cannot be s ized accurate ly. A 5/~mdiamete r d rop le t can be measu red wi th an accurac y o f1 #m. As f0 # gets smal ler, the focal length usual ly

decreases and hence the work ing range becom es ve rysmall.

F igure 3 shows the dep th o f focus as a func t ion o ff l # . The w ork ing range is de f ined by the space underthe curve. Figure 4 shows the depth of focus as afunc t ion o f r eso lu t ion R~ wi th the w ork ing pa ramete r sly ing be low the curve . Fur the r magni f i ca t ion can beadde d to the s ingle lens system by ad ding a re lay lens .Ho wev er, the resolut ion, d epth of field, e tc . wil l bede te rmined by the f i r s t l ens . When coheren t o rpar t ia l ly coherent i l luminat ion is used, the edgesharpness (acu tance) i s improved , bu t mensura t ion

errors can arise.

Magni f i ca t ion

D e p t h o f f o c u s

Res olut ion in object space R0

De pth of f ie ld Au

Effective f # in object space f~ #

Effective f # in image space f i #

I TI = V / U .

A ~ = R ~ / ~

1.22u22 d

= 2R~/2.

u/2d.

v/2d.

/

?o

gg

eIlg

10 600 nm

s o o ~6 J 400 nm

2

0 I I0 5 10

E f f e c t i v e f

FIG. 2. Resolution as a functiono effective # .


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1.3. Stil l Photograph),

S t il l p h o t o g r a p h y o f f er s t h e m o s t i n e x p e n si v em e t h o d f o r p r o v i d i n g h i g h c o n t r a s t , h i g h r e s o l u t i o ni m a g e s o f s p r a y s a n d p a r t i c le c o n f i g u r a t i o n s . R e s u l t sa r e p r o d u c e d i n t h e f o r m o f s l id e s o r n e g a t iv e s in

b l a c k a n d w h i t e o r i n c o l o r. Ta k i n g o f p h o t o g r a p h sa n d d e v e l o p m e n t a n d p r o c e s s in g o f f il m s c a n b eh a n d l e d q u i c k l y a n d i n e x p e n s iv e l y u s i n g t h e a p p r o -p r i a t e c a m e r a s a n d d a r k r o o m f a c il it i es . D a t a c a n b ee x t r a c t e d d i r e c t ly f r o m n e g a t iv e s o r p r i n t s f o r a n a l y s i so r r e p r o d u c t i o n . T h e v i s u al im a g e o f t h e c o m p l e t es p r a y o r a s e c t io n o f t h e s p r a y p r o v i d e s i n f o r m a t i o non s t ruc tu re an d g eom et ry o f pa r t i c l e s tha t i s d i f f icu l tt o o b t a i n b y o t h e r m e a n s . N o z z l e e x i t c o n d i t i o n s ,l i q u i d b r e a k u p , d i s i n t e g r a t i o n o f l i g a m e n t s , d r o pf o r m a t i o n , c l u s t e ri n g a n d c o a l e s c e n c e o f d r o p s c a n b em a d e v i s i b le . T h e g e n e r a l q u a l i t y o f a t o m i z a t i o n c a n

b e g a u g e d b y c o m p a r i s o n o f p h o t o g r a p h s o f d i ff e r e n ts p r a y s a n d b y r a n k i n g t h e s p r a y s a c c o r d i n g t o t h e i rr e l a t ive f ineness o r co a r seness . I t i s ve ry necessa ry top h o t o g r a p h s e c t i o n s o f s p r a y s t o a s c e r t a i n t h a tp a r t i c l e s a r e s p h e r i c a l o r t r a n s p a r e n t b e f o r e u s i n gl a s e r d i a g n o s t i c t ec h n i q u e s. M e a s u r e m e n t e r r o r s a r ese r ious i f t he pa r t i c l e s a re no t sphe r i ca l .

1.4. Cameras

3 5 m m c a m e r a s h a v i n g t h e f o l l o w i n g a d v a n t a g e s : (i )low cos t , ( i i ) conv en ien t fo rm a t , ( i ii ) ex tens ive sys t emswi th accesso r i e s can b e cons t ruc ted , ( iv ) h igh mag -n i f i ca t ion , and (v ) w ide dep th o f f i e ld . The m oree x p e n s i v e 35 m m c a m e r a s h a v e h i g h q u a l i t y l e n s es a n de x t e n s io n s t h a t a l l o w p h o t o g r a p h s t o b e m a d e o v e r aw i d e r a n g e o f c o n d i t i o n s . T h e m a i n d i s a d v a n t a g e o f3 5 m m c a m e r a s i s t h e s m a l l n e g a t i v e s i z e( 2 4 m m x 3 6 m m ) . F o r m i c r o p h o t o g r a p h y , t h i sposes spec ia l p rob lems s ince the r e so lu t ion o f sma l lpa r t i c l e s i s o f t en l im i t ed by the f i lm g ra in s ize .

P l a t e c a m e r a s w i t h l a rg e f i l m a r e a h a v e s p e c i a la d v a n t a g e s i n m i c r o p h o t o g r a p h y. H i g h r e s o l u t i o n

Standard 4 x 5W/300 mm I, en~

/

4 x 5 C a m e r a

12.5 x m agnification

SoLid ex tens ion

10 f e e t

FiLm plane

a g n i f i c a t i o n

M a g n i f i c a t i o n M ) :

m a g e s i z eM

Object s ize

Film to Lens d i s t a n c eM

Lens to subject d i s t a n c e

Sub jec

Lens to subject Lens to fi lmd i s t a n c e d i s t a n c e

m a g e

FIG. 6. Magnification.

w i t h f in e l y d e t a i l e d im a g e s a r e o b t a i n e d . N a r r o wd e p t h s o f f ie l d ca n b e a c h i e v e d a l l o w i n g i s o l a t i o n o f 't h in ' s l i ce s ' o f t he f low f i e ld . One d i sad van tage i s t heg r e a t e r b u l k i n e s s o f t h e e q u i p m e n t m a k i n g i t m o r ed i ff icu l t t o m an ipu la t e . E ach shee t o f f i lm mu s t bei n s e rt e d a n d r e m o v e d f o r e a c h p i c t u r e a n d t h e c o s t o ff i lm is r e l a t ive ly h igh . F ig ure 5 show s a ca me ras p e c ia l l y b u i l t f o r s p r a y p h o t o g r a p h y. A 1 0 f t h o l l o ws q u a r e s e c t i o n e x t e n s i o n i s m o u n t e d b e t w e e n t h eb e l lo w s a n d t h e f il m h o l d e r. A 4 x 5 W / 3 0 0 m m

lens is used, and th e f ilm s ize i s 4 x 5 . Th is sys temprov ides a magn i f i ca t ion o f 12 .5 . Th i s p re -magn i f i -ca t ion p rov ides s ign i f i can t i nc reases in the r e so lu t iono f p a r t i c l e s iz e . F u r t h e r m a g n i f i c a ti o n c a n b e o b t a i n e db y g r e a t e r e x t e n si o n o f t h e t u b e b e t w e e n t h e b e l l o w sand f i lm as shown in F ig . 6 .

W hen a l ens is focused on an ob jec t O (F ig . 7 ) , t he rei s a zone beh ind and in f ron t o f t he ob jec t wh ich i sr e l a t ive ly sha rp . Th i s zone (be tween Y and Z) i sk n o w n a s t h e d e p t h o f f ie l d. D e p t h s o f f ie l d c a n b ec a l c u l a t e d a n d v e r i fi e d b y c a l i b r a t i o n . T h e d e p t h o ff i e l d i s g o v e r n e d b y t h e i m a g e m a g n i f i c a t i o n r a t h e rthan by the l ens foca l l eng th . The h ighe r the mag-n i f i ca t ion , t he m ore l i gh t i s r equ i red fo r ade qua tee x p o s u r e . T h e d e p t h o f f i el d c a n b e i n c r e a s e d b y u s i n gs m a l l e r l e ns a p e r t u r e o p e n i n g s .

Depth o f f i e ld

When a Lens is focused o n a n o b j e c t 0 ) , t h e r e i s azone behind and in f ront o f t h e o b j e c t t h a t i s r endered

a c c e p t a b l y s h a rp . T h i s z o n e b e t w e e n Y a n d Z ) is knowna s t h e d e p t h o f f i e l d .

Lens [

z > < ~

D e p t h o f f i e l d F i l m p l a n e

FIG. 5. High magnification camera for spray photo graphy . FIG. 7. Dep th of field.


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216 N. CHIGIER

1.5. Flash Units

Sc ien t if i c f l a sh u n i t s a re spec i f i ca lly des igned top r o v i d e c o n t r o l l e d s h o r t d u r a t i o n a n d h i g h i n t e n s i t yspa rks . The f l a sh un i t s a re e l ec t ron ica l ly synch ron izedt o p a r t i c u l a r e v e n t s i n t h e s p r a y o r t r i g g e r e d b y t h ec a m e r a . T h e E G & G 5 49 -11 h i g h s p e e d f la s h u n i t h a sa f la s h d u r a t i o n o f 0 .5 ~ s e c a n d a l i g h t o u t p u t o f50 x 106cande la . Th i s p rov ides a day l igh t ba l an cedl i g h ti n g c o l o r t e m p e r a t u r e o f 5 0 00 K . H i g h s p e e d fl a shu n i t s a r e r e q u i r e d i n o r d e r t o f r e e z e t h e m o t i o n o f h ig hspeed pa r t i c l e s .

E lec t ron ic f l a sh un i t s can be used fo r sp rays wi thi n t e r m e d i a t e p a r t i c l e v e l o c i t i e s . S t a n d a r d x e n o np h o t o g r a p h i c f l a s h u n i t s a r e a v a i l a b l e w i t h f l a s hdur a t ion s be twe en 1 /1000 sac and 1 /50 000 sec. Th el i g h t o u t p u t d e c r e a se s a s t h e f la s h d u r a t i o n b e c o m e sshor t e r ma k ing i t i nc reas ing ly d if f i cu lt t o o b ta in su ff i -

c i en t con t ra s t . T h i s i s a r e l a t ive ly inexpens ive so lu t ionfo r sp rays where pa r t i c l e s can be f rozen a t t hese f l a shd u r a t i o n s .

Lighting arrangements.Ba ck Lighting can consist of eith er a) bright field

iLLumination or b)dark f i e ld iLLumination.

Spray/ / / / / / / / /

Subject is silhouettedagainst brightbackground

Spray

ILLuminated su bjec t withdark background

I \ \

S p r a y

/ - = . , ,I \ \

pray

1.6. Lighting

L i g h t i n g c o l o r te m p e r a t u r e n e e d s t o b e c o n s i d e r e dw h e n u s i n g c o l o r f i l m o r b l a c k a n d w h i t e f i l m w i t hse l ect ive co lo r spec t r a l r e spons iveness . Pho to grap h icl i g h ti n g h a s a c o l o r t e m p e r a t u r e r a n g e f r o m 2 8 0 0 t o6 0 0 0 K . T h e m o s t f r e q u e n t ly e n c o u n t e r e d t e m -p e r a t u r e s a r e 3 2 00 K f o r t u n g s t e n b a l a n c e d l i g h ti n ga n d 5 00 0 K f o r d a y l i g h t b a l a n c e d . F i l m s n e e d t o b eb a l a n c e d w i t h t h e l i g h t in g t e m p e r a t u r e . F i l t e r s c a n b ei n t r o d u c e d t o a d j u s t t h e b a l a n c e .

D i r e c t l ig h t f r o m a b a r e b u l b , o p e n s p a r k , o r f l a s ht u b e i s s p e c u la r. T h i s m a y c a u s e u n w a n t e d r e f le c t io nor spec t r a l h igh l igh t s . Fo r sub jec t s w i th h igh r e f l ec -t i o n , p o l a r i z e r s m a y b e r e q u i r e d . I n d i r e c t l i g h tr e f le c t ed o f f a m a t t e s u r f ac e o r t r a n s m i t t e d t h r o u g h at r ans luce n t m a te r i a l i s d i f fuse . D i ffused l igh t p rod ucesl es s c o n t r a s t a n d m o r e e v e n i l l u m i n a t i o n a t t h ee x p e n s e o f r e d u c e d i n t e n si t y.

C o n t i n u o u s l i g h t i n g c a n b e u se d f o r p h o t o g r a p h i n g

Backgrounddark for Lights u b j e c t )

, ~ S p ra y

/1111\\\I I I ~X \ \

X Direct ord i f f u s e d~ . i g h t sources

n

Camera

FIG. 8. Front and side lighting.

R] J a m e r a [ a m e ra

FIG. 9. Back lighting.

s l o w e r m o v i n g p a r t i c l e s w h e r e e x p o s u r e U m e s a r ed e t e r m i n e d b y t h e c a m e r a s h u t t e r. T h e f a s t e s ts t anda rd shu t t e r s have exposure t imes o f 1 /2000 sac .C o n t i n u o u s l i g h t i n g p r o d u c e s h e a t a n d b r i g h t n e s sw h i c h m a y b e c o m e u n c o m f o r t a b l e . T h i s l i g h t i n g i sg e n e r a l l y n o t a p p l i c a b l e fo r h i g h m a g n i f i c a t io n s a n ds i t u a t i o n s o f h i g h p a r t i c l e s p e e d w h e r e h i g h l i g h ti n t e n s it y a n d s h o r t t i m e d u r a t i o n s a r e r e q u i r e d .P a r t i c le t r a c k s o r s t re a k s c a n b e o b t a i n e d f r o m w h i c hv e l o c it i es c a n b e c a l c u l a t e d b y d i v i d i n g t h e l e n g t h o ft h e t r a c k b y t h e t i m e d u r a t i o n o f t h e f la s h .

L i g h t i n g a r r a n g e m e n t s c a n m a k e b i g d i f f er e n c es t ot h e q u a l it y a n d r e s o l u t i o n o f p h o t o g r a p h s . F i g u r e 8s h o w s t h e a r r a n g e m e n t f o r f r o n t a n d s i d e l i g h t i n gus ing e i the r d i r ec t o r d i f fused l igh t sou rces . A da rk

background i s gene ra l ly used wi th th i s l i gh t inga r r a n g e m e n t . E x p o s u r e o n t h e f i lm is d e p e n d e n t o nre f l ec t ion o f l i gh t f rom pa r t i c l e s in the sp ray. Backl i g h ti n g a r r a n g e m e n t s a r e s h o w n i n F i g . 9 . I n b r i g h tf i e ld i l l umina t ion , t he camera f aces d i r ec t ly in to thel i g h t s o u r c e w i t h t h e s p r a y i n b e t w e e n . T h e d r o p l e t sa p p e a r a s d a r k o b j e c t s a g a i n s t a b r i g h t b a c k g r o u n d .F o r d a r k f ie l d i l l u m i n a t i o n , a d a r k s u r f a c e is p la c e db e t w e e n t h e s p r a y a n d t h e l i g h t so u r c e w h i l e al l o w i n gl igh t t o r each the sp ray ind i r ec t ly. The f ina l r e su l t i si l l u m i n a t e d p a r t i c l e s o n a d a r k b a c k g r o u n d .E x a m p l e s o f p h o t o g r a p h s a r e s h o w n i n F i g s 1 0 - 1 5 .

1.7. Microphotography

I n m i c r o p h o t o g r a p h y, t h e t h e o r e t i c a l l i m i t o n t h em i n i m u m r e s o l v a b l e d i m e n s i o n s i s t h e w a v e l e n g t h o f


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Optical imag ing of sprays 217

FIG. 10. W ate r spray: 500 1b/hr, 12psig A ir: 41 psig.

t he sou rce o f l i gh t . Vi s ib l e l i gh t can , i n p r inc ip l e ,r e so lve mic ron s i zed pa r t i c l e s , bu t , i n p rac t i ce , a s ac o n s e q u e n c e o f g e o m e t r ic a n d c h r o m a t i c a b e r r a t i o n sin t he op t i ca l sys t em, i t i s no t poss ib l e t o r e so lveo b j e c ts s m a l l e r t h a n 1 0 # m .

The expos u re t ime , / exp , i s u sua l ly de f ined a s t het ime r equ i r ed fo r a pa r t i c l e t o move a sma l l f r ac t iono f i t s o w n d i a m e t e r

K E d/ e x p < v~

where K E i s a con s t an t w i th a va lue o f 0 .1 , d i s t hed i a m e t e r o f t h e s m a l l e st p a r ti c l e to b e m e a s u r e d , a n d

V~ i s t he pa r t i c l e ve loc i ty. Thus fo r 10 / lm d rops wi tha ve loc i ty o f 100 m/sec , a n e xposu re t im e o f 10 nsec i sr equ i r ed .

T h e t o t a l a m o u n t o f l ig h t , E , f a l li n g o n a n y p a r t o fthe pho tog raph ic p l a t e i s g iven by :

E = I / exp

where E i s t he exposu re and I i s t he l i gh t i n t ens i ty.The l i gh t s ca t t e r ed f rom a sma l l sphe re (10 -

1000/~m) is g iven by

I = loF O)d z

where I0 i s the i nc id en t i l l um ina t ion and F (0 ) i s a


8/52

218 N. CHIGIER

FIG. 11. Water spray: 500 lb/hr, 26 psig Air: 153 psig.

function of the direction of observation relative tothat of the illumination.

The blac keni ng o f photo graphi c film B is given by

B = Kf log E

where Kf is a constant.The high resolution films that are required for

resolving very small particles are generally not verysensitive i.e. Kf is relatively low. When exposure timesare less tha n 1 msec, the effective bla ckeni ng ofphotographic emulsion is lower than that obtainedunder normal conditions of exposure time and il-lumination. Hence for microphotography, higher il-

lumina tion intensities are required than for normalphotography. If E* is the value of exposure requiredfor sufficient effective blackening B* of film in orderto obta in clear images, the inciden t light intensit y I 0 is

E*VpI0~ d3F(0)

i.e. the higher the velocity and the smaller thediameter of the particle, the greater the illuminationthat is required. F(0) increases with 0, the anglebetween the illuminatio n and the observing lightpaths.

Resolution can be improved by magnification in the


9/52


FIG . 12. Coal water slurry: 500 lb/h r, 48 psig A ir: 45 psig.

c a m e r a t o p r o v i d e d i r e c t m a g n i f i c a t i o n o n t h enega t ive . Images o f t he sma l l e s t pa r t i c l e s to bea n a l y z e d s h o u l d b e a t l e a s t a n o r d e r o f m a g n i t u d el a rg e r in s i z e t h a n t h e g r a i n s i z e o f t h e p h o t o g r a p h i cf i lm . The use o f h igh magn i f i ca t ion c rea t e s fu r the rp r o b l e m s . T h e d e p t h o f f i el d o f th e l e n s D i s g i v e n b y

2 F f ( M + 1)D 103M 2

where F i s t he foca l l eng th o f t he cam era l ens , f t hea p e r t u r e , a n d M t h e m a g n i f i c a t io n . D d e c r e a s e s w i t hM a n d h i g h e r f v a l u e s a r e r e q u i r e d t o k e e p D w i t h i nr e a s o n a b l e l i m i t s . T h e o p t i c a l s y s te m i s r e q u i r e d t o b ee x t e r n a l to t h e s p r a y c h a m b e r i n o r d e r t o a v o i d d e p o -s i ti o n a n d c o n t a m i n a t i o n o n l en s es . H e n c e l e n s es w i thcons id e rab le foca l l eng th need to be used , r e su l t ing in

r e d u c e d l u m i n o s i t y. M i c r o p h o t o g r a p h y f o r s p r a yana lys i s r equ i re s l i gh t sou rces wi th bo th ve ry h ighi n t e n s it y a n d v e r y s h o r t d u r a t i o n .

1 .8 . O p t i c a l Sys t e ms

T h e o p t i c a l s y s t e m u s e d b y Yu l eet al. 1f o r s p a r kp h o t o g r a p h y o f f u el s p r a y s i s s h o w n i n F i g . 1 6. A1 0 k V s p a r k u n i t i s e i t h e r f o c u s s e d o n a 1 m m d i a m e t e ra p e r t u r e t o p r o v i d e a ' p o i n t ' l i g h t s o u r c e , o r a l t e r -na t ive ly the ape r tu re i s r ep lac ed by a d i f fus ion sc reenwhich e ff ec t ive ly r educes the d ep th o f f i eld when h ighdens i ty sp rays a re s tud ied . The d i s t ance , u3 , be tweent h e c a m e r a l e n s a n d t h e m e a s u r e m e n t p o s i t i o n in t h es p r a y, sh o u l d b e m i n i m i z e d a n d i s d e t e r m i n e d b y t h e


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220 N. CHIGIER

; ,~Q

FIG. 13. Coal water slurry: 500 lb/hr, 72 psig Air: 162 psig.

?

geometry of the spray system e.g. the width of thespray chamber. The length of the camera v3, and thusthe focal length of the camera lens, is determined bythe magnification vf lu3 required to produce measur-able images of the smallest particles which are ofinterest. The images of these smallest particles shouldhave diameters at least an order of magnitude largerthan the grain size of the photographic film.

The condenser lens L2, which focuses the lightsource on the camera lens, should be of sufficientlylarge diameter to provide illumin ation of most of thefilm and also provide uniform background illumi-

nati on across the film which is particularly impor tan t

for automatic analysis. Both condenser lenses shouldhave relatively short focal lengths to maximize theamount of spark energy which is utilized and tominimize the overall length of the system. Table 1gives values of the optical param eters used by Yule e tal.~ to photograph three different types of spray.

An AMP4 technical camera was used with aPolaroid 5 x 4 film holder. T55 Positive/Negative

film was found to be particularly suitable because ofits wide tonal range and fine grain size. High co ntra stfilms must be avoided as these produce misleadinglysharp images of particles which are not in focus.Automatic analysis techniques rely on measurement


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FIG. 14. Ethylene glycol-water mixture (equal volume portions, liquid pressure 70 psig, nozzle~elavan45 deg. ho llow cone).

TABLE 1. Optical geometries used to study different sprays (see Fig. 16)

Type of spray Low throu ghput Air blast atomized Heavy fuel oil,ultrason ically kerosene or off-design

atomized steam blast conditions,kerosene, atomized heavy atomized by coldwater or fuel oil air, high throu ghpu t

heavy fuel oil

Particle dias. of interes t (~m) 7-90 15-300 70-900Mass median diameter (/~m) 25 80 350Camera magnif ication 11 5 1Measurement depth of field (mm) 1 3 10Light source point point diffusion screen

Diameter of lens L 2 (mm) 80 120 95Focal length of L2 (mm) 80 220 150v 2 (mm) 300 490 1050Focal length of L 3 (mm) 90 150 360u 3 (mm) 100 180 720v 3 (camera length)(mm) 1100 900 720


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222 N. CHIGIER

FIG. 15. Ethylene glyco l-water mixture (equal volume portions, l iquid pressure 21 psig, n oz zle ~e la va n60 deg. solid cone).

o f l i g h t i n t e n s i ty g r a d i e n t s a t t h e e d g e s o f o u t - o f - f o c u s e n e rg y. T h e e l e c tr o n i c c o n t r o l s y s te m p r o d u c e s ai m a g e s , a n d h i g h c o n t r a s t f i lm r e d u c e s t h e a c c u r a c y o f d e l a y b e t w e e n t h e t w o s p a r k s i n t h e r a n g e 5 00 n s e c -me asur in g these g rad ien t s . The use o f Po la ro id f i lm 15 / tsec .r educes the t ime r eq u i red fo r d eve lop ing fi lms .

1 .9 . H igh Speed Mic ropho tog raphy Sys t e m

A h i g h s p e e d m i c r o p h o t o g r a p h i c s y s t e m h a s b e e nd e v e l o p e d f o r s p r a y d i a g n o s t i c s b y C h e l l aet al. 2T h eou t l ine o f t h i s app a ra tus i s shown in F ig . 17. Thep o w e r u n i t ( a ) is c o n n e c t e d t o o n e ( o r t w o ) d i s c h a rg eun i t s (b ) , a spa rk gap ( c ) , one (o r two) f l a sh heads (d )and a t r igge r un i t ( e ) . A l l t hese un i t s a re sepa ra t e . Thed i s c h a rg e u n i t a n d f l a s h h e a d w e r e d e s i g n e d o n t h eb a s is o f p u l s e d l a s e r t e c h n o l o g y. T h e s y s t e m p r o v i d e sa n e x p o s u r e t i m e o f a b o u t 2 0 n s e c w i t h h i g h i r r a d i a t e d

1.9.1. Spark l ight source

I n c o m m e r c i a l s p a r k l i g h t so u r c e s, t h e s p a r k t i m ea n d t h e i r r a d i a t e d e n e rg y a r e n o t i n d e p e n d e n t . S p a r kl i g h t s o u r c e s o p e r a t i n g i n t h e n a n o s e c r a n g e u s u a l l yh a v e c o m p a r a t i v e l y w e a k l i g h t o u t p u t s . T h e F i s h e rn a n o l i t e3 h a s s p a r k t i m e s o f 9 - 1 8 n s e c w i t h e n e rg y i nt h e r a n g e 5 - 2 5 m J , u s i n g 0 . 5 - 3 n F a r a d c a p a c i t a n c e s.T h i s a m o u n t o f i l l u m i n a t i o n i s i ns u f fi c ie n t f o r m i c r o -p h o t o g r a p h y. H i g h e n e rg y s o u r c e s s u c h a s t h o s ee m p l o y e d b y J o n e s a n d S a r j e a n t4 p r o v i d e p u l s ed u r a t i o n s a t h a l f h e ig h t o f 1 5 0 - 2 6 0 n se c . T h i s t i m e


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Optical imaging of sprays

[I- a~O O m m lOOmrnu2 - I uS V3

~F~a( r~'do(~ammmexad 2 5 m m )

omero ens L 310 kVspark source

Atomizer Ape rture A2

Combust ionc h a m b e r

FIG. 16. Optical system for spark ph otograph y.

223

d u r a t i o n i s n o t s h o r t e n o u g h t o p r e v e n t i m a g e b l u r i nv e r y f a s t m o v i n g o r / a n d f i n e s p r a y s . I n t h e s e s p a r ksources , a h igh vo l t age d i f f e rence i s app l i ed to thee l e c tr o d e s . W h e n t h e s p a r k i s tr i g g e r e d , a p a r t i a l i o n i -z a t i o n o f t h e g a s b e t w e e n t h e m a i n e l e c t r o d e s o c c u r s ,f o l l o w e d b y t h e d i s c h a rg e o f t h e m a i n s p a r k t o t h eg r o u n d . I n s u c h a c o n f i g u r a t i o n , p r e i o n i z a t i o n o c c u r sa n d t h e t i m e a t w h i c h t h e s p a r k s t a r t s i s i n d e t e r m i n a t ed u e t o t h e i n h o m o g e n e i t y o f t h e e l e c tr i c f ie l d. I n o r d e rt o p r e v e n t s p o n t a n e o u s d i s c h a rg e a t h i g h v o l ta g e s , t h ee lec t rodes a re kep t a t a r e l a t ive ly l a rge d i s t ance apa r t .

M a n y o f t h e a b o v e m e n t i o n e d p r o b l e m s a r e s o l ve dby u s ing a B lum le in M olec t ron 3 c i r cu i t (F ig . 18 ). Int h is s y s t e m , t h e e l e c t ro d e s a r e c h a rg e d o n l y d u r i n g t h es p a r k t i m e a n d n o t b e f o r e . T h e c a p a c i t o r s C 2 a r eu n l o a d e d w h e n a h i g h v o l t a g e is a p p l i e d a t c a p a c i t o r sC~ . As the c i r cu i t is c losed v ia a spa rk -gap ion ized bya t r igge r pu l se , capac i to r s C] d i scha rge , r e su l t ing int h e i n s t a n t a n e o u s c h a rg i n g o f C 2. I f t h e v a l u e o f t h eindu c tance L i s su i t ab ly chosen , t he n C2 i s d i scha rg edac ross the e l ec t rodes o f t he f l a sh head .

T h e f o l l o w i n g w e re f o u n d t o d e c r e a s e c ir c u i t

impedance : ( i ) capac i to r s wi th low in t r in s i c induc -t ance ( a f ew nHenrys ) , ( i i ) a c i r cu i t des igned to

JPECS 17:3-C

min im ize the r e l a t ive d i s t ance be tw een cap ac i to r s , ( i i i)t h e u s e o f c o a x c a b l e s a n d c o a x i a l s t r u c tu r e s i n t h ed i scha rge un i t s and f l a sh heads (F ig . 19 ) , and ( iv )s i l v e r -p l a t e d c a p a c i t o r e l e c t r o d e s a n d f l a s h - h e a ds t ruc tu re .

T h e t w o g r o u p s C~ a n d C 2 h a v e t e n c a p a c i t o r s o f2 .2 p F a r a d i n p a r a l l e l, p l a c e d o n c i r c u l a r p la t e s . T h ev o l t a g e a p p l i e d i s 20 k V p r o v i d i n g 4 J f o r e a c h s p a r k .L i g h t o u t p u t i s m e a s u r e d b y m e a n s o f a p y r o e l e c t r i cc r y s t a l a n d a h o l o m e t e r. U s i n g a p h o t o d i o d e w i t h asubna nosec r i s e t ime and a Tek t ron ic 7503 osc i l lo -s c o p e , p u l s e d u r a t i o n a t h a l f h e i g h t w a s m e a s u r e d t ob e a b o u t 2 0 n s e c w i t h a t o t a l d u r a t i o n o f le s s t h a n6 0 n s e c . T h e f l a s h h e a d e l e c t r o d e s a r e C h a m p i o nC J 7 Y s p a r k p l u g s , t h e d i s t a n c e b e tw e e n t h e m i s 2 m m .A 'po in t ' l i gh t sou rce i s ob ta ined which i s easy tof o c u s. T h e s p a r k c h a m b e r i s f i ll e d w i t h a rg o n . T h em a g n e t i c c o n f i n e d p l a s m a h a s a m a g n e t i c f i e l d o f

1 00 k G a u s s a n d a p e a k c u r r e n t o f ~ 1 0 k A m p w i t ha c o l o r t e m p e r a t u r e o f > 1 0 0 0 0 K . D i s c h a rg e o c c u r sm a i n l y i n t h e b l u e a n d n e a r u l t r a v i o l e t . T h e s p a r koccur s in the cen te r o f a hem isphe r i c su r face . Thea s p h e r i c a l l e n s ( f = 0 . 9/ 3 5 ) tr a n s f e r s m o s t o f th el i g h t p r o d u c e d b y t h e s p a r k . T h e s p a r k i s f o c u se d o n

( a ) ( b ) ( d ]

( e )

D1 J. D2 . 1 .

NozzL(

( g )

A t o m i z a t i o n r i g

D3 J

I n i t i a t i n g s i g n a l

FIG. 17. High speed microp hotograp hic system.2


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T R

R

H V

0

L

EL

f lashes i s ad jus t ed to min imize in t e r f e rence wi thimages o f o the r pa r t i c l e s . A doub le f l a sh l i gh t sou rcew i t h v a r i a b l e i n t e r s p a r k t i m e i n t e r v a l o f 0 . 5 - 1 5 / t s e ccan be r ead i ly ach ieved (F ig . 20 ). The sequence o feven t s i s : open ing the camera shu t t e r ; f i r i ng the f i r s tspa rk ; a f t e r a sho r t and va r i ab le t ime in t e rva l , f i r i ng

t h e s e c o n d s p a r k ; c l o s i n g t h e c a m e r a s h u t t e r. T h eb l o c k d i a g r a m o f t h e e l e c t r o n i c s y s t e m i s s h o w n i nF i g . 20 . T h e p u l s e g e n e r a t o r a n d d e l a y u n i t a r e m a d eup o f an im pu l se shape r, a de l a y c i rcu i t w i th ap o t e n t i o m e t e r f o r d e l a y ti m e r e g u l a t i o n , a n d t w o a m -p l i f i ca t ion c i r cu i ts . B y inse r t ing f i l t e rs o f d i f f e ren tco lo r s in f ron t o f t he spa rks an d us ing co lo r f i lm e .g .A g f a c h r o m e 1 00 R S , p a i r e d i m a g e s o f d r o p s a r e e a s il yrecogn ized .

C 5OOPF R c I MD, TR TriggerC 2.2 nF R 500 M,t /, E FLash head

C2 2.2 nF SG Spark gap L Inductance

FIG. 18. Blumlein Molectron circuit.

t h e m e a s u r e m e n t r e g i o n w i t h a n a c h r o m a t i c c o n d e n s -i n g l en s b y u s i n g t h e a p p r o p r i a t e f o c a l l e n g th .

1.10. Doub le F la sh Sys tem fo r Ve loc i tyM e a s u r e m e n t

By us ing two f l a shes wi th a co n t ro l l e d t ime in t e rva l ,t w o i m a g e s o f t h e s a m e p a r t i c l e a r e f o r m e d o n t h es a m e n e g a t iv e . M e a s u r e m e n t o f t h e d i s ta n c e b e t w e e nt h e t w o i m a g e s a n d t h e t i m e b e t w e e n t h e t w o f l as h e sy i e ld s t h e v e l o c i ty o f t h e p a r t i c le . T h e t i m e b e t w e e n

1.11. Pho tograph ic and O pt ica l Sys tem

A 3 5 m m P e n t a x S u p e r A c a m e r a w i t h m o t o r - d r i v ew a s u s e d b y C h e l l aet al. 2 S y n c h r o n i z a t i o n w i t h aspa rk l i gh t sou rce i s ach ieved a t a camera shu t t e rspeed o f 1 /125 sec the reb y ensu r ing m in im a l exposu ret o a m b i e n t l i g h t . L e n se s a r e c h o s e n o n t h e b a s i s o f th efo l lowing f ac to r s : necessa ry magn i f i ca t ion , r equ i redd e p t h o f f i e l d , m a i n t a i n i n g l e n s e s o u t s i d e t h e s p r a ycham ber. Va lues o f D~ , D2 , and D 3 (F ig . 17) a rec h a n g e d a c c o r d i n g to t h e m e a s u r e m e n t r e q u i r e m e n t s .D 2 i s r e d u c e d t o a m i n i m u m a n d i s u s u a l l y d e t e r m i n e db y t h e w i d t h o f t h e s p r a y c h a m b e r. D 3 i s d e t e r m i n e db y t h e m a g n i f i c a t io n r e q u i r e d i n t h e c a m e r a .

N e g a t i v e f i lm s p r o v i d e a c o m p r o m i s e b e t w e e n h i g hr e s o l u t io n , g o o d s e n s it i v it y a n d h i g h c o n t r a s t . H i g hr e s o l u t i o n f i l m s m a k e i t p o s s i b l e t o m a i n t a i n m a g -n i f i ca t ion on the neg a t ives to wi th in r easo nab le l imi t s .

Sparking- plug

n s

FIG. 19. Discharge u nits an d flashheads.


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Optical imaging o f sprays

Trigger

4 ,:;, , - 2 q_l-

Trigger

Pu segenerotordeLoy

u n t

FIG. 20. Dou ble flash light source.

~Osci t toscope

225

T h e r e s o l u t i o n o f I l f o r d P a n F - 5 0 A S A f i l m i s a b o u t1 5 0 1 in e s /m m w h i l e t h a t o f A G F A P a n 2 5 A S A i s185 l ines /mm . To ph o to gra ph 10 /~m pa r t i c l e s , a mag-n i f i ca t i o n o f a b o u t x 6 i s r e q u i r e d . H i g h c o n t r a s tf i l m s a r e n o t r e c o m m e n d e d s i n c e t h e y p r o d u c em i s l e a d i n g l y s h a r p i m a g e s o f p a r t i c le s w h i ch a r e n o ti n f o cu s . W h e n a u t o m a t e d a n a l y s i s i s u s e d , t h is c a nresu l t in s ign i f i can t e r ro r s . P ro jec t io n o f nega t iveso n t o s c r ee n s w i th m a g n i f i c a t io n o f u p t o 1 00 c a nfasc i l i t a t e m anu a l ana lys i s .

F i g u r e 2 1 sh o w s p h o t o g r a p h s o f l i q u i d b r e a k u p i n

a s i m p l e p r e ss u r e a t o m i z e r w i t h p r o g r e s s i v e in c r e a s ein l i qu id f low ra t e . A t v e ry low ve loc i ty (3 m/sec )b r e a k u p o c c u r s a t 11 4 d i a m e t e r s d o w n s t r e a m f r o mt h e n o z z l e w i t h i n i t ia l d i a m e t e r o f 0 . 3 5 m m ( F i g .21 (a ) ). F ig u re 21 (b ) shows a w a te r- g lyc e ro l j e t , 45d i a m e t e r s d o w n s t r e a m w i t h a v e l o c i t y o f 3 0 m / s e c ,wh i l e F ig . 21 (c ) i s fo r a j e t ve loc i ty o f 65m/sec .F o r m a t i o n a n d b r e a k u p o f l ig a m e n t s c a n b e c l e a rl ys ee n . I m a g e s c a n b e a n a l y z e d a n d d r o p s iz e c a n b em e a s u r e d .

1.12. Photographic Materials

o

O

o) b) c)

FIG. 21. (a) Grow th of axisymmmetric oscil lation andbreaku p of a w ater jet V = 3 m/sec (original magnificationx 10); (b) drop ejection from the jet by b reaku p of l igamentsV = 30m/sec (orig inal magnification x 15); (c) sho rt wave-length surface waves on the jet and subsequent stripping of

ligaments V = 65m/sec (original magnification x 15).

M a t e r i a l s t h a t h a v e b e e n u se d f o r p h o t o g r a p h i cr e c o r d i n g i n c l u d e : d i c h r o m a t e d g e l a t in , p h o t o r e s i s t s ,e l e c t r o d e f o r m a b l e t h e r m o p l a s t i c s , f e r r o e l e c t r i cc r y s ta l s , v a r i o u s o rg a n i c a n d i n o rg a n i c p h o t o c h r o m i cm a t e r i a l s , p h o t o c o n d u c t o r s i n d e v i c e s w i t h o t h e rm a t e r i a l s , m a g n e t o - o p t i c f i l m s a n d v e r y t h i n m e t a lf i l m s . A m o n g a l l t h e c h e m i c a l a n d p h y s i c a lp h e n o m e n a w h i c h h a v e b e e n i n v e s t ig a t e d t h u s f a r , n o te v e n o n e a p p r o a c h e s s i lv e r h a l i d e ' s u n i q u e c o m b i -n a t i o n o f p r o p e r t i e s w h i c h c o m b i n e s e n s i t i v it y a n ds tab i l i t y wi th va r i e ty and ve r sa t i l i t y. S i lve r ha l idep h o t o g r a p h i c m a t e r i a l s a r e t h e m o s t w i d e l y u se dm e d i a f o r i m a g e r e c o r d i n g a n d r e t r ie v a l f o r co u n t l e s sa p p l i c a t i o n s i n c l u d i n g h o l o g r a p h y. P h o t o g r a p h yinc ludes the fo l lowing p rocesses : P l a c e m e n t o f t h e p h o t o s e n s i t iv e m e d i u m ( t h e

e m u l s i o n ) E x p o s i n g th e e m u l s i o n w i t h p h o t o n s t o p r o d u c e a n

inv i s ib l e ( l a t en t ) im age D e v e l o p i n g t h e e x p o s e d e m u l s i o n t o r e n d e r a

v i s ib l e image ( ampl i f i ca t ion )

F i x i n g t o r e n d e r t h e i m a g e p e r m a n e n t .

1.12.1. Emulsion

The pho tosens i t ive emuls ion i s a t h in f i lm d i spe r-


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s i o n o f s i l v e r h a l i d e m i c r o c r y s t a l s i n a p r o t e c t i v ec o l l o id , t y p i c a l l y g e l a ti n . T h e t e r m ' e m u l s i o n ' i s u s edt o d e s c r i b e b o t h t h i s d i s p e r s i o n a n d t h e c o a t i n g o n as u p p o r t b a s e s u ch a s p a p e r, g l a s s , m e t a l o r p o l y m e r i cf il m w h i c h p r o v i d e s m e c h a n i c a l s t r e n g t h .

S i l v er c h l o r i d e , b r o m i d e , i o d i d e , o r t h e i r c o m b i -

n a t i o n s a r e u s e d d e p e n d i n g o n t h e s e n s i ti v i ty a n do the r cha rac te r i s t i c s des i r ed . S i lve r ch lo r ide i s u sedf o r e m u l s i o n s o f l o w e r s e n s it iv i t y ; c h l o r i d e / b r o m i d ea n d b r o m i d e f o r s o m e w h a t g r e a t e r s e n s it i vi t y ; a n db r o m i d e / i o d i d e f o r t h e g r e a t e s t s e n s i t i v i t y. I o d i d es e l d o m e x c ee d s 5 % a n d a l o n e h a s n o p r a c t i c a l v a l u ea s a p h o t o g r a p h i c e m u l s i o n .

The s i lve r ha l ide mic roc rys t a l s r ange in s i ze ands h a p e f o r v a r i o u s e m u l s i o n s , a l t h o u g h w i t h i n a n yp a r t i c u l a r e m u l s i o n t h e d i s t r i b u t i o n i n s i ze s i s c o m -para t iv e ly na r row. The ave rage s i ze in a ve ry f ineg r a i n L i p p m a n n e m u l s i o n m a y b e 0 . 0 5 # m w i t h a

range o f 0 .03 -0 .08 ~m, wh i l e in a ve ry f a s t nega t ive -t y p e e m u l s i o n t h e g r a i n s i z e m a y b e o n t h e o r d e r o fseve ra l mic rons .

The s i lve r ha l ides have a cu b ic c rys t a l s t ruc tu re inw h i c h e a c h s i lv e r i o n A g i s s u r r o u n d e d b y s i x n e a r e s tn e i g h b o r h a l i d e i o n s X - a n d v i ce v e r s a. T h e c r y s t a lh a s e x c e ss h a l i d e i o n s ( o r i g i n a t in g f r o m t h e e m u l s i o n -m a k i n g p r o c e s s ) s t r o n g l y a b s o r b e d t o i t s s u r f a c e sa long w i th ge la t in , s ens it i z ing dyes , and o the r spec ie s ,a l l o f wh ich p lay c r i t i ca l ro l e s in s t ab i l i z ing thee m u l s i o n a n d t h e l a t e n t i m a g e , a n d i n d i r e c t i n g t h er e s u lt s o f d e v e l o p m e n t .

1.12.2. Latent image formation

T h e s i l v e r h a l i d e c r y s t a l i s a n n - t y p e p h o t o -c o n d u c t o r w i t h a v a l e n c e b a n d o f l o c a l iz e d e l e c t r o n sa n d w i t h a c o n d u c t i o n b a n d i n w h i c h i n j e c t e de l e c t ro n s a r e f re e t o m i g r a t e t h r o u g h o u t t h e c r y s t a l,a n e l e c t r o n i s p r o m o t e d t o t h e c o n d u c t i o n b a n d ,l eav ing beh ind a pos i t i ve ho le r ep resen ted by a f r eeh a l o g e n a t o m :

A g + X - ( c r y s t a l ) +hv ~ A g + X ( c r y s ta l ) + e - .

The f r ee e l ec t ron m ig ra t e s un t i l t r ap ped a t a l a t t i ced e f e c t w h i c h , a m o n g o t h e r t h i n g s , c a n b e a s i l v e r a t o mA g . Th i s t r apped e l ec t ron can then r educe a ne igh-b o r i n g s i lv e r io n A g t o p r o d u c e a s i l v e r a t o m :

A g + X - ( c ry s t a l) + e ~ A g X - ( c r y st a l ).

The s ing le si lve r a to m i s no t s t ab le , however, andhas a l i f e t ime es t ima ted to b e abo u t 1 sec . I f du r ing i t sl i fe t im e it t r a p s a n o t h e r e l e c tr o n , a t w o a t o maggrega te i s fo rmed , and i t i s s t ab le :

A g , X - ( u n s t a b l e ) + e - ~ A g ( e - ) ,

X - A g ( e - ) , X - + A g , X - ~ A go 2 X - .

T h i s t w o - a t o m a g g r e g a t e a l t h o u g h s t a b l e d o e s n o tcons t i tu t e a l a t en t im age . I t w i l l no t r e nde r the c rys t a ld e v e l o p a b l e . C u r r e n t t h e o r y c o n t e n d s t h a t a t l e a s t af o u r - a t o m a g g r e g a t e ( A g, x /> 4) i s req uire d to

r e n d e r a c r y s t a l d e v e l o p a b l e . A t w o - a t o m a g g r e g a t e ,b e i n g s ta b l e , c a n t r a p a d d i t i o n a l m i g r a t i n g e l e c t r o n sto p roduce a l a t en t image , and i t , t he re fo re , con-s t i t u t e s a s t ab le nuc leus fo r l a t en t image fo rma t ion :

A g ( A g + X - ) , + x ( e - ) ~ A g + ., .( A g +X - ) ..... ( X ) .

po ten t i a l l a t en t imag e l a t en t imag e fo r x ~> 2

Because o f t he shor t l i f e t ime o f the s ing le s i lve ra t o m , t h e r m a l e v e n t s a n d p h o t o e v e n t s o f v er y lo wp r o b a b i l i t y, s u c h a s o c c u r d u r i n g d a r k s t o r a g e a n dd u r i n g v e r y l o w i r r a d i a n c e e x p o s u r e s m a y l e a v e li t tl eo r n o h i s t o r y o f t h e i r h a v i n g o c c u r r e d . T h i s ' r e l a x -a t i o n ' o r r e v e r s i b il i t y a c c o u n t s f o r t h e e x c e l le n ts to rage l i f e o f s i l ve r ha l ide emuls ions and fo r t heexce l l en t s t ab i l i t y o f t he l a t en t image , pa r t i cu la r ly inc o m p a r i s o n t o o t h e r p h o t o i m a g i n g s y s t e m s .

T h e f r e e h a l o g e n a t o m ( p o s i t i v e h o l e ) w h i c h w a sf o r m e d a l o n g w i t h t h e f r e e el e c t r o n c a n m i g r a t e a n dre t r ap the e l ec t ron ,

X + e - --* X -

or ox id ize s i lve r back to s i lve r ha l ide

X + Ago - -. A g + X - .

To i m p r o v e t h e e f fi ci e nc y o f l a t e n t i m a g ef o r m a t i o n , t h i s h a l o g e n a t o m m u s t b e t r a p p e d , a n dt h is c a n b e a c c o m p l i s h e d b y c h e m i c a l re d u c i n g a g e n t sw h i c h a r e a d d e d d u r i n g e m u l s i o n p r e p a r a t i o n . S u l f u rc o m p o u n d s a r e p a r t i c u l a r l y e f fe c ti v e w h i c h e x p l a i n st h e u n i q u e r o l e p l a y e d b y g e l a t i n , w h i c h i s a n a t u r a lp r o t e i n - c o n t a i n i n g s u l f u r - b e a r i n g a m i n o a c i d . T h el a t e n t im a g e t h e n i s a t l e a s t a f o u r - a t o m a g g r e g a t e o fm e t a l l i c s i lv e r e m b e d d e d s o m e w h e r e w i t h i n o r o n t h esu r face o f t he s i lve r ha l ide c rys t a l .

1.12.3. Developing agents

C h e m i c a l d e v e l o p e r s c o n t a i n c h e m i c a l r e d u c i n ga g e n t s w h i c h d i s t i n g u i s h b e t w e e n e x p o s e d a n du n e x p o s e d s i l v e r h a l i d e a n d c o n v e r t t h e e x p o s e dha l ide to s i lve r. Deve lope r s in gene ra l u se a re com-p o u n d e d f r o m o r g a n i c r ed u c i n g c o m p o u n d s , a n a l k a lito g ive des i r ed ac t iv i ty, sod ium su l f it e wh ich ac t s a s ap r e s e r v a t iv e a n d p o t a s s i u m b r o m i d e u s e d to s u p p r e s st h e d e v e l o p m e n t o f u n e x p o s e d c r y s t a l s ( f og ) . M o s td e v e l o p i n g a g e n t s a r e p h e n o l s o r a m i n e s u s u a l l y c o n -t a i n i n g t w o h y d r o x y l g r o u p s o r t w o a m i n o g r o u p s , o ro n e h y d r o x y l a n d o n e a m i n o g r o u p a t t a c h e dortho- o rpara- t o e a c h o t h e r o n a b e n z e n e n u c l e u s . A l k a l i e sg e n e r a l l y u s e d a r e s o d i u m c a r b o n a t e , s o d i u mh y d r o x i d e , a n d s o d i u m m e t a b o r a t e . S u l f it e i n ad e v e l o p e r l o w e r s th e t e n d e n c y f o r o x i d a t i o n b y a i r.O x i d a t i o n p r o d u c t s o f d e v e lo p e r s h a v e a n u n d e s i r a b l e

i n f l u e n c e o n t h e c o u r s e o f d e v e l o p m e n t a n d m a yresu l t i n s t a in .

D e v e l o p i n g a g e n t s a n d f o r m u l a e a r e s e l e c t e d f o ruse wi th speci f ic emu ls ions and pu rposes . F ine g ra ind e v e l o p e r s r e d u c e t h e a p p a r e n t g r a i n i n e s s o f


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Optical imaging of sprays

n e g a t i v e s . C o n v e n t i o n a l c o m p o n e n t s a r e a d j u s t e d t os lo w a c t i v i t y a n d c o n t a i n a s o l v e n t f o r s i lv e r b r o m i d e .I n a n u m b e r o f p r o d u c t s , t h e d e v e l o p i n g a g e n t i si n c o r p o r a t e d i n t h e e m u l s i o n , d e v e l o p m e n t b e i n gi n i t i a t e d b y p l a c i n g t h e f i l m o r p a p e r i n a n a l k a l i n eso lu t ion . When f i lm i s deve loped , t he re i s u sua l ly ap e r i o d d u r i n g w h i c h n o v i s ib l e e f fe c t a p p e a r s . A f t e rth i s , t he dens i ty inc reases r ap id ly a t f i r s t and thenm o r e s l o w l y, e v e n t u a l l y r e a c h i n g a m a x i m u m . T h er e l a t i o n b e t w e e n d e n s i t y a n d d e v e l o p m e n t t i m e i sg iven by

D ----- D ~(1 -- e k')

where D i s t he dens i ty a t t a ined in t ime t , Do~ i s t hem a x i m u m d e v e l o p a b l e d e n s i ty, a n d k i s t h e v e l o c i tyc o n s t a n t o f d e v e l o p m e n t .

S i l v e r h a l i d e s c a n b e c h e m i c a l l y re d u c e d t o m e t a l li cs i lv e r b y a l a rg e n u m b e r o f r e d u c i n g a g e n t s . T h e n e t

r eac t ion fo r t h i s r educ t ion i s :2 A g + X - + H O C 6 H 4 O H ~ -

2 A g + O = C 6 H 4 = O + 2 H X

f o r th e c o m m o n l y u s e d d e v e l o p e r, h y d r o q u i n o n e . T h ep r o d u c t s o f r e d u c t i o n a r e m e t a l li c si lv e r , t h e o x i d i z e df o r m o f t h e d e v e l o p e r ( q u i n o n e ) a n d h a l o g e n a c i d .T h e p r e s e n c e o f a l k a l i d u r i n g d e v e l o p m e n t a c c e l er a t e st h e r e d u c t i o n a n d c o n s u m e s t h e c o p r o d u c e d a c id . T h es i lv e r h a l i d e g r ai n s i n a n u n e x p o s e d p h o t o g r a p h i ce m u l s i o n c a n a l s o b e r e d u c e d b y t h e d e v e l o p e r t op r o d u c e u n i m a g e d d e n s i t y c a l l e d ' f o g ' , b u t t h i sr e d u c t i o n i s c o m p a r a t i v e l y s lo w i n r e l a t i o n t o l a t e n t -i m a g e d g r a i n s. T h e p r e s e n ce o f m e t a l l i c si lv e r ( A gox > / 4 ) ca t a lyzes the chem ica l r educ t ion , acce le ra t ingt h e r a t e o f c h e m i c a l r e a c t i o n t o s u c h a d e g r e e t h a t al a t e n t - i m a g e d c r y s t a l c a n b e c o n v e r t e d t o m e t a l l i cs i lve r be fo re an unex posed c rys t a l can r eac t .

S ince on ly a f ou r-a to m aggreg a te o f s i l ve r i s su ff i-c i en t t o conver t t he en t i r e c rys t a l o f s i l ve r ha l ide tome ta l l i c s il ve r, a t r em end ous am pl i f i ca t ion is r ea l ized .A s low speed , f ine g ra in em uls ion co n ta in in g 0 .05 ~ tmcubes o f s i lve r b ro m ide con ta in s ab ou t 2 .6 x 106s i lve r ions pe r c ry s t a l , wh i l e a coa r se -g ra in , h igh speed

em uls ion con ta in in g 1 #m cubes con ta ins 2 x 101s i lve r ions pe r c rys t a l . Conse quen t ly, on ly a r e l a t ive lyfew pho to ns , su ff ic i en t t o p rodu ce an Ag~ aggreg a te ,can p r odu ce 106-t05 a t om s o f me ta l l i c s i l ve r.

S ince ph o to gra ph ic sens i t i v i ty ( speed) is a func t iono f t h e d e v e l o p e d s i l v e r ( i m a g e d e n s i t y ) p r o d u c e d p e rg iven exposure , t he re i s a r ec ip roca l r e l a t ionsh ipb e t w e e n e m u l s i o n s p e e d a n d r e s o l u t i o n . A c o a r s e -g r a i n e m u l s i o n p r o v i d e s m o r e s i lv e r p e r d e v e l o p a b l eg r a i n t h a n a f i n e - g r a i n e m u l s i o n . I f h i g h r e s o l u t i o n i sd e s i r e d , p h o t o g r a p h i c s p e e d m u s t b e r e d u c e d ,e m u l s i o n m u s t b e f i n e - g r a i n a n d s l o w e r w h i l e i r -

r a d i a n c e e x p o s u r e s m u s t b e l o n g e r a n d h i g h e r.

1.12.4. F ix ing

A f t e r t h e i m a g e i s d e v e l o p e d , th e u n c h a n g e d h a l i d ei s r e m o v e d u s u a l l y i n w a t e r s o l u t i o n s o f s o d i u m o r

227

a m m o n i u m t h i o s u l f a t e . T h i s r e m o v a l o f u n c h a n g e dha l ide i s ca l l ed fix ing. P r io r t o f ix ing , a d i lu t e ac id s topb a t h i s o f te n u s e d t o n e u t r a l i z e t h e a l k a l i c a r r ie d o v e rf r o m t h e d e v e l o p e r. T h e r a t e o f f ix i n g d e p e n d s l a rg e l yo n t h e c o n c e n t r a t i o n o f th e f i xi n g a g e n t a n d t h e t e m -p e r a t u r e . I n t h e c a s e o f s o d i u m t h i o s u l f a te , t h e r a t e o ff ix i n g i s m o s t r a p i d a t 2 0 - 4 0 % c o n c e n t r a t i o n a n d a t6 0 - 7 5 F. N e g a t i v e s a n d p r i n t s a r e w a s h e d i n w a t e ra f t e r f ix ing to r emove the so lub le s i lve r ha l ide - f ix inga g e n t c o m p l e x e s w h i c h m i g h t r e n d e r t h e p h o t o g r a p hu n s t a b l e w h e n s t o r e d o r m i g h t c a u s e st a i n . T h e r a t e o fr e m o v a l o f c o m p o u n d s b y w a s h i n g c a n b e a c c e l e ra t e db y n e u t r a l s a l t s o l u ti o n s k n o w n a s h y p o c l e a n i n g a i d s .A f t e r w a s h i n g , t h e m a t e r i a l s m u s t b e d r i e d a su n i f o r m l y a s p o s s i b l e , p r e f e r a b l y i n m o v i n g w a r m a i r.P a p e r p r i n t s a r e f r e q u e n t l y d r i e d o n h e a t e d m e t a ld r u m s w h i c h m a y a l s o g iv e g lo s s to t h e p r i n t s i f th e ya re d r i ed wi th the i r em uls ion s ide to the me ta l su r face .

I m a g e s a r e i n t e n s if i e d b y i n c r e a s in g t h e d e n s i t y o ft h e i m a g e , u s u a l l y b y d e p o s i t i o n o f s i lv e r , m e r c u r y, o ro t h e r c o m p o u n d s , t h e c o m p o s i t i o n b e i n g s e l e c t e da c c o r d i n g t o t h e d e g r e e o f i n t e n s i f i c a t i o n r e q u i r e d .E n l a rg e r s a r e o p t i c a l p r o j e c t o r s c o n s i s t i n g o f al am pho use an d l igh t sou rce , a ho lde r fo r a nega t ive , acondense r o r d i f fus ing shee t , a p ro jec t ion l ens , and am e a n s f o r f o c u s in g f o r t h e d e s i r e d m a g n i f i c a ti o n .

A s a c o n s e q u e n c e o f s il v e r h a l i d e ' s u n i q u e c o m -b i n a t i o n o f s o l i d s t a te a n d c h e m i c a l p r o p e r t i e s , s i lv e rh a l i d e p h o t o g r a p h y r e m a i n s u n p a r a l l e l e d a s a no p t i c a l r e c o r d i n g m e d i u m . S i l v e r h a l i d e e m u l s i o n s

have the fo l lowing spec ia l p rope r t i e s :(1) excel len t shel f l i fe,(2 ) exce l l en t l a t en t im age s t ab i l i t y,(3 ) ca n b e spec t r a l ly sens i ti zed ,(4 ) g rea t f l ex ib i l i ty fo r t a i lo r ing to spec if i c app l i -

ca t ions , and( 5 ) f l e xi l ib i ty n t h e q u a l i t y a n d t y p e o f i n f o r m a t i o n

tha t can be r e t r i eved by p rocess ing .

1 .13. Sensi tometry

S e n s i t o m e t r y d e s c ri b e s h o w t h e i m a g e o f a p h o t o -g r a p h i c e m u l s i o n v a r ie s a s a f u n c t i o n o f t h e e x p o s u r e .S e n s i t o m e t r y is d e p e n d e n t u p o n , ( i) t h e e m u l s i o n ' scha rac te r i s t i c cu rve , ( i i) i ts spec t r a l cu rve , a nd ( i ii ) i t sr e c i p r o c i t y f a i l u r e c u r v e. T h e e m u l s i o n ' s m o d u l a t i o nt r a n s f e r f u n c t i o n ( M T F ) c u r v e d e s c r i b e s i t s r es o l v i n gp o w e r.

E x p o s u r e ( E ) E = l . t

w h e r e I i s th e r a d i a n t f l u x p e r u n i t a r e a ( i r r a d i a n c e o r

i l l uminance ) inc iden t on the f i lm , and t i s t he expo suret ime .

Sens i t iv i ty (S ) S =1 /E

Tr a n s m i t t a n c e(T ) T -~ I / Io


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228 N. HIGIER

where I i s the t ransm it ted f lux an d I0 is the incidentf l u x .

Am pli tude t ransm it tance (TA) TA = T m

Opa c i ty (O) O = Io / I

A b s o r b a n c e (A ) A = l o g ( I / T )Dens i ty (D) D = log ( I /T ) .

The radiant f lux incident on the emulsion ismeasured in e rgs /cm 2 - sec o r t tW /cm 2 . Expo sure i smea sured in ergs /cm 2 or joules /m 2. I llumin ance is theradiant f lux incident on the emulsion in terms o f whi tel ight as i t affects human visual response. I t i smeasured in pho tomet r i c un i t s a s foo t -cand les o rmeter-candles . Ph otom etr ic uni ts re la te only to vis iblel ight in the range 400-7 00 nm . Th e sensi t iv i ty of thehuman eye var ies across this range reaching amax imum a t 555 nm.

Every deve loped emuls ion exh ib i ts a b ackg rounddensi ty which is cal led 'base plus fog ' . The emulsionsupport (base) s l ight ly a t tenuates the readout as afunct ion o f i t s th ickness , c lar ity, and spectra l charac-teristics. F or typ ical base mater ials su ch as glass,polyester, and cel lulose t r iaceta te , the a t tenuat ion isnorm al ly spectra l ly fla t over the visible po r t ion of thespectrum, but i t can vary s ignif icant ly in the ul t ra-v io le t and in f ra red reg ions . An unexposed emuls ionhas an inheren t t endency to deve lop dens i ty to somedegree depend ing on deve loper compos i t ion , t em-

pera tu re , t ime , type o f emuls ion , dev e lopmenttechn ique , emuls ion age and cond i t ions o f ag ing . Baseplus fog combines these two effects as a s ingle mea-surement .

'Co ntra s t ' i s the visual di fference between two levelsof l ight . The con tras t ra t io is def ined as

C = I , /I2 (I , > I2).

The ' speed ' o f a pho togra ph ic emuls ion i s desc r ibedby a speed number which re la tes the sensivi ty of theemuls ion to a pa r t i cu la r type o f app l i ca t ion such asday l igh t pho tography. The speed number i s an index

which p rov ides the pho tographer wi th a means o fde te rming the min imum shu t t e r speed o r smal les taper tu re requ i red to p roduce a des i red pho tograph icresu lt wi th a g iven amo unt o f l igh t . Examples o f speedindexes are : Am er ican S tandard s A ssoc ia t ion (ASA) ,Deutsche Indus t r i e Nomen (DIN) , and Aer ia l F i lmSpeed (AFS) .

For emulsions whose speed is def ined for whi te-l ight exposures , i t i s very important to def ine thespectra l com pos i t ion o f the whi te l ight used toproduce the charac te r i s t i c cu rve because mos temulsions do not exhibi t equal sensi t iv i ty over the

ent i re vis ible spectrum. The spectra l composi t ion ofwhite l ight is def ined in terms of i ts colo r tempe raturewhich i s approx im ate ly 5500 K fo r day l igh t and whichtyp ical ly ranges f rom 3200 to 340 0K for tungs tenlamps . Day l igh t co lo r t empera tu re i s then s imula tedf rom a tungs ten source by means o f a day l igh t co r rec -

t ion f i l ter which at tenuates the longer wavelengths toprov ide the p rop er b lue - to - red co lo r ba lance .

1.14. Process ing

Process ing is dependen t on the type o f deve loperand the t ime and t empera tu re o f deve lopment .Dev eloper fo rm ulat ion s differ in their abi l ity to effectdifferent degrees o f contras t , effect ive speed, g ranular-i ty, fog and the t ime o f deve lopment . Al l these charac -ter is t ics are interre la ted, and a given developer for-mula t ion i s des igned to p rov ide the des i red combi -nat ion of character is t ics as def ined by the specif ica-t ions of the emulsion. A f ine-grain developer, forexample, wi ll usual ly red uce the effective speed w hilea high con tras t d evelope r wil l usual ly increase graini-ness . Dev elopm ent t ime is determ ined from a ser ies of

character is t ic curves . Development t ime decreaseswi th inc reas ing t empera tu re o f the deve lop ing ba th .Too high a temperature wil l general ly increase fog,p roduce a coarse r g ra in , and shor ten the work ing l if eo f the deve loper so lu t ion w hi le too low a t emp era tu rewil l s ignif icant ly reduce contras t . The temperature-t ime re la t ionship is cr i t ical to producing specif icimage qua l i ty. F igure 22 shows a typ ica l t ime-tempera tu re g ra ph fo r deve lopment . The s i lve r imageof a p roper ly p rocessed and w ashed emuls ion remainss tab le fo r man y years and i s r e s is t an t to cor ros ion bywater an d n onre act ing sal ts a t a ll pH levels , but i t canbe oxidized by oxidizers such as hydrogen peroxide.

1.15 Densi tometry

The image in s i lve r ha l ide pho tography i scom pos ed o f f inely divided p ar t ic les o f metal lic s i lvergrains , and their effective con cen trat ion per un i tsurface area is re la ted to the exposure . The incidentf lux ( inf lux) on passing through the developed

75

,To

7 0

P88

6 5

I I I2 3 4 5 6 7 8 9 1 0

eveLopment t i m e [ m i n

FIG. 22. Time-temperature graph for development.


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Optical imaging o f sprays 229

emu ls ion i s s ca t t e red by the s i lve r g ra ins so tha t t heemerg ing f lux ( e ff lux ) i s a t t enua ted . The degree o fa t t e n u a t i o n i s d e s c ri b e d b y t h e d e n s i t y, a n d i t s e v a l u -a t i o n d e p e n d s o n t h e m a n n e r b y w h i c h t h e e f fl ux i sm e a s u r e d . W h e n a n i n f l ux e n t e rs t h e e m u l s i o n n e a r l yp e r p e n d i c u l a r t o i t s s u r fa c e , a p o r t i o n o f t h i s in f l u xw i l l e m e rg e m o r e o r l e ss o n a x i s a s th o u g h i t h a d n o tb e e n s c a t t e r e d o r o n l y v e r y s l i g h tl y s c a t te r e d . A n o t h e rp o r t i o n w i l l b e s c a t t e r e d t o t h e e x t e n t t h a t i t n e v e re m e rg e s . T h e r e m a i n d e r i s s c a tt e r e d s o t h a t i t em e rg e sf rom the em uls ion a t a l l ang les ove r 180 Th a tp o r t i o n w h i c h e m e rg e s o n o r n e a r l y o n a x i s i s c a l l e d' specu la r ' , an d the to t a l e fftux ove r 180 i s ca l l ed'd i f fuse ' . The specu la r dens i ty wi l l be g rea te r t han thed i ffuse dens i ty. When the in f lux en te r s the emuls iono v e r t h e f u l l a n g u l a r r a n g e o f 1 8 0 , th a t p o r t i o n w h i c hen te r s a t l a rge ang les to the ax i s is m ore l i ke ly to bes c a t t e re d w i d e l y a n d n e v e r e m e rg e f r o m t h e e m u l s i o n .

Con sequ en t ly th e e ff lux wi l l be l e s s, wh e the r specu la ro r d i f fuse , t han when the in f lux i s specu la r. De ns i ty i smeasured by : specu la r / specu la r, specu la r /d i f fuse , d i f -fuse / specu la r, and d ou b ly d i f fuse . A typ ica l den -s i tome te r g ives doub ly d i f fuse dens i t i e s . Con tac tp r i n t i n g w i l l b e e i t h e r s p e c u l a r / d i f f u s e o r d o u b l yd i ff u se d e p e n d i n g u p o n t h e e x p o s u r e s o u r c e . P r o j e c -t ion p r in t ing wi l l be be tween specu la r and d i ffused e p e n d i n g o n d i s t a n c e a n d f o r m a t .

1.16. Reciprocity

T h e l a w o f r e c i p r o c i t y f o r p h o t o g r a p h i c e m u l s i o ns t a te s t h a t i m a g e d e n s i t y ( D ) i s a f u n c t i o n o n l y o f t h et o t a l e x p o s u r e ( I . t ) a n d i s i n d e p e n d e n t o f t h em a g n i t u d e o f e it h e r I o r t . B e c au s e o f t h e m e c h a n i s ma n d k i n e t ic s o f la t e n t i m a g e f o r m a t i o n , t h e r e c i p r o c i t yl a w d o e s n o t h o l d t r u e f o r e x p o s u r e s o f h i g h i r-r a d i a n c e ( s h o r t d u r a t i o n ) a n d f o r e x p o s u r e s o f l o wi r r a d i a n c e ( l o n g d u r a t i o n ) . I n g e n e r a l , e v e r y e m u l s i o nh a s an o p t i m u m c o m b i n a t i o n o f I . t f o r p r o d u c i n g ag i v e n d e n s i t y w i t h a l l o t h e r c o m b i n a t i o n s p r o d u c i n g a

l o w e r d e n s it y, b u t f o r m a n y e m u l s i o n s , r e c ip r o c i t yh o l d s e f f e ct iv e ly o v e r a b r o a d r a n g e o f I . t c o m b i n a -t i o n s . A n e m u l s i o n w h i c h e x h i b i t s l o w - o r h i g h -in t ens i ty r ec ip ro c i ty f a i lu re can be used e ff ec tive ly ine i t h e r o f t h e s e r e g io n s b y e m p l o y i n g d o u b l e e x p o s u r et e c h n i q u es r e f e r r e d t o a s h y p e r s e n s i t iz a t i o n a n dla t ens i f i ca t ion . Low- in tens i ty r ec ip roc i ty f a i lu re canb e l a rg e l y o v e r c o m e b y f i r s t su b j e c t i n g t h e e m u l s i o n t oa u n i f o r m , b l a n k e t e x p o s u r e w i t h h i g h i r r a d i a n c e o fs u ff i ci e n tl y s h o r t d u r a t i o n t o p r o d u c e n o d e n s i t y o f i tso w n . T h i s h y p e r s e n s i ti z e d e m u l s i o n c a n t h e n b es u b j e c te d t o a l o w i r r a d i a n c e , im a g e w i s e e x p o s u r e a n dm a n i f e s t l i t tl e o r n o r e c i p r o c i t y f a i lu r e . O n t h e o t h e rh a n d , i f v e r y b r i e f im a g e w i s e e x p o s u r e t o h i g h i r -r a d i a n c e i s i m m e d i a t e l y f o l l o w e d b y a u n i f o r m ,b l a n k e t e x p o s u r e t o l o w i r r a d i a n c e ( i n s u f f i c i e n t t op r o d u c e d e n s i t y o f it s o w n ) t h e o t h e r w i s e a t t e n d a n trec ip roc i ty f a i lu re can be l a rge ly ove rcome .

1.17. Image Characteristics

1.17.1. Halation

A p o r t i o n o f l i g h t i n c i d e n t o n a n e m u l s i o n p a s s e st h r o u g h i n t o t h e s u p p o r t w h e r e i t c a n b e s c a t t e r e d a n d

r e f l e c t e d b a c k i n t o t h e e m u l s i o n t o p r o d u c eu n w a n t e d , s p u r i o u s e x p o s u r e s . R e f l e c t io n s c a n o c c u ra t b o t h t h e e m u l s i o n / s u p p o r t i n t e r f a c e a n d a t t h es u p p o r t / a i r i n t e r f a c e , r e s u l t i n g i n d e g r a d e d i m a g eq u a l i t y w h i c h i s p a r t i c u l a r l y u n d e s i r a b l e w h e r e h i g h e rr e s o l u t i o n is r e q u i r ed . W i t h c o h e r e n t r a d i a t i o n , t h e s er e f le c t io n s p r o d u c e u n w a n t e d w a v e - i n t e r f e re n c ep a t t e r n s . P o i n t i m a g e s p r o d u c e d b y s u f f i ci e nt l y h i g he x p o s u r e c a n b e s ee n s u r r o u n d e d b y a h a l o ( h a l a t i o n ) .E m u l s i o n s c a n b e p r o v i d e d w i t h a n t i h a l a t i o n l a y e r sw h i c h c o n t a i n d y e s o r p i g m e n t s t o a b s o r b t h e l i g h twhich wou ld o the rwise be r e f l ec t ed . B lack l acque r s

a r e a p p l i e d t o t h e b a c k o f t h e s u p p o r t a n d a r er e m o v e d d u r i n g p r o c e s s i n g o r b y s u b s e q u e n ts t r i p p i n g . A n t i h a l a t i o n l a y e r s m a y a l s o b e p l a c e db e t w e e n e m u l s i o n a n d s u p p o r t , e m p l o y i n g d y e s w h i c ha r e b l e a c h e d o r r e m o v e d d u r i n g p r o c e s s in g .

1.17.2. Image relief

T h e c h e m i s t r i e s o f b o t h t h e d e v e l o p m e n t a n db l e a c h i n g p r o c e s s e s p r o d u c e b i p r o d u c t s w h i c h c a nh a r d e n t h e g e l a t i n in t h e v i c i n it y o f t h e d e v e l o p i n g o rd i s s o l v in g s il v e r i m a g e . T h i s h a r d e n i n g r e s u l ts f r o m

c h e m i c a l c r o s s li n k i n g o f t h e g e l a t in w h i c h r e n d e r s i tl es s s o l u b l e o r i n s o l u b l e . T h i s p h e n o m e n o n , r e f e r r edt o a s t a n n i n g , v a r i e s i n d e g r e e , d e p e n d i n g u p o n t h ec h e m i c a l n a t u r e o f t h e d e v e l o p e r o r b l e a c h . T h eha rdened ge la t in becomes l e s s swol l en and l e s sp e r m e a b l e t o w a t e r a n d p r o c e s s i n g c h e m i c a l s ,r e su l t ing in in t e rna l s t r es ses . These s t r e sses a re in t en -s if ie d d u r i n g d r y i n g b e c a u s e t h e h a r d e n e d g e l a t inc o n t a i n s l e s s w a t e r a n d d r i e s m o r e r a p i d l y t h a n t h em o r e s w o l l e n , u n h a r d e n e d g e l a t i n . T h e r e s u l t i n gs t r a in m an i fe s t s a s a su r face r e l i e f and a s d i f f e rences inin t e rna l r e f r ac t ive ind ices .

1.17.3. Grain

A n u n d e v e l o p e d e m u l s i o n i s f i n e , m e d i u m , o rc o a r s e g r a i n e d a c c o r d i n g t o t h e s iz e o f t h e s i l v e r h a l i d ec r y s ta l s , b u t w h e n d e v e l o p e d a n d f ix e d , t h e e m u l s i o nc o n t a i n s o n l y p a r t i c l e s o f m e t a l l i c s i l v e r c a l l e dd e v e l o p e d g r a in . T h e t e r m s g r a i n i n e s s o r g r a n u l a r i t ya r e u s e d t o d e s c r i b e t h e q u a l i t y o f t h e i m a g e . I f a v e r ys m a l l a r e a i s s c a n n e d w i t h a m i c r o d e n s i t o m e t e r , t h es i lve r g ra ins , va ry ing in s i ze and shape ca n b e seen tob e d i s t r i b u t e d s o m e w h a t r a n d o m l y, c l u s t e r i n gt o g e t h e r t o f o r m c o n g l o m e r a t e s a n d o v e r l a p p i n g t of o r m a g g l o m e r a t e s . A n y s i lv e r p i c t u r e w h i c h i s su f -f i ci e n tl y e n l a rg e d w i ll a p p e a r g r a i n y a n d i s d e s c r ib e di n t e r m s o f g r a in i n e s s. Va r i a t i o n i n m i c r o s c o p i c g r a i nd i s t r i b u t i o n d e p e n d s o n t h e p a r t i c u l a r e m u l s i o n a n di t s d e v e l o p m e n t c o n d i t i o n s . T h e f l u c t u a t i o n i n


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230 N. CHIGIER

measured dens i ty depends on the s ize o f the scann ingaper ture . The term granular i ty, a t ) , i s def ined by

Na ~ = ~ D ~ - - - D ) 2 / N - 1)

i l

where Di i s the mic rodens i ty a t po in t i and /5 i s theme an den si ty over the scan. O r, preferably, i f the s izeof the scann ing aper tu re i s taken in to accoun t , thenwe def ine the Selwyn granular i ty G as

G = ( 2 A ) ] / 2 a D

where A is the area o f the scann ing aper ture . I f thenum ber of s ilver par t ic les in the scan ning aper tu re islarge, then G is independ ent of A.

1.17.4. N o i s e

The no i se in t roduced by a pho tograph ic emuls ioncan be a t t r ibuted to three sources:

(1) defects and nonuniformit ies in the support ,(2) r an dom sca t te r ing o f the inpu t s igna l by the

si lver hal ide grains dur ing exposure; and(3) r and om sca t te r ing o f the ou tpu t s igna l due to

the gran ular i ty of the m etal lic s ilver image.Theories have been developed to descr ibe the

pho tograph ic p rocess due to the ou tpu t s igna l(densi ty ) be ing a n on l inea r func t ion o f the inpu ts ignal and because the granular i ty (noise) isdependent on the input s ignal . The s ignal- to-noiserat io and hence the inform at ion co ntent o f a s ilverhal ide emulsion can be improved by using a lesssensi t ive , f ine grain emulsion with longer exposuret imes and by enhanc ing the ou tpu t s igna l wi th ab lanke t pos t -exposure .

1.17.5. R e s o l u t i o n

The reso lv ing power o f a pho tograph ic emuls ion i si ts abi l i ty to dis t inguish f ine detai ls of the subject orinput signal and is expressed in line pairs per milli-meter. An emulsion is unable to resolve detai l f inerthan the s ize of the s i lver grains com pris ing the image.

Reso lu t ion i s dependen t on fac to r s such as g ranu la r-i ty, contras t , sharpness of image, and clar i ty of theemuls ion . The reso lv ing pow er o f an emuls ion i s de -termined exper imental ly using a resolut ion targetsuch as shown in Fig. 23. Resolut ion increases withincreasing contras t between adjacent pic tor ia le lements . The c ontra s t of the target must , therefore ,be specif ied. T he target is imag ed by the em ulsion, an dthe smal les t se t of bars , which is jus t barely discern-able , def ines the resolut ion o r resolving pow er of thatemulsion for a specif ied exposure and development .

1.17.6. L a t e r a l d i m e n s i o n a l s t a b i l i t y

Late ra l d imens iona l s t ab i l i ty i s an impor tan t con-s ide ra tion fo r m a in ta in ing accuracy o f spa t ia l r e la -t ionships . Lateral d imensional changes are a funct ion

I

= I I I2

l I l l 4 -= 111~ ' m - |6 I I 1 ~4 i I I I s - -m m - - 2 ram , 5

i I l l ~ ' ' = ' 111- -- -- -'6

5 I I I 0I 1 - -I I I I

FIG. 23. Resolving power o f an emulsion.

o f the emuls ion type and the base mate ri a l. These a reaffected by thickness , temperature , re la t ive humidi ty,pressure , mechanical s t ress , and photographic pro-cessing. Mater ia ls which are used as support for theemulsion include glass , metals , pa pe r, polyester,polystyrene, polycarbonate , and cel lulose t r iaceta te .

Glass possesses r igidi ty and excel lent dimensionals tabi l i ty which is unaffected by photographic process-ing. Glass is unaffected by tensile forces, but i t isbr i t t le and breaks easi ly. During manufacture ,stresses in glass plate emulsions are relieved bysoak ing in wa te r o r by equ i l ib ra t ing the mois tu recon ten t a t h igh humid i ty.

Polyester f i lm supports possess super ior mechanicalproper t ies to cel lulose t r iaceta te . I t i s s t ronger, moreresilient, an d less adv ersely affected by tem per atureand re la t ive humid i ty. Low tempera tu res o r lowrelat ive humidi ty wil l cause br i t t leness which canresul t in i r revers ible damage, and high temperaturescan cause i r revers ible dimensional changes f romstresses . Process ing phot ogr aph ic f ilms always causespermanen t change in l a t e ra l d imens ions due toshr inkages . Very smal l d imens iona l changes a lwaysoccur in the emuls ion l ayer, typ ica l ly abou t 0 .02%shrinkage.

Tempera tu res and t empera tu re changes p r imar i lyaffect the support . Emulsions are re la t ively unaffect-ed. There are different coeff ic ients of thermalexpans ion fo r the l eng th and w id th o f t r iace tate , an dthey are two to three t imes greater than the coeffic iento f expans ion fo r po lyes te r. Lo w tempera tu res cancause br i tt leness and i r revers ible dam age on h andl ing.Higher temperatures can cause s t icking and resul t in

mechan ica l ly induced , pe rmanen t d i s to r t ions .Re la t ive humid i ty a ffec t s mois tu re con ten t and con-sequen t ly the l a te ra l d imens ions o f bo th supp or t andemulsion. Bo th increase in s ize with increasing re la t ivehumidi ty.


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Optical imaging o f sprays 231

2 . H I G H S P E E D C I N E M AT O G R A P H Y

M o t i o n - p i c t u r e c a m e r a s u se s e r ia l f r a m e s e p a r a t i o na n d c a m e r a s w i t h i n t e r m i t t e n t m o v e m e n t o f f i lm a r eusua l ly l imi t ed to 128 f r ames / sec ( s t and a rd r a t e s a re16 o r 24 ) . Fo r h ighe r f r aming r a t e s , con t inuous f i lm

m o v e m e n t i s u s e d w i t h o p t i c a l c o m p e n s a t i o n f o ri m a g e m o t i o n , s u c h a s a r o t a t i n g p l a n e - p a r a l l e l gl a ssb l o c k o r a r i n g o f m i r r o r s o r l e n se s . P i c t u r e s m a d e a tt h e s e h i g h f r e q u e n c i e s , w h e n p r o j e c t e d a t n o r m a lr a t e s, s l o w d o w n t h e m o t i o n t o t h e e x t e n t o f t h e r a t i oo f t h e t a k i n g a n d p r o j e c t i n g s p ee d s . H i g h e r f r a m i n gr a t e s a r e o b t a i n e d b y w r a p p i n g f i l m r o u n d t h ep e r i p h e r y o f a r a p i d l y r o t a t i n g d r u m , u s i n g s p e c i a ldev ices fo r f r aming . For ve ry h igh speeds the f i lm i sk e p t s t a t i o n a r y w h i l e th e i m a g e i s m o v e d b y m e a n s o fa r a p i d l y r o t a t i n g m i r r o r. I n a f r a m i n g c a m e r a , t h em i r r o r s w e e p s t h e i m a g e a c r o s s a n a r c o f l e n se s w h i c h

p r o j e c t a s e q u e n c e o f i m a g e s o n a s t a t i o n a r y f il m . I nt h e s t r e a k c a m e r a , t h e m i r r o r s w e e p s t h e i m a g e a s as m e a r a c r o s s t h e f i l m o v e r a t i m e p e r i o d . C o m b i n a -t i o n s o f f r a m i n g a n d s t r e a k c a m e r a s a r e a l so u s e d .

H i g h s p e e d c i n e m a t o g r a p h y i s u s e d t o r e c o r dp r o c e s se s t h a t o c c u r s o r a p i d

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