Development and construction of an interferometer for measuring density fields by optical means

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Pages

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17 F i g u r e s

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1 5 Ottawa, Canada TECHNICAL TRANSLATION Techo Trans, TT-18 Date

-

20 June, 1947, F i l e

-

12-R4-22

T i t l e : Entwicklung und Bau e i n e s I n t e r f e r a n z g e r a e t e s z u r o p t i s c h e n Messung von D i c h t e f e l d e ~ n

By: Tho Zobel

Reference: German A e r o n a u t i c a l Research I n s t i t u t e ,

E,Vo,

I n s t i t u t e of Gas-dynamics, B e r l i n A1dersh0.S~ 30,6,1938, ZWB F o r s c h u n g s b e r i c h t N r , 1808 S u b j e c t : Development and C o n s t r u c t i o n of a n I n t e r f s ~ 2 -

ometer f o r Measuring u e n s i t y F i e l d s by O p t i c a l Means

Submitted by:

W,

F, Campbell T r a n s l a t e d by: Aerodynamics S e c t i o n

D o

A , S i n e l a i r

Approved by: J,

H,

Parkin D i r e c t o ~ A b s t r a c t :

I n t h e f o l l o w i n g r e p o r t an i n t e r f e r e n c e pro-' cedure i s d e s c r i b e d which promises a u s e f u l a p p l i c a t i o n i n a e r o n a u t i c a l - r e s e a r c h , The p h y s i c a l b a s i s of t h e pro- cedure and a simple method of adjustment a r e e x p l a i n e d , F u r t h e r a s p e c i a l t e c h n i c a l ' d e ~ i ~ n of t h e a p p a r a t u s i s d e s c r i b e d , i n s u r i n g I t s u s a b i l i t y even where shocks a r e p r e s e n t i n the a r e a , and p e r m i t t i n g t h e s t u d y of non- 3 t e a d y phenomena, The p o s s i b i l i t y i s e s t a b l i s a e d of measuring o p t i c a l l y t h e s m a l l d e n s i t y v a r i a t i o n s sup-,* rounding b o d i e s i n a flow even a t low speeds ( 4 0 m e t e s p6r s e c , ) u s i n g t h e i n t e r f e r e n c e procedure,

Page (if)

Tech. Trans, TT-18

TABLE OF CONTENTS

VII

VIII

Introduction

Pages

1

Basic Physical Construction of the

Interferometer

2

Method of Adjustment using a Pentaprism

5

Monochromatic

Light

of Greater Intensity

8

The Influence of Monochromatic Quality on

the Interf

e ~ e n c e

Image

10

Possible Technical Applications of the

Interf

srome

ter

12

Special Technical Features

13

Swnmary

15

Tech, Trans, TT-18

O p t i c a l measuring techniques a r e being more and rriore f r e q u e n t l y a p p l i e d i n modern technology, The r e a s o n

f .sr tills i s the r a p i d development of t h e s e p a r a t e f i e l d s of a p p l i c a t i o n and the s t e a 6 y improvement i n a p t i c a h app- ~ r t i t u s ,

Foremost among tl e measuring techniques which lravrn a t t a i n e d g r e a t s i g n i f i c a n c e f o r extreme by f i n e and extj:

t

q u a n t i t a t i v e measurei,len%s i s t h e i n t e r f ence m t h a d ,

W i + : l sfmple, but h i g h l y ef Pective o p t i c a l a i d s , such a s

m l r r o r s and plane p a r a l l e F g l a s s p l a t e s , l i g h t i s s p l i t up i n t o coherent wave t r a i n s which a r e brought i n t o i n t e r - f e r u n c e , The i n t e r f e r e n c e s t r i p s s o obtained

enable

uk

t:

measure t h e s m a l l e s t changes of d e n s i t y i n t h e med%um btai i-ig t e a bed,

I n t h i s r e p o r t an i n t e r f e r o m e t e r i s d e s c r i b e d which was intended e s s e n L i a l l y f o r t e s t f n g t h e 3 e n s i t y f i e l d s a d j a c e n t t p bodies i n s u b - c r i t i c a l "lows, ? h i s

i s a p a r t i c u l a r l y d i f f i * a l t a s s i g s r e n t , s i l ~ c e , a s i s w e l l known,. t h e d e n s i t y changes i n flows of low v e l o c i t y up t o about 100 meters per second a r e s o s l i g h t t h a t the a i r

s a n be t r e a t e d almost a am incompressible medium, Sinze

f n : uch a i r flows t h e r e a r e no temperature v a r i a t i o n s due

t r ' r l ~ a i e s t h a t g i v e o f f h e a t , only those s m a l l d e n s i t y d: f t r e n c e s a r i s i n g frcm d i f f e r e n t v e l o c i t i e s a t v a r i o u s

o i n t 3 on the body e x i a t a s measur*able q i l a n t i t i e s o Although the i n t e ~ f e r e n c e technique permi t s the ;lieasurement of these s m a l l d e n s i t y changes w t a r e c l o s e t o t h e l i m i t s of i t s measur?.ng accuracy, A l l s t e p s

m u s t t h e r e f o r e be taken t 9 i n c r e a s e t h e measuring s e n s i -

t f v f t y st111 f u r t h e r ,

Hand i n hand with t h i s demcna f o r extreme accuracy I n the i n t e r f e r o m e t e r goea t h e demand l o r very

f i n e a d j u s t a b i l i t y , A b r i a f d e s c r f p ~ i c n n w i l l be given c_P" an a d j u s t i n g method whfch h a s been t e s t e d and

,

oven i n many of the authoros experiments ( c f n r e f o l l ) , and which makes i t p o s s i b l e t o a d j u s t such an a p p a r a t u s i n a r e l a t f v e l y s h o r t time, 1- a d d i t i o n t h e r e w i l l be a r e - p a r t of s t e p s t a k e n t o prevent deformation of t h e appa- r a t u s through temperature i n f l u e n c e s and t o make i t l e s s s e n s i t f v e t o e x t e r n a l mechanical d i s t u r b a n c e s ( v i b r a t i o n s ) , s o t h a t t h e method may a l s o be used i n t e c h n i c a l measur- i n g p r a c t i c e ,

Page

-

2

Tech, Trans, T T - 1 8

Still unsolved was t h e problem of monochromatfc

l i g h t s o u r c e s wf t h g r e a t e r i n t e n s i t y , These a r e r e q u i r e d i n t h e i n v e s t i z a t i o n of n e n - s ~ ~ e a d y o c c u r r e n c e s , t o o b t a i n a s u f f i c i e n t l y b r i e f exposure time, from h/100 t o 1/200 second, when t h e l f g h t pa,th i s r e l a t i v e l y l a r g e and when i n t e r m e d i a t e o p t f c a l elements a r e brought I n t o p l a y , I1

-

B A S I C P H Y S I C A L CONSTRUCTION OF THE INTERFEROMETER

The occurrence of s t r a f g h t , p a r a l l e l i n t e r f e r - ence s t r i p s ( i n c o n t r a s t t o i n t e r f e r e n c e ' r i n g s ), t o which t h e f o l l o w i n g d i s c u s s i o n i s l i m i t e d , i s brought about through t h e e f f e c t of a wedge-shaped g l a s s p l a t e ( F i g , 3 ) , To t h e l i g h t r a y L, which i s r e f l e c t e d i n t h e wedge p l a t e a t p o i n t A , i s added a second r a y from Lo, which, a f t e r r e f r a c t i o n and pe:'l6ction i n t h e wedge p l a t e , emerges from

i t e x a c t l y a t t h e p o i n t A , These two r e f l e c t e d r a y s of l i g h t , whose d i f f e r e n c e i n p a t h depends on t h e t h i c k n e s s of p l a t e P a t t h e p o i n t i n q u e s t i o n and on t h e a n g l e of i n c i d e n c e , K

,

i n t e ~ f e r e i n t h e image p l a n e B1 of t h e wedge p l a t e a s f a c e , When t h e a n g l e of i n c i d e n c e

d

_{i s} c o n s t a n t " f n t e ~ f e r e n c e s t r i p s of e q u a l t h i c k n e g s , " s t r a i g h t s t r f p s of l f g h t and d a r k , occur p a r a l l e l t o t h e o b l i q u e edee of t h e wedge p l a t e , The e f f e c t of such p l a t e s v a r i e s depending on whether t h e i n c i d e n t l i g h t r a y i s d e f l e c t e d towards o r away from t h e wedge, Accordingly, a l s o , t h e p a r t i a l r a y s of a l i g h t group r e f l e c t e d s e v e r a l times i n

t h e wedge p l a t e w i l l be d i v e r g e n t o r convergent,

This b a s i c p r i n c i p J e was f i r s t u s e d e f f e c t i v e l y , i n modi f Led f o r m , f o r producOng s t r a i g h t i n t e r f e r e n c e

s t r i p s i n c l e a ~ b y d e f i n e d p l a n e s and f o r observing spa- t i a l l y extended b o d i e s by t h e f o u r - p l a t e system shown i n f i g u r e 4 , I n t h i s o p t i c a l arrangement, developed by Mach and Zelmder, t h e two l i g h t p a t h s a r e s e p a r a t e d f a r enough from each o t h e r t o avoid a r e c i p r o c a l i n f l u e n c e , when

one

OP t h e o t h e r of t h e p a t h s

travceraea

he d e n s i t y . f i e l d

which i s b e i n g examined, Suen a s e p a r a t i o n of t h e l f g h t p a t h s i s of d e c i s i v e importance i n o b s e r v i n g phenomena of flow p h y s i c s and thermodynamics,

Two of t h e f o u r p a r a l l e l m f r r o r s , S1 and S2,

a r e c o a t e d f o r f u l l r g f l e c t i o n , The o t h e r two, P and PZD a l s o c a l l e d d i v i d i n g p l a t e s , a r e p l a t i n u m p l a ed by cathode spu-% S O as t o be s e m i - t r a n s p a r e n t

,1j

S i l v e r p l a t i n g t h e m i r r o r s does n o t prove s a t i s f a c t o r y , a s t h e s u r f a c e s cannot be zapon v a r n i s h e d and t h e l a y e r s of s i l v e r q u i c k l y d i s i n t e g r a t e under atmospheric i n - f l u e n c e s and a r e a l s o v e r y s e n s i t i v e m e h a n - i c a l l y , The p l a t i n u m s u ~ f a c e s , on t h e o t h e r hand, a r e c h e m i c a l l y d u r a b l e , b u t t h e y , t o o , a r e very s e n s i t f v e m e c h a n i c a l l y , A c o a t i n g of s i l i c o n , which has been developed by DP,

Hochheim of 1 , ~ , Fapben by a new and r e c e n t l y t e s t e d p r o m s s , fa recommended f o r t h e m i r ~ o r s ,

Tech, T r a n s , TT-18 A l i g h t beam

L

s t r i k i n g t h e p l a t e P i s p a r t l y

f

r e f l e c t e d , p a r t l y t r a n s m i t t e d through t h e p l a e , The f i r s t beam i s r e f l e c t e d a t m i r r o r S and p a s s e s t h r o u g h

t

p l a t e P2, The second p a r t i a l beam s r e f l e c t e d a t m i r r o r S2 and p l a t e P2, These two l i g h t groups a r e c o h e r e n t , i , e , t h e y come from one and t h e same s o u r c e of l i g h t and have t h e same p l a n e of v i b r a t i o n , They a r e used f o r i n -

t e r f e r e n c e a f t e r t r a v e r s i n g l i g h t p a t h s of e q u a l l e n g t h i n t h e a p p a r a t u s , I n p l a t e s P1 and P2, however, s t i l l o t h e r p a r t i a l r e f l e c t i o n s o c c u r which a l s o cause i n t e r - f e r e n c e phenomena, However, owing t o t h e s e m i - t r a n s p a r e n c y of t h e p l a t e s t h e s e r e f l e c t i o n s become s o weak, t h a t a

d i s t u r b i n g i n f l u e n c e on t h e f i r s t l i g h t beam i s avoided, With t h i s arrangement a v e r y complicated s h i e l d i n g of t h e p a r t i a l r e f l e c t i o n , a s would be needed i f p l a n e p a r a l l e l

g l a s s p l a t e s were used f o r PI and P2, i s made u n n e c e s s a r y , With t h i s a p p a r a t u s ( f i g u r e 5 ) i n t e r f e r e n c e can be produced a t any d e s i r e d d i s t a n c e from t h e m i r r o r s ,

I n t e r f e r e n c e always t a k e s p l a c e a t t h e p o i n t where c o h e r e n t l i g h t r a y s i n t e r s e c t a f t e r t r a v e r s i n g s e p a r a t e , b u t e q u a l , l i g h t p a t h s I n t h e a p p a r a t u s ( r e f e r e n c e 71, I f t h e appa- r a t u s i s i n e x a c t a d j u s t m e n t , i , e , , i f a l l m i r r o r s a r e ex- a c t l y p a r a l l e l , and t h e s u m s of t h e d i s t a n c e s from PI t o P3 over t h e r e s p e c t i v e p a t h s v i a S1 and S2 a r e e x a c t l y e q u a l , 80 t h a t t h e phase d i s p l a c e m e n t i n t h e two wave t r a i n s i s

j u s t z e r o , t h e n a l l m a n i p u l a t i o n s of t h e i n t e r f e r e n c e nn-- d u c t i o n can be made w i t h t h e two p l a t e s PI and P2 a l o n e ,

Each of t h e 4 p l a t e s and m i r r o r s i s s o constructeci t h a t i t c a n be t u r n e d about two axes a t r i g h t a n g l e s t o

each o t h e r , I n a d d i t i o n p l a t e P2 a l o n e c a n be moved a l o n g a guide s l o t i n a d i r e c t i o n a t r i g h t a n g l e s t o i t s p l a n - , s o t h a t a f t e r f i n e adjustment any f u r t h e r n e c e s s a r y c o r r e c - t i o n i n t h e d i s t a n c e can be made, A f t e r complete a d j u s t - ment of t h e a p p a r a t u s t h e two f u l l y r e f l e c t i n g m i r r o r s S 1

and 32 remain unchanged f o r a l l f u r t h e r o p e r a t i o n s , _{( I f} p o s s i b l e t h e y should be f i r m l y f i x e d i n p o s i t i o n ) , The r e l a t i o n s h i p between t h e r o t a t i o n of p l a t e s PI and Pzp t h e p o i n t of i n t e r f e r e n c e , and t h e b r e a d t h of t h e i n t e r f e r e n c e s t r i p s i s g i v e n a s f o l l o w s : If t h e p l a t e PI i s r o t a t e d through t h e a n g l e

M

on3y t h e p a t h of t h e f i r s t p a r t i a l r a y v i a Sl and S2 i s a l t e r e d and t h e o r i g i n a l i n t e r s e c t i o n p o i n t B D on t h e s c r e e n s h i f t s by a n amount

Page

-

4

Tech,. Trans, T T - 88

(The s i n e may be replaced by the angle i t s e l f where t h e s e small angles are concerned), The second p a r t i a l r a y via

S2 and

PZ

remains unchanged, I n o r d e r t o o b t a i n i n t e r - f e r e n c e a t B 3 , p l a t e P2 must be r o t a t e d i n t h e opposite d i r e c t i o n through t h e angle

XI

The width of t h e I n t e r f e r e n c e s t r i p s i s then given by the

-

_{between the wave normals, and the s t r i p d i s t a n c e between}

the two f n t e r e f e r e n e e s t r i p s i s then:

U

where i s the wave l e n g t h of the l i g h t used,

4

t h e d i s t a n c e 0% t h e i n t e r f e r e n c e plane from the l a s t p l a t e P

a1 and a2 a r e the d i s t a n c e s between the m i r r o r s , and

a

2 " f s t h e r o t a t i o n of the f i r s t p l a t e

PI

( c f c r e f e r e n c e 7 ) "

The e x t e n s i o n of the p r i n c i p l e shown i n f i g u r e 3 of u s i n g wedge+hAp@d ;;Bass p l a t e s t o produce s t r a i g h t i n t e r f e r e n c e s t r i p s , c o n s i s t s i n t h e forming of t h e wedge angle by t i l t i n g plane p a r a l l e l g l a s s p l a t e s , These b r i n g l i g h t rays of e q u a l energy i n t o i n t e r f e r e n c e , With t h e e n t f ~ e mirror supface they g e n e r a t e i n t e r f e r e n c e s t r i p s whfch lie in a e l e a r l y defined plane, With the two p l a t e s P1 and P2 and t h e i r f o u r possible ways of adjustment, i n - t e r f e r e n c e s may be produced fn any d e s i r e d p'lane, p o s f t f o n and width, Henee the remarkable adaptability of the i n t e n - f e r e n c e procedure t o the most v a r i e d requirements of t e e h - n i c a l measurement, Renee, too, t h e change of s e n s f t f v f t y through correapondfng qhoiee i n t h e width of s t r i p ,

I f t h e two p l a t e s a r e r o t a t e d about axes dhich a r e perpendicular t o the plane of t h e page ( f i g m e 51, then, usfng a pfn-point s o w e e of l i g h t , we g e t i n t e r - f erence s t r f pa p a r a l l e l

t'oY

t h e s e two axes, R o t a t i on about h o ~ i z o n t a l axes y i e l d 8 horizontal i n t e r f e r e n c e s t r i p s ,

A l l i n t e r m e d i a t e values betweenethe v e r t f e a l and h o r i z o n t a l p o s i t f o n s r e q u f r e t h e c o p r e l a t i o n of t h e two p l a t e s with corresponding p o t a t i o n about a l l f o m axes, The i n t e r - f e r e n c e s t r i p s a r e alwaya p a r a l l e l t o t h e oblfque edge of

the wedge,

--

P .-- -

*

T r a n s P s t o ~ s note: I n t h i s formula

i t

appears t h a t Qaab) should have been

Techo Trans, TT-18

111 METHOD OF ADJUSTWNT U S I N G A PENTAPRISM

From the f o r e g o i n g e x p l a n a t i o n of t h e b a s i c working p r i n c i p l e s of t h e i n t e r f e r e n c e r e f r a c t o m e t e r , i t

i s a l r e a d y c l e a r t h a t the alignment of the f o u r p l a t e s and m i r r o r s r e q u i r e s a p r e c i s i o n of the o r d e r of magni- tude of the wave l e n g t h s of the l i g h t employed, The

adjustment of t h e apparatus t h e r e f ore demands t h e utmost c a r e and c a l l s f o r a s p e c e a l method of a l i g n i n g t h e m i r r o r s which r e a l l y w i l l meet the requirements of p r e c i s i o n ,

I n many f i e l d s of a p p l i c a t i o n where t h e i n t e r - f e r e n c e method could have c o n t r i b u t e d t o t h e c l a r i f i c a t i o n of important q u e s t i o n s , i t s employment -las prevented by the d i f f i c u l t i e s of a d j u s t i n g t h e apparatus i t s e l f ,

The method of adjustment found i n t h e l i t e r a - t u r e , ( r e f e r e n c e 5 ) p t h e p r i n c i p l e of which w i l l be b u t b r i e f l y d e s c r i b e d h e r e , r e q u i r e s c e r t a i n c o n d i t i o n s ob-

t a i n a b l e only under l a b o r a t o r y c o n d i t i o n s by mounting t h e m i r r o r s on o p t i c a l benches, The m i r r o r s and p l a t e s ape

secured t o an o p t i c a l bench and qach i s made t o r o t a t e on two axes a t r i g h t a n g l e s t o each o t h e r , The p a r a l l e l alignment of mfrror t o p l a t e i n each i n s t a n c e i s accom- p l i s h e d by o b s e r v a t i o n of a p o i n t s e v e r a l k i l o m e t e r s away ( f f g u r e 6 ) , The two images, r e f l e c t e d i n p l a t e and m f r r o r , a r e brought i n t o e x a c t coincidence by a t e l e s c o p e , a l s o secured t o t h e o p t i c a l bench and s e t p a r a l l e l t o i t ,

The second p a i r of images i s a l i g n e d i n t h e same manner, Since the image of the d i s t a n t objec reaches both m i r r o r s , t h e e r r o r i n t h e angular d e v i a t i o n i n c r e a s e s with

g r e a t e r d i s t a n c e s between m i r r o r s and r e q u i r e s c o r r e c t i o n by s i g h t i n g on an astronomical o b j e c t , cog, a b r i g h t s t a r , When both p a f r s of mfrror and p l a t e have been a l i g n e d t h e

two optfcaP benches a r e f i x e d p a r a l l e l t o each o t h e r and, by the use of ad j u s t f n g screws on t h e mounting, a r e ad-

justed s o t h a t i l l u m i n a t e d c r o s s - h a i r s placed i n f r o n t of p l a t e P a r e brought i n t o cofncidence i n the e n t i r e mirror s y s

%

em, The disadvantages of t h i s method of ad-

justment do not need t o be d i s c u s s e d any f u r t h e r ,

The urgent need was f o r a method which would make p o s s i b l e t h e r a p i d and a c c u r a t e adjustment of t h e i n t e r f e r o m e t e r w i t h o u t s p e c i a l d i f f i c u l t y a t t h e p l a c e where i t was t o be used, Such a method, u s i n g a penta- prfsm, has been t r f e d f n many experiments and h a s proven

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A p r o p e r t y of t h e pentaprisnl ( f i g u r e 7 ) , i s t h a t a l l l i g h t s a y s s t ~ f k f n g f t a r e r e f l e c t e d a t an a n g l e of 90°, Thus through i t s e n t i r e range of r e f l e c t i o n , i t

i s i n s e n s i t i v e t o a n g l e s , Ffgv-re 8 shows t h e p r f n c f p l e of a d j u s t m e n t , The ground p l a n e of t h e whole a p p a r a t u s , c o n t a f n f n g a l l f o u ~ m l r r o r s , i s f % r s t le v e l l e d h o r f z o n t - a l l y and t h e m l r r o ~ s a r e p l a c e d a t an a n g l e of 45' t o t h e l f n e s j o f n f n g t h e i r b vertical a x e s , An a u t o - c o l l f m a t o r t e l e s c o p e

F

Is t h e n i n s t a l l e d i n e x a c t h o r f z o n t a l p o s i - t f o n wfth I t 8 o p t i c a l a x f s p a r a l l e l t o t h e m f r r o r s u r f a c e s , I n t h e d i r e c t i o n of t h i a s p t f c a l a x f s f s an o p t i c a l t r a c k a l o n g which r u n s a moveable p l a t f o r m h o l d i n g t h e p e n t a - prism, The c r e s s - h a f r s i n the oculak of t h e t e l e s c o p e a r e now p r o j e c t e d I n t o t h e pentaprfsm and through i t o n t o t h e m f r r o r , The miprop i s t h e n t u r n e d and t f l t e d u n t f l t h e Image of t h e c r o s s - h a f r s r e f l e c t e d from i t s f r o n t s g r f a e e r e t u r n s v i a t h e o p t f e a x f s of t h e t e l e s c o p e and c o i n c i d e s e x a c t l y w i t h t h e o r i g i n a l c r o s s - h a i r s , Wfth t h f s procedure a t t e n t i o n must be p a i d t o a pre-adjustment of t h e p e n t a p l ~ f s m i t s e l f , i f we a r e t o succeed i n g e t t f n g f n % e r f e r e n c e a , The p e n t a p r i s m f s n o t c o n s t r u c t e d wfth t h e h i g h d e g ~ e e of accuracy h e r e r e -

quired,

A s a consequence t h e t w o fmages r e f l e c t e d from

%ts

I k o n % and bask s u r f a c e s a r e n o t e x a c t l y e o f n c i d e n t ,

We t h e r e f o r e choose one of t h e two p a r t f a l images f o r our purpose and keep ts t h e same one i n t h e adjuatment of a l l t h e m f r r o r s , The undersupface of t h e pentaprfam f s s e t h o r f z o n t a l t o b o t h l f g h t p a t h s by means of an a c c u r a t e s p i r i t l e v e l , The m f r r o ~ concerned f s t h e n f n p e r f e c t adjustment when t h e r e f l e c t e d c r o s s - t h r e a d s from b o t h

m f r r o ~ and pentapkism c o i n c i d e w i t h t h a t of t h e t e l e s c o p e , The pentaprfsm i s now moved a l o n g t h e o p t f c a l t r a c k and used I n t h e same way f o r t h e a d j u s t m e n t of t h r e e s f t h e m f r r o r s , The f o u r t h m f r r o r can t h e n e a s f l y be a l i g n e d w i t h t h e eye by p l a c i n g luminous cposa t h r e a d s i n flpont of p l a t s

Pg,

o b s e ~ v f n g i t from p l a t e P2 and

b r i n g i n g f

t

I n t o axact alfgnment wfth t h e o t h e r m i r r o r Images, I t Is a good I d e a , however, t o do t h f s w i t h a t e l e s c o p e , The a91 gnment i s t h e r e b y made e a s i e r and f n a d d i t f o n one can aecomrnodate o n e s e l f b e t t e r w i t h a t e l e - scope t h a n wfth t h e eye t o a p a r t f c u l a r p l a n e , This

l a s t i s Important l a t e r on i n a d j u s t i n g t h e i n t e r f e r e n c e , I f t h e aZfgnfng of t h e m f r r o r s h a s been done a c c u r a t e l y , a monochromatie l i g h t s o u r c e may now be switched on b e f o r e p l a t e P and i n t e r f e r e n c e s w f l l be s e e n , These wfll be e x c e p 3 f o n a l l y narrow because when t h e m f r r o r s a r e a b s o l u t e l y p a r a l l e l t h e I n t e r f e r e n c e

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plane l i e s a t i n f i n i t y , The i n t e r f e r e n c e s t r i p s a r e a l s o p r e s e n t i n monochromatic l i g h t when t h e two s e p a r a t e l i g h t paths a r e not e x a c t l y e q u a l b u t d i f f e r by a m u l t i p l e of

t h e wave l e n g t h ,

The c r i t e r i o n of complete adjustment of t h e apparatus i s not f u l f i l l e d , however, u n t i l t h e two l i g h t paths a r e e x a c t l y e q u a l , Adjustment of t h e e x a c t d i s t a n c e of t h e m i r r o r s can be accomplished by moving p l a t e

P2

along i t s guide s l o t , We do t h i s by seeking t h e so-called zero- i n t e r f e r e n c e i n o r d i n a r y white l i g h t , This

is

th8,pheno-

menon

* h a r a c t e r i z e d by a s h a r p l y d e f i n e d interference s , r i p I n which the s t r i p s corresponding t o a p a t h d i f f e r e n c e of

zero f o r a l l c o l o u r s c o i n c i d e , On e i t h e r s i d e of the zero- i n t e r f e r e n c e t h e s t r i p s l o s e t h e i r sharpness due t o t h e overlapping of d i f f e r e n t wave l e n g t h s , and a f t e r a few s t r i p widths they vanish completely, ( c f , f i g , 9), The presence of t h i s z e r o - i n t e r f e r e n c e i n white l i g h t i s always a c r i t e r i o n of the most e x a c t adjustment of the apparatus, even where monochromatic l i g h t i s being used and t h e zero- i n t e r f e r e n c e can no longer be recognized,

It i s a d v i s a b l e always t o s t a r t with t h i s b a s i c adjustment, s i n c e under c e r t a i n circumstances, where some- what complicated d e n s i t y f i e l d s a r e b e i n g evaluated, t h e r e s o - i n t e r f e r e n c e must be introduced t o i d e n t i f y i n d i v i d u a l a t r f p s of l i g h t ,

I n t h e experiment we a r e d e a l i n g with, t h e i n t e r - ferometer was b u i l t f o r use i n v e r t i c a l p o s i t i o n , The

adjustment took p l a c e i n h o r i z o n t a l p o s i t i o n according t o t h e procedure described; t h e n t h e apparatus was r o t a t e d through 90° and used as shown i n f i g u r e s 1 and 2 ,

When the apparatus i s a d j u s t e d and i n t e r f e r e n c e s a r e obtained, the next s t e p i s t o f o c u s the i n t e r f e r e n c e s t r i p s on the c e n t r a l plane of the o b j e c t t o be t e s t e d

( f i g u r e

l o ) ,

A c l e a r l y o u t l i n e d o b j e c t i s placed a t t h i s p o i n t and a t e l e s c o p e i s focused s h a r p l y upon i t , By man-

i p u l a t i o n of t h e p l a t e s P1 and PZs t h e p o s i t i o n of the i n t e r f e r e n c e s i s now a l t e r e d u n t i l i t coincides with t h a t of the o b j e c t and a c l e a r image i s obtained of both, This p i c t u r e of the i n t e r f e r e n c e f i e l d i s an apparent m e ,

Actually t h e i n t e r f e r e n c e occurs o u t s i d e t h e bpparatus

a t t h e p o i n t where t h e two coherent wave groups i n t e r s e c t , The t e l e s c o p e can be replaced by a photographic l e n s and t h e i n t e r f e r e n c e s t r i p image reproduced, ( f i g u r e 12; f i g u r e s 15-17 )

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MONOCHROYATIC L I G H T O F GREATER I N T E N S I T Y -

S s f a r , i n speaking of i n t e r f e r e n c e s t r i p f i e l d s , we have t a c i t l y assumed a s o u r c e of l i g h t a s n e a r l y pin- p o i n t i n s i z e a s p o s s i b l e and e m i t t i n g o n l y a s i n g l e wave- l e n g t h

A

.

U n f o r t u n a t e l y , no such a b s o l u t e l y mono-chro- matie l i g h t s s u r c e e x l s t s , On t h e c o n t r a r y t h e r a d i a t i o n from m y source w i l l c o n t a i n a whole complex of v i b r a t i o n s , As everyone knows, t h e s p e c t r a l range of an i n c a n d e s c e n t lamp i n c l u d e s a l l t h e wave l e n g t h s from u l t r a - v i o l e t through t h e v i s i b l e range t o the h e a t r a d i a t i o n of i n f r a - r e d ,

On t h e o t h e r hand $umfnescent vapours o r gases emit o n l y a narrow a p e c t ~ a l range and a r e t h e r e f o r e used a s more o r l e s s homogeneous s o u r c e s of l i g h t f o r i n t e r - f e r e n c e purposes, For t h e most p a r t , however, i f t h e i n - t e r f e r e n c e s t r i p images a r e t o be u t i l i z e d , i t < i s n e c e s s a r y t o narrow t h e s p e c t r a l r m g e a t f l l f u r t h e r , u s i n g c o l o u r f i l t e r s t o a b s o r b t h o s e wave l e n g t h s which a r e s t i l l caus- i n g d i s t u r b a n c e s , However t h e s e measures cause such a g r e a t l o s s i n l i g h t p r o d u s t i o n a s t o p r e c l u d e t h e t a k i n g of h i g h speed photographs and t h u s t o r e n d e r i m p o s s i b l e t h e inves- t i g a t i on s f non-s teady phenomena,

The q ~ e s t i o n a s t o what l i g h t s o u r c e s a r e s u i t - a b l e t o produce a h i g h degree of l i g h t i n t e n s i t y and a t t h e same time t o s a t i s f y t h e requirements i n r e g a r d t o s p e c t r a l q u a l l t f e a was a l s o s t u d f ed w i t h i n t h e framework of t h i s p r o j e c k ,

The agidfum s p e e t ~ a l lamp, with D l and D2 l i n e s

h

-

_{5890/96 A,U, e m i t s such a narrow s p e c t r a l range t h a t} i t can be t a k e n a s a monoehromatfc l i g h t , T h i s lamp i s t h e r e f o r e e s p e c i a l l y s u i t a b l e f o r i n t e r f e r e n c e purposes and f o r l o c a t i n g i n t e r f e r e n c e s t r i p s a f t e r adjustment of t h e a p p a r a t u s , s f n e e t h e e n t f r e image f i e l d i s always

covered w i t h f u l l y s a t u r a t e d i n t e r f e r e n c e s t r i p s , T h e i n - t e n s i t y of t h i s lamp, however, i s o n l y somewhere between 25 and 30 ab*, which f s 8 0 low t h a t photographs w i t h s h o r t exposures a r e impossible even when panchromatic emulsions a r e employed,

A light of c o n s i d e r a b l y g r e a t e p i n t e n s i t y and a t t h e same time w i t h s h a r p e r s p e c t r a l l i n e s i s OsramQs mercury s p e c t r a l lamp having a n i n t e n s i t y of about 1000 s b , The wave l e n g h t s which a r e e m i t t e d i n the v i s u a l l y and

photographi e a l l y a c t i ve range w i t h r e l a t i v e l y g r e a t e ~ spec- t r a l energy a r e t h e l h n e s

h

-

4080, .4360, 5460 and 5778190

w ~ I f g hg ~ producin$ f

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Tech, Trans, TT-18

i t s own i n t e r f e r e n c e s t r i p system, t h e v a r i o u s systems over- l a p and p l a c e s occur i n t h e i n t e r f e r e n c e s t r i p p i c t u r e where t h e d i f f e r e n c e i n p a t h between t h e s e p a r a t e l i g h t waves

l e a d s t o t h e e x t i n g u i s h i n g of i n d i v i d u a l s t r i p s , The r e - s u l t i s a s t r i p image c o n s i s t i n g of s e v e r a l , p e r i o d i c a l l y r e c u r r i n g groups of i n t e r f e r e n c e s t r i p s ( f i g , 1 2 ) which a r e unusable f o r s p e c t r a l s t u d i e s , Hence, through t h e u s e of

s u i t a b l e f f l t e r s , t h e monochromatic q u a l i t y must be i n c r e a s e d s o t h a t a s naprow a range of wave l e n g t h s a s p o s s i b l e i s

e m i t t e d , Various f f l t e r s from t h e f i r m s S c h o t t and Gen,, Jena, and C a r l Z e f s s were t r i e d and t h e f i l t e r chosen which gave t h e h i g h e s t monochromatic q u a l i t y a l o n g w i t h t h e g r e a t - e s t p h o t o g r a p h i c e f f e c t i v e n e s s ,

The l o s s of l i g h t through f i l t e r i n g i s e x c e p t i o n - a l l y l a r g e , Moreover i t i s a p r o p e r t y of most f i l t e r s t h a t t h e y absorb l i g h t w e l l up t o 5000 AoU, For g r e a t e r wave- l e n g t h s t h e i r t r a n s m i s s i o n i n c r e a s e s u n t i l , i n t h e i n f r a - r e d region,, i t becomes complete,

I n i t s e l f t h e b l u e l i n e

1

=

4360 i s v e r y e f f e c - t i v e p h o t o g r a p h i c a l l y , However i t s o h a n d i c a p s v i s u a l ob- s e r v a t i o n t h a t i t cannot be used,

Panchromatic emulsfon i s n o t s e n s i t i v e enough t o t h e green l i n e

=

5460 A o U a t o o b t a i n a s u f f i c i e n t l y s h o r t exposure time even w i t h t h e t r a n s m i s s i o n of a Z e f s s B f i l t e r s f 67% of t h e r e l a t i v e s p e c t r a l energy,

The b e s t r e s u l t s were o b t a i n e d w i t t h e f i l t e r . - OG 2 ( S c h o t t and Gen,) f o r t h e yellow l i n e s

%

= 5770/90. I t s p e r m e a b i l i t y amounts t o 94% of t h e r e l a t i v e s p e c t r a l energy of' t h e s e l i n e s which s t a n d o u t a s t h e b r i g h t e s t i n t h e v i s i b l e range of t h e mercury lamp spectrum w i t h 80% r e l a t i v e energy, (Standard of energy of t h e s p e c t r a l l i n e i s t h a t of t h e u l t r a - v i o l e t l i n e with 100$),

This f i l t e r OG 2 i s e s p e c i a l l y u s e f u l i n a o f a r a s

i t a b s o r b s e n t i r e l y t h e many wave l e n g t h s s m a l l e r t h a n

=

5460 A , U , c o n t a i n e d i n t h e h i g h p r e s s u r e mercury d i s c h a r g e

( e f , f i g ,

1 1 1 ,

The p e r m e a b i l i t y t h e n mounts r a p i d l y , For t h e l i n e

A

5460 A , U , i t amounts t o o n l y 14%; f o r t h e de- s i r e d l i n e s = 5470/90, 94%, and f o r a l l g r e a t e r wave l e n g t h s up t o t h e f n f r a - r e d r a y i t i s 100$,

Within t h e v i s i b l e range no l o n g e r wave l e n g t h s a r e e m i t t e d , The few i n f r a - r e d r a y s a r e absorbed n any c a s e by t h e g l a s s l e n s e s and m i r r o r s , Thus t h e l i n e s

3-

= 5770/90 a r e p r a c t i c a l l y t h e o n l y ones t h a t r e e f f e c t i v e and p o s s e s s

g r e a t energy f o r producing a n i

%

t e r f e r e n c e f i e l d w i t h s a t u r - a t e d i n t e r f e r e n c e s t r i p s , When a t e l e s c o p e l e n s was used t h e exposure times o b t a i n e d were i n the neighborhood of 1/22 s e c , f o r good, p r a c t i c a b l e i n t e r f e r e n c e photographs, This l o n g exposure tixhe, however, i s u s e l e s s f o r s t u d y i n g non- s t e a d y phenomena,

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The next s t e p i n i n c r e a s i n g t h e b r i g h t n e s s was

taken when Osram3s newly-developed u l t r a h i g h p r e s s u r e mop- cury l a n p , type HgB 500 was used, A t a vapour p r e s s u r e of

50 atm,, t h i s lamp develops

am

i n t e n s i t y of X I , O Q Q s b , The l i n e spectrum of t h e mercury vapour d i s c h a r g e a l t e r s funda- mentally with i n c r e a s i n g vapour p r e s s u r e , The l i n e s become c o n t i n u a l l y b r o a d e r and a t very h i g h p r e s s u r e s , around 200 atm,, t h e y merge i n t o one a n o t h e r a s c o l o u r bands, Experi- m n t s had t o be undertaken t o d i s c o v e r whether a t the g r e a t - e s t i n t e n s i t y of t h i s L a p t h e l f n e s f o r

=

5770190 had. a l r e a d y broadened t~ a d i s t u r b i n g degree, It t u r n e d o u t , f o r t u n a t e l y , t h a t t h e broadening i n t o bands extended only t o t h e s p e c t r a S i n e s of s h o r t e r wave l e n g t h s , while t h e chosen l i n e s = 5770/00 maintained almost t h e same width. By t r a n s m i s s i o n s through t h e f i l t e r OG 2 t h e d i s t u r b i n g range sf wave l e n g t h s can thus be made i n e f f e c t i v e and t h e g r e a t i n t e n s i t y of Yfght can be made u s e f u l f o r i n t e r f e r - ence purposes, mith telescopic camera t h e exposure time f o r p i c t u r e s sf waln s a t u r a t e d i n t e r f e r e n c e s t r i p s i s from

P]lQQ t o P]200 s e c , W i t h t h e s e l i g h t f a c t o r s t h e p o s s i b i l i t y of s t u d y i n g nonc-steady phenomena is a i r e a d y e s t a b l i s h e d ,

V, THE IEJFENC_E OF_ M O N O E O M A T I C QUALITY ON THE INTERFERENCE

ID.IAGE,

The a b f l i t y of t h e l i g h t t o produce an i n t e r f e r - enee system depends upon t h e f o l l o w f n g two b a s i c c o n d i t i o n s :

1,

The two superimposed wave t r a i n s v i b r a t e in one plane i n t h e same phase, and undergo no r e e i p r s ~ a f change d u r i n g t h e occurrence, and

I

2, The l i g h t s o u r c e emits i n the same s t a t e of v i - b r a t i o n d u r i n g t h e time r e q u i r e d t o t r a v e r s e t h e d i f - f e r e n c e i n p a t h between t h e two i n t e r f e r i n g wave

groups ( c o h e r e n c e ) ,

The l i g h t g s c a p a c i t y f o r i n t e r f e r e n c e i s l i m i t e d by t h e magnitude of t h e g r e a t e s t p a t h d i f f e r e n c e which s t i l l permits t h e o b s e r v a t i o n of i n t e r f e r e n c e s , This p a t h d i f - f e r e n c e depends on t h e width of t h e s p e e t r a i l i n e s and,

t h e r e f o r e on t h e p u r i t y of t h e c o l o u r (monochromatic q u a l i t y ) ,

If

a l i g h t s o u r c e e m i t s , a s i n t h e p r e s e n t i n s t a n c e , two a d j a c e n t , homogeneous s p e c t r a l l i n e s of d i f f e r e n t wave l e n g t h s , t h e n each wave can produce i t s own i n t e ~ f e r e n c e s t r i p system, The two s t r i p systems o v e r l a p , and when t h e p a t h d i f f e r e n c e s a r e s m a l l t h e y f a l l s o c l o s e t o one a n o t h e r

Page

-

1.1

Tech, Trans, TT-18 However, if we now i n c r e a s e t h e path d i f f e r e n c e we observe t h a t a f t e r a c e r t a i n number of i n t e r f e r e n c e s t r i p s t h e s e become p r o g r e s s i v e l y weaker and f i n a l l y d i s - appear, They vanish a l t o g e t h e r when the i n t e n s i t y maximum of one c o l o u r system c o i n c i d e s e x a c t l y with the minimum of t h e o t h e r , By changing t h e l o n g i t u d i n a l d r i v e s e t t i n g a t p l a t e P2 we ,can observe t h i s phenomenon, because t h e p l a t e b r i n g s about a l t e r a t i o n of the p a t h d i f f e r e n c e s of both

i n t e r f e r i n g r a y s ,

I n t h e p r e s e n t i n s t a n c e the u l t r a high p r e s s u r e mercury lamp, a f t e r f i l t r a t i o n through f i l t e r OG 2 emits t h e two a d j a c e n t wave l e n g t h s = 5770/90 A o U o which d i f - f e r from each o t h e r by 20.A6Uo The s t r i p i n t e r v a l s of t h e two c o l o u r s a r e t h e r e f o r e d i f f e r e n t , f o r t h e wave l e n g t h s a r e p r o p o r t i o n a l t o the s t r i p i n t e r v a l s :

The d i f f e r e n c e i n t h e s t r i p i n t e r v a l of t h e two-

.

i n t e r f e r e n c e f i e l d s i s , t h e r e f o r e ,

This d i f f e r e n c e i s s o s m a l l t h a t i t can' be completely

d i s r e g a r d e d I n t h e i n t e r p r e t a t i o n M c t h e i n t e r f e r e n c e images, Owing t o the emission of t h e two d i f f e r e n t wave l e n g t h s a displacement occurs i n t h e -Joe-at ion of tihe i n t e r - f e r e n c e s t r i p s a f t e r k s t r i p s , This displacement can be c a l c u l a t e d , It i s c h a r a c t e r i s t i c of t h e r e g i o n i n which t h e s t r i p s , c a l c u l a t e d from z e r o - I n t e r f e r e n c e , juqt dig- appear, This circumstance occurs when t h e d f f f e r e n c e i n t h e s t r i p i n t e r v a l a f t e r k s t r i p s i s e x a c t l y one h a l f of a s t r i p width:-

e , the number of s t r i p s k, a f t e r vrfiich t h i s c h a r a c t e r i s - t i c r e g i o n appears i n t h e s t r i p image

's

Independent 2 t h e s t r i p width f o r which t h e a p p a r a t u s i s a d j u s t e d :

When

A1

=

5790 AoU, and

h 2

=

5770 A o U o , k w i l l be 145 A o U o

Page

-

1 2 Tech, Trans, TT-18 i o e o , t h e i n t e r f e r e n c e s v a n i s h 1 4 5 s t r i p s a f t e r z e r o - i n t e r - f e r e n c e , o r , a f t e r each 2 k = 298 s t r i p s e c t i o n complete s a t u r a t i o n of t h e s t r i p s reappeaps, A s i m i l a r occurrence of groups o f i n t e r f e r e n c e s t r i p s can be s e e n i n f i g u r e s 12, %2a and B2b0

I f we use t h e l f g h t from a continuous s o r c e , e , g , an o r d i n a r y i n c a n d e s c e n t lappp, and s u b s t i t u t e f o r

1

t h e mean wave l e n g t h of t h e e f f e c t i v e s p e c t r a l range s f about 4000 t o 6000 A and f o r

Xl

-

A2

w e p u t h a l f t h e d i f - f e r e n c e between t h e two extreme wave l e n g t h s , we can t h u s e s t a b l i s h t h e boundaries up t o whfcb t h e i n t e r f e r e n c e s t r f p s _- a r e s t i l l w e l l s a t u r a t e d , k

=

= 5000 -. 2,5- P

i?-YmRm

s t r i p s t e l l s us t h a t t h e r e a r e o n l y about 5 web1 s a t u r a t e d i n t e r f e r e n c e s t r i p s I n t h e e n t i r e f i e l d , This i s confirmed i n f i g u r e 9, FOP t h e i n t e r p r e t a t i o n of i n t e r f e r e n c e s t r i p images t h e r e a d e r i s r e f e r r e d t o t h e l i t e r a t u r e ( r e f , 8 and 9) i n whfch t h e laws governfng t h e ~ e l a t f o n s h f p s be- tween t h e o p t i c a l l y a s c e r t a i n e d d e n s i t y and' t h e magnitudes

! of t h e othela cond3tfons, p r e s s u r e and temperature, a r e de-

ducedo

VI.

POSSIBLE TECHNICAL APPLICATIONS OF THE INTERFEROMETER

The e x t r a o r d i n a r y s e n s i t i v i t y

of

t h e i n t e r f e r e n c e method p e r m i t s t h e measurement, i n a measuring f i e l d l e n g t h

of 1 metre, of changes of 9 i n t h e r e f r a c t i v e in-

Tqmoo,

ooi7

dex of t h e medium, By choosing t h e w i d t h of s t r i p , which can be c o n t r o l l e d a s d e s i r e d with t h e two p l a t e s PI and P2

( f i g , 51, t h e a r e a of s e n s i t i v i t y can be a l t e r e d w i t h i n wide l i m i t s , For example, when t h e d e n s i t y changes a r e l a r g e (fig, 151, v e r y narrow s t r f p s w i l l be chosen because of t h e l a r g e b u l g i n g s t o be expected i n them, Small changes i n d e n s i t y , on t h e o t h e r hand, r e q u i r e wider s t r f p s , I n e x t ~ e r n e c a s e s v e r y s m a l l d e n s f t y f f e l d s c a n be scanned o n l y w i t h a few v e r y broad s t r f p s ,

We g e t a n i d e a of t h e s e n s i t i v i t y of t h i s measur- i n g technique when we c o n s f d e r t h a t , when t h e i n t e r f e r e n c e s t r i p s a r e broad, s m a l l f r a e t f o n s of t h e f r - w i d t h s c a n s t i l l be measured a c c u r a t e l y , while a bulge amounting t o an en- t i r e s t r i p width corresponds t o a change in t h e ' o p t i c a l p a t h of only one wave l e n g t h $0,578/1800

mm.1

of t h e l f g h t employed,

Page

-

13

Tech, Trans, TT-18 The main fields in which the interference method is employed are thermodynamics, flow physics, gas dynamics, the determination of densities in motionless gaseous and liquid substances, of transparent solid bodies and the dis- covery of tension fields in transparent solid and liquid research media,

The application of the interference method gains a special significance in the study of bodies in high-speed subsonic flows, Owing to the compressibility of flowing air, local sound waves appear on the airfoil profile at the high flying speeds which are already being planned today, and these waves affect considerably the lift and drag characteristics of the flying body and conseqvently also its flight behnviour, In this gritical flow region, where the use of measuring methods proven in wind tunnel practice at lower air speeds, is no longer possible, our optical method proves itself particularly valuable, Further, it permits the study of non-steady flow phenomena on the main plane, as when spoilers or slots are suddenly extended, slots are opened, or the like, The change in the interfer- ence strip field, which provides a measure for the change in the density field of the research medium occurs entirely free from inertia,

VIl SPECIAL TECHNICAL FEATURES

The fact that light can be brought to interference

by an optical system of

l a t e s

and mirrors in particularly

delfcate adjustment stfl

T.

provides no certainty that such

an apparatus is suitable for technical measurements, The known interferometers of this type, of which, moreover, there were only a few fn Germany until recent years, neces- sitated the fulfilment of certain requfrements of construc- tion that are impossible where technical application is in- tended, They were built horizontally on very heavy, solid ground plates, in shock-proof rooms, or on specially con- structed foundatfons, and in places where the temperature was partially controlled by thermostat, Nevertheless in certain instances heat radiated from the body of a person near the interferometer sufficed to cause the delicately adjusted interferences to vanish, by reason of small defor- mations of the instrument which were attributable to tem- perature influences, In some cases, too, a light touch of the finger on a ground plate weighing several hundredweight was enough to change the interferences,

From these observations it was apparent that sen- sitivity to temperature and to mechanical influences had to

Page

-

1 4 Tech, T r a n s , TT-18 be e l i m i n a t e d a s f a r as p o s s i b l e through b e t t e r t e c h n i c a l c o n s t r u c t i o n , The f i r s t s t e p , t h e r e f o r e , was t o d e s i g n a l a t t i c e g i r d e r t o c a r r y t h e m f r r o r assembly, It was of c a n t i l e v e r c o n s t r u e t i o n , f o l l o w i n g c e r t a i n p r i n c i p l e s of bkidge c o n s t r u c t i o n , b u t was i n i t s e l f r e s i l i e n t ( f i g , 1 m d 210 For temperature e q u a l f z a t i o n a s p e c i a l i n v a r s t e e l a l l o y was chosen, h a v i n g a P f n e a ~ sxpansion c o e f f i c i e n t

( a t

looo

C ) of*

d

=

0,000009, t h u s showing a g r e a t s u p e r i o r - f t y over o r d f n a r y s t e e l w i t h &

'

0,00165, The l a t t i c e

work could n o t be weldedu l e s t t h e s e o u t s t a n d i n g q u a l i t i e s sf i n v a r s t e e l be l a s t through structural change,

The a p p a r a t u s was f u ~ t h e , r desfgned f o r v e r t i c a l arrangement and suspended by s p r f n g s s o a s t o f l o a t i n a pfpe frame-work p e r m f t t i n g freedom of movement i n a l l t h r e e d f m e n s i s n ~ , n a t u r a l s s c f l l a t i o n frequency of t h e ap-

when i t s mass m 9 s given, can be a d j u s t e d by a c h o i c e of t h e s p r i n g c o n s t a n t c

.

I t f s a good i d e a t o choose t h e n a t p r a l fkequency of t h e a p p a r a t u s much s m a l l e r t h a n t h a t of t h e ' f r e q u e n c y of t h e v i b r a t i o n s which a f f e c t i t from w i t h o u t , The v i b r a t i o n s a r e t h e n n e u t r a l 1 zed and s u f f i e i e n t l y damped f o r t h e f n t e r f e r e n c e s t r f p s t o remain unchanged and f o r measurements t o be pos- s i b l e with t h e a p p a r a t u s even i n p l a c e s which a r e s u b j e c t t o s e v e r e shocks,

Another s p e c i a l f e a t u r e of t h e a p p a r a t u s I s i t s mobile amangemen%, e n a b l i n g t h e s u r v e y of l a r g e d e n s i t y f f e l d s a t d i f f e r e n t p l a c e s with t h e s m a l l r n l r ~ o r s employed,

It

i s a d v i s a b l e t o f i x the camera used t o t a k e the i n t e r f e r e n c e p i c t u r e s on a s m a l l o p t i c a l t ~ a c k a t t a c h e d t o t h e i n t e r f e r o m e t e r i t s e l f , a s t h a t i t f o l l o w s e v e r y move- ment of t h e a p p a r a t u s m d the e n t f r e f n t e k f e k e n c e image is

i n f o e u s wherever one may be i n t h e d e n s f t y f i e l d t o be s t u d i e d ,

1% t h e body befng s t u d i e d h a s a r a t h e r l a r g e l o n g i t u d i n a l expanse, e , g , & h e a t e d p f p e , o r t h e span of a

main wfng model, an s p t i e a l d i s t o r t i o n i s f n t ~ o d u c e d , be-

cause t h e two boundary p l a n e s of the body a r e a t d f f f e ~ e n t

d f s t a n c e s from t h e l e n s , This must be t a k e n I n t o aeeount when f o c u s i n g t h e camera on t h e o b j e c t , whfch must be p l a c e d s o t h a t two f i x e d p o f n t s a t corresponding p o s i t i o n s on t h e

Page

-

15

Tech, Trans, TT-18 This distortion can be made very slight by first diverting the light from the interferometer through prisms and thus increasing the object distance, The great advan- tage in this is that one can work with large focal distan-

ces of an optical system, knd in spite of the great dis-

tance from the object of study one can fasten the camera directly to the interferometer, Since we have at our dis- posal nearly parallel light of such intensity that exposure times up to B/200 sec are possible, the small loss of light through absorbtion by the air over the artificially exten- ded path outside the apparatus can be disregarded,

It remains to be mentioned that the use of thick glass plates for the boundary planes of the density field under study creates no dffficulty, It is merely essential

to use optically perfect glass, free from stresses and striae, No special requirement as to plane parallelism is made in respect to these,

The important thing in using such plates is to obtain the correct compensation of the phase displacement, If thfs is attempted by varying the distance of the appara- tus (longitudinal drive at P2) then it is necessary at

times to ad,just the mirrors, since the plates are not quite exactly plane parallel,

A

much simpler way, however, is to take a glass

plate of about the same thickness as the two plates to- gether, which deffne the boundaries of the densfty field,

and place it in the path of the second beam, whf ch has

thus far been undisturbed, If this plate can be turned and tflted, we find that the interferences are of good quality again without changing the exact basic adjustment of the apparatus, When these glass plates are in use the interferences can still be altered as required at the two mirrors PI and P2,

VIII S U P R " RY-

The physical basis of an interference procedure was described together with an interferometer which was

developed for the opti cal measurement of density f ielda,

The particular points of view stressed in this work were the technical construction of the apparatus itself and a method of adjustment which made it possible to overcome

the difffcultfes, previously very great, of adjusting and using such instruments,

P a g e , - 16: Tech, T ~ a n s , T T - 1 8 A n i n v e s t i g a t i o n of t h e l f g h t s o u r c e s most u s e f u l f o r s p e c t r a l s t u d i e g and o f a s u f t a b l e f i l t e r f o r good monochromatic q u a l i t y and p h ~ t o ~ z r a p h i c e f f e c t i v e n e s s l e d t o a t t a i n m e n t of wel; s a t u r a t e d i n t e r f e r e n c e s t r i p p i c t u r e s ( s e e f i g , 13

-

1 5 ) w i t h e x p o s u r e t i m e s of B/POO t o 1/200 s e c , , e v e n when t h e l i g h t was f i l t e r e d and t r a v e r s e d l a r g e p a t h s , With t h e r e s u l t s of t h i s s t u d y , t h e p o s s i b i l i t y of u s i n g i n t e r f e ~ o m e t e r s f c r Rhe f i n e s t d e n e f t y measurements, e v e n i n t e c h n i c a l m e a s u r i n g p r a c t i c e , is e s t a b l i s h e d a s w e l l a s t h e p o s s i b f li t y o f s t u d y i n g noncys t e a d y phenomena, Thus t h e way i s opened f a r t h e i n t r o d u c t i o n o f t h e i n t e r f e r e n c e method I n t o many t e c h n i c a l f i e l d s of a p p l i c a t i o n ,

, The o b s e r v a t i o n o f t h e i n t e r f e r e n c e f i e l d on a wing model o f a b o u t PO cm, c h o r d and 40 cm, s p a n , r e v e a l e d c h a r a c t e r i s t i c b u l g i n g s of t h e I n t e r f e r e n c e s t r i p s c l o s e t o t h e uppreer p r o f i l e s u r f a c e w i t h a f l o w v e l o c i t y a s low a s 40 m e t e s p e r s e c , The s e n s i t i v i t y of t h e m e a s u r i n g method i n c r e a s e s I n l i n e a r p r o p o r t i o n t o t h e l o n g i t u d f n a l e x p a n s e ( s p a n ) o f t h e f i e l d o f f n v e s t i g a t i o n ; t h e d e n s f t y c h a n g e s t o be measured i n c r e a s e w i t h t h e s p e e d o f f l o w , When t h e s p a n l s a b o u t 2 met&s and t h e s p e e d s a r e from

EOO t o 2 0 0 m e t r e s p e r s e c o n d , c a l c u l a b l e b u l g i n g s o f t h e i n t e r f e r e n c e s t r f p s o c c u r , and a r e s f s u c h s f z e t h a t v e r y s m a l l s t r i p w i d t h s must b e u s e d , Thus t h e r e q u i r e m e n t s f o r d e t e r m f n f n g t h e d e n s f t g f f e l d s u r r o u n d i n g submerged b o d i e s and f o r i n v e s t i g a t i n g a g r e a t number s f i m p o r t a n t q u e s t i o n s of f l o w p h y s i c s a r e r e a l l z e d , A t t h e t i m e when t h e s e c b s e r v a t i o n s I n t h e wind t u n n e l were b e i n g made, t h e information a b o u t t h e s p e c t r a l l i g h t s o u r c e of g r e a t e s t i n t e n s i t y had n o t y e t b e e n ob- t a i n e d , s o t h a t t h e s e I n t e r f e r e n c e f i e l d s c o u l d n o t b e p h o t o g r a p h e d ,

H ~ w e v e r , a f t e r t h e problem o f i l h u m f n a t i o n h a d b e e n s o l v e d , t h e wind t u n n e l , owfng t o a t e c h n i c a l mishap, was p u t o u t o f a c t i o n f o r s e v e r a l months, s o t h a t p u b l i c a - t i o n of s u c h i n t e r f e r e n c e p i c t u r e s In t h e n e f g h b o r h o o d of submerged b o d i e s i s p o s t p o n e d ' u n t f l a l a t e r d a t e ,

Page

-

17

Techo Trans, TT-18 BIBLIOGRAPHY

i,

H,

Dvorak, Efne neue einfache Art der Schlierenbeobachtung

(A

New, Simple Method of Schlieren Observation) ; Wiedemanns

Ann,; Vol, 9 (1880) P o 502,

2, G, Schmfdt; achlierenaufnahmen des Ternperaturfeldes in der

Naehe waermeabgebender Koerper (Schlieren Photographs of

the Temperature b'ield Near Heat Radiating Bodies ); Forschg,

Ingo-Wes,; Vol, 5 (19321, P o 18,

3, A, Toepler: Beobachtungen nach einer neuen optischen Meth-

ode (Observations by a New Optical Method);: Ostwalds Klas-

siker exakt, Wiss,;: Vol, 157 and 158; Leipzfg, 1926,

4,

H,

Schardin; Das Toeplersche Schlierenverfahren (Toepler Ps

Schlieren Method) ;: V D I Forschg,; No, 367 ;: Berlin, 1934,

5, C, Cranz; Lehrbuch der Ballistik (Textbook of ~allistics);

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E,

Gehrekep Die Anwendung der Interferenzen in der Spek-

troscopie und Metrologie

he

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to Spectroscopy and Metrology); Braunschweig, 1906,

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techno Phys,;

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H,

Schardin; Theorie und Anwendung des Mach-Zehndersehen

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Mach-Zehnder Interferenca Refractometer);

Z,

Instrumenten-

kde,; Vol, 53 (1933),

T o

396, 424 and 425.0,

9, A, v, Dechend; Ueber d ~ e genaue Messyrqg ~ e r Lfchtbrechung

in Gasen (The Accurate Measurement of Refra,ctfon of Light ;

in Gases ) ;. Hefdelberg, 1915,

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J o Mueller and CP, S o

M,

Pouillet; Lehrbuch der Physik;

11, Auflage (Textbook of Physics; 11th Edition);; Vol, 2;. Braunschweig, 1926; P o 666,

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No,

18, P, 505,

Fig. 1 & 2

Tech. Trans. TT-18

Fig* 1

Fig* 2

Fig. 1 & 2. Mobile interferometer with floating suspension

Tech. Trans. TT-18

I / -

_screen

'Glass p l a t s

Fig. 3. Occurrence of interferences of uniform thickness on the wedge plate (ref, 12)

Fig. 4. Mach-Zehnder four-plate system (ref, 11)

Fig. 5 & 6 Tech. Trans. TT-18 Lens !I

1

_{S c r e e n} Fig. 5. P r o d u c t i o n o f i n t e r f e r e n c e a t t h e d e s i r e d d i s t a n c e from t h e a p p a r a t u s b y m i r r o r a d j u s t m e n t ( r e f . 12)

(B'

i s t h e l o c a t i o n o f t h e i n t e r f e r e n c e )

I

t e l e s c o p e F i g . 6. Method o f a l i g n i n g e a c h p a i r of m i r r o r s by o b s e r v a t i o n of a d i s t a n t o b j e c t

Tech. Trans. T T - 1 8

Fig. 7. Path of l i g h t r a y s i n a p e n t a p r i s m

Fig. 8. Adjustment of t h e m i r r o r s u s i n g a p e n t a p r i s m ( r e f . 11, 1 2 )

Fig. 9 & 10

Tech. Trans. TT-18

Fig. 9. Zero interference in continuous white light

Light source

7-

H

lens

screen

Fig. 10. Construction of the interferometer

Tech, Trans. TT-14

Fig. 11. Distribution of relative spectral energy in the high pressure mercury discharge

(From Zeitschrift h e r Technische Physik; 14; 1933, 393)

Fig. 12a. Fig. 12b.

Without density field With density field!

Fig. 12. Groups of interference strips

in

unfiltered mercury light Exposure time: 1/100 sec.

F i g . 13, 14 & 15 Tech. Trans

.

TT-18 F i g . 13. S t r i p d i s p l a c e m e n t around a h e a t e d body. Exposure time: 1/100 s e c . F i g . 14. S t r i p d i s p l a c e m e n t F i g . 15. Marked d i s t o r t i o n due t o d e n s i t y v a r i a t i o n s i n of t h e i n t e r f e r e n c e s t r l p s a s u p e r s o n i c flow ( r e f . 11). due t o wide d e n s i t y v a r i a t i o l Exposure time: 1/100 s e c . i n t h e t e s t medium.