Apparatuses for Measuring the Thickness of Thin Coatings

Publisher’s version / Version de l'éditeur:

Technical Translation (National Research Council of Canada), 1952

READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.

https://nrc-publications.canada.ca/eng/copyright

Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la

première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.

Questions? Contact the NRC Publications Archive team at

PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.

NRC Publications Archive

Archives des publications du CNRC

For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.

https://doi.org/10.4224/20386523

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at

Apparatuses for Measuring the Thickness of Thin Coatings Monch, G. C.

https://publications-cnrc.canada.ca/fra/droits

L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.

NRC Publications Record / Notice d'Archives des publications de CNRC:

https://nrc-publications.canada.ca/eng/view/object/?id=ed432dc0-f4f9-4690-97f3-123e09ee38f5 https://publications-cnrc.canada.ca/fra/voir/objet/?id=ed432dc0-f4f9-4690-97f3-123e09ee38f5

T i t l e :

NATIONAL

RESEARCH

COUNCIL O F CANADA

Tecnniodl T r a n s l a t i o n TT-315 Apparatuses f o r measuring t h e t h i c k n e s s of t h i n c o a t i n g s , ( ~ n o r d n u n g e n z u r Messung k l e i n e r S c h i c h t d i c k e n . ) By: G. C . MBnch. Reference : Optik 8: 550-560, 1951. T r a n s l a t e d by: D. A . S i n c l a i r

.

APPARATUSEg FOR L'IEASURING p TME THICKNESS OF THIN COATINGS

O p t i c a l s e t - u p s u s i n g K d s t e r s l double-prism i n t e r f e r e n c e p r i n c i p l e f o r measuring t h e t h i c k n e s s o f t h i n c o a t i n g s a r e des- c r i b e d , c o n s t r u c t i o n drawings of mountings f o r t h e double p r i s m s a r e p r e s e n t e d , and methods of d e t e r m i n i n g t h e t h i c k n e s s of m e t a l c o a t i n g s d e p o s i t e d by t h e v a p o r i z a t i o n p r o c e s s and of c e l l u l o i d c o a t i n g s manufactured f'rom Zapon v a r n i s h a r e s e t f o r t h .

1, The Double Prism: D e f i n i t i o n o f t h e

--

Problem

A s a f u r t h e r development of t h e method developed i n t h e P h y s i k a l i s h - T e c h n i s c h e n R e i c h s a n s t a l t f o r d e t e r m i n i n g l e n g t h s w i t h t h e a i d of t h e i n t e r f e r e n c e f r i n g e phenomenon w i t h v i s i b l e l i g h t , a n a p p a r a t u s was used f o r detemnining t h e t h i c k n e s s of l a y e r s a l s o by employing ~ o s t e r s ' p r i s m . The l a t t e r w i l l now be d e s c r i b e d .

~ d s t e r s ' ~ ) double prism i s a n e x c e e d i n g l y u s e f u l d e v i c e f o r many o p t i c a l . p r e c i s i o n i n s t r u m e n t s , S i n c e no f i r m manu- f a c t u r e s t h e prism w i t h a h o l d e r , a u s a b l e i n s t r u m e n t c a n be o b t a i n e d o n l y by o r d e r i n g a double p r i s m a c c o r d i n g t o o n e s s own ~ ~ e c i f i c a t i o n s , ~ " ) and d e s i g n i n g a mounting f o r i t which may t h e n be c o n s t r u c t e d o However, t h e d e s i g n i t s e l f r e q u i r e s

some e x p e r i e n c e w i t h p r i s m s . I t i s because of t h i s t h a t t h e double p r i s m has n o t been employed a s e x t e n s i v e l y a s i t should be i n view o f i t s i m p o r t a n c e , The f o l l o w i n g r e p o r t d e s c r i b e s i t s o p t i c a l c o n s t r u c t i o n , a s w e l l a s two mounting d e s i g n s f o r

*)w.

K 8 s t e r s 9 W e r k s t a t t e c h n i k und W e r k s l e i t e r 3 2 . 527 ( 1 9 3 8 ) . C f .

F.

Kohlrausch, P r a k t i s c h e Physik 1 9 t h e d , 1944 p o 41,

%*)

C , Z e i s s , J e n a , has manufactured two sizes., Schmidt &

Haensch, 33 N ~ W M S t , , Berlin-Schoeneberg, make them t o o r d e r .

t h e prisms and t h e method of u s i n g them i n o r d e r t o d e t e r m i n e t h e t h i c k n e s s of t h i n l a y e r s ,

The double prism c o n s i s t s of two i d e n t i c a l semi-prisms

(PI

and P 2 i n F i g , 1 t o 3) w i t h t h e a n g l e s 30°, 60' + a p p r o x ,

5 ' and 89' + approx, 5 5 ' , Thus t h e s h o r t l e g i s n o t e x a c t l y

p e r p e n d i c u l a r t o t h e l o n g one, and d i s t u r b i n g r e f l e x i o n s a r e t h e r e f o r e e l i m i n a t e d . The l o n g l e g f a c e of one of t h e p r i s m s i s c o a t e d i n t h e vacuum w i t h m e t a l vapour ( s i l v e r , aluminum, rhodium) t o make i t s e m i - t r a n s p a r e n t , T h e , p r i s m s a r e p u t t o - g e t h e r w i t h a drop of v i s c o u s o i l , T h i s manner o f j o i n i n g makes it p o s s i b l e t o i n s t a l l t h e two prisms i n a h o l d e r which keeps one of them i n a f i x e d p o s i t f on b u t p e r m i t s t h e o t h e r t o move.

2 , Designs of Prism IiTountinga

--

The prism mounting ( F i g . 1 ) r e s t s on a b o l t Ba of 20 mrn. d i a m e t e r , which i s s u i t a b l e f c r i n s e r t i o n i n r i d e r s w i t h 20 mm. h o l e s f o r t h e Z e i s s r a i l , A s l o t t e d p l a t e PI1 i s f a s t e n e d

o n t o b o l t B, and c a r r i e s a screw S1, w i t h s p ~ i n g b o l t F1 a s a c o u n t e r s u p & ~ i , t , The p l a t e P I 2 above t h i s i s f a s t e n e d t o a c o n i c a l p i n

Bi

which f i t s i n t o a j a c k e t which s i m u l t a n e o u s l y forms t h e o u t e r b o l t

B

,

.

To p r e v e n t t h e c o r e ( B i ) from jamming i n i t s j a c k e t ( B a ) i t r e s t s w i t h a s t e e l b a l l o n a s t e e l p l a t e which c l o s e s o f f B, a t t h e bottom. Adjustment of screw S 1 which c a u s e s arm A, f a s t e n e d t o PI2 t o move back ,and f o r t h

w i t h o u t back l a s h a l s o c a u s e s p l a t e P I 2 t o r o t a t e , t h e r e b y f o r c f n g Bi t o t u r n i n B,, The f r a m e w r k f o r t h e a c t u a l h o l d e r of the two semi-prisms P1 and P2 i s mounted on p l a t e P I 2 . The frame c o f i s i s t s of a base p l a t e and a cover p l a t e , PI2 and PI3 ~ e s p e c t i v e l y , joined t o g e t h e r by a s q u a r e rod S t l and round r o d s S t 2 and St3. The f i x e d semi-prism

P2

r e s t s on a b r a s s p h t b which i s s e c u r e d t o b a s e p l a t e P12 by round b r a s s r o d s (columns). The p l a t e l y i n g on t o p of semi-prism

P2

i s secured by h e a d l e s s screws

S6

which p a s s t h r o u g h P l g J and t h i s h o l d s P2.

The movable semi-prism P1 i s clamped i n a h o l d e r H having t h e shape of a r i g h t - a n g l e d t r i a n g l e c o r r e s p o n d i n g t o t h a t of t h e semi-prism which i s r e t a i n e d by s e t - s c r e w S5.

H

i s moved by screws S2,

Sg

and S 4 0 Two a n g l e b r a c k e t s a r e f a s t e n e d t o P I 2 t o hold screws S2 and S g o Turning t h e s e screws moves t h e t o g g l e l e v e r s and these, i n t u r n , cause semi-prism

PI

t o move up and down, Two s p r f n g r e t a i n e r s F2 and F3 w i t h s p h e r i c a l t i p s i n t h e top p l a t e P I 3 keep t h e p r i s m frame c o n s t a n t l y p r e s s e d a g a i n s t b o l t s B which a l s o have s p h e r i c a l t i p s , and a r e o p e r a t e d by t h e t o g g l e l e v e r s , .Thus t h e h o l d e r i s a b l e t o move up and down w i t h i n wide

l i m i t s between upper and lower b o l t s (F2, Fg and B ) . Screw S4 i s r e t a i n e d i n s q u a r e p o s t S t l and e x e r t s p r e s s u r e a g a i n s t h o l d e r H through a c o n n e c t i n g l i n k G which has two b a l l s

Thus t h e f o u r screws

Si,

S2, Sg and S4 p e r m i t a u n i v e r s a l adjustment of the prism, S1 t u r n s the e n t i r e double p r i s m about t h e a x i s of Be and Bi. S2 and S3 move semi-prism P1 up and down, and when t h e y a r e a d j u s t e d u n e q u a l l y t h e y t i l t i t . S4 d i s p l a c e s P1 h o r i z o n t a l l y i n r e l a t i o n t o P2, So t h a t t h e l e g f a c e s of P1 and P2 s l i d e i n r e l a t i o n t o each o t h e r . I n a l l movements of P1 r e l a t i v e t o P2 t h e two semi- p r i s m s c o n t a c t one a n o t h e r o n l y t h r o u g h t h e o i l f i l m between t h e i r n e i g h b o r i n g f a c e s , A l l screw p r e s s u r e s e x e r t e d a g a i n s t P1 permit o n l y f r i c t i o n l e s s s l i d i n g of t h e o i l c o a t e d , l a r g e l e g f a c e s of t h e prisms over one a n o t h e r ,

The second prism mounting t o be d e s c r i b e d here i s f o r a double prism of t h e s p e c i a l t y p e shown i n F i g , 2 ( l a s t p i c - t u r e ) , It f s a l s o mounted on a 2 0 mrn, p i n , and t h e base p l a t e , PI1 a g a i n r o t a t e s on t h e i n n e r c o n f c a l p i n Bf i n s i d e t h e o u t e r b o l t Bao The lower and upper p l a t e s , PI1 and PI2, r e s p e c t i v e l y , a r e r i g i d l y connected by t h r e e p o s t s , The f i x e d semf-prism P2 i s s e c u r e d between PI1 and PI2 by s e t screws mounted i n t h e l a t t e r . The movable p a r t P1 i s a g a i n h e l d i n a movable h o l d e r H and i s clamped i n p l a c e by screws S3.

Holder H w i t h i t s base p l a t e PI4 i s movably mounted on p l a t e P13. Post S t which is r i g i d l y mounted on P l g has a t h r e a d e d h o l e t o r e c e i v e screw S2, by which

H

can be moved back and f o r t h on P13. To e n a b l e i t t o do t h i s H r e s t s and

s l i d e s on two s t e e l b a l l s lodged i n P l g n e a r t h e c e n t r e of t h e double prisms a r e a d e f i n e d by P1 and P2, An a n g l e b r a c k e t Yf w i t h an i n s e t s t e e l b a l l p r e s s e s from above on t h e r i m of p l a t e P I 4 . P l a t e P l g w i t h h o l d e r H can be t i l t e d up o r down by means of screw S1. The hinge f o r t h i s t i l t i n g motion i s t h e l e a f s p r i n g f a s t e n e d t o t h e s h o r t l e g f a c e s of PI1 and P l g o The t i l t i n g i s c a r r i e d out by t h e p r e s s u r e e x e r t e d by S1 on S t ( w i t h c o v e r p l a t e of p o l i s h e d s t e e l ) , a c t i n g a g a i n s t t h e p r e s s u r e from a s p r i n g r e t a i n e r mounted below in PIlo During t h i s o p e r a t i o n S2 moves f r e e l y i n a n opening i n t h e p o s t S c o n n e c t i n g PI1 and P I Z e

The d e g r e e s o f freedom i n t h i s p r i s m mounting a r e r e s - t r f c t e d t o t h e t f l t i n g of one semi-prism r e l a t i v e t o t h e o t h e r by means of S1 and t h e h o r i z o n t a l d i s p l a c e m e n t of P1 w i t h r e s p e c t t.o

P2

by means of S2,

3 , O p t i c a l Set-up

The e n t i r e o p t i c a l arrangement i s shown i n F i g o 3 . A p e n c i l of l i g h t e n t e r s t h e i n s t r u m e n t t h r o u g h a n opening Spl,

c o n s i s t i n g o f a round o r square s t o p , and i s made p a r a l l e l by a n o b j e c t i v e 01, I t t h e n s t r i k e s p e r p e n d i c u l a r l y a g a i n s t t h e f a c e of semi-prism PI and i s d i v i d e d a t t h e s e p a r a t i n g p l a n e E, h a l f b e i n g r e f l e c t e d , t h e o t h e r h a l f b e i n g allowed t o p a s s t h r o u g h i n t o semi-prism P2, A f t e r d u a l r e f l e c t i o n i n P1 and s i n g l e r e f l e c t i o n i n P2, a s d e p i c t e d i n F i g , 3 a , t h e r a y s l e a v e b o t h semi-prisms t h r o u g h t h e s h o r t l e g f a c e s a s two

beams w h i ~ h a r e p a r a l l e l t o one a n o t h e r , When t h e s e s t r i k e m i r r o r s I and

I1

which a r e s i t u a t e d i n a p l a n e and a r e ap- p r o x i m a t e l y p e r p e n d i c u l a r t o t h e p a t h of t h e r a y s , t h e l a t t e r a r e r e f l e c t e d back on themselves, p a s s t h r o u g h P1 and P2, a s shorn i n F i g , 3 b, and u n i t e a g a i n , emerging a t r i g h t a n g l e s

from t h e hypotenuse f a c e o f semi-prism P2, where t h e y i n t e r - f e r e w i t h each o t h e r . With t h i s prism, t h e r e f o r e , p l a n e s I and I1 a r e o p t i c a l l y reproduced over one a n o t h e r . Outside t h e double p r i s m t h e emerging l i g h t i s c o l l e c t e d by a n oh-

j e c t i v e 02, The examiner l o o k s t h r o u g h t h e narrow, round o r square s t o p (opening s p 2 ) and s e e s t h e superimposed m i r r o r s e c t i o n s s t r i p e d w i t h i n t e r f e r e n c e f r i n g e s ,

Employing a white l i g h t s o u r c e , t h e o b s e r v e r can a d j u s t e i t h e r t h e movable semi-prism i n t h e manner d e s c r i b e d o r t h e m i r r o r w i t h s u r f a c e h a l v e s I and 11, by t i l t i n g i t on i t s t h r e e - p o i n t suspension, s o a s t o modify t h e i n t e r f e r e n c e and obtafn a f i e l d of view c o n t a i n i n g s e v e r a l h o r i z o n t a l , p a r a l l e l i n t e r f e r e n c e s f r i n g e s of s u i t a b l e s i z e w i t h t h e z e r o f r i n g e a t 2: ) t h e c e n t r e , F i g , 4 shows a n a p p a r a t u s c o n s t r u c t e d on t h e p r i n c i p l e of F i g , 3, u s i n g t h e c o l l i m a t o r w i t h l i g h t s o u r c e , t h e obser- v a t i o n t u b e and t h e a d j u s t a b l e p l a t f o r m .

' ) ~ f . F. Kohlrausch: " P r a k t i s c h e Physik", 1 9 t h ed., 1944, p , 40, f i g . 1 5 . Ui. ~ 8 s t e r s : " ~ f e r k s t a t t e c h n i k und

4 , Determining t h e t h i c k n e s s o f a c o a t i n g a p p l i e d b x v a p o u r i ' z a t f

-

on

A p l a n e g l a s s p l a t e was used a s t h e base f o r t h e c o a t i n g t o be i n v e s t i g a t e d . I t s e n t i r e s u r f a c e had been c o a t e d by v a p o u r i z a t i o n i n a vacuum w i t h t h e same m e t a l , and i t t h e r e - f o r e formed a m i r r o r w i t h t h e s u r f a c e p a r t s I and I1 a s i n Figo 3 "

Now, i f t h e h a l f of t h e m i r r o r d e s i g n a t e d by I1 i s given a n a d d i t i o n a l c o a t i n g by v a p o u r i z a t i o n of m e t a l i n

t h e vacuum, p l a c f n g a s h i e l d i n f r o n t of i t , t h e n t h i s narrow s u r f a c e I11 c o n s t i t u t e s a p r o j e c t i o n from I1 which i s i n d i - c a t e d on t h e i n t e r f e r e n c e p i c t u r e by a p a r a l l e l d i s p l a c e m e n t of t h e h o r i z o n t a l f r i n g e s i n t h e c e n t r a l f i e l d c o r r e s p o n d i n g t o a r e a 111.

S i n c e t h e p a t h of t h e wave r e f l e c t e d a t I11 i s now 2d s h o r t e r t h a n t h a t r e f b c t e d a t IT, where d i s t h e t h i c k n e s s

of t h e vapourized-on l a y e r , t h e n t h e d i s p l a c e m e n t of t h e cen- t r a l system of f r i n g e s r e l a t i v e t o t h e boundary r e g i o n s c o r - r e s p o n d s t o a p a t h d i f f e r e n c e of 2d. I f d = X/2, i . e . , i f t h e p a t h d i f f e r e n c e i s e q u a l t o

1,

a d i s p l a c e m e n t e q u a l t o one f r i n g e width t a k e s p l a c e . A d i s p l a c e m e n t of one f r i n g e p t h e r e f o r e , c o r r e s p o n d s t o a l a y e r of t h i c k n e s s d = h/2, i , e . f o r t h e v i s i b l e r e g i o n a p p r o x i m a t e l y 0 , 3 If t h e f r i n g e d i s p l a c e m e n t i s e s t i m a t e d t o be 1/20 of t h e f r i n g e i n t e r v a l , t h e t h i c k n e s s of t h e vapourized-on l a y e r 111 i s found t o be a p p r o x i m a t e l y 0.015

_{P a}

Since estimates can never be carried out as accurately as compensations which cancel the displacement, a compensator was installed. It consisted of a narrow pressure chamber

K

(Fig. 3 b) enclosed in plane parallel glass plates.

The pressure increase was accomplished by'means of a small hand pump, which forced air into the chamber m d into the glass container of approximately 2 liters capacity attached to it, An elbow manometer tube filled with mercury was used to measure the pressure. Since the fringe displacements were very small the temperature changes accompanying the changes in pressure were not a disturbing factor, Since the thickness of the layer and the fringe displacement are proportional, and since the fringe displacement and the increase in pressure, at constant tempera- ture, are also proportional, it follows that the thickness is proportional to the change in pressure. Thus an increase in pressure equal to

1

Torr corresponds to 1/100 of a fringe width, or 0.003p layer thickness. To be sure a compensation to 1/100 of a strip width cannot be carried out accurately, since a dis- placement of this amount is too small to be observed, Actually the compensator increases the sensibility, compared with the estimation method, only to 1/30th of a strip interval. With the compensator, therefore, layer thicknesses can be determined to an accuracy of approximately

2

0.05p. This limit of sensi- bility can only be achieved in favourable circumstances with the aid of a monochromatic light source, e,g. an Osram thallium

The t h i c k n e s s e s of m e t a l and semi-conductor c o a t i n g s a p p l i e d by v a p o u r i z a t f o n have been measured by the method d e s c r i b e d h e r e a s w e l l a s t h e t h i c k n e s s of p l a t e s t h a t a r e s u f f i c i e n t l y p l a n e ,

The f o l l o w i n g s e c t i o n d e a l s w i t h t h e s e t - u p f o r d e t e r - mining t h e t h i c k n e s s of t r a n s p a r e n t m a t e r i a l such a s g l a s s

o r t h i n c e l l u l o i d f i l m .

5. Determinfng t h e Thickness of C e l l u l o i d Films

Measuring t h e t h i c k n e s s e s of c e l l u l o i d f i l m s produced by p o u r i n g zapon v a r n i s h on water and f i s h i n g o u t t h e l a y e r l e f t f l o a t i n g on t o p by means of a bow a f t e r e v a p o r a t i o n of t h e amyl a c e t a t e i s p o s s i b l e a c c o r d i n g t o t h e f o l l o w i n g t h r e e methods, F i r s t method: I n p l a c e of t h e vapourized-on p r o t r u b e r - ance ( F i g . 3, 5) t h e c e l l u l o i d f i l m i s a p p l i e d t o t h e h a l f of t h e m i r r o r p l a t e d e s i g n a t e d by I1 and immediately a d h e r e s t o t h e s u r f a c e . I t s edge i s shaved w i t h a sharp wooden rod u n t i l a s u i t a b l e r e c t a n g u l a r p i e c e t h e w i d t h of t h e compensator

chamber r e m a i n s , Then t h e e n t i r e p l a t e i s exposed i n a vacuum t o t h e vapour of an e a s i l y vapourized m e t a l s u c h a s bismuth, s i l v e r o r aluminum, The p a t h d i f f e r e n c e o b t a i n e d , which i s measured a s a n i n t e r f e r e n c e f r i n g e displacement, g i v e s d i r e c t l y

Thus : where a

-

f r i n g e displacement caused by t h e f i l m , a d i s - placement from f r l n g e t o f r i n g e b e i n g e q u a l t o 1; p

-

t h e p r e s s u r e i n t h e chamber vhich c a n c e l s t h i s displacement ;

s

-

t h e p r e s s u r e which causes a displacement e q u a l t o one f r i n g e , and

A

-

nave l e n g t h of the l i g h t employed.

The method works s a t i s f a c t o r i l y whenever t h e r e i s no i n t e r v e n i n g space between t h e g l a s s s u r f a c e and t h e a p p l i e d f i l m , and provided t h e f i l m does n o t s h r i n k i n t h e vachum due t o l o s s of water o r o t h e r l o s s , o r if t h e atoms of vapour- i z e d m e t a l do n o t p e n e t r a t e more deeply i n t o t h e c e l l u l o i d f i l m t h a n i n t o t h e s u r f a c e of t h e g l a s s b a s e .

Second method: A f t e r p r e p a r i n g t h e p l a t e and s u b j e c t i n g i t t o t h e metal v a p o u r i z a t i o n p r o c e s s a s i n t h e f i r s t method,

i t

i s placed i n t h e r e v e r s e d p o s i t i o n i l l u s t r a t e d i n F i g . 6 .

i n t h e p a t h of t h e l i g h t . Here t h e p a t h d i f f e r e n c e i s g r e a t e r t h a n i n t h e f i r s t case, s i n c e t h e wave v h i c h i s r e f l e c t e d from 111 moves, i n t h e l a y e r of t h i c k n e s s d , i n a medium of r e - f r a c t i v e index n

>

1. Then

The second method g i v e s c o r r e c t v a l u e s under t h e same s e t of c o n d i t i o n s a s t h e f i r s t . I n a d d i t i o n t h e g l a s s p l a t e must have p r a c t i c a l l y t h e same index of r e f r a c t i o n a s t h e

u

f i l m and.mudt be s l i g h t l y t a p e r e d t o prevent t h e r e f l e x i o n s a t the f r o n t f a c e from causing supplementary, d i s t u r b i n g

i n t e r f e r e n c e s , The r e f r a c t i v e index of t h e c e l l u l o i d m a t e r i a l must be

known.

Third method: E i t h e r t h e g l a s s p l a t e w i t h t h e f i l m a t t a c h e d t o i t , but without a vapourized-on metal c o a t i n g , i s placed between t h e p r e s s u r e chamber and t h e m i r r o r 1-11, o r t h e f i l m i s s t r e t c h e d on a wire frame and i s placed i n t h e p a t h of t h e r a y s a s shown i n F i g , 7.

The t h f c k n e s s of t h e f i l m i s given by d = a

--

X

=

-

P

X

2 (n-1) s 2(n-1) (3) This method, t o o , assumes t h a t t h e r e f r a c t i v e index i s known. The o t h e r c o n d i t i o n s of t h e two previous methods a r e a b s e n t h e r e , but because of t h e value of n, which i s approxi-

r3

mately 1.5 (hence n-1

=

0.5) t h e f a c t o r (n-1) reduces t h e measuring accuracy t o about h a l f .

Of t h e t h r e e methods, the' f i r s t and t h i r d were most g e n e r a l l y used, The t h i r d method had t h e advantage t h a t l i t t l e time was r e q u i r e d f o r s e t t i n g i t up and it was pos- s i b l e t o r e t a i n m a l a r g e number of p r e p a r a t i o n s , which could

n o t be done i n t h e e a s e o f t h e o t h e r methods because of t h e h i g h c o s t of p l a n e p a r a l l e l p l a t e s , Furthermore, w i t h t h e p r e s e r v e d p r e p a r g t i o n s f o r t h e t h i r d method i t was always p o s s i b l e t o determine t h e i n t e r f e r e n c e c o l o u r , whereas when t h e f i l m was a t t a c h e d t o t h e g l a s s p l a t e and a m e t a l c o a t i n g was a p p l i e d over i t by v a p o u r i z a t i o n subsequent d e t e r m i n a t i o n cf t h e e o l o u r proved f m p o s s i b l e .

To t h e c o n d i t i o n s appended t o t h e d e s c r i p t i o n of t h e f i r s t and second methods i t must be added t h a t where t h e f i l m s a r e t h i n , f r e s h and have n o t been exposed t o d u s t , t h e y a d h e r e s t r o n g l y t o t h e c l e a n ' g l a s s s u r f a c e . T h e i r adherence i s comparable t o t h a t of an end b l o c k which has been s p r a y e d , except t h a t i n t h e c a s e of t h e f i l m t h e spray- i n g i s accomplished more e a s i l y by r e a s o n of t h e movable surf a c e ,

There I s no r e a s o n t o suppose t h a t a s e p a r a t i n g l a y e r e x i s t s between t h e g l a s s and c e l l u l o i d , Any t h i n l a y e r of w a t e r which may e x i s t on t h e g l a s s s u r f a c e is absorbed by

t h e f i l m and p r o b a b l y g i v e n o f f a g a i n i n t h e vacuum, The i n t e r f e r e r i c e e o l o u r can be t a k e n as c h a r a c t e r i s t i c of t h e t h i c k n e s s of a f r e e , s t r e t c h e d f i l m . S i n c e t h e c o l o u r d i d n o t change d u r i n g t h e e v a c u a t i o n , t h e r e i s no r e a s o n t o suppose t h a t any d i s t u r b i n g d e c r e a s e of t h i c k n e s s o c c u r s a s a r e s u l t of l o s s of w a t e r o r t h e e v a p o r a t i o n o f a

. component during t h i s p r o c e s s , Otherwise, any decrease i n t h i c k n e s s would have t o be accompanied by an i n c r e a s e i n t h e r e f r a c t i v e index such t h a t t h e l e n g t h of t h e o p t i c a l p a t h , and hence t h e colour of t h e f i l m , d i d not change,

It was a180 found by experiment t h a t t h e e f f e c t of h e a t i n g t h e f i l m i n connection with t h e vapourization of metal i n t h e vacuum ( a s a r e s u l t of r a d i a t i o n from t h e glowing v a p o u r i z a t i o n v e s s e l ) was n e g l i g i b l e , Under c e r t a i n circumstances a change i n the zapon v a r n i s h f i l m s t r u c t u r e can be expected a f t e r i t has been subjected t o m e t a l c o a t i n g by vapourization i n t h e vacuum, s i n c e i t has been found from experience t h a t f i l m s coated by v a p o u r i z a t i o n are more b r i t t l e t h a n uncoated ones and l o s e t h e i r g r e a t s t r e n g t h , The degree t o which t h i s causes a change i n t h i c k n e s s i s d f f f i c u l t t o determine, Some r e s u l t s appear t o i n d i c a t e t h a t t h e at.oms of metal p e n e t r a t e more deeply i n t o the c e l l u l o i d t h a n i n t o g l a s s . I n p r i n c i p l e t h e d i f f e r e n c e can be determined by measurement of t h e t h i c k - ness, f i r s t l y by t h e t h i r d method, and afterwards by t h e f i r s t o r second one, The t h i r d method measures t h e t h i c k n e s s i n t h e a i r wfthout a m e t a l c o a t i n g while t h e o t h e r

t w o

measure i t

a f t e r i t has been coated, With c a r e f u l t r e a t m e n t i n t h e vacuum, using only metals which f u s e

a t

low temperature and vapourize e a s i l y , e s p e c i a l l y bismuth, t h e change i n t h i c k n e s s of t h e c e l l u l o i d f i l m caused by t h e c o a t i n g p r o c e s s f s n e g l i g i b l e i n

The thickness measurements were carried out partly

with white light and partly with the green thallium line.

Displacements of one fringe width were taken to be

A

=

0 . 2 8 0 ~

for white light and

=

0.267525p for

Z

green TI-light.

A

further article will report on the

test results.

I

wish to thank Dr. E o

Engelhard and his technical

assistant

Mr.

Ho

Feyerabend for their aid in designing

and building the prism mountings.

Halle, Institut fuer Angewandte Physik (11. Physikalisches

Insti

tut) der Universitaet

f l F i g .

3 :

O p t i c a l s e t - u p f o r t h e K o s t e r s d o u b l e p r i s m . P1, P2: s e m i - p r i s m s ; E: p l a n e of s e p a r a t i o n between t h e s e m i - p r i s m s w i t h s e m i - t r a n s p a r e n t c o a t i n g a n d o i l f i l m ; Spl, 01, 0 2 , Sp2: a p e r t u r e s and t e l e s c o p i c o b j e c t i v e s ; I , 11: t h e h a l v e s of a p l a n e m i r r o r ; S

,

S2: s e t - s c r e w s f o r t h r e e - p o i n t s u p p o r t o f t h e p l a n e m i r r o r ; 111: l a y e r o f m e t a l a p p l i e d t o

I1

by v a p o u r i z a t i o n i n t h e vacuum; K : p r e s s u r e chamber o f t h e c o n p e n s a t o r . F i g .

4:

A p p a r a t u s

f g r

: r , e a s u r i n g t h e t h i c k n e s s o f t h i n c o a t i n g s u s i n g t h e K o s t e r s t y p e d o u b l e p r i s m . An Osram t h a l l i u m lamp i s n o u n t e d i n f r o n t o f t h e e n t r a n c e s t o p . - .- - - - -

Fig.

5 :

Arrangement f o r f i r s t method

F i g .

6:

P o s i t i o n

of

t h e m i r r o r f o r t h e s e c o n d method