An inexpensive schlieren apparatus

An

inexpensive

schlieren

apparatus

N. R. Bronson*

B e c a u s e of the usual s i z e and c o s t of a s c h l i e r e n a p p a r a t u s , few m e d i c a l i n v e s t i g a t o r s a r e p r e s e n t l y able to employ this valuable method of d e t e r m i n i n g the b e a m p a t t e r n of diagnostic u l t r a s o n i c equipment. D e s c r i b e d h e r e a r e the c o n s t r u c t i o n

d e t a i l s of an i n e x p e n s i v e yet highly s e n s i t i v e s c h i i e r e n s y s t e m suitable for u s e in m e d i c a l u l t r a s o n i c r e s e a r c h . The light path i s c o n s t r u c t e d so that the beam p a t t e r n of the t r a n s d u c e r being t e s t e d i s displayed in actual size.

The diagnostic applications of u l t r a s o u n d a r e being actively studied by many i n v e s t i g a t o r s . In evaluating the p o t e n t i a l i t i e s and l i m i t a t i o n s of a new diagnostic p r o c e d u r e , it i s v e r y valuable for the i n v e s t i g a t o r to know the c h a r a c t e r i s t i c s of the sound b e a m e m i t t e d by the p a r t i c u l a r t r a n s d u c e r s he i s using. The p e r f o r m a n c e of p u l s e echo equipment depends d i r e c t l y on the shape and d i a m e t e r of the b e a m at v a r i o u s depths and on the p r e s e n c e o r a b s e n c e of side lobes. In Doppler s y s t e m s , the angle between the t r a n s m i t t e d b e a m and the axis of the r e c e i v i n g c r y s t a l d e t e r m i n e s the t i s s u e depth at which m a x i m u m s e n s i t i v i t y will o c c u r . D i f f e r e n c e s in b e a m p a t t e r n s among t r a n s d u c e r s can effect the r e p r o d u cibility of findings and i s One of the v a r i a b l e s that should be checked w h e n e v e r i n v e s t i g a t o r s c o m e to conflicting conclus i o n s a f t e r p e r f o r m i n g s e e m i n g l y identical p r o c e d u r e s .

p a s s a g e through the s c h l i e r e n field and d i r e c t s it to a knife edge placed at the focal point of the light b e a m . Additional m i r r o r s t r a n s m i t the i m a g e to a ground g l a s s located at a d i s t a n c e calculated so that the s c h l i e r e n field i s shown 1 : 1.

The s c h l i e r e n technique, o r i g i n a l l y d e s c r i b e d by T o e p l e r a s e a r l y as 1867,1 is an optical method of illuminating s m a l l g r a d i e n t s in the r e f r a c t i v e index of a medium. Since u l t r a sound c a u s e s slight a l t e r a t i o n s in the r e f r a c t i v e index of water, the s c h l i e r e n m e t h o d i s applicable to u l t r a s o n i c s . This technique i s a highly effective method of v i s u a l i z i n g and thus analyzing u l t r a s o n i c b e a m p a t t e r n s . At p r e s e n t , however, v e r y few m e d i c a l i n v e s t i g a t o r s have a c c e s s to such equipment b e c a u s e of i t s s i z e and cost. This a r t i c l e d e s c r i b e s a compact, e a s i l y built s c h i i e r e n a p p a r a t u s with e x c e l lent s e n s i t i v i t y and r e s o l u t i o n that can be c o n s t r u c t e d u s i n g p a r t s costing l e s s than sixty d o l l a r s .

The e n t i r e s y s t e m i s housed in a box 72in x 18in x 6in. Even though it i s p o s s i b l e to m e r e l y a r r a n g e the components on a bench top, it i s much m o r e convenient to house them in a l i g h t - p r o o f box so that t e s t s can be p e r f o r m e d u s i n g s u b dued r o o m lighting r a t h e r than in total d a r k n e s s . The c a s e was m a d e of 3/4in p a r t i c l e b o a r d which i s e a s i l y w o r k e d and will not warp. The inside was painted flat black to e l i m i n a t e s p u r i o u s r e f l e c t i o n s . To f u r t h e r r e d u c e s t r a y light, the light s o u r c e was i s o l a t e d in a c o m p a r t m e n t . CONSTRUCTION DETAILS Light s c a r c e The s o u r c e of illumination for the s y s t e m c o n s i s t s of a 45W q u a r t z iodine lamp (Q6.6A/T21/2/CL). The f i l a m e n t i s

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Knife edge

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BASIC DESIGN ELEMENTS The e s s e n t i a l e l e m e n t s of t h i s s c h l i e r e n s y s t e m a r e shown in Fig la. An i n t e n s e beam of light is f o c u s s e d through a lens o r a concave m i r r o r onto a knife edge. So long a s the beam is undisturbed, all the light i m p i n g e s on the knife below the edge and i s p r e v e n t e d f r o m t r a v e l l i n g f u r t h e r . When an u l t r a s o n i c b e a m i s d i r e c t e d a c r o s s the light path in the r e f r a c ting medium, any light that is f u r t h e r r e f r a c t e d by the u l t r a sonic e n e r g y will p a s s above the knife edge and be p r o j e c t e d onto the ground g l a s s a s an i m a g e plane w h e r e it may b e o b s e r v e d and photographed. The light path of the i n e x p e n s i v e s y s t e m p r e s e n t e d h e r e is shown d i a g r a m m a t i c a l l y in Fig lb. Fig 2 is an o v e r h e a d photograph depicting the a r r a n g e m e n t of c o m p o n e n t s . The light s o u r c e , a q u a r t z iodine lamp, is f o c u s s e d on the pin hole. The light b e a m i l l u m i n a t e s a 3-inch t e l e s c o p i c m i r r o r d i r e c t l y behind the s c h l i e r e n field (Fig 3). The b e a m r e f l e c t e d f r o m the t e l e s c o p e m i r r o r is a l m o s t p a r a l l e l to the incident b e a m . This a r r a n g e m e n t c a u s e s both b e a m s to p a s s t h r o u g h the s c h l i e r e n field, thus doubling the s e n s i t i v i t y of the s y s t e m . Continuous input power a s low a s 20roW h a s been r e a d i l y s e e n with t h i s s y s t e m . A plane m i r r o r r e c e i v e s the r e f l e c t e d b e a m a f t e r its second

* 186 Old Town Road, Southampton, New York 11968, USA

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Concave mirror

Fig 1 Schlieren systems (a) E s s e n t i a l e l e m e n t s of the s c h l i e r e n s y s t e m . The u l t r a s o u n d t r a n s d u c e r i s i m m e r s e d in the r e f l e c t i n g m e d i u m behind the s c h l i e r e n l e n s and the sound b e a m t r a v e l s t r a n s v e r s e l y through the light b e a m within the s c h l i e r e n field (b) D i a g r a m m a t i c r e p r e s e n t a t i o n of the a u t h o r ' s s c h U e r e n s y s t e m . The concave m i r r o r r e f l e c t s the t r a n s m i t t e d light in a path n e a r l y p a r a l l e l to the incident b e a m , c r e a t i n g a double p a s s through the s c h l i e r e n field which doubles the s e n s i t i v i t y of the s y s t e m . The length of the light path, g o v e r n e d by the p l a c e m e n t of the m i r r o r s , r e s u l t s in a s a m e - s i z e i m a g e on the ground glass ULTRASONICS J a n u a r y 1969

67

Fig 2 View of the s c h l i e r e n s y s t e m looking t o w a r d s the light s o u r c e . The w a t e r tank is in the f o r e g r o u n d . M i r r o r M1 is the b r i g h t l y i l l u m i n a t e d m i r r o r i m m e d i a t e l y left of the pinhole. The light path then extends a c r o s s the knife edge and is f u r t h e r d i r e c t e d by t h r e e plane m i r r o r s to the ground g l a s s , which l i e s under the c a m e r a mounting r i n g at left f o r e g r o u n d

Fig 4 Typical b e a m p a t t e r n f r o m an opthalmologic t r a n s d u c e r (T-31). Scale is shown at lower r i g h t . The faint p a t t e r n above the s c a l e indication is c a u s e d by r e f l e c tion f r o m the bottom of the tank

f o c u s s e d by two condensing l e n s e s , 35mm in d i a m e t e r and having a focal length of 36mm, mounted in line with a pinhole of v a r i a b l e d i a m e t e r . During use, the s i z e of the a p e r t u r e is a d j u s t e d f o r the b e s t c o m p r o m i s e between illumination and r e s o l u t i o n . Our original light s o u r c e was a 2W z i r c o n i u m a r c l a m p which gave good r e s o l u t i o n but which could not p r o v i d e illumination as i n t e n s e or as constant a s we l a t e r a c h i e v e d with the q u a r t z iodine lamp. ( l m m = 0. 039in) Mirrors

As our p r i n c i p a l intended u s e of the s c h l i e r e n s y s t e m was to e x a m i n e the b e a m p a t t e r n s of the s h o r t - r a n g e t r a n s d u c e r s u s e d in ophthalmology, a 3in t e l e s c o p e m i r r o r was used. The m i r r o r is mounted on a s t a n d a r d f r a m e and has t r i a n g u lating s c r e w s f o r c o n v e n i e n c e in making final a d j u s t m e n t s . A 41~in m i r r o r i s available, at s o m e w h a t g r e a t e r c o s t , but u s e of a l a r g e r m i r r o r e n t a i l s a l o n g e r light path. Since l a r g e m i r r o r s can be v e r y e x p e n s i v e , for a p p l i c a t i o n s involving the t e s t i n g of t r a n s d u c e r s with l o n g e r b e a m p a t t e r n s , such a s the l o w - f r e q u e n c y t r a n s d u c e r s u s e d in e c h o - e n c e phalography, u l t r a s o u n d c a r d i o g r a p h y and o b s t e t r i c a l e x a m i n a t i o n s , it is s u g g e s t e d that the s a m e s y s t e m could be used but with a l a r g e r tank; the b e a m p a t t e r n could then be o b s e r v e d in two o r m o r e s e g m e n t s . Any m i r r o r of good quality can be u s e d so long a s it is s p h e r i c a l r a t h e r than p a r a b o l i c . Fig 3 View of the s c h l i e r e n s y s t e m looking t o w a r d s the w a t e r tank. F o r d e m o n s t r a t i o n p u r p o s e s , the w a t e r tank is p a r t i a l l y filled. The concave m i r r o r , which r e f l e c t s the light b e a m so that it m a k e s a s e c o n d p a s s through the s c h l i e r e n field, can be s e e n behind the w a t e r tank (round white disc). The knife edge with adjusting cont r o l s is s e e n in the f o r e g r o u n d 68

ULTRASONICS J a n u a r y 1969

Tank Any optical g r a d e of plate g l a s s can be u s e d to make the tank, but it should be thick enough so that its shape i s not d i s t o r t e d by the weight of the w a t e r . We found Silastic adh e s i v e m o s t s a t i s f a c t o r y for s e a l i n g the tank and reducing the chance of leakage. A m e t a l or g l a s s bottom may give

Fig 6 B e a m p a t t e r n s obtained with the s a m e T-31 t r a n s d u c e r at t h r e e d i f f e r e n t u l t r a s o n i c p o w e r l e v e l s . The patternE r e v e a l a s t r o n g i n n e r b e a m and w e a k e r side l o b e s . The tip of the t r a n s d u c e r is p h o t o g r a p h i c a l l y s u p e r i m p o s e d on the film with the s c h l i e r e n image

Fig 5 Beam pattern from a focussed transducer otherwise s i m i l a r in c o n s t r u c t i o n to the T-31 t r a n s d u c e r yielding the p a t t e r n shown in F i g 4

annoying r e f l e c t i o n s ; t h i s p r o b l e m can be avoided by using p a r t i c l e b o a r d o r o t h e r a b s o r b e r s in building the bottom of the tank. Plane mirrors Each of the plane m i r r o r s was glued into a dado cut f r o m a piece of p a r t i c l e b o a r d . T h i s method of mounting o f f e r s the advantage that the plane m i r r o r s can be moved during final a d j u s t m e n t b e f o r e being p e r m a n e n t l y s c r e w e d into the c a s e . Shims may be u s e d for m i n o r a d j u s t m e n t s , and the s i d e s and back of the m i r r o r s should be painted flat black to e l i m i n a t e reflections. Knife

edge

It w a s found that a blued r a z o r blade would s e r v e a d m i r a b l y as a knife edge. It is mounted on the m e c h a n i c a l stage of a m i c r o s c o p e to allow fine a d j u s t m e n t s in height and focus. The g r e a t e s t s e n s i t i v i t y i s a c h i e v e d when the m a i n beam i s just cut off by the knife edge. Baffles In c o n s t r u c t i n g such a s y s t e m , one o r m o r e b a f f l e s may be helpful. The need for b a f f l e s depends on w h e r e the m i r r o r s a r e placed. The b a f f l e s a r e mounted in a m a n n e r s i m i l a r to that u s e d for the plane m i r r o r s and a r e also painted flat black.

Fig 7 B e a m p a t t e r n obtained with an U l t r a s o n o l u x t r a n s d u c e r . Splitting of the b e a m i s c l e a r l y shown. Again the t r a n s d u c e r i s shown on the s a m e f i l m for identification p u r poses

Ground g l a s s

The ground g l a s s should be located n e a r the g l a s s tank and within r e a c h of the knife edge a d j u s t m e n t s .

It w a s found that the s c h l i e r e n i m a g e could be o b s e r v e d m o r e c o m f o r t a b l y and photographed m o r e conveniently if the ground g l a s s w e r e horizontally placed. The i m a g e was f o c u s s e d on the ground g l a s s by m e a n s of a 45 ° plane m i r r o r .

Any s o u r c e of continuous-wave power at the a p p r o p r i a t e f r e quency i s s a t i s f a c t o r y to d r i v e the t r a n s d u c e r . We h a v e u s e d

T r a n s d u c e r p o w e r smtrce

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a pulse g e n e r a t o r , but o r d i n a r y radio t r a n s m i t t e r s have been e m p l o y e d s u c c e s s f u l l y in s i m i l a r s y s t e m s . We obtained a b r i l l i a n t i m a g e using a p o w e r level of about 10W cw without finding it n e c e s s a r y to d a r k e n the r o o m .

Variations A c o n s i d e r a b l e n u m b e r of v a r i a t i o n s f r o m the s p e c i f i c d e s i g n d e s c r i b e d h e r e a r e p o s s i b l e . A pulsed light s o u r c e may be used. Two t e l e s c o p e m i r r o r s may be i n s t a l l e d to c r e a t e a longer s c h l i e r e n field. It is s u g g e s t e d that the c o m plete s y s t e m be aligned on a bench top b e f o r e it i s i n s t a l l e d in the c a s e so that the i n v e s t i g a t o r has an opportunity to o p t i m i z e the c a p a b i l i t i e s of the a p p a r a t u s f o r his p a r t i c u l a r n e e d s while all the c o m p o n e n t s a r e still a c c e s s i b l e .

RESULTS S c h l i e r e n p h o t o g r a p h s of v a r i o u s t r a n s d u c e r s f r o m two m a n u f a c t u r e r s a r e shown in F i g s 4-7. F i g u r e 6 also d e m o n s t r a t e s the effect of i n c r e a s i n g the s e n s i t i v i t y setting of the transducer power source. It is i m p o r t a n t to r e a l i z e that any s c h l i e r e n photograph shows the e n t i r e beam longitudinally, r a t h e r than a c r o s s section. A careful check for side lobes o r splitting of the beam, t h e r e f o r e , r e q u i r e s that the t r a n s d u c e r be r o t a t e d through s e v e r a l p o s i t i o n s in its h o l d e r and the b e a m p a t t e r n o b s e r v e d at each position. It should also be r e a l i z e d that s c h l i e r e n photography is not a s a c c u r a t e as a b e a m plot s i n c e the s y s t e m p e r m i t s i n s p e c t i o n only of the t r a n s m i t t e d

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b e a m and does not p r o v i d e i n f o r m a t i o n on the behaviour of sound r e t u r n i n g to the c r y s t a l f r o m a r e f l e c t o r . However, o f f - s e t t i n g t h e s e l i m i t a t i o n s is the i m p o r t a n t fact that the s c h l i e r e n method is an e a s y tool to use and it does provide convenient and s e n s i t i v e i n f o r m a t i o n on u l t r a s o n i c b e a m patterns. EQUIPMENT The optical and mechanical components for this system were obtained from the Edmund Scientific Corporation, Barrington, New Jersey. The to~al cost of parts was slightly less than sixty dollars. An excellent description of schlieren systems with practical points on construction is available from Eastman Kodak. 2 The transducer in Fig 4 is No T-31 from Smith Kline Instrument Company. In Fig 7 an Ultrasonolux transducer produced by Kretztechnik, Austria is used. ACKNOWLEDGE MENTS The author g r e a t l y a p p r e c i a t e s the help of Dr. John M. Reid and Mr. Donald J. Hart in the d e s i g n of the equipment. REFERENCES 1 T o e p l e r , A. M., Annalen d e r Physik, No 131, (1867) p 180 2

' S c h l i e r e n photography' E a s t m a n Kodak P a m p h l e t No P-11