OPTICS IN OPHTHALMOLOGY E. S. PERKINS*

Institute of Ophthalmology, University of London.

Optical methods are used in ophlhalmology f o r the diagnosis and treatment o f eye disease and in experimental studies on the eye. The optical principle of opthalmos- copy keratometry , slitlamp microscopy and the use of contact lenses and high inten- sity light sources are described. Examples of optical methods in research are given.

SINCE THE INVENTION of the ophtha lmoscope in the middle of the l a s t cen tu ry , ophthalmology, m o r e than any o ther b r a n c h of med i c ine , has used opt ical me - thods in d i agnos i s , in r e s e a r c h and even in t h e r a p e u t i c s .

DIAGNOSIS

The ba s i c p r i n c i p l e of d i r e c t ophtha lmoscopy is s imp le (Fig. 1); if the eye to be obse rved has no r e - f r ac t ive e r r o r and is focused at in f in i ty , r ays f rom an i l l u m i n a t e d a r e a of the r e t i n a wil l e m e r g e f rom the eye in p a r a l l e l . S i m i l a r l y , p a r a l l e l r ays of l ight wil l be focused on the o b s e r v e r ' s r e t i na if he is a l so a m e t r o p i c i .e . with no r e f r a c t i v e e r r o r , and focused on the d i s t ance . Thus an image of the i l l u m i n a t e d r e t i na is f o r m e d on the o b s e r v e r ' s r e t i na if the v i sua l axes of the two eyes a re co inc ident .

The m a i n p r o b l e m is to i l l u m i n a t e the s u b j e c t ' s fund- us, and th is was o r i g i n a l l y ach ieved by v iewing through a s m a l l hole in a m i r r o r which re f l ec ted l ight f rom a sou rce p laced behind and to one s ide of the pa t ien t . The i n t e r p o s i t i o n i n g of an a r r a y of plus and minus l e n s e s behind the m i r r o r a l lowed for the c o r r e c t i o n of any r e f r a c t i v e e r r o r s in the pa t i en t o r o b s e r v e r .

The ma in i m p r o v e m e n t in m o d e r n i n s t r u m e n t s has been in p e r f e c t i n g the i l l u m i n a t i n g s y s t e m and com- b in ing it with a high i n t ens i t y l ight, such as an e l ec - t r on i c f lash for the p u r p o s e s of photographing the fundus. A g rea t i n c r e a s e in c o n t r a s t and def in i t ion of the r e t i n a l v e s s e l s can be obta ined by in j ec t ing f l u o r e s c e i n into the c i r c u l a t i o n and v iewing or photo- g raph ing the fundus, us ing su i t ab le exc i t ing and b a r - r i e r f i l t e r s . This technique has g rea t ly extended the value of r e t i n a l photography in the study of v a s c u l a r d i sease .

In i n d i r e c t ophtha lmoscopy a convex lens (+ 13 diop- t r e s ) is held in f ron t of the p a t i e n t ' s eye so that the e m e r g e n t r a y s f rom the fundus a re brought to a focus as a r ea l i n v e r t e d image . This a e r i a l image is v iew-

*Professof E x p e r i m e n t a l Optha lmology

Observed eye

,•Mir ro r

IV~,

% e

ht source

Fig. 1. Principle o f direct ophthalmoscopy. Light rays f rom illuminated area of observed fundus emerge as parallel beam and are brought to a focus on observer ' s retina.

ed by the o b s e r v e r , and the g r e a t advantage of the method is that b inocu la r s t e r e o s c o p i c v iewing i s poss ib l e .

The s i m p l e s t method of m e a s u r i n g the r e f r a c t i o n of the eye is to use an ophtha lmoscope type of m i r r o r held at a d i s tance of 1 m e t r e f r o m the eye and to t i l t the m i r r o r s l ight ly while o b s e r v i n g the red r e f l ex in the pa t i e n t ' s pupil . The shadow on the fundus cas t by the edge of the pupil wil l appea r to move a c r o s s the pupil as the m i r r o r is moved, and l e n s e s , e i the r : s phe r i c a l or c y l i n d r i c a l , can be p laced in a t r i a l f r a me in f ron t of the eye unt i l the m o v e m e n t is jus t r e v e r s e d in a l l d i r e c t i o n s . The l ens , or combina t ion of l e n s e s , r e q u i r e d to achieve these condi t ions co r - r e c t s the r e f r a c t i o n of the eye for a d i s t ance of one m e t r e . Sub t r ac t ing 1 d iopt re gives the r e f r a c t i o n for inf in i ty . F i n a l a d j u s t m e n t s a r e made by us ing sub- jec t ive t e s t s . More r e f ined opt ica l methods , such as the F i n c h a m co inc idence o p t o m e t e r , have been de- veloped, but have so fa r fa i led to r ep l ace s imple me- thods in rou t ine c l in ica l use on two grounds , F i r s t l y , these i n s t r u m e n t s a r e expens ive , Secondly, they achieve a degree of a c c u r a c y which is h igher than that r e q u i r e d for rou t ine c l in i ca l work. The r e s u l t s s t i l l have to be checked by sub jec t ive tes t , as the mos t a c c u r a t e c o r r e c t i o n is not a lways the mos t

Optics Technology February 1969 59

comfor t ab l e for the pa t ien t . It i s p robab le that a r e - l a t ive ly i nexpens ive i n s t r u m e n t that was quick and easy to use would become popular .

The m a i n r e f r a c t i v e power of the eye r e s u l t s f rom the c u r v a t u r e of the co rnea . That due to the l ens is only about one- f i f th of the total r e f r a c t i o n (approxi - ma te ly 60 d iop t res ) of the eye. M e a s u r e m e n t of the r a d i u s of c u r v a t u r e of the co rnea , and p a r t i c u l a r l y of i ts v a r i a t i o n in d i f fe ren t m e r i d i a n s , g ives an approx i - mate e s t i m a t i o n of the r e f r a c t i o n and is a l so n e c e s - s a r y for the manu fac tu r e of contact l e n s e s .

K e r a t o m e t e r s employ i l l u m i n a t e d t a r g e t s , the d i s - t ance be tween which can be v a r i e d , with a t e l e scope for v iewing the i m a g e s of the t a r g e t s f o r m e d by the su r f ace of the co rnea . The i n s e r t i o n of an i m a g e - doubl ing device a l lows co inc idence methods to be used to d e t e r m i n e the c o r r e c t end-po in t ; two i m a g e s of the two t a rge t s a re s een behind the co rnea and the t a r g e t s s e p a r a t e d unt i l the two c e n t r a l i m a g e s coin- cide. The i n s t r u m e n t can be c a l i b r a t e d to given r e a d - ings of r ad ius of c u r v a t u r e or equ iva len t diopt ic power.

If a fine s l i t of l ight is focused on the t r a n s p a r e n t media of the eye mos t of the l ight is t r a n s m i t t e d , but s ince the r e f r a c t i v e i nd i ces of the s t r u c t u r e s a re d i f fe ren t , p a r t of the inc iden t l ight is r e f l ec t ed and d i s p e r s e d . The effect i s that of a sec t ion of a so l id object , and such an opt ica l sec t ion of the co rnea and lens can be viewed by a b i n o c u l a r m i c r o s c o p e mounted ho r i zon ta l l y .

The s l i t - l a m p m i c r o s c o p e , f i r s t developed by Gul l - s t r a n d in 1911 and brought to pe r f ec t ion by many subsequen t w o r k e r s , enab le s o b s e r v a t i o n of l iv ing t i s s u e s unde r high magn i f i ca t ion and has helped e n o r m o u s l y in the u n d e r s t a n d i n g of pa thologica l p ro - c e s s e s in d i s e a s e s of the eye. Many r e f i n e m e n t s have been added and s l i t - l a m p photography is becoming a rou t ine p r o c e d u r e .

Special methods have to be used to obta in photo- graphs of a comple te opt ica l sec t ion of the a n t e r i o r s egmen t of the eye if a l l the s t r u c t u r e s a re to be in focus at the f i lm p lane . Th i s can however be ach iev- ed by applying Sche impf lug ' s p r i nc ip l e and t i l t ing e i the r the l ens or f i lm plane (Figs 2 & 3).

In o r d e r that the eye may funct ion as an ef f ic ien t opt ica l i n s t r u m e n t it m u s t be able to r e s i s t d e f o r m a - t ion r e s u l t i n g f rom con t r ac t i on of the e x t r a - o c u l a r m u s c l e s when the eye moves . To achieve th is the eye is pumped up to a p r e s s u r e of be tween 10 and 20 t o r r by the cont inuous fo rma t ion and d ra inage of i n t r a - ocu l a r fluid. In the group of d i s e a s e s ca l led g lau- coma, d r a inage of f luid f rom the eye b e c o m e s ob- s t r u c t e d and the p r e s s u r e in the eye r i s e s enough to i n t e r f e r e with the en t ry of blood with consequent v i sua l f a i lu re . It i s i m p o r t a n t to be able to m e a s u r e the i n t r a - o c u l a r p r e s s u r e , and two methods have been used. In one, inden ta t ion t o n o m e t r y , the amoun t of inden ta t ion which can be p roduced by a known weight, is m e a s u r e d . Unfor tuna te ly the app l ica t ion of th is type of t o n o m e t e r r a i s e s the i n t r a - o c u l a r p r e s s u r e and the t o n o m e t e r can only m e a s u r e th is r a i s e d p r e s s u r e .

A much m o r e e legan t method is that of applana t ion t o n o m e t r y , in which a s m a l l s e g m e n t of the co rnea is f la t tened . The amount of f l a t t en ing is so s m a l l that

Fig. 2. Photograph of an optical sect ion through anterior segment of an eye (by courtesy of N. A. P. Brown, Moorfields Eye Hospital).

x I / , ," I mQ9 e pl, a n e ~'\ I /

\ I / , ,

\ I / / \ I /

Object prone

Fig. 3. Scheinpflug's principle. When the object plane, the lens plane and the image plane con- verge on one point, all points on the object will be brought to a focus in the plane of the image.

the r i s e in p r e s s u r e produced on applying the tono- m e t e r can be ignored . The a r e a of f l a t t en ing is p ro - por t iona l to the force appl ied and to the i n t r a o c u l a r p r e s s u r e (Fig. 4). The only p r o b l e m is to m e a s u r e the a r e a (or d i a m e t e r ) of the f la t tened a r e a with suf- f ic ien t a c c u r a c y . Hans Goldmann , of B e r n e , who has been r e s p o n s i b l e for many of the s u p e r b i n s t r u m e n t s ava i lab le to m o d e r n oph tha lmolog i s t s , dev i sed an in - genious opt ica l so lut ion; it cons i s t s of a p a i r of p r i s m s , with the i r b a s e s in opposi te d i r e c t i o n s , which a re appl ied to the co rnea .

The f ield of view seen under magn i f i ca t i on with a s l i t - l a m p m i c r o s c o p e is thus sp l i t into two ha lves , and if the edge of the c i r c l e of contact i s out l ined with f l u o r e s c e i n the force with which the p r i s m is applied, can be v a r i e d unt i l the i n n e r edges of the two fluo- r e s c e i n s e m i - c i r c l e s a re in l ine . This wil l occur when the c i r c l e of app lana t ion is 3 . 0 6 m m in d i a m e t e r

60 Optics Technology February 1969

and under these c i r c u m s t a n c e s the i n t r a - o c u l a r p r e s - su re i n , t o r r wil l be equal to the force appl ied in g r a m s × 10.

THERAPEUTICS The t he r apeu t i c app l i ca t ions of opt ics inc lude l e n s e s to c o r r e c t r e f r a c t i v e e r r o r s , spec ia l l ens s y s t e m s for the p a r t i a l l y s ighted and the use of powerfu l l ight s o u r c e s to c r e a t e con t ro l l ed b u r n s for welding b r e a k s in the r e t i na . Spectac le l e n s e s a r e so f a m i l i a r as to r e q u i r e l i t t l e f u r t h e r d e s c r i p t i o n , except to note that g r e a t e r use now is made of m u l t i - f o c a l l e n s e s , e i t he r as b i - or t r i - f o c a l s or with a con t inuous ly v a r i a b l e power .

Contact l e n s e s ac t by abo l i sh ing the r e f r a c t i v e power of the a n t e r i o r su r f ace of the c o r n e a and r ep l ac ing this with a new c u r v a t u r e des igned to give the c o r r e c t power . An i r r e g u l a r co rnea l su r f ace p roduces com- plex r e f r a c t i v e effects which cannot be c o r r e c t e d by spec t ac l e s but which a re cance l l ed by contac t l e n s e s . As a subs t i t u t e for s p e c t a c l e s in c a se s of s imp le r e - f r ac t ive e r r o r , contact l e n s e s a r e , at the p r e s e n t t ime , only jus t i f i ed on occupat iona l o r c o s m e t i c grounds .

Other methods of c o r r e c t i n g r e f r a c t i o n at p r e s e n t under i nves t i ga t i on a r e the i m p l a n t a t i o n of a c r y l i c l e n s e s into the subs t ance of the co rnea and the g r ind - ing of the f rozen co rnea into a new shape.

Unfo r tuna te ly some ocu la r d i s e a s e s r e s u l t i n g in r e - duced v i s u a l acui ty cannot be ef fec t ive ly t r e a t e d by p r e s e n t med i ca l or s u r g i c a l methods , but if the ocu l a r media a r e c l e a r c o n s i d e r a b l e help can be given to such pa t i en t s by s o - c a l l e d low v i s ion a ids . These may be s imp le plus l e n s e s or , m o r e commonly , com- pound l e n s e s p roduc ing a t e l e scop ic effect . The ap- p l i ances a r e m o s t usefu l for n e a r v i s ion as the f ield of v i s ion is l im i t ed by the l ens s y s t e m .

If a p e r s o n with a n o r m a l eye looks at the sun it has been ca l cu la t ed that some 113 × 106erg s e c / c m 2 of r a d i a n t ene rgy wil l fal l on a s m a l l a r e a of the r e t i na . Not s u r p r i s i n g l y , th is will cause a s e v e r e l e s ion in a Very sho r t t ime . The damag ing effects of such so l a r b u r n s a r e wel l known but it was not unt i l 1949 that M e y e r - S c h w i c k e r a t h r e c o g n i s e d the t he rapeu t i c pos- s i b i l i t i e s of us ing f i r s t sunl ight and then i n t ense a r t i f i c i a l l ight s o u r c e s in the t r e a t m e n t of r e t i na l d e t a c h m e n t (Meye r -Schwicke ra th , 1949, 1954).

In this condi t ion a t e a r develops in the r e t i n a a l lowing fluid to c r eep benea th it and detach i t f rom the u n d e r - ly ing choroid which n o u r i s h e s the ou te r l a y e r s . Suc- cess fu l t r e a t m e n t r e l i e s on sea l ing the hole in the r e t i n a and p rov ided that th is a r e a of the r e t i n a is c lose to the choroid an in t ense beam of l ight d i r e c t e d through the pupil c r e a t e s suf f ic ien t i n f l a m m a t o r y r e - ac t ion in the r e t i n a and choroid to p romo te adhes ion of the m a r g i n s of the hole to the choroid .

A ruby l a s e r can be used for the s a m e purpose and can be i n c o r p o r a t e d into a hand -he ld ophtha lmoscope , giving g r e a t e r f r e edom of man ipu la t ion than that of the convent iona l l ight coagula to r . The use of a ruby l a s e r for ope ra t i ons on the i r i s i s be ing eva lua ted on e x p e r i m e n t a l a n i m a l s at the m o m e n t (Hal lman et a l . , 1968; H a l l m a n et a l . , in p r e s s ) .

/ IW

(Area = A)

, ,!/ Fig. 4. Principle of applanation tonometry . When

f la t surface is p r e s s e d with f o r c e W against spherical container having internal p re s su re P, equilibrium is attained when PA = W, where A is the area o f f lat tening.

RESEARCH

The app l i ca t ions of opt ical methods to r e s e a r c h in ophthalmology cover such a wide f ie ld that i t i s only poss ib l e to men t ion a few example s f rom r e s e a r c h with which I am p e r s o n a l l y f a m i l i a r .

One type of g l aucoma occur s only in eyes with a p a r t i c u l a r a n a t o m i c a l conf igura t ion of the a n t e r i o r s egmen t : the d i s t ance be tween the c o r n e a and the i r i s i s s m a l l e r than n o r m a l and the vo lume of f luid in the space bounded a n t e r i o r l y by the co rnea and p o s t e r i o r - ly by the i r i s and l ens (the a n t e r i o r c h a m b e r ) is a l so reduced. The depth of the a n t e r i o r c h a m b e r can be m e a s u r e d by v iewing a s l i t - l a m p sec t ion with a m i c r o s c o p e and us ing an opt ica l doubl ing device to d i sp lace on half of the image unt i l the p o s t e r i o r s u r - face of the c o r n e a is a l igned with the a n t e r i o r s u r - face of the l ens . The volume of the a n t e r i o r c h a m b e r is m o r e di f f icul t to e s t i m a t e , b u t a r e l a t i v e l y s imp le method, us ing a photograph of an opt ica l sec t ion , has been dev i sed by Jones and M a u r i c e , (1949).

In o r d e r to m e a s u r e the r e s i s t a n c e to outflow of f luid f rom the eye in e x p e r i m e n t a l a n i m a l s i t i s n e c e s s a r y to pe r fuse the eye at p r e s s u r e s s l igh t ly above the n o r m a l i n t r a - o c u l a r p r e s s u r e and to m e a s u r e the r a t e of flow of fluid into the eye. The r a t e s of flow a r e of the o r d e r of a few m i c r o l i t r e s pe r minu te , and one so lu t ion of the p r o b l e m has been to coil thin ny lon tubing on the su r f a c e of an i l l u m i n a t e d photocel l and f i l l the tubing with the p e r f u s i o n f luid s e p a r a t e d by a bubble f r o m a so lu t ion of a d a r k dye. As the p e r - fus ion s t a r t s , the co lumn of dye advances in the coil and cas t s a shadow on the photocel l , the drop in out- put of which can be ampl i f i ed and r e c o r d e d con t inuous ly , F ig . 5 (Edwards et. a l . , 1967).

E x t r a c t s of p igmen t s f rom the v i sua l ce l l s have been s tudied in vi lro for many y e a r s , b u t r e c e n t l y re f ined methods of r e f l e c t o m e t r y have enab led the s p e c t r a l ab so rp t i on c h a r a c t e r i s t i c s of these p igmen t s to be m e a s u r e d in the l iv ing eye. A s m a l l a r e a of r e t ina is s canned rap id ly with l ight of d i f f e ren t wavelengths and the amoun t of l ight of each wavelength r e f l ec t ed m e a s u r e d us ing an oph tha lmoscopic opt ica l s y s t e m and a pho tomul t ip l i e r (Weale & Rushton, 1956). A s i m i l a r s y s t e m can be used to de tec t changes in the

Optics Technology February 1969 61

Evans blue reservoir

Cal ibrat ion reservoi r

A i r bubble

Pressure J ~ Pressure t ransducer t ransducer

Oclip

Condenser sorbing l e n s ~ / / ~ ~ ~ s s f i l t e rs

EA.V'~a n~" ~ "'J Perspex

/ 'li' cllibro.tion Nylon tube coil 'i'To eye mask

Fig. 5. (a)

(b)

Apparatus for recording inlra-ocular pressure and rate of perfusion o f fluid into an eye.

Details of f lowmeter showing coiled nylon tubing, calibration mask and photocell.

amount and oxygenation of the blood in the choroid, Fig. 6 (Gloster , 1967).

TEACHING

As in other branches of medic ine ,visual aids are widely used in routine teaching but ophthalmic sur- gery posed special problems. The operation site is so small that only the surgeon and one ass is tant can real ly see the details of surgery , par t icu lar ly now that operating microscopes are becoming more wide- ly used. Closed-c i rcui t television has proved of great value in this respec t and the new operat ing thea t res at Moorfields Eye Hospital are being provided with a comprehensive television system, including video- recording faci l i t ies , which wilt be invaluable for teaching the minutiae of surgical techniqde to postgraduate students.

REFERENCES

Edwards, J . , Hallman, V. L. , & Perkins , E. S. (1967), Exp. Eye Res . ,6 ,316 .

Gloster , J . (1967), Exp. Eye Res. , 6,187-212.

Hallman, V. L., Perk ins , E. S., Watts, G. K., & Wheeler , C. B. (1968), Exp. Eye Res. , 7,481.

Hallman, V. L., Perk ins , E. S., Watts, G. K., & Wheeler , C. B. (In press ) .

Jones, R. F. , and Maurice, D. M. (1963), Exp. Eye Res. , 2,233.

Meyer-Schwickerath (1949), Ber . dtsch, ophthal. Ges. 55,256.

Meyer-Schwickerath (1954), van Grafes Arch. Ophthal., 156,2.

Weale, R. A., and Rushton, W. A. H. (1956), J . Physiol. , 134, 11.

Weale, R. A., and Rushton, W. A. H. (1961 ), Vision Re s., 1,354.

/ 131 \ C h--'-~ ch°nnel /, Yk , , r=,Oe, I ^ / 1 . . ~ 1 II . . . . . UHighspeed Meters

CRmm-utator(~ / _ ,~ (~)-----I Eht suppl I C p ~ ) - - - - - I Eht supply I n,~h t : ~ k n I "Zn:~r l l~7 / 11Phoiomutiipile,. ~ vv . . . . . . . . . . . . v-# L, rhannet ..tl n .ll.-llll_.l i lamp ~ '~E'e amplif iers, ~

- - i' rect i f iers and 'backing'

Fig. 6. Apparatus for fandus reflectometry. Light of different wavelengths is projected onto funclus and intensity of reflected light measured with a photomultiplier.

62 Optics Technology February 1969