Holography Using a Fundus Camera

R. L. Wiggins, K. D. Vaughan, and G. B. Friedmann

Modifications to a Zeiss fundus camera to enable production of holograms of selected sites of an opticfundus are described. Resolution near the resolution limit of the camera has been obtained in hologramsof the fundi of anaesthetized cats.

Introduction

Two methods have been previously reported, 1 2 forproduction of holograms of the eye. The first, that ofVan Ligten et al. uses a lens to form an image hologramas does our method. To date Van Ligten et al., usingthis method, have published holograms of model eyesonly. The second method, that of Calkins andLeonard, 2 has been used on living eyes. It does notinvolve image holography.

We have devised a method whereby repeatableholograms of selected sites of the fundus of a living eyecan be obtained. In essence, our method employs amodified Zeiss fundus camera to construct the hologram.A feature of our method is the ability to visualize,through this common ophthalmological camera, theretinal area of which the hologram is made. Theoptical modifications of this camera will be describedin this paper.

Modification of the Fundus CameraThe schematic diagram of the fundus camera is

shown in Fig. 1. (Complete details of this camera aregiven by Littman. 3 ) We replaced the usual xenonflash source X and its condensor L (Fig. 1) with 37-mm diam, 200-mm focal length biconvex lensLD (Fig. 2). A hole cut in the camera casing in linewith lens L2 (Fig. 2) allowed a cw argon laser beamto enter the camera optics in the desired direction.

In the unmodified camera (Fig. 1), the white lightflash passes through lens L 4 , which has a small centralstop on its rear surface so that the central hole inmirror 2 is not illuminated: rather, this mirrorreflects an annular light spot into the eye. Theaspheric objective of the camera (L 5 ) forms in the

R. L. Wiggins is with the Ophthalmology Department, St.Josephs Hospital, Victoria, B.C., and the other two authors arewith the Department of Physics, University of Victoria, Victoria,B.C.

Received 29 June 1971.

Laser intensities on the retina are below injury levels.

plane of the patient's pupil an image of the hole in themirror M2. The light returning from the eye thenpasses through the central hole in M 2 to produce thedesired image on the photographic emulsion F. Theplacement of our biconvex lens LD (Fig. 2) was suchthat the light path of the laser beam followed veryclosely the light path of the xenon flash. The camerawas then positioned so that the laser beam was focusedjust to one side of the central stop on lens L4. Thelaser beam then diverged to form a small spot of lightjust at the side of the hole in mirror 112. This allowedthe laser light returning from the eye to trace verynearly the same path as the white light through thesystem L 5, 1112, L 6, C, L 7 (Fig. 2).

The auxilliary viewing light W and its attendantlenses L9 as well as the glass plate P (Fig. 1) wereretained so that visualization of the fundus could beaccomplished through the auxilliary viewing microscopeR, J14, L8, although we found it advantageous to turnthis microscope through 900. The fundus cameracould thus be approached from the side without inter-fering with the various lenses and mirrors required forthe holographic reference beam.

The spring loading on the viewing mirror M 3 (Fig. 1)was removed and replaced with a long handle. Themirror was thus normally out of the way; it could beraised (13, Fig. 2) when the auxilliary microscope wasbeing used.

The camera attachment CA (Fig. 1) was removed,and an aperture was cut in the collar to take a smallplane mirror ME (Fig. 2) set at an angle to the lastsurface of L7. This mirror reflected the reference beamonto the holographic emulsion placed just in front of thefocal plane F of the system.

Hologram Construction GeometryThe reference beam was obtained through a variable

beam splitter (VBS) (Fig. 3) placed some distance fromthe camera and so angled that a single plano-convex

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Fig. 1. Schematic diagram of unmodified Zeiss fundus camera.X = xenon flash bulb, P = glass plate, W = double filamentbulb, C = compensator for astigmatism, R = eyepiece reticule,Ml, M2, M3, M4 = mirrors, Il condensor lens system, L,2 = lensand field diaphragm, L3 to L,8 = camera lens components, F =photographic emulsion plane, CA = camera housing. Raysshown represent light incident on the eye (---) and reflected from

the eye (-).

Fig. 2. Schematic diagram of the modified Zeiss fundus camera.SF = spatial filter, CL = collimating lens, VBS = variable beamsplitter, LA, LB, LD = lenses, ME = mirror, HP = hologramplate. All other components are as in Fig. 1. Rays shownrepresent laser light incident (---) on and reflected (-) from the

eye.

- I I M 14*

lens (L,) collimated it, and a series of four mirrors(MA, BB, 310, MD) produced the desired angle of inci-dence on the mirror MlE. The reference beam just filledthis mirror and produced a spot (1.4 cm in diameter)on the holographic emulsion HP (Fig. 3). Further-more, the optic pathlength of the reference beam frombeam splitter to emulsion varied no more than 1 cm, withthe optic path of the object beam from the beam splitterthrough the camera optics to the eye reflected from theeye and thence to the emulsion.

Construction Procedure

The desired region of the fundus was first visualizedthrough the auxilliary microscope using the auxilliarywhite light viewing mechanism of the camera. Then,using a very low intensity laser object beam into thecamera from the variable beam splitter, this fundusregion was checked safely in laser light. The holo-graphic exposure was then made with a large objectbeam intensity. The beam ratio of the referenceto object beam intensities at the hologram plate variedbetween 2: 1 and 3: 1.

The angle between the reference and object beamat the hologram plate was 110. The argon laserenergy density at the plate was measured as 15 ergs/cm2 .Exposure times of 8 msec were obtained by singlepulsing an acoustic shutter. The emulsion was shieldedagainst the fluorescent glow of the laser between pulses.

Agfa Gevaert Scientia 10E56 plates were used.Development, stop bath, hardening, and fixing wereaccording to Schultze.4 Final running water wash wasat 20'C for 20 min. Plates were dried at room tem-perature. Forced air was not used. Bleaching wasnot performed.

Reconstruction Geometry

During reconstruction without the fundus camera,a collimated reconstruction beam duplicated the con-struction reference beam. A real image of the originalimage produced by the fundus camera was observed.This image could be examined and photographed

Fig. 3. Schematic diagram of the construc-tion reference beam geometry. MA, MB,MC, MD, and ME = reference beam mirrorsfor construction, Lc = lens. All othercomponents are as in Fig. 2. Rays shown

represent construction reference beam.

j= LASER

7 Lc LA

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Fig. 4. Photograph of reconstructed hologram of a cat fundustaken with a 2-W cw argon ion laser.

through a conventional microscope and clearly showedthe characteristic three-dimensional nature of a holo-graphic image. To reduce speckle, we placed a rotatingdiffuser in the plane of the reconstructed image andviewed the image on the diffuser through a microscope.To examine the image throughout its depth with agiven magnification, the microscope and diffuserwere moved so as to maintain constant separationbetween them. Using the diffuser we have beenable to reach very nearly the resolution limit of thefundus camera, some 20 .

Results and DiscussionA photograph of a typical reconstruction from a

hologram of a cat fundus taken with a 2-W cw argon ionlaser is shown in Fig. 4. The maximum power densityat the retina in our experiments was calculated to be 2W/cm 2 on a retinal spot 0.3 cm in diameter. Vassiliadiset al.5 found the threshold for 50% probability ofdamage to monkey retinas with a cw argon laseroperating at 514.5 nm and irradiating small (approxi-mately 50 ,u in diameter) retinal areas to be a total powerof 20 mW entering the eye during 10 msec. Calculationsshow this to be equal to a retinal energy density of103 W/cm 2 . The American Conference of GovernmentIndustrial Hygienists6 (ACGIH) endorses 1 X 10-5W/cm 2 for cw lasers as a safe irradiance at the cornea forthe worst case, i.e., pupil diameter of 7 mm and acollimated beam. Since the lenticular effect of theeye causes the laser power density at the retina to be6 X 105 times that at the cornea, a safe retinal powerdensity would be 6 W/cm2 . This safe level is, in turn,at least two orders of magnitude below the level for50% probability of damage. The retinal powerdensity in our experiment was below this safe level.

Anaesthetized cats were used7 ; nevertheless, wefeel that safe holograms of human fundi can be obtained

using this method, even allowing for the involuntaryintention tremors of unanaesthetized eyes. Worktoward this end is in progress.

Theoretically this method might be improved bythe use of the Toth and Collins8 reversed field techniquefor reconstruction. In this case, during reconstruction,the developed hologram plate would be replaced in itsoriginal position and the conjugate of the originalreference beam plus all the fundus camera optics used.A lens with optical transfer function identical to thatof the patient's lens would be made (Ohzu9 ) and insertedin the plane occupied by the patient's lens during con-struction. The reconstructed image would be free ofany aberrations caused by the patient's lens. Therewould, however, be no canceling of any aberrations inthe image due to the patient's cornea.

We wish to express our indebtedness to HarveyE. Richardson and to Robert E. Brook for help inmodifying the Zeiss fundus camera and to G. Littmann(II) of Carl Zeiss for his most generous response to allour questions concerning the optics of the camera.

This work was supported by Grant MA 2870 of theMedical Research Council of Canada.

References1. R. F. Van Ligten, B. Grolman, and K. Lawton, Am. J. Optom.

Arch. Optom. 43, 351 (1966).

2. J. L. Calkins and C. D. Leonard, Invest. Ophthal. 9, 458(1970).

3. H. Littmann, Die Zeiss Funduskamera Ber. 59. Zusam-menkunft Deutsch. Ophthalmolog. Gesellsch., Heidelberg1955 (Verlag Bergmann, Munchen), p. 318.

4. D. Schultze, Laser Focus 4, 23 (1968).

5. A. Vassiliadis, H. C. Zweng, N. A. Zweng, and R. C. Honey,"Laser Ocular Damage Thresholds," Stanford ResearchInstitute, Menlo Park, California (March 1969).

6. Threshold Limit Values for Physical Agents, AmericanCouncil of Government Industrial Hygienists, Cincinnati(1968), Suppl. 7; Laser Focus, 4, 50 (1968).

7. R. L. Wiggins, K. D. Vaughan, and G. B. Friedmann, Arch.Ophthalmol (in press).

8. L. Toth and S. A. Collins, Appl. Phys. Lett. 13, 7 (1968).

9. H. Ohzu, in Applications of Holography, E. S. Barrekette,W. E. Kock, T. Ose, J. Tsujiuchi, and G. W. Stroke, Eds.(Plenum, New York, 1971), pp. 365-376.

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