Vol. 57

JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME 57, NUMBER 6 JUNE 1967

Closed-Circuit Television PupillometerDANIEL G. GREEN AND F RODE MAASEIDVAAG

Department of Ophthalmology, University of Michigan, Ann Arbor, Michigan 48104(Received 18 January 1967)

This paper describes a new digital pupillometer based on an infrared closed-circuit television system. Theinstrument displays the instantaneous pupil diameter in digital and/or analog mode. The sensitivity isbetter than 0.02 mm of full scale and linearity is better than 1%. Its frequency response is flat to 15 cps witha rise time less than 25 msec.INDEX HEADING: Vision.

TO aid in the study of the pupillary light reflexesTof the eye, a number of instruments which auto-matically register the size of the pupil have been de-veloped. In general, these instruments have used infra-red so that measurements could be made undetected bythe eye under examination. Infrared is particularlysuitable since melanin, the brown pigment of the iris,has relatively high reflectance in the infrared part ofthe spectrum. It is, in fact, possible simply to monitorthe total amount of infrared reflected from the eye andto obtain a signal which varies approximately as thearea of the pupil.1-3 The shortcoming of this scheme isthat, besides being nonlinear, to obtain the absolutepupil size the output must be calibrated on each indi-vidual eye by taking infrared photographs. In addition,the output is particularly sensitive to changes in theintensity of the source or the sensitivity of the monitor-ing photocell and to eye and head movements.

A number of scanning pupillometers which overcomemost of these difficulties have been reported in theliterature.4 -7 In those instruments, a small spot ofinfrared is repeatedly swept across the eye. When thespot falls on the iris or sclera, a measurable flux isreflected. Very little is reflected when the spot entersthe pupil. The output of the photocell monitoring thereflected flux is a series of negative-going pulses whosewidth is proportional to the width of the pupil along the

lL. Stark, F. W. Campbell, and J. Atwood, Nature 182, 857(1958).

2 L. Stark, Proc. IRE 47, 1925 (1959).3B. L. Zuber and D. Miller, Vision Res. 5, 695 (1965).0. Loewenstein and I. E. Loewenfeld, Arch. Ophthalmol. 59,

352 (1958).6 G. W. King, Proc. Natl. Elect. Conf. 16, 672 (1960).

V W. B. Clark, C. Nelson, and H. A. Knoll, Arch. Ophthalmol.76, 355 (1966).

7 A. Troestra, J. Boogaard, and H. van Doorne, Med. Res.Eng. 5, 3 (1966).

line of scan. Information about the size of the pupilcan be electronically extracted in a number of waysfrom this train of pulses.

This paper describes the use of a closed-circuit tele-vision as a digital pupillometer. Recent technical ad-vances which have led to the development of infrared-sensitive television-camera tubes have made this devicefeasible.

All the components used in the pupillometer are com-mercially available; and, with the possible exception ofthe special infrared vidicon tube, the instrument canbe constructed with components which are practicallystandard pieces of laboratory apparatus.

METHODS

A diagrammatic representation of the instrument isshown in Fig. 1. A photograph of the actual apparatusis shown in Fig. 2. The eye is illuminated withtungsten source S in whose beam an infrared trans-mitting filter F has been interposed. An infrared sheet

p-F

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VIDO SIGNAL |electronic |PUPILEye p) o Ves IR VnICOg reading

>/ B L Pz

C ElTELEV[SION

MONITOR

FIG. 1. Diagrammatic representation of the instrument. Notethat the placement of the optical components is not to scale.

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CLOSED-CIRCUIT TELEVISION PUPILLOMETER

polarizer PI is placed in front of the light source. Asecond sheet polarizer F2, with its axis crossed withrespect to the first, is placed before the lens system.These two polarizers serve to remove the flux directlyreflected from the cornea. The lens system focuses aninfrared image of the pupil and the iris onto the phot-sensitive surface of the television camera tube (Hama-matsu, type N156). This image is scanned by a beam ofelectrons. As the scanning spot moves across the imageon the sensitive surface, it generates a signal which isproportional to the irradiance of each point. This signal,the video output, is displayed on a television monitor.Figure 3 shows a photograph of the monitor display.The reflectance of the iris relative to that of the pupilis evident. The television picture on the monitor iscomposed of 525 horizontal lines. It takes 1/30 of asecond for the electron beam in the picture tube toscan a single 525-line frame of the picture. The size ofthe pupil can be automatically determined by processingthe video signal using the method suggested by Schou-ten, 8 i.e., count the number of lines in the televisiondisplay which cross the pupil. This count is a numberproportional to the diameter of the pupil measured inthe vertical meridian.

The signal flow and electronic data processing isshown in Fig. 4. The camera outputs are a compositevideo signal and separate horizontal and vertical syn-chronization pulses. The composite video-output signalconsists of the signal representing the image on thevidicon and the synchronization pulses. The dark (non-reflecting) pupil appears in the video as negative-goingpulses between the synchronization pulses. All thenegative-going pulses appearing in the composite videoare detected by a Schmitt trigger and fed to an ANDgate. The horizontal synchronization pulses from thecamera drive a flip-flop through a capacitor-diode net-work and cause the flip-flop to trigger on the laggingedge of the horizontal pulse. The flip-flop output is

FIG. 2. Photograph of the apparatus.

8 H. Bouma, thesis, Eindhoven, Holland (1964).

FIG. 3. A photograph of the pupil as seen on the monitor display.

also fed to the AND gate. In this manner, the AND gateeliminates the synchronization pulses and yields a pulseoutput only when the scan crosses the pupil. The re-sulting pulse is then shaped and resets the flip-flop sothat only one count per horizontal line can be registeredin the 12-bit BCD counter. Thus, for each pictureframe a count proportional to the number of linescrossing the pupil is registered in the counter. The out-put from the shaper can be used to intensity modulatethe TV monitor display. When the Schmitt-trigger levelis set to detect all scans crossing the pupil, these pulsesform a bright semicircular ring around the left half ofthe pupil edge on the monitor. This feature is not usedduring an actual experiment.

After each frame, the vertical synchronization pulsedirects the stored information in the BCD counter to a12-bit STORE register and resets the BCD counter. Anew frame can now be registered in the BCD counterwhile the digital information in the STORE register canbe fed to a digital output device or converted to ananalog output via a digital-to-analog converter.

RESULTS

Figure 5 summarizes the results of a test in which aseries of artificial pupils of various sizes were placedbefore the TV camera. The number of counts registered

TV. MONITOR

FIG. 4. Block diagram of electronic processing.

June 1967 831

D. G. GREEN AND F. MAASEIDVAAG

10

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PUPIL DIAMETER - IN MILLIMETERS

FIG. 5. Linearity of instrument measured with a seriespupils placed before the TV camera.

by the counting circuitry is plotted on anscale vs the diameter of the holes. It is evidentoutput is a linear function of the diameter official pupil. The device has a linearity of bel1% of full scale. The linearity is essentially deby the linearity with which the electron bea:placed vertically across the photocathode ofcamera tube.

The sensitivity of the instrument is deterrthe number of lines making up the scan. If, forthe size of the image on the vidicon tube is adthat 400 lines will cross a pupil of 8-mm dianisensitivity of the instrument is 0.02 mm/cotsensitivity is several times greater than that rerany of the existing pupillometers and could, cbe increased still further by increasing the D

lines in the scan. With the above method of pthe video output, the measured pupil diamettained as a digital number which can be fedinto an on-line computer for further calculcan be directly printed out using a high-speeprinter (such as a Franklin model 1000). In thment, an analog output is also obtained by genvoltage which is proportional to the numberregistered per frame. The analog output can beon an x-y plotter or strip-chart recorder.

A useful instrument must be not only sensilinear but, in addition, it must be able to redynamic changes in pupil size with speed andThe step response of the pupillometer was mea

momentarily presenting a 5.5-mm artificial pupil to theinstrument. The output follows the stimulus with a risetime of less than 25 msec (Fig. 6). From the speed ofthe step response, we estimate that the frequency re-sponse of the device is flat to at least 15 cps. As examplesof the recordings that can be obtained of transientchanges of the size of the pupil, three pupil contractionsare shown in Fig. 7. These contractions were producedby brief electronic flashes of increasing intensity.

The performance of this instrument was compared tothat of a more conventional pupillograph, similar to onedescribed elsewhere by Green.9 Pupillary contractions asa function of light intensity were recorded on the samesubject. To obtain equal signal-to-noise ratios, singleresponses from this instrument were compared with theaverages of 16 responses from the conventional device.As far as could be determined, the outputs of the twoinstruments were identical in amplitude and time course.

DISCUSSION

As has been indicated, the television pupillometer hasX j X excellent linearity and sensitivity. If standard TV cam-

7 e eras are used, which are locked to the 60-cps line fre-quency and have a 2:1 interlaced scan, the speed of

of artificial response is limited by the fact that a measurement ofthe diameter of the pupil can be obtained only every1/60 of a second. For most applications, this is not a

arbitrary serious limitation since contractions and dilations of thetthat the pupil are rather slow. Even 30 samples per second shouldthe arti- be fully adequate to define the shapes of the responses.Lter than However, if the latencies of the responses had to betermined measured with an error of no more than a few milli-m is dis- seconds, it would be necessary to build a special TV-the TV- camera system which scans the eye at several hundred

frames per second.mined by In any instrument of this kind, it is particularly im-example, portant that the instrument not confuse head and eyejusted so movements with changes of the size of the pupil. Theieter the readings of this pupillometer are virtually unaffected bynt. This head and/or eye movements so long as these movements

inrted fnr do not carry the image of the pupil outside of the

)f course,.umber ofrocessingter is ob-1 directlyLtions or:d digital

is instru-erating a)f countsread out

tive and!cord the* fidelity.sured by

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FIG. 6. Measured step response of pupillometer.

9 D. G. Green, Ph.D. thesis, Northwestern University (1964).

Io

Vol. 57

CLOSED-CIRCUIT TELEVISION PUPILLOMETER

scanned area. Lateral head movements may cause prob-lems since they tend both to blur the image on thevidicon and to change its size. The magnitude of thisproblem depends on a number of factors, such as thedepth of focus of the lens and the angular subtense ofthe pupil in the image. The problem is minimized inthis instrument, because counting the number of linescrossing the pupil is less sensitive to lateral movementsthan some other methods of measuring pupil size. How-ever, in practice it has been necessary to remove thispossible source of error by fixing the head with either abite bar or with a chin- and forehead-rest combination.

Aligning and focusing the optics on the subject's pupilis conveniently accomplished by using the TV monitorto determine the position and quality of the infraredimage formed on the photocathode of the televisioncamera tube. By continuously viewing the magnifiedpicture of the pupil on the monitor during experiments,it is quite easy to observe the pupillary responses. Inaddition, a number of possible artifacts can be detectedand corrected as they occur. For example, it is im-mediately evident when the eyelids of the subjectobscure part of the pupil or when a gross eye move-ment takes the image of the pupil outside of thescanned region.

The instrument is easily calibrated by placing a pupilof known size before the apparatus and taking all otherreadings in proportion to the output obtained. Havingthe output in both digital and analog form makes thispupillograph compatible with both small- and large-scale data-acquisition systems. The digital nature ofthe output makes it possible, by adding some logiccircuitry, to extract and record automatically variousattributes of the response. For example, area under theresponse curve can be obtained by simply not resettingthe counter at the end of each frame. Latency can bemeasured by counting the number of frames whichelapse between the start of the stimulus and the timewhen the output changes by some predetermined num-ber of counts. The possibilities are practically endless

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FIG. 7. Typical recordings of pupil contractions produced bybrief electronic flashes. A determination of pupil diameter wasmade every 1/60 of a second. The records labeled b and c havebeen displaced downward by 1.25 mm and 2.5 mm, respectively.The contractions are the responses to flashes which were in (a)1 log unit, in (b) 2 log units, and in (c) 3 log units above pupil-lary threshold.

and, in many instances, only simple modifications arenecessary to extract the significant variables.

While this has not been done, it is possible to modifythe present monocular pupillometer so that the pupilsof both eyes are measured simultaneously. One way ofdoing this is to arrange the optics of the instrument sothat images of the two pupils are formed side by side onthe photocathode of the camera tube. By using appro-priate gate circuitry, the outputs corresponding to eachof the two pupils could be sorted out and channeled toseparate counting circuits. In this way, both analog anddigital readings of the pupils of the two eyes would beobtained.

ACKNOWLEDGMENTS

The work was supported in part by grants from thePublic Health Service, NB 01578-09 and FR 05383-05and the Snyder Ophthalmic Foundation. We wish tothank Dr. M. Alpern for many helpful discussions andfor having the demanding requirements which providedthe stimulus for developing this instrument.

Mary Corning, National Library of Medicine, Wallace Brode,and William F. Meggers, at 1964 meeting in New York, at whichMeggers and G. R. Harrison received the joint award of C. E. K.Mees Medal.

June 1967