Medical Engineering & Physics 20 (1998) 109–113

Photo-pachometer: an instrument for the objective non-contactcorneal thickness measurement

Andrew W. SiuDepartment of Optometry and Radiography, Faculty of Health and Social Sciences, The Hong Kong Polytechnic University, Hung Hom,

Hong Kong

Received 21 June 1997; accepted 8 December 1997

Abstract

A new instrument to measure corneal thickness is described. The design was based on the well-established optical principle ofpachometry and its operation required only basic clinical skills. The instrument measures corneal thickness using a newly developedcomputer-aided paradigm in order to minimise the inter- and intra-subject variability commonly associated with the original opticaltechnique. Preliminary clinical trials showed that it provides fast, repeatable and objective measurements comparable to ultrasonicpachometry, and no physical contact with the eye was required throughout the procedure. It combines the advantages of the twocurrently employed techniques and it has potential clinical applications in monitoring the dynamic corneal hydration responseinvivo. 1998 IPEM. Published by Elsevier Science Ltd. All rights reserved.

Keywords:Pachometry; Cornea; Instrumentation

1. Introduction

1.1. Corneal thickness

Since the hydration level of the cornea is highly corre-lated with its volume changes expressed by anterior–pos-terior expansion [1], measurement of corneal thicknessis a simple method by which corneal physiology can bestudiedin vivo [2,3]. This kind of measurement has beenapplied clinically to the investigation of various factorsaffecting corneal physiological functions, such as contactlens wear [4] and the effect of age [5,6].

1.2. Clinical measurement

Blix is believed to be the first person who conductedthe thickness measurement of a living cornea usingoptical principles [7]. In the original design, apparentthickness of the cornea was gauged by a measuringmicroscope. Knowing both the refractive index of thecornea and the air, the real corneal thickness was determ-ined by a simple mathematical conversion. Since then,optical pachometry has become a standard clinical pro-cedure to characterise the water-regulatory functions ofthe cornea.

1350-4533/98/$19.00 1998 IPEM. Published by Elsevier Science Ltd. All rights reserved.PII: S1350-4533 (98)00005-8

Modern optical pachometers adopt Blix’s idea andtheir performance is enhanced using a slit-lamp bio-microscope [8]. The advantage of using a slit lamp is toprovide a high magnification of the corneal optical sec-tion. The corneal image is duplicated by a pair of bi-prisms which split the images into two half-sections. Byadjusting the relative displacement of these optical sec-tions, the instrument determines the thickness of the cor-nea.

Nowadays, the accuracy of the optical pachometer hasbeen improved by a few modifications [9], e.g. instal-lation of fixation and alignment LED targets, prior cali-bration against a set of contact lenses of known thicknessand alteration of the viewing angle, etc. The measure-ment is further assisted by the installation of a poten-tiometer which accurately determines the displacementof the duplication system leading to a correct separationof the images [10].

Although the modified optical pachometer offers goodcontrol of the corneal location to be measured, subjectivealignment of the optical sections by the examiner intro-duces a significant amount of intra- and inter-subjectvariability [11]. This variability limits the use of opticalpachometry in practice.

The other technique commonly employed is ultra-

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sound. Ultrasonic pachometers use pulsed echo signalsto determine the physical separation of the anterior andposterior corneal surfaces. It has been widely used owingto its relative simplicity and objectivity [12]. In ultra-sonic pachometry, the examiner does not require muchtraining to accomplish the measurement and it requiresno subjective judgements in deriving the end points.

Despite its relative ease and accuracy, ultrasonic pach-ometry has a few inherent operational disadvantages andlimitations. For example, the measurement requires top-ical application of corneal anaesthesia to abolish painsensation when the probe touches the cornea. It was pos-tulated that the application of local anaesthetics intro-duced a sudden influx of fluid into the cornea [13] andthe possible effect of mechanical force of the probe oncorneal physiology remains unclear. For this reason, theuse of ultrasonic pachometry in any repetitive cornealthickness measurement, where a sequential applicationof local anaesthetics is indicated, may render it clini-cally unacceptable.

Many studies have been conducted to evaluate the per-formance of optical and ultrasound pachometers [14,15].Giasson and Forthomme [14] compared the clinical per-formance of central thickness measurements betweenoptical and ultrasound pachometers. There was a sig-nificant correlation between the results obtained by thetwo techniques. However, the relatively superior repeat-ability and objectivity of ultrasonic pachometry hasmade it more popular in practice [16]. It seems that theoptical pachometer remains the only instrument suitablefor continuous monitoring of the corneal hydration con-trol response. However, its variability makes it not sensi-tive enough to assess subtle changes in corneal thicknessover time. Therefore, there is a need to develop a newinstrument which can measure corneal thickness withgood repeatability and objectivity, and does not requirethe application of local anaesthetics nor any physicalcontact with the cornea. The purpose of this paper is todescribe the design of a modified optical pachometerusing computer-aided image analysis. This techniqueutilises photographic image processing and is thusnamed photo-pachometry.

2. Instrument description

2.1. General design

Corneal optical section was carefully taken by a slitlamp bio-microscope (Fig. 1) under× 40 magnification.The width of the optical slit was maintained at 0.1 mm.An elongated slit aperture was placed in front of thereflecting mirror. The slit aperture was constructed byfixing two razor blades side-by-side onto a piece of thickcardboard, leaving a parallel gap of 1 mm in the middle.It was painted black to minimise stray light reflecting

Fig. 1. Optical section of the cornea. The apparent corneal thicknessis taken as the distance between the anterior and posterior corneal sur-faces, denoted A and A9, respectively. Also note the circular LEDreflex which indicates the mid corneal position.

onto the cornea. The light source of the slit lamp wasplaced at 40° lateral to the observation system. A smallLED (red) was affixed to the side of the illuminationsystem (fixation LED). Another LED (green) was placed40° from the fixation LED, opposite the observation tele-scope (alignment LED). The installation of colouredLEDs assisted the examiner to align the optical imagesquickly and accurately.

The subject was asked to look at the red fixation LEDand a sharp corneal optical section was taken by focusinga narrow beam of light onto the cornea. The central cor-neal location was determined by adjusting the bio-micro-scope until the green LED reflection was superimposedon the optical section and assumed a central locationthrough the eyepiece.

A small digital video camera (CCD Camera CCIR4541) was attached to the image outlet of the slit lamp(Topcon 7-F). An intermediate extension tube (Pentax,5 mm) was fitted between the camera and the slit lampimage outlet junction. The camera was electrically con-nected to a personal computer (P5, 100 MHz). Videoimages were acquired by a Mach DT3155 frame grabberinstalled within the computer. Fine alignment of the bio-microscope was achieved by monitoring the magnifiedimage on the computer display screen. The camera pro-cessed black and white images and the signals (greyscale values) were fed into the computer.

When optimum alignment had been achieved, a singleimage frame was selected and saved. Three consecutivepictures were taken for each measurement and the wholeprocedure could be completed within 30 seconds.

3. Image analysis

The optical image was processed and analysed usinga computer software program (Global Lab Image v. 3.1,Data Translation Inc.).

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3.1. Noise filtering

Prior to analysis, the image was processed by a built-in filter that minimised the noise disturbance to theimage quality. Median intensity in the neighbourhoodwas used to replace the pixel with an extreme value.Such a procedure minimised the nuisance interferencefrom light scattering. This preserved the brightness dif-ference of the edge [17] and thus enhanced its sharpness(Fig. 2).

3.2. Magnification of the image

The central portion of the corneal image was selected(with reference to the LEDs) and magnified (three times)to achieve optimum visibility. The distribution of bright-ness across the central optical image was interpreted bythe computer software program. The program analysedthe grey scale values of the optical image and the datawere plotted as a function of position (pixels). The distri-bution of brightness across the optical image indicatedthe apparent thickness of the cornea.

3.3. Calibration procedure

The actual corneal thickness was determined by refer-ring the apparent thickness to a prior calibration func-tion. The function correlated the apparent optical thick-ness of a set of rigid contact lenses (Oxycon Co. Ltd.,HK) with their known thickness [9], as taken by the cur-rent technique. All lenses had the same overall diameter

Fig. 2. Graphical determination of the optical band width. A50 andA509 represent the 50% intensity level of light measured at the anteriorand posterior corneal surfaces, respectively.

of 9.5 mm and the same front optic radius of 8.00 mm,except that they had different pre-set central thickness(Table 1). The apparent thickness of a corneal opticalsection was derived from the known thickness of thecontact lens set.

The relative distribution of light across the cornealimage is shown in Fig. 2. At the air–cornea/cornea–aqueous interfaces, the image experienced a spread oflight energy at the border. The width of the optical sec-tion as captured by the system was in fact the apparentthickness of the corneal optical section plus the lightscattering at the interfaces. It was assumed that thereexisted a similar line spread function in the calibrationprocedure when the instrument was calibrated againstthe contact lens set. Therefore, using a set of pre-set con-tact lenses to quantify the actual thickness of the corneahad already accounted for this error and thus the pro-cedure could effectively minimise the effect of light scat-tering.

3.4. Thickness determination

The relative thickness of the optical section wasdetermined by measuring the bandwidth of the image.The bandwidth was taken as the width of optical sectionin pixels at the 50% level of maximal light intensity ofthe respective interfaces. The location corresponding to50% intensity was determined by referring to the dataset deriving from the brightness analysis which specifiedthe brightness distribution of light across the optical sec-tion. Figure 2 illustrates the determination of bandwidth.

4. Clinical trial

Clinical trials were conducted on a young subject totest the performance of the equipment. Three cornealoptical sections were captured from the left eye and theirvalues averaged to determine the baseline corneal thick-ness. After that, the subject was fitted with a 0.3-mm-thick soft contact lens (HEMA, 38% water content; Oxy-con Co. Ltd., HK). The function of wearing the contactlens was to decrease the oxygen availability to the cor-nea. After a few minutes of lens adaptation, the eye was

Table 1Parameters of the contact lenses for calibration. A correction factor of0.925 was included to compensate for the difference in refractive indexbetween the lens material and the cornea

Physical thickness (mm) Equivalent optical thickness(mm)

0.4 0.370.5 0.460.6 0.560.7 0.65

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patched under the contact lens, which further decreasedthe oxygen supply to the cornea [18,19].

Under hypoxic stress, the cornea increased in thick-ness as a result of disruption to the fluid regulatorymechanism [20–22]. At 20-minute intervals, the eyelidwas opened briefly and the corneal thickness measuredagain [23]. The procedure of eyelid opening and re-patching was completed within 40 seconds and the briefintervention of atmospheric oxygen supply to the corneathrough the thick contact lens was ignored.

Figure 3 depicts the increase in corneal thickness(corneal oedema) during the period of hypoxic stress.The results showed that the increase in corneal thicknessfollowed a non-linear mode. The rate of increase wasvery fast within the first 40 minutes and then started toslow down until it approached a more or less constantlevel. A similar mechanism, although in the oppositedirection, has been established in the deswelling functionwhen the cornea was recovering from hypoxia [24].

5. Discussion

The accuracy of thickness determination depended onthe resolution of the optical image and edge localisation.Using the current method, each pixel represented a thick-ness of 2.8mm. The maximum resolving power of thetechnique was therefore limited by the resolution of eachpixel and the detection of edge profile. Such perform-ance is currently lower than that of the ultrasonic pacho-meter which has a resolution of 1mm. However, it isanticipated that the instrument could be improved bychanging the video camera to a digital imaging systemwhich will provide a better resolving power.

Fig. 3. The increase in corneal thickness with time. The data werecollected from one young subject wearing a thick soft contact lenswhile the cornea was continuously exposed to hypoxic stress.

The design of a non-contact photo-pachometer whichrequires a minimal amount of operational skills has beenintroduced. The instrument provides objective data oncorneal thickness that do not need the topical applicationof local anaesthetics (LA). This feature eliminates thepossible effects of LA on corneal hydration control. Pre-liminary results suggest that this instrument providesfast, non-contact and objective corneal thicknessmeasurement. Its resolving power is currently lower thanthe ultrasonic pachometer but it could be improved inpace of the advancements in image processing andanalysis. The measuring errors introduced by light scat-tering might compromise the validity of the instrument.However, the relatively constant nature of light scat-tering renders the error insignificant when the interest isin the change of corneal thickness rather than its absolutevalue. This technique will be particularly useful in thesequential assessment of corneal hydration controlinvivo. This information cannot be provided effectively byany of the current measurement techniques.

The results showed that photo-pachometry mightserve as a viable clinical measurement of the dynamiccorneal oedema responsein vivo. It solves the problemof repetitive drug instillation necessary in ultrasonicpachometry. A potential application of the photo-pacho-meter is corneal thickness assessment during the surgicalprocedure of photo-refractive keratotomy (PRK). Withproper alignment, corneal thickness could be constantlymonitored throughout the procedure. Quick and reliabledata, comparable to those provided by ultrasonic pacho-metry, can be obtained without causing further physicaldisturbance to the cornea. This may help improve theaccuracy of PRK and might lead to a better refractivecorrection [25].

Acknowledgements

The author would like to thank C. M. Kan for techni-cal advice. This work was supported by Polytechnic Uni-versity Research Grants 351/122.

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