AU746383B2 - Method and apparatus for surface profiling of materials and calibration of ablation lasers - Google Patents

Description

WO 99/04220 PCT/AU98/00568 1 METHOD AND APPARATUS FOR SURFACE PROFILING OF MATERIALS AND CALIBRATION OF ABLATION LASERS The present invention relates to the surface profiling of materials, for example as in the laser processing or ablation of materials, or as needed in the calibration and performance checking of the laser apparatus used in operations on the corneal tissue of the eye for the correction of refractive errors.

The invention will be described by reference to two operations for the correction of refractive errors, photorefractive keratectomy (PRK) and laser in-situ keratomeleusis (LASIK), but the invention may be used to measure the surface profile of a wide range of materials or to calibrate lasers for a variety of medical and industrial applications.

To ensure that the correct profile is etched onto a patient's cornea during PRK or LASIK, the surgical laser must first be calibrated. This process imparts an accurate picture of how the laser will ablate the cornea. The corneal surface may be ablated to effect a myopic, hyperopic or astigmatic correction. Myopic corrections should produce a new, flatter curvature, while hyperopic corrections should remove more material around the edge of the area to be ablated.

One of the current methods used to perform the calibration procedure involves etching the surface of a plastic polymer such as polymethyl-methacrylate (PMMA). The etched surface is examined by an instrument known as a lensometer. This instrument determines the power of the resultant 'lens' in diopters. The reading can then be compared to the desired refractive correction. Discrepancies between the desired and achieved readings indicate that the laser needs to be adjusted, by a factor proportional to the difference WO 99/04220 PCT/AU98/00568 2 between the lensometer reading and the desired surgical correction (see US Patent No. 5,261,822).

Another method of calibration is described in US Patent No.

5,261,822. This patent illustrates the use of a calibration block that can be examined by visual inspection. It teaches the use of a plurality of thin coatings of PMMA of progressively increasing thickness, layered over a solid substrate of the same material. Each layer may be doped with a differently coloured or fluorescent material. When the cavity of material ablated by the laser is viewed from above, a pattern of circles is visible. A correctly calibrated laser should produce patterns of concentric circles, whereas patterns of eccentric circles indicate that the laser is not correctly calibrated. However, the result is usually judged subjectively and this technique provides only a crude prediction of the shape created during a refractive correction.

The above laser calibration methods suffer from a number of disadvantages. PMMA does not necessarily mimic the ablation characteristics of corneal tissue, and different brands of PMMA ablate at different rates van Saarloos and I.J. Constable, J. Appl. Phys. 68(1) (1990) 377).

Further, different brands of lasers ablate at different fluences, where the ratio of ablation rates of tissue and plastic are different. Nor does the lensometer provide an accurate reading of the ablation surface. The shape desired to be etched on the cornea does not necessarily produce an accurate lens shape when ablated into plastic.

The ablated surface is usually aspheric, and may be inaccurately read. This means that a lensometer reading does not give an absolute measure of laser performance, and in some cases the measurement is meaningless. This method can therefore only give an approximate reading of surface curvature. Lensometer readings are also time consuming.

WU"U34f1 3 Other known methods to measure ablated surface profiles include the use of interferometry, or include scanning the ablated surface with a scanning electron microscope, a confocal microscope or surface contact needles. Devices according to these known methods are, however, costly and of prohibitive size, and impractical to cover the range of shapes produced by refractive lasers. There exists, therefore, a demand for an accurate, low cost device for performance analysis and calibration of refractive lasers to ensure appropriate shapes are etched onto the surface to be ablated.

It is an object of the present invention to provide a new and improved method and apparatus for surface profiling of materials and calibration of ablation lasers that can more accurately and reliably examine the surface of an ablation.

According to the present invention, therefore, there is provided a method for calibrating laser ablation apparatus, including ablating a sample; measuring the oooo surface profile of said sample by directing light from a light source through a beam-splitter to form two split beams, directing said split beams respectively onto the ablated surface of said sample and also onto a reference surface, reflecting the split beams from said ablated surface and said reference surface, respectively, and forming an interference signal from said reflected split beams, and detecting I.:i :said interference signal and therefrom determining the surface profile of said sample surface; and calibrating said laser ablation apparatus on the basis of said determined surface profile.

o:i The method may include reflecting said light from a mirror and/or focussing said light to minimise space requirements.

WO 99/04220 PCT/AU98/00568 4 Preferably the light source includes a light emitting diode.

Alternatively the light source is a source of white or near infra-red light.

Preferably the sample surface is a plastic polymer that ablates at a substantially constant fraction of the ablation rate of said material over the range of fluences used in ablating said material, and preferably the fraction equals The material may be biological material.

The biological tissue may be corneal tissue, and the method include ablating said material in a surgical procedure, in which case the fluences are preferably in the range 50 800 mJ/cm 2 and more preferably in the range 120 250 mJ/cm 2 Preferably the reference surface is a flat mirror or a flat surface.

The reference surface may be mounted on a pendulum including a plurality of substantially parallel sheets of flexible material.

The method may include moving the reference surface by means of a speaker or voice coil.

Preferably the imaging system includes a CCD video camera.

The method may include measuring said surface profile, comparing said measurement with a predicted profile, and determining an indicator of the safety or predictability of ablation performed on said sample for use in a surgical procedure.

UU3OU4 I Preferably the reference surface positioning means includes a voice coil driver and a position sensor.

The method may include transferring the calibration profile information ascertained by said method into a laser system control computing device, to allow the self correction of the calibration and shape controls of the laser system.

The method may also include communicating with a topography measuring device for measuring the topography of the front surface of a human or animal eye in order to combine the results of a calibration measurement in plastic and the results of a topography measurement, and predicting from said calibration and topography results the post laser treatment shape of the eye.

The present invention also provides a surface profiling apparatus for measuring the surface profile of a sample, said apparatus including a light source o for generating a source beam, beamsplitter means positioned in the path of the *o source beam for splitting said source beam into split beams, a reference surface io 15 positioned to reflect one of said split beams back to said beamsplitter means for S:.."forming an interference signal with another of said split beams reflected back to said beamsplitter means by a surface of a sample, reference surface positioning means including a voice coil driver for positioning the reference surface, means to o• detect the position of said reference surface, to which detection means the voice 20 coil driver is responsive; and means for imaging said interference signal and therefrom determining the surface profile of said sample surface.

The invention still further provides apparatus for calibrating laser ablation apparatus, including a laser source means for generating a source laser beam, beamsplitter means positioned in the path of the source beam for splitting said source beam into split beams, a reference surface positioned to reflect one of said split beams for forming an interference signal with another of said split beams reflected by a surface of a sample ablated by said laser ablation apparatus, reference surface positioning means for positioning the reference surface; and means for imaging said interference signal and therefrom determining the surface profile of said sample surface, from which said laser ablation apparatus may be calibrated.

The apparatus may include focussing optical elements to concentrate the intensity of said light, and a mirror, said optical elements and said mirror located between said light source and said beam splitting means.

*o WO 99/04220 PCT/AU98/00568 6 Preferably the light is white light or near infra-red light.

The light source may include a halogen bulb, or a light emitting diode (LED).

Preferably said LED has a maximum intensity in the red to infra-red portion of the spectrum.

The reference surface may be a flat mirror or a flat surface.

The imaging system preferably includes a CCD video camera.

Preferably the reference surface positioning means includes a voice coil driver and a position sensor.

Preferably the position sensor includes a known sample.

Preferably the position sensor includes a mirror or optical element that allows both the known sample and the plastic sample being measured to be viewed by means of the imaging system simultaneously or alternately.

In one form of the invention, the position sensor is a capacitance or inductance position sensor.

Preferably the voice coil driver is similar to that used in a loud-speaker.

The position sensor may be an opto-electric sensor including a photodiode with an amplification system and an additional LED, wherein the sensor uses the intensity of the additional LED, and said additional LED is positioned to reflect light at an angle from the reference surface, or any surface moving with the reference surface, to the photodiode.

WO 99/04220 PCT/AU98/00568 7 Preferably the position sensor is one of a plurality of position sensors.

Preferably the plurality of position sensors includes a plurality of types of position sensor.

In one embodiment, the reference surface positioning means includes a loud-speaker.

Preferably the loud-speaker is used as or constitutes a displacement driver for the reference surface.

Preferably the reference surface is mounted on a pendulum including a plurality of substantially parallel sheets of flexible material.

The invention also provides an apparatus for calibrating a laser for the ablation of a material including the surface profiling apparatus described above.

The sample surface may comprise a plastic polymer that ablates at a substantially constant fraction of the ablation rate of said material over the range of fluences used in ablating said material, and preferably the fraction equals The material may be biological material, including for example corneal tissue, and the apparatus may be for ablating the material in a surgical procedure (such as PRK or LASIK). In these cases the fluences are preferably in the range 50 800 mJ/cm 2 and more preferably in the range 120 250 mJ/cm 2 In one particular embodiment, the apparatus includes a laser means, wherein the apparatus is for calibrating and/or checking the laser means, and includes communication WO 99/04220 PCT/AU98/00568 8 means for communicating with, a computer controlled laser means, whereby the laser means can use calibration profile information obtained by the calibration apparatus to self correct the calibration and shape controls of said laser means. In this embodiment, the laser means may be for use in PRK or LASIK operations of the cornea of the eye to correct refractive errors.

The apparatus may include a corneal topography measuring means for measuring the topography of the front surface of a human or animal eye and communication means for communicating with said topography measuring means, for predicting post laser treatment eye topography from calibration measurements in plastic and topography measurements of the eye, and may further include display means for displaying the post laser treatment corneal topography predicted by means of the apparatus.

In order that the invention may be more fully explained, some preferred embodiments will be described, by way of example, with reference to the accompanying drawings in which: Figure 1A is a diagrammatic plan view of an ablation pattern formed by a laser source directed onto the surface of a plastic sample surface; Figure IB is a cross section through A-A of figure 1A; and Figure 2 is a schematic view of a calibration apparatus according to a preferred embodiment of the present invention.

Figures 1A and IB show a typical myopic or myopic/astigmatic ablation pattern etched onto the surface of a plastic sample surface. The ablation pattern may have been etched by an excimer, solid state or other type of laser suitable for refractive correction.

WO 99/04220 PCT/AU98/00568 9 Referring to Figure 2, the first arrangement of the apparatus includes a red light source in the form of light emitting diode 2. Alternatively the light source may be a general purpose halogen bulb. The light 4 passes through a beam splitter 6 where two separate beams are formed. Some of the light is directed onto the ablated sample 8, which is a plastic polymer that ablates at the same rate as corneal tissue over the range of laser fluences used in corneal ablation procedures, 120 to 250 mJ/cm 2 The rest of the light is directed onto a reference surface comprising a mirror or other flat surface which is scanned back and forth. Both the ablated plastic sample 8 and the reference surface 10 reflect or scatter the light back to the beam splitter 6. Some of the reflections from the sample 8 and the reference surface 10 bounce off the beam splitter 6 and disappear. The remaining combined beam is directed through the beam splitter 6 towards a CCD video camera 12, for example a COHO 1100 video camera or the like. The reference surface 10 is scanned to adjust the beam path length of the light going back to the camera 12.

When the light beam path length from the sample surface 8 matches the path length to the reference surface interference patterns will be formed.

For an ablated sample such as that of figure 1, when viewed through the camera 12, circular interference patterns are imaged for good, non-astigmatic, myopic ablations. A smaller circular pattern is produced at the deepest point of the ablated surface, when the reference surface 10 is further away from the beam splitter 6. Progressively larger circular patterns are produced as shallower ablations are encountered.

However, interference patterns can only be produced when the reference surface 10 and a point on the ablated sample 8 are at the same optical path distance from the video camera 12. The reference surface 10 must therefore be WO 99/04220 PCT/AU98/00568 10 movable to allow the imaging of different ablation depths.

A voice coil driver 14 moves the reference surface 10 back and forth, while an opto-electronic sensor 16 (or, in other embodiments, a capacitance or inductance position sensor), such as a photodiode with an amplification system, senses the spatial positioning of the reference surface 10. Voice coil driver 14 and position sensor 16 therefore allow positioning, with feedback from the reference surface 10 in relation to the ablated sample 8.

An alternative embodiment involves the use of optical rather than mechanical position measurement. In this embodiment, a known sample in the form of a wedge shaped object 18, and a small mirror 20, are used to detect the positioning of the reference surface 10. Voice coil 14 is again used to drive the reference surface 10. In this embodiment, the known sample 18 features a sloping surface 22 that reflects the minimum and maximum movement of the reference surface 10. However, position sensor 16 may additionally be used in this embodiment.

The calibration device as described above is preferably connected to a computer 24. This computer 24 can calculate the shape of the ablated sample surface 8, display the shape in a three dimensional form, compare the actual shape to a desired shape and issue a "go/no go" message, indicating that a good calibration or a laser problem has been detected, respectively. The computer may also be joined to a laser system or corneal topography device 26.

The calibration device can therefore exchange information concerning the ablated profile with the laser system. The information provided about the measured profile produced can then be interpreted, and used to alter the parameters of the laser system so that the desired corneal profile is produced in its next ablation.

Apparatus for performing topographic profiling of the ii WO 99/04220 PCT/AU98/00568 11 cornea may also be included in a preferred embodiment.

This apparatus may be used to measure the original profile of a corneal surface and then import the measured ablation profile from the calibration apparatus of the present invention. The corneal topography that may be expected if a laser ablation procedure were performed on a cornea, based on the calibration data, may then be calculated and displayed. Alternatively, the calibration apparatus may read the corneal topographic data, and calculate and display on computer 24 the resultant corneal shape that would be created if the laser was used on the eye.

Thus, the present invention may be used to calibrate lasers used, for example, in the improvement of eyesight or other medical, dental or cosmetic procedures where the accurate ablation of tissue is required.

Modifications within the spirit and scope of the invention may be readily effected by a person skilled in the art.

Such modifications may include swapping positions of the sample and reference surfaces. It is to be understood, therefore, that this invention is not limited to the particular embodiments described by way of example hereinabove.

Claims (1)

120-250 mJ/cm 2 13 A method as claimed in any one of the preceding claims, wherein said reference surface is a flat mirror or a flat surface. 14 A method as claimed in any one of the preceding claims, wherein said reference surface is mounted on a pendulum including a plurality of substantially parallel sheets of flexible material. A method as claimed in any one of the preceding claims, including moving said reference surface by means of a speaker or voice coil. 16 (amended) A method as claimed in any one of the preceding claims, 1s'/ wherein said interference signal is detected with an imaging system that includes i 9 AMENDED SHEET (Article 34) (IPEA/AU) 13 003995471 14 a CCD video camera. 17 A method as claimed in any one of the preceding claims, including measuring said surface profile, comparing said measurement with a predicted profile, and determining an indicator of the safety or predicability of ablation performed on said sample for use in a surgical procedure. 18 A method as claimed in any one of the preceding claims, including adjusting the calibration and shape controls of the laser ablation apparatus. 19 A method as claimed in any one of the preceding claims, including communicating with a topography measuring device for measuring the topography of the front surface of a human or animal eye in order to o combine the results of a calibration measurement in plastic and the results of a topography measurement, and predicting from said calibration and o* topography results the post laser treatment shape of the eye. 20 A surface profiling apparatus for measuring the surface profile of a sample, said apparatus including: a light source for generating a source beam; e* beamsplitter means positioned in the path of the source beam for splitting said source beam into split beams; a reference surface positioned to reflect one of said split beams back to said beamsplitter means for forming an interference signal with another of said split beams reflected back to said beamsplitter means by a surface of a sample; 003995471 reference surface positioning means including a voice coil driver for positioning the reference surface; means to detect the position of said reference surface, to which detection means the voice coil driver is responsive; and means for imaging said interference signal and therefrom determining the surface profile of said sample surface. 21 An apparatus as claimed in claim 20, including focussing optical elements to concentrate the intensity of said light, and a mirror, said optical elements and said mirror located between said light source and said beamsplitter means. 22 An apparatus as claimed in either claim 20 or 21, wherein said light is white S light or near infra-red light. g 23 An apparatus as claimed in either claim 20 or 21, wherein said light source includes a halogen bulb, or a light emitting diode (LED). 15 24 An apparatus as claimed in claim 23, wherein said LED has a maximum goo* :intensity in the red to infra-red portion of the spectrum. 0 a* 0 An apparatus as claimed in any one of claims 20 to 24, wherein said reference surface is a flat mirror or a flat surface. 26 An apparatus as claimed in any one of claims 20 to 25, wherein said imaging means includes a CCD video camera. imaging means includes a COD video camera. 003995471 16 27 An apparatus as claimed in any one of claims 20 to 26, further including a sample having a surface positioned to reflect said other split beam back to said beamsplitter means for forming, with said one reflected split beam, said interference signal. 28 An apparatus as claimed in any one of claims 20 to 26, wherein said position detection means includes a known sample. 29 An apparatus as claimed in claim 28, wherein said position detection means includes a mirror or optical element that allows both the known sample and said sample being measured to be viewed by means of the imaging system simultaneously or alternately. An apparatus as claimed in any one of claims 20 to 28, wherein said fposition sensor is a capacitance or inductance position sensor. o *o o S"31 An apparatus as claimed in any one of claims 20 to 30, wherein said voice i coil driver is similar to that used in a loud-speaker. 32 An apparatus as claimed in any one of claims 20 to 31, wherein said position detection means is an opto-electric sensor including a photodiode with an amplification system and an additional LED, wherein the sensor uses the intensity of the additional LED, and said additional LED is positioned to reflect light at an angle from the reference surface, or any surface moving with the reference surface, to the photodiode. 33 An apparatus as claimed in any one of claims 20 to 32, wherein said position detection means comprises a plurality of position sensors. 003995471 17 34 An apparatus as claimed in claim 33, wherein said plurality of position sensors includes a plurality of types of position sensor. An apparatus as claimed in any one of claims 20 to 30, wherein said reference surface positioning means includes a loud-speaker. 36 An apparatus as claimed in claim 35, wherein said loud-speaker is used as or constitutes a displacement driver for the reference surface. 37 An apparatus as claimed in any one of claims 20 to 36, wherein the reference surface is mounted on a pendulum including a plurality of substantially parallel sheets of flexible material. so °o, 10 38 An apparatus for calibrating laser ablation apparatus, including the surface ~profiling apparatus as claimed in any one of claims 20 to 37, said sample S°o being a sample ablated by said laser ablation apparatus, further including s *means for determining, from said imaged reference signal, the surface profile of said sample surface and for calibrating said laser ablation o :00 15 apparatus on the basis of said determined surface profile. 39 An apparatus as claimed in claim 38, wherein said sample surface comprises a plastic polymer. S An apparatus as claimed in claim 38 or 39 for calibrating said laser ablation apparatus for use in ablating a predetermined material, wherein said sample has an ablation rate that is a substantially constant fraction of the ablation rate of said material to be ablated over the range of fluences used in ablating said material. 003995471 18 41 An apparatus as claimed in claim 40, wherein said material is biological tissue. 42 An apparatus as claimed in claim 41, wherein said biological tissue is corneal tissue. 43 An apparatus is claimed in either claim 41 or 42, in combination with apparatus for laser ablating said biological tissue in a surgical procedure. 44 An apparatus as claimed in claim 43, wherein said surgical procedure is PRK or LASIK. An apparatus as claimed in any one of claims 41 to 44, wherein said 10 fluences are in the range 50-800 mJ/cm 2 el 0= go 46 An apparatus as claimed in claim 45, wherein said fluences are in the range 120-250 mJ/cm 2 ii!i47 An apparatus as claimed in any one of claims 20 to 46, including a laser *fee means, wherein said apparatus is for calibrating and/or checking said laser see: 15 means, and includes communication means for communicating with a computer controlled laser means, whereby the laser means can use calibration profile information obtained from said sample to self correct the calibration and shape controls of said laser means. 48 An apparatus as claimed in claim 47, wherein the laser means is for use in PRK or LASIK operations of the cornea of the eye to correct refractive _errors. 003995471 19 49 An apparatus as claimed in any one of claims 20 to 48, including a corneal topography measuring means for measuring the topography of the front surface of a human or animal eye and communication means for communicating with said topography measuring means, for predicting post laser treatment eye topography from calibration measurements in plastic and topography measurements of the eye. An apparatus as claimed in claim 49, including display means for displaying the post laser treatment corneal topography predicted by means of the apparatus. 51 A method of calibrating laser ablation apparatus, utilising apparatus according to any one of claims 20 to 52 A method according to any one of claims 1 to 19 wherein said reflected split beams are directed back through said beamsplitter means to form said U 0: interference signal. 15 53 Apparatus for calibrating laser ablation apparatus, including: a laser source means for generating a source laser beam; beamsplitter means positioned in the path of the source beam for splitting said source beam into split beams; a reference surface positioned to reflect one of said split beams for forming an interference signal with another of said split beams reflected by a surface of a sample ablated by said laser ablation- apparatus; reference surface positioning means for positioning the reference surface; and 003995471 means for imaging said interference signal and therefrom determining the surface profile of said sample surface, from which said laser ablation apparatus may be calibrated. 54 An apparatus as claimed in claim 53, including focussing optical elements to concentrate the intensity of said light, and a mirror, said optical elements and said mirror located between said light source and said beam-splitter means. An apparatus as claimed in either claim 53 or 54, wherein said light source provides white light or near infra-red light. 56 An apparatus as claimed in either claim 53 or 54, wherein said light source includes a halogen bulb, or a light emitting diode (LED). a. a 0 go ea a S,57 An apparatus as claimed in claim 56, wherein said LED has a maximum "intensity in the red to infra-red portion of the spectrum. go 0 e* gin 58 An apparatus as claimed in any one of claims 53 to 57, wherein said 15 reference surface is a flat mirror or a flat surface. ,a lal a.e. 59 An apparatus as claimed in any one of claims 53 to 58, wherein said imaging means includes a CCD video camera. An apparatus as claimed in any one of claims 53 to 59, wherein said reference surface positioning means further includes a position sensor. 003995471 21 61 An apparatus as claimed in claim 60, wherein said position sensor includes a known sample. 62 An apparatus as claimed in claim 61, wherein said position sensor includes a mirror or optical element that allows both the known sample and said sample being measured to be viewed by means of the imaging system simultaneously or alternately. 63 An apparatus as claimed in any one of claims 60 to 62, wherein said position sensor is a capacitance or inductance position sensor. 64 An apparatus as claimed in any one of claims 53 to 63, wherein said voice coil driver is similar to that used in a loud-speaker. S.. .65 An apparatus as claimed in any one of claims 60 to 64, wherein said °position sensor is an opto-electric sensor including a photodiode with an amplification system and an additional LED, wherein the sensor uses the intensity of the additional LED, and said additional LED is positioned to 15 reflect light at an angle from the reference surface, or any surface moving with the reference surface, to the photodiode. •S 0 0" i• 66 An apparatus as claimed in any one of claims 60 to 65, wherein said S: position sensor is one of a plurality of position sensors. 67 An apparatus as claimed in claim 66, wherein said plurality of position sensors includes a plurality of types of position sensor. 003995471 22 68 An apparatus as claimed in any one of claims 53 to 67, wherein said reference surface positioning means includes a loud-speaker. 69 An apparatus as claimed in claim 68, wherein said loud-speaker is used as or constitutes a displacement driver for the reference surface. 70 An apparatus as claimed in any one of claims 53 to 69, wherein the reference surface is mounted on a pendulum including a plurality of substantially parallel sheets of flexible material. 71 An apparatus as claimed in any one of claims 53 to 70, wherein said sample surface comprises a plastic polymer. 10 72 An apparatus as claimed in claim 70 or 71 for calibrating said laser ablation ::.apparatus for use in ablating a predetermined material, wherein said sample has an ablation rate that is a substantially constant fraction of the ablation rate of said material to be ablated over the range of fluences used e S.in ablating said material. 15 73 An apparatus as claim 72, wherein said fraction equals 74 An apparatus as claimed in claim 72 or 73, wherein said material is biological tissue. An apparatus as claimed in claim 74, wherein said biological tissue is corneal tissue. 003995471 23 76 An apparatus is claimed in either claim 74 or 75, in combination with apparatus for laser ablating said biological tissue in a surgical procedure. 77 An apparatus as claimed in claim 76, wherein said surgical procedure is PRK or LASIK. 78 An apparatus as claimed in any one of claims 74 to 77, wherein said fluences are in the range 50-800 mJ/cm 2 79 An apparatus as claimed in claim 78, wherein said fluences are in the range 120-250 mJ/cm 2 An apparatus as claimed in any one of claims 53 to 79, wherein the laser 10 source means is for use in PRK or LASIK operations of the cornea of the eye to correct refractive errors. 81 An apparatus as claimed in any one of claims 53 to 80, including a corneal topography measuring means for measuring the topography of the front i surface of a human or animal eye and communication means for communicating with said topography measuring means, for predicting post laser treatment eye topography from calibration measurements in plastic and topography measurements of the eye. 0*o 82 An apparatus as claimed in claim 81, including display means for displaying the post laser treatment corneal topography predicted by means of the apparatus. UUJ3b4/1 24 83 An apparatus as claimed in any one of claims 53 to 82, further including a sample having a surface positioned to reflect said other split beam for forming, with said one reflected split beam, said interference signal. Q-Vis Limited By its Registered Patent Attorneys Freehills Carter Smith Beadle 14 February 2002