WO2018011466A1 - Device and method for measuring an optical thickness of a layer - Google Patents

Device and method for measuring an optical thickness of a layer Download PDF

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Publication number
WO2018011466A1
WO2018011466A1 PCT/FI2017/050524 FI2017050524W WO2018011466A1 WO 2018011466 A1 WO2018011466 A1 WO 2018011466A1 FI 2017050524 W FI2017050524 W FI 2017050524W WO 2018011466 A1 WO2018011466 A1 WO 2018011466A1
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WIPO (PCT)
Prior art keywords
optical
measurement result
layer
transparent material
optical path
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PCT/FI2017/050524
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French (fr)
Inventor
Ivan Kassamakov
Risto MONTONEN
Edward HÆGGSTRÖM
Antti Kontiola
Ari Salmi
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Photono Oy
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Publication of WO2018011466A1 publication Critical patent/WO2018011466A1/en

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
    • G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B3/00—Apparatus for testing the eyes; Instruments for examining the eyes
    • A61B3/10—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions
    • A61B3/1005—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for measuring distances inside the eye, e.g. thickness of the cornea
    • A—HUMAN NECESSITIES
    • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
    • A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
    • A61B3/00—Apparatus for testing the eyes; Instruments for examining the eyes
    • A61B3/10—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions
    • A61B3/102—Objective types, i.e. instruments for examining the eyes independent of the patients' perceptions or reactions for optical coherence tomography [OCT]
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
    • G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
    • G01B11/0608—Height gauges
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B11/00—Measuring arrangements characterised by the use of optical techniques
    • G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
    • G01B11/06—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material
    • G01B11/0616—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating
    • G01B11/0675—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness for measuring thickness ; e.g. of sheet material of coating using interferometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02015—Interferometers characterised by the beam path configuration
    • G01B9/02017—Interferometers characterised by the beam path configuration with multiple interactions between the target object and light beams, e.g. beam reflections occurring from different locations
    • G01B9/02021—Interferometers characterised by the beam path configuration with multiple interactions between the target object and light beams, e.g. beam reflections occurring from different locations contacting different faces of object, e.g. opposite faces
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02015—Interferometers characterised by the beam path configuration
    • G01B9/02027—Two or more interferometric channels or interferometers
    • G01B9/02028—Two or more reference or object arms in one interferometer
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02041—Interferometers characterised by particular imaging or detection techniques
    • G01B9/02044—Imaging in the frequency domain, e.g. by using a spectrometer
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02083—Interferometers characterised by particular signal processing and presentation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/0209—Low-coherence interferometers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/41—Refractivity; Phase-affecting properties, e.g. optical path length
    • G01N21/45—Refractivity; Phase-affecting properties, e.g. optical path length using interferometric methods; using Schlieren methods
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B26/00—Optical devices or arrangements for the control of light using movable or deformable optical elements
    • G02B26/06—Optical devices or arrangements for the control of light using movable or deformable optical elements for controlling the phase of light

Definitions

  • the disclosure relates to a device for measuring an optical thickness of a layer consisting of one or more materials.
  • the layer can be, for example but not necessarily, a cornea.
  • the disclosure relates to a method for measuring an optical thickness of a layer.
  • the disclosure relates to a computer program for estimating an optical thickness of a layer.
  • an optical thickness of a layer consisting of one or more materials can be measured using interferometry where optical radiation is directed to the layer under consideration and to a reference reflector.
  • the layer to be measured can be e.g. a cornea.
  • the measurement of the optical thickness is based on interference between optical waves reflected from the reference reflector, optical waves reflected from a first surface of the layer, and optical waves reflected from the second surface of the layer.
  • An estimate of the optical thickness can be obtained for example with the aid of a wavelength spectrum of optical radiation that comprises the above- mentioned optical waves reflected from the reference reflector and from the surfaces of the layer.
  • the wavelength spectrum can be measured with e.g. a spectrometer or another suitable device.
  • the process for estimating the optical thickness may comprise detecting wavelengths where constructive interference takes place and/or detecting wavelengths where destructive interferences take place. This can be carried out for example by computing an inverse Fourier transform of the wavelength spectrum.
  • the computed inverse Fourier transform comprises peaks corresponding to components of the wavelength spectrum which alternate as a function of a distance. These peaks can be used to estimate the optical thickness of the layer under consideration.
  • the above-described measurement method is not free from challenges.
  • One challenge is that the optical measurement range is limited according to the following equation: where r ma x is the maximum optical measurement range, ⁇ is the center wavelength of the optical radiation, and ⁇ is the spectral resolution of the wavelength spectrum.
  • the maximum optical measurement range is about 300 ⁇ .
  • This maximum optical measurement range is insufficient for example in cases where the cornea thickness is about 600 ⁇ which is significantly beyond the maximum optical measurement range 300 ⁇ .
  • a method for measuring an optical thickness of a layer that consists of one or more materials can be, for example but not necessarily, a cornea.
  • a method according to the invention comprises:
  • - producing a first measurement result indicative of a position of a first surface of the layer, the first measurement result being based on interference between optical waves reflected from the first surface of the layer and optical waves reflected from a reference reflector, - changing an optical length, i.e. the propagation time, of at least one optical path each being either a reference optical path comprising the reference reflector or a measurement optical path comprising the layer,
  • the change in the above-mentioned optical length makes the layer appear optically thinner in the interference-based measurement than in a case where there is no change in the optical length. Therefore, the limitation related to the optical measurement range can be circumvented.
  • the change in the optical length is computationally compensated for in order to estimate the real optical thickness of the layer. In cases where the layer is homogenous so that the refractive index is same in all points of the layer, an estimate of the physical, i.e. geometrical, thickness of the layer can be obtained based on the refractive index and the estimate of the optical thickness.
  • optical waves is not limited to mean only visible light but the scope of the term “optical waves” includes also invisible radiation such as e.g. ultraviolet and infrared light, as well as any other electromagnetic radiation which is suitable for interference based measurements.
  • a device according to the invention comprises:
  • an optical radiation source e.g. a light emitting diode
  • a modifier section for changing the optical length of at least one optical path each being either a reference optical path comprising the reference reflector or a measurement optical path comprising the layer
  • a processing system for computing an estimate of the optical thickness of the layer based on i) a first measurement result indicative of a position of a first surface of the layer and measured with a first value of the optical length, ii) a second measurement result indicative of a position of a second surface of the layer and measured with a second value of the optical length, and iii) a difference between the first and second values of the optical length.
  • a computer program for obtaining an estimate of an optical thickness of a layer consisting of one or more materials.
  • a computer program according to the invention comprises computer executable instructions for controlling a programmable processor to compute the estimate of the optical thickness of the layer based on i) a first measurement result indicative of a position of a first surface of the layer, ii) a second measurement result indicative of a position of the second surface of the layer, and iii) first and second values of an optical length of at least one optical path each being either a reference optical path comprising a reference reflector or a measurement optical path comprising the layer, wherein:
  • the first measurement result is based on interference between optical waves reflected from the first surface of the layer and optical waves reflected from the reference reflector when the optical length has the first value
  • the second measurement result is based on interference between optical waves reflected from the second surface of the layer and optical waves reflected from the reference reflector when the optical length has the second value different from the first value.
  • the computer program product comprises a non-volatile computer readable medium, e.g. a compact disc "CD", encoded with a computer program according to the invention.
  • figures 1 a, 1 b, and 1 c illustrate devices according to exemplifying and non-limiting embodiments of the invention for measuring an optical thickness of a layer consisting of one or more materials
  • figure 2 illustrates a device according to an exemplifying and non-limiting embodiment of the invention for measuring an optical thickness of a layer consisting of one or more materials
  • figure 3 shows a flowchart of a method according to an exemplifying and non-limiting embodiment of the invention for measuring an optical thickness of a layer consisting of one or more materials.
  • Figure 1 a shows a schematic illustration of a device according to an exemplifying and non-limiting embodiment of the invention for measuring an optical thickness of a layer 1 10 consisting of one or more materials.
  • the device comprises an optical radiation source 101 , a beam splitter "BS" 1 1 1 , a reference reflector 102, and a measurement section 103.
  • the beam-slitter 1 1 1 reflects a part of the optical radiation emitted by the optical radiation source 101 to the layer 1 10 and allows another part of the optical radiation to travel to the reference reflector 102.
  • the beam splitter 1 1 1 is a cube type beam splitter. It is also possible that the beam splitter is a dichroic mirror.
  • the measurement section 103 is configured to produce measurement results which are based on interference between optical waves reflected from the layer 1 10 and optical waves reflected from the reference reflector 102.
  • the optical radiation source 101 may comprise for example a light emitting diode, and the measurement section 103 may comprise for example a spectrometer for measuring the intensity of the received optical radiation as a function of wavelength.
  • the center wavelength ⁇ of the optical radiation emitted by the optical radiation source 101 can be for example 780 nm and the wavelength range can be for example from 775 nm to 785 nm.
  • the device further comprises a processing system 105 for post-processing the measurement results produced by the measurement section 103.
  • the processing system 105 can be configured to compute an inverse Fourier transform of the measured spectrum.
  • the computed inverse Fourier transform comprises peaks corresponding to components of the spectrum which alternate as a function of distance. These peaks are indicative of the positions of the surfaces of the layer 1 10.
  • the positions of the surfaces of the layer 1 10 are measured along the z- direction of a coordinate system 199, i.e. in the direction that is substantially perpendicular to the surfaces under consideration.
  • the inverse Fourier transform can be computed for example using the non-uniform fast inverse Fourier transform "NUIFFT" algorithm.
  • the device further comprises a modifier section 104 for changing an optical length of a reference optical path that comprises the reference reflector 102.
  • the modifier section 104 comprises transparent material for changing the optical length of the reference optical path.
  • the transparent material can be for example glass.
  • transparent is not limited to mean only penetrability by visible light but the term “transparent” covers also situations where material under consideration is not penetrable by visible light but is penetrable by invisible electromagnetic radiation suitable for interference based measurements. In other words, the term “transparent” is to be understood from the viewpoint of the electromagnetic radiation used in interference based measurements.
  • the above-mentioned optical length is dependent on the physical length of a part of the reference optical path constituted by the transparent material and on a difference between the refractive index of the transparent material and the refractive index of the surroundings of the transparent material.
  • the transparent material of the modifier section 104 is surrounded by air and therefore the refractive index of the surroundings is substantially one.
  • the modifier section 104 comprises a flat plate 106 made of the transparent material.
  • the optical length, i.e. the propagation time, of the reference optical path can be increased by adding the flat plate 106 on the reference optical path.
  • the position of the flat plate 106 on the reference optical path is illustrated with a dashed line 109. Mechanical structures for supporting the flat plate 106 are not shown in figure 1 a.
  • the processing system 105 is configured to compute an estimate d of the optical thickness of the layer 1 10 based on i) a first measurement result rt indicative of a position of a first surface of the layer 1 10 and measured with a first value of the optical length of the reference optical path, ii) a second measurement result rb indicative of a position of the second surface of the layer and measured with a second value of the optical length, and iii) a difference between the first and second values of the optical length taking in to account the optical path length of the flat plate 106.
  • the above-mentioned first surface of the layer 1 10 can be the front surface which faces towards the arriving optical radiation, and the above-mentioned second surface of the layer can be the back surface on the opposite side of the layer.
  • the first value of the optical length of the reference optical path can correspond to a situation in which the flat plate 106 is absent from the reference optical path, whereas the second value of the optical length of the reference optical path can correspond to a situation in which the flat plate 106 is present on the reference optical path.
  • the processing system 105 is configured to compute an estimate of the physical thickness D of the layer 1 10 based on the refractive index ni_ and the estimate d of the optical thickness of the layer 1 10.
  • Figure 1 b illustrates a part of a device according to an exemplifying and non-limiting embodiment of the invention.
  • the device is otherwise similar to the device illustrated in figure 1 a, but a modifier section 1 14 is different from the modifier section 104 shown in figure 1 a.
  • the modifier section 1 14 comprises two wedge-shaped pieces 1 16 and 1 17 which are made of transparent material, e.g. glass.
  • the physical thickness H, and also the optical thickness, of the modifier section 104 can be changed by sliding the wedge-shaped pieces 1 16 and 1 17 with respect to each other.
  • the optical length of the reference optical path can be changed by sliding the wedge-shaped pieces 1 16 and 1 17 with respect to each other.
  • Mechanical structures for supporting the wedge-shaped pieces 1 16 and 1 17 are not shown in figure 1 b.
  • Figure 1 c illustrates a part of a device according to an exemplifying and non-limiting embodiment of the invention.
  • the device is otherwise similar to the device illustrated in figure 1 a, but a modifier section 124 is different from the modifier section 104 shown in figure 1 a.
  • the modifier section 124 comprises a disk 126 made of transparent material e.g. glass.
  • the disk 126 comprises sectors having different physical thicknesses and/or different refractive indices so that the optical length of the reference optical path can be changed by rotating the disk.
  • the disk 126 comprises sectors whose physical thicknesses are Ha, Hb, and He. Mechanical structures for supporting the disk 126 are not shown in figure 1 c.
  • the above-mentioned Hi is the physical length of the part of the reference optical path constituted by the transparent material when measuring the first measurement result rf
  • H2 is the physical length of the part of the reference optical path constituted by the transparent material when measuring the second measurement result rb.
  • the above-mentioned nn is the refractive index of the transparent material that is in the reference optical path when measuring the first measurement result rf
  • the above-mentioned nt2 is the refractive index of the transparent material that is in the reference optical path when measuring the second measurement result rb.
  • the refractive index nn differs from the refractive index nt2 from example in an exemplifying case where the above-mentioned disk 126 comprises sectors having different refractive indices. In cases where the transparent material is surrounded by air or vacuum, the refractive index n s is substantially one or exactly one.
  • the above-mentioned H2 is the physical thickness H of the flat plate 106 and the above-mentioned Hi is zero because the flat plate 106 is absent from the reference optical path when the first measurement result rf is measured.
  • the above-mentioned Hi is the physical thickness H of the modifier section 1 14 when the first measurement result rf is measured.
  • the above-mentioned h is the physical thickness H of the modifier section 1 14 when the second measurement result rb is measured.
  • the above-mentioned Hi is the physical thickness, e.g.
  • H2 is the physical thickness, e.g. Ha, of another sector of the disk 126 which is on the reference optical path when the second measurement result rb is measured.
  • Figure 2 shows a schematic illustration of a device according to an exemplifying and non-limiting embodiment of the invention for measuring an optical thickness of a layer 210 consisting of one or more materials.
  • the device comprises an optical radiation source 201 , a beam splitter 21 1 , a reference reflector 202, and a measurement section 203.
  • the measurement section 203 is configured to produce measurement results which are based on interference between optical waves reflected from the layer 210 and optical waves reflected from the reference reflector 202.
  • the device comprises a processing system 205 for post-processing the measurement results produced by the measurement section 203.
  • the device further comprises a modifier section 204 for changing an optical length of a reference optical path that comprises the reference reflector 202.
  • the modifier section 204 comprises mechanical support structures 208 for supporting the reference reflector 202 so that the position of the reference reflector is changeable.
  • the optical length of the reference optical path can be changed by changing the position of the reference reflector 202 in a stepwise manner.
  • the position of a lens 213 changes together with the position of the reference reflector 202.
  • the processing system 205 is configured to compute an estimate d of the optical thickness of the layer 210 based on i) a first measurement result indicative of a position of a first surface of the layer 210 and measured with a first value of the optical length of the reference optical path, ii) a second measurement result indicative of a position of the second surface of the layer 210 and measured with a second value of the optical length, and iii) a difference between the first and second values of the optical length taking in to account the change in the position of the reference reflector 202.
  • the above-mentioned first surface of the layer 210 can be the front surface which faces towards the arriving optical radiation, and the above- mentioned second surface of the layer can be the back surface on the opposite side of the layer.
  • the first value of the optical length of the reference optical path can correspond to a situation in which the reference reflector 202 is in a first position shown in figure 2, whereas the second value of the optical length of the reference optical path can correspond to a situation in which the reference reflector 202 is in a second position depicted with a dashed line 209.
  • ⁇ _ is the distance from the first position of the reference reflector 202 corresponding to the first measurement result rt to the second position of the reference reflector 202 corresponding to the second measurement result rb.
  • the processing system 105 shown in figure 1 a as well as the processing system 205 shown in figure 2 can be implemented with one or more processor circuits, each of which can be a programmable processor circuit provided with appropriate software, a dedicated hardware processor such as, for example, an application specific integrated circuit "ASIC”, or a configurable hardware processor such as, for example, a field programmable gate array "FPGA”.
  • processor circuits each of which can be a programmable processor circuit provided with appropriate software, a dedicated hardware processor such as, for example, an application specific integrated circuit "ASIC”, or a configurable hardware processor such as, for example, a field programmable gate array "FPGA”.
  • ASIC application specific integrated circuit
  • FPGA field programmable gate array
  • the optical length of the reference optical path is changed in a stepwise manner between measuring the first and second measurement results. It is, however, also possible that the optical lengths of both the reference optical path and the measurement optical path are changed, or that only the optical length of the measurement optical path is changed.
  • the modifier section comprises both a) mechanical support structures for supporting the reference reflector so that the position of the reference reflector is changeable and b) transparent material for changing the optical length of the reference optical path and/or the measurement optical path.
  • Figure 3 shows a flowchart of a method according to an exemplifying and non- limiting embodiment of the invention for measuring an optical thickness of a layer, e.g. a cornea, consisting of one or more materials.
  • the method comprises the following actions:
  • - action 301 producing a first measurement result indicative of a position of a first surface of the layer, the first measurement result being based on interference between optical waves reflected from the first surface of the layer and optical waves reflected from a reference reflector,
  • - action 302 changing an optical length of at least one optical path each being a reference optical path comprising the reference reflector or a measurement optical path comprising the layer
  • - action 303 producing a second measurement result indicative of a position of the second surface of the layer, the second measurement result being based on interference between optical waves reflected from the second surface of the layer and optical waves reflected from the reference reflector
  • - action 304 computing an estimate of the optical thickness of the layer based on i) the first measurement result, ii) the second measurement result, and iii) a difference between the optical length related to the first measurement result and the optical length related to the second measurement result.
  • a method according to an exemplifying and non-limiting embodiment of the invention further comprises estimating the physical thickness of the layer based on the refractive index of the material of the layer and the estimate of the optical thickness of the layer.
  • a method according to an exemplifying and non-limiting embodiment of the invention comprises using transparent material for changing the optical length of the optical path .
  • the optical length depends on the physical length of a part of the optical path constituted by the transparent material and on the difference between the refractive index of the transparent material and the refractive index of the surroundings of the transparent material .
  • the optical length is changed by adding, to the optical path, a flat plate made of the transparent material.
  • the optical length is changed with the aid of two wedge-shaped pieces made of the transparent material.
  • the optical length is changed by sliding the two wedge-shaped pieces with respect to each other.
  • the optical length is changed with the aid of a disk made of the transparent material and having sectors featuring different physical thicknesses and/or different refractive indices.
  • the optical length is changed by rotating the disk so that a sector of the disk located on the optical path is changed.
  • the optical length is changed by changing the position of the reference reflector.
  • a method according to an exemplifying and non-limiting embodiment of the invention comprises both a) moving the reference reflector and b) using the above- mentioned transparent material so as to change the optical length.
  • a method comprises measuring a spectrum of the optical radiation constituted by the optical waves reflected from the layer and the optical waves reflected from the reference reflector.
  • the spectrum is indicative of the intensity of the optical radiation as a function of wavelength.
  • a method according to an exemplifying and non-limiting embodiment of the invention comprises computing an inverse Fourier transform of the spectrum.
  • the above-mentioned first and second measurement results are obtained on the basis of peaks appearing in the computed inverse Fourier transform.
  • a computer program according to an exemplifying and non-limiting embodiment of the invention comprises computer executable instructions for controlling a programmable processor to carry out actions related to a method according to any of the above-described exemplifying embodiments of the invention.
  • a computer program according to an exemplifying and non-limiting embodiment of the invention comprises software modules for obtaining an estimate for an optical thickness of a layer consisting of one or more materials.
  • the software modules comprise computer executable instructions for controlling a programmable processor to compute the estimate of the optical thickness of the layer based on i) a first measurement result indicative of a position of a first surface of the layer, ii) a second measurement result indicative of a position of the second surface of the layer, and iii) first and second values of an optical length of at least one optical path each being either a reference optical path comprising a reference reflector or a measurement optical path comprising the layer, wherein: - the first measurement result is based on interference between optical waves reflected from the first surface of the layer and optical waves reflected from the reference reflector when the optical length has the first value, and
  • the second measurement result is based on interference between optical waves reflected from the second surface of the layer and optical waves reflected from the reference reflector when the optical length has the second value different from the first value.
  • a computer program further comprises computer executable instructions for controlling the programmable processor to compute an estimate of the physical thickness of the layer based on the refractive index of the material of the layer and the estimate of the optical thickness of the layer.
  • a computer program product according to an exemplifying and non-limiting embodiment of the invention comprises a computer readable medium, e.g. a compact disc "CD", encoded with a computer program according to an exemplifying embodiment of invention.
  • a signal according to an exemplifying and non-limiting embodiment of the invention is encoded to carry information defining a computer program according to an exemplifying embodiment of invention.

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Abstract

A device for measuring an optical thickness of a layer (110)consisting of one or more materials comprises a measurement section (103) for producing measurement results based on interference between optical waves reflected from the layer and optical waves reflected from a reference reflector (102). The device comprises a modifier section (104) for changing the optical length of a reference optical path comprising the reference reflector and/or the optical length of a measurement optical path comprising the layer. The optical thickness is estimated based on: a first measurement result indicative of a position of a first surface of the layer and measured with a first value of the optical length, a second measurement result indicative of a position of the second surface of the layer and measured with a second value of the optical length, and a difference between the first and second values of the optical length.

Description

Device and method for measuring an optical thickness of a layer
Technical field
The disclosure relates to a device for measuring an optical thickness of a layer consisting of one or more materials. The layer can be, for example but not necessarily, a cornea. Furthermore, the disclosure relates to a method for measuring an optical thickness of a layer. Furthermore, the disclosure relates to a computer program for estimating an optical thickness of a layer.
Background In many cases, an optical thickness of a layer consisting of one or more materials can be measured using interferometry where optical radiation is directed to the layer under consideration and to a reference reflector. The layer to be measured can be e.g. a cornea. The measurement of the optical thickness is based on interference between optical waves reflected from the reference reflector, optical waves reflected from a first surface of the layer, and optical waves reflected from the second surface of the layer. An estimate of the optical thickness can be obtained for example with the aid of a wavelength spectrum of optical radiation that comprises the above- mentioned optical waves reflected from the reference reflector and from the surfaces of the layer. The wavelength spectrum can be measured with e.g. a spectrometer or another suitable device. The process for estimating the optical thickness may comprise detecting wavelengths where constructive interference takes place and/or detecting wavelengths where destructive interferences take place. This can be carried out for example by computing an inverse Fourier transform of the wavelength spectrum. The computed inverse Fourier transform comprises peaks corresponding to components of the wavelength spectrum which alternate as a function of a distance. These peaks can be used to estimate the optical thickness of the layer under consideration. The above-described measurement method is not free from challenges. One challenge is that the optical measurement range is limited according to the following equation:
Figure imgf000003_0001
where rmax is the maximum optical measurement range, λο is the center wavelength of the optical radiation, and Δλ is the spectral resolution of the wavelength spectrum.
For example, in a case where the center wavelength of the optical radiation is 780 nm and the spectral resolution is 0.5 nm, the maximum optical measurement range is about 300 μιτι. This maximum optical measurement range is insufficient for example in cases where the cornea thickness is about 600 μιτι which is significantly beyond the maximum optical measurement range 300 μιτι.
Summary
The following presents a simplified summary to provide basic understanding of some aspects of different invention embodiments. The summary is not an extensive overview of the invention. It is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. The following summary merely presents some concepts of the invention in a simplified form as a prelude to a more detailed description of exemplifying and non-limiting embodiments of the invention. In accordance with the invention, there is provided a new method for measuring an optical thickness of a layer that consists of one or more materials. The layer to be measured can be, for example but not necessarily, a cornea. A method according to the invention comprises:
- producing a first measurement result indicative of a position of a first surface of the layer, the first measurement result being based on interference between optical waves reflected from the first surface of the layer and optical waves reflected from a reference reflector, - changing an optical length, i.e. the propagation time, of at least one optical path each being either a reference optical path comprising the reference reflector or a measurement optical path comprising the layer,
- producing a second measurement result indicative of a position of the second surface of the layer, the second measurement result being based on interference between optical waves reflected from the second surface of the layer and optical waves reflected from the reference reflector, and
- computing an estimate of the optical thickness of the layer based on i) the first measurement result, ii) the second measurement result, and iii) a difference between the optical length related to the first measurement result and the optical length related to the second measurement result.
In the above-described method, the change in the above-mentioned optical length makes the layer appear optically thinner in the interference-based measurement than in a case where there is no change in the optical length. Therefore, the limitation related to the optical measurement range can be circumvented. In the last- mentioned method phase, the change in the optical length is computationally compensated for in order to estimate the real optical thickness of the layer. In cases where the layer is homogenous so that the refractive index is same in all points of the layer, an estimate of the physical, i.e. geometrical, thickness of the layer can be obtained based on the refractive index and the estimate of the optical thickness.
In this document, the term "optical waves" is not limited to mean only visible light but the scope of the term "optical waves" includes also invisible radiation such as e.g. ultraviolet and infrared light, as well as any other electromagnetic radiation which is suitable for interference based measurements. In accordance with the invention, there is provided also a new device for measuring an optical thickness of a layer consisting of one or more materials. A device according to the invention comprises:
- an optical radiation source, e.g. a light emitting diode,
- a reference reflector, - a measurement section for producing measurement results based on interference between optical waves reflected from the layer and optical waves reflected from the reference reflector,
- a modifier section for changing the optical length of at least one optical path each being either a reference optical path comprising the reference reflector or a measurement optical path comprising the layer, and
- a processing system for computing an estimate of the optical thickness of the layer based on i) a first measurement result indicative of a position of a first surface of the layer and measured with a first value of the optical length, ii) a second measurement result indicative of a position of a second surface of the layer and measured with a second value of the optical length, and iii) a difference between the first and second values of the optical length.
In accordance with the invention, there is provided also a new computer program for obtaining an estimate of an optical thickness of a layer consisting of one or more materials. A computer program according to the invention comprises computer executable instructions for controlling a programmable processor to compute the estimate of the optical thickness of the layer based on i) a first measurement result indicative of a position of a first surface of the layer, ii) a second measurement result indicative of a position of the second surface of the layer, and iii) first and second values of an optical length of at least one optical path each being either a reference optical path comprising a reference reflector or a measurement optical path comprising the layer, wherein:
- the first measurement result is based on interference between optical waves reflected from the first surface of the layer and optical waves reflected from the reference reflector when the optical length has the first value, and
- the second measurement result is based on interference between optical waves reflected from the second surface of the layer and optical waves reflected from the reference reflector when the optical length has the second value different from the first value. In accordance with the invention, there is provided also a new computer program product. The computer program product comprises a non-volatile computer readable medium, e.g. a compact disc "CD", encoded with a computer program according to the invention. A number of exemplifying and non-limiting embodiments of the invention are described in accompanied dependent claims.
Exemplifying and non-limiting embodiments of the invention both as to constructions and to methods of operation, together with additional objects and advantages thereof, are best understood from the following description of specific exemplifying embodiments when read in connection with the accompanying drawings.
The verbs "to comprise" and "to include" are used in this document as open limitations that neither exclude nor require the existence of un-recited features. The features recited in dependent claims are mutually freely combinable unless otherwise explicitly stated. Furthermore, it is to be understood that the use of "a" or "an", i.e. a singular form, throughout this document does not exclude a plurality.
Brief description of figures
Exemplifying and non-limiting embodiments of the invention and their advantages are explained in greater detail below with reference to the accompanying drawings, in which: figures 1 a, 1 b, and 1 c illustrate devices according to exemplifying and non-limiting embodiments of the invention for measuring an optical thickness of a layer consisting of one or more materials, figure 2 illustrates a device according to an exemplifying and non-limiting embodiment of the invention for measuring an optical thickness of a layer consisting of one or more materials, and figure 3 shows a flowchart of a method according to an exemplifying and non-limiting embodiment of the invention for measuring an optical thickness of a layer consisting of one or more materials. Description of exemplifying and non-limiting embodiments
The specific examples provided in the description below should not be construed as limiting the scope and/or the applicability of the accompanied claims. Lists and groups of examples provided in the description below are not exhaustive unless otherwise explicitly stated.
Figure 1 a shows a schematic illustration of a device according to an exemplifying and non-limiting embodiment of the invention for measuring an optical thickness of a layer 1 10 consisting of one or more materials. The device comprises an optical radiation source 101 , a beam splitter "BS" 1 1 1 , a reference reflector 102, and a measurement section 103. The beam-slitter 1 1 1 reflects a part of the optical radiation emitted by the optical radiation source 101 to the layer 1 10 and allows another part of the optical radiation to travel to the reference reflector 102. In this exemplifying case, the beam splitter 1 1 1 is a cube type beam splitter. It is also possible that the beam splitter is a dichroic mirror. The measurement section 103 is configured to produce measurement results which are based on interference between optical waves reflected from the layer 1 10 and optical waves reflected from the reference reflector 102. The optical radiation source 101 may comprise for example a light emitting diode, and the measurement section 103 may comprise for example a spectrometer for measuring the intensity of the received optical radiation as a function of wavelength. The center wavelength λο of the optical radiation emitted by the optical radiation source 101 can be for example 780 nm and the wavelength range can be for example from 775 nm to 785 nm.
The device further comprises a processing system 105 for post-processing the measurement results produced by the measurement section 103. In an exemplifying case where the measurement section 103 comprises a spectrometer, the processing system 105 can be configured to compute an inverse Fourier transform of the measured spectrum. The computed inverse Fourier transform comprises peaks corresponding to components of the spectrum which alternate as a function of distance. These peaks are indicative of the positions of the surfaces of the layer 1 10. The positions of the surfaces of the layer 1 10 are measured along the z- direction of a coordinate system 199, i.e. in the direction that is substantially perpendicular to the surfaces under consideration. The inverse Fourier transform can be computed for example using the non-uniform fast inverse Fourier transform "NUIFFT" algorithm.
The device further comprises a modifier section 104 for changing an optical length of a reference optical path that comprises the reference reflector 102. In the exemplifying device illustrated in figure 1 a, the modifier section 104 comprises transparent material for changing the optical length of the reference optical path. The transparent material can be for example glass. In this document, the term "transparent" is not limited to mean only penetrability by visible light but the term "transparent" covers also situations where material under consideration is not penetrable by visible light but is penetrable by invisible electromagnetic radiation suitable for interference based measurements. In other words, the term "transparent" is to be understood from the viewpoint of the electromagnetic radiation used in interference based measurements. The above-mentioned optical length is dependent on the physical length of a part of the reference optical path constituted by the transparent material and on a difference between the refractive index of the transparent material and the refractive index of the surroundings of the transparent material. In many cases, the transparent material of the modifier section 104 is surrounded by air and therefore the refractive index of the surroundings is substantially one. In the exemplifying device illustrated in figure 1 a, the modifier section 104 comprises a flat plate 106 made of the transparent material. The optical length, i.e. the propagation time, of the reference optical path can be increased by adding the flat plate 106 on the reference optical path. In figure 1 a, the position of the flat plate 106 on the reference optical path is illustrated with a dashed line 109. Mechanical structures for supporting the flat plate 106 are not shown in figure 1 a.
The processing system 105 is configured to compute an estimate d of the optical thickness of the layer 1 10 based on i) a first measurement result rt indicative of a position of a first surface of the layer 1 10 and measured with a first value of the optical length of the reference optical path, ii) a second measurement result rb indicative of a position of the second surface of the layer and measured with a second value of the optical length, and iii) a difference between the first and second values of the optical length taking in to account the optical path length of the flat plate 106. The above-mentioned first surface of the layer 1 10 can be the front surface which faces towards the arriving optical radiation, and the above-mentioned second surface of the layer can be the back surface on the opposite side of the layer. The first value of the optical length of the reference optical path can correspond to a situation in which the flat plate 106 is absent from the reference optical path, whereas the second value of the optical length of the reference optical path can correspond to a situation in which the flat plate 106 is present on the reference optical path.
In cases where the layer 1 10 is homogenous so that the refractive index ni_ is same in all points of the layer, the physical, i.e. geometrical, thickness D of the layer 1 10 can be estimated based on the refractive index ni_ and the estimate d of the optical thickness. In a device according to an exemplifying and non-limiting embodiment of the invention, the processing system 105 is configured to compute an estimate of the physical thickness D of the layer 1 10 based on the refractive index ni_ and the estimate d of the optical thickness of the layer 1 10.
Figure 1 b illustrates a part of a device according to an exemplifying and non-limiting embodiment of the invention. The device is otherwise similar to the device illustrated in figure 1 a, but a modifier section 1 14 is different from the modifier section 104 shown in figure 1 a. The modifier section 1 14 comprises two wedge-shaped pieces 1 16 and 1 17 which are made of transparent material, e.g. glass. As illustrated in figure 1 b, the physical thickness H, and also the optical thickness, of the modifier section 104 can be changed by sliding the wedge-shaped pieces 1 16 and 1 17 with respect to each other. Thus, the optical length of the reference optical path can be changed by sliding the wedge-shaped pieces 1 16 and 1 17 with respect to each other. Mechanical structures for supporting the wedge-shaped pieces 1 16 and 1 17 are not shown in figure 1 b.
Figure 1 c illustrates a part of a device according to an exemplifying and non-limiting embodiment of the invention. The device is otherwise similar to the device illustrated in figure 1 a, but a modifier section 124 is different from the modifier section 104 shown in figure 1 a. The modifier section 124 comprises a disk 126 made of transparent material e.g. glass. The disk 126 comprises sectors having different physical thicknesses and/or different refractive indices so that the optical length of the reference optical path can be changed by rotating the disk. In the exemplifying case illustrated in figure 1 c, the disk 126 comprises sectors whose physical thicknesses are Ha, Hb, and He. Mechanical structures for supporting the disk 126 are not shown in figure 1 c.
In a device according to an exemplifying and non-limiting embodiment of the invention, the processing system 105 shown in figure 1 a is configured to compute the estimate d of the optical thickness of the layer 101 according to the following equation: d = H2 *(nt2 - n8) - Η (η« - ns) + (rb - rf), (2) where ns is the refractive index of the surroundings of the transparent material, rt is the first measurement result indicative of the position of the first surface of the layer 1 10, and rb is the second measurement result indicative of the position of the second surface of the layer 1 10. The above-mentioned Hi is the physical length of the part of the reference optical path constituted by the transparent material when measuring the first measurement result rf, and correspondingly H2 is the physical length of the part of the reference optical path constituted by the transparent material when measuring the second measurement result rb. The above-mentioned nn is the refractive index of the transparent material that is in the reference optical path when measuring the first measurement result rf. Correspondingly, the above-mentioned nt2 is the refractive index of the transparent material that is in the reference optical path when measuring the second measurement result rb. The refractive index nn differs from the refractive index nt2 from example in an exemplifying case where the above-mentioned disk 126 comprises sectors having different refractive indices. In cases where the transparent material is surrounded by air or vacuum, the refractive index ns is substantially one or exactly one.
In the exemplifying case illustrated in figure 1 a, the above-mentioned H2 is the physical thickness H of the flat plate 106 and the above-mentioned Hi is zero because the flat plate 106 is absent from the reference optical path when the first measurement result rf is measured. In the exemplifying case illustrated in figure 1 b, the above-mentioned Hi is the physical thickness H of the modifier section 1 14 when the first measurement result rf is measured. Correspondingly, the above-mentioned h is the physical thickness H of the modifier section 1 14 when the second measurement result rb is measured. In the exemplifying case illustrated in figure 1 c, the above-mentioned Hi is the physical thickness, e.g. He, of a sector of the disk 126 which is on the reference optical path when the first measurement result rt is measured. Correspondingly, the above-mentioned H2 is the physical thickness, e.g. Ha, of another sector of the disk 126 which is on the reference optical path when the second measurement result rb is measured.
Figure 2 shows a schematic illustration of a device according to an exemplifying and non-limiting embodiment of the invention for measuring an optical thickness of a layer 210 consisting of one or more materials. The device comprises an optical radiation source 201 , a beam splitter 21 1 , a reference reflector 202, and a measurement section 203. The measurement section 203 is configured to produce measurement results which are based on interference between optical waves reflected from the layer 210 and optical waves reflected from the reference reflector 202. The device comprises a processing system 205 for post-processing the measurement results produced by the measurement section 203. The device further comprises a modifier section 204 for changing an optical length of a reference optical path that comprises the reference reflector 202. In this exemplifying device, the modifier section 204 comprises mechanical support structures 208 for supporting the reference reflector 202 so that the position of the reference reflector is changeable. Thus, the optical length of the reference optical path can be changed by changing the position of the reference reflector 202 in a stepwise manner. In the exemplifying device illustrated in figure 2, the position of a lens 213 changes together with the position of the reference reflector 202.
The processing system 205 is configured to compute an estimate d of the optical thickness of the layer 210 based on i) a first measurement result indicative of a position of a first surface of the layer 210 and measured with a first value of the optical length of the reference optical path, ii) a second measurement result indicative of a position of the second surface of the layer 210 and measured with a second value of the optical length, and iii) a difference between the first and second values of the optical length taking in to account the change in the position of the reference reflector 202. The above-mentioned first surface of the layer 210 can be the front surface which faces towards the arriving optical radiation, and the above- mentioned second surface of the layer can be the back surface on the opposite side of the layer. The first value of the optical length of the reference optical path can correspond to a situation in which the reference reflector 202 is in a first position shown in figure 2, whereas the second value of the optical length of the reference optical path can correspond to a situation in which the reference reflector 202 is in a second position depicted with a dashed line 209.
In the exemplifying device illustrated in figure 2, the processing system 205 is configured to compute the estimate d of the optical thickness of the layer 210 according to the following equation: d = AL + rb - rf, (3) where rt is the first measurement result indicative of the position of the first surface of the layer 210 and rb is the second measurement result indicative of the position of the second surface of the layer 210. The above-mentioned ΔΙ_ is the distance from the first position of the reference reflector 202 corresponding to the first measurement result rt to the second position of the reference reflector 202 corresponding to the second measurement result rb.
The processing system 105 shown in figure 1 a as well as the processing system 205 shown in figure 2 can be implemented with one or more processor circuits, each of which can be a programmable processor circuit provided with appropriate software, a dedicated hardware processor such as, for example, an application specific integrated circuit "ASIC", or a configurable hardware processor such as, for example, a field programmable gate array "FPGA".
In the exemplifying cases illustrated in figures 1 a-1 c and in figure 2, the optical length of the reference optical path is changed in a stepwise manner between measuring the first and second measurement results. It is, however, also possible that the optical lengths of both the reference optical path and the measurement optical path are changed, or that only the optical length of the measurement optical path is changed.
In a device according to an exemplifying and non-limiting embodiment of the invention, the modifier section comprises both a) mechanical support structures for supporting the reference reflector so that the position of the reference reflector is changeable and b) transparent material for changing the optical length of the reference optical path and/or the measurement optical path.
Figure 3 shows a flowchart of a method according to an exemplifying and non- limiting embodiment of the invention for measuring an optical thickness of a layer, e.g. a cornea, consisting of one or more materials. The method comprises the following actions:
- action 301 : producing a first measurement result indicative of a position of a first surface of the layer, the first measurement result being based on interference between optical waves reflected from the first surface of the layer and optical waves reflected from a reference reflector,
- action 302: changing an optical length of at least one optical path each being a reference optical path comprising the reference reflector or a measurement optical path comprising the layer, - action 303: producing a second measurement result indicative of a position of the second surface of the layer, the second measurement result being based on interference between optical waves reflected from the second surface of the layer and optical waves reflected from the reference reflector, and - action 304: computing an estimate of the optical thickness of the layer based on i) the first measurement result, ii) the second measurement result, and iii) a difference between the optical length related to the first measurement result and the optical length related to the second measurement result. A method according to an exemplifying and non-limiting embodiment of the invention further comprises estimating the physical thickness of the layer based on the refractive index of the material of the layer and the estimate of the optical thickness of the layer. A method according to an exemplifying and non-limiting embodiment of the invention comprises using transparent material for changing the optical length of the optical path . The optical length depends on the physical length of a part of the optical path constituted by the transparent material and on the difference between the refractive index of the transparent material and the refractive index of the surroundings of the transparent material .
In a method according to an exemplifying and non-limiting embodiment of the invention, the optical length is changed by adding, to the optical path, a flat plate made of the transparent material.
In a method according to an exemplifying and non-limiting embodiment of the invention, the optical length is changed with the aid of two wedge-shaped pieces made of the transparent material. The optical length is changed by sliding the two wedge-shaped pieces with respect to each other.
In a method according to an exemplifying and non-limiting embodiment of the invention, the optical length is changed with the aid of a disk made of the transparent material and having sectors featuring different physical thicknesses and/or different refractive indices. The optical length is changed by rotating the disk so that a sector of the disk located on the optical path is changed.
In a method according to an exemplifying and non-limiting embodiment of the invention, the estimate d for the optical thickness of the layer is computed according to the following equation: d = H2*(nt2 - ns) - Η (η« - ns) + (rb - rf), where Hi is the physical length of the part of the optical path constituted by the transparent material when measuring the first measurement result, h is the physical length of the part of the optical path constituted by the transparent material when measuring the second measurement result, , ns is the refractive index of the surroundings of the transparent material, rt is the first measurement result, rb is the second measurement result, nn is the refractive index of the transparent material that is in the reference optical path when measuring the first measurement result rt, and nt2 is the refractive index of the transparent material that is in the reference optical path when measuring the second measurement result rb.
In a method according to an exemplifying and non-limiting embodiment of the invention, the optical length is changed by changing the position of the reference reflector. In a method according to an exemplifying and non-limiting embodiment of the invention, the estimate d for the optical thickness of the layer is computed according to the following equation: d = AL + rb - rf, where rt is the first measurement result, rb is the second measurement result, and AL is the distance from a position of the reference reflector corresponding to the first measurement result to another position of the reference reflector corresponding to the second measurement result.
A method according to an exemplifying and non-limiting embodiment of the invention comprises both a) moving the reference reflector and b) using the above- mentioned transparent material so as to change the optical length.
A method according to an exemplifying and non-limiting embodiment of the invention comprises measuring a spectrum of the optical radiation constituted by the optical waves reflected from the layer and the optical waves reflected from the reference reflector. The spectrum is indicative of the intensity of the optical radiation as a function of wavelength.
A method according to an exemplifying and non-limiting embodiment of the invention comprises computing an inverse Fourier transform of the spectrum. The above-mentioned first and second measurement results are obtained on the basis of peaks appearing in the computed inverse Fourier transform. A computer program according to an exemplifying and non-limiting embodiment of the invention comprises computer executable instructions for controlling a programmable processor to carry out actions related to a method according to any of the above-described exemplifying embodiments of the invention. A computer program according to an exemplifying and non-limiting embodiment of the invention comprises software modules for obtaining an estimate for an optical thickness of a layer consisting of one or more materials. The software modules comprise computer executable instructions for controlling a programmable processor to compute the estimate of the optical thickness of the layer based on i) a first measurement result indicative of a position of a first surface of the layer, ii) a second measurement result indicative of a position of the second surface of the layer, and iii) first and second values of an optical length of at least one optical path each being either a reference optical path comprising a reference reflector or a measurement optical path comprising the layer, wherein: - the first measurement result is based on interference between optical waves reflected from the first surface of the layer and optical waves reflected from the reference reflector when the optical length has the first value, and
- the second measurement result is based on interference between optical waves reflected from the second surface of the layer and optical waves reflected from the reference reflector when the optical length has the second value different from the first value.
A computer program according to an exemplifying and non-limiting embodiment of the invention further comprises computer executable instructions for controlling the programmable processor to compute an estimate of the physical thickness of the layer based on the refractive index of the material of the layer and the estimate of the optical thickness of the layer.
The above-mentioned software modules can be e.g. subroutines and/or functions implemented with a programming language suitable for the programmable processor under consideration. A computer program product according to an exemplifying and non-limiting embodiment of the invention comprises a computer readable medium, e.g. a compact disc "CD", encoded with a computer program according to an exemplifying embodiment of invention. A signal according to an exemplifying and non-limiting embodiment of the invention is encoded to carry information defining a computer program according to an exemplifying embodiment of invention.
The non-limiting, specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims. Furthermore, any list or group of examples presented in this document is not exhaustive unless otherwise explicitly stated.

Claims

What is claimed is:
1 . A device for measuring an optical thickness of a layer consisting of one or more materials, the device comprising:
- an optical radiation source (101 , 201 ), - a reference reflector (102, 202),
- a measurement section (103, 203) for producing measurement results based on interference between optical waves reflected from the layer and optical waves reflected from the reference reflector, and
- a modifier section (104, 1 14, 124, 204) for changing an optical length of at least one optical path each being a reference optical path comprising the reference reflector or a measurement optical path comprising the layer, characterized in that the device further comprises a processing system (105, 205) for computing an estimate of the optical thickness of the layer on the basis of i) a first measurement result indicative of a position of a first surface of the layer and measured with a first value of the optical length, ii) a second measurement result indicative of a position of a second surface of the layer and measured with a second value of the optical length, and iii) a difference between the first and second values of the optical length.
2. A device according to claim 1 , wherein the modifier section (104, 1 14, 124) comprises at least one transparent material for changing the optical length of the optical path, the optical length being dependent on a physical length of a part of the optical path constituted by the transparent material and on a difference between a refractive index of the transparent material and a refractive index of surroundings of the transparent material.
3. A device according to claim 2, wherein the modifier section (104) comprises a flat plate (106) made of the transparent material, the flat plate being in the optical path when the second measurement result is being measured and the flat plate being absent from the optical path when the first measurement result is being measured.
4. A device according to claim 2, wherein the modifier section (1 14) comprises two wedge-shaped pieces (1 16, 1 17) made of the transparent material, the modifier section changing the optical length when the two wedge-shaped pieces are slid with respect to each other.
5. A device according to claim 2, wherein the modifier section (124) comprises a disk (126) made of the transparent material and having sectors having different physical thicknesses and/or different refractive indices, the disk changing the optical length when being rotated so that a sector of the disk located in the optical path is changed.
6. A device according to any of claims 2-5, wherein the processing system is configured to compute the estimate d for the optical thickness of the layer according to the following equation: d = H2 *(nt2 - ns) - Η (η« - n8) + (rb - rf), where Hi is the physical length of the part of the optical path constituted by the transparent material when measuring the first measurement result, h is the physical length of the part of the optical path constituted by the transparent material when measuring the second measurement result, nn is the refractive index of the transparent material that is in the optical path when measuring the first measurement result, nt2 is the refractive index of the transparent material that is in the optical path when measuring the second measurement result, ns is the refractive index of the surroundings of the transparent material, rt is the first measurement result, and rb is the second measurement result.
7. A device according to any of claims 1 -6, wherein the modifier section (204) comprises mechanical support structures (208) for supporting the reference reflector so that a position of the reference reflector is changeable so as to change the optical length of the reference optical path.
8. A device according to claim 7, wherein the processing system is configured to compute the estimate d of the optical thickness of the layer according to the following equation: d = AL + rb - rf, where rt is the first measurement result, rb is the second measurement result, and AL is a distance from a first position of the reference reflector corresponding to the first measurement result to a second position of the reference reflector corresponding to the second measurement result.
9. A device according to any of claims 1 -8, wherein the measurement section comprises a spectrometer for measuring a spectrum of optical radiation received at the measurement section, the spectrum being indicative of intensity of the optical radiation as a function of wavelength.
10. A method for measuring an optical thickness of a layer consisting of one or more materials, the method comprising: - producing (301 ) a first measurement result indicative of a position of a first surface of the layer, the first measurement result being based on interference between optical waves reflected from the first surface of the layer and optical waves reflected from a reference reflector, characterized in that the method further comprises: - changing (302) an optical length of at least one optical path each being a reference optical path comprising the reference reflector or a measurement optical path comprising the layer,
- producing (303) a second measurement result indicative of a position of a second surface of the layer, the second measurement result being based on interference between optical waves reflected from the second surface of the layer and optical waves reflected from the reference reflector, and
- computing (304) an estimate of the optical thickness of the layer based on i) the first measurement result, ii) the second measurement result, and iii) a difference between the optical length related to the first measurement result and the optical length related to the second measurement result.
1 1 . A method according to claim 10, wherein the method comprises using at least one transparent material for changing the optical length of the optical path, the optical length being dependent on a physical length of a part of the optical path constituted by the transparent material and on a difference between a refractive index of the transparent material and a refractive index of surroundings of the transparent material.
12. A method according to claim 1 1 , wherein the optical length is changed by adding, to the optical path, a flat plate made of the transparent material.
13. A method according to claim 1 1 , wherein the optical length is changed with the aid of two wedge-shaped pieces made of the transparent material, the optical length being changed by sliding the two wedge-shaped pieces with respect to each other.
14. A method according to claim 1 1 , wherein the optical length is changed with the aid of a disk made of the transparent material and having sectors having different physical thicknesses and/or different refractive indices, the optical length being changed by rotating the disk so that a sector of the disk located on the optical path is changed.
15. A method according to any of claims 1 1 -14, wherein the estimate d of the optical thickness of the layer is computed according to the following equation: d = H2 *(nt2 - ns) - Hi*(nti - ns) + (rb - rf), where Hi is the physical length of the part of the optical path constituted by the transparent material when measuring the first measurement result, h is the physical length of the part of the optical path constituted by the transparent material when measuring the second measurement result, nn is the refractive index of the transparent material that is in the optical path when measuring the first measurement result, nt2 is the refractive index of the transparent material that is in the optical path when measuring the second measurement result, ns is the refractive index of the surroundings of the transparent material, is the first measurement result, and rb is the second measurement result.
16. A method according to any of claims 10-15, wherein the optical length is changed by changing a position of the reference reflector.
17. A method according to claim 16, wherein the estimate d of the optical thickness of the layer is computed according to the following equation: d = AL + rb - rf, where rt is the first measurement result, rb is the second measurement result, and AL is a distance from a first position of the reference reflector corresponding to the first measurement result to a second position of the reference reflector corresponding to the second measurement result.
18. A method according to any of claims 10-17, wherein the method comprises measuring a spectrum of optical radiation constituted by the optical waves reflected from the layer and the optical waves reflected from the reference reflector, the spectrum being indicative of intensity of the optical radiation as a function of wavelength.
19. A method according to any of claims 10-18, wherein the layer is a cornea.
20. A computer program for obtaining an estimate of an optical thickness of a layer consisting of one or more materials, characterized in that the computer program comprises computer executable instructions for controlling a programmable processor to compute the estimate of the optical thickness of the layer based on i) a first measurement result indicative of a position of a first surface of the layer, ii) a second measurement result indicative of a position of a second surface of the layer, and iii) first and second values of an optical length of at least one optical path each being a reference optical path comprising a reference reflector or a measurement optical path comprising the layer, wherein: - the first measurement result is based on interference between optical waves reflected from the first surface of the layer and optical waves reflected from the reference reflector when the optical length has the first value, and
- the second measurement result is based on interference between optical waves reflected from the second surface of the layer and optical waves reflected from the reference reflector when the optical length has the second value different from the first value.
21 . A computer program product comprising a non-volatile computer readable medium encoded with a computer program according to claim 20.
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