Ophthalmological length measurement by means of dual-beam space-time domain wavelength tuning low-coherence interferometry

Abstract

Measurement of intraocular lengths by dual-beam Fourier low-coherence interferometry on the basis of Fresnel-zone-type space-time domain interferograms of the Purkinje-Sanson reflexes. The eye is illuminated by parallel, monochromatic dual beams having wavelengths which differ in temporal sequence. Wavelength spectra of space-time domain interferograms are imaged onto a photodetector array and registered. Viewing direction and position of the eye are fixed by optical aids and are monitored by acoustic and optical aids. A zoom optical unit in the output beam of the ophthalmological interferometer makes it possible, by simple focusing, to image virtual Fresnel-zone-type space-time domain interferograms from contrast-optimized positions onto the photodetector array or onto an image intensifier that is arranged in front of the photodetector array such that the position-dependent size change of the space-time domain interferograms is compensated by the scale change of this imaging.

Claims

1. An arrangement for ophthalmological length measurement by dual-beam space-time domain wavelength tuning interferometry, comprising a double-beam interferometer that illuminates a patient's eye with a measuring dual-beam; a tunable laser that illuminates the double-beam interferometer and thus generates a tunable measuring dual-beam made up of paired monochromatic, temporally offset coaxial components at an output of the interferometer, which illuminates the patient's eye after being reflected at a beam splitter; and wherein the structure of the eye is calculated from a spectrum of scattered-field intensity data I.sub.ξ,Ψ(k) of a scattered and/or reflected scattered-field that is emitted from the patient's eye; wherein the scattered-field intensity data I.sub.ξ,Ψ(k) that is necessary for calculation of structure of the eye is imaged onto an image intensifier or a photodetector array via a lens from the space-time domain interferogram (RZI) of an output beam of the patient's eye localized in transversal ξ-Ψ-positions in longitudinal direction in front or behind the cornea, and are then transferred to a computer; and optics designed as a zoom lens arranged within the output beam of the interferometer, wherein the zoom lens functions as an inverse magnifying glass and images differently positioned space-time domain interferograms in z-direction via a focal length setting thereof along the visual axis of the eye onto a fixed position of the image intensifier or of the photodetector array in z-direction thereby imaging the space-time domain interferograms of the eye from contrast-optimized positions onto the photodetector array.

2. The arrangement for the ophthalmological length measurement by dual-beam space-time domain wavelength tuning interferometry according to claim 1, wherein the scattered-field intensity data I.sub.ξ,Ψ(k) that is necessary for calculation of structure of the eye is imaged onto the image intensifier and wherein the image intensifier comprises a photocathode at an input thereof, a microchannel plate that amplifies the image and a phosphor screen at an output thereof.

3. The arrangement for the ophthalmological length measurement by dual-beam space-time domain wavelength tuning interferometry according to claim 1, wherein the space-time domain interferograms are positioned on the image intensifier or wherein the optics, which is imaging the photodetector array, is positioned within the output beam on the z-axis such that an increase of a ring diameter of a Fresnel zone-like space-time domain interferograms along the visual axis of the eye is compensated by a scale of the image of the RZI on the photodetector array or on the image intensifier, which is decreasing in z-direction with increasing distance from the eye.

4. The arrangement for the ophthalmological length measurement by dual-beam space-time domain wavelength tuning interferometry according to claim 1, wherein the monochromatic dual-beams of different wavelengths are generated by the tunable laser and a second beam splitter in a Michelson interferometer.

5. The arrangement for the ophthalmological length measurement by dual-beam space-time domain wavelength tuning interferometry according to claim 1, further comprising two fixation light beams that illuminate the patient's eye wherein the two fixation light beams are coaxial with regards to the measuring dual-beams and feature different colors for direction of the patient's eye.

6. The arrangement for the ophthalmological length measurement by dual-beam space-time domain wavelength tuning interferometry according to claim 5, wherein the fixation light beams for the direction of the patient's eye are implemented by an imaging of an guiding light conductor output surface of a light-guiding light conductor through a lens onto a fundus of the eye.

7. The arrangement for the ophthalmological length measurement by dual-beam space-time domain wavelength tuning interferometry according to claim 5, wherein the fixation light beams for the direction of the patient's eye are implemented by a light ring that is projected on the patient's eye by imaging an aperture that is illuminated in a circular ring-shaped manner onto the sclera or onto the area of the pupil of the patient's eye.

8. The arrangement for the ophthalmological length measurement by dual-beam space-time domain wavelength tuning interferometry according to claim 1, wherein one of the partial beams of the measuring dual-beam is reflected in the Michelson Interferometer, which generates the dual-beam, by utilization of a retro-reflector, which moves back and forth periodically.

9. The arrangement for the ophthalmological length measurement by dual-beam space-time domain wavelength tuning interferometry according to claim 8, wherein the retroreflector moves back and forth by a distance of λ/4, at an audio frequency, wherein λ, is an average wavelength of the paired monochromatic beams.

10. The arrangement for the ophthalmological length measurement by dual-beam space-time domain wavelength tuning interferometry according to claim 1, further comprising a second beam splitter that is arranged within the output beam of the ophthalmological interferometer and that directs a portion of the output beam onto the photodetector comprising an amplifier and a speaker.

11. The arrangement for the ophthalmological length measurement by dual-beam space-time domain wavelength tuning interferometry according to claim 1, wherein measurements of intraocular distances are possible at each point of the pupil (x, y).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1a: Purkinje-Sanson images P1, P2, P3, P4,

(2) FIG. 1b: Fresnel-zone like space-time domain interferogram (RZI) embedded in the center of the speckle structure in front of the eye;

(3) FIG. 1c: Enlarged section of the RZI from FIG. 1b and

(4) FIG. 2: Beam path in an arrangement according to an example embodiment of the invention.

DETAILED DESCRIPTION

(5) The invention is hereby explained in more detail on the basis of an example embodiment according to FIG. 2.

(6) In the beam path of an arrangement according to the invention as it is shown in FIG. 2, the ophthalmological interferometer in the narrower sense [A], the fixation light components [B] and the device for checking and detecting of the measuring signal [C] are delimited by dash-dotted lines for clarity.

(7) 4A. Ophthalmological Interferometer:

(8) The measuring method described here uses interferometers in an unconventional way in that the eye is not illuminated by a single measuring beam, but rather by a measuring dual-beam generated by a Michelson interferometer. The Michelson interferometer in the narrower sense comprises the components 10 to 27. 9 is the hand of the observer (medical assistant) that is carrying out the measuring of the length. 10 is the light source for the measuring dual-beam 11. The measuring light source 10 is a spatially coherent light source (DSL) that can be tuned in its wavelength, e.g. a tunable, spatially coherent laser. The light beam 10 that is emitted by the measuring light source 12 is collimated by the optics 13 and divided by the beam splitter 14 into two partial beams 15 and 16.

(9) The partial beam 15 is reflected by the measuring mirror 17 that can be moved parallel to the beam direction (y-direction) as a partial beam 24 of the measuring dual-beam 11 towards the beam splitter 30. The retroreflector 17 is mounted on a moveable table 18 which is adjustable in a controlled manner by the hand 9 or by means of an electric drive 19 and its y-position can be read out by use of a built-in position sensor or it can be read by use of a nonius 20 by an observer. Similar to the space-domain dual-beam LCI interferometry technology, a path difference ΔL that is intrinsic to the dual-beam 11 is also set in relation to the partial beam 16 in this case. This path difference plays an important role in tackling the auto correlation problem that was mentioned in section 4D-a.

(10) The partial beam 16 impinges on a retroreflector 21, which is mounted on a piezoelectric actuator 22 and, powered by an AC voltage “U˜”, it periodically moves back and forth in the beam direction (z-direction) in small increments, e.g. by λ/4, with an audio frequency f. As a result, the herein reflected partial beam 23 of the measuring dual-beam 11 experiences a temporally periodic phase shift with reference to the partial beam 24. This serves for the monitoring of the adjustment by use of an audio signal; it is interrupted upon initiation of the measurement or at the start of the signal registration by the photodetector signal. Both reflected partial beams 23 and 24 are finally reflected or transmitted by the beam splitter 14 as a measuring dual-beam 11 in the direction of the beam splitter 30 and are reflected by the latter onto the eye (1).

(11) The beam expander consisting of zoom-ocular 26 and lens 27 serves for the adjusting of the measuring beam diameter to the size of the pupil. The zoom-ocular 2 in FIG. 26 is not shown by the drawn combination of a converging lens and a diverging lens in its actual structure, but only symbolically. (The same applies to the zoom optics 52 and 56)

(12) 4B. Beam Parameters and Fixation of the Test Person's Eye:

(13) The basic parameters of the measurement beam in the pupil of the test person are the beam intensity, the beam diameter and the wavelength as well as the beam direction and beam position in relation to the direction and position of the visual axis of the eye. Beam intensity: The admissible limit value for the radiation strength (radiation power with reference to the beam cross-section area) of the light rays that reach the eye is limited by safety regulations and depends, in addition to the wavelength, on the radiation duration that is to be expected.

(14) Beam Diameter:

(15) On the one hand, it determines the overall radiation power that is entering into the eye and thus the achievable sensitivity of the length measurement. On the other hand, the beam diameter limits the illumination of the pupil and thus e.g. the maximum expansion of the measurable transfer function of the eye.

(16) Wavelength:

(17) The herein dominating Rayleigh-Scattering increases by 1/λ.sup.4. The use of light with a greater wavelength is therefore advantageous, but it is increasingly restricted by the absorption in the tissue water starting at approx. λ=1.4. (currently DSL's in the wavelength range of approximately 680 nm up to ˜3 μm are commercially available)

(18) Beam Direction with Reference to the Visual Axis:

(19) It is defined by the direction of the fixing light beam 31, wherein the test person looks at the light spot that was generated by the fixing light beam on his retina. The fixing light beam 31 is generated by an image of the output surface 32 of the light conductor 33 through optics 34 and the optics of the patient's eye 1 via the beam splitters 40, 36 and 30 on his fundus. The optical fiber 33 is illuminated by a light-emitting diode (LED) 38, which emits e.g. green light, whose light is bundled by an optics 35 onto the input surface 37 of the optical fiber 33. The x-y position of the output surface 32 of the optical fiber 33 determines the direction of the fixing light beam 31; it is positioned by use of a 2-coordinate adjusting device 39 in 2 directions that are orthogonal towards each other, e.g. in x- and y-direction.

(20) Measuring Position with Reference to the Visual Axis:

(21) This is controlled by use of a light ring that is projected onto the patient's eye. To generate this light ring, a circular ring-shaped aperture 41 is reflected by use of a fixing light beam 44 through the optics 34, reflected by the beam splitter 36, and through the beam splitter 30 onto the sclera or onto the environment of the pupil 2 of the patient's eye 1. The aperture opening 41 is illuminated with the aid of a lens 42 by use of LED 43 which emits e.g. red light. In this way the patient can have an active part in the positioning. The position of the eye 1 is adjusted by operation of a 2-coordinate adjusting device 45 in 2 directions that are orthogonal to each other, e.g. in x- and y-direction. As a criterion for the standard position of the patient's eye, it is possible to use the symmetrical brightness perception of the patient for the respective wavelength or color of the light emitted by the LED 43.

(22) 4C. Position Monitoring and Registration of the RZI:

(23) A monitoring of the positioning of the input pupil 2 of the patient's eye in relation to the light ring image is made possible by use of an image of the pupil layer via the zoom optics 52 at the interferometer output via the image intensifier (BV) in the focal plane 60 of the ocular 61. To accomplish this, the focal length and/or z-position (the origin of z is located in the center point of the curvature of the cornea, about 8 mm away from the cornea vertex on the inner side of the anterior chamber of the eye) of the zoom optics 52 are set in such a way, that the pupil 2 is imaged onto the photocathode of 53 of the BV.

(24) Such a BV can be based e.g. on a micro channel plate technology, made up of photocathode 53 at the input, micro channel plate 54 for amplification in the narrower sense and phosphor screen 55 at the output of the BV. The spectral sensitivity of this BV is determined by the respective photo cathode material. However, beyond a wavelength of 1 μm, the detectivity of the herein available solid-state photo receiver decreases by 1/λ.sup.2. As an alternative, it is also possible to use a digital camera, based on CCD technology, electron multiplying CCD or intensified CCD technology instead of such a BV. In the latter cases, the sensor of the digital camera replaces the photocathode, the phosphor screen 55 is replaced by the electronic viewfinder.

(25) Visual Monitoring of the Position:

(26) The reinforced RZI occurring at the phosphor screen 55 of the BV simplifies the finding of the RZI by the observer 100 (with the help of the microscope-analogous optics 56 and 61) or by the observer 101 (by imaging onto the array 70 and transfer by computer 90 onto monitor 91), in particular when there is an irregular optics of the eye. There is furthermore a reticle 58 as an aid for the visual monitoring of the present RZI position in the layer of the phosphor screen 55 of the BV. The observation of the image at the output of the BV on the reticle that is arranged on it makes it easier to position and align the eye of the test person. (The latter can be further adjusted by use of the fixing light beam 31)

(27) Alignment Monitoring Via Audio Signal:

(28) A more general checking of the adjustment status of the ophthalmological interferometer is supported by use of an acoustic observation. For this purpose, a beam splitter 80 is located within the output beam 49 of the ophthalmological interferometer, which directs a portion of the output beam onto a photodetector 81 comprising an amplifier 82 and a speaker 83. A sound with the frequency f that occurs at the actuator 22 when the AC voltage U˜ is switched on, indicates that light from the ophthalmological interferometer reaches the BV via the eye.

(29) By varying of the focal length of the zoom optics 52, the contrast of an RZI 51 that is imaged on the BV is first of all optimized for the interferometric measuring, the RZI is then imaged onto the photodetector array 70 by means of the portion of the image beam 57 that is reflected by the beam splitter 59 and it is then measured or “scanned” by its photodetector grid.

(30) Fresnel-Zone-Like Interferograms RZI 51:

(31) The light beam bundle 50, which is reflected by the eye, in this case characterized by the “output beam” 49 of the ophthalmological interferometer, contains a series of 3-dimensional interference phenomena in form of interference hollow cones with increasing diameter along the optical axis of the eye. In the case of a regular anatomy of the eye, all these interferograms superimpose, wherein the interferogram of the strong reflexes of the fundus and of the two corneal reflexes dominate. On the other hand, the corneal reflexes and the reflex of the rear lens surface feature almost the same curvature radius and therefore almost the same interference state across the entire pupil of the eye, so that the (monochromatic) interferograms that are formed by these when they are illuminated with light of a higher coherence length can hardly be recognized as being separate when visually inspected. In the case of an irregular anatomy, e.g. in the case of a cataract, it could be possible under certain circumstances that no complete or only speckle-like interferograms can be observed.

(32) The interesting high-contrast RZI's are primarily present as real interferences in the area in front of the cornea (z>0) and as virtual interferences behind the cornea (z<0). Real high-contrast interferences are localized on the z-axis (several cm to dm) in front of the eye. For the zoom optics 52 that is arranged in front of the eye, these are virtual objects and they are imaged by it—by way of a reverse-operating magnifying glass—as real images onto the BV-input or onto a photodetector array that is positioned there.

(33) These interferograms form the basis for the ophthalmologic WT-interferometry variant of the LCI length measurement. It is thus basically possible to measure a series of intraocular distances in each point of the pupil (x, y). In the case of a regular anatomy of the eye, due to the dominating of the regular reflexes for the RZI, there is a definite allocation of the interferogram positions for the corresponding x-y-positions in the EP of the eye. In this way access to e.g. the distribution of the optical length of eye cornea/fundus or of the eye's anterior chamber depth cornea/lens of the eye within the pupil is gained. In each case, it is possible to add the measured lengths across the entire pupil and to thus obtain a signal with a very high sensitivity—however, due to the sum of different lengths, these are obtained at the price of a reduced accuracy.

(34) Incidentally, the RZI 51 that is used for the measurement is located—when observed by the test person—behind the zoom optics 52 (at z>D). Thus, the distance of the RZI that is used for the measuring of the zoom optics does not have any influence of enlarging the interferometer in z-direction. The zoom optics 52 can also be directly adjacent to beam splitter 80 or, when the adjusting aids are omitted, to beam splitter 36.

(35) 4D. Signal Processing.

(36) 4D-a. Intensity Spectra of the Light Waves that are Emitting from the Eye:

(37) The herein used WT-interferometry or Swept-Source LCI interferometry—there are several techniques that are based on the same optical principles but with different and yet synonymous names—is based on the intensity spectrum I.sub.ξΨ(k) of the light waves that are emitting from the eye at the transversal object position (ξ,Ψ). These are generated by use of spectrally tunable lasers as light source, they are then forwarded by a detector to a computer 90 where they provide via Fourier transformation and auto correlation decoding the object structure in the transversal object position (ξ,Ψ) along the measuring beam in the ophthalmological interferometer [A].

(38) In an example method according to the invention, the scattered-field intensity spectrum l.sub.x,y(k) of the patient's eye that is required for the calculation of the subsurface structure of the eye is registered by a photodetector array (70) from transversal positions of an RZI (51) which is localized in longitudinal z-direction some cm to dm outside the patient's eye, and is then transferred to a computer (90). Due to the regular reflexes that are also dominating at these distances even in moderate cataracts, a definite allocation of the RZI positions to the transverse pupillary coordinates is largely provided.

(39) 4D-b. Fourier Transformation of the RZI Array Data:

(40) The computer 90 saves the spectrum of the ocular scattered-field intensity data I.sub.x,y(k) of the individual array photodetectors, calculates from these the transversal positions (x, y) of the lengths data of the partial sections corresponding to the array photodetectors and displays these on the connected monitor 91.

(41) The array data refers to data matrices l.sub.x,y(k) with
l.sub.x,y(k)˜|FT.sub.z{F.sub.x,y(z)}|.sup.2  (2)
wherein FT.sub.z=Fourier transformation with reference to the z-coordinate; k=2π/λ is the wave number, λ the wavelength of the light beam that is emitted from the DSL. l.sub.x,y(k) refers to the spectral intensity data that is allocated to the individual array photodetectors and registered by light propagation outside of the patient's eye's pupil (via the optics 52 and 56 as well as the BV). The DSL 10 is hereby tuned by a spectrum Δλ, the size of which determines the depth resolution, see equation (1).
F.sub.x,y(z)˜n.sup.2(x,y,z)−1
is the scattering potential of the eye or its “structure”, n(x,y; z) is the corresponding refractive index.

(42) The signal strength of the used light that is hereby scattered back is high at locations with adjacent tissues with great scattering potential differences. Therefore, the z-positions of tissue boundaries z.sub.Gi/Gj(x, y) along each respective light beam in the eye pupil position (x, y) can be determined by use of the signal intensity peaks that are occurring there (the index Gi/Gj refers to “tissue boundary between tissue Gi and tissue Gj” with e.g.: G1/G2=cornea anterior surface/corneal interior surface, G1/G3=corneal anterior surface/lens anterior surface, G4/G5=lens interior surface/fundus). The corresponding lengths of the partial sections are obtained as the difference Δz.sub.Gi/GJ of the z-values of these signal intensity peaks along the beams through the pupil position (x, y).

(43) The following variants can hereby be used: (a) The use of the measured lengths of the partial sections Δz.sub.Gi/Gj in the pupil for the determination of the optometric data or the transfer function of the patient's eye. (b) The use of the measurable partial amounts of the lengths of the partial sections Δz.sub.Gi/Gj(x,y) with segments of the pupil, in particular in the case of an advanced cataract. (c) The forming of average values of the measurable lengths of the partial sections Δz.sub.Gi/Gj(x,y) across several or all points of the pupil for an increase of the sensitivity.

(44) Resolution and Measurement Range:

(45) The transversal resolution, by use of which the RZI is registered, is provided in accordance with the classic Abbe resolution formula by use of the wavelength and numeric aperture of the image through the optics 52. The depth resolution is provided by a tuning of the DSL 10 via a spectrum coherence length of the imaging light beam.

(46) However, the size of the measuring range is determined by the density of the sample values on the k-axis.

(47) Auto Correlation:

(48) However, an inverse FT of the intensity data of the photodetector array 70 does not—automatically—provide the object structure, but rather its auto correlation function. To solve this problem, there are a number of techniques, which are described in detail in the literature (e.g. Fercher et al., Opt. Commun. 117 (1995) 43-48 or Seelamantula et al., J. Opt. Soc. Am A, 25 (2008) 1762-1771) and which can also be used in this case.

(49) 4D-c. Size and Contrast of the RZI; Role of the Sampling Theorems:

(50) Both, the position D and the focal length of the zoom optics 52 determine the imaging scale of the RZI that is projected onto the photocathode 53 of the BV and further onto the array 70—or onto a photodetector array 70 that is localized without the interposition of a BV. Generally, the size of the RZI at the photodetector array via zoom optics 56 is of course to be selected in such a way, that it is “sampled” by the array detectors in the correct distance—in accordance with the sampling theorem. The central interferogram circular surface of an RZI and furthermore also its ring structure up to the 4. interference ring—in accordance with the sampling theorem—can be sampled by means of e.g. a 32×32 photo detector array. In contrast to a mere detection of the central interferogram circular surface, this leads to a sensitivity gain of already 6 dB for the sum signal (when disregarding the Gaussian profile and with a homogeneous transparency of the eye media, the size of the sum signal of the photodetector-array increases with the surface of the registered RZI). A photodetector array comprising 1000×1000 pixel furthermore features a sensitivity potential in the range of 20 dB when used in accordance with the sampling theorem.

(51) 4D-d. RZI Size and Sensitivity:

(52) However, an optimal RZI contrast is given in individually different z-positions (within a few cm to dm in front of the patient's eye). If different z-positions of the RZI 51 (via the optics of 52 and 56 and the BV) are imaged onto the detector array 70 to optimize the contrast, this is done due to the necessary tuning of the focal length of the zoom optics 52 via individually different imaging scales in dependence of parameters of the eyes. The sampling of the image of the RZI 51, which is varying in its size, onto photodetector array 70 thus often leads to an “undersampling” or “oversampling”, due to which the image quality and sensitivity is impaired.

(53) Homogenization of the Size of the RZI.

(54) 2 steps are thus necessary for the signal optimizing. 1. The locating of an RZI with an optimal contrast along the z-axis. 2. An image of the RZI from the phosphor screen 55 of the BV by the zoom optics 56 onto the photodetector array 70 which is in accordance with the sampling theorem.

(55) This is a 2-dimensional diversity of possibilities and is therefore not practicable.

(56) Since the ring diameter of the RZI, which is caused by the increase in the curvature radii of the corneal reflexes that are expanding, increase with growing distance z from the patient's eye, whereas the imaging scale of the RZI on the photocathode of the BV reduces with increasing distance from the optics 52, it is possible to compensate the size variation of the RZI along the z-axis by a suitable selection of the RZI imaging scale—or by a suitable selection of the position and/or focal length of the zoom optics 52. As an example, a positioning of the zoom optics 52 at z=D=60 mm in front of eye 1 is assumed. In this way it is possible to achieve—by means of a corresponding zoom-optics 52 focal length—a consistency of the RZI ring diameter that is imaged on the photocathode 53 by more than +/−5% for the z-positions between z=100 mm-600 mm.

(57) Alternatively, it is possible to move the apparatus for the detection of the measuring signal [C] along the z-axis instead of a tuning of the focal length of the zoom optics 52 (e.g. towards position C′, see open arrow 71).

(58) 4D-e. Strongly Inhomogeneous Transparency of the Patient's Eye's Media:

(59) In such cases, an advantage of the method according to the invention is, that the detector array, which is possibly much larger in comparison to the single detector, provides a considerable relief with regard to the otherwise standard point-for-point spectral interferometry comprising only one single tiny detector which requires a much more laborious signal search and also a segmentation of histologically interrelated areas.