Filed on 16 May 2011 Instrument for examining or machining a human eye The invention relates to an instrument for examining or machining a human eye. Laser radiation is used in numerous techniques for treatment of the human eye. In some of these techniques focused laser radiation is utilised for the purpose of ablat ing (resecting) eye tissue. In this case it is necessary to direct the beam focus onto the eye in controlled manner, so that the ablation takes place at the desired position o on the eye. But by virtue of movements of the eye during the treatment the eye may change in its position in relation to the treatment laser beam. This may then result in a discrepancy between a specified position and an actual position of the ablation. 5 For this reason it is desirable to track the movements of the eye and to take them into account in the control of the beam focus. For the purpose of acquiring the eye movements, use is made of an eye-tracker. At present, a two-dimensional eye track ing is generally conventional, which is based on the acquisition of the pupillary mar gin of the eye by only one camera. From the light/dark jump in contrast at the a pupillary margin (iris), the pupillary centre is calculated which then serves as orienta tion coordinate for the laser ablation. Control of the treatment laser radiation is then effected by taking into account the position of the pupillary centre ascertained by the eye-tracker. However, the position of the pupillary centre does not always lie on the axis of symmetry of the eye or on the optical visual axis of the patient (for example, 5 by virtue of asymmetrical displacement of the pupillary centre in the case of varying pupillary size, or deviation from circular symmetry in many patients). Such a devia tion may result in suboptimal treatment outcomes. In order to avoid such inaccuracies in the laser treatment resulting from the shift of a position of the pupillary centre, the tracking of the pupil can be supplemented by a tracking of the limbus, which is oriented with respect to the invariable light-dark transition of the white sclera (sclerotic coat of the eye) to the iris. Prominent dis placements of the pupillary centre can consequently be detected and taken into account in the ablation program as a so-called pupil-centre-shift correction (PCSC). 5 Overall, for the purpose of tracking the eye movement (eye tracking) the state of the art has hitherto utilised two-dimensional camera-image acquisition. Positional de- Filed on 16 May 2011 -2 faults derived therefrom may, however, be faulty, since the actual eye movements take place in three-dimensional space and consequently three translational move ments as well as three rotational movements have to be described. Furthermore, eye-trackers that are based on camera-based two-dimensional image-recordings s enable only the indirect acquisition of three-dimensional data by computationally intensive reconstruction. Meanwhile, camera-based eye-trackers have become avail able that enable a five-dimensional or six-dimensional eye tracking. In this connec tion, by an additional projection onto the eye of a pattern of light consisting of fringes and by the acquisition of these fringes (registration of the curvature, position o and deformation of the fringes), a locational position and orientational position of the eye are inferred. But, here too, the registration process is intensive in terms of com putation and time. Therefore the image-rate of eye-trackers used hitherto is limited in its speed and is often too slow for a correction of position in the course of the treatment of the eye with laser light. In addition, the camera systems utilised for 5 this purpose merely detect light that has been scattered or reflected by the eye of the patient, which is why it is necessary to ensure appropriate illumination of the eye (which, however, may also have a disturbing effect on the treatment) and at the same time to avoid incidence of light from other secondary illuminations from the room onto the eye. 0 It is an object of the invention to make available an instrument for examining or treating a human eye, said instrument including an eye-tracker that can provide results about eye movements with great speed and precision. s With a view to achieving this object, in accordance with the invention an instrument is provided for examining or treating a human eye, with an eye-tracker for acquiring eye movements and for outputting a signal that is representative of the acquired eye moments, the eye-tracker including an interferometric image-acquisition device that has been set up for time-resolved acquisition of sectional images of the eye and that operates on the basis of three-dimensional optical coherence tomography, and also an evaluating module ascertaining the eye movements solely from the sectional images. The interferometric image-acquisition device may, for example, be characterised by a s scan rate of > 200,000 line scans per second, wide lateral scan ranges >= 15 mm, depth ranges > 8 mm to 12 mm, digital high-speed cameras (CCD, CMOS) with up to 12,000 pixels per line or more, high read-out speeds of about 140 kHz, high detec- Filed on 16 May 2011 -3 tion sensitivities of > -90 dB, and high resolutions within the range from < 1 pm to 10 pm. Three-dimensional image-rates with 500 frames per second or more are possible. s The invention has the advantage that through the use of the interferometric measur ing method the incidence of light from other secondary illuminations from the room onto the eye does not have a disturbing effect on the eye tracking, since only coher ent light, but not incoherent light, enters into the measuring process. o The invention makes it possible, in addition, to use the high resolution, the high measuring speed and also the high sensitivity of interferometric image-acquisition devices that operate on the basis of three-dimensional optical coherence tomography not only, as hitherto, for the purpose of tomography, i.e. for structural surveying of a particular portion of the eye, but also for the purpose of measuring the position and s the orientation of the eye in space. In this connection it is crucial that for the ascer tainment of the position signals, orientation signals and movement signals only data from (one or more) interferometric image-acquisition devices are drawn upon that in each instance enable an image acquisition with assignment of X-, Y- and Z coordinates to each image point. 0 With the aid of the time-resolved acquisition of the sectional images of the eye it is consequently possible to ascertain corresponding eye movements. Eye movements of such a type include cyclotorsional movements (flat rotations about the optical axis of the eye), rolling movements (rotations of the eye about an axis perpendicular to 5 the optical axis of the eye) and also translational movements in all three directions in space. By reason of the use of the interferometric image-acquisition devices, the accuracy of measurement of the six-dimensional eye movements may be distinctly higher than that of an eye-tracker based on cameras. Similarly, the instrument ac cording to the invention offers a distinctly higher speed in connection with the acqui o sition of the eye movements. According to a further development of the invention, the interferometric image acquisition device may have been set up to acquire at least two mutually orthogonal sectional images of the eye that each represent a section substantially along the s visual axis of the eye. The sectional images may, however, also extend along the visual axis of the eye, parallel to the visual axis of the eye, or substantially along the central (optical) axis of the eye, along the central axis of the eye, parallel to the Filed on 16 May 2011 -4 central axis of the eye, or oriented with respect to the apex of the eye. The two sectional images arranged mutually orthogonally permit an acquisition of orientations and positions and also of rolling and translational movements in the X- and Y directions (in this connection the conventional notation is to be understood, in which s the direction of propagation of the measuring beam of the interferometric image acquisition device runs along Z, and X and Y jointly with Z complete the three dimensional Cartesian coordinate system). A translational shift and/or translational movement in the Z-direction can also be determined from sectional images of such a type. The acquisition may, for example, be effected by a comparison with one or o more sectional images determined previously (for instance, prior to the start of treatment). In particular, for this purpose highly resolved three-dimensional com plete sectional images may be drawn upon for the purpose of comparison. Furthermore, the interferometric image-acquisition device has preferentially been set s up to acquire at least one sectional image of the eye that represents a section sub stantially along the margin of the iris of the eye. On the basis of this sectional im age, prominent structures (for example, within the iris) can then be identified and can be utilised for a determination of the cyclotorsion of the eye. A section of such a type can consequently be understood as a face-on recording of the eye. C The interferometric image-acquisition device and the evaluating module have prefer entially been set up to acquire a plurality of sectional images of the eye in time resolved manner and to ascertain from the sectional images a time-resolved topog raphy of a subregion of the eye as the signal that is representative of the eye move ment. The plurality of sectional images of the eye may, for example, correspond to cross-sections of the eye that are offset parallel to one another. The subregion of the eye may, for example, entirely or partly include the cornea, the human lens, the anterior-chamber region, the sclera, the iris, the apex of the cornea, the centre of the lens of the human lens and/or the fovea. The invention consequently makes it possible, on the basis of the time-resolved to pography of the subregion of the eye (e.g. on the basis of the time-resolved topog raphy of the cornea), to acquire translational and rotational movements of the eye and at the same time to orient a treatment of the eye with respect to an arbitrary point within the subregion (e.g. the apex of the cornea). The arbitrary point may be any outstanding feature of the subregion of the eye. This makes it possible to choose an outstanding feature of the subregion of the eye, on the basis of which a Filed on 16 May 2011 -5 particularly exact orientation and precise implementation of a concrete treatment of the eye is possible. Accordingly it makes sense, for example, in the case of an abla tive laser treatment of the cornea to choose the apex of the cornea by way of out standing feature and to orient with respect thereto. An orientation with respect to s the pupil/iris, as in the state of the art, is also possible with the invention but is not absolutely essential. The instrument for examining or machining a human eye preferentially further in cludes components for making available focused treatment laser radiation and for ID directing the same onto the eye, and also a control arrangement that has been set up to set the focus location of the treatment laser radiation in a manner depending on the signal that is representative of the acquired eye movement. The present invention consequently makes it possible to take into account, in the 5 course of the treatment of the eye, the position data, gained from the sectional im ages, pertaining to particular features of the eye and also the eye movements ascer tained from the sectional images. In this context, features of the eye may be constituted by, for example, the apex of the cornea, a point on the inside of the cornea, the midpoint of the cornea, the midpoint of the human lens or the fovea !0 (place of best vision). The interferometric image-acquisition device and the control device may also have been set up to detect a deviation of an actual focus location of the treatment laser radiation from a specified focus location of the treatment laser radiation on or in the Ls eye and to output a notification signal, in which case upon output of the notification signal the control device can, for example, interrupt or stop the emission of the treat ment laser radiation onto the eye. According to a further aspect, in accordance with the invention a process for examin 0 ing or machining a human eye is furthermore provided, including the following steps: - time-resolved acquiring of sectional images of the eye on the basis of three dimensional optical coherence tomography, - ascertaining eye movements solely from the sectional images, and - outputting a signal that is representative of the acquired eye movements. 5 Filed on 16 May 2011 -6 Also in connection with the process aspect, in the course of acquiring the sectional images at least two mutually orthogonal sectional images of the eye can be acquired that each represent a section substantially along the visual axis of the eye. s Furthermore, in the case of the process it is also conceivable that in the course of acquiring the sectional images at least one sectional image of the eye is acquired that represents a section substantially along the margin of the iris of the eye. It is also conceivable that a plurality of sectional images of the eye are acquired in o time-resolved manner and from the sectional images a time-resolved topography of a subregion of the eye is ascertained as the signal that is representative of the eye movement. The process preferentially includes the following additional steps: 5 - making available focused treatment laser radiation and directing the same onto the eye, and - setting the focus location of the treatment laser radiation in a manner depending on the signal that is representative of the acquired eye movements. Alternatively or additionally, the process may include the following step: detecting a deviation of an KO actual focus location of the treatment laser radiation from a specified focus location of the treatment laser radiation on or in the eye and outputting a notification signal, in which case upon output of the notification signal the emission of the treatment laser radiation is, for example, interrupted or stopped. 5 In the following the invention will be elucidated further on the basis of the appended drawings, in which Figure 1 shows schematically an exemplary embodiment of an instrument for exam ining or machining a human eye, Figure 2 shows a schematic representation of the human eye in section. Figure 3 shows a schematic representation of a centring error as a consequence of a rolling movement of the eye, and Figures 4a and 4b show schematic representations relating to the progression of sectional images in the eye.
Filed on 16 May 2011 -7 Figure 1 represents, in greatly schematised manner, an exemplary embodiment of an instrument for examining or machining a human eye. The instrument is denoted generally by 10. The instrument includes an eye-tracker 12. The eye-tracker 12 s includes an interferometric image-acquisition device 14 and also an evaluating mod ule 16 connected to the image-acquisition device 14. The interferometric image acquisition device 14 takes the form, for example, of an OLCR measuring device (OLCR: optical low coherence reflectometry) and emits a measuring beam which, by means of a (semi-transmitting or dichroic) deflecting mirror 18 or other suitable o beam-guiding components, reaches an eye 22 to be treated along an optical beam path 20. The measuring beam emitted by the image-acquisition device 14 passes through a measuring scanner 38 which makes it possible to deflect the measuring beam. Consequently an external as well as internal scanning of the eye 22 by the measuring beam is possible at varying points of the eye tissue. The image 5 acquisition device 14 causes the generated measuring beam to interfere with a re flected beam coming back from the eye 22. From the measured interference data gained in this way, sectional images of the eye 22 can be acquired in time-resolved manner. The image-acquisition device 14 operates in this case on the basis of two dimensional or three-dimensional optical coherence tomography. The evaluating 2 module 16 receives data from the interferometric image-acquisition device 14 that include the acquired sectional images, and computes, solely from these sectional images, besides the position and the orientation of the eye in three-dimensional space, also the movements of the eye 22. The eye movements in this case repre sent translational movements along the three directions in space X, Y, Z as well as Is rotational movements about the three spatial axes X, Y, Z. A coordinate system which has been drawn in illustrates the three directions in space X, Y, Z, whereby the Z-axis defines the direction of the beam path 20. The eye-tracker 12 yields signals via the interface 24 that are representative of the acquired eye movements. o The instrument 10 further includes a laser-surgical apparatus 26. Said apparatus includes a laser 28 which emits a suitably intense (highly repeating or continuous wave) laser radiation. The laser radiation propagates along an optical beam path 30 and then impinges on the eye 22 to be treated. In the beam path 30 various com ponents for guiding and shaping the laser radiation are arranged. In particular, 5 these components include a focusing objective 32 as well as a scanner 34 situated upstream of the objective 32, by means of which the focus, generated by the objec tive 32, of the laser radiation made available by the laser 28 is capable of being de- Filed on 16 May 2011 -8 flected along the X-, Y- and Z-directions. A control arrangement 36 controls the scanner 34 in accordance with a predetermined control program which implements an ablation profile to be generated within the eye 22. In the region between the mirror 18 and the eye 22 the measuring beam of the image-acquisition device 14 and s the treatment laser beam of the laser 28 run collinearly or at least substantially col linearly. Alternatively or additionally, the laser-surgical apparatus 26 may have been designed in such a manner that the laser 28 takes the form of an ultra-short-pulse laser which emits pulsed laser radiation with pulse durations within the range of, for example, picoseconds, femtoseconds or attoseconds and which is suitable for cutting o within the eye tissue, such as is required, for example, for LASIK or in the case of a cataract operation. The laser-surgical apparatus permits, for example, cutting accu racies of ±10 pm or even ±1 pm. A sectional image of the eye 22 acquired by the image-acquisition device 14 is, for 5 example, represented schematically in Figure 2. In this picture the apex (i.e. the point furthest removed from the centre of the eye) of the cornea 42, the iris 44, the pupil 46 and the lens 48 can be discerned. In addition, in Figure 2 an axis 50 has been drawn in which, for example, represents the central (optical) axis of the eye 22 or the visual axis of the eye. In the present case the axis 50 is represented by the 0 position of the apex 40 and the position of the pupillary centre 52. With a view to the ablation of eye tissue, the appropriate tissue part has to be aligned precisely in position and orientation relative to the instrument 10. However, it cannot be ruled out that during the treatment the eye moves or rotates with re 5 spect to the coordinate system X, Y, Z (and consequently with respect to the instru ment 10). Eye-trackers are known in the state of the art that are based on the acquisition of the position of the pupillary centre 52 of the pupil 46 in the eye 22 with the aid of cam o eras for the purpose of two-dimensional acquisition of the position of the pupillary centre 52 within the X-Y plane. Now if there is provision to ablate eye tissue at a position that is different from the position of the pupillary centre 52, suboptimal treatment outcomes by reason of rotations of the eye may occur, as elucidated in more detail in Figure 3. Figure 3 shows a schematic representation of the pupil 46 and also of the cornea 42 of the eye 22 in the position and orientation thereof within the coordinate system X, Filed on 16 May 2011 -9 Y, Z. The straight line 54 runs in this case parallel to the Z-axis and along the optical beam path 20. Now if eye tissue is to be ablated at the specified position 56 on or within the cornea 42 (for example, after exposing tissue within the cornea by folding aside a tissue lamella (flap) in LASIK, i.e. laser in-situ keratomileusis), a rotary S movement of the eye 22 into the orientation of the eye 22 represented by the straight line 60 results in a deviation of the actual position 58 of the ablation relative to the specified position 56 of the ablation if - as in the state of the art - the eye tracking is based merely on a two-dimensional acquisition of the pupillary centre 52. The rotary movement leads in this case to a transverse displacement of the specified o position 56 in relation to the actual position 58 along the X- and Z-axes. An orientation of the ablation procedure with respect to the pupillary centre may therefore be undesirable. On the other hand, an orientation of the ablation proce dure may preferably be with respect to the visual axis. The visual axis lies close to S the optical axis of the eye and runs approximately through the apex of the cornea and the lens centre of the human lens. The eye tracking according to the invention is based on the time-resolved acquisition of sectional images of the eye by interferometric image acquisition on the basis of a three-dimensional optical coherency tomography. By virtue of the three-dimensional image information the evaluating module 16 can ascertain in time-resolved manner the spatial position and also orientation of the portion of eye tissue to be ablated or of the portion of eye tissue to be machined (e.g. within the scope of a cataract op eration) as well as the translational and rotational movements thereof, and on the s basis of the interface 24 can communicate a signal that is representative of these data to the control arrangement 36 of the laser 28, in order to set the focus location of the treatment laser radiation in a manner depending on the signal that is repre sentative of the acquired data. Furthermore, for the purpose of controlling the focus location the eye tracking according to the invention may be oriented, for example, a with respect to the apex 40, the position of which does not change - in contrast to the pupillary centre 52 - in the case of a variable illumination situation. In addition, the invention offers the possibility of choosing from the sectional images an orienta tion centre for the laser ablation that is positioned close to the tissue to be treated that is to say, for example, the apex 40 by way of orientation centre for a treatment s of the cornea 42. In this case it is expedient to acquire at least two mutually or thogonal sectional images 62, 64 of the eye 22 that each represent a section sub stantially along the visual axis 50 or some other suitable axis of the eye 22, see - 10 Figure 4a. Alternatively or additionally, it is expedient to acquire at least one sec tional image 66 of the eye 22 that represents a section substantially along the mar gin 68 of the iris 44 (that is to say, the pupillary margin) of the eye 22, see Figure 4b. A three-dimensional, in particular time-resolved, complete-image acquisi 5 tion (three-dimensional tomography) consisting of a plurality of sectional images parallel to sectional image 62, a plurality of sectional images parallel to sectional image 64 and/or a plurality of sectional images parallel to sectional image 66 is also conceivable. This complete-image acquisition may, for example, represent a time resolved 3D topography of the cornea 42, on the basis of which the translational and 10 rotational eye movements can be ascertained and an orientation of the treatment laser radiation with respect to a point in this 3D topography (such as the apex 40 of the cornea 42) is possible. Comprises/comprising and grammatical variations thereof when used in this is specification are to be taken to specify the presence of stated features, integers, steps or components or groups thereof, but do not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. 20