Patent Publication Number: US-2002013573-A1

Title: Apparatus and method for tracking and compensating for eye movements

Description:
REFERENCE TO RELATED APPLICATION  
     [0001] This application is a continuation-in-part of co-pending patent application Ser. No. 08/549,385, filed on Oct. 27, 1995. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] The present invention generally relates to a system and method for tracking a moving object. More specifically, this invention relates to a system and method for tracking movement of an eye during diagnostic analysis or during a surgical procedure wherein a laser beam is directed on the eye, and compensating for such movement so as to maintain a substantially centered condition between the laser beam and the eye.  
       [0003] Surgical procedures are known which aim to correct refractive disorders of a human eye through ablation of the cornea of the eye using laser radiation. Such procedures include Photorefractive Keratectomy (PRK), Phototherapeutic Keratectomy (PTK), and Laser In Situ Keratomileusis (LASIK). Typically, according to these procedures, laser pulses are scanned in sequence over centralized circular areas of the cornea to cause localized tissue ablation (what may be called “scanning laser” ablation) or are used to simultaneously irradiate similar centralized circular areas of the cornea (commonly referred to as wide area ablation). The treated areas are typically between 6 and 9 mm in diameter.  
       [0004] Scanning laser systems for use in corneal surgery were taught, for example, by L&#39;Esperance in U.S. Pat. No. 4,665,913 and by Lin. in U.S. Pat. No. 5,520,679. Both of these patents deal with methods using 193 nm wavelength radiation from an excimer laser. An alternative scanning system invokes a photospallation mechanism to perform corneal ablation using a mid-infrared laser as described in U.S. patent application Ser. No. 08/549,385, of which the present application is a continuation-in-part.  
       [0005] Typically, the above-referenced (and other) scanning techniques for corneal sculpting involve rapidly moving a relatively small spot of laser radiation over a specific central portion of the corneal surface in a predefined pattern. This allows selective removal of tissue at various points within the scanned region, thereby cumulatively re-shaping the surface of the cornea into the desired geometry in a predictable fashion.  
       [0006] A problem which has plagued the art is that, during corneal refractive surgery, the eye which is receiving the laser pulses is subject to various involuntary and voluntary movements. The movements of the eye vary in type and in degree and may occur simultaneously. For example, one type of involuntary eye movement is known as a “saccade”. Saccades generally involve rapid eyeball rotations of up to 600 deg/sec and occur typically on a 10-30 msec time scale with amplitudes ranging from 1 to 10 degrees. See Bahill et al,  Invest. Ophthalm. Vis. Sci.,  21, 116, 198 1. A second type of involuntary eye movement involves tremors. Tremors may occur at rates of 10 to 200 Hz and with amplitudes on the order of 0.5 arc min. See Carpenter,  Movements of the Eyes,  2 nd  ed., 1988 and Findlay, “Frequency Analysis of Human Involuntary Eye Movement”,  Kybernetik,  8, 207, 1971. Another type of involuntary eye movement involves drifts which can occur at velocities of about 4 arc min/sec and with significantly larger amplitudes than tremors. See Ditchburn,  Eye Movements and Visual Perception,  1973. Studies of eye movements, such as one reported by Bahil et al (referenced above), indicate that extremely high accelerations of up to 40,000 deg/sec 2  may be involved in the fastest movements.  
       [0007] Eye movements often lead to misalignments, i.e., decentrations, of all or portions of the ablated region on the cornea. The treatment area decentrations are particularly harmful in the above mentioned surgical procedures since they may result in irregular astigmatism, glare phenomena, decreased visual acuity and lower contrast sensitivity. Such eye movements cause uneven distribution of tissue ablation patterns and must be minimized in order to achieve requisite surface smoothness. Implementation of such improved means for suppressing eye motion, while important in wide area ablation, is especially important in scanning laser delivery systems, which require precise execution of specific scanning algorithms, and spot placement accuracy on the order of 5 to 50 μm.  
       [0008] It is standard practice during corneal laser surgery for the patient&#39;s head to be securely restrained so movements of the eye being treated result only from roll of the eyeball within its socket. These movements cause the center of the cornea to shift position in the vertical and/or horizontal directions, usually by no more than 5 mm.  
       [0009] In some prior art apparatus for corneal surgery, the eyeball itself is further immobilized by clamping, suction rings or other means, such as stitching the eye to an eyelid retractor (called a speculum), such as that disclosed in U.S. Pat. No. 5,556,417 to Scher, so as to suppress movements of the eye. However, ever this further immobilization of the eye is not completely effective in suppressing all involuntary eye movements. These physical constraints also may be uncomfortable for the patient and may lead to infection, as in the case where invasive techniques such as stitching are used. The availability of a technique for tracking movements of the eye and compensating therefor would eliminate the need for immobilization of the eye during laser surgery.  
       [0010] Means for tracking an object typically involve an optical system for imaging the object or a portion thereof onto some form of sensor such as a video camera or an array of light detectors. It is essential that the object be illuminated so the image is sufficiently bright for detection. It is important for this tracking illumination to come from a source or sources under the control of the operator so that factors such as intensity, color, propagation direction, etc. can be optimized. Other sources, such as room lights, are not so optimized hence any light from these extraneous sources which reaches the image sensor will tend to obscure the ability of the tracker to sense the motion of the object.  
       [0011] Certain prior art techniques for tracking eye movement are based on pattern recognition of various features in the eye, such as localized variations in iris coloration or the circular shape of the pupil. These techniques are fundamentally digital in nature. For example, U.S. Pat. No. 5,231,678 to Cleveland et al teaches a digital method for detecting the edges of the pupil and analytically locating the pupil&#39;s center in reference to the first Purkinje point (the reflection from the anterior surface of the cornea). Other techniques rely on different reference points or alternative features of the eye. Because these techniques are digital, they require point-by-point acquisition of target features using video cameras and frame grabbers, as well as complex edge detection algorithms and sophisticated signal processing methods.  
       [0012] In such techniques, the response of the tracking system is limited by the video scanning rate of 60 Hz. This rate is not sufficient for tracking the fastest eye movements and also translates into an electronically complex system due to high sampling rate requirements which leave less than a millisecond for processing the signals. Furthermore, techniques predicated upon digital correlation processing of video signals derived from an optical image are often deficient due to unfavorable trade-offs between image size (or field of view) and spatial resolution due to limits on pixel size. In view of the foregoing, it is readily apparent that such digital techniques are unattractive for addressing the needs of refractive corneal laser surgery.  
       [0013] Other techniques for providing eye tracking are based on optical point trackers, such as the system taught by Crane and Steele in U.S. Pat. No. 4,287,410 and by Crane et al in U.S. Pat. No. 4,443,075. These systems utilize the lens-like properties of the eye to compare the displacements, over time, of the first and fourth Purkinje points (the latter is the reflection from the rear surface of the lens). These techniques purport to be able to distinguish between rotational and translational movements of the eye and to possess, in principle, sufficient speed to follow the fastest eye movements. Importantly, however, they cannot be utilized in conjunction with a surgical laser device which aims to modify the very anterior surface of the cornea which provides the specular reflection forming the first Purkinje point. Since the fourth Purkinje point is observed through the corneal surface, it would be severely degraded by the surgical intervention and hence rendered useless as a tracking aid. Even for diagnostic applications, the high eye-illuminating light levels needed to distinguish the low-reflectance fourth Purkinje point may provide unacceptable interference with other illumination means used in such diagnosis.  
       [0014] Yet other prior art techniques rely on tracking of the outer or inner edge of the iris, by detecting light scattered from such naturally occurring boundaries of the eye to measure differences in illumination from such boundaries. Such “differential reflection techniques”, as they are sometimes known, have the advantage of allowing for analog signal processing techniques which are known to be simpler, faster and have higher accuracy than the above-mentioned digital techniques.  
       [0015] One such naturally occurring boundary for use with differential reflection techniques is the limbus, which is the approximately circular intersection of the eye&#39;s transparent cornea with the translucent and white-colored sclera. The limbus also corresponds to the outer boundary of the colored iris which can be seen through the cornea. The limbus is a particularly attractive tracking landmark for corneal surgery, constituting, as it does, an integral part of the eyeball structure itself. It moves in the same manner as the central cornea area which is to be modified surgically, yet is located far enough away from the surgical site as not to interfere with the surgical procedure itself or for that procedure to affect the tracking landmark.  
       [0016] Such differential reflection prior art arrangements have been successful in sensing horizontal eye movements over a wide range of 15-25 degrees. However, sensing movements along the eye&#39;s vertical axis has been especially troublesome due to partial obscuration of the limbus by the upper and lower eye lids. One approach to overcoming this difficulty was disclosed by Knopp et al in PCT Patent Application Serial No. WO94/18883. The Knopp application teaches a differential light reflection technique using off-axis illumination of the eye and a pair of position sensors, each consisting of a multiplicity of segments. The sensors detect and measure both horizontal and vertical displacements of the limbus by continuously monitoring variations in the relative image illumination among the various segments. The technique taught by Knopp suffers from a major problem—that is, it does not provide the same high sensitivity in the vertical direction as in the horizontal direction. This is due to the much smaller differentials between illuminated areas on the detector elements produced by small vertical displacements as compared with those differentials produced by equivalent displacements in the horizontal direction. The resulting lower sensitivity characteristics of the system in the vertical displacement direction make the technique taught by Knopp difficult to implement in practice and reduce its ability to respond to small eye movement in the vertical direction. Furthermore, the disclosure of Knopp et al does not appreciate complications due to spurious signals which may be generated by ambient illumination or specular reflections from the eye. Such spurious signals may be especially troublesome when the eye is subject to off-axis illumination, which off-axis illumination is taught by Knopp et al.  
       [0017] Alternative differential reflection techniques use the pupil, which is the aperture in the iris, as the feature to be tracked. For e example, the technique taught by Cornsweet et al in U.S. Pat. No. 5,410,376 uses a quadrant detector to sense saccadic movement of the eye in both the vertical and horizontal directions. However, the technique of pupil backlighting taught by Cornweet et al requires illumination from a direction nearly coincident with the axis of the eye. Thus, this illumination would necessarily pass through the central area on the cornea. Since this region on the cornea is precisely that which would be ablated during PRK, PTK, or LASIK, the technique taught by Cornsweet et al would not be compatible with use during those surgical procedures relating to the cornea. Similarly, the pupil tracking methods taught by Taboada and Robinson in U.S. Pat. No. 5,345,281 are deficient for use with corneal surgical procedures in that they also rely upon nearly on-axis illumination of the eye through the region to be ablated on the cornea.  
       [0018] Still another differential reflection technique is taught by Frey et al in U.S. Pat. No. 5,632,742. In that reference, the eye is tracked using a natural feature, such as the limbus or the pupil of the eye, or a circular ink mark manually added thereto. The tracking is accomplished using a single light source focused to a plurality of positions on the feature of choice. By temporally sequencing the light pulses, a single detector can be used for sensing differences between light reflected or scattered from the various locations on the eye, such differences being indicative of eye movement in two orthogonal directions. This technique of Frey is limited in its dynamic range by the sizes of the illuminated light spots on the eye since the desired proportional error signal at each sampled location can be derived only while the chosen feature of the eye (inner or outer edge of the iris or the ink mark) lies within the appropriate spot. As described, the technique taught by Frey et al would also require fast signal detection, i.e., in less than 1 msec response time. While present technology can track such fast detection, such means are typically more costly and add complexity to the system by imposing stricter signal processing requirements. Further, the technique of Frey et al is sensitive to ambient illumination which may reach the eye and be reflected into the detector where it would tend to reduce detectability of the light pulses.  
       [0019] In view of the above, what is needed is a system and method for tracking movement from eye in both the horizontal and vertical directions which is fully compatible with laser surgery procedures, has fast response, and is insensitive to ambient illumination.  
       SUMMARY OF THE INVENTION  
       [0020] One aspect of the present invention is directed to a system for facilitating tracking of a moving object. The object has a feature, associated therewith which is illuminated with ambient light. The system includes illumination means for illuminating at least the feature of the object with a tracking light. The system also includes detection means for detecting an image of the feature and for outputting signals corresponding to movement of the image. The signals have a first component due to the tracking light and a second component due to the ambient light. Further, the system includes filter means for filtering the second component from the signals and for outputting the first component of the signals so that the ambient light is discriminated from the tracking light and the moving object can be tracked using the first component of the signals.  
       [0021] Another aspect of the present invention is directed toward a system for compensating for movement of an eye of a patient during a surgical procedure. The eye has a feature and a visual axis associated therewith, wherein the feature is illuminated with ambient light. The surgical procedure includes directing a laser beam upon the eye using a mirror. The laser beam has an optical axis associated therewith. The system includes illumination means for illuminating at least the feature of the object with a tracking light. The system also includes detection means for detecting an image of the feature and for outputting signals corresponding to movement of the image, wherein the signals have a first component due to the tracking light and a second component due to the ambient light. A filter means is also included for filtering the second component from the signals and for outputting the first component of the signals so that the ambient light is discriminated from the tracking light. The system further includes logic means for receiving the filtered signals and for generating tracking signals based thereon. The system also includes means for directing the laser beam upon the eye based on the tracking signals to maintain a substantially centered condition between the optical axis of the laser beam and the visual axis of the eye.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0022] Representative embodiments of the present invention will be described with reference to the following figures:  
     [0023] FIGS.  1 ( a ) and  1 ( b ) are diagrammatic views of the present invention.  
     [0024]FIG. 2 illustrates an alternative embodiment of the tracking light source  1005 .  
     [0025]FIG. 3 illustrates the overlapping pattern of the light beams  1480  on the eye  900 .  
     [0026]FIG. 4 is a diagrammatic view of the detector  1580 .  
     [0027]FIG. 5( a ) illustrates an image of the eye in an aligned position with respect to the detector elements  1620 A- 1620 D.  
     [0028]FIG. 5( b ) illustrates an image of the eye in an unaligned position with respect to the detector elements  1620 A- 1620 D.  
     [0029]FIG. 6 is a diagrammatic view of the filter  1780 .  
     [0030]FIG. 7 a  illustrates a means for adjusting the size of the tracking feature image and the detector  1580 .  
     [0031]FIGS. 7 b  and  7   c  show an embodiment of the means for adjusting the size of the tracking feature image.  
     [0032]FIG. 7 d  shows an image of the eye in which the image size has not been adjusted.  
     [0033]FIG. 7 e  shows an image of the eye in which the image size has been adjusted.  
     [0034]FIG. 7 f  depicts an embodiment of the tracker subsystem  4000 , including the means for adjusting the size of the image.  
     [0035]FIG. 8 is a block diagram of the system  1000  into which the present invention has been integrated.  
     [0036]FIG. 9 is a diagram of the system  1000  in which the laser subsystem  2000  and the microscope subsystem  3000  are shown in detail.  
     [0037]FIGS. 10 a  and  10   b  are an expanded schematic diagram and a detail view of embodiments of components shown in FIG. 9.  
     [0038]FIG. 11 illustrates components of the eye tracking subsystem  4000  for use in the system  1000 .  
     [0039]FIGS. 12 a  and  12   b  are an expanded schematic diagram and a detail view of embodiments of components shown in FIG. 11.  
     [0040]FIG. 13 is a block diagram showing the interrelationship of the laser subsystem  2000  and the eye tracking subsystem  4000  that allows compensation for eye movements. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0041] Reference is now made to the accompanying Figures for the purpose of describing, in detail, the preferred embodiments of the present invention. The Figures and accompanying detailed description are provided as examples of the invention and are not intended to limit the scope of the claims appended hereto.  
     [0042]FIG. 1 a  depicts a system  1  for facilitating tracking of a moving object. Also shown there is a multiplicity of potential ambient light sources  1002 A and  1002 B that, without the system and method of the present invention, would interfere detrimentally with the tracking of the moving object. Ambient light sources such as fluorescent or incandescent room lights, microscope illuminators, or lights used for photography (including flash lamps) all can radiate light onto the object. Part of this ambient illumination is reflected or scattered by the object into and through the lens  700  of the tracker and can irradiate the detector  1600  of that tracker. This portion of the irradiation at the detector is called “stray light”. In general, this stray light will generate an electrical signal that may not be well correlated with the movement of the object. A portion of the illumination from the tracker illuminator  1005  also reflects or scatters from the object  900  into and through the lens  700  of the tracker and irradiates said detector array. This we term “signal light” because the electrical signal it generates is correlated with the object motion. Superposition of stray light onto the signal light reduces the signal-to-noise ratio at the detector array by increasing the noise level. Any reduction in signal-to-noise ratio will tend to interfere with the tracking system&#39;s ability to track the object.  
     [0043] In one embodiment of the present invention and as shown in FIG. 1 b,  the object to be tracked is a human eye  900 . Tracking movement of the eye  900  is particularly useful during laser surgical procedures on the cornea  930  and diagnostic applications involving the eye. Eye tracking also is useful during automated refractometry or measurement of corneal topography. However, while the following description is set forth for tracking movement of the eye  900 , it is contemplated that the present invention may be readily adapted for tracking the movement of other objects. For example, the system and method disclosed herein may be used to track the movement of objects such as selected skin tissue site relative to a dermatology surgical laser, an object of interest relative to a robotic machine vision system, or a docking site on a spacecraft relative to a remotely controlled probe. In each case, the object would bear a distinctive natural or artificial fiduciary marking to facilitate the tracking function.  
     [0044] Referring to FIG. 1 b,  as is well known, the eye  900  includes a transparent cornea  930  and a translucent and white-colored sclera  960 . The eye  900  also includes a limbus  950 , which is the intersection of the cornea  930  and the sclera  960 . The limbus  950  also corresponds to the outer boundary of the colored iris of the eye  900  (not shown), which can be seen through the cornea  930  and is a characteristic feature of all human eyes.  
     [0045] The present invention uses a differential reflection technique to track the movement of the eye  900 . Thus, the system and method disclosed herein involve detecting light scattered from the region of a naturally occurring feature of the eye  900  to measure differences in illumination from that region. The naturally occurring feature of the eye  900  should be substantially circular in shape; larger in diameter than the site to be surgically treated, i.e., 10 to 14 mm in diameter; constitute a boundary between sub-regions having differences in light reflectivity of at least 10 percent; and fixed to the object of interest, i.e., the eye. The naturally occurring feature of the eye  900  that is used for tracking the movement of the eye  900  may also be referred to herein as the “tracking feature.” 
     [0046] In our preferred embodiment, the limbus  950  is used as the tracking feature to track the movement of the eye  900 . The limbus  950  is a desirable tracking feature because it is an integral part of the eyeball structure itself. Also, the limbus  950  moves in the same manner as the central area of the cornea  930 . Thus, in frontal view, transition at the circular limbus  950  from the colored or tinted circular area and the white sclera  960  offers photometric contrast due to significant differences in light reflectivity of the iris and sclera in an axi-symmetric feature of the eye  900  that lends itself to tracking by the means described herein. Alternatively, the movement of the eye  900  may be tracked with reference to other naturally occurring boundaries of the eye  900 , including the pupil, as well as non-natural features, such as colored ink markings on the sclera  960 .  
     [0047] The light scattered from the eye  900  that will be detected is illustrated in FIG. 1 a  as light rays  1006 S. The light rays  1006 S are generated from light that is incident on a predetermined portion of the eye  900 . The predetermined portion of the eye  900  includes the tracking feature and the areas surrounding the tracking feature. Thus, in the embodiment in which the limbus  950  is the tracking feature, the predetermined portion of the eye  900  that is illuminated is the iris (not shown) inside the cornea  930 , the limbus  950 , and the sclera  960  surrounding the limbus  950 .  
     [0048] As is seen in FIG. 1 a,  the tracking light source  1005  and the ambient light sources  1002  provide light that is incident on the predetermined portion of the eye  900  to generate the superimposed light rays  1003 S and  1006 S. The tracking light source  1005  receives synchronization signals  1009  from modulator  1008 . The tracking light source  1005  illuminates the predetermined portion of the object  900  with the tracking illumination  1006  based on the synchronization signals  1009 . Thus, the synchronization signals  1009  cause the tracking light source  1005  to illuminate the predetermined portion of the eye  900  at a predetermined frequency, which differs from the frequency of the ambient light rays  1003 A and  1003 B originating at any of the stray light sources  1002 A and  1002 B.  
     [0049] The ambient light sources  1002 A and  1002 B represents one or more light sources that deliver unwanted light  1003 A and  1003 B to the predetermined portion of eye  900 . The ambient light source  1002  thus is any light source, aside from the tracking light source  1005 , that may be present when the system or method of the present invention is practiced. For example, the ambient light source  1002 A or  1002 B may be an illuminator associated with a microscope or ambient room illumination. Some ambient light sources  1002 A or  1002 B, such as fluorescent lights, typically have a known frequency, such as 60 or 120 Hz, which is significantly different from the predetermined frequency, of, for example, 200 to 300 Hz, at which the tracking light source  1005  illuminates the predetermined portion of the eye  900 . Others, like tungsten bulbs, produce continuous illumination.  
     [0050] Thus, from the above, it is seen that the tracking illumination  1006  and the ambient light  1003 A and  1003 B are incident on, and scatter from, the predetermined portion of the eye  900  to generate the light rays  1003 S and  1006 S. As such, the light rays  1003 S and  1006 S may be viewed as two superimposed components. The first component is due to the tracking illumination  1006  and the second is due to the ambient light  1003 .  
     [0051] The system  1  also includes a lens  700  that is positioned to receive the light rays  1003 S and  1006 S and to focus them in the form of light rays  1003 F and  1006 F onto certain detector elements of the detector  1600 . In this way, an image  1670  (not shown) of the tracking feature, here, the limbus  950 , is formed at and can be detected by the detector  1600 . The image  1670  may be viewed as having two components, one due to the tracking illumination  1006  and the other due to the ambient light  1003 A and  1003 B.  
     [0052] The detector  1600  outputs detector signals  1006 X and  1003 X which are characterized by eye movement in the X direction and  1006 Y and  1003 Y which relate to eye movement in the Y direction. The detector signals output from the detector  1600  may be therefore viewed as having two X components and two Y components, the first due to the tracking illumination  1006  and the second due to the ambient light  1003 A and  1003 B.  
     [0053] As will be described in more detail below, the demodulator  1800  filters the signals and outputs only the component of the detector signals corresponding to the tracking illumination  1006  as tracking signals  1810 X and  1810 Y. The demodulator  1800  rejects the second component of the detector signals that are due to the ambient light  1003 . By synchronizing the tracking light source  1005  to the synchronizing signals  1009 , the tracker is rendered insensitive to the stray light originating at ambient light sources  1002 A and  1002 B and ultimately imaged upon detector  1600 . Since the, signals  1810  output from filter  1780  result only from light received from tracking light source  1005 , variations in intensity, color, direction of propagation, etc. in ambient light source  1002  do not compromise the operation of the tracker system. The signal-to-noise ratios of the output signals  1810 X and  1810 Y are thus substantially increased as is the tracker&#39;s ability to track the eye. The tracking signals  1810  may be used to track the movement of the eye  900 , for example, by using them to measure the lateral displacement of the apex of the eyeball while performing visual tasks such as reading or observing a video screen. These measurements may be of interest to visual scientists studying the behavior of the human eye. Other ways, in which the present eye tracker might prove valuable would be in measuring the ability of the human eye to follow rapid motions of targets in simulated military encounters or in measuring eye displacements during exposure to accelerations during flight training.  
     [0054] The tracking light source  1005  and its illumination of the eye  900  is next described in more detail. The tracking light source  1005  generates tracking illumination  1006 , which illuminates the predetermined portion of the eye  900  substantially uniformly.  
     [0055] In one embodiment, the tracking light source  1005  is an individual light generating element that is positioned to illuminate the portion of the eye  900  from a substantially axial direction with respect to the axis a  810 . For example, the individual light generating element may be positioned a few degrees off of the axis  810  so that the predetermined portion of the eye  900  is substantially uniformly illuminated with light. In this embodiment, the individual light generating element may comprise a light emitting diode (LED), a diode laser, or the like. We prefer that the individual light generating element emit monochromatic light at a near-infrared wavelength of about 0.88 μm because of its low visibility to the human eye.  
     [0056] An alternative embodiment of the tracking light source  1005  is depicted in FIG. 2. There, it is seen that the tracking light source  1005  may comprise a plurality (e.g.,  8 ) of individual light generating elements  1420 . The individual light generating elements  1420  are positioned in a ring-like manner, equidistant from, and off of, the axis  810 . Each of the plurality of elements  1420  generates a light beam  1480  which illuminates a predetermined portion of the eye  900  as follows.  
     [0057] In this alternative embodiment, the predetermined portion of the eye  900  is illuminated such that a light beam  1480  from one of the plurality of elements  1420  overlaps with light beams  1480  from adjacent elements  1420 . Thus, as shown in FIG. 2 and more clearly in FIG. 3, each light beam  1480  from an element  1420  illuminates an area  1720  on the predetermined portion of the eye  900 . This overlap and the radial extent of the illuminated region of the eye  900  ensure that the predetermined portion of the eye  900  remains substantially uniformly illuminated when it moves.  
     [0058] If the radius  1421  of the ring of sources  1420  is a significant fraction of their distances to the eye  900 , the angle of convergence  1430  of the beams  1480  with respect to the axis  810  may be large enough that the beams  1480  striking the cornea  930  or the nearby sclera  960  might reflect specularly into the lens  700 . This light might then reach the detector elements of detector  1600  and adversely affect the performance of the tracker.  
     [0059] To illustrate, in an embodiment of the invention with light sources  1420  at an angle  1430  of approximately 56 degrees to the axis  810  (FIG. 2), rays specularly reflected from the human cornea  930  of radius of curvature approximately 8 mm would be imaged within 2 mm square detector elements if the magnification produced by lens  700  is about unity and an array of detector elements is located 6.35 mm from the axis  810 . This specularly-reflected light would be superimposed upon the image of the tracking feature and, being brighter than the scattered light from the eye  900 , would adversely affect the ability of the tracker to measure true eye movements. If the sources  1420  were to be moved significantly closer to the axis  810 , this potential problem is reduced or alleviated. For instance, using the example just described, the spurious images reflected specularly from the cornea or the adjacent stroma would not be seen by the detector elements if the sources were located no more than 30 degrees off the axis  810 .  
     [0060] The importance of using near-coaxial illumination in avoiding interference from spurious signals due to specular reflections has not been appreciated by some of the prior art, including the methods represented by PCT Application No. WO 94/18883 due to Knopp et al. The dual light source arrangement therein described may not be symmetric enough to ensure illumination uniformity over the full predetermined area of the eye. Further, it may allow substantial interference from specular reflections that enter the detector means thereby degrading measurement of the eye&#39;s motions.  
     [0061] The tracker illumination subsystem utilized in a pupil-tracking version of the present invention would need to be designed so that the specularly-reflected light therefrom does not interfere with the tracking function. This design would follow the principles just described for the case of the limbus-tracking system. The light sources  1420  should, in a design for tracking a pupil, be located no more than 10 degrees off the axis  810 .  
     [0062] In the alternative embodiment of tracking light source  1005  as depicted in FIG. 2, the wavelength of the light beams  1480  from the elements  1420  are chosen to lie in the near-infrared range of wavelength approximately 0.8 to 1.0 μm. To achieve this, light-emitting diodes (LEDs), such as the DPI-E805 type units manufactured by Photonic Detectors, Inc., may be used. We prefer to use such a wavelength because the sensitivity of the human eye is extremely low at the 0.88 μm emission wavelength of these devices so the observed intensity of any portion of the light beam  1480  reflected or scattered by a cornea surface will be so small as not to affect observation of the patient&#39;s eye by the surgeon. In addition, because of its low visibility to the human eye, the light beams  1480  will not interfere with fixation of the eye  900  by the patient upon a visible light target located within a fixation target device.  
     [0063] The tracking light source  1005  is modulated at a predefined frequency. This is done using the synchronization signals  1009  received from the modulator  1008 . More specifically, the modulator  1008  varies the synchronization signals  1009  between zero and X volts at the predefined frequency. The value of X is the maximum operating voltage of the tracking light source  1005 .  
     [0064] In yet another embodiment of the tracking light source  1005 , the light beam  1006  many emanate from a tungsten filament lamp that provides for both visual observation of the eye  900  and tracking the movement of the eye  900 . This beam  1006  would be intensity modulated by a mechanical chopping device such as the type available from Oriel Corporation as their Model 75155 Enclosed Optical Chopper with motor-driven, 30-aperture, slotted wheel. This chopping device is capable of modulating the beam  1006  at frequencies up to 3000 Hz. The light beam  1006  would appear to be of constant intensity to the surgeon&#39;s eye so it would serve well for visual alignment of the eye  900  to an alignment reference (reticle) pattern in the microscope.  
     [0065] Referring next to FIG. 4, the detector  1600  is described in more detail. As is seen there, the detector  1600  receives the light rays  1006 F and  1003 F, which collectively form image  1670 , and outputs the detector signals  1006 X,  1003 X,  1006 Y and  1003 Y which collectively are designated as  1610 . The detector  1600  outputs signals  1610  to the demodulator  1800  and then to amplifier  1700  (not shown).  
     [0066] As is shown in FIG. 5 a,  the detector array  1600  comprises a plurality of detector elements  1620 A- 1620 D. The detector elements  1620 A and  1620 C are positioned opposite each other on the X-axis  164 . The detector elements  1620 B and  1620 D are positioned opposite each other on the Y-axis  163 . In one embodiment, the detector elements  1620 A- 1620 D each comprise a dual-element PIN silicon photodetector, such as the PIN SPOT-2DM1 manufactured by United Detector Technologies.  
     [0067] The light rays  1006 F and  1003 F each contribute to image  1670  of the tracking feature (e.g., the limbus  950 ) on the detector elements  1620 A- 1620 D. The opposing pairs of detector elements  1620 A/ 1620 C and  1620 B/ 1620 D produce varying electrical outputs as the image  1670  of the tracking feature moves with respect to the X and Y axes. The arithmetic difference between signals from each pair of opposing detectors  1620 A/ 1620 C and  1620 B/ 1620 D is substantially proportional to the displacement of the image  1670  from the centered position in the corresponding axis.  
     [0068] When the cornea  930  of eye  900  is perfectly centered with respect to the axis  810 , the image  1670  is centered on the detector elements  1620 A- 12620 D, as shown in FIG. 5 a.  In this centered condition, the four detector elements  1620 A- 1620 D receive essentially equal amounts of light energy from the image  1670 . In this case, the detector array  1600  outputs voltage signals  1610  indicative of the centered condition.  
     [0069] When the cornea  930  is not perfectly (entered with respect to the axis  810 , the image  1670  is not centered on the detectors  1620 A- 1620 D; an example of which condition is shown in FIG. 5 b.  In this uncentered condition, at least two of the detector elements  1620 A- 1620 D receive unequal amounts of light energy from the image  1670 . In this case, the detector array  1600  outputs voltage signals  1610  that are proportional to the movement of the image  1670  relative to the axis  810 .  
     [0070] It should be readily apparent from the foregoing to those skilled in the art that, in the absence of stray light  1003 F, as the image  1670  of the tracking feature moves across the detectors  1620 A- 1620 D, signals proportional to image displacement are produced as voltage signals  1006 X and  1006 Y. Once the image  1670  moves sufficiently for diametrically opposite detectors  1620 A/ 1620 C or  1620 B/ 1620 D to receive light only from the sclera  960  or the iris and not partially from both, the voltage signals  1610  cease to be linear with image displacement. The detector sizes can, however, be chosen so as to provide an appropriate linear range magnitude in both orthogonal directions, thereby ensuring dynamic tracking adequate to cover the anticipated lateral displacement of the cornea  930  in each direction.  
     [0071] As previously discussed, the highest accelerations of movements of the eye  900  occur during the saccades, so these eye motions would be the fastest and hardest to track. A typical saccade corresponds to a motion of up to about 5 degrees in 10 to 20 msec, which corresponds to about 1 mm of corneal translation, assuming a 1 in. diameter globe. Hence, the system  1  preferably is able to sense and respond to the eye&#39;s motion in 3-5 msec in order to provide real-time tracking. This corresponds to a response frequency of 200 to 300 Hz, which is 2-3 times faster than the eye and is easily achieved with standard electronics if the signal-to-noise ratio at the detector elements is high enough.  
     [0072]FIG. 6 illustrates the signal filtering action in more detail. As is seen there, the filter  1780  includes a modulator  1008  and a demodulator  1800  as well as signals  1009  to tracking light source  1005 . The modulator  1008  outputs timing signals  2003  to the demodulator  1800 , as well as signals  1009  to tracking light source  1005 . The timing signals  2003  temporally synchronize the demodulator  1800  with the modulation frequency of the tracking light source  1005  used to illuminate the eye  900 . This ensures that only light of an appropriate frequency is allowed to produce the tracking signals  1810 X and  1810 Y. As previously indicated, this synchronization constitutes a means for temporal discrimination of light  1006 S used for tracking from light  1003 S originating at ambient light sources  1002 A and  1002 B.  
     [0073] None of the prior art concerned with eye tracking has appreciated the unique advantages derived by modulating the light source  1005  so that the signals detected therefrom can be filtered from those due to other unwanted or stray light sources. Hence, a distinct advantage of the present invention over the prior art is clearly seen.  
     [0074] The diameter of the human eye limbus  950  is not constant for all the population; it typically varies, in adults, from approximately 10 to 14 mm. In some individual eyes, the vertical and horizontal dimensions of the limbus may differ slightly so the frontal aspect thereof may appear somewhat elliptical. Ideally, the rim of the image  1670  of the limbus  950  should be substantially coincident with the centers of the detectors  1620 A- 1620 D in the array  1600  as indicated in FIG. 5 a.  For this to occur with varying limbus diameters, it is desirable to incorporate into the system a means for adjusting the size of the image  1670 . This can be done by varying the optical magnification of the lens system forming the image  1670 . While, theoretically, anamorphic magnification of the image could be provided so a slightly elliptical limbus could be aligned perfectly with the detector centers, this added complexity is not essential. Proper function of the present eye tracking system requires only that the rim of the limbus image  1670  be symmetrically disposed with respect to the centers of opposing detectors ( 1620 A and  1620 C in the vertical direction and  1620 B and  1620 D in the horizontal direction). Slight mismatch of image size in orthogonal directions due to an elliptical nature of the limbus image will not reduce the ability of the system to sense displacement of that image in each direction, and hence eye motion, as described above.  
     [0075] A means for varying the size of the image  1670  of the limbus  950  is described with reference to FIGS. 7 a - 7   e.  FIG. 7 a  depicts an adjuster  1550  that is positioned between the lens  700  and detector  1600  so as to receive the light rays  1006 F. The adjuster  1550  introduces variable magnification into the beam comprising the light rays  1006 F and outputs them in the form of rays  1006 M as follows.  
     [0076]FIGS. 7 b  and  7   c  show one embodiment of an optical system that is used to vary magnification of the tracking feature image  1670  at the plane of the detector array  1600 . Here, the image-forming component comprises a set of three refracting (lens) elements  112 - 114 , at least two of which are axially moveable by external means such as a motorized drive mechanism. With two moving components, the magnification can be changed as required and sharp focus of the image  1670  at the detector array  1600  maintained. Note that the image-forming components of FIG. 7 b  may comprise single elements or multiple-elements, such as cemented doublets, for aberrational control reasons.  
     [0077] In one embodiment shown in FIG. 7 c , each of the two moveable lens elements  113  and  114  is independently driven along axially-oriented tracks or rails by an electric motor  118  of the type commonly known as a “stepper” motor that turns a lead screw  117 . A nut  116  attached to the moveable lens&#39;s mount ( 124   a  or  124   b ) engages said lead screw and moves along said screw as the screw is turned in a forward or reverse direction by the motor. Starting at a reference or “zero” position established by an encoder  119  attached to the motor  118 , the motor  118  receives a series of pulsed drive signals from associated electronics (not shown). The motor  118  turns a fixed angular amount in response to each pulse received. In order to move the lens by a predetermined axial distance corresponding to a specific magnification change, the electronics delivers a corresponding specific number of pulses. An algorithm within a computer in communication with the encoder  119  may be used to control the electronics (not shown) driving both stepper motors so the movements of the two lenses  113  and  114  always remain synchronized.  
     [0078] One embodiment for automating the function of the magnification-change feature of this invention is described below. Following alignment of the patient to the axis  810 , a computer routine is initiated that commands the magnification feature optics to adjust to a minimum value such that the image  1670  of the tracking feature  950  located at a predetermined distance in front of lens  700  will lie inside the centers of the detectors in array  1600  as shown in FIG. 7 d.  In this figure, the detectors  1620 A/ 1620 C are shown for the X-axis only for purposes of clarity. The geometry relating to the detectors  1620 B/ 1620 D in the Y axis would be similar to that shown. Each detector of FIGS. 7 d  and  7   e  is of the dual-element type as mentioned earlier. The magnification is increased and focus maintained under computer algorithm control while the electrical signals  133  (S 1 A),  134  (S 1 B),  135  (S 2 A), and  136  (S 2 B) are monitored. When the photometrically darker region of the image inside the limbus  950  reaches the junction between adjacent elements in each detector  1620 A or  1620 C, the signals from the outermost elements (S 1 A and S 2 A) will begin to decrease because progressively smaller areas of the image  1670  of the brighter-appearing sclera  960  will fall into those detector elements. When this change in signals is recognized by the computer, the stepper motors are stopped and electrically locked in place. The same condition would occur simultaneously in the Y-axis direction if the image of the limbus  950  is symmetrical, which is generally the case, and the condition shown in FIG. 7 e  would prevail. To allow for minor differences between end-points measured by the detector pairs in the X and Y axes, averages of the received signals can be derived and used by the computer. With the magnification now properly adjusted, tracking of the eye&#39;s motions can proceed as described earlier.  
     [0079] If the present invention were to be configured for tracking the pupil of the eye instead of the limbus thereof, this automatic magnification-adjusting feature would be advantageous in compensating for pupil diameter changes due to changes in illumination level and/or the effects of medication administered by the physician to facilitate the surgical procedure. The nominal magnification of the image-forming optics comprising lens  700 , lens  112 , lens  113 , and lens  114  would need to be appropriately adjusted as would the dynamic range of the adjuster  1550 .  
     [0080] A semiautomatic method for setting the magnification of system  1  also could be implemented as follows. Since the average limbus diameter of the patient&#39;s eye is easily measured during preparation for surgery, this dimension could be entered into an algorithm in a computer and the stepper motors  118  commanded to reposition the moveable lenses in accordance with a prior calibration to the proper locations to produce the properly sized, in-focus image of the limbus  950  at the detector array  1600 . Once this adjustment is made, the stepper motors  118  can be electrically locked and tracking can proceed as described earlier.  
     [0081]FIG. 7 f  shows an embodiment of the tracker subsystem  9000  comprising tracking light  1005 , modulator  1008 , lens  700 , adjuster  1550 , detector  1600 , demodulator  1800 , and amplifier  1700  as it might be configured for use in tracking an object (here shown as eye  900 ) for use in, for example, a diagnostic application. Computer subsystem  5000  and microscope subsystem  3000  are depicted in their roles as means for controlling the magnification adjuster  1550  and observing the eye, respectively. The amplified output signals  1710 X and  1710 Y provide information as to the lateral movements of the cornea  930  relative to the line of sight of microscope  100  within microscope subsystem  3000 . The beamsplitter  80  provides simultaneous optical access to the eye by the tracker and the microscope. Through the filtering action of modulator  1008  combined with tracking light  1005  and demodulator  1800  through connection  2003 , these signals are not affected by the presence, absence, or nature of ambient light sources such as is represented by ambient light  1002 B. This insensitivity to ambient illumination provides a distinct advantage of the present invention over prior art.  
     [0082]FIG. 8 depicts a system  1000  for surgical treatment of the eye with a laser into which the present invention has been integrated. The system  1000  includes a microscope subsystem  3000  through which a surgeon can view an eye  900  via beam  3500 . Under the control of the surgeon who observes the eye  900  and the surgical treatment thereof through microscope subsystem  3000 , the laser subsystem  2000  delivers a beam  2500  preferably comprising a sequence of short pulses of mid-infrared light to the cornea  930  of the eye  900 . These pulses are moved over the cornea  930  in a predetermined pattern in accordance with predetermined commands from the computer subsystem  5000  that is coupled to the laser system  2000  through connection  5200 .  
     [0083] Motions of the eye  900  during the surgical procedure are sensed by the tracking subsystem  4000  via a light beam  4500  that includes an image of the tracking feature of the eye  900 . Commands to deviate the laser beam  2500  to compensate for such motions are delivered from the tracking subsystem  4000  to electronics subsystem  6000  via connection  5610 , and then to the computer subsystem  5000  via connection  5600 . The commands to deviate the laser beam  2500  are then delivered from the computer  5000  to the laser subsystem  2000  via connection  5200 . In this way, the laser beam  2500  is deviated as required to center the pattern of laser pulses to the displaced cornea. The result is that the pattern of laser pulses at the eye  900  remains centered on the cornea as if the eye had not moved.  
     [0084]FIG. 9 is a block diagram of the system  1000  in which the laser subsystem  2000  and the microscope subsystem  3000  are shown in more detail and in relationship to the computer subsystem  5000 . Commands  11  are sent to control  20  from computer  5000  to control the laser  30  via connection  21 . The laser  30  is preferably a mid-infrared laser generating short laser pulses which yield a tissue removal mechanism based on photospallation as disclosed by Telfair et al in U.S. patent application Ser. No. 08/549,385.  
     [0085] The laser beam  31  passes through a safety shutter  40  as beam  41 . The intensity of the beam  41  is controlled by variable attenuator  50  whose output is beam  51 . The beam  51  is deviated through small angles in two orthogonal directions upon reflection from scanning mirror  60  to form beam  61 . This beam  61  is focused by lens  700  into a small circular spot of laser light at the cornea  930  of the patient&#39;s eye  900  as  2500 A. The lens  700  may comprise single or multiple refracting and/or reflecting elements.  
     [0086] The beam  2500 A is incident upon and reflected by beamsplitter  80  as beam  2500 B. The laser beam  2500 B is preferably scanned over a specific centralized region of the surface of the cornea  930  in a predefined manner so as to selectively remove tissue at various points within the cornea  930  and thereby cause the curvature of the cornea  930  to change in a predictable and controlled fashion (PRK) or, in the case of a therapeutic intervention, to remove tissue substantially uniformly over the treated area (PTK).  
     [0087] The system  1000  also can be utilized to perform the procedure called LASIK in which controlled tissue removal occurs after a flap of anterior tissue has temporarily been lifted from the surface of the eye  900 . By virtue of the scanning motion introduced by the mirror  60  as driven by a set of actuators  66  through connection set  33  to the scan control electronics  22 , the focused beam  2500 B traces a prescribed pattern on the cornea as directed by the computer  5000  via drive signals  36  and  33 . Feedback as to the instantaneous position of the mirror  60  is given to the computer  5000  via a set of position transducers  67  and associated connection set  34  and  35 . It should be noted that two actuators  66  (operating in push-pull fashion) and one transducer  67  are required for each axis of motion of the mirror  60 .  
     [0088] Alignment of the eye  900  to the system  1000  is initially established and subsequently monitored by the surgeon who observes the eye  900  via reflected beams  91 ,  92 , and  101  passing through beamsplitter  80  and magnified by microscope  100 . The pulse energy monitor  120  measures the intensity of the laser beam  2500 A via transmitted beam  72  and feeds this measurement to the control electronics  20  via connections  121  and  11  by way of computer  5000  to ensure that sufficient energy is deliverers to the eye  900  for the intended surgical procedure, but that safe limits on the energy are not exceeded.  
     [0089]FIG. 10 a  illustrates schematically an assemblage of certain components from FIG. 9 in order to clarify their mutual spatial relationships. The scanning mirror  60  is shown as a two-axis gimbaled assembly which tilts about orthogonal axes  62  and  63  to affect movement of the focused spot of laser light at the cornea  930  in the coordinate system depicted as  140 .  
     [0090] To correlate the reference frame of the eye  900  to that of the system  1000  as shown in FIGS. 9, 10 a,  and  10   b,  the line-of-sight of the patient&#39;s eye  900  is substantially coincident with the propagation axis of the undeviated incident laser beam  2500 B. As used herein, in accordance with customary definition, the term “line-of-sight” or “principal line of vision” refers to the chief ray of the bundle of rays passing through the pupil of the eye  900  and reaching the fovea, thus connecting the fovea with the fixation point through the center of the entrance pupil. It will therefore be appreciated that the line-of-sight constitutes an eye metric defined directly by the patient, rather than through some external measurement of the eye&#39;s position and further, that the line of-sight can be defined without ambiguity for a given eye  900  and is the only axis amenable to objective measurement using cooperative patient fixation.  
     [0091] It is generally acknowledged that, for best post-surgery visual performance, the point marking the intersection of the line-of-sight with the cornea establishes the desired center for the optical zone of refractive procedures seeking to restore visual acuity. It is noted that the orientation of the line-of-sight of the eye  900 , shown in FIGS. 9, 10 a,  and  10   b,  may be vertical, horizontal, or intermediate to those extremes as befitting comfortable positioning of the patient for surgery without affecting the effectiveness of the invention.  
     [0092] Visual access to the eye  900  by the surgeon&#39;s eyes through microscope  100  is by means of beamsplitter  80 . The beamsplitter bears, on its side nearest to the eye  900 , a thin-film coating that maximally reflects mid-infrared radiation in the wavelength region of 2.7 to 3.1 μm while partially transmitting visible light. The wavelength-preferential, or dichroic, nature of this coating serves to separate the functions, of the surgical laser  30  from that of the microscope  100  and, hence, to facilitate the surgeon&#39;s observation and control of the surgical process. The side of the beamsplitter  80  nearest to said microscope is conventionally anti-reflection coated to maximize transmission of visible light.  
     [0093] During preparation for laser surgery on the cornea  930 , the line-of-sight of the eye  900  is aligned to coincide with the axis of the undeviated laser beam  2500 B by two-axis lateral-translational adjustments, in a known manner, as directed by the surgeon. The surgeon observes the eye  900  by way of beams  91 ,  92 , and  101  through the surgical microscope  100 . In this way, the surgeon judges the degree of centration of the frontal image of the cornea  930  with respect to a crosshair or other fixed reference mark (not shown) internal to microscope  100  indicating, as a result of prior calibration, the location of the axis of undeviated laser beam  2500 B.  
     [0094] The axial location of the cornea  930  can also be judged by the surgeon&#39;s eyes by virtue of the observed degree of focus of the image of corneal features relative to the previously calibrated and fixed object plane of best focus  94  for microscope  100 . See FIG. 9. Directions from the surgeon allow adjustment of the axial position of the cornea of eye  90  to coincide with said plane of best focus  94 .  
     [0095] We now describe in more detail the constituent parts of microscope subsystem  3000  as depicted in FIG. 9. Frontal illumination of the eye  900  to facilitate visual observation thereof by the surgeon viewing beams  101  exiting from microscope  100  in preparation for and during surgical procedures is provided by a light source  102  attached to or integral with the microscope  100 . The light beam  103  from light source  102  typically emanates as beam  103  from a tungsten filament lamp therein and is incident upon the eye  900  as beam  104 . The beams  103  and  104  propagate at a small angle, typically of the order of 0-10 degrees, with respect to the axis of the microscope  100 . Such illumination is frequently termed coaxial or near-coaxial because of its angular proximity to that axis.  
     [0096] The angular orientation of the line-of-sight of eye  900  is preferably established by directing the patient to observe and focus attention, i.e., fixate, on beam  132  which is a continuation of beam  131  from an illuminated target (not shown) projected into the eye  900  by an optical fixation target device  130 , which is preferably integrated into microscope  100  as indicated in FIG. 9. The target will appear to be located at a sufficient axial distance from the eye  900  of the patient so it can be observed and will have been previously aligned coaxially with the axis of the microscope  100 .  
     [0097] As shown in FIGS. 9, 10 a,  and  10   b,  the laser subsystem  2000  preferably includes a safety shutter  40  which closes automatically if the laser beam  2500 B fails to follow a prescribed path, if pulse energy-monitoring means  120  indicates a malfunction of laser  30 , or if the eye tracker subsystem  4000  described below cannot adequately follow the eye motion (as might happen it the eye inadvertently moves beyond the dynamic range of the tracker). The surgeon also can close the shutter  40  by actuating a nearby emergency stop switch (not shown).  
     [0098] Lateral motions of the patient&#39;s cornea  930  (preferably less than about 5 mm in either orthogonal lateral direction X or Y) that occur after the initial alignment performed in the manner described above, or throughout the surgical treatment, are rendered inconsequential by virtue of the function of the eye tracker subsystem  4000  shown in more detail in FIGS. 11, 12 a,  and  12   b.  The eye tracker subsystem  4000  functions as described below to sense the motion of the cornea  930  and to provide electrical signals  1810 X and  1810 Y that are proportional to the lateral misalignment of the cornea  930  relative to the axis of the incident undeviated laser beam  2500 B. The influence of stray light from ambient sources such as  1002 B of FIG. 11 are here ignored because of the filtering action described earlier.  
     [0099] The signals  1810 X and  1810 Y are processed by demodulator  1800 , amplifier set  1700 , logic circuit  1900 , and X- and Y-servo drivers  1930  and  1950  to cause tracking mirror  150  to restore centration of the image of cornea  930  formed by the lens  700  (and the magnification-adjusting lenses  1550  if used) at detector array  1600 .  
     [0100] The eye tracking subsystem  4000  is integrated with the above-described laser system  2000  so any sensed eye movements can be quantitatively fed back to the laser system in such a manner as to compensate for the eye movements. This function is accomplished as indicated in FIG. 13. Scanned laser beam  61  is incident upon tracking mirror  150  as beam  61 A after passing through beamsplitter  84 . After reflection from the tracking mirror  150 , the laser beam  61 B passes through and is focused by lens  700  and proceeds to eye  900  as described above. An image of a selected feature of said eye, such as the limbus  950 , is formed by the combined action of lens  700  and adjuster lenses  1550  located within eye tracking subsystem  4000  in front of detector  1600 . This image is formed by a beam following the path  1006 S,  1006 F,  1006 A by way of tracking mirror  15 C and beamspliter  84 . When the eye tracker subsystem  4000  senses and measures an eye movement, it sends signals  197  and  203  that cause tracking mirror  150  to tilt about its X and Y axes thereby compensating for the movement and reducing the signals  1810 X and  1810 Y to negligible values. Since the laser beam also reflects from tracking mirror  150 , the reflected laser beam  61 B and  61 C is deflected so as to align the pattern of pulses caused by action of scanning mirror  60  to the center of the cornea  930 . The surgical procedure therefore is accomplished as if the eye remained stationary.  
     [0101] It is noted, from FIG. 13, that beamsplitter  84 , tracking mirror  150 , lens  700 , and beamsplitter  80  are common to both laser subsystem  2000  and eye tracker subsystem  4000 . These components serve distinctive functions in each subsystem. For example, lens  700  focuses the mid-infrared laser beam  61 B as beam  61 C at or near the cornea and images the near-infrared beam  1006 S bearing eye feature motion information into the adjuster  1550  and thence to detector  1600 .  
     [0102] The functions of the single mirrors  60  (used for laser beam scanning) and  150  (used for eye tracking) as illustrated in FIGS. 9, 10 a,    11 , and  12   a  and  13 , also could be accomplished by two mirrors independently scanning, about mutually orthogonal X and Y axes and intercepted in sequence by the laser beam enroute to the eye  900  as illustrated in FIGS. 10 b  and  12   b.  If two mirrors are used for scanning, the mirrors tilt as commanded by input X and Y drive signals  33  (shown in FIG. 9) about axes  62   a  and  63   a  (shown in FIG. 10 b ) to controllably move the reflected laser beam  61 . Similarly, if two mirrors are used for tracking, the mirrors tilt as commanded by input X and Y drive signals  197  and  203  (shown in FIG. 11) about axes  152   a  and  153   a  (shown in FIG. 12 b ) to controllably move the reflected tracking beam  1006 F.  
     [0103] Regardless as to whether single or double mirrors are used for scanning and tracking, the tracking mirror means ( 150  or  152  in combination with  153 ) are located closer to the patient&#39;s eye  900  than the scanning mirror ( 60  or  64  in combination with  65 ) so as to separate the functions thereof and to allow the scanned laser beam  2500 B to be synchronized with measured movements of the eye  900 .  
     [0104] It may be observed from FIGS. 9 and 10 a,  as well as FIGS.  11 , and  12   a  in conjunction with FIG. 13, that the reflecting natures of tilting mirrors  60  (or  64  and  65  of FIG. 10 b ) and  150  (or  152  and  153  of FIG. 12 b ) play important roles when the present invention is used as part of the system  1000 . In both cases, laser radiation in beams  51  and  61 A is reflected. Near-infrared light in beam  1006 F also is reflected by mirror  150  (or  152  and  153  of FIG. 12 b ). This can be accomplished through use of common aluminum or silver thin-film coatings protected by overcoats of suitable dielectric materials such as silicon monoxide. Multiple-layer dielectric coatings also could be employed for these purposes.  
     [0105] Similarly, the substrate of and coating on beamsplitter  84  of FIG. 13 would preferably be selected to have high transmittance at the mid-infrared wavelength of laser  30  and high reflectance at the visible and/or near-infrared wavelengths used by the eye tracker subsystem  4000 . This dichroic coating, of a type frequently called a “cold mirror,” is commercially available from several suppliers, such as Optical Coating Laboratory, Inc., or Denton Vacuum, Inc. The other side of beamspliter  84  would preferably be antireflection coated for the wavelength of laser  30 . The latter coating can be omitted if said beamsplitter is oriented at Brewster&#39;s angle of incidence for the wavelength of said laser  30 .  
     [0106] Other arrangements of lenses, beamsplitters and mirrors could be incorporated into the optical system of this invention to accomplish the functions described herein. For example, beamsplitting prisms, typically in the form of cemented two-element cubes, each with a partially-reflecting, dichroic coating on an internal surface, might be employed to provide the functions of beamsplitters  80  and  84 .  
     [0107] As shown in FIG. 13, at the beamsplilters  80  and  84  the transmitted beams  92  and  61 A undergo small lateral displacements due to oblique incidence and the finite thickness of the component substrates. These fixed displacements are easily compensated for in the design of the apparatus, as would be apparent to a person of ordinary skill in the art.  
     [0108] As shown in FIG. 9, the computer subsystem  5000  communicates with and controls the laser source  30  through control  20  by means of connections  11  and  21 . In addition, the computer  5000  provides commands to scan control electronics  22  via connection  36  which drives the scanning mirror  60  by means of connection  33  and a set of actuators  66  in accordance with stored scanning patterns and commands input to the computer  5000  by the surgeon or an assistant. A connection  12  between the computer  5000  and the safety shutter  40  provides means for affecting maximum safety of the patient, the surgeon, and attending personnel in the following manner. As shown in FIG,  11 , the computer  5000  continually monitors the operation and status of the eye tracker subsystem  4000  by means of a connection  107  to the logic circuit  1900 . If malfunction of the tracking mirror  150  occurs or if the signals  1710 X and  1710 Y received from detector  1600  through demodulator  1800  and amplifier  1700  fall outside allowable limits, the computer issues a command to close safety shutter  40  through the connection  12  (See FIG. 9). If monitor  120  senses laser energy outside predetermined limits, a signal  121  also commands computer subsystem  5000  to close shutter  40 .  
     [0109]FIG. 11 also shows one embodiment of a servo system comprising detector  1600 , demodulator  1800 , amplifier set  1700 , logic circuit  1900 , X- and Y-servo drivers  1930  and  1950 , actuator set  200  (X-axis not shown), and position transducer set  201  (X-axis not shown), as well as associated connections, used to drive the tracking mirror  150 .  
     [0110] In one embodiment, the four detectors, collectively labeled set  1620  in FIG. 12 a,  each comprise a dual-element PIN silicon photodetector such as the PIN SPOT-2DM1 manufactured by United Detector Technologies. As indicated in FIG. 11, voltage signals  1006 X and  1006 Y, respectively, received from the detectors associated with the X- or Y-motion-sensing axis are sent to demodulator  1800  with the filtered signals  1810 X and  1810 Y then channeled directly into amplifier  1700 .  
     [0111] The logic circuit  1900  converts the demodulated and amplified signals from the detector  1600  corresponding to limbus image position, into commands for controlling the tracking mirror  150 . Diametrically opposing pairs of detectors  1620  produce varying electrical outputs as the image  1670  of the limbus  950  moves with respect to the X and Y axes.  
     [0112] The arithmetic difference between signals from each pair of opposing detectors is substantially proportional to the displacement of the image from the centered or null position in the corresponding axis. The signal differences produced within logic circuit  1900  and further processed by the logic circuit  1900  constitute mirror tilt commands indicated by control signals  1910 X and  1910 Y. The commands are relayed to the servo drivers  1930  and  1950  which, in turn, drive sets of actuators  200  which are mechanically linked to mirror  150 , thus causing said mirror to pivot about one or both of its axes. In this manner, the angular orientation of the mirror  150  may be modified as required to follow the limbus image motion in two orthogonal lateral directions.  
     [0113] A set of transducers  201  are also mechanically connected to mirror  150  to provide feedback to logic circuit  1900  via connections  198  and  202  in the Y and X directions respectively. The transducers  201  generally comprise, position-sensing elements which, in one embodiment, are simple, readily-available capacitive sensors such as are made by Kaman Instrumentation Corp. In another embodiment, they may be optical encoders integral with actuator set  200 . The transducers  201  facilitate stabilization of the motion of the tracking mirror  150 , referenced to a pre-selected default position. In addition, the transducers  201  sense when the tracking mirror  150  is at the end of its, range and will no longer track the eye&#39;s motion. By connection  108 , the logic  1900  commands the computer subsystem  5000  to close shutter  40 , if the tracker is no longer able to follow the eye motion.  
     [0114] In one embodiment, the reference position of the mirror  150  corresponds to alignment of the patient&#39;s line-of-sight with the optical axes of the instrument and of the undeviated laser beam  2500 B, as previously discussed. This reference position can be selected by the computer  5000 , when the surgeon indicates that the patient&#39;s eye  900  is properly aligned.  
     [0115] The servo system shown in FIG. 11 preferably is an off-null measurement system based on returning the error signals to zero. There may be alternative implementations of a servo control system other than the one depicted in this figure which would allow the accurate measurement and/or control of eye displacements at sufficiently high rates.  
     [0116] Although the particular embodiments shown and described above will prove to be useful in many applications relating to the arts to which the present invention pertains, further modifications of the present invention herein disclosed will occur to persons skilled in the art. All such modifications are deemed to be within the scope and spirit of the present invention as defined by the appended claims.