Abstract:
A method for tracking lateral movement of an object is disclosed employing a retro-reflective disk as a positioning reference and a simplified positioning detection to determine the position of the reference disk. The reference disk having a retro-reflective surface is affixed onto the object to be tracked. An illumination beam illuminates the reference disk from one direction and imaging optics forms a bright image spot of the reference disk with the backward scattering from the reference disk. A two-dimensional positioning detector detects the position of the bright image spot and an electronic circuit then generates positioning signals of the object for tracking applications. The bright image spot of the reference disk enables the use of single element two-dimensional positioning detector and thus enable fast detection (&gt;1 kHz) of the object&#39;s position. An embodiment of the method on an eye-tracking system is described.

Description:
REFERENCES 
     U.S. Patent Documents 
     5,620,436 Lang and Clouts April 1997 
     Method and apparatus for providing precise location of points on the eye 
     5,632,742 Frey, et al May 1997 
     Eye movement sensing method and system 
     5,410,376 Cornsweet, et al April 1995 
     Eye tracking method and apparatus 
     5,430,505 Katz July 1995 
     High speed eye tracking device and method 
     5,345,281 Taboada, et al September 1994 
     Eye tracking system and method 
     This application claims the benefit of U.S. provisional application No. 60/093,618, filed on Jul. 21, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an optical device for tracking lateral movement of an object. In particular, the present invention relates to an optical tracking device to track the eye movement during a laser surgery on the cornea. 
     BACKGROUND OF THE INVENTION 
     In a cornea surgery with a small laser beam, such as photo-refractive keratectomy or laser assisted in-situ keratomileuses, a fast and accurate eye tracking device is usually required to track the position of a patient&#39;s eye under surgery. With such a tracking device, the surgical laser beam can be delivered to predetermined positions on the eye even when the eye moves during the surgery. An example of this tracking device is described in U.S. Pat. No. 5,632,742 to Frey, et al. Another example is presented in U.S. Pat. No. 5,620,436 to Lang and Clouts. 
     These devices are optical systems operated in the infrared spectral range. To achieve a reliable eye tracking with high accuracy and high speed, it is essential to obtain on the cornea a well-defined reference of which the position can be determined precisely and quickly by an optical imaging system. Natural structures of the eye, such as the pupil and the limbus, do not always provide a reliable reference for this purpose. For instance, the centroid of the pupil moves when the pupil changes its size. In addition, these natural structures may be disturbed during the surgery. 
     High contrast masks have been proposed to apply onto the cornea to provide reliable references for a variety of eye-tracking devices. Frey et al. disclosed in U.S. Pat. No. 5,632,742 an ink ring affixed on the patient&#39;s eye. Lang and Clouts described in U.S. Pat. No. 5,620,436 a ring-shape aiming-fixture applied onto the patient&#39;s eye. 
     The main advantage of a ring shape mask is that it can be easily placed around the center of the cornea, where the laser surgery is taken place. On the other hand a ring shape mask is not convenient in many situations. One example is for laser assisted in-situ keratomileuses. In this type of surgery, a mechanical device called automated microkeratone laminates a thin layer of the cornea from the central part of the patient&#39;s eye. This layer is attached to the cornea by an uncut hinge and is flapped over to allow laser ablation on the corneal bed. This flap makes it difficult to apply a ring shape mask on the eye. 
     SUMMARY OF THE INVENTION 
     The present invention contemplates a small reference disk with a retro-reflective surface to enable fast and reliable tracking of lateral movement of an object such as an eye. The reference disk is affixed onto an object to be tracked. The disk has a retro-reflective surface to enhance significantly the backward scattering of an incident beam. An illumination beam illuminates the reference disk from a direction. An imaging optics collects the backward scattering to form a bright image spot of the reference disk. The lateral position of such a bright image spot can be detected by a single-element positioning-detector. An electronic circuit coupled to the positioning detector can then generate positioning signals of the reference disk and thus enable the tracking of the lateral movement of the object. 
     In the embodiments presented in this application, a retro-reflective disk and a single-element positioning-detector are implemented to provide fast eye tracking for refractive laser surgery on the cornea. In these embodiments, a retro-reflective disk of a few millimeters in diameter is affixed on the cornea near and outside the surgery area. An infrared light source located near the visual axis illuminates the eye and the reference disk from a working distance of about 25 cm. An imaging optics forms an image of the cornea area on a single-element positioning-detector. The strong backward scattering from the retro-reflective disk produces a bright spot in a basically dark background image. When the eye moves, the reference disk moves with the eye and the bright image spot moves on the positioning detector. An electronic circuit that reads in the output from the positioning detector generates positioning signals of the bright image spot and, thus, provides information on the eye movement. A control circuit can then use these positioning signals to control a beam steering mechanism to direct the surgical laser beam to follow the movement of the eye. 
     The reference disk may include a substrate and a retro-reflective surface according to one embodiment. The second surface of the disk is attachable to the cornea without slipping. The substrate can be made of paper or other materials, which are harmless to the cornea and durable for sterilization. The reference disk is preferably a disposable item. 
     The application of a retro-reflective reference disk simplifies the positioning detection of the eye to the positioning detection of a bright image spot. Consequently, single-element positioning-detector and simple electronics can be used to achieve fast and sensitive tracking of the eye movement. A prototype shows that positioning detection with a retro-reflective disk and a single-element positioning-detector can be faster than 10 kHz and sensitive to a few microns. In comparison, a CCD camera based positioning detection requires expensive frame grabber and sophisticate data-processing electronics. The up-date rate of a CCD camera is typically limited to 30 to 60 Hz. These and other aspects and advantages of the present invention will become more apparent in the following drawings, detailed description, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing one embodiment of an eye-tracking system in accordance with the present invention (open loop). 
     FIG. 2 shows another embodiment of an eye-tracking system in accordance with the present invention (closed loop). 
     FIG. 3 is a schematic diagram of a reference disk with a retro-reflective surface. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a schematic diagram showing one preferred embodiment of an eye-tracking system  100  in accordance with the present invention. The eye-tracking system  100  includes a position-detection device  20 , control electronics  50 , and a scanner  60 . 
     To track the eye movement during a photo-refractive surgery, the tracking system  100  incorporates with a reference  10  affixed on the eye  11 . The position-detection device  20  projects an infrared illumination beam  4  on the reference  10 , detects the position of the reference  10 , and thus detects any displacement of the subject&#39;s eye  11 . Using the positioning signal  26  from the output of the position-detection device  20 , the control electronics  50  controls the scanner  60  to steer a surgical laser beam  62  to follow the movement of the subject&#39;s eye  11 . 
     The reference  10  is a retro-reflective disk with a diameter of a few millimeters. The reference disk  10  should be attached onto the cornea of the eye  11  and located near and outside the surgical area. Practically, the reference disk  10  can be attached onto the cornea simply by moisture. The reference disk  10  has a retro-reflective surface to enhance significantly the backward scattering of an illumination beam. 
     The position-detection device  20  consists of an infrared light source  21 , a beam splitter  22 , a focal lens  23 , a single-element positioning-detector  24 , and an electronic circuit  25 . The position-detection device  20  and the other part of the tracking system  100  are located some 25 cm away from the patient&#39;s eye  11  so that enough working distance is available for the surgeon to handle the surgery. 
     The infrared light source  21  projects an infrared illumination beam  4  onto the eye  11  and the reference disk  10 , via the beam splitter  22  and a turning mirror  40 . This infrared light source  21  is preferably within the near infrared spectrum ranging from 750 nm to 1300 nm. The wavelength in this spectrum range is long enough to avoid-disturbing the surgeon and the patient and short enough for commonly available-photo-detector to have good responsibility. The intensity of the infrared illumination beam  4  on the eye  11  is preferably below 1 mW/cm 2  to avoid discomfort to the subject&#39;s eye  11 . 
     A simple embodiment of an infrared light source  21  is a diffused laser beam from a laser diode operated around 800 nm. A diffuser may be used to reduce the spatial coherence of the laser beam to produce a more uniform and safer illumination beam on the eye. For an open-loop tracking system, the size of the illumination beam on the eye should be big enough to cover the desired tracking range. 
     The backward scattering  5  from the reference disk  10 , as well as from the eye  11 , traces backward into the position-detection device  20 . This backward scattering beam  5  passes partially the beam splitter  22  and is then focused by the lens  23  to form an image onto the positioning detector  24 . The image spot size of the reference disk  10  on the positioning detector  24  should be 1 mm or smaller for good spatial resolution of the disk position. 
     Because of the retro-reflective surface of the disk  10 , the backward scattering from the disk  10  is many orders of magnitude stronger than that from the tissue of the eye  11 . The image of the disk  10  on the positioning detector  24  is thus a bright spot over a basically dark background. The output signals from the positioning detector  24  are resolvable for the position of the bright spot and, consequently, can be used to determine the position of the reference disk  10 . The electronic circuit  25  converts these output signals from the positioning detector  24  into positioning signals  26  of the centroid of the bright spot. 
     The control electronics  50  uses the positioning signals  26  as feedback to control the scanner  60  to follow the movement of the eye  11 . The control electronics  50  can be either an analog circuit or a computer-based digital circuit. A digital circuit is discussed below, as it is preferable for its flexibility. 
     From the positioning signals  26  of the bright image spot, the computer  50  calculates the position of the reference disk  10  with a scale factor from calibration. An initial position of the disk  10  is registered and stored. A real time position of the disk  10  is then registered and compared with the initial position to determine the displacement of the disk  10 . The computer  50  further then generates a signal  51  to drive the scanner  60  to deflect a surgical laser beam  62  to follow the movement of the eye  11 . 
     It is the retro-reflective disk  10  leads to a bright spot to be formed on the image plane. Such an image of a bright spot over a basically dark background enables the use of the single-element positioning-detector  24  and a simple electronic circuit  25  to resolve the position of the patient&#39;s eye  11 . 
     The tracking system  100  of FIG. 1 is an open-loop tracking system, in which only the surgical laser beam  62  follows the movement of the eye  11 . In a close-loop tracking system, both the surgical laser beam  62  and the tracking illumination beam  4  follow the eye movement. 
     FIG. 2 is a schematic diagram showing another preferred embodiment, a close-loop eye-tracking system  200 , in accordance with the present invention. Similar to the embodiment of FIG. 1, this tracking system  200  includes a position-detection device  20 , control electronics  50 , and a scanner  60 . Differently, the input surgical laser beam  61  and the infrared illumination beam  4  are combined through a diachronic mirror  70  and both reflect from scanner  60 . 
     The backward scattering  5  from the disk  10  retraces back to the position-detection device  20  and forms a bright image spot on the single-element positioning detector  24 . The electronic circuit  25  converts the output signals of the detector  24  into positioning signals  26  indicating the centroid of the bright image spot. These positioning signals  26 , however, depend on not only the position of the disk  10  but also the angular position of the scanner  60 . 
     To track the eye  11  with this close-loop tracking system  200 , a computer  50  first registers and stores an initial position of the bright image spot. A real time position of the bright image spot is then registered and compared with its initial position to determine a displacement of the bright image spot on the positioning detector  24 . This displacement is treated as an error signal. The computer  50  then generates a signal  52  to drive the scanner  60  to bring the error signal toward zero. This way the scanner  60  serves as a part of a negative-feedback servo loop to keep the bright image spot of the reference disk  10  stationary on the detector  24 , while the disk  10  may move with the eye  11 . A similar servo loop is described in U.S. Pat. No. 5,410,376 to Cornsweet et al., in which the pupil is used as a reference for tracking the eye movement. (The eye tracking device described there is not suitable for a photo-refractive surgery because it tracks the pupil and it needs to use an eyepiece in front of the eye. The beam pass to the pupil will be interrupted in a photo-refractive surgery and the eyepiece will interfere with the surgical laser beam.) 
     When the close-loop tracking is established, the eye  11  looks steady when it is viewed along the input surgical laser beam  61 . The input surgical laser beam  61  is inserted by a diachronic mirror  70  into the scanner  60  and is then projected onto the eye  11  to serve as the surgical laser beam  62 . The diachronic mirror  70  has a high reflectivity on the input surgical laser beam  61  and a high transmission to the infrared illumination beam  4 . With the close-loop tracking, this input surgical laser beam  61  could be directed to any predetermined position on the eye as if the eye remains constant. 
     In either the tracking system  100  or  200 , the real time position of the disk  10  can be registered at a high repetition rate so as to achieve fast response to the eye movement. For example, with a bright image spot of 1 μW, the single-element positioning detector  24  and the electronic circuit  25  can be easily operated faster than 500 Hertz, the speed required for tracking the involuntary eye movement. Detection speed higher than 10 kHz has been achieved with a prototype. 
     For a close loop tracking system  200 , good linearity is not required and a quadrant detector can be used as the positioning detector  24 . High-speed operation is also achievable with a quadrant detector. 
     If a CCD camera is used as a positioning detector  24 , attention should be given to increase the readout speed of the camera. It is difficult to handle a readout rate of 500 Hertz from a two-dimensional CCD camera. To overcome this difficulty, the readout could be grouped into one sweep along x-direction and one sweep along y-direction. This way the two-dimensional CCD camera services as two orthogonal linear CCD cameras. 
     A high-speed scanner should be used for fast eye tracking. Galvanometer type scanners can have a good responsibility for small-step signal of 500 Hertz or higher. Fast scanners are commercially available from General Scanning Inc. or Cambridge Technology Inc.; both located in Watertown, Mass. Other type of beam steering mechanism may be used to replace the scanner  60  for various tracking requirements. 
     The eye-tracking system  100  or  200  is to incorporate into a laser surgical system to achieve more reliable and accurate surgical result. The tracking system is installed to track a patient&#39;s eye at a predetermined location and orientation relative to the surgical system. To operate, the surgeon should apply a retro-reflective disk  10  at a proper position on the patient&#39;s eye  11 . Turn on the infrared light source  21  and the tracking system. Align the patient&#39;s eye  11  to the predetermined location and orientation (under a microscope  30 , for instance). Activate the eye tracking and then start the laser surgery. Any eye movement during the surgery will be automatically compensated by the tracking system. The laser surgery can thus be performed on the eye as if the eye remains at its initial position. 
     The infrared light source  21  in FIG. 1 or  2  is for the tracking system only. In a surgical system, illumination for observation may be used. The illumination light for observation is in the visible spectrum range and can be decoupled from the tracking system by using optical filters. 
     FIG. 3 is a schematic diagram of a retro-reflective disk  10  in accordance with the present invention. The disk  10  consists of a substrate  1 , a first surface  2 , and a second surface  3 . 
     The substrate  1  is made of paper or other materials, which are harmless to the cornea and are durable for sterilization. The substrate  1  should be light in weight. It has a diameter in the order of a few millimeters and a thickness of a fraction of the diameter. 
     The first surface  2  of the disk  10  is retro-reflective. The second surface  3  should be attachable to the cornea without slipping. Practically, the disk  10  can be attached on the cornea simply by moisture and can be removed easily after the surgery. 
     The retro-reflective surface  2  should have a strong backward scattering to incident illumination light. The backward scattering is much more condensed within a small cone angle around the incident illumination light. One way to produce a retro-reflective surface is to embed a large number of tiny glass or plastic spheres in a layer of paint and to make the spheres partially uncovered as the paint dries (S. R. Milk, Optics &amp; Photonics News, December 1993, 6-7, Optical Society of America). These spheres should be transparent to the predetermined illumination light, preferable in the near infrared spectrum range for eye-tracking application. The paint should be oil-based and not degrade in water. It should also be durable for sterilizing. A practical substitute of this paint is to use a layer of reflective tape, available from hardware or auto-part stores. 
     The above figures and description are intended for illustrating the present invention. It is understood that various modifications can be made without departing from the scopes of the invention as defined in the appended claims.