EYE TRACKING EMPLOYING A RETRO-REFLECTIVE DISK

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 (4) illuminates the reference disk from one direction and imaging optics forms a bright image spot (5) of the reference disk with the backward scattering from the reference disk. A two-dimensional positioning detector (24) detects the position of the bright image spot and an electronic circuit (25) then generates positioning signals (26) 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 enables fast detection (> >1 kHz) of the object's position. An embodiment of the method on an eye-tracking system is described.

Description: EYE TRACKING EMPLOYING A RETRO-REFLECTIVE DISK This application claims the benefit of US provisional application No. 60/093,618, filed on July 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 required to track the position of the patient's eye. With such a tracking device, the surgical laser beam can be delivered to predetermined positions on the eye even the eye moves during the surgery. An example of this tracking device is described in US patent 5632742 to Frey, et al. Another example is presented in US Patent 5620436 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 US Patent 5632742 an ink ring affixed on the patient's eye. Lang and Clouts described in US Patent 5620436 a ring-shape airning-fϊxture applied onto the patient'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'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 mfrared light source located near the visual axis illuminates the eye and the reference disk from a working distance of about 25cm. 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 consists of a substrate and a retro-reflective surface. 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 Figure 1 is a schematic diagram showing one embodiment of an eye-tracking system in accordance with the present invention (open loop). Figure 2 shows another embodiment of an eye-tracking system in accordance with the present invention (closed loop). Figure 3 is a schematic diagram of a reference disk with a retro-reflective surface. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 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'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'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 ' 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'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's eye 11. The tracking system 100 of Figure 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. Figure 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 Figure 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 US patent 5410376 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, Massachusetts. 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'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's eye 11. Turn on the infrared light source 21 and the tracking system. Align the patient'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 Figure 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. Figure 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 & Photonics News, 12/1993, 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.

Claims: THE CLAIMS What is claimed is: 1. A position-detection apparatus for tracking eye movement comprising: An infrared illumination beam projected onto a reference affixed on an eye, said reference has a retro-reflective surface; An optical means disposed to collect backward scattered light from said reference to form a bright image spot of said reference; A two-dimensional positioning detector detecting the position of said bright image spot of said reference, An electronic circuit coupled to said two-dimensional positioning detector to produce positioning signals of said reference; Wherein, said positioning signals can be used to steer a surgical laser beam or an optical path to follow the movement of said eye. 2. A position-detection apparatus as defined in Claim 1 wherein said infrared illumination beam has a wavelength ranging from 750 to 1500 nm. 3. A position-detection apparatus as defined in Claim 1 wherein said infrared illumination beam is generated from a laser diode. 4. A position-detection apparatus as defined in Claim 1 wherein said reference is a retro-reflective disk with a diameter of a few millimeters. 5. A position-detection apparatus as defined in Claim 1 wherein said two- dimensional positioning detector includes a single-element positioning- detector. 6. A position-detection apparatus as defined in Claim 1 wherein said two- dimensional positioning detector includes a quadrant detector. 7. A method for tracking eye movement comprising the steps of: Affixing onto the eye a reference having a retro-reflective surface; Projecting an infrared illumination beam onto said reference; Providing an optical means to collect backward scattered illumination light from said reference to form a bright image spot of said reference; Providing a two-dimensional positioning detector to detect the position of said bright image spot of said reference, Providing an electronic circuit coupled to said two-dimensional positioning detector to produce positioning signals of said eye; Wherein, said positioning signals can be used to steer a surgical laser beam or an optical path to follow the movement of said eye. 8. A method as defined in Claim 7 wherein said infrared illumination beam has a wavelength ranging from 750 to 1500 nm. 9. A method as defined in Claim 7 wherein said infrared illumination beam is generated from a laser diode. 10. A method as defined in Claim 7 wherein said reference is a retro- reflective disk with a diameter of a few millimeters. 11. A method as defined in Claim 7 wherein said two-dimensional positioning detector includes a single-element positioning-detector. 12. A method as defined in Claim 7 wherein said two-dimensional positioning detector includes a quadrant detector. 13. A method for tracking lateral movement of an object comprising the steps of: Affixing onto the object a reference having a retro-reflective surface; Projecting an illumination beam onto said reference; Providing an optical means to collect backward scattered light from said reference to form a bright image spot of said reference; Providing a two-dimensional positioning detector to detect the position of said bright image spot of said reference, Providing an electronic circuit coupled to said two-dimensional positioning detector to produce positioning signals of said object; Wherein, said positioning signals can be used to track the lateral movement of said object. 14. A method as defined in Claim 13 wherein said illumination beam has a wavelength range from 3Q0 to 1500 nm. 15. A method as defined in Claim 13 wherein said reference is a retro- reflective disk with a dimension equal or smaller than said object. 16. A method as defined in Claim 13 wherein said two-dimensional positioning detector includes a single-element positioning-detector or a quadrant detector. 17. A reference disk for eye tracking comprising: A substrate made from material harmless to human corneas and durable for sterilizing; A first surface having a retro-reflective coating operable at inferred illumination light; and A second surface attachable onto a human cornea. 18. A reference disk as defined in Claim 17 wherein said substrate is made of materials including paper and plastic. 19. A reference disk as defined in Claim 17 wherein said retro-reflective coating is formed by a layer of reflective tape. 20. A reference disk as defined in Claim 17 wherein said disk is a disposable item. OTHER REFERENCES S. R. Wilk, Optics & Photonics News, 12/1993, Optical Society of America, How retro-reflectors really work

Inventors: LAI MING, LAI, MING
Applicants: LAI MING, LAI, MING
Application year: 1999
Main classification: A61N5
IPC codes: A61B3, A61F9, G01B11