Patent Publication Number: US-6334683-B2

Title: Eye illumination system and method

Description:
This is a continuation of application Ser. No. 09/472,951, filed on Dec. 27, 1999, is now 6,193,373 by Howard P. Apple, Martin P. Nevitt and Xiaofeng Han, which in turn was a continuation of application Ser. No. 09/176,755, filed on Oct. 22, 1998, is now 6,007,202 by the same inventors, which claimed priority from Provisional Application No. 60/062,696, filed on Oct. 23, 1997, by the same inventors entitled “Eye Tracker Lighting System for Laser Vision Correction.” 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a lighting system for illuminating the eye and more particularly to a lighting system for video based tracking and correcting for eye movement during vision correction treatments. 
     2. Description of Related Art 
     Pupil position data obtained by eye tracking systems is used to detect eye motion during vision correction treatments. Conventional video based eye tracking systems automatically recognize and track the position of eye positions based on landmarks present within an image of a human eye. Such equipment requires illumination of the eye by infrared (IR) light. IR light typically 850 to 930 nanometers (nm) is used because it provides a good picture contrast between the pupil and iris. Additionally, the use of IR light decouples this lighting source from other visual sources which do not contain the infrared wavelengths. 
     The eye, illuminated by invisible IR light, is scanned by an infrared sensitive video camera. Under normal conditions, the pupil of the eye appears as a dark hole to the illumination. The dark pupil image is input to a real-time eye tracking system consisting of a digital image processor that outputs pupil size and position coordinates relative to the scan of the camera. The eye tracking system includes a circuit and processor designed to acquire and track the dark pupil position even in the presence of shadows or other clutter normally found in images of the eye. 
     FIG. 1 shows a conventional eye tracking system including illumination of the eye during laser vision correction surgery. The conventional illumination system includes one or two infrared light bundles  10 , mounted on a central hub  12 , to illuminate the eye for tracking by the eye tracking system. The path of a visible light beam used during vision correction treatments is shown at  14 . A camera  18 , sensitive to IR illuminations and fixed with respect to the subject&#39;s head, scans the eye to provide a video image for tracking the position of the eye. 
     Conventional illumination systems such as that shown in of FIG. 1 require that light bundles  10  be relatively close to the eye in order to achieve an evenly distributed illumination. Generally, the light bundles  10  are about 80 millimeters (mm) from the eye being treated, as shown by the dimension A in FIG.  1 . This narrow spacing is a significant disadvantage because the light bundles  10  may interfere with the physician&#39;s hands either prior to or during the actual vision correction treatment. Additionally, the physician&#39;s hands can inadvertently block light emitted from one or both of the light bundles  10  causing a system efficacy and/or safety problem. Under certain circumstances, the patient&#39;s brow or nose can block light emitted from the light bundles  10  causing uneven illumination of the eye. 
     Additionally, conventional illumination systems such as that shown in FIG. 1 require the light bundles  10  to be within 0 to +/−20 degrees to the visual axis in order to achieve generally presumption of an even illumination. If not, the light bundles  10  must be painstakingly adjusted for each patient at the time of treatment to achieve the best possible illumination based on the shape of that particular patient&#39;s face, eyes, etc. This is a time-consuming process and may result in errors. 
     However, even if the light bundles  10  are positioned within the conventionally preferred 0 to +/−20 degrees to the visual axis, there is an additional problem caused by specular reflections. FIG.  2  and FIG. 3 show specular reflections  21  at an eye  17  caused by any illumination system composed of point sources, e.g. light bundles  10 . Because of the relatively narrow angle (0 to +/−20 degrees) at which the light beams  19  (FIG. 3) are delivered, the specular reflections  21  can occur either within the pupil  18  or even worse at the pupil/iris border  20 . This makes the machine vision computer task of defining the pupil/iris border  20  much more complex and error prone. 
     The system shown in FIG. 1 was originally designed for research studies of normal eyes in which the epithelium is intact with a tear layer providing distinct specular reflections. These conditions no longer hold true for current laser vision correction techniques. In particular, either the epithelium is removed under a technique called surface Photorefractive Keratectomy (PRK), or a flap is cut with a microkeratome, the flap is folded back, and the treatment is performed on the underlying stromal layer (called LASIK). Both LASIK and PRK markedly affect Illumination for eye tracking systems. The eye, rather than being shiny and smooth, becomes dryer and more diffusely reflective. This often leads to additional time-consuming physician adjustment of the light bundles  10  during a critical time period of the surgery. 
     Another problem exists with conventional eye tracking lighting systems when used with laser vision correction treatments. An ablating laser beam dries and roughens the eye surface further obscuring the camera&#39;s view of the pupil/iris border  20  and the peripheral limbal border. Empirically, this disadvantageous effect is accentuated by relatively coaxial lighting provided by singular light sources placed close to the eye at angles of less that +/−20 degrees. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a lighting system for illuminating the eye during vision correction treatments that does not physically interfere with preoperative and postoperative procedures, requires little or no physical adjustment, is insensitive to inadvertent light blockage, and minimizes or eliminates disadvantageous specular reflections at the pupil/iris border. These objects are attained in accordance with the principles of the present invention by providing a method and system which includes a generally arcuate main body. The main body is constructed and arranged to be mounted in spaced relation to an eye to be tracked. An infrared light source is carried by the main body on at least a portion of its inner circumferential surface to direct infrared light toward the eye being treated at an angle from about 20 to 45 degrees with respect to an iris base plane of the eye being tracked. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings, in which. 
     FIG. 1 is a perspective view of a conventional lighting system for illuminating the eye during vision correction treatment, shown disposed above an eye being treated; 
     FIG. 2 is an enlarged plan view of specular reflections which occur on a pupil of an eye being illuminated with the conventional system of FIG. 1; 
     FIG. 3 is an elevation view of specular reflections which occur on an eye being illuminated with the conventional system of FIG. 1; 
     FIG. 4 is a perspective view of a lighting system for illuminating the eye during laser vision correction treatment provided in accordance with the principles of a first embodiment the invention and shown disposed above an eye being treated; 
     FIG. 5 is a plan view of the lighting system of FIG. 4, showing light sources in the form of a light pipe or in the form of a plurality of light emitters 
     FIG. 6 is a perspective view of a lighting system for illuminating the eye during laser vision correction treatment provided in accordance with the principles of a second embodiment of the invention and shown disposed above an eye being treated; 
     FIG. 7 is an elevation view of the lighting system of FIG. 6 shown disposed above an eye to be treated; and 
     FIG. 8 is a plan view of an eye showing the location of specular reflections outside the pupil/iris border, which result from the lighting system of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to FIG. 4, a first embodiment of an eye illumination system is shown, generally indicated at  100 , which embodies the principles of the present invention. 
     The eye illumination system  100  includes an illuminating device in the form of a generally arcuate main body  110  constructed and arranged to be mounted in spaced relation to an eye being treated. As shown in FIG. 5, the main body  110  is in the form of a continuous ring having an inner diameter generally sufficient to ensure that adequate space is provided above the patient&#39;s face for the surgeon to maneuver. It has been determined that an inner diameter greater that about 140 mm is sufficient to ensure adequate space for maneuverability. 
     In the illustrated embodiment, the inner diameter of the ring-shaped main body  110  is approximately 280 mm. This 280 mm diameter permits the main body  110  to be disposed away from the eye  120  a distance of approximately 165 mm, as shown by the dimension B in FIG.  4 . The main body  110  includes an infrared light source  125  for directing infrared light towards the eye being treated. The light source  125  may comprise a plurality of infrared light emitters  130  (FIG. 5) that are evenly or randomly spaced along an inner circumferential surface  135  of the main body  110 . The light emitters  130  may be fiber bundles, light emitting diodes, lasers, electro luminescent panels, etc. It has been determined that for the disclosed embodiment  180  light emitters  130  spaced evenly about the inner circumferential surface  135  provides sufficient illumination, although more or less light emitters  130  may be employed. Further, the number of light emitters  130  may be reduced by eliminating light emitters  130 , e.g., at the 3, 6, 9, and/or 12 o&#39;clock locations, without noticeable illumination degradation. Eliminating light emitters in these regions also simplifies eye tracker measurements. 
     Alternatively, the light source  125  may be an infrared light pipe  140  disposed on the inner circumferential surface  135 , a portion of which is shown in FIG.  5 . The light pipe  140  is preferably provided along the entire inner circumferential surface  135 , but may be disposed only along significant portions thereof. 
     In the illustrated embodiment, a hub or mount  150  is coupled to the main body  110  by a plurality of spokes  155  so that the hub  150  is generally concentric with the ring-shaped main body  110 . The hub may be used for mounting the main body  110  to a fixed location and/or for mounting a visible light source (not shown) thereto. The hub  150  includes a passage  160 , through which a visible light beam  165  may be delivered to the eye  120  being treated. 
     An eye imaging camera  170  (FIG. 4) may be provided in the conventional manner to scan the eye  120 . The camera  170  may be connected to a conventional pupil tracking system, such as, for example, the RK-416PC Pupil Tracking System manufactured by ISCAN INC. of Cambridge, Massachusetts. This conventional ISCAN Tracking System comprises a real time digital image processor that automatically tracks the center of the patient&#39;s pupil and measures pupil size and pupil position from a video image of the patient&#39;s eye. Another example of an eye imaging camera and digital imaging system that may be employed with an eye illumination system in accordance with the principles of the present invention is disclosed in U.S. Pat. No. 5,684,562, the content of which is hereby incorporated into the present specification by reference. 
     In operation, the eye is illuminated with a low-level (preferably 850 to 930 nm) infrared eye illumination source according to the principles of the invention. The pupil acts a sink to the IR light and while surrounding areas of the pupil reflect the IR light back to the camera  170 , yielding “dark pupil” eye images. A bright comeal reflection corresponds to the reflection of the IR light off of the cornea. The eye imaging camera  170  is fitted with an optics package having an infrared pass filter to obtain a clear, in-focus image of the eye. 
     With reference to FIG. 6, a second embodiment of an eye illumination system of the present invention is shown generally at  200 . The main body  210  is of the second embodiment is generally C-shaped. All other components of the eye illumination system  200  are as described with respect to the eye illumination system  100  shown in FIG.  4 . 
     The inner diameter of the main body  210  is preferably greater than about 140 mm, as in the first embodiment of the invention. The embodiment shown in FIG. 6 has an inner diameter of about 280 mm. The C-shaped main body  210  has an open portion  220  to allow an unobstructed view for the physician. The C-shaped main body  210  is typically configured with respect to the patient such that the open region is toward the top of the patient&#39;s head, where illumination is typically blocked by the patient&#39;s brow. Thus, even though the embodiment of FIG. 6; does not encompass a full 360 degrees, there is generally no compromise in eye illumination. 
     With the eye illumination system in accordance the principles of the present invention, as shown in FIG. 7, light rays  230  are emitted towards the eye at an angle θ which is approximately between 20 to 45 degrees, and preferably between 25 to 40 degrees to an iris base plane  121 . The iris base plane  121  is considered to be the plane which is tangential to the outer edge of the iris. 
     As shown in FIG. 8, an illumination system in accordance with the principles of the present invention eliminates specular reflections at the pupil/iris border  20 . Thus, eye tracking performance can be improved significantly. 
     The main body of a ring-shaped or C-shaped eye illumination system works equally well in treating left and right eyes with no adjustment. The lights become automatically centered under conventional laser vision correction centering methods. Thus, advantageously, no further adjustment of the eye illumination system is required during surgery. 
     With an eye illumination system in accordance wit the principles of the present invention, the main body may be mounted sufficiently distance from the patient&#39;s face so as to not interfere with the physician&#39;s hands. If a physician&#39;s hand should inadvertently block some of the light from the eye illumination system, there is sufficient light from other portions of the main body to achieve adequate and even illumination of the eye. 
     Moreover, the infrared light beams are directed at a sufficiently oblique angle such that specular reflections caused by the light source do not interfere with the pupil/iris border, particularly during eye movement encountered during patient fixation. This leads to greater increased accuracy and reliability of eye tracking. 
     It can be appreciated that the eye illumination system of the present invention provides an effective way of illuminating an eye, particularly for the purposes of tracking and correcting eye movements during vision correction treatments. Since the eye illumination system is constructed and arranged to be significantly spaced from the patient&#39;s face, there is less likelihood of the physician&#39;s hand interfering with the overall light source. Further, the angle at which the light is directed to the eye ensures that specular reflection interference at the iris/pupil border is minimized or eliminated. 
     While the invention has been described in accordance with what is presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.