Abstract:
A system for tracking eye movement is disclosed. A first light source, emitting light including a first wavelength, is adapted to illuminate corneal tissue of an eye. The first wavelength is selected such that it is strongly absorbed by corneal tissue. A position sensitive detector is adapted to detect a reflection of light at the first wavelength off the corneal tissue. Signal may be generated in response to the detected reflection which are representative of the position of the reflection, and therefore the position of the corneal tissue, with respect to the position sensitive detector.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The field of the present invention is systems and methods for tracking eye movement. 
   2. Background 
   Four different types of non-contact eye trackers are generally known in the prior art. Of the four, the limbus eye tracker is the simplest and least expensive. The limbus eye tracker aims two pulsed infrared photo-emitters at the iris-sclera boundary on either side of the eye. Because of the difference in color between the iris and sclera, differing amounts of light are reflected depending on the position of the eye relative to the emitter. A pair of infrared detectors picks up the reflected light from the emitters, and the differential signal from the emitters is demodulated and filtered to provide a signal proportional to horizontal eye position. This technique provides a relatively easy-to-use recording method, but it does have substantial drawbacks. First, it is limited to tracking horizontal eye movement, and cannot track vertical eye movement. In addition, the two photo emitters and the pair of infrared detectors must be held immobile with respect to the user&#39;s head. Any relative movement between the head and the photo emitters or the infrared detectors can create substantial errors in the tracking data. 
   Another technique, which is very similar to limbus tracking, tracks the boundary between the pupil and the iris. As with limbus tracking, the emitters and detectors of the apparatus must be immobilized in relation to the head. However, this technique does offer the advantage of being able to track vertical movement of the eye because the pupil is far less covered by the eyelids than the limbus. Another advantage is that the border of the pupil is often sharper than that of the limbus, thus yielding a higher resolution. The advantage of using the pupil boundary for tracking purposes, however, also carries an inherent disadvantage in that the difference in contrast is lower between the pupil and iris than between the iris and sclera, thus making detection of the boundary more difficult. 
   In another technique, the relative positions between reflections from the cornea and lens are used to track eye movement. When light is shone into an eye, several reflections occur at the boundaries of the lens and cornea. These are the so-called Purkinje images, the first of which is also called the “glint”. An image of light, typically referred to as the “bright-eye”, is simultaneously reflected off the retina. Images of the bright-eye and glint can be recorded, using an infrared sensitive camera, as a very bright spot and a less bright disc, respectively. When the eye is panned horizontally or vertically, the relative positioning of the glint and the center of the bright-eye change with the movement. From the relative positions of the glint and the bright-eye, the direction of gaze can be determined. 
   A fourth eye tracking technique also uses Purkinje images. Such eye trackers measure the relative displacement of the images formed by the reflection of a light source at the anterior corneal surface and the posterior lens surface, which are known as the 1 st  and 4 th  Purkinje images, respectively. Rotation of the eye results in a greater displacement of the 1 st  Purkinje image relative to the 4 th  Purkinje image, thereby providing a signal proportional to eye position. The primary drawback of such eye trackers is that they require precise alignment and are not suitable under circumstances where the subject is permitted to have relatively free head movement. 
   SUMMARY OF THE INVENTION 
   The present invention is directed toward a system and a method for tracking eye movement. The system includes a light source and a position sensitive detector. The light source emits light which includes a first wavelength, wherein light at the first wavelength is strongly absorbed by corneal tissue, and is adapted to illuminate corneal tissue of the eye. The position sensitive detector is adapted to detect a reflection of light at the first wavelength off the corneal tissue. With such a system, the rotational position of the cornea, with respect to the position sensitive detector, may be determined by using the signal output from the position sensitive detector. 
   The system may be enhanced by including a second light source emitting light which includes a second wavelength, wherein light at the second wavelength is strongly absorbed by scleral tissue. The second light source is adapted to illuminate scleral tissue of the eye, and the position sensitive detector is adapted to detect a second reflection of light off the scleral tissue. With this enhancement, the position of the eye, with respect to the position sensitive detector, may be determined. Optionally, the first and second wavelengths may be the same wavelength, with the corneal tissue and the scleral tissue being alternately illuminated. This may be accomplished through the use of multiple light sources, with the phase of the pulses differing by 180°. 
   The method for tracking eye movement involves first illuminating corneal tissue with light including a first wavelength, this first wavelength being one that is strongly absorbed by the corneal tissue. A reflection of light at the first wavelength off the corneal tissue is then detected, and a signal representative of the position of the first reflection is then generated. With this signal, the rotational position of the cornea may be determined. Similar to the system, the method may be enhanced by also illuminating scleral tissue with light including a second wavelength, wherein light at the second wavelength is strongly absorbed by scleral tissue. A second reflection of light, this one being at the second wavelength and off the scleral tissue, is then detected. As with the first reflection, a signal representative of the position of the second reflection may be generated. Optionally, the corneal tissue and the scleral tissue may be alternately illuminated, thereby facilitating use of the same wavelength for the first wavelength and the second wavelength. 
   Accordingly, an improved system and an improved method of tracking eye movement are disclosed. Advantages of the improvements will appear from the drawings and the description of the preferred embodiment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, wherein like reference numerals refer to similar components: 
       FIG. 1  schematically illustrates a system for tracking eye movement from a top elevation view of the eye; 
       FIG. 2  illustrates a system for tracking eye movement incorporated into a helmet; 
       FIG. 3  illustrates a fiber bonded between two layers of a visor; and 
       FIG. 4  illustrates a sensor module. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Turning in detail to the drawings,  FIG. 1  illustrates a basic system  11  for tracking eye movement. Two light sources  13 ,  15  are positioned to direct light toward the eye  17 , with the first light source  13  directing light toward the cornea  19 , and the second light source  15  directing light toward the sclera  21 . Both light sources emit light at a wavelength which is strongly absorbed by the tissues of the eye. While the system  11  may be constructed with the two light sources  13 ,  15  having different wavelengths, in the illustrated system  11  both construction and operation are simplified by pulsing the two light sources  13 ,  15  with a phase difference of 180° and operating them at the same wavelength. Alternatively, a single light source could be used in combination with appropriate light guides and optical switches. 
   By selecting a wavelength that is strongly absorbed by tissues of the eye, no significant reflections are seen from internal structures of the eye. In particular, reflections from the iris and retina are preferably minimized to the extent that the intensities of reflections from these internal structures are less than 1% of the intensity of reflections from outer surfaces of the eye. While many different wavelengths may be used, wavelengths in the range of 1300 nm to 1600 nm, and in particular a wavelength of 1450 nm, are appropriate choices when emitted from low-power sources, as these wavelengths are recognized as being non-harmful to tissues at low powers, even with constant exposure over a period of several hours. Other wavelengths outside this range, if found to be strongly absorbed and eye-safe, might also be used. By eliminating reflections from internal structures, such as the iris and the retina, the eyeball appears as a black featureless orb with a bulge representing the cornea. Without interference from reflections off internal surfaces, the reflections from both the cornea and the sclera easily tracked by a single position sensitive detector (PSD)  23 , although multiple PSDs could also be used. 
   The PSD  23  is positioned to receive both the reflection off the cornea  19  from the first light source  13  and the reflection off the sclera  21  from the second light source  15 . The spatial relationship between the PSD  23  with respect to each of the two light sources  13 ,  15 , is fixed. As the gaze of the eye moves, the reflection off the cornea  19 , and its position on the PSD  23 , likewise moves. By knowing the position of the first light source  13  and the PSD  23  with respect to the eye  17 , the position of the reflection off the cornea  19  on the PSD  23  enables the direction of gaze to be determined. Depending upon the type of PSD used, it is estimated that the direction of gaze with respect to the PSD  23  can be determined to better than 1 milliradian accuracy. While a system in which the first light source  13  and the PSD  23  are in fixed positions with respect to the eye are easily achieved in a laboratory, inclusion of the second light source  15  facilitates real world applications for the eye tracker. The position of the reflection off the sclera  21  enables the position of the PSD  23  with respect to the eye  17  to be determined. As the relative position between the eye  17  and the PSD  23  moves, the position of the reflection off the sclera  21  moves on the PSD  23 . This enables tracking of the spatial relationship between the eye  17  and the PSD  23 . This tracking information is utilized to correct for movement of the PSD with respect to the eye, so that the direction of gaze may be determined with consistent accuracy. 
   While the system described above is capable of tracking movement, and thus the gaze, of a single eye, two such systems may be used in combination to track movement of both an individual&#39;s eyes, thereby allowing more accurate tracking of the direction of the individual&#39;s gaze over a larger field of regard. For a single eye, the above system is capable of tracking the horizontal position of an eye from about −20° to about +50° and the vertical position from about −30° to +30°. By tracking both eyes the horizontal tracking capabilities extend from about −50° to about +50°. In addition, tracking both eyes allows a measurement of the vergence between the gaze of each eye, and the vergence can be used to estimate the range of the object being viewed. 
     FIG. 2  shows a flight helmet  31  with tracking systems for both of the wearer&#39;s eyes. Two Infrared diodes  33  are mounted to the helmet  31  to serve as light sources for illuminating the cornea. Two more infrared diodes  35  (only one is shown in this profile view) are similarly mounted to the helmet  31  to serve as light sources for illuminating the sclera of each eye. Each of the infrared diodes  33 ,  35  is electronically linked to a controller  37 , also mounted to the helmet  31 , which controls the rate and timing of light pulses from each diode. For this purpose, a simple oscillating circuit suffices for the controller  37 . Fibers  39 ,  41  are embedded into, or applied to a surface of, the visor  43 , with each fiber  39 ,  41  receiving light from one of the laser diodes  33 ,  35 . The two fibers  39  used for illuminating cornea tissue of each eye are shown mounted in the visor  43  over the nose bridge, although other positions could also be used. The other two fibers  41 , used for illuminating sclera tissue, extend down to the outer corners of each eye, although here, too, other positions could be used. The end  49  of each fiber is cut at an angle, as is shown in  FIG. 3 , and a reflective coating is applied to the cut surface  51 . The end of the fiber and the reflective coating  51  are positioned within the visor  43  to direct light toward each respective eye. 
     FIG. 3  also shows one method of embedding the fibers in the visor  43  that uses refractive index matching to render the embedded fiber virtually invisible to the wearer. A fiber  45  is placed in an adhesive layer  53  between two layers of polycarbonate  55 ,  57 , with the adhesive layer  53  having an index of refraction that is closely matched to the index of refraction of the fiber cladding. 
   Referring to  FIGS. 2 &amp; 4 , two sensor tubes  59  are mounted to the helmet  31 , one on each side of the visor  43 , and positioned to receive light emitted from the fibers  39 ,  41  as reflected off both the cornea and sclera, respectively. By placing the sensor tubes  59  on the side of the user&#39;s face, the system is still capable of tracking gaze even when the eyelids droop or lower to partially cover the eyes. Both sensor tubes  59  are constructed with the same design. 
   The sensor tube  59  includes a mirror  61  positioned to direct light into the sensor tube  59  and cylindrical imaging lenses  63  which focus an image of the reflected light onto the PSD  65 . The PSD  65  measures the position of the image in the x and y directions on the sensor plane and generates output signals corresponding to the position. The (x,y) position of the focused spot on the PSD  65  correlates to the direction of light reflected by the eye, and this position moves as the direction of gaze shifts. The circuitry  67  within the sensor tube  59  measures and digitizes the signals from the PSD  65  and is electronically coupled to the controller  37  so that the reflections can be sampled synchronously with the respective diode pulses. In a flight helmet, the PSD  65  and associated circuitry  67  preferably sample the spot position at about 200 Hz. Other applications may use greater or lesser sampling rates. The digitized signals from the circuitry  67  are sent to a processor  71  via a wireless infrared communication link  73 . Using the known geometry of the tracking system and calibration data, the processor is enabled to track the gaze of the individual wearing the helmet  31 . Calibration data is obtained and used to provide full eye tracking accuracy since eyes vary in size, spacing, and position amongst different individuals. 
   The processor  71  may be a personal computer, the computerized systems on board an aircraft, or any other type of programmable processor, and the infrared communication link  73  is of a type commonly known in the prior art. Depending upon the environment in which the eye tracker is used, the communication link may use other forms of wireless technology, or alternatively, a wired link may be used. 
   Thus, a system and a method of tracking eye movement are disclosed. While embodiments of this invention have been shown and described, it will be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the following claims.