Abstract:
An optical tracking device and method are disclosed for tracking lateral movement of an object. A scanning probe beam and a time-resolved detection are implement in the disclosed technique. A particular application is for tracking the eye movement during a laser surgery.

Description:
This application is a continuation of the U.S. application Ser. No. 09/300,194, is now U.S. Pat. No. 6,179,422, filed on Apr. 27, 1999, which claims the priority from the U.S. Provisional Application No. 60/083,248, filed on Apr. 27, 1998. 
    
    
     TECHNICAL FIELD 
     The present invention relates to tracking an object by optical means, and more specifically, to automatic monitoring and tracking a movable object such as an eye. 
     BACKGROUND 
     Monitoring and tracking a laterally movable object are important in many applications. In certain applications, it is desirable to have a tracking device not only to monitor the displacement of the object but also to follow the movement of the object without a significant delay. Tracking and following the eye movement during a laser eye surgery is an example of such applications. 
     Many eye-tracking devices have been developed for eye surgery with lasers, in particular, for photo-refractive surgery. A typical photo-refractive surgery scans an UV laser beam on the cornea to sculpture the profile of the corneal outer surface, one layer at a time. This procedure can correct various refractive disorders of the eye, including nearsightedness, farsightedness, and astigmatism. 
     Any eye movement during the surgery may adversely affect the outcome of refractive correction. Immobilizing the eye movement during a surgery has been proven difficult in practice. A device automatically tracking and compensating the eye movement is an attractive approach. For the nature of photo-refractive surgery, the tracking device needs to be fast, accurate, and reliable. 
     U.S. Pat. No. 5,620,436 discloses use of a video camera to monitor the eye&#39;s movement and to determine the position of an aiming beam on the eye. U.S. Pat. No. 5,632,742 teaches projecting four laser spots on the eye and using a set of peak-and-hold circuits to determine the position of the eye. In these designs, a ring shape reference is used for determining the eye position, and spatial stationary infrared beams are applied to illuminate the reference. Sophisticated imaging system and electronics, such as a CCD camera or four peak-and-hold circuits are implemented to determine the position of the reference. The ring shape references are practically either the limbus or the iris of the eye and the whole ring is needed as the reference for determining the eye position. 
     SUMMARY 
     Generally, any optically identifiable reference mark or indicator affix to an object can be used to indicate the position and movement of the object. The devices and methods disclosed herein apply an optical probe beam scanning repeatedly and rapidly over such a reference mark. A change in the position of the reference mark can then be determined by measuring the change in the delay between a predetermined reference time and the detected time at which the optical probe beam intercepts the reference mark. The reference mark can be artificially formed on the object, or alternatively, can be an inherent mark on the object. 
     For the application of eye tracking, a reference mark may be the limbus of the eye, which is the natural boundary between the transparent cornea and the white sclera. Optical scattering changes from one side of the limbus to the other significantly. Therefore the position of the limbus can be detected by measuring the timing of the change in the scattered light of the probe beam as the probe beam scans across the limbus. The devices and methods of the present disclosure will be described by examples of eye tracking using a section of the limbus as the reference mark. 
     In one embodiment, a section of the limbus is used as the reference mark and the x-y positions of the limbus are determined by two sets of linear positioning devices. The two linear positioning devices are set for measurement along two mutually orthogonal axes. 
     Each linear positioning device includes a scanning beam generator, a detection assembly, and a processing electronics. The scanning-beam generator projects an infrared probe beam onto the eye and scans the probe beam across a section of the limbus repetitively. The detection assembly detects the infrared light scattered from the eye. The detected scattered-light signal is a time-resolved signal and has a sequence of sharp steps corresponding to the probe beam repeatedly across the limbus. The timing of each sharp step depends on the limbus position at the corresponding scan. The processing electronics converts the timing of the sharp steps into the positioning signal indicating the position of the eye. 
     With the positioning signal, a system computer can then generate a control signal to steer the surgical laser beam to follow the movement of the eye. Hence, accurate laser surgery can be achieved even though the eye may move during the surgery. 
     In this embodiment, about a quart of the limbus is used to determine the x and y positions of the eye. This is particularly important for a new type of refractive surgery so called LASIK, in which part of the limbus is obstructed during the surgery. This embodiment can use the limbus section that is not blocked and thus it can use the limbus as a reliable reference mark for LASIK. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing one embodiment of an open-loop optical monitoring and tracking system; 
     FIG. 1 a  shows timing diagrams of the scattered-light signal from the eye and the reference signal generated by a scanning beam generator. 
     FIG. 2 is a schematic diagram showing an embodiment of a close-loop optical monitoring and tracking system; 
     FIG. 3 a  is a schematic diagram showing one embodiment of a scanning-beam generator; 
     FIG. 3 b  is a schematic diagram showing another embodiment of a scanning-beam generator; 
     FIG. 3 c  is a schematic diagram showing a third embodiment of a scanning-beam generator; 
     FIG. 4 is a block diagram showing a processing electronics for the optical monitoring and tracking systems of FIGS. 1 and 2; 
     FIG. 5 a  is a schematic diagram illustrating simultaneous tracking of an eye in two different directions by two scanning probe beams projected on the limbus. 
     FIG. 5 b  shows two scanning probe beams projected on a partially obscured limbus to track the eye movement in two different directions in a LASIK surgery. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a schematic diagram of one embodiment of an optical monitoring and tracking system  100  for an eye  10 . The system  100  implements an open loop configuration that includes a position sensing module  101 , a system computer  80 , and a beam steering module  60  (e.g., a x-y scanner). The position-sensing module  101  projects a scanning probe beam  4  and monitors the position of the eye  10 . The system computer  80  controls the beam steering module  60  to guide a surgical laser beam  62  to a desired position on the eye  10 . As an open loop configuration, the scanning probe beam  4  dose not follow the movement of the eye  10  and only one beam steering module  60  is needed. 
     For illustration purpose, the position-sensing module  101  shown in FIG. 1 is only a linear positioning device and is for monitoring one-dimensional eye movement only (e.g., along x-direction). To determine the eye&#39;s movement in two dimensions, a second set of linear positioning device is needed to monitor the movement of the eye  10  along a second different direction, e.g., the y-direction orthogonal to the x-direction. 
     The position-sensing module  101  comprises a scanning beam generator  30 , a collection lens  6 , a photo-detector  7 , and a processing electronics  50 . The limbus  11  of the eye  10  is used as a reference mark  20 . The scanning-beam generator  30  projects the scanning probe beam  4  across the reference mark  20 . The scanning probe beam  4  may repeatedly start from a fixed point and is scanned at a constant speed over a predetermined tracking range. The scanning-beam generator  30  also produces a reference signal  31  to indicate a reference point of the scanning. 
     The lens  6  is disposed at a proper position relative to the eye  10  to collect the scattered light  5 . The photo-detector  7  receives and converts the scattered light  5  into an electrical signal, i.e., the scattered-light signal  8 . The scattering from the sciera side  13  of the eye  10  is approximately 20 times stronger than that from the transparent cornea side  14 . Hence, the intensity of the scattered light  5  exhibits a significant change when the probe beam  4  scans across the limbus  11 . This intensity change of the scattered light  5 , in turn, generates a sharp step in the scattered-light signal  8 . The timing of this sharp step depends on the position of the eye  10 . 
     In one implementation, an infrared laser beam (at 830 nm) of about 100 μW is used as the scanning probe beam  4  and the collection lens  6  having an aperture of about 18 mm is located about 30 cm away from the eye  10 . Detector  7  receives a scattered-ligtht power of about 20 nW when the probe beam  4  is on the sclera side. 
     FIG. 1 a  shows timing diagrams of the scattered-light signal  8  and the reference signal  31 . The scattered-light signal  8  has a sequence of sharp steps and each sharp step  9  corresponds to a scan of the probe beam  4  across the limbus  11 . The sharp step  9  has a time delay Td with respect to the reference point  31   s  of the scanning. This time delay Td depends on the position of the limbus  11  and varies as the eye  10  moves. The processing electronics  50 , which may include a microprocessor, processes the reference signal  31  and the scattered-light signal  8  to determine this time delay Td for each scan. This time delay Td is then used to determine the position of the limbus  11 . The lines Vth represent the threshold voltage for triggering. 
     To operate the tracking device  100 , an initial time delay Td 0  or eye position is first registered and stored in the system computer  80 . The time delay Td of subsequent scans is then compared with the initial time delay Td 0  to calculate a displacement of the eye  10 . With this calculated displacement, the system computer  80  can generate a control signal  81  to drive the beam steering module  60  to steer the surgical laser beam  62  to follow the movement of the eye  10 . 
     As an open loop device, the scanning probe beam  4  does not move with the eye  10 . The beam steering module  60  can be used simultaneously to compensate the eye movement and to scan the surgical laser beam  62  on the eye  10 . In this case, the control signal  81  may include of a scanning signal and an offset signal. The scanning signal scans the surgical laser beam  62  in a predetermined pattern while the offset signal offsets the scanning to compensate for the eye movement. This open-loop device is relatively simple and can be used to accurately track small movement of the eye  10 . 
     FIG. 2 shows a schematic diagram of a close-loop tracking device  200 . In the close-loop configuration, both the scanning beam  4  and the surgical beam  62  are steered to the eye  10  by a common steering module  60 . Consequently, both the scanning probe beam  4  and the surgical laser beam  62  follow the movement of the eye  10 . 
     In implementation, the scanning probe beam  4  is directed into the beam steering module  60  and reflected onto the reference mark  20  (i.e. the limbus  11 ). A dichromatic mirror  70  is placed in the path of the scanning probe beam  4  to couple the surgical laser beam  62  into the beam steering module  60 . The dichromatic mirror  70  reflects light at the wavelength of the surgical laser beam  62  but transmits light at the wavelength of the scanning probe beam  4 . The surgical laser beam  62  is reflected from the beam steering module  60  and projected onto the eye  10 . 
     Again, the scattered light  5  from the reference mark  20  is collected by a lens  6  and detected by a photo-detector  7 , which produces an output of scattered-light signal  8 . Similar to the open loop device  100 , the scatted-light signal  8  has a sharp step  9  corresponding to each scan of the probe beam  4  across the boundary of the reference mark  20 . The sharp step  9  has a time delay Td with respect to the reference point  31   s  of corresponding scan. A processing electronics  50  determines this time delay Td for each scan. 
     To operate the tracking device  200 , an initial time delay Td 0  or eye position is first registered and stored by the system computer  80 . The time delay Td of later scans is then compared with the initial time delay Td 0 . Any deviation of Td from Td 0  is used as an error signal to drive the beam steering module  60  such that to bring the error signal toward zero. Through this process, the beam steering module  60  deflects the scanning probe beam  4  to follow the movement of the eye  10 . Seeing the same deflection as the scanning probe beam  4 , the surgical laser beam  62  can thus impinge on any predetermined position of the eye  10  as if the eye remains stationary. 
     As a close loop device, the relative position between the trace of the scanning probe beam  4  and the reference mark  20  is kept constant during the operation. The beam steering module  60  is thus used solely for compensating the eye movement. A second beam steering module  90  is used to scan the surgical laser beam  62  on the eye  10  for surgery purpose. In this case, the control signal  81  to beam steering module  60  is simply the driving signal to compensate the eye movement. The control signal  82  to beam steering module  90  is simply the programmable signal to scan the surgical laser beam  62 . The close loop device  200  is relatively more complicate but it can track a relative large displacement of the eye  10 . 
     FIG. 3 a  shows one embodiment of a scanning-beam generator  30   a  that produces a scanning probe beam  4   a.  The generator  30   a  includes an infrared-light source  32   a,  which produces an infrared-light beam  33   a  projected onto a rotating blade  35   a.  The blade  35   a  has a set of pinholes  36   a  evenly distributed on a circle. A motor  34   a  drives the blade  35   a  at a constant rotation speed. The pinholes  36   a  are thus scanned across the infrared-light beam  33   a  at a constant speed. 
     A lens  37   a  focuses onto a reference ring  20  (i.e. the reference mark) the infrared-light beam  38   a  that is transmitted through the pinhole  36   a.  As the pinhole  36   a  is scanned across the infrared beam  33   a,  the image of the pinhole  36   a  is scanned across the reference ring  20 . Thus, the transmitted infrared beam  38   a  may serve as the scanning probe beam  4  of FIG.  1 . 
     A beam splitter  39   a  directs a small portion of the beam  38   a  onto a reference photo-detector  40   a.  This reference photo-detector  40   a  has a tiny light-sensitive area and the detected signal is thus a sequence of spikes as the split beam scans across the reference detector repetitively. The output signal from the photo-detector  40   a  defines a reference point of the scanning and serves as the reference signal  31  of FIG.  1 . 
     In this embodiment, the infrared-light source  32   a  can be simply a light emitted diode. The repetition rate of the scanning probe beam  4  can be up to the kilohertz range. For example, the motor  34   a  may run at 100 rotation per second and the blade  35   a  may have 10 pinholes  36   a  on it. 
     FIG. 3 b  shows another embodiment of a scanning-beam generator  30   b  producing a scanning probe beam  4 . The generator  30   b  includes an infrared-light source  32   b,  which produces an infrared-light beam  33   b  directed onto a disk  35   b.  The disk  35   b  holds a set of identical lenses  36   b  evenly distributed on a circle. A motor  34   b  rotates the disk  35   b  and the lenses  36   b  are scanned across the infrared-light beam  33   b  at a constant speed. 
     The infrared-light beam  38   b  transmitted through a lens  36   b  is focused onto a reference ring  20 . As the lens  36   b  is scanned across the infrared-light beam  33   b,  the focused beam  38   b  is scanned across the reference ring  20 . Thus, the focused infrared-light beam  38   b  may serve as the scanning probe beam  4  of FIG.  1 . 
     Again, a beam splitter  39   b  directs a small portion of the beam  38   b  onto a reference photo-detector  40   b.  The output signal from the photo-detector  40   b  defines a reference point of the scanning and serves as the reference signal  31  of FIG.  1 . In this embodiment, the infrared-light source  32   b  is preferably either a pre-focused beam or a point source. 
     FIG. 3 c  is a schematic diagram showing a third embodiment of a scanning-beam generator  30   c  producing a scanning probe beam  4 . The generator  30   c  includes an infrared-light source  32   c,  which produces an infrared-light beam  33   c  directed into a lens  37   c.  The transmitted infrared beam  38   c  is reflected by a mirror  36   c  and focused onto a reference ring  20 . The mirror  36   c  is driven by a scanner head  34   c  to scan the infrared beam  38   c  across the reference ring  20 . Thus, the transmitted infrared beam  38   a  may serve as the scanning infrared beam  4  of FIG.  1 . 
     Similarly, a beam splitter  39   c  directs a small portion of the beam  38   c  onto a reference photo-detector  40   c.  The output signal from the photo-detector  40   c  defines the reference point of the scanning and serves as the reference signal  31  of FIG.  1 . The scanner  34   c  scans the beam  38   c  back and forth. A synchronized signal from the scanner  34   c  can also be used as a reference point of the scanning. In this embodiment, the infrared-light source  32   c  can be either a collimated beam or a point source. 
     FIG. 4 is a block diagram showing one embodiment of the processing electronics  50 . This processing electronics  50  includes a first trigger circuit  52 , a second trigger circuit  54 , and a microprocessor  58 . The reference signal  31  from the scanning beam generator  30  is fed into the first trigger circuit  52  to produce a TTL output signal  53  carrying the timing of the reference signal  31 . The scattered-light signal  8  from the photo-detector  7  is fed into the second trigger circuit  54  to produce a TTL output signal  55  carrying the timing of the scattered-light signal  8 . 
     The microprocessor  58  reads in the signal  53  and signal  55  to calculate a time delay Td between the two signals. This time delay Td indicates the relative position of the reference mark  20  to the scanning probe beam  4 . This delay Td can be compared with an initial delay Td 0  registered and stored by the system computer  80  at the very beginning of the tracking. 
     For an open loop device  100 , any change of the delay Td from its initial value Td 0  can be used to determine a displacement of the eye  10  from its initial position. The determined displacement can then be converted into an offset signal combined in the control signal  81  to deflect the surgical laser beam  62  to follow the movement of the eye  10 . 
     For a close loop device  200 , any deviation of the delay Td from its initial value Td 0  is used as an error signal to drive the beam steering module  60  such that to bring the error signal toward zero. The beam steering module  60  thus deflects both of the scanning probe beam  4  and the surgical laser beam  62  to follow the movement of the eye  10 . 
     The above-described operation of the processing electronics  50  is repetitively for every scan of the probe beam  4 . The first trigger circuit  52  and the second trigger circuit  54  should be reset automatically after the signal  53  and signal  55  are read by the microprocessor  58 . 
     The processing electronics  50  shown in FIG. 4 is for one axis tracking. To track the two-dimensional movement of the eye  10 , another pair of the trigger circuit should be used. 
     FIG. 5 a  shows schematically two scanning probe beams  4   x  and  4   y  projected on a reference ring  20  (the limbus  11 ) for two-dimension positioning detection. The two scanning probe beams  4   x  and  4   y  are set along two approximately perpendicular directions and occupy about one quart of the limbus  11 . 
     FIG. 5 b  shows how the tracking device remains fill performance for LASIK. In a LASIK surgery, a disk shape flap is laminated from the cornea and about one quart of the perimeter is uncut to maintain the flap attached to the cornea. The flap is folded over during the surgery to allow laser ablation on the corneal bed. The folded flap  15  covers about one third of the limbus  11  and may disable those eye tracking devices which rely on the whole limbus as the reference. The corneal bed after the flap is folded becomes less smooth and the scattered light from the corneal bed may disturb those tracking devices that use the pupil as a reference. 
     As illustrated in FIG. 5 b,  the two scanning beams  4   x  and  4   y  use only the limbus section that is not covered by the cornea flap  15 . Therefore, the limbus  11  remains as a good reference for the tracking device of the present invention. 
     In all the above description, the tracking device is to steer a surgical laser beam  62  to follow the eye movement. Obviously, the same tracking mechanism can guide any other light beam or simply an optical path to follow the eye movement. Therefore, the above technique can be used to other surgical or diagnosis application in which compensating the eye movement is desirable. 
     Although the above embodiments are described with a specific reference to eye tracking, the techniques can be generally used to track lateral movement of other object with an optical reference mark. Various modifications can be made without departing from the scopes of the appended claims.