Patent Publication Number: US-2003225398-A1

Title: Zoom device for eye tracker control system and associated methods

Description:
FIELD OF THE INVENTION  
       [0001] The invention relates generally to eye tracking devices for ophthalmic laser surgical systems, and more particularly to such a device that has a zoom capability.  
       BACKGROUND OF THE INVENTION  
       [0002] The use of lasers to erode a portion of a corneal surface is known in the art to perform corrective surgery. In the field of ophthalmic medicine, photorefractive keratectomy (PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileus (LASIK), and laser epithelial keratomileusis (LASEK) are procedures for laser correction of focusing deficiencies of the eye by modification of corneal profile.  
       [0003] In these procedures, surgical errors due to application of the treatment laser during unwanted eye movement can degrade the refractive outcome of the surgery. The eye movement or eye positioning is critical since the treatment laser is centered on the patient&#39;s theoretical visual axis which, practically speaking, is approximately the center of the patient&#39;s pupil. However, this visual axis is difficult to determine, owing in part to residual eye movement and involuntary eye movement, known as saccadic eye movement. Saccadic eye movement is high-speed movement (i.e., of very short duration, 10-20 milliseconds, and typically up to 1° of eye rotation) inherent in human vision and is used to provide a dynamic scene to the retina. Saccadic eye movement, while being small in amplitude, varies greatly from patient to patient due to psychological effects, body chemistry, surgical lighting conditions, etc. Thus, even though a surgeon may be able to recognize some eye movement and can typically inhibit/restart a treatment laser by operation of a manual switch, the surgeon&#39;s reaction time is not fast enough to move the treatment laser in correspondence with eye movement.  
       [0004] A system for performing eye tracking has been described in U.S. Pat. Nos. 5,632,742; 5,752,950; 5,980,513; 6,302,879; and 6,315,773, which are commonly owned with the present application, and the disclosures of which are incorporated hereinto by reference.  
       SUMMARY OF THE INVENTION  
       [0005] It is an object of the present invention to provide an eye tracking method and system that is used in conjunction with a laser system for performing corneal correction.  
       [0006] Another object is to provide such a method and system that includes a zooming feature for changing a separation of light spots incident upon the eye, collectively called the probe beam.  
       [0007] A further object is to provide such a system and method in which use of the zooming feature does not change a size of the probe beam.  
       [0008] In accordance with the present invention, a zooming mechanism for use in an eye tracking system is disclosed that, in a first embodiment, comprises a pyramidal prism having a plurality of reflective facets meeting at an apex, oriented so that the apex points along an optical axis. Means are provided for directing an incident light beam onto each facet of the prism. Each incident light beam is reflected away from the prism in a direction pointing toward the apex. The directing means is adapted to produce a plurality of reflected beams that, when incident upon a planar surface substantially normal to the optical axis, form a plurality of light spots arrayed about the optical axis.  
       [0009] A second embodiment of the zooming mechanism comprises a pyramidal transmissive prism that has a plurality of facets meeting at an apex, the apex pointing along an optical axis. Means are provided for directing an incident light beam onto each facet of the prism. Each incident light beam is refracted within the prism to form a refracted beam in a direction pointing toward the apex. When the plurality of refracted beams are incident upon a planar surface substantially normal to the optical axis, a plurality of light spots are formed that are arrayed about the optical axis.  
       [0010] In both embodiments, means are provided for translating the prism along the optical axis between a first position wherein the light spots are separated by a first spacing and a second position wherein the light spots are separated by a second spacing that is smaller than the first spacing. The light spots thereby, in a preferred embodiment, have a substantially equal size with the prism in the first and the second positions.  
       [0011] In a system incorporating the zoom mechanism of the present invention, a light source generates a modulated light beam, for example, in the near-infrared 905-nanometer wavelength region. An optical delivery arrangement including the zoom mechanism converts each laser modulation interval into the plurality of light spots, which are focused such that they are incident on a corresponding plurality of positions located on a boundary whose movement is coincident with that of eye movement. The boundary can be defined by two visually adjoining surfaces having different coefficients of reflection. The boundary can be a naturally occurring boundary (e.g., the iris/pupil boundary or the iris/sclera boundary) or a manmade boundary (e.g., an ink ring drawn, imprinted or placed on the eye, or a contrast-enhancing tack affixed to the eye). Energy is reflected from each of the positions located on the boundary receiving the light spots. An optical receiving arrangement detects the reflected energy from each of the positions. Changes in reflected energy at one or more of the positions is indicative of eye movement.  
       [0012] One aspect of the method of the present invention comprises a method for sensing eye movement. This method comprises the steps of directing a plurality of light beams onto a plurality of positions on a boundary defined by two adjoining surfaces of the eye to form a plurality of light spots. The two surfaces are selected to have different coefficients of reflection. Reflected energy from each of the plurality of positions is detected, wherein changes in the reflected energy at one or more of the positions is indicative of eye movement. In order to retain the light spots on the boundary, a size of a pattern formed by the plurality of light spots is adjusted on the plurality of positions. This adjustment, in a preferred embodiment, is performed without substantially changing a diameter of the individual light spots. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013]FIG. 1 is a block diagram of an eye movement tracking system in accordance with the present invention.  
     [0014]FIG. 2 is a block diagram of an optical arrangement for the focusing optics in the eye tracking system.  
     [0015]FIG. 3 is a block diagram of an optical arrangement for the focusing optics in the eye tracking system using a pyramidal zoom device.  
     [0016]FIG. 4 is a schematic diagram of a translatable reflective prism being used in a zoom mechanism in a first position.  
     [0017]FIG. 5 is a schematic diagram of the translatable reflective prism of FIG. 3 in a second position.  
     [0018]FIG. 6 is a schematic diagram of a translatable transmissive prism being used in a zoom mechanism in a first position.  
     [0019]FIG. 7 is a schematic diagram of the translatable transmissive prism of FIG. 5 in a second position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0020] A description of a preferred embodiment of the present invention will now be presented with reference to FIGS.  1 - 7 .  
     [0021] A preferred embodiment system, referenced generally by numeral  100 , for carrying out the method of the present invention will now be described with the aid of the block diagram shown in FIG. 1. System  100  may be broken down into a delivery portion and a receiving portion. The delivery portion projects light spots  21 ,  22 ,  23 , and  24  onto eye  10 , while the receiving portion monitors reflections caused by light spots  21 ,  22 ,  23 , and  24 .  
     [0022] The delivery portion includes a laser  102  transmitting light through optical fiber  104  to an optical fiber assembly  105  that splits and delays each pulse from laser  102  into preferably four equal-energy pulses. An exemplary laser  102  comprises a 905-nanometer pulsed diode, although this is not intended as a limitation. Assembly  105  includes a one-to-four optical splitter  106  that outputs four pulses of approximately equal energy into optical fibers  108 ,  110 ,  112 ,  114 . Such optical splitters are commercially available (e.g., model HLS2X4 manufactured by Canstar and model MMSC-0404-0850-A-H-1 manufactured by E-Tek Dynamics). In order to use a single processor to process the reflections caused by each pulse transmitted by fibers  108 ,  110 ,  112 , and  114 , each pulse is uniquely multiplexed by a respective fiber optic delay line (or optical modulator)  109 ,  111 ,  113 , and  115 . For example, delay line  109  causes a delay of zero, i.e., DELAY=Ox where x is the delay increment; delay line  111  causes a delay of x, i.e., DELAY=1x; etc.  
     [0023] The pulse repetition frequency and delay increment x are chosen so that the data rate of system  100  is greater than the speed of the movement of interest. In terms of saccadic eye movement, the data rate of system  100  must be on the order of at least several hundred hertz. For example, a system data rate of 4 kHz is achieved by (1) selecting a small but sufficient value for x to allow processor  160  to handle the data (e.g.,  250  nanoseconds), and (2) selecting the time between pulses from laser  102  to be  250  microseconds (i.e., laser  102  is pulsed at a  4 -kHz rate).  
     [0024] The four equal-energy pulses exit assembly  105  via optical fibers  116 ,  118 ,  120 , and  122 , which are configured as a fiber optic bundle  123 . Bundle  123  arranges optical fibers  116 ,  118 ,  120 , and  122  in a manner that produces a square (dotted line) with the center of each fiber at a corner thereof.  
     [0025] Light from assembly  105  is passed through an optical polarizer  124  that attenuates the vertical component of the light and outputs horizontally polarized light beams as indicated by arrow  126 . Horizontally polarized light beams  126  pass to focusing optics  130 , where the spacing between beams  126  is adjusted based on the boundary of interest. Additionally, a zoom capability can be provided to allow for adjustment of the size of the pattern formed by spots  21 - 24 . This capability allows system  100  to adapt to different patients, boundaries, etc. In particular embodiments, the spots  21 - 24  are focused on a boundary between the iris and the sclera or on a boundary between the iris and the pupil.  
     [0026] While a variety of optical arrangements are possible for focusing optics  130 , one such arrangement is shown by way of example in FIG. 2. In FIG. 2, fiber optic bundle  123  is positioned at the working distance of microscope objective  1302 . The numerical aperture of microscope objective  1302  is selected to be equal to the numerical aperture of fibers  116 ,  118 ,  120 , and  122 . Microscope objective  1302  magnifies and collimates the incoming light. Zoom lens  1304  provides an additional magnification factor for further tunability. Collimating lens  1306  has a focal length that is equal to its distance from the image of zoom lens  1304  such that its output is collimated. The focal length of imaging lens  1308  is the distance to the eye such that imaging lens  1308  focuses the light as four sharp spots on the corneal surface of the eye.  
     [0027] The zoom lens  1304  as described above changes the probe beam geometry, that is, the inscribed circle that contains all the probe beams, in order to accommodate varying object sizes and boundaries. A standard zoom lens  1304  may be used for this purpose; however, the dynamic range for laser tracking devices using standard zoom lenses is limited because the individual probe beam size is changed in direct proportion to the overall probe beam geometry.  
     [0028] In order to optimize dynamic range, the magnification of the overall probe beam geometry, that is, the inscribed circle of spots  21 - 24 , would preferably be decoupled from that of the individual beam size. Two embodiments of a system and method for achieving such a decoupling will now be presented with reference to FIGS.  3 - 7 , with FIG. 3 representing a block diagram of an optical arrangement for the focusing optics  130 ′ in the eye tracking system using a pyramidal zoom device.  
     [0029] A first embodiment of the zoom mechanism  30  comprises a pyramidal prism  31  having a plurality of, in a preferred embodiment four, reflective facets  32  (FIGS. 4 and 5). It will be understood by one of skill in the art that FIGS. 4 and 5 (and subsequently discussed FIGS. 6 and 7) are highly schematic representations in two dimensions for ease of presentation, four-sided pyramidal prisms being well known in the art.  
     [0030] The facets  32  meet at an apex  33  that points along an optical axis  34 . It will also be understood by one of skill in the art that by “apex” is meant herein the point or sector at which the facets reach their smallest dimension, and that the prism may in fact comprise a truncated pyramid without a pointed apex.  
     [0031] An incident light beam  35  is directed onto each facet  32  of the prism  31  by an optical arrangement comprising a focusing lens  36  that is positioned to receive an incident light beam  35  and is adapted to image the respective incident light beam  35  to an image plane.  
     [0032] In a preferred embodiment a generally planar mirror  37  is disposed in the optical pathway to receive the respective incident light beam  35  downstream of the respective focusing lens  36  and to reflect the respective incident light beam  35  onto a selected prism facet  32 . Preferably the mirror  37  is oriented substantially parallel to the selected prism facet  32 . The mirror  37  is present in a preferred embodiment to serve as a “folding” mirror for reducing a size of the mechanism  30 .  
     [0033] Each incident light beam  35  is then reflected away from the prism  31  in a direction pointing toward the apex  33 , producing a plurality of reflected beams  38 . When the reflected beams  38  are incident upon a planar surface substantially normal to the optical axis  34  to form the plurality of light spots  21 - 24  (FIG. 1) arrayed substantially on an inscribed circle  39  about the optical axis  34  substantially in a square pattern.  
     [0034] A second embodiment of the zoom mechanism  40  comprises a pyramidal transmissive prism  41  having a plurality of, in a preferred embodiment four, facets  42  (FIGS. 6 and 7). The facets  42  meet at an apex  43  that points along an optical axis  44 .  
     [0035] An incident light beam  45  is directed onto each facet  42  of the prism  41  by an optical arrangement comprising a focusing lens  46  that is positioned to receive an incident light beam  45  and is adapted to image the respective incident light beam  45  to an image plane.  
     [0036] Each incident light beam  45  refracted within the prism  41  to form a refracted beam  48  in a direction pointing toward the apex  43 . The plurality of refracted beams  48 , when incident upon a planar surface substantially normal to the optical axis  44 , form the plurality of light spots  21 - 24  arrayed substantially in a square on an inscribed circle  49  (FIG. 1) about the optical axis  44 .  
     [0037] The zooming mechanisms  30 ,  40  further comprise a mechanism  50 ,  60  for translating the prism  31 ,  41  along the optical axis  34 ,  44  between a first position (FIGS. 4 and 6) wherein the light spots  21 - 24  are separated by a first spacing  51 ,  61  and a second position (FIGS. 5 and 7) wherein the light spots  21 - 24  are separated by a second spacing  52 ,  62  smaller than the first spacing  51 ,  61 . In this arrangement, the light spots  21 - 24  advantageously have a substantially equal size with the prism  31 ,  41  in the first and the second positions. The translating mechanism  50 ,  60  may comprise, for example, a motorized translating stage such as is known in the art that is under processor  160  control.  
     [0038] Referring again to FIG. 1, polarizing beam splitting cube  140  receives horizontally polarized light beams  126  from focusing optics  130 . Polarization beamsplitting cubes are well known in the art. Byway of example, cube  140  is a model 10FC16PB.5 manufactured by Newport-Klinger. Cube  140  is configured to transmit only horizontal polarization and reflect vertical polarization. Accordingly, cube  140  transmits only horizontally polarized light beams  126  as indicated by arrow  142 . Thus it is only horizontally polarized light that is incident on eye  10  as spots  21 - 24 . Upon reflection from eye  10 , the light energy is depolarized (i.e., it has both horizontal and vertical polarization components), as indicated by crossed arrows  150 . The vertical component of the reflected light is then directed/reflected as indicated by arrow  152 . Thus cube  140  serves to separate the transmitted light energy from the reflected light energy for accurate measurement.  
     [0039] The vertically polarized portion of the reflection from spots  21 - 24  is passed through focusing lens  154  for imaging onto an infrared detector  156 . Detector  156  passes its signal to a multiplexing peak detecting circuit  158 , which is essentially a plurality of peak sample- and-hold circuits, a variety of which are well known in the art. Circuit  158  is configured to sample (and hold the peak value from) detector  156  in accordance with the pulse repetition frequency of laser  102  and the delay x. For example, if the pulse repetition frequency of laser  102  is 4 kHz, circuit  158  gathers reflections from spots  21 - 24  every 250 microseconds.  
     [0040] By way of example, infrared detector  156  is an avalanche photodiode model C30916E manufactured by EG&amp;G. For a given transmitted laser pulse, the detector output will consist of four pulses separated in time by the delays associated with optical delay lines  109 ,  111 ,  113 , and  115  shown in FIG. 1. These four time-separated pulses are fed to peak-and-hold circuits. Input enabling signals are also fed to the peak-and-hold circuits in synchronism with the laser fire command. The enabling signal for each peak and hold circuit is delayed by delay circuits. The delays are set to correspond to the delays of delay lines  109 ,  111 ,  113 , and  115  to allow each of the four pulses to be input to the peak-and-hold circuits. The reflected energy associated with a group of four spots is collected as the detector signal is acquired by all four peak and hold circuits. At this point, an output multiplexer reads the value held by each peak-and-hold circuit and inputs them sequentially to processor  160 .  
     [0041] The values associated with the reflected energy for each group of four spots (i.e., each pulse of laser  102 ) are passed to a processor  160 , where horizontal and vertical components of eye movement are determined. For example, let R 21 , R 22 , R 23 , and R 24  represent the detected amount of reflection from one group of spots  21 - 24 , respectively. A quantitative amount of horizontal movement is determined directly from the normalized relationship  
           (       R   21     +     R   24       )     -     (       R   22     +     R   23       )           R   21     +     R   22     +     R   23     +     R   24                     
 
     [0042] while a quantitative amount of vertical movement is determined directly from the normalized relationship  
           (       R   21     +     R   22       )     -     (       R   23     +     R   24       )           R   21     +     R   22     +     R   23     +     R   24                     
 
     [0043] Note that normalizing (i.e., dividing by R 21 +R 22 +R 23 +R 24 ) reduces the effects of variations in signal strength.  
     [0044] Once processed, the reflection differentials indicating eye movement (or the lack thereof) can be used in a variety of ways. For example, an excessive amount of eye movement may be used to trigger an alarm  170 . In addition, the reflection differential may be used as a feedback control for tracking servos  172  used to position an ablation laser. Still further, the reflection differentials can be displayed on display  174  for monitoring or teaching purposes.  
     [0045] Additionally, the detected reflected energy from light spots  21 - 24  may be analyzed in the processor  160  to determine a change in pupil size as determined by the reflection differentials and the spacing of the light spots  21 - 24 . As it is desired to retain the light spots  21 - 24  on a selected eye surface boundary, here coincident with the circle  39 ,  49 , means are provided under direction of the processor  160  for directing the translating mechanism  50 ,  60  to translate the prism  31 ,  41  in a direction for retaining the light spots  21 - 24  on the selected boundary  39 ,  49 , without substantially altering the diameters of the light spots  21 - 24 .  
     [0046] The advantages of the present invention are numerous. Eye movement is sensed in accordance with a non-intrusive method and apparatus. The present invention will find great utility in a variety of ophthalmic surgical procedures without any detrimental effects to the eye or interruption of a surgeon&#39;s view. Further, data rates needed to sense saccadic eye movement are easily and economically achieved.  
     [0047] Although the invention has been described relative to a specific embodiment thereof, there are numerous variations and modifications that will be readily apparent to those skilled in the art in the light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced other than as specifically described.