Method and apparatus for determining position of an eye

An embodiment of the present invention is an apparatus to determine position of an eye that includes: (a) two off-axis, radiation emitter-photodetector pairs, wherein an emitter of a pair is disposed to transmit radiation toward the eye and a photodetector of the pair is disposed to receive radiation reflected by the eye; and (b) a controller that analyzes output from the photodetectors to determine the position of the eye.

TECHNICAL FIELD OF THE INVENTION
 The present invention pertains to the field of determining position of an
 eye, for example, a human eye.
 BACKGROUND OF THE INVENTION
 A common design issue associated with ophthalmic instruments is that
 significant diagnostic errors can be introduced whenever a patient's eye
 is not positioned within predetermined bounds. Some prior art ophthalmic
 instruments rely on an operator's judgment and skill to visually monitor
 the position of the patient's eye, and manually to place it in an
 "acceptable" location. An Acuitus Model 5000 available from Carl Zeiss,
 Inc. of Dublin, Calif. is one such manually positioned, prior art
 ophthalmic instrument. In using this ophthalmic instrument, an operator
 must judge the position of the patient's eye using a video image thereof.
 To do this, the operator centers the pupil of the patient's eye on a video
 screen; the operator infers the position of the eye from the degree of
 focus of the video image of the pupil. As can be readily appreciated from
 this, eye position is problematic because the degree of focus of the video
 image is subjective, and it is generally not very sensitive. Thus, some
 error in eye position is inevitable in such a manually positioned
 ophthalmic instrument because of variation in operator judgment and skill.
 As is also well known to those of ordinary skill in the art, ophthalmic
 instruments can use eye position measurement data to help correct for
 diagnostic measurement errors associated with residual eye position offset
 errors. For example, one type of prior art ophthalmic instrument uses eye
 position measurement data to compensate for refractor errors caused, for
 example, by range offset. Range offset refers to errors in positioning the
 instrument in the correct position along the patient's line of sight.
 However, despite an ophthalmic instrument's being designed to minimize
 diagnostic measurement errors caused by eye position offset errors, some
 eye position offset error sensitivity still occurs.
 As one can readily appreciate from the above, a need exists in the art for
 a method and apparatus to determine position of an eye.
 SUMMARY OF THE INVENTION
 Embodiments of the present invention advantageously satisfy the
 above-identified need in the art and provide method and apparatus to
 determine position of an eye.
 A preferred embodiment of the present invention is an apparatus to
 determine position of an eye that comprises: (a) two off-axis, radiation
 emitter-photodetector pairs, wherein an emitter of a pair is disposed to
 transmit radiation toward the eye and a photodetector of the pair is
 disposed to receive radiation reflected by the eye; and (b) a controller
 that analyzes output from the photodetectors to determine the position of
 the eye.
 In addition, one embodiment of the present invention is a simple, modular,
 stand-alone alternative to video image processing schemes, which simple,
 modular, stand-alone alternative does not impact the design of the rest of
 an ophthalmic instrument with which it is associated and is easy to
 manufacture and install. Advantageously, the one embodiment provides good
 accuracy, sensitivity, range, and cycle rate in a modular package.

DETAILED DESCRIPTION
 FIG. 1 shows embodiment 1000 of the present invention. As shown in FIG. 1,
 embodiment 1000 comprises two collimated radiation sources, infrared
 ("IR") emitters 100 and 110, and two radiation detectors, IR quadrant
 photodetectors 120 and 130, all of which are held in fixture 140. As
 further shown in FIG. 1, frame 140 is configured with viewport 150 for use
 by an ophthalmic instrument with which embodiment 1000 may be used in
 viewing a patient's eye 160. As still further shown in FIG. 1, emitters
 100 and 110 and detectors 120 and 130 are configured as two IR
 emitter-detector pairs (pair 1 comprises off axis IR emitter 100 and
 diagonally opposed off-axis quadrant photodetector 130 and pair 2
 comprises off axis IR emitter 110 and diagonally opposed off-axis quadrant
 photodetector 120).
 Embodiment 1000 also comprises electronic circuitry (not shown for clarity
 and ease of understanding the present invention) that: (a) drives emitters
 100 and 110 in accordance with any one of a number of methods that are
 well known to those of ordinary skill in the art; (b) reads outputs from
 quadrant photodetectors 120 and 130 in accordance with any one of a number
 of methods that are well known to those of ordinary skill in the art; and
 (c) interfaces with a controller (not shown) in accordance with any one of
 a number of methods that are well known to those of ordinary skill in the
 art. As is well known to those of ordinary skill in the art, electronic
 circuitry that performs these functions can also be a part of the
 controller, which controller can be, for example, a computer. In
 accordance with a preferred embodiment of the present invention, and as
 will be described in detail below, a software routine that operates in a
 manner to be described in detail below converts outputs from detectors 120
 and 130 to an X,Y,Z position of the vertex of the cornea of patient's eye
 160.
 FIG. 2 shows a diagram of light paths involved in using embodiment 1000 of
 the present invention for emitter-detector pair 1 which comprises off axis
 IR emitter 100 and diagonally opposed, off-axis, quadrant photodetector
 130. As shown in FIG. 2, patient's eye 160 is disposed at a predetermined
 location with respect to fixture 150 (not shown in FIG. 2) by use of head
 seating fixture (not shown), which predetermined location provides
 placement of the cornea of the patient's eye at a nominal corneal position
 (all of this being done in accordance with any one of a number of methods
 which are well known to those of ordinary skill in the art). In
 conjunction with this, the patient may be asked to gaze at a fixation
 device to provide a reasonably steady choice for a nominal position. The
 fixation device may be fabricated in accordance with any one of a number
 of methods that are well known to those of ordinary skill in the art.
 As further shown in FIG. 2, collimated IR emitter 100 comprises source LED
 101 and lens system 103, and quadrant photodetector 130 comprises lens
 system 131 and photodetector 133. In accordance with a preferred
 embodiment of the present invention, lens systems 103 and 131 are
 configured so that IR emitter 100 and quadrant photodetector 130 have
 narrow fields-of-view; both of which fields of view are centered on the
 nominal corneal vertex position. As shown in FIG. 2, radiation beam 105
 emitted by LED 101 is directed to impinge upon patient's eye 160 and,
 after reflection thereby, is captured (as radiation beam 107) by
 photodetector 133. In addition, fixture 150 is designed in accordance with
 any one of a number of methods that are well known to those of ordinary
 skill in the art so that quadrant photodetector 130 sees an image of IR
 emitter 100 along a line-of-sight that passes through the nominal vertex
 of the cornea of patient's eye 160.
 As can readily be appreciated by one of ordinary skill in the art, a change
 in position of the vertex of the cornea of patient's eye 160 will produce
 a related change in the line-of-sight between the vertex of the cornea of
 patient's eye 160 and the quadrant photodetectors of embodiment 1000. In
 accordance with the present invention, output from each of quadrant
 photodetectors 120 and 130 enables the position of the vertex of the
 cornea to be measured in two dimensions. Then, in accordance with the
 present invention, and as will be described in detail below, measurements
 of the line-of-sight using outputs from both quadrant photodetectors 120
 and 130 are combined to measure the vertex of the cornea in three
 dimensions (the X,Y,Z position). As one can readily appreciate,
 measurement of the vertex of the cornea in three dimensions is made
 possible because outputs from quadrant photodetectors 120 and 130,
 respectively, measure lines-of-sight from two different points of
 reference.
 FIGS. 3A-3F illustrate the manner in which quadrant photodetectors used to
 fabricate embodiment 1000 of the present invention operate to indicate
 corneal vertex displacement of patient's eye 160. As shown in FIG. 3A,
 when one of quadrant photodetectors 120 and 130 receives radiation
 reflected from a centered corneal vertex, radiation pattern 200 is
 balanced, i.e., equal illumination is received in the four quadrants of
 the photodetector. As further shown in FIG. 3A, when one of quadrant
 photodetectors 120 and 130 receives radiation reflected from an offset
 corneal vertex, radiation pattern 210 is unbalanced, i.e., there is
 unequal illumination in the four quadrants of the detector. FIG. 3B shows
 the illumination received by quadrant photodetectors 120 and 130,
 respectively, for a centered corneal vertex. FIG. 3C shows the
 illumination received by quadrant photodetectors 120 and 130,
 respectively, for a corneal vertex with an X offset. FIG. 3D shows the
 illumination received by quadrant photodetectors 120 and 130,
 respectively, for a corneal vertex with an Y offset. FIG. 3E shows the
 illumination received by quadrant photodetectors 120 and 130,
 respectively, for a corneal vertex with an Z offset.
 In accordance with a preferred embodiment of the present invention,
 quadrant photodetectors 120 and 130 are initially aligned to a nominal
 corneal vertex position, i.e., a position at which both quadrant
 photodetectors 120 and 130 exhibit a bias which is substantially zero or
 which differs therefrom by a predetermined amount. For example, this may
 be done by placing an artificial eye, for example, a glass eye, at a
 nominal origin (for example, 0,0,0) to align the apparatus. Then, as will
 described in detail below, in accordance with the present invention,
 horizontal and vertical biases of quadrant photodetectors 120 and 130
 provide a measure of X,Y,Z displacement from the nominal corneal vertex
 position with high sensitivity. Advantageously, in accordance with the
 present invention, the method used to determine the position of the
 corneal vertex is insensitive to the radius of curvature of the cornea.
 FIG. 3F shows how regions A, B, C, and D are defined for quadrant
 photodetectors 120 and 130. Using these definitions, the horizontal bias
 of quadrant photodetector 120 is given by:
 ##EQU1##
 and the vertical bias of quadrant photodetector 120 is given by:
 ##EQU2##
 Likewise, the horizontal bias of quadrant photodetector 130 is given by:
 ##EQU3##
 and the vertical bias of quadrant photodetector 130 is given by:
 ##EQU4##
 In accordance with the present invention, for "small" displacements, the
 X,Y,Z coordinates of the corneal vertex are linearly related to the
 horizontal and vertical biases of quadrant photodetectors 120 and 130. For
 the simple case shown in FIGS. 3C through 3E:
EQU X=C.sub.X (H.sub.1 +H.sub.2)
EQU Y=C.sub.Y (V.sub.1 +V.sub.2)
EQU Z=C.sub.Z (H.sub.1 -H.sub.2)
 where C.sub.X, C.sub.Y, and C.sub.Z are constants that are determined by
 geometry and/or calibration in accordance with any one of a number of
 methods that are well known to those of ordinary skill in the art.
 In a more general case, quadrant photodetectors 120 and 130 may be oriented
 with an arbitrary polar orientation. In such a case, the X,Y,Z coordinates
 of the corneal vertex are given by the following matrix relationship:
 ##EQU5##
 where C.sub.XH1, C.sub.YH1, C.sub.ZH1, C.sub.XV1, C.sub.YV1, C.sub.ZV1,
 C.sub.XH2, C.sub.YH2, C.sub.ZH2, C.sub.XV2, C.sub.YV2, and C.sub.ZV2 are
 constants that are determined by geometry and/or calibration in accordance
 with any one of a number of methods that are well known to those of
 ordinary skill in the art.
 In accordance with a further embodiment of the present invention,
 cross-talk between emitter-photodetector pairs (100, 130) and (110, 120),
 respectively, can be minimized by alternately running only one
 emitter-photodetector pair at a time in accordance with signals that are
 generated in the electronic circuitry in accordance with any one of a
 number of methods that are well known to those of ordinary skill in the
 art. In addition, in a still further embodiment of the present invention,
 the electronic circuitry includes synchronous detection apparatus that
 operates in accordance with any one of a number of methods that are well
 known to those of ordinary skill in the art to reject unwanted signals in
 the photodetectors. For example, in accordance with such an embodiment,
 LED emitters 100 and 110 do not operate continuously, but flash in
 response to input from energizer portions of electronic circuitry which
 are fabricated in accordance with any one of a number of methods that are
 well known to those of ordinary skill in the art. Then, in accordance with
 the present invention, photodetector inputs are analyzed at times which
 correspond to times at which radiation reflected from the patient's eye is
 expected to be received, which times can be readily synchronized with the
 times during which the emitters are energized.
 Advantageously, in accordance with the present invention, emitters 100 and
 110 are configured so that on-axis images thereof, i.e., images that are
 reflected from patient's eye 160 and pass through viewport 150 of frame
 140 are minimized. This is done to reduce interference with ophthalmic
 instruments with which embodiments of the present invention are used.
 The output from embodiment 1000 which measures the X,Y,Z position data
 associated with the corneal vertex can be used to cause an ophthalmic
 instrument to make measurements whenever the vertex is sufficiently close
 to a predetermined position or the vertex position measurement data may be
 supplied to the ophthalmic instrument for use in determining or correcting
 diagnostic errors produced thereby.
 In addition, the output from embodiment 1000 which measures the X,Y,Z
 position data associated with the corneal vertex can be used to cause an
 motorized system to drive a servomechanism to move the corneal vertex
 toward a predetermined position, or to cause a feedback cue to be given to
 an operator to prompt corrective positioning action.
 FIG. 4 shows a block diagram of an embodiment of the present invention used
 in conjunction with an ophthalmic instrument. As shown in FIG. 4,
 emitter-photodetector pairs (420, 450) and (430, 440) are affixed to frame
 410 which provides a line of sight between patient's eye 400 and
 ophthalmic instrument 500. As shown in FIG. 4, emitters 420 and 430 are
 connected to emitter circuitry 510, which emitter circuitry 510 operates
 in response to signals from controller 530 to transmit electrical pulses
 to energize emitters 420 in accordance with any one of a number of methods
 that are well known to those of ordinary skill in the art. As further
 shown in FIG. 4, photodetectors 440 and 450 receive radiation reflected
 from patient's eye 400 and transmit signals to detector circuitry 520,
 which detector circuitry 520 transmits detector signals to controller 530
 in accordance with any one of a number of methods that are well known to
 those of ordinary skill in the art. Controller 530 analyzes the detection
 signals in accordance with the methods described above. Further, the
 synchronization of emitter output and detector signal analysis may be
 performed by signals sent from controller 530 to emitter circuitry 510 and
 detector circuitry 520. Alternatively, the synchronization may be
 performed within controller 530.
 As discussed above, controller 530 may send information to be displayed on
 operator console 540 in accordance with any one of a number of methods
 that are well known to those of ordinary skill in the art, which
 information indicates a corneal vertex position and which information may
 further indicate a position correction movement that may be used to alter
 the position to a predetermined position. Alternatively, controller 530
 may send a signal to a positioning device (not shown) for moving the
 position of either patient's eye 400 or ophthalmic instrument 500 in
 accordance with any one of a number of methods that are well known to
 those of ordinary skill in the art. Lastly, if controller 530 determines
 that the position of the corneal vertex of patient's eye 400 is at a
 predetermined position, controller 530 can sent a message in accordance
 with any one of a number of methods that are well known to those of
 ordinary skill in the art to ophthalmic instrument 500. In response,
 ophthalmic instrument can take an appropriate action such as making a
 measurement of patient's eye 400.
 Those skilled in the art will recognize that the foregoing description has
 been presented for the sake of illustration and description only. As such,
 it is not intended to be exhaustive or to limit the invention to the
 precise form disclosed. For example, although embodiments of the present
 invention were discussed in terms of using quadrant photodetectors,
 embodiments may be fabricated using, for example, video detectors such as
 for example, CCD video detectors. In such a case, the video photodetector
 outputs are analyzed in accordance with any one of a number of methods
 that are well known to those of ordinary skill in the art to determine a
 horizontal and vertical bias of the radiation reflected by the patient's
 eye.