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Claims  |
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What is claimed is:
1. Apparatus for directing a first beam of light energy onto the fundus of
an eye and for stabilizing the beam during eye movement, such apparatus
comprising
means defining an optical path to the fundus for directing a first beam of
light energy to be stabilized on the fundus,
means for producing and directing at the fundus a tracking beam of light
energy, distinct from the first beam, for illuminating a region
surrounding a microscopic tissue feature constituting a narrow tracking
target extending along a line,
angular correction means responsive to a control signal for controllably
redirecting the optical path to the fundus,
imaging means for focusing light reflected from the fundus through the
angular correction means so as to form a fundus image including an image
of the tracking target,
alignment means in the optical path for aligning the image of the tracking
target to place said line in a selected position, and
tracking means responsive to movement of the image of the tracking target
transverse to said line for developing a control signal representative of
eye movement, said control signal being provided to said angular
correction means to redirect the optical path so that the first beam is
stabilized on the fundus in a fixed position during eye movement.
2. Apparatus according to claim 1, further comprising
a photosensor array for developing an electronic output signal indicative
of spatially distributed light energy impinging thereon, and wherein said
imaging means forms an image of a region of the fundus including the
tracking target on said photosensor array.
3. Apparatus according to claim 2, wherein the tracking means further
comprises
electronic scanning and processing means for electronically scanning said
photosensor array so as to develop a feature signal representative of the
tracking target position, and
comparing means for comparing the feature signal at successive times so as
to develop said control signal.
4. Apparatus according to claim 3 further comprising an image intensifier
between said imaging means and said photosensor array.
5. Apparatus according to claim 3 adapted for tracking a blood vessel
target, wherein said tracking beam is a green light beam for enhancing
contrast of a target image, and wherein said alignment means includes an
image rotator for rotating the image to align it orthogonal to a scan
direction of said scanning means.
6. Apparatus according to claim 1, wherein the apparatus is an ophthalmic
instrument further comprising means for producing said first beam of light
energy with a wavelength distinct from the tracking beam.
7. Apparatus for directing a first beam of light energy onto the fundus of
an eye so that the beam strikes the eye at a fixed position during eye
movement, such apparatus comprising
means defining an optical path to the fundus for directing a first beam of
light energy to be stabilized on the fundus,
means for producing and directing at the fundus a second beam of light
energy,
angular correction means for controllably redirecting the optical path so
as to controllably aim the first beam at the fundus,
imaging means in the optical path for focusing through the angular
correction means light of the second beam reflected from the fundus, so as
to form a fundus image,
a photosensor array for receiving the image of a selected portion of the
fundus including a microscopic target structure having a characteristic
luminance and extending along a line of elements of the array, and for
developing an electronic output representing such image, and
means for electronically tracking the imaged target structure by processing
the electronic output so as to develop a tracking signal representative of
eye movement transverse to said line of elements,
said tracking signal being provided to said angular correction means so
that the optical path is redirected to maintain the image of the target
structure in a fixed position on the photosensor array, thereby
stabilizing the first beam on the fundus.
8. In an ophthalmic imaging or treatment device of the type which includes
optical path defining means for directing or receiving illumination along
an optical path to the eye fundus, the improvement comprising a stabilizer
for maintaining the optical path directed at a fixed fundus position, such
stabilizer including
means for producing and directing at the fundus a tracking beam of light
energy,
angular correction means in the optical path for controllably redirecting
the optical path to the fundus,
imaging means for focusing through the angular correction means light
reflected from the fundus so as to form a fundus image,
alignment means in the path for moving the image of the fundus to align in
a selected position an image of a fundus structure having a characteristic
luminance and extending along a line,
means for tracking motion of the image of the fundus structure transverse
to said line so as to develop a signal representative of eye movement, and
means for controlling the angular correction means in accordance with said
signal to move the optical path by a fixed amount in a direction opposite
to said motion, said tracking and controlling means being actuated at a
rate greater than fifty times per second so that the optical path
intersects the fundus in a stabilized position.
9. A stabilization system for an ophthalmic illumination instrument, such
system comprising
means defining an optical path for conducting to the fundus illumination of
the ophthalmic illumination instrument,
means for producing and directing at the fundus a tracking beam of light
energy for illuminating at one time a region about a microscopic tracking
target of fundus tissue,
angular correction means responsive to a control signal for controllably
redirecting the optical path to the fundus,
imaging means for focusing light reflected from the fundus, back through
the angular correction means so as to form a fundus image including an
image of the tracking target which extends along a line,
alignment means in the optical path for aligning the image of the tracking
target to place said line in a selected position, and
tracking means responsive to movement of the imaged tracking target for
developing a control signal representative of eye movement transverse to
said line, said control signal being provided to said angular correction
means to redirect the optical path whereby the illumination from the
ophthalmic instrument is stabilized on the fundus in a fixed position
during eye movement.
10. Apparatus according to claim 9, wherein
said angular correction means includes first and second
galvanometer-controlled front-surface mirrors mounted for pivotal motion
about respective first and second orthogonal axes for controllably
redirecting the optical path in two dimensions, and wherein said tracking
means includes a photodetector for generating a spatially resolved
electronic signal representative of said image, and further includes
scanning and processing means operative on said spatially resolved signal
for determining said control signal.
11. Apparatus according to claim 10, wherein said processing means
comprises
means for summing the spatially resolved electronic signals from the
photodetector over a selected group of spatial elements and for providing
a movement signal representative of movement of an imaged fundus structure
in two orthogonal directions, and
step control means responsive to plural movement signals for providing a
selected magnitude step correction signal as the control signal to the
first and second galvanometer-controlled mirrors.
12. Apparatus according to claim 10, further comprising a lens system
positioned with respect to the two galvanometer-controlled mirrors such
that the fundus of the subject's eye is imaged in two planes, each of
which is conjugate to the subject's fundus, and further such that the
nominal plane of the axis of rotation of each galvanometer-controlled
mirror contains a virtual image of the center of rotation of the subject's
eye.
13. Apparatus according to claim 10, including first and second
galvanometer drive members for rotating said first and second mirrors
respectively, said first and second drive members being responsive to said
control signal.
14. Apparatus according to claim 9, wherein the tracking means further
includes means for adaptively determining a control signal magnitude as a
step-signal which varies in accordance with the magnitude and direction of
at least one previous control signal and with the movement of the imaged
tracking target.
15. Apparatus according to claim 9, wherein said instrument illumination
and said tracking beam are coaxial along a portion of said optical path
including said angular correction means, and lying between a beam junction
point and the eye.
16. Apparatus according to claim 9 wherein said angular correction means
comprises means for deflecting along two orthogonal axes said light beams
traveling in forward and reverse directions on the optical path.
17. Apparatus according to claim 9, wherein said alignment means comprises
a low-loss front-surface mirror optically aligned with an image of the eye
fundus for angularly rotating the image of the tracking target.
18. Apparatus according to claim 9 further comprising
means for providing observation illumination, and
optical filter means for separating reflected light from the eye fundus
into three components corresponding to
(i) said tracking beam;
(ii) said illumination from the instrument; and
(iii) said observation illumination
so as to separately divert said three light components to the image
intensifier, to a diagnostic beam output port, and to an optical viewing
system, respectively.
19. Apparatus for directing a first beam of light energy onto the fundus of
an eye and for stabilizing the beam during eye movement, such apparatus
comprising
means defining an optical path to the fundus for directing a first beam of
light energy to be stabilized on the fundus,
means for producing and directing at the fundus a tracking beam of light
energy for illuminating a microscopic tracking target on the fundus,
angular correction means responsive to a control signal for controllably
redirecting the optical path to the fundus,
a photosensor array having a plurality of photosensitive surface elements
for developing a two dimensional spatially resolved electronic signal
representative of light energy impinging on a surface of the array,
imaging means for focusing through the angular correction means light
reflected from the fundus so as to form a fundus image on the photosensor
array, such image including an image of the tracking target,
means for aligning the image of the tracking target in a selected position
across a line of photosensitive elements on said photosensor array, and
tracking means responsive to movement of the imaged tracking target for
electronically scanning the spatially resolved electronic signal so as to
develop a control signal representative of eye movement, said control
signal being provided to said angular correction means to redirect the
optical path so that the first beam is stabilized on the fundus in a fixed
position during eye movement.
20. Apparatus according to claim 19, wherein the means for aligning
includes an image rotator.
21. A method of stabilizing an optical instrument on a selected region of
the fundus of an eye of a subject, wherein the optical instrument is aimed
at the fundus along a path including an optical steering system, such
method comprising the steps of
(i) directing a tracking beam to illuminate a microscopic tissue structure
serving as a target feature on the fundus, 48
(ii) imaging the target feature through the steering system across a line
of sensing elements of a photosensor array,
(iii) scanning the line of sensing elements of the photosensor array to
detect motion of the imaged target feature along said line of elements,
and
(iv) developing control signals indicative of image motion and providing
the control signals to the steering system for controlling the steering
system to maintain the imaged target feature stationary, so that the
instrument remains aimed at the selected fundus region. |
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Claims |
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Description |
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This invention relates generally to instruments for tracking eye movements
and to instruments for directing beams of light into the eye. In
particular, the invention includes an electro-optical eye tracking and
concatenated beam stabilizing system which fixes, by iterative
corrections, the position on the fundus of a beam directed at the eye. The
system for tracking eye movements described here involves the
electro-optical tracking of retinal blood vessels and other structures
with light absorption characteristics sufficiently different from the
surrounding tissue of the eye fundus.
BACKGROUND
Applications requiring a beam fixed on a particular structure of the fundus
to date have been hampered by the subject's normal eye movements.
Uncontrolled eye movement introduces complications in such clinical
treatments as laser photocoagulation. Thus a photocoagulation procedure
could be improved using an accurate eye tracker to stabilize the
photocoagulation beam at a particular site.
Successful prior art methods for tracking eye movements have evolved along
two principal approaches. One early approach was to attach a tightly
fitting contact lens to the surface of the eye, and to either attach a
test object to the lens or to reflect an image from a front-surface mirror
attached to the lens and through an optical system to produce an image of
the fundus stabilized against eye movements. The degree of stabilization
in opto-mechanical systems of this type is inherently limited by the
slippage that occurs between the contact lens and the angular movements of
the visual axis of the eye. Stabilization of this kind has been
demonstrated to be insufficient for precise work.
A second procedure for image stabilization involves a two-component system,
including a device for tracking the movements of the eye and a mechanism
for moving the object or image proportionally. Historically, tracking eye
movements in this manner involved following the movements of a contact
lens attached to the eye, so that this method suffers the same limitation
as the one first described.
The earlier efforts required accurately fitted individual lenses for each
subject. Although the contact lens systems offer the best resolution of
any system down to 10 arc sec, they do so in general at the sacrifice of
range. They are normally applicable for the study of small eye movements.
The expense and discomfort of the contact lens makes it a technique more
suitable for use on a few subjects.
Another prior art class of instruments uses corneal reflections and
reflections from other optical curvatures in the eye (Purkinje images),
and measures translation, as well as rotation. However, these instruments
track movements at the surface of the eye, and are less accurate for
stabilizing an image at the back of the eye.
Recently, emphasis has been placed on tracking structures at the fundus.
One fundus tracking method involves projecting a scanning pattern onto the
eye fundus, and detecting the translational and rotational movements of
the reflected pattern by means of high-speed correlation processing of the
video signal. Scanning systems of this type have generally been "light
starved". That is, the light intensity required to provide a good image
signal-to-noise ratio exceeds acceptable retinal illumination levels.
Furthermore, scanning systems require extremely regular and fast moving
optical deflectors. As a whole these systems require complex electronic
processing, limiting their response time.
OBJECTS OF THE INVENTION
Principal objects of the invention are therefore, to provide an
electro-optical eye movement tracker for directing onto the fundus a light
beam stabilized against eye movement, having improved response time,
having improved accuracy, which uses a relatively low light level, which
preferably illuminates only that portion of the fundus to be tracked,
which contains few moving parts, and which provides a minimum of
discomfort to the patient.
Other objects of the invention are to provide an electro-optical eye
movement tracker which allows independent viewing of the fundus, which
provides sampling of the reflected stabilized beam, which is reliable, and
which provides the user with operational flexibility adapted to stabilize
diverse diagnostic, treatment, measurement and observational instruments.
SUMMARY OF THE INVENTION
A device for tracking eye movements according to the invention includes a
laser source which projects a tracking strip of light on the fundus,
optics for producing an image of reflected light from the tracking strip
(image strip) onto a detecting element, equipment for repeatedly and
rapidly scanning the intensity profile of the image strip, and processing
electronics for analyzing the scanned intensity profile and providing
correction signals to direct the optical path of both the tracking laser
beam and an additional diagnostic beam to a fixed position on the fundus.
A subject is positioned with the eye to be illuminated placed in optical
alignment with light deflectors so as to allow the maximal range of beam
movement about a feature on the fundus, the image of which serves as a
tracking target.
This device for tracking eye movement follows differential change in the
position of a spatially variant intensity feature of the fundus. The
fundus is illuminated by the tracking strip and the intensity feature is
detected in the image strip. The position of the intensity feature is
related to a reference mark relative to a position stationary with respect
to the device. In particular, the image strip falls on a
position-sensitive detector placed at the image plane. The image strip is
scanned, either mechanically or electronically, to obtain scan data
containing information of light intensity as a function of position. A
preferred scanning technique electronically samples within the detector
the electronic signals caused by the image light. Very accurate correction
is achieved due to the scan rate, and the performance of an ophthalmic
system using the tracker is not degraded, as the illumination intensity,
and the resolution are neither determined nor limited by the detecting
element.
The detecting element provides an electric signal whose amplitude is
proportional to the magnitude of the light intensity of the image strip
produced by illumination of a portion of the eye fundus by the laser
source. The processing electronics interpret the electronically encoded
intensity profile produced by the image strip at the detector, sense
differential movement of the eye, and produce a control signal which is
provided to the optical deflectors for correcting three laser beam
position on the eye fundus. The image strip is scanned rapidly and
repeatedly providing fast-response iterative correction of the beam
position with exceptional accuracy.
Further preferred embodiments include one or more refinements, including
electrical shaping of the control signal provided to the optical
deflectors; an image intensifier to improve detected contrast and
intensity level of the fundus image at the detection element; an image
rotator for aligning an image fundus structure in a preferred orientation
with respect to a detector scan direction; an optical viewing path; and
correction magnitude setting circuitry within the processing device for
adaptive selection of different magnitude correction signals corresponding
to detected large and small eye movements.
A tracking laser source of specific wavelength is used providing maximal
sensitivity and minimal interference or distortion from the diagnostic
beam, with good image contrast of the intensity feature and the
surrounding background. The image strip of the tracking laser source is
separated from the diagnostic laser source by selected filters and pupil
arrangements.
DESCRIPTION OF THE DRAWINGS
These and other objects, features, and advantages of the invention will
appear from the following description of preferred embodiments of the
invention, taken together with the drawings.
FIG. 1 is a diagrammatic representation of an eye movement tracker
according to the invention and showing the optical light path from the
laser sources to the subject's eye and from the subject's eye to the
detecting element and observer's eye;
FIG. 2 shows the optical arrangement of the beam steering mechanism;
FIG. 3 illustrates the motion tracking of an imaged fundus structure; and
FIG. 4 is a flow chart illustrating adaptive motion tracking according to a
preferred embodiment of the invention.
DESCRIPTION OF ILLUSTRATED EMBODIMENT
The light stabilization device according to the invention will be
understood by reference to a detailed description for a presently
preferred embodiment of the stabilization device incorporated in an
optical illumination instrument which aims a treatment beam of light at
the fundus, which, illustratively, is shown as a helium-neon 632.8 nm red
laser light beam.
Referring to FIG. 1, a device 1 which follows eye fundus movement according
to the invention has a green (543.5 nm) laser energy illumination source
10 which produces a narrow, slightly divergent, highly directed light
output beam 11. Light output beam 11 passes through beam shaping optics 12
which attenuate the beam to an acceptable eye irradiance level and produce
a preferably rectangular beam 2. A second laser source 20 produces a red
beam which is folded into the beam path 2 at point 3 by means of a fifty
per cent deflection element 21 to produce a combined beam 4 in which the
two laser beams are coaxial and traverse substantially overlapping optical
paths to the subject's eye. The locus of points containing these two beams
is denoted as virtual pupil P1. The positions, within the optical setup,
of laser sources 10 and 20 may be interchanged. The combined beam 4
encounters a fifty per cent deflection element 22 whereby fifty per cent
of its intensity is deflected an angle .alpha. and stopped at optical
absorber 23, and fifty per cent is transmitted along beam path 5. The
transmitted light beam along path 5 encounters a red dichroic filter 24.
Ninty-nine per cent of the red component and fifteen per cent of the green
component of beam 5 are deflected an angle .beta. and stopped at optical
absorber 25. The transmitted light beam 6 passes through an achromatic
lens 27 which corrects for the natural divergence of the combined output
beam 6 and produces a slightly convergent beam 7.
Beam 7 enters a two-dimensional image stabilization subsystem 30 indicated
by the dashed line. The preferred system includes two open loop iron-core
galvanometer scanners 31 and 32 oriented orthogonally with their planes of
angular deflection coincident with the xy and yz system planes,
respectively. Both galvanometers 31 and 32 are controlled by signals from
a processing device 40 during an automated iterative correction sequence
described below. The control signals to the galvanometers are shaped to
minimize oscillations in the mirror position after rotation, in a manner
known in the scanning control art. A joystick 41 is also attached to the
processing controller which produces mirror control signals for manual
control of the mirror position. Galvanometer scanning assemblies such as
those manufactured by General Scanning of Watertown, Massachusetts are
suitable for driving and controlling the positions of the mirrors. Lenses
33, 34 between the galvanometers position the axis of rotation of each
scanner mirror at the center of rotation of the virtual eye image, as
discussed below.
Beam 7 is deflected by galvanometer-controlled mirror 31, passes through
lenses 33 and 34, and is deflected by galvanometer 32. It then passes
through lenses 35 and 36 to exit the stabilization subsystem 30 as a
deflected beam 8. The emergent beam 8 passes through lens 13 and into an
image rotator 50. Within image rotator 50 the beam is reflected by
adjustable front-surface mirrors 51, 52 and 53 and emerges from the image
rotator as beam 9 along a path collinear with beam 8. The beam 9 passes
through lens 14 and ophthalmoscopic lens 15, enters the eye and strikes
the fundus F. The fundus scatters a portion 16 of the incident light to
exit from the interior of the eye through the pupil. The scattered light
travels back through the ophthalmoscopic lens 15 and forms an image at
image plane IP1 which in turn is imaged by lens 14 onto adjustable mirror
53 of image rotator 50. The scattered beam 16 is reflected out of the
image rotator by mirror 51 and passes through lens 13. Lenses 13, 36 and
35 image the fundus in planes IP2 and IP3, both of which are conjugate to
the subject's fundus. The reflected beam passes through the
two-dimensional image stabilization subsystem 30 and emerges as a
divergent beam 17. A lens 27 converges beam 17. At dichroic filter 24 the
red component of the reflected beam is ninety-nine per cent deflected an
angle .beta. emerging at the diagnostic output port as a beam 17a. The
green component of beam 17 is eighty per cent transmitted by a filter 24
and fifty per cent deflected by an angle .alpha. by a deflector 22 to
produce a return tracking beam 17b. Beam 17b strikes a green dichroic
filter 28. Eighty per cent of the green component of beam 17b is
transmitted through filter 28 emerging as a beam 17c, and the remainder is
deflected an angle .gamma. as a residual return beam 17d.
Beam 17c enters an image intensifier subsystem 60 indicated by the dashed
line. One preferred intensifier system includes an image intensifier tube
61 coupled by a fiber optics minifier 62 to a two-dimensional
charge-coupled device (CCD) 63. Image intensifiers such as those
manufactured by ITT's electro-optical division of Roanoke, Virginia are
suitable for this low intensity imaging application. The CCD transmits
electronically, serially encoded light intensity as a function of spatial
information to an output processor 40. A lens 64 forms an image IP4 of
beam 17c on the photocathode of the image intensifier tube 61. The image
intensifier amplifies the intensity of the fundus image IP4 with a spatial
resolution comparable with that of the CCD. The fiber optics minifier
couples the light output of the image intensifier to the charge-coupled
device 63, which has an active surface several millimeters square, while
increasing the spatial resolution by reducing the size of image IP4. The
illustrated fiber optic minifier employs a coherent tapered fiber bundle
which permits the quality reduction of high resolution images. One
manufacturer of minifiers is Galileo Electro-Optics of Sturbridge,
Massachusetts. For the CCD, charge-coupled devices and supporting
electronics such as those manufactured by Fairchild of Palo Alto,
California are suitable for this application requiring electronic encoding
of two-dimensional intensity profiles.
The CCD 63 converts the photo-signal to an analog electrical signal which
is clocked into the processing device 40. The processing device analyses
this electrical signal, and provides a correction signal via scanner
control 42, 44 which repositions the front-surface mirrors of
galvanometers 31, 32 and hence repositions the incoming combined
treatment/tracking beam 7 on the eye fundus at F.
Beam 17d deflected from filter 28 encounters a front-surface mirror 29, is
reflected by an angle .delta. and enters an optical viewing subsystem 70.
The optical viewing subsystem contains a lens 71 for focusing the image of
the eye fundus F on the retina of the observer's eye 19. Two filters 72
and 73 together produce a narrow band-pass filter passing yellow optical
illumination light which is provided by an optical illumination source 74.
A small turning mirror 75 injects the yellow observation beam along the
reverse of the optical path just described to the subject's fundus for
observation illumination.
As described in more detail below, apparatus according to the invention
produces a signal at the two-dimensional charge-coupled device 63 with a
signal-to-noise ratio which is comparatively high, especially considering
the high turbidity of the media within the eye through which the optical
tracking energy must pass. The contrast of spatially variant intensity
features which the apparatus is designed to follow is preferably optimized
by employing the tracking laser light source in combination with a
small-aperture collection pupil, thus reducing signal degradation due to
scattered light.
In the above described apparatus, the laser light sources 10 and 20 are
selected to serve distinct purposes. Source 10 provides a small strip of
laser light which is directed onto a particular retinal structure, the
image of which is tracked to provide eye-motion correction signals.
Preferably, the tracking source 10 is a green helium-neon laser from which
energy is available at a single power level with wavelength 543.5 nm.
Green light is useful both for examining the superficial layers of the eye
fundus, and for providing high contrast demonstration of the retinal
vasculature, retinal pigment epithelium, and the choroidal vessels in less
pigmented peripheral fields. There are no operational requirements on the
intensity or wavelength of laser source 20, provided the wavelengths from
sources 10 and 20 are physiologically safe and spectrally separable. For
the purpose of illustration, the diagnostic beam is provided by a red
helium-neon laser source 20. The invention provides a stabilizer for a
wide variety of fundus-illuminating instruments; for other sources,
different filters may be substituted in the optical path.
The geometry and alignment of the tracking assembly will be understood with
reference to FIG. 2. The source beam 10 is positioned on a retinal
structure of choice (denoted by X) by joystick 41 which manipulates the
galvanometer-controlled mirrors 31, 32. Magnified detail view 84 shows the
strip 80 positioned on a vessel denoted X of the fundus F. Once the
structure X is centered in the green tracking strip 80 of the laser source
10, activation of the automated iterative correction procedure is
initiated by pushing a mechanical push button switch on the joystick. Both
laser light sources 10 and 20 follow a coaxial optical path to the eye
fundus, so that corrections made to the position of the beam originating
at source 20 are coincident with corrections made to the beam originating
from source 10. That is, corrective motions of mirrors 31, 32 redirect the
common optical path followed by the combined beam, formed by tracking
source 10, treatment source 20 and observation source 74, discussed above.
In FIG. 1 the two beams from sources 10, 20 are referred to collectively
as a single beam, starting at position 4. Source 20, the diagnostic beam,
is thus stabilized on the eye fundus F. It will be appreciated that source
20 need not produce a beam but, as discussed more fully below, may produce
a pulsed or continuous pattern of illumination which is directed along the
common path. More generally, the eye tracker of this invention achieves
two-dimensional stabilization of an arbitrary light stimulus pattern at
the eye fundus position. The stimulus pattern can be offset by an
arbitrary angle from the tracking target structure of the fundus without
compromising its stability characteristics.
Still referring to FIG. 2, the two-dimensional path corrector of the
presently preferred image stabilizing system is shown in relation to the
subject's eye. The fundus of the subject's eye F is imaged in planes IP2
and IP3, both of which are conjugate to the fundus. The optical system
contains two open loop iron-core galvanometer-controlled mirrors 31 and 32
that rotate in response to signals from the processing device 40. The two
mirror axes of rotation are mounted orthogonally along the x- and y-axes
of the overall coordinate system. In the plane of each axis of rotation is
a virtual image of the center or rotation 19 of the eye, illustrated by
virtual eye images 81 and 82. When the galvanometer-controlled mirrors are
driven by the appropriately scaled correction signals, they redirect the
optical path so as to compensate for the subject's normal eye movements.
The orthogonal orientation of the mirrors provides stabilization in two
dimensions to keep the tracking strip 80 directed on a fixed target on the
fundus and to keep the image of the targeted fundus structure aligned in a
fixed position on CCD 63. The imaging optics include two high quality
relay-lens pairs 33, 34 and 35, 36 with the lens pairs separated by twice
their focal length.
Optimal positioning of the image of the tracking target is achieved as
follows. First, the tracking strip is directed at the fundus and a
suitable tracking target is identified in the small region illuminated by
the strip, which region may be, for example, a rectangular area of
approximately one by five millimeters. The long axis of the strip serves
as a direction-indicating marker which is visible on a macroscopic level
to facilitate the setting up. The tracking target preferably includes two
linear vessels which are spaced close to each other and oriented
substantially orthogonally, with one vessel parallel to the length or
width of the strip. The image rotator is then rotated to align the long
axis of the tracking beam with a scan line (e.g. with the length
dimension) of the CCD, so that the images of the two vessels cut across
the orthogonal scan lines (or extensions thereof) of the CCD. The joystick
is then used to "steer" the tracking beam (hence, also the image of the
tracking structure), so that the two linear vessels move to a position
such that one vessel cuts perpendicularly across each scan line on the
face of the CCD. In this position the scanning planes of the
galvanometer-controlled mirrors are aligned with the x-y coordinates of
the CCD, maximizing the sensitivity of the tracker to eye motion, and
minimizing the chance that a large rapid eye movement will remove a
targeted vessel or other spatially variant intensity fundus structure from
the field of scan.
A number of image rotating devices are known; the preferred embodiment
consists of three front-surface mirrors arranged in the configuration
illustrated in FIG. 1. A surface reflecting device is preferred to an
internally reflecting device such as a Dove prism because the light loss
of a surface reflecting device is less. As shown in FIG. 1, the rotation
axis of the image rotator 50 coincides with the optical axis of the
apparatus.
In addition to beam alignment, the detection of fundus illumination
requires some care. The strip of green light produced by laser source 10
is referred to as the "tracking strip" when shown on the fundus, as
distinguished from the "image strip" which is the green light exiting the
eye and producing an image of the tracking strip at the front surface of
the position-sensitive charge-coupled device 63.
Light is scattered from the eye fundus F in all directions. The low light
levels permitted for illumination of the retina are attenuated further
within the eye, resulting in an image strip which is close to the
intensity threshold of available position-sensitive semiconductor
detectors. The position-sensitive detector electronically scans the image
strip and provides an analog signal containing spatially resolved
intensity information to the output processor 40.
Of the three commercially available position-sensitive semiconductor
devices CCD, CID, and Reticon diode array, the CCD was chosen for its low
noise characteristics during low-light level illumination. The CCD is a
charge-transfer device which permits the movement of packets of
photo-induced electrons from one location in its silicon substrate to
another without losing spatial information. The spatial resolution of such
devices is approximately 13 .mu.m. Such devices have a linear response
over several decades of light intensity. At low light levels the device is
preferably cooled to reduce thermal noise and to improve the
signal-to-noise ratio.
A CCD contains many photo-active sites physically separated and
electronically isolated in a dense array. In this invention a
two-dimensional array is specified. The photo-active sites convert the
incident photons into electronic signals which are subsequently serially
clocked out. Each site contributes a packet of electrons separated from
adjacent packets in time, and a charge-to-voltage amplifier converts the
charge signal into a voltage modulated signal with voltage amplitude
representing light intensity, and its variation in time providing spatial
information. For clarity of illustration, the CCD is treated here as a
black box requiring at its input a spatially resolved image of sufficient
intensity and producing at its output an electronic signal containing
spatial and intensity information of the input.
The low intensity backscattered green image strip is close to the intrinsic
noise level, and the image strip intensity is preferably increased by
directing the two-dimensional image strip onto the photocathode of an
image intensifier (61, FIG. 1), which provides a spatially resolved
reproduction of the original image at a far higher intensity. The
intensified output of the image intensifier couples via a fiber optics
minifier 62 to the front surface of the CCD 63. The signal-to-noise ratio
of the CCD is further enhanced by attaching to its back surface a Peltier
effect thermoelectric cooler 65 which maintains the CCD at a temperature
below ambient. The thermoelectric cooler 65 generates a magnetic field
which interferes with the proper operation of the CCD 63. To permit
shielding against this effect, a suitably thick copper heat conductor 65
connects the two devices a short distance apart, and a thin sheet of mu
metal magnetic shielding (not shown) is then placed between them to block
magnetic fields.
To track eye movement, a presently preferred processing method samples the
image strip intensity distribution along two orthogonal lines (x- and
y-scan lines) each consisting of a single row of pixels.
FIG. 3 illustrates the tracking of the illustrated embodiment and shows one
suitable orientation of the image strip on the face of CCD 63. Two
orthogonal scan lines 91, 92 touch only at one end, and preferably to
maximize their length they form half the perimeter of the rectangular
two-dimensional CCD 63. For purposes of illustration, the number of
spatial cells (pixels) illustrated is substantially less than the actual
number of the CCD. An intensity feature, e.g., a retinal vessel 95 is
chosen such that one portion of the retinal vessel image intersects the x
scan line 92 and one portion intersects the y scan line 91, preferably
crossing the scan lines at right angles. This selection of an intensity
feature and positioning within the tracking strip is accomplished using
the joystick and image rotator as described above, and is done in a manner
which ensures only one intensity extremum per scan line field. The system
magnification and the CCD area are such that a single vessel intersects
each scan line, and there is no other vessel in the field which might
cause the tracker to jump to a different target. The tracking strip is
aimed such that the image of a blood vessel intersects each scan line
once, and only one blood vessel crosses each scan line. Once the desired
orientation is achieved with the image rotator, and the position is set
using the joystick, pressing the push button switch on the joystick 41
locks the extrema positions along the scan lines into memory. Thereafter
the processor 40 scans the photodetector and detects eye movement by
tracking the change in position of the imaged vessel.
By employing a continuous tracking beam which illuminates a region of the
fundus, and electronically scanning a fixed photodetector array, the
illustrated system achieves a scan rate of 1000 or more complete image
strip scans (frames) per second. The electrical processing portion 40
includes timing, memory processing, and control sections 64, 67, 68
discussed below. The clock rates and timing required to achieve the
1000/sec scan rate are known in the photosensing art and are provided by
timing and synchronization circuit 66. The electronic signals proportional
to the light intensity for each one of the pixels of the charge-coupled
device are serially applied to a memory processing and storage circuit 67.
Circuit 67 stores an active two-dimensional array of the electronically
encoded intensity profile, refreshing each memory location as new pixel
data arrives. The memory processing circuit 67 also selects the two
orthogonal lines of pixel data, preferably intersecting at one corner of
the CCD as described above. These two lines coincide with the x- and
y-axes of the overall coordinate system and are referred to as the x- and
y- scan lines, respectively. The x- and y- scan lines are treated
independently. Position information is encoded as a differential. The
intensity profile of each scan line is summed over two half-scans yielding
.SIGMA.x.sub.1, .SIGMA.x.sub.2, for the x-scan line, and .SIGMA.y.sub.1,
.SIGMA.y.sub.2 for the y-scan line. During the automated iterative
correction sequence the ratio .SIGMA.x.sub.1 /.SIGMA.x.sub.2 and the ratio
.SIGMA.y.sub.1 /.SIGMA.y.sub.2 are preserved by controlling the
galvanometer mirrors to redirect the tracking beam. For each of the two
scan lines processing circuit 67 sums the first half of the total pixels
per line (.SIGMA..sub.1), and compares this value with the sum of the
second half of pixels from the previous frame (.SIGMA..sub.2). Control
circuit 68 generates a unit step in the position of the appropriate
galvanometer in response to a change in .SIGMA..sub.1 /.SIGMA..sub.2. The
direction of the step depends on whether the previous frame sum
.SIGMA..sub.2 is greater than or less than the present inferred frame sum
.SIGMA..sub.2 '.
In the preferred embodiment, circuit 68 includes a variable step setting
circuit for providing coarse, medium and fine corrections to the beam
position, so as to achieve a smoother, more continuous tracking action.
This adaptive control signal magnitude setting is preferably accomplished
by a configuration of resettable counters in software.
FIG. 4 shows a flow chart for the step size correction signal determination
according to this aspect of the invention. The | | |
