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Description  |
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FIELD OF THE INVENTION
This invention relates in general to optical instruments and methods, and
more particularly to an instrument for scanning a surface or other
structure with an optical beam, detecting the light emitted from the
structure, and generating either a two-dimensional representation of an
image of the structure or a set of stored data representing such an image.
BACKGROUND OF THE INVENTION
In the art of optical instruments, it is known to scan a surface to be
imaged with a small light source, collect the light reflected from the
illuminated spot and direct it to a detector which provides an output
signal varying in time in correlation with the scanning of the illuminated
spot across the surface. The detector output can be stored in a permanent
storage medium or provided directly to a scanning display device, such as
a television raster or a cathode ray tube display. By synchronizing the
scanning operation of the illuminating source with the scanning of the
display signals, a two dimensional image is produced.
One such instrument is a scanning ophthalmoscope which produces an image of
the fundus of the eye. It has been found that the use of a laser light
source provides improved imaging in an ophthalmoscope. A laser scanning
ophthalmoscope is described in U.S. Pat. No. 4,213,678.
In a device as described in the noted patent, the entrance pupil for the
scanning laser beam has a small cross sectional area within the pupil of
the eye, typically 0.8 mm in diameter, whereas the exit aperture for the
reflected light is the overall pupil of the eye, which typically is nine
mm in diameter. The detector is placed in a plane conjugate to this exit
aperture.
An improved technique is described in pending U.S. application Ser. No.
876,230 filed June 19, 1986 and U.S. application Ser. No. 876,231 filed
June 19, 1986.
SUMMARY OF THE INVENTION
Broadly speaking, in the present invention a confocal scanning
ophthalmoscope which scans along only one coordinate is constructed
utilizing a laser source, an asymmetrical focusing element, such as a
cylindrical lens, together with a deflection galvanometer or other
scanning element for scanning on the same axis for which the asymmetrical
element focuses. The laser beam which is of generally circular cross
section and small compared to the diameter of an eye pupil is directed
onto the cylindrical lens, which focuses on the vertical axis but does not
focus along the horizontal axis so that what is produced at the focal
point of the cylindrical lens is a vertically focused horizontal extended
rectangular beam characterized by a low vertical to horizontal aspect
ratio. In other words it appears to be a horizontal line beam. This beam
is directed by a small turning mirror onto a deflection galvanometer or
other vertical scanning means which scans it along a vertical coordinate.
The scanning beam is directed by means of another focusing element,
preferably a mirror, through the eye pupil and onto the fundus of the eye,
the focal length being arranged such that the beam as it passes through
the pupil is focused to a narrow waist, substantially smaller than the
diameter of the eye pupil, and then expands back to the width of the
horizontal beam for scanning the fundus. The overall input beam system
then scans the line beam vertically over the fundus, thereby scanning an
area of the fundus.
The light reflected from the area of the fundus illuminated by the beam is
collected by the focusing mirror and directed back to the vertical
deflection mirror, which is positioned so that its face is approximately
conjugate with the plane of the eye pupil. The turning mirror is placed in
the center of the reflected beam and since its diameter is very small
compared to the cross sectional area of the beam as it leaves the
galvanometer mirror, it intercepts only a very small portion of the
reflected light. The major portion of this reflected output beam then
passes by the turning mirror to a lens placed at the pupillary conjugate
which focuses it onto a horizontally distributed line of detectors located
at a retinal conjugate plane. The detectors produce a plurality of
electrical signals representing the time variation of light arriving at
each one of the horizontally distributed detectors. This electrical signal
can then be used to develop a raster display or for optical pattern
recognition.
DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 is a diagrammatic representation of one embodiment of a scanning
instrument according to the principles of this invention;
FIGS. 2 and 3 are explanatory ray diagrams of optical beam features of the
embodiment illustrated in FIG. 1;
FIG. 4 is an explanatory ray diagram of optical scan features of the
embodiment of FIG. 1;
FIGS. 5 and 6 are also explanatory ray diagrams of optical beam features of
the invention illustrated in FIG. 1;
FIG. 7 is an explanatory ray diagram of the reflected optical beam of the
embodiment illustrated in FIG. 1; and
FIG. 8 is a diagrammatic illustration of a second embodiment of an optical
instrument constructed in accordance with the principles of this invention
.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows an embodiment of the invention in the form of an
ophthalmoscope. A laser illumination source 11 produces a narrow incident
light beam which passes through an anamorphic beam shaping system 13, 15
which produces a beam focused along a first axis and diverging along an
axis normal to the first axis. This beam, in turn, impinges on a small
turning mirror 14. The mirror 14 directs the incident laser beam onto the
reflecting surface of a galvanometer deflection scanner 17 to produce a
vertical scanning motion. From the galvanometer deflection scanner 17, the
laser input beam is directed onto a focusing mirror 18, for conjugating
the galvanometer 17 to the pupil 19c. The incident beam also passed
through the crystalline lens of the eye 19b.
The reflected light from the fundus 19a is directed back over a common
portion of the foregoing optical input path, which includes focusing
mirror 18, and vertical scanner 17. Both of these common elements can be
mirrors and hence do not contribute reflections of the input beam back to
the detector as noise background. The reflected output beam from the
scanner 17 in large part passes by the turning mirror and hence separates
from further traverse along the input optical path. Instead the output
beam is directed through focusing lens 20 onto an optical detector array
21.
The detector 21 is electrically connected to an electrical instrumentation
unit 22 which provides electrical control signals to the laser source 11
and electrical drive signals to the scanning deflection element 17. In
essence, the instrumentation unit provides synchronization of the signals
received at the scanning element 17 so that the temporal order of the
signals produced by the detector 21 can be correlated with the location of
the scanned incident laser beam on the surface of the fundus. The detector
21 is a multi-element detector having discrete detection elements
dispersed along the horizontal axis. It responds to incident light by
providing from each discrete horizontal element a time varying electrical
signal. These signals are provided to a signal processor 16, which
processes the data representing simultaneous reflections from a
horizontally extended beam correlated with the variation in time as that
beam is scanned vertically to produce signals suitable for creating a
raster display. The control and synchronization which the processor and
instrumentation unit provide enables a display device 23, such as a
television raster device, to form a two-dimensional display of an image of
the eye fundus 19a, in response to the reflected optical energy it
receives. The detector signal may be applied to a long term storage
element 24, such as a video tape recorder, for subsequent readout and
display. Alternatively the output signal may be compared to predetermined
patterns of signal for eye identification, disease screening or the like.
These patterns may be stored time varying signals from specific detector
locations. For a description of a suitable electrical timing and control
circuit, reference is made to U.S. Pat. No. 4,213,678 which is
incorporated herein by reference. If the detector is a charge coupled
detector, it may integrate for only 63 microseconds. This requires
different, but well understood, timing circuitry.
THE LASER GENERATOR
The laser 11 can be any suitable laser light source which provides emission
at frequencies yielding appropriate contrast for the fundus, or other
target. Typically, the laser 11 is an Argon-Krypton laser or Helium-Neon
laser operated at a power level to produce an illumination irradiance of
one hundred microwatts per square centimeter or substantially less at the
fundus. The laser 11 may also be selected to emit in the infrared
wavelength region to provide a scanning beam which is not perceptible to
the subject. For these irradiances the eye pupil need not be medically
dilated to obtain an imaqe of the fundus. For color imaging two lasers of
different wavelengths may be employed and converted into a single beam
with a dichroic beam splitter.
THE INPUT OPTICAL SYSTEM
The purpose of the input optical system is to scan the fundus along a first
axis with a rectangular optical beam having a low "height to width" aspect
ratio to illuminate a "vertical" sequence of these line-like rectangular
areas across the fundus surface in a known pattern so that the reflected
light detected in time sequence can be electrically converted to a
two-dimensional representation of the reflection characteristics of the
fundus. Of course the first axis could be horizontal so that it would be
the "width to height" aspect ratio which would be low. In one illustrative
instrument, the input optical sytem forms the incident laser beam with a
cross sectional area of substantially 0.9 mm diameter at the entrance
pupil of the eye and focused on the fundus to produce an illuminated
segment approximately twelve microns by 6 mm.
The vertical scanning motion in the illustrated preferred embodiment is
introduced by a deflection galvanometer 17 that provides a scan action
which corresponds with the television vertical scan of 60 Hz. Galvanometer
controls, such as those manufactured by General Scanning of Watertown,
Mass., are suitable for driving and controlling the position of th
galvanometer mirror. The mirror 17 can, for example, be a type G120D or
G325D General Scanning mirror. The deflection galvanometer could be
replaced by a slow rotating polygon.
The shaped laser beam must be in (vertical) focus at the retina, and the
scan waist must be located (approximately) at the pupil of the eye. Under
these circumstances the beam cross section on the retina is appropriate
for the available resolution, and the image will appear in focus at the TV
screen even if it is not in focus at the confocal aperture. It is the
focus of the incident beam which determines the picture's resolution and
the focus of the return beam (at the confocal stop) which controls
contrast. The system, however, is confocal only in the scanning (vertical)
dimension, hence the statement applies only to that dimension. The fact
that these controls are largely orthongonal is what allows flexibility as
to mode of view.
The turning mirror 14 preferably is a stationary mirror reflector. It is
small in size in order to produce a minimal shadow in the output beam, and
hence preferably is only large enough to intercept the input beam which
the focusing element 13 and cylindrical lens 15 direct, via the turning
mirror, to the scanner 17. In the configuration shown the turning mirror
acts as the beam separator between the input and reflected return beam.
FIGS. 2 and 3 illustrate features of the input optical system. FIG. 2
represents the vertical aspect of the input beam with the scanner 17
assumed to be stationary in a neutral, non-deflecting, position. The
narrow collimated incident beam from the laser is shaped by lens 13 and
directed onto cylindrical lens 15. The cylindrical lens is positioned such
that it focuses on the vertical axis (which is the axis illustrated in
FIG. 2). The focused beam from the cylindrical lens 15 is then reflected
from turning mirror 14 onto deflecting galvanometer mirror 17 which
directs it onto the face of relay mirror 18 which focuses the cross
sectional beam on the retina 19a of the eye 19. It will be understood
that, while the scanning axis is the vertical axis and the extended beam
from the cylindrical lens is horizontal, this is an arbitrary choice, and
the system could be arranged in the opposite fashion.
FIG. 3 is again a beam diagram of the same optical configuration as FIG. 2,
representing however the view along the horizontal axis. Thus, along this
axis, the beam from the cylindrical lens 15 is focused on the galvanometer
reflecting surface 17 and on the pupil of the eye. (Thus, while in FIG. 2,
the foci are at the optical conjugates of the retina, in FIG. 3 they are
at the conjugates of the pupil.) The turning mirror 14 which is small
compared to the pupil of the eye, typically being less than 0.9 mm, is
positioned sufficiently close to the cylindrical lens so that the
horizontal extension of the beam location of the turning mirror is not
greater than the dimension of that mirror. What is reflected from the
turning mirror 14 is then, in the horizontal dimension, an extended line
which is in turn focused by the relay mirror 18 onto the eye's pupil 19c.
It spreads into a line at the retina. The beam cross section as it arrives
at the retina has a generally rectangular shape with a very low aspect
ratio of vertical dimension to horizontal dimension (a horizontal line).
This horizontal line beam is scanned in a vertical direction over the
retina surface by the action of the deflecting galvanometer mirror 17.
Since the line at the retina may have a gaussian profile, it will be
necessary to put in a stop at 18a to give it crisper ends.
FIG. 4 which represents scan features of the input system, illustrates the
input beam instantaneously as a single ray which the scanning element
moves in the vertical direction as a function of time. The drawing shows,
in effect, the time exposure on the vertical axis which, for the scanned
input beam includes the entrance pupil. The scan angle is the full angle
of this envelope in the plane of the scan.
The mirror 18 is spherical and large so that even at f/2 (for the scan) the
eye's pupil is far back from the optics. With human subjects there are
some inflexible dimensions. The mirror is spherical because no aspheric is
correct for both beam and scan systems at all points. That constraint can
be understood by noting that the beam on one side of this mirror may be
always collimated, no matter where it hits the mirror. So the mirror must
have everywhere the same local curvature-which implies a sphere.
THE OUTPUT OPTICAL SYSTEM
As described, a major portion of the output optical system has a common
optical path with the input system. This common path includes both the
scanning element 17 and the focusing mirror 18. In the output system light
reflected from the galvanometer mirror 17 passes around the turning mirror
14 and is incident on the detector system which includes lens 20 and
detector 21.
FIG. 5 represents the output beam along the vertical axis in the same
manner as the representation in FIG. 2, while FIG. 6 represents that same
output beam along the horizontal axis in the same manner as FIG. 3.
As illustrated in FIG. 5, the reflected beam from the fundus has an exit
aperture large compared to the vertical dimension of the scanning beam,
preferably substantially the entire pupil of the eye, with a diameter of
as much as nine mm. The image of this aperture at its conjugate plane also
is nine mm. Absent magnification, the reflected output beam from the
illuminated area on the fundus likewise is approximately nine mm in
diameter at any conjugate of the exit pupil, which is where the scan
element 17 is located.
In this configuration the central region of the eye's pupil is used as an
entrance pupil and the remaining annulus an exit pupil, thus conforming to
Gulstrand's principle. This means that scanner 17, optically conjugate to
the pupil, needs to be big enough to intercept that larger return beam.
For the vertical scanner which moves as a 60 Hz sawtooth, a 10-15 mm
mirror is suitable.
The ophthalmoscope can have a small entrance pupil, as described above, due
to the large radiance of the incident beam. The output beam, however, has
relatively low radiance, and hence the provision of this large output
pupil is desired to collect a maximal amount of output light energy. The
large exit aperture hence enhances the high efficiency of the instrument.
It also facilitates viewing a large portion of the eye fundus.
FIG. 5 also illustrates, with exaggerated scale, that the output beam
passes around the turning mirror 14, which hence casts a small shadow
generally of low significance.
It is desirable to separate the incident and return beams as close to the
scanning mirror as possible in order to place the incident beam in the
center of the return beam and thus stop direct reflection from the cornea
(and spectacles if desired) from reaching the detector.
FIG. 6 illustrates the reflected beam from the fundus along its horizontal
axis. In both figures the field lens 20 is placed at the pupillary
conjugate plane while the detector 21 is placed at a retinal conjugate
plane. Thus the image of the retina at the plane of the detector 21 is the
portion of the illuminated area which at any instant in time has an
extended width and a very low height. An ideal detector 21 is then an
array of very small discrete elements dispersed horizontally and having a
low vertical height. One suitable detector for this configuration is a
series of charged coupled detectors providing, for example, 512 discrete
horizontal elements. The output signals are then taken in parallel from
each of the elements. The time variance at each element represents the
change in the retinal image as the line of illumination is scanned in the
vertical direction. The output electrical signals can be transmitted to a
processor 16 which can transfer the processed information into a storage
unit 24, to a display 23, which would typically be a television raster, or
to further pattern recognition means.
FIG. 7 is a ray diagram of the scanning envelope of vertical dimension of
the output reflected beam.
Although specific block diagrams have not been provided for the circuitry
components and for the process and logic, it is believed that
synchronizing the raster scan with the galvanometer mirror oscillation and
the processing of the time variant signals to produce a raster scan is
well known to those skilled in the art. Reference is also made to
copending U.S. application Ser. No. 876,230, which is incorporated herein
by reference.
The system described herein has many of the advantages of a double scanning
confocal ophthalmoscope. It is confocal in one dimension, and has the
advantage of using the identical optical path for the reflected beam,
which is descanned at the reflecting galvanometer mirror. The positioning
of the turning mirror as a small centrally located mirror in the reflected
beam provides that very little light intensity is lost and that corneal
reflections are blocked. Since the contrast enhancement is in the ratio of
observed to illuminated retina, this system improves contrast by 512,
while a fully double scanning optimum improves it by (512).sup.2. One
clear advantage of the system as illustrated is the simplicity and cost
effectiveness resulting from including only one scanning element.
While the embodiments as described have generated a rectangular raster
scan, it is possible by employing a rotational optical element to generate
a polar scan rather than a vertical deflection. Thus, in a configuration
is shown in FIG. 8 in which the vertical galvanometer is replaced with a
dove prism or a Dove mirror (the mirror analogue of a Dove prism) which is
rotated at a predetermined speed to produce at the retina, a polar
scanning line shaped beam and, at the output detector, a signal which
varies in time in accordance with the polar scan.
While the system has been described in terms of presenting a visual image
of the fundus. The apparatus has other uses, for example, for eye
recognition, the detected information in either the polar scan or the
rectilinear scan configuration can be matched against previously recorded
information for an individual retina, thus providing determination of the
identity or lack of identity of the person. Similarly, patients can be
screened to determine whether there are specific characteristics of the
retina indicating broad categories of disease, or change of condition. In
these applications information developed by the detector and processor
would be either visually screened or processed electronically to determine
whether specific areas of the retina are characterized by specific images
or changes in images.
Other embodiments of the invention including modifications of and deletions
from these disclosed embodiments will accordingly be apparent to those
skilled in the art.
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