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Description  |
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BACKGROUND OF THE INVENTION
The invention relates to techniques for examining objects that are subject
to small movements which interfere with the examination process,
particularly when magnification is used. For example, when structures
within the eye are examined at high magnification, the limiting factor
often is the rapid, involuntary motion of the eye. Not only do such small
eye movements interfere with detailed examination, they also frustrate the
accurate focusing of magnification devices, such as a specular microscope
and, therefore, make flash photography difficult. Furthermore, in
treatments such as laser photocoagulation of the retina, and in laser
treatments of the trabecular meshwork, eye movements can interfere with
effective treatment.
A number of optical instruments such as ophthalmoscopes, biomicroscopes, or
specular microscopes are used for examination of detail within the eye.
For low magnification applications, such as the indirect ophthalmoscope,
small eye movements, of the kind which are involuntary and cannot be
controlled even by a cooperative patient, tend to be of little
consequence.
However, for instruments with higher magnification, such eye movements
become increasingly troublesome as the magnification is increased. These
small, sudden eye movements often occur at the rate of about one per
second. The excursion may be only a few minutes of arc, but this can cause
severe blurring of an image of corneal cells, for example. Moreover, the
changes in the cornea position with such movements tends to defocus the
image of interest.
Various techniques have been proposed in the past for decreasing the motion
of the eye, or reducing its deleterious effect on detailed examinations.
For example, one method is to contact the cornea with a so-called dipping
cone objective. The outermost element of such an objective has a flat,
polished glass surface which, when in contact with the cornea with a
slight pressure, substantially prevents motion of the cornea along the
examination axis and inhibits, although it does not completely eliminate,
rotational motion of the eye. This contact can be uncomfortable for the
patient, and can cause slight abrasion and consequent adverse effect on
the quality of the images which can be obtained. It is also common
practice to use diagnostic contact lenses, which are generally handheld in
contact with the cornea and, as a result, do not move with the eye as it
rotates. Their primary function has been to allow the fundus of the eye
and certain other interior regions to be viewed at low magnification, e.g.
through the biomicroscope. Such lenses may reduce the eye motion somewhat
due to the contact with the eye and the inertia of the contact lens.
However, when the eye moves, the image seen through such a lens still
moves.
Another proposal has been to use a contact lens designed to move with the
eye during its small, frequent rotation, and designed to form a virtual
image of the plane of interest at the center of rotation of the eye.
Because such a virtual image is located at the eye's center of rotation,
the image will not translate either laterally or longitudinally as the eye
rotates, so long as the contact lens moves with the eye. This virtual
image is then reimaged by a stationary optical system for visual
examination or photography. The plane of interest may be the endothelial
or epithelial cell layers of the cornea, the epithelial cell layer of the
lens, regions within the depth of the lens or the vitreous, or the retina.
The optical power of the contact lens, along with its thickness, determine
the depth within the eye of the plane which is imaged. This proposal is
described in a prior U.S. patent application of the inventor herein, filed
on Apr. 14, 1980 and now issued as U.S. Pat. No. 4,410,245, dated Oct. 18,
1983, hereby incorporated by reference. While this technique helps reduce
the effect of involuntary eye movements, difficulties can arise in keeping
the eyelids from interfering with the positioning of the contact lens on
the eye. A mild suction can be applied to keep the contact lens firmly
attached to the cornea, but this can introduce other complications. The
system relies on the center of rotation of the eye, which may be at
different locations in various patients, and which may have a slightly
different location in a given eye, depending on the nature of the eye
motion. Other difficulties can arise in that the optics of the contact
lens, being fixed with respect to the patient's eye, may not remain
centered with the optical system of the viewing device, due to patient
movement.
In contrast with the known prior art proposals, this invention provides an
apparatus and a method which stabilize the image of an object such as the
eye, when the motions of that object are small, and when the object can be
contacted. Exemplary apparatus embodying the invention includes a contact
element structure comprising a contact element that contacts the eye and
moves with the eye, and a lens that is fixed with respect to the contact
element and forms an image of a selected region of the eye. The contact
element structure is supported to pivot freely about a point outside the
eye, and the image that forms is stabilized in that it does not move
laterally with said small eye movements. Of course the image will, in
general, rotate with the rotational motion of the image stabilizing
system. But the image has no lateral motion; points located away from the
center of the image will have a slight longitudinal motion either toward
or away from the observer. When the rotational motions are small, these
longitudinal motions will generally be negligible.
Thus, one of the advantages of using the invention is that there is no need
to place a contact lens on the eye. Another is that the center of rotation
of the contact element structure is established by the invented apparatus
rather than by the eye structure of the particular patient. Therefore, the
image is in a predetermined, stable position, and the optics remain
centered with respect to the examining instrument. A further advantage is
that in examining the fundus of patients with various refractive errors,
image stabilization can be achieved by adjustment of the position of a
lens. Numerous other advantages will become apparent to those skilled in
the art from the remaining disclosure herein and the illustrative
embodiments of the invention.
SUMMARY OF THE INVENTION
The invention concerns a system for forming a stabilized image of a region
of a patient's eye comprising a support and a contact element structure
which is supported by the support so as to move with small rotational eye
movements transverse to an optical axis about a pivot point which is
outside the eye and defined by the intersection of two axes orthogonal to
the optical axis. The contact element structure comprises a transparent
plate having a surface shaped so as to move with a rotational action in
response to the eye movements and positioned so as to contact the cornea
of the eye or a lens or film covering the cornea of the eye and form the
stabilized image wherein the image does not move with the eye movements.
In the preferred embodiment, the contact element structure further
comprises a lens, such as a positive or negative objective lens.
Additionally, the system further comprises means for illuminating a
selective region of the eye with a light beam which passes through the
contact element structure and continues to strike the selective region
despite the eye movements.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates schematically a nonlimiting example of utilizing the
invention as an image stabilization system for examining the cornea.
FIG. 2 shows a portion of the system shown in FIG. 1 and illustrates
pivoting of a contact element structure with eye movement.
FIG. 3 illustrates another nonlimiting example embodying the invention as
an image stabilizing system for the fundus.
FIG. 4 illustrates pivoting of the contact element structure of FIG. 3 with
eye movement.
FIG. 5a is a partly sectional and party elevational view of an exemplary
eye examining system in accordance with the invention suitable for the
cornea.
FIG. 5b is an elevational view showing the arrangement of gimbal rings and
bearings.
FIG. 5c is similar to FIG. 5a, but with the addition of a centering key.
FIG. 6 is a sectional view of the contact element structure of an exemplary
image stabilizing system suitable for the fundus.
FIG. 7 is a sectional view of the contact element of an exemplary image
stabilizing system with an adjustable lens position, suitable for
examining eyes with various degrees of refractive error.
FIG. 8 illustrates a nonlimiting embodiment of the invention in which a
laser beam is direct through the image stabilizing system to a region
within the eye, while at the same time permitting the examiner to monitor
the location of the laser beam in the eye.
FIG. 9 illustrates another example of the invention in which an image is
presented to the eye of an observer through the optical system of the
invention so that the image is stabilized with respect to the retina of
the observer.
FIG. 10 is an illustration of an embodiment of the invention in which the
image of the posterior capsule of the eye lens is stabilized.
FIG. 11 illustrates the use of the invention for stabilizing the image of
the angle of the anterior chamber of the eye.
FIG. 12 illustrates the application of the invention to examination of
objects other than the eye. In studies on animals the motion of the tissue
due to respiration and/or heartbeat can sometimes interfere with detailed
observation or measurements.
FIG. 13 is a further embodiment utilizing a positive lens.
FIG. 14 illustrates an extension of the embodiment of FIG. 13 on which the
stabilized image is not located at the pivot point.
FIG. 15 illustrates still a further embodiment of the invention for the
posterior portion of the eye.
FIG. 16 illustrates an embodiment of the invention which does not use
lenses.
FIG. 17 illustrates an alternative form of the embodiment in FIG. 16 with
the addition of a negative lens surface.
FIG. 18 illustrates still another alternative configuration of the
embodiment in FIG. 16 with the addition of a negative lens.
FIG. 19 illustrates an image stabilization system which incorporates a
contact lens for placement on the cornea of the eye.
FIG. 20 illustrates an image stabilization system which incorporates a low
vacuum system.
DETAILED DESCRIPTION OF THE INVENTION
The invention concerns a system for forming a stabilized image of a region
of a patient's eye comprising a support and a contact element structure
which is supported by the support so as to move with small rotational eye
movements transverse to an optical axis about a pivot point which is
outside the eye and defined by the intersection of two axes orthogonal to
the optical axis. The contact element structure comprises a transparent
plate having a surface shaped so as to move with a rotational action in
response to the eye movements and positioned so as to contact the cornea
of the eye or a lens or film covering the cornea of the eye and form the
stabilized image wherein the image does not move with the eye movements.
In the preferred embodiments, the contact element structure further
comprises a lens, such as a positive or negative objective lens.
Additionally, the system further comprises means for illuminating a
selective region of the eye with a light beam which passes through the
contact element structure and continues to strike the selective region
despite the eye movements.
In one exemplary embodiment, best suited for examining the cornea, a
stabilized image is formed at the pivot point. When the eye moves, the
contact element structure follows its motion by rotating about its pivot
point, but the image stays substantially stationary at the pivot point,
and can be examined with an instrument such as a biomicroscope focused at
the pivot point. Another exemplary embodiment is best suited for examining
the fundus, and forms the stabilized image at a region anterior to the
center of rotation of the eye, and an examining instrument can be focused
at the image. The longitudinal position of the image must be chosen so
that it is stationary when the eye and the contact element move. In
principle, the refractive error of the eye will determine the precise
optical characteristics of the image stabilizing system necessary to
produce a stationary image. It has been found that a lens having an
adjustable position can achieve a stabilized image for a range of
refractive errors.
In a further embodiment the contact element structure includes a contact
element that contacts the eye and moves with the eye, and a lens that is
fixed with respect to the contact element and forms a real image of a
selected region of the eye. As with the previously discussed embodiment,
this embodiment is also best suited for examining the cornea, and the
stabilized real image is formed at the pivot point. When the eye rotates,
the contact element follows the cornea, and since the contact element and
the lens both pivot about the pivot point the image remains at the same
position in the image plane. The image remains in focus at the same plane
as long as the contact element remains in contact with the cornea. The
image plane of course will rotate with respect to the axis of a
biomicroscope used to examine the eye, but the image will not seriously be
affected by small eye motions.
In yet another embodiment the contact element structure is a thick layer of
a material such as glass or plastic, and no lenses are used. This simple
system is best used for stabilizing the image of the cornea (epithelium,
stroma, or endothelium) so that it may be examined by a microscope. The
material of the contact element is desirably selected to have an index of
refraction which allows the pivot point to be located approximately at the
center of the block. This allows the block to follow eye movements more
precisely, but the advantage of having the pivot point near the center of
the block is lost if the material also has a high density.
In yet another embodiment the contact element structure is made up of a
solid block of material as previously discussed but includes a negative
lens. By the addition of a negative lens to the viewing end of the block
the pivot point can be moved to the center of the block or even further to
the right. The negative lens can be ground and polished into the material
of the block, or a glass lens of an appropriate power can be cemented to
the end of the block. The glass lens provides the advantage that a
multi-element lens can be used to correct for chromatic aberrations
introduced by the thick block of refractive material.
Referring to the schematic representation of FIGS. 1 and 2, a contact
element structure generally indicated at 10 contacts a patient's eye and
moves, with small eye movements, about pivot point P which is outside the
eye, and forms a stabilized virtual image V of an object 0. The virtual
image remains substantially fixed with respect to the pivot point, and for
practical purposes does not move with movements of the imaged object O,
such as the movement thereof illustrated in FIG. 2. A microscope
schematically illustrated at M can therefore be focused on that virtual
image, and object 0 can thus be reimaged for viewing by the observer's eye
indicated at E. In this case the contact element structure 10 comprises a
contact element A which is a flat glass plate that touches the eye and
moves with it, a negative lens L positioned as illustrated, and a support
structure sleeve SS which rigidly connects the contact element and lens to
each other. The support structure sleeve is pivotally mounted on another
support, not shown in FIGS. 1 and 2, which allows it to pivot freely about
point P which is on the optical axis 12 and is intermediate contact
element A and lens L. Referring to FIG. 2, when the patient's eye S is
rotated as indicated about its center of rotation C, the contact element A
has remained in contact with the cornea, and the contact element structure
10 has correspondingly rotated about pivot point P. Although the object 0
has moved as indicated, the virtual image V remains at the pivot point P,
and microscope M can remain focused at the pivot point, thereby allowing a
detailed and accurate examination of object O (e.g., the corneal
epithelial or endothelial cell layer).
In using an exemplary embodiment of the invention to examine the ocular
fundus, as illustrated in FIGS. 3 and 4, the contact element and the lens
can be combined into a single plano-concave lens L, which contacts the
cornea as illustrated and is pivoted, by means not shown in FIGS. 3 and 4,
about a pivot point P located some distance from the negative lens along
the optical axis 12. In this case the position of the virtual image V of
object 0 is in the eye and anterior to the center of rotation of the eye.
Also in this case the pivot point is shown to be located within the
stationary microscope viewing system, M. The precise location of the pivot
point and the required power of the negative lens can be calculated by the
method described below.
A particular and nonlimiting example of apparatus using the invention for
cornea examinations is illustrated in FIGS. 5a-c, wherein components
corresponding to what is shown in FIGS. 1 and 2 are identified by
corresponding reference numerals or letters. In FIGS. 5a-c, contact
element structure 10 again includes a contact element A and lens L but,
instead of the conical sleeve SS shown in FIG. 1, they are rigidly
connected to each other by means of a step-down sleeve 14 serving the same
function. Step-down sleeve 14 can be adjusted longitudinally (along axis
12) by sliding it within ring 16. This adjustment serves to locate the
pivot point at the optimum position. In order to allow contact element
structure 10 to pivot freely about point P with small eye movements, ring
16 is mounted in a gimbal bearing comprising rings 16 and 18, along with
bearings 21, 22, 24, and 26. The gimbals permit free rotation about two
orthogonal axes. For purposes of illustration cone bearings are shown in
FIG. 5b. Support post 20 remains stationary during an examination. A
stationary microscope can be supported at a position stationary with
respect to support post 20, and along the same optical axis 12 which
passes through pivot point P, and can be used to examine the stationary
image.
Illustrated in FIG. 5c is a centering key 30, supported on an extension 20'
of stationary post 20 to pivot about a horizontal axis 32. Centering key
30 includes a downwardly projecting portion 34 which moves forwardly when
the horizontal portion of the centering key is pushed down, and engaged
gimbal bearing rings 16 and 18 so as to center contact element structure
10 along optical axis 12. When the horizontal portion of the centering key
is then moved up, either manually or by means of a spring bias (not
shown), contact element structure 10 is again free to pivot about point P.
In operation, the centering key is pushed down to center the contact
element 10 on the optical axis, and contact element A and the patient's
eye are brought into contact as illustrated in FIG. 5a. While the
centering key is depressed, the microscope, together with the image
stabilizing system, may be adjusted in position so as to bring details of
interest into the field of the microscope. When this has been accomplished
the centering key may be released to allow the contact element structure
10 to pivot freely. The stabilized virtual image of the selected corneal
layer is then viewed through a microscope, or a similar instrument focused
at pivot point P.
An exemplary contact element structure for use in fundus examination is
illustrated in FIG. 6, where again elements corresponding to those shown
in FIGS. 3 and 4 are identified by the same reference numerals or letters.
In FIG. 6, contact element 10 comprises a plano-concave lens L at the
smaller end of a step-down sleeve 14. The contact element structure shown
at FIG. 6 is supported for pivoting about point P by a gimbal arrangement
which is not shown expressly in FIG. 6 but matches both in structure and
function that illustrated in detail in FIGS. 5a-c, and uses a
corresponding centering key and a corresponding microscope or similar
instrument. A counterweight ring is illustrated at W.
An exemplary contact element structure for use with eyes having various
refractive errors is illustrated in FIG. 7. The plano-concave lens L is
mounted in a sleeve T that can be adjusted in longitudinal position (along
axis 12). In position L the lens is located so as to form a stabilized
image of the fundus of a patient with no refractive error. In position M
the lens forms a stabilized fundus image of a myopic patient, and in
position N the lens forms a stabilized fundus image of a hyperopic
patient, e.g., an aphakic patient. A counterweight, W, is adjustable in
longitudinal position (along axis 12) to maintain balance for various
positions of the lens.
FIGS. 8 and 9 illustrate how the invention can be used in reverse, that is,
to send light into the eye in such a way that it travels to the same point
in the eye regardless of small eye movements. In the example shown in FIG.
8 a laser shown at A generates a light beam, B, that is directed by mirror
M into the image stabilizing system. The image stabilizing system in this
case comprises lens L mounted as earlier described so that it contacts the
cornea of eye S, and so that it pivots about point P. In this example, the
laser beam is directed by mirror M toward the virtual image V of the
fundus. This assures that beam location on the fundus will be stabilized.
The actual path of the beam after passing through the lens L is shown by
the solid line. M represents a microscope such as the biomicroscope, used
by the observer E to examine the eye S and to monitor the position of the
laser beam on the fundus.
A principal application of the example in FIG. 8 is laser photocoagulation
of the retina, for treatment of detached retinas, diabetic retinopathy,
macular degeneration and certain other conditions. However, laser beams
may be directed into the eye for diagnostic purposes as well. Laser
Doppler velocimetry is a recently developed technique that measures blood
cell velocity in selected retinal vessels. It requires that a low power
laser beam be directed at a specific retinal vessel and remain at a fixed
position on that vessel long enough to detect the light scattered back to
detectors. Eye movement is a critical limitation on the technique. It is
believed that this invention can ease the problem of directing the laser
beam to a specific blood vessel and maintaining its position on that
vessel.
FIG. 9 illustrates use of the invention to project a stabilized light
pattern or image on the retina of the subject's eye, S. Light from a
source, F, is collected by a condenser lens C, and illuminates a target or
reticle denoted by R. Light from this target is directed by objective lens
J to a focus at the virtual image of the retina at V. As in the previous
example, the image stabilizing system comprises the lens L mounted as
earlier described so that it contacts the eye S and pivots at point P.
Since the retinal image at V is stabilized by the action of the image
stabilizing system, the pattern of light from the target will be
stabilized on the retina.
FIG. 10 illustrates the use of one form of the invention for examination of
another region within the eye, the posterior capsule, B, of the lens. In
cataract surgery, one technique is to remove the cataractous lens material
while leaving the posterior portion of the capsule in place. A possible
complication of this technique is the later opacification of this capsular
membrane. It is believed that this invention can help both in the
examination related to this condition and in the laser treatment.
The system in this case comprises positive lens L1 contacting the cornea,
negative lens L2, and housing H that fixedly connects these two optical
elements and that pivots about P. Letter I designates the iris of the eye.
M is the microscope used by observer E to examine the stabilized image,
which in this case, appears at virtual image V, anterior to the cornea.
FIG. 11 illustrates the use of the invention for examination of the angle
of anterior chamber (AC) of the eye. The angle is, generally speaking, the
narrow region between the peripheral cornea, K, and the iris, I, and
includes the trabecular meshwork, the structure through which aqueous
humor flows in order to exit from the eye. The image stabilizing system in
this case comprises contact element A, which contains mirrors Ml and M2,
and attached to which is positive lens L1, also negative lens L2, and the
housing H that holds the previously mentioned elements and that pivots
about point P. Microscope M is used by observer E to examine the virtual
image. In both FIGS. 10 and 11 a laser can be combined with the image
stabilizing system, in the manner described in connection with FIG. 8, for
purposes of laser treatment.
In order that the contact element move precisely with the eye as the eye
undergoes its small rotational motions, the contact element should be
pressed against the cornea with a sufficient but not excessive force. Such
a force can be provided by the operator as he moves the instrument,
including the contact element, into contact with the eye. A preferred
method is to mount the contact element and the gimbal arrangement on a
spring loaded linear bearing (not shown). When the contact element is
placed against the cornea, the spring tension will determine the force on
the cornea; this can be pre-adjusted to an appropriate level, strong
enough to assure accurate tracking of the contact element with the cornea,
but not so strong as to cause indentation of the cornea or other adverse
effects to the cornea.
It has been assumed that contact of the distal end of the image
stabilization system to the cornea will be sufficient to assure that the
image stabilization system follows the motion of the eye. In some
circumstances it may be desirable to place a contact lens on the eye and
to have the image stabilization system touch and move with the contact
lens. One reason would be to protect the cornea against a possible corneal
abrasion if the patient should move suddenly. If the epithelial cell layer
is being examined the contact lens would protect the cells that are being
examined, and it may help them to become more visible (as in the case of
specular microscopy of the epithelial cell layer). Second, a contact lens
material (or surface treatment) may be chosen that provides a more secure
grip to the image stabilization system then does the cornea with its tear
film. Presumably the contact lens would move with the eye because of the
large area of contact between the eye and the contact lens. If necessary,
a low vacuum could be applied under the contact lens to assure that it
moves precisely with the eye. Third, it may be desirable to mechanically
connect the contact lens to the image stabilization system. One way would
be to bond the external surface of the contact lens to the distal element
of the image stabilization system (FIG. 19). Another possibility would be
to incorporate a low vacuum system into the contact element of the image
stabilization system (FIG. 20).
FIG. 12 illustrates the use of the image stabilizing system with a specimen
R that is vibrating or otherwise moving through small excursions in
directions perpendicular to the axis X. An example would be the ear or
other structure on the surface of an animal, where respiration and heart
pulse will cause motion of the structure being examined. The contact
element A, shown as a thin transparent member, contacts the specimen and
moves with it when the specimen translates. Motion of the specimen in the
direction of axis X is prevented by the contact element. The lens L and
the contact element A are mounted in a housing H, which is free to pivot
at point P. Observer E views the specimen through microscope M, focused on
the virtual image of the specimen at V, generally located at the pivot
point P. The contact element A may be omitted in cases where the specimen
can be contacted by the housing.
The position of the pivot point and the required power of the lens are
found as follows. For the configurations shown in FIG. 1 and 2, the
procedure is straight forward for the examination of the cornea or other
object plane very close to the contact element. The location of the pivot
point is chosen to be at a convenient distance from the eye, typically 20
to 50 mm. The power of the negative lens is then selected so that the
virtual image is formed at the pivot point. This virtual image will then
be stationary and can be viewed through a stationary biomicroscope or
other examining instrument.
For the examination of the fundus the calculation is more involved. First,
the object (the fundus) is moving with a different velocity and direction
than the contact element. Second, as the cornea and the contact element
each rotate about their respective centers of rotation, an effective prism
is formed. For example, if the cornea and contact element touch at just
one point, then the tear layer between the two surfaces forms a prism. The
angle of the prism increases as the rotational motion of the eye
increases. As in the previous design, the first step is to select a
convenient pivot position relative to the eye. With the eye and the image
stabilizing system rotated slightly from their original, aligned
positions, a ray is traced from the posterior pole of the fundus, passing
through the center of rotation of the eye, through the prism formed at the
cornea/contact element interface, and through the optics of the image
stabilizing system. When this ray emerges from the image stabilizing
system, it is extended backwards toward the fundus until it crosses the
axis. The optics of the image stabilizing system are selected so that the
virtual image is formed at the intersection point. When this is
accomplished the virtual image of the posterior pole will be stationary.
FIG. 13 illustrates a contact element structure indicated at 10' which
contacts a patient's eye and moves, with small eye movements, about pivot
point P' which is outside the eye, and forms a stabilized real image V' of
an object. In this embodiment the contact element structure 10' is made up
of a contact element A' which is a flat glass plate that touches the eye
and move with it, a positive lens L', and a support structure sleeve SS'
which rigidly connects the contact element and lens together. The support
structure sleeve is pivotally mounted to a support (not illustrated) which
allows it to pivot freely about point P, which is on the optical axis 12
and beyond both the contact plate and positive lens. When the patient's
eye is rotated about its center of rotation C, the contact element A
remains in contact with the cornea, and the contact element structure 10'
correspondingly rotates about pivot point P'. The virtual image V' remains
at the pivot point P' allowing the microscope to remain focused at that
point.
This embodiment utilizing a positive lens has certain advantages over the
embodiment in FIG. 1 which uses a negative lens. Firstly, the positive
lens system allows for magnification of unity or greater whereas the
negative lens system produces a demagnification. In addition, the positive
lens may be small in diameter thus light in weight while still allowing a
substantial numerical aperture for image formation. Finally, because a
real image is formed by the positive lens system the microscope can have a
relatively short working distance. The negative lens system on the other
hand requires a working distance greater than the distance between the
virtual image and the outer surface of the negative lens.
The embodiment illustrated in FIG. 13 can also be used for examining
specimens that are vibrating or otherwise moving through small excursions
in directions perpendicular to the axis as was discussed previously in
connection with FIG. 12.
FIG. 14 illustrates an embodiment of the invention which can be used in
place of the embodiment shown in FIGS. 10 and 11. In this embodiment a
positive lens is used alone without a negative lens. Thus, instead of the
image being formed precisely at the pivot plane, the image is formed at a
point along the axis of the image stabilizing system where the rotation of
the system just compensates the effects of the prism formed at the
cornea/system interface and of the movement of the object about the center
of rotation.
In FIG. 14 the eye is aphakic, i.e. the crystalline lens has been removed.
Point A represents the object to be visualized, e.g. the posterior capsule
or a region within the vitreous. From the exterior surface of the cornea
the distance to point A is b, and the distance to the center of rotation
of the eye is q. In this system the stabilized image is located at a
distance z from the pivot point, as given by the equation:
##EQU1##
If the z is positive and finite, then the stabilized image is located to
the right of the pivot point, along the horizontal axis. This is a real
image which can be directly photographed or video recorded. It is also
possible to choose the parameters (c and e) so that the denominator is O,
and z is infinite. In this case the stabilized image is at infinity and
can be viewed with a telescope.
FIG. 15 illustrates a positive lens system for the posterior portion of the
eye, including the fundus. In order to stabilize the image of the
posterior region, the positive lens is advantageously placed to the right
of the pivot point, as shown in FIG. 15. For an image stabilized at
infinity the position of the lens is given by:
##EQU2##
Where a' is the apparent distance to the fundus, as seen through the
flattened cornea. This distance a' can be estimated for a given eye
provided that the power of the cornea (PC) and the refractive error (R) of
the patient are known or can be measured. If R is expressed as the
correction required to achieve emmetropia then the distance a' is given
by:
##EQU3##
When the power is expressed in diopters, the distance a' will be in
meters. The lens focal length is defined by the equation:
f=k+g+a'
FIG. 16 illustrates a system which does not use lenses. This system works
on the principle that if an object is viewed through a thick layer of
glass, plastic, or water the object appears to be closer than it actually
is. If the object is at a depth t, the image (dashed) appears to be at a
depth of t/n, where n is the index of refraction. If the object is either
fixed to the end of the block or moves with the end of the block, the
virtual image will always be at the same position within the block. When
the block is mounted so that it can pivot about a point located at this
virtual image the image will not move laterally, and thus will be
stabilized.
This simple, no-lens system is useful for stabilizing the image of the
cornea so that it can be microscopically examined. If the block is made of
a plastic material having an index of refraction of 1.49, the pivot point
would be approximately t/3 from the cornea for a block of thickness t.
Generally this is not a favorable position since two thirds of the length
of the block is beyond the pivot point so that the moment of inertia makes
it difficult for the block to precisely follow eye movements. It is more
desirable to use a material of a higher index of refraction since this
would cause the pivot point to be closer to the center of the block.
However, such advantage would be lost if the material is of a high
density, such as flint glass.
In the embodiment in FIG. 16 viewing angles must always be small so that
image location can be specified. Small viewing angles are necessary in
that if the object is viewed at a very oblique angle the image will appear
to be at a shallower depth. This is due to the fact that for large angles
the law of refraction is nonlinear.
FIG. 17 illustrates a variation of the block illustrated in FIG. 16. In
this embodiment a negative lens is provided at the viewing end of the
block. By providing this negative lens the pivot point can be moved to the
center of the block or even farther to the right. The lens power required
to produce an image at the center of a block of length t and index n is
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