This application is a continuation-in-part of U.S. Serial No. 590,414, filed March 16, 1984, now allowed, the contents of which are hereby incorporated by reference into this application.

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 hand¬ held 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 rei aged 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 April 14, 1980 and now issued as U.S. Patent No. 4,410,245, dated October 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 g 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 _Q 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 5 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 0 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 5 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

30 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

35

become apparent to those skilled in the art from the remaining disclosure herein and the illustrative embodiments of the invention.

Sum ary 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 nonli iting 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. l 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 gi bal 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 •JO 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

15 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

20 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

25 and/or heartbeat can sometimes interfere with detailed observation or measurements.

Fig. 13 is a further embodiment utilizing a positive lens.

30

Fig. 14 illustrates an extension of the embodiment of Fig. 13 on which the stabilized image is not located at the pivot point.

35

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, l and 2, a contact element structure generally indicated at 10 contacts a patient's eye and moves, with small ^e Y ^e 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 0, 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

028

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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 0 (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 O 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

exa ination related to this condition and in the laser treatment.

The system in this case comprises positive lens LI 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 LI, 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 (Figure 19) . Another possibility would be to incorporate a low vacuum system into the contact element of the image stabilization system (Figure 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 50mm. 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 Figure 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 as in the previous embodiments. 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 effect of the prism formed at the cornea/system interface and the movement of the object about the center of rotation. In the positive lens system for the anterior portion of the eye illustrated in Fig. 14, the location of the stabilized image is given by the equation:

z = eb(c+e+q) q (c+b/n)-b(c+e+q)

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 0, 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:

8

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k = 1 (a'-q)

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:

= 1

PC+R

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 principal 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 — _M a ~t the center of a block of length t and index n is P=-r—. For other desired positions of the image, the required power can be calculated using the vergence equation which is well known in the art.

The negative lens can be ground and polished into the material of the block as indicated in Fig. 17, or a

glass lens of appropriate power can be fixed to the end of the block, as shown in Fig. 18. The construction of Fig. 18 has the advantage that a multi-element lens can be used to correct for chromatic aberrations introduced by a thick block of refractive material.

Fig. 19 illustrates an image stabilization system to which a soft contact lens has been bound with transparent cement or by other means. This combination is of use when the patient's tear layer and eye motion is such that the image stabilization system does not move precisely with the eye movements, due to slipage of the contact element structure on the cornea. The contact lens is in close contact with the cornea (and in some cases to the sclera as well) and therefore will have greater adhesion to the cornea than will the contact element structure alone.

Fig. 20 illustrates an image stabilization system with a contact element structure which incorporates a vacuum system to ensure adhesion between the cornea and contact element structure. The vacuum is applied by means of a flexible tube. The vacuum is of low tension, sufficient to keep the contact element structure firmly in place, but not so strong to produce discomfort or injury to the eye. The groove is the contact element structure distribute the vacuum over a circular region of the contact area.

Other regions within the eye can be examined using image stabilizing systems designed specifically for those regions, for example by combining known optical systems such as the Goldmann gonioprism with the image stabilizing principles described herein.

While examination of the eye has been used as the example in describing the operation of the image stabilizing system, the system works equally well in reverse. That is, a beam or pattern of light can be projected through the image stabilizing system, and that beam or pattern will be stationary with respect to the selected region of the eye. In laser photocoagulation of the retina, for example, if the beam is focused to a point immediately adjacent to a retinal blood vessel, the beam will remain focused at the point on the retina even if the eye undergoes a small rotational motion. Other laser treatments requiring high accuracy in positioning the laser beam include: photocoagulation in and near the macula, laser trabeculoplasty, and neodymium YAG laser capsulotomy and membranotomy.

Other diagnostic procedures involve sending low energy laser beams into the eye. Laser doppler velocimetry and laser ophthalmoscopy both require precise localization of the laser beam on the retina. The image stabilizing system offers a relatively inexpensive, easily applied method for neutralizing the effect of small eye rotations.

A final but nonlimiting example of the image stabilizing system used in reverse is the presentation of stabilized images on the retina of an observer, to test the response of the observer to such an image. Such tests have been performed in well known psychophysical experiments to demonstrate the fading of an image that is stabilized on the observer's retina. Special contact lenses have been used in these experiments. The image stabilizing system presents the opportunity to perform such tests without the fitting

of special contact lenses. Additionally, clinical tests can be devised to test vision under precisely controlled conditions. Visual acuity of pre-selected regions of the retina may be determined. In some cases, it can be useful to identify visual field defects using the image stabilizing system. Many patients experience difficulty in maintaining fixation, i.e. their direction of gaze, during such testing. The image stabilizing system neutralizes the effects of small eye motions so that they do not affect the locating and plotting of visual field defects. This can be of value in the plotting of field defects at or near the macula.

Some ophthalmic measurements systems, such as the applanation tonometer for determining intra-ocular pressure, utilize a split image viewing device to permit measurements to be made in spite of eye movements. The image stabilizing system offers an alternative approach; stabilization of the image of the applanated area, so that the split image approach is not necessary.

Another application is the microscopic examination of the surface of objects that are vibrating in a linear fashion. For example, a component may need to be examined under conditions in which it vibrates along a straight line. The vibration could also be in a plane, e.g. a circular or elliptical motion of the object.

As in the case of the eye examination, the front element of the system contacts the object and moves with it. The system is mounted so as to pivot at some point at a predetermined distance from the object. The power of the lens (or lenses) and the location of the

pivot point are chosen so that the image of the surface is stabilized. That image can then be magnified and examined using a stationary optical system of conventional design.

In this case the front element is designed so that it will move precisely with the object being examined. One technique is to provide a compliant material with a high coefficient of friction to contact the vibrating object. A rubber gasket material, or an "O Ring" will be suitable in cases of small amplitude vibration.

The optical system comprises a negative lens or a positive lens positioned so that the image of the object under study is located at the pivot point. A viewing device in the form of a long working distance microscope can then be used to examine the virtual image.