Abstract:
An apparatus and method for measuring the shape of the cornea utilize only one reticle to generate a pattern of rings projected onto the surface of a subject&#39;s eye. The reflected pattern is focused onto an imaging device such as a video camera and a computer compares the reflected pattern with a reference pattern stored in the computer&#39;s memory. The differences between the reflected and stored patterns are used to calculate the deformation of the cornea which may be useful for pre-and post-operative evaluation of the eye by surgeons.

Description:
ORIGIN OF INVENTION 
     The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 USC 202) in which the Contractor has elected to retain title. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to keratometry, that is, measurement of the cornea of the human eye and more specifically, to a laser keratometer having an optical subsystem designed to impose a reticle-generated series of rings on the eye and capture a series of reflected rings from the eye. A computer-stored reference image is effectively superimposed on the image reflected from the subject&#39;s eye so that computer processing provides means for generating either real time or near real time information on the eye shape. 
     BACKGROUND ART 
     Corneal surgery is currently undergoing rapid evolution with improvements designed to minimize or eliminate astigmatism following penetrating keraplasty (corneal transplants), as well as to correct refractive error. Because the cornea is the most powerful refracting surface of the eye, numerous procedures have been devised to incise, lathe, freeze, burn and reset the cornea to alter its shape. Currently practiced keratorefractive surgical techniques include: cryorefractive techniques (keratomileusis, keratophakia, ipikeratophakia), radialkeratotomy, thermal keratoplasty, corneal relaxing incisions and wedge resections. 
     When preparing the patient for any of these surgical techniques, it is essential to accurately measure the corneal curvature. Existing methods to measure corneal curvature include central keratometry and photokeratoscopy with central keratometry. However, with these methods only the central three millimeters of the cornea is measured. Recently, photokeratoscopy has been adapted to provide a topographic map of the cornea. However, this technique in its present form provides only a qualitative assessment of the cornea. This is because while photographs can be analysed by computer techniques or manual tracing, the time delay and effort in producing such data reduces the utility of the method for measuring corneal curvature preoperatively and for evaluating the effect of surgical techniques post-operatively. Thus, there is an ongoing need for a real time keratometer system for medical diagnosis and for preparation of the corneal contour for eye surgery as well as for post-operative analysis of completed eye surgery. One example of a computerized laser keratometer of the prior art utilizes a computer to analyse the moire patterns generated by laser excitation of the corneal surface and the resulting projecting and reflected beams. Unfortunately, such prior art devices have alignment problems as well as problems due to fringes that result from misalignment. 
     SUMMARY OF INVENTION 
     The system of the present invention provides a configuration that is somewhat similar to that of a classic keratometer, but with a novel optical system that illuminates through a shuttered light source and a focusing assembly and a sensor in the form of a solid state imager arranged to accept the image created by reflections from several zones of the cornea. A reference image is stored in a computer memory. The numerical superposition of the stored image on the reflected image from the subject&#39;s eye is displayed in real time or processed by the computer to yield numerical information regarding deformation of the eye. The novel approach of the present keratometer requires only one optical reticle which results in a substantial simplification in the optical system. Only the reflected ring pattern is required to be processed by the optical system. Since a reference reticle is generated and stored in the computer, the computer determines the center of the reflected ring pattern and overlays it precisely on the center of the reference ring pattern. Fringes due to misalignment are thus obviated in the present invention and only fringes due to deformation appear. 
     The present optical system imposes a series of rings generated by the reticle on the eye and captures a series of reflected rings. If no cornea deformation exists there is no displacement of the rings reflected from the eye from the reference set of rings stored in the computer. Any deformation causes some or all of the rings to be displaced slightly from the reference set and the computer determines the amount of deformation. In one particular embodiment disclosed herein, the apparatus comprises a helium-neon laser, a shutter, a reticle, a beam splitter, a quarter-wave plate and focusing assemblies. The reticle comprises a chrome deposited array of circular rings. Furthermore, in the particular embodiment disclosed herein the optical sensor or imager comprises a CCD video camera the output of which is eventually applied to a computer which is programmed to carry out the software routine disclosed herein for numerically analyzing the deformation of the observed cornea surface based upon displacement between the reflected image and the stored reference image. 
     OBJECTS OF THE INVENTION 
     It is therefore a principal object of the present invention to provide a computer driven optical keratometer as an evaluation tool for assessing the shape of the cornea primarily for pre-and post-operative evaluation in conjunction with corneal eye surgery. 
     It is an additional object of the present invention to provide a computer driven optical keratometer in which a reticle-induced series of rings is reflected off the surface of the cornea of the subject being tested and compared in a computer against a reference set of rings stored in the computer as a representation of a non-deformed corneal surface. 
     It is still an additional object of the present invention to provide a computer driven optical keratometer which may be advantageously used by opthamologists, optometrists and other such eye-related medical personnel for the purpose of evaluating the shape of the corneal surface and wherein a laser light source is used in conjunction with a single reticle pattern to produce a series of rings on the surface of the cornea which is reflected back into an optical focusing assembly and onto an optical sensor which transmits corresponding data to a computer which compares the reflected image with a stored reference image for evaluating the deformation of the cornea. 
     It is still an additional object of the present invention to provide a novel and advantageous method for numerically evaluating the shape of the cornea using only one reticle-generated ring pattern, the reflection of which is compared on a point-by-point basis with a stored reference pattern. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aforementioned advantages and objects of the present invention, as well as additional advantages and objects thereof, will be more fully understood hereinafter as a result of a detailed description of a preferred embodiment when taken in conjunction with the following drawings: 
     FIG. 1 is a block diagram representation of the optical subsystem of the present invention; 
     FIG. 1a illustrates the pattern of a reticle used in the preferred embodiment of the invention; 
     FIG. 2 is a schematic diagram of the final focusing lens of the optical system of FIG. 1 shown relative to the eye being measured; 
     FIG. 3 is a graphical representation illustrating variation in reticle radius and eye viewing radius at various distances from the eye; 
     FIG. 4 is a schematic diagram illustrating the interrelationship between the eye being measured and rays emanating from the reticle; 
     FIGS. 4a and 4b are two enlargements of a portion of FIG. 4 illustrating the mathematical analysis of the deformation measurement process of the present invention; 
     FIG. 5 is a graphical representation of expected deflection angles and actual deformation for various ring systems and viewing radii; 
     FIG. 6 is a schematic representation of a portion of the numerical process of the present invention specifically illustrating how the measured angle of deflection of laser generated rings is also dependent upon the focusing distance of the invention; 
     FIG. 7 is a graphical representation illustrating the relationship between vertical displacement on the focusing lens and total deformation for various ring geometries and focusing distances; 
     FIG. 8 is an additional schematic diagram illustrating the behavior of certain rays of the apparatus of the present invention; and 
     FIG. 9 is a graphical representation illustrating the relationship between the deflection angle and eye deformation for various ring systems and lens distances. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now first to FIG. 1 it will be seen that the optical subsystem 10 of the present invention comprises a laser 12, a mirror 14, a shutter 16, a microscope objective lens 18, a pinhole 19, a collimating lens 20, a reticle 22, a beam splitter 24, a quarterwave plate 26, focusing assemblies 28 and 30, a mirror 32 and a video camera 34, the latter being connected to a computer 36. Although laser 12 may be any one of numerous lasers it has been found that it is preferable to use a laser operating in the visible light spectrum such as a helium-neon laser. Mirror 14 is be used to bend the light beam emitted by laser 12 so that it enters the microscope objective lens 18 along the proper optical path through a shutter 16. The shutter is designed to provide the optical subsystem with a pulse of laser light of the appropriate duration for the measurement and may be synchronized, by appropriate electronics (not shown), with the timing of computer 36. Objective lens 18 and pinhole 19 act in combination to provide a relatively narrow uniform light beam which is then appropriately shaped an redirected by collimating lens 20 to fill reticle 22 with a relatively intense, uniform light source in which all the rings of the reticle 22 (see FIG. 1a) receive roughly the same magnitude of incident light energy. 
     The rings of light produced by reticle 22 are transmitted through a beam splitter 24, a quarterwave plate 26 and a first focusing assembly 28 which focuses the rings on the eye 5 being measured over a selected circular region having a viewing radius h. The rings of light incident on the eye 5 are reflected by the surface of the cornea. The reflected light passes through focusing assembly 28, quarterwave plate 26 and enters beam splitter 24 where it is redirected at a 90 degree angle relative to the incident light path into focusing assembly 30. Focusing assembly 30 is designed to focus the reflected light or ring pattern onto mirror 32 which redirects the reflected light energy into the lens of video camera 34. Quarterwave plate 26 is designed to direct the light reflected from the eye 5 to the video camera by changing the polarization of the outgoing and incoming light by 90 degrees. The video camera 34 generates a corresponding signal replicating the reflected ring pattern from the eye 5 and transmits it to the computer 36. Electronics between video camera 34 and computer 36 may be used to configure the video camera electronic signal in an appropriate data output suitable for use by computer 36. The operation of the video camera 34 as well as the operation of any necessary electronics to configure the corresponding electrical signal to be compatible with a computer are well-known in the art and need not be disclosed herein in any detail. 
     As previously indicated, computer 36 is provided with a reference pattern, that is, with appropriate data corresponding to a set of reflected rings which would otherwise be received by video camera 34 if the corneal surface of the eye 5 were precisely spherical without any deformation whatsoever anywhere on its surface. It will be recognized that by simply altering the contents of the signal stored in the memory of computer 36, which signals correspond to the reference pattern to which the reflected pattern is compared by the computer, one can readily alter the reference pattern to any desired configuration. Thus, the optical system 10 of the invention imposes a series of rings generated by passage of light through a reticle onto the eye and captures a series of reflected rings reflected from the surface of the eye. If no deformation exists, then there is no displacement of the rings from a reference set stored in the computer. If there is deformation, it will cause some or all of the rings to be displaced slightly from the reference set and the computer determines the amount of such deformation. The computer numbers and locates the center of each ring and compares it to the corresponding reference ring. The output of computer 36 may be designed to provide different forms of information depending upon the application of the invention. Thus for example, computer 36 may be programmed to simply provide a read out in either diopters or millimeters of the relative radius of curvature of the cornea indicative of a refractive deficiency. On the other hand, computer 36 may be programmed to provide a detailed topographical map which may either be displayed in the form of a set of numerals or as an actual simulated presentation of the cornea shape. The first type of output of computer 36 may be readily used to correct a refractive deficiency of the eye, while the latter is preferable particularly for surgeons who wish to know precisely what the shape of the eye is before or after corrective surgery. Both types of computer outputs are generated in response to a detailed comparison between the reflected rings and the stored reference rings by computer 36 to determine the extent of corneal deformation. The program used in an embodiment of the invention that has been reduced to practice is designed to produce an output corresponding to the relative radius of curvature of the eye, but could be modified by those having skill in the relevant art to provide a topological map of the eye. 
     Reference will now be made to FIGS. 2-9 primarily to illustrate the method of the present invention and more specifically, to demonstrate by mathematical analysis at the interface of the rays of light and the real eye surface that the system of the present invention is capable of measuring deformations with necessary accuracy and resolution and to show that the reflected rings are sufficiently displaced by deformation to be analyzed by computer 36. Table I below defines the nomenclature used in FIGS. 2-9. 
     
                       TABLE I______________________________________Nomenclature:d =     Spacing of rings on the eye, μmh =     Height above horizontal eye center for   viewing (viewing radius), mmH.sub.1 =   Height above central horizontal ray for   Ring N, mmh.sub.2 =   Height above central horizontal ray for   Ring N+1, mmH =     Reticle radius, mml =     Distance from center of eye to point where   reflected ray intersects central horizontal   ray, mmL =     Distance from edge of focusing lens to center   of eye, mmr =     Radius of eye, assumed to be approximately   .75 inch = 19.05 mm______________________________________Points:A -    Intersection of N ring and radius of eyeB -    Intersection of N+1 ring and radius of eyeD -    Intersection of Real surface and horizontal line  from point BE -    Intersection of Normal to line AD (from point F)  and either line AB or BDF -    Intersection of Incident of ray of ring N+1 and real  surfaceG -    Intersection of Reflected ray (from point F) and  either line AB or ADI -    Intersection of Reflected ray projected back and  central horizontal rayM -    Intersection of Reflected ray and focusing lensO -    Center of eyeP -    Intersection of incident ray from ring N  ring and focusing lensQ -    Intersection of incident ray from ring N+1  ring and focusing lensS -    Intersection of vertical projection down  from point Q and central rayT -    Intersection of vertical projection down from  point M and central ray______________________________________Greek:α =  Angle between incident ray from ring N and  central ray, Degreesβ =  Angle between incident ray from ring N+1 and  central ray, Degreesγ =  1/2 (α+β), Degreesδ =  Difference between reference and actual points,  measured horizontally, μmΔ =  Difference between incident and reflected rays,  measured vertically, on focusing lens, mmη =  Angle between N+1 and N rings, DegreesΘ =  Total viewing angle, Degreesξ = Angle between reflected ray and incident ray at  same point on focusing lens, DegreesΦ =  Angle between reference normal and actual  normal, Degrees______________________________________ 
    
     FIG. 2 is a schematic diagram of the final focusing lens of focusing assembly 28 and the eye 5 being subjected to evaluation by the invention. FIG. 2 illustrates the lens at a distance L from the center of the eye. In order to view the exterior surface or cornea of the eye up to a height h from the central ray of the eye (represented by the horizontal line in FIG. 2), the lens radius must be at least height H. FIG. 3 is a graphical representation showing the variation of eye viewing height h with optical radius or reticle radius H for various distances between the final lens and the center of the eye. More specifically, the variation of h with H is shown for distances of 2, 3, 4 and 6 inches respectively. Thus it will be seen that in FIG. 3 that if, for a distance L of 4 inches between the center of the eye and the final lens of the focusing assembly, one desires an eye viewing height h of 8 millimeters, it is necessary to have a reticle or lens height of 50 millimeters. Precise dimensions for the optical section may be varied based on parameters such as cost and preferences of medical personnel. 
     Referring now to FIGS. 4, 4a and 4b, it will be seen that the eye and the rays from the n and n+1 rings are shown schematically therein. The ray from the n ring intersects the actual surface of the eye at A which corresponds with the reference surface. No distortion is assumed at position A. The ray from the n+1 ring intersects the reference surface at B. However, it is assumed for purposes of demonstration that, due to distortion, the actual point of contact between the n+1 ring and the eye surface is at point F. The length of line BD is a measure of the bulging or flatness of the eye at that point. 
     The triangle ABD is shown in detail in FIG. 4b. Line AD is assumed to be the actual surface which ray QB intersects at F. Ray QB is reflected at an angle equal to the angle between QF and the normal to line AD. This angle Φ is crucial because once it is known, the value of line BD or the distortion can be determined. The length of line BD is also dependent upon the length of line AB as well as on the angles α and β which are known from the geometry of the optical system. The length AB is the distance between successive rings. 
     Examples of deflection angles for various actual deformations for different ring systems and viewing radii are shown in FIG. 5. As illustrated therein, for reticles in which there are 10, 25 and 50 rings respectively, total deformations up to 100 micrometers per millimeter produce an eye reflection angle or deflection angle Φ less than 6 degrees. Thus, the deflection angles are of the appropriate order of magnitude to be easily measured but are not large enough to escape the optical system. FIG. 5 also shows that the greater the number of rings, the greater is the deflection angle Φ for the same amount of eye deformation. 
     FIG. 6 demonstrates how the deflection angle Φ corresponds to a vertical displacement on the focusing lens. Displacement is the difference between incident and reflected rays measured vertically on the focusing lens and is also dependent on the focusing distance L. The graph of FIG. 7 shows vertical displacement Δ as it varies with total deformation or the various ring geometries shown in FIG. 5 as well as for different focusing distances L. As seen in FIG. 7, the larger the focusing distance L, the greater is the displacement for all ring geometries. Also seen in FIG. 7, for two geometries having the same number of rings, the one with the smaller viewing radius produces the greater displacement. It is preferable to choose a focusing distance and a ring geometry which will prevent the displaced rays from escaping the optical limits of the system and which will also avoid the necessity for requiring larger and more expensive optics. 
     Referring again to FIG. 6 it will be seen that the reflected ray FM, if projected back towards the center of the eye, crosses the central horizontal ray at some distance from the focus. Because ray IM does not originate at the focus, it will not be reflected back from the focusing lens in a horizontal line as would ray AP or ray BQ. Consequently, this will cause the reflected ray to diverge slightly from the horizontal at M. This diverging angle ξ is shown in FIG. 8. This diverging angle can be approximated as equal to the angle made by the ray MF and ray MO where MO is the ray drawn from point M to the focus of the eye. This angle adds additional displacement from the incident ray, but should be kept small to keep all rays within the limits of the optical system. 
     In this regard, FIG. 9 illustrates this diverging angle ξ as it varies with total deformation for the previously indicated ring system and lens distances. FIG. 9 illustrates that although the longer distances yield higher displacement values, they also produce higher diverging angles. FIGS. 5, 7 and 9 together demonstrate how an actual eye surface, which differs from a reference surface, will give rise to a reflected ray which is displaced from the incident ray on the focusing lens. This displacement is a function of the geometry of the system and its difference from the true surface. In an actual measurement using the present invention, the displacement and deflection angles are measured quantities and the reflection angle and the deformation are generated based upon the following formulas. 
     
         ______________________________________Θ =    arcsin(h/r)H =      L * tan Θη =  Θ/(Number of Rings)α =    N * ηβ = (N+1) *ηh.sub.1 =    r*sin αh.sub.2 =    r*sin βγ =    Y.sub.2 (α+β)Ring Spacing = h.sub.2 - h.sub.1δ =    Total Deformation * Ring SpacingAB =     [2 * r.sup.2 (1 - cosη)].sup.1/2AD =     [AB.sup.2 + δ.sup.2 - 2 * AB * δ *cos(90+γ)].su    p.1/2≮ 6 =    arcsin[(AB/AD)sin(90° + γ)]≮ 5 =    180° - (≮6 + β)]Φ =  90° - ≮5Δ =    MT - QSQS =     L * sinβMT =     MI * sin(2 * Φ +β )MI =     MF + FIFI = (λ * sinβ)/sin(2 *Φ) where λ =                FO * sin(2 *Φ)                sin(180° - (2 *Φ +β)          and                FO = r - δ sin.sub.≮ 6                sin(90° +Φ)MF =     MG + GFSince GF &lt;&lt; MG, then MF = MGMG =     L - rξ =   2 * Φ+β - arcsin(MT/L)______________________________________ 
    
     The computer software for carrying out the numerical analysis in accordance with the equations above for measured deflection angles and displacement is provided herein in Table II. ##SPC1## 
     It will now be understood that what has been disclosed herein comprises a laser beam keratometer having only one optical reticle. The keratometer provides an optical subsystem designed to impose a series of rings generated by a reticle on the surface of the eye and to capture a series of reflected rings from the eye. The image of reflected rings is transmitted to a computer which effectively superimposes a computer stored reference image on the image reflected from a subject&#39;s eye. Data processing numerical analysis then provides a real time display or numerical information on the condition of the eye. When no deformation exists there is no displacement of the rings from the reference set stored in the computer. However, any deformation that is observed causes some or all of the rings to be displaced slightly from the reference set and the computer analyzes the amount of deformation to produce either a detailed topology of the eye or a simpler numerical representation of the eye&#39;s refractive condition. The real time or near real time capabilities of the present invention are particularly advantageous for use in medical diagnosis and evaluation of the corneal contour for eye surgery as well as for evaluation of the corneal contour post-operatively. The present system provides corneal contour evaluation over a much larger surface area than previously possible using prior art keratometers. Furthermore, the novel use of one reticle-produced image, the eye&#39;s reflection of which is compared against a reference image in a computer, alleviates prior art alignment problems and resulting fringes. 
     Those having skill in the art to which the present invention pertains will now, as a result of the disclosure herein, perceive various modifications and additions which may be made to the invention. Thus for example, while FIG. 1 demonstrates an illustrative embodiment of an apparatus configured to accomplish the method of the invention, it will be understood that other optical subsystems may be utilized to generate the reflected image used by the computer in calculating the deformation and surface characteristics of the cornea. Furthermore, while it will be observed that a particular reticle geometry has been used herein, other reticle geometries and corresponding modified numerical analyses may be readily employed to accomplish essentially the same method as disclosed herein or a method substantially equivalent thereto. Consequently, it will be understood that all such modifications and additions are contemplated as being within the scope of the invention which is to be limited only by the claims appended hereto.