CA2127426C - Ophthalmic pachymeter and method of making ophthalmic determinations - Google Patents

Abstract

An ophthalmic pachymeter which is highly effective in aiding in the determination of thickness and the optical density of the cornea of an eye an a real-time basis. The pachymeter therefore lends itself to effective employment in aiding in radial keratotomy and other surgical procedures With respect to the eye. The ophthalmic pachymeter of the invention has three major subsystems which include a television camera, a multiple slit projector and an associated processing and display system. In a broad form, the invention comprises illuminating a selected portion of the cornea, moving a slit across the cornea and generating Tyndall image ray paths for enabling, analysis of the optical density of the cornea and the thickness of the cornea. This is accomplished through a series of digitally-encoded television images of the optical section of the cornea produced by a multiple slit projector and which images are then subjected to digital analysis. A locus of each of the significant elements of the reflected image of the anterior portion of the eye is defined. In this way, the optical character of the cornea/air interface is compared with the corrected reflectance of the stroma and the endothelium, in order to determine relative transparency. A density map may be constructed in parallel slices far display as a three-dimensional plot of the frontal surface shape of the cornea and for defining and displaying the local thickness, the posterior surface contour, and the optical density of the cornea.

Description

BACKGROUND OF THE INVENTLON
1. Field off, t~e_ I,~g~ntion:
This invention relates in general to certain new and useful improvements in ophthalmic pachymeters for aiding in the determination o:~ the thickness and relative optical density of the cornea of the eye on a real-time basis and to an improved ophthalmic pachymeter of the type which primarily relies upon a television camera, a multiple slit projector and an associated processing and display systems which cooperate in a unique manner to provide a three-dimensional map of the cornea.

2. Brief Description of the Prior Art:
The measurement of optical density of cataracts has been a subject which is becoming more widely addressed in recent times.
one of the recent teachings of cataract optical density measurement is set forth in U.S. Patent No. 4,863,261 to J. Flammer, "Method and Apparatus for Measuring the Extent of Clouding of the Human Eye." The prior art relating to the measurement of optical density is also exemplified in U.S. Patent No. 4,019,813 for ''Optical Apparatus fox Obtaining Measurements of Portions of the Eye.~~
Planning for anterior segment surgery is also a topic which has received increasing attention in recent years, as, for example, in a paper by Lehrman, et al., ''Measurement of Anterior Chamber Diameter'' and ~~Biometry of Anterior Segment by Scheimpflug Slit Lamp Photography", reported in Investigative Ophthalmology and Visual Science, Volume 32, No. 3, March 1991, pages 529-532.
The slit lamp is an instrument employed by many optometrists and ophthalmologists for examination of the anterior portion of the eye. Many different versions of the instrument have been produced over the last century, but all of the slit lamps have three major elements in common and which include a projector for providing a collimated image of an optical slit focused an the eye, a bio-microscope or camera for viewing the image and a mechanical support system. Tn this slit lamp system, the bio-microscope or camera is designed to view the image formed by the projector and is confocal with the projector. The mechanical support system must be elaborate to at least support the subject and the projector and viewing system. Furthermore, the elements must be positioned relative to one another for appropriate examination of the eye, Pachymetry attachments are available for the slit lamp to be used in clinical environments. These attachments operate so as to displace half the image by a plane parallel glass block interposed in the viewing path. In this way, corneal thickness is measured at a single point. The reading of the drum farming part of this attachment is then recorded by hand as the local corneal thickness .
While these modified forms of slit lamps, which operate as pachymeters, can accurately define corneal thickness at an unknown location, they are slow, expensive and fragile. Moreover, they are quite d,if f icult to operate, and require substantial training on the part of the operator.

One of the most common corneal thickness measurements used in clinical practise today is that of ultrasound. The A-scan ultra sound probe, much the same as with the optical pachymeter, produces a single-point measurement at an unknown locus of the cornea. In addition, this unknown single point is, in reality, the average thickness of an area of several square millimeters in e~ttent.
Because the location of the measurement is not repeatable, the data is variable as well.
one of the principal problems of the prior art systems is that the plots which one generated to provide a cornea mapping were not accurate and more importantly, were not repeatable. Thus, the prior art did not provide the ophthalmic surgeon with the data required for planning radial keratotomy or other refractive surgical processes.
Some of the other deficiencies in the prior art techniques used for determining corneal thickness and mapping is hereinafter described in the following Overview of The Invention. In this overview of The Invention, the prior art is, in some cases, also contrasted with the principles of the present invention in order to mare fully show the substantial advantages achieved by the present invention. Also, and to some extent, background theory ~.s set forth in order to mare fully aid in the understanding of the present invention.

OVER~,TEW OF THE INVEN'~TON
Densitometry is a term applied to measurement of the optical density of areas of photographs. Densitometers commonly measure the log of the reciprocal of the percentage of light transmission for a defined area at a stated wavelength or wave band.
Measurement of the relative reflection of scattered light is commonly employed to define turbidity in water samples. The amount of light scattered and thus retro-reflected is thereupon compared to a known reference value in this process, in order to determine scattered light reflection.
The densitometer of the present invention measures the relative amount of reflected light from scattering within ocular tissue as an indication of the optical density. In the strictest sense, this is not a true density measurement but is a measure of the relative transparency of the corneal tissues. The corneal interface with the air is not affected by stromal opacification and serves as a relative density reference fox the measurement, The minimum reflectance value fox calibration is derived from the signal representing the anterior chamber over the pupil Where the average reflectance is the smallest. Optical density of small and large areas of the cornea often provides diagnostic data for deciding the need for surgical intervention. Tn addition, surface contour and the thickness of the cornea are quantified for producing a complete, three dimensional thickness map of the cornea including local thickness of the membrane.

The present invention utilixes light from an incandescent lamp for analysis of the thickness and optical dens3,ty of the Cornea at one or more wavelengths. By making successive exposures with a small linear movement of a slit image between each e~tposure, a series of slice density images can be generated. These images are then stored in a digital format. The displayed reconstruction of these slices is similar to that used in computer axial tomography and light processes which are now well known in the art. From this three dimensional display, the anterior chamber depth arid local corneal thickness may also be seen, and this can be used in planning anterior segment surgery in accordance with the proposal of Lehrman, et al., supra.
The present invention is also effective for mapping localized opacities and used in planning corneal replacement surgery.
corneal wounds and ulcers can also be mapped to provide accurate diagnostic information to the physician by use of the pachymet.er of the invention. Changes in index of refraction associated with scaring or ulceration create foci of Light scattering and loss of transparency. The index of refraction in the bulk of the cornea is less in the fluid than in the fiber cytoplasm and scattering results from the optical discontinuities. The cornea may be damaged by disease processes, mechanical forces or foreign object penetration. Tn these cases the opacification is probably due to the changes in protein molecule orientation at the injury site 3n healing. The degree of opacification can be monitored by loss of visual acuity but the loss of night vision due to loss of image contrast may be debilitating occasionally where the Snellen acuity is only slightly affected.
The ability of the present invention to quantify the degree of ogacification and measure the locus of any localized changes provides a tool for assessment of corneal disease and scatting.
Sometimes, the localized changes in refractive index may be located away from the optical center of the cornea and may not interfere with vision to any great extent. Tn these cases there is a high probability of degeneration of visual acuity over time, although the traditional Snellen test does not indicate any current loss of visual acuity. Localized opacifications may also produce monocular diplopia due, in part, to diffraction at the edges of the opacification that may be masked by the constricted pupil in conventional testing methods. Testing of contrast acuity and testing of low light level, when such opacities exist, tends to show the loss of night vision adequate for safe driving and precedes the loss of Snellen acuity.
slit lamp examination will reveal the presence of these abnormalities but direct visual examination does not provide accurate and repeatable assessment of the lesion location, density, area and any changes in size or opacity with time. The present invention provides a tool for repeatable assessment of potential vision loss under adverse lighting conditions.
A beam of sunlight entering a darkened room through a hole in a curtain or shining through a hole in a cloud forms a visible path due to dust particles, water droplets and smoke in the air.

The same principle is used in slit lamp examination of the eye where the scattering of light in nearly transparent tissues renders visible structures that cannot otherwise 1~e seen. Focal illumination is employed in the present invention by a modified conventional I~tihler projector. The object at the focal plane is one or more optical slits that may be moved either manually or by an associated computer controlled mechanism. The image of each slit is made confocal with a television camera that forms a part of the pachymeter of the present invention so that the instrument may examine various areas without repositioning the instrument. The focal length of the projection lens is as long as possible to reduce the convergence. or divergence of the beam over small axial distances. Projection lens focal ratio is calculated by well-known techniques for producing optimal brightness and sharpness of the illuminated area, The projector and the camera are mounted on a common vertical member to allow the projected thin sheets of light to enter the eye at an angle to the camera axis and to permit horizontal, vertical and a~cial alignment With the eye. The beams of light produce diffuse reflection from each successive portion of the anterior part of the eye through Which it passes. Proper selection of slit width and position produces the illusion of a cross section of tae cornea as a luminous band against a dark pupil.
The "Tyndall phenomenon" is the .term employed to describe this method for generation of an optical section of the eye that is well known in the ophthalmic art. The diffuse reflection from the first of the layers of the cornea that provides the optical section image is used in the present invention as a reference against which the diffuse reflection from the other portions of the cornea are, compared for determining the relative transparency.
The Camera system commonly used to make slit lamp photographs is a single lens reflex, 35mm camera back. The camera back receives an image through a beam sputter attached to a slit lamp microscope, For some types of anterior photography this system works well and is simple to operate. A motor film drive in the camera back and direct viewing capacity, in particular, are of great value in this type of instrument. It can be seen that there are inherent. limitations in the prior art camera system which are addressed in and solved by the system of the present invention.
The lack of depth of field prevents adequate effective resolution far meaningful measurement of the image., In addition, the optical system of the bio-microscope degrades the image in both resolution and light gathering power.
The present invention does not use the conventional bio-microscope of the common slit lamp, but rather employs a television camera system for producing digital images for analysis of the anterior portion of the eye. The correlation of a sequence of fixed image individual photographs is difficult. This is due to the change of fixation of gaze relative to the optical axis of the camera caused by the long time required to reposition the camera system for each image acquired. The resuxting non-orthogonal picture series is difficult to interpret, The present invention solves this problem by e~onploying a rectilinear span system to provide rapid density arid thickness measurement of the entire cornea in an easily comprehended format.
The Width of the slit may be increased to provide greater illumination to compensate for the exposure loss due to small lens apertures. The slit width is preferably near 0.8 millimeters.
When photographs of objects close to the camera are made, the depth of focus is dependent upon the lens focal length and the iris opening. In ophthalmic photography with a conventional slit lamp camera there is little choice over iris opening and none over focal length. The design of the instrument imposes severe light losses and provides at best about an f~16 lens opening. , The use of a single lens system in the present invention permits focal ratios of f-2.s which, in turn, permits a considerable reduction in illumination levels and consequent reduction in the possibility of photo-toxicity. Tn the preferred embodiment, the magnification ratio far the slit projectors is 1:1 and the camera magnification is .25;1. These provide the desired image for analysis with good overall focus.
The ophthalmic pachymeter of the present invention measures the elemental brightness of a selected portion of a Tyndall image of the eye. Selection of a portion of the field of view of a slit lamp system is accomplished with computer control through the use of a table of valued fiducial marks_delineating selected areas in the video display. The instant slit location and angle are computer controlled and the highest Tyndall arc is definable in terms of known maximum curvature of the human corneal surface.
These data permit the selection of a band of picture containing the Tyndall image to be digitized, stored aid used for construction of the corneal surface models for display. The elemental amplitude i within the delineated area is determined by subdividing the video image line segments selected into small picture elements or "pixels" which are examined and c,~uantified for brightness information.
Tn the system disclosed in the present invention, a "clock"
signal is derived from a highly accurate crystal controlled oscillator to provide subdivision c~f the raster lines into well-defined time/size elements. The preferred embodiment uses a medium resolution solid state television camera. The lower inherent resolution, as compared to a 35mm film, is adequate for mapping the eye for most clinical applications and provides better long term stability than tube type television cameras, while requiring minimal storage and processung time.
The present invention provides a system for further reducing the number of loci used in the calculations involved in the mapping by circumscribing portions of the visible frame to encompass the area of interest. In this way, the resultant information is stored in more compact form without loss of resolution or accuracy. The gamma curve for film images shows tyke relationship between log exposure and log density of the image.' In a video based system the gamma curve may be tailored by methods that are well known in the television signal processing art, for 'optimal data der~.vation. Tn the video amplifier used for the television camera interface the gamma curve, can be made to vary with amplitude that is a function of log exposure. The high inherent sensitivity of modern television cameras, together raith the ability to shape the gamma curve for calibration in the present invention, thereby provides a considerable improvement in raw data quality over the photographic process of the prior art.
Because the brightness of the tyndall image of the cornea differs from the almost constant anterior corneal surface reflex, a simple numerical amplitude discrimination process is used to define points within the circumscribed fiducial area. These brightness values together with the X, Y locus of these pixels is the only data stared for the pachymetric measurements. The numerical values for the pixels so defined are then used to define the optical density in terms of relative scattering of light by location within the cornea. The optical density is further corrected by compensating the measured brightness of each pixel for the losses due to the density of portions of the path through which the illuminating and scattered light traverses and by conventional gamma aorreotion circuitry. Histogram correction or other well-knoran techniques may also be employed for enhancing the values defined far the Tyndall image which is stored in digital format.
The shape of both anterior and posterior surfaces and the thickness of the cornea of the human_eye can be mapped by means of the slit projection system of this invention. The line of gage fixation is made coincident with the optical axis of the camera by l .l a target viewed by the subject via a beam splitter. The beam sputter and fixation target are so positioned as to cause the desired alignment of the eye and camera, and thus, the slit beam.
The coaxial location of the fixation target insures that the visual axis of the eye being examined is coincident With the optical axis of the television camera. Beams of light formed by projection of an optical slit or slits are focused at or slightly behind the corneal surface. The beams are projected from known points located on a line at a fixed angle, preferably 45 degrees, from the optical axis of a camera and in the same plane.
At normal incidence, the reflection of light at a boundary between media of differing index of refraction is calculated as follows:
= ( na"W ) 2 / ~ na+n, ) 2 Where R is the reflected percentage of the incident beam, and, nI
and n2 are the indexes of refraction of the two media respect~.~ely.
From this it follows that the air to cornea and cornea to aqueous interfaces will reflect a definable portion of the slit beam diffusely. The stroma or interior structure of the cornea comprises large muco-saccharide molecules in lamellas arrangement with surrounding saline solution. This causes a similar diffuse reflection of part of the incident beam and produces the so called '"Tyndall" phenomenon. The tyndall image is therefore the result of diffuse reflection at the optical discontinuit3.es in the slit beam path. Because the locus of origin of the slit beam relative to the optical axis of the camera is knocm, the shape of the Tyndall image as vie~ted describes the shape of the cornea. Taking a point on the Tyndall image, there is a displacement from Where the slit beam would have intersected the reference plane that corresponds to a function of the height of the surface at the datum point.
After generating the anterior surface of the cornea the posterior surface can be defined using the refractive index of the cornea and the angle of incidence for each ray derived from the anterior surface data by application of knell's law. The beam is refracted into the denser medium by an amount calculated from the relative slope at each calculated ray entrance. The light from the slit projector is also reflected by the corneal/air interface in a specular manner. This reflex provides a check of the contour generated from the Tyndall data. The local slope of the cornea can be taken to be equivalent to a sphere of some rad3.us tahen the surface area is very small.. The resulting spherical mirror formed will act to reflect the incoming ray from the projector(s), The location of beam source and relative angle provides the basis for calculation. The image of the reflection is located in X, Y
coordinates from the digitized image and the slope at the surface is calculated by the methods used by Placido that are well known in the art. This independent method slope derivation at points on both sides of the cornea, generally,midway betvTeen the center and the limbus, provides quality assurance for the Tyndall image derived surface shape.

Calculation of the surface shape in the present invention is performed by means of analysis of similar triangles. The anterior surface contour is defined first.
The method involves simple geometric analysis of the Tyndall image. The angle between the slit beam and the optical axis is fixed at 45 degrees at center. The distance between the mirror and the eye is also fixed at a known distance. The image point for each pixel is displaced from the optical axis as a direct function of point height above the reference base plane.
In the preferred embodiment of the present invention the image to be analyzed is produced by projection of narrow bands of light into the eye by optical projectors of conventional "Kohler" design.
In the conventional slit lamp, the bio-microscope and slit projector are mounted on pivoted arms with a common bearing center so that the microscope and projector may be independently rotated in the horizontal plane while remaining confocal. In the present invention this arrangement of elements may be replaced by two slit beams from opposing sides with a constant angular relationship to the eye and camera. The image of Tyndall illuminated areas of the slit or slits are formed in the plane of focus of the camera. The slit beams are angled to produce edge convergence of the Tyndall images when the axial distance between the central point of the cornea is at the desired distance and the camera focal range lies from this plane at or near the iris to a more anterior plane beyond the corneal anterior surface. This construction provides the requisite known distance for all calculations and provides a simple 1 '~

operator clue to proper centering and focus.
The grajectors of the present invention are fitted with internal filters for establishing the spectral content of the beam of light projected into the eye. The use of selectable filters to limit the spectral distribution of the illuminating beam provides the ability to quantify discoloration of portions of the cornea associated with age, scars or disease processes while limiting any potential photo-~to~cicity from the light energy within the globe.
In an alternative embodiment, the filters are designed to pa s only the near infra-red portion of the spectrum which makes the slit beams invisible to the subject.
The projector used with the present invention is add3.tionally provided with a mechanism for providing slit motion. In the preferred embodiment, the slit assembly is moved under computer control in a direction perpendicular to the long axis of the slit when sequential image data slices are required. The slits are initially centered and projected from both sides simu:lta~.eously.
The resultant dual. Tyndall image is used for determining instrument position to assure accurate focus and Known magnification. The operator positions the instrument so that the center of the two arcuate images is coincident and cantered in the pupil. The construction of the instrument then provides the proper focus for the image series that comprises the pachymetric measurent~nt.
T.n one of the preferred embodiments, as previously- mentioned, a pair of projectors are employed and each move an individual slit assembly. Further, each slit assembly is moved under Computer control perpendicular to the longitudinal axis of the slit. Each slit is also moved from an opposite side of the eye.
In another alternative embodiment the optical slits are formed by ferro-fluidic type liquid crystal devices. The liquid crystal system is faster and dissipates less power but provides less flexibility of slit locus for special diagnostic use.
The signal from the television camera is used to derive the relative image brightness of the corneal optical sections While the optical figure is being digitized and also viewed on the television monitor. Monitors far viewing both pictorial and computer signals are well known in the art and are not described herein.
Repositioning the slit beam by incremental motion perpendicular to the slit axis permits measurement of almost all of~
the cornea in detail and the composite time domain image data sequence is,stored for analysis and display. The slit motion is interlocked in time with the vertical interval of the camera so that the successive images are stored over a very short interval and so that loss of fixation or micro saccadic motion of the eye does not degrade the data.
The television sync signals cause an internal oscillator in the display system to be synchronous with the computer and camera and consequently, the data sequence. Each video lane re~qui.res a fixed, known time. The action of a video analog tea digital converter, called a "frame grabber", serves to quantify the instantaneous brightness related voltage amplitude of each image element and to store the sequence in a memory for later use.

Subdivision of the video line into small spatial elements or pixels is by time interval selection. The intervals are defined by a stable clock oscillator that provides a series of pulses which correspond to image loci. The line rate and the clack rate define the size of a pixel in terms of pixels per line or pixels per second. ~n conventional broadcast television, the signal is limited to 4.5 Megahertz, which yields a pixel rate of only 236 for the active or visible line of some 52.4 microseconds, although the overscan of the display means that even fewer pixels make up the actual image viewed by the user. A computer generated display is structured in similar fashion to make use of components that are in volume production. _ The present invention makes use of a common commercial camera and monitor apparatus without modification. Because the cornea of the eye is essentially circular in form, the normal television picture aspect ratio of a 4:3 ratio or a 5:4 ratio, employed by many computer displays, provides na advantage. In operation, the use of computer displays for television pictures deletes same of the picture in the horizontal axis. This '~cropping'~ is of no consequence in the present invention. The most contunon aam~era tube is two-thirds inches (approx. 18 millimeters) in diamater. The desired image is roughly eleven millimeters in diagonal or somewhat less. This assumes the diameter is equal to the height of the image. The image sizes for the other common tubes ors, 1&mm for the one inch tube and 8mm for the one-half inch type. The focal length of the lens and the size and location of the fiducial mark generation is determined on the basis of a square area that nearly fills the frame with the image in the vertical direction. The analog to digital conversion is usually, although not necessarily, limited to a square area with a Width equal to the largest oommonly found limbus that is about 22 millimeters. The inner edge of the iris opening is also visible in the captured images and because the light does not always enter the pupil, the iris changes size during the measurement. The shape and rate of iris motion are often of clinical interest and can be generated through an additional computer program in an alternative embodiment that is not illustrated or described.
For highest accuracy, each instrument must be calibrated after assembly to compensate for minor differences in system magnifica-tion and linearity to obtain maximum accuracy of the derived data.
For this reason.calibration objects and programs are provided as a part of the computer software so the user may check the calibration and reset the table values at any time.
The actual measurement of the image is by the process of subdividing the television image raster line into small.elements of time that are examined for brightness information. A relative magnitude decision is based on these bits of data and represents relative brightness for each information pixel within the selected area. A signal is generated by the computer display driver to control the display system used for operator interface. The signal complex comprises horizontal and vertical synchronizing signals, blanking signals and the pixel brightness data for the information l~

to be displayed, as described above. The synchronizing signals are used in the present invention along with the ba is timing signal called "dot clock". The dot clock is derived in the computer or frame grabber from a crystal controlled oscillator and, together with the synchronizing pulses, defines the locus of each pixel in the image. These signals are used to assure the synchronism in t~.me arid therefore spatially of the data to be analyzed in the present invention.
One of the display functions commonly available from computers is the so-called graphic format in Which lines may be positioned on the display screen under software control. The present invention makes use of this ability to define part of the image from a camera synchronized to the computer display that contains the im$ge of the cornea to be defined. The computer controlled slit movement signal is coordinated with the data masking function to define the portion of the image far computer analysis. Only those pixel loci that define brightness within the defined margin are stored. This technique reduces the number of data points to compute, increases speed and reduces artefact signal induced error in the analysis.
The present invention may also use shading corrector circuits of conventional design to eliminate artefact signals that can be generated by the "black" level offset and the nanlinearities introduced into the video signal by some types of camera protective circuits. The shading signals are the first and second derivatives of the synchronizing pulses with adjustahie polarity and phase that are algebraically added to the raw video signal before quantizing.
A table is constructed to provide a to kup technique of conversion for a range of measured brightness re resenting a range of known optical density. This table may be constructed with values to obtain any required degree of. prec sion and thereby assure accurate output data for the intended application. The lookup table is constructed in a histogram analysis step Where anterior surface reflectance is measured and t~e relative values which are obtained are then stored. Extrapolation between table entries is quite practical and reduces the numbed of table entries needed to assure accurate measurements.
The micro computer has provisions for graph'c displays of bit mapped information in terms of X, Y coordinates on one of several pages of display memory. These data may take th form of pictorial and/or text type images in storage. Whole pag s or portions of I
pages may be called for display on a frame by frame basis. The magnification to be used for the camera of the pr sent invention is fixed and consequently the area encompassed by the optical section and the constituent pixels are definable i~ terms of X, Y
t coordinates for all elements of the areas defi ed, The initial data sample is masked by selection of only th t section of the image data that defines the corneal image, Each of the subsequent data samples that represent corneal sections are defined by a second range of loci that encompasses only the a ea of the Tyndall image. The pixel clock from the computer display system defines each pixel in storage and in the display.

Computer generated alignment points, or so called "fiducial points", are displayed with the picture of the ey . These fiducial points are placed over the corneal image by movement of the instrument by the user. The operator moves the instrument until the corneal image lies within the defined area, acuses the camera by axial motion until the desired focus figure 's centered on the limbos within the frame, and then operates a sw'tch to accept the data sample and to initiate the data collection sequenoe.
The light reflected from an optical discont3~uity, such as the corneal surface, can be calculated by use of the~formula set forth above for determining a reflected percentge of light. In this case, the values of index of refraction, n, a a 1.000 for air, 1.333 far the tear film and approximately x..376 ~or the cornea and Z.34 far the anterior chamber fluid. Thus, it can be seen that only a small fraction of the incident light wil be reflected by the normal cornea. The losses are calculate in the computer program to normalize the corneal image data fo determination of the effective optical density of image point! in the optical section.
In an alternative embodiment, a color filt r is inserted in the projection path far reducing the blue end o the spectrum to enhance the contrast ratio due to blue or grey 'ris coloration if present. In another alternative embodiment, the filters pass only near infra-red light so that the subject is unaware of the measurement and so that the iris reflex is not stimulated.
The radius of curvature of the cornea is measured in the typical clinical practice by an ophthalmometer or keratometer. The image of an object of known size is observed as a reflection from the corneal surface. This convex mirror provides the data required for the calculations. In practice, the value of the sagittal height V, is small relative to the axial distance a and is ignored.
The use of cylindrical targets in some devices complicates the calculations and, in general, produces rather poor resu~,ts due to the number of assumptions and approximations in the derivations.
By referring to Figures l3.and 14 of the present invention, as hereinafter described, it can be seen that the object AB is an erect virtual image ab in the plane defined by sagittal height V.
The magnification is negative since the mirror formed by the corneal surface is convex. Then, ab/AB = I/O =V/u and since i/f =
1/~r/2) then 1/V + 1/u = 2/r. Eliminating V by solving for 1/V and substituting produces: O/Iu + 1/u = 2/r, from which the radius of surface curvature at the paint of image measurement is calculated as r = 2~u~I/O+x. Because the ratio of object size to image size is large the simplified calculation r + 2uI/~ is used in most calculations.
There are several fallacies in the calculation even before the curvature is transformed into dioptric terms for use by the physician. The inherent assumption is that the surface is spherical and that in constructing a corneal surface map the Central point is accurately defined. sn foot ne3.ther of these conditions obtain in the clinioal setting, The a~entral, point cannot be derived because the instrument ha.s a ca,a~tera lens centered, in the. object that is a series of concentric circles illuminated from behind. The presence of the lens prevents measttre~ents near the center of the cornea but the map is ~canstxucted as a, set of e~qu,iva~.ent radius points from Which slopes are defined relative to the central po3,nt that is not measured. This reqwires that the central surface 1be assumed to be perfectly spherical ~tha,c~h is rarely, if ever, the case.
Iz~. addition to the fact that the surface is nvt usually spherical, the lensmakers formula used as the basis for the computation is only true for paraxial rays and the error increases as the marginal rays are considered for surfaces removed from the center of the .corneal mirror. The results are com~tc~a,Zy e~p,rs,ssed ,gin d.~optr~,c farm that introduces an additional error in that the cc~rnea.~. thickness and rear surface are unJ~x~o~an. The aanversic~n of approxa,~:a.tedl surface shapes to focusing pav~ter in dioptrie form uses a constant to adjust the value for better representat~,on of the true focusing pouaer. ~awe.ve.r, the fact that several, manufacturers of ~eratox~eters and aphthalmometers use differing constants and, in some cases, tables of correction that axe quite non-linear, demonstrates the inefficacy of the method. To express the fca~cus3.ng poorer of the cornea in dioptric terms the area so desari?~ed must be a spherical surface and the corneal thickness and posterior curvature must be known ? Even ~rhen the true cornes:l ga~~e~y~ ,~s as known, the non spherical surface over the area of the entrance pupil of the eye means that any dioptric representation is probably only an approximation and cannot serve as a predictor of focusing power.

S ~ D T D S. fJ~~
~enera~.ly speaking, the present invention generally grcavides both a new system for a~easur3.ng corneal th~.ckness and c~pt3.cal density, as well as a grracess fear measuring the corneal thi,cl~ness and op~tacal density. The system and the mt~thod both provide relative transparency mapping of may types of lasions of the cornea. The invention further provides pachyxnetry With an sl~~ost ~.nstant display of the data to permit a,ss~essment of need for surgery. 'Thus, rune of the important contributions of tha present invent~.an is that it essentially operates on a real-~t~,me basis, rendering alm~c~st immediate results.
The ~aachymeter of the present inventions as ind,~,cated previously, constitutes three major subsystems t~thxch are a ~adj,fied slit lamp for projecting beams of light onto the eye to be e~camined, a television caanera and. lens system for obta3n~.ng the ~.mage of the eye and electronic exr~cuity for def,i.ning and quantifying a. pprtion of the television a,mage. Drive eechar~a.sms are clap provided for moving the optical slit to pradu-ce s~;~,oc.essive images for analysis. Associated computer soft~ta,.re gexfo~.s the requisite drive mechanism control, image se3.ectxon, d~,g~,tal conversion of the analog television signals for comp ter proc~ssi.ng and numerical analysis. This converts the information into a viewak?le measurement of surface shape, thickness, optional c~;ensity area and even a display of der~,ved ,informa,t~,on fox cl~.n.~cal use, Obviously, a video cautput could be connected for displa:~ tea;cning and/or reec~rd k~~pir~g.

The cJLinical pachymeter of the invention also includes amplitude detection circuitry to derive the locus ~of image brightness discontinuities which may be a.ssac~iated with a yesi4n.
The a.nvention inc3.udes znexaory storage for these image po3~nts t~oh3oh are digital representations of a magnitude of the image l~r~.ghtness discontinuities. These ~.mage brightness d.~.soc~nt~.nuita.~es exist 3,n pia~el terms. A. conventional electronic aamputer is used to derive the r~e~,ata.t~e optical density and thickness prrafatle of the o~arnea.
The pachyaneter of the present invention also generates a, $~,splay of the deri~red. density and thickness information for im~ued3.ate use.
xn broad terms the in~rention can be descri~red as an c~phtha~.mic pachymeter for aiding in the determinatitan of the thickness, surface cant~a~zr and transparency of the anterior segment of an eye, The ophthalmic pachymeter comprises, in broad ter~ts, a, ~.~.ght pr~~ecting means, such as a projector, fc~r ilhami.nating a, def3,ned area of the cornea. An fmaging means, such as a telav3.sian oamera, pro~rides for a tele~rision image of selected port~,on o;~ the i7.luminated area of the eye, A video means, such as a ~r,ide:a amp~.ifa.ex~~ raceiWes the image of the eye and, generates and transma.ts a ~t~.de~a signal representing the image of the ere.
The video signals, which are deri.Wed bar the ~t3.deo means, .operates a.n con junction with a converter aneans for con~erti.ng these ~rideo signals into a digital format. The converter means may adopt the form of an analng~-to-digital, _ .converter ~rh.~ch operates i.n conjuncti~an with a computer having a data meanory. A fid~c~,a,a m~eax~s delineates portions of the video image for facusi~g a~,,~ ,ai~~,en,~, A suitable computer program de~.ineates these portions of the. video signal to ?ae converted into this digital for~ma.t. Finally, an analysis oceans is provided for dr~tecting and storing the re~,ative br~,ghtness levels within the delineated areas in tl~e aforesaid data memory. An address counter is also employed to enab3.e s~tk~sec~uent addressing .of the stored information.
'The present invention also relates to several. prose ses asso~.iated with the finding of thickness axtd relative optical density of the cornea of the eye, as well a.s ascertaa,na.ng extent of lesions or other optical discontinuity. Ire ~ne aspect, the invention compr~.ses a method for ascerta~.ning the een,t of the les~.on or ~athex optical discontinuity of the t,iss~u~e of tha eye.
This method ut~.~.i~es the steps of illuminating the tissue area to be analysed. An image is generated and this image is ,~a,nt~fi,ed.
Qortions of the ~.a~age which contain the lesion are del,3neated and a reference area is also delineated. Thereafter, the. delineated areas of the image are converted into digital form for subseguently analyzing the numerical magnitude of the digital form data.
Tn another aspect, the present invention prova,des for a method of ascertaining the surface sh~rpe and local thi~l~neas of the cornea of the eye. In this ease, the method inZto~,~:'es seleotively illum5.nating the tissue area of the eye and quaritifyinq the, image c~f tha.t tissue. Again, portions of the cc~rneaZ a~xaage are delineated. and a reference area _ is also del5,raeated. These de~.~.neated areas are then converted intca dxgitay f,cy~ for subsequent analysing ~af the relative nu~enerica~, mag~tit~de of the digital. fore data r The ~.nventa.c~n may further be described as a system for producing surfacr~ contour maps of the ct~rnea of the eye and comprises proaeotion illzamination means for psodue3:ng a def3.na.'~aale spacia~. c~:el~.neat3.on oaf ~ecarneal contour. A television c~a~t~era means renders the illurna.nated areas into electrical analog fort; signa~.s.
The anal.oc~ sir~nals arc converted in a d,iga;tiza.ng means for cQZ~puter~~.ega:ble operation ~ A computer means ca3,culates the corneas surface shape from the digital signals.
The preferred embodiment of the pachym,eter of the present invention makes use of "bus--cards'" in a F~ type ~,~:.croco~p~ter to provide fast and aocurate measure~aents and requa,s3.~e mt~tian contro2 caithout the apparatus associated limitations of the griar~art sys-~
terns m zn an alternative construction, the prr~c-essnr and displa~r can lie constr~.tcted as a dedicated instrument at lr,~wer totals, oust to fabricate lout with J.esser capacity to do othex tasks. W~.tIa. careful use the system ~ra.7.l consistently provide inf~s~ntat~.s~n to the physician to c~u,an.tify corneal thickness and optical dens3.ty and to map the surface shape of the eye. several computer generated data d,isp~.ay formats are made available from a numerical area or average density and thickness to computer monitor displa,~rs such as a Mire frame pseudo three-dimensional rapresentatian~ ooZor o,~d;,ed thickness map or other depiction of the area cxf ~.r~texest allc~,ts the ph.ys~.oian to seheet an appropriate type of infvrmati.on presentations This a.nvention possesses many other a,d,vantaqes and has other a~

ola~ acts and advantages whioh will become z~ore o~:ear~.~r a;~rparent from a consideration of the f~rrm:s in which it ma:~r be em~b~da,ed. The following detailed description and the aGC~atnpan~r~,n~ dra~r~.ngs ~ ~,~.ustrate cane of the practical embad~.~nez~ts of the inaention, alths~~gh a.t is to be ~an:derstoad that this dets:~.~;ed deserip~t3.on, arid the aoaompan~r~.ng drawings are set forth anl~ for pposes of allustx~ating the general principles of the inw~entxon. 'thus, it is to be underst.orad that the detailed descripta.on a,nd the dratvtings are not tc~ be t.a~en in a limiting sense.
~q BR,~E DES~RIFTICtN of FiE D' .
laving thus described the invention in general, terms, reference wi~.~, now be made to the accampan~ri~zg drawings (seven sheets) in which:
Figure 2 is a schematic view showing same of the mayor components of the system of the present inventir~n;
Figure ~ is a toga plan view, partially in hari2ahtal section, of the optical pachymeter constructed in accordance with and embodying the present invention;
F~.gure ~ is a perspective view of an alternate canstrt~~ct,xr~n of a focus aid mech,anis~n for~rning part of the system of the present invention;
Figure ~ is~ a front elevati~anal view of the r~phtha~,xnic pach~rmeter of the present invention;
Figure .~ is a side elevational view of the ophthalmic pachyxneter Qx the present invention, partiall~t in section, and illustx~ata~ng the major components in the interior thereof;
Figure ~ is a schematic view showing the optatas a,nd optical paths invra~.ved in producing a slit image at the eye of a sul~~eat;
Figure "~ is a plan view of a television screen and sb:awing, in enlarged detail, a portion of the television raster th;erefr~r;
Figure S is a schematic diagram of a portion ef the elaatric circuitry empl.a~red in the system of the present rove-nt.ion;
F~,gure ~ is a graphical illustration showing a tele.~is~,on waste form which mar be. prcaduced in the system of the present. i.nventian;

Figure 1aA illustrates a front ele~at3,ox~a1 view of the eye with half~Tyndal~. images for focus and al,ign~tent super~.~gQSed;
Figure ~.t~8 illustrates a front elevational view 4f the ere, similar to FS.gure lo.~.~ ~r~.th the half Tyndall ia~a~es ala.gned and in the position where they would be centered in a fidu~ia~, mark;
Figure l1 is a schematic view shotaing the fiduoial figure employed for alignment in the present in~tentir~n;
Figure ~.2 is. an il7.ustration of a horizontal crass section of the eye fear reference purposes;
Figure 1~ is a graphical illustration showing the c~~e-om~etry of the iz~age analysis employed in the Placido methad and in the present invention;
Figure 14 is a schematic illustration of an opt~.oa~. ray trace for deriving thickness of a transparent ~embex, aid Figure 15 is a schematic illustration sht~~~,ng the s~~~r~etr~.c relationship of the imagre o?atained in accordance with the present invention with respect to a projector lens and camera lens.
~l DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
Referring now in more detail and by reference characters to the drawings, and particularly to Figures 13 to I S, in calculating the posterior surface, the displacement of the projected ray by the refraction of the cornea must be included. Referring now to Figure 14, the virtual image of the slit at the posterior corneal surface is laterally displaced by an amount that is related to the angle of incidence and the local tissue thickness. The rear surface distance D' is derived from the incident ray angle ~; .
The entering ray from A (see Figures 13-IS) exists as a reflection at G along path 1. The ray also enters the medium n and is deflected to point H by refraction.
The ray from H, in turn, exists at I along path 2. Looking back, the object appears to be located at both C and F. The distance desired, t, is derived from the following:
from triangles ABG and BGC, QAGB = dBGC because they are both complements of ~;.
Therefor, BC = D.
Then BG = D tan ~;, BI = BF tan ~; and GI = 2t tan ~~.
BIF and BGC are also equal to ~; and so, it follows that;
(BG + GI)/BF = BG/D since BI = BG + GI and BC = D. Rearranging, (BGBF)-(BG/D) -(GI/BF) and substituting for GI, (BG/BF) - (BG/D) _ (-2t tan~r)BF = (-2t sin~~)/(BF cos~r).
Using Snell's law, (sin~;lsin~~ = n);
(BGBF) - (BG/D) - (2t sin~;)/(n BF cos~~) and, BG = D tan $; _ (D sin~;/cos ~;).
Which leads to ;
( 1 +(2t/ND)) ~ (cos $; /cos fir) ( 1 BF) = I /D and BF = D( 1+(2t/nD) (cos ~1/cos fir) The distance required is, JF which is derived, in turn, by, BF-B J = BF - t.
Then, substituting, t = D (( 1 + 2t/nD) ((cos ~;/cos fir) -JF
Classic mathematical image analysis requires that groups of "pixels" or picture elements be examined as a matrix to find the location of each portion of the reflection in two dimensions and a numerical value or relative image brightness for each.
With suitable computer programs, this technique provides the data for the generation of a three dimensional model of the cornea or lesion to be measured.
The surface contour of the cornea is also generated by the pachymeter of the present invention. The use of ultra-sound measurements in conjunction with the date derived from the present invention provides information to the cataract surgeon from intraocular lens selection. The main light focusing power of the eye is provided by the cornea and the fluid filled anterior chamber in front of the crystalline lens.
The posterior surface of the cornea forms a lens which adds to the focusing power of the corneal anterior surface to air lens. For this reason the anterior corneal curvature provided by conventional keratometers cannot provide an accurate measure of corneal focusing power even though the measurements are expressed in dioptric terms. The present invention provides both surface shapes so that an accurate measure of focusing power in diopters is made available to the user for surgical planning.
The present invention provides a repeatable record of the corneal contour and thickness for inclusion in the patient records and improvements in accuracy and speed that are requisite for clinical use.
.33 Referring to Figure l, it can be seen that the ophthalmic pachymeter of the present invention comprises a television camera 20 having a conventional lens and which is aligned with and receives an image of the eye 22 of a subject through a beam splitter 24 for later quantification and for providing a television image of the eye for analysis.
Referring to Figure 3, it can be seen that the apparatus comprises a conventional incremental motor 26 for positioning an elongate aperture, e.g. a slit 28, of a slit form 30 in the focal plane of light projector represented in Figure 1 by a lamp 32.
The slit form 30 may be operatively connected to a suitable slide assembly 31, as also best illustrated in Figure 3 of the drawings.
The slit form 30 and particularly the slit 28 thereof, in conjunction with the lamp 32, will produce an image at the eye 22 through the action of a projection lens 34 as shown in Figure 1 for selecting sequential images for analysis. Referring now to Figure 6, there is illustrated a Kohler arrangement 36 which comprises the lamp 32 having a dense filament 38. In this case, by further reference to Figure 6, it cari be seen that the image of the filament 38 is formed at the entrance pupil of the projection lens 34 by means of a condenser lens 40.
Adjacent to the condenser lens 40 and in optical alignment with the condenser lens 40 is the slit form 30 carrying the optical slit 28. This slit form 30 is preferably mounted on a carrier (not sh4t~n a~n d,etaa.la , The carrier is moveable in a direction perp~enda.c~lar to the slit ~8 by the aforesaid 3norementa~. Factor ~6.
The ~.mages of the slats are brought into focus in the same p~,an~e as the television camera 20 ?ay the projection lens 34 and a s',~stetn of mirrors and pa~i.sms, designated sehematieall~ by reference numeral ~2 in Figures ~., 2, ~4 and 5, A fixation la~ctp 49~, sametimes referred to as a "target lamp" or "fixation target lamp°, as shown in Figure 1, is also provided for operation with t~xe beam sputter 24 p as shorn, This cc~~nbination of the lamp 32, lenses 3~ and and m~.rrors and prisms ~42, as well as the slit form 3~, funotion as a sli~G lamp proj~ect~or, p,eferring again to Figure 1, it can be seen that the television .camera 2o generates a signal representat~a.~te of the image of the eye which is transmitted to a video ampl~.fa.er 4~ for axnpla.;~ication and mixing video signals for anai,~rsi.s. A flash analogxto~-digital converter 48 receives the output of the video amplifier ~G for processing and digitizing anal~xg signals rec~eatved from the television camera 20. A data buffer 50 reoei~es an output fx~am the ana~.cag~to-digital converter ~48 for directing tie d3.g5.tal data to and from a storage in the form of a da,gital data memory 52.
For example, the data bv.ffer 50 and the data memory 52 raay form part of a conventional computer which is z~ot i~.~.ustrated a,n detail herein, In this respect, it can be observed that manor Qf the components are shown in schematic. form (rectangular bo~es~ in Figure 1, The digital data which is directed to the digital data memory 52 constitutes a storage of the numerical brightness of each element within the fiducial boundary. An address counter 54 is provided for determining the location and storage of the pixel brightness data for each pixel in the image. A mode controller 56 is connected to the address counter 54 and is provided for determining the sequence of operations of the system. The mode controller 56 receives an input from a computer interface 58 which, in turn is connected to the data buffer 50 and is also connected to the analog-to-digital converter 48, as illustrated in Figure 1.
The computer interface 58, operating in conjunction with a computer, controls the system elements through the associated computer. In this case, a display driver 60 is provided for controlling the aforesaid fixation target lamp 44 visible by reflection by the beam splitter 24. This serves to render the apparent location of the fixation target lamp 44, namely from the beam splitter 24 coincident with the optical center of the television camera 20 and its associated lens system.
Turning now to Figures 10A and IOB, it can be observed that there is a representative image of the eye 22. The slit beam 28 illuminated anatomical features are visible as a Tyndall image 62 representing those portions of the eye, such as the corneal epithelium, the stroma and the endothelial layer which scatter the light. An iris 64 in the eye is not the area to be measured and therefore, the illumination of this area is an artefact of Tyndall illumination. The iris image 64 may further be diminished by limiting the spectral distribution of the slit beam 2s through the use of a color filter knot sh~wn~a In addition, a slit projector, or slit projectors if more than one is used, produce spewu.lar reflections 66, as shc"wn in Figure 10, and which are boated in ""X, Yn coordinate space, depending upon the surface curvatux'e of the cor~aea of the eye ~2. The associated computer, through the action of the computer interface 58 and the mode controller 56 and display driver 60 see Figure 1~, restrict the sample data to the Corneal section ~~ of the eye made visible by slit illu~n,ination, The iris 64 may be dilated maximally to provide a uniform dark baokgrot~nd for the optical section and the slit height, which is restricted to eliminate bright reflections above and below th-e area of interest.
The operator of the ophthalmic pachymeter is provided with a computer-generated (figure which is used as a fiduoial mark system, ~.llustrated bar reference number 68 in Figure ~.1. The fiduoial marks in this fiducial mark system 6s are located arowmd the center of the display monitor, The figure is preferably aoft~rare~-controlled to coincide with operation of the incremental or ""stepper"" motcar ~~ which moves the slit form ~0 and hen~~e, the a half slit 160 also fn this farm 30. Thus, the oomguter generated fiducial marks are designed to be coincident with the .syit pas5,tion so that the operator is provided with a focus anal. al3.gr~t~nt aid.
Referring now to Figures .~ and 5, it can be seen that th;e slit lamp illuminator components, e,g, the Lamp 32 and slit form 30, as well as the te~.evisa.on camera system 20 are mou~xted on a ~tovea,ble base 7~ which comprises a frame casting. A vertical, ~,s~,t,~,oni~.
=r el.e~ment, ix~ the nature of a vertically arranged support shaft 7~ is operative mounted on the base castings a0, as i~,lustrated.
~enerall~r a~~.es "~.~ so~ha,ch support toothed wheels (not shown) axe located in the base casting ?0, as best shown in Fic"~ure 5, fpr motion toward and away from the subject. The device also comprises dust covers '78, which cover the toothed wheels . A friction cre~a'ting z~ne~mber '~6 is operated bar a lever 82 against the table surface 8a tca cause the instrument to be moved by the operator for ft~cus~.ng and, ala,c~nxnent. Th.S.s arrangement allows for motion toward and at~a~ from the subject, as indicated. The base casting "~4 is provided with internal bearings knot shov~m) to permit the assemaay~r tra move transversel~r, that is perpendicular to the f~arward and backward motion parallel to the optical axis of the instru~aent. The toothed wheels '~6, located under the dust covers 78, serve to constrain the motion relata.ve to a table 8o and hence, the patient so that mcwe~nent occurs only in a specif ied area .
The vertical positioning element, such as the support shaft raises and lowers the instrument relative to the subject to per~ni.t centera,ng of the image in the televis-ion picture, The subaect is positioned at the table 8~ with a ta,~ale mat~nted ohin and brow rest of conventional design for positioning and stabilizing the head during the xn:easurexnent. Inasmuch as this chin and brow rest is of a c~rnvent3,onal construction, it is neither illustrated nor described in any further detail herein. Hcweve~r, the base casting '~d is provided with an upstanding handle 82 for manual manipulat~.ar~ bar an operator of the apparatus to enable posit3.oning of the instrument With respect to a subject a.~d which is also h~:reinafter described in more detail.
"the beam; sputter 2 ~ may k~e mounted on the base plate 8~ Qf a housing 86 which, hr~uses many oaf the components oaf the ophthalmic pach~rmeter, such as, far example, the televis.ic~n camera 20 the condenser lenses 4U, the slit form 30f the ~,a~tp 32 and the mirrors and,~or prisms .42. Located beneath the beam sputter 24 is a printed ca.rcuit assembl~t (not shown) . This printed c~,rce~3t assembly may c~antain the fixation lamp 44. Othert~tise, the f3.xat3.c~n lamp 4~ z~a~ be mounted alcove the beam splatter ~4 in the maz~ner as best illustrated in Figures 4 and 5 of the drawings. This beam sputter ~4 and the fixation target lamp 4..~ pr~svide a bright target for determining the paint of gaze for the sula~ect. The brightness of this target. may be controlled to permit persona with ~,otar v~aual acuity to pexceive it and to fixate upon it.
In a more preferred embodiment of the invention, the fi~ration lamp ~4 is preferably a light-emitting diod:e~-t~rpe lamp and is preferably bi-~co~.ored with pulse drive to present a ~risib~.e pulse stream of alternative colors at about a one second interval race.
The use of this type of fixation lamp 44 and the associated drive provides a wide ranr~e of brightness so that the tarr~et can be fixated upon by the subject irrespective of visual acui,t~ of the subject. The co-axial location of the ffixation target assures maximal ability to accurately reconstruct the three d~:mensional ,data. ~~he f~.a~~rt~,on lamp ~4, which causes the iris 64 and the sclera to be illuminated, snot only provide for an im-aga of the eke, but also enable an image to be generated for recoa:~d-keeping purposes.
The normal illumination levels, when slit 3z~ages are being arecorr~ed, a.s usually inadec;uate to cause surrounding tissue to tae well. defined for overall viewing. The common sl~.~t lamp aaa~era uses the optical system of the bio-microscope and due to the length of the focal ratio of these systems. A large amount of flash enexg~r is required for e~pasures. The present invention, ho~e~rer, prouides a much more efficient optical design and thus, the flash enere~y is reduced by orders of magnitude, when compared to conventional slat lamp photography. The reducta.on of light exaerc~r entering into the eye isf of course, a desirable feature for subaect safety and comfortF and also ensures more rel3.able data.
The operator of the ophthalmic pachymeter will position the half slit a.z~ages ~~ a as shown in Figure 7.oA, into co~.ncidence so that, in effect, the two half-slit images form somewhat of an ~~5~~
shape. This will occur with reference to the fidu~c~,a~. f~.gure 68 of Figure 11, wh,~.ch is displayed for the operator, to thereby al3.gn and thereby focus the instrument. The half-sl3as, as shown, are effectively positioned by the computer in the optical center l3.ne of each projector. The operator moves the instrument, preferably by ~aanual manipulation of the handle 82 in order to obta~.n this coincidence, as hereinafter described, in order to form this ~-~t~tpe image arrans~ement. T~lhen the s-type-Tyndall im$ge has been formed of the half Tyndall imagesf the operator may then take the necessary data.

'The motors 26 which move the slit form 3o will sles~ the full length slits 2~ of Figure 3 across the eye from, each side sequentially to provide the data sequence wh3,ch will ultimately be stored for analysis, The data is masked blr software i.n order to elimanat~e extraneous material. The arc of the Tlrndall 3;mages lies oza only c~x~e side of the iris section illuminated by t~a,e l3.ght which is passed through the cornea and has a definable maximu~t number Qf pixel loGa~ at the apex from the iris Line. The area of th,$s p~.xel loci is defined day software within the system far each frame and only the data which falls within this defined area is stored for analysis. As a possible exception, a small area at the center which contains the reflection of the fixation lamp ~:ay also be stored for compensation of involuntary movements of the eye.
As indicated previously, the operator of the pa.Ghy~m~,et.er can position the half-slits images 62. This can be acct~2pl~.sed by "
m~anua~. xaanipulation of the handle 82 in carrier to gas~.tian the teleu~.si~an camera ~a in three dimensional space relat3,t~e to the eye. The desired ala:gnr~ent is obtained by viewing the d3,splay before recordation of the data to be anal~rzed. The get~~ation and positioning of the box, circle or other limiting fid~.<c3.al marking is by well kn:~awn computer techniques that are not detailed herein.
The caperator simply adjusts the controls so that the optical sections coincide at the center of the display mcrnitar. Th3,s acta.c~n assures the operator that the. focus and area be3,n~ ~measu,rgd are cnrreet. The focus and image location are sim~u~,taneously adjusted by the operator with reference to the display that shows, V!

the i~aage froxa the cannera with the fiducial ma.rk3,ngs saaperimgosed.
A picture formation of a Tyndall image 62 is generated i.n the television oaa~exa 2fl. A given point on the Tyndall image ~2 is pr~a~ acted, onto the photo--sensitive area of the television Ga~nera 20~. The datum of this given paint on the Tyndall ,image, after an analog~tca~d~.gital conversion, represents an X, Y locus with associated brightness. The slit 28 pas3.t3.oned, under co~tput~er ~contro?~ by the incremental motor 2s is at a kncawn location relative to the optical centerla.ne of the camera 2~D.
The pr~r~ectoac optical axis relating to the camera axis ~,s established in manufacture at a known angular relationship. Since that .angle is known, the magnification is known: and that the skit posit.ivn is also known, the angle ~ is thexeby d~ef3ned~ in the associated aomputeac soft~rare. The height of the datum above the reference plane .oh is then calculated. Each raster line intersection with the Tyndall image 62 is used to calculate the associated height value. After the series of i~tages which ctm~prise a complete measurement are so defined and stored in the computer memcary~ the surface contour far both surfaces of the cornea and the local thickness are displayed for use, The image which is generated nay be identified as either a left eye image or a right eye image by means of a switch (not shown and which can be located in the instruament base and whl,ch is else interfaced to the computer. With this identificat,3.on, the location of i~h:e cursor in the fiduoial image 68 is determined in the aomputex softtaare. A transducer (not shown) xnalr be vxta.l~.zed to ~a pro~ride a signal representative of instrument lateral d3.splaceaaent and :~s~ interpreted to determa.ne the eye being axt.axn;aned, due to the fact that the slit lamp 3~ is always displayed in th.e t.entpora~.
direction for use"
Tn the preferred embodiment, a point at the vertical center of the cursor an the fiducial figure 68, displaced a feat piaceZs tamard one side, is identified in the software and can serve as a sample for black c~.amp~.~:g of the video signal. and which ~,s usually accomplished by conventional circuitry. 'The ~.mage the cornea as 7.~acated and stared by computer softvaare, based øn known characterastaas of the aerneal image. All initial p3.xel,values for the enclosed. lane segments of the corneal a.x~taga, so 5.den,tified, are averaged for reflected light intensity in terms of pixel bric~htn~ss and the resultant numerical constant is used to determine the optical ehara.cter of the remainder of the Tyndall image X62. After determination of the corneal pixel loci, the corneal thaokness is derived by known magnification projection angle, surface shape and pixel. patchy The data are then stored by loc~.tian in an area of the memory for latex uses Referring again to Figure 6 which indicates a ~~5hler proje~ct~ars it can be seen. that the image of the slits are brought onto focus in th~~ same p~.ane as the television camera 20 by the projection lens 3~ and the system of mirrors or prisms ~2, as previausl~r described The beam path is folded lay the m~,rrors or prisms ~4~ a~n order to achieve compact assembly. The focal length, c~f the pr~~ectian lenses 34 is made to be as lsang as possi~sle to reduce beam convergence ox divergence at the eye which would otherwise degrade the Tyndall image 62.
In general, the projection lens 34 is selected to provide an aperture sine function on the order of .OS or less for best results. The aperture sine is calculated from the optical components by the formula; f/d2 where f is the focal ratio of the lens and d is the distance from the slit to the exit pupil. The brightness of the slit image E is calculated by the formula, E = (f/d')DB, where D is the optical transmission factor for the lens and B is the luminance of the filament source, e.g. the filament 38. The use of aspheric condenser lenses, optical coatings for all surfaces and a low ratio beam splitter for the fixation target permit the use of lamps in the range of 20 Watts that provide over 400 Lumens as the light source. The minimum brightness level of the slit image reflection is dependent upon the sensitivity of the camera employed. The reflected light is on the order of 4% or less of the incident light and the greater the illumination level of the diffuse reflection, the better the signal to noise ratio of the resultant television signal.
The use of halogen cycle lamps improves the stability of light output with time and provides the best available lamp design. In addition one ar more optical filters 90 located on a support pivoted to the housing 86 by a pivot pin 92 and positionable by a handle 94 (Figure 5), or computer controlled mechanism (not illustrated), are included in the illumination path for selected illumination wave band determination. The optical filter 90 also __._ _~__~_.___ CA 02127426 2005-08-22 serves i~.o limit energy delivered to the eye 22 to reduce the possa.~aility of phot.a-toxic reaction hazard to the sub~eate The apfi.a,aa~. filter 9~ for th~.s purpose has little or no ultra-violet or a.n~xa-red transparency a The television display is in the forxa of a raster as showta in lEigure '~ m The television has a monitor 10a which df.sp~:a~rs the v~aua~. ~tnfax~ati~on in time sequence ~ The beam curreant is ~.ow for k~lac~ areas ~.4~~ and high for white areas ~.0~ and soaled in magnitude to recreate the brightness range of the original soene~
The televisi~an camera 2~o generates the voltage analog c~f scene illumination that is provided with synchrc~ni~3.ng signals to assure that the ta.xc~e sequence as reproduced is a faithful recreation c~f the scene bei~ag photagraphed~
S

T~-AGE PROCESSING AND CdPERAT~C~~I' The following section more speci-f icall~r describes the process employed in determining thickness and topagraphy of the c~arnea.
However, and while the circuitry as shown in Figure 8, ,literally constitutes a part of the apparatus, it is neverthelesss de~saribred in~connection with this image processing and operation, since it is integrally related to the image processing and operation.
Tn Figure 13, the relationship between the T~ndall image 62 and the t~apographir of the Cornea is shown. Along each raster lane in i~he television display, there is a detectable edge of the Tyndall image which has a virtual image location displaced b!~ delta d (add. This displacement distance is from the point at,which the beam would have intersected the optical axis, if untieflected, as best shown in Figure 15. Fram this image pixel laces the height of the datum above the reference plane, delta h (eh), can be calculated. The calculations are performed for all intercepts in all data frames to provide a n~:atrix of X coordinate loci ~roxn 'sthiah the topographx can be plotted.
Referring naw to Figure 9, the voltage waveform produced b~
the te~.evi,s~.on r~amera of the pachymeter, is illustrated. As indicated previously, the beam is low for lblack areas 1~~ and high.
for brighter areas 10~. The brightness amplitude ratio of the anterior edge of the corneal section to the dark papillary area representing the anterior chamber is. used as a reftren~ce value for ~.ens~ ref~,ection ~.ssess~nent. The pixel amplitudes for a~.~. elements of the reference areas are averaged to provide the baseline reflectance value.
The television signal voltage gave farm, as sh~tam in Fa.gure 9 is a single raster line of video information in trrhxeh there are bright areas 10~ from the image of the cornea and a brighter image of the iris 6.4 (represented by the bright a,re-as x.0:4) illum3:nated by the slit beam after the latter gasses through the cor.naa. A sync pulse signal 1.~.~ precedes each line of pictorial information carrying voltage levels. After the sync pulse ~.~.2 a sh~srt period of a low level blanking pulses 114 follows. The ?~lankirtg pulse 1~.4 insures that the displa~r is off while the beam is retraced to the start c~f a neca line, The black level, represented by reference numeral 1~.~, is the most negative of the pictorial data vo!~.tages in the video com~pcasa.te signal. This level. is determined by a keyed clamp ~cirou~,t of conventional design ~rhere a selected spot, in the image representing the anterior chamber signal, is sa~p3,ed and used as .a azzini.mum brightness determinant. As the vo~.tage 3ncreaaes, the brightness also increases in the displayed image from b~.a~k to peak white 118 representing saturation of the signal. The v~a~.tage 3.eve1 produced at saturation by a "White" image x,1.8 is sh~a~ by the dotted line at the top of the illustration. The brightness profile Qf the corneal image will vary as the local optical dens3a~r and index of refraction varies.
At the leading edge in time of the corneal reflex signal, the signal. rises to .a peak 2~.~ which ,represents thc~ c~rrnea to air interface a The amplitude of this signal is c~u,ite constant from subject to subject and froze time to time. This constant interface signal is used for signal reference again;~t which aee~flex measurements are made to quantify corneal transparency. Each succc~edfng raster line will then provide a dens~,t~t profile for a different portion of the cornea.
In the preferred embodiment of the present inventi~n, the optical slit form 30 is moved in small lateral increments by the incremental motor 26 for sequential data sampling. Tn an alternative embodiment, the optical slit for, 3~ and 3ncrea~tenta:l motox 26 are replaced by rhonchi rulings of suitable pattern dimension to provide several parallel slit beams in a single e~p~asure. The plural beam system reduces the time reqt~,ired. for data acquisition but complicates the computer processing of the data from the Tyndall images 62. Tn a further alternative embodiment, the slit 28 can be replaced by a liquid cr~;tstal display element so structured as to form electronically selected transparent areas substantially equivalent to the var~.!ous slit positions in the preferred embodiment of this inventa.an.
Each exposure containing the Tyndall image or images 62, is converted to digital form by the analog to digital canvsrter 48.
Thrcaugh the action of the data buffer 50, the mode ac~ntroller 56 the address ce~unter 54 and the digital data memo~~.~y 52 these sequential amplitude values are stored for use,. The data in stoacage represents the pixel brightness versus locus for each slice of the cornea to be analyzed. Each successa~ve pix~aJ. of each successive frame is then multiplied by a constant dera.~~sd from the cornea to air interface signal average and the opt~,cal constant that corrects far the lower normal brf.ghtn~ss. As each point is calculated, it is returned to storage in the same sequence for later Computation and display. Tyndall ~.l~.urn:inatic~n prcavi.des three-dimensional data sequences of data that are tran7sferred to the oo~n~suter by the action of the computer interface 5~.
figuxe ~ represents a schematic diagram of part of 'the ele~ctrc~n~c° circuitr~r emp~.o~red in the preferred embad~.mea~t of the present invention. The composite video sa.gnal from .the television camera 20 is applied to the input 120 c~f a signal c~andit~.r~n3.ng amplifier. The terminated signal is buffered by ~:n emi.tte~r fo~.lo~er l~~ s~aha.ch drives DC restoration and sync str~.pper netv~rtarats 1~4 and 1~~. The DC restored and limited video a,s bruffered by a seGOnd emitter fo~.~.otaer and serves to drive clamping and m3~x3ng amplifiers 1~8 and ~.30~
computer derived black. reference tima~ng s.i.gnal ~.3~ i,s generated in temporal synchronism with the area of the pa~cture from the television camera 20 ~rhi.ch defines the pupillary ~;rea near the center of the pa.cture. This pulse is condit~.aned by ~tt~n4-stab~.e circuits 134 to provide a constant amplitude and constant wra.dth sampling pulse. This sampling pulse, via a capacitor x.36 a~,lo~ts the capacitor to store a voltage sample of the raw vxdeQ that represents the ""black'" level. The black reference level thus generated ~aa.ases the amplifier 130 for use in the analr~g to digital converter ~$. ..
~igna~.s fra~n the Computer are used for regeration of the te7.evision t3.ming in a conventional integrated circuit device which makes use of a coznpr~site sync signal 14~ and dot clocl~ signal ~.9:~ from the computer display driver. The computer generated fiducial signal x.46 and regenerated composite sync are smiled via resistors ~.~4~ with the tideo signal from an emitter followex x.50 fcr pa~aavid.ing the monitor signal. The m~rnit~ar signal 3.s used to drive a c~onvent~.onal CRT display fax use as a viewf~,~txder blr the user oaf tha. pachymeter of the present in~rent3.on.
The display of the data can take the farm of a single fraiae~s ~.nf~armat~.on that can be displayed as false colored areas for relative transparency, fc~r example. The entire set of frames may be combined to form a virtual three-dimensional display of surface contour or memk~rane thickness as needed. T~:a data also malt bt presented simp~.y as a numerical value for average optical d;~ens3,ty, density area or rather f4rms that the user finds mast useful, by the use of well-known display techniques. In me t, if not all, cases, the exact optical density profile is of less interest to the clinician than the shape. and thickness, Fear exa~ctple, in rr~fractive surgery, the in~cisisan depth. should be at least ninety percent of the local corneal thic~tness without total penetration.
When a sua.table image ar sequence of images has been stored and the requisite computations performed in the computer, the digital. infor~n:ation that defines the cornea can be d3.splayed in some arbitrary ~colr~r upon the monitor together ~rith the alphanumeric 3,nfarmation image from, computer by conwentic:~al video mixer means. .A,lternatively the data may be presented, ft~r use S,n any of several formats such as plotted graphs, tabular ner3.ca1 for~n~ pseudo three dimensional shaded surface plots or other formats that are well known in the art.
~eferra.ng again Figure ~ , the motion imparted to the skit form.
gar sa~-cal~.ed slit carrier member is aontro~.~,ed b~ the cc~mguter through the action c~f the ,incremental stepper mflto~ ~6. In an alternative embodiment a second slit 160 4f ~.esser length than the slit ~8 is prtavided in both beam projection paths far the rpose caf focusing the instrument ~ The two half sl3.ts so priced are placed by the. operator into contact at the point of ref~,ecta.on: of the fa.~ataon lamp 4~ to establish proper alignment prior to recordation of the image sequence.
Thus p there has been illustrated and descr3,bed a u;n,iqta.e and novel ophthalmic pach~zneter which enables determ5.nati,on of the thickness and relative optical density Qf the cornea; o~n a a~ea~.-time basis and which thereby fulfills all of the ~bjeots and a;~l~antages which hare been sought. Tt should be understood that many changes, xncadifica.tir~ns, variations and ether uses and applicat~.~ns ~til1 become apparent to those skilled in the art after ct~n-sider~.nt~ this specification and the accompanying drawings. Therefore, and and all such changes modifications, variati~ans and other uses and applications which do not dApart Pram the spirit anr~ scope of the inuention~ are deemed to be covered by the in~rent~.on.

Claims (13)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An ophthalmic analysis system which can be used in determining one or more physical characteristics of the anterior segment of the eye, comprising:
a) light projection means (32) including a slit means (28), for illuminating defined areas of an anterior segment of corneal tissue;
b) imaging means (20), for providing a television image of selected portions of the illuminated areas as illuminated by the projector means (32);
c) means for causing a movement of the slit means relative to and transverse to the anterior segment to obtain a series of selected images;
d) video means (46) cooperatively located with respect to the projector means for receiving the selected images of the selected portions of the corneal tissue and for generating and transmitting a video signal representative of these images;
e) converter means (48) for converting portions of the video signal into digital format; and f) an analysis means (58) for detecting and storing relative brightness levels within the defined areas and which brightness levels are directly correlated to the one or more desired physical characteristics which can be determined, characterized in that the axis of the slit means is held in a fixed position relative to the cornea of the eye and that the movement of the slit means is transverse across the anterior segment of corneal tissue.

2. An ophthalmic analysis system according to Claim 1 further comprising a masking means (58) for delineating a portion of the video signal to be converted into digital format.

3. An ophthalmic analysis system according to Claim 1 capable of functioning as a densitometer for aiding in determining the thickness and relative optical density of corneal tissue on a real-time basis and wherein:
a) said imaging means (20) generates a series of digital encoded television images of sequential individual segments the portion of the cornea of the eye illuminated by the projector means (28, 32); and b) the light projector means (32) illuminates individual preselected areas of the corneal tissue in which the digitally encoded images are to be generated and which operated in conjunction with the slit projector means; and c) the analysis means (58) comprises processing means for receiving the digitally encoded images from the slit projector means and generating data used in the determination of thickness and of the optical density, said processing means generating the data related to the digitally encoded images at substantially the same time that the digitally encoded television images are being generated.

4. An ophthalmic system according to Claim 3 wherein the system further comprises as part of said analysis means:
a) storage means (52) associated with the processing means for receiving and storing the processed digitally encoded television images in digital format;
and b) means (56, 54) operatively connected to the storage means for regenerating the images which were stored in the storage means.

5. The system according to Claim 4 wherein the digitally encoded images are comprised of a plurality of digital data points and the system further comprises discriminator means for reducing the number of digital data points in the digitally encoded images processed by the processing means.

6. The system according to Claim 3 wherein a corneal-air interface is compared with a corrected reflectance of certain portion of the corneal tissue in order to determine relative transparency.

7. The system according to Claim 3 further characterized in that an analog to digital converter means (48) is provided for converting the television images into digital format.

8. The system of Claim 3 comprising a fiducial means (58) for delineating portions of the television images which are to be converted into digital signals.

9. A system according to Claim 1 which will also produce a surface contour map of the cornea of the eye and which also comprises:
a) the light projector means (32) for illuminating areas of the cornea for producing a definable spatial delineation of corneal contour;
b) slit image means (20, 28) being movable across and relative to the surface of the cornea in which a contour map is to be generated;
c) the video for rendering the illuminated areas into electrical analog signals; and d) computer means (58) for processing the digital signals from the computer means and generating data to provide a determination of the corneal surface shape from the said digital signals and generating control signals for generating a map of the surface contour of the cornea of the eye.

10. The system according to Claim 9 wherein said system comprises program control means (56) for controlling the computer means and to enable the computer generated control signals to be generated control signals to be generated into a visibly displayed surface contour.

11. The system of Claim 1 further comprising an improved circuit arrangement which comprises:
a) computer interfacing means (58) for connection to a digital computer;

b) a data memory (52) operatively connected to said interfacing means and said converter means 48 for storing the corresponding digital signals;
c) a mode controller (56) operatively connected to said computer interfacing means for determining and controlling sequence of operations;
d) driving means (60) operatively connected to said computer interfacing means for driving a fixation light means for maintaining a fixation of the eye of a subject in relation to a slit means (28) moving with respect to the plane of the cornea.

12. A method practiced with the system of Claim 1 for calculating the shape of the anterior and posterior surfaces of the cornea and determining the thickness and, hence, the distance between a surface of a first eye tissue and a surface of a second eye tissue which is spaced from the first eye tissue and where at least one of said surfaces is posterior to the anterior surface of the eye, said method comprising the steps of a) moving a first slit across a portion of the eye and illuminating a portion of the first eye tissue during movement of said first slit thereacross to scan said first eye tissue and at a first preselected angle with respect to the first eye tissue;
b) moving a second slit across a portion of the eye and illuminating a portion of the second eye tissue during movement of the second slit thereacross to scan said second eye tissue and at a second preselected angle with respect to the second eye tissue;
c) generating an image of the first eye tissue scanned with movement of the first slit;
d) generating an image of the second eye tissue scanned with movement of the second slit; and e) enabling the determination of the distance between the first and second eye tissues at selected points with said images.

13. The method of Claim 12 further characterized in that the first surface is an anterior surface of the cornea and the second surface is a posterior surface of the cornea.