Abstract:
A device and method for fitting contact lenses to a patient that matches the anterior curve of the lens to the measured cornea of the patient improving the optical performance of the lens on the eye.

Description:
The present invention is a continuation-in-part of application Ser. No. 09/173,346 filed on Oct. 15, 1998 now abandoned, which is a division application based on U.S. patent application Ser. No. 08/799,031 filed on Feb. 10, 1997 now U.S. Pat. No. 5,838,811. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to apparatus for measuring transparent aspheric surfaces. Examples of such surfaces include the cornea of the eye and the surface of a contact lens. More particularly, the invention combines a optical system with a video image analysis system to generate a mathematical expression describing the shape of a transparent aspheric surface. 
     2. Description of the Art 
     In the prior art it has been recognized that accurate characterization of the shape of the surface of the cornea would aid in the fitting of contact lenses. However, initially it was believed that the cornea had a substantially spheric shape and that the “fit” between the cornea and the lens need not be exact, for the comfort of the wearer or for operation of the lens. In general, it was believed that “soft” contact lens material would be compliant enough to conform to the corneal shape, and that only a few “base curves” would suffice to fit the majority of the population. Lindmark et al. demonstrated that the corneal shape is not spherical and that failure to accommodate the complex surface of the cornea can result in injury to the cornea itself, see “The Correction of Atypical Ametropia with Flexlens”, Vol. 13, 1979, of the  Contact Lens Journal , R. C. Lindmark, et al. 
     In the prior art, there have been two principal techniques used for estimating the “sphericity” of the cornea. The earliest systems involve the projection of Placido&#39;s rings onto a cornea. The practitioner observes the clarity and spacing of the projected rings and compares the resultant image pattern with reference curves to estimate which one of a collection of spherical curves most closely approximates the surface of the cornea. The earliest systems of this type relied on direct observation of the projected rings on the eye. More recent versions of this system photograph the ring pattern on the eye producing a karotograph which may be evaluated with the use of a computer system. Examples of this approach are taught by U.S. Pat. No. 4,685,140 to Mount; U.S. Pat. No. 4,978,213 to El Hage; In each of these systems a television camera is utilized along with a computer to process the Placido&#39;s ring data. 
     An alternate approach develops an image of the cross section image of the meridian of cornea of the eye with an optical system. This approach is typified by the Corneoptor system developed by Scientific Advances Incorporated of Columbus, Ohio in the late 1960&#39;s. This system utilizes a slit illuminator to develop a photograph of the cross section of the cornea. In use the operator would compare the test curves to determine the “best fit” representation of the contour. 
     It is now well recognized that the cornea of the eye has a very complex aspheric shape, and that more exact knowledge of this shape for particular individuals would be an aid to fitting contact lenses. Such a system would also be useful for evaluating the corneal surface for surgical procedures and for evaluating the base curve and front curve of contact lenses as an aid to fitting them on the human eye. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides a flexible measurement tool for assessing the shape of complex aspheric transparent surfaces. For purposes of this disclosure the term “subject surface” may include any transparent surface, of which the cornea and contact lens and a contact lens on a cornea are only examples. 
     The apparatus of the invention includes four subsystems. 
     The subject surface itself is held by a positioning subsystem which preferably is a wet cell. A measurement reticle located near the subject surface may be presented along with the subject surface as an aid to calibration of the system. However it is preferred to substitute a lens with known curved surfaces into the preferred wet cell to calibrate the system. 
     The illumination subsystem projects a slit of light onto the subject surface along an illumination axis. An image formation subsystem is aligned along an observation axis which is orthogonal to the illumination axis. Together these two subsystems present a cross-section image of the subject surface to a camera which is associated with the image formation subsystem. The illumination subsystem and image formation subsystem can be moved with respect to the subject surface lens so that cross-sectional data sets can be taken at several positions. The camera generates a cross-sectional image of the subject surface referred to as the “raw data” This signal digitized to form a “pixel data set”. The conversion process may occur within the data processing subsystem which includes a computer. Within the data processing subsystem the pixel data set converted to a silhouette image data set. This is accomplished by an image contrast enhancement process which applies a threshold criteria to the pixel data set and generates a white-on-black silhouette of the particular crossection selected by the orientation of the illumination subsystem. Next Cartesian coordinates are applied to the silhouette image data set. The coordinates are applied to the silhouette image to define a first surface data set and a second surface data set. This process may involve editing the data sets to exclude non-lens areas of the subject surface. The first and second surface data sets are compared with regular conic section equations to ascertain whether they adequately represent the data sets. Assuming the conics are not suitable, the data processor computes a higher order polynomial expression for the first and second surface data sets which is displayed to the user along with an image of subject surface. Preferably a least means squares algorithm is applied to the data set to generate a polynomial expression for the curve displayed by the subject surface although a finite element analysis may be used as well. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic system level block diagram depicting in vivo measurement of the cornea; 
     FIG. 2 is a schematic system level block diagram depicting wet cell measurement of a hydrated contact lens; 
     FIG. 3 is a system level flowchart for the measurement process; 
     FIG. 4 is schematic diagram depicting the process of evaluating a corneal shape, and depicting a display of the calulated data. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hardware Description 
     In FIG. 1 the test subject is a human eye and the positioning subsystem  12  has been placed human cornea in position along the illumination axis  22 . For corneal measurements a standard head positioning assembly such as “chin rest” structure taught by the Corneopter product can be arranged to stabilize the eye position with respect to illumination subsystem  14 . 
     The illumination subsystem  14  projects light from a light source  11  onto a slit  13 . The image of the slit  13  is focused on the cornea by a lens  15 . This optical train defines the illumination axis  22 . It should be appreciated that a laser could also be used to scan the cornea as an alternative to the incandescent filament and slit system depicted in FIG.  1  and FIG.  2 . 
     The image forming subsystem  16  is provided to view the first surface  28  and the second surface  30  of the cornea along an observation axis  24  that is orthogonal to the illumination axis  22 . The user can adjust the relative orientation of the illumination axis  22  and the observation axis  24  to the eye to realize a good image. The user may rotate the entire optical subassembly about the illumination axis  22  to several positions to fully characterize the cornea. The image forming subsystem  16  includes a CCD or other type video camera  26  which is used to generate an image of the crossection of the cornea  20 . The RCA model CC285 camcorder is one suitable device for carrying out the invention. 
     A reticle  29  may be positioned on the same plane as the image of the cornea  20  to facilitate system calibration. Typically the camera  26  will transmit an analog video signal of the subject surface and reticle to the data processor subsystem  18 . IBM 286 AT class computers or equivalent may be used for carrying out the invention. The data processor subsystem  18  will include an appropriate analog to digital converter or a frame grabber utility to capture a camera image. A suitable PC compatible board for carrying out these functions is the Supervision PC Image Capture Board available from IDEC, Inc. of Fountainville, Pa. It is preferred to have a minimum resolution of 512×400 pixels, and higher resolution is desirable. 
     The data processor subsystem  18  processes this video data to calculate and display a mathematical expression characterizing the first surface  28  and second surface  30  of the cornea  20 . 
     In FIG. 2 the lens positioning subsystem  12  includes a wet cell  34  which stabilizes and positions the contact lens  32  along the illumination axis  22 . The wet cell  34  will contain an equilibriating saline solution to hydrate or stabilize the shape of contact lens  32 . Detail cell design must ensure that the structure is properly viewed and is free of adherent air bubbles. The remainder of the physical system is similar to the system used for taking measurements from the cornea. However, certain software routines may differ between in vivo corneal measurements and wet cell measurements. 
     The measurements taken and represented to the user on the data display screen as seen in FIG.  1  and FIG. 2 can take many forms. As discussed above, the physician may make a judgement concerning the fit of the lens by noting the distance between the displayed surfaces as represented by the distances typified by distance  7  in FIG.  1 . (amended drawing) Since the display is of a single slice or plane of measurement the physician may wish to note the fit in several planes. In practice the physician may collect a sequence of such measurements and create a composite graph (not shown) that represent the distances  7  between the two surfaces. The ability to measure and display this data using the apparatus of the invention makes it is possible to measure the shape change that is induced by a test lens having a known “as manufactured” base curve. Then, by creating a modified base curve test lens that more nearly matches the actual corneal curve a better fit is achieved. This process results in a lens that is specifically tailored to particular subject with a base curve that is created to achieve an optimal fit on the eye. The measurement and display process may be enhanced by adding fluorescent dyes to the eye to more fully reveal the edges of the volume defined between the cornea and the test lens. With the curvature of the cornea known and the curvature of the base curve of a hard test lens known it is possible to compute the distances between the two surfaces which is useful when the corneal shape is surgically modified. In that instance the bars in FIG. 1 typified by bar  7  represent a graphical image of the computed distance. By rotating the system and computing the distance bars for each plane a 3D contour can be built up which represents the “fit” between the hard test lens and the surgically modified cornea. This graphically presented information can be used to decide whether to increase or decrease the curvature of the base curve of the lens. The fitting process can be used to enhance surgical procedures which alter the shape of the cornea. For example the patient can be fitted with a test lens before and after the procedure to test the accuracy of the surgical procedure. 
     The system of the present invention can be used in connection with corrective surgery for the eye. At the present time physicians are making cuts in the peripheral area of the cornea, the healing process changes the shape of the surface of the cornea and “corrects” vision without the use of a corrective lens. In some instances the correction is better than 20/20. This effect appears to be the result of the elimination of aberrations in the cornea. In use the physician will use the system to extract measurements in several planes. Typically the limbus area of the cornea will be of great interest. Unlike conventional systems the present invention can characterize the peripheral area of the cornea which lie close to the area of surgical intervention. After the completion of the surgery the corneal shape is checked again to confirm the surgical procedure. More recently corrective surgery has been accomplished by the insertion of a filament into the eye to apply force to the cornea. In general a flattening takes place that can correct certain vision defects. The present invention may be used to assist these procedures as well. Once again before and after measurement can be made to follow the shape change of the cornea. 
     Data Processing Description 
     FIG. 3 sets forth the system level flowchart for the software processes used to carry out the invention. Several commercially available “off-the-self” software products can be used to execute the steps of the process. Catenary Systems Victor Imaging Procesing Library, Auto Desk Auto Cadd, Generic Cadd, Zsoft Corporation PC Paintbrush IV Plus, are suitable PC products which have been used to prototype the invention. However, incompatible file formats between software products renders this approach cumbersome and custom software code should be developed to facilitate use of the system. 
     The process begins at start process block  35 . Process block  36  is entered and the test specimen or subject surface is positioned. In process block  38  the subject surface is illuminated along the illumination axis  22 . For each angular position about the illumination axis, unique cross-section of the subject surface is illuminated and evaluated. Typically, the amount of illumination will be in a user adjustable parameter, which can be optimized to ensure adequate illumination intensity to generate an adequate gray scale image of the test surface. In process block  39 , the camera and illumination subsystem  14  may be rotated about the illumination axis  22  so that additional cross-sections of the test surface may be acquired. Typically, at least two separate cross-section images will be developed, although a larger number may be required to accurately characterize a complex shape. In process block  40 , a raw image is acquired of the cross-section of the subject surfaces along the orthogonal observation axis  24 . Although any of a variety of camera systems can be used to acquire the image, the preferred structure is a video camera generating an analog video output for delivery via connection  19  to a computer or data processor subsystem  18 . In process block  42 , the analog cross-sectional image is converted to a pixel data set. This conversion process requires the analog-to-digital conversion of the video image, and conventional image capture hardware can be utilized within the data processor subsystem  18  to achieve this result. Turning to process block  44 , the gray scale image of the subject surface is converted from gray scale to black and white scale pixels generating the silhouette image of the transparent portion of the test surface. This process is achieved by establishing a gray scale threshold, and assigning black pixels to values below the threshold, and white pixels to values above the threshold. This procedure generates the silhouette, and will exclude opaque structures from the composite image. In the case of the human cornea, this contrast enhancement structure may generate silhouettes of internal structures of the eye which are not of interest in characterizing the shape of the cornea. The internal structure may be enhanced to better examine corneal anatomy or pathological conditions, and this data recorded for future comparison and use. With the silhouette image generated, file conversion utilities may be used to convert the silhouette image data set to a computer automated design program (CAD). The CAD software permits overlaying cartesian coordinates on the image in process block  46 . Typically such software permits the user to edit the image to eliminate limbus points, and other portions of the image which do not need to be characterized. This step of the process is depicted as process block  50 . The CAD software is utilized to collect a set of “X”, “Y” cartesian coordinates for the anterior transparent surface of the cornea referred to as the first surface data set, as well as for the posterior surface, referred to as the second surface data set. Through suitable conversion utilities, these coordinates can be delivered to mathematical modeling software such as the “Mathematica” product published by Wolfram Research. The mathematical function software is applied to the reduced data sets in process block  52  and is used to compare the coordinate sets with regular conic sections. For some surface, for example, contact lenses conic section data may adequately represent the surface contour, and the computational overhead required for this comparison is relatively low. This testing procedure, may generate an output to be displayed to the user as represented by process block  53 , if the conic curves adequately represent the data set information. If, however, the subject surface is very irregular, the mathematical software in process block  54  can be used to compute a higher order polynomial expression for the first surface and second surface curves. Upon completion of this curve fitting routine, the polynomial expressions can be displayed for the user in process block  53 . This process is repeated via transition  51  for each of the cross-sections which returns data flow for additional cross-sections of the subject surface. This process step may include manually reorienting the observation and illumination structures to different positions. 
     FIG. 4 represents schematic applications of the algorithm to images captured by the camera. In FIG. 4, the black on white silhouette of the cornea  20  has been digitized and imported into CAD software where the cartesian coordinates  23  and  25  have been manually positioned to exclude non-lens features of the image. The CAD system has been used to find the midpoint of the X axis, which is defined as the Y axis coordinate. The reduced data sets drawn from the CAD program are imported to the mathematical processes set forth on FIG. 3, resulting in the computation of the polynomial expression for the anterior and posterior surfaces. 
     Although the invention has been described in connection with an illustrative embodiment, it should be understood that the teaching is illustrative and that various changes can be made without departing from the scope of the invention.