CORNEAL TOPOGRAPHER WITH CENTRAL AND PERIPHERAL MEASUREMENT CAPABILITY

Abstract: A videokeratograph has a placido disc (12) with rings (14) concentric about a central axis (16), a center positioned on the central axis, and an observation opening at the center. A plurality of fixation points (34, 36, 38 and 40) are positioned in the placido disc spaced from the center opening and distributed circumferentially around the central axis, whereby rings on the placido disc spaced from the center are reflected from the central zone of the cornea. A video camera positioned on the center axis behind the placido disc observes an image of the placido disc reflected from the cornea. A processor (48) coupled to the video camera determines the shape of the central zone of the cornea.

Description: CORNEAL TOPOGRAPHER WITH CENTRAL AND PERIPHERAL MEASUREMENT CAPABILITY BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the field of corneal topographers, and particularly to corneal topographers determining cornea topography from an image of a placido disc reflected from an eye and observed through the center of the disc. 2. Background of the Related Art Corneal topographers are instruments that measure the anterior surface of the cornea. The most commonly used topographer is the videokeratograph, which is based on what is known as a placido disc design. A placido disc consists of a series of concentric circles of alternating black and white rings or other colors. Originally, the placido disc rings were positioned in a flat plane, but now the rings usually have a three dimensional configuration. The disc is illuminated, typically from the rear and placed in front of the cornea to be measured. A reflection of the rings is obtained from the corneal surface when observed, such as by viewing, photographing or videographing the corneal image through a hole in the center of the disc. A principal deficiency of this method is that the central hole through which the target reflection is measured contains no object that is reflected from the central cornea, and hence this corneal area cannot be measured. Additionally, areas immediately adjacent to the central cornea also cannot be measured because the distance is too small to provide a reliable measurement based upon the optical laws of object-to-image relationships. There is therefore a need for a method of accurately measuring the central zone of a cornea using a placido disc based corneal topographer. As used herein, the central zone of the cornea is a region containing the entrance pupil. SUMMARY OF THE INVENTION The present invention provides a method and apparatus for accurately measuring the topography of the central zone of a cornea, as well as other regions of the cornea, using a placido disc based corneal topographer. In the method of the invention this is provided by (a) positioning an eye on a central axis; (b) illuminating the eye through a placido disc having a center and a center opening at the center, the center being on the central axis and the disc having rings concentric about the central axis; (c) fixating the eye along a first viewing axis to a first fixation point positioned in the placido disc spaced from the center opening; (d) observing an image of the placido disc reflected from the eye along the central axis, whereby the central zone of the cornea of the eye reflects images of rings on the placido disc spaced from the center; and (e) determining from the observed image of the cornea the shape of at least the central zone of the cornea. The preferred method of the invention further includes sequentially fixating the eye to a plurality of fixation points spaced radially from the central axis and circumferentially around the placido disc. At each fixation point, an image of the placido disc is reflected from a different region of the cornea. Accordingly, the shape of at least the central zone of the cornea from each observed image is then determined. In an apparatus according to the present invention, a placido disc has rings concentric about a central axis, a center positioned on the central axis, and a center opening at the center. At least a first fixation point is positioned in the placido disc spaced from the center opening, whereby the central zone of the cornea of an eye fixated on the fixation point reflects images of rings on the placido disc spaced from the center. A means is positioned on the center axis for observing through the center opening an image of the placido disc reflected from the cornea of an eye positioned at an observation station. Finally, a means is coupled to the observing means for determining the shape of at least the central zone of the cornea from the observed image. The preferred embodiment of the invention includes a plurality of fixation points distributed around the center of the placido disc. A particularly advantageous feature of this invention is that the central zone of the cornea is positioned on peripheral rings of the placido disc image. Accurate measurements of the central zone of the cornea are thereby made, whereas it is impossible to measure the corneal apex when the fixation point is on the central axis, as is conventional. Further, the off-axis fixation point provides imaging of a radially larger area of the cornea, and the entire cornea is measured by compiling a plurality of measurements taken when a plurality of fixation points distributed around the central axis are used. These and other features and advantages of the present invention will be apparent from the preferred embodiment described in the following detailed description and illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified top plan view of a corneal topographer with a placido disc made according to the invention. FIG. 2 is a front view of the placido disc of FIG. 1. FIG. 3 is an idealized diagram of a reflected concentric ring image observed by the corneal topographer of FIG. 1. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS As has been mentioned, the invention provides a method and apparatus for accurately measuring the central zone of a cornea using a placido disc based corneal topographer. A videokeratograph made according to the invention is shown generally at 10 in FIG. 1. Videokeratograph 10 includes a conventional placido disc 12 shown formed as a cone having an inside surface 12a with rings 14 concentric about a central axis 16. The rings have alternating contrasting colors, such as black rings 18, represented by the dashed lines, and white rings 20. Disc 12 is typically backlit, but other forms of creating the alternating ring light source may be used, all of which are included in the common term placido disc. A person's head is located to position an eye 22 to be observed at an observation station 24 with the cornea 26 of the eye located on central axis 16. A videokeratograph typically has a fixation point 28, such as an LED light source. This fixation point is aligned with the central axis with a conventional beam splitter 30 and viewable through a hole 32 in the center of the disc. Videokeratograph 10 however has a plurality of fixation points 34, 36, 38, 40 positioned to be visible to eye 22 and located at 90° intervals in one of the disc rings, as shown. When eye 22 is fixated on a fixation point along what is referred to as a viewing axis 42, the viewing axis is usually at an angle A of 5° to 20° to central axis 16. A conventional video camera 44 observes the illuminated cornea along central axis 16. The image is focused on a detector 46 that is coupled to a processor 48, typically in the form of a personal computer. A representative observed image 50 is shown in FIG. 3. Processor 48 determines the shape of the cornea from this image. The rings 14 are represented only by the lines separating them, for clarity. The position of the entrance pupil 52 having a center E on the viewing axis corresponds to fixation of the eye on one of the off-axis (off the central axis) fixation points 34, 36, 38, 40. Point A corresponds to the center of the placido disc image, which is on the central axis. Point B is an arbitrary point on a ring spaced from points A and E, and is used as an example for computation of the curvature of any point on the central zone of the cornea from its approximate center at point E along line BE. Line 54 represents the edge of the cornea, or limbus, which is now also measurable with the method of the present invention. It is seen that with the fixation point mounted off of the center of the placido target, the eye is correspondingly rotated a known amount of approximately 5-20 degrees off of the central axis and the center of the videokeratograph target, shown as point A, is displaced from the center of the corneal image, shown as point E. The image of the more peripheral rings are thereby positioned over the central cornea. Hence the central region of the cornea is now to be measured with respect to a fixed, known peripheral position as shown in FIG. 3. The important region of the cornea that must be measured is that corresponding to the entrance pupil of the eye and is about 3-6 mm in diameter. In use, the central corneal measurements with respect to point A are transposed to a new reference point that is meaningful with respect to the optics of the eye. The logical position for the new reference point is the center E of the entrance pupil 52. Using a peripheral fixation point 34, 36, 38 or 40 to find reference A, a point B on any ring image in any direction is identified. The power of the cornea at point B is then obtained using the usual methods of videokeratography. A line BE is then defined from point B to the center of the entrance pupil at point E. The power of the cornea at point B is then transposed to the effective power along the axis BE. This method is useful for measuring the central corneal region. A number of sophisticated methods which are not limited to paraxial optics may be used to produce an accurate measurement of the central cornea. The modification of the videokeratograph as provided by the present invention allows a very sensitive and accurate measurement of the central corneal region. If need be, the accuracy can be improved further by repeating the measurement using different fixation points that are mounted on the videokeratograph, as shown in FIGS. 1 and 2. The four fixation points shown, i.e., two in the horizontal (points 34, 38) and two in the vertical (points 36, 40) meridian are adequate for these measurements. However, it may be desirable to use only one, two or three fixation points, or in some cases more than four fixation points. A target (placido disc) that consists of a multiplicity of fixation points could also be used to align the cornea with the point of greatest curvature (corneal apex) or the line of sight. As an alternative, it may be possible to vary the position of a single fixation point, either using mechanical means or by an optical technique involving a video monitor. By changing to a peripheral fixation point, it is also found that the videokeratograph rings will now reflect from a peripheral corneal region which includes the corneal limbus 54. However, it is only possible to measure one quadrant of the corneal surface by using this technique. Nevertheless, changing the fixation point to different directions, all of the different regions of the peripheral corneal surface can be measured. Similar techniques, such as is disclosed in U.S. Patent No. 4,420,228 issued to Humphrey, only give a single measurement at one peripheral point corresponding to each of the peripheral fixations. It does not provide any measurement of the central region as can be accomplished by the present invention. The present invention is also especially useful in detecting central corneal irregularities, such as central corneal islands and other effects that may result from refractive surgery. It is also useful in determining the true refractive corneal power with respect to the line of sight. Although the present invention has been described in detail with reference to a particular preferred embodiment, persons possessing ordinary skill in the art to which this invention pertains will appreciate that various modifications and enhancements may be made without departing from the spirit and scope of the claims as written and as judicially construed according to principals of law. The above disclosure is thus intended for purposes of illustration and not limitation.

Claims: WHAT TS CLAIMED IS: 1. An apparatus for monitoring an observation distance between an eye and a reflection target of a system for observing an eye, comprising: a light source that emits a beam of electromagnetic radiation towards the eye; and a detector configured to receive electromagnetic radiation from the emitted beam that is reflected by the eye, wherein the detector generates position information indicative of the observation distance. 2. The apparatus of claim 1, wherein the light source and the detector are attached to a system for observing an eye, and wherein the light source and detector are arranged on opposite sides of a reflection target, and are spaced approximately equally from a center of the reflection target. 3. The apparatus of claim 1, wherein the light source and the detector are attached to a system for observing an eye that includes a reflection target, and wherein the light source is arranged on the reflection target. 4. The apparatus of claim 3, wherein a pattern is formed on the reflection target, and wherein the reflection target is configured so that the pattern is reflected from the eye. 5. The apparatus of claim 4, wherein the pattern comprises light and dark areas, and wherein the light source is positioned in a dark area. 6. The apparatus of claim 5, wherein the light and dark areas are concentric circles. 7. The apparatus of claim 3, wherein a source opening is formed in a surface of the reflection target, and wherein the light source is configured to emit the beam through the source opening. 8. The apparatus of claim 1, wherein the light source and the detector are attached to a system for observing an eye that includes a reflection target, and wherein the detector is arranged on the reflection target. 9. The apparatus of claim 8, wherein a pattern of light and dark areas is formed on the reflection target, and wherein the detector is positioned in a dark area. 10. The apparatus of claim 8, wherein a detection opening is formed in a surface of the reflection target, and wherein the detector is configured to receive the reflected electromagnetic radiation through the detection opening. 11. The apparatus of claim 1, wherein the detector comprises a CCD array. 12. The apparatus of claim 1, wherein the detector comprises a linear photodiode. 13. The apparatus of claim 1, wherein the detector comprises a photoreceptor array. 14. The apparatus of claim 1, wherein the light source emits infrared light, and wherein the detector comprises an infrared pass filter. 15. The apparatus of claim 1, further comprising a display screen coupled with the detector and configured to display the position information. 16. A system for observing an eye of a patient, comprising: a reflection target having a reflection surface that is centered around a viewing axis; a patient station opposite the reflection target, the patient station admitting the patient's face and supporting it in such a way that the patient's eye that is to be observed looks towards the reflection surface; a control device, responsive to position commands, configured to cause relative movement between the reflection target and the patient station; a light source configured to emit a beam of electromagnetic radiation towards a patient's eye when the patient's head is supported in the patient station; and a detector configured to receive a portion of the emitted electromagnetic radiation that is reflected from the patient's eye, wherein the detector is also configured to generate position information indicative of an observation distance between the patient's eye and the reflection target. 17. The system of claim 16, wherein the light source emits infrared light, and wherein the detector comprises an infrared pass filter. 18. The observation system of claim 16, wherein the control device is configured to maintain an optimum observation distance based on the position information generated by the detector. 19. The apparatus of claim 18, wherein an angle between propagation directions of the emitted and reflected electromagnetic radiation is approximately 90 ° + 15° when the observation distance equals the optimum. 20. The observation system of claim 18, further comprising a display screen, coupled to the detector, for displaying the position information. 21. The observation system of claim 18, further comprising a processor configured to monitor the position information and to generate the position commands used by the control device based on the position information. 22. The observation system of claim 16, further comprising a user input device that generates the position commands used by the control device.

Inventors: RORABAUGH DALE A, RORABAUGH, DALE, A.
Applicants: VISMED INC, VISMED, INC.
Application year: 1998
Main classification: A61B3
IPC codes: A61B3