Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Patent
1995-01-09
1996-05-28
Sikes, William L.
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
351221, 351246, A61B 310
Patent
active
055216579
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to measuring the shape of surfaces. Whereas other uses can be envisioned, the invention has with particular applicability to measurement of the corneal surface and to facilitating treatments of the eye.
Corneal topography measurements are valuable for planning, performing, and evaluating the effects of surgical procedures. Measurements of the corneal surface are needed for keratorefractive procedures, which correct a refractive power of the eye by changing the curvature of the corneal surface. In addition, corneal topography can also be used to predict the results of radial keratotomy, evaluate the design of epikeratophakia for myopia, diagnose the stage of keratoconus, and guide suture removal following corneal transplantation.
There are several methods to test and characterize the optical power of the eye and the cornea in particular. One of the oldest methods is the Snellis diagram test, wherein a patient is asked to read letters or to recognize shapes from a standard distance. This is a subjective method which requires the patient's cooperation.
Since the corneal curvature and its dioptric power account for about three quarters of the refractive power of the eye, it is important, however, to measure the corneal surface with greater accuracy than the Snellis diagram test provides.
One class of methods of measuring the corneal surface is based on the deflectometry principle, which utilizes reflection of light from the smooth corneal tear film (i.e., the lower, oily part of the tear film). In this method, a system of rings is optically projected onto the surface of the eye. A doctor directly observes the symmetry of the reflected rings and judges the condition of the eye. This qualitative technique is quite reliable; however, it is dependent upon the doctor's experience.
In recent years, automatic measurement devices which measure the shiny surface of the cornea using deflectometry have been introduced. These are computerized systems which analyze distortion of the images of a system of rings optically projected towards the eye and detected by a camera detection system. The spatially defined system of rings is projected onto the smooth eye surface from a precisely positioned source governed by a computer. The reflected pattern is detected by a camera and stored in the memory of a computer. Using the well-defined characteristics of the incident and detected light, the geometric position of the source and the detector, and the shapes of the incident and detected pattern the computer calculates the shape of the reflecting sphere.
Such a computerized topographer can be used as a principal guide in a laser system performing corneal sculpturing surgery, to provide the necessary pre-operative and post-operative corneal measurements, or to provide the measurements to guide post-operative manipulation of the cornea for reduction of astigmatism. However, during and after eye surgery, once the epithelium is removed from the eye surface, the local microtopology of the eye surface has changed so that the surface of the eye is only partially a specular reflector, and now partially a diffuse reflector. Since a diffuse reflector has no fixed relationship between the incident angle and the reflected angle of the projected light, the described deflectometry-based topographer is no longer useful. Furthermore, since both the corneal topographer using deflectometry and a laser beam delivery system of a laser sculpturing system require positioning on the optical axis of the eye, there is difficulty in incorporating both systems into one unit designed for intraoperative use. In addition, since the vision of a patient during surgery or after de-epithelization is significantly impaired, it is difficult to achieve proper eye alignment necessary for deflectometric measurement.
Rasterography or fringe phase shifts are methods of determining topography of the cornea which are well suited for diffusive surfaces. The method does not require smooth reflective surfaces and images can be
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Dudek James A.
Sikes William L.
Summit Technology Inc.
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