Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Reexamination Certificate
2001-01-25
2002-08-27
Manuel, George (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
Reexamination Certificate
active
06439720
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and optical instrumentation for objectively measuring the aberrations of the human eye and specifically to an instrument capable of measuring not only the focus (spherical) and astigmatism (cylindrical) characteristics and aberrations of a person's eye but also all of the lower and higher order optical aberrations that are derived from a measured wavefront utilizing a wavefront curvature sensor.
BACKGROUND OF THE INVENTION
Measuring the aberrations of an optical system, including a human eye, is an important part of working with any optical system. Existing methods of measurement include various interferometric techniques, the Shack-Hartman wavefront sensor, and various systems involving the projection of patterns through the optical system. These systems are typically complex and expensive and most require access to the focal plane.
The human eye, although comprised of only a few optical components, may manifest a wide variety of optical aberrations that vary from person to person and over time. These aberrations may result from surface contour shape, lens thickness factors, axial alignment of refractive surfaces, axial length of the eye, and even localized refractive index variations. The fact that the human eye possesses optical aberrations has been known for centuries. Nevertheless, the measurement and characterization of these aberrations, primarily the monochromatic aberrations, has remained a problem and has fostered much research in physiological optics over the years. Finding the proper prescription, even in modern times, has been primarily based on the subjective responses to the viewing of eye charts by the person being tested, whereas recent advances in corrective methods have emphasized objective measurement of these aberrations.
Current methods of optical correction of the human eye to allow clear vision include spectacles, contact and intraocular lenses, and refractive surgery. Spectacles, the most commonly used method, only allow correction of sphere (defocus) and regular or symmetric cylinder (astigmatism). None of the present methods allow for correction of other aberrations and thus do not maximize the optical potential of the visual system, leaving the images generated to be less than optimal. In addition, not only is the view out of the eye not optimal but so is the view in. Thus, the examination of the eye's interior is also limited by these aberrations, and in some clinical situations, is severely handicapped.
Recently, great interest in this area has been kindled by the development of laser technologies, such as the excimer laser, whereby the refractive or optical errors of the eye, such as myopia (nearsightedness), hyperopia (farsightedness) and astigmatism, can be corrected by laser abelation or sculpting of the cornea. Such treatment creates a new corneal contour, or curvature, designed such that the image becomes clearly focused on the retina of the eye. Many degrees of myopia and hyperopia, with or without astigmatism, and astigmatism alone can now be corrected by such laser corneal surgery.
Although the clinical results of such surgery are good, it has been postulated, on the basis of experiment, that improved results could be obtained if the surgery were customized fully to correct all the optical aberrations of the eye, not just the sphere and cylinder. Super vision at the diffraction limit set by the aperture is possible. This could be accomplished by a computer-directed small spot scanning laser and sophisticated algorithm that takes into consideration all the aberrations of the eye. Also, many subject's have irregularly shaped corneas, not currently treatable. In addition, the asphericity of the modified cornea is often significantly increased. Clinical studies have indicated that current autorefractors, when used to determine the refraction, or optical prescription, of surgically modified eyes may provide less reliable data in such cases. Even in normal eyes, their accuracy is such that the information cannot be routinely relied upon but must be verified by further subjective testing.
It is apparent that a complete diagnosis and understanding of the eye's optical function, as the organic optical instrument, is currently very limited. A full evaluation should provide a complete description of the optical characteristics and aberrations in a quantitative format. Only then can there arise the possibility of correcting the abnormalities.
Theoretically, light arriving at the eye from a point source at infinity arrives in the form of a plane (flat) wave, whereas light from closer objects provide a wave with a convex spherical shape. This wave, in an ideal eye, would be focused as a discrete point limited only by diffraction on the retina of the eye. However, because of the optical aberrations of the eye, a degraded or blurred image is created on the retina. This concept can be appreciated in the reverse direction with resultant utility.
A plane wave, directed into the eye, would form a spot on the retina. In reverse this spot scatters light which escapes through the same optical path from which it came in. Because this light originates from a scattering process the incoming wavefront information is lost, resulting in a new source which originates from the back of the eye. This emergent wavefront now processes only the aberrations of the eye on a single pass. The present inventors have discovered that the distorted shape of this source, caused by incoming aberrations, can uniquely be eliminated with the differential curvature wavefront sensing method. Measurement and characterization of this wavefront allows one to describe the aberrations of the eye mathematically. Presently, some of these concepts are taken advantage of in ground-based telescope systems that are typically coupled with adaptive optical elements in a closed loop system. They can rapidly neutralize the wavefront aberrations induced by atmospheric turbulence and produce images that are limited only by diffraction and the aperture of the telescope.
Unfortunately, current subjective clinical methodology and instrumentation, such as the phoropter and objective devices such as autorefractors, do not avail themselves of this understanding and are based on concepts and techniques that restrict measurements to defocus and astigmatism only. During the past decade, this limitation has been appreciated and devices called corneal topographers, utilizing images reflected by the cornea, have been developed to obtain more optical information about the eye. However, they gather optical information about only one surface in the eye's refractive system and reveal nothing about the system as a whole.
A number of investigators have attempted or suggested means whereby the wavefront, either explicitly or implicitly, was recognized as an entity to be captured and determined. These studies were interested primarily in determining the monochromatic aberrations of the eye rather than the development of autorefractor-like devices for routine clinical use or methods of correction.
A number of approaches have been taken to measure the monochromatic aberrations of the eye. Some used projecting rays or patterns of light into the eye and analysis of the images by subjective or objective means. Initially this work, such as present by M. S. Smirnov (“Measurement of the Wave Aberration of the Human Eye”, Biophysics, 1961; 6:776-94) was carried out using subjective sequential subject testing, which was inaccurate and time consuming. More studies, however, have been performed using a modification of the principle first presented by Tscherning in 1894. One approach employed a device called the crossed cylinder aberroscope (Howland B and Howland H C: Subjective measurement of high-order aberrations of the eye. Science 1976; 193:580-02 and Howland H C and Howland B: “A subjective method for the measurement of monochromatic aberrations of the eye”, J. Opt Soc. Am 1977; 67(11): 1508-1518). Initially, this device was used in a subjecti
Graves J. Elon
Northcott Malcolm J.
Aoptics, Inc.
Lyon & Lyon LLP
Manuel George
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