Optics: eye examining – vision testing and correcting – Eye examining or testing instrument – Objective type
Reexamination Certificate
2001-11-13
2003-05-20
Manuel, George (Department: 3737)
Optics: eye examining, vision testing and correcting
Eye examining or testing instrument
Objective type
Reexamination Certificate
active
06565209
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to systems and methods for performing and analyzing data from objective measurements of a visual system, and, more particularly, to such a system and method for enhancing data from a Hartmann-Shack image and for extending a range of Hartmann-Shack-based wavefront sensors.
2. Description of Related Art
Optical systems having a real image focus can receive collimated light and focus it at a point. Such optical systems can be found in nature, e.g., human and animal eyes, or can be manmade, e.g., laboratory systems, guidance systems, and the like. In either case, aberrations in the optical system can affect the system's performance. By way of example, the human eye will be used to explain this problem.
A perfect or ideal eye diffusely reflects an impinging light beam from its retina through the optics of the eye, which includes a lens and a cornea. For such an ideal eye in a relaxed state, i.e., not accommodating to provide near-field focus, reflected light exits the eye as a sequence of plane waves. However, an eye typically has aberrations that cause deformation or distortion of reflected light waves exiting the eye. An aberrated eye diffusely reflects an impinging light beam from its retina through its lens and cornea as a sequence of distorted wavefronts.
One method of measuring ocular refractive errors is disclosed in U.S. Pat. No. 5,258,791 to Penney et al. for “Spatially Resolved Objective Autorefractometer,” which teaches the use of an autorefractometer to measure the refraction of the eye at numerous discrete locations across the corneal surface. Penney '791 further teaches the use of autorefractometer measurements in determining an appropriate corneal surface reshaping to provide emmetropia, a condition of a normal eye when parallel beams or rays of light are focused exactly on the retina and vision is perfect.
By way of example, one method and system known in the art are disclosed by Junzhong Liang et al. in “Objective Measurement of Wave Aberrations of the Human Eye with the Use of a Hartmann-Shack Wave-Front Sensor” [J. Opt. Soc. Am. 11(7), July 1994, pp 1949-57]. Liang et al. teach the use of a Hartmann-Shack wavefront sensor to measure ocular aberrations by measuring the wavefront emerging from the eye by the retinal reflection of a focused laser light spot on the retina's fovea. The actual wavefront is reconstructed using wavefront estimation with Zernike polynomials. A parallel beam of laser light passes through beam splitters and a lens pair, which brings the beam to a focus point on the retina by the optics of the eye. Possible myopia or hyperopia of the tested eye is corrected by movement of a lens within the lens pair. The focused light on the fovea is then assumed to be diffusely reflected and acts as a point source located on the retina. The reflected light passes through the eye and forms a distorted wavefront in front of the eye that results from the ocular aberrations. The aberrated wavefront is then directed to the wavefront sensor.
The Hartmann-Shack wavefront sensor disclosed by Liang et al. includes two identical layers of cylindrical lenses with the layers arranged so that lenses in each layer are perpendicular to one another, as further disclosed in U.S. Pat. No. 5,062,702 to Bille. In this way, the two layers operate as a two-dimensional array of spherical lenslets that divide the incoming light wave into subapertures. The light through each subaperture is brought to focus in the focal plane of the lens array where a charge-coupled-device (CCD) image module resides.
The system of Liang et al. is calibrated by impinging an ideal plane wave of light on the lenslet array so that a reference or calibrating pattern of focus spots is imaged on the CCD. Since the ideal wavefront is planar, each spot related to the ideal wavefront is located on the optical axis of the corresponding lenslet. When a distorted wavefront passes through the lenslet array, the image spots on the CCD are shifted with respect to a reference pattern generated by the ideal wavefront. Each shift is proportional to a local slope, i.e., partial derivatives of the distorted wavefront, which partial derivatives are used to reconstruct the distorted wavefront, by means of modal wavefront estimation using Zernike polynomials.
However, the system disclosed by Liang et al. is effective only for eyes having fairly good vision. Eyes that exhibit considerable myopia (near-sightedness) cause the focus spots to overlap on the CCD, thereby making local slope determination practically impossible for eyes having this condition. Similarly, eyes that exhibit considerable hyperopia (farsightedness) deflect the focus spots such that they do not impinge on the CCD, thereby again making local slope determination practically impossible for eyes having this condition.
Various embodiments of a method and system for objectively measuring aberrations of optical systems by wavefront analysis have been disclosed in commonly owned application Ser. No. 09/566,668, “Apparatus and Method for Objective Measurement and Correction of Optical Systems Using Wavefront Analysis,” filed May 8, 2000, which is hereby incorporated by reference herein. In this invention, an energy source generates a beam of radiation. Optics, disposed in the path of the beam, direct the beam through a focusing optical system (e.g., the eye) that has a rear portion (e.g., the retina) that provides a diffuse reflector. The beam is diffusely reflected back from the rear portion as a wavefront of radiation that passes through the focusing optical system to impinge on the optics. The optics project the wavefront to a wavefront analyzer in direct correspondence with the wavefront as it emerges from the focusing optical system. A wavefront analyzer is disposed in the path of the wavefront projected from the optics and calculates distortions of the wavefront as an estimate of ocular aberrations of the focusing optical system. The wavefront analyzer includes a wavefront sensor coupled to a processor that analyzes the sensor data to reconstruct the wavefront to include the distortions thereof.
A perfectly collimated light beam (i.e., a bundle of parallel light rays, here a small-diameter, eye-safe laser beam) incident on a perfect, ideal emmetropic eye, focuses to a diffraction-limited small spot on the retina. This perfect focusing is true for all light rays passing through the entrance pupil, regardless of position. From the wavefront perspective, the collimated light represents a series of perfect plane waves striking the eye. The light emanates from an illuminated spot on the retina as wavefronts exiting as a series of perfect plane waves, which are directed onto a wavefront analyzer for measuring distortions from ideality.
In one embodiment, the radiation is optical radiation and the wavefront sensor is implemented using a plate and a planar array of light-sensitive cells. The plate is generally opaque but has an array of light-transmissive apertures that selectively let impinging light therethrough. The plate is disposed in the path of the wavefront so that portions of the wavefront pass through the light-transmissive apertures. The planar array of cells is arranged parallel to and spaced apart from the plate by a selected distance. Each portion of the wavefront passing through one of the light-transmissive apertures illuminates a geometric shape covering a unique plurality of cells.
The wavefront optical path relays the re-emitted wavefront from the corneal plane to an entrance face of a Hartman-Shack wavefront sensor. The wavefront incident on the sensor is received by a sensitive charged-coupled-device (CCD) camera and an optical plate containing an array of lenslets. The lenslet array is parallel to the CCD detector face, with a distance therebetween approximately equal to the focal length of each lens in the lenslet array. The lenslet array divides the incoming wavefront into a matching array of “wavelets,” each of which focuses to a s
Alcon Universal Ltd.
Manuel George
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