The present invention is generally related to measurements of optical systems and surfaces. The invention provides devices, systems, and methods for measurement of optical errors of optical systems, and is particularly well-suited for determining a refractive correction of optical errors of the eye. The invention is also particularly well suited to the measurement of optical surfaces such as lenses, spectacles and contacts, including lenses ablated during calibration of a laser eye surgery system.
Known laser eye surgery procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of stromal tissue from the cornea of the eye. The laser typically removes a selected shape of the corneal tissue, often to correct refractive errors of the eye. Ultraviolet laser ablation results in photodecomposition of the corneal tissue, but generally does not cause significant thermal damage to adjacent and underlying tissues of the eye. The irradiated molecules are broken into smaller volatile fragments photochemically, directly breaking the intermolecular bonds.
Laser ablation procedures can remove the targeted stroma of the cornea to change the cornea's contour for varying purposes, such as for correcting myopia, hyperopia, astigmatism, and the like. Control over the distribution of ablation energy across the cornea may be provided by a variety of systems and methods, including the use of ablatable masks, fixed and moveable apertures, controlled scanning systems, eye movement tracking mechanisms, and the like. In known systems, the laser beam often comprises a series of discrete pulses of laser light energy, with the total shape and amount of tissue removed being determined by the shape, size, location, and/or number of laser energy pulses impinging on the cornea. A variety of algorithms may be used to calculate the pattern of laser pulses used to reshape the cornea so as to correct a refractive error of the eye. Known systems make use of a variety of forms of lasers and/or laser energy to effect the correction, including infrared lasers, ultraviolet lasers, femtosecond lasers, wavelength multiplied solid-state lasers, and the like. Alternative vision correction techniques make use of radial incisions in the cornea, intraocular lenses, removable corneal support structures, and the like.
Known corneal correction treatment methods have generally been successful in correcting standard vision errors, such as myopia, hyperopia, astigmatism, and the like. However, as with all successes, still further improvements would be desirable. Toward that end, wavefront measurement systems are now available to measure the refractive characteristics of a particular patient's eye. By customizing an ablation pattern based on wavefront measurements and providing improved laser system calibration, it may be possible to correct minor refractive errors so as to reliably and repeatably provide visual accuities greater than 20/20.
Known methods for calculation of a customized ablation pattern using wavefront sensor data generally involves mathematically modeling an optical surface of the eye using expansion series techniques. More specifically, Zernike polynomial series and Fourier series have each been employed to model the optical surface. For example, U.S. patent application Ser. No. 10/872,107, filed on Jun. 17, 2004, and entitled “Iterative Fourier Reconstruction for Laser Surgery and Other Optical Applications”, the full disclosure of which is incorporated herein by reference, describes application of a Fourier transform algorithm to measured gradients to reconstruct an optical surface. Coefficients of the Zernike or Fourier series are derived through known fitting techniques, and the refractive correction procedure is then determined using the shape of the optical tissue surface the eye indicated by the mathematical series expansion model.
Work in connection with the present invention suggests that the known methodology for calculation of an optical surface and a laser ablation treatment protocol based on wavefront sensor data may be less than ideal. The known methods which determine a gradient field from several beams of light can be sensitive to errors and “noise” which can arise while finding a location of a light beam, which can lead to a less than ideal refractive correction. Furthermore, the known surface modeling techniques based on gradients can be somewhat indirect, and may lead to unnecessary calculations.
In light of the above, it would be desirable to provide improved optical measurement techniques, particularly for use in measurements of the eye for refractive correction purposes.