Optics: measuring and testing – By polarized light examination – With light attenuation
Reexamination Certificate
1998-11-05
2001-02-27
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By polarized light examination
With light attenuation
C356S124000, C219S121600
Reexamination Certificate
active
06195164
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed to systems, methods, and apparatus for calibrating laser ablation systems. In particular, the invention relates to methods and apparatus for measuring the refractive power, shape and quality of a laser test ablation on a test surface. The present invention is particularly useful for calibrating excimer lasers used during laser ablation procedures of the eye, such as photorefractive keratotomy (PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileusis (LASIK), or the like.
Ultraviolet and infrared laser based systems and methods are known for enabling ophthalmological surgery on the external surface of the cornea in order to correct vision defects. These procedures generally employ an ultraviolet or infrared laser to remove a microscopic layer of an anterior stromal tissue from the cornea to alter its refractive power. In ultraviolet laser ablation procedures, the radiation ablates corneal tissue in a photodecomposition that does not cause thermal damage to adjacent and underlying tissue. Molecules at the irradiated surface are broken into smaller volatile fragments without substantially heating the remaining substrate; the mechanism of the ablation is photochemical, i.e. the direct breaking of intermolecular bonds. The ablation penetrates into the stroma of the cornea to change its contour for various purposes, such as correcting myopia, hyperopia, and astigmatism.
In such laser based systems and methods, the irradiated flux density and exposure time of the cornea to the laser radiation are controlled so as to provide a surface sculpting of the cornea to achieve a desired ultimate surface change in the cornea. To that end, ablation algorithms have been developed that determine the approximate energy density that must be applied to remove a certain depth of tissue from the cornea. At ultraviolet wavelengths, for example, a cumulative energy density of about 1 joule/cm2 will typically ablate corneal tissue to a depth of about one micron when applied in a series of pulses of about 100 to 400 millijoules/cm2. Accordingly, the ablation algorithms are tailored for each procedure depending on the amount and the shape of corneal tissue which will be removed to correct a particular individual's refractive error.
In order to properly use these laser ablation algorithms, the laser ablation system typically should be calibrated. Calibration of the laser system helps ensure removal of the intended shape and quantity of the corneal tissue so as to provide the desired shape and refractive power modification to the patient's cornea. In addition, it is usually desirable to test for acceptable levels of system performance. For example, such tests can help ensure that internal optics are aligned, that laser fluence is accurate, etc.
Ablations of plastic test materials are often performed prior to excimer laser surgery to calibrate the energy density and ablation shape of the laser. During these tests, a lens is ablated into the test plastic, and the refractive power of the test lens is read by a standard lensometer. The reading from the lensometer is then entered back into the laser system so that the system can make appropriate calibration adjustments. The test lens may also be visually evaluated under a magnifying glass or with the microscope of the laser system, and test samples are sometimes sent to a laboratory for accurate evaluation to help determine beam homogeneity and quality.
Although known laser ablation calibration techniques are fairly effective, these methods still suffer from certain disadvantages. For example, delaying each surgery while obtaining accurate laboratory evaluations of a test lens may be impractical. On the other hand, requiring specialized test lens evaluation equipment at each site could add significantly to equipment costs and overall system complexity. Nonetheless, some information beyond refractive power and a visual evaluation of the test lens would be helpful to improve the accuracy of regular calibrations, whether they are performed monthly, daily, or before each ablation procedure.
In light of the above, it would be desirable to provide improved systems, methods, and apparatus for calibrating laser ablation procedures. It would be particularly desirable if such improvements enhanced calibration accuracy without significantly increasing the overall system costs and complexity. It would further be desirable if such improvements could provide quantifiable data which might be used in an automated calibration feedback and adjustment system.
SUMMARY OF THE INVENTION
The present invention is directed to improved systems, methods, and apparatus for calibrating a laser ablation system, such as an excimer laser system for selectively ablating the cornea of a patient's eye. The present invention provides apparatus and methods for measuring the shape of a test surface that has been ablated by energy delivered from a laser, such as an excimer laser. In addition, the present invention provides apparatus and methods for monitoring the quality of the ablated test surface to minimize undesirable system performance, such as flawed internal optics, misalignment, poor laser fluence and the like. Calibration accuracy is generally enhanced by analyzing distortions of a geometrical pattern projected onto and/or viewed through the ablation test surface. Conveniently, the interaction of the pattern and the lens can be analyzed using existing components of the laser ablation system such as a microscope, video camera, computer processor, and other system components. These improved calibration techniques allow enhanced quantitative evaluations of the test surface at low cost and with little delay, and can also be used to accurately and automatically adjust the laser system.
In one aspect, a method is provided for calibrating a laser ablation system. The method includes the steps of selectively ablating a test surface with the laser, superimposing a geometrical test pattern and the ablated test surface to generate a resulting pattern, digitizing at least a portion of the resulting patterns and analyzing the digitized pattern to determine the ablation characteristics of the laser ablation system. The geometric pattern will usually include a plurality of uniformly spaced elements, such as lines, dots, circles, or the like. The test surface is often ablated into a lens that will refract the geometric elements viewed through the lens such that the resulting pattern will provide information regarding the refractive power, shape and quality of the test surface. Advantageously, the effects of the lens on the geometrical pattern may be quantified. Further, by using the digitized images and the laser system computer, adjustments to the laser ablation may be made based on the quantitative measurements of the surface.
Preferably, the test pattern will include a peripheral portion that is disposed around the ablated test surface, and which is not refracted by the test surface. An inner portion is aligned with the test surface and is refracted. The spacing between the geometrical elements can be compared to determine the distribution of refractive power of the test surface, and geometrical elements which extend between the peripheral and inner portions will indicate a contour and quality of the ablation near its edge. If desired, the refractive power may be determined by comparing the ratios of the spacing of the geometrical elements in the center to those at the periphery.
The test surface will typically include a lens, the lens generally comprising a polymer material that can be ablated with an excimer laser in a repeatable, predictable manner. Suitable test ablation materials include polymethylmethacrylate (“PMMA”), VISX calibration media (available from VISX, Incorporated), and the like. The plastic is ablated into a lens having a distribution of refractive power and a particular shape. Typically, the plastic lens will be ablated with approximately the same treatment that will be applied to a patient&apos
Caudle George
Clapham Terrance N.
Thompson Angelina
Barrish, Esq. Mark D.
Font Frank G.
Rodriguez Armando
Townsend & Townsend & Crew LLP
VISX Incorporated
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