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ABSTRACT
PURPOSE: Currently, refractive surgical excimer laser systems are calibrated by ablating plastic lenses, which are measured by lensometer and analyzed by a technician. The accuracy of this method is approximately 0.25 diopters (D) in sphere and cylinder power. Theoretically, objective calibration using wavefront technology would be significantly more accurate, thereby improving surgical outcomes. This study describes a Shack-Hartmannbased instrument, which has been developed to measure ablated plastic lenses for calibration and quality control of the excimer laser.
METHODS: A calibration instrument comprising an LED source at 640 nm, a lenslet array, beam-guiding optics, and a CCD camera was designed to perform full wavefront analysis. The measurement plane is conjugate to the lenslet array plane, and the diameter of the pupil is 5 mm. Accuracy was determined by measuring a set of well-calibrated spherical and cylindrical glass lenses. Plastic lenses were ablated, and high-precision measurements were performed by surface profile scanner.
RESULTS: In the power range of -6.00 to +4.00 D, repeatability exceeded 0.01 D, accuracy of measurement exceeded 0.04 D, and 1° for the axis of cylinder lenses. The measurement of excimer-ablated plastic lenses agreed with high-precision surface profile scanner measurements within 0.10 D, and repeatability exceeded 0.01 D.
CONCLUSIONS: Wavefront technology-based, highprecision measurement of calibration lenses can more accurately set the energy of the excimer laser, which enhances the accuracy of refractive laser correction. In automating calibration, the new instrument removes operator subjectivity and decreases the time needed for calibration. [J Refract Surg. 2006;22:938-942.]
Accurate delivery of laser energy to the corneal surface is one of the elements that are crucial to the accuracy of refractive surgery. Numerous technical advancements, making customized treatments possible, have been made in refractive surgery over the past decade. Among these advancements are aberrometers to measure the wavefront of the eye, advanced algorithms to ablate complex shapes, precise registration of the customized ablation to the corneal surface, and eye trackers that adjust the beam to compensate for tiny movements of the eye. One area where accuracy and precision can be further enhanced is in the calibration of laser energy.
The current procedure for calibrating the fluence (energy per unit beam area) of STAR (AMO-VISX, Santa Clara, Calif) excimer laser systems is by manual measurement and analysis...