Shack-Hartmann–Based Wavefront Analyzer for Calibrating Excimer Lasers

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-Hartmann-based 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, high-precision 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.]

AUTHORS

From VISX Incorporated, Santa Clara, Calif (Somani, Zickler).

The patent for the calibration device described in this article is owned by Advanced Medical Optics, Inc.

Drs Somani and Zickler are employees of VISX Incorporated, a subsidiary of Advanced Medical Optics Inc. Mr Gross and Dr Cheng are former employees of VISX Incorporated.

Correspondence: Seema Somani, PhD, VISX Incorporated, 3400 Central Expressway, Santa Clara, CA 95051. Tel: 408.773.7353; E-mail: seema.somani@amo-inc.com