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S.M. Pantanelli, G. Yoon, T. Jeong, S. MacRae; Large Dynamic Range Shack-Hartmann Wavefront Sensor for Highly Aberrated Eyes . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2536.
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Purpose: One limitation of a Shack-Hartmann wavefront sensor is the dynamic range of measurement. Although this problem can be avoided by decreasing the focal length of the lenslet array at a fixed lenslet spacing, this change reduces the measurement sensitivity. Our goal was to develop a large dynamic range Shack-Hartmann wavefront sensor without sacrificing sensitivity. Methods: A Shack-Hartmann wavefront sensor was built with a 5.2 mm focal length and a 0.4 mm lenslet spacing. Without reducing the focal length, the dynamic range was increased by placing a translatable plate (sub-aperture) at a plane conjugate with the pupil. Holes in the plate allow light to pass through every other lenslet in both the horizontal and vertical directions. A single measurement consists of four exposures, one from each position of the plate. Using this setup, the spacing between spots in each exposure is effectively increased by a factor of two. The plate was tested in our Shack-Hartmann wavefront sensor with two normal eyes and one eye with penetrating keratoplasty (PK). The normal eyes were measured with and without the plate. The PK eye could be measured only with the plate in place because of the amount of the aberration. Up to 10th order Zernike coefficients were calculated for a 6.7 mm pupil. Results: The dynamic range of our wavefront sensor with and without the plate was approximately +/- 9 D and +/- 18 D, respectively. The sensitivity of the sensor with and without the plate was 0.125 D. For the normal eyes having a low (<-1D) and high (-9D) refractive error, the difference in the RMS values calculated with and without the plate was less than 2% on average. Using the plate, Zernike coefficients were successfully calculated for the PK eye over a 6.7 mm pupil. Total RMS error was 13.4 um and the most dominant higher order aberrations were spherical aberration and trefoil. Conclusions: The dynamic range of a Shack-Hartmann wavefront sensor can be increased without losing measurement sensitivity by using a translatable plate. By increasing the spacing between holes in the plate and the number of translations required, the dynamic range can be increased even more. An extreme configuration of this idea would be to fully scan the exit pupil using a single hole as the sub-aperture. The developed large dynamic range Shack-Hartmann wavefront sensor is a powerful diagnostic tool to apply customized correction methods to eyes with large wavefront errors due to corneal pathology or surgery.
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