Aberration Characterization of Abnormal Eyes using the Large Dynamic Range Shack–Hartmann Wavefront Sensor

Abstract

Abstract: : Purpose: Abnormal eyes, such as those having keratoconus (KC) or penetrating keratoplasty (PK), have an unusually large amount of higher order aberration (HOA) that can be difficult to measure with a conventional Shack–Hartmann wavefront sensor. The purpose of this study is to characterize the aberrations in a large number of abnormal eyes using the large dynamic range Shack–Hartmann wavefront sensor (Pantanelli et al., ARVO 2003). Methods:The wavefront sensor used in this study had a lenslet array with a 10.7 mm focal length and 0.4 mm lenslet spacing. A translatable plate placed in conjugate with the lenslet array has holes that allow light to pass through every other lenslet in both the horizontal and vertical directions. Using this setup, the spacing between spots in each exposure is effectively increased by a factor of two. This doubled the dynamic range of the wavefront sensor without sacrificing sensitivity. This sensor had a dynamic range of approximately ±10 D without any pre–compensation for defocus and sensitivity better than 0.06 D. One complete measurement (four pictures) was acquired in about 200 ms. The aberrations in eleven abnormal eyes (5 with KC and 6 PK) were measured over a pupil size ≥ 6 mm (and ≤ 7.09 mm). At least 3 measurements were averaged for each eye. The Zernike coefficients were computed for a 6 mm pupil for all subjects. Results: The 5 KC eyes exhibited an abnormal amount of HOA, 2.02 ± 1.26 µm (mean ± standard deviation) RMS vs. an average value of 0.41 ± 0.16 µm for the normal population (208 myopic eyes). The most dominant HOA was vertical coma (1.76 µm), accounting for 75.7% of the total variance of HOA. Other significant HOA included secondary astigmatism Z(2,4) (5.6%), secondary coma Z(–1,5) (3.2%), and tertiary astigmatism Z(2,6) (1%). The PK subjects had an average higher order RMS of 0.96 ± 0.65 µm. Particularly high amounts of trefoil and spherical aberration accounted for 28% and 38% of the total higher order variance, respectively. For PK and KC eyes, the RMS wavefront error higher than 5^th order was 0.31 µm and 0.2 µm, respectively. Conclusions: The large dynamic range Shack–Hartmann wavefront sensor can be used to effectively measure the wavefront aberration of abnormal eyes. Although normal eyes can be accurately characterized with up to and including 5^th order aberrations, 6^th and 7^th order aberrations are still significant in eyes having either keratoconus or PK. These observations are important not only in understanding the optical quality of abnormal eyes, but also when considering the significance of customized correction methods.

Keywords: optical properties • keratoconus • refractive surgery