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Aberration Coefficient

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T.j.j. Denteneer – One of the best experts on this subject based on the ideXlab platform.

Tiegen Liu – One of the best experts on this subject based on the ideXlab platform.

Alan Tomlinson – One of the best experts on this subject based on the ideXlab platform.

  • corneal optical Aberrations induced by photorefractive keratectomy
    Journal of Refractive Surgery, 1997
    Co-Authors: Katherine Oliver, R P Hemenger, Melanie C Corbett, David P S Obrart, Seema Verma, John Marshall, Alan Tomlinson
    Abstract:

    BACKGROUND: Photorefractive keratectomy causes marked alteration to anterior corneal topography, and is likely to induce major changes to the optical Aberrations of the eye. METHODS: Six diopters (D) of myopia correction was attempted on one eye of 50 patients, randomly allocated to one of three different treatments: 5-mm or 6-mm single ablation zone, or a double ablation (multizone; -5.00 D correction over 4.6 mm and -1.00 D over 6 mm). Topographic data was used to estimate corneal Aberration Coefficients. These were compared for effect of ablation zone, before and 1 year after photorefractive keratectomy. The Coefficients were used to derive modulation transfer functions for the anterior corneal surface. RESULTS: Corneal spherical Aberrations and coma-like Aberrations both increased significantly following photorefractive keratectomy (p < 0.001). The mean spherical Aberration Coefficient increased from 0.36 +/- 0.11 before, to 0.91 +/- 0.37 after treatment, while the mean coma-like Aberration Coefficient changed from 0.28 +/- 0.16 before, to 0.60 +/- 0.31 after treatment. Ablation zone form had a significant effect on spherical Aberration (p = 0.030), but not for coma (p = 0.96). The spherical Aberration Coefficient increased least for the 6-mm ablation (by 0.38 +/- 0.17), compared with the 5-mm ablation (0.69 +/- 0.45) and the multizone (0.62 +/- 0.38). Corneal modulation transfer functions were reduced significantly following the photorefractive procedure. The effect was greatest for large pupil diameters and for spatial frequencies between 2 and 15 cycles per degree. CONCLUSIONS: Corneal modulation transfer function calculations suggest that a significant loss of visual performance should be anticipated following photorefractive keratectomy, the effect being greatest for large pupil diameters. Results for three ablation zones show that induced Aberrations are least for the largest (6 mm) ablation zone.

W.m.j. Coene – One of the best experts on this subject based on the ideXlab platform.

  • Improved methods for the determination of the spherical Aberration Coefficient in high-resolution electron microscopy from micrographs of an amorphous object
    Ultramicroscopy, 1991
    Co-Authors: W.m.j. Coene, T.j.j. Denteneer
    Abstract:

    New and improved statistical procedures are proposed for determination of the spherical Aberration Coefficient Cs in high-resolution electron microscopy (HREM) from micrographs of an amorphous object. A least-squares technique applied to the experimental zeros of the microscope’s transfer function with a parabola instead of a straight line as fitting curve represents the major key to improved determination of the spherical Aberration Coefficient. A smaller further improvement is realised by application of a second iteration step for the parabola method. Another improvement is obtained by using a focal series of HREM images instead of a single micrograph. With the multifocus iterated parabola method, a standard deviation (sd) below 1% can be reached for the spherical Aberration Coefficient. The methods are applied to the determination of Cs for the Philips CM30-SuperTWIN and the Philips CM20-UltraTWIN microscopes. Apart from an error in the calibration of the magnification, which leads to an estimated error of 1% on Cs, we obtained for the CM30-Super- TWIN Cs = 1.210 mm with sd = 0.008 mm, and for the CM20-UltraTWIN Cs = 0.494 mm with sd = 0.004 mm.

Yimo Zhang – One of the best experts on this subject based on the ideXlab platform.