The Wavefront Aberration Function of Normal Human Corneas (original) (raw)
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Spherical aberration of the anterior and posterior surfaces of the human cornea
Journal of The Optical Society of America A-optics Image Science and Vision, 2006
A ray-tracing procedure was applied to corrected Scheimpflug photography measurements to determine the spherical aberration of the anterior and posterior surfaces of the cornea. It was found that the total spherical aberration of the cornea increases slightly with age. The spherical aberration of the posterior corneal surface is negative at a young age and becomes positive at an older age. To make an accurate description of the spherical aberration for the whole eye, the posterior surface must also be measured.
The spherical aberration of the crystalline lens of the human eye
Vision Research, 2001
The in vivo spherical aberration of the lenses of 26 subjects was estimated from the measured total aberration of the eye and that predicted from the measured shape of the anterior corneal surface. Since it was only possible to estimate the aberration contribution from the posterior corneal surface, its value led to an uncertainty in the final aberration level of the lens. For all the subjects and for a wide range of possible aberration levels at the posterior corneal surface, the spherical aberration of the relaxed lens was found to be negative.
Contributions of the cornea and the lens to the aberrations of the human eye
Optics Letters, 1998
The relative contributions of optical aberrations of the cornea and the crystalline lens to the f inal image quality of the human eye were studied. The aberrations of the entire eye were obtained from pairs of double-pass retinal images, and the aberrations of the cornea were obtained from videokeratographic data. Third-order spherical aberration and coma were signif icantly larger for the cornea than for the complete eye, indicating a signif icant role of the lens in compensating for corneal aberrations. In a second experiment retinal images were recorded in an eye before and after we neutralized the aberrations of the cornea by having the subjects wear swimming goggles f illed with saline water, providing a direct estimate of the optical performance of the crystalline lens.
Purpose: The purpose of this study was to theoretically model the contributions of corneal asphericity (Q) and anterior chamber depth to peripheral wavefront aberrations. Methods: Ray-tracing was performed on a model eye using a customised MatLab program to calculate Zernike aberrations up to the 5th order across AE60° of the horizontal visual field. The corneal Q was varied from À0.5 to 0.8, and the anterior chamber depth was changed from 2.05 to 4.05 mm while axial length was held constant. Spherical equivalent refractive error derived from Zernike defocus was used to estimate peripheral refraction. Results: Relative to axial Zernike aberrations, both defocus and astigmatism in the peripheral field increased with the corneal Q value, but the increases in relative peripheral astigmatism were much smaller in amplitude than relative peripheral defocus. Anterior chamber depth shortening caused the relative peripheral defocus and astigmatism to increase toward more positive values, although the changes in relative peripheral astigmatism with anterior chamber depth were small. Combination of the variations in both corneal Q and anterior chamber depth does not produce linear sum of the changes in relative peripheral defocus. The relative peripheral refractive error was more myopic when either the corneal Q was increased or the anterior chamber depth was shortened. The changes in relative peripheral x-axis coma, trefoil and spherical aberration with corneal Q value were complex but were barely changed with anterior chamber depth within the central 60° visual field. Conclusions: Both corneal asphericity and anterior chamber depth play important roles in determining peripheral wavefront aberrations. The two factors nonlinear-ly interact to affect peripheral aberrations. Higher corneal Q and/or shorter anterior chamber depth tend to produce relatively more myopic peripheral refraction. Increasing the Q value of the anterior surface of a contact lens might provide an interesting intervention to slow myopia progression.
Optical aberrations of the human anterior cornea
Journal of Cataract & Refractive Surgery, 2003
Purpose: (1) To investigate the distribution of anterior corneal higher-order aberrations (HOAs, 3rd to 6th orders) in the population; (2) to evaluate the symmetry of anterior corneal aberrations between right and left eyes of individuals; and (3) to study the variations in anterior corneal aberrations with aging.
Journal of the Optical Society of America A, 2006
Generally, the analysis of corneal topography involves fitting the raw data to a parametric geometric model that includes a regular basis surface, plus some sort of polynomial expansion to adjust the more irregular residual component. So far, these parametric models have been used in their canonical form, ignoring that the observation (keratometric) coordinate system is different from corneal axes of symmetry. Here we propose, instead, to use the canonical form when the topography is referenced to the intrinsic corneal system of coordinates, defined by its principal axes of symmetry. This idea is implemented using the general expression of an ellipsoid to fit the raw data given by the instrument. Then, the position and orientation of the three orthogonal semiaxes of the ellipsoid, which define the intrinsic Cartesian system of coordinates for normal corneas, can be identified by passing to the canonical form, by standard linear algebra. This model has been first validated experimentally obtaining significantly lower values for rms fitting error as compared with previous standard models: spherical, conical, and biconical. The fitting residual was then adjusted by a Zernike polynomial expansion. The topographies of 123 corneas were analyzed obtaining their radii of curvature, conic constants, Zernike coefficients, and the direction and position of the optical axis of the ellipsoid. The results were compared with those obtained using the standard models. The general ellipsoid model provides more negative values for the conic constants and lower apex radii (more prolate shapes) than the standard models applied to the same data. If the data are analyzed using standard models, the resulting mean shape of the cornea is consistent with previous studies, but when using the ellipsoid model we find new interesting features: The mean cornea is a more prolate ellipsoid (apical power 50 D), the direction of the optical axis is about 2.3°nasal, and the residual term shows three Zernike coefficients significantly higher than zero (third-order trefoil and fourthand sixth-order spherical). These three nonzero Zernike coefficients are responsible for most of the higherorder aberrations of the average cornea. Finally, we propose and implement a simple method for threedimensional registration of corneal topographies, passing from the general to the canonical form of the ellipsoid.
Change in corneal shape and corneal wave-front aberrations with accommodation
Journal of Vision, 2003
This study investigated the change in corneal curvature and corneal wave-front aberrations with accommodation. The corneal curvature of the right eyes of 12 young adults was measured using a corneal topography system, while subjects fixated far (4.0 m) and near (0.2 m) targets with their left eyes. Convergence was controlled. Both the mean corneal radius at the vertex and the shape parameter significantly increased from the far to the near viewing condition. No significant change in root mean square of wave-front aberrations with accommodation was observed for the group, but there was individual variation in the change of wave-front aberration. A significant mean change for the group in both x-axis coma and spherical aberration was found. The change in corneal surface with accommodation suggests an increase in peripheral curvature with flattening at the vertex.
Correlation between corneal and total wavefront aberrations in myopic eyes
Journal of Refractive Surgery, 2003
PURPOSE: Corneal topography data expressed as corneal aberrations are frequently used to report corneal laser surgery results. However, the optical image quality depends on all optical elements of the eye, including the human lens. We investigated correlations between corneal and total wavefront aberrations and the relevance of corneal aberrations for representing the optical quality of the total eye. METHODS: Thirty-three eyes of 22 myopic patients were measured using a corneal topography system and a Tscherning-type wavefront analyzer. Pupils were dilated to at least 6 mm in diameter. All measurements were centered with respect to the line of sight. Corneal and total wavefront aberrations were calculated up to the 6th Zernike order in the same reference plane.