Comparison of Monochromatic Ocular Aberrations Measured with an Objective Cross-Cylinder Aberroscope and a Shack-Hartmann Aberrometer (original) (raw)
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Clinical and Experimental Optometry, 2009
Background: Clinical aberrometers are accurate, robust instruments for measuring wavefront aberrations for foveal vision but several practical concerns arise when performing aberrometry of the peripheral field. The purpose of this study was to evaluate these concerns experimentally using a physical eye model. Methods: A physical model eye was constructed to provide a stable test case that resembled a human eye. Aberrations were measured with a commercial Shack-Hartmann aberrometer along lines-of-sight ranging from zero to 45°of eccentricity. Commercial software for wavefront reconstruction and Zernike analysis was adapted for use with elliptical entrance pupils encountered off-axis. Results: Pupil dimensions estimated from the array of Shack-Hartmann spots captured by the wavefront sensor followed geometrical optics predictions up to 30°eccentricity. With careful attention to detail, aberration analysis over an elliptical pupil was verified with alternative software. Retinal image quality declined slowly as eccentricity increased due to the eye model's spherical aberration. The total RMS computed from Zernike coefficients overestimated the total RMS computed based on the wavefront error of the elliptical pupil. Conclusion: Measurement of off-axis wavefront aberrations of a model eye over a restricted range of eccentricities is possible with the COAS clinical wavefront aberrometer and auxiliary lenses to correct astigmatism. When central image quality is good, the off-axis aberrations will have a powerful effect on peripheral image quality. When central image quality is poor, the additional effect of off-axis aberrations will be minor.
Optometry and Vision Science, 2001
To compare quantitatively three techniques to measure the optical aberrations of the human eye: Laser Ray Tracing (LRT), Hartmann-Shack wavefront sensor (HS) and Spatially Resolved Refractometer (SRR). LRT and HS are objective imaging techniques whereas the SRR is psychophysical. METHODS: Wave aberrations were measured in two normal subjects with all three techniques, as implemented in two different laboratories. RESULTS: We compared the experimental variability of the results obtained within each technique with the overall variability across the three methods. For the two subjects measured (RMS wavefront error 0.5 µm and 0.9µm respectively), we found a close agreement; the average standard deviation of the Zernike coefficients within a given method was 0.07 µm, whereas the average global standard deviation across techniques was 0.09µm, which is only slightly higher.
Objective technique for the determination of monochromatic aberrations of the human eye
Journal of the Optical Society of America A, 1984
The subjective crossed-cylinder aberroscope method of Howland and Howland [J. Opt. Soc. Am. 67, 1508 (1977)] has been modified by the addition of a beam splitter and a camera to permit direct photographic recording of the distorted retinal image of the aberroscope grid. The ocular aberration can then be deduced from direct measurements of the grid distortion. Preliminary results on 11 subjects confirm earlier findings that (1) comalike, thirdorder aberrations are more important than spherical or other fourth-order aberrations in degrading the retinal image and (2) for the average subject, the diffraction-limited pupil size is approximately 3 mm. This new objective method for measuring wave aberration yields significantly less variance in population estimates of the coefficients of the wave-aberration polynomial than that of the previous subjective method.
Component contributions to ocular aberrations
2016
PURPOSE: To determine component contributions to ocular aberrations. METHODS: Anterior and posterior corneal contributions were obtained from Pentacam topography images and data, and ocular aberrations were obtained from iTrace aberrometer images and data. Compensation was made for decentration of corneal data relative to aberrometry data. Lenticular contributions were given as differences between ocular and corneal aberrations. RESULTS: Data were presented for right eyes of 56 adults. The signs of coefficients for ocular and total corneal aberration were usually the same (8/11 coefficients). Total corneal and lenticular aberrations usually had opposite signs (8/11 coefficients) and similar magnitude. There was compensation of total corneal aberrations by lenticular aberrations for 5/11 coefficients. CONCLUSION: In contrast to previous studies using the aberrations provided by the Pentacam, anterior corneal aberrations were much higher than posterior corneal aberrations.
A new approach to the study of ocular chromatic aberrations
We measured the ocular wavefront aberration at six different visible wavelengths (between 450 and 650 nm) in three subjects, using a spatially resolved refractometer. In this technique, the angular deviation of light rays entering the pupil at different locations is measured with respect to a target viewed through a centered pupil. Fits of the data at each wavelength to Zernike polynomials were used to estimate the change of defocus with wavelength (longitudinal chromatic aberration, LCA) and the wavelength-dependence of the ocular aberrations. Measured LCA was in good agreement with the literature. In most cases the wavefront aberration increased slightly with wavelength. The angular deviations from the reference stimulus measured using a magenta filter allowed us to estimate the achromatic axis and both optical and perceived transverse chromatic aberration (TCA), (including the effect of aberrations and Stiles-Crawford effect). The amount of TCA varied markedly across subjects, and between eyes of the same subject. Finally, we used the results from these experiments to compute the image quality of the eye in polychromatic light.
Ophthalmic and Physiological Optics, 2009
The purpose of this work is to analyze the contribution of eye movements to the variability of the Zernike coefficients as determined with a Hartmann–Shack aberrometer. In order to isolate this effect we considered static aberrations tied to the eye pupil. We used several eye movements of different magnitude, both synthetic and corresponding to actual series recorded in our laboratory with different subjects. Our results show the relevance of the modal coupling induced by the estimation process and the benefit of correcting eye movements in order to get a better estimate of the ocular aberrations. They also show that eye movements during aberrometric measurements are an important source of apparent wavefront variability.
BMC ophthalmology, 2004
Recently, instruments for the measurement of wavefront aberration in the living human eye have been widely available for clinical applications. Despite the extensive background experience on wavefront sensing for research purposes, the information derived from such instrumentation in a clinical setting should not be considered a priori precise. We report on the variability of such an instrument at two different pupil sizes. A clinical aberrometer (COAS Wavefront Scienses, Ltd) based on the Shack-Hartmann principle was employed in this study. Fifty consecutive measurements were performed on each right eye of four subjects. We compared the variance of individual Zernike expansion coefficients as determined by the aberrometer with the variance of coefficients calculated using a mathematical method for scaling the expansion coefficients to reconstruct wavefront aberration for a reduced-size pupil. Wavefront aberration exhibits a marked variance of the order of 0.45 microns near the edge...
Repeatability of corneal and ocular aberration measurements and changes in aberrations over one week
Clinical and Experimental Optometry, 2009
Background: Both Shack-Hartmann aberrometry (IRX3, Imagine Eyes, Orsay, France) and Scheimpflug photography (Oculus Pentacam, Oculus Inc Wetzlar, Germany) are known to provide repeatable measurements. The variability in measurements of corneal and ocular aberrations obtained with these instruments over one week had not been assessed. The aim was to study the variability in corneal and ocular aberrations in the human eye over one week and to determine the impact of age on corneal and crystalline lens aberrations and on the variability of these measurements. Methods: Monochromatic ocular aberrations were measured with the IRX3 and monochromatic corneal aberrations were measured using Scheimpflug photography on one eye of 23 normal subjects at periods of a few seconds, one hour and one week after the initial measurement. Results: No significant differences were found between the initial aberrations and aberrations measured at 59 Ϯ 24 seconds, 1.10 Ϯ 0.24 hours and 7.11Ϯ 0.31 days later. Analysis with Bland and Altman plots showed that measurements with both instruments were highly repeatable over the times studied. There was no relationship between age and the variance of corneal and ocular aberrations (higher order, spherical and coma aberrations). Corneal spherical aberration did not show a significant correlation with age, whereas the lens aberrations changed from being negative in the younger age groups to positive in the older age group, however, these differences failed to reach statistical significance (p > 0.05). Conclusion: The variability found in all the measurements was small and not clinically significant and could be attributable to instrument noise, changes in the tear film and to small fixational eye movements.
Optics Express, 2007
We present data analysis for ocular aberrations of 60 normal eyes measured with a Hartmann-Shack (HS) wavefront sensor (WFS). Aberration measurements were made on-axis and at 5 degree field angles in the nasal, inferior, temporal and superior semi-meridians. Particular attention is given to aberration distributions and possible strategies for aberration correction are discussed. A versatile HS WFS was designed and constructed with features of simultaneous pupil centre determination, offaxis capability, real-time data displays, and efficient lenslet sampling orientation. The subject alignment is achieved by the use of a parallel channel that is recombined with the sensing channel to simultaneously image the eye and the HS spots onto a single CCD. The pupil centre is determined using this image of the eye (iris edge), rather than the HS spots. The optical design includes a square lenslet array positioned with its diagonals aligned with the most typical principal astigmatic meridians of the eye. This favourable orientation helps to enlarge the dynamic range of the WFS. The telecentric re-imaging of the HS spots increases the robustness of the system to defocus in the event of CCD misalignment.
Unfocused effect on the measurement of ocular aberrations
Journal of the Medical Association of Thailand = Chotmaihet thangphaet, 2005
To evaluate the effect of unfocused measurement in the value of ocular aberrations. The present study was conducted at Siriraj Hospital, Mahidol University. Ocular aberrations of 20 eyes in 10 myopic patients (myopia less than -6.00 diopters (D) and astigmatism less than -2.00 diopters (D)) were analyzed by using Zywave aberrometer (Bausch and Lomb, USA). The measurements were done at the pupillary plane (focus) and 6 other different planes (unfocus); 5, 10 and 15 mm inside and outside pupillary plane respectively. The value of each measurement was analyzed to find the effect of unfocused measurement on ocular aberrations. The magnitude of error (root mean square, RMS) of the spherical component of refraction at 5, 10, 15 mm inside and outside the pupillary plane was 0.16, 0.44, 0.57 D and 0.21, 0.38, 1.35D respectively. The RMS of astigmatic component of refraction inside and outside the pupillary plane at the same distance was 0.19, 0.50, 0.80 and 0.18, 0.52, 1.55 D respectively. ...