Stefan Goelz

Bio: Stefan Goelz is an academic researcher from Heidelberg University. The author has contributed to research in topics: Wavefront & Ablation. The author has an hindex of 5, co-authored 7 publications receiving 1590 citations.

Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor

Abstract: A Hartmann-Shack wave-front sensor is used to measure the wave aberrations of the human eye by sensing the wave front emerging from the eye produced by the retinal reflection of a focused light spot on the fovea. Since the test involves the measurements of the local slopes of the wave front, the actual wave front is reconstructed by the use of wave-front estimation with Zernike polynomials. From the estimated Zernike coefficients of the tested wave front the aberrations of the eye are evaluated. It is shown that with this method, using a Hartmann-Shack wave-front sensor, one can obtain a fast, precise, and objective measurement of the aberrations of the eye.

Analysis of the performance of the Hartmann–Shack sensor in the human eye

Abstract: A description of a Hartmann–Shack sensor to measure the aberrations of the human eye is presented We performed an analysis of the accuracy and limitations of the sensor using experimental results and computer simulations We compared the ocular modulation transfer function obtained from simultaneously recorded double-pass and Hartmann–Shack images The following factors affecting the sensor performance were evaluated: the statistical accuracy, the number of modes used to reconstruct the wave front, the size of the microlenses, and the exposure time

In-vivo measurement of the retinal birefringence with regard to corneal effects using an electro-optical ellipsometer

Abstract: The phase retardation of light induced by the birefringent parts of the human retina in vivo is measured with an electro-optical ellipsometer using the principle of confocal imaging. A scanning unit allows to examine an area of 25 degrees by 12.5 degrees on the retina with a resolution of 256 by 128 points with both, incident and exit beam transmitting the cornea at a fixed position. Due to this fixed beam position the Muellermatrix of the cornea can be calculated using light which is specularly reflected on blood vessels lying above the nerve fiber layer of the retina. The obtained images show a homogenous radial distribution of the retinal retardation around the fovea without the appearance of the so called Haidinger brushes. The areas with the thickest nerve fiber layers, the arcuate bundles, appear in their typical arc and were measured quantitatively. In addition, an alternative method for the compensation of corneal birefringence is evaluated by focusing the light beam onto the surface of the lens. Hereby, the measured area in the center of the cornea is 3 X 0.75 mm2.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Hartmann-Shack Sensor as a Component in Active Optical System to Improve the Depth Resolution of the Laser Tomographic Scanner

Abstract: This paper presents the measurement of the wave aberrations of human eyes with a Hartmann-Shack Sensor (HSS), used in a system for active optical depth resolution improvement of the Laser Tomographic Scanner(LTS). A least-squares algorithm of modal wavefront estimation from the tested derivatives is described. The noise propagation of this algorithm is examined. And the experimental results of tested living eyes are presented.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

New wavefront sensor for metrology of spherical surfaces

Abstract: In this paper an extension of the principle of the Hartmann-Shack wavefront sensor to the neasureirtent of the sphericity of a specular reflection surface is presented. The features of this widely usable system are discussed by means of the application of this approach to the modeling of surface aberrations of the cornea of the eye. The results of measurements performed with a first prototype on a set of stainless steel calibration spheres of different radii are shown.

Supernormal vision and high-resolution retinal imaging through adaptive optics

Abstract: Even when corrected with the best spectacles or contact lenses, normal human eyes still suffer from monochromatic aberrations that blur vision when the pupil is large. We have successfully corrected these aberrations using adaptive optics, providing normal eyes with supernormal optical quality. Contrast sensitivity to fine spatial patterns was increased when observers viewed stimuli through adaptive optics. The eye's aberrations also limit the resolution of images of the retina, a limit that has existed since the invention of the ophthalmoscope. We have constructed a fundus camera equipped with adaptive optics that provides unprecedented resolution, allowing the imaging of microscopic structures the size of single cells in the living human retina.

Aberrations and retinal image quality of the normal human eye.

Abstract: We have constructed a wave-front sensor to measure the irregular as well as the classical aberrations of the eye, providing a more complete description of the eye's aberrations than has previously been possible. We show that the wave-front sensor provides repeatable and accurate measurements of the eye's wave aberration. The modulation transfer function of the eye computed from the wave-front sensor is in fair, though not complete, agreement with that obtained under similar conditions on the same observers by use of the double-pass and the interferometric techniques. Irregular aberrations, i.e., those beyond defocus, astigmatism, coma, and spherical aberration, do not have a large effect on retinal image quality in normal eyes when the pupil is small (3 mm). However, they play a substantial role when the pupil is large (7.3-mm), reducing visual performance and the resolution of images of the living retina. Although the pattern of aberrations varies from subject to subject, aberrations, including irregular ones, are correlated in left and right eyes of the same subject, indicating that they are not random defects.

Statistical variation of aberration structure and image quality in a normal population of healthy eyes.

Abstract: A Shack-Hartmann aberrometer was used to measure the monochromatic aberration structure along the primary line of sight of 200 cyclopleged, normal, healthy eyes from 100 individuals. Sphero-cylindrical refractive errors were corrected with ophthalmic spectacle lenses based on the results of a subjective refraction performed immediately prior to experimentation. Zernike expansions of the experimental wave-front aberration functions were used to determine aberration coefficients for a series of pupil diameters. The residual Zernike coefficients for defocus were not zero but varied systematically with pupil diameter and with the Zernike coefficient for spherical aberration in a way that maximizes visual acuity. We infer from these results that subjective best focus occurs when the area of the central, aberration-free region of the pupil is maximized. We found that the population averages of Zernike coefficients were nearly zero for all of the higher-order modes except spherical aberration. This result indicates that a hypothetical average eye representing the central tendency of the population is nearly free of aberrations, suggesting the possible influence of an emmetropization process or evolutionary pressure. However, for any individual eye the aberration coefficients were rarely zero for any Zernike mode. To first approximation, wave-front error fell exponentially with Zernike order and increased linearly with pupil area. On average, the total wave-front variance produced by higher-order aberrations was less than the wave-front variance of residual defocus and astigmatism. For example, the average amount of higher-order aberrations present for a 7.5-mm pupil was equivalent to the wave-front error produced by less than 1/4 diopter (D) of defocus. The largest pupil for which an eye may be considered diffraction-limited was 1.22 mm on average. Correlation of aberrations from the left and right eyes indicated the presence of significant bilateral symmetry. No evidence was found of a universal anatomical feature responsible for third-order optical aberrations. Using the Marechal criterion, we conclude that correction of the 12 largest principal components, or 14 largest Zernike modes, would be required to achieve diffraction-limited performance on average for a 6-mm pupil. Different methods of computing population averages provided upper and lower limits to the mean optical transfer function and mean point-spread function for our population of eyes.

Monochromatic aberrations of the human eye in a large population.

Abstract: From both a fundamental and a clinical point of view, it is necessary to know the distribution of the eye's aberrations in the normal population and to be able to describe them as efficiently as possible. We used a modified Hartmann-Shack wave-front sensor to measure the monochromatic wave aberration of both eyes for 109 normal human subjects across a 5.7-mm pupil. We analyzed the distribution of the eye's aberrations in the population and found that most Zernike modes are relatively uncorrelated with each other across the population. A principal components analysis was applied to our wave-aberration measurements with the resulting principal components providing only a slightly more compact description of the population data than Zernike modes. This indicates that Zernike modes are efficient basis functions for describing the eye's wave aberration. Even though there appears to be a random variation in the eye's aberrations from subject to subject, many aberrations in the left eye were found to be significantly correlated with their counterparts in the right eye.

Method and apparatus for improving vision and the resolution of retinal images

Abstract: A method of and apparatus for improving vision and the resolution of retinal images is described in which a point source produced on the retina of a living eye by a laser beam is reflected from the retina and received at a lenslet array of a Hartmann-Shack wavefront sensor such that each of the lenslets in the lenslet array forms an aerial image of the retinal point source on a CCD camera located adjacent to the lenslet array. The output signal from the CCD camera is acquired by a computer which processes the signal and produces a correction signal which may be used to control a compensating optical or wavefront compensation device such as a deformable mirror. It may also be used to fabricate a contact lens or intraocular lens, or to guide a surgical procedure to correct the aberrations of the eye. Any of these methods could correct aberrations beyond defocus and astigmatism, allowing improved vision and improved imaging of the inside of the eye.