Alexey Belyakov

Human eye anisoplanatism: eye as a lamellar structure

In this paper we consider anisoplanatism effect as a fundamental limitation on the size of high resolution area (isoplanatic patch) of retinal images obtained using fundus cameras equipped with adaptive optics. Isoplanatic patch size was estimated using experimental results for on-axis and off-axis eye aberrations measured by Shack-Hartmann technique. Isoplanatic patch size varied among examined subjects in the range from 1.5o to 2.5o which is in good agreement with results obtained using ray-tracing technique1. We estimated isoplanatic patch size for Gullstrand eye model and found it to be close to the values obtained from experimental results for subjects with good vision. We also discuss the possibilities of Gullstrand eye model modifications for modeling anisoplanatism effect for each particular subject. We also estimated the efficiency of multibeacon correction method and found out that this method allows us to almost twice increase the area with high resolution.

Custom-oriented wavefront sensor for human eye properties measurements

The problem of correct measurement of human eye aberrations is very important with the rising widespread of a surgical procedure for reducing refractive error in the eye, so called, LASIK (laser-assisted in situ keratomileusis). In this paper we show capabilities to measure aberrations by means of the aberrometer built in our lab together with Active Optics Ltd. We discuss the calibration of the aberrometer and show invalidity to use for the ophthalmic calibration purposes the analytical equation based on thin lens formula. We show that proper analytical equation suitable for calibration should have dependence on the square of the distance increment and we illustrate this both by experiment and by Zemax Ray tracing modeling. Also the error caused by inhomogeneous intensity distribution of the beam imaged onto the aberrometers Shack-Hartmann sensor is discussed.

Human retina imaging: widening of high resolution area

In this paper we consider different methods of widening high resolution retinal image area. The first method is based on compensation of an average phase of two beacons formed on human retina within the isoplanatic patch. The second one is based on compensation of external corneal surface refraction with the help of immersion liquid. In both methods we use a single wavefront corrector conjugated to the pupil plane. The immersion method was found to be the most appropriate as it allows one to increase the area of high resolution almost twice without loss of image quality.

Anisoplanatism in human retina imaging

All extended optical systems with aberrations suffer from anisoplanatism effect. In this presentation we investigate anisoplanatism in human eye. For that purpose we use a reference source (beacon) obtained by focusing of a dim laser beam on the retina and consider increasing the retina resolution within anisoplanatic angle by means of ideal wavefront corrector and a real bimorph flexible one. The numerical simulations of isoplanatic patch size of human retina were made for different beacon positions and based on the aberrations measured by means of custom wavefront-guided aberrometer. We found out that in particular human eye the existence of optimal correction directions is possible. As the behavior of Zernike coefficients varies from subject to subject the existence of optimal correction angle is a feature of a particular eye. We also estimated the contribution of low-order and high-order aberrations in anisoplanatism effect for the subjects we measured. We found out that aberrations with strongly variable amplitude across the visual field have effect on the isoplanatic patch size most. In this paper we illustrated the isoplanatic patch enlargement with variation of beacon position by presenting two-dimensional retina and test object images. Also anisoplanatism in two-layer human eye model has been discussed. As the main part of the eyes aberrations is induced by the surfaces of the cornea and the crystal lens, our model consists of two thin phase screens that correspond to the cornea and the lens. Then we used such two-layer model to minimize residual mean-square error of correction by means of just one applied corrector.

Isoplanatism of the optical system of the human eye

The size of the isoplanatism zone is determined on the basis of experimental measurements of aberrations of the eye for various positions of the test point on the retina of the eye. For the patients examined here, the angular size of the zone varied from 1.5° to 2.5°.

Creating a model of the human eye by the methods of adaptive optics

This paper proposes a simulator of the aberrations of the human eye that is capable of reproducing monochromatic aberrations of the human eye and their temporal dynamics.

Double-pass measurement of human eye aberrations: limitations and practical realization

The problem of correct eye aberrations measurement is very important with the rising widespread of a surgical procedure for reducing refractive error in the eye, so called, LASIK (laser-assisted in situ keratomileusis). The double-pass technique commonly used for measuring aberrations of a human eye involves some uncertainties. One of them is loosing the information about odd human eye aberrations. We report about investigations of the applicability limit of the double-pass measurements depending upon the aberrations status introduced by human eye and actual size of the entrance pupil. We evaluate the double-pass effects for various aberrations and different pupil diameters. It is shown that for small pupils the double-pass effects are negligible. The testing and alignment of aberrometer was performed using the schematic eye, developed in our lab. We also introduced a model of human eye based on bimorph flexible mirror. We perform calculations to demonstrate that our schematic eye is capable of reproducing spatial-temporal statistics of aberrations of living eye with normal vision or even myopic or hypermetropic or with high aberrations ones.

The influence of human eye aberrations on the resolution and field of view of fundus-cameras

In this paper we estimate isoplanatic patch size of human eye using experimental results of human eye aberrations measurements. For examined subjects it was found to be in the range 1.1° to 2.5°. The size of isoplanatic patch for Gulistrand-Navarro model was calculated, it was found to be close to values obtained experimentally. We also measured contribution of corneal surface and internal optics into total aberrations of the eye. We modified Gullstrand-Navarro eye model to reproduce on-axis and off-axis performance of the eyes of each measured subject. The distribution of aberrations between optical elements of the eye was taken into account when modeling. We also investigated isoplanatic patch size widening methods such as average phase correction using 2 beacons and immersion method based on compensation of external corneal surface refraction with immersion liquid. Immersion method was found to be the most appropriate for isoplanatic patch widening as it allows us to increase isoplanatic patch size almost twice without loss of image quality at the center of the image.

Human retina imaging: anisoplanatism considerations

In this presentation we report our results of investigation of anisoplanatism effect in human eye. We measured aberrations of human eye depending on the location of beacon source on the retina and determined the value of the isoplanatic patch. We show that the size of isoplanatic patch depends on the direction of compensation and determine an optimal direction for every patient. To enhance quality of retina image correction we introduce lamellar eye model where aberrations of human eye are considered to be induced in two thin phase screens corresponding to cornea and crystal lens locations. For that model we found the optimal corrector location which differs from the one for the one-layer eye model.

Anisoplanatism in adaptive optics compensation of human eye aberrations

The areas of adaptive optics application have increasingly expanded beyond astronomy over past ten years. One of the most striking examples is visual science. Fundus camera equipped with adaptive optics has been extensively investigated over past few years and employed with great success in obtaining fine images of the human retina in real time. But even if the aberrations of the human eye are corrected with adaptive optics the quality of retinal images is still degraded by anisoplanatism effect. We can obtain high-resolution image only if decorellation of the phase that is incident from the beacon on the retina and the point being imaged is small. The wavefront compensation is effective only within a finite area - the isoplanatic patch. On the basis of Zernike decompositions of the aberrated wavefront for different retinal angles we have been able to calculate the residual mean-square error for the corrected wavefront. We estimated the isoplanatic angle in human eye as the angular distance between the two sources where the mean-square error is equal to 1 square rad . Computer simulations illustrating the degrading effects of anisoplanatism on retinal imaging performance of adaptive optics system are presented. In the paper we discuss the limitations of isoplanatic patch enlargement by examining an ideal adaptive corrector that provides compensation of all Zernike modes. We simulated the blur of the retinal image induced by the eyes aberrations and the compensation of these aberrations by the corrector thus illustrating the performance of anisoplanatism-limited adaptive optics systems.