Showing papers on "Wavefront published in 2000"

Corneal first surface optical aberrations and visual performance.

Abstract: PURPOSE: Wavefront analysis has demonstrated that refractive surgery-induced corneal first surface aberrations are large, are dominated by symmetric aberrations (spherical-like aberrations), and are correlated to measures of visual performance. It is not clear whether the correlation between corneal first surface aberrations and visual performance can be generalized to other corneal conditions where large asymmetric aberrations (coma-like aberrations) may dominate the aberration structure. The purpose of the research reported here was to determine the general utility of corneal first surface wavefront analysis in predicting visual performance. METHODS: Patients were 13 normals and 78 patients with a variety of corneal conditions including surgically removed pterygia, penetrating keratoplasty, keratoconus, radial keratotomy, laser in situ keratomileusis, and others. Videokeratographs were taken for all patients and used to calculate corneal first surface wavefront variance for 3 and 7 mm pupils. Similarly, visual performance was quantified by measurements of contrast sensitivity and high and low contrast acuities through both 3 and 7 mm pupils. RESULTS: Statistically significant correlations existed between all three measures of visual performance and the corneal wavefront variance. All relationships were stronger for the 7 mm diameter-pupil condition than the 3 mm pupil. CONCLUSION: Regardless of the cause, corneas with increased wavefront variance showed a quantifiable decrease in visual performance that was pupil size dependent.

Introduction to Adaptive Optics

Abstract: A Long Time Ago, in a Laboratory Far, Far, Really Far, Far Away Adaptive Optics Systems - Optics is Our Middle Name Speaking the Language - a Few Definitions Atmospheric Turbulence - Bad Air... Bad, Bad Air Laser Guide Stars - a Beacon in the Wilderness Systems -Putting it All Together Wavefront Sensors - the Eyes Deformable Mirrors - the Hands Control Computers and Reconstructors - the Brains.

New modal wave-front sensor: a theoretical analysis

Abstract: We present a new design of a modal wave-front sensor capable of measuring directly the Zernike components of an aberrated wave front. The sensor shows good linearity for small aberration amplitudes and is particularly suitable for integration in a closed-loop adaptive system. We introduce a sensitivity matrix and show that it is sparse, and we derive conditions specifying which elements are necessarily zero. The sensor may be temporally or spatially multiplexed, the former using a reconfigurable optical element, the latter using a numerically optimized binary optical element. Different optimization schemes are discussed, and their performance is compared.

Adaptive aberration correction in a two-photon microscope

Abstract: We demonstrate aberration correction in two-photon microscopy. Specimen-induced aberrations were measured with a modal wavefront sensor, implemented using a ferro-electric liquid crystal spatial light modulator (FLCSLM). Wavefront correction was performed using the same FLCSLM. Axial scanned (xz) images of fluorescently labelled polystyrene beads using an oil immersion lens show restored sectioning ability at a depth of 28 mm in an aqueous specimen.

Performance and Scalability Analysis of Teraflop-Scale Parallel Architectures Using Multidimensional Wavefront Applications

Abstract: The authors develop a model for the parallel performance of algorithms that consist of concurrent, two-dimensional wavefronts implemented in a message-passing environment. The model, based on a LogGP machine parameterization, combines the separate contributions of computation and communication wavefronts. The authors validate the model on three important supercomputer systems, on up to 500 processors. They use data from a deterministic particle transport application taken from the ASCI workload, although the model is general to any wavefront algorithm implemented on a 2-D processor domain. They also use the validated model to make estimates of performance and scalability of wavefront algorithms on 100 TFLOPS computer systems expected to be in existence within the next decade as part of the ASCI program and elsewhere. In this context, the authors analyze two problem sizes. Their model shows that on the largest such problem (1 billion cells), interprocessor communication performance is not the bottleneck. Single-node efficiency is the dominant factor.

Adaptive-optics corrections available for the whole sky

Abstract: Adaptive-optics systems can in principle allow a telescope to achieve performance at its theoretical maximum (limited only by diffraction), by correcting in real time for the distortion of starlight by atmospheric turbulence1. For such a system installed on an 8-m-class telescope2,3, the spatial resolution and sensitivity could be up to 100 times better than conventional imaging4,5. Adaptive-optics corrections have hitherto been achieved only for regions of the sky within a few arcseconds of a bright reference source. But it has been proposed theoretically that by using multiple guide stars, the tomography of atmospheric turbulence could be probed and used to extend adaptive-optics corrections to the whole sky6,7. Here we report the experimental verification of such tomographic8 corrections, using three off-axis reference stars ∼15 arcsec from the central star. We used the observations of the off-axis stars to calculate the deformations of the wavefront of the central star, and then compare them with the real measured values. This tomographic approach is found to reduce variations in the wavefront by ∼92%. Our result demonstrates that a serious barrier to achieving diffraction-limited seeing over the whole sky has been removed.

System and method of integrating corneal topographic data and ocular wavefront data with primary ametropia measurements to create a soft contact lens

Abstract: A system and method is provided for integrating corneal topographic data and ocular wavefront data with primary ametropia measurements to create a soft contact lens design. Corneal topographic data is used to design a better fitting soft contact lens by achieving a contact lens back surface which is uniquely matched to a particular corneal topography, or which is an averaged shape based on the particular corneal topography. In the case of a uniquely matched contact lens back surface, the unique back surface design also corrects for the primary and higher order optical aberrations of the cornea. Additionally, ocular wavefront analysis is used to determine the total optical aberration present in the eye. The total optical aberration, less any corneal optical aberration corrected utilizing the contact lens back surface, is corrected via the contact lens front surface design. The contact lens front surface is further designed to take into account the conventional refractive prescription elements required for a particular eye. As a result, the lens produced exhibits an improved custom fit, optimal refractive error correction and vision.

Diffusion Waves and their Uses

Abstract: The concept of waves is an integral part of our scientific culture and has nourished physicists, pure and applied alike, for centuries. Many important discoveries in physics, including quantum mechanics, have involved wave phenomena. The wave concept owes some of its scientific success to its mathematical tractability. Linear wave equations—the sort that describe the transmission of sound and radio waves through air—contain a nonzero second‐order time derivative, which gives rise mathematically to the rich and familiar array of properties we associate with waves, such as wavefront propagation, reflection, and refraction.

Solar adaptive optics

Abstract: High resolution observations of the Sun are of key importance in understanding fundamental astrophysical processes. Adaptive optics (AO) is an important tool that allows solar astronomers to achieve diffraction limited observations from existing ground based telescopes. AO is also a key technology required for a future 4m-class Advanced Solar Telescope (AST) that the international community of solar astronomers is planning to build. The history of the development of solar AO is reviewed and results from recent successful demonstrations of solar AO systems are presented. The main difference between solar AO and night time AO is the different, and more elaborate wavefront sensing technique that has to be applied in order to measure wavefront aberrations using solar granulation as a target. Different approaches to this problem are discussed. Multi-conjugate AO has been proposed as a technique to achieve diffraction limited resolution over a field-of-view (FOV) significantly larger than the isoplanatic patch. The Sun is an ideal object for the development and application of MCAO.

A low cost adaptive optics system using a membrane mirror

Abstract: A low cost adaptive optics system constructed almost entirely of commercially available components is presented. The system uses a 37 actuator membrane mirror and operates at frame rates up to 800Hz using a single processor. Numerical modelling of the membrane mirror is used to optimize parameters of the system. The dynamic performance of the system is investigated in detail using a diffractive wavefront generator based on a ferroelectric spatial light modulator. This is used to produce wavefronts with time-varying aberrations. The ability of the system to correct for Kolmogorov turbulence with different strengths and effective wind speeds is measured experimentally using the wavefront generator.

Apparatus and method for improving vision and retinal imaging

Abstract: A method for improving the visual performance of a person involves correcting higher-order monochromatic aberrations in combination with the correction of chromatic aberration. Such correction results in a visual benefit greater than that realized by correcting only the higher-order monochromatic aberrations or the chromatic aberration alone. The higher-order monochromatic aberrations are corrected by introducing appropriate phase profiles to compensate for the wavefront aberrations of the eye. This compensation can be provided by contact lenses, IOLs, inlays and onlays having appropriate surface shapes or by corneal shaping achieved through refractive surgery or other techniques. Chromatic aberration can be corrected by spectral filtering or artificial apodization. An apodization filter is described that provides a non-uniform amplitude transmission across the pupil of the eye. Contact lenses or other ocular devices for correcting higher-order monochromatic aberrations may include an appropriate apodization filter for correcting chromatic aberration, or an external optical device for correcting chromatic aberration may be used in combination with a contact lens, etc. for correcting the higher-order monochromatic aberrations. A device and method for improved retinal imaging is also described.

Adaptive optics for 100-m-class telescopes: new challenges require new solutions

Abstract: For giant, 100 m-class, telescopes, a completely new class of problems arises. Trying to solve them adopting the techniques used for the classical 4 ... 10 m-class telescope systems is indeed a reductive approach. Because of the cone effect and of the difficulties in retrieving the tilt due to the use of LGSs, tomography and multiconjugation will play a central role in the development of these systems. We review the tomographic concept and we report about recent experiments to validate the concept using sky observations. The Field of View requirement is another crucial step in the design of the optical system of these telescopes and hence it needs to be attacked in a detailed and as wide as possible approach. The adoption of classical wavefront sensors looking to different stars is here overcome with wavefront sensors directly conjugated to different layers, with big advantages in term of intrinsic complication of the system, both from the optomechanical point of view (less complexity and saving of a huge amount of light) and from the computational point of view. The adoption of these new sensing techniques (that, gaining from the close loop situation, can be efficiently coupled to a number of very faint stars, whose complexity does not scale with the number of references but, rather, to the number of sensed layers) is here reviewed. This concept of multiple object wavefront sensing leads to an example of a system with a minimum number of independent adaptive optics loop, in which each layer represented in a wavefront sensor can be conjugated to a specifically conjugated deformable mirror.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Method for determining and correcting vision

Abstract: A method for enhancing vision of an eye includes a laser delivery system having a laser beam for ablating corneal material from the cornea of the eye Measurements are made to determine an optical path difference between a plane wave and a wavefront emanating from the retina of the eye for a location at a surface of the cornea An optical correction is provided to the laser delivery system for the location based on the optical path difference and refractive indices of media through which the wavefront passes The optical correction includes dividing the optical path difference by a difference between an index of refraction of corneal material and an index of refraction of air The laser beam is directed to the location on the surface of the cornea and corneal material ablated at the location in response to the optical correction to cause the wavefront to approximate the shape of the plane wave at that location

Laser illumination with speckle reduction

Abstract: A speckle reduction system divides pulses of coherent radiation into successions of temporally separated and spatially aberrated pulselets. One or more beamsplitters divide the pulses into the successions of pulselets that are circulated through delay lines. Spatial aberrators located along the delay lines modify wavefront shapes of the pulselets. Together, the temporal separation and spatial aberration of the pulselets produce a succession of different speckle patterns that can be averaged together within the integration interval of a detector to reduce speckle contrast.

Palomar adaptive optics project : status and performance

Abstract: We describe the current performance of the Palomar 200 inch (5 m) adaptive optics system, which in December of 1998 achieved its first high order (241 actuators) lock on a natural guide star. In the K band (2.2 micrometer), the system has achieved Strehl ratios as high as 50% in the presence of 1.0 arcsecond seeing (0.5 micrometer). Predictions of the system's performance based on the analysis of real-time wavefront sensor telemetry data and an analysis based on a fitted Kolmogorov atmospheric model are shown to both agree with the observed science image performance. Performance predictions for various seeing conditions are presented and an analysis of the error budget is used to show which subsystems limit the performance of the AO system under various atmospheric conditions.

Principles of Tscherning aberrometry.

Abstract: Higher-order optical errors of the human eye are often responsible for reduced visual acuity in spite of an optimal spherical or cylindrical refraction. These optical aberrations are of natural origin or can result from operations on the eye involving optical structures. The presented wavefront ana- lyzer is based on Tschernings aberroscope. A colli- mated laser beam illuminates a mask with regular matrix pin holes which forms a bundle of thin par- allel rays. These rays form a retinal spot pattern on the retina that is more or less distorted according to the optical errors of the eye. This retinal spot pattern is imaged onto the sensor of a low-light CCD camera by indirect ophthalmoscopy. The devi- ations of all spots from their ideal regular positions are measured by means of a personal computer, and from these values the optical aberrations are com- puted in the form of Zernike polynomials up to the 8th order. (J Refract Surg 2000;16:S570-S571)

Fast computation of Fresnel holograms employing difference

Abstract: For practical holographic video system, it is important to generated holographic fringe as fast as possible. We have proposed an approximation method that can calculate the Fresnel hologram fast. To compute the hologram, an object is assumed as a collection of self-illuminated points and the fringes from each object point are superposed. To determine the fringe, a distance between object point and sampling point on the hologram is used to obtain phase of the light. Since sampled hologram usually has small pixel intervals, the difference of the distance values between adjacent pixels is also small and its n-th order difference becomes a constant. Therefore, the distance value at certain pixel can be obtained from the neighbor pixel with simple additions. We have investigated approximation errors and computational speed of the method. The numerical results show that the proposed method is quite effective. The distance error can be reduced less that one wavelength with practical parameters and the computational speed becomes 16 times faster than conventional method. With the proposed method, a hologram, which has horizontal parallax only, 1.3 mega- pixels and 1,000 object points, can be calculated less than on second with a personal computer.

Infrared scanning interferometry apparatus and method

Abstract: The invention features an interferometry system for a measuring a surface profile or thickness of a measurement object In one aspect, the interferometry system includes: a broadband infrared source which during operation generates broadband infrared radiation including central wavelengths greater than about 1 micron; a scanning interferometer which during operation directs a first infrared wavefront along a reference path and a second infrared wavefront along a measurement path contacting the measurement object, and, after the second wavefront contacts the measurement object, combines the wavefronts to produce an optical interference pattern, the first and second infrared wavefronts being derived from the broadband infrared radiation; a detector producing data in response to the optical interference pattern; and a controller which during operation causes the scanning interferometer to vary the optical path difference between the reference and measurement paths over a range larger than the coherence length of the broadband source and analyzes the data as a function of the varying optical path difference to determine the surface profile

Closed-loop aberration correction by use of a modal Zernike wave-front sensor

Abstract: We describe the practical implementation of a closed-loop adaptive-optics system incorporating a novel modal wave-front sensor. The sensor consists of a static binary-phase computer-generated holographic element, which generates a pattern of spots in a detector plane. Intensity differences between symmetric pairs of these spots give a direct measure of the Zernike mode amplitudes that are present in the input wave front. We use a ferroelectric liquid-crystal spatial light modulator in conjunction with a 4–f system and a spatial filter as a wave-front correction element. We present results showing a rapid increase in Strehl ratio and focal spot quality as the system corrects for deliberately introduced aberrations.

Customized corneal profiling

Abstract: A customized corneal profile is provided by combining corneal topography data with captured wavefront aberration data to form a course of refractive treatment of the eye. In one embodiment, the captured wavefront data is employed within the area of a pupil, while the corneal topography data is employed in the area outside of the pupil. In other embodiments, the topography data is adjusted based on the wavefront data, a course of refractive treatment is simulated and displayed upon the topography data, and an initial evaluation of the suitability of a patient for treatment is performed based on the topography data.

A wavefront generator for complex pupil function synthesis and point spread function engineering

Abstract: We describe a simple method to produce an arbitrary complex optical field using a ferroelectric liquid crystal spatial light modulator. The system is configured so as to act as a pupil plane filter in a confocal microscope. The ability to tune the complex pupil function permits the system to be used both to modify the imaging performance by effectively engineering the point spread function as well as to remove optical aberrations present in the optical system.

Phase-diversity wave-front sensing with a distorted diffraction grating

Abstract: We describe a novel wave-front sensor comprising a distorted diffraction grating, simple optics, and a single camera. A noniterative phase-diversity algorithm is used for wave-front reconstruction. The sensor concept and practical implementation are described in detail, and performance is validated against different Zernike modes and a representative atmospheric phase map.

Positioning tolerances for phase plates compensating aberrations of the human eye

Abstract: The positioning tolerances for phase plates used to compensate human eye aberrations are analyzed. Lateral displacements, in-plane rotations, and axial translations are considered, describing analytic and numerical procedures to compute the maximum degree of compensation achievable in each case. The compensation loss is found to be dependent both on the kind and the amount of misalignment and on the particular composition of the aberration pattern of each subject in terms of Zernike polynomials. We applied these procedures to a set of human eye aberrations measured with the laser ray-tracing method. The general trend of results suggests that lateral positioning, followed by angular positioning, are the key factors affecting compensation performance in practical setups, whereas axial positioning has far less stringent requirements.

Diffractive Alvarez lens

Abstract: A diffractive Alvarez lens is demonstrated that consists of two separate phase plates, each having complementary 16-level surface-relief profiles that contain cubic phase delays. Translation of these two components in the plane of the phase plates is shown to produce a variable astigmatic focus. Both spherical and cylindrical phase profiles are demonstrated with good accuracy, and the discrete surface-relief features are shown to cause less than λ/10 wave-front aberration in the transmitted wave front over a 40 mm×80 mm region.