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At least 1,225 records · Page 68Linked to original sources
PubMed2006

Optical tomography of transparent objects with phase-shifting interferometry and stepwise-shifted Ronchi ruling.

An experimental setup for tomographic inspection of phase objects is presented. The system uses a common-path interferometer consisting of two windows in the input plane and a translating grating as its pupil. In the output, interference of the fields associated with replicated windows is achieved by a proper choice of the windows' spacing with respect to the grating period. With a rotating object in one window and a plane wave in the second one, the phase distribution of each projection is encoded as a corresponding digital image row, which, in turn, constructs a composite interferogram over the plane of a traditional sinogram. Phase stepping of composite interferograms can be achieved by a proper translation of the grating in order to obtain the unwrapped phase distribution as the corresponding sinogram. This sinogram allows tomographic reconstruction of phase slices by standard procedures. Composite interferograms and reconstructions for some transparent samples are shown.

Equipment Design↗
PubMed2006

Phakometry and lens tilt and decentration using a custom-developed Purkinje imaging apparatus: validation and measurements.

We present a Purkinje imaging system for phakometry and measurement of tilt and decentration of crystalline and intraocular lenses (IOLs). Crystalline lens radii of curvature were estimated by using both a merit function and the equivalent mirror approaches. Tilts and decentrations were estimated by using Phillips's linear analysis. We present a complete validation of the technique through exhaustive computer simulations and control experiments, and measurements in 17 normal eyes (mean age 26.67 +/- 2.31) and nine postcataract surgery eyes (mean age 74 +/- 2.3). Crystalline lens radii ranged from 12.7 to 8.81 mm and from -5.64 to -7.09 mm for anterior and posterior surfaces, respectively. Crystalline lens tilt ranged from 2.8 to -2.87 deg horizontally and from 2.58 to -1 deg vertically. Crystalline lens decentration ranged from 0.09 to 0.45 mm horizontally and from 0.09 to -0.22 mm vertically. IOL tilt ranged from 3.6 to -1.51 deg horizontally and from 5.97 to -1.85 deg vertically. IOL decentration ranged from 0.53 to -0.31 mm horizontally and from 0.13 to -0.96 mm vertically.

Corneal Topography↗
PubMed2006

Scaling Zernike expansion coefficients to smaller pupil sizes: a simpler formula.

In a recent paper [J. Opt. Soc. Am. A 19, 1937 (2002)] a recursive analytical formula was derived to calculate a set of new Zernike polynomial expansion coefficients from an original set when the size of the aperture is reduced. In the current paper I describe a more intuitive derivation of a simpler, nonrecursive formula, which is used to calculate the instantaneous refractive power.

Algorithms↗
PubMed2006

Spherical aberration of the anterior and posterior surfaces of the human cornea.

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.

Adolescent↗
PubMed2006

Differential geometry of normalized Stokes vector trajectories in anisotropic media.

Trajectory of the normalized Stokes vector on the Poincaré sphere corresponding to light propagation in anisotropic tissues with birefringence and biattenuance is derived. Analytic expressions are determined from the Serret-Frenet formulas and derivatives of arc length for five quantities including the tangent, normal, and binormal vectors with curvature and torsion. Depth variation of curvature and torsion of normalized Stokes vector trajectories corresponding to light propagating in rodent tail tendon are given. Use of analytic expressions for depth variation of curvature and torsion of the normalized Stokes vector trajectories on the Poincaré sphere is discussed for analysis of polarization-sensitive optical coherence tomography data recorded from anisotropic biological tissues with birefringence and biattenuance.

Algorithms↗
PubMed2006

Impact of Zernike-fit error on simulated high- and low-contrast acuity in keratoconus: implications for using Zernike-based corrections.

This study examines the extent to which a Zernike-based optical correction can restore acuity in keratoconus as a function of disease severity and contrast level. Increasingly complete Zernike corrections in the presence of Zernike-fit error were simulated. Acuity for keratoconic eyes with <60 D maximum corneal power reached 20/13 with a fifth-order Zernike correction under high-contrast conditions and exhibited a loss of 0.1 logMAR (minimum angle of resolution) (from 20/32 to 20/40) for low-contrast conditions. Acuity for keratoconic eyes with >60 D maximum corneal power approached 20/13 with sixth-tenth-order corrections under high-contrast conditions but did not return to similar levels with a tenth-order correction for low-contrast conditions. The results suggest that fit error affects visual performance for more difficult tasks and that restoring high-contrast acuity (20/16 or better) using a fifth-order Zernike correction is not limited by Zernike=fit error for over 88% of keratoconus cases.

Computer Simulation↗
PubMed2006

Interocular high-order corneal wavefront aberration symmetry.

The interocular symmetry of the high-order corneal wavefront aberration (WA) in a population of myopic eyes was analyzed before and after photorefractive keratectomy (PRK). The preoperative and one-year postoperative corneal aberration data (from third to seventh Zernike orders) for 4- and 7-mm pupils from right and left eyes were averaged after correcting for the effects of enantiomorphism to test for mirror symmetry. Also, the mean corneal point-spread function (PSF) for right and left eyes was calculated. Preoperatively, a moderate and high degree of correlation in the high-order corneal WA between eyes was found for 4- and 7-mm pupils, respectively. Myopic PRK did not significantly change the interocular symmetry of corneal high-order aberrations. No discernible differences in the orientation PSF between eyes were observed one year after surgery in comparison with the preoperative state over the two analyzed pupils.

Adult↗
PubMed2006

Analysis of wavefront-guided corrections to see if they fully correct ocular aberrations.

In a recent paper [J. Opt. Soc. Am. A22, 1473 (2005)], the authors concluded, as a result of analysis using an optical model of the eye, that wavefront-guided ablative correction of the refractive errors of the eye does not efficiently remove those errors. An analytical method is given that analyzes the effect of a wavefront-guided ablative treatment of the aberrations of the eye using only the measured wavefront error of the eye and a measured or assumed anterior corneal surface shape and shows that in fact a wavefront-guided ablative treatment does remove all clinically significant error of the eye if only the anterior surface of the cornea is physically altered as planned as a result of treatment. The use of a variation of the method to analyze the effect of treatment when other changes occur as a result of treatment is also given.

Computer Simulation↗
PubMed2006

Refractive power of a multilayer rotationally symmetric model of the human cornea and tear film.

Optical models of the human cornea and tear film typically employ a single homogeneous cornea with an average refractive index. I propose to use a more realistic multilayer model based on morphological data from the literature. The mathematical methodology to derive the refractive power equation of this model is presented. Special attention is given to the axial gradient index of the refraction structure of the stroma layer because of its optical implications. The importance of considering this multilayer model is quantified in a specific example (orthokeratology) with the help of the derived power equation.

Computer Simulation↗
PubMed2006

Selective observation of starch in a water plant using optical sum-frequency microscopy.

The photosynthesis, transfer, and storage of starch are the most important biogenic processes occurring in plants. In order to observe the colorless and transparent starch granules in a plant, a chemical pretreatment such as staining of the starch is currently required, which seriously damages the tissue cells in the plant. We demonstrate that nondestructive chemical analysis of starch granules in a plant can be performed by using optical second-harmonic and sum-frequency microscopy. These techniques for in vivo analysis will provide extremely useful information about saccharides in a plant and can be extended to the analysis of many other materials, from living tissue to semiconductors.

Chara↗
PubMed2006

Matrix approach to quantitative refractive index analysis by Fourier domain optical coherence tomography.

The rapid development in the field of optical coherence tomography has demanded increasingly sophisticated numerical models to enable the interpretation of image data and extract quantitative results. We use a matrix formulation of Fresnel's equations for multilayered media to extract layer-dependent thickness and refractive index directly from Fourier domain optical coherence tomography spectrograms. An eigenanalysis spectral decomposition approach is used to constrain the least squares fitting algorithm, avoiding the need for initial estimates of the parameter values. We demonstrate this novel quantitative analysis approach by using a multilayered phantom and show good agreement with the known layer parameter values. This approach introduces a powerful tool for the analysis of layer-dependent optical properties that could have an important role in the differentiation of healthy and diseased tissue.

Algorithms↗
PubMed2006

Perturbation approach to the time-resolved transmittance for a spatially varying scattering inclusion in a diffusive slab.

In the framework of the perturbation approach to the diffusion equation, an analytical expression is derived to describe the effects on the time-resolved transmittance due to the presence of a spatially varying scattering inclusion hidden inside a diffusive slab. This formula assumes that the reduced scattering coefficient of the inclusion is spatially Gaussian distributed and complements that obtained for the absorptive case. The accuracy and the application range of the perturbed transmittance are investigated through comparisons with the numerical solutions of the time-dependent diffusion equation given by using the finite-element method. The proposed perturbation model is validated through a fitting procedure that determines the relative error in retrieving the scattering perturbation parameter of the inclusion located at the midplane of the slab.

Algorithms↗
PubMed2006

Experimental and theoretical study of skylight polarization transmitted through Snell's window of a flat water surface.

The celestial polarization pattern may be scrambled by refraction at the air-water interface. This polarization pattern was examined in shallow waters with a submersible polarimeter, and it was calculated by using land measurements ('semiempirical predictions') and models of the skylight polarization. Semiempirically predicted and measured e-vector orientations were significantly similar. Conversely, predicted percent polarization was correlated but lower than measurements. Percent polarization depended on wavelength, where at high sun altitudes maximal percent polarization generally appeared in the UV and red spectral regions. The wavelength dependency of polarization may lead to differential spectral sensitivity in polarization-sensitive animals according to time and type of activity.

Computer Simulation↗
PubMed2006

Direct transformation of Zernike eye aberration coefficients between scaled, rotated, and/or displaced pupils.

In eye aberrometry it is often necessary to transform the aberration coefficients in order to express them in a scaled, rotated, and/or displaced pupil. This is usually done by applying to the original coefficients vector a set of matrices accounting for each elementary transformation. We describe an equivalent algebraic approach that allows us to perform this conversion in a single step and in a straightforward way. This approach can be applied to any particular definition, normalization, and ordering of the Zernike polynomials, and can handle a wide range of pupil transformations, including, but not restricted to, anisotropic scalings. It may also be used to transform the aberration coefficients between different polynomial basis sets.

Computer Simulation↗
PubMed2006

Polarization-based and specular-reflection-based noncontact latent fingerprint imaging and lifting.

In forensic science the finger marks left unintentionally by people at a crime scene are referred to as latent fingerprints. Most existing techniques to detect and lift latent fingerprints require application of a certain material directly onto the exhibit. The chemical and physical processing applied to the fingerprint potentially degrades or prevents further forensic testing on the same evidence sample. Many existing methods also have deleterious side effects. We introduce a method to detect and extract latent fingerprint images without applying any powder or chemicals on the object. Our method is based on the optical phenomena of polarization and specular reflection together with the physiology of fingerprint formation. The recovered image quality is comparable to existing methods. In some cases, such as the sticky side of tape, our method shows unique advantages.

Biometry↗
PubMed2006

Tomographic method for measurement of the gradient refractive index of the crystalline lens. II. The rotationally symmetrical lens.

In the first part of this paper we presented a tomographic method to reconstruct the refractive index profile of spherically symmetrical lenses. Here we perform the generalization to lenses that are rotationally symmetrical around the optical axis, as is the ideal human lens. Analysis of the accuracy and versatility of this method is carried out by performing numerical simulations in which different magnitudes of experimental errors and two extreme case scenarios for the likely shape of the refractive index distribution of the human lens are considered. Finally, experimental results for a porcine lens are shown. Conceptually simple and computationally swift, this method could prove to be a valuable tool for the accurate retrieval of the gradient index of a broad spectrum of rotationally symmetrical crystalline lenses.

Animals↗
PubMed2007

Phase retrieval from noisy data based on minimization of penalized I-divergence.

We study noise artifacts in phase retrieval based on minimization of an information-theoretic discrepancy measure called Csiszár's I-divergence. We specifically focus on adding Poisson noise to either the autocorrelation of the true image (as in astronomical imaging through turbulence) or the squared Fourier magnitudes of the true image (as in x-ray crystallography). Noise effects are quantified via various error metrics as signal-to-noise ratios vary. We propose penalized minimum I-divergence methods to suppress the observed noise artifacts. To avoid computational difficulties arising from the introduction of a penalty, we adapt Green's one-step-late approach for use in our minimum I-divergence framework.

Algorithms↗
PubMed2007

Polarization of 'water-skies' above arctic open waters: how polynyas in the ice-cover can be visually detected from a distance.

The foggy sky above a white ice-cover and a dark water surface (permanent polynya or temporary lead) is white and dark gray, phenomena called the 'ice-sky' and the 'water-sky,' respectively. Captains of icebreaker ships used to search for not-directly-visible open waters remotely on the basis of the water sky. Animals depending on open waters in the Arctic region may also detect not-directly-visible waters from a distance by means of the water sky. Since the polarization of ice-skies and water-skies has not, to our knowledge, been studied before, we measured the polarization patterns of water-skies above polynyas in the arctic ice-cover during the Beringia 2005 Swedish polar research expedition to the North Pole region. We show that there are statistically significant differences in the angle of polarization between the water-sky and the ice-sky. This polarization phenomenon could help biological and man-made sensors to detect open waters not directly visible from a distance. However, the threshold of polarization-based detection would be rather low, because the degree of linear polarization of light radiated by water-skies and ice-skies is not higher than 10%.

Algorithms↗