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At least 865 records · Page 48Linked to original sources

PubMed1990

Rotational characteristics and stability of soft toric lenses.

Toric soft contact lenses of a double slab-off design were fitted to 72 patients (128 days) with varying refractive requirements. Rotational characteristics and lens stability were studied as related to sphere power, cylinder power, and axis orientation. The patients were followed over a 1-month period. Although rotation of all lenses was skewed nasally, the total number of all lenses showing no rotation or temporal was somewhat greater than those with nasal rotation. In addition lenses of oblique axis displayed the least amount of rotational instability. Finally this study suggests some differences when comparing the rotational characteristics between with-the-rule and against-the-rule astigmatism correction, low vs. high myopia, and myopia vs. hyperopia.

Adult↗

PubMed2004

Intraocular lens power calculation after corneal refractive surgery.

PURPOSE OF REVIEW: Keratorefractive procedures designed to decrease refractive errors have gained enormous popularity among ophthalmologists and patients. As the post-refractive surgery patient population ages, visually significant cataracts will develop. With advances in techniques for cataract extraction and intraocular lens implantation, cataract surgery has evolved into a refractive surgical procedure as well as an operation to improve best corrected visual acuity. This raises expectations in terms of desired postoperative refractive status and uncorrected visual acuity. Although performing modern cataract surgery in post-refractive surgery eyes is technically no more complicated than operating on virgin eyes, the calculation of intraocular lens power for a desired refractive target can be challenging and complicated. This has become increasingly apparent as case reports of "refractive surprises" after cataract surgery appear in the literature more frequently. RECENT FINDINGS: This paper reviews the current clinical experience with intraocular lens power determination after cataract surgery in post-keratorefractive patients, provides an overview of possible sources of error in intraocular lens power calculation in these patients, and analyzes methods to minimize intraocular lens power errors. SUMMARY: The clinical and routine methods of intraocular lens power determination after keratorefractive surgery need to be modified to improve accuracy. Our knowledge of this subject is still evolving. Given the enormous impact of this problem on clinical practice, awareness of the shortcomings and suggested methods to improve accuracy can be valuable to clinicians.

Cataract↗

PubMed1984

Comparison of Dioptron Nova refractive data with conventional refractive data.

Refractive data obtained by means of the Dioptron Nova Diagnostic Eye Computer were compared to data obtained by conventional clinical refraction on a group of 236 clinic patients. Spherical equivalent power data were found to average -0.32 D more for Dioptron Nova data than for clinical data, whereas spherical power data averaged -0.25 D more for Dioptron Nova data than for clinical data, and cylinder power averaged -0.12 D more for Dioptron Nova data. Dioptron and clinical data were found to be within +/- 0.50 D of each other for 74% of eyes for spherical equivalent power, for 83% of eyes for spherical power, and for 91% of eyes for cylinder power. Cylinder axis data were found to be within +/- 10 degrees for the two methods for 78% of eyes.

Accommodation, Ocular↗

PubMed1997

Refractive outcome of radial keratotomy: does the result of the first eye predict outcome in the second eye?

PURPOSE: To present a method of quantifying variability in the outcome of a refractive surgical procedure using the SD of the difference between achieved and expected refractive changes. We used this method to determine whether the refractive outcome of radial keratotomy in a first eye is predictive of outcome in the second eye. METHODS: We retrospectively identified patients who underwent eight-incision radial keratotomy in the first eye from February 1993 through April 1994, with follow-up refraction 2.5 to 5 months postoperatively. This group consisted of 129 eyes of 81 patients. Thirty-nine patients had bilateral surgery with appropriate follow-up. Achieved refractive change was analyzed by multivariate linear and nonlinear regression to yield an expected refractive change for each eye based on patient age and optical zone size. RESULTS: Residuals, defined as the difference between the achieved and expected refractive change, were normally distributed. The SD of the residuals was 0.68 diopter and was independent of the expected correction. The prediction of second-eye refractive change was not significantly improved by incorporating the residual from the first eye into the regression prediction. CONCLUSIONS: The SD of the difference between the achieved and expected refractive change is an appropriate measure of the variability in refractive outcome following a refractive surgical procedure. Surgeons who perform bilateral simultaneous radial keratotomy do not sacrifice refractive accuracy in the second eye.

Adult↗

PubMed2002

Absolute refractive indices and thermal coefficients of CaF2, SrF2, BaF2, and LiF near 157 nm.

We present high-accuracy measurements for wavelengths near 157 nm of the absolute index of refraction, the index dispersion, and the temperature dependence of the index for the ultraviolet optical materials with cubic symmetry: CaF2, SrF2, BaF2, and LiF. Accurate values of these quantities for these materials are needed for designs of the lens systems for F2 excimer-laser-based exposure tools for 157-nm photolithography. These tools are expected to use CaF2 as the primary optical material and possibly one of the others to correct for chromatic aberrations. These optical properties were measured by the minimum deviation method. Absolute refractive indices were obtained with an absolute accuracy of 5 x 10(-6) to 6 x 10(-6).

Journal Article↗

PubMed2004

Performance evaluation of iterative tomography algorithms for incomplete projection data.

Projection data obtained through optical techniques for tomographic measurements, such as interferometry for refractive-index-based measurements, are often incomplete. This is due to limitations in the optical system, data storage, and alignment and vignette issues. Algebraic iterative reconstruction techniques are usually favored for such incomplete projections. A number of iterative algorithms, based on additive and multiplicative corrections, are used with a known simulated phantom and noise source to assess the reconstruction performance of incomplete data sets. In addition, we present reconstructions using experimental data obtained from a coherent gradient sensing interferometer for a steady temperature field in a fluid medium. We tested the algorithms using the simulated data set for incompleteness conditions similar to those found in the experimental data, and the best-performing algorithm is identified.

Journal Article↗

PubMed1996

Refractive surgery.

In recent years, there has been increasing interest in refractive surgery among patients and physicians. Refractive surgery lessens or eliminates the need for contact lenses or glasses. The goal of refractive surgery is to allow an individual more independence from optical devices. Technologic innovations have improved the predictability and safety of surgical correction of refractive errors and actually revolutionized treatment.

Humans↗

PubMed1989

Prevalence and variation of astigmatism in a military population.

Astigmatism is a common refractive anomaly. Observations on 1112 consecutive patients from a military optometric clinical population were taken to establish the prevalence of astigmatism and the incidence of its different variations. Astigmatism was found to exist in about 63% of the eyes. It was found that with-the-rule (WTR) and against-the-rule (ATR) astigmatism were the predominant types of astigmatism, and that approximately 70% of astigmatism found required 1.00D of correcting cylinder power or less. One objective of the study was to discover if a statistically significant difference existed between right and left eyes when comparing the mean correcting minus cylinder axes for WTR, ATR, and oblique types of astigmatism. This was shown to exist for WTR and ATR astigmatism. Further studies to determine the etiology of this difference are indicated.

Adult↗

PubMed2000

Two year results: sharp versus rounded optic edges on silicone lenses.

To evaluate the role of optic edge design of 2 silicone intraocular lenses (IOLs) in 2 year clinical results. Department of Ophthalmology, University of Vienna, Medical School, Allgemeines Krankenhaus, Austria. In this comparative clinical study, 50 eyes had phacoemulsification and implantation of a high-refractive 3-piece silicone IOL with sharp optic edges (CeeOn model 911F) (n = 25) or a 3-piece silicone lens with rounded optic edges (CeeOn model 920) (n = 25). Biomicroscopic findings, including those of specular microscopic examination of the anterior lens surface, were documented and the results analyzed. After 2 years, a significant between-group difference in posterior capsule opacification (PCO) but not in anterior capsule alterations was observed. Behind all CeeOn 911F sharp-edge IOLs, the capsule remained clear; in 2 of 23 capsules behind the CeeOn 920 rounded-edge, a neodymium: YAG laser capsulotomy had to be performed for dense central fibrotic PCO. Seven of 21 of the remaining eyes had first-degree central fibrotic PCO, 14 of 23 had peripheral mixed fibrotic and slender Elschnig pearl PCO, and 8 of 23 had second-degree peripheral PCO. Specular microscopic findings did not differ between the 2 groups. No severe IOL decentration occurred in any eye; 25% in the sharp-edge group and 40% in the rounded-edge group had minimal decentration. The silicone IOL with the sharp optic edge design was associated with significantly reduced PCO 2 years postoperatively.

Aged↗

PubMed2002

Optical measurement of ultrasonic Poynting and velocity vector fields.

This report describes a method for estimating several wide bandwidth ultrasonic field parameters from optical measurements of the local, acoustically induced, refractive index perturbation in water. These parameters include Poynting and particle velocity vector fields as well as pressure and density fields at any temporal delay under mild (forward-propagating) assumptions on the angular plane-wave spectrum of the ultrasound field. A sampling theorem is derived stating that two complete measurements of the three-dimensional pressure field separated in time by delta t allow release of the forward-propagating assumption for every acoustic wave number k satisfying k not = n pi/(c delta t), where c is the acoustic wave speed in the medium and n an integer greater than zero. The approach provides detailed measurements of very general ultrasound fields. Two optical measurement methods that acquire the Radon transform of the three-dimensional refractive index perturbation are briefly reviewed. It is shown that the Radon transform of the field itself satisfies a two-dimensional wave equation and may be propagated independently forward or backward in time under a source-free model. Conversely, the Radon transform of the ultrasound field measurement at several known time delays provides a means of applying a filter to the data based on known ultrasound propagation models. Each two-dimensional distribution may be propagated to a common time point and the ensemble averaged, thus incorporating the propagation model into the measurement. We support the presented theory with several experiments.

Journal Article↗

PubMed2003

Refractive errors, intraocular pressure, and glaucoma in a white population.

OBJECTIVE: To examine the relation of refractive errors to glaucoma and intraocular pressure (IOP) in a defined white population. DESIGN: Population-based cross-sectional and follow-up study. PARTICIPANTS: Persons aged 43 to 86 years living in Beaver Dam, Wisconsin (n = 4926). METHODS: All participants received a standardized assessment of refraction, IOP, and glaucoma at baseline (1988-1990), with IOP remeasured 5 years later (1993-1995). Refraction was defined at baseline as follows: myopia as spherical equivalent of -1.00 diopters (D) or less, emmetropia as -0.75 to +0.75 D, and hyperopia as +1.00 D or more. MAIN OUTCOME MEASURES: Relation of baseline refraction to prevalent glaucoma (defined from IOP, optic disc, and visual field criteria) and incident ocular hypertension (defined as IOP more than 21 mmHg at the 5-year examination in eyes with IOP of 21 mmHg or less at baseline). RESULTS: A myopic refraction was correlated with increasing IOP at baseline (P < 0.001). After controlling for age and gender, persons with myopia were 60% more likely to have prevalent glaucoma than those with emmetropia (odds ratio [OR], 1.6; 95% confidence interval [CI], 1.1, 2.3). In contrast, controlling for age, gender, and baseline IOP, persons with hyperopia were 40% more likely to have incident ocular hypertension than those who were emmetropic at baseline (OR, 1.4; 95% CI, 1.0, 2.0). Myopia was not related to incident ocular hypertension. CONCLUSIONS: In these population-based data, there was a cross-sectional association of myopia with higher IOP and prevalent glaucoma. Similar associations have been found in previous studies. Hyperopia may be associated with 5-year risk of ocular hypertension, a finding that needs further investigation.

Adult↗

PubMed2001

Statistical analysis when dealing with astigmatism: assessment of different spherocylindrical notations.

Ophthalmic epidemiological studies frequently deal with ocular refractive errors, which are commonly expressed in the form sphere/cylinder x axis. However, this representation has been shown not to be the most suitable one for performing statistical analysis. Although alternative analytical and graphic methods to represent this kind of data have been developed, these formalisms have often gone unnoticed by researchers, despite their usefulness and versatility. Besides, there has been no discussion of how each of them fits in with a particular type of study. In this paper, several mathematical representations of dioptric power are revisited in a comprehensive way. The aim is to encourage researchers in ophthalmology and optometry to use these formalisms in their epidemiological studies, thus profiting from their exactitude and simplicity. Consequently, the emphasis is not on complicated mathematical derivations but on how to use these representations. Their potential and suitability in different applications is analyzed in detail. In addition, some examples are presented to illustrate the mathematical methods considered.

Astigmatism↗

PubMed2005

Negative refraction in ferromagnet-superconductor superlattices.

Negative refraction, which reverses many fundamental aspects of classical optics, can be obtained in systems with negative magnetic permeability and negative dielectric permittivity. This Letter documents an experimental realization of negative refraction at millimeter waves, finite magnetic fields, and cryogenic temperatures utilizing a multilayer stack of ferromagnetic and superconducting thin films. In the present case the superconducting YBa2Cu3O7 layers provide negative permittivity while negative permeability is achieved via ferromagnetic (La:Sr)MnO3 layers for frequencies and magnetic fields close to the ferromagnetic resonance. In these superlattices the refractive index can be switched between positive and negative regions using external magnetic field as tuning parameter.

Journal Article↗

PubMed1969

Change in the absorbancy of bacterial suspensions before initiation of growth.

The apparent absorbancy of suspensions of stationary-phase cells of Streptococcus lactis strain 354/07 decreased immediately after being placed in fresh media. This optical effect also occurred in defined mixtures of buffer glucose and KCl. CaCl(2) caused the absorbancy to increase. CaCl(2) and KCl together had about the same effect as KCl alone. SrCl(2) could replace CaCl(2), but it was less effective by a factor of 10(2). MnCl(2), MgCl(2), and NaCl were without effect. The absorbancy did not change when cells were first killed by p-chloromercuribenzoate or when the reaction was carried out at 0 C. The rate of the reaction was dependent on temperature and concentration of glucose and salts. Gradient centrifugation suggests that this optical effect was caused by change in the refractive index of the test organism rather than by change in volume. Nine other organisms representing four additional genera gave the same optical effect as S. lactis 354/07. Two other organisms reacted feebly whereas another strain of S. lactis reacted in the opposite way, the absorbancy of the suspension increasing instead of decreasing. Spores of Bacillus cereus did not respond.

Anti-Bacterial Agents↗

PubMedDate not supplied

[Skiascopy. Some reflexions on the prescription of optical correction].

The skiascopy allows determination of the objective eye refraction, but this exam should not be interpreted mechanically. The eye is a living organ and several subjective factors should be considered, mainly the accommodation and oculomotor equilibrium. Based on her own experience, the author discusses the difficulties she had in prescribing optical corrections for 7 patients, after she had done the skiascopy. Transcription of data objectively acquired using skiascopy must consider also the psycho-physiological state of the subjective patient.

Adolescent↗

PubMed1999

Intraocular lens power prediction in a vietnamese population. A study to establish a limited set of intraocular lens powers that could be used in a vietnamese population.

AIM: To determine a range of dioptric powers for Intraocular Lenses (IOLs) that would cover at least 95% of the Vietnamese population attending hospital for cataract removal and IOL implantation. OBJECTIVE: To determine the ocular axial lengths and keratometric powers of people attending for cataract surgery at a hospital in Vietnam and hence predict the intraocular lens powers required for emmetropia for this population. HYPOTHESIS: 95% of the population attending a Vietnamese hospital for cataract and intraocular lens surgery would be within +/- 4.5 dioptres of the mean predicted emmetropic intraocular lens power. STUDY DESIGN: A case series. STUDY FACTORS: Axial length, keratometric power, age, sex, preoperative refractive status, preoperative vision, ethnicity. OUTCOME: Intraocular lens power for emmetropia. RESULTS: 346 adults attending for cataract surgery at the Hue Central Hospital, Vietnam were examined. The mean axial length was found to be 22.96 mm +/- 0. 99 (range 20.37-27.25), median 23.01 mm. The mean corneal power was 44.24 dioptres +/- 1.53 (range 39.50-49.00), median 44.5 dioptres. Using the SRK/T formula, with an "A" constant of 118.3, the mean predicted intraocular lens power required for emmetropia was 21.44 dioptres +/- 2.77 (range 10.87-27.25), median 21.56 dioptres. The mean predicted IOL power for females was 21.94 dioptres +/- 2.60 (range 13.84-29.68), median 22.13 dioptres. The mean predicted IOL power for males was 20.72 dioptres +/- 2.84 (range 10.87-28.95), median 20.66 dioptres. conclusion 95% of the population attending for cataract surgery were within 5.5 dioptres of the mean predicted emmetropic IOL power.

Adolescent↗

PubMedDate not supplied

Refractive astigmatism acts predominantly as a source of high spatial frequency image distortion: the associated lineal distortions can be overcome by using a low pass spatial filter!

PURPOSE: Surprisingly, an important characteristic of astigmatism has been overlooked by ophthalmic and clinical scientists. Apparently, refractive astigmatism is due largely to a form of high spatial frequency image distortion. METHODS: Characteristic astigmatic image distortion can be minimized or eliminated by using a low-pass spatial filter (here, a ground glass plate was employed for this purpose). The ground glass is placed a short distance in front of a visual acuity chart, or it may be used with other tests, such as vernier acuity. RESULTS: This test has been performed by us on large numbers of patients and test subjects. A clinician can try this test for himself/herself. Place a +2.00 D.C. (any axis) lens in front of the eye; the usual distortions will be observed. Locate the ground glass plate as described. The usual distortions associated with the conoid of Stürm will not be visible or will be virtually eliminated, although some image blur will remain. CONCLUSIONS: This technique has significance, e.g., in visual screening programs in developing nations, or in assessing patients with media disorders prior to ophthalmic surgery.

Astigmatism↗

PubMed1979

Speckle movement in laser refraction. I. Theory.

Slowly rotating, cylindrical, diffusing drums have been widely used in laser refraction. A simple geometrical evaluation of the optical path lengths involved is used to derive expressions for the position of the plane of stationarity and the apparent speed of speckle motion as a function of the characteristics of the illuminating wavefront, the drum, the refraction of the observer, and other system parameters. With apparatus of known characteristics, determination of the apparent speckle velocity allows calculation of the ametropia of the observer.

Humans↗