Search results for "Refraction"
showing 10 items of 200 documents
The effect of spherical aberration on visual performance and refractive state for stimuli and tasks typical of night viewing
2018
Purpose: The aim of this work was to examine the impact of Seidel spherical aberration (SA) on optimum refractive state for detecting and discriminating small bright lights on a dark background. Methods: An adaptive-optics system was used to correct ocular aberrations of cyclopleged eyes and then systematically introduce five levels of Seidel SA for a 7-mm diameter pupil: 0, ± 0.18, and ± 0.36 diopters (D) mm−2. For each level of SA, subjects were required to detect one or resolve two points of light (0.54 arc min diameter) on a dark background. Refractive error was measured by adjusting stimulus vergence to minimize detection and resolution thresholds. Two other novel focusing tasks for si…
Visual Acuity in Simple Myopic Astigmatism: Influence of Cylinder Axis
2006
Purpose. The relationship between astigmatic refractive errors and their associated visual acuity has been studied in recent years in the context of refractive power space. The influence of the axis of astigmatism remains a matter of controversy. Our goal in this study is to provide additional experimental evidence to clarify this subject. The influence of the simulated axis orientation was compared with other factors that affect visual acuity such as the particular design of the test and the differences between eyes. Methods. Simple myopic astigmatism from 0 to 3.00 D, in steps of 0.25 D, and with five different axes between 0° and 90°, were simulated on four healthy eyes of young observer…
Avaliação da resposta de acomodação após tarefas visuais próximas usando diferentes dispositivos eletrônicos portáteis
2015
ABSTRACT Purpose: To assess the accommodation response after short reading periods using a tablet and a smartphone as well as determine potential differences in the accommodation response at various stimulus vergences using a Hartmann- Shack aberrometer. Methods: Eighteen healthy subjects with astigmatism of less than 1 D, corrected visual acuity of 20/20 or better, and normal findings in an ophthalmic examination were enrolled. Accommodation responses were obtained under three different conditions: accommodation system of the eye relaxed and visually stressed with a tablet and an smartphone for 10 min, at a distance of 0.25 m from the subject's eyes. Three measurements of accommodation res…
Femtosecond laser versus mechanical microkeratome laser in situ keratomileusis for myopia: Metaanalysis of randomized controlled trials.
2011
Purpose To examine differences in efficacy, accuracy, safety, and changes in aberrations between femtosecond and mechanical microkeratome laser in situ keratomileusis (LASIK) for myopia. Setting Department of Ophthalmology, Shanghai First People's Hospital Affiliated to Shanghai Jiaotong University, Shanghai, China. Design Evidence-based manuscript. Methods Data sources, including PubMed, Medline, EMBASE, and Cochrane Controlled Trials Register, were searched to identify potentially relevant prospective randomized controlled trials. Primary outcome measures were efficacy (uncorrected distance visual acuity ≥20/20), accuracy (±0.50 diopter mean spherical equivalent), and safety (loss of ≥2 l…
Effects of LASIK on Corneal Endothelium Using the 15-kHz IntraLase Femtosecond Laser
2011
PURPOSE: To assess the effects of LASIK with the 15-kHz IntraLase femtosecond laser on corneal endothelium. METHODS: In a prospective, single-center clinical trial, 138 patients (mean age: 32.0±7.1 years [range: 21 to 42 years]) underwent femtosecond LASIK for the correction of myopia −0.75 to −9.00 diopters (D) and cylinder up to 3.25 D. Patients were divided into two groups: contact lens group (n=76) and non-contact lens group (n=62). Pre- and 12-month postoperative specular microscopy of the central corneal endothelium was performed. The integrity of the central endothelium was assessed in terms of endothelial cell density and percentage of hexagonality. RESULTS: In the contact lens gro…
Collagen copolymer posterior chamber phakic intraocular lens for hyperopia correction: three-year follow-up.
2013
Purpose To assess the efficacy, predictability, safety, and stability of the Visian V3 Implantable Collamer Lens posterior chamber phakic intraocular lens (PC pIOL) for moderate to high hyperopia. Setting Private clinic, Oviedo, Spain. Design Prospective nonrandomized clinical study. Methods The uncorrected (UDVA) and corrected (CDVA) distance visual acuities, manifest refraction, endothelial cell density, intraocular pressure (IOP), central vault, and adverse events were evaluated for 3 years postoperatively. Results The study comprised 73 eyes of 44 patients. The mean spherical equivalent decreased from +4.84 diopters (D) ± 1.6 (SD) (range +1.00 to +7.75 D) preoperatively to −0.13 ± 0.39 …
Behaviour of the phakic iris-claw intraocular lens (Artisan /Verisyse) during accommodation: an optical coherence biometry study.
2007
Purpose To evaluate variations in the position of the phakic iris claw lens (Artisan®, Verisyse®) and the natural crystalline lens during the accommodation process. Methods We measured changes in position of the iris claw lens and the crystalline lens during the accommodation using optical coherence biometry (AC Master®/Carl Zeiss Meditec, Germany) in 17 patients (28 eyes) with a phakic iris claw lens implanted for high myopia and/or myopic astigmatism. Accommodative effort was obtained using an internal optical target within the measuring device. Results There was a forward shift of the phakic iris claw lens with a mean of 70 μm (8–178 μm) of the optical path length (OPL). At the same time…
Chromatic and achromatic visual fields in relation to choroidal thickness in patients with high myopia: A pilot study
2018
Purpose. - To analyse the relationship between the choroidal thickness and the visual perception of patients with high myopia but without retinal damage. Methods. - All patients underwent ophthalmic evaluation including a slit lamp examination and dilated ophthalmoscopy, subjective refraction, best corrected visual acuity, axial length, optical coherence tomography, contrast sensitivity function and sensitivity of the visual pathways. Results. - We included eleven eyes of subjects with high myopia. There are statistical correlations between choroidal thickness and almost all the contrast sensitivity values. The sensitivity of magnocellular and koniocellular pathways is the most affected, an…
Joint interpretation of electrical and seismic data aimed at modelling the foundation soils of the Maredolce monumental complex in Palermo (Italy)
2020
The monumental complex of Maredolce (Palermo) includes an Arab-Norman castle that stands on the banks of an artificial lake carved out of the calcarenitic rock that rests on impermeable clays. The lake was originally filled with water, thanks to the channelling of a spring. Over time, the lowering of the water table and the high hydraulic permeability of the calcarenites have caused the lake to dry up. A project to renovate the monumental complex has recently been launched. It also provides the study of a possible restoration of the original conditions of the lake. To this end, a multidisciplinary geophysical study was carried out to investigate the geotechnical conditions of permeability o…
New Objective Refraction Metric Based on Sphere Fitting to the Wavefront
2017
Purpose. To develop an objective refraction formula based on the ocular wavefront error (WFE) expressed in terms of Zernike coefficients and pupil radius, which would be an accurate predictor of subjective spherical equivalent (SE) for different pupil sizes.Methods. A sphere is fitted to the ocular wavefront at the center and at a variable distance,t. The optimal fitting distance,topt, is obtained empirically from a dataset of 308 eyes as a function of objective refraction pupil radius,r0, and used to define the formula of a new wavefront refraction metric (MTR). The metric is tested in another, independent dataset of 200 eyes.Results. For pupil radiir0≤2 mm, the new metric predicts the equ…