Search results for "Power vector"
showing 5 items of 5 documents
Intrastromal corneal ring segment implantation in paracentral keratoconus with coincident topographic and coma axis.
2012
To evaluate the efficacy, predictability, and safety of implantation of Ferrara-type intrastromal corneal ring segments (ICRS) for refractive correction of paracentral keratoconus.Fernández-Vega Ophthalmological Institute, Oviedo, Spain.Cohort study.Snellen uncorrected (UDVA) and corrected (CDVA) distance visual acuities and residual refractive errors were recorded before and 6 months after ICRS implantation for keratoconus. The tunnels for ICRS implantation were created with a femtosecond laser. The power vector and the Alpins method were used to analyze postoperative refractive outcomes.The study evaluated 56 eyes of 49 patients. The mean UDVA was 0.17 ± 0.14 (SD) preoperatively and 0.41 …
Visual acuity with computer simulated and lens-induced astigmatism
2014
[EN] The relationship between spherical and astigmatic refractive errors and their associated visual acuity is investigated in this work by means of two different approaches. In the first one, different refractive errors were induced in normal subjects by trial lenses. In the second one, defocused images were simulated numerically by the optical transfer function of a model eye and then judged by the same subjects. The amount of defocus (measured in terms of the modulus of the dioptric power vector) necessary to reduce the visual acuity to 0.1 logMAR and to 0.4 logMAR was computed with each method and then compared. We found that the visual system is clearly more tolerant to lens-induced de…
Digitālās refrakcijas pielietojums labākā koriģētā redzes asuma novērtēšanai
2021
Darbs ir uzrakstīts angļu valodā uz 36 lappusēm, satur 20 attēlus, 4 tabulas un 35 literatūras avotus. Darba mērķis ir pārbaudīt redzes asumu ar trim refrakcijas metodēm: autorefraktometriju, subjektīvo refraktometriju un digitālo refraktometriju ar ierīci ESSILOR Vision R-800. Tika salīdzināta refrakcija 15 subjektiem vecumā no 12 līdz 40 gadiem. Refrakcijas dati tika pārveidoti optiskā stipruma vektora formā. Rezultāti: Statistiski nozīmīgas atšķirības tika konstatētas redzes asumam, bet ne optiskā stipruma vektoriem, kas bija ļoti līdzīgi viens otram. Secinājums: digitālā refrakcija ar precizitāti 0,01 D pie nepārtrauktas optiskā stipruma maiņas parādīja labāku redzes asumu pat pie zema …
Femtosecond laser–assisted intrastromal corneal ring segment implantation for high astigmatism correction after penetrating keratoplasty
2013
PURPOSE: To assess the visual and refractive outcomes of femtosecond laser-assisted implanta- tion of Ferrara-type intrastromal corneal ring segments (ICRS) in post-penetrating keratoplasty (PKP) eyes. SETTING: Instituto OftalmolFernOviedo, Spain. DESIGN: Cohort study. METHODS: Patients with previous PKP had ICRS implantation after femtosecond laser tunnel creation. The uncorrected (UDVA) and corrected (CDVA) distance visual acuities and residual refractive errors were recorded before and 6 months after ICRS implantation. The power vector method was used to analyze refractive errors preoperatively and postoperatively. RESULTS: This study enrolled 32 eyes of 30 patients. The mean UDVA (Snell…
Step-along power vector method for astigmatic wavefront propagation
2013
Purpose To propose both a new algebraic solution and a graphical monitoring method for astigmatic wavefront propagation in the framework provided by power vectors. Methods The generalised propagation equation describing the propagation of astigmatic wavefronts from one plane to another is adapted to the power vectors formalism using a novel algorithm based on a step-along method. The step-along procedure is directly applied to the tuple of power vectors [M, J0, J45] representing an arbitrary astigmatic wavefront and it permits the calculation of the tuple of power vectors [M′, J′0, J′45] after a given propagation distance. This is achieved mathematically first by temporarily rotating the as…