PubMed Health⌕ Search

SEARCH · PubMed Health

Results for “Optics and Refraction”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 649 records · Page 36Linked to original sources
PubMed1997

Blood flow velocity measurements in the optic nerve of glaucomatous patients by elaboration of a constant to evaluate the evolutive risk.

METHODS: We performed Echo color Doppler examinations on 100 glaucomatous patients (197 eyes). Ophthalmic artery and central retinal artery were particularly considered. The study included eyes in every level and type of glaucoma, and in various types of treatment (topical therapy, Argon laser trabeculoplasty, Argon/YAG/surgical iridectomy, trabeculectomy). The parameters we considered were: ocular tension, anterior chamber angle, optic disc excavation, visual field, refraction and vascular risk factors. We measured the flow values of the ophthalmic artery and of the central retinal artery, i.e. maximal systolic velocity, diastolic velocity and vascular resistance index. RESULTS: We observed that if we divide the value of ocular tension by the systolic velocity and multiply the result by the resistance index, we obtain a number which may be considered as a prognostic factor of evolutive risk. We compared our cases with a population free from glaucomatous and cardiovascular problems.

Adult↗
PubMed2005

Silica nanorod-array films with very low refractive indices.

The refractive-index contrast is an important figure of merit for dielectric multilayer structures, optical resonators, and photonic crystals. This represents a strong driving force for novel materials that have refractive indices lower than those of conventional optically transparent materials. Silica nanorod-array dielectric films with unprecedented low refractive indices of 1.08 are demonstrated and shown to have viable optical properties including enhanced reflectivity of a single-pair distributed Bragg reflector.

Birefringence↗
PubMed2002

Screening for amblyopia and strabismus with the Lang II stereo card.

PURPOSE: To evaluate the effectiveness of the Lang II stereo card as a screening test for amblyopia and/or strabismus. METHODS: A total of 1046 children aged 12-13 years were examined in a field study in the Göteborg area, Sweden. In addition to the Lang II stereo card, the examination included visual acuity, cover testing, cycloplegic refraction, and inspection of the optical media and posterior pole. RESULTS: If every incorrect subject response was considered a reason for referral, the Lang II test would have correctly identified 82% (23 subjects) of the 28 children with manifest strabismus and 38% (11 subjects) of the 29 children with amblyopia. The test failed to refer 45% (21 subjects). Of all subjects referred, 44 (63%) were found to be ophthalmologically normal. CONCLUSIONS: The Lang II stereo card is neither a reliable nor an efficient method of screening for amblyopia and/or strabismus.

Adolescent↗
PubMed2001

Development of the refractive state in eyes of ostrich chicks (Struthio camelus).

OBJECTIVE: To follow the development of the refractive error in the eyes of ostrich chicks from age 0 to day 37 after hatching. ANIMALS: 35 ostrich chicks. PROCEDURES: Spot retinoscopy was conducted to assess refractive error in ostrich chicks. Seventy eyes of 35 ostrich chicks were examined. Of these, 18 chicks were followed over time. At least 4 serial measurements (at 2- to 7- day intervals) were conducted in each of these chicks from day 1 to 37 after hatching. Seventeen additional chicks were examined on days 0, 3, 12, and 19 after hatching. RESULTS: Ostrich chicks were myopic at hatching, with a mean +/- SD refractive error of -4.47 +/- 0.15 diopters (D). The refractive error rapidly decreased during the first week of life, and by day 7 after hatching, chicks were slightly hyperopic, with a mean refractive error of 0.42 +/- 0.12 D. After day 7, there were no significant differences in the mean refractive error. CONCLUSIONS: The development of optics in the ostrich eye appears to be unique among animals and is characterized by myopia at hatching, rapid onset of emmetropia, and minimal variation in refractive error among chicks.

Animals↗
PubMed1991

[Development of impaired vision in mentally handicapped children].

The aim of the following study was 1) to show the incidence of ophthalmic disorders in severely visually and mentally handicapped children retrospectively 2) to follow up the change in visual acuity over at least 2 years prospectively 3) to look at the effect of therapeutic concepts. 270 children of the "Blindeninstituts-stiftung Würzburg" were followed up between 1960 and 1987. ad 1) Optic atrophy was the leading cause of visual impairment (24%) followed by cataract and retinopathy of prematurity (both found in 17%), malformations of the anterior segment (12%), cortical amblyopia (8%) and refractive error (6%). Strabismus was an additional finding in 38% of the children often associated with nystagmus. Convergent and divergent strabismus had the same incidence. ad 2) Visual acuity improved in 30% of cases of cortical amblyopia, in 40% of refractive errors and in 30% of optic atrophy. In cases of cataract there was a slight improvement in 20%. In 24% of aphakia a secondary glaucoma was observed. In cases of malformations of the anterior segment and ROP the visual acuity remained stable on a low level. The cases of ROP were advanced and had not received any surgical treatment. A deterioration in vision as often seen after surgical intervention was observed in 20% of ROP. In respect of the high incidence of refractive errors and orthoptic problems in multiple handicapped children they should be seen as early as possible by an ophthalmologist. Detection of a congenital cataract soon after birth as well as complications of other disorders should help to prevent blindness. The remaining visual perception is especially important for the handicapped child to communicate and move about.

Adolescent↗
PubMed1994

A prospective evaluation of Unilens soft multifocal contact lenses in 100 patients.

One-hundred patients were fit with Unilens soft multifocal contact lenses over a 2-year period. The Unilens was selected for two reasons: the initial comfort of a soft contact lens and Unilens' purported ability to correct a wide range of refractive errors with simultaneous vision optics. The goal of this study was to find a consistent and simplified multifocal contact lens system that would satisfy a majority of the patients in the study. Forty-eight patients had no previous contact lens experience, whereas 52 patients were wearing some type of single-vision contact lens. All patients were charged the clinic's customary fee for multifocal contact lens fitting with no incentives provided for participating in the study. Success rates were very similar between first time wearers and previous contact lens wearers (71% for first time wearers and 76% for previous contact wearers). Of the four base curves available, the Unilens soft multifocal contact lenses in the 8.4 base curve demonstrated the highest success rate (81%), especially in patients with up to +2.25 D add and 1.50 D or less of cylinder.

Adult↗
PubMed1993

Refractive power calculations for intraocular lenses in the phakic eye.

To correct a severe refractive error and obviate thick spectacles or contact lenses, an intraocular lens can be placed in the anterior chamber in front of the normal crystalline lens as an alternative to keratorefractive procedures. The power of the intraocular lens may be determined from the recipient's spectacle correction, its vertex distance, and the corneal power in diopters. These measurements, combined with an intraocular lens constant based on its location within the anterior chamber, provide the appropriate power of the intraocular lens. These formulas were applied to seven Baikoff anterior chamber lenses and three Momose anterior chamber lenses in phakic eyes. The mean absolute prediction error was 0.42 (standard deviation, +/- 0.60) for the Baikoff lenses and 0.57 (+/- 0.64) for the Momose lenses. The accuracy of the measurements is limited by the accuracy of the preoperative refraction, the measurement of the corneal power, and the restoration of the preoperative refractive power of the cornea.

Anterior Chamber↗
PubMed2003

Corneal aberrations and reading.

PURPOSE: To investigate the effects of eyelid pressure on corneal shape and corneal aberrations during reading. METHODS: Twenty young subjects with normal ocular health were recruited for the study. The experiment was conducted early in the morning, with subjects instructed not to perform any prolonged reading before the experiment. Corneal topography of one eye was measured with a videokeratoscope before reading and then again after a 60-min reading task. The natural position of the eyelids was photographed in primary gaze and during the reading task. RESULTS: Twelve of the 20 corneas showed significant changes in central topography immediately after reading. The location of the changes corresponded closely to the position and angle of the subject's eyelids during reading. The change in shape was best described as a wave-like distortion that significantly altered some corneal wavefront Zernike coefficients. There was a significant correlation between the changes in primary vertical coma and trefoil (along 30 degrees). Within the central 6 mm of the cornea, there were significant changes in the root mean square error, overall refractive power, and astigmatism. CONCLUSIONS: The changes we observed in corneal topography appear to be directly related to the force exerted by the eyelids during reading. Because the cornea is the major light-refracting surface of the eye, the optical characteristics of some eyes can be significantly changed during reading by the force of the eyelids. These findings may have important implications for the definition of refractive status and may also aid in the understanding of the relationship between reading and the development of refractive errors.

Adult↗
PubMed1988

Correlations between ocular optical components, height and head circumference.

Correlations between the ocular optical components, height and head circumference were investigated in 144 subjects. Refraction had a higher correlation with axial length than with corneal curvature. On the other hand, height and head circumference had a higher correlation with corneal curvature than with axial length. These results express the dependence of refraction on the axial length and suggest a similarity between corneal and skeletal growth, compared with the relatively independent elongation of the eye. Refraction correlated significantly with head circumference but not with height, relations which may be explained by considering some of the optical components that determine refraction, rather than treating refraction as a single factor.

Adolescent↗
PubMed2002

Customized corneal ablation.

Refractive surgery is still one of the most innovative and evolving fields in ophthalmology. With the advent of customized ablation, a change in paradigm has been established: the primary goal of refractive surgery is not only to eliminate spectacles but to improve or at least prevent deterioration of the optical performance of the eye. Wavefront-guided refractive surgery has, as a new goal, to correct or at least minimize all optical aberrations of the eye, and consequently to improve or preserve visual performance, especially under scotopic conditions.

Cornea↗
PubMed2004

Tunable soliton self-bending in optical lattices with nonlocal nonlinearity.

We address the phenomenon of soliton self-bending in Kerr-type nonlocal nonlinear media with an imprinted transverse periodic modulation of the linear refractive index. We show that the imprinted optical lattice makes possible to control the mobility of soliton by varying the depth and the frequency of the linear refractive index modulation.

Journal Article↗
PubMed1983

[A graphic method for determining the refractive power of intraocular lenses].

A nomogram has been developed for determining the refractive power of emmetropic intraocular lenses. Individual deviations of optical dimensions from mean values, such as axis length, anterior chamber depth and corneal curvature are used to find a graphical solution. Errors of measurement of optical dimensions are analyzed with regard to their influence on the refractive power of an intraocular lens; errors in measuring the length of the axis have the greatest influence. The nomogram represents a simple method of determining the refractive power of an emmetropic lens. Furthermore, residual refraction can be determined if an intraocular lens of a given power is used or if postoperative ametropia is desired.

Eye↗
PubMed2005

Dielectric optical elements for surface plasmons.

Basic optical elements for surface plasmons are fabricated and their functionality (focusing, refraction, and total internal reflection) is demonstrated experimentally. The optical elements consist of dielectric structures of defined geometry on top of a gold film. The working principle of these structures is discussed on the basis of calculated surface plasmon dispersion relations.

Electric Conductivity↗
PubMed1996

Problems of refraction after photorefractive keratectomy.

BACKGROUND: Excimer laser photorefractive keratectomy (PRK) is a refractive procedure to correct myopia that is gaining worldwide acceptance. PRK has been performed as an open-ended clinical trial since January 1992, and specific problems have been encountered in refracting PRK patients. METHODS: A review of all cases of PRK performed with at least 1-year follow-up was conducted. Nine hundred and seventy-one eyes with myopia ranging from -2.00 to -15.00 diopters had preoperative manifest and cycloplegic refraction, and postoperative refraction at 3 months after treatment. RESULTS: Refractive difficulty due to degradation of the retinoscopic reflex was encountered in patients who developed moderate to severe haze (seven eyes). Small ablation zones (5 mm) and decentered ablations also contributed to confusion in interpretation of the retinoscopic reflex. In addition, excessive accommodation, profound early myopic and hypermetropic shifts, postoperative irregular astigmatism and inaccurate autorefractometer readings contributed to difficulty in refraction. CONCLUSIONS: Laser ablation of the central cornea produces alterations in optical homogeneity of the cornea that may cause significant refractive difficulties. Refracting PRK patients requires careful and consistent refraction by an experienced refractionist, both before and after laser treatment.

Accommodation, Ocular↗
PubMed1985

[Free-vision refraction determination].

A refraction process is called free vision if the patient's visual field is not restricted at all, or at most by the same amount as when using normal spectacles. Free-vision refraction measurement is performed with the help of virtual lenses, which are real optical images of material lenses. In certain circumstances virtual lenses act as normal optical lenses. The advantage of free-vision refraction measurement lies above all in the fact that remote refraction measurement can also be performed from a distance of less than five meters. Moreover, it permits over-refraction, which enables the duration of the measuring procedure to be shortened considerably.

Eyeglasses↗
PubMed1970

The optics of the compound eye of the honeybee (Apis mellifera).

The optical system of the compound eye of the worker honeybee, as a representative of the closed-rhabdom type of eye, was investigated and its function analyzed. Measurements of refractive indices of the elements of the optical system were made with an interference microscope. With the use of the resulting measurements, the optical system was analyzed by means of a ray-tracing procedure implemented for the IBM 7094 digital computer, and by means of the Gaussian thick lens formulae. The more detailed results of the ray-tracing technique were used for further analyses. Direct visual confirmation of the focal point was obtained. The rhabdom and the surrounding zone of lower refractive index act together as a wave guide, as demonstrated by the presence of several wave guide modes in the rhabdom. An admittance function was defined as the percentage of the rays reaching the rhabdom with respect to those entering the ommatidium. Good agreement with experimental results was found. The characterization of the visual field of an ommatidium by means of an admittance function permits the analysis of the influence of different stimuli on the eye.

Animals↗
PubMed1993

Theoretical effect of refractive error and accommodation on longitudinal chromatic aberration of the human eye.

Simple formulas based on reduced eyes have been developed to predict the variation in longitudinal chromatic aberration with variation in ametropia or accommodation. Two formulas were developed, one for axial ametropia and one for refractive ametropia. The latter also served as a model for accommodation. The results using the formulas are in close agreement with results obtained using raytracing through more sophisticated models. Combining the results of different methods gives the following predictions of change in chromatic difference of focus, between wavelengths of 400 and 700 nm, with change in each diopter of refractive error or accommodation: axial ametropia 0.012 to 0.017 D (0.6 to 0.9%), refractive ametropia 0.05 D (2.2 to 2.4%), and accommodation 0.04 to 0.05 D (2.1 to 2.6%). The chromatic aberration effects of correcting lenses with low dispersion are intermediate in effect and opposite in sign to the effects of corresponding degrees of axial ametropia and refractive ametropia.

Accommodation, Ocular↗