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 739 records · Page 41Linked to original sources

PubMed1983

The significance of ocular morbidity in very-low-birthweight infants to the Australian health service.

The survival rate of very-low-birthweight (VLBW) infants has greatly increased due to the introduction of intensive-care methods to neonatal nurseries. It was feared that this would also cause an increase in the amount of ocular morbidity associated with prematurity. In order to estimate this, 111 very-low-birthweight infants (birthweights less than or equal to 1500 g) were reviewed at two years of age. They comprised 63% of the total number of long-term surviving babies born at, or transferred in the neonatal period to, the Royal Women's Hospital, Melbourne, in 1977 and 1978. In 33% a significant ocular problem was detected; 19% had strabismus, 17% had a significant refractive error, 10% had cicatricial retrolental fibroplasia (RLF), and 2.7% were blind, due to optic atrophy associated with cerebral palsy. Other studies have shown that 7% of VLBW infants develop severe (Stage III) RLF, and 18% of these (1.26% of VLBW infants) will be socially or totally blind. In order to estimate the significance of VLBW infants to the ophthalmic health services, and to the organisations for the care of the visually handicapped, these figures can be extrapolated. Based on 1980 figures, it would be expected that approximately 1105 VLBW infants would survive annually, and nine would become blind from RLF, while 110 would have been affected by RLF. Thirty-three children would be blind from optic atrophy associated with cerebral palsy, 210 would have strabismus, and at least 187 would have a significant refractive error. VLBW infants will contribute significantly to the number of children requiring ocular care, and because of the high incidence of ocular abnormalities, it is recommended that routine screening of all VLBW infants be carried out at one year and two years of age.

Australia↗

PubMed1996

Calculating relative retinal image sizes of eyes.

I derive simple formulas which accurately give the relative retinal image sizes of corrected eyes. These are useful in calculations involving anisometropia and/or refractive surgery. Using the formulas, I derive simple equations for particular situations and provide two examples. I compare the formulas with the approach adopted by Applegate and Howland (1993) and Applegate and Chundra (1995) to determine relative retinal image sizes before and after refractive surgery.

Eyeglasses↗

PubMed1989

Corneal asphericity and refractive error.

The relationship between corneal curvature measured by classical keratometry and refractive error raises the question of a possible systematic relationship between refractive error and the degree of corneal asphericity. A recomputation and analysis of previously published data leads to the conclusion that the different refractive groups have similar values of the parameter 'p' and differ only in the apical radius of the cornea.

Cornea↗

PubMed1989

The effect of standardized keratoplasty technique on IOL power calculation for the triple procedure.

Twenty-one consecutive triple procedures (keratoplasty, cataract extraction, lens implantation) performed by one surgeon using identical suturing technique, donor size, and donor/recipient size disparity were analyzed for visual outcome and refractive error. Ninety-five percent of all grafts were clear with an average follow-up of 11.8 months. Of patients with good preoperative visual potential, 84% achieved 20/40 or better visual acuity, and the majority of these patients obtained 20/40 acuity within 6 months of surgery. Sixty-three percent of eyes with 20/40 or better acuity had refractions within +/- 2 diopters of the predicted post-operative refraction. The most recent 14 eyes in this series had IOL power calculations performed utilizing the SRK regression formula and 43.00 K's (surgeon's average post-keratoplasty keratometry values). Within this group, 79% achieved 20/40 or better vision. Eighty-two percent of these eyes had refractions within +/- 2 diopters, and 100% were within +/- 3 diopters of the predicted value. These findings demonstrate that a single surgeon using standardized keratoplasty can achieve good refractive results in the triple procedure.

Cataract Extraction↗

PubMed2005

Angular dependence of the reflectance from an isotropic polydomain medium: effect of large domain size on total reflection.

We investigate the angular dependence of the reflectance from an isotropic medium consisting of optically large and anisotropic, randomly oriented domains, assuming a highly refractive, isotropic, and homogeneous incidence medium, which is presumed to have a higher refractive index than any of the domains' principal indices of refraction. By employing average reflectance and transmittance theory, we are able to show that the onset of total reflection is considerably shifted to higher angles of incidence compared with an isotropic medium with domains that are small compared with the wavelength. The onset of total reflection for a random medium with large domains is found to be dependent only on the largest principal index of refraction of the domains, assuming that all domains have the same optical properties. Therefore the shift of the onset depends on the magnitude of the optical anisotropy of the domains. Even in the case of a small optical anisotropy, large cross-polarization terms are predicted in the vicinity of the onset of total reflection. These terms show a pronounced maximum near that onset and extend beyond it.

Journal Article↗

PubMed1996

A computer model for predicting image quality after photorefractive keratectomy.

BACKGROUND: Accurately predicting visual performance remains a concern in refractive surgery. The effects of the eye's optics on retinal image quality were investigated using computer ray tracing to model the human eye after photorefractive keratectomy (PRK). METHODS: Ray-tracing analysis was used with an anatomically realistic model of the human eye including aspheric surfaces and crystalline lens gradient index distributions. The contribution of corneal curvature to refractive error was investigated using data of axial length, corneal power, anterior chamber depth, and lens power from 318 eyes from the literature. The computer interface was specifically designed for use with PRK and provides graphical plots of the remodeled eye, ray paths and retinal image formation. RESULTS: Modeling the optical contribution of corneal curvature resulted in an improvement in predicted refractive state of the eye as a function of axial length expressed as the R2 value of the regression analysis from 0.88 to 0.96. Subsequently, analyses were conducted for single and multizone treatment areas of differing diameter and with varying pupil size. Retinal image quality following PRK for the human cornea was found to be affected by not only the corneal parameters of anterior curvature and thickness, but also by axial length, pupil size, and anterior chamber depth. CONCLUSIONS: The inclusion of multiple interdependent optical parameters showed differences from conventional methods in predicting refractive outcome following PRK and revealed factors affecting image quality may account for some imperfections in visual performance based on simpler optical modeling.

Anterior Chamber↗

PubMed1997

[Visual acuity and use of eyeglasses after implantation of a diffractive multifocal lens].

BACKGROUND: The implantation of a diffractive multifocal lens (dMIOL) as alternative to a monofocal lens is justified if after surgery there is practically no need to wear glasses. PATIENTS AND METHODS: 31 patients had an implantation of a total of 35 dMIOLs (3M 815 LE). We evaluated the visual acuity, the refractive data and the patients' attitude to wearing glasses. The average age was 67.0 +/- 11.8 years. Follow up took place after 18.7 +/- 5.4 months. RESULTS: The mean value of the uncorrected distance acuity was 0.59 +/- 0.17 and the corrected distance acuity 0.96 +/- 0.13. The uncorrected near acuity amounted to Jg 2.40 +/- 0.94, best distance correction was Jg 1.49 +/- 0.55. The patients still accepted an average of 0.68 +/- 0.37 dpt for the best near correction (near vision over the diffractive near focus) and thus achieved Jg 1.46 +/- 0.55. At the best distance correction plus 3.5 dpt, the near visual acuity was improved to 1.03 +/- 0.17. 54.8% of the patients indicated that they did not use glasses at all. 32.3% stated that they only used glasses for reading. 9.7% wore bifocals all the time, and 3.2% always used glasses for the distance. CONCLUSIONS: Regarding distance vision, the dMIOL is equivalent to monofocal lenses. Without any correction the results of the dMIOLs for the near vision are superior to monofocal lenses. Glasses can be dispensed with if the uncorrected visual acuity of the operated eye is at least 0.6 Jg 2-3 after surgery and the other eye too, does not need any correction. In case the postoperative visual acuity is worse, good visual acuity of the other eye may render glasses unnecessary. Part of the patients put up with a correctable loss of visual acuity in order not to become dependent on glasses. The need to wear glasses may be decreased considerably by implanting a dMIOL in both eyes and by avoiding postoperative refraction errors.

Adult↗

PubMed2006

Temperature dependence of optical anisotropy of holographic polymer-dispersed liquid crystal transmission gratings.

We measured the angular dependence of the 0th, +/-1 st, and +/-2 nd optical diffraction orders from a 50 microm thick transmission grating recorded in a UV-curable holographic polymer-dispersed liquid crystal (HPDLC) made from commercially available constituents. The analysis was performed for two orthogonal polarizations of the probe beams. The emphasis was laid on the temperature dependence of the grating anisotropy. Above the nematic-isotropic phase transition, the grating is optically isotropic. At lower temperatures the grating strength for the optical polarization perpendicular to the grating vector decreases with decreasing temperature, while for orthogonal polarization it increases with decreasing temperature. As a consequence, a regime of diffraction with strongly overmodulated gratings is observed. Our investigations indicate that the anisotropy of the refractive-index modulation scales with the optical anisotropy of the liquid crystal medium forming the phase-separated domains. We further demonstrate that light scattering effects, which are profound only in the nematic phase, must not be neglected and can be taken into account via a Lorentzian line-shape broadening of the probing wave vector directions in the framework of the diffraction theory for anisotropic optical phase gratings.

Journal Article↗

PubMed1986

Screening of infants for strabismus and refractive errors with two-flash photorefraction with and without cycloplegia.

35 1/2-year-old and 31 1-year-old children were screened by two-flash photorefraction for strabismus and refractive errors with and without cycloplegia. The sensitivity of the method to detect refractive errors was tested with an optical demonstration eye. All the children were examined clinically to compare the sensitivity of the method. Every child co-operated with the photography, but 4 children did not co-operate in the clinical examination and were thus excluded from the refractive material, as were also two cases of esotropias which were found. There were no false positive or negative strabismus cases. The total refractive material consisted of 120 eyes. The method was clearly more sensitive for refractive errors with cycloplegia. Even one hyperopia of +5.25 D(OD) and +6.0 D(OS) was underestimated without cycloplegia. In the material there were no cases of anisometropia of over 1.0 D in spherical equivivalent that would cause a potential risk for amblyopia. A rather good correlation of refractive results existed with the method in cycloplegia. With partial overlapping of emmetropic and moderate hyperopic cases. The screening of children aged 1/2-1 year with two-flash photorefraction is simple to perform. The underestimation of symmetrical hyperopias should be accepted when cycloplegia is not used. Only one successful photograph of each child is necessary for the interpretation.

Accommodation, Ocular↗

PubMed1985

Intraocular lens power calculation--the selection of formula.

The postoperative refraction of the eye after intraocular lens (IOL) implantation is an important aspect of the quality of aphakia correction. The postoperative refraction cannot be guessed with consistent accuracy as the refractive power of an eye is multifactorial. Calculation of IOL power is based upon measurements of corneal curvature and axial length and an estimation of postoperative anterior chamber depth. There are a number of formulae available for the calculation of IOL power. The optical formulae, of which those of R. D. Binkhorst are the most popular, give results which are very similar and which all differ from the mathematical regression SRK formula by the indication of a stronger IOL power for short eyes. A statistical analysis of postoperative results confirms that the R. D. Binkhorst formula gives the author an over-powered IOL for short eyes when calculated for emmetropia but not when calculated for planned ametropia. Results with the R. D. Binkhorst formula will be improved by the modification of the anterior chamber depth according to the axial length. Both formulae give satisfactory results for the range of axial lengths commonly encountered in clinical practice. Whichever formula is used, it is important for each surgeon to analyse postoperative results and to modify the selection of IOL power according to this feedback to correct for consistent errors of instrumentation or technique.

Anterior Chamber↗

PubMed2006

Correcting the corneal power measurements after myopic laser in situ keratomileusis.

To describe and evaluate a refraction-derived method and a clinically derived method to calculate the correct corneal power for intraocular lens (IOL) power calculations after laser in situ keratomileusis (LASIK) and to compare the results to the commonly used history-derived method. Retrospective analysis were conducted in consecutive case from clinical practice. For each patient, we established the pre-LASIK and post-LASIK spectacle refraction, the pre-LASIK (K(pre)) and post-LASIK K readings (K(post)). We then calculated the pre- and post-LASIK refraction at the cornmeal plane and the amount of correction obtained by the refraction surgery (deltaSEQco). The cases were divided into two groups. Group I was used to derive two formulas. The values obtained with the two methods were compared with the K by history-derived method (K(c.hd) in group II to validate the results. The K values calculated by using the refraction-derived method (K(c.rd)) and the K values calculated using the clinically derived method (K(c.cd)) correlated highly with K(c.hd). The correct corneal power for intraocular lens (IOL) power calculations after LASIK can use refraction-derived method and clinically derived method instead of history-derived method when some refractive parameters are not available.

Adolescent↗

PubMed2003

The optical properties of the crystalline lens and their significance.

We do not yet have definitive data on the optical structure of the lens, including surface shapes and gradient refractive index distribution. Some data do exist but we need much more data to complete our knowledge of this structure. However, using current and sparse knowledge, we can use mathematical modelling to study the effects of surface shapes and gradient refractive index structure on the Gaussian and aberration properties of the lens. The equations extracted from this modelling show some interesting results, for example, the way the gradient index structure provides some of the refractive power and how this structure possibly predicts a negative spherical aberration of the lens, which is observed in real eyes. This modelling will provide a firm theoretical foundation for analysing more accurate and extensive lenticular shape and refractive index data as they become available.

Humans↗

PubMed1995

The equivalent refractive index of the crystalline lens in childhood.

Despite the importance of crystalline lens power in ocular development, schematic refractive index values used to calculate lens power have been validated for children. We measured refractive error and ocular component dimensions in 519 schoolchildren, calculating lens power using phakometrically measured lens radii and three different refractive index profiles: (1) Gullstrand-Emsley schematic indices [Gullstrand-Emsley lens power (GELP)]; (2) a 10-shell gradient index model [gradient index lens power (GILP)]; and (3) the equivalent refractive index (IND) needed to bring calculated and measured refractive error into agreement [calculated lens power (CLP)]. GELP was significantly lower than either GILP or CLP, indicating the Gullstrand-Emsley refractive index of 1.416 is too low for use in children. Variation in IND cannot be explained by measurement error alone. GILP and CLP also differed as a function of lens shape, with GILP greater than CLP at steeper external curvatures and less than CLP at flatter external curvatures. Variation in equatorial gradient index profile as a function of lens shape is proposed as an explanation for this bias. Equivalent index appears to be a useful tool for encompassing individual variation in lens gradient profiles as well as for assessing the relative role of lens surface curvature and refractive index changes during lens power development in childhood.

Adolescent↗

PubMed1976

Reflections on retinoscopy.

It has often been stated that static retinoscopy findings are not affected by the examiner's refractive correction. To what extent this statement remains true is the subject of this statement remains true is the subject of this paper. The basic principle of retinoscopy is the Foucault test. In this test, a knife edge placed on the principal axis of an optical system (S) intercepts a bundle of rays coming out of (S). Depending on the position of the knife edge, various distributions of light and shadow can be observed on the anterior surface of (S). In retinoscopy it is either the entrance pupil of the examiner or the peephole of the instrument that plays the role of the knife edge. Therefore, any change in conditions that bring about a change in the "knife-edge-pupil" position will result in errors in static retinoscopy. Nevertheless, calculations show that these errors do not become significant under usual clinical conditions of static retinoscopy.

Evaluation Studies as Topic↗

PubMed2005

Topography-based intraocular lens power selection.

PURPOSE: To provide mathematical tools for selecting intraocular lens (IOL) power for normal eyes and for "odd" eyes, particularly after corneal refractive surgery. SETTING: Universitats-Augenklinik, Mainz, Germany. METHODS: First, IOL power is selected based on the radii and numerical eccentricity of the cornea, extracted from corneal topography in a consistent numerical model of the cornea. To fine-tune the result, the visual impression is simulated by blurred Landolt rings superimposed on the retinal receptor grid. The calculation uses numerical ray tracing of the whole pseudophakic eye comprising all monochromatic errors. The error contributions of the influencing parameters, such as anterior and posterior corneal shape and corneal thickness, are quantified in detail. The method is verified in IOL power selection for normal eyes and for eyes after corneal refractive surgery. RESULTS: The main difference between normal corneas and corneas after refractive surgery results from different asphericities. Normal corneas are prolate, with typical numerical eccentricities of 0.5, whereas corneas after laser surgery for myopia are oblate. This causes the main difference (hyperopic shift up to 2.0 diopters) in IOL power selection. Shifts in the posterior corneal radius and corneal thickness are of minor importance. CONCLUSION: Intraocular power selection after corneal refractive surgery should be based on all the information corneal topography provides.

Cataract Extraction↗

PubMed1981

The effect of spherical aberration of contact lens to the wearer.

To determine the influence of the spherical aberration of a contact lens on the total spherical aberration of the eye-contact lens system, appropriate formulas are derived. By comparing the total spherical aberration of the eye without a contact lens with that found when a contact lens is in place, it is shown that the spherical aberration of the eye is determined, almost entirely, by the first refracting surface (either cornea or contact lens). The amount of aberration when converted to a dioptric value shows the relative effects clearly.

Contact Lenses↗

PubMed1986

Comparison of subjective lens preference to objective refractive error measurements in pigtail macaques.

Two measurements can be taken to determine the refractive error in human beings: a subjective, lens preference measurement and objective retinoscopy. This study determined the subjective refractive error in eight pigtailed monkeys by placing lenses of different powers in front of peepholes in a solid wall cage and recording the amount of time each subject used the holes. Retinoscopy measurements were then taken and compared to the subjective findings to determine the degree of similarity between these two refraction methods. The results showed a high correlation between the subjective measurements and the vertical ocular refraction (VOR) and spherical equivalent (SE) (rho = 0.96), further supporting generalizations made from nonhuman primate vision studies to human beings.

Animals↗

PubMed1990

Effects of dioptric blur on Snellen and grating acuity.

We compared the effects of dioptric blur on Snellen acuity and grating acuity. Dioptric blur had a strong negative effect on Snellen acuity, consistent with previous studies, but had little effect on grating acuity. Between 1 and 12 D both types of acuity were reduced as a linear function of blur. However, 12 D of blur reduced grating acuity to only 6/24 (20/80), whereas letter acuity was worse than 6/300 (20/1000). We suggest that these differences are due to the presence of "spurious resolution" in which phase-reversed gratings are readily detectable. But the phase reversals so distort the relative positions of linear segments within the letters that the letters become unrecognizable. These results indicate that Snellen letters are more sensitive than gratings to a patient's refractive errors, emphasize the differences between Snellen and grating acuity, and indicate that the minimum angle of resolution (MAR) concept is not applicable to letters.

Adult↗