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Results for “Optics and Refraction”

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At least 829 records · Page 46Linked to original sources

PubMed2004

Correlation of aberrations with visual acuity and symptoms.

Standard refractive surgery (photorefractive keratectomy and laser in situ keratomileusis)induces optical changes in the wavefront aberrations of the eye. Higher-order optical changes are induced, especially spherical aberration and coma. Wavefront sensing performed after refractive surgery has revealed a decrease in low-contrast acuity and contrast sensitivity, comprising the patient's night vision and leading to symptoms such as halos, starburst, and glare.

Adult↗

PubMed1986

Estimates of primary implant power using an intraocular lens table.

Intraocular lens (IOL) power data have been compiled in tabular format as a function of axial length measurements, corneal powers, and desired refractive errors. The table is presented to assist the clinician in performing IOL power estimates and as an independent source for checking preoperative IOL power calculations. Ophthalmologists who do not own a personal computer will find the lens power table a valuable adjunct to their present repertoire of analytical skills.

Forecasting↗

PubMed1998

A generalization of Prentice's law for lenses with arbitrary refracting surfaces.

A generalization of the Prentice's law is presented in this paper. The idea consists of removing some (but not all) of the approximations that comprise the paraxial approach. In that way, we obtain a new formulation that permits us to compute the prismatic power of a lens made up of arbitrary refracting surfaces, and to improve the precision obtained by Prentice's law when applied to monofocal lenses. The resulting formalism is simple and manageable and its derivation leads us to a precise definition of the local dioptric power matrix, introduced in a previous paper, as well as a better understanding of the same.

Humans↗

PubMed1993

On the possible function of the proximal lens unit of Notonecta glauca.

The water bug Notonecta glauca has doublet lenses corrected for spherical aberration. The proximal lens unit constitutes a refractive index transition between the distal lens unit and the crystalline cone. It inevitably reduces the reflectivity of the eye. Such correction for spherical aberration, however, could also be simply achieved by means of an aplanatic singlet lens. Therefore it seems possible to suppose that the proximal lens unit might produce both decreased reflection and increased transmission. To test this idea, the reflectivity of the doublet in Notonecta is calculated and compared with the reflectivity of two corresponding aplanatic singlets of different refractive indices for contact with air and water. Because of the lower lens unit, the absolute reflection suppression delta R in the dioptric apparatus is about 2% in water and the relative reflectivity reduction delta R/R amounts to 65%. In air this effect is negligible. The question of the evolutionary importance of this phenomenon is briefly discussed.

Air↗

PubMed1981

[Myopic astigmatism for increased depth of focus in pseudophakia (author's transl)].

The optimal refraction after lens implantation is a slight simple myopic astigmatism for patients wishing to see without glasses. The depth of focus is increased in myopic astigmatism because the shape, rather than the size, of the blurred retinal image changes as Sturm's conoid moves over the retina. Such a refraction can be obtained in pseudophakia if the corneal power change induced by surgery is taken into consideration when the intraocular lens. The induced corneal ametropia is a spherocylinder with a very low equivalent power. The results in a series of 50 successive implants employing these principles are described. The depth of fucus is best in simple myopic astigmatism.

Accommodation, Ocular↗

PubMed2004

Correlation between a disc hemorrhage and peripapillary atrophy in glaucoma patients with a unilateral disc hemorrhage.

PURPOSE: To investigate the correlation between a disc hemorrhage and peripapillary atrophy in glaucoma patients with a unilateral disc hemorrhage. METHODS: The 44 glaucoma patients (7 with primary open-angle glaucoma and 37 with normal-tension glaucoma) with a unilateral disc hemorrhage from June 1997 to November 2002 were selected randomly and included sequentially. The topographic measurements were performed using Heidelberg Retina Tomograph (HRT) within 3 months of detecting the disc hemorrhage. The zone beta parameters of the peripapillary atrophy were analyzed by the Atrophy Zone Analysis software. The intraocular pressure, refractive error, visual field parameters, and optic disc parameters were compared between both eyes. Univariate and multivariate regression analysis were performed. RESULTS: The area, angular and radial extent of the zone beta, and the ratio of the zone beta area to the disc area were significantly greater in the hemorrhagic eyes than in the contralateral eyes (P < 0.001). The prevalence of peripapillary atrophy was significantly higher in the hemorrhagic eyes (84%) than in the contralateral eyes (66%) (P = 0.034, chi2 test). The rim area and the rim volume of the hemorrhagic eyes were significantly smaller than those of the contralateral eyes (P = 0.02, < 0.001, respectively). In multivariate regression analysis, the peripapillary atrophy area was the independent significant factor associated with disc hemorrhage (P = 0.03, Odds Ratio = 1.51). The refractive error, intraocular pressure, Mean Deviation (MD), and Corrected Pattern Standard Deviation (CPSD) of the visual fields in both eyes were similar. CONCLUSION: The area and extent of the peripapillary atrophy was significantly greater and more prevalent in the eyes with a disc hemorrhage than in the contralateral control eyes. Peripapillary atrophy is closely associated with a disc hemorrhage in glaucoma patients irrespective of small neuroretinal rim area and volume.

Aged↗

PubMed1998

Partial coherence interferometry: a novel approach to biometry in cataract surgery.

PURPOSE: To compare biometry performed by an enhanced version of dual beam partial coherence interferometry and applanation ultrasound in a prospective study of 85 cataract eyes to improve refractive outcome of cataract surgery due to a more accurate calculation of intraocular lens power. METHODS: The SRK II formula using ultrasound biometry data was employed. Three months after surgery, partial coherence interferometry biometry was repeated and refractive outcome was determined. Preoperative partial coherence interferometry biometry data were used to determine the refractive power of the intraocular lenses retrospectively and to calculate the possible refractive outcome. RESULTS: Precision of partial coherence interferometry biometry was more than 10 times better than that of ultrasound. Therefore, the possible mean absolute error for postoperative refraction achieved with partial coherence interferometry biometry was 0.49 diopters (compared with 0.67 diopters with ultrasound biometry), resulting in an improvement of 27%. Axial eye length measured with the two techniques differed by a mean of 460 microm. The difference in lens thickness measured with partial coherence interferometry and ultrasound significantly correlated with cataract grade. A mean shortening of 120 microm of axial eye length following cataract surgery was also detected by partial coherence interferometry. CONCLUSIONS: The enhanced version of partial coherence interferometry offers biometry with unprecedented precision (<10 microm) and resolution (approximately 12 microm), therefore improving the refractive outcome in cataract surgery. This noninvasive technique provides a high degree of comfort for the patient, with no need for local anesthesia or pupil dilation and minimized risk of corneal infection.

Adult↗

PubMed1995

Aberrations associated with rigid contact lenses.

A rigid contact lens on an eye can produce levels of spherical aberration very different from those produced by a spectacle lens in front of the eye. These levels are considerably affected by contact lens surface asphericity. Change in longitudinal spherical aberration associated with aspherizing a contact lens surface is well predicted by a simple equation for change in sagittal power of the surface. Displacing an aspheric contact lens on the eye can produce considerable defocus, which is well predicted by simple equations for change in sagittal and tangential surface powers. The best refractive correction with contact lenses can be determined only by overrefraction with a patient wearing a contact lens of power and characteristics similar to that which will be prescribed. An aspheric contact lens that moves to a considerable extent on the eye will cause more unstable vision than will a spherical lens that moves to the same extent.

Contact Lenses↗

PubMed2001

Optical phase contrast measurement of ultrasonic fields.

This report describes an optical phase contrast imaging technique for the measurement of wide bandwidth ultrasound fields in water. In this method, a collimated optical wavefront (lambda(l) = 810 nm) impinges on a wide bandwidth ultrasound pulse. The method requires that refractive index perturbations induced by the ultrasound field be sufficiently small. Specifically, on exit from the acoustic field, the phase of the optical wavefront must be proportional to the ray sum of local density taken in the direction of propagation of the incident optical wave. A similar restriction is placed on the dimensions of the ultrasound pulse. Repeated measurement of this phase as the ultrasound field is rotated through 180 degrees about an axis normal to the direction of propagation of the incident optical wave generates the Radon transform of the ultrasonically induced refractive index perturbation. Standard tomographic reconstruction techniques are used to reconstruct the full three-dimensional refractive index perturbation. A simple two-lens imaging system and an optical signal processing element from phase contrast microscopy provide a method of directly measuring an affine function of the desired optical phase for small optical phase shifts. The piezo- and elasto-optic coefficients (the first partial derivatives of refractive index with respect to density and pressure) relate refractive index to density and pressure via a linear model. The optical measurement method described in this paper provides a direct, quantitative measurement of the piezo- and elasto-optic coefficients (from the density or pressure fields).

Journal Article↗

PubMed2006

Rugate filters fabricated by a radio frequency magnetron sputtering system by use of an optical in situ monitoring technique.

We propose, for the first time to our knowledge, a new feedback fabrication technique for rugate filters with sinusoidal refractive index distribution. The technique uses an in situ optical monitoring system, in contrast to conventional techniques for rugate filters that are based on time control, which is generally unsuitable for accurate fabrication of a continuous index distribution. We employed a-SiOx:H thin film as the material for the rugate filters because its refractive index can be successively controlled. Using the proposed technique and material, we fabricated near-infrared rugate minus filters having multiple and continuous refractive index distributions. The experimental and calculated spectra were well correlated as a result of applying the proposed feedback fabrication technique.

Journal Article↗

PubMed1989

Optical constancy of the chick lens during pre- and post-hatching ocular development.

Embryonic and post-embryonic development of the ocular lens is associated with the continual production of new secondary lens fibers by the mitotic activity of equatorial epithelial cells. This continual development affects lens size and shape and refractive index distribution. Study of embryonic lens optical function has been largely ignored. The optical characteristics of the developing chick lens, including paraxial and eccentric focal lengths, were measured during the embryonic period of development and up to 15 days after hatching. Measurements were made with an automated scanning laser system in which the video image of a helium-neon laser beam refracted by an excised lens in solution is digitized. Focal length is measured for beams moving in small steps on either side of that center. Measurements were made on excised lenses as well as with the lens in situ within the anterior segment of the eye. The results, collected from a study of a total of 80 lenses, indicates that embryo lenses at 6-7 days of incubation have long and very variable focal lengths. At the tenth embryo day, focal length drops by more than one-half and focal variations, between lenses and for different beam positions within a single lens, is reduced. Further measures for 14- and 17-day embryo lenses, as well as for lenses from hatchling and 5-, 10- and 15-day-old chicks, indicate that there is little change in focal length, either paraxially or for eccentric beam positions.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1991

Relation between the chromatic difference of refraction and the chromatic difference of magnification for the reduced eye.

We provide a simple model for calculating the chromatic difference of magnification for the human eye. Spectral differences in image size are proportional to the eye's longitudinal chromatic aberration and the axial distance between the entrance pupil and nodal point. We verify that the model provides magnification estimates equal to previously published predictions, and we show the significant increase in this aberration produced by experimental use of artificial pupils.

Humans↗

PubMed1994

Integrated iseikonic equation for intraocular lens power calculations.

I present a simplified equation to calculate the iseikonic intraocular lens power and the resultant refractive error needed to achieve iseikonia with 5% tolerance limits. Iseikonic lens power calculations are indicated in the presence of a unilateral cataract and a phakic fellow eye. Iseikonia is often achieved by leaving the operated eye more myopic than the fellow eye. The induced myopia, with 5% tolerance limits, ranges from -0.75 +/- 2.75 diopters (D) for a posterior chamber lens to -2.40 +/- 2.75 D for an anterior chamber lens.

Humans↗

PubMed2006

[Study on measurement of liquid refractive index by analyzing SPR spectrum character].

The principle of optic fiber surface plasmon resonance sensor is discussed in detail based on the electromagnetic theory and radial theory in the present paper. The relationship between SPR spectrum resonance wavelength and liquid refractive index is also analyzed. Through relative spectrum measurement technology, SPR spectrum curves, whose resonance wavelength increases with increasing liquid refractive index, were obtained for eight different kinds of chemical liquids. The measurement of concentration of mixed solution including ethanol and glycol was performed by analyzing the spectrum attribute. The experimental results show that the sensing system features simple structure and all light transmitted by fiber. It not only realizes rapid, accurate and real time measurement for target medium, but also can be used in some special situations and easily realize long distance measurement.

English Abstract↗

PubMed2003

Computerised calculation scheme for ocular magnification with the Zeiss telecentric fundus camera.

Littmann (1982) described a method to determine the magnification of the eye in order to relate the size of a retinal feature to its measured image size on a telecentric fundus camera film. This required information only about ametropia and corneal curvature. Several other methods have been reported since then which consider other biometric data to enhance the accuracy of this classical method. The purpose of this study is to describe a numerical calculation scheme to determine the magnification q of the eye in two cardinal meridians using paraxial raytracing. Our calculation scheme is based on ametropia, keratometry, as well as biometric data such as axial length, anterior chamber depth and thickness of the crystalline lens. It is described step-by-step in order (1) to determine the refractive powers of both surfaces of the crystalline lens, which are not directly measurable in vivo, (2) to derive the retinal image conjugate to a circular object using paraxial raytracing, (3) to fit an ellipse to the retinal image, (4) to determine the secondary principal points (Gaussian length) separately for both cardinal meridians and (5) to calculate the ocular magnification q. The power of the crystalline lens is estimated to compensate for the spherocylindrical refraction at the spectacle plane and the corneal refraction with an astigmatic component thus creating a sharp image focused at the retinal plane. The capabilities of this computing scheme are demonstrated with five clinical examples and are related to the respective values of the classical Littmann formula as well as to enhanced methods described by Bennett (1988), Bennett et al. (1994) and Garway-Heath et al. (1998).

Anterior Chamber↗

PubMed2003

Comparison of the Zeiss IOLMaster and applanation A-scan ultrasound: biometry for intraocular lens calculation.

PURPOSE: A comparison of axial length estimates using applanation A-scan ultrasound and the Zeiss IOLMaster was conducted. The accuracy in predicting postoperative refraction determined by each method was also compared. METHODS: A cross-sectional study was performed on 51 eyes in 46 patients presenting to clinical practice for cataract surgery assessment. Preoperative measurement of axial length was performed with applanation ultrasound and the IOLMaster. The IOLMaster measurements were used to determine the intraocular lens power based on the SRK/T formula. Postoperative refractive outcomes were obtained and spherical equivalent calculated and compared to the predicted refractive error with each biometric method. RESULTS: On average the axial lengths measured by the IOLMaster were longer by 0.15 mm compared to ultrasound biometry (P < 0.01). Using the IOLMaster over applanation ultrasound biometry significantly improved the postoperative refractive outcome from 0.65 D to 0.42 D (P = 0.011). CONCLUSIONS: The IOLMaster provides an accurate axial length measurement and results in accurate intraocular lens power calculation based on the SRK/T formula. It is quick and easy to use and provides a non-contact technique with no risk of infection or corneal abrasion.

Adult↗

PubMed1993

Understanding the diffractive bifocal contact lens.

How a diffractive bifocal contact lens works is not intuitively obvious. This is because the diffractive bifocal operates by an unusual blend of refraction and interference. The present article seeks to clarify how it achieves its two focal powers. Most of this article, however, is devoted to the conceptually simpler case of diffraction gratings in order to develop an intuition for how a double focus is produced. Single slit, double slit, and multiple slit gratings are considered using two formalisms: Feynman's "arrow" method (also called phasors) and the Fourier method. When dealing with linear diffraction gratings we are interested in the light intensity in the focal plane. By a simple redefinition of variables, all of the mathematics for diffraction gratings can be applied to circular lenses. For this latter case instead of looking at the light intensity in the focal plane, we look at light intensity on the optic axis. The appearance of two focal points is a simple consequence of a combined interference and diffraction pattern. The goal of this article is to present an introduction to Fourier optics and an analysis of the bifocal contact lens in a reader-friendly manner.

Contact Lenses↗

PubMed2000

Visual complaints associated with the AcrySof acrylic intraocular lens(1).

PURPOSE: To describe the visual complaints of a series of patients implanted with the AcrySof(R) (Alcon Surgical) acrylic intraocular lens (IOL) that resolved with IOL exchange. SETTING: Jules Stein Eye Institute, Los Angeles, California, and John A. Moran Eye Center, Salt Lake City, Utah, USA. METHODS: This was a retrospective review of patients who had AcrySof IOL exchange from January 1997 to December 1998. RESULTS: Eight patients (9 eyes) with bothersome visual symptoms following AcrySof IOL implantation were identified. Problems included glare, halos around point light sources, and peripheral arcs of light, often worse at night. In each case, the IOL was well-centered in the capsular bag and there was no significant posterior capsule opacification. Six patients (7 eyes) had the MA30BA model with a 5.5 mm optic, and 2 patients (2 eyes) had the MA60BM model with a 6.0 mm optic. No extralenticular reasons for the patients' complaints could be identified. Exchanging the AcrySof IOLs with silicone or poly(methyl methacrylate) IOLs alleviated most symptoms. In 5 of 8 patients, dysphotopsias resolved completely. CONCLUSIONS: A small number of patients implanted with AcrySof IOLs have specific complaints of glare, halos, and peripheral arcs of light. Optical considerations that may help explain these symptoms include the high refractive index of the IOL material and the truncated design of the optic. Patients who are highly observant and those with large pupils may be particularly symptomatic. Intraocular lens exchange may be necessary in some cases.

Acrylates↗