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PubMed2005

Brillouin gain analysis for fibers with different refractive indices.

We studied an anomalous result in the measured stimulated Brillouin scattering threshold for single-mode optical fibers with different refractive index profiles and effective areas. It was found that, in addition to the optical effective area, the refractive-index-dependent interaction between the optical and the acoustic fields must be considered in order to account for the differences in the stimulated Brillouin scattering threshold between fiber types. A detailed analytical formalism describing the acousto-optic interaction was derived to accommodate the influence of the index profile design on Brillouin scattering.

Journal Article↗
PubMed2007

Reflection and refraction of narrow Gaussian beams with general astigmatism at tilted optical surfaces: a derivation oriented toward lens design.

The formulas for the reflection and refraction of a narrow Gaussian beam with general astigmatism at a tilted optical surface are derived by ray-tracing techniques. The propagation direction of the reflected and refracted beams is computed by tracing the central ray of the incident beam, and the characteristic parameters of the respective wavefronts are worked out by applying the formulas developed for the generalized ray tracing. Moreover, the Gaussian form of the reflected and refracted amplitude distributions along the transverse coordinates is determined by requiring the matching of the incident, reflected, and refracted light spots on the optical surface. No limiting assumptions are made regarding the form of the optical interface or the orientation of the incident astigmatic wavefront. In the end, to illustrate a simple application of these formulas, the reflection of a Gaussian beam at a conicoid is considered, and a simple property of the conicoidal mirrors is reported.

Journal Article↗
PubMedDate not supplied

Pattern of childhood blindness at a referral center in Saudi Arabia.

BACKGROUND: An understanding of the causes of blindness and the magnitude of the problem is crucial in designing effective intervention and prevention programs. We undertook this retrospective review to determine the causes of childhood blindness at an eye referral center. PATIENTS AND METHODS: We reviewed charts of children who presented between August 1997 and August 2003. All children had a complete ophthalmologic examination. Blindness was defined as a visual acuity < 20/400, visual impairment as visual acuity between 20/400 and 20/60, and visual loss as a visual acuity < 20/60. RESULTS: A total of 5217 children included 220 (59%) males and 152 (41%) females (age range 2 to 18 years, mean age, 10 years). One hundred twelve (2%) were blind and 260 (5%) had visual impairment. The most common causes of bilateral blindness included optic nerve diseases, retinal disorders, and cataract. The most common causes of unilateral blindness included trauma, retinal diseases, refractive errors, and optic nerve diseases. The most common causes of bilateral visual impairment included refractive errors, corneal diseases, retinal disorders, cataract, and congenital nystagmus. Genetically determined disorders were observed in 37 (70%) of 53 patients with bilateral blindness, and in 52 (56%) of 93 patients with bilateral visual impairment. The incidence of consanguinity among parents of children with acquired causes was 2 (3%) of 59 patients compared to 34 (38%) of 89 among genetically determined causes (P < 0.001). CONCLUSION: Genetically determined disorders continue to play an important role in the causation of childhood blindness among patients attending our referral center in Saudi Arabia. Genetic counseling, early eye screening of children and public education may help in the prevention of visual disorders in children.

Adolescent↗
PubMed1980

Axial lengths and refractive errors in kittens reared with an optically induced anisometropia.

An anisometropia was simulated in kittens during the critical period of development by securing a high-powered negative lens in front of one eye. Refractive error measurements obtained with an objective infrared optometer indicated that the deprived eyes of the anisometropic kittens were significantly more myopic than the normal eyes. A-scan ultrasonography showed that these differences in refractive error were correlated with an increase in the axial dimensions of the deprived eyes. The results of this experiment demonstrate that form deprivation associated with a habitually defocused retinal image produces an experimental myopia which is similar in nature to the refractive error changes produced by lid fusion and corneal opacification.

Age Factors↗
PubMed1992

[Surgical anatomy, physiology, biochemistry and questions on the inlay technique].

The chemical identification of collagens and proteoglykans adds to the knowledge of the fine structure of the cornea as seen in histological sections and with the electron microscope. Chemical properties of collagens and proteoglykans help to understand the microscopical appearance of the tissue and some special functions of the cornea, such as transparency, stromal hydration, cohesion, elasticity and shear force resistance. Understanding mechanical properties of the cornea may be valuable for corneal and especially refractive surgery. A review of recent literature on intrastromal implants to be used for optical correction of refractive errors showed, that the resulting problems of corneal nutrition and also those of hydration may be solved by the time. But presumably proteolytic processes around intrastromal implants still prevent longterm tolerance of such implants.

Biomechanical Phenomena↗
PubMed2005

[Calculation of the IOL power with the numerical simulation program].

PURPOSE: Realisation of a numerical simulation program for the optic system of human eye. Simulation of the refractive errors and their correction with optic system. Calculation of the IOL power with this program. METHOD: The program is accomplished in Pascal programming language. RESULTS: Applying the formulas derived from Abbe's invariant we can calculate all the paraxial characteristics of human eye. With this program we can calculate the power of an air lens, of a contact lens and power of an IOL. CONCLUSIONS: This program simulate the optic system and refractive errors of the human eye. Its necessary to know all the parameters of the optic system of the eye to calculate the power of an IOL. This program can be used in an educational and research purposes.

Algorithms↗
PubMed2004

[Corneal asphericity in a young adult population. Clinical implications].

PURPOSE: To determine the relevance of the different ocular optical components in the refractive state of young adults, paying special attention to the corneal topography represented by the asphericity value. SUBJECTS AND METHODS: Corneal topographies and ultrasonic biometries were obtained from 109 university students with different refractive errors (spherical equivalent range: +3.25 D to -11.00 D). A regression study was performed in order to establish the relationships between corneal asphericity and refractive error, as well as other ocular optical components related to the emmetropization mechanism of the eye. RESULTS: The mean asphericity values were -0.23 (SD 0.08, range: -0.42 to -0.03). All the values correspond to the mathematical description of the prolate ellipse, most commonly accepted for the normal human cornea. The statistical correlation between asphericity and equivalent refractive error was not significant, but a significant correlation was found for the asphericity with respect to the radius of curvature, vitreous chamber depth and axial length. CONCLUSIONS: 1) The asphericity values support the generalised morphology of the prolate cornea as the standard. The influence of this configuration on the contact lens fit, refractive surgery or the visual performance of the eye are discussed. 2) Results suggest that, although a relationship between axial length and corneal topography actually exists, it is not likely that the latter has implications for the emmetropization mechanisms which determine the refractive state of the adult eye.

Adult↗
PubMed1995

The refractive index and protein distribution in the blue eye trevally lens.

BACKGROUND: The relationship between structure (crystallin distribution) and function (refractive index) in the lens is not understood and can be studied by comparing biochemical and optical properties. Such a comparison has been made using a blue eyed trevally lens. METHODS: The optical parameter of refractive index distribution was determined using a nondestructive ray tracing technique. The distributions of the various classes of proteins in the lens were determined by dissolving lenses in concentric layers and using biochemical protein assay. HPLC and SDS-PAGE electrophoresis were used to investigate the proportion of proteins in each layer. RESULTS: The refractive index distribution, from center to edge, follows a second order polynomial. The proteins do not vary in their proportions over most of the lens; only in the inner-most regions is there a rapid increase in insoluble protein and a concomitant decrease in the soluble protein classes. The smallest proteins (gamma crystallins) become insoluble later than the alpha- and beta-crystallins. CONCLUSIONS: There are no similarities in the distributions of any of the protein classes to that of the refractive index in the fish lens. This result indicates that a quantitative relationship cannot be derived by comparing protein to refractive index distributions. However, the findings are consistent with those made in other species: a high content of gamma-crystallins is always found in lenses which have steep refractive index gradients and high index magnitudes.

Animals↗
PubMed1983

Predicting the results of radial keratotomy.

Radial keratotomy on 251 patient eyes was performed. Patients traversing the one-year gate after surgery have demonstrated a refractive change best predicted by the optical zone size, the number of incisions, the corneal dissymmetry, and the age of the patient. One year after surgery, a 16-incision radial keratotomy with 3.0-mm optical zone produced 4.66 diopters of refractive change if the limbus was traversed (N = 13), and 5.18 diopters of refractive change if all incisions remained in clear cornea (N = 39). An eight-incision, 3.0-mm optical zone radial keratotomy produced 5.21 diopters of refractive change at one year (N = 37). A 3.5-mm optical zone, eight-incision radial keratotomy produced 3.66 diopters of change at one year (N = 6). A 4.0-mm optical zone produced 2.67 diopters of change with 16 incisions (N = 8), and 3.45 diopters of change with eight incisions at one year (N = 9). Although older patients appear to have significantly greater refractive change with radial keratotomy at the three-month gate, this difference is not yet confirmed at one year after surgery. The shape of the cornea before surgery contributes to the predictability of the refractive results when the horizontal and vertical keratometer measurements are analyzed. Corneal dissymmetry produces greater refractive results with steeper horizontal K when against-the-rule astigmatism is present before surgery.

Adult↗
PubMed2003

[Identification of asbestos in bulk materials by polarised light microscopy and phase contrast--method and results].

The implementation of the 1997 law on the banned use of asbestos, entailing a successive elimination of asbestos products, is planned for 30 years. Asbestos identification in samples of bulk materials is essential before starting dismantling works. The presented method of asbestos identification by polarized light microscopy and phase contrast involves the study of the following optical parameters of asbestos fibers: color, morphology, refractive index, anisotropy, pleochroism, optic nature and extinguish angle. The major advantages of this method are: small material samples, short time of analysis, high asbestos sample detectability (0.1%) and high reliability of results. This paper presents the results of asbestos identification in over a dozen of samples of bulk materials derived from various buildings and installations.

Asbestos↗
PubMed2004

Ocular shape and myopia.

INTRODUCTION: To learn if eye shape might be a useful parameter in refractive research. MATERIALS AND METHODS: Laboratory research on eye growth mechanisms is summarised. The available clinical literature relating refraction to eye shape and peripheral refraction is critically assessed in the context of the laboratory research on refractive development. RESULTS: Almost all refraction research assesses optical and length parameters exclusively along the visual axis. Contemporary laboratory research demonstrates a remarkable phylogenic conservation of the neural mechanisms regulating refractive development. On-axis image quality regulates central refractive development in animals and probably, to some extent, in humans. Off-axis image quality at the retina depends on anterior segment geometry and optics, and on the 3-dimensional conformation of the retina. In chicks, eye shape is a predictable parameter linked to the underlying neural mechanisms modulating eye development. Based on the sparse clinical literature in human adults and children, the eye shapes induced in chicks are also seen in human subjects in patterns suggesting that eye shape may be a useful parameter in clinical studies. CONCLUSION: The diverse findings suggest that incorporating the 3-dimensional conformation of the eye into future clinical studies may help resolve many of the ambiguities in contemporary refractive research.

Animals↗
PubMed2003

Corneal tissue ablation depth and the Munnerlyn formula.

PURPOSE: To determine whether the error in ablation depth produced by approximations inherent in the Munnerlyn formula are clinically significant when estimating residual corneal stromal depth for the evaluation before refractive surgery. SETTING: Department of Ophthalmology, University of Texas Medical Branch, Galveston, Texas, USA. METHODS: Using identical geometric assumptions, the exact ablation depth was calculated and compared to the approximate ablation depth predicted by the Munnerlyn formula. An adjustment factor was then derived for large optical zones and corrections. RESULTS: The exact ablation depth is always larger than the ablation depth predicted by Munnerlyn's formula. Analysis found the error in ablation depth varied as the fourth power of the optical zone and linearly with correction. The initial corneal radius had little effect on the difference. The ablation depth could be reasonably approximated by adding an adjustment factor for large optical zones and refractive corrections. CONCLUSIONS: In patients with large optical zones, it may be preferable to calculate tissue ablation depth using the exact formula. Alternately, the Munnerlyn formula can be used to calculate ablation depth and then an adjustment factor can be added.

Cornea↗
PubMed2002

The National Blindness and Low Vision Prevalence Survey of Bangladesh: research design, eye examination methodology and results of the pilot study.

PURPOSE To describe the research design and eye examination protocol of The National Blindness and Low Vision Prevalence Survey of Bangladesh and to present the main results of the rural pilot study. METHODS A thorough description of the sampling strategy, eye examination protocol and operational definitions are presented. Multi-stage stratified (rural/urban) cluster random sampling, with probability proportional-to-size procedures, will be used for selection of a cross-sectional, nationally representative sample (12,900 subjects) of the population aged > or = l30 years. Each subject will be tested for visual acuity, auto-refracted and undergo optic disc examination. Those with <6/12 visual acuity in either eye will be re-tested with their refractive correction, dilated and examined for anterior and posterior segment disease. A preliminary, separate rural pilot survey sample was conducted according to the eye examination procedures, with results reported herein. RESULTS Two-hundred-and-four (73.1%) of 279 eligible subjects were examined for the rural pilot. Forty-eight persons had presenting visual acuity worse than 6/12 in either eye. The presenting visual acuity of the better eye was used to group the subjects into the following WHO categories (brackets enclose the number of subjects after refractive correction): Blind: 4 [4]; Visually impaired: 29 [14]; Sighted: 171 [186], of whom 3 [3] were unilaterally blind. Cataract was the main cause of visual acuity of less than 6/12, followed by refractive error, and age-related macular degeneration. CONCLUSIONS The pilot survey demonstrated that the proposed examination process for the main survey was both feasible and appropriate for the purposes of this study. Particular strengths of the pilot and subsequent main survey include the use of logMAR visual acuity testing and auto-refraction of all subjects. The pilot study reveals the burden of cataract and refractive error, which are two of the five diseases specifically targeted by the WHO global blindness initiative 'Vision 2020'.

Adult↗
PubMed1999

Ultrasonographic determination of the dimensions of ocular components in enucleated eyes of the one-humped camel (Camelus dromedarius).

The refractive status and visual capabilities of the one-humped camel are not currently known. Schematic eyes are important tools in studying the optical characteristics and refractive status of an eye. The variables required in developing a schematic eye for a particular animal species include the dimensions of the optical components. We have employed A-scan ultrasonography to measure the dimensions of optical components in freshly enucleated eyes of the one-humped camel. Measurements were made in eyes immersed in distilled water kept at 20 degrees C. The average values of the optical components were as follows: anterior chamber depth = 5.27 mm, lens thickness = 10.93 mm, vitreous chamber depth = 14.85 mm and axial length = 31.05 mm. The uncorrected average corneal thickness was 0.76 mm. These values would have to be confirmed in live animal subjects before they can be used to design a schematic eye for the one-humped camel.

Animals↗
PubMed2003

Effect of large refractive index gradients on the performance of absorption optics in the Beckman XL-A/I analytical ultracentrifuge: an experimental study.

An analytical centrifuge cell was modified to detect refraction of light transmitted through the cell caused by refractive index gradients formed by sedimenting solute during centrifugation. Sedimentation velocity and sedimentation equilibrium experiments were carried out in this cell on solutions containing high concentrations of protein and polysaccharide in a Beckman-Coulter XLA analytical ultracentrifuge. Analysis of the results indicates that in the absence of an optical artifact easily recognized as a "black band," the dependence of apparent absorbance upon radial position reported by the instrument may be considered a reliable measure of the solute concentration gradient.

Ovalbumin↗
PubMed2001

Accommodative esotropia.

Accommodative esotropia is the most common type of strabismus with a very favorable prognosis if the appropriate treatment is initiated promptly. Spectacles remain the preferred treatment, but both contact lenses and refractive surgery may be helpful in some situations to correct the associated hyperopia. Strabismus surgery should only be used if optical correction or refractive surgery are ineffective in restoring normal ocular alignment.

Accommodation, Ocular↗
PubMed2005

Modulation of multiply scattered coherent light by ultrasonic pulses: an analytical model.

We present an analytical solution for the acousto-optical modulation of multiply scattered light in a medium irradiated with a train of ultrasound pulses. Previous theory is extended to cases where the ultrasound-induced optical phase increments between the different scattering events are strongly correlated, and it is shown that the approximate similarity relation still holds. The relation between the ultrasound induced motions of the background fluid and the optical scatterers is generalized, and it is shown that correlation exists between the optical phase increments that are due to the scatterer movement and the optical phase increments that are due to the modulation of the optical index of refraction. Finally, it is shown that compared with the spectrum of ultrasound pulses, the power spectral density of acousto-optically modulated light is strongly attenuated at the higher ultrasound frequencies.

Journal Article↗