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PubMed1976

The variation and covariation of cup and disc diameters.

The variation and covariation of cup and disc diameters were studied in a material derived from a population survey and consisting of 2,334 fundus photographs from as many eyes in 1,322 subjects. A simple device was used to facilitate focusing of the camera. The colour slides were projected on to a screen at a fixed distance and measured on ruled paper. The effect of refraction on the magnification in the eye-camera system was compensated by the use of a simple correcting factor. Some apparently quite normal discs, nevertheless, had an area more than four times larger than that of other equally normal ones. The sizes of discs and cups covaried, however, to a suprisingly great extent (r = 0.8) and changes in disc diameter were in general parallelled by similar changes in cup diameter. The amount of tissue in the optic nerve head therefore varied somewhat less than the disc size. Cup diameters were widely dispersed, unevenly distributed and heavily dependent on disc size. The "average rim breadths", on the other hand, were much less dispersed, normally distributed and independent on the disc diameter. By taking the covariation of cup and disc diameters into account the detection of any enlargement or diminution of the optic cup ought to be facilitated.

Adolescent↗
PubMed1989

Geometric optics of trilobite eyes: a theoretical study of the shape of the aspherical interface in the cornea of schizochroal eyes of phacopid trilobites.

A general method is given to determine theoretically the shape of the aspherical interface that eliminates spherical aberration in an optionally shaped thick lens. The theory is applied to trilobite eyes. On the basis of the geometric optical method presented, the refractive indices and focal length of the original corneal lenses of trilobites can be determined. The shape of the aspherical interface in the cornea of some phacopid trilobites with schizochroal eyes is investigated. The theoretical aspherical interfaces agree well with the real ones.

Animals↗
PubMed2006

Characteristics of highly myopic eyes: the Beijing Eye Study.

OBJECTIVE: To evaluate factors associated with high myopia (defined as a myopic refractive error exceeding -8 diopters) in a population-based study. DESIGN: Population-based prevalence study. PARTICIPANTS: The Beijing Eye Study included 4439 participants from among 5324 individuals from a rural area and an urban region of Greater Beijing, > or =40 years old and invited to participate (response rate, 83.4%). METHODS: Interview and detailed ophthalmic examination. MAIN OUTCOME MEASURES: Refractive error, microvascular retinal abnormalities, optic disc morphometry, amount of cataract, and age-related macular changes. RESULTS: Fundus photographs and data for refractive error were available for 4319 participants (97.3%; 8484 eyes). In binary logistic regression analysis, prevalence of high myopia was significantly associated with low best-corrected visual acuity (P<0.001; 95% confidence interval [CI], 0.15-0.40), large optic disc size (P<0.001; 95% CI, 1.64-2.25), large size of beta zone (P = 0.31; 95% CI, 1.45-1.75) and alpha zone of peripapillary atrophy (P<0.001; 95% CI, 1.20-1.58), and lower macular drusen count (P = 0.020; 95% CI, 0.81-0.98). The highly myopic group had a smaller mean size of macular drusen (P = 0.03; 95% CI, 0.02-0.26) and a smaller area covered by drusen (P = 0.01; 95% CI, 0.03-0.22). In the highly myopic group, the predominant drusen type was significantly (P = 0.01; 95% CI, 0.05-0.41) more often the hard distinct drusen type than the soft drusen type, and visual field defects were significantly more common (P<0.001; odds ratio [OR], 24.0; 95% CI, 13.9-41.4) and larger (P<0.001; 95% CI, -1.67 to -1.13). The frequencies of early macular degeneration (P = 0.03; OR, 3.0; 95% CI, 1.21-7.51) and late macular degeneration (P<0.001; OR, 6.33) were significantly lower in the highly myopic group than in the non-highly myopic group. High myopia was not significantly associated with gender (P = 0.18; 95% CI, 0.76-1.05), focal arteriolar thinning (P>0.35), arteriolar sheathing (P>0.45), arteriovenous crossing abnormalities (P>0.20), self-reported diagnosis of diabetes mellitus (P = 0.54; OR, 1.36; 95% CI, 0.48-3.80), or arterial hypertension (P = 0.34; OR, 0.66; 95% CI, 0.32-1.34). CONCLUSIONS: In the adult Chinese population, high myopia is associated with a lower number, smaller, size and less advanced type of macular drusen, a larger optic nerve head, and decreased best-corrected visual acuity. The risk of early and late macular degeneration was lower for highly myopic participants than for non-highly myopic participants.

Adult↗
PubMed1995

[The differences in corneal shape between myopic and normal eyes].

The corneal shapes of 35 eyes with moderate and mild degrees of myopia compared with those of 35 normal eyes were studied by a computerized video keratoscope (TMS-1). The results show that the surface regularity index (SRI), the refractive power at central optical zone, the simulated keratoscopic reading (Simk), the average corneal power of zones 3mm, 5mm, 7mm in diameter centered at the corneal center of the eyes with moderate and mild degrees of myopia are greater than those of the normal eyes (P < 0.05). However, the surface asymmetry index (SAI), the differences of corneal powers between the first and the 15th ring and between the first and the 25th ring in both groups are of no statistical significant differences (P > 0.05). It is suggested that the corneal refractive power of myopic eye be much higher than that of the normal eye. The authors propose the first time that the pathogenesis of some eyes with moderate and mild myopia may be the increase of corneal refractive power which results from traction of sclera near the limbus due to accommodation and discuss it in detail.

Adolescent↗
PubMed2001

The relationship between the difference in prismatic refractive power of an eye-and-face protector and its thickness, radius of curvature, and material.

With the help of the research results presented here and on the basis of a graphic analysis we aim to prove the existence of a relationship between the difference in prismatic refractive power and the thickness, curvature radius, and type of material used for panoramic oculars in protective spectacles, goggles, and face shields. The difference in the prismatic refractive power is a fundamental optical characteristic of a protective ocular without corrective effect. According to Standard No. EN 165:1995 (European Committee for Standardization, 1995) the difference in the prismatic refractive power is a difference in the prismatic effect at 2 observation points of an eye-protector.

Eye Protective Devices↗
PubMed1993

[Optical power of the corneal epithelium].

The refracting power of the corneal epithelium is relatively unknown. We measured the keratometric refracting power of the cornea in two series of 10 fresh bank eyes and on five cat eyes with and without epithelium. The readings were performed at the 2 and 3.6 mm optical zones with an automated keratometer and with a videokeratoscope (TMS-1). The mean change was +1.30 D (0.55-1.85 D) at the 2 mm optical zone and 0.85 D (0.2-1.6 D) at the 3.6 mm optical zone in the first series of 10 eyes. The optical power of the epithelium was most significant within the central cornea. By removing the epithelium there was an increase in corneal refracting power and a change in the power and axis of astigmatism. We discuss the consequences of these findings and how they might affect the outcome of photorefractive keratectomy or other similar keratorefractive procedures.

Animals↗
PubMed1987

Epidemiology of intraocular pressure in a population screened for glaucoma.

This study evaluated the association between intraocular pressure and various sociodemographic characteristics, ocular findings, and cardiovascular risk factors in a population screened for glaucoma. A total of 2594 subjects older than 40 years residing in three urban areas of southern Israel participated. Of those screened 6.1% had a raised intraocular pressure (IOP greater than or equal to 21). The mean IOP increased with age, was higher among persons born in Africa or Asia than those born in Europe or America, higher among myopes than hypermetropes and among those with an enlarged cup-disc ratio (CDR). Analysis of variance tests indicated that refractive status, CDR, age, country of birth, and diabetes were each independently associated with IOP. In addition, associations between raised intraocular pressure and age, country of birth, myopia, CDR, diabetes, and glaucoma in the family were found.

Adult↗
PubMed1980

Eye problems of the elderly.

The most frequent changes of the anterior eye observed in senescence include changes in refraction which may be toward the hyperopic or the myopic side and a diminished tear secretion which may lead to the sicca syndrome. The corneal endothelium often shows degenerative changes which may lead to a decompensation of the cornea and corneal edema. A senile cataract is a frequent finding in older people as is chronic open angle glaucoma. One of the most serious senile changes in macular degeneration which may lead to extremely poor central vision though the peripheral vision may still be satisfactory. A similar effect has the less frequently seen anterior ischemic optic neuropathy.

Aged↗
PubMed2000

A photorefractive organically modified silica glass with high optical gain.

Photorefractive materials exhibit a spatial modulation of the refractive index due to redistribution of photogenerated charges in an optically nonlinear medium. As such, they have the ability to manipulate light and are potentially important for optical applications including image processing, optical storage, programmable optical interconnects and simulation of neural networks. Photorefractive materials are generally crystals, polymers and glasses with electro-optic or birefringent properties and noncentrosymmetric structure. Here we report the photorefractive effect in both non-centrosymmetric and centrosymmetric azo-dye-doped silica glasses, in which refractive index gratings that are spatially phase-shifted with respect to the incident light intensity pattern are observed. The effect results from a nonlocal response of the material to optical illumination, and enables the transfer of energy between two interfering light beams (asymmetric two-beam coupling). Although the writing time for the present grating is relatively slow, we have achieved a two-beam coupling optical gain of 188 cm(-1) in the centrosymmetric glasses, and a gain of 444 cm(-1) in the non-centrosymmetric structures. The latter are fabricated using a corona discharge process to induce a permanent arrangement of azo-dye chromophores.

Journal Article↗
PubMed2000

Effects of experimentally induced ametropia on the morphology and optical quality of the avian crystalline lens.

PURPOSE: To examine the effects of refractive error on avian lens morphology and optical quality. METHODS: Hatchling white leghorn chicks were unilaterally goggled for 7 days with either a form-deprivation goggle (n = 12), a -10 D defocus goggle (n = 12), or a +10 D defocus goggle (n = 12) to induce myopia and hyperopia. Optical quality of lenses (focal length and focal length variability) from treated and contralateral control eyes was assessed using a scanning laser apparatus. Lens morphology was examined by light and electron microscopy. RESULTS: Although the induction of refractive errors did not significantly alter lens size, shape, paraxial focal length, or average focal length, average focal length variability increased. Lenses from eyes goggled with form-deprivation and +10 D defocus goggles demonstrated a twofold increase in average focal length variability, when compared with their contralateral controls. The morphology of the lens is not altered by these experimental manipulations. CONCLUSIONS: This study provides evidence that the refractive development of the chick lens is not independent of the refractive development of the ocular globe and that chick lenticular development is influenced by both genetics and visual experience.

Animals↗
PubMed2001

Histopathologic ocular examination after lensectomy in a Macacus rhesus monkey, an unexpected event of secondary glaucoma.

In a Macacus rhesus monkey an unilateral lensectomy was performed on the 15th day of life to study the emmetropization proces. During a period of 20.5 months refractional state, axial length, corneal dioptric power and intraocular pressure were measured in the aphakic and fellow eye. Results showed that axial elongation of the aphakic eye was larger in comparison to the phakic fellow eye. Histopathologic examination of the aphakic eye revealed glaucomatous changes due to obstruction in the anterior chamber angle.

Animals↗
PubMed2000

Optical Trapping of Titania/Silica Core-Shell Colloidal Particles.

Manipulation of colloidal systems via optical trapping techniques requires a refractive index mismatch between particles and solvent which leads to strong interparticle van der Waals interactions. Investigation of the behavior of systems without such strong attractive interactions, however, requires the uncoupling of particle refractive index and particle-particle interactions. To accomplish this, the synthesis of core-shell titania/silica particles has been performed. By index matching a silica shell on a titania core using a mixture of toluene and propanol, the van der Waals interactions between particles can be minimized. Due to the mismatch of the refractive index between the solvent and titania core, however, a strong trapping force can be generated, making optical manipulation feasible. In order to confirm that the silica shell was indeed matched, pure silica particles were synthesized by the method of Stöber (1968) and added to the core-shell system. In these mixed systems of core-shell and pure silica particles in silica-index-matching solvents, only the core-shell particles were trappable. Copyright 2000 Academic Press.

Journal Article↗
PubMed2004

Sources of normal and anomalous motion in retinoscopy.

PURPOSE: Besides the classic "with," "against," and "neutral" absence of motion, retinoscopic reflexes can display anomalous "with" motion in myopia. A model is presented that explains the source of this anomalous motion, as well as quantifies the appearance of retinoscopic motion in myopia and hyperopia. METHODS: Various 2 x 2 matrices were created to describe schematic eyes for a +20 D trial lens, a Gullstrand #1 schematic eye, and an infant schematic eye. Rays from the retinoscope were traced paraxially through these matrices over a full transit of the retinoscope beam across the pupil. Retinal position of the edge of the reflex visible to the observer was plotted as a function of pupil sizes from 2 mm to 16 mm for -5.00 D and +2.00 D refractive errors for the +20 D trial lens. RESULTS: The edge of the retinoscopic reflex could be formed by one of two sources: the edge of the retinoscope beam itself, or the shadow cast by the beam against the edge of the pupil. Anomalous "with" motion arose in myopia when the edge of the reflex was formed by the edge of the beam. The edge of the beam was also visible in hyperopia but did not create anomalous motion. The retinoscope peephole was not involved in the formation of the edge of the reflex. The degree of anomalous motion increased with greater myopia and pupil size. Measured pupil sizes needed to completely eliminate anomalous motion agreed well with those predicted by the model, except at the largest pupil sizes. The limit for anomalous motion depended only on pupil size, refractive error, and working distance but not on whether the system matrix represented the trial lens, a Gullstrand #1 eye, or an infant eye. CONCLUSIONS: Seeing the edge of the beam at large pupil sizes during retinoscopy creates anomalous "with" motion in myopia but may make the reflex easier to see in hyperopia. Anomalous "with" motion in myopia can be managed by adjustment of pupil size, working distance, or net corrected refractive error. Aside from possible effects of aberrations, retinoscopic motion appears to be consistent across various paraxial optical systems for a given refractive error and pupil size.

Artifacts↗
PubMed1974

Identification and quantitation of asbestos in talc.

The currently used analytical methods for identification, characterization and quantitation of asbestos fiber in consumer talcum products include polarized light microscopy, x-ray diffraction analysis, transmission electron microscopy with selected area electron diffraction and electron microprobe techniques. Light microscope methods have severe limitations imposed by the ultimate size resolution of the light-optical system. Small particles go unresolved; those marginally resolved may possess optical properties different from those properties cited in the literature; most optical properties, e.g., indices of refraction, are difficult to measure on small particles. In addition to these difficulties, talc fibers often possess optical properties different from those of talc plates, which further confound analysis. Light microscopy is recommended for use only as a preliminary tool on limited, large-sized, samples. Transmission electron microscopy is a good standard technique for visualization of contaminant asbestos fibers. Together with selected area electron diffraction, talc fibers may be easily differentiated from amphibole asbestos fibers on the basis of both morphological and structural characterization. Chrysotile fibers are easily distinguished on this basis as well. The amphibole asbestos minerals require chemical characterization to differentiate among the different fiber types. Probe analysis is mandatory for such fibers. The major drawbacks to electron beam instrumentation for the mineralogical characterization of talcum products are the time and effort required for data acquisition. These techniques do not lend themselves to routine study.X-ray diffraction analysis, utilizing the step-scan method, offers a relatively rapid, quantitative technique for gross fiber analysis. Based on comparison with standard specimens the fiber content of talcs may be quantitatively determined. It is essential to employ a specimen preparation technique which yields homogeneously dispersed particles. Tremolite may be determined at levels as low as 0.10% by weight, chrysotile 0.25%, and anthophyllite at 2.0% by weight occurrence in talc. The variance of these values depends upon many factors, including the mass absorption coefficient of the fiber types as compared to talc and selected diagnostic reflections and their relative intensities. Each of the above techniques is described in detail. A method for routine analysis of consumer talcum products is suggested.

Asbestos↗
PubMed2000

Vision-dependent changes in the choroidal thickness of macaque monkeys.

PURPOSE: To determine whether changes in the eye's effective refractive state produce changes in the thickness of the choroid in infant monkeys. METHODS: Normal developmental changes in choroidal thickness were studied in 10 normal rhesus monkeys. Hyperopia or myopia was induced by rearing 26 infant monkeys with either spectacle or diffuser lenses secured in front of one or both eyes. The treatment lenses were worn continuously beginning at approximately 3 weeks of age for an average of 120 days. Refractive status and ocular axial dimensions, including choroidal thickness, were measured by retinoscopy and high-frequency A-scan ultrasonography, respectively. RESULTS: Three lines of evidence indicate that the normal increase in choroidal thickness that occurs during early maturation can be altered by the eye's refractive state. First, in monkeys experiencing form deprivation or those in the process of compensating for imposed optical errors, choroidal thickness and refractive error were significantly correlated with eyes developing myopia having thinner choroids than those developing hyperopia. Second, the choroids in eyes recovering from binocularly induced myopia increased in thickness at a faster rate than the choroids in recovering hyperopic eyes. Third, monkeys recovering from induced anisometropias showed interocular alterations in choroidal thickness that were always in the appropriate direction to compensate for the anisometropia. These changes in choroidal thickness, which were on the order of 50 microm, occurred quickly and preceded significant changes in overall eye size. CONCLUSIONS: Changes in the eye's effective refractive state produce rapid compensating changes in choroidal thickness. Although these choroidal changes are small relative to the eye's refractive error, they may play an important role in the visual regulation of axial growth associated with emmetropization.

Animals↗
PubMed2000

Optical gap solitons: Past, present, and future; theory and experiments.

Optical gap solitons refer to nonlinear waves propagating in optical fibers whose linear refractive index has a periodic variation. Stationary gap solitons came to light first in 1987 [Chen and Mills, Phys. Rev. Lett. 58, 160 (1987)]; two years later, they re-emerge in Christodoulides and Joseph [Phys. Rev. Lett. 62, 1746 (1989)] and are first extended to a more general traveling wave form in Aceves and Wabnitz [Phys. Lett. A 141, 37 (1989)]. But it was not until seven years later, that the first experimental demonstration [Eggleton et al., Phys. Rev. Lett. 76, 1627 (1996); J. Opt. Soc. Am. B 14, 2980 (1997)] was reported. Since then, there has been an increase in the study of the dynamics and applications of such solitons. This paper is a brief survey of some of the ongoing and future research on optical gap solitons. (c) 2000 American Institute of Physics.

Journal Article↗
PubMedDate not supplied

Biomechanics of the cornea and wavefront-guided laser refractive surgery.

PURPOSE: Wavefront-guided laser refractive surgery induces postoperative optical aberrations that are not explained by the ablation profile. METHODS: A conceptual model is presented for a potential mechanism of surgically-induced corneal shape change that is dependent on structural modification, rather than ablation profile. RESULTS: Examples are provided from clinical and basic studies that are consistent with the proposed model. CONCLUSION: Corneal biomechanical response to ablative surgery may significantly affect outcomes, and should be taken into account when planning customized procedures.

Biomechanical Phenomena↗