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At least 487 records · Page 27Linked to original sources
PubMed1989

Growth and ageing effects on the refractive index in the equatorial plane of the bovine lens.

The refractive index profile in the equatorial plane of bovine lenses from over a wide age range is presented. The form of the profile is parabolic and the shape, already apparent in lenses from early prenatal age, is maintained throughout the span investigated. With age the magnitude of the refractive index increases at all points. An empirical formula which relates the value of the refractive index to any point along a radial distance from the centre is derived. This is applicable to lenses of all sizes in the range studied.

Aging↗
PubMed1990

The refractive structure and optical properties of the isolated crystalline lens of the cat.

Direct measurements of the shape and internal refractive index distribution of the isolated cat lens were used to construct individual optical models of the lenses from the left eyes of five cats. The right eyes were used in a companion study of the optics of the cat eye. The lens refractive index spatial distribution and dispersion were measured with a specially designed Pulfrich areal refractometer. Agreement of calculated ray paths through these models with the observed paths of laser beam fans incident parallel to, and at an oblique angle to the lens axis indicates that the models, which contain no freely adjustable parameters, are good physical and optical representations of the isolated (accommodated) crystalline lenses.

Accommodation, Ocular↗
PubMed1990

Theory and measurement of ocular chromatic aberration.

We have determined the transverse chromatic aberration of the human eye by measuring the apparent offset of a two-color vernier viewed foveally through a displaced, pinhole aperture. For the same subjects, we also determined the longitudinal chromatic aberration for foveal viewing by the method of best focus. In both cases, the results were closely predicted by a simple, reduced-eye optical-model for which transverse and longitudinal chromatic aberration are directly proportional, with the constant of proportionally being the amount of displacement of the pinhole from the visual axis. Further measurements revealed that the natural pupil was closely centered on the visual axis for two subjects and slightly displaced in the temporal direction for three other subjects. One implication of these results is that, although the eye has substantial chromatic aberration, the pupil is positioned so as to minimize the transverse component of the aberration for central vision, thereby optimizing foveal image quality for polychromatic objects.

Color Perception Tests↗
PubMed1992

The optics of the spherical fish lens.

The optical design of the fish eye is particularly simple because immersion renders the cornea optically ineffective and the lens is nearly spherical in shape. Measurements have shown that an approximately parabolic gradient of refractive index exists within the lens. If full internal and external spherical symmetry of the lens applies, the geometrical-optical behaviour of the lens is then a function only of the refractive index of the surrounding medium, that of the lens core and cortex, and of the form of the index gradient. The theoretical optical performance of models of the spherical fish lens is calculated by means of the ray-tracing program Drishti as a basis for understanding the optical design of real fish and aquatic eyes. Models based on the gradients proposed by earlier workers are shown to be unable to predict reported spherical aberration and image quality. A model of the fish lens with a polynomial gradient is proposed that yields spherical aberration, image quality and chromatic aberration similar to that reported for the fish.

Animals↗
PubMed1993

Ocular development and visual deprivation myopia in the common marmoset (Callithrix jacchus).

The normal postnatal ocular development of the common marmoset (Callithrix jacchus) and the effects of visual deprivation on eye growth and refractive state are described. The marmoset normally undergoes a developmental process of emmetropization from high hyperopia at birth. This emmetropization is easily disrupted by visual deprivation produced by lid-suture. Myopia and axial elongation of the vitreous chamber are induced by visual deprivations of 12, 5, and 3 weeks duration. The development of axial myopia after 3 weeks of visual deprivation differs from longer duration deprivations in that the experimental eyes are initially shorter than normal and hyperopic at the end of the visual deprivation period, but subsequently become longer than normal and myopic. Visual deprivation myopia in the marmoset persists even after the deprivation is discontinued and a visual signal is restored. In all experimental groups, the development of the eye in response to the cessation of visual deprivation shows no slowing of vitreous chamber enlargement; the axial enlargement relative to the control eye is either maintained or increases and produces significantly greater myopia. These results suggest that the visual control of postnatal eye growth in the marmoset may be unidirectional in its response to visual experience and able only to increase the growth rate of the vitreous chamber, possibly after an initial delay.

Animals↗
PubMed1994

Refractive index of decapsulated bovine lens surfaces measured with a reflectometric sensor.

The surface refractive index values from 16 decapsulated bovine lenses were determined with a reflectometric optical fibre sensor. The refractive index is calculated from the amount of light reflected at the interface created by the fibre and the sample. Measurements were made on anterior, posterior and equatorial surfaces. There was no difference found between values on the anterior and posterior surfaces and no variations along either surface. However index values at the equator were lower. Measurements were also conducted on sucrose solutions of graded concentrations and compared with those made using the Abbe refractometer. There was good agreement between results from the two methods.

Animals↗
PubMed1993

An esthetic glass-ceramic for use in composite restoration inserts.

The objective of the preliminary work reported here was to prepare an improved formulation of intrinsically colored microcrystalline glass-ceramic. Applications could include "megafillers" for direct composite restorations, precision castings, and CAD-CAM prostheses. The experimental glass-ceramic reported here contained SiO2 56.9, AI2O3 19, LiO2 7, ZnO 6, MgO 5, TiO22, ZrO22, P2O52, and CeO20.1 mole%. The batch materials were melted and stirred at 1,610 degrees C for 2 h, quenched in water and also formed into a block of a clear, slightly yellow glass. To identify the crystalline phases that developed during transformation of the glass to the ceramic, x-ray diffraction was used on ten aliquots taken during 15 h of stepwise heating from 750 to 1050 degrees C. With heating, the yellow color deepened to a very translucent "dark yellow" dental shade, then lightened with gradually increasing opacity during formation of secondary crystalline phases. X-ray opacity was approximately equivalent to that of dental enamel. The refractive index of the glass, nD1.554, increased during nucleation and growth of the crystalline phases to a maximum of 1.586. Intrinsic coloration of these glass-ceramic materials can be controlled by varying the heat treatment and/or composition to match typical dental shades.

Ceramics↗
PubMed1993

In vitro characterization of visible light-cured glass ionomer liners.

The purpose of this study was to characterize the in vitro performance of three visible light-cured glass ionomer liners. The liners containing 2-HEMA showed significantly higher compressive strength, diametral tensile strength and Vickers hardness than the liner without 2-HEMA. Statistically significant differences were found in the optical properties of the materials tested. The liners containing 2-HEMA manifested higher shear strength at liner/composite than at dentin/liner interfaces. The acid-base reaction was significantly delayed after light exposure in all the liners tested. The liner without 2-HEMA presented the greatest maximum gap at the liner/dentin interface. Significant correlations were detected between the optical properties of the liners and the top-bottom differences in the extent of conversion, as well as between the maximum marginal gap at the dentin/liner interface and the bond strength with dentin and composite. These correlations suggest the following areas for further developments: incorporation of low molecular weight polyalkenoic acids or accelerating agents to increase the initial carboxylate yield, increase in total transmittance and reduction in diffuse reflectance to improve the in-depth conversion, and reinforcement of the dentin liner interface to prevent gap formation.

Analysis of Variance↗
PubMed1994

Development of high-toughness resin for dental applications.

OBJECTIVES: The purpose of this study was to develop a heat-cured resin with improved toughness. Polyurethane dimethacrylate (PUDMA) with hard and soft segments in the chemical structure was synthesized using polyurethane diisocyanate and 2-HEMA. METHODS: The bulk polymerization of PUDMA, Bis-GMA, TEGDMA and MMA was carried out in a glass tube with 0.5 wt% of BPO at 60 degrees C for 24 h and 110 degrees C for 3 h. The physical and mechanical properties of these polymers were measured, i.e., the polymerization shrinkage, water sorption, transverse strength, modulus of elasticity, deflection and Knoop hardness of these polymerized methacrylates were determined. RESULTS: The volume shrinkage and the water sorption values of PUDMA were lower than those of Bis-GMA. The transverse strength and the modulus of elasticity of PUDMA were close to those of PMMA. The deflection of PUDMA, as determined by a bending test, was higher than any of the other cured resins tested. SIGNIFICANCE: These results suggest that PUDMA is an excellent toughened polymer. The fracture toughness of PUDMA is sufficient for crown and bridge resins and dental composites.

Acrylic Resins↗
PubMed1987

Automated interference refractometer: an algorithm for locating an irregular fringe pattern.

The interference refractometer is potentially a valuable instrument for the measurement of gas concentrations, but its usefulness has been limited by the necessity to locate visually a pattern of light and dark bands in order to obtain the reading. The instrument is therefore liable to human error and is unsuitable for continuous monitoring. An improved design has been patented, in which this location process is automated by the use of a microprocessor and an array of light-sensitive diodes. To implement this improvement it was necessary to design an algorithm which would reliably locate the pattern. This paper describes the approaches which were considered and the successful algorithm. Some examples are given to illustrate the power of the final algorithm to locate patterns even after severe distortion.

Algorithms↗
PubMed1991

Dynamic light scattering by kappa- and lambda-carrageenan solutions.

The intensity correlation functions of kappa- and lambda-carrageenan in various salt solutions and at different concentrations have been determined with the help of dynamic light scattering. From the first cumulant of these correlation functions the values of the translational diffusion coefficients D have been derived. They increase with macromolecular concentration. The extrapolated values to infinite dilution of the diffusion coefficients increase with increasing salt concentration as expected from the salt concentration dependence of the r.m.s. radii of gyration determined previously by static light scattering. The translational diffusion coefficient of lambda-carrageenan in 0.1 M NaCl is smaller than the corresponding value for the kappa species. This is consistent with the difference in contour length and linear charge density of the two samples used. No satisfactory interpretation for the concentration dependence of the diffusion coefficient seems to be possible at present. Although current theories for the macromolecular and salt concentration dependence of D, taking into account charge effects, seem to be applicable, they do not allow for a consistent interpretation of the data. No specific difference between the solution behaviour of kappa- and lambda-carrageenan has been detected.

Carrageenan↗
PubMed1988

Quantitative analysis of immunological reactions on silicon surfaces by multiple-angle Brewster angle reflectometry.

An optical biosensing instrument has been developed for the quantitative analysis of immunological reactions on biochemically sensitized surfaces. It is based on the measurement of reflectance changes of polarized laser light incident on high refractive index substrates at angles close to the pseudo-Brewster angle. Multiple-angle Brewster angle reflectometry (MABAR) has been used to investigate the binding of anti-human serum albumin (a-HSA) to human serum albumin (HSA) coated silicon surfaces. The concentration dependence and reaction kinetics of antibody-antigen complex formation have been studied using red and green He-Ne laser light. A significant increase in sensitivity has been observed with green light.

Antibodies↗
PubMed1992

Refractive index and molar refraction of methacrylate monomers and polymers.

The refractive indices of a number of methacrylate monomers have been measured and corresponding molar refraction data calculated. Similar determinations were made on a number of methacrylate polymers. The molar refraction values determined were in excellent agreement with the values calculated from the published molar refraction values of the chemical groups involved. There was little difference between the molar refraction values of monomers and that of the repeat unit in the corresponding polymers, in marked contrast to the molar volumes. The ratio of refractive index to molar volume was reasonably consistent for all methacrylates studied (0.25-0.30 approximately).

Chemical Phenomena↗
PubMed1995

Preferential interactions in the H2O/lysozyme/AlCl3 system.

The preferential interactions of lysozyme with solvent components is studied in aqueous solutions of AlCl3. The interaction parameter is negative at large salt concentrations, indicating that the interaction process of both salt and protein is thermodynamically favorable. The transfer free-energy parameter and the solubility data show that aluminum chloride is a salting-in agent for lysozyme. Moreover, these preferential interactions also are correlated with both protein solubility in the solvent medium and the influence of salt on the lysozyme structure. Viscometric and refractometric studies show that lysozyme can undergo a conformational change at 1 mM of salt, and spectrophotometric studies indicate a protein activity of approximately 75% at 10 mM of salt. Therefore, neither the interaction of AlCl3 with the lysozyme nor the conformational change undergone directly affect the catalytic amino acid residues of the active site.

Adsorption↗
PubMed1979

Role of nucleic acids and cytoskeleton in nuclear refringence phenomenon (nrp) of human peripheral blood lymphocytes.

Nuclear refringence is a phenomenon visible by light microscopy that is correlated with lymphocyte maturation and activation. The present study, in employing isolated nuclei as well as intact lymphocytes, confirms the nuclear basis of the phenomenon. Enzymatic degradation of nucleic acids in isolated nuclei attenuates refringence, providing presumptive evidence for the involvement of chromatin in refringence. Cytoplasmic events involving RNA and protein synthesis and microtubular, but not microfilament, function are necessary for the maintenance of refringence. These studies provide the basis for evaluating refringence within the context of nuclear protein changes known to occur during lymphocyte activation and for determining its usefulness as a simple test of the dynamic state of the lymphocyte nucleus.

Amanitins↗
PubMed1994

Real-time detection of DNA during gel electrophoresis using a Zeeman refractive index detector.

We have developed a new method for detecting unstained, untagged, unlabeled DNA bands in real-time during gel electrophoresis. This technique is based on the use of a Zeeman refractive index detector with lambda/12,000 resolution and outstanding stability. Current sensitivity in a polyacrylamide gel is 0.2 ng of DNA in the probe beam. We believe this can be significantly improved. We don't understand why the DNA initially decreases, then dramatically increases, the local refractive index as it passes through the gel, although we propose three possible mechanisms.

Computer Systems↗
PubMed1994

Properties and applications of a new type of iodinated gradient medium.

This paper reports on the properties of a new iodinated density-gradient medium, Iotrolan, and its potential application for separating dense cells. Iotrolan is a dimeric, nonionic, iodinated compound which is distinguished by its ability to form dense, non-viscous gradients of low osmolarity. Isotonic gradients of up to 1.30 g/ml can be formed, allowing even dense cells such as sperm to be separated under isotonic conditions.

Animals↗