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At least 91 records · Page 5Linked to original sources
PubMed1979

The theory and computation of optical modifications to the cornea in refractive keratoplasty.

The optical theory of keratophakia employing implant lenticules with a refractive index different from that of the cornea is presented. This generalized theory also embraces keratophakia employing donated stromal lenticules and keratomileusis, with equal accuracy. The method of solution of the formulas is indicated. The effects of the various parameters upon the radius of curvature to be applied to the cornea are illustrated and discussed.

Cornea↗
PubMed2003

Experimental evaluation of online optical coherence pachymetry for corneal refractive surgery.

PURPOSE: Online optical coherence pachymetry (OCP) allows to monitor central changes of the corneal cross section intraoperatively. In this experimental evaluation the validity of the optical measurements for corneal refractive surgery was assessed. METHODS: Online OCP based on low-coherence interferometry with a wavelength of 1310 nm and a measurement frequency of 74 Hz was directly integrated in a clinical excimer laser. In 16 patients the central corneal thickness was measured with online OCP and ultrasound pachymetry (US). Furthermore, the ablation characteristics were assessed in corneoscleral discs unsuitable for transplantation (n=12) and PMMA samples (n=18). RESULTS: Online OCP was possible in all patients and materials studied. The mean central corneal thickness was 537+/-31 microm (OCP) and 546+/-33 microm (US). The corneal reproducibility was +/-4.3 microm (coefficient of variation [CV] 0.8%) with online OCP and +/-3.7 microm (CV 0.68%) with US. The reproducibility in PMMA samples was +/-1.0 microm (CV 0.16%). There was a significant correlation between online OCP and US measurements (r=0.93, P<0.001). The mean difference was 9.1 microm or 1.69% (P=0.01), and the limits of agreement (95% CI) ranged from -15 microm to 33 microm. There was a significant linear relationship (r=0,95; P<0.001) between the calculated and the optically determined ablation depth with online OCP. Also ablation depth measurements in PMMA correlated positively with spectrophotometric values (r=0.98; P<0.001). CONCLUSION: In this experimental evaluation, online OCP revealed to be a precise and reproducible method to assess the central corneal thickness and its changes intraoperatively. This could be important to monitor incisional and excimer laser-based corneal refractive procedures, such as PRK or LASIK.

Anatomy, Cross-Sectional↗
PubMed1988

Correlation of optical components with ocular refraction among teen-agers in Taipei.

From 2 senior-high and 2 vocational schools in Taipei, 3251 children with age 16-19 were randomly selected to study the relationship between ocular refraction and its main optical components, corneal curvature and axial length. While the corneal curvature was found to play only a minor role in the determination of ocular refraction, the measurements of axial length parallelled the degree of myopia (r = 0.74). A two-years' longitudinal study of additional 411 children from one junior-high school (age 13 through 15) further pointed to the importance of axial length in the production and progression of myopia.

Adolescent↗
PubMed2006

Nanosphere dispersed liquid crystals for tunable negative-zero-positive index of refraction in the optical and terahertz regimes.

An analysis of aligned nematic liquid crystal cells containing core-shell nanospheres shows that it is possible to devise a new type of metamaterial whose index of refraction is tunable from negative, through zero, to positive values. The design parameters for the constituents can be scaled for application in the optical as well as very long wavelength (e.g., terahertz and microwave) regions.

Journal Article↗
PubMed2006

Spectrometric determination of the refractive index of optical wave guiding materials used in lab-on-a-chip applications.

The design and optimization of light-based analytical devices often require optical characterization of materials involved in their construction. With the aim of benefiting lab-on-a-chip applications, a transmission spectrometric method for determining refractive indices, n, of transparent solids is presented here. Angular dependence of the reflection coefficient between material-air interfaces constitutes the basis of the procedure. Firstly, the method is studied via simulation, using a theoretical algorithm that describes the light propagation through the sample slide, to assess the potentially attainable accuracy. Simulations also serve to specify the angles at which measurements should be taken. Secondly, a visible light source and an optical fiber spectrometer are used to perform measurements on three commonly used materials in optical lab-on-a-chip devices. A nonlinear regression subroutine fits experimental data to the proposed theoretical model and is used to obtain n. Because the attainable precision using this method of refractive index determination is dictated by the uncertainty in the transmission measurements, the precision (with 95% confidence) for mechanically rigid samples, namely glass and poly(methyl methacrylate) (PMMA), is higher than those estimated for the elastomer sample (in-house-molded poly(dimethylsiloxane) (PDMS)). At wavelengths with the highest signal-to-noise ratio for the spectrometer setup, the estimated refractive indices were 1.43+/-0.05 (580 nm) for PDMS, 1.54+/-0.02 (546 nm) for glass, and 1.485+/-0.005 (656 nm) for PMMA. Accurate refractive index estimations with an average precision equal to 0.01 refractive index units (RIU) were obtained for PMMA and glass samples, and an average precision of 0.09 RIU for the PDMS molded slide between 550 and 750 nm was obtained.

Combinatorial Chemistry Techniques↗
PubMed2006

[Optical quality after refractive corneal surgery].

Correction of myopia, hyperopia and astigmatism within its indicated margin by means of refractive corneal surgical procedures such as LASIK and surface ablation (e.g. PRK) is one of the standard procedures in ophthalmology. Now that advances in the fields of surgical techniques and the technical devices employed have further progressed in terms of safety and predictability, research also focuses on optical quality. "Optical quality" is not a clearly defined parameter, but can be captured indirectly by means of directly measured data. One has to start with the anatomical properties of the eye, which determine the optical images on the retinal level. The quality of the retinal image influences the eye's function, i.e. acuity and contrast perception. Finally, there is the subjective perception of the image we receive. "Optical quality" as such is reflected by the patient's evaluation of this image perception. Three phenomena are especially responsible for deterioration of the quality of the retinal image: diffraction, aberrations and dispersion. Some of the methods for measuring optical quality are subjective questionnaires, functional testing procedures for measuring visual acuity and contrast sensitivity, optical measuring procedures for the determination of optical quality, as well as biomicroscopy, aberrometry and corneal topography for assessing anatomical changes.

Cornea↗
PubMed2004

Effect of hyperbaric oxygen on guinea pig lens optical quality and on the refractive state of the eye.

The main objective of this study was to investigate the effect of in vivo hyperbaric oxygen (HBO) treatment of albino guinea pigs on ocular refractive state and optical properties of the lens in vitro, as well as on the integrity of the mitochondria of the lens. The animals were treated 30-35 times (2.5-3 months) or 70 times (6 months) with HBO. An increased level of lens nuclear light scattering was evident by slit-lamp at 30 treatments, and this increased at 70 treatments. After 30-35 HBO treatments a myopic shift in refractive state of the eye was seen in two separate studies with two different refractionists. Also, the average back vertex distance of the lens was significantly shorter after 35 HBO treatments while spherical aberration (focal variability) increased after 70 treatments. No difference in refractive state was noted after 70 HBO treatments (a reversal of the initial myopic effect). The mitochondrial distribution and morphology of the lens epithelium and the superficial cortical fibre cells were normal after both 35 and 70 HBO treatments, highlighting that HBO treatment does not affect the superficial cortex of the lens. The results of the in vitro lens optical analysis carried out in this study correlate with the myopia observed after 30-35 HBO in vivo treatments. A similar reversible myopia and increase in lens nuclear light scattering is known to occur in humans treated with HBO for extended periods and the results suggest that the myopia was caused by a change in the refractive index of the lens. The significant loss of sharp focus after 70 HBO treatments can be correlated with previous reports of biochemical and morphological changes associated with HBO-induced loss of lens nuclear transparency in mature guinea pigs. The guinea pig HBO model may be a useful approach for the study of lens development and refractive error.

Animals↗
PubMed2002

Remediation of refractive amblyopia by optical correction alone.

Amblyopia--the commonest vision abnormality of childhood--is characterized by a loss of visual acuity usually of one eye only. Treatment aims to promote function of the amblyopic eye and does this by restricting, usually through occlusion, the competitive advantage of the fellow eye. Recent experimental evidence demonstrates that the recovery of vision following early deprivation is facilitated by increasing visually evoked activity. An analogous approach in humans is to minimise image blur by correcting refractive error prior to treatment--a practice which may account for the poorly quantified improvements in visual acuity sometimes attributed to 'spectacle adaptation'. Here we describe clinically significant gains in visual acuity obtained over a period of 4-24 weeks in a group of amblyopic children arising solely in response to the correction of refractive error. Consequences for the clinical management of refractive amblyopia are discussed.

Amblyopia↗
PubMed2002

Effect of CO2 laser irradiation on the refractive-index change in optical fibers.

The effect of CO2 laser irradiation on the refractive-index change in optical fibers is investigated by measuring the interference fringe shift formed by a long-period fiber grating pair. The refractive-index decrease on CO2 laser irradiation was due to relaxation of the residual stress, which was formed in optical fibers during the drawing process, and the refractive-index decrease was found to increase linearly with the drawing force. The effect of the CO2 laser output power on residual-stress relaxation, and fiber elongation was also studied.

Journal Article↗
PubMed2004

First octameric ellipsoid lanthanide(III) complexes: crystal structure and nonlinear optical absorptive and refractive properties.

The ligand N,N'-bis(1,3,4-thiobiazolyl)-2,6-pyridyldicarboxamide (btapca) has been synthesized and its coordination properties toward La(III) and Ce(III) in the presence of air have been investigated. The complexes [Ln8(tbzcapc)12(H2O)24].6DMF (Ln = La, 1, Ce, 2; tbzcapc = 6-[2-N-(1,3,4-thiabiazolyl)carboxamido]-2-pyridylcarboxylate) show octameric ellipsoid structures. We found the starting ligand btapca had been altered into tbzcapc during the formation process of [Ln8(tbzcapc)12(H2O)24].6DMF. The NLO properties of complexes 1 and 2 were investigated via Z-scan techniques. It is interesting that the two isostructural complexes show completely different NLO properties. Complex 1 shows NLO refractive effects without absorption, while complex 2 possesses NLO absorptive behavior without refraction.

Journal Article↗
PubMed1997

Use of liquid-crystal adaptive-optics to alter the refractive state of the eye.

We evaluated the potential of wavefront shaping with liquid-crystals for modulating the eye's refractive state. A spatial light modulator with 127 liquid crystals cells was imaged in the entrance pupil of the eye and programmed to induce prismatic, spherical, and astigmatic refractive changes. Psychophysical evaluation of these optical effects was in agreement with expectations for prisms up to approximately 0.08 D and for lenses up to approximately 1.5 D. These maximum dioptric values represent wavefront retardation of about 3 to 4 wavelengths of 584 nm light across a 3-mm diameter pupil. Optical aliasing of high-power prisms was traced to spatial undersampling of the wavefront retardation function by the discrete array of liquid crystal cells. Undersampling may also be the factor which limits the useful dioptric range of the technique.

Eyeglasses↗
PubMed1995

The refractive index along the optic axis of the bovine lens.

This study aimed to measure the refractive index values along the optic axes of adult bovine lenses. An optic fibre reflectometric sensor was used for this purpose. Seventeen lenses were examined and over the weight range studied there were no apparent age-related trends in the index gradient. The peak index values varied from 1.432 to 1.442. There was a greater variation in the edge index values, which may have reflected differences in hydration on lens surfaces or may be indicative of a slight decrease in surface index with age.

Animals↗