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At least 307 records · Page 17Linked to original sources
PubMedDate not supplied

Increase in corneal asphericity after standard laser in situ keratomileusis for myopia is not inherent to the Munnerlyn algorithm.

PURPOSE: Standard refractive surgery for myopia induces a shift in corneal asphericity toward positive values, resulting in an increase of spherical aberration. Analytical studies of changes in theoretical corneal shape after application of standard algorithms have yielded controversial conclusions. This study tries to resolve this controversy and discusses causes of optical degradation after refractive surgery. METHODS: Computationally, we subtracted from real preoperative corneas the ablation depth given by the Munnerlyn equation and the parabolic approximation of the Munnerlyn equation. We compared the predicted postoperative corneal asphericity (and corneal spherical aberration) with real postoperative corneal asphericities of the same eyes, after laser in situ keratomileusis (LASIK). RESULTS: Corneal asphericity increased after LASIK in real eyes, with an increase proportional to the amount of correction. This increase was not predicted by the computational application of the Munnerlyn algorithm, which predicted a slight decrease of corneal asphericity. The parabolic approximation of the Munnerlyn algorithm produced an increase in corneal asphericity that correlated with the amount of correction, but was less than the clinical findings. CONCLUSION: Potential causes for increased asphericity (radial changes in laser efficiency, epithelial healing, and biomechanical response) are discussed. These conclusions are important for the design of optimized and customized ablation algorithms, since the theoretical performance of a given ablation algorithm (ie, Munnerlyn algorithm) can differ drastically from real outcomes.

Adult↗
PubMedDate not supplied

[Why is the eye round?].

By an unsophisticated experiment, published data on the anatomy of the eye, wave theory of light, and the laws of geometrical optics the author tries to prove that a spherical concave screen is the optimal variant of a screen on which spherical wave front is projected, formed by a single lens. The shape of the wave front and of the concave screen are in complete agreement. Such optical imperfections as the field curvature, distortion, coma (partially), and astigmatism of the oblique light beams are compensated for. These optical drawbacks are undetectable on a concave screen, and hence they are proposed to be the shortcomings of a flat screen. The author compares this to a spherical shape of the eye, the posterior wall of which represents a concave screen, an indisputable fact, and assumes that the before-named optic imperfections are completely compensated for in human eye due to concavity of the posterior optic wall. Based on the anatomy and practical results of using aspherical refracting surfaces in artificial optic devices, the author draws a parallel with the anatomical structures of the eye possessing aspherical refracting surfaces in order to demonstrate that higher and lower-order spherical aberrations (Zeidel's aberrations) are fully compensated for in human eye, as are the position and enlargement chromatism. Analysis of published data permitted the author to assert that the eye of man is characterized by a method for compensation of spherical aberrations, which has no analogs in man-made practice. Hence, the Nature has imparted the eye with such a collection of compact, universal, and highly effective methods for compensation of optic imperfections, which is not to be found in any of the man-made devices of today.

Eye↗
PubMed2004

Graphical analysis of mass and anisotropy changes observed by plasmon-waveguide resonance spectroscopy can provide useful insights into membrane protein function.

Plasmon-waveguide resonance spectroscopy is a recently developed optical method that allows characterization of mass and structural changes in two-dimensionally ordered thin films (e.g., proteolipid membranes) deposited onto a sensor surface. Full analysis of these systems involves fitting theoretical curves (obtained using Maxwell's equations) to experimental spectra measured using s- and p-polarized excitation. This allows values to be obtained for refractive indices and optical extinction coefficients in these two directions, as well as a value for film thickness, thereby providing information about mass density and anisotropy changes. This is a time-consuming process that works well for simple systems in which only a single conformational event occurs, but cannot distinguish between events involving multiple conformations that proceed either sequentially or in a parallel series of events. This article describes a graphical method that can distinguish between mass density and anisotropy changes in a simpler, more rapid procedure, even for processes that proceed via multiple conformational events. This involves measurement of plasmon-waveguide resonance spectral shifts obtained upon molecular interactions occurring in deposited films with both s- and p-polarized excitation, and transforming these from an (s-p) coordinate system into a (mass-structure) coordinate system. This procedure is illustrated by data obtained upon the binding of a small peptide, penetratin, to solid-supported lipid bilayer membranes.

Anisotropy↗
PubMed2004

Determination of refractive indices of opaque rough surfaces.

The refractive indices of optical materials are usually determined from spectrophotometric and ellipsometric measurements of specular beams. When the roughness of the interfaces increases, the energy in the specularly reflected and transmitted beams decreases and scattering becomes predominant. For strong roughness (compared to the incident wavelength) a surface does not exhibit specular reflection or transmission, making difficult the determination of the refractive index. We describe two techniques, based on scattering measurements, that one can use to determine the refractive indices of opaque inhomogeneous media.

Journal Article↗
PubMed2006

Determination of the refractive index and size distribution of aerosol from dual-scattering-angle optical particle counter measurements.

A method is presented for inferring both the refractive index and the size distribution of aerosol from observations of a dual-scattering-angle optical particle counter (OPC). An existing prototype of an OPC with 60 degree and 90 degree dual-scattering angles was used for the experiments. Based on the high sensitivity of the OPC response to the refractive index of particulates, two families of size distribution curves may be calculated. The solution of the refractive index corresponds to the superposition of the two size distributions. This method was applied to the simulation and to the field measurements conducted in Beijing and Hefei, and the results of both are presented.

Journal Article↗
PubMed1998

[No effect of diabetes mellitus on the morphology of the optic nerve papilla in primary open angle glaucoma].

PURPOSE: To evaluate the effect of diabetes mellitus on the optic disc morphology in primary open-angle glaucoma. PATIENTS AND METHODS: Within a group of 364 patients with primary open-angle glaucoma, a subgroup of 52 patients with diabetes mellitus and a second subgroup of 312 patients without diabetes mellitus were compared with each other. Eyes with proliferative diabetic retinopathy had been excluded. Both subgroups were matched for age, gender, and refractive error. The optic disc was morphometrically evaluated using stereo disc photographs. RESULTS: The two subgroups did not vary significantly (p > 0.10) in mean size of the optic disc, neuroretinal rim and zones Alpha and Beta of the parapapillary chorioretinal atrophy, and frequencies of disc hemorrhages. CONCLUSIONS: Patients with or without diabetes mellitus do not differ significantly in the morphology of the optic disc. One may conclude that diabetes mellitus does not markedly influence planimetric assessed optic disc damage in patients with primary open-angle glaucoma, if eyes with proliferative diabetic retinopathy have been excluded.

Adult↗
PubMed1981

The optics of vitreous surgery.

We calculated the refractive power of the human eye in the various optical configurations that may occur during vitreous surgery. We found that a biconcave corneal contact lens with an anterior surface refractive power of -63 to -98 diopters allowed the fundus to be seen when the vitreous cavity was filled with air, as did a weak minus lens attached to the bottom of the operating microscope. Our tests showed that the unsymmetrical biconcave lenses were superior.

Contact Lenses↗
PubMed1998

Coupled plasmon-waveguide resonance spectroscopy studies of the cytochrome b6f/plastocyanin system in supported lipid bilayer membranes.

The incorporation of cytochrome (cyt) b6f into a solid-supported planar egg phosphatidylcholine (PC) bilayer membrane and complex formation with plastocyanin have been studied by a variant of surface plasmon resonance called coupled plasmon-waveguide resonance (CPWR) spectroscopy, developed in our laboratory. CPWR combines greatly enhanced sensitivity and spectral resolution with direct measurement of anisotropies in refractive index and optical extinction coefficient, and can therefore probe structural properties of lipid-protein and protein-protein interactions. Cyt b6f incorporation into the membrane proceeds in two stages. The first occurs at low protein concentration and is characterized by an increase in total proteolipid mass without significant changes in the molecular order of the system, as demonstrated by shifts of the resonance position to larger incident angles without changing the refractive index anisotropy. The second stage, occurring at higher protein concentrations, results in a decrease in both the mass density and the molecular order of the system, evidenced by shifts of the resonance position to smaller incident angles and a large decrease in the membrane refractive index anisotropy. Plastocyanin can bind to such a proteolipid system in three different ways. First, the addition of plastocyanin before the second stage of b6f incorporation begins results in complex formation between the two proteins with a KD of approximately 10 microM and induces structural changes in the membrane that are similar to those occurring during the second stage of complex incorporation. The addition of larger amounts of plastocyanin under these conditions leads to nonspecific binding to the lipid phase with a KD of approximately 180 microM. Finally, the addition of plastocyanin after the completion of the second phase of b6f incorporation results in tighter binding between the two proteins (KD approximately 1 microM). Quantitation of the binding stoichiometry indicates that two plastocyanin molecules bind tightly to the dimeric form of the cyt b6f complex, assuming random insertion of the cytochrome into the bilayer. The structural basis for these results and formation of the proteolipid membrane are discussed.

Cyanobacteria↗
PubMed2005

[Influrence of age, gender, refraction, keratometry and disc area on the topographic parameters of the optic nerve head].

Optic nerve head (ONH) examination using the Heidelberg Retina Tomograph II was carried out in a study group of 39 healthy persons aged 42-79 years. Refraction and keratometry were measured by means of the autorefractometer. The following topographic parameters of the ONH were tested: disc area, rim area and cup area, cup/disc ratio, rim/disc ratio, rim volume, cup volume, mean and maximum cup depth, height variation contour, cup shape measure, mean RNFL thickness and RNFL cross sectional area. Influence of gender on the topographic parameters of the ONH was not found. The age significantly influenced values of the disc area, rim area, rim volume and RNFL cross sectional area. Refraction significantly influenced the parameters disc area and rim area, keratometry significantly correlated with the disc area and cup shape measure. Disc area significantly influenced most parameters of the ONH (cup area, rim area, C/D ratio, R/D ratio, cup volume, rim volume, mean and maximum cup depth, cup shape measure, RNFL cross sectional area). Our study showed that refraction, keratometry, age and disc area significantly influence many topographic parameters of the optic nerve head in the Heidelberg Retina Tomograph ONH analysis.

Adult↗
PubMed1965

DIFFRACTION.

Explore the source record for details and available documents.

Eye↗
PubMed1993

[Streak retinoscopy. Optical principles and practical recommendations].

The optical principles of streak retinoscopy are demonstrated by diagrams and by optical models. The phenomena observed in the proband's pupil are being explained in two ways: In the first explanation, the image on the proband's retina and its movements are considered, or rather their projection into space; this explanation corresponding to the term "retinoscopy" is commonly used in English textbooks. In the second explanation, the limitations of the light bundle by the proband's and the observer's pupils are considered; hence the term skiascopy (sigma kappa iota alpha = shadow), commonly used in the German literature. The practical hints include retinoscopy through the phoropter, suggestions for the examination of children both with and without cycloplegia, the measurement of accommodation, the detection of opacities and irregularities in the optical media, and the analysis of visual field defects caused by optical anomalies, such as refraction scotomas.

Accommodation, Ocular↗
PubMed2005

Closed-form inversion of the reflection ellipsometric function of a film-substrate system: absorbing-substrate optical constant.

A closed-form inversion expression for obtaining the optical constant (complex refractive index) of the substrate of an absorbing-film-absorbing-substrate system from one reflection ellipsometry measurement is derived. If, in addition, the film thickness is to be determined, a second measurement at another angle of incidence may or may not be used. The derived formula does not introduce errors itself, and tolerates errors in input variables very well. Random and systematic errors in the measured ellipsometric parameters do not affect the value obtained for the optical constant of the substrate: it is always the true value to two decimal places. Two examples in ellipsometry and in the design of reflection-type optical devices, one each, are presented and discussed. In addition, experimental results for a commercially available wafer are also presented. Two other closed-form inversion expressions for obtaining the optical constant of the substrate from two and three measurements are also presented and briefly discussed.

Journal Article↗
PubMedDate not supplied

On the computer-aided and optimal design of keratorefractive surgery.

BACKGROUND: Several recent papers have discussed the use of engineering-based computer methods for the analysis of keratorefractive surgical procedures. What has been lacking is a broader view of the role of engineering analysis in keratorefractive surgery. This article demonstrates how these various analysis methods can be coupled to provide a comprehensive methodology for the design of refractive surgical procedures. METHODS: A structural model of the eye, based on a linearly elastic, transversely isotropic finite element formulation is coupled to a full-eye optical model. The optical errors due to refractive keratotomy are estimated by ray tracing through the optical model and measuring the position of the resulting focal plane relative to the retina. Computer-based optimization methods are employed to determine the surgical parameters necessary to correct myopia for a given set of surgical design goals. RESULTS: Results based on a hypothetical eye demonstrate agreement with clinical trends. Radial keratotomies are designed that eliminate refractive error while minimizing invasiveness in one case and maximizing the optical zone size in another. It is also shown that there is significant potential to customize this process on a patient-by-patient basis using clinically measured data. CONCLUSIONS: We present an overview of the research necessary to bring this approach to fruition. While only a first step, the methodology presented in this article has the potential to increase the predictability of keratorefractive surgery by substantially increasing both the quality and the quantity of the information available to the refractive surgeon preoperatively.

Biomechanical Phenomena↗
PubMed1999

The early fluidic and optical physics of cytometry.

All forms of cytometry, depend on the basic laws of physics, including those of fluidics, optics, and electronics, most of which were established centuries ago. Flow cytometry depends critically on the fluidics presenting each individual cell with precision to the sensing volume. This is intersected by a high-intensity light source, and light scattering and fluorescence from suitably stained constituents in each cell are captured by the light-collecting optics and measured. The works and observations of Bernoulli and Euler in the 18th century, Reynolds in the 19th century, and Crosland-Taylor in the 20th century in the field of fluid dynamics laid the foundations for hydrodynamic focussing, which is the primary prerequisite for presenting individual cells to the sensing volume. In addition, electrostatic cell sorters must have the ability to generate stable droplet formation in the jet-stream issuing from the flow chamber nozzle. The origins here can be traced to work carried out in the early to mid-19th century by Savart, Magnus, and Thomson. Flow, image, and confocal cytometry are all dependent on the laws of optics, including those of reflection and refraction as well as numerous other optical principles. The observations and works of Socrates, Ptolemy, Snel, and Descartes between about BC 370 and 1637 were of seminal importance in developing the laws of reflection and refraction. In the mid-17th century Hooke illustrated the power of magnifying glasses and microscopy in his Micrographia and Newton was responsible for explaining colours in the spectrum. Huygens, toward the end of the 17th century, put forward the concept of point source light propagation contributing to a wave front. Finally, Thomas Young, early in the 19th century, established the wave form of light from interference patterns. Most people will be familiar with some of these discoveries and the investigators who carried out the work; some people will be familiar with all of these. However, very few people are likely to have had the opportunity and privilege to access the very early works and the original data and manuscripts, or translations thereof, which laid the foundations of physics that enabled our discipline to be established. It is always important for any discipline to remember its roots and to appreciate the seed from which those roots grew, for it is much easier to learn and fully understand when we have a knowledge of the source and the logical progressions that lead from one discovery to the next. This knowledge lends perspective to our current endeavours as the past, after all, created the present, which in turn contributes to the future. In this article, which was presented as an invited lecture at the 9th Canadian Consensus meeting on AIDS, I have attempted to trace the origins of the early work on the physics of fluidics and optics, which laid the foundations.

Flow Cytometry↗
PubMed1995

Light transmittance as an index of cell volume in hippocampal slices: optical differences of interfaced and submerged positions.

Light transmittance (T) in the CA1 region of hippocampal slices was measured during exposure to media of various osmolarities to determine the utility of optical measurements as an index of changes in cell volume. In slices positioned at the gas-liquid interface, hypo-osmotic medium consistently produced a decrease in T and hyperosmotic medium produced an increase in T. The magnitude of deltaT was graded as a function of the strength of osmotic change. All changes in T were reversible upon return to isosmotic medium. In contrast, osmotically induced changes in T in submerged slices were consistently opposite in direction to those observed in slices at the interface. The magnitude and direction of deltaT could be altered by systematic variation of the level of the bathing medium within the same chamber, indicating that both extrinsic optical properties of various interfaces, such as refraction and reflection, and intrinsic optical properties of the tissue contribute to the observed T. Spectral measurements eliminated the possibility that osmotically induced deltaT was the result of changes in light absorbance by intrinsic chromophores such as cytochromes or hemoglobin. The results show that measurements of deltaT can be a useful index of changes in cell volume in brain slices, provided that the level of the bath remains constant.

Animals↗
PubMed1991

Acuities through annular and central pupils after radial keratotomy.

The corneal radius of curvature after radial keratotomy (RK) increases centrally in the surgery-free area while remaining relatively unchanged paracentrally and peripherally in the surgical areas. These corneal topography changes suggest that the imaging properties of the cornea should vary with the area of the cornea allowed to participate in image formation. To test this hypothesis visual acuities were measured both through a central and an annular pupil for normals and RK patients as a function of time after surgery. Annular acuities were decreased significantly after RK and remained decreased over time. Best corrected central acuity increased as a function of time after surgery, becoming significantly better than presurgery acuities but not as good as normals with similar low refractive corrections. Clinical implications include: (1) variations in visual performance (e.g., acuity, contrast sensitivity, glare) and optical quality measures (e.g., refraction, higher-order aberrations) as a function of pupil size; (2) use of a large a surgery-free area as possible; (3) careful centering of the surgery-free area on the natural pupil; (4) new contact lens designs for correcting RK patients' residual refractive error; and (5) counseling patients in general, and patients with naturally large pupils in particular, concerning possible variation in visual function with pupil size. In summary, this study indicates that postsurgery RK paracentral/peripheral corneal optics experience a loss in optical quality as compared to either normal eyes with a low refractive correction or the same eye before surgery.

Adult↗
PubMed2004

Posterior capsule opacification after implantation of CeeOn Edge 911A, PhacoFlex SI-40NB, and AcrySof MA60BM lenses: one-year results of an intraindividual comparison multicenter study.

PURPOSE: To perform an intraindividual comparison of posterior capsule opacification (PCO) with 2 foldable intraocular lenses (IOLs) and a foldable acrylic IOL 1 year after in-the-bag implantation. SETTINGS: Seven German ophthalmology centers. METHODS: In an open prospective randomized multicenter study, each center intraindividually compared a high-refractive-index, sharp-edged optic silicone IOL (CeeOn Edge 911A, Pharmacia) with a high-refractive-index, round-edged optic silicone IOL (PhacoFlex SI-40NB, Allergan) or a sharp-edged optic acrylic IOL (AcrySof MA60BM, Alcon). Of 288 randomized patients, 247 had standard phacoemulsification with in-the-bag IOL implantation in both eyes by the same surgeon. One eye of each patient received a CeeOn Edge IOL and the fellow eye, an AcrySof or PhacoFlex IOL. A morphologic evaluation of PCO was performed using the Evaluation of Posterior Capsule Opacification (EPCO) system 1 to 2 weeks and 11 to 14 months after surgery. The digital pictures were evaluated by an independent investigator who was blind to the type of IOL. Intraindividual differences in EPCO scores were statistically evaluated by a 1-sided binomial test at an alpha-level of 5%. RESULTS: One year after surgery, 127 patients with the AcrySof IOL and 102 patients with the PhacoFlex IOL in the control eye were reexamined. Functional results, safety, and handling were not significantly different between the 3 IOLs. All reexamined eyes had a very low PCO grade. The EPCO values revealed less PCO in eyes with the CeeOn Edge IOL than in eyes with the AcrySof or PhacoFlex IOL, but the difference was not statistically significant. A neodymium:YAG laser capsulotomy was performed in 1 eye with a CeeOn Edge IOL, 1 eye with an AcrySof IOL, and 2 eyes with a PhacoFlex IOL. CONCLUSIONS: The EPCO PCO grade was low 1 year after implantation of CeeOn Edge 911A, PhacoFlex SI-40NB, and AcrySof MA60BM IOLs; there was no statistically significant difference between the IOLs. The impact of IOL material and edge design on PCO development might be relevant in a long-term follow-up of this study.

Acrylic Resins↗