SEARCH · PubMed Health

Results for “Optics and Refraction”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 343 records · Page 19Linked to original sources

PubMed2006

Comparison of optical coherence tomography and fundus photography for measuring the optic disc size.

PURPOSE: To assess the agreement and repeatability of optic nerve head (ONH) size measurements by optical coherence tomography (OCT) as compared to conventional planimetry of fundus photographs in normal eyes. METHODS: For comparison with planimetry the absolute size of the ONH of 25 eyes from 25 normal subjects were measured by both OCT and digital fundus photography (Zeiss FF camera 450). Repeatability of automated Stratus OCT measurements were investigated by repeatedly measuring the optic disc in five normal subjects. RESULTS: Mean disc size was 1763 +/- 186 vertically and 1632 +/- 160 microm horizontally on planimetry. On OCT, values of 1772 +/- 317 microm vertically (p = 0.82) and a significantly smaller horizontal diameter of 1492 +/- 302 microm (p = 0.04) were obtained. The 95% limits of agreement were (-546 microm; +527 microm) for vertical and (-502 microm; +782 microm) for horizontal planimetric compared to OCT measurements. In some cases large discrepancies existed. Repeatability of automatic measurements of the optic disc by OCT was moderately good with intra-class correlation coefficients (ICC) of 0.78 horizontally and 0.83 vertically. The coefficient of repeatability indicating instrument precision was 80 microm for horizontal and 168 microm for vertical measurements. CONCLUSIONS: OCT can be used to determine optic disc margins in moderate agreement with planimetry in normal subjects. However, in some cases significant disagreement with photographic assessment may occur making manual inspection advisable. Automatic disc detection by OCT is moderately repeatable.

Adult↗

PubMed1976

Further data on biometric correlations of central corneal thickness.

Central corneal thickness measured in 126 young men aged from 18-21 years was correlated to a number of ocular and other parameters. The frequency distribution was skewed towards the lower end, but the deviation could not be statistically supported. No correlation between corneal thickness and birth weight was found. Other characteristics not correlated in this material to corneal thickness were corneal astigmatism, refraction, visual acuity, optic disc and retinal abnormalities, red-green colour vision defects, ABO and rhesus blood-groups, EEG abnormalities, chromosomal abnormalities, hearing defects, ear abnormalities and quality of the hair. The findings in this study, although mainly of negative character, stress the relative independence of the central corneal thickness as a biometric parameter.

ABO Blood-Group System↗

PubMed2025

Causal association between different types of ametropia and risk of diabetic retinopathy: a two-sample Mendelian randomization study.

OBJECTIVE: To investigate the causal link between ametropia and diabetic retinopathy, as well as to offer genetic support for the association between these two conditions. METHODS: This study employed a methodology involving the utilisation of genome-wide association studies data that are publicly accessible. Specifically, single nucleotide polymorphisms (SNPs) that exhibit a strong association with ametropia were employed as instrumental variables, and a two-sample Mendelian randomization (MR) approach was employed to examine the causal relationship between different types of ametropia and diabetic retinopathy. The main findings were derived from the utilisation of inverse variance weighted (IVW), while supplementary results were obtained through the utilisation of MR Egger, weighted median, simple mode and weighted mode. Additionally, a sensitivity analysis was conducted using the 'leave-one-out' method. Cochran's Q statistics were also used to quantify the heterogeneity of SNPs. RESULTS: 38 SNPs were finally included. The results of the IVW analysis indicate that myopia may exert an inhibitory effect on the development of diabetic retinopathy (OR=0.596, 95% CI (0.371, 0.957), p<0.05). Conversely, hypermetropia (OR=8.882, 95% CI (0.389×10-3, 2.06×105), p>0.05) and astigmatism (OR=1.004, 95% CI (0.888, 1.135), p>0.05) do not exhibit a causal relationship with the risk of diabetic retinopathy. CONCLUSION: This two-sample Mendelian randomization study provides evidence that myopia may impede diabetic retinopathy occurrence, while hypermetropia and astigmatism show no significant causal effects. However, our analysis treats refractive errors as independent entities, which may not reflect their clinical interdependence. Further investigations are warranted to elucidate myopia's protective mechanisms.

Humans↗

PubMed2006

Movies of cellular and sub-cellular motion by digital holographic microscopy.

BACKGROUND: Many biological specimens, such as living cells and their intracellular components, often exhibit very little amplitude contrast, making it difficult for conventional bright field microscopes to distinguish them from their surroundings. To overcome this problem phase contrast techniques such as Zernike, Normarsky and dark-field microscopies have been developed to improve specimen visibility without chemically or physically altering them by the process of staining. These techniques have proven to be invaluable tools for studying living cells and furthering scientific understanding of fundamental cellular processes such as mitosis. However a drawback of these techniques is that direct quantitative phase imaging is not possible. Quantitative phase imaging is important because it enables determination of either the refractive index or optical thickness variations from the measured optical path length with sub-wavelength accuracy. Digital holography is an emergent phase contrast technique that offers an excellent approach in obtaining both qualitative and quantitative phase information from the hologram. A CCD camera is used to record a hologram onto a computer and numerical methods are subsequently applied to reconstruct the hologram to enable direct access to both phase and amplitude information. Another attractive feature of digital holography is the ability to focus on multiple focal planes from a single hologram, emulating the focusing control of a conventional microscope. METHODS: A modified Mach-Zender off-axis setup in transmission is used to record and reconstruct a number of holographic amplitude and phase images of cellular and sub-cellular features. RESULTS: Both cellular and sub-cellular features are imaged with sub-micron, diffraction-limited resolution. Movies of holographic amplitude and phase images of living microbes and cells are created from a series of holograms and reconstructed with numerically adjustable focus, so that the moving object can be accurately tracked with a reconstruction rate of 300ms for each hologram. The holographic movies show paramecium swimming among other microbes as well as displaying some of their intracellular processes. A time lapse movie is also shown for fibroblast cells in the process of migration. CONCLUSION: Digital holography and movies of digital holography are seen to be useful new tools for visualization of dynamic processes in biological microscopy. Phase imaging digital holography is a promising technique in terms of the lack of coherent noise and the precision with which the optical thickness of a sample can be profiled, which can lead to images with an axial resolution of a few nanometres.

Animals↗

PubMed2005

Physically based reflectance model utilizing polarization measurement.

A surface bidirectional reflectance distribution function (BRDF) depends on both the optical properties of the material and the microstructure of the surface and appears as combination of these factors. We propose a method for modeling the BRDF based on a separate optical-property (refractive-index) estimation by polarization measurement. Because the BRDF and the refractive index for precisely the same place can be determined, errors cased by individual difference or spatial dependence can be eliminated. Our BRDF model treats the surface as an aggregation of microfacets, and the diffractive effect is negligible because of randomness. An example model of a painted aluminum plate is presented.

Journal Article↗

PubMed2003

Infrared reflection spectroscopy of thin films on highly oriented pyrolytic graphite.

The properties of highly oriented pyrolytic graphite (HOPG) as a substrate for external reflection infrared spectroscopy in the mid-infrared region were investigated. Clean HOPG substrates, physisorbed hydrocarbon multilayers, and chemisorbed monolayers of p-substituted aryl radicals on HOPG were used as samples, and the experimental spectra were compared and complemented with the results of spectral simulations. From reflectivity measurements of clean HOPG surfaces with polarized light as a function of the light incidence angle and the frequency, the anisotropic optical constants n (refractive index) and k (absorption index) were determined for in-plane and out-of-plane directions with respect to the graphite basal plane. These constants express the semimetallic properties of HOPG, indicated by an intermediate reflectivity between a typical metal and a dielectric substrate and by asymmetric, distorted peak shapes in adsorbate film spectra, which represent a transition state between symmetrical, positive absorptions on metals and inverted, negative peaks on dielectric substrates. Regarding spectral sensitivity and surface selection rules, HOPG behaves much like a metal and is therefore an equally suitable substrate for external reflection infrared (IR) measurements.

Journal Article↗

PubMedDate not supplied

Combined surgery to correct high myopia: iris claw phakic intraocular lens and laser in situ keratomileusis.

PURPOSE: To evaluate the results of combined surgery, implantation of an Artisan phakic iris claw intraocular lens (IOL) followed by laser in situ keratomileusis (LASIK) to correct high myopia. METHODS: A prospective study of 6 patients (8 eyes) with high myopia who had Artisan phakic IOL implantation followed by LASIK was undertaken. The IOL was a standard -15.00-D, 6-mm diameter optical zone. Residual refractive error was corrected by LASIK. Mean follow-up was 16 +/- 4 months (range, 12 to 20 mo). RESULTS: After the second procedure (LASIK), uncorrected visual acuity ranged from 0.4 to 0.63 (mean, 0.5 +/- 0.07) at 1 month and from 0.6 to 0.7 (mean, 0.62 +/- 0.04) at 12 months. Spectacle-corrected visual acuity improved 2 or more lines in 62.5% (5 eyes) from preoperative values. Mean postoperative spherical equivalent refraction was -0.68 +/- 0.23 at 1 month and -0.35 +/- 0.22 at 12 months after LASIK. All eyes were within +/-1.00 D of emmetropia following the LASIK portion of the two-stage procedure and 5 eyes were within +/-0.50 D. We had no major complications. No significant endothelial damage occurred. CONCLUSIONS: The accurate refractive outcome, absence of major complications, stability of results, and most important, improvement in quality of vision (defined as no change in vision when illumination conditions varied, eg, at night) experienced by these highly myopic patients are reasons to continue using and improving this combined technique.

Adult↗

PubMed1990

[Eyeglasses].

Spectacles belong to the most ancient inventions in cultural history. Their development, however, cannot be regarded as final. New impetus has resulted in the creation of non-spherical lenses and, through the development of refractive lenses, in a more progressive optic effort.

Eyeglasses↗

PubMed1987

Ophthalmic complications of water sports.

Those who participate in aquatic sports are at risk for some special ocular problems. These have been outlined here and include optical problems and refractive errors, pressure related disease, ocular trauma, infections and irritations, toxins, and complications of exposure to sun and wind. Awareness of these disorders may allow better management and prevention.

Eye Diseases↗

PubMedDate not supplied

[Oculomotor and visual sensory changes in the neurologically lesioned child].

Pathological changes in the visual system are often present in children with neurological handicaps. Strabismus, refractive errors, amblyopia, optic atrophy are the more frequent findings. The ophthalmological evaluation in these children includes: eliciting of pupillary light reflex; occlusion test; cover-test; observation of symmetry of corneal light reflexes; retinoscopy and ophthalmoscopy. Strabismus is the most frequent alteration. The so-called "caotic" strabismus, with very variable angle of squint and abnormalities of the vertical movements, is peculiar in children with neurological handicaps. Sensorial adaptation to strabismus can lead to: suppression, lack of fusion power, anomalous retinal correspondence, amblyopia; these sequelae are the same as those found in strabismic patients without neurological lesions. The basic aims of therapy are an energetic treatment of sensorial anomalies of strabismus, and the development of the best possible visual acuity in each eye. Surgical treatment has mainly cosmetic purposes. It tends to be delayed in time because the results are rather impredictable, owing to the variations of the angle of squint. It is stressed that surgical and non-surgical treatment of strabismus in children with neurological handicaps has no influence on their psycho-motor performances.

Brain Damage, Chronic↗

PubMed1982

Resolution of gratings oriented along and across meridians in peripheral vision.

Grating resolution was measured at various locations of the visual field for four grating orientations. As an instance of the oblique effect, vertical and horizontal gratings produced the highest resolution values in the central area. At eccentricities larger than about 20 deg, the oblique effect was replaced by a meridional resolution effect, in which resolution was systematically best for meridionally oriented grating bars and worst for grating bars perpendicular to the visual-field meridians. The origin of the effect seems to be neural because it was not caused by peripheral refractive errors or optical distortion of the peripheral retinal image.

Humans↗

PubMed2001

Refractive state and accommodation in the eyes of free-swimming versus restrained juvenile lemon sharks (Negaprion brevirostris).

Optical measurements of the refractive state of the eyes of various shark species typically have depicted sharks as hyperopic (far-sighted) with little evidence of accommodation (i.e. the ability to change focus for visualizing objects at different distances from the eye). In this study, we used infrared video retinoscopy to measure the refractive state in juvenile lemon sharks (Negaprion brevirostris). This technique allows dynamic measurement of refractive state in free-swimming animals as they pass by an aquarium window. We found that unrestrained lemon sharks are focused emmetropically relative to a 1-m distant photorefractor for the lateral visual field. However, when restrained either right side up or upside down (the latter inducing tonic immobility), the sharks become increasingly hyperopic, an artifact also reported in some other vertebrates. In addition, unrestrained lemon sharks display small amplitude accommodative excursions. Thus, refractive state measurements on restrained sharks in general may not reflect the natural, resting state of the shark eye, but rather, an induced hyperopia and lack of accommodative function. Such an artifact may be present in other vertebrate species, underscoring the need to obtain measurements of refractive state in unrestrained animals.

Accommodation, Ocular↗

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↗

PubMed2001

Effect of rotation and translation on the expected benefit of an ideal method to correct the eye's higher-order aberrations.

An ideal correcting method, such as a customized contact lens, laser refractive surgery, or adaptive optics, that corrects higher-order aberrations as well as defocus and astigmatism could improve vision. The benefit achieved with this ideal method will be limited by decentration. To estimate the significance of this potential limitation we studied the effect on image quality expected when an ideal correcting method translates or rotates with respect to the eye's pupil. Actual wave aberrations were obtained from ten human eyes for a 7.3-mm pupil with a Shack-Hartmann sensor. We computed the residual aberrations that appear as a result of translation or rotation of an otherwise ideal correction. The model is valid for adaptive optics, contact lenses, and phase plates, but it constitutes only a first approximation to the laser refractive surgery case where tissue removal occurs. Calculations suggest that the typical decentrations will reduce only slightly the optical benefits expected from an ideal correcting method. For typical decentrations the ideal correcting method offers a benefit in modulation 2-4 times higher (1.5-2 times in white light) than with a standard correction of defocus and astigmatism. We obtained analytical expressions that show the impact of translation and rotation on individual Zernike terms. These calculations also reveal which aberrations are most beneficial to correct. We provided practical rules to implement a selective correction depending on the amount of decentration. An experimental study was performed with an aberrated artificial eye corrected with an adaptive optics system, validating the theoretical predictions. The results in a keratoconic subject, also corrected with adaptive optics, showed that important benefits are obtained despite decentrations in highly aberrated eyes.

Eye↗

PubMed1996

Analysis of astigmatic keratotomy with a 5.0-mm optical clear zone.

PURPOSE: To investigate the surgically induced refractive change after astigmatic keratotomy with a 5.0-mm optical clear zone in patients with severe naturally occurring astigmatism. METHODS: We analyzed surgically induced refractive change in 40 consecutive eyes undergoing astigmatic keratotomy with a 5.0-mm optical clear zone. The surgically induced refractive change was calculated according to the method of Holladay, Cravy, and Koch with some modifications. Eyes were divided into two groups: Group I included eyes with mixed astigmatism and a maximum preoperative spherical equivalent of +1.25 diopters that underwent arcuate keratotomy, and Group II included eyes with compound myopic or simple myopic astigmatism (maximum preoperative spherical equivalent of -3.00 diopters) or mixed astigmatism with low minus spherical equivalent that underwent four-incision radial keratotomy and arcuate keratotomy. RESULTS: After refractive surgery, all eyes experienced a reduction of astigmatism when examined three months or more after surgery. Only one eye from each group was overcorrected. The magnitude of refractive astigmatism of surgically induced refractive change was 3.25 +/- 0.90 diopters for eyes in Group I and 3.25 +/- 0.80 diopters for eyes in Group II. There was minimal axis deviation. Eyes in Group I had more flattening with the wound than steepening against the wound, with a coupling ratio of -0.86 +/- 0.36, whereas in Group II, there was flattening both with the wound and against the wound. The net effect of surgery (sum of the primary and secondary effects) was more flattening than steepening in Group I eyes, and flattening of the circumference of the eye in Group II. A decrease of one line of best-corrected visual acuity was observed in one (6.6%) of 15 eyes in Group I and in two (8%) of 25 eyes in Group II. CONCLUSIONS: A 5.0-mm optical clear zone arcuate keratotomy is an effective method for correcting moderate to severe naturally occurring astigmatism. Further investigation of this optical clear zone size on glare and contrast sensitivity testing is necessary and is underway.

Adult↗

PubMed2000

Complete photonic bandgaps in 12-fold symmetric quasicrystals

Photonic crystals are attracting current interest for a variety of reasons, such as their ability to inhibit the spontaneous emission of light. This and related properties arise from the formation of photonic bandgaps, whereby multiple scattering of photons by lattices of periodically varying refractive indices acts to prevent the propagation of electromagnetic waves having certain wavelengths. One route to forming photonic crystals is to etch two-dimensional periodic lattices of vertical air holes into dielectric slab waveguides. Such structures can show complete photonic bandgaps, but only for large-diameter air holes in materials of high refractive index (such as gallium arsenide, n = 3.69), which unfortunately leads to significantly reduced optical transmission when combined with optical fibres of low refractive index. It has been suggested that quasicrystalline (rather than periodic) lattices can also possess photonic bandgaps. Here we demonstrate this concept experimentally and show that it enables complete photonic bandgaps--non-directional and for any polarization--to be realized with small air holes in silicon nitride (n = 2.02), and even glass (n = 1.45). These properties make photonic quasicrystals promising for application in a range of optical devices.

Journal Article↗

PubMedDate not supplied

Examination and refractive management of patients with nystagmus.

Patients with nystagmus present unique challenges to the ophthalmologist. These patients can be difficult to examine and refract. Treatment options to improve vision or reduce disturbing visual symptoms are limited, which is disappointing to the patient and frustrating to the clinician. This paper will provide the clinician with one method of clinically organizing nystagmus, describe the patients who may benefit from optical treatments, and discuss the methodology used in their implementation. Techniques that will be discussed include patient examination and objective and subjective refraction. Optical treatments discussed include spectacles, prisms, contact lenses, and retinal image stabilization.

Contact Lenses↗

PubMed2006

Myopia, axial length, and OCT characteristics of the macula in Singaporean children.

PURPOSE: The purpose of this study was to evaluate the associations between macular volume and thickness, as assessed by optic coherence tomography (OCT), with refraction and axial length (AL) in children. METHODS: A total of 104 Chinese school children (51 girls and 53 boys) 11 to 12 years of age were randomly selected from one school during the 2005 examination in the Singapore Cohort Study of the Risk Factors for Myopia (SCORM). Cycloplegic autorefraction was performed to obtain refraction (defined as spherical equivalent [SE]) and ultrasound biometry performed to obtain the AL. Macular volume and thickness were then measured (StratusOCT3; Carl Zeiss Meditec, Dublin, CA). RESULTS: Children with moderate myopia (SE at least -3.0 D) tended to have smaller total macular volume and thinner quadrant-specific macular thickness (except in the inferior and superior inner quadrants), followed by children with low myopia (-0.5 -0.5 D). Total macular volume was positively associated with SE (beta=1.58, 95% CI, 0.84 to 2.32, standardized beta=0.14, P<0.001) and negatively associated with AL (beta=-1.20, 95% CI, -1.62 to -0.79, standardized beta=0.45, P<0.001) in multiple linear regression models controlling for age and gender. CONCLUSIONS: In children, increasing axial myopia was associated with reduced macular volume and thickness. These findings suggest that early anatomic changes may be present in the retinas of children with axial myopia.

Anthropometry↗