Refractive Variation Research Articles

Yearly, Seasonal and Daily Variations of Radio Refractivity

Yearly, seasonal and daily variations of radio refractivity have been analyzed. The method proposed in the recommendation of International Telecommunication Union ITU has been used. The local meteorological data have been used in calculation of radio refractivity. The highest values of the radio refractivity have been observed in Klaipėda (in Seacoast) in the year 2009. In July, the values of the radio refractivity were highest in all localities investigated here and over all the time of the day in the year 2009. In the continental part of Lithuania (in Vilnius and Kaunas), analysis of radio refractivity has been made by using the meteorological data of longer period (starting from April 2005 up to July 2010). Five-year meteorological data collected in February, April, July and October have been used. It was obtained that the values of refractivity in the year 2010 are much higher than ones obtained in all the years of the period investigated here.

Stable refraction and visual acuity in diabetic patients with variable glucose levels under routine care

To investigate how refraction and visual acuity may vary in patients with diabetes under routine care. Fifty-three eyes of 53 patients with various degrees of diabetic retinopathy were examined prospectively on four different occasions within a month. Refraction, best-corrected visual acuity (expressed as logMAR score) and blood glucose were measured on each occasion. Intraindividual variability was calculated as the range between the highest and lowest measurements. Associations between blood glucose levels and each of the other variables were tested by linear regression analysis for each patient. Refraction was completely stable in 43 patients and changed only slightly in 10, in whom the mean intraindividual variability of the spherical equivalent was 0.4 dioptres. Visual acuity test results were also highly reproducible. Mean intraindividual variability in visual acuity was 0.08 logMAR. Mean haemoglobin A1c (HbA1c) was 7.3 ± 1.5% but individual blood glucose levels ranged from 2.8 to > 22.2 mmol/l. Intraindividual variability ranged from 0.5 to 18.1 mmol/l, with a median of 6.0 mmol/l for the entire group. There were no associations between refraction or visual acuity and blood glucose levels or inter- or intraindividual glucose variations. Refraction and visual acuity test results were highly reproducible and stable in patients with reasonably well controlled diabetes but variable blood glucose levels under routine care.

Pd nanoparticles formation by femtosecond laser irradiation and the nonlinear optical properties at 532 nm using nanosecond laser pulses

Pd nanoparticles (NPs) were prepared by focused femtosecond laser irradiation of PdCl2 dissolved in ethanol. Transmission electron microscopy (TEM) analysis revealed that Pd NPs show certain crystalline microstructure, and the average diameter is 3.4 nm with narrow size distribution from 2.0 to 6.0 nm. The nonlinear optical absorption and refraction of Pd NPs solution were investigated with nanosecond laser pulses at 532 nm. The nonlinear absorption of Pd NPs is saturable at low intensity of 3.28×1011 W/m2 but it is changed to reverse saturable with the intensity increased to 7.96×1011 W/m2, which accordingly indicates the nonlinear refraction is changed from self-defocusing to self-focusing. The transition of the nonlinear absorption with the increase in pulse intensity is analyzed by an empirical model which includes mostly saturable absorption (SA) and two-photon absorption (TPA). The intensity of saturable Is is obtained, along with TPA coefficient β. SA and TPA are both originated from the interband transition between the d band and s-p conduction band. The SA possess less occupied density of states in the ground state and less unoccupied density of states in the excited state than that of TPA, so the TPA dominates the nonlinear absorption when the pulse intensity is high, leading to as-observed transition from SA to TPA. The refraction variation with increase in pulse intensity is attributed to the interband transition of electrons from d band to s-p conduction band in the Pd NPs.

Tracking refractivity from radar clutter using extended Kalman filter and unscented Kalman filter

Since the traditional statistical and physical algorithms in the inversion of refractivity from radar clutter (RFC) cannot track omni directions in real time, a new filter algorithm (extended Kalman filter and unscented Kalman filter) is proposed. The parameter equation of atmospheric duct, observation operator, state equation of filter arithmetic are derived separately. Finally, the implementation of the iterative inversion filter algorithm is derived. On the theoretical basis, two algorithms above are tested separately with or without considering the variation of refractivity with time. The experimental result indicates that unscented Kalman filter is suited to solve the nonlinear inversion problem, which has significance in theoretical foundation and technological support for practical applications for the future.

DIURNAL AND SEASONAL VARIATION OF SURFACE REFRACTIVITY OVER NIGERIA

The diurnal and seasonal variation of surface refractivity over Nigeria was studied using four years in-situ meteorological data from eight location over Nigeria. At all the stations studied, it was observed that the diurnal refractivity variation was caused majorly by the dry term in the rainy season and the wet term is the major cause of refractivity variation in dry season except Sokoto and Jos. In Sokoto the result was found to be opposite and it is attributed to the fact that in dry season the humidity is almost close to zero while in rainy season the pressure seems to be almost constant but the temperature ∞uctuates rapidly and consequently the humidity. The variation pattern in Jos is as observed because of the altitude (» 1000m above sea level). At this altitude pressure variation seems to be insigniflcant. The result also show that the surface refractivity generally have higher value during rainy season than dry season at all location studied. The result also show that the value of surface refractivity increases from arid region in the north to the coastal area in south. The result also show that the diurnal refractivity variation is basically a function of local meteorology and while seasonal variation is caused follows the climatic condition.

Nature and nurture: the complex genetics of myopia and refractive error

The refractive errors, myopia and hyperopia, are optical defects of the visual system that can cause blurred vision. Uncorrected refractive errors are the most common causes of visual impairment worldwide. It is estimated that 2.5 billion people will be affected by myopia alone within the next decade. Experimental, epidemiological and clinical research has shown that refractive development is influenced by both environmental and genetic factors. Animal models have showed that eye growth and refractive maturation during infancy are tightly regulated by visually guided mechanisms. Observational data in human populations provide compelling evidence that environmental influences and individual behavioral factors play crucial roles in myopia susceptibility. Nevertheless, the majority of the variance of refractive error within populations is thought to be because of hereditary factors. Genetic linkage studies have mapped two dozen loci, while association studies have implicated more than 25 different genes in refractive variation. Many of these genes are involved in common biological pathways known to mediate extracellular matrix (ECM) composition and regulate connective tissue remodeling. Other associated genomic regions suggest novel mechanisms in the etiology of human myopia, such as mitochondrial-mediated cell death or photoreceptor-mediated visual signal transmission. Taken together, observational and experimental studies have revealed the complex nature of human refractive variation, which likely involves variants in several genes and functional pathways. Multiway interactions between genes and/or environmental factors may also be important in determining individual risks of myopia, and may help explain the complex pattern of refractive error in human populations.

Ge-codoped fibers for mitigating stimulated Brillouin scattering in high power fiber amplifiers

Rare-earth doped fibers for high power fiber amplifiers normally have a small refractive index difference between the core and inner cladding, but have a larger core diameter than conventional telecom fibers. We take advantage of this feature as well as the difference between the optical and acoustic waveguides and proposed several fiber designs to mitigate Stimulated Brillouin scattering (SBS) to produce higher output powers. The numerical modeling shows that an increase in SBS threshold by 3–6dB can be achieved using Ge-codopant while maintaining diffraction limited beam quality. The key parameters that can affect the SBS performance are specified and optimal parameters for SBS suppression are obtained. The study also highlights the need to take into account the effects of minor refractive index profile variation in evaluating SBS performance.

Changes in objective refraction after pharmacological pupil dilation

Abstract Purpose To analyze the variation of the objective refraction induced by pharmacological pupillary dilation in a normal population. Methods The objective refraction obtained by an auto‐kerato‐refractometer was analyzed before and after maximum pupillary dilation obtained by 0.5% Tropicamide and 10% Phenylephrine. 200 phakic eyes of 100 patients aged from 15 to 76 years were prospectively included. The refractive data were expressed in a dioptric space by 3 Cartesian coordinates (X, Y, Z). Results After pupil dilation, the mean refraction ranges from ‐0.08(+0.11)×90.2° to +0.41(+0.10)×90.3° this equal to an astigmatic vectorial variation of (+0.03)×89.6°. The spherical equivalent increases significantly (hyperopization of +0.49D: +0.24D vs +0.74D p<0.001) contrary to the mean keratometry (43.48D vs 43.49 p=0.40) and the cylinder (0.64D vs 0.64D p=0.49) which are unchanged. Change in spherical equivalent is correlated with the age (r=‐0.34 p<0.001) but not with the change in keratometry (r=0.06 p=0.20) or the initial spherical equivalent (r=‐0.05 p=0.23). The mean change in axis between both exams is 15.4° for the ocular astigmatism and 12.2° for the corneal astigmatism. The modification in spherical equivalent of both eyes of a patient are not different (p=0.26) and are strongly correlated (r=0.85 p<0.001) contrary to the modification in axis and in cylinder. Conclusion Pharmacological pupil dilation modifies primarily the spherical equivalent without significant modification of keratometry and astigmatism. Finally, hyperopization is probably due to the cycloplegic effect of eye drops.

Atmospheric diurnal variations observed with GPS radio occultation soundings

Abstract. The diurnal variation, driven by solar forcing, is a fundamental mode in the Earth's weather and climate system. Radio occultation (RO) measurements from the six COSMIC satellites (Constellation Observing System for Meteorology, Ionosphere and Climate) provide nearly uniform global coverage with high vertical resolution, all-weather and diurnal sampling capability. This paper analyzes the diurnal variations of temperature and refractivity from three-year (2007–2009) COSMIC RO measurements in the troposphere and stratosphere between 30° S and 30° N. The RO observations reveal both propagating and trapped vertical structures of diurnal variations, including transition regions near the tropopause where data with high vertical resolution are critical. In the tropics the diurnal amplitude in refractivity shows the minimum around 14 km and increases to a local maximum around 32 km in the stratosphere. The upward propagating component of the migrating diurnal tides in the tropics is clearly captured by the GPS RO measurements, which show a downward progression in phase from stratopause to the upper troposphere with a vertical wavelength of about 25 km. At ~32 km the seasonal variation of the tidal amplitude maximizes at the opposite side of the equator relative to the solar forcing. The vertical structure of tidal amplitude shows strong seasonal variations and becomes asymmetric along the equator and tilted toward the summer hemisphere in the solstice months. Such asymmetry becomes less prominent in equinox months.

The Split-Step-Fourier and Finite-Element-Based Parabolic-Equation Propagation-Prediction Tools: Canonical Tests, Systematic Comparisons, and Calibration

Powerful propagation-prediction tools, based on the split-step Fourier-transform and the Finite-Element-Method (FEM) solutions of the parabolic equation (PE) are discussed. The parabolic equation represents one-way propagation, and is widely used in two-dimensional (20) groundwave propagation modeling. It takes the Earth's curvature, the atmospheric refractivity variations, non-flat terrain scattering, and the boundary losses into account. MA TLAB-based numerical split-step parabolic-equation and Finite-Element-Method parabolic-equation routines were developed. These were used in canonical tests and comparisons to illustrate that the parabolic equation accounts for all of these effects. Both tools were calibrated against an analytical exact solution.

Association of Matrix Metalloproteinase Gene Polymorphisms with Refractive Error in Amish and Ashkenazi Families

Matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) are involved in scleral extracellular matrix remodeling and have shown differential expression in experimental myopia. The genetic association of refractive error and polymorphisms in MMP and TIMP genes in Old Order Amish (AMISH) and Ashkenazi Jewish (ASHK) families was investigated. Individuals from 55 AMISH and 63 ASHK families participated in the study. Ascertainment was designed to enrich the families for myopia; the mean spherical equivalent (MSE) refractive error (SD) was -1.61 (2.72) D in the AMISH, and -3.56 (3.32) D in the ASHK. One hundred forty-six common haplotype tagging SNPs covering 14 MMP and 4 TIMP genes were genotyped in 358 AMISH and 535 ASHK participants. Association analyses of MSE and the spherical component of refraction (SPH) were performed separately for the AMISH and the ASHK. Bonferroni-corrected significance thresholds and local false discovery rates were used to account for multiple testing. After they were filtered for quality-control, 127 SNPs were included in the analyses. No polymorphisms showed statistically significant association to refraction in the ASHK (minimum P = 0.0132). In AMISH, two SNPs showed evidence of association with refractive phenotypes: rs1939008 (P = 0.00016 for SPH); and rs9928731 (P = 0.00026 for SPH). These markers were each estimated to explain <5% of the variance of SPH in the AMISH sample. Statistically significant genetic associations of ocular refraction to polymorphisms near MMP1 and within MMP2 were identified in the AMISH but not among the ASHK families. The results suggest that the MMP1 and MMP2 genes are involved in refractive variation in the AMISH. Genetic and/or environmental heterogeneity most likely contribute to differences in association results between ethnic groups.

Ten years analysis of Tropospheric refractivity variations

The refractivity variations of the troposphere are responsible for various effects on radio wave propagation, such as refraction, bending, radio-station interference, etc. In this work, the refractivity variations of the Hellenic troposphere are studied using data from Helliniko airport of Athens/Greece. The data were analyzed using various interpolation procedures, i.e. in a day-by-day manner for temperature and relative humidity, to transform the data according to a reference height common for the whole dataset and finally for the refractivity N with respect to time, using piece-wise hermite interpolation polynomials for the low and medium altitudes and linear interpolation factors for the high altitudes. Since refractivity varies with time and height, two height independent basic parameters were computed and analyzed: the refractivity at station height N0 and scale height Hs. These parameters can be used to calculate the refractivity profile. Using statistical tools as the moving average, the s0 parameter and the monthly mean values, together with the corresponding standard deviations, useful results were obtained for the variations of the refractivity with respect to observation hour, height, season, month, day and level.

A Modified Technique for Rectus Muscle Plication in Minimally Invasive Strabismus Surgery

Purpose: To present a novel, minimally invasive strabismus surgery (MISS) technique for faster and less invasive rectus muscle plication. Methods: The new plication method is performed without spatula and, therefore, instead of a total dissection (TODI) only a marginal dissection (MADI) of the plicated part of the muscle is performed. Forty patients operated with combined, horizontal MISS rectus muscle recession/MADI plication are retrospectively compared to a similar group of 49 patients, who had MISS recession/TODI plication. The following outcome parameters were analyzed: alignment, binocular single vision, variations in vision, refraction, conjunctival abnormalities, dose-response relationship, and number and types of complications during the first 6 postoperative months. Results: Postoperatively, the conjunctiva over the plicated muscle was less swollen and red with MADI compared to TODI plication (p < 0.01). In esodeviations, there was no significant difference in the dose-response relationship between MADI and TODI plication (1.38 ± 0.44 vs. 1.44 ± 0.50°/mm, p > 0.1). In exodeviations, the difference between MADI and TODI plications was not significant either (1.41 ± 0.38 vs. 1.42 ± 0.50°/mm, p > 0.1). Also final alignment, binocular single vision, visual acuities, refractive changes, or complications did not differ between the two techniques. Conclusions: This study demonstrates that MADI plication is feasible and equally effective in comparison with the TODI plication technique when performing MISS. MADI plication, which is performed without spatula, allows to considerably further reduce postoperative conjunctival swelling and redness.

Eikonal acceleration technique for studying of the Earth and planetary atmospheres by radio occultation method

[1] Lately introduced a phase (eikonal) acceleration technique extends an applicable domain of radio occultation (RO) method and can be applied for identification of the plasma layers in the lower ionosphere. This technique can also convert the eikonal excess or Doppler frequency changes measured in RO experiments into the refractive attenuation variations. From these derived refractive attenuation and amplitude data one can estimate the integral absorption of radio waves. This is important for study of the radio wave propagation effects in the atmospheric telecommunication links and for remote sensing of the atmosphere and ionosphere. The advantages of the eikonal acceleration technique are validated by analyzing the RO data from the Challenging Minisatellite Payload (CHAMP) and Venus missions.

Thermal sensitivity of photonic crystal fibers in opto-electronic oscillators

Stable opto-electronic oscillators (OEOs) are realized using long fiber delay lines and changes in the index of refraction of high quality factor delay line results in temperature sensitivity of OEOs. Temperature sensitivity of various OEOs is measured to compare index of refraction variation of standard (SMF-28) and photonic crystal fiber (PCF). Both hollow-core (HC) and solid-core (SC) versions of PCF are quantified. SC-PCF exhibited a factor of three reductions in the rate of index of refraction change (about +4.7 ppm/°C) with temperature over SMF-28 (about 12 ppm/°C) based OEO. Although HC-PCF have a greater attenuation per unit length, but those fibers have demonstrated a negative rate of change (about − 0.6 ppm/°C) in the effective index of refraction with temperature and prospect of thermal stability in the OEO using passive techniques is great when a combination of HC-PCF and SMF-28 are employed as fiber delay lines.

Phase acceleration: a new important parameter in GPS occultation technology

Based on 40 years of radio-occultation (RO) experiments, it is now recognized that the phase acceleration of radio waves (equal to the time derivative of the Doppler shift), derived from analysis of high-stability Global Positioning System (GPS) RO signals, is as important as the Doppler frequency. The phase acceleration technique allows one to convert the phase and Doppler frequency changes into refractive attenuation variations. From such derived refractive attenuation and amplitude data, one can estimate the integral absorption of radio waves. This is important for future RO missions when measuring water vapor and minor atmospheric gas constituents, because the difficulty of removing the refractive attenuation effect from the amplitude data can be avoided. The phase acceleration technique can be applied also for determining the location and inclination of sharp layered plasma structures (including sporadic E s layers) in the ionosphere. The advantages of the phase acceleration technique are validated by analyzing RO data from the Challenging Minisatellite Payload (CHAMP) and the FORMOSA Satellite Constellation Observing Systems for Meteorology, Ionosphere, and Climate missions (FORMOSAT-3/COSMIC).

Monochromatic aberrations of the human eye and myopia

Background: The relationship between monochromatic aberrations of the human eye and myopia has been investigated extensively over the past decade or so but the results are inconclusive. Wide age range, narrow refractive error range and large individual variations are common confounding factors in previous studies. To address these shortcomings, we conducted a two‐part study. The first part set out to determine whether the monochromatic aberrations in myopic eyes are different from those in the non‐myopic eyes in adults. Subjects were drawn from a narrow age band to minimise the confounding effect of age. The second part of this study compared the monochromatic aberrations of the more myopic and less myopic eyes of anisometropes.Methods: Monochromatic aberrations were measured for 5-mm pupils using the Complete Ophthalmic Analysis System (COAS) Wavefront Analyzer in 116 subjects with refractive spherical equivalent between ‐10.38-D and +1.38-D and in 26 anisometropes. Measurements were done in a dark room with natural pupils. The refractive errors, corneal curvatures and axial lengths of the eyes were measured under natural accommodation.Results: Highly myopic eyes had significantly smaller root mean square (RMS) values of fourth order (p = 0.015) and spherical aberrations (p = 0.009) than non‐myopic eyes. The correlation coefficient was 0.2354 (p = 0.011) between fourth‐order aberrations and refractive spherical equivalents and 0.2817 (p = 0.002) between spherical aberration and refractive spherical equivalents. Less myopic eyes of the anisometropes showed significantly larger total higher‐order, third‐order and spherical aberrations than the more myopic eyes (p < 0.05, p < 0.05 and p < 0.01, respectively).Conclusions: Our study shows that spherical aberration is associated with refractive error. More myopic eyes tended to have smaller amounts of spherical aberration, however, the ‘cause or effect’ question remains. Longitudinal studies are needed to further investigate the relationship between monochromatic aberrations and refractive development.

Field optimization and CCD data simulation for the antarctic International Concordia Explorer Telescope (ICE‐T)

AbstractWe performed extensive data simulations for the planned ultra‐wide‐field, high‐precision photometric telescope ICE‐T (International Concordia Explorer Telescope). ICE‐T consists of two 60 cm‐aperture Schmidt telescopes with a joint field of view simultaneously in two photometric bandpasses. Two CCD cameras, each with a single 10.3k × 10.3k thinned back‐illuminated device, would image a sky field of 65 square degrees. Given a location of the telescope at Dome C on the East Antarctic Plateau, we searched for the star fields that best exploit the technical capabilities of the instrument and the site. We considered the effects of diurnal air mass and refraction variations, solar and lunar interference, interstellar absorption, overexposing of bright stars and ghosts, crowding by background stars, and the ratio of dwarf to giant stars in the field. Using NOMAD, SSA, Tycho‐2 and 2MASS‐based stellar positions and BVIJH magnitudes for these fields, we simulated the effects of the telescope's point‐spread‐function, the integration, and the co‐addition times. Simulations of transit light curves are presented for the selected star fields and convolved with the expected instrumental characteristics. For the brightest stars, we showed that ICE‐T should be capable of detecting a 2 REarth Super Earth around a G2 solar‐type star, as well as an Earth around an M0‐star – if these targets were as abundant as hot Jupiters. Simultaneously, the telescope would monitor the host star's surface activity in an astrophysically interpretable manner (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Variation of radio refractivity with respect to moisture and temperature and influence on radar ray path

In this study, the variation of radio refractivity with respect to temperature and moisture is analyzed. Also, the effects of vertical gradients in temperature and moisture on the propagation paths of electromagnetic waves of weather radar are examined for several sites across the United States using several years of sounding data from the National Weather Service. The ray path is important for identifying storm characteristics and for properly using the radar data in initializing numerical weather prediction models. It is found that during the warm season the radio refractivity gradient is more sensitive to moisture gradients than to temperature gradients. Ray paths from the commonly accepted vertical ray path model are compared to a ray path computed from a stepwise ray tracing algorithm using data from actual soundings. For the sample of about 16 000 soundings examined, we find that only a small fraction of the ray paths diverge significantly from those calculated using a ray path model based on the US Standard Atmosphere. While the problem of ray ducting in the presence of a temperature inversion is fairly well known, we identify the presence of a strong vertical moisture gradient as the culprit in the majority of the cases where significant deviations occurred.

Profiling the Atmosphere Using the Airborne GPS Radio Occultation Technique: A Sensitivity Study

Global Positioning System (GPS) radio occultation (RO) sounding, with its high vertical resolution temperature and humidity profiling capability, is revolutionizing atmospheric science, particularly through assimilation in numerical weather prediction (NWP) models. Currently, the observations are derived from GPS receivers onboard low Earth orbiting satellites. However, with the current number of satellites, it is difficult to provide dense sounding measurements in a specific region within a limited time period. With a GPS receiver onboard an airplane, the GPS RO technique offers such an opportunity while retaining the high vertical resolution sounding capability. The GNSS Instrument System for Multistatic and Occultation Sensing is currently under development for the National Science Foundation's High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) aircraft. This paper presents a sensitivity analysis of the airborne occultation technique that will be used for the HIAPER system. The results demonstrate an anticipated overall accuracy of better than 0.5% for the retrieved refractivity from the surface to about 1 km below the airplane, where the expected airplane velocity errors of up to 5 mm/s limit the accuracy. The effects on the retrievals due to horizontal variations in atmospheric refractivity are significant, and retrieval errors may reach several percent inside frontal systems when the front is perpendicular to the ray paths and within 200 km of the tangent point. In general, the airborne GPS RO system provides a promising new data source for NWP and targeted observational studies.