Refraction Patterns Research Articles

Modeling of reflected ultrasonic fields in composed samples

Ultrasonic nondestructive testing involves the study of propagation, reflection and refraction patterns of elastic waves excited by contact or non-contact piezoelectric transducers in the inspected object. The finite element modeling usually requires high computational costs and additional postprocessing to select individual waves from the total solution. When probing joints of homogeneous materials, such as turbine blades made of heat-resistant monocrystalline alloys, the joint boundary is low-contrast, and the reflected signals are relatively weak. This causes additional difficulties for their separation from the total wave field and correct interpretation of the information they bring. To solve this problem, explicit asymptotic representations for reflected and transmitted waves in a two-layer elastic half-space with a surface source are proposed in the present work, which allow fast parametric analysis. They can be used to analyze ultrasonic probing data, for example, to estimate the state of the junction zone or to determine the mutual orientation of the crystals’ principal axes.

Pseudosteady shock refractions over an air–water interface

Shock refraction in a gas–liquid interface is ubiquitous in nature and engineering. This study investigates the shock refraction phenomena in air–water interfaces for various inclination angles. The interface inclination angles are achieved using a tiltable vertical shock tube. The time-resolved schlieren images are compared with numerical simulations performed using the BlastFoam solver in the OpenFOAM software. The stiffened gas equation of state is used to model water in the simulations. The shock polar analysis using modified shock relations for a stiffened gas is used to elucidate the refraction patterns. A regular refraction pattern with a reflected shock wave and a bound precursor refraction with a regular reflection are observed experimentally for the first time in an air–water system. Further, a new free precursor refraction pattern with a Mach reflection is observed. The transition criteria and the corresponding boundaries for each refraction pattern are demarcated in the ( $M_S, \theta _w^c$ )-plane. The refraction sequence and the range for various incident shock strength regimes are also identified.

Peripheral Refraction Using Ancillary Retinoscope Component (P-ARC).

To assess the agreement of retinoscope-based peripheral refraction techniques with the criterion standard open-field autorefractor. Fifty young adults (mean age, 24 ± 3years) participated in this study. Two masked, experienced senior examiners carried out central refraction and peripheral refraction at the temporal 22° (T22°) and nasal 22° (N22°) eccentricities. Peripheral refraction techniques were (a) peripheral refraction using ancillary retinoscope component (P-ARC), (b) retinoscopy with eye rotation, and (c) open-field autorefractor. Peripheral refraction with retinoscopy values was compared with an open-field autorefractor (Shinn Nippon NVision-K) to assess the agreement. All measurements were taken from the right eye under noncycloplegic conditions. The mean difference ±95% limits of agreement of peripheral refraction values obtained using P-ARC from T22° (+0.11 diopters [D] ± 1.20 D; P=0.20) or N22° (+0.13 D ± 1.16 D; P = 0.13) were comparable with open-field autorefractor. The eye rotation technique compared to autorefractor showed a significant difference for T22° (+0.30 D ± 1.26 D; P=0.002); however, there was an agreement for N22° (+0.14 D ± 1.16 D; P=0.10). With respect to the identification of peripheral refraction patterns, examiners were able to identify relative peripheral hyperopia in most of the participants (77%). Peripheral refraction with P-ARC was comparable with open-field autorefractor at T22° and N22° eccentricities. Peripheral retinoscopy techniques can be another approache for estimating and identifying peripheral refraction and its patterns in a regular clinical setting. Retinoscope with P-ARC has high potential to guide and enable eye care practitioners to perform peripheral refraction and identify peripheral refraction patterns for effective myopia management.

A Forward Model and Inversion Algorithm for Near-Surface Soil Moisture Estimation With GNSS Refraction Pattern Technique

A Forward Model and Inversion Algorithm for Near-Surface Soil Moisture Estimation With GNSS Refraction Pattern Technique

Modelling changes in the prevalence of childhood myopia.

To re-examine the changes with time in the underlying patterns of individual refraction at different ages, which have led to an increased prevalence of myopia in a population of Asian children. Using published cross-sectional longitudinal data, the frequency distributions of spherical equivalent refractive error (SE) in yearly cohorts of 6- and 12-year-old Japanese children during the period 1984-1996 were modelled in terms of ex- and bi-Gaussian distributions. Both models suggested that over the period of the study, little change occurred in the SE frequency distributions for 6-year-olds, with most children having SEs near emmetropia. In contrast, in each annual cohort of 12-year-olds, although the SE of some children remained near-emmetropic, a sub-set failed to maintain emmetropia. Most of this group became more myopic between 6 and 12 years of age. The proportion of children showing myopic progression increased over the period of study. The observed increase in mean levels of myopia in older Japanese children in the late 20th century is due to a greater proportion of children failing to maintain emmetropisation between the ages of 6 and 12, rather than to myopic shifts in all children. Some children, with small SE changes between 6 and 12 years of age, would not have benefitted from any treatment intended to slow myopia progression.

The Relationship Between Feeding Pattern and Measurements of Refraction in Preschool Children

Objective: To compare the effect of feeding human milk as opposed to formula and human milk on refractive error in preschool children aged 3 to 6 years. Material and Methods: One hundred and seventy six eyes of 88 children were evaluated. According to the feeding pattern, the patients who fed only breast-fed were classified as Group I, and the patients who fed with combination of breast-fed and formula were classified as Group II. Results were retrospectively compared between groups. Results: A total of 88 children undergone detailed ophthalmological examinations, of which 40 (45.5 %) were males and 48 (54.5 %) were females. Maternal birth age of 27.3% (n=24) of the children included in the study was over 30 years and birth weight of 96.6% (n=85) was over 2500 grams. The mean ± standard deviation of the spherical, cylindrical and spherical equivalent (SE) values were 1.61±1.46 D; -0.98 ± 0.69 D and 1.12±1.43 D in group I and 1.00±2.57 D; -0.90 ± 0.81 D and 0.55 ± 2.72 D in group II, respectively (p=0.507 for spherical; p=0.299 for cylindrical and p=0.799 for SE). Although myopic refraction was less common in the breastfed group, there was no significant difference in myopic refraction between the groups (p= 0.507). Conclusion: In conclusion, mean refractive error were different according to feeding pattern, but this study did not show significant differences between groups (p<0.050). There is a need to confirm this finding by performing more studies with a larger sample sizes.

Technical notes on peripheral refraction, peripheral eye length and retinal shape determination.

To give an overview of the misconceptions and potential artefacts associated with measuring peripheral refractive error and eye length, the use of these measures to determine the retinal shape and their links to myopia development. Several issues were identified: the relationship between peripheral refractive error and myopia development, inferring the retinal shape from peripheral refraction or eye length patterns, artefacts and accuracy when measuring peripheral eye length using an optical biometer. A theory was developed to investigate the influence of artefacts in measuring peripheral eye length and on using peripheral eye length to make inferences about retinal shape. When determining peripheral axial length, disregarding the need to realign instruments with mounted targets can lead to incorrect field angles and positions of mounted targets by more than 10% for targets placed close to the eye. Peripheral eye length is not a good indicator of the effects of myopia or of treatment for myopia development because eyes of different lengths but with the same retinal shape would be interpreted as having different retinal shapes; the measurement leads to overestimates of changes in retinal curvature as myopia increases. Determining peripheral eye length as a function of estimated retinal height rather than field angle will halve the magnitude of the artefact. The artefact resulting from the peripheral use of biometers with an on-axis calibration is modest and can be ignored. There are significant issues with peripheral measurements of the refractive error and eye length that must be considered when interpreting these data for myopia research. Some of these issues can be mitigated, while others require further investigation.

Effects of the parameters of inner air cylinder on evolution of annular SF6 cylinder accelerated by a planar shock wave

Based on the compressible Navier–Stokes equations combined with the fifth-order weighted essentially non-oscillatory scheme, this paper discusses the interaction of a planar shock wave with an annular SF6 cylinder. The influence of the position and radius of inner cylinder on the evolution of the annular cylinder is examined in detail. Numerical schlieren results clearly show the evolution of the inner and outer interfaces induced by the Richtmyer–Meshkov instability and reveal the evolution of complex shock wave structures as the incident planar shock interacts with the annular cylinder. Shock transformation from the free precursor refraction pattern to the free precursor von Neumann refraction pattern occurs when the inner cylinder position shifts forward, while the shock transformation from the twin von Neumann refraction pattern to the free precursor refraction pattern and the shock transformation from free precursor refraction pattern to the free precursor von Neumann refraction pattern occur when the radius of the inner cylinder gradually becomes larger. The generation and transportation of vorticity on the interfaces are also analyzed, revealing that changes to the inner cylinder play a significant role. The distribution and evolution of vorticity on the interfaces influence the formation of the primary vortex structure at later stages. Quantitative analysis of the circulation and enstrophy indicates that the smaller the inner radius, the larger the value of circulation and enstrophy at the later stage.

On-chip valley phononic crystal plates with graded topological interface

In this work, we construct on-chip valley phononic crystal plates by building up triangular silicon pillars on the silicon plate. We introduce the graded interface sliding for two typical interfaces ψBA and ψAB, by modulating the horizontal lattice constant in a selected region in PC plate. Based on simulation and laser ultrasonic technique, we present the frequency range evolution of topological edge states from the gapless feature to gapped one. We demonstrate the displacement distribution of topological edge states in two graded interfaces, magnifying numerically the topological rainbow phenomena. In particular, we characterize the plate wave propagation through both types of graded interfaces, observe the abnormal refraction pattern behind the truncated interfaces, and underline abnormal refraction to the splitting of wave vector in PC plate. Our graded interfaces provide new clue to control elastic wave propagation based on valley degree of freedom.

Dynamics of laser-induced tunable focusing in silicon

We report here on focusing of a probe IR (λ = 1.55 μm) laser beam in silicon. The focusing is done by a second pump laser beam, at λ = 0.775 μm and 30 ps pulse width, with a donut shape that is launched collinearly and simultaneously (with some delay time) with the IR beam pulse. The pump beam pulse is absorbed in the silicon and creates, temporally, a free charge carriers (FCCs) donut pattern in the silicon. Following the plasma dispersion effect, the donut FCCs shapes a complex index of refraction pattern in the silicon that serves as a sort of dynamic GRIN lens for the probe beam due to the diffusion of the FCCs towards the donut center. This lens can be tuned to its focal point by the pump-probe delay time to reduce the point spread function (PSF) of the IR probe beam. We start seeing the focusing of the probe beam at pump-probe delay time of mathrm{Delta t }approx 100mathrm{ ps}. The best focusing (results in PSF < 2mathrm{ mu m}) was observed at mathrm{Delta t}approx 350mathrm{ ps} and it slowly degrades before the FCCs full recombination at mathrm{Delta t }sim 12mathrm{ ns}. We propose this beam shaping method to overcome the diffraction resolution limit in silicon microscopy on and deep under the silicon surface dependent on the pump wavelength and the pulse width. We also proposed this technique for direct measurement of the FCCs dynamics.

Magnetohydrodynamic shock refraction at an inclined density interface

Shock wave refraction at a sharp density interface is a classical problem in hydrodynamics. Presently, we investigate the strongly planar refraction of a magnetohydrodynamic (MHD) shock wave at an inclined density interface. A magnetic field is applied that is initially oriented either perpendicular or parallel to the motion of incident shock. We explore flow structure by varying the magnitude of the magnetic field governed by the non-dimensional parameter β∈(0.5,106) and the inclination angle of density interface α∈(0.30,1.52). The regular MHD shock refraction process results in a pair of outer fast shocks (reflected and transmitted) and a set of inner nonlinear magneto-sonic waves. By varying magnetic field (strength and direction) and inclination interface angle, the latter waves can be slow shocks, slow expansion fans, intermediate shocks, or slow-mode compound waves. For a chosen incident shock strength and density ratio, the MHD shock refraction transitions from regular (all nonlinear waves meeting at a single point) into irregular when the inclined density interface angle is less than a critical value. Irregular refraction patterns are not amenable to an analytical solution, and hence, we have obtained irregular refraction solutions by numerical simulations. Since the MHD shock refraction is self-similar, we further explore by converting the initial value problem into a boundary value problem (BVP) by a self-similar coordinate transformation. The self-similar solution to the BVP is numerically solved using an iterative method and implemented using the p4est adaptive mesh framework. The simulation shows that a Mach stem occurs in an irregular MHD shock refraction, and the flow structure can be an MHD equivalent to a single Mach reflection irregular refraction and convex-forwards irregular refraction that occur in hydrodynamic case. For Mach number M = 2, both analytical and numerical results show that perpendicular magnetics fields suppress the regular to irregular transition compared to the corresponding hydrodynamic case. As Mach number decreased, it is possible that strong perpendicular magnetics promote the regular to irregular transition, while moderate perpendicular magnetics suppress this transition compared to the corresponding hydrodynamic case.

Practising refraction in ophthalmology: instructive or outdated? A prospective study and literature review

ABSTRACTClinical RelevanceMany ophthalmologists preform clinical refactions, although little is known of the perceptions and practise of refraction by ophthalmologists and key barriers preventing this aspect of ophthalmic practice.BackgroundAlthough there are numerous studies on visual acuity in ophthalmology, there is no study to date on the practice of refraction by ophthalmologists. This study evaluates the practice patterns of ophthalmologists in current practice. It specifically addresses perceptions of ophthalmologists about (a) the importance of refraction in clinical practice, and (b) barriers to performing refraction. The methodology and frequency of performing refraction by ophthalmologists is also assessed.MethodsThis cross-sectional study was conducted at the Annual Scientific Congress of the Royal Australian and New Zealand College of ophthalmologists in 2017, held in Perth, Australia. All attending ophthalmologists and ophthalmology trainees were invited to participate. Participants completed a 17-variable questionnaire on the perceptions of practitioners about refraction and their practice of it. Data were analysed using Microsoft Excel and IBM SPSS Version 24.ResultsAt this Congress, 213 attendees completed the survey, with most being consultant general ophthalmologists (85%). Twenty-six percent of participants either ‘really loved’ or ‘liked’ refracting patients. Those who reported feeling competent with refraction were more likely to perform it themselves (p = 0.001). Individuals most commonly reported taking 3-5 minutes to refract a patient (38%). Participants under the age of 65, and participants practising paediatric ophthalmology, were more likely to perform a refraction.ConclusionsThe literature indicates that this is the first study to describe the practice patterns of refraction by ophthalmologists. Although ophthalmologists found refraction important, the majority preferred patients to be refracted by others. Key barriers to ophthalmologists performing refraction included the time required to perform the refraction, a busy clinic, and the availability of alternative providers.

Refractive geometry for underwater domes

Underwater cameras are typically placed behind glass windows to protect them from the water. Spherical glass, a dome port, is well suited for high water pressures at great depth, allows for a large field of view, and avoids refraction if a pinhole camera is positioned exactly at the sphere’s center. Adjusting a real lens perfectly to the dome center is a challenging task, both in terms of how to actually guide the centering process (e.g. visual servoing) and how to measure the alignment quality, but also, how to mechanically perform the alignment. Consequently, such systems are prone to being decentered by some offset, leading to challenging refraction patterns at the sphere that invalidate the pinhole camera model. We show that the overall camera system becomes an axial camera, even for thick domes as used for deep sea exploration and provide a non-iterative way to compute the center of refraction without requiring knowledge of exact air, glass or water properties. We also analyze the refractive geometry at the sphere, looking at effects such as forward- vs. backward decentering, iso-refraction curves and obtain a 6th-degree polynomial equation for forward projection of 3D points in thin domes. We then propose a pure underwater calibration procedure to estimate the decentering from multiple images. This estimate can either be used during adjustment to guide the mechanical position of the lens, or can be considered in photogrammetric underwater applications.

Peripheral refraction of myopic eyes with spectacle lenses correction and lens free emmetropes during accommodation

PurposeTo measure axial and off-axis refraction patterns in myopic eyes with spectacle lenses correction and lens free emmetropes in young healthy subjects at different target distances from 2.00 m (0.50 D) to 0.20 m (5.00 D) in terms of sphere, astigmatism, and spherical equivalent refraction.MethodsRefraction was measured at the center, 20 and 40 degrees from the line of sight both nasally and temporally in 15 emmetropic and 25 myopic young healthy subjects with an open field, binocular, infrared autorefractor (Grand Seiko WAM-5500, Hiroshima, Japan). Fixation target was a Maltese cross set at 2.00, 0.50, 0.33 and 0.20 m from the corneal plane. Changes in off-axis refraction with accommodation level were normalized with respect to distance axial values and compared between myopic eyes with spectacle lenses correction and lens free emmetropes.ResultsOff-axis refraction in myopic eyes with spectacle lenses correction was significantly more myopic in the temporal retina compared to lens free emmetropes except for the closest target distance. Relative off-axis refractive error changed significantly with accommodation when compared to axial refraction particularly in the myopic group. This change in the negative direction was due to changes in the spherical component of refraction that became more myopic relative to the center at the 0.20 m distance as the J0 component of astigmatism was significantly reduced in both emmetropes and myopes for the closest target.ConclusionAccommodation to very near targets (up to 0.20 m) makes the off-axis refraction of myopes wearing their spectacle correction similar to that of lens free emmetropes. A significant reduction in off-axis astigmatism was also observed for the 0.20 m distance.

Shock wave refraction patterns at a slow–fast gas–gas interface at superknock relevant conditions

Shock wave refraction theory and high-resolution numerical simulations were employed to predict the refraction pattern under superknock relevant conditions at slow–fast gas–gas interfaces which are characterized by a higher acoustic impedance in the incident phase than in the transmitted phase. First, our theoretical and computational methodologies were validated against results from the literature for planar shock–straight oblique interface interactions. Second, our framework was applied to planar shock-/cylindrical shock–cylindrical interface interactions. The theoretical regime diagram agrees well with the numerical predictions for the former configuration whereas significant discrepancies were observed for the latter. Numerical results show the formation of temperature and pressure peaks as the refraction structure transits from a free precursor refraction to a twin von Neumann refraction. This change in thermodynamic state can induce a significant reduction in ignition delay time, potentially leading to detonation onset.

Refraction of Oblique Shock Wave on a Tangential Discontinuity

The refraction of an oblique shock wave on a tangential discontinuity dividing two gas flows with different properties is considered. It is shown that its partial reflection occurs with the exception of the geometrical diffraction of an oblique shock. Another oblique shock, expansion wave or weak discontinuity that coincides with the Mach line can act as a reflected disturbance. This study focuses on the relationships that define the type of reflected discontinuity and its parameters. The domains of shock wave configurations with various types of reflected discontinuities, including characteristic refraction and refraction patterns with a reflected shock and a reflected rarefaction wave, are analyzed. The domains of existence of various shock wave structures with two types of reflected disturbance, and the boundaries between them, are defined. The domains of parameters with one or two solutions exist for the characteristic refraction. Each domain is mapped by the type of refraction with regard to the Mach number, the ratio of the specific heat capacities of the two flows and the intensity of a refracted oblique shock wave. The conditions of the regular refraction and the Mach refraction are formulated, and the boundaries between the two refraction types are defined for various types of gases. Refraction phenomena in various engineering problems (hydrocarbon gaseous fuel and its combustion products, diatomic gas, fuel mixture of oxygen and hydrogen, etc.) are discussed. The result can be applied to the modeling of the shock wave processes that occur in supersonic intakes and in rotating and stationary detonation engines. The solutions derived can be used by other researchers to check the quality of numerical methods and the correctness of experimental results.

Shallow lacustrine versus open ocean coastal clastic deposits: Morphosedimentary diagnostic indicators and interpretation

Lacustrine coasts, especially those associated with shallow highly fluctuating lakes, do not evolve in exactly the same way as marine coasts. Climate variability can significantly modify the morphology of the water body, affecting the lake surface area, the available fetch, and the water depth, resulting in possible changes to wave generation and propagation. To look more closely at the nature of coastal sediments generated under such specific environments, we selected examples of both shallow lacustrine and open ocean beach deposits. The first location was Gallocanta Lake, a shallow body of water in the Iberian Chain in central Spain; and the second was Mazagón Beach, in the Gulf of Cádiz, a mesotidal sandy beach exposed to Atlantic waves and tides. Both deposits are formed by beach and ridge and runnel systems. The sedimentary structures of the two deposits were compared, together with a genetic scheme for these. In both cases, applying wave propagation models facilitated the interpretation of the sedimentary processes responsible for their genesis. Although the general morphosedimentary characteristics of the two deposits are roughly similar, a more detailed analysis revealed significant differences, some of which could be used as diagnostic indicators of shallow lacustrine environments in geoarchives. Effective fetch seemed not to be a determining factor for explaining the textural differences between the two deposits. Local factors such as wind strength and direction, and approaching wave refraction patterns are important in controlling the final energy reaching both types of coasts, and, hence, the resulting geometry of the clastic coastal deposits. The type and maturity of the available sediment also influence the final textural properties of the deposits. Open ocean beaches receive sporadic high energetic storms; these partially erode the sedimentary record and reduce the preservation potential of coastal deposits compared with those from shallow lake beaches. Comparing lacustrine deposits with marine analogues and identifying similarities and differences between the two coastal environments may help to infer the main factors underlying the formation and development of coastal deposits in non-tidal, fetch-limited environments.

How to Calculate Crystallite Size from X-Ray Diffraction (XRD) using Scherrer Method

X-Ray Diffraction (XRD) has been developed to analyze a crystal structure in the material. XRD is then improved for determining and identifying the presence of a compound, crystallinity, and crystal properties. XRD works by observing the ray refraction pattern as a result of the beam being refracted by a material that has an atomic arrangement in its crystal lattice. Until now, there is less information regarding detailed information on how to read the XRD spectrum and peak. This study aims to demonstrate how to calculate and interpret the crystallite size based on the XRD spectra. The step-by-step method in the calculation based on the XRD data is presented, which can help especially students and first-time users in understanding XRD results. This study is prospective to be used as standard information on how to read and interpret XRD spectra.

Abnormal topological refraction into free medium at subwavelength scale in valley phononic crystal plates

In this work we propose a topological valley phononic crystal plate and we extensively investigate the refraction of valley modes into the surrounding homogeneous medium. This phononic crystal includes two sublattices of resonators (A and B) modeled by mass-spring systems. We show that two edge states confined at the AB/BA and BA/AB type domain walls exhibit different symmetries in physical space and energy peaks in the Fourier space. As a result, distinct refraction behaviors, especially through an armchair cut edge, are observed. On the other hand, the decay depth of these localized topological modes, which is found to be solely determined by the relative resonant strength between the scatterers, significantly affects the refraction patterns. More interestingly, the outgoing traveling wave through a zigzag interface becomes evanescent when operating at deep sub-wavelength scale. This is realized by tuning the average resonant strength. We show that the evanescent modes only exist along a particular type of outlet edge, and that they can couple with both topological interface states. We also present two designs of topological functional devices, including an elastic one-way transmission waveguide and a near-ideal monopole/dipole emitter, both based on our phononic structure.

Global implications of surface current modulation of the wind-wave field

The influence of ocean surface currents on the global wind-wave field is revisited. State-of-the-art numerical spectral wave model simulations with and without surface currents taken from an eddy resolving global ocean reanalysis were compared. As a global average, simulations forced with currents display significantly better agreement with altimeter derived wave heights. The bias and root mean square error in significant wave heights are mostly reduced when including current forcing, especially in the Southern Ocean. An overall improvement in wave periods and wave direction is also seen when comparing model outputs with the Australian and United States buoy network observations. Including surface ocean current forcing in wave simulations reduces the simulated wave heights in most areas of the world, due to a decreased relative wind given by co-flowing winds and currents. Current-induced refraction generates important changes in wave direction in western boundary current and tropical regions. Furthermore, large and broad changes in friction velocity, atmosphere-to-ocean energy flux, whitecap cover and Stokes drift velocities are observed in equatorial regions. Finally, the importance of the wave model resolution for representing wave–current interactions was tested by comparing results from eddy-permitting (lower resolution) and eddy-resolving (higher resolution) configurations. We conclude that the main patterns of current-induced refraction are well represented in both cases, albeit that the higher resolution simulation represents these in a more detailed manner. Finally, the implications that the observed wave–current interactions have on several ocean processes are discussed.