Wavefront Angle Research Articles

Real-time monitoring of adaptive lenses with high tuning range and multiple degrees of freedom

Adaptive lenses offer fast and flexible scans without mechanical movement. However, driving the lens to achieve the desired behavior is challenging and requires online monitoring. The demand for monitoring techniques can be high when lenses with large tuning ranges or multiple degrees of freedom are employed. We analyze the performance of a partitioned aperture wavefront (PAW) as a tool for in situ and in-process monitoring of adaptive lenses. PAW has the advantage of enabling measurements of large input wavefront angles and thus high-tuning-range lenses with just a single beam path. PAW is used to characterize an adaptive lens with a high tuning range of ± 20 d p t and for controlling the behavior of a novel adaptive lens with two actuators that allow for the tuning of focal length and spherical aberrations simultaneously.

Differential effect with septal and apical RV pacing on ventricular activation in patients with left bundle branch block assessed by non-invasive electrical imaging and in silico modelling

PurposeIt is uncertain whether right ventricular (RV) lead position in cardiac resynchronization therapy impacts response. There has been little detailed analysis of the activation patterns in RV septal pacing (RVSP), especially in the CRT population. We compare left bundle branch block (LBBB) activation patterns with RV pacing (RVP) within the same patients with further comparison between RV apical pacing (RVAP) and RVSP.MethodsBody surface mapping was undertaken in 14 LBBB patients after CRT implantation. Nine patients had RVAP, 5 patients had RVSP. Activation parameters included left ventricular total activation time (LVtat), biventricular total activation time (VVtat), interventricular electrical synchronicity (VVsync), and dispersion of left ventricular activation times (LVdisp). The direction of activation wave front was also compared in each patient (wave front angle (WFA)). In silico computer modelling was applied to assess the effect of RVAP and RVSP in order to validate the clinical results.ResultsPatients were aged 64.6 ± 12.2 years, 12 were male, 8 were ischemic. Baseline QRS durations were 157 ± 18 ms. There was no difference in VVtat between RVP and LBBB but a longer LVtat in RVP (102.8 ± 19.6 vs. 87.4 ± 21.1 ms, p = 0.046). VVsync was significantly greater in LBBB (45.1 ± 20.2 vs. 35.9 ± 17.1 ms, p = 0.01) but LVdisp was greater in RVP (33.4 ± 5.9 vs. 27.6 ± 6.9 ms, p = 0.025). WFA did rotate clockwise with RVP vs. LBBB (82.5 ± 25.2 vs. 62.1 ± 31.7 op = 0.026). None of the measurements were different to LBBB with RVSP; however, the differences were preserved with RVAP for VVsync, LVdisp, and WFA. In silico modelling corroborated these results.ConclusionsRVAP activation differs from LBBB where RVSP appears similar.Trial registration(ClinicalTrials.gov identifier: NCT01831518)

Variations in the dynamic response of structures founded on piles induced by obliquely incident SV waves

SummaryAlthough the seismic actions generally consist of a combination of waves, which propagates with an angle of incidence not necessarily vertical, the common practice when analyzing the dynamic behavior of pile groups is based on the assumption of vertically incident wave fields. The aim of this paper is to analyze how the angle of incidence of SV waves affects the dynamic response of pile foundations and piled structures. A three‐dimensional boundary element‐finite element coupling formulation is used to compute impedances and kinematic interaction factors corresponding to several configurations of vertical pile groups embedded in an isotropic homogeneous linear viscoelastic half‐space. These results, which are provided in ready‐to‐use dimensionless graphs, are used to determine the effective dynamic properties of an equivalent single‐degree‐of‐freedom oscillator that reproduces, within the range where the peak response occurs, the response of slender and nonslender superstructures through a procedure based on a substructuring model. Results are expressed in terms of effective flexible‐base period and damping as well as maximum shear force at the base of the structure. The relevance and main trends observed in the influence of the wavefront angle of incidence on the dynamic behavior of the superstructure are inferred from the presented results. It is found that effective damping is significantly affected by the variations of the wave angle of incidence. Furthermore, it comes out that the vertical incidence is not always the worst‐case scenario.

Determinants of atrial bipolar voltage: Inter electrode distance and wavefront angle

BackgroundLocal bipolar electrogram (EGM) peak-to-peak voltage (Vpp) is currently used to characterise mapped myocardial substrate. However, how interelectrode distance and angle of wavefront incidence affect bipolar, Vpp values, in the current era of multi-electrode mapping is unknown. ObjectivesTo elucidate the effects of tissue and electrode geometry on bipolar Vpp measurements, when mapping healthy versus diseased atrial regions. MethodsA bidomain model of human atrial tissue was used to quantify the influence on Vpp values of various electrode configurations in healthy tissue, and tissue containing an unexcitable region. The orientation angle and interelectrode spacing of a surface bipole, and thickness and depth of the unexcitable core were serially varied. Results were validated with data obtained from isolated porcine hearts. ResultsIn healthy tissue, bipolar Vpp values increased with increasing interelectrode spacing and plateaued beyond a spacing of approximately 4 mm. The bipolar Vpp values in healthy tissue were relatively less sensitive to wavefront orientation angle with large interelectrode spacing. In diseased tissue, on the contrary, with increasing interelectrode spacing, bipolar Vpp values increased linearly without a plateau and were more sensitive to orientation angle. The bipolar Vpp values decreased with increasing thickness of the scar, with larger relative decrease in small bipoles than larger ones. Bipolar Vpp values increased with a progressively intramural location of fixed-size scar and became less distinguishable from healthy tissue especially for smaller interelectrode spacings. ConclusionsThe scalable relationship established for interelectrode distances favour an electric-field-based assessment as opposed to traditional Vpp values as a tool for physiologically relevant measurement for mapping catheters with interelectrode spacing up to 4 mm. This will allow for universal assessment of myocardial health across catheters with varied spacing.

Dual-Phase Transmit Focusing for Multiangle Compound Shear-Wave Elasticity Imaging.

Shear-wave elasticity imaging (SWEI) enables the quantitative assessment of the mechanical properties of tissue. In SWEI, the effective generation of acoustic radiation force is of paramount importance. Consequently, several research groups have investigated various transmit beamforming and pulse-sequencing methods. To further improve the efficiency of the shear-wave generation, and therefore, to increase the quality of SWEI, we introduce a technique referred to as "multiangle compound SWEI" (MAC-SWEI), which uses simultaneous multiangular push beams created by dual-phase transmit focusing. By applying a constant phase offset on every other element of an array transducer, dual-phase transmit focusing creates both main and grating lobes (i.e., multiangular push beams for pushing) to simultaneously generate shear waves with several wavefront angles. The shear waves propagating at different angles are separated by multidirectional filtering in the frequency domain, leading to the reconstruction of multiple spatially co-registered shear-wave velocity maps. To form a single-elasticity image, these maps are combined, while regions associated with known artifacts created by the push beams are omitted. Overall, we developed and tested the MAC-SWEI method using Field II quantitative simulations and the experiments performed using a programmable ultrasound imaging system. Our results suggest that MAC-SWEI with dual-phase transmit focusing may improve the quality of elasticity maps.

Scanning high-power continuous wave laser-generated bulk acoustic waves.

The ultrasonic bulk waves generated by a high-power continuous laser scanning along the surface of aluminum material were theoretically investigated. Although the temperature rise generated by this scanning laser irradiation was small, it provided a large temperature gradient, which was able to generate measurable ultrasonic waves. Detailed discussions were given to the influence of scanning speed on the generation propagation direction and the amplitude of the wavefront. The longitudinal and transverse waves would be generated in the material only when the scanning speeds reached a certain range. What's more, the amplitude of the wavefronts were significantly enhanced if the wavefront angle controlled by the scanning speed matched with the propagation direction of the ultrasound. In summary, it expounded a method to obtain the ultrasonic signal of direction, controlled from the perspective of numerical simulation, as long as the scanning speed met the requirements.

Polarisation properties of [SQUID]2horizontal components at the LSBB (Laboratoire Souterrain à Bas Bruit)

Polarisation properties of the geomagnetic signal are computed using the coherence matrix of horizontal components (EW and NS) of the [SQUID]2 datasets, in a relatively small bandwidth. Wavefront ellipticity, signal-to-noise ratio (snr ) and wavefront arrival angle of the magnetic quasi-monochromatic waves are determined. From the variation of ellipticity extremum position in the vicinity of 8 Hz, the temporal variation of the peak frequency is traced for the LSBB and two Northern American stations distant from the LSBB by ∼8,000 km. The spectra of the peak frequency variation display the daily, half daily and third-daily harmonics at all stations, which are characteristic of the first Schumann resonance. Ellipticity spectrograms also unveil a type of chirping local nighttime resonances, known as ionospheric Alfven resonances (IAR), observed at all stations. Thanks to the snr spectrograms, components of the signal which are local to the LSBB station are cancelled from the output, particularly the 50-Hz power grid signal which is minimised in the snr spectra.

A new approach to compensate the geometric distortion in the synthetic aperture ultrasonic imaging system.

In the field of ultrasonic imaging technology, the problem of geometric distortion is often encountered, especially in the ultrasonic near-field. In this study, a new approach is proposed to compensate for geometric distortion in the synthetic aperture ultrasonic imaging system. This approach is based on the synthetic aperture ultrasonic holographic B-scan (UHB) imaging system, which is a combination of ultrasonic holography based on the backward propagation principle and the conventional B-scan technique. To solve the geometric distortion problem, the operation of the spatial compression and resampling in the frequency domain are introduced. The main advantage of the approach is that the real holographic value can be calculated without distortion by using the spatial interpolation function after the spatial frequency compression. After the compensation for geometric distortion is performed, the synthetic aperture technique based on the backward propagation principle is then applied in the process of the two-dimensional numerical imaging reconstruction. Both the simulation and measurement experiment show that the approach is promising. The geometric distortion that is dependent on the wave front angle can be effectively compensated. The spatial resolution is practically uniform throughout the depth range and close to the theoretical limit in the experiments.

Theoretical Analysis of Interferometer Wave Front Tilt and Fringe Radiant Flux on a Rectangular Photodetector

This paper is a theoretical analysis of mirror tilt in a Michelson interferometer and its effect on the radiant flux over the active area of a rectangular photodetector or image sensor pixel. It is relevant to sensor applications using homodyne interferometry where these opto-electronic devices are employed for partial fringe counting. Formulas are derived for radiant flux across the detector for variable location within the fringe pattern and with varying wave front angle. The results indicate that the flux is a damped sine function of the wave front angle, with a decay constant of the ratio of wavelength to detector width. The modulation amplitude of the dynamic fringe pattern reduces to zero at wave front angles that are an integer multiple of this ratio and the results show that the polarity of the radiant flux changes exclusively at these multiples. Varying tilt angle causes radiant flux oscillations under an envelope curve, the frequency of which is dependent on the location of the detector with the fringe pattern. It is also shown that a fringe count of zero can be obtained for specific photodetector locations and wave front angles where the combined effect of fringe contraction and fringe tilt can have equal and opposite effects. Fringe tilt as a result of a wave front angle of 0.05° can introduce a phase measurement difference of 16° between a photodetector/pixel located 20 mm and one located 100 mm from the optical origin.

Long spatio-temporally stationary filaments in air using short pulse UV laser Bessel beams

The formation of long stationary filaments resulting in uniform high density plasma strings in air using short pulse UV laser Bessel beams is shown. The length and the electron density of the plasma strings can be easily tuned by adjusting the conical Bessel wavefront angle. It is shown that in this regime the length of the plasma string can be extended over meter-long scales without any compromise in the string uniformity or any temporal evolution of the filamented laser pulse.

Hybrid modeling of electrical and optical behavior in the heart

Optical mapping of transmembrane potential using voltage-sensitive dyes has revolutionized cardiac electrophysiology by enabling the visualization of electrical excitation waves in the heart. However, the interpretation of the optical mapping data is complicated by the fact that the optical signal arises not just from the surface, but also from some depth into the heart wall. Here, we review modeling efforts, in which the diffusion of photons is incorporated into the computer simulations of cardiac electrical activity (“hybrid” modeling), with the goal of improving our understanding of optical signals. We discuss the major accomplishments of hybrid modeling which include: (i) the explanation of the optical action potential upstroke morphology and prediction of its dependence on the subsurface wave front angle, (ii) the unexpectedly low magnitudes of optically recorded surface potentials during electrical shocks, and (iii) the “depolarization” of the core of the spiral wave and odd dual-humped optical action potentials during reentrant activation. We critically examine current optical mapping techniques and controversies in our understanding of electroporation during defibrillation. Finally, we provide a brief overview of recent theoretical studies aimed at extending optical mapping techniques for imaging intramural excitation to include transillumination imaging of scroll wave filaments and depth-resolved optical tomographic methods.

Measurements of profiles of the wavefront outer scale using observations of the limb of the Moon

The wavefront outer scale is an important parameter to use when evaluating the experimental performance of large aperture telescopes. Knowledge of three-dimensional atmospheric wavefront distortions is required for specifications of multiconjugate adaptive optics systems, which are essential for observations using Extremely Large Telescopes with a wider field of view. We aim to estimate the vertical distribution of the wavefront outer scale. We analyse the angular correlation of the fluctuations of the wavefront angle of arrival, deduced from image motion of the limb of the Moon. We use a simulated annealing algorithm to deduce the height dependence of the wavefront outer scale with given C 2 N profiles simultaneously measured with the scintillation detection and ranging (SCIDAR) instrument. We present results obtained during two campaigns of observation at the Mauna Kea Observatory (Hawaii) and the Observatoire de Haute Provence (France). Estimated profiles of the outer scale exhibit smaller values in the boundary layer than in the free atmosphere. Comparisons with measurements of the outer scale using the Generalized Seeing Monitor (GSM) are possible and give good agreement. We consider some implications for adaptive optics systems.

Analytic solution of ray-tracing equations for a linearly inhomogeneous and elliptically anisotropic velocity model

We study wave propagation in anisotropic inhomogeneous media. Specifically, we formulate and analytically solve the ray-tracing equations for the factorized model with wavefront velocity increasing linearly with depth and depending elliptically on direction. We obtain explicit expressions for traveltime, wavefront (phase) angle, and ray (group) velocity and angle, and study these seismological quantities for a model that successfully describes field measurements in the Western Canada Basin. By considering numerical examples, we also show that the difference between the wavefront and ray velocities depends only slightly on the anisotropy parameter, whereas the difference between the wavefront and ray angles is, in a first-order approximation, linear in the anisotropy parameter.

Effects of image restoration on target acquisition

We focus on modeled atmospheric and image restoration effects on the human acquisition of a target. The image restoration is by filtering atmospheric effects, such as blur caused by absorption, scattering from aerosols, and distortion caused by turbulence that changes the wave front angle, moves the image on the image plane, and blurs it. The restoration method using an atmospheric wiener filter is intended to correct the atmospheric effects. This filter is based on the fact that the modulation transfer function (MTF) of the turbulence is composed of a mean value and a random component while the aerosol MTF changes slowly. The atmospheric wiener filter is more advantageous than an ordinary wiener filter in high turbulence situations. Using the atmospheric wiener filter, we consider the target acquisition limitations of the filter through perception experiments and statistical analysis of the results. We found that the atmospheric wiener filter can noticeably improve target acquisition probability at low noise levels. As the noise increases, however, the improvement becomes more limited because the restoration increases the noise level in the image.

Performance of acoustic sensor arrays operating in atmospheric turbulence: Geometric acoustics regime

The performance of acoustic sensor arrays operating in atmospheric turbulence with fluctuations described by a von Kármán spectrum has been investigated for strong diffraction in the presence of strong or weak scattering [Collier and Wilson, ASA June 2001 Meeting, ASA Dec. 2001 Meeting, J. Acoust. Soc. Am. (in submission)]. Here the previous analysis is extended to the geometric acoustics regime, i.e., to the weak diffraction and weak scattering limit. A model for the probability likelihood function is developed and the coherence function calculated. The Cramer–Rao lower bounds of the wavefront angles of arrival are investigated for geometric acoustics and are shown to reduce to the expected expression for the angle-of-arrival variance in the limit of zero noise.

Numerical Analysis of Structure of Free-Surface Shock Wave About a Wedge Model

Detailed characteristics of the free-surface shock wave (FSSW) are numerically studied for the case of waves generated about a wedge model. The MAC-type finite-difference solution method called TUMMAC-VIII is employed. The density function method is used for implementation of the nonlinear free-surface condition in the framework of a rectangular coordinate system. The computed results show that the wave front angle varies systematically and that the shock condition is fulfilled in the thin layer near the free surface, which is one of the most typical characteristics of the FSSW. Some important mechanisms of wave breaking accompanied by the FSSW, such as overturning of wave front, vortex generation and energy dissipation, are also approximately simulated.

Detection of direct and indirect sonic booms from space shuttle landings

Two hundred seismic stations covering over 50 000 square kilometers in Southern California were used to analyze the sonic boom patterns from the landing of STS-42 in November 1993 and STS-58 in January 1994 at Edwards Air Force Base (EAFB). The instrument ground motion traces show separate arrivals which can be identified as the primary boom, creeping waves generated at the edge of the primary boom carpet, and multiple indirect sonic booms refracted from high altitude. The measured arrival times are compared with the results of linear ray tracing calculations through reference temperature and wind profiles for EAFB. The ray calculations accurately predict the measured arrival times and wavefront angles for the primary sonic boom and the creeping waves in shadow regions, and accurately predict the wavefront angles for the indirect booms. Ray theory using the reference atmosphere fails to predict indirect boom arrival time, observed multiple booms within the first shadow region, and extensive overlap of the multiply refracted sonic booms. These results suggest that boom exposure under the real atmosphere may be larger than previously expected.

Highly efficient sonochemical reaction with a switched spiral focal field

This paper describes an efficient method of inducing sonochemical reactions with focussed ultrasound. A sector-vortex transducer (a phased array transducer with multiple sectors and a geometric focus) is used to produce focal ultrasound fields with spiral-shaped wavefronts. Clockwise and counter-clockwise spiral focal fields, with basically the same ultrasound power distribution but having different wavefront angles, are produced by using the same array transducer. The oxidation of the iodide ions induced from the periodic switching between such a pair of spiral focal fields is investigated at an ultrasound frequency of 750 kHz. The oxidation rate at the optimum switching period (3 ms) is about ten times higher than that at switching periods of 1 s. The optimum switching period and the sonochemical reaction rate are unchanged when the viscosity was adjusted to that of human blood.

White-light optical Fourier transform device

The construction and operation of a new optical phase plate assembly is described that takes the Fourier transform of an intensity image in white light. It consists of symmetrical wedges of birefringent crystals that induce an angle between the wave fronts of the polarized components of the output beam. The wave-front angle is shown to be linearly proportional to the angle of incidence. This is sufficient to generate the necessary phase factor for a Fourier transform.

Analytic design of optimum holographic optical elements

A method is developed for analytically determining the holographic optical element phase function that is optimum for transforming a set of input wave fronts into a corresponding set of output wave fronts. These sets are allowed to be infinite in the sense that the wave-front phases can be given as functions of continuous parameters. The method can be tolerant of specified wave-front aberrations, with the optimum amount of these aberrations determined as part of the solution process. For many practical design problems, the phase function and its first derivatives will be continuous. The method is applied to the design of a one-dimensional Fourier-transform holographic element with the input wave-front angle of arrival as a continuous parameter and with the optimum distortion of the output plane determined by the solution. The resulting design compares favorably with other work using damped-least-squares optimization.