Fresnel zone

About: Fresnel zone is a research topic. Over the lifetime, 2337 publications have been published within this topic receiving 37650 citations.

Comparative analysis on response of ground penetrating radar wave field to crack width

Abstract: Through theoretical calculation,numerical simulation and physical experiments,we studied the variation of amplitudes of the diffracted wave at location of the top point of the crack with different width of vertical crack which is in the underground medium.The results show that:(1) Width of crack less than the diameter of the first Fresnel zone is not able to be measured directly from the image of ground penetrating radar.But with the increase of the width of crack,the amplitude of the diffracted wave is continuously enhanced.Especially,when the width of crack is less than half the diameter of the first Fresnel zone,the amplitude of the diffracted wave versus the width of crack has the approximate linear relationship.(2) When the width of crack is approximately equal to the diameter of first Fresnel zone,the amplitude reaches the maximum.(3) After that the amplitude decrease gradually and until it be stable to a constant value as width of crack is far more than the first Fresnel zone and the width crack is able to be measured directly from the image of ground penetrating radar.The research results can be used in recognizing the underground crack and quantitatively estimating the width of the crack.

APD-based X-ray imaging telescope using fresnel zone plates for extremely high Spatial Resolution

Abstract: A method for constructing an x-ray telescope with exceedingly hgh spatial resolution is to use a pair of coaxial, Fresnel zone plates aligned with an imaging x-ray detector. This combination allows the high sensitivity imaging of x-ray and gamma-ray sources ranging in energy from 1 keV to several hundred keV over a field of view of several degrees with spatial resolution of a fraction of an arc minute. We have implemented a version of such a telescope using several relatively new technologies. These include specialized techniques for constructing Fresnel zone plates from thin sheets of tungsten, a 64-element, avalanche photodiode (APD) array coupled to a matching, segmented, CsI(T1) scintillator, a new ASIC which provides 16-channels of low noise amplification, and image processing software that provides the user not only with localized intensity information, but also with localized spectral information.

Peak accelerations from subshear and supershear radiation of kinematic dislocation models

Abstract: The field of accelerations of a kinematic fault-model are evaluated in terms of fundamental interference-integrals that depend upon the source-observer geometry and the various spatial and 'temporal source elements. The dependence of the accelerations at various distances r from the fault's center, on the fault's major dimension L and the radiation's wavelength λ are scaled to the three dimensionless 'regionalization-indices': 2πr/λ, 2L2/λr and 0.62 × (L/r) √L/λ. These determine the limits of the far-field, the outer Fresnel-zone and the inner Fresnel-zone respectively. The interference integrals are then evaluated through the stationary-phase approximation and the Fresnel approximation both in the time and frequency domains. In the Fresnel zone they are expressed in terms of the Lommel functions of two variables. The ensuing acceleration field is shown to depend strongly on the shear Mach-number. In subshear-rupture, the acceleration in the near-fault and Fresnel zones decreases exponentially in a direction normal to the fault. In supershear-rupture the acceleration is a Mach-wave that propagates without attenuation along the Mach-lines. The theoretical scaling law for the acceleration in each region is determined. We assert: (1) peak accelerations in the near-fault zone are essentially independent of the earthquake's magnitude; (2) peak accelerations in the near-fault and Fresnel zones are proportional to the particle velocity on the fault; (3) accelerations in the near-fault and Fresnel zones are determined by the radiation from the nearest fault-segment. This explains the elliptical shape of isoseismals, which are locii of equidistant points from the fault.

Safe range of free space light propagation calculation in single precision.

Abstract: Calculations of free space light propagation, such as those used in digital holography, deal with distances much longer than a wavelength. Computer representation of real numbers must therefore provide enough precision to handle this situation. We show that single precision must be used with the utmost care, which is especially important in GPU calculations. We also show that Fresnel approximation significantly improves single precision calculations for distances bigger than about one metre.

Modification of Fresnel zone light field spectral imaging system for higher resolution (Conference Presentation)

Abstract: Recent interest in building an imaging system using diffractive optics that can fit on a CubeSat (10 cm x 10 cm x 30 cm) and can correct severe chromatic aberrations inherent to diffractive optics has led to the development of the Fresnel zone light field spectral imaging system (FZLFSI). The FZLFSI is a system that integrates an axial dispersion binary diffractive optic with a light field (plenoptic) camera design that enables snapshot spectral imaging capability. This system suffered from poor resolution and a modified FZLFSI based on full resolution light field rendering has been built and tested. The modified FZLFSI shifts the optical elements to different positions which change the way the light field is encoded on the focal plane array. The new encoding increases the available spatial information at the expense of some spectral information. The system was tested for different internal system parameters, at a range of wavelengths, and the resulting tradeoffs between spatial and spectral performance were studied. The performance of the modified FZLFSI was compared to that of the conventional FZLFSI and optimal internal system parameters identified for different imaging scenarios.