Spatial filter

About: Spatial filter is a research topic. Over the lifetime, 6170 publications have been published within this topic receiving 100451 citations.

Cross Pattern Coherence Algorithm for Spatial Filtering Applications Utilizing Microphone Arrays

Abstract: A parametric spatial filtering algorithm with a fixed beam direction is proposed in this paper. The algorithm utilizes the normalized cross-spectral density between signals from microphones of different orders as a criterion for focusing in specific directions. The correlation between microphone signals is estimated in the time-frequency domain. A post-filter is calculated from a multichannel input and is used to assign attenuation values to a coincidentally captured audio signal. The proposed algorithm is simple to implement and offers the capability of coping with interfering sources at different azimuthal locations with or without the presence of diffuse sound. It is implemented by using directional microphones placed in the same look direction and have the same magnitude and phase response. Experiments are conducted with simulated and real microphone arrays employing the proposed post-filter and compared to previous coherence-based approaches, such as the McCowan post-filter. A significant improvement is demonstrated in terms of objective quality measures. Formal listening tests conducted to assess the audibility of artifacts of the proposed algorithm in real acoustical scenarios show that no annoying artifacts existed with certain spectral floor values. Examples of the proposed algorithm can be found online at http://www.acoustics.hut.fi/projects/cropac/soundExamples.

Coherent x-ray zoom condenser lens for diffractive and scanning microscopy.

Abstract: We propose a coherent x-ray zoom condenser lens composed of two-stage deformable Kirkpatrick-Baez mirrors. The lens delivers coherent x-rays with a controllable beam size, from one micrometer to a few tens of nanometers, at a fixed focal position. The lens is suitable for diffractive and scanning microscopy. We also propose non-scanning coherent diffraction microscopy for extended objects by using an apodized focused beam produced by the lens with a spatial filter. The proposed apodized-illumination method will be useful in highly efficient imaging with ultimate storage ring sources, and will also open the way to single-shot coherent diffraction microscopy of extended objects with x-ray free-electron lasers.

Programmable on-focal plane signal processor

Abstract: A programmable on-focal plane signal processor having analog to digital conversion, nonuniformity correction, gamma spike compensation all digitally performed on a single silicon substrate inside a dewar. The architecture supports time delay integration and spatial filtering to increase the signal to noise ratio of the focal plane array data. The processor has programmable coefficients which may be changed while the system is operating or eliminated. The architecture provides increased signal noise ratio for infrared data and decreases the output data bandwidth from the infrared focal plane array by two orders of magnitude.

Measurement of an aeroacoustic dipole using a linear microphone array

Abstract: It is shown that the standard beamformer technique is inadequate for both the source location and the measurement of a simple dipole and that this is due to the assumption of monopole propagation in the calculation of the phase weights used to steer the focus of the array. A numerical simulation is used to illustrate the problem and to develop a correction to the signal processing algorithm to account for the dipole propagation characteristic. This is then applied to array measurements for an aeroacoustic dipole produced by a cylinder in a cross flow. The resulting source map and the beamformed spectrum are shown to give a true representation of the source energy and frequency content. A secondary effect of this correction is that the array becomes insensitive to other source types so that in addition to acting as a spatial filter, the array can perform as a source filter. This work also demonstrates how an array measurement can be misinterpreted if applied without consideration of the source mechanism.

Chromatic confocal setup for displacement measurement using a supercontinuum light source

Abstract: Chromatic confocal microscopy is a technique to measure distances by analyzing the spectrum of the light reflected by a sample. The key element of the confocal setup is a dispersive lens, which focuses different wavelengths at different distances from the lens. In this paper, a novel setup realized with a supercontinuum light source and a spatial filter composed by reflective elements is described. The supercontinuum source is implemented by injecting high power pulses from a microchip laser into a Ge-doped microstructured optical fiber. The usage of metallic parabolic mirrors, for the focusing and collimation required in the spatial filter, lets the dispersive lens be the only dispersive element of the confocal setup and improves the efficiency of the spatial filter itself. A silicon-based spectrometer is used for the acquisition of the spectra, which are normalized and Gaussian-fitted before extracting the displacement information. A complete calibration is performed, and the set of wavelengths from 500 nm to 900 nm can be mapped into a 280 μm measuring range. The obtained relative accuracy of 0.36% shows an enhancement of almost one order of magnitude when compared to other supercontinuum-based confocal systems.