Showing papers on "Spatial filter published in 2006"

Spiral phase contrast imaging in microscopy

Abstract: The present invention provides a microscopic device and a method for enhancing the edge contrast of an image observed with microscopic device. The microscopic device of the invention comprises a spiral phase element which is positioned in one of the focal or Fourier planes in the light path through the microscope thereby filtering the whole light field in said plane.

Spatiotemporal filtering using principal component analysis and Karhunen-Loeve expansion approaches for regional GPS network analysis

Abstract: Spatial filtering is an effective way to improve the precision of coordinate time series for regional GPS networks by reducing so-called common mode errors, thereby providing better resolution for detecting weak or transient deformation signals. The commonly used approach to regional filtering assumes that the common mode error is spatially uniform, which is a good approximation for networks of hundreds of kilometers extent, but breaks down as the spatial extent increases. A more rigorous approach should remove the assumption of spatially uniform distribution and let the data themselves reveal the spatial distribution of the common mode error. The principal component analysis (PCA) and the Karhunen-Loeve expansion (KLE) both decompose network time series into a set of temporally varying modes and their spatial responses. Therefore they provide a mathematical framework to perform spatiotemporal filtering.We apply the combination of PCA and KLE to daily station coordinate time series of the Southern California Integrated GPS Network (SCIGN) for the period 2000 to 2004. We demonstrate that spatially and temporally correlated common mode errors are the dominant error source in daily GPS solutions. The spatial characteristics of the common mode errors are close to uniform for all east, north, and vertical components, which implies a very long wavelength source for the common mode errors, compared to the spatial extent of the GPS network in southern California. Furthermore, the common mode errors exhibit temporally nonrandom patterns.

Static two-dimensional aperture coding for multimodal multiplex spectroscopy

Abstract: A class of aperture coded spectrometer is optimized for the spectral characterization of diffuse sources. The instrument achieves high throughput and high spatial resolution by replacing the slit of conventional dispersive spectrometers with a spatial filter or mask. A number of masks can be used including Harmonic masks, Legendre masks, and Hadamard masks.

Radial Hilbert transform with Laguerre-Gaussian spatial filters

Abstract: We analyze the point spread function (PSF) of the image processing system for radial Hilbert transform and propose a novel spiral phase filter, called the Laguerre-Gaussian spatial filter (LGSF). Theoretical analysis and real experiments show that the LGSF possesses some advantages in comparison with the conventional spiral phase plate (SPP). For example, the PSF of the imaging system with a LGSF presents smaller suboscillations than that with the conventional SPP, which allows us to realize a radial Hilbert transform for achieving a high contrast edge enhancement with high resolution.

Angular momentum and geometrical phases in tight-focused circularly polarized plane waves

Abstract: Calculations for the field at the focal plane of a high numerical aperture lens focusing a circularly polarized plane wave are presented. The calculations show that the polarization of the wave front in the focal plane is space varying, and that a geometrical phase is added to the wave front. Calculation of the angular momentum at the focal plane reveals that it depends on the numerical aperture of the lens. It is shown that this dependence is directly connected to the lens acting as a spatial filter.

High power coherent beam combination from two fiber lasers

Abstract: Phase locking of two fiber lasers is demonstrated experimentally by the use of a self-imaging resonator with a spatial filter. The high-contrast interference strips of the coherent beam profile are observed. The coherent output power of the fiber array exceeds 12W and the efficiency of coherent power combination is 88% with pump power of 60W. The whole system operates quite stably and, for the spatial filter, no thermal effects have been observed, which means that we can increase the coherent output power further by this method.

Spiral interferogram analysis

Abstract: Interference microscopy using spatial Fourier filtering with a vortex phase element leads to interference fringes that are spirals rather than closed rings. Depressions and elevations in the optical thickness of the sample can be distinguished immediately by the sense of rotation of the spirals. This property allows an unambiguous reconstruction of the object's phase profile from one single interferogram. We investigate the theoretical background of "spiral interferometry" and suggest various demodulation techniques based on the processing of one single interferogram or multiple interferograms.

Optical configuration for imaging-type optical encoders

Abstract: A displacement measuring device including a scale, and an optical readhead including an index pattern and a light receiving element is provided. A bright/dark pattern arising from a scale grating is detected by the readhead to measure displacement. In various embodiments, a magnification of the pattern is adjusted by the spacing between at least a lens element, aperture element, and detection plane of the readhead. An aperture can be designed to provide a diffraction-limited telecentric imaging configuration that filters an image of the scale grating to provide a sinusoidal intensity pattern that supports highly interpolated measurements. An aperture dimension, selected in relation to the grating pitch and other parameters, can provide a desirable combination of readhead operating characteristics including one or more of a desired depth of field; degree of spatial filtering; and optical signal power.

Deep nulling of laser light with a single-mode-fiber beam combiner

Abstract: The detection of faint companions to bright stars requires the development of very-high-contrast-ratio detection techniques such as nulling interferometry. Here we present, to our knowledge, the first experimental test of a novel beam-combination method, which is based on coupling the light at the center of a dual-aperture interference pattern into a single-mode fiber. Using such a "fiber nuller" approach, a visible He-Ne laser has been stably nulled to as low as 1.3×10−6, thus validating the approach.

Integrated optical filter arrays fabricated by using the combinatorial etching technique

Abstract: A combinatorial etching technique is developed to fabricate integrated narrow bandpass filters on a single substrate. It is highly efficient for fabrication of integrated filter arrays in optical regions. A monolithic filter array has been fabricated by using the technique with a two-step deposition process. The filter contains 32 elements in the near-infrared region. The relative full width at half-maximum (FWHM) deltalambda/lambda of the filter elements is less than 0.2%. Such a narrow bandpass filter array can be utilized in many optical applications.

Efficient array beam forming by spatial filtering for ultrasound B-mode imaging

Abstract: This paper proposes an efficient array beam-forming method using spatial matched filtering (SMF) for ultrasonic imaging. In the proposed method, ultrasonic waves are transmitted from an array subaperture with fixed transmit focus as in conventional array imaging. At receive, radio frequency echo signals from each receive channel are passed through a spatial matched filter that is constructed based on the system transmit-receive spatial impulse response. The filtered echo signals are then summed without time delays. The filter concentrates and spatially registers the echo energy from each element so that the pulse-echo impulse response of the summed output is focused with acceptably low side lobes. Analytical beam pattern analysis and simulation results using a linear array show that this spatial filtering method can improve lateral resolution and contrast-to-noise ratio as compared with conventional dynamic receive focusing (DRF) methods. Experimental results with a linear array are consistent but point out the need to address additional practical issues. Spatial filtering is equivalent to synthetic aperture methods that dynamically focus on both transmit and receive throughout the field of view. In one common example of phase aberrations, the SMF method was degraded to a degree comparable to conventional DRF methods.

Variable-radius focused optical vortex with suppressed sidelobes

Abstract: We propose a design for a phase mask for generating an optical vortex with suppressed sidelobes in the focal plane where the radius of the intensity ring is variable. A radial modulation added to conventional phase mask exp(ilθ) projects the light diffracted from different annular zones into a single intensity ring in the focal plane.

Compact and integrated 2-D photonic crystal super-prism filter-device for wavelength demultiplexing applications.

Abstract: A two-dimensional photonic crystal (PhC) super-prism integrated with one-dimensional photonic crystal microcavity filters has been designed using the plane wave expansion (PWE) and 2-D Finite Difference Time Domain (FDTD) methods based on Silicon-on-Insulator (SOI) technology. The super-prism operates as a coarse spatial filter with an average response bandwidth of 60 nm, while the 1-D PhC microcavity filters operate as narrow band-pass transmission filters with an average filter response line-width of 10 nm. This work demonstrates the simultaneous operation of two photonic devices for de-multiplexing applications on a single platform that could be useful in future Photonic Crystal Integrated Circuits (PCICs).

Performance scaling in flat imagers

Abstract: A performance scaling formulation for flat form-factor cameras is introduced The analysis follows from basic geometric and sensitivity constraints found in low-profile imaging sensors A capacity metric is proposed and used to estimate performance cost scaling as a function of the width-to-height aspect ratio in the optics of thin imagers Two basic flat imaging sensor classes are considered—one folds the optical path of an annular telescope within the volume of a central obscuration, and the other uses spatial multiplexing and filtering across an array of low-resolution small cameras to generate an estimate of the high-resolution image Scaling trends are highlighted that enable general performance comparisons at the optical signal collection level, thereby providing conclusions that are independent of the computational aspects of any particular approach The results indicate that thin imagers face significant costs in physical size and sampling requirements if they are to match the performance of conventional cameras in the basic parameters of field of view, resolution, dynamic range, and sensitivity

A Survey on Image Enhancement Techniques: Classical Spatial Filter, Neural Network, Cellular Neural Network, and Fuzzy Filter

Abstract: Present day applications require various kinds of images and pictures as sources of information for interpretation and analysis. Whenever an image is converted from one form to another, such as, digitizing, scanning, transmitting, storing, etc., some form of degradation occurs at the output. Hence, the output image has to undergo a process called image enhancement which consists of a collection of techniques that seek to improve the visual appearance of an image. In this paper, a classical spatial filter, neural network (NN), cellular neural network (CNN) and fuzzy filters are presented for the noise reduction of images that are corrupted with additive noise. A three layer neural network is trained for few test images and is used to filter the corrupted colour images. A single layer CNN is developed to reduce the noise in the colour image and compared with that of the classical spatial filter. A new fuzzy filter technique is studied with respect to noisy gray scale images. All the techniques produce convincing results when applied to additive (Gaussian) noisy images. Experimental results are obtained based on the mathematical models of expert systems and compared by numerical measures and visual inspection. It is envisaged to train CNN using gradient descent back-propagation algorithm for better results and extend fuzzy filter technique to reduce noise in colour images.

Method and apparatus for bilateral high pass filter

Abstract: A target pixel and surrounding pixels corresponding to the target pixel are obtained from a digitally represented image. A bilateral high pass filtering kernel is determined based at least in part upon the target pixel and the surrounding pixels. A high pass spatial filtering kernel is provided and multiplied with the high pass photometric filtering kernel to provide a bilateral high pass filtering kernel. The resulting bilateral high pass filtering kernel is thereafter applied to the target pixel and the surrounding pixels to provide a filtered pixel. When it is desirable to combine noise filtering capabilities with sharpening capabilities, the bilateral high pass filter of the present invention may be combined with a bilateral low pass filtering kernel to provide a combined noise reduction and edge sharpening filter. The present invention may be advantageously applied to a variety of devices, including cellular telephones that employ image sensing technology.

Macroscopic control of high-order harmonics quantum-path components for the generation of attosecond pulses

Abstract: We present measurements of the different quantum path contributions to the high-order harmonic emission. Through spatial and spectral filtering, we evidence the strong correlation between the spatial and spectral distributions, which allows us to quantify the contribution of each quantum path. A systematic analysis as a function of the generating parameters has been done to identify the conditions for efficient generation and selection of a single quantum path. We show that combining phase matching and spatial filtering allows maximizing and selecting the short quantum path contribution, condition for the generation of ``clean'' and intense attosecond pulses.

Resonant modes in continuous metallic grids over ground and related spatial-filtering applications

Abstract: We use the theory of periodic structures, full-wave electromagnetic, and microwave circuit simulations to explain the resonant modes that propagate in metallic grids having rectangular unit cells constructed over a ground plane. We show that these metallic grids can support two types of resonant modes that have rectangular and hyperbolic isofrequency dispersion contours. By exploiting the spatial dispersion properties of these modes, a microwave 3GHz∕6GHz harmonic splitter and a highly selective 5.8GHz∕6.2GHz diplexer are designed and simulated. Furthermore, we provide experimental results for the diplexer and for the harmonic splitter, synthesized in microstrip technology. The proposed metallic grids utilize continuous unloaded transmission-line segments thus leading to spatial-filtering devices that are easy to fabricate and are scalable to terahertz frequencies and beyond.

Design of a tangential phase contrast imaging diagnostic for the TCV tokamak

Abstract: A core density fluctuation imaging diagnostic is being developed for the TCV tokamak, employing a 7 cm wide CO2 laser beam transmitted through the plasma in a near-toroidal direction. The proposed system employs the phase contrast method and can resolve wavelengths ranging from 7 down to 0.1 cm, with a minimum measurable line-averaged density of 3 x 10(15) m(3)/MHz(1/2). The broad range of microinstabilities that can be at play in the strongly electron-cyclotron-resonance heated TCV plasmas, from ion to electron spatial scale lengths, widely known as ion-temperature-gradient, trapped-electron-mode, and electron-temperature-gradient modes, would thus be accessible. The use of an imaging technique overcomes the difficulties faced by traditional scattering diagnostics in investigating highly inhomogeneous regions, such as internal transport barriers. Wavelengths and correlation properties can be recovered from the spatial mapping. The tangential configuration, combined with appropriate spatial filtering techniques, provides an excellent spatial resolution, of the order of 1% of the minor radius. In view of the extreme plasma shaping and positioning flexibility of the TCV tokamak, the beam positioning will also be flexible, with translatable mirrors enabling measurements close to the magnetic axis in some configurations. (c) 2006 American Institute of Physics.

Optimizing spectral filters for single trial EEG classification

Abstract: We propose a novel spectral filter optimization algorithm for the single trial ElectroEncephaloGraphy (EEG) classification problem. The algorithm is designed to improve the classification accuracy of Common Spatial Pattern (CSP) based classifiers. The algorithm is based on a simple statistical criterion, and allows the user to incorporate any prior information one has about the spectrum of the signal. We show that with a different preprocessing, how a prior knowledge can drastically improve the classification or only be misleading. We also show a generalization of the CSP algorithm so that the CSP spatial projection can be recalculated after the optimization of the spectral filter. This leads to an iterative procedure of spectral and spatial filter update that further improves the classification accuracy, not only by imposing a spectral filter but also by choosing a better spatial projection.

Wide spatial frequency topography and roughness measurement

Abstract: In one embodiment, a system to inspect a surface comprises an assembly to direct a first radiation beam onto a surface in a first plane of incidence, a first detector to generate a first signal from a portion of the radiation reflected from the first radiation beam, a first spatial filter interposed between the surface and the first detector, a first ellipsoidal mirror to collect scattered light, a second detector to generate a second signal from the scattered portion of the beam, and a processor to generate, from the first and second signals, a data set representing one or more characteristics of the surface using the first and second signals.

Real-time monitoring of vibration fringe patterns by optical reconstruction of digital holograms: mode beating detection

Abstract: A new technique capable of monitoring two-dimensional vibration fringe patterns in real-time is presented. It is based on optical reconstructing of time-averaged digital holograms. Recording of the holograms is realized in a quasi-Fourier off-axis setup whilst reconstructing in an extended Fourier transform setup capable of spatial filtering of the hologram reconstructions. The effectiveness of the proposed device, tested on a silicon wafer, was verified by performing operations such as monitoring of vibration modes, adjusting the hologram recording parameters, or searching for resonant frequencies. Additionally, mode beating between two distant vibration frequencies is observed and reported. The proposed technique is suitable for various scientific and industrial applications.

Retrieval of Moisture from Slant-Path Water Vapor Observations of a Hypothetical GPS Network Using a Three-Dimensional Variational Scheme with Anisotropic Background Error

Abstract: A three-dimensional variational (3DVAR) scheme is developed for retrieving three-dimensional moisture in the atmosphere from slant-path measurements of a hypothetical ground-based global positioning system (GPS) observation network. It is assumed that the observed data are in the form of slant-path water vapor (SWV), which is the integrated water vapor along the slant path between the ground receiver and the GPS satellite. The inclusion of a background in the analysis overcomes the under-determinedness problem. An explicit Gaussian-type spatial filter is used to model the background error covariances that can be anisotropic. As a unique aspect of this study, an anisotropic spatial filter based on flow-dependent background error structures is implemented and tested and the filter coefficients are derived from either true background error field or from the increment of an intermediate analysis that is obtained using an isotropic filter. In the latter case, an iterative procedure is involved. A set ...

Bow-tie effect: differential operator

Abstract: We propose to use a differential operator for representing the influence of phase-only filters on the defocused modulation transfer function of the clear pupil aperture. We present a phase-only filter that implements optically Taylor's theorem in phase space. We show numerical simulations of the modulation transfer functions and the images that can be obtained by using the proposed filter.

Microphone array post-processor using instantaneous direction of arrival

Abstract: In this paper we describe a novel algorithm for postprocessing a microphone array’s beamformer output to achieve better spatial filtering under noise and reverberation. For each audio frame and frequency bin the algorithm estimates the spatial probability for sound source presence and applies a spatio-temporal filter towards the look-up direction. It is implemented as a real-time post-processor after a timeinvariant beamformer and it substantially improves the directivity of the microphone array. The algorithm is CPU efficient and adapts quickly when the listening direction changes. It was evaluated with a linear four element microphone array. The directivity index improvement is up to 8 dB, the suppression of a jammer 40° from the sound source is up to 17 dB.

Detection and tracking of the backreflection of potassium dihydrogen phosphate images in the presence or absence of a phase mask

Abstract: The potassium dihydrogen phosphate (KDP) crystals present in the final optics assembly at the National Ignition Facility (NIF) are used for conversion of an infrared laser light beam into an ultraviolet beam. The conversion is highest for a certain incident angle, the alignment of which is determined from the position of the backreflection beam, which exhibits a distinct characteristic shape. When a phase-plate device is introduced before the final assembly to increase the uniformity of the beam, the backreflection pattern changes drastically. The algorithm that is best for tracking the special-shaped beam is no longer suitable for tracking the phase-modified beam. We discuss our detection schemes for both situations. In particular, we demonstrate how the algorithm senses the modified beam by using a newly proposed criterion of correlation peak pedestal area and executes an alternate algorithm in real time without operator intervention. This new algorithm continuously tracks the beam pattern to guarantee reliable and repeatable sensing. Results from simulation and real-world implementation of the algorithm at the NIF facility are presented.

Nanofabrication of a space-variant optical transmission filter.

Abstract: A space-variant optical transmission filter is demonstrated for which a simplified process is used to tailor the spatial response of the filter across the surface of a single wafer. A multilayer stack, of alternating high or low refractive index dielectric materials, was used to produce a narrow transmission notch in the center of a wide stop band. Subsequent patterning and etching of arrays of holes through the volume of the dielectric stack was performed to control the fill factor of the dielectric in the layers. The position of the transmission notch within the reflection spectrum was varied across the device surface by adjusting the hole diameter of the hole arrays. Experimental and numerical simulation were used to confirm the space-variant transmission characteristics of a single-wafer sample with two zones of different hole diameter arrays in the 1550 nm wavelength regime.

Defect inspection device and method for it

Abstract: PROBLEM TO BE SOLVED: To reduce noise due to a circuit pattern for performing highly sensitive detection of foreign matter or a defect causing real damage on a test object having a transparent film such as an oxidation film. SOLUTION: In this defect inspection device provided with a stage part, which is used for mounting a substrate specimen and can be moved in the X-direction, Y-direction, and Z-direction respectively, a lighting system lightening the circuit pattern diagonally, and an image focusing optical system forming an image of a lightened test area from the upper side and the diagonal side on a detector, scattered rays and diffracted rays generated on the circuit pattern by radiation by means of the lighting system are condensed. The defect inspection device is also provided with a spatial filter arranged on a Fourier transformation face for shutting out diffracted rays from the linear part of the circuit pattern. The scattered reflection rays received from the radiated specimen by the detector are transformed into electric signals, and when comparison of the transformed electric signals between chips results in disagreement, a foreign matter is determined on the specimen and inspected. COPYRIGHT: (C)2007,JPO&INPIT