Spatial Spectrum Distribution Research Articles

Adaptive reconstruction imaging based on K-means clustering in off-axis digital holography

An adaptive filtering method for off-axis digital holographic reconstruction is presented. The spatial spectrum distribution of an off-axis digital hologram is clustered based on the K-means clustering algorithm of unsupervised machine learning . The spatial-spectrum filtering can be carried out by the quantitative comparison of different filtering interception windows. The first-order spectrum including the object information can be automatically located and intercepted after clustering the spatial spectrum distribution of the hologram. The experiment results demonstrate that the reconstruction images with better resolution and imaging quality can be achieved by the adaptive filtering algorithm. This adaptive filtering reconstruction based on K-means clustering provides a new way of automatic reconstruction imaging for off-axis digital holography.

High-resolution international LOFAR observations of 4C 43.15

It has long been known that radio sources with the steepest spectra are preferentially associated with the most distant galaxies, the α − z relation, but the reason for this relation is an open question. The spatial distribution of spectra in high-z radio sources can be used to study this relation, and low-frequency observations are particularly important in understanding the particle acceleration and injection mechanisms. However, the small angular sizes of high-z sources together with the inherently low resolution of low-frequency radio telescopes until now has prevented high angular resolution low-frequency observations of distant objects. Here we present subarcsecond observations of a z = 2.4 radio galaxy at frequencies between 121 and 166 MHz. We measure the spatial distribution of spectra, and discuss the implications for models of the α−z relation. We targeted 4C 43.15 with the High Band Antennas of the International LOFAR Telescope with a range of baselines up to 1300 km. At the central frequency of 143 MHz we achieve an angular resolution of ~0.3″. By complementing our data with archival Very Large Array data we study the spectral index distribution across 4C 43.15 between 55 MHz and 8.4 GHz at resolutions of 0.4″ and 0.9″. With a magnetic field strength of B = 5.2 nT and fitted injection indices of αinjnorth = −0.8 and αinjsouth = −0.6, fitting a Tribble spectral ageing model results in a spectral age of τspec = 1.1 ± 0.1 Myr. We conclude that our data on 4C 43.15 indicates that inverse Compton losses could become comparable to or exceed synchrotron losses at higher redshifts and that inverse Compton losses could be a viable explanation for the α−z relation. Statistical studies of these objects will become possible in the future when wide-area subarcsecond surveys start.

Full-aperture chirped-pulse grating compression with a non-uniform beam

A novel chirped-pulse compression scheme is proposed to pursue higher pulse peak power of chirped-pulse amplifiers based on state-of-the-art gratings. It is characterized by the spectral non-uniformity of the beam cross section as well as by the uniform size of the beam projections on the four compression gratings. The spatial distribution of spectra is well designed so that the beam can remain generally collimated in the compressor and the gratings can be employed with a full work area. The maximum possible output pulse energy can therefore be increased within the damage threshold limit. A detailed design is developed for a scheduled 50-PW optical parametric chirped-pulse amplifier. Results of the numerical simulation on the focused output show that the spot size and pulse duration in the focal plane are close to those of conventional grating compression. Further, it is a promising method for upgrading existing chirped-pulse amplifiers.

Terahertz Spiral Spatial Filtering Imaging

In this paper, we propose a terahertz (THz) spiral spatial filtering (SSF) imaging method that can enable image contrast enhancement. The related theory includes three main steps: (1) the THz image of the target is Fourier transformed to the spatial spectrum distribution; (2) the spatial spectrum is modulated by a spiral phase at the Fourier plane; (3) the filtered spatial spectrum is inverse Fourier transformed to the desired THz image. Meanwhile, analytic expression of the final THz image is derived. Due to the unique nature of the spiral phase, THz image contrast enhancement can be achieved and verified by various simulated target images with different contrasts. In our designed THz SSF imaging system, Fourier transform is carried out by the lens, and the spiral phase is acquired by the spiral phase plate (SPP). Proof-of-principle experiments with three different types of targets (carved metal letters, a high-density polyethylene (HDPE) piece with a scratch, and a leaf) were carried out, and the effectiveness of contrast enhancement and edge extraction on the THz reconstruction images was validated.

Sparse aperture arrangement for periodic LED array sampling in a Fourier ptychography microscopy system.

In a traditional Fourier ptychography microscopy (FPM) system, multiple low-resolution (LR) intensity images have to be sequentially captured under variable illumination angles and then stitched together in the Fourier domain, which is time-consuming. This paper proposes a sparse aperture arrangement method in which the relationship between the spatial spectrum distribution and the light-emitting diodes array sampling pattern is investigated. Mathematical models of the proposed method are developed, and numerical simulations and optical experiments are demonstrated to verify its feasibility. The results show that the number of LR images could be decreased significantly without sacrificing image reconstruction quality. To the best of our knowledge, this is the first time that the sparse aperture arrangement has been exploited for FPM. The proposed method may provide new insights for improving the efficiency of the FPM platform and discovering more applications in digital pathology.

Evaluating the Effects of Autofluorescence during Raman Hyperspectral Imaging

Raman hyperspectral imaging is becoming a popular technique to analyse geological materials. Autofluorescence can affect the quality of the spectra that comprise hyperspectral data sets. Few studies have addressed potential misinterpretation of Raman images from hyperspectral data sets affected by autofluorescence. Additionally, little work has been done to develop methods for identifying the spatial distribution of spectra affected by autofluorescence. This study illustrates how autofluorescence may lead to misinterpretation of the distribution of materials based on intensity at a point images. A method is proposed utilising signal to axis analysis to create images that identify regions affected by autofluorescence. Post‐processing baseline correction is often used to address autofluorescence, and most software programs utilise a form of partial least squares regression modelling based on a subjective choice of polynomial order. This study shows that an inappropriate choice of polynomial order can introduce error, which may lead to misinterpretation of Raman images. A signal to axis analysis method is proposed to statistically compare seemingly ‘appropriate’ baseline correction trials. Although post‐processing of hyperspectral data sets and creating Raman images seem simple, data quality issues such as autofluorescence must be considered. If baseline correction is deemed necessary, it should be addressed as an experiment involving statistical comparison.

Spectral interpolation of long-travelling predicted waves

If a swell propagating over a long distance is dominant, existing spectral wave models will produce unphysical oscillations in the model-predicted wave height field as a result of finite spectral resolution. This is called the Garden Sprinkler Effect (GSE), which is more apparent with high-accuracy propagation schemes. Here, an interpolation method to remove the GSE is developed. This method involves retrieving the higher-resolution wave spectra from lower-resolution wave spectra that are predicted not from a GSE-alleviating scheme but from a shape-preserving advection scheme, by considering the spatial distribution of spectra. Using this method both the removal of the GSE and the preservation of the spatial distribution of spectral values can be attained almost completely.

Development of Shock Tube for Ground Testing Reentry Aerothermodynamics

A free-piston double-diaphragm shock tube with 70×70 mm cross-section at test section is newly developed to investigate thermochemical nonequilibrium phenomena behind a shock wave. This paper presents the performance of the shock tube and the measurement system newly developed. Experimental investigations to clarify its characteristics are conducted for various operational parameters, such as rupture pressure of the first diaphragm and initial pressures in the low-pressure tube, the high-pressure tube, and the compression tube. Based on the characteristics experiment, a performance map of the shock tube is obtained by changing the operation parameters. The result shows that the shock tube can generate the post-shock condition corresponding to super-orbital reentry flight conditions. The newly developed measurement system enables us to obtain a spatial distribution of spectra behind a shock wave with high spatial and time resolution. A description of the measurement system and typical examples of the measured spectra are presented.

Analysis of Laser-MAG Hybrid Welding Coupling Mechanism Based on Spectrum

The radiation of laser-MAG hybrid welding plasma is collected with fiber spectrometer.The spatial distribution of spectra from 200 to 1 100 nm is studied and spectral bands are selected.The selected spectral bands include ultraviolet radiation(Fe II dominated),visible radiation(Fe I dominated) and infrared radiation(Ar I dominated).The radiation in the selected spectral bands is acquired to compare the radiation intensity of different scanning surface.Furthermore,high speed video is also applied to studying the coupling of laser-MAG hybrid welding plasma.The result shows that there is a high radiation intensity zone with great gradient in the center of laser hybrid welding plasma.The spectral bands of FeI and FeII have a broader radiation range for laser-MAG welding.This is significant near the laser focusing position.The radiation increases in the middle and lower part of the plasma,especially near the welding pool.However,it decreases in the upper part of welding plasma.The welding process with laser hybrid will cause the plasma energy to focus on the center of welding arc and the welding pool.The broader radiation range of Fe ion provides ionizing duct for welding arc,thus makes the welding arc stable.The coupling effect provides better bead formation and more stable arc in the laser-MAG hybrid welding process.

Mapping of radiation sources by multiple sensor arrays

The paper develops a method for mapping of emitters which includes determining the location and spectrum of each source for discrete sources or the spatial distribution of spectra for extended (distributed) sources. A two step procedure is suggested. At first, the received signal is decomposed into spectral modes, and beams are formed in every direction of interest. Then the beams from all the arrays are backprojected. The method used here for the backprojection is an algebraic technique by which a system of ray equations is constructed and solved. The solution constitutes the spatial distribution of each spectral mode.