Ling Ma

A fast variable selection method for quantitative analysis of soils using laser-induced breakdown spectroscopy

Variable selection is widely utilized in spectral data analysis. However, for the quantitative analysis of laser-induced breakdown spectroscopy (LIBS), conventional variable selection methods could be inapplicable due to too many dense and informative variables leading to unreliable modeling and a bad explanation effect. To solve this problem, we propose a fast variable selection method combining interval partial least squares (iPLS) and modified iterative predictor weighting-partial least squares (mIPW-PLS) and apply it to the LIBS quantitative analysis of soils. Our method is validated by detecting the concentrations of Cu, Ba, Cr, Mg and Ca in different kinds of soil samples. The results demonstrate that variables with maximal relevance were selected effectively and efficiently. Compared with other methods, our method establishes a more simplified model with high accuracy and robustness as well as improved predictive ability. The number of variables used for calculation was reduced significantly. The root mean square error of prediction (RMSEP) and the R2 of prediction showed more satisfactory results compared with conventional methods. The limits of detection (LODs) obtained for Cu, Ba, Cr, Mg and Ca were 11.4, 4.3, 3.6, 529.5 and 307.6 mg kg−1.

Astigmatism-corrected echelle spectrometer using an off-the-shelf cylindrical lens

As a special kind of spectrometer with the Czerny-Turner structure, the echelle spectrometer features two-dimensional dispersion, which leads to a complex astigmatic condition. In this work, we propose an optical design of astigmatism-corrected echelle spectrometer using an off-the-shelf cylindrical lens. The mathematical model considering astigmatism introduced by the off-axis mirrors, the echelle grating, and the prism is established. Our solution features simplified calculation and low-cost construction, which is capable of overall compensation of the astigmatism in a wide spectral range (200-600xa0nm). An optical simulation utilizing ZEMAX software, astigmatism assessment based on Zernike polynomials, and an instrument experiment is implemented to validate the effect of astigmatism correction. The results demonstrated that astigmatism of the echelle spectrometer was corrected to a large extent, and high spectral resolution better than 0.1xa0nm was achieved.

A Robust method for constructing rotational invariant descriptors

Abstract Unlike most existing descriptors that only encode the spatial information of one neighborhood for each sampling point, this paper proposed two novel local descriptors which encodes more than one local feature for each sampling point. These two local descriptors are named as MIOP (Multi-neighborhood Intensity Order Pattern) and MIROP (Multi-neighborhood Intensity Relative Order Pattern), respectively. Thanks to the rotation invariant coordinate system, the proposed descriptors can achieve the rotation invariance without reference orientation estimation. In order to evaluate the performance of the proposed descriptors and other tested local descriptors (e.g., SIFT, LIOP, DAISY, HRI-CSLTP, MROGH), image matching experiments were carried out on three datasets which are Oxford dataset, additional image pairs with complex illumination changes, and image sequences with different noises, respectively. To further investigate the discriminative ability of the proposed descriptors, a simple object recognition experiment was conducted on three public datasets. The experimental results show that the proposed local descriptors exhibit better performance and robustness than other evaluated descriptors.

Bio-Inspired Covert Active Sonar Strategy

The covertness of the active sonar is a very important issue and the sonar signal waveform design problem was studied to improve covertness of the system. Many marine mammals produce call pulses for communication and echolocation, and existing interception systems normally classify these biological signals as ocean noise and filter them out. Based on this, a bio-inspired covert active sonar strategy was proposed. The true, rather than man-made sperm whale, call pulses were used to serve as sonar waveforms so as to ensure the camouflage ability of sonar waveforms. A range and velocity measurement combination (RVMC) was designed by using two true sperm whale call pulses which had excellent range resolution (RR) and large Doppler tolerance (DT). The range and velocity estimation methods were developed based on the RVMC. In the sonar receiver, the correlation technology was used to confirm the start and end time of sonar signals and their echoes, and then based on the developed range and velocity estimation method, the range and velocity of the underwater target were obtained. Then, the RVMC was embedded into the true sperm whale call-train to improve the camouflage ability of the sonar signal-train. Finally, experiment results were provided to verify the performance of the proposed method.

A rotationally invariant descriptor based on mixed intensity feature histograms

Abstract This paper proposes a novel method to build a rotation invariant local descriptor by mixed intensity feature histogram. Most existing local descriptors based on intensity ordinal information typically encode only one local feature for each sampling point in the image patch. To address this problem, we proposed a method to encode more than one different local features for each pixel in the image patch and construct a 2D mixed intensity feature histogram, from which our proposed MIFH descriptor is then constructed. Since the rotation invariant coordinate system is adopted, the MIFH descriptor does not need to estimate the reference orientation. In order to evaluate the performance and the robustness of the MIFH with other well-known local descriptors (e.g., SIFT, GLOH, DAISY, HRI-CSLTP, LIOP, MROGH), image matching experiments were carried out on standard Oxford dataset, additional image pairs with complex illumination changes and image sequences with different noises. To further investigate the discriminative ability of the MIFH descriptor, a simple object recognition experiment was conducted on three public datasets. The experimental results demonstrate that our descriptor MIFH exhibits better performance and robustness than other evaluated local descriptors.

Towed Array Shape Estimation Based on Single or Double Near-Field Calibrating Sources

Towed array shape estimation is a crucial step for most array processing algorithms, since the position uncertainty of hydrophone elements seriously degrades the algorithm performance. Different from conventional methods using far-field calibrating sources, this paper introduces two kinds of towed array shape estimation methods, the single near-field source-dependent method (S-NFSDM) and the double near-field source-dependent method (D-NFSDM), respectively. Under the assumption that the frequency and direction of calibrating sources are known, the proposed methods can obtain the position of every hydrophone element based on eigenvector technique and the geometric relationship between the calibrating sources and hydrophone elements. In S-NFSDM, the shape of each segment between adjacent array elements is assumed to be straight, which ensures a low computational load and high real-time capability. But in this way, S-NFSDM can only estimate the distortion of uniform linear arrays. To address this, the D-NFSDM is proposed to eliminate the linear constraint of adjacent elements in S-NFSDM. Not only can it ensure the estimation accuracy of element positions, but also estimate distorted non-uniform linear arrays. The Cramer–Rao lower bounds of the proposed S-NFSDM and D-NFSDM are derived in this paper. Numerical simulations demonstrate that the S-NFSDM and D-NFSDM have better estimation performance than the classical method and strong robustness for different array shapes. Moreover, the estimation accuracy of Multiple Signal Classification algorithm can be improved obviously through the proposed methods in the direction of arrival estimation.

Stability design of a light-emitting diode light source for trace constituent concentration measurement

Optical measurement is widely applied in determining the concentration of trace constituents in oilfield injection water. The performance of the light source directly affects the accuracy of the whole optical measurement system. To guarantee the stability of the infrared light source and the ultraviolet (UV) light source, an infrared light-emitting diode (LED) with the central wavelength of 860xa0nm and a UV LED with the central wavelength of 365xa0nm were chosen. Both LEDs were modulated to high frequency to shield the stray light. Through the analysis and comparison among the double-load current-source drive mode, single-load voltage-source drive mode, and single-load current-source drive mode, we designed and theoretically deduced a closed-loop control system for the single-load current-source driving LED. The experiment validated that this system could effectively eliminate the light intensity instability caused by the drift of ambient temperature and light emitting efficiency, and ensure the intensity stability of the light source in the determination of trace constituents.