Determination of the conversion gain and the accuracy of its measurement for detector elements and arrays

Standard statistical theory is used to calculate how the accuracy of a conversion-gain measurement depends on the number of samples. During the development of a theoretical basis for this calculation, a model is developed that predicts how the noise levels from different elements of an ideal detector array are distributed. The model can also be used to determine what dependence the accuracy of measured noise has on the size of the sample. These features have been confirmed by experiment, thus enhancing the credibility of the method for calculating the uncertainty of a measured conversion gain.     

Accuracy analysis in laser keratopography

We analyze the accuracy of a laser keratopographer in the evaluation of corneal topography for non-smooth corneal surfaces and when some noise is introduced into measured data. Through some numerical simulations, cosinesoidal deformations with different amplitudes and spatial periods are introduced on theoretical surfaces. Gaussian noise is introduced on the simulated x and y position coordinates for the measured position of the reflected beam in order to simulate detection errors that are due to vibrations or electric and other noise on the position-sensing detector. We found that the topography of the surface could be obtained with reliable accuracy if the height-to-width ratio of the deformations of the surface is smaller than 0.02 and the error in the detection of position at the position-sensing detector is under 0.5 mm.     

Laser vibrometry from a moving ground vehicle

We investigated the fundamental limits to the performance of a laser vibrometer that is mounted on a moving ground vehicle. The noise floor of a moving laser vibrometer consists of speckle noise, shot noise, and platform vibrations. We showed that speckle noise can be reduced by increasing the laser spot size and that the noise floor is dominated by shot noise at high frequencies (typically greater than a few kilohertz for our system). We built a five-channel, vehicle-mounted, 1.55 μm wavelength laser vibrometer to measure its noise floor at 10 m horizontal range while driving on dirt roads. The measured noise floor agreed with our theoretical estimates. We showed that, by subtracting the response of an accelerometer and an optical reference channel, we could reduce the excess noise (in units of micrometers per second per Hz(1/2)) from vehicle vibrations by a factor of up to 33, to obtain nearly speckle-and-shot-noise-limited performance from 0.3 to 47 kHz.     

Thermally Induced Losses in Ultra-Cold Atoms Magnetically Trapped Near Room-Temperature Surfaces

We have measured magnetic trap lifetimes of ultra-cold ⁸⁷Rb atoms at distances of 5–1000 µm from surfaces of conducting metals with varying resistivity. Good agreement is found with a theoretical model for losses arising from near-field magnetic thermal noise, confirming the complications associated with holding trapped atoms close to conducting surfaces. A dielectric surface (silicon) was found in contrast to be so benign that we are able to evaporatively cool atoms to a Bose–Einstein condensate by using the surface to selectively adsorb higher energy atoms.     

涡桨飞机机体外表面噪声源计算与实验验证

针对某型涡桨飞机在巡航状态下机体表面的主要噪声源（附面层噪声和螺旋桨噪声）分别进行了计算及结果验证。利用VA-One软件开展了机身表面不同位置处附面层噪声数值计算,同时利用半经验公式进行了螺旋桨近场噪声的计算研究。综合考虑两种噪声源的计算结果,与该型涡桨飞机的机体外表面噪声实测结果进行了对比研究。结果表明,仿真和理论计算相结合的计算结果与实测结果吻合较好,验证了算法的正确性。该算法可应用于涡桨飞机机身外表面噪声载荷及舱内噪声预计。     

Electrical model for high-power-density laser diodes

We present an electrical model for modulation and noise of laser diodes that takes spectral-hole burning into account and, unlike previous models, is accurate at the quantum level. The active part of the laser diode is represented by a capacitance-expressing carrier storage and a series resistance 1 + β, where β is proportional to the spectral-hole depth. These two elements are followed by a negative impedance converter. The modulation rate measured on this electrical model is in excellent agreement with the theoretical expression. Amplitude noise is simulated by two independent noise sources whose spectral densities are independent of the nonlinearity.     

Magnetization Rate Dependence of the Barkhausen Noise in JRQ Steels

The magnetic Barkhausen noise (MBN) in JRQ reference steel was measured under varying magnetization rate. The scaling properties of the avalanche size distribution were determined experimentally as a function of the excitation frequency. We have found that the exponent decreases and the scaling region extends as the excitation frequency increases in accordance with theoretical expectations. The average avalanche energy, size, and frequency were found to have power law dependence on the excitation rate. The temporal analysis has shown that the MBN noise generation starts earlier and develops a more symmetric character with the increase of the excitation frequency. These changes with the excitation rate can not be accounted for only by the overlap of individual MBN avalanches, but reveal a change of the dynamics of the MBN process.      

Understanding TES microcalorimeter noise and energy resolution

We have performed numerical calculations of the noise in voltage-biased superconducting transition edge-based X-ray microcalorimeters, using a finite-element model. Details of the model are discussed, as well as results for different absorber geometries. The results are in agreement with an analytical model and show that the amount of internal thermal fluctuation noise can be reduced by using a segmented absorber. The simulation also agrees well with experimental data, which, for our detectors, contain no major unidentified noise sources. Furthermore, the discrepancy between the (small-signal) theoretical and the measured energy resolutions for 5.9 keV X-rays, for our sensors typically a factor of 2, is explained by a more accurate modelling of the sensor responsivity, taking into account the large signal behaviour.     

Relationship of surface roughness of video tape to its magnetic performance

The surface roughness, known to influence the recording and play-back properties of magnetic recording media, was interferometrically measured of a series of VHS video tapes. A strong correlation (r>.95) was found between both the signal outputs and noise of these tapes with rms surface roughness. The measured short waveloength rf output was shown to decrease with increasing surface roughness in a manner consistent with three theoretical models. A multiplier of 2.7 times the interferometrically measured rms surface roughness is equal to the calculated head-to-tape spacing of these tapes.     

Phase Behaviour in Diffuse Light Bands from Diffraction Gratings

Abstract

We have measured the phase in diffuse light bands generated by the surface electromagnetic waves on diffraction gratings. We confront the result with a simple theoretical model.     

Theoretical analysis of ultrasonic vibration spectra from multiple particle-plate impacts

Many industrial processes involve particles in a carrier fluid, and it is often of interest to monitor the size of these particles noninvasively. The aim of this paper is to develop a theoretical model of multiple particle-wall impact vibrations that can be used to recover the particle size from experimental data. These vibrations have been measured by an ultrasonic transducer attached to the exterior of a vessel containing a stirred-particle-laden fluid. A linear systems model is derived for the response of the piezoelectric ultrasonic transducer, which has a single matching layer. The acceleration power spectrum of these vibrations has been shown experimentally to contain information on the size of the impacting particle. In particular, the frequency of the main spectral lobe is inversely proportional to the particle size. We present a theoretical model that agrees with this empirically observed phenomenon. The theoretical model is then used to simulate multiple particle-wall impacts, with each particle impacting at a randomly chosen location. A set of theoretical vibration spectra arising from multiple particle-wall impacts are integrated and compared with the experimental data. The ability of this approach to distinguish between different particle sizes is clearly shown.     

Is motion processing unitary?

Motion discrimination space is conventionally categorized into motion detection, speed discrimination, and direction discrimination tasks. But an ideal observer uses a unitary motion mechanism that is affected only by the noise level and the difference in speed (or displacement) between two stimuli. We tested whether human performance in the various motion tasks showed the working of a unitary mechanism or the combined outputs of more than one mechanism. We examined the whole motion discrimination space, using random dots that underwent a sudden jump or displacement. The discriminability was measured as a function of the standard and comparison displacements. Both the ideal observer model and a nonideal observer model that contains additive internal noise predict a planar response surface. When the dot motion was noiseless, the planar surface fitted well except for much higher than expected sensitivity for motion detection. This is consistent with a purely temporal mechanism that uses flicker or a purely spatial mechanism that uses the length of time-averaged streaks. It is also consistent with a Weber's law device. When motion noise was added to the displays, the planar response surface again fitted well, although the residuals showed the presence of a speed energy mechanism. We conclude that a unitary motion mechanism exists (nonideal observer model), although its performance may be supplemented by other mechanisms whose main impact is on discrimination of speeds near zero.     

动力吸振器对有轨电车弹性车轮的减振降噪影响分析

为了评价有轨电车弹性车轮动力吸振器的减振降噪效果,通过实验室测试方法对动力吸振器进行振动噪声测试,并结合理论和仿真来分析其降噪特性。首先,在半消声室分别测试弹性车轮在有无动力吸振器情况下的振动声辐射,测试结果表明：动力吸振器对弹性车轮轴向振动有明显的抑制作用。径向激励下,动力吸振器的降噪量为0.6 dB(A),轴向激励下,动力吸振器的降噪量为2.6 dB(A)。进而,基于动力吸振原理探究动力吸振器的降噪性能,并结合测试图纸建立动力吸振器有限元模型,分析表明：动力吸振器在车轮固有频率2 066 Hz、2 245 Hz和3 837 Hz处降噪效果较好,原因是降噪频率差值在2 %以内,调谐频率和理论最优频率相吻合。动力吸振器在车轮固有频率899 Hz处降噪效果较差,其降噪频率差值为6.26 %,由于调谐减振频率偏离最优同调条件,导致降噪性能的恶化。     

基于声源监测的厂界噪声预测及贡献分析方法研究

噪声监测是环境噪声预测和治理的重要技术方法。提出了一种基于声强测量的声源监测方法并应用于电厂环境噪声预测和厂界噪声贡献分析。在电厂主要设备噪声源附近布置测点测量并计算设备厂房的辐射声强，将设备厂房简化为面声源建立噪声预测模型，并以测量计算的声强级作为声源模型的源强。利用该模型计算厂界预测点A声级，与实验值具有良好的一致性，验证了该声源监测方法数据的可靠性与噪声预测模型的正确性。通过该模型计算分析了电厂主要噪声源对厂界噪声排放的贡献和影响，为电厂噪声治理提供技术依据。     

高速轿车气流噪声模拟研究

采用大涡模拟（LES）方法计算了某轿车模型的瞬态外流场，并研究了车辆表面脉动压力和流态，然后采用FW-H声学模型，预测了车外场点的噪声特性。根据流场和声学模拟结果对某轿车模型进行修改，并进行了噪声测量，模型修改后气流噪声有显著降低。     

Metallicity and ferromagnetism in nanosystem of charge ordered Nd0.5Sr0.5MnO3

The fascinating phenomenon of destabilization of charge/orbital order in Nd0.5Sr0.5MnO3 with the reduction of grain size is critically investigated. Based on our magnetic and transport experiments followed by a theoretical analysis, we analyze various possible mechanisms and try to delineate a universal scenario behind this phenomenon. We revisit this issue carefully and discuss various evidences from experiments in nano and bulk manganites on the absence of correlation between size reduction and pressure effects on manganites. We propose a phenomenological model based on enhanced surface disorder to explain the appearance of weak ferromagnetism and metallicity in nanosize Nd0.5Sr0.5MnO3 system. All evidence seems to suggest that the transport is mediated through the surface via enhanced density of states in the nanometric grains. We provide theoretical support for this by performing an ab-initio electronic structure calculation as well as from a recent numerical simulation and argue that the mechanism is likely to be general in all nanosize charge ordered manganites.     

Surface state dependence of PbS and PbSe infrared noise and detectivity

It is established that the surface state is one of the most important mechanisms limiting the performance of photoconductors. In a previous work, we have presented the theoretical platform of surface state model exhibiting the analysis of the space charge region at the free surface of semiconductors. To know that recombination effects directly influence the quantum efficiency of detector, the dependence of some detector parameters on surface defect densities and the comparison of this model with experimental data of responsivity are evaluated. The present papers use this model to explore the dependence of surface parameters on signal, noise and detectivity. The results of the model are compared with the experimental results for p-PbS case.     

Correlation Between AC Core Loss and Surface Magnetic Barkhausen Noise in Electric Motor Steel

Core loss is a significant source of energy loss in electric motor steel laminates. Therefore, there is interest in monitoring the quality and consistency of laminates at various stages of manufacturing. The purpose of this study was to investigate the feasibility of using surface magnetic Barkhausen noise for the evaluation of AC core loss, and further, to examine potential origins of magnetic loss in non-oriented electrical steel. Core loss values were measured by a single sheet tester and Barkhausen noise measurements were performed using pole flux control on eight laminates with various grain size, texture and composition. Magnetocrystalline energy was calculated from X-ray diffraction data to quantify texture. Results demonstrated higher surface Barkhausen emissions for samples with lower core loss. Barkhausen noise analyses were used to examine the interplay among core loss, grain size, magnetocrystalline energy and B–H characteristics. The inverse correlation between core loss and Barkhausen noise emissions was qualitatively explained in terms of the orthogonal vector contribution of microscopic eddy currents to losses associated with bulk currents arising in the sample during magnetization.     

Quantum noise analysis in correlated two-photon imaging

The theoretical analysis of quantum noise is presented in a correlated two-photon imaging system with entangled photons. The noise is strongly influenced by the size and the entanglement degree of the source. An increase of the source size results in a remarkable decrease of the amplitude of the noise when the size is much smaller than the aperture of the lens, whereas the change will not be obvious if the size is close to the aperture. The difference of the effects from the source between correlated imaging formed with entangled photons and with partially coherent light is also discussed.     

Measurement Problems in Optical Monitoring Systems

A probabilistic image model is proposed. According to the model, the noise of the boundaries has the principal effect on the precision with which the dimensions of optical monitoring objects are measured. Results of theoretical and experimental investigations of measurement precision are presented. An approach to the determination of monitoring reliability and the selection of the resolution of a machine vision system based on structural requirements is formulated.