Influence of geometry on polychromatic speckle contrast

Understanding speckle behavior is very important in speckle metrology application. The contrast of a polychromatic speckle depends not only on surface roughness and the coherence length of a light source, as shown in previous works, but also on optical geometry. We applied the Fresnel approach of diffraction theory for the free-space geometry and derived a simple analytical relationship between contrast, coherence length, size of illuminated spot, and distances between source, object, and observation plane. The effect of contrast reduction is found to be significant for low-coherence light sources.     

Surface roughness measurement by means of polychromatic speckle elongation

A new approach for determining the roughness of engineering surfaces that is applicable to industrial in-process measurements is introduced. Laser speckle patterns, arising from light scattered from rough surfaces that are illuminated by polychromatic laser light, are detected in the far-field region. The incoherent superposition of these light intensities and the angular dispersion cause the effect of speckle elongation. This is characterized by increasing speckle widths and leads to a radial structure of the speckle patterns. With increasing surface roughness, the elongation is replaced more and more by the decorrelation of the monochromatic speckle patterns for the different wavelengths. Such effects were detected with the CCD technique and analyzed by local autocorrelation functions of intensity fluctuations that were calculated for different areas of the speckle patterns. The results of surface-roughness determination by means of the speckle elongation effect are presented.     

Statistics of local speckle contrast

In describing the first-order properties of laser speckle under polarized illumination conditions, it is almost an article of faith that the contrast is unity. In many processing schemes, however, the contrast defined as the quotient of the standard deviation and the mean is calculated over a localized spatial region. In such cases, this local contrast displays a distribution of values that can depart substantially from unity. Properties of this distribution depend on details of the data acquisition and on the size of the local neighborhood over which the contrast is calculated. We demonstrate that this local contrast can be characterized in terms of a log-normal distribution. Further, we show that the two defining parameters of this model can in turn be expressed in terms of the minimum speckle size and the extent of the local neighborhood. Performance of the model is illustrated with some typical optical coherence tomography data.     

Quantitative temporal speckle contrast imaging for tissue mechanics

We demonstrate through a series of simulations that by parameterizing the temporal speckle contrast statistic from a sequence of translating speckle images on a number of experimental constants, the local temporal contrast can be used to quantitatively assess local motion, provided that the spatial and temporal Nyquist sampling criteria are both met. We develop a simple exponential model for quantifying speckle motion for speckle patterns that display arbitrary intensity statistics and provide suggestions for optimizing both the experimental acquisition of speckle data and the temporal contrast analysis of the data. The confounding effects of uncorrelated noise are also discussed. The model is demonstrated by applying it to an optical coherence tomography image sequence of an engineered tissue construct undergoing dynamic compression. Applications to tissue mechanics are shown, although the discussion is equally relevant for fluid motion studies.     

Speckle contrast of the sum of N partially correlated speckle patterns

In this paper a general method is presented for calculating the theoretical speckle contrast of a sum of correlated speckle patterns, motivated by the need to suppress the presence of speckle in laser projection displays. The method is applied to a specific example, where correlated speckle patterns are created by sequentially passing light through partially overlapping areas on a diffuser, before being projected onto a screen. This design makes it possible to find a simple expression for the correlation between speckle patterns. When the set of correlations involves symmetry, it is shown that the expression for the speckle contrast becomes simpler. The difference in performance between discretely and continuously varying speckle patterns is also investigated. In an example with speckle reduction by a rotating sinusoidal grating, it is found that continuous variation gives a speckle contrast that is 0.61 times the contrast obtained by discretely summing the maximum number of independent patterns.     

Determination of optimal exposure time for imaging of blood flow changes with laser speckle contrast imaging

Laser speckle contrast imaging is becoming an established method for full-field imaging of cerebral blood flow dynamics in animal models. The sensitivity and noise in the measurement of blood flow changes depend on the camera exposure time. The relation among sensitivity, noise, and camera exposure time was investigated experimentally by imaging the speckle contrast changes in the brain after electrical forepaw stimulation in rats. The sensitivity to relative changes in speckle contrast was found to increase at longer exposure times and to reach a plateau for exposure times greater than approximately 2 ms. However, the speckle contrast noise also increases with exposure time and thus the contrast-to-noise ratio was found to peak at an exposure time of approximately 5 ms. Our results suggests that approximately 5 ms is an optimal exposure time for imaging of stimulus-induced changes in cerebral blood flow in rodents.     

Versatile polychromatic dynamic testbed for optical disks

A dynamic testbed for the evaluation of optical disks has been designed and constructed. The system is achromatic within the wavelength range 440-690 nm, allowing any light source in this range to be utilized for read-write-erase experiments. In addition, the system accepts disks with substrate thicknesses ranging from 0 to 1.7 mm. The polarization handling capabilities of the testbed are such that, with the turn of a knob, one can generate either linearly polarized or circularly polarized light at the disk surface. This feature permits the testing of both magneto-optical and phase-change disks, in addition to compact disks and digital versatile disks, without any modifications to the system. A leaky polarizing beam splitter (LPBS) has been specially designed and built for this tester. The LPBS allows continuous adjustment of the ratio between p- and s-polarized components of the reflected beam that reach the detectors. This feature is especially useful for magneto-optical disks, where one can achieve an optimum signal-to-noise ratio by adjusting the relative amounts of the two components of polarization at the detection module. Focus-error detection is based on the astigmatic method, and the primary track-error detection scheme is the push-pull method, although other focusing and tracking schemes may also be implemented. The rf data signal and the focusing and tracking servo signals are all derived from the same detectors, thus allowing the optical power returning from the disk to be used in its entirety for these multiple purposes. The detection channel consists of two high-speed quad detectors mounted on the two arms of a differential detection module. By combining the various outputs of these detectors it is possible to generate the astigmatic focus-error signal, the push-pull track-error signal, the differential magneto-optical readout signal, the conventional sum signal for phase-change disk readout, and the differential edge-signal for mark-edge detection on various types of optical media.     

Color encoding for polychromatic single-channel optical pattern recognition

The common multichannel system for recognizing colored images is replaced by a color-encoded single-channel system. Amethod inspired by the Munsell color system is used for encoding the different colors as phase and amplitude functions. It is shown that for many practical cases the phase information part of the color code is sufficient for obtaining good results. An implementation based on a liquid-crystal television panel that works in a phase-modulation mode is suggested. Computer simulations that demonstrate the capabilities of the suggested method are given as well as a comparison with previously published multichannel performance.     

颗粒物质表面波动的简单模型

实验表明 ,垂直振动下的颗粒物质呈现类似流体的行为 ,例如隆起、对流、分层及表面波动 [1] 。将颗粒物质置于容器内。在外加激励下 ,容器作垂直振动。当无量纲的激励加速度振幅 Γ=4 π2 f2 A/ g<1时 (其中 A为激励振幅 ,f 为激励频率 ,g为重力加速度 ) ,颗粒呈压紧状态 ,并随着容器一起振动 ,颗粒层表面是平坦的。当Γ >1时 ,在每个振动周期的部分时间内 ,Γ sin2 πft<- 1,颗粒层与容器底面分离并自由飞行 ,颗粒层厚增加 ,但表面仍保持平坦 ,这就是所谓“热膨胀”现象。随着 Γ 的进一步增加 ,至某一临界加速度 Γc,颗粒层厚度增加到 h0…     

Simple lattice model with inertia for sand piles

In this article we make a critical analysis of the model proposed by Weber, present an improvement that gives the logarithmic correction on the shape of sandpiles properly, and study the normal forces on the surface supporting a pile using the improved model.     

Calculation of retinal image quality for polychromatic light

Although the retinal image is typically polychromatic, few studies have examined polychromatic image quality in the human eye. We begin with a conceptual framework including the formulation of a psychophysical linking hypothesis that underlies the utility of image quality metrics based on the polychromatic point-spread function. We then outline strategies for computing polychromatic point-spread functions of the eye when monochromatic aberrations are known for only a single wavelength. Implementation problems and solutions for this strategy are described. Polychromatic image quality is largely unaffected by wavelength-dependent diffraction and higher-order chromatic aberration. However, accuracy is found to depend critically upon spectral sampling. Using typical aberrations from the Indiana Aberration Study, we assessed through-focus image quality for model eyes with and without chromatic aberrations using a polychromatic metric called the visual Strehl ratio. In the presence of typical levels of monochromatic aberrations, the effect of longitudinal chromatic aberration is greatly reduced. The effect of typical levels of transverse chromatic aberration is virtually eliminated in the presence of longitudinal chromatic aberration and monochromatic aberrations. Clinical value and limitations of the method are discussed.     

Monitoring thermal-induced changes in tumor blood flow and microvessels with laser speckle contrast imaging

Laser speckle contrast imaging (LSCI) was used to monitor thermal-induced changes in the blood flow and the diameter of mesenteric microvessels of normal and tumor bearing mice under 60 min treatment at different constant temperatures between 41 degrees C and 45 degrees C. The results show that the blood flow and the diameter increase at the beginning and then reach a plateau and finally start to decrease. The lower the temperature, the longer the plateau stays. A t-test indicates that there is no significant difference in plateau values of relative blood flow and relative diameter for the same group. For normal mice, the relative increases in the blood flow and the diameter are 1.26 and 1.41, respectively, while for tumor-bearing mice they are 1.08 and 1.13, respectively. At higher treatment temperature or under longer heat treatment, there are decreases in the blood flow and the diameter, such changes in tumor-bearing mice are more obvious than those in normal mice, which means tumor microvessels are more sensitive to heat than normal. Moreover, thermal induced shrink of microvessel usually occurs sooner than the decrease in blood flow, and the relative change in diameter is larger than that in blood flow. Therefore we may conclude that deformation of vessel is a main factor for changing the blood perfusion of a microvessel.     

Simple method for simultaneously measuring the magnitude and direction of 2D in-plane displacement in white-light speckle photography

A simple method for simultaneously measuring the magnitude and direction (or argument) of a 2D in-plane displacement vector is presented. The theoretical mechanism for extracting the desired displacement signal from random noises produced by the nonoverlapping area of the recorded specklegrams of the test specimen surface is derived in detail. This procedure is realized by establishing a 1D displacement model in a white-light speckle photographic system using discrete Fourier transform shifting theorem. Results of the computer simulation and experiment show that the present method exhibits advantages such as convenient displacement direction determination, better robustness and wider measurement range than those of the traditional fringe analysis-based method. These characteristics make the proposed method a potential approach in practical applications.     

Simple correlated channel model for ultrawideband multiple-input multiple-output systems

A simple correlated channel model for ultrawideband (UWB) multiple-antenna systems is proposed. The authors show that a single numerical value of the spatial correlation coefficient is sufficient to accurately model the performance of UWB spatial multiplexing systems in an indoor environment. The appropriate value of the correlation coefficient is selected by ensuring a close match between the bit error rate results achieved on the proposed correlated channel and those on the measured indoor channel. The authors also experimentally confirm that the performance substantially degrades in the presence of high values of spatial correlation for a range of spatial multiplexing receivers, and quantify the relationship between this degradation and the value of the spatial correlation coefficient. Thus, a route for the development of the existing standards for single- antenna UWB channels to the multiple-antenna regime is provided here.     

Simple lattice model ford-type gap anisotropy for layered cuprates

Double electron exchange between Cu4s and O2p_σ orbitals is analyzed as mechanism which creates traditional cooper pairing in layered cuprates. The conductivity band is obtained in the oversimplified model which considers only hopping between O2p_σ orbitals, and the amplitude of the Cu4s-O2p_σ hybridization is accounted as perturbation. The proposed toy model possesses most of the distinguishable features of the experimental data: angle dependence of the superconducting gap, the shape of the Fermi surface, dispersion law for conductivity band and the electron spectrum of the superconducting phase. The influence of the Cu3d_x ²−y² orbital, electron doping and disorder for transition to s-type superconducting gap is briefly discussed.     

Simple model for scanning calorimeter with pyroelectric heat sensor

The heat-conduction equation is solved for a simplified model of a scanning heat-conducting calorimeter with a working cell in the form of a spherical pyroelectric thermometer. The optimum material and dimensions for the pyroelectric sensors are determined by numerical analysis of the solution.     

Simple physical model for the fragmentation of rock avalanches

Rock avalanches are the largest granular flows on Earth. In contrast to artificial, small-scale granular avalanches, they exhibit a large degree of fragmentation with reduction of average grain volume by a factor of up to 10¹⁵–10¹⁸. Even though fragmentation likely affects the whole dynamics of the rock avalanche, as yet the basic mechanics of the process is poorly known. In this work, a simple model is presented for the fragmentation of rock avalanches, assuming that most of the fragmentation occurs along force chains in the granular medium. The landslide motion is simulated along a curved bumpy profile path. The predicted grain spectra are found to agree reasonably with field data.     

Simple model for wet granular beds subjected to tapping

We present a simple model to describe the response of a granular bed to a tapping-like excitation when grains can form capillary bridges due to the presence of interstitial liquid. We implement a pseudodynamic simulation of adhesive hard disks. The packing fraction and coordination number after the steady state of the tapping process has been reached are compared for different tapping intensities and liquid contents. We find some contrasting behavior with dry systems and qualitative agreement with experimental data.     

Simple model for the parametric analysis of grid amplifiers

A simple method for parametric analysis of grid amplifier performances is presented here. The proposed approach is based on the development of an equivalent transmission line model where each element of the grid is represented by an S-matrix derived from a full-wave simulation in an infinite array environment. The proposed technique makes it possible to investigate the dependency of grid amplifier performance on its major parameters taking into account the actual geometry of each part of the circuit while preserving the simulation efficiency.     

Simple model for austenite grain growth in microalloyed steels

Abstract

Previous models for grain growth are usually based on Beck's formula, which are inadequate for quantitative prediction of austenite grain growth during reheating of as cast microstructures in microalloyed steels. The applications of these empirical grain growth models are limited to some particular categories of steels, such as Nb, Nb–Ti and Ti–V microalloyed steels, etc. In this study, a metallurgically based model has been developed to predict the austenite grain growth kinetics in microalloyed steels. This model accounts for the pinning force of second phase particles on grain boundary migration, in which the mean particle size with time and temperature is calculated on the basis of the Lifshitz–Slyozov–Wagner (LSW) particle coarsening theory. The volume fraction of precipitates is obtained according to the thermodynamic model. The reliability of the model is validated by the agreement between theoretical predictions and experimental measurements in the literature.