Autocalibrating Stokes polarimeter for materials characterization

The design and construction of an ellipsometer based on a fixed-wavelength rotating-retarder Stokes polarimeter is described. Details are provided for an automated calibration scheme that provides two advantages for its operation. The first allows the phase of the lock-in amplifiers to be set based on the raw data, without a known calibration sample. The second illustrates that the relative amplitude of the acquired signals may also be calibrated in a similar manner. As an illustration, the refractive index and thickness of a glass cover slide are determined over a range of incident angles.     

Data analysis for a rotating quarter-wave, far-infrared Stokes polarimeter

Data analysis techniques are reviewed and extended for the measurement of the Stokes vector of partially or completely polarized radiation by the rotating quarter-wave method. It is shown that the conventional technique, based on the Fourier analysis of the recorded signal, can be efficiently replaced by a weighted least-squares best fit, so that the different accuracy of the measured data can be taken into account to calculate the measurement errors of the Stokes vector elements. Measurement errors for the polarization index P and for the azimuth and ellipticity angles psi and chi of the radiation are also calculated by propagation error theory. For those cases in which the above technique gives a nonphysical Stokes vector (i.e., with a polarization degree of P>1) a constrained least-squares best fit is introduced, and it is shown that in this way a Stokes vector with P = 1 (rather than P相似文献   

Target detection with a liquid-crystal-based passive Stokes polarimeter

We present an imaging system that measures the polarimetric state of the light coming from each point of a scene. This system, which determines the four components of the Stokes vector at each spatial location, is based on a liquid-crystal polarization modulator, which makes it possible to acquire four-dimensional Stokes parameter images at a standard video rate. We show that using such polarimetric images instead of simple intensity images can improve target detection and segmentation performance.     

Concurrent measurement of linear and circular birefringence using rotating-wave-plate Stokes polarimeter

A novel technique is presented for obtaining concurrent measurements of the linear and circular birefringence properties of an optical sample by using a rotating-wave-plate Stokes polarimeter to extract the 2x2 central elements of the corresponding Mueller matrix via two linearly polarized probe lights. For a compound sample comprising a half-wave plate in series with a quarter-wave plate, the measured values of the principal angle and retardance of the quarter-wave plate are found to have average normalized errors of 0.56% and 1.16%, respectively, while the measured value of the rotation angle of the half-wave plate has an error of just 0.39%. When analyzing glucose solutions with concentrations ranging from 0-1.2 g/dl positioned in front of a half-wave plate, the average normalized errors in the principal axis angle and retardance measurements of the half-wave plate are 0.69% and 2.65%, respectively, while the error in the rotation angle measurements of the glucose solutions is 2.13%. The correlation coefficient between the measured rotation angle and the concentration of the glucose solution is determined to be 0.99985, while the standard deviation is just 0.0022 deg. Overall the experimental results demonstrate the ability of the proposed system to obtain highly accurate measurements of the linear and circular birefringence properties of an optical sample and to decouple the relationship between the principal axis angle and the rotation angle.     

General method for calibrating Sun photometers

A general method for calibrating Sun photometers that relaxes the constraints on atmospheric conditions is described. Instead of requiring constant extinction conditions the method requires only that the relative aerosol size distribution remains constant during observations over a range of air masses during a morning or afternoon. Provided that the relative aerosol extinction component [m(a)(t)δ(t, λ(0))] can be obtained at wavelength λ(0), the calibration at wavelength channel λ can be calculated with simple least-squares techniques. A variant of the method in which a Sun photometer is used to provide [m(a)(t)δ(t, λ(0))] is detailed and is verified with both model atmospheres and Sun-photometer data for 1988-1991 from Cape Grim, Tasmania (41°S). The method produces calibration data having sample variances more than 5 times smaller than Langley method calibration results.     

标定十字结构光传感器的新方法

建立了基于十字结构光的视觉传感器的数学模型,提出了一种基于自由移动平面靶标的十字结构光传感器参数的简易标定方法。在自由移动的平面靶标上建立局部世界坐标系,将通过交比不变方法获得的两个光平面上特征点的局部世界坐标,变换到摄像机坐标系,从而获得已知三维的标定特征点。利用构建的分别位于两个光平面上标定特征点,可以实现工作状态的十字结构光传感器参数的优化估计。该标定方法降低了标定设备的成本,简化了标定过程,为十字结构光传感器的工程化应用奠定了基础。通过对圆的直径和中心坐标的测量实验,结果表明,该方法切实可行。     

A hybrid method for calibrating acoustic arrays

A method to calibrate the elements of large arrays devoted to underwater applications is presented. The goal is to measure the sensitivity and directivity of the elements over their full bandwidth. The main constraint comes from the bounded geometry of the experimental setups that limits the duration of the time windows available for analyzing the received signals. Using a short wideband pulse is detrimental to obtaining high signal-to-noise ratios. A classical method for handling this problem is time-delay spectrometry (TDS), which is based on the transmission of a linear frequency- modulated signal combined with a sliding frequency filter. An alternative, hybrid method based on the transmission of a sequence of time-frequency-limited signals is proposed. This hybrid method is shown to provide the same spectral density as TDS in the frequency scanning, but the filtering process is quite different. The transmitted signals are designed to take advantage of the coherent sums of the received signals to track the time of flight of the direct paths between the source and the elements. In addition, a fitting process based on the calibration geometry of data acquisition enables the boundaries of the interference-free time windows to be precisely delineated. An example of the application is described.     

A method of calibrating a stripline contact device

A method of calibrating a stripline contact device, which increases the accuracy of its calibration using a matched reference stripline calibrator and a short circuit or a single short circuit, is considered. __________ Translated from Izmeritel’naya Tekhnika, No. 5, pp. 59–61, May, 2006.     

Homography-based method for calibrating an omnidirectional vision system

We present a homography-based method for calibrating an omnidirectional vision system with a parabolic mirror. Assuming that the intrinsic parameters of the camera are known a priori, we focus on finding the solution for the mirror parameter and its positions. We first estimate the homographic matrix partially using six or more point correspondences. Then the rotation matrix and two components of the translation vector can be estimated. Finally, the remaining parameters are computed. In this method, a closed-form solution for all the variables is obtained using the homographic matrix. Another advantage is the enhanced robustness in implementation via the use of two over-constrained linear systems. Numerical simulations and real data experiments are also performed to validate the proposed algorithm.     

A method of calibrating magnetometers on a spinning spacecraft

This paper describes a procedure for quantifying and compensating for angular offsets and slow-varying drifts associated with a flux-gate magnetometer onboard a spinning spacecraft. Such magnetometers have been down on numerous spacecraft such as IMP-8, DE-1, and ISTP/GEOTAIL, and similar instruments are currently proposed to fly on the ISTP/WIND spacecraft. Consequently, slight geometric misalignment of the sensor from their chosen axes will create errors in the measured signal. These misalignments can be quantified using perturbation theory and compensated in the data analysis process. The technique is applicable to magnetometers on spinning spacecraft. Also, over long time periods, the magnetometer will develop slight drifts in the electronic components, and these too can be quantified in the data analysis process. A technique is discussed that quantifies and compensates for such perturbations from the measurements. The technique has been successfully applied for the last sixteen years to IMP-8 magnetometer data and more currently to the GEOTAIL magnetometer data, and can be applied in a general way to measurements from any magnetometer onboard a spinning spacecraft. Further improvements in the technique are also discussed