Full-field retardation measurement of a liquid crystal cell with a phase shift polariscope

A novel method is developed to provide high-resolution, full-field retardation measurement of the liquid crystal cell using a phase shift polariscope. The phase retardation of a liquid crystal cell is influenced by its thickness, and hence an accurate measurement of the full field retardation gives the thickness distribution of the liquid crystal cell. In the phase shift polariscope, phase shift images obtained by rotating the analyzer are recorded and analyzed to obtain the phase retardation and hence the whole field thickness distribution of the liquid crystal cells. The results obtained using this method are compared with those using a commercial pointwise sensor with good agreement. The proposed method yields accurate and repeatable full-field cell gap measurement of the liquid crystal cell with a simple experimental setup.     

Simultaneous absolute measurements of principal angle and phase retardation with a new common-path heterodyne interferometer

This study demonstrates a new method for simultaneously measuring both the angle of the principal axis and the phase retardation of the linear birefringence in optical materials. We used a circular common-path interferometer (polariscope) as the basic structure modulated by an electro-optic (EO) modulator. An algorithm was developed to simultaneously measure the principal axis and the phase retardation of a lambda/4 or lambda/8 plate as a sample. In the case of a lambda/4 plate, the average absolute error of the principal axis is approximately 3.77 degrees, and that of the phase retardation is approximately 1.03 degrees (1.09%). The retardation error is within the 5% uncertainty range of a commercial wave plate. Fortunately, the nonlinear error caused by the reflection phase retardation of the beam splitter dose not appear in the new system. Therefore the error could be attributed to misalignment and defects in the EO modulator or the other optical components. As for the repeatability of this new common-path heterodyne interferometer, the average deviation for the principal axis is 0.186 degrees and the phase retardation is 0.356 degrees. For the stability, the average deviation for the principal axis is 0.405 degrees and the phase retardation is 0.635 degrees. The resolution of this new system is estimated to be approximately 0.5 degrees, and the principal axis and phase retardation could be measured up to pi and 2pi, respectively, without ambiguity.     

Laser heterodyne method for high-resolution gas-density measurements

A new method for noncontact, high-resolution measurement of gas density is described. The method uses a two-frequency Zeeman-split He-Ne laser and cumulative phase-measuring electronics. The measurement is resolved in two dimensions and provides density that is averaged only along the length of the laser beam that passes through the test section. The technique is based on highly accurate measurement of the optical path-length change of the laser beam as it passes through a test cell (in principle, to within 0.001lambda, where lambda is the wavelength of the laser). The technique also provides a very large dynamic range (again, in principle, up to 10(10)), which makes the method additionally attractive. Although the optical path length through the test section is directly related to the index of refraction, and hence to the density of the gas, the method can also be used to measure temperature (if the gas pressure is known) or pressure (if the temperature is known).     

Polariscope for simultaneous measurement of the principal axis and the phase retardation by use of two phase-locked extractions

A novel polariscope with electro-optic modulation that is capable of simultaneous measurement of the principal axis and the phase retardation of an optical linear birefringent medium by means of two phase-locked extractions is described. A phase compensator is used to suppress the transmission phase-retardation effect of the beam splitter, thereby enhancing the precision of the measuring performance. The validity of the proposed design is demonstrated by measurement of the principal axis and phase retardation of a quarter-wave plate sample. There are absolute errors of 0.25 degrees on average and 0.58 degrees at maximum in the principal-axis measurement and of 0.75 degrees (0.83%) on average and 3.11 degrees at maximum in the phase-retardation measurement. Meanwhile, the retardation error lies within a 5% uncertainty range of a commercial wave plate. The root-mean-square resolutions for the principal-axis angle and phase-retardation measurements are 0.042 degrees and 0.081 degrees, respectively. Finally, the dynamic ranges of the principal-axis angle measurement and the phase-retardation measurement extend as far as 180 degrees.     

Homodyne and heterodyne signal processing assisted phase resolved optical technique for latent fingerprint imaging: a theoretical study

A general theory for phase-resolved fingerprint imaging and background suppression based on laser-induced fluorescence is developed and presented in this paper. Novel approaches of incorporating an even-step phase shifting method with a homodyne-assisted phase-resolved method as well as the camera exposure control approach for the heterodyne-assisted phase-resolved method are proposed, theoretically formulated and discussed. Theoretical results imply that the fluorescence from latent fingerprints can be extracted effectively, irrespective of whether its lifetime is longer than that of the background or not. Furthermore, it is shown that there exists an optimum modulation frequency, which is dependent on the fluorescence lifetimes of both the background and the fingerprint, to obtain a fingerprint image with better contrast.     

Two-wave-plate compensator method for single-point retardation measurements

The two-wave-plate compensator (TWC) technique is introduced for single-point retardation measurements. The TWC method uses a known wave plate together with a wave plate of unknown retardation and produces a linearly polarized output that allows a null of intensity to be detected. The TWC method is compared both theoretically and experimentally with the existing Brace-K?hler and Sénarmont methods. The resolution of the TWC is shown to be 0.02 nm. TWC enables the measurement of a sample retardation with as little as 0.13% error and thus is more accurate than either the Brace-Kohler or the Sénarmont method.     

Two-wave-plate compensator method for full-field retardation measurements

The two-wave-plate compensator (TWC) method is expanded for full-field retardation measurements by use of a polarization microscope. The sample image is projected onto a CCD camera connected to a computer, allowing the retardation to be measured at all pixels. The retardation accuracy of this implementation of the TWC is evaluated to be 0.06 nm. The method is applied to polarization-maintaining fibers and long-period fiber gratings. The measured retardation is in good agreement with the crossed-polarizer images of the fibers. The method achieves a spatial resolution of 0.45 microm and a retardation resolution of 0.07 nm. The full-field TWC method can thus be a useful tool for characterizing and monitoring the fabrication of optical devices.     

Novel method for determining the fast axis and phase retardation of a wave plate using Fresnel Rhomb

In this study, an optical scheme based on the measurement of light intensity and a proper orientation of the Fresnel Rhomb is successfully developed for simultaneous determination of the phase retardation and fast axis of a wave plate. The theoretical analysis of the principle of measuring the reflection-induced retardance of the Fresnel Rhomb and the phase retardation of a wave plate is given taking Stokes–Mueller Formalism as a mathematical tool. In this method, the fast axis position of the wave plate need not be determined in advance. In addition, the measured result is free of the intensity fluctuation of light source. An application example is also demonstrated. The measured result of the example is verified with an experiment. Some of the merits of the method, such as, a simple optical set-up, high stability and high accuracy, low cost and easier operation, are presented, and the validity of the method is demonstrated.     

Full-field phase error compensation method based on relationship between unwrapped phase and phase error

The three-dimensional reconstruction in phase-measuring profilometry (PMP) usually involves the phase error caused by the gamma effect of the projector. In this study, the relationship between the unwrapped phase and the phase error of every pixel is analysed, and an effective full-field phase error compensation method based on this relationship is proposed for the reduction of every pixel error. In our optimized PMP system, the full-field phase error can be detected by directly fitting the unwrapped phase of the reference plane. In addition, the relationship between the unwrapped phase and the phase error can be established by creating a phase-error lookup table for the phase error compensation of every pixel. The experimental results demonstrate the effectiveness of the proposed method in practical PMP, and the measurement errors can be reduced by a factor of least 10.     

Full-field analysis and image forces for piezoelectric bimaterials

Summary In this study, the two-dimensional explicit full-field solutions of transversely isotropic piezoelectric bimaterials subjected to mechanical and electrical loads are derived by using the Fourier-transform technique. The major objective of this study is to analyze the physical meaning and the structure of the solution. One of the novel features is that Green’s functions for bimaterials consist of Green’s functions for the infinite plane. The complete solutions of this problem include Green’s function of originally applied singularities in an infinite medium and nine image singularities which are induced to satisfy interface continuity conditions. It is shown that the physical meaning of the solution is the image method. The mathematical method used in this study provides an automatic determination for the locations of image singularities. The locations of image singularities are dependent on the roots of the characteristic equation for bimaterials. According to the characteristic roots, the number and distribution for image singularities are discussed in detail. The expressions for image forces acting on edge dislocations are given explicitly with the aid of the generalized Peach–Koehler formula. Numerical results for the full-field distributions of stresses, electric fields in bimaterials and image forces for edge dislocations are presented. Dedicated to Professor Franz Ziegler on the occasion of his 70th birthday     

Instantaneous phase method for readout signal processing of body dithered ring laser gyro

The readout signal of a body dithered ring laser gyro contains both useful information and the dither component. The dither component must be removed to get the useful information. The dither stripping method can get the useful information without latency. But the quadrature demodulator only has 1/4 pulse resolution, which means that the quantization noise will limit the short-term accuracy. The effect and the property of quantization noise are analyzed in detail. Taking advantage of high-speed analog-to-digital conversion (ADC), the analog values of the two beat frequency signals in quadrature are sampled. A novel instantaneous phase method is introduced, which can get the residual phase besides 4× resolution and the quantization noise can be removed radically. The Allan variance analysis of experimental results shows that the quantization noise coefficient using this method is only 1/7 of that using the original 4× resolution method.     

归一化算法实现畸变不变图像识别研究

在光电混合联合变换相关器图像识别系统中,对与参考图像完全相同的真目标图像和不同的假目标图像能实现完全识别,但对于与参考图像间存在旋转和比例变化的目标图像却很难识别,甚至无法识别。由此,提出了基于图像质心和最长边的归一化图像处理新算法:根据图像像素总值求取图像质心,根据质心和图像边缘轮廓点之间的长度求出图像的最长边和旋转角度,然后根据统一标准将目标图像和参考图像调整至同一长度和旋转方向的完全相同的图像。最后给出了模拟和实验的相关结果,结果表明,该方法能实现存在旋转和比例变化的图像的畸变不变处理,从而提高了系统的识别能力。     

Digital signal processing techniques for high accuracy ultrasonicrange measurements

Several digital signal processing (DSP) methods are analyzed and compared with respect to the expected errors for an ultrasonic range measurement arrangement. These include L1, L2 norms and correlation with different approaches for envelope extraction. The influence of different factors such as signal-to-noise ratio (SNR), sampling frequency, and digitizing resolution on measurement errors is analyzed using a synthetic approach through nearly 40000 simulations. Results show different performance levels involving accuracy, computing time, and cost for the studied methods, although all of them allow reduction of errors by several orders of magnitude     

Hybrid optical-digital signal processing applied to an optimal nonlinear phase estimator

An electrooptical realization of an optimal phase estimator for phase modulated communication signals is described in this work. The realization uses an electrooptical processor to perform a 1-D convolution in a 2-D space and a computer to complete the calculations. The processor is a realization of the time domain recursive nonlinear filter. This paper describes the nonlinear filter theory, the electrooptical realization,and the performance of the processor.     

Wigner-related phase spaces for signal processing and their optical implementation

Phase spaces are different ways to represent signals. Owing to their properties, they are often used for signal compression and recognition with high discrimination abilities. We present several recently introduced Wigner-related sets of representations that have improved signal processing performance, and we introduce an optical implementation. This study deals with the generalized Wigner spaces, the fractional Fourier transform, and the x-p and the r-p representations. The optical implementations are demonstrated and discussed.     

Applications of the image processing method on the structure measurements in porous GaN

Porous GaN films on sapphire (0001) prepared by ultraviolet-assisted electrochemical etching and their quantitative structural characteristics based on mathematical morphology analysis using scanning electron microscope (SEM) images are reported in this study. The evaluation of porous GaN quality can be performed through a non-destructive investigation of its nanostructures using adapting image analysis techniques to obtain rapid, objective and quantitative information. The algorithm used for this work was implemented using the MATLAB software. The distributions of the maximum, minimum and average radii of the pores were obtained. Moreover, the porosity of the structures was obtained by calculating the areas occupied by the pores. SEM micrographs indicated that the shapes of pores for all porous samples were nearly hexagonal. The quantitative results were obtained and related with the characteristics of the fabrication process, showing reliability and promise in the control of pores over the formation process.     

Transform method of processing for speckle strain-rate measurements

We have developed a highly sensitive method for measuring thermal expansion, mechanical strain, and creep rates. We use the well-known technique of observing laser speckle with a pair of linear array cameras, but we employ a data-processing approach based on a two-dimensional transform of the speckle histories from each camera. This technique can effect large gauge sizes, which are important in the assessment of the spatial statistics of creep. Further, the algorithm provides simultaneous global estimates of the strain rates at both small- and large-scale sizes. This feature may be of value in the investigation of materials with different short- and long-range orders. General advantages of our technique are compact design, modest resolution requirements, insensitivity to slow surface microstructure changes (as seen with oxidation), and insensitivity to zero-mean-noise processes such as turbulence and vibration. Herein we detail the theory of our technique and the results of a number of experiments. Thesetests are intended to demonstrate the performance advantages and limitations of the transform method of processing speckle strain-rate data.     

Invariant operators for processing the results of measurements with a finite-dimensional image for increased reliability

An operator for processing data from a specific experiment is considered to transform the data into an output function. The requirement for this function to be independent of the way of presenting the input data is formalized. Some well-known methods for processing information with a finite-dimensional image are investigated for invariance.Translated from Izmeritel'naya Tekhnika, No. 6, pp. 5–6, June, 1994.     

一种自适应的图像双边滤波方法

提出一种利用双边滤波的图像平滑滤波方法,即在滤除图像中高频噪声的同时,按照图像亮度变化保持图像中处于高频部分的边缘信息的自适应滤波过程。该滤波方法将传统的Gauss滤波器的权系数优化成Gauss函数和图像的亮度信息乘积的形式,优化后的权系数再与图像作卷积运算。这样,滤波时就可以考虑到图像的亮度信息,在滤除图像噪声的同时尽量保持了图像的边缘。由于双边滤波的方法可以使滤波器的权系数随着图像的亮度变化而改变,所以在滤波过程中能达到自适应滤波的目的。     

A method for single image restoration based on the principal ergodic

We present a method to extract from a single image both object and point spread function using low contrast features of an extended field of view. Invoking the principal ergodic on stochastic turbulent phenomena, we show that the aberration parameters, characteristics of the earth's turbulence, can be recovered from multiple features within an isoplanatic patch. The ensemble statistics is replacing the spatial statistics of a single realization to derive an equivalent modulation transfer function and to apply usual deconvolution techniques such as Richardson-Lucy algorithms. The reliability of this postprocessing treatment has been tested on synthetic data, on solar granulation observations performed at La Lunette Jean Rosch du Pic du Midi, and during the event of the Venus transit at La Tour Solaire de Meudon.