Image processing for particle characterization

A highly efficient and automatic technique of particle characterization using digital image processing and analysis is presented. The process, centered on the selection of individual objects, is composed of five general steps: image acquisition, preprocessing, segmentation, extraction, and representation of the characteristic parameters. The implementation of this technique is done using the Khoros system. Synthetic images are used to test the segmentation algorithms and the form-factor process. The proposed technique can deal with several limiting problems such as background heterogeneity, touching or partially overlapping particles and a wide range of particle sizes and shapes. The performance of the method is illustrated by application to zirconia powder and to organic adipic acid particles on images obtained by light optical microscopy. The implemented method produces a statistical determination of various morphologic parameters including area, diameter, eccentricity, and form factor of the particles.     

Border processing in digital holography by extension of the digital hologram and reduction of the higher spatial frequencies

When a digital holographic reconstruction is performed, digital diffraction effects occur at the borders when the hologram amplitudes at the two opposite border points are different on each vertical or horizontal line. We propose a method of digital hologram extension to reduce such diffraction effects. The method consists of extending the size of the digital hologram and of filling the extended part by complex values that minimize, according to a numerical criterion, the highest spatial frequencies. The theoretical aspects of the method are given and the results from a demonstration are provided.     

Analysis of homomorphic processing for ultrasonic grain signal characterization

A model for the grain signal is presented, which includes the effect of frequency-dependent scattering and attenuation. This model predicts that the expected frequency increases with scattering and decreases with attenuation. Homomorphic processing was used for spectral smoothing, and the selection of parameters for optimal performance was examined. Experimental results are presented that show both the upward shift in the expected frequency with grain boundary scattering and the downward shift with attenuation. Furthermore, it is shown that the expected frequency shift can be correlated with the grain size of the material. It is important to point out that the quantitative relationship between the average grain size and the expected frequency shift (either upward or downward) is dependent on the type of material, the quality of grain boundaries, and the characteristics of the measuring instruments.     

Method for electrical characterization of nanowires

The ability to control conductivity in semiconductor nanostructures is often challenged by surface states trapping the majority of the charge carriers. Addressing this challenge requires a reliable method for assessing electrical properties such as carrier concentration and mobility. Unfortunately, here we are facing another challenge, as the Hall effect is geometrically inapplicable to nanowires while the field effect model is also challenged by the geometry of the common nanowire field effect transistor, and can only yield channel mobility which is very different from Hall mobility. In this paper, we propose a method that combines resistivity and photovoltage measurements with a chemical perturbation to the surface to measure carrier concentration and mobility, as a function of wire diameter, and also to measure the surface state density and the surface band bending before and after the chemical treatment. We apply this method to CVD grown GaN nanowires, before and after a mild HCl etch. Using transmission electron microscope and x-ray photoelectron spectrometry we find that HCl removes the native gallium oxide. The etch is found to reduce the surface state density from 1 × 10(12) to 5.3 × 10(11) cm(2), which is calculated from a reduction of the critical radius for full depletion from 7.6 to 4 nm, and a reduction of the surface band bending from 0.53 to 0.29 eV, observed using surface photovoltage. On the average, the values of carrier concentration we obtain are about ten times smaller, and the mobility about ten times greater, than values obtained using field effect transistors. Interestingly, the weak size dependence of the mobility disappears after etching, suggesting a causal linkage between the as-grown size dependence of the mobility and the density of surface states. The proposed method provides an experimental handle to the study of surface states and their effects on the electrical properties of nanowires.     

Method for characterization of filter radiometers

We have developed a new method for characterizing the irradiance responsivity of filter radiometers. The method is based on a spatially uniform, known irradiance, generated by combining several identical laser beams. The measurement setup and the experimental demonstration at one wavelength are presented. The diffraction correction related to the generated irradiance is studied experimentally. The uncertainty analysis of the method indicates a relative standard uncertainty of 1 x 10(-3). The results with the new method are compared with the characterization measurements based on our present spectral-irradiance scale. The results have a relative deviation of 1 x 10(-3), which is well within the combined standard uncertainty of the comparison.     

Method of hologram multiplexing by use of a fiber bundle with rotary movement

A simple method of rotating an optical fiber bundle is presented to apply the random-pattern referencing scheme to hologram multiplexing. In the theoretical study, a dependence of the diffraction efficiency on the number of spatial frequencies in the reference pattern is estimated with the Bragg diffraction theory. In the experiment, hologram multiplexing is performed in which 30 holograms are recorded in a LiNbO3 crystal and are read out by rotating a fiber bundle. The result shows that this simple approach enables us to perform the hologram multiplexing and also contributes to the building of a compact optical setup.     

Gray-level computer-generated hologram filters for multiple-object correlation

Synthesis of gray-level computer-generated holograms allows for an increase of the information storage capability that is usually achieved with conventional binary filters. This is mainly because more degrees of freedom are available. We propose to profit from this feature by synthesizing complex filters formed by many superimposed holograms, each with a different carrier frequency. We apply these gray-level filters to perform multichannel correlation and in this way enhance the capability of optical correlators to process the information in parallel and simultaneously. First, we analyze the behavior of some performance criteria on the impulse response and on the correlation as a function of the number of holograms that are multiplexed. Then we show the results of two experiments: In the first a composed phase-only filter is used in a multiple-object recognition process. In the second a composed synthetic discriminant function filter is used to implement an object classification by means of a binary code.     

Method of correcting stitching errors in reconstructing a synthetic-aperture digital hologram with seams

We propose a correction method based on the shift theorem of the Fourier transform for the non-integer pixel stitching error in a synthetic-aperture digital hologram with seams. After being Fourier transformed, the sub-holograms stemming from the dislocated recording planes are multiplied by a phase factor and then superposed onto in-place sub-holograms. Finally, the entire hologram is inverse Fourier transformed for reconstruction. This method is applied to simulated particle detection. The results show that the correlation coefficient (CC) between the reconstructed image and the simulated target image is 1.5 times, by comparison with an image that does not involve the phase-factor correction, and particles at the seams are clearly visible. In addition, the effect of noise on the method is small. These verify the feasibility of the method.     

Stochastic approach in the efficient design of the direct-binary-search algorithm for hologram synthesis

An algorithm incorporating a stochastic approach is proposed for reducing the computation time of the direct-binary-search algorithm for hologram synthesis. Two variants of this new algorithm are considered for a number of hologram-generation problems. Both variants can reduce the computation time significantly with a very small increase in the reconstruction error on average. The effectiveness of the proposed algorithm is found to improve with the increasing computational complexity of the design problem. Also, the algorithm is able to generate the hologram in a time that is relatively independent of the initial conditions.     

Binary representation of interference patterns for phase-shifting digital hologram

Phase-shifting digital holograms can completely record the complex (amplitude and phase) wavefront information, containing three-dimensional object shape and relative position. In this study, we examine a binary representation for a phase-shifting digital hologram and apply it to three-dimensional object recognition and reconstruction. For this purpose, we derive an optimal threshold and quantized value for the binary representation of the interference patterns. The recognition results indicate that even with only one bit to represent the digital hologram, there is still enough information for us to recognize the three-dimensional objects. By using the proposed algorithms, one can easily implement the overall recognition process in real-time applications.     

Comparison of methods for determining the bias index of a dichromated gelatin hologram

Determination of the bias refractive index of a holographic emulsion before exposure and after development is an important factor in the design of holographic optical elements. Several experimental methods are discussed for determining the bias index of a volume hologram in dichromated gelatin, and the results for each technique are presented. It is shown experimentally that these measurement methods yield different results for the same hologram, and the cause of the differences is proposed to be a variation of the bias index with depth in the hologram. An index measurement technique is also presented that accounts for variation in the bias index and is shown to yield an accurate value for the bias index.     

Accelerated algorithm for three-dimensional computer generated hologram based on the ray-tracing method

An accelerated algorithm for three-dimensional computer generated holograms (CGHs) based on the ray-tracing method is proposed. The complex amplitude distribution from the center point of an object is calculated in advance and the field distributions of rest points on the hologram plane can be given by doing a small translation and an aberration to the pre-calculated field. A static two-dimensional car, a three-dimensional teapot, and a dynamic three-dimensional rotating teapot are reconstructed from CGHs calculated with the accelerated algorithm to prove its validity. The simulation results demonstrate that the accelerated algorithm is eight times faster than the conventional ray-tracing algorithm.     

飞机抖振响应数据处理及分析方法

为了编制抖振疲劳谱、估算抖振疲劳寿命等,必须预先对时域的抖振响应数据进行处理与分析。考虑到抖振响应的随机特性,特别是其显著的分散性,建立了统计模型来对其分析处理。针对同一飞行状态下数据仓中的抖振响应数据,将其划分为若干子数据块,以子数据块中数据统计特征来描述对应子数据块均方根下的响应分布情况,而以对应飞行状态下各子数据块的均方根分布情况来描述该状态下抖振响应的总体分布趋势以及选择其关键响应状态水平。首先,采用威布尔分布假设,运用极大似然估计法对子数据块的数据进行分布参数估计,并给出了分布假设的检验方法;然后,采用“三步进”经验函数来描述抖振响应均方根的分布规律。在本文研究的基础上,根据给出的抖振数据处理与分析结果使用流程,即只需根据确定的几个关键均方根水平,定位到相应的子数据块,再结合子数据块数据的统计模型得到对应飞行状态下的响应分布,就可用于飞机设计与强度校核。由实际飞行试验抖振数据的处理与分析表明,此方法具备一定的合理性     

Three-dimensional display of a horizontal-parallax-only hologram

We propose a three-dimensional (3D) holographic display by converting an optically recorded complex full-parallax (FP) hologram to an off-axis horizontal-parallax-only (HPO) hologram. First, we record the complex FP hologram of an object using optical scanning holography. We then convert the complex FP hologram to an off-axis HPO hologram through fringe-matched Gaussian low-pass filtering and with the introduction of an off-axis reference. Finally, we reconstruct the off-axis HPO hologram optically using an amplitude-only spatial light modulator. Until now, only computer-generated HPO holograms have been displayed optically. To the best of our knowledge, this is the first demonstration of a 3D display of an optically recorded HPO hologram.