Minimum theoretical requirements for three-dimensional scanning-laser Doppler anemometry

An investigation of the minimum number of intersecting beams that is required for laser Doppler anemometry (LDA) incorporating only a single detector is presented. We aim to provide decisive arguments for using four beams as the minimum requirement for complete three-dimensional velocity reconstruction even though three beams supply three velocity components. We derive expressions for the detected signals of the most general LDA system. From a matrix analysis of these expressions, we conclude that there is no physically realizable arrangement of three beams that results in complete three-dimensional velocity reconstruction and that four beams is the minimum number of beams required. We also determine the optimal arrangement of the four incident beams for unambiguous LDA and for best signal separation and immunity to minor optical alignment errors. To ascertain the velocity components, we scan the specimen in a precise manner relative to the point of focus of the beams, whereas some other researchers alter the frequency of the incident beams. The results obtained with these two methods are equivalent. However, scanning is mechanically simpler than frequency shifting and also allows for the formation of velocity images-images of the flow velocity over a region in two- or three-dimensional space. In particular, we examine systems that are limited by the common practice of using only a single high-numerical-aperture objective for both focusing and detection. We show that using high-numerical-aperture objectives results in the best signal differentiation and immunity to minor alignment errors.     

Fabrication and characterization of a three-dimensional feed-horn infrared antenna for an infrared detector

A three-dimensional feed-horn antenna for the 10-microm-wavelength infrared region has been suggested, characterized, and fabricated. It is applied to an infrared detector for efficient collection of infrared radiation and to reduce background noise. The optimum size of the horn antenna was designed for maximum antenna directivity. The three-dimensional feed-horn antenna mold was fabricated by rotating and tilting illumination, whereas the antenna plate was acquired through electroplating. Antenna characteristics were measured by coupling of the antenna with a microbolometer. Measurement results show that the directivity of the antenna is 16.1 dB and the background noise is reduced by a factor of approximately 2 compared with an open-structure infrared detector.     

Noise-cancellation-based nonuniformity correction algorithm for infrared focal-plane arrays

The spatial fixed-pattern noise (FPN) inherently generated in infrared (IR) imaging systems compromises severely the quality of the acquired imagery, even making such images inappropriate for some applications. The FPN refers to the inability of the photodetectors in the focal-plane array to render a uniform output image when a uniform-intensity scene is being imaged. We present a noise-cancellation-based algorithm that compensates for the additive component of the FPN. The proposed method relies on the assumption that a source of noise correlated to the additive FPN is available to the IR camera. An important feature of the algorithm is that all the calculations are reduced to a simple equation, which allows for the bias compensation of the raw imagery. The algorithm performance is tested using real IR image sequences and is compared to some classical methodologies.     

AIDA: an adaptive image deconvolution algorithm with application to multi-frame and three-dimensional data

We describe an adaptive image deconvolution algorithm (AIDA) for myopic deconvolution of multi-frame and three-dimensional data acquired through astronomical and microscopic imaging. AIDA is a reimplementation and extension of the MISTRAL method developed by Mugnier and co-workers and shown to yield object reconstructions with excellent edge preservation and photometric precision [J. Opt. Soc. Am. A21, 1841 (2004)]. Written in Numerical Python with calls to a robust constrained conjugate gradient method, AIDA has significantly improved run times over the original MISTRAL implementation. Included in AIDA is a scheme to automatically balance maximum-likelihood estimation and object regularization, which significantly decreases the amount of time and effort needed to generate satisfactory reconstructions. We validated AIDA using synthetic data spanning a broad range of signal-to-noise ratios and image types and demonstrated the algorithm to be effective for experimental data from adaptive optics-equipped telescope systems and wide-field microscopy.     

Kubik: An automatic three-dimensional finite element mesh generator

This paper describes an automatic mesh generator for finite element problems in three dimensions, called KUBIK, KUBIK is based on a modular concept. First, a set of modules is generated, each with its own mesh inside. Then, the modules are placed in contact, and a matrix describing the connectivity of the entire mesh is determined. Finally, the geometry of the mesh is specified and all nodal co-ordinates are determined by the computer. The user's labour required to specify a mesh which represents a certain physical configuration resumbles the labour involved when the configuration has to be specified to the machine shop workers who are supposed to build it. The modules are independent, except for the compatibility requirements between modules which are to be placed in contact. Structures of any degree of complexity can in principle be obtained using KUBIK. Only two types of modules are required, except when helicoidal structures are to be generated, in which case it is convenient to use a third type. The finite elements used are distorted cubes, but they can be divided into tetrahedra, prisms of pyramids. Nodes are placed at the corners of the cubes, and additional nodes can be introduced if desired in order to obtain other types of finite elements in use. Infinite elements of quadrilateral cross-section are also a standard feature of KUBIK, and can be divided in order to obtain infinite prisms of triangular cross-section. KUBIK can also generate two-dimensional meshes, including meshing of a three-dimensional surface for use with boundary elements.     

Multimodel Kalman filtering for adaptive nonuniformity correction in infrared sensors

We present an adaptive technique for the estimation of nonuniformity parameters of infrared focal-plane arrays that is robust with respect to changes and uncertainties in scene and sensor characteristics. The proposed algorithm is based on using a bank of Kalman filters in parallel. Each filter independently estimates state variables comprising the gain and the bias matrices of the sensor, according to its own dynamic-model parameters. The supervising component of the algorithm then generates the final estimates of the state variables by forming a weighted superposition of all the estimates rendered by each Kalman filter. The weights are computed and updated iteratively, according to the a posteriori-likelihood principle. The performance of the estimator and its ability to compensate for fixed-pattern noise is tested using both simulated and real data obtained from two cameras operating in the mid- and long-wave infrared regime.     

一种改进的红外焦平面非均匀性校正算法

针对红外焦平面探测器现有非均匀性校正算法在实际应用中存在的问题,结合实际系统的开发,提出了一种改进算法——一点加两点校正算法。先求两点校正算法的校正增益和校正偏置,再求一点校正算法的校正偏置,求取一点校正算法的偏置参数时的图像数据来自前面求得的两点校正之后的数据,即最后的校正结果是两点校正后的再校正。理论分析和应用表明该算法与目前流行的算法相比具有实时性好、误差小、处理效果好等特点。     

Solution for the nonuniformity correction of infrared focal plane arrays

Based on the S-curve model of the detector response of infrared focal plan arrays (IRFPAs), an improved two-point correction algorithm is presented. The algorithm first transforms the nonlinear image data into linear data and then uses the normal two-point algorithm to correct the linear data. The algorithm can effectively overcome the influence of nonlinearity of the detector's response, and it enlarges the correction precision and the dynamic range of the response. A real-time imaging-signal-processing system for IRFPAs that is based on a digital signal processor and field-programmable gate arrays is also presented. The nonuniformity correction capability of the presented solution is validated by experimental imaging procedures of a 128 x 128 pixel IRFPA camera prototype.     

Kalman filtering for adaptive nonuniformity correction in infrared focal-plane arrays

A novel statistical approach is undertaken for the adaptive estimation of the gain and bias nonuniformity in infrared focal-plane array sensors from scene data. The gain and the bias of each detector are regarded as random state variables modeled by a discrete-time Gauss-Markov process. The proposed Gauss-Markov framework provides a mechanism for capturing the slow and random drift in the fixed-pattern noise as the operational conditions of the sensor vary in time. With a temporal stochastic model for each detector's gain and bias at hand, a Kalman filter is derived that uses scene data, comprising the detector's readout values sampled over a short period of time, to optimally update the detector's gain and bias estimates as these parameters drift. The proposed technique relies on a certain spatiotemporal diversity condition in the data, which is satisfied when all detectors see approximately the same range of temperatures within the periods between successive estimation epochs. The performance of the proposed technique is thoroughly studied, and its utility in mitigating fixed-pattern noise is demonstrated with both real infrared and simulated imagery.     

Surface temperature correction for active infrared reflectance measurements of natural materials

Land surface temperature algorithms for the moderate resolution imaging spectroradiometer satellite instrument will require the spectral bidirectional reflectance distribution function (BRDF) of natural surfaces in the thermal infrared. We designed the spectral infrared bidirectional reflectance and emissivity instrument to provide such measurements by the use of a Fourier transform infrared spectrometer. A problem we encountered is the unavoidable surface heating caused by the source irradiance. For our system, the effects of the heating can cause a 30% error in the measured BRDF The error caused by heating is corrected by temporally curve fitting the radiance signal. This curve-fitting technique isolates the radiance caused by reflected irradiance. With this correction, other factors dominate the BRDF error. It is now ~5% and can be improved further. The method is illustrated with measurements of soil BRDF.     

Image correction algorithm for functional three-dimensional diffuse optical tomography brain imaging

We outline a computationally efficient image correction algorithm, which we have applied to diffuse optical tomography (DOT) image time series derived from a magnetic resonance imaging (MRI)-based brain model. Results show that the algorithm increases spatial resolution, decreases spatial bias, and only modestly reduces temporal accuracy for noise levels typically seen in experiment, and produces results comparable to image reconstructions that incorporate information from MRI priors. We demonstrate that this algorithm has robust performance in the presence of noise, background heterogeneity, irregular external and internal boundaries, and error in the initial guess. However, the algorithm introduces artifacts when the absorption and scattering coefficients of the reference medium are overestimated--a situation that is easily avoided in practice. The considered algorithm offers a practical approach to improving the quality of images from time-series DOT, even without the use of MRI priors.     

Active aberration correction for the writing of three-dimensional optical memory devices

We describe an active optical system that both measures and corrects the aberrations introduced when writing three-dimensional bit-oriented optical memory by a two-photon absorption process. The system uses a ferroelectric liquid-crystal spatial light modulator (FLCSLM) configured as an arbitrary wave-front generator that is reconfigurable at speeds as great as 2.5 kHz. A method of aberration measurement by the FLCSLM wave-front generator is described. The same device is also used to correct the induced aberrations by preshaping the wave fronts with the conjugate phase aberration as well as to scan the focal spot in three dimensions. Experimental results show the correction of both on- and off-axis aberrations, allowing the writing of data at depths as great as 1 mm inside a LiNbO3 crystal.     

Maximum entropy deconvolution of infrared spectra: use of a novel entropy expression without sign restriction

Absorbance and difference infrared spectra are often acquired aiming to characterize protein structure and structural changes of proteins upon ligand binding, as well as for many other chemical and biochemical studies. Their analysis requires as a first step the identification of the component bands (number, position, and area) and as a second step their assignment. The first step of the analysis is challenged by the habitually strong band overlap in infrared spectra. Therefore, it is useful to make use of a mathematical method able to narrow the component bands to the extent to eliminate, or at least reduce, the band overlap. Additionally, to be of general applicability this method should permit negative values for the solution. We present a maximum entropy deconvolution approach for the band-narrowing of absorbance and difference spectra showing the required characteristics, which uses the generalized negative Burg-entropy (Itakura-Saito discrepancy) generalized for difference spectra. We present results on synthetic noisy absorbance and difference spectra, as well as on experimental infrared spectra from the membrane protein bacteriorhodopsin.     

Face tracking and pose estimation with automatic three-dimensional model construction

A method for robustly tracking and estimating the face pose of a person using stereo vision is presented. The method is invariant to identity and does not require previous training. A face model is automatically initialised and constructed online: a fixed point distribution is superposed over the face when it is frontal to the cameras, and several appropriate points close to those locations are chosen for tracking. Using the stereo correspondence of the cameras, the three-dimensional (3D) coordinates of these points are extracted, and the 3D model is created. The 2D projections of the model points are tracked separately on the left and right images using SMAT. RANSAC and POSIT are used for 3D pose estimation. Head rotations up to plusmn45deg are correctly estimated. The approach runs in real time. The purpose of this method is to serve as the basis of a driver monitoring system, and has been tested on sequences recorded in a moving car.     

Calibration procedures and correction of detector signal relaxations for the CRISTA infrared satellite instrument

Remote sensing from space has become a common method for deriving geophysical parameters such as atmospheric temperature and composition. The Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) instrument was designed to sound the middle and the upper atmosphere (10-180 km) with high spatial resolution. Atmospheric IR emissions were measured with Si:Ga bulk or Si:As blocked impurity band detectors for a wavelength interval of 4-17 microm and Ge:Ga bulk detectors for 56-71 microm. An overview of the calibration of the instrument and the correction of detector signal relaxations for the Si:Ga detectors are given, both of which are necessary to provide high-quality IR radiance data as input for the retrieval of atmospheric temperature and trace gas mixing ratios. Laboratory and flight data are shown to demonstrate the quality of the results.     

Hybrid holographic microscopy: visualization of three-dimensional object information by use of viewing angles

One of the attractive features of hybrid holographic microscopy, in which the hologram of a microscopic object recorded by an image sensor is numerically reconstructed with a computer, is that the three-dimensional (3-D) information of a recorded object is obtained. The 3-D information has often been extracted by means of changing the reconstruction distance in the numerical reconstruction process, but here we describe an alternative technique that allows for variable viewing angles. That is, the perspective from which the object is viewed can be varied. The approximation used enables use of the fast-Fourier-transform algorithm for numerical reconstruction even in the high-resolution case in which the Fresnel approximation is no longer valid. The resolution of the proposed technique is also discussed.     

Improved neural network based scene-adaptive nonuniformity correction method for infrared focal plane arrays

An improved scene-adaptive nonuniformity correction (NUC) algorithm for infrared focal plane arrays (IRFPAs) is proposed. This method simultaneously estimates the infrared detectors' parameters and eliminates the nonuniformity causing fixed pattern noise (FPN) by using a neural network (NN) approach. In the learning process of neuron parameter estimation, the traditional LMS algorithm is substituted with the newly presented variable step size (VSS) normalized least-mean square (NLMS) based adaptive filtering algorithm, which yields faster convergence, smaller misadjustment, and lower computational cost. In addition, a new NN structure is designed to estimate the desired target value, which promotes the calibration precision considerably. The proposed NUC method reaches high correction performance, which is validated by the experimental results quantitatively tested with a simulative testing sequence and a real infrared image sequence.     

Temperature correction of radiometric and geometric models for an uncooled CCD camera in the near infrared

This paper presents radiometric and geometric models for both temperature and displacement noncontact measurements using an uncooled charge-coupled device (CCD) video camera. Such techniques ("one sensor-two measures") represent an interest in many industrial low cost applications and scientific domains. To benefit from both measurements, we have to use the camera's spectral response in the near infrared spectral band from 0.75 to 1.1 /spl mu/m. In this spectral band, the temperature variations of an uncooled CCD camera are taken into account in the radiometric and geometric models. By using physical models for CCD camera, we quantify detector's quantum efficiency, sensor noise and spatial resolution as a function of the wavelength and of the detector temperature. These models are confirmed by experimental results of calibration with a low cost uncooled camera based on a Sony detector and operating over the detector temperature range of -30 to -50/spl deg/.