Electronic Speckle Interferometer Research Articles

Development of a new piezoelectric galvanometer scanner: fabrication, operation, and measurements

Our objective in this study was to overcome the size and complexity of existing galvanometers by developing a direct-drive galvanometer based on the deformation of a double-layer ceramic piezoelectric disc with mirrored surface. The resulting device is extremely compact (0.41 mm), light weight, and highly responsive. The performance of the galvanometer was evaluated using an electronic speckle interferometer to characterize vibration mode characteristics. The voltage and input frequency can be adjusted to match the vibration amplitude and vibration mode required for practical galvanometer operations. Using a position sensitive detector, we recorded changes in the position of a light spot generated by a single-point laser by which to track changes in the optical path imposed by the galvanometer. Experiments using graphics drawn by an open-loop control system verified the performance of the device.

Use of electronic speckle interferometry for recording nanodisplacements

Advancement of nanomechanics and nanotechnologies assumes creation of adequate tools for measuring displacements in the nanoscale range. In the present paper, for these purposes we propose to use the method and hardware of electronic speckle interferometry, which have several advantages over the other known measurement means. We present an idea based on which the method of electronic speckle interferometry, primarily designed to be used to measure displacements in the submicron range, can be used to measure displacements that are hundreds of times smaller, i.e., of the order of 1 nanometer. We consider the theoretical justification of this idea and the program algorithm for its implementation and describe the methods, the test specimens, and the results of experimental metrological test of the possibility of measuring displacements in the nanoscale range by using the existing model of the electronic speckle interferometer.

Noise reduction in interferometric fringe patterns with mean curvature diffusion

Dual beam electronic speckle interferometers provide raw data in the form of maps of wrapped relative phase or fringe patterns. Interpretation of such fringe patterns is complicated by aliased and random speckle noise. This noise can result in misidentification of the phase at a given point in the image. Automated determination of the loci of fringe extrema, useful for quantitative evaluation, are particularly affected. A nonlinear image filtering technique referred to as mean curvature diffusion is applied to overcome this difficulty. This technique essentially smooths the image without a substantial reduction in the magnitude of the underlying trends that here represent the fringes. Mean curvature diffusion uses calculations analogous to those for the diffusion of heat with the difference that the diffusion coefficient, reminiscent of thermal diffusivity, varies spatially within the image with a value given by the reciprocal of the local surface gradient. At a given point in the image, the rate of surface diffusion depends only on the average value of the normal curvature in any two orthogonal directions and not on its magnitude; this allows the lower frequency underlying components of the image structure to be retained. The method is tested on both calculated and real speckle interferograms to highlight the effectiveness of this smoothing technique relative to more standard smoothing algorithms.

Speckle interferometry: three-dimensional deformation field measurement with a single interferogram

An electronic speckle interferometer, arranged for out-of-plane sensitivity and with an off-axis reference beam to produce spatial phase bias, is used for three-dimensional deformation field measurements. The complex amplitude of the object wave is calculated by application of a Fourier-transform method to a single interferogram. The change in phase after object deformation yields the out-of-plane component of the displacement field. The two in-plane components are obtained by cross correlation of subimages of the reconstructed object wave's intensity, a method that is also referred to as digital speckle photography. The Fourier-transform algorithm is extended and modified, leading to random measurement errors that are below widely accepted theoretical limits and also to an extended measuring range. These properties and the mutually combined information improve the accuracy of both methods compared with their usual single implementation. The performance is evaluated in experiments with pure out-of-plane, pure in-plane, and combined deformations and compared with theoretical values. An example of a practical application is given.

Introduction of the Impulse Response Function in Common-Path Interferometers with Fourier Plane Filters

In the present paper it will be shown how the introduction of a Fourier plane filter can create various types of common-path interferometers for measuring changes in surface tilt or curvature of an object surface. This is obtained by placing a holographic optical element in the Fourier plane of a 4-f optical system. The interferometers are analysed by using the paraxial approximation of the Huygens-Fresnel integral formalism, and the interferometer functions are given by a novel formalism using impulse response functions. Based on this technique, an interferometer for measuring dedicated changes in surface deflection is presented. This interferometer is insensitive to rigid surface rotations and displacements. The interferometer can be embedded in systems based on single point measurement of a time dependent deflection, i.e. vibrometers, as well as in full-field measurements such as electronic speckle interferometers.

Robust optical systems for nondestructive testing based on laser diodes and diffractiveoptical elements

Two robust systems for vibration measurements are presented. Both systems are based on low-cost laser diodes. The stability of the optical systems is achieved by implementing the optical system as a holographic optical element (HOE) while using a common-path interferometer concept for the measurement scheme. The former system facilitates real-time simultaneous tracking of vibrations about two axes perpendicular to the optical axis. The latter system is an electronic speckle interferometer working in a differential mode providing whole-field information on the angular deformation about one axis between two states of the object. It will be argued that these concepts will provide compact, self-aligning systems for industrial use due to the inherent possibility of mass fabrication.

Optimization of fringe pattern calculation with direct correlations in speckle interferometry

A direct correlation technique is used to calculate correlation fringe patterns from consecutive speckle patterns acquired with a dual-beam electronic speckle interferometer. Although more calculations are required than in standard image differencing routines, an advantage of the method is that the illumination over the surface of the object need not be uniform. In the method, Pearson's coefficient of correlation between the intensities within a set of adjacent pixels is calculated. This has the added advantage of being directly related to the theoretical phase-dependent correlation. A mapping of this measure of correlation results in the correlation fringe pattern. Laboratory tests were carried out with in-plane translations ranging from 5 to 45 mum. The correlation calculations were carried out by using cells (square sets of pixels) in the raw speckle images with dimensions ranging from 2 pixels x 2 pixels to 19 pixels x 19 pixels. Both cell dimension and translation magnitude dependent decorrelation effects influence the quality of the correlation fringe patterns.

Electronic Correlation Speckle Interferometer for Flatness Measurement of Glossy Surfaces

Abstract Correlation electronic speckle interferometer enabling distortion measurement of flat glossy objects is described. By means of this interferometer it is also possible to test flatness of optically glossy surfaces and to measure changes in phase undergone in one arm of the interferometer.