Aberration-free lateral shear holographic interferometry

Lateral shear interferometry is considered as a possible means for optical correction of wave fronts reconstructed from singly and doubly exposed holograms. Optical aberrations are eliminated in a moiré pattern by superposing two lateral shear interferograms using waves diffracted by original holograms. It is shown that the singly exposed hologram allows compensation for only optical inhomogeneities due to the substrate, while the doubly exposed hologram also excludes aberrations due to the recording system. The experimental results supporting the efficiency of the method as applied to imperfect substrates of holograms are presented.     

Classification of holographic fringe patterns inherent in through hole drilling in residual stress field

Comparative analysis of actual fringe patterns, which are induced by combined implementing the hole drilling method and reflection hologram interferometry for residual stresses determination, is presented. Involved considerations are related to plane thin-walled structural elements. A set of interferograms of perfect (ideal) form is selected proceeding from one-side measurements. A base for recognising each specific ideal configuration is a fine coincidence between actual interferograms and analogous reference fringe patterns constructed for the same stress state. Perfect (ideal) both actual and reference fringe patterns are defined as a response of pure membrane 2D stress field on through hole drilling between exposures. Main principles of creating the regular catalogue of reference fringe patterns inherent in through hole drilling in thin-walled components are formulated. Emphasis is made on a careful collecting and classifying actual interferograms with clear indications of bending stress presence in total residual stress field. Evidences needed for a reliable classification of the type of residual stresses field of interest are established and verified. A response of superimposed residual stress field, which consists of both membrane and bending components, is characterised by various deviations of each specific fringe pattern from an ideal form. More deep analysis of fringe patterns related to superimposed residual stress field is based on specially designed technique. The main essence of the approach developed is simultaneous measurements of through hole distortions in two principal strain directions on opposite sides of thin plane specimen. These sides are faces of the drill entrance and exit. Sophisticated optical set-up that is capable of obtaining high-quality fringe patterns in the course of two-side measurements is developed and implemented. Typical set of fringe patterns obtained for single probe hole on opposite specimen faces is presented.     

干涉图特征信息自动采集方法

介绍了一种用于计算全息数字波面干涉仪中采用数字图像处理技术采集干涉图的特征信息的方法。它分为图像二值化、条纹细化、条纹修补、跟踪、标记、采样等步骤,对于没有闭合条纹的简单干涉图和有闭合条纹的复杂干涉图,都能得到较好结果。     

Fringe analysis and enhanced characterization of sub-surface defects using fringe-shifted shearograms

A technique for fringe analysis using variance of directly fringe-shifted shearograms is demonstrated experimentally. Statistical variations in each of the fringe-shifted shearograms are calculated to obtain a contrast map which is then used to characterize sub-surface defects and mechanical loading points. The technique offers reduced computational load compared to Fourier transform-based fringe processing algorithms. The technique is implemented using both phase difference maps and interferograms. The latter allows the extension of the technique to physical (i.e., non-reconstructed) interferograms for the analysis of dynamic systems.     

Spherical reflectors as laser-beam expanders

Lasers emit a narrow beam of light which, for many applications, is often required to be expanded and then collimated. A common type of device used for this purpose is a lens telescope in which a pin-hole is placed at the common focal point of the two lenses. In dye laser cavities, dispersion through a number of prisms gives an expanded beam. The divergence of a laser beam can also be achieved by a convex reflecting surface like a polished steel ball. Here the capability of a small steel ball as an inexpensive, simple and convenient laser-beam expander has been investigated experimentally. It has been shown, with the help of shearing interferograms and Ronchigrams, that the aberrations introduced from on-axis as well as off-axis illumination of the ball are less than 1/4 of a fringe.     

Multi-parameter measuring method of image intensifier based on Fourier transform phase measurement

A new quality parameter measuring method of image intensifier based on Fourier transform phase measurement is proposed. A standard sinusoidal fringe pattern is projected into the measuring system by spatial light modulator and the corresponding reference pattern and deformed pattern are captured by CCD. The Fourier transform and inverse Fourier transform are applied in order to extract the phase distribution of deformed pattern. The magnification, pillow-shaped distortion, barrel-shaped distortion, snake-shaped distortion and shear distortion of image intensifier are obtained at the same time. Experiment has proved its feasibility with real-time performance and without manual intervention.     

Measurement of dynamic crack tip displacement field by speckle photography and interferometry

Speckle interferometry has been used to measure the transient displacement field around a crack due to stress wave loading. Double exposure interferograms are recorded with a pulsed ruby laser, and correlation fringes formed by spatial filtering of the developed film. A new technique is presented for reducing the noise level in spatially filtered speckle interferograms. Independent fringe patterns, obtained from one interferogram but with the filtering aperture sampling different regions of the Fourier transform plane, are digitised by a CCD camera and combined numerically. It is demonstrated that accurate analysis of fringe patterns by the Fourier transform method can be carried out, even if no carrier fringes are present. The measurements are compared with those obtained by speckle photography. Least-squares fitting of the theoretical displacement field is used to calculate the stress intensity factor at the crack tip as a function of time.     

Two-dimensional cell parameters measurement of nematic liquid crystal using optical interferometry and Fourier transform fringe analysis technique

We report a method for the measurement of nematic liquid crystal (NLC) cell parameters i.e., switching voltage, birefringence, retardation, dielectric anisotropy, average tilt angle and change in refractive index with applied DC voltage to LC material. The proposed method is based on optical interferometry and Fourier transform fringe analysis technique, in which we obtain 2-dimensional (2D) phase map of the interferograms as a function of applied voltage. Mach-Zehnder interferometer (MZI) was used for the study of cell parameters and interferograms were recorded at different applied DC voltages to NLC cell using CCD camera. From the phase map, 2D-refractive index distribution of the LC cell with applied voltage was reconstructed. Analytical equations are derived based on optical interferometry and then solved to obtain cell parameters. The present method is fast and can give 2D-cell parameters from only two quick interferograms.     

Extension of the controlled sensitivity range in holographic interferometry with a lateral shear

The possibility of controlling the measurement sensitivity in shearing interferometry with a small lateral shear is considered. A method based on recording several holographic interferograms with different magnitudes of lateral shear and with different carrier frequencies of holographic fringes during a single exposure is proposed. The measurement sensitivity of the interference patterns is controlled at the stage of optical processing of the combined holographic interferograms with lateral shear, when the interferograms are illuminated by two coherent light beams and the filtering of the corresponding diffraction orders is performed. The results of the experimental testing of the proposed method by using it for the visualization of the melting zones of planar polymethylmethacrylate samples under heat treatment are presented.     

Evaluation of a method to determine interferometric phase derivatives

A recently published method for the determination of phase derivatives maps by direct manipulation of three or more phase-shifted interferograms is tested for accuracy. The method is evaluated on computer-simulated holographic and electronic speckle pattern interferometry fringes and the results are assesed through the calculation of two comparative parameters: the relative image difference and the linear correlation coefficient. It is shown that errors in the phase derivative strongly depend on the filtering method used for smoothing the fringe patterns. An experimentally recorded ESPI fringe pattern is used to confirm the predictions of the numerical simulations.     

Knowledge-based hybrid expert system for automated interferometric data reduction

In order to accomplish the ultimate goal of automated interferogram evaluation, the utilization of artificial neural networks or expert systems in artificial intelligence has been expected. In this paper, a new PC-based fringe evaluation technique, which is based on hybridization of an expert system and a conventional fringe tracking method, is presented together with some test results of noise-ridden interferograms in high-speed aerodynamics. This technique incorporates algorithmic image processing with expert-system-driven knowledge manipulation in a single structure to provide an optimal solution for noisy interferogram evaluation. A new approach for regional phase unwrapping based on the technique is also proposed. This concept is applicable to interferometry such as phase shifting and Fourier transform that provides phase-wrapped information. It is based on conservative decision making in defining phase-wrapped regions by utilizing global/regional knowledge in the expert system. The approach is thus believed to reduce computation time and memory space in processing complex or noisy interferograms that require frequent operator interactions.     

Constant radial shearing interferometry with two spiral phase gratings

Two spiral phase gratings consisting of triangular grooves are used for arranging a simple constant radial shearing interferometer with small light loss. The fringe formation is described and a method for analyzing the interferograms is presented.     

A method for increasing the sensitivity of determination of the wavefront distortion in optical processing of holographic lateral shear interferograms

A method for obtaining interference patterns of increased sensitivity in optical processing of holographic lateral shear interferograms recorded under linear conditions at a shear exceeding the size of a phase object under study is proposed. Cases of reconstruction of interference patterns of increased sensitivity using matched and optically conjugate holographic lateral shear interferograms are considered. It is shown that two-and fourfold increases in sensitivity, respectively, are obtained in these cases. The validity of the method proposed is confirmed experimentally     

Optical methods for precision measurements

Contactless measuring techniques are becoming increasingly important for industrial applications. The use of a laser, solid-state detector arrays and powerful small computers leads to a very efficient fringe analysis in holography as well as in Moiré and speckle techniques. Due to the computer analysis, much more information can be extracted from interferograms, leading to higher sensitivities and accuracies. The application of different fringe analysis procedures is discussed, together with some potentials of the application of interferometry, holography, and speckle and Moiré techniques.     

Control of the tuning of reference fringes in the interference pattern of changes in the object state in the optical processing of holograms and photographic images with the same spatial carrier

The condition of obtaining undistorted reference fringes when processing a pair of superimposed holograms or photographic images with a constant carrier frequency of bands due to their relative rotation or tilt is obtained. It is shown that the degree of fringe distortion is determined by the combined action of aberrations and the value of the parameter being measured. The technique of an arbitrary tuning of reference fringes is described, which makes it possible to eliminate the relative shift of holograms or photographic images. Experimental results obtained by the optical processing of photographic images of projected fringes are presented.     

Removing harmonic distortion of measurements of a defocusing three-step phase-shifting digital fringe projection system

Binary defocusing method was adopted in 3D profilometry as it allows real-time measurement and does not need to handle the luminance nonlinearity of a projector. Current patch-based binary fringe patterns are periodic and carry strong harmonic distortion as compared with the ideal sinusoidal fringe patterns, which affects the measuring performance remarkably. In this paper, we propose a framework for generating aperiodic fringe patterns based on optimized patches. The produced fringe patterns can significantly lower the noise floor and suppress the harmonic distortion in the constructed phase map. Accordingly, the achieved depth measuring performance can be significantly improved. Special care is also taken during the optimization of the patches in our framework such that the depth measuring performance is robust to fringe period and defocusing extent.     

Collimation testing using wedge plate lateral shearing interferometry and Fourier fringe analysis

For checking the collimation of an optical beam Fourier fringe analysis has been incorporated into the wedge plate interferometric setup. Typical interferograms corresponding to ‘in-focus’, ‘at-focus’ and ‘out-of-focus’ positions of an optical beam have been recorded. As per the testing procedure, FFT of the recorded interferometer is computed digitally, and necessary processing for direct determination of phase is undertaken. Finally, the phase data is unwrapped and plotted as a function of pixel position along the direction perpendicular to the shear. The slope of the phase provides the information regarding collimation position of the collimator. As the collimation position is detected by the direct measurement of the phase over the whole area of the interferogram, high accuracy, reliability and precision are achieved.     

High-precision technique for in-situ testing of the PZT scanner based on fringe analysis

A technique based on fringe analysis is presented for the in-situ testing of the PZT scanner, including the end rotation analysis and displacement measurement. With the interferograms acquired in the Twyman-Green interferometer, the testing can be carried out in real time. The end rotation of the PZT scanner and its spatial displacement deviation are analyzed by processing the fringe rotation and interval changes; displacement of the PZT scanner is determined by fringe shift according to the algorithm of template-matching, from which the relation between the driving voltage and displacement is measured to calibrate the nonlinearity of the PZT scanner. It is shown by computer simulation and experiments that the proposed technique for in-situ testing of the PZT scanner takes a short time, and achieves precise displacement measurement as well as the end rotation angle and displacement deviation measurement. The proposed method has high efficiency and precision, and is of great practicality for in-situ calibration of the PZT scanner.     

Submillimeter laser interferometry with pyroelectric image recording

The interferometer with a submillimeter laser and standard pyroelectric TV camera for visualization of interference fringe pattern is described. Problems typical for the development of such interferometer are solved by choosing of a proper arrangement and a mutual spacing of its elements with respect to laser. The interferograms of a teflon wedge and an arcing plasma are presented as an example.