Quantitative evaluation of digital shearing interferogram using the spatial carrier method

Applications of digital speckle pattern shearing interferometry in characterization of fluids

Digital speckle pattern shearing interferometry is a robust optical technique mostly used for measuring small deformations in solid objects. In this paper we focus on applications of this technique in characterization of liquid and gaseous samples. We demonstrate this by a few examples: measurement of the density (temperature) profiles inside a candle flame and around a hot wire in air. Also, we study the diffusion process in a binary mixture and measure the diffusion coefficient. The results of these studies confirm that this highly sensitive, nondestructive, and real-time technique is ideal for investigation of fluid specimens.

Phase retrieval using bidirectional interference

In this paper we describe a phase retrieval algorithm using constraints given by diffraction patterns and phase difference obtained from bidirectional interference. Wave propagation and linear phase ramps are used to connect the recordings. At least three patterns are recorded and processed (two diffraction patterns and one interference pattern). The quality of the results can be improved when recording and processing more patterns. The method works well with non-sparse samples and short (few millimeter) recording distances. Simulations, comparisons with other methods, and experimental validations are presented.

Spatial-light-modulator-based dual shearing direction shearography

We propose a dual shearing shearography system based on a spatial light modulator (SLM). Compared to spatial phase shift shearography, the advantages of this system include its simple structure, relatively high light efficiency, and good phase map quality. Digital shearography is a fast, practical, non-contact, whole-field, and anti-turbulent optical approach to non-destructive testing (NDT) and strain measurement. Because the shearing direction determines the strain direction being measured, tests using multiple shearing directions are sometimes required to obtain strain in different directions and detect all defects. Various setups, based on the spatial phase shift method, have been proposed to solve the issue. While some of these setups perform well, they may also introduce new problems, such as poor phase map quality and low light efficiency. We present a sequential dual shearing shearographic system with good phase map quality and high light efficiency. Due to the SLM's high-speed response, capable of reaching hundreds of hertz, SLM-based dual shearing direction shearography allows for fast temporal phase shifting and shearing direction switching while providing very good phase map quality. Unlike the spatial phase shift method, which has low light efficiency due to its need for a small aperture to enable a relatively large speckle size to cover multiple pixels, the proposed method is based on a fast temporal phase shift and does not have this limitation. In addition, SLM can provide a programmable and adjustable shearing method in any direction and distance, which is beneficial for strain measurements and NDT requiring strain measurements in different directions using a small and precise shearing distance. We describe in detail the theory derivation and non-destructive testing application results for the SLM-based dual shearing direction shearography system.

Digital speckle shearography setup to measure the field-induced strain map in piezoelectric materials

Residual or induced strains are important factors in the performance of electronic devices, actuators, and sensors. In this paper, we report the application of digital speckle shearography to obtain the two-dimensional field-induced out-of-plane strain maps in a piezoelectric slab under a varying electric field. Both the free-standing and loaded (pinned) states are investigated. The results show field-dependent strain maps with parabolic profiles on the order of 10^-4 and 10^-3 in the free-standing and pinned states, respectively, in agreement with typical values for piezoelectric ceramics. This study provides a simple, non-destructive, and full-field method to characterize these materials.

Pixelated Carrier Phase-Shifting Shearography Using Spatiotemporal Low-Pass Filtering Algorithm

Shearography has been widely used in non-destructive testing due to its advantages in providing full-field, high precision, real-time measurement. The study presents a pixelated carrier phase-shifting shearography using a pixelated micropolarizer array. Based on the shearography, a series of shearograms are captured and phase maps corresponding to deformation are measured dynamically and continuously. Using the proposed spatiotemporal filtering algorithm in the complex domain, the set of phase maps are simultaneously low-pass filtered in the spatial and temporal domains, resulting in better phase quality than spatial low-pass filtering. By accumulating the temporally adjacent phase, the phase corresponding to large deformation can be evaluated; thus, large deformations can be accurately measured and protected from speckle noise, allowing internal defects to be easily identified. The capability of the proposed shearography is described by theoretical discussions and experiments.

Measurement of In-Plane Displacement in Two Orthogonal Directions by Digital Speckle Pattern Interferometry

The measurement of in-plane displacement in two orthogonal directions is of considerable significance for modern industries. This paper reports on a spatial carrier phase-shift digital speckle pattern interferometry (DSPI) for the simultaneous measurement of in-plane displacement in two orthogonal directions. The object is illuminated from a single direction and observed from four symmetrical directions simultaneously. One pair of the four observation directions is sensitive to in-plane displacement in one direction, and the other pair is sensitive to in-plane displacement in the perpendicular direction, resulting in the displacement in two directions being measured independently. The polarization property of light is used to avoid cross-interference between the two pairs of beams. Spatial carrier frequencies are generated by aperture misalignment, and the displacement in two directions is modulated onto the same interferogram. With a spatial carrier phase-shift technique, the displacement can be separated in the frequency domain and the phase can be evaluated from a single interferogram in real time. The capability of DSPI is described by theoretical discussions and experiments.

Dual-directional shearography based on a modified common-path configuration using spatial phase shift

This paper describes a dual-directional shearography system to address the issue of two-dimensional characterization of the surface strain. A common-path configuration coupled with an additional light path is used to provide the shearing in two directions. One of the three interfering beams is shared by both directional shearograms to improve the light efficiency and enhance the robustness of the system. The two directional shearograms are carried by different spatial carriers to distinguish one from the other. The spatial carrier is introduced by the single-aperture-lens Wollaston prism configuration. Rather than the conventional method in which the aperture is fixed at the front focal point of the imaging lens, a general case is considered by introducing a variable distance between the aperture and the imaging lens. The influence of the aperture-lens distance on the spatial carrier is then analyzed, which enables the separate control of the shearing amount and the spatial carrier. Two types of dual-directional shearography are presented to demonstrate the feasibility and the flexibility of the system. Type I is the simultaneous dual lateral shearography in orthogonal directions, and Type II is the simultaneous lateral and radial shearography. The spatial carrier introduced by the single-aperture-lens Wollaston prism configuration is discussed, and a configuration in which the Wollaston prism and the aperture are located at different sides of the lens is recommended for further shearography applications.

Digital Shearography for NDT: Phase Measurement Technique and Recent Developments

Composite materials have seen widespread use in the aerospace industry and are becoming increasingly popular in the automotive industry due to their high strength and low weight characteristics. The increasing usage of composite materials has resulted in the need for more effective techniques for nondestructive testing (NDT) of composite structures. Of these techniques, digital shearography is one the most sensitive and accurate methods for NDT. Digital shearography can directly measure strain with high sensitivity when combined with different optical setups, phase-shift techniques, and algorithms. Its simple setup and less sensitivity to environmental disturbances make it particularly well suited for practical NDT applications. This paper provides a review of the phase measurement technique and recent developments in digital shearographic NDT. The introduction of new techniques has expanded the range of digital shearography applications and made it possible to measure larger fields and detect more directional or deeper defects. At the same time, shearography for different materials is also under research, including specular surface materials, metallic materials, etc. Through the discussion of recent developments, the future development trend of digital shearography is analyzed, and the potentials and limitations are demonstrated.

Soft tissue elastography via shearing interferometry

Early detection of cancer can significantly increase the survival chances of patients. Palpation is a traditional method in order to detect cancer; however, in minimally invasive surgery the surgeon is deprived of the sense of touch. We demonstrate how shearing elastography can recover elastic parameters and furthermore can be used to localize stiffness imhomogenities even if hidden underneath the surface. Furthermore, the influence of size and depth of the stiffness imhomogenities on the detection accuracy and localization is investigated.

Online 3D Displacement Measurement Using Speckle Interferometer with a Single Illumination-Detection Path

Measurement systems for online nondestructive full-field three-dimensional (3D) displacement based on the single-shot and multiplexing techniques attract more and more interest, especially throughout the manufacturing industries. This paper proposes an accurate and easy-to-implement method based on an electronic speckle pattern interferometer (ESPI) with single illumination-detection path to realize the online nondestructive full-field 3D displacement measurement. The simple and compact optical system generates three different sensitivity vectors to enable the evaluation of the three orthogonal displacement components. By applying the spatial carrier phase-shifting technique, the desired information can be obtained in real time. The theoretical analysis and the measurement results have proven the feasibility of this ESPI system and quantified its relative measurement error.

Polarized digital shearography for simultaneous dual shearing directions measurements

The selection of the direction of sensitivity for digital shearography is determined by its shearing direction. As a result, directionally shaped defects could be missed in non-destructive testing using a digital shearography system with only one shearing direction. This paper reports a polarized digital shearography system based on two Mach-Zehnder interferometers, which can create two orthogonal shearing directions and record shearograms in the two orthogonal directions simultaneously. The two shearograms are separated from each other by proper polarization design so that no cross interference occurs. The phase maps of the shearograms are generated by spatial phase shift methods through the introduction of different carrier frequencies in the two orthogonal shearograms and use of the Fourier transform method. This enabled simultaneous dual directional non-destructive testing during continuous loading. Theory derivation, spectrum analysis, and non-destructive testing application results are shown in detail.

Simultaneous displacement and slope measurement in electronic speckle pattern interferometry using adjustable aperture multiplexing

This paper suggests the use of adjustable aperture multiplexing (AAM), a method which is able to introduce multiple tunable carrier frequencies into a three-beam electronic speckle pattern interferometer to measure the out-of-plane displacement and its first-order derivative simultaneously. In the optical arrangement, two single apertures are located in the object and reference light paths, respectively. In cooperation with two adjustable mirrors, virtual images of the single apertures construct three pairs of virtual double apertures with variable aperture opening sizes and aperture distances. By setting the aperture parameter properly, three tunable spatial carrier frequencies are produced within the speckle pattern and completely separate the information of three interferograms in the frequency domain. By applying the inverse Fourier transform to a selected spectrum, its corresponding phase difference distribution can thus be evaluated. Therefore, we can obtain the phase map due to the deformation as well as its slope of the test surface from two speckle patterns which are recorded at different loading events. By this means, simultaneous and dynamic measurements are realized. AAM has greatly simplified the measurement system, which contributes to improving the system stability and increasing the system flexibility and adaptability to various measurement requirements. This paper presents the AAM working principle, the phase retrieval using spatial carrier frequency, and preliminary experimental results.

Michelson interferometer based spatial phase shift shearography

This paper presents a simple spatial phase shift shearography based on the Michelson interferometer. The Michelson interferometer based shearographic system has been widely utilized in industry as a practical nondestructive test tool. In the system, the Michelson interferometer is used as a shearing device to generate a shearing distance by tilting a small angle in one of the two mirrors. In fact, tilting the mirror in the Michelson interferometer also generates spatial frequency shift. Based on this feature, we introduce a simple Michelson interferometer based spatial phase shift shearography. The Fourier transform (FT) method is applied to separate the spectrum on the spatial frequency domain. The phase change due to the loading can be evaluated using a properly selected windowed inverse-FT. This system can generate a phase map of shearography by using only a single image. The effects of shearing angle, spatial resolution of couple charge device camera, and filter methods are discussed in detail. The theory and the experimental results are presented.

From speckle pattern photography to digital holographic interferometry [Invited]

Speckles are inherently an interference phenomenon produced when an optically rough surface or a turbulent medium introduces some degree of randomness to a reflected or a transmitted electromagnetic field. Speckles are often nuisance in coherent image formation. Speckle patterns are however a useful tool for displacement and deformation as well as vibration and stress analysis. The development of speckle photography to speckle interferometry and digital holographic interferometry is described in this paper.

Spatial phase shifting for pure in-plane displacement and displacement-derivative measurements in electronic speckle pattern interferometry (ESPI)

Hitherto no method, to our knowledge, was known to incorporate spatial phase shifting for the measurement of pure in-plane displacements. We demonstrate that the modified Duffy two-aperture configuration [Opt. Lett. 22, 1958 (1996)], which is sensitive to only the in-plane displacement component and offers increased sensitivity, lends itself to measurement with spatial phase shifting. The configuration can also be used for obtaining displacement derivatives by the introduction of shear with the tilt of a mirror.