Electronic Speckle Pattern Interferometer System Research Articles

Building an electronic speckle pattern interferometer to visualize modal shapes in an educational setting

An electronic speckle pattern interferometer (ESPI) is an optical system used to investigate vibrating objects and can be a great educational tool for visualizing mode shapes. At the Pennsylvania State University, the Acoustical Society of America student chapter acquired funds to build an ESPI for classroom and outreach use. The design of the ESPI was based on previously published work using equipment purchased for less than $5000. To approach this project, students were split into two groups where one developed a MATLAB application to collect and process images while the other arranged the optical components of the ESPI. To test the system, interferometry was performed on a banjo head which matched mode shapes found using a scanning Laser Doppler Vibrometer (LDV). Future work includes adapting our ESPI system to obtain images in real-time, developing educational demos involving the ESPI, and further refining the MATLAB application. This presentation provides a model for a multi-disciplinary project that could be implemented at the undergraduate or graduate level.

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.

Vibration phase mapping using holographic optical element-based electronic speckle pattern interferometry

An out-of-plane sensitive electronic speckle pattern interferometer (ESPI) using holographic optical elements (HOEs) for vibration phase mapping is reported. Constant phase contours across a sinusoidally vibrating circular metal plate are mapped using reference beam phase modulation in the unbalanced HOE based ESPI system. Reference phase modulation is implemented by modulating the drive current of a laser diode. The correlation fringes were obtained using the time average ESPI method. The phase measurement accuracy depends on the vibration amplitude of the object.

Interferometric Techniques in Structural Damage Identification

This paper describes several interferometric techniques and their applications in structural damage identification. With that objective in mind, damaged aluminum beams with clamped-free and free-free boundary conditions are analyzed. Different damages cases are inflicted by creating small cuts perpendicular to the beams longitudinal axis, being the damage, therefore, characterized by the dimensions of these cuts. The out-of-plane displacement field of modal response is measured with an electronic speckle pattern interferometric system. The static and dynamic rotation fields, defined as the spatial derivative of the displacement field, are measured with two different speckle shearography systems. Second and third order spatial derivatives of the displacement field, which are related to the bending moment and shear force, are computed by differentiation techniques. The cuts locations are determined by looking for maximum values and/or perturbations of damage indicators based on bending moments and shear forces. This method is validated by comparing its results with numerical ones, from which the most suitable interferometric technique is chosen.

Interferometric techniques in structural damage identification

This paper describes several interferometric techniques and their applications in structural damage identification. With that objective in mind, damaged aluminum beams with clamped-free and free-free boundary conditions are analyzed. Different damages cases are inflicted by creating small cuts perpendicular to the beams longitudinal axis, being the damage, therefore, characterized by the dimensions of these cuts. The out-of-plane displacement field of modal response is measured with an electronic speckle pattern interferometric system. The static and dynamic rotation fields, defined as the spatial derivative of the displacement field, are measured with two different speckle shearography systems. Second and third order spatial derivatives of the displacement field, which are related to the bending moment and shear force, are computed by differentiation techniques. The cuts locations are determined by looking for maximum values and/or perturbations of damage indicators based on bending moments and shear forces. This method is validated by comparing its results with numerical ones, from which the most suitable interferometric technique is chosen.

Static experimentations of the piezoceramic d15-shear actuation mechanism for sandwich structures with opposite or same poled patches-assembled core and composite faces

Static d15-shear actuated smart composites consisting of glass fiber/epoxy layers sandwiching piezoceramic shear patches-assembled cores were investigated experimentally and numerically. The piezoceramic cores were formed by connecting two or three patches with the same or opposite polarization directions. For each cantilevered benchmark the shear-induced transverse tip deflection, under increasing actuation voltage, ranging from 61.5 V to 198 V, was measured by an electronic speckle pattern interferometer system. The performance of the shear actuated smart composites was characterized by their shear-induced transverse deflection per length per voltage. It was found that this performance is much better at high voltages for which the response becomes nonlinear. For verification of the experimental results the proposed benchmarks were simulated within ABAQUS® commercial code using three-dimensional piezoelectric finite elements. The comparison of the obtained experimental and simulation results show a nonlinear dependence of the transverse deflection for voltages higher than around 92 V.

Rapid high spatial resolution imaging of ground vibration for buried landmine detection using ESPI

Recent work has shown that many landmines exhibit a multimode vibration pattern. To fully map the vibration pattern of these modes requires spatial resolutions on the order of millimeters. Any practical field instrument to detect landmines that cues on such modal patterns must take advantage of parallel optical processing. An optical technique that lends itself to such vibration sensing is an electronic speckle pattern interferometer (ESPI). The double-pulsed ESPI system has been used for the vibration measurement of the ground surface. The first and second laser pulses synchronized with the vibration peak and the vibration valley, respectively, illuminate the object. The imaged object wavefronts are combined with a reference wavefront and recorded in two video frames by using a CCD camera. The spatial distribution of the 2D vibration amplitude is obtained by subtracting two speckle patterns and processing the corresponding fringe pattern. The displacement sensitivity of the ESPI is about 30 nm. Here, we not only consider airborne sound sources to excite vibrations in the ground, but also a mechanical shaker to significantly increase the vibration amplitudes at the spot of interest. The vibrational pattern at the surface of the ground over buried antitank and antipersonnel landmines is studied.

Temperature dependence of fluid mixtures diffusivity by ESPI endoscopy

An electronic speckle pattern interferometric system combined with an endoscopic probe capable of measuring the diffusion coefficient of fluid mixtures as a function of temperature is presented. Its use for an aqueous solution LiBr is considered as an example in the temperature range 298–333 K.

Increased sensitivity to in-plane displacements in electronic speckle pattern interferometry

We describe an optical arrangement that increases the sensitivity to in-plane displacement in an electronic speckle-pattern interferometric system. This is accomplished by oblique illumination and observation along the direction of illumination. An anamorphic prism placed in front of the object is used to correct for the eccentricity in the image caused by the oblique observation. The sensitivity to in-plane displacement can be increased to a maximum of approximately λ/2. Experimental results including phase stepping are presented.

Quasi-equal-path electronic speckle pattern interferometric system

Limitations of a quasi-equal-path electronic speckle pattern interferometric system are discussed. We show that by replacement of the reference glass plate with a plano-concave lens the quasi-equal-path electronic speckle pattern interferometric system can be made more versatile. An arrangement to reduce the aberration in the reference beam is also presented.

Electronic speckle pattern interferometry using diode lase stroboscopic illumination

An electronic speckle pattern interferometer (ESPI) using stroboscopic illumination from a diode laser, and its application in vibration measurement, are described. Diode lasers are readily frequency (FM) and power modulated (AM) via control of their injection current; they are inexpensive and compatible with the single-mode optical fibres used in the described system. However, their modest peak power output demands the use of high-duty-cycle pulses, so that it was necessary to synchronize the illumination with the turning points of the vibration. This was achieved by operating the ESPI system in conjunction with a laser vibrometer. Injection-current-induced FM of diode lasers is non-uniform for modulation frequencies less than about 10 MHz, and to achieve successful stroboscopic illumination an electronic equalization stage was included to shape the current pulse, and hence compensate for the non-uniform FM.

Quasi-equal-path electronic speckle pattern interferometric system

A quasi-equal-path electronic speckle pattern interferometric system that uses the polarization property of a diffusing object is presented. The fringe pattern is extremely stable with high visibility. Phase stepping in this arrangement is accomplished by translating the glass plate used for obtaining the reference beam. The only alignment that is necessary is that for obtaining a uniform field of view. This experimental arrangement demonstrates high efficiency of laser power usage. An equation for longitudinal alignment tolerance is also obtained.

Optically phase-locked electronic speckle pattern interferometer system performance for vibration measurement in random displacement fields

The performance of the optically phase-locked electronic speckle pattern interferometer (OPL-ESPI) in the presence of random noise displacements is analyzed. It is shown that a sufficient condition for high visibility of signal speckle contours is that the signal pattern lie within a region of completely spatially coherent noise, i.e., where the random components of the surface vibrations have equal amplitudes and are in phase, and that the region contain the OPL-ESPI system lock point. Under these conditions the effects of the noise vibrations within the region are canceled by the action of the OPL-ESPI optical phase-locked loop, and the complete signal speckle contour pattern is visible. The single-frame speckle contrast and the root-mean-square exposure for sequentially subtracted frames are evaluated for stationary, coherence-separable noise displacements that obey Gauss–Markov temporal statistics. It is shown that these functions decrease as the noise temporal bandwidth and the mean-square displacement difference between the observation point and the lock point increase. Experimental results that illustrate the theoretical development are presented.

Electronic speckle pattern interferometric system based on a speckle reference beam

A new electronic speckle pattern interferometric (ESPI) technique is introduced. The technique is based on a reference beam combined with a large aperture optical system. The basic principles are described and compared with conventional ESPI setups. The new interferometer is easy to adjust, it is invulnerable to dust and scratches on the optical components, and is very compact. It is well-suited for practical engineering applications. Light sensitivity and fringe quality are comparable with the conventional ESPI features. Superior fringe pattern can be obtained by use of a new speckle reduction technique to be described.