Speckle Pattern Interferometer Research Articles

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.

Age-Related Differences in the Elasticity of the Human Cornea

The goal of this study was to determine age-related variation in the elasticity of the human cornea using nondestructive means. Organ cultured human corneoscleral buttons were studied. Changes in strain were measured with a radial shearing speckle pattern interferometer after an increase in intraocular pressure from 15.0 to 15.5 mm Hg. Changes in central corneal displacement were calculated by integration, and a bulk corneal Young's modulus was derived by mathematical analysis. Fifty corneas, including 17 pairs, were studied. Donors were aged between 24 and 102 years (mean, 73.1); 29 (58%) specimens were from male donors and 21 from female donors. Young's modulus of the cornea increased with age, with the line of best fit indicating an approximate doubling from 0.27 MPa at age 20 years (95% confidence interval, 0.22-0.31) to 0.52 (0.50-0.54) MPa at age 100 years (R² = 0.70). The stiffness of the human cornea increases by a factor of approximately two between the ages of 20 and 100 years. This variation is relevant to the algorithms used to predict the response to incisional and ablative refractive surgery and will also affect the formulas used to calculate intraocular pressure by applanation.

High-speed, sub-Nyquist interferometry

The velocity measurement limit in dynamic interferometry is vNyq, the velocity at which the interferogram is sampled at the Nyquist limit. We show that vNyq can be exceeded by assuming continuity of the surface motion and unwrapping the velocity modulo 2vNyq. The technique was demonstrated in a high-speed speckle pattern interferometer with spatial phase stepping. Surface velocities of 4vNyq were measured experimentally. With a reduced exposure, high-speed sub-Nyquist interferometry could be implemented up to a maximum acceleration of vNyq/ts, where ts is the detector frame period.

Optical methods in stringed instrument testing.

Stringed instruments present an experimental challenge for several reasons. Radiated sound is a strong function of dynamic response, so particularly accurate measurements of structural and acoustic resonant frequencies are desirable. However, the structures tend to be very light and sensitive to the additional mass of contacting sensors. Thus, optical methods are attractive. The ready availability of lasers, inexpensive digital cameras, and powerful, inexpensive computers has made optical testing practical. As a result, a range of methods has been applied to musical instruments. There are many ways to organize the methods, but an attractive one is whether they require coherent light for interference effects. A hierarchy of methods is presented along with representative results. The simplest methods simply track motion of a reflected beam across an optical sensor. Coherent methods start with laser vibrometry and move through holography and speckle pattern interferometry. We pay particular attention to an particular application of laser vibrometry and to a low cost speckle pattern interferometer.

In-plane sensitive electronic speckle pattern interferometer using a diffractive holographic optical element

We describe a student project in electronic speckle pattern interferometry. The project includes holographic recording of diffraction gratings in thick self-processing photopolymer layers made from off-the-shelf chemicals. The gratings are employed in a simple electronic speckle pattern interferometer to measure in-plane rotation.

Mechanical stress measurement by an achromatic optical digital speckle pattern interferometry strain sensor with radial in-plane sensitivity: experimental comparison with electrical strain gauges

This paper shows the optical setup of a radial in-plane digital speckle pattern interferometer which uses an axis-symmetrical diffractive optical element (DOE) to obtain double illumination. The application of the DOE gives in-plane sensitivity which only depends on the grating period of the DOE instead of the wavelength of the laser used as illumination source. A compact optical layout was built in order to have a portable optical strain sensor with a circular measurement area of about 5 mm in diameter. In order to compare its performance with electrical strain sensors (strain gauges), mechanical loading was generated by a four-point bending device and simultaneously monitored by the optical strain sensor and by two-element strain gauge rosettes. Several mechanical stress levels were measured showing a good agreement between both sensors. Results showed that the optical sensor could measure applied mechanical strains with a mean uncertainty of about 5% and 4% for the maximum and minimum principal strains, respectively.

Evaluation of the piezoelectric behaviour produced by a thick-film transducer using digital speckle pattern interferometry

This paper presents an interferometric measurement of the out-of-plane deflections produced by a piezoelectric transducer, manufactured by thick-film deposition of a ceramic paste over an alumina substrate, when is subjected to a DC electric voltage. It is shown that a digital speckle pattern interferometer with an incorporated phase-shifting facility allows the measurement of nanometer displacements generated by the piezoelectric device. These measurements are used to evaluate the effective piezoelectric charge constant along the polarization direction ( d 33) eff that characterizes the thick-film transducer.

Electronic speckle-pattern interferometer using holographic optical elements for vibration measurements

We report a simple, compact electronic speckle-pattern interferometer (ESPI) incorporating holographic optical elements (HOEs) for the study of out-of-plane vibration. Reflection and transmission HOEs provide reference and object beams in the interferometer. The alignment difficulties with conventional ESPI systems are minimized using HOEs. The time-average ESPI subtraction method is used to generate the fringe pattern and remove background speckle noise by introducing a phase shift between consecutive images. The amplitude and phase maps are obtained using path-difference modulation.

Achromatic digital speckle pattern interferometer with constant radial in-plane sensitivity by using a diffractive optical element

We report on a digital speckle pattern interferometer that applies a binary diffractive optical element (DOE) to generate double illumination and radial in-plane sensitivity. The application of the DOE ensures independence on the wavelength of the laser used as an illumination source. Furthermore, in-plane sensitivity only depends on the grating period of the DOE. An experimental setup was built allowing the measurement of a set of radial in-plane displacement fields either using a red laser as a light source or a green one. When displacement fields computed from the measured optical phase maps obtained with a red or a green laser were compared, two main results were observed: (a) deviations between mean values ranged only up to 7 nm and (b) phase maps presented the same amount of fringes. In addition, phase maps measured with the red laser were processed as they were obtained with green light. For this case, deviations have ranged only up to 0.5 nm. On the other hand, a set of measurements performed changing the DOE by a conical mirror showed clearly that radial in-plane sensitivity increased when the red laser was changed by the green one.

Opto-mechatronic configurations to maximize dynamic range and optimize resolution of optical instruments

We developed a new mechanical configuration which combines various optical techniques to maximize dynamic range and to optimize resolution in optical instruments. We assessed various optical configurations by looking at such variables as electronic signal processing and mechanical construction. Previous configurations developed over the last 10 years are discussed. In our new configuration, we adopted and integrated devices such as a microscopic, an interferometer, an electronic speckle pattern interferometer, and a photon tunneling microscope. We examined the impact of our new configuration on future opto-mechatronic systems design. Our new configuration can be an effective and versatile optical metrology tool for enhancing the performance of MEMS and NEMS devices.

Applying electronic speckle pattern interferometry to the study of musical instruments.

An overview of the electronic speckle pattern interferometer as a tool for the study of structural vibrations will be presented, highlighting both the theory and construction of the device. Two specific applications to the study of musical instruments will then be discussed: time-averaged interferometry of harmonic motion and time resolved interferometry of transient motion.

Constructing and using a low cost speckle pattern interferometer for analysis of musical instruments.

Following the design of Moore [Am. J. Phys. 72, 1380 (2004)], a speckle-pattern interferometer for use in exploring the mode shapes of vibrating structures, especially musical instruments, has been constructed. The interferometer was built primarily by undergraduates with mostly common optical elements found in any undergraduate optics laboratory. The interferometer uses a FireWire (IEEE 1394) camera, which has a built-in frame-grabbing capability, eliminating the need for expensive frame-grabbing hardware. The interferometer has been used to study vibrating structures such as snare drum heads, mandolins, and a tennis racket. The interferometer has also been used to study the the vibrational properties of a cured carbon fiber disk. The design and implementation of an affordable speckle-pattern interferometer along with a brief overview of some of the results of research projects conducted with undergraduate students will be presented.

Time-averaged electronic speckle pattern interferometry in the presence of ambient motion Part I Theory and experiments

An electronic speckle pattern interferometer is introduced that can produce time-averaged interferograms of harmonically vibrating objects in instances where it is impractical to isolate the object from ambient vibrations. By subtracting two images of the oscillating object, rather than the more common technique of subtracting an image of the oscillating object from one of the static objects, interferograms are produced with excellent visibility even when the object is moving relative to the interferometer. This interferometer is analyzed theoretically and the theory is validated experimentally.

Two Dimensions Residual Stresses Analysis Through Incremental Groove Machining Combined with Electronic Speckle Pattern Interferometry

In this study a new residual stress determination method in two directions simultaneously is presented. This method is based on stresses relaxation in a groove that is machined incrementally. The residual stresses relaxation occurs simultaneously from both the depth and the length of the groove. Thus, measuring the surface strain field generated by the relaxation enables to determine the stress gradient both along the depth and the length of the groove. To measure the surface strain in a direction perpendicular to the groove, a digital speckle pattern interferometer is used. This method is suitable when the residual stress field in the structure varies in the depth as well as along the surface of the part, like for example in a welded structure. The method is tested here on an aluminium plate in which a central band has been shot peened.

High-speed electronic speckle pattern interferometry of a struck flat plate

An electronic speckle pattern interferometer capable of imaging 33,000 frames per second has been used to study the transient motion of a flat plate which was struck at a point near the edge. Multiple interferograms, each with an exposure time of 25 microseconds, were combined to construct a video of the plate motion that can be used for both qualitative and quantitative analysis. The interferograms reveal the expected evolution of the bending waves created when a thin circular plate is struck near the edge; however, anomalous displacement of the surface of the plate is also observed. This unusual displacement of the surface takes the form of a transverse wave that precedes the motion directly attributable to the normal propagation of the initial bending wave.

Measurement of large cryogenic structures using a spatially phase-shifted digital speckle pattern interferometer

The James Webb Space Telescope (JWST) Backplane Stability Test Article (BSTA) was developed to demonstrate large precision cryogenic structures' technology readiness for use in the JWST. The thermal stability of the BSTA was measured at cryogenic temperatures at the Marshall Space Flight Center (MSFC) X-Ray Calibration Facility (XRCF) and included nearly continuous measurements over a six-week period in the summer of 2006 covering the temperature range from ambient down to 30 Kusing a spatially phase-shifted digital speckle pattern interferometer (SPS-DSPI). The BSTA is a full size, one-sixth section of the JWST primary mirror backplane assembly (PMBA). The BSTA, measuring almost 3 m across, contains most of the prominent structural elements of the backplane and is to our knowledge the largest structure ever measured with SPS-DSPI at cryogenic conditions. The SPS-DSPI measured rigid body motion and deformations of BSTA to nanometer-level accuracy. The SPS-DSPI was developed specifically for the purposes of this test and other tests of large cryogenic structures for JWST.

Calibration of spatially phase-shifted DSPI for measurement of large structures

We present a method for the calibration of a spatially phase-shifted digital speckle pattern interferometer (SPS-DSPI), which was designed and built for the purpose of testing the James Webb space telescope (JWST) optical structures and related technology development structures. The need to measure dynamic deformations of large, diffuse structures to nanometer accuracy at cryogenic temperature is paramount in the characterization of a large diameter space and terrestrial based telescopes. The techniques described herein apply to any situation, in which high accuracy measurement of diffuse structures are required. The calibration of the instrument is done using a single-crystal silicon gauge. The gauge has four islands of different heights that change in a predictable manner as a function of temperature. The SPS-DSPI is used to measure the relative piston between the islands as the temperature of the gauge is changed. The measurement results are then compared with the theoretical changes in the height of the gauge islands. The maximum deviation of the measured rate of change of the relative piston in nm/K from the expected value is 3.3%.

Electronic speckle pattern interferometry applied to the displacement measurement of sandwich plates with two ‘fully potted’ inserts

The experiments of electronic speckle pattern interferometer (ESPI) successfully applied to sandwich plates with two inserts has been earliest presented in this paper. Proposed ESPI has advantages of full-field and non-destructive testing, which can measures microscopic out-of-plane displacement in the elastic region without wasting specimen. For validation purpose, the finite element method (FEM) analysis was conducted. By comparing the results of ESPI and FEM displacement fields around the inserts, a convincing agreements is revealed. The effect of insert spatial interval on the deformation of two-inserted sandwich plates was obtained by the ESPI and FEM: the wider the spatial interval of the two inserts is, the bigger the out-of-plane displacement at the insert center of sandwich loaded by the same force is.

Imaging vibrations and flow using electronic speckle pattern interferometry

Electronic speckle pattern interferometry has been used to measure sub-micrometer displacements and visualize the deflection shapes of harmonically vibrating objects for many years; however, to date the technique has not been widely used within the community of musical acousticians. The cost of the equipment, the need for effective vibration isolation, and the complexity of the hardware are usually cited as the reasons why this technique has not been more widely utilized in the study of musical instruments. To alleviate these concerns, a speckle pattern interferometer has been designed that is relatively inexpensive, easy to assemble, and very robust. Construction of the interferometer will be discussed, and applications to problems in musical acoustics will be presented.

Residual stress measurement using indentation and a radial electronic speckle pattern interferometer — recent progress

A new kind of electronic speckle pattern interferometer (ESPI) was developed by the authors' group using conical mirrors to measure the radial in-plane displacement component. True radial in-plane sensitivity is achieved by double illumination. This device is used in the present work to measure residual stresses in combination with the indentation method. A semi-empirical mathematical model is developed to quantify the residual stresses. Several tests were made on a specimen with different levels of stress imposed by mechanical loading. Empirical constants were computed from these tests and are used in combination with the developed model to predict the residual stress levels. The tests presented here are preliminary as they are restricted to only one material, a uniaxial stress state, one geometry of the indentation tip, and only one value of indentation force. In these conditions the results are very encouraging, a typical measurement error of about 5 per cent being found for the principal residual stress.