Pramod Rastogi

Digital Speckle Pattern Interferometry & Related Techniques

From the Publisher: Digital Speckle Pattern Interferometry (DSPI) is the generic name for a class of important interferometric techniques such as TV holography, electronic holography or phase shifting pattern interferometry, all of which involve similar optical and electronic principles. These techniques are increasingly used in a wide range of fields including experimental mechanics, vibration analysis and non-destructive testing. Digital Speckle Interferometry and Related Techniques provides a single source of information in this rapidly progressing field. Containing contributions from leading experts, it provides the key background information, including the fundamental concepts, techniques, and applications, and presents the major technological progress that has contributed to revitalization in the field over the past fifteen years, including digital speckle photography and digital holographic interferometry. This is an invaluable text for practising engineers in industry and research institutes, and academic and development researchers in scientific and engineering disciplines who use, or are interested in, DSPI. This title would also be of interest to teachers and final year undergraduate/postsgraduate students of physics, optics, experiemental mechanics, photomechanics, optical metrology, engineering metrology, and non-destructive testing.

Relative humidity sensor with optical fiber Bragg gratings

A novel concept for an intrinsic relative humidity (RH) sensor that uses polyimide-recoated fiber Bragg gratings is presented. Tests in a controlled environment indicate that the sensor has a linear, reversible, and accurate response behavior at 10-90% RH and at 13-60 degrees C. The RH and temperature sensitivities were measured as a function of coating thickness, and the thermal and hygroscopic expansion coefficients of the polyimide coating were determined.

Micromechanics of wood subjected to axial tension

SummaryThe behaviour of a small group of wood fibers of Sitka spruce during tensile loading is investigated. The load-extension curves for both early and late wood fibers consist of three distinct segments. The first segment is almost a straight line, at some stage of loading a yield point is observed. Beyond this point the specimen becomes less stiff and undergoes a large, mainly irreversible deformation. As the load is increased further, the curve exhibits the third segment showed by a significant change in slope. These curves look different from those obtained on thick specimens. In this respect, the behaviour of a thin wood specimen subjected to cyclic type tensile loading along its longitudinal direction is also illustrated. Based on wood microstructure, a model is presented to interpret the evolution of the Youngs modulus of a wood fiber during tensile loading. The model considers wood as an assembly of cylindrical fibers pasted together in a longitudinal direction. We have assumed the cell wall to comprise only an S2 layer made of a composite material consisting of a lignin and hemicellulose matrix reinforced by helical microfibrils along the fiber. Furthermore, it is assumed that the microfibril angle a in the S2 layer is not uniform along the fiber axis and matrix degradation occurs in the zones where the microfibril angles are bigger. The validity of this assumption is verified by using holographic interferometry to visualize the displacement field of the specimens surface under tension.

Speckle motions induced by rigid-body movements in free-space geometry: an explicit investigation and extension to new cases

In line with the current interest in speckle motion in free-space propagation, it is proposed to investigate new aspects of the induced speckle motion due to rigid-body movements of a model using as a starting point the well-known concept of homology, first stated in holographic interferometry, the importance of which has only recently been realized in speckle metrology. At the outset, stating the homology conditions leads to a general expression relating the speckle shift to the geometrical parameters at the recording and to the six degrees of movement of the object surface. The expression presented in this explicit form is found to be true for both opaque and thin transparent diffusing models and for different conditions of illumination and observation. The study is then extended to (1) when the object is at rest, subjecting the illuminating source to a small displacement and (2) when the wavelength of the illuminating beam is changed between the two exposures. The case of the rigid-body displacement of the object is experimentally verified and the results are found to be in good agreement with theory. The viability of the method to make measurements is assessed, several applications are envisaged, and some advantages are pointed out.

Holographic moiré in real time

The body of knowledge necessary to observe holographic-moiré patterns in real time is introduced. The basic factors influencing fringe visibility in holographic moiré are analyzed and expressions to evaluate fringe visibility for any given displacement and deformation are given. The application of the introduced theory in the case of real-time observation is discussed. It is shown that the maximum benefits of this technique are achieved by combining it with closed-circuit TV. Several examples of application are given.

Recent developments in digital speckle pattern interferometry

Reference IMAC-ARTICLE-2003-007View record in Web of Science Record created on 2007-06-13, modified on 2016-08-08

High efficient superresolution combination filter with twin LCD spatial light modulators

A comparative study of pupil filters for transverse superresolution is presented in this article. We propose to combine the advantages of amplitude and phase filters in one complex filter that performs better than either phase or amplitude filters designed so far. The performance here refers to having a smaller spot size along with higher peak to side lobe intensity ratio. Using numerical simulation the limitations of phase and amplitude filters are assessed. The experimental verification of the designed combination filter is performed with two LCD spatial light modulators used for displaying separately the phase and amplitude part of the filter. Results obtained from this setup confirm the simulation.

Measurement of difference deformation using speckle interferometry

An application of speckle interferometry is presented that allows one to compare the resistance to stress of two nominally identical specimens. The interference pattern gives contours of equal differences in displacements of the two stressed specimens. The method is briefly developed, and its experimental feasibility is demonstrated by comparing the deflections of two square plates clamped along the edges and subjected to centrally concentrated loads.

Holographic Interferometry Principles and Methods

This study provides an up-to-date source of information on a rapidly expanding field. The contributions cover the principles and methods currently in use. The scope of the book has been limited to the study of opaque objects and ample space has been devoted to a comprehensive treatment of the phenomena of fringe formation and a quantitative evaluation of the holographic interference fringe patterns. The emergence of computer-aided fringe analysis and phase-shifting techniques have considerably simplified the quantative real-time measurements of object shapes and deformations. This volume also includes an overview of full-field holographic methods for the measurement of shapes, displacements, derivatives, difference displacements and vibrations.

Phase shifting applied to four-wave holographic interferometers.

A phase-shifting procedure that is applicable to four-wave holographic inteferometry is described. The technique can be used to obtain whole-field phase maps with all methods that use holographic moire fringes.