N. Krishna Mohan

Electronic speckle pattern interferometry for simultaneous measurement of out-of-plane displacement and slope.

An optical configuration is presented for simultaneous measurement of out-of-plane displacement and slope change in electronic speckle pattern interferometry. Experimental results for a rectangular plate clamped along the edges and loaded at the center are presented.

Deformation and shape measurement using multiple wavelength microscopic TV holography

Characterization of deformation and surface shape is an important parameter in quality testing of micro-objects in view of the functionality, reliability, and integrity of the components. Single-wavelength TV holography is widely used for deformation analysis. However, the single-wavelength TV holographic configuration suffers from overcrowding of fringes for large deformation that sets a limitation due to speckle decorrelation for quantitative fringe analysis. Furthermore, shape cannot be determined when using single wavelength. In this paper, we describe a multiple-wavelength microscopic TV holographic configuration that uses sequentially recorded phase-shifted frames at three different wavelengths before and after deformation of the specimen for evaluation of relatively large deformation fields at the effective wavelengths. Use of multiple wavelengths for deformation and shape evaluation is discussed. The design of the system along with the experimental results on small-scale rough specimens under static load is presented.

Digital speckle pattern interferometry using spatial phase shifting: influence of intensity and phase gradients

Spatial phase shifting technique in digital speckle pattern interferometry (DSPI) and digital shearography (DS) provides the phase information due to the object displacement from two images, one stored before and other after loading. The technique needs a carrier fringe system. The double aperture mask in front of the imaging system is one of the methods for introducing the spatial carrier frequency for phase evaluation. The size of the apertures and their separation are important criteria to obtain appropriate phase shift/column within the desired size of the speckle for phase retrieval. The assumptions of constant intensity and phase on adjacent pixels of the camera while calculating phase in spatial phase shifting (SPS) are not met as the speckled object wave contains intensity and phase gradients, resulting in distortions in the calculated phase profiles. In this paper we discuss a strategy to overcome these problems. The contrast of the correlation fringe obtained using this approach is much improved. It also eliminates the distortion in the unwrapped phase map like wave ripples. The experimental results on an edge clamped circular plate loaded at the center are presented.

Optical configuration for measurement in speckle interferometry.

An optical configuration for measurement of in-plane displacement and for contouring is reported. In this method an object point is viewed symmetrically with respect to surface normal and combined coherently at the image plane of the imaging system. Because the beams are combined by small apertures at the image plane, decorrelation sets in rather slowly. Owing to low decorrelation, fringes have been obtained for large in-plane deformations and large angular tilts. The method is simple to implement, and its sensitivity can be varied over a wide range. The configuration, therefore, extends the range of measurement.

Electronic shearography applied to static and vibrating objects

Abstract An optical shearography configuration is presented which can be used for the measurement of slope change for static and vibrating objects in real time. Displacement and vibration amplitude fields are extracted from the shear data, which is an useful property in noise environments. In a Michelson interferometer set-up the two arms are provided with a phase-shift mirror and a bias modulation mirror to carry out the numerical evaluation of the measured deformation. Experimental results for an edge- clamped aluminium plate are presented.

In-plane displacement measurement configuration with twofold sensitivity

An optical configuration is suggested that will achieve a twofold increase in sensitivity when one measures an in-plane displacement component of a deformation vector compared with the Leendertz two-beam illumination method. A theory and experimental demonstration of the method are presented.

Microscopic TV shearography for characterization of microsystems

We demonstrate a microscopic TV shearographic configuration for characterization of microsystems by measuring the slope under relatively large out-of-plane deformation. In the optical arrangement, a long working distance zoom imaging system is combined with a conventional Michelson shear interferometer. The experimental results on a microelectromechanical system pressure sensor subjected to external pressure load are presented.

Fringe formation in multiaperture speckle shear interferometry

Multiaperture speckle shear interferometry exhibits sensitivity to in-plane displacement components. The response of the interferometer to in-plane displacement, however, depends on the location of the shear element on an aperture in front of the imaging lens. Two configurations with three apertures, with the shear element positioned at two different locations, are examined theoretically and experimentally.

Multiple wavelength interferometry for surface profiling

Interferometry is a well established technique for surface profiling. The conventional interferometric surface profilers using a single wavelength offer excellent vertical resolution, but a serious limitation to their use is that they can only handle smooth profiles and step heights less than half a wavelength. In the situation where the surface profile is discontinuous, white light interferometry has been applied with great success. However the scanning white light interferometry requires large number of frames to be recorded, whereas in spectrally resolved white light interferometry only a line profile of the object is obtained, although the requirement on number of frames is similar to the single wavelength phase shifting interferometry. In this paper we discuss three wavelength interferometry in which a limited number of frames suitable for phase shifting technique are recorded at three laser wavelengths. The phase evaluation at the three wavelengths gives wrapped phase at any pixel corresponding to these wavelengths. The fringe order is obtained considering the fact the variation of phase with wavenumber for a given profile height is linear. The slope of the phase verses wavenumber line gives the absolute value of the profile height and is used to ascertain the fringe order. The fringe order along with the wrapped phase gives the profile height with a resolution given by phase shifting technique. Experimental results on etched silicon samples are presented.

A holographic speckle shearing technique for the measurement of out-of-plane displacement , slope and curvature

Abstract A holographic speckle shearing technique combining image plane holography is presented which enables the measurement of out-of-plane displacement, slope and curvature for flexural members. Slope is obtained both as moire of displacement fringes as well as by speckle shear interferometry. The curvature fringes occur as moire of two speckle slope patterns. The image plane holography enables the use of white light or quasi-monochromatic light for filtering, thereby resulting in reduction of speckles in the fringe pattern. The results are presented for a centrally loaded circular diaphragm with its edge rigidly clamped.