Saba Mirza

Surface profiling using phase shifting Talbot interferometric technique

We implement a phase-shifting Talbot interferometric technique to measure step heights of a single-step and channel (dip) objects. Experimentally recorded interferograms include noise due to grating lines and speckles. To make the phase map, four interferograms are recorded with /2 phase shifts. To remove noise due to grating lines and speckle, interferogram data are first filtered using Fourier filtering. Height variations per profile of the objects is calculated from the phase map made using Fourier-filtered data. Results obtained from this technique are compared with the results acquired from a profile projector having a coordinate measurement facility with a resolution of 1 µm and an accuracy of ±1 µm. The results are in good agreement.

Study of various preprocessing schemes and wavelet filters for speckle noise reduction in digital speckle pattern interferometric fringes

A new filtering scheme is investigated for the removal of speckle noise in the recorded fringe pattern by a digital speckle pattern interferometric technique (DSPI). The scheme consists of a preprocessing scheme i.e., averaging, sampling, thresholding, and again averaging, followed either by a symlet or biorthogonal wavelet filter. The preprocessing scheme improves the contrast between the dark and bright fringes, and the implementation of the biorthogonal wavelet filter, which is symmetrical and purely linear in phase, solves the problem of border effect and discontinuity. The developed scheme is implemented on recorded fringe patterns of a vibrating cantilever beam and vibrating square plate with all ends fixed. The performance and effectiveness of the proposed scheme in reducing speckle noise is evaluated and compared in terms of reduction in speckle index and the improvement of the signal-to-noise ratio (SNR) of speckle interferograms. The new scheme has the ability to improve the speckle index and SNR significantly.

Distance measurement with extended range using lateral shearing interferometry and Fourier transform fringe analysis

We propose a distance measuring system based on lateral shearing interferometer (LSI) with extended range of measurement and improved resolution. The sensor is based on a wedge-shaped plate LSI and due to the presence of tilt, a finite number of fringes parallel to the direction of shear appear and hence significant spatial carrier frequency is generated at the focus position. On varying the distance of the object surface from the focus position, the orientation, fractional and integer order, and the width of the interference fringes change. It is shown that the orientation of the fringes is the most sensitive parameter, by means of which one can detect small changes in distance. The Fourier transform method for fringe analysis is used, and from the position of the maxima of the first-order Fourier spectrum the orientation of the fringes and hence the distance is measured. Experimental results of distance measurement by changing the focal length of the lens are presented. A large range of measurement without any fringe ambiguity problem can be achieved using the system.

Two-wavelength lateral shearing interferometry

We propose a two-wavelength interferometric system based on a single optical element, i.e., a shear plate. The observation of beat formation phenomenon and the measurement of phase at synthetic wavelength are demonstrated for the first time using two wavelengths simultaneously in a lateral shearing interferometer. Shearing interfero- grams are recorded both at individual wavelength and at synthetic wave- length. The phase map at synthetic wavelength is obtained by means of subtracting the phase maps at individual wavelength as well as directly from the synthetic interferogram, and the results are found in good agreement. The validity of the principle is applied for collimation testing and reconstructing the phase map of transparent objects. The main ad- vantages of the present system over the previous systems based on two-wavelength interferometry are high stability, nearly common path in- terferometry, single optical element shear plate requirement, compact- ness, and low cost.

Different preprocessing and wavelet transform based filtering techniques to improve Signal- to-noise ratio in DSPI fringes

Experimental results reveal that using appropriate pre-processing scheme followed by Biorthogonal wavelet filter reduced speckle index and increase the SNR significantly. This results in enhancement of contrast between dark and bright fringes. The Fig 5(d) shows that the implementation of appropriate pre-processing scheme followed by Biorthogonal wavelet filter gives more clear fringe pattern as comparison to filtering schemes results shown in Fig. 5(a), Fig. 5(b) and Fig. 5(c).

Profiling of gas turbine blade using phase shifting Talbot interferometric technique

In this paper we present phase shifting Talbot interferometry for the measurement of surface topography of the gas turbine blades. Interferograms of the different steps are recorded and displayed on the computer monitor using digital techniques. Presence of the harmonic components in the phase map due to the Ronchi gratings are removed by using Fourier filtering. The variation of the surface height at the different points of the objects is obtained by generating the phase map. The results obtained by phase shifting Talbot interferometric techniques are in good agreements with that of the measured by the manually controlled co- ordinate measuring machine. The critical analysis of results alongwith error analysis is presented.

Surface profiling of turbine blade using phase-shifting Talbot interferometric technique

In this paper we present phase shifting Talbot interferometric technique to draw the profile of the gas turbine blade. In Talbot interferometry phase shifting is implemented by giving in-plane translation to first grating by using precision translation stage. To obtain the phase connected with the object, the additional phase change stepwise, with a step of π/2 at least three times is introduced and the intensity patterns corresponding to these steps are recorded as Talbot interferometric fringes pattern. Experimentally recorded interferograms have noise due to grating lines and speckles. These noises are filtered by using Fourier filtering technique. The phase map made by using Fourier filtered data. Height variations/profile of the gas turbine blades made by using phase shifting Talbot Interferometry are compared with the results obtained from Co-ordinate Measuring Machine (CMM) having coordinate measurement facility with the resolution of 1μm. The results are in good agreement.

Three-dimensional range sensing and surface profilometry using lateral shearing interferometry

A distance measuring system and surface profiler with extended range based on collimation testing technique using lateral shearing interferometry is proposed. The sensor works on the principles of focus sensing and interferometry and the depth gating is achieved by collimation testing. A wedge-shaped shearing plate was used for the interferometer and when the object surface is exactly at the focus of the collimating lens the interference fringes are perfectly parallel to a reference line with a finite spatial frequency. If the object surface is out of focus of the collimating lens the fringe pattern is oriented with a change in the spatial frequency. The orientation of the interference fringes and their spatial-frequency increases as we move the object farther from the focused position. Fourier transform method for fringe analysis is used and from the maximum of the Fourier spectrum the distance and three-dimensional surface profile of the objects is measured. Since the surface profile is reconstructed from the maxima of the first-order Fourier spectrum, a large range of measurement without any fringe ambiguity problem can be achieved using the system. The system works both for smooth as well as diffuse objects and is compact, robust and inexpensive. A high-depth of resolution of the order of 10 micrometer achieved with a range measurement 10mm.

Study of mode shapes and measurement of vibrations in square plates using DSPI and wavelet transform

In this paper the study of out-of-plane or transverse vibrations in a square plate using digital speckle pattern interferometry (DSPI) is presented. To improve the accuracy of measurement, earlier we have suggested filtering scheme based on proper use of average/median filters followed by Symlet wavelet filter. In this paper we present a different filtering scheme to reduce the speckle noise and improve the accuracy of measurement of DSPI fringes by proper choice of average filter, sampling and thresholding followed by Symlet/Biorthogonal wavelet filter. The speckle index of filtered pattern is calculated and compared with speckle index of unfiltered fringe pattern. It is observed that speckle index is significantly reduced after filtering the DSPI fringe pattern. Experimentally obtained resonance frequencies for the square plate for the boundary condition fixed at all edges were compared with that of classical theory for thin plates. The resonance frequencies obtained from DSPI show good agreement with that of obtained from the classical theory.

Application of digital speckle pattern interferometry in measurement of large deformation

In this paper an algorithm to measure large deformations using digital speckle pattern interferometry is presented. It facilitates recording of large number of frames and can subtract them to display speckle correlation fringes with improved signal-to-noise ratio. To further improve signal-to-noise ratio a filtering scheme is also presented by using average/median filtering followed by wavelets filtering. Experimental results and analysis are presented in detail. For a typical displacement of 200 μm, error in measurement was less than 1.5 percent. Range in measurement is governed by change in scattering property of the surface of the object.