Biomedical subjects

Dalip Singh Mehta

Publications and source records attributed to Dalip Singh Mehta.

5 recordsLinked to original sources

PubMed2006

Two-wavelength Talbot effect and its application for three-dimensional step-height measurement.

The phenomenon of Talbot self-image shift by changing the wavelength of the illuminating light is described and demonstrated experimentally. A periodic grating is illuminated by light with wavelengths lambda1 and lambda2 generated by two lasers, and the Talbot self-images are recorded along the longitudinal direction at individual wavelengths. The Talbot self-image shift due to the change in the wavelength of light is implemented for the measurement of the three-dimensional step height of a large discontinuous object without any phase ambiguity problem. Fourier-transform fringe analysis was used to determine the maximum contrast of the high-visibility bands for the measurement of the step height of the object. The main advantages of the proposed system are nonmechanical scanning, high stability because of its common path geometry, compactness, and a wide range of measurement as compared to interferometric three- dimensional profilers.

Journal Article↗

PubMed2005

Simple multifrequency and phase-shifting fringe-projection system based on two-wavelength lateral shearing interferometry for three-dimensional profilometry.

We propose a simple multifrequency spatial-carrier and phase-shifting fringe-projection system based on two-wavelength lateral shearing interferometry (LSI). In this system a wedge-shaped plate lateral shearing interferometer is used and, owing to the presence of tilt, a finite number of fringes parallel to the direction of the shear appears; hence a significant spatial-carrier frequency is generated at the focus position. We further enhance the spatial-carrier frequency either by changing the wavelength of the laser light or by slight defocusing. A synthetic interferogram with low spatial-carrier frequency is obtained by use of laser light of two wavelengths simultaneously in the lateral shear interferometer. We obtain the phase-shifted fringe patterns from the same setup by simply moving the wedge plate in an in-plane parallel direction, using a linear translator. The fringe projection system was tested for measurement of the three-dimensional shape of a discontinuous object. The present system has many advantages; e.g., it is a common-path interferometry and hence is insensitive to external vibrations, is compact in size, and is relatively inexpensive.

Journal Article↗

PubMed2003

Spectral interference Mirau microscope with an acousto-optic tunable filter for three-dimensional surface profilometry.

A nonmechanical scanning Mirau-type spectral interference microscope has been developed for the measurement of three-dimensional surface profiles of discontinuous objects. An acousto-optic tunable filter (AOTF) is used as a high-resolution spectral filter, which scans the optical frequency of the broadband light emitted from a superluminescent diode. To generate spectral fringes that make full use of the limited coherence length of the filtered light we unbalanced the Mirau interferometric system by positioning the reference mirror nearly halfway between the top and the bottom of the step height. When the frequency of the broadband light source is scanned by an AOTF, the interference fringes move in opposite directions on the top and the bottom of the object. To uniquely determine the sign of the fringe movement over the large area of the object, we developed a three-dimensional Fourier-transform technique, and from the detected sign of the fringe movement and phase information, we determined the three-dimensional step height. Experimental results of the measurement of 100-microm step height are presented. The main advantages of the proposed system are that it provides nonmechanical scanning and a large measurement range without ambiguity in the sign of the phase.

Journal Article↗

PubMed2003

Spectral interferometric microscope with tandem liquid-crystal Fabry-Perot interferometers for extension of the dynamic range in three-dimensional step-height measurement.

The maximum measurable range of a spectral interference microscope depends on the coherence length of the light transmitted by its tunable spectral filter. To achieve a large range in step-height measurement we have developed a new tunable spectral filter that uses tandem liquid-crystal Fabry-Perot interferometers (LC-FPIs), which can simultaneously attain both a high spectral resolution and a large tuning range. Fringe visibility measurements were carried out, and it was found that the coherence length of the light transmitted through tandem LC-FPIs is two times larger than that transmitted through a single LC-FPI. Using this novel tunable spectral filter, we developed a new spectral interference microscope for the measurement of three-dimensional shapes of discontinuous objects. Experimental results of step-height measurements both with a single LC-FPI and with tandem LC-FPIs are presented for a combination of standard steel gauge block sets with 1-, 99-, and 100-microm steps. A large range (1-100 microm) of measurement with submicrometer resolution was achieved with tandem LC-FPIs that was not possible with our previous system in which a single LC-FPI was used.

Journal Article↗

PubMed2002

Simultaneous three-dimensional step-height measurement and high-resolution tomographic imaging with a spectral interferometric microscope.

A noncontact, nonmechanical scanning, wide-field spectral interference microscope is developed for simultaneous measurement of three-dimensional step-height of discontinuous objects and tomographic imaging. A superluminescent diode (SLD) is used as a broadband light source and a liquid-crystal Fabry-Perot interferometer (LC-FPI) as a frequency-scanning device. By means of changing the injection current to the SLD, the spectral profile of the SLD is equalized, and a constant light input to the interferometer is achieved over the entire frequency-scan range. The Fourier-transform technique is used to determine both the amplitude and the phase of spectral fringe signals. Three-dimensional height distribution of a discontinuous object is obtained from the phase information, whereas optically sectioned images of the object are obtained either from the amplitude information alone or from the combination of both the amplitude and phase information. Experimental results with submicrometer resolution are presented for both step-height measurement and tomographic sectioning.

Imaging, Three-Dimensional↗