Ph.D. Thesis Colloquium
DEPARTMENT OF INSTRUMENTATION AND APPLIED PHYSICS
Ph.D. Thesis Colloquium
NAME OF THE CANDIDATE : Mr. Manish Saxena
DEGREE : Ph.D.
TITLE OF THE THESIS : Structured illumination techniques for performance
Enhancement in Optical metrology
SUPERVISOR : Prof. Sai Siva Gorthi
DATE & TIME : 22nd July, Monday @ 11:00 A.M.
VENUE : Seminar Hall, Dept. of Instrumentation and
Optical means of metrology provides advantage over the conventional mechanical means in terms of non-contact measurement, high speed interaction, high resolution, and whole-field information. Optical metrology finds application in micro- and meso-scale measurement domains like quantitative microscopy and active stereo vision. Various optical metrology systems have attempted to improve upon the performance parameters related to throughput and resolution. There exist inherent trade-offs which preclude improvement in one of these parameters without affecting the other. Attempts at increasing the throughput of a fluorescence-based imaging flow cytometer by increasing the flow rate of cells cause degradation in spatial resolution due to the higher motion-blur incurred. Similarly, attempts at capturing higher range of whole-field deformation by reducing the modulation frequency of projected pattern in active stereo vision techniques lead to reduction in the achievable resolution. This dissertation describes the approach of structuring the illumination, temporally and spatially, for achieving performance enhancement in quantitative microscopy and active stereo vision techniques respectively.
Compact hand-held fluorescence imaging flow cytometer is developed using miniature optics, LED, and off-the-shelf inexpensive camera. Low frame rate of an off-the-shelf camera leads to considerable motion blur leading to loss of spatial features of the objects being investigated. Figure-of-merit based algorithm using hybrid coarse and fine searches is applied to optimally deconvolve the images and restore the spatial content. Throughput of about 2900 beads per minute is achieved, and is ultimately limited by the camera frame rate. Further, the issues of throughput enhancement and improving the signal-to-noise ratio (SNR) are addressed by temporally structuring the illumination / excitation pattern. Excitation is broken up in time-domain using a series of light pulses within the exposure period of the camera. This causes the motion-blur of the fluorescing cells or beads to be modulated as per the excitation pattern. The pattern is selected such that it avoids nulls in the frequency spectrum of the motion-blur point spread function. Throughput enhancement by a factor of 5 is demonstrated in comparison to constant illumination. This method opens up the possibility of using low frame-rate cameras for achieving high throughput, and is amenable for deployment in handheld point-of-care devices.
The range of whole-field out-of-plane displacements measurable using fringe projection is improved by using new method of circular fringe projection. This method is able to measure whole-field out-of-plane deformation without phase ambiguity which is encountered in linear fringes. New methods of circular fringe analysis and phase reconstruction are introduced by making use of co-ordinate transformation. Both shape reconstruction as well as whole-field translation of dynamic objects is demonstrated using circular grating based fringe projection system. This method paves the way for measurement of increased range of out-of-plane displacements without compromising on the resolution of the system. Single-shot measurement of both in-plane and out-of-plane displacement is addressed by using colour encoding of the circular fringes in conjunction with speckle pattern. Speckles and fringes are separated using frequency domain filtering rather than colour-based separation, thereby allowing all three colour sub-frames to be utilized for fringe phase analysis. This method is able to offer a four-fold improvement over Fourier fringe analysis, while retaining single-shot acquisition capability. The capability to make unambiguous single-shot 3D measurements with higher range of out-of-plane displacement opens up the possibility of using cameras of lower frame rates than conventionally required for dynamic deformation measurements.
ALL ARE WELCOME
Date(s) - 22/07/2019
11:00 am - 12:00 pm
Seminar Hall, Dept. of Instrumentation and Applied Physics
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