Towers, David Peter (1991) The automatic and quantitative analysis of interferometric and optical fringe data. PhD thesis, University of Warwick.

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Abstract

Optical interference techniques are used for a wide variety of industrial measurements. Using holographic interferometry or electronic speckle pattern interferometry, whole field measurements can be made on diffusely reflecting surfaces to sub-wavelength accuracy.

Interference fringes are formed by comparing two states of an object. The interference phase contains information regarding the optical path difference between the two object states, and is related to the object deformation. The automatic extraction of the phase is critical for optical fringe methods to be applied as a routine tool. The solution to this problem is the main topic of the thesis.

All stages in the analysis have been considered: fringe field recording methods, reconstructing the data into a digital form, and automatic image processing algorithms to solve for the interference phase.

A new method for reconstructing holographic fringe data has been explored. This produced a system with considerably reduced sensitivity to environmental changes. An analysis of the reconstructed fringe pattern showed that most errors in the phase measurements are linear. Two methods for error compensation are proposed. The optimum resolution which can be attained using the method is lambda/90, or 4 nanometers.

The fringe data was digitised using a framestore and solid state CCD camera. The image processing followed three distinct stages : filtering the input data, forming a 'wrapped' phase map by either the quasi-heterodyne analysis or Fourier transform method, and phase unwrapping. The primary objective was to form a fully automatic fringe analysis package, applicable to general fringe data.

Automatic processing has been achieved by making local measurements of fringe field characteristics. The number of iterations of an averaging filter is optimised according to a measure of the fringe’s signal to noise. In phase unwrapping it has been identified that discontinuities in the data are more likely in regions of high spatial frequency fringes. This factor has been incorporated into a new algorithm where regions of discontinuous data are isolated according to local variations in the fringe period and data consistency.

These methods have been found to give near optimum results in many cases. The analysis is fully automated, and can be performed in a relatively short time, « 10 minutes on a SUN 4 processor.

Applications to static deflections, vibrating objects, axisymmetric flames and transonic air flows are presented. Static deflection data from both holographic interferometry and ESPI is shown.

The range of fringe fields which can be analysed is limited by the resolution of the digital image data which can be obtained from commercially available devices. For the quantitative analysis of three dimensional flows, the imaging of the fringe data is difficult due to large variations in localisation depth. Two approaches to overcome this problem are discussed for the specific case of burner flame analysis

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics T Technology > TA Engineering (General). Civil engineering (General)
Library of Congress Subject Headings (LCSH):	Holographic interferometry, Speckle -- Industrial applications, Diffraction patterns, Algorithms
Official Date:	1991
Dates:	Date Event 1991 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	Department of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Extent:	327 leaves : charts, illustrations
Language:	eng

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