Peter John Bryanston-Cross

A review of phase unwrapping techniques in fringe analysis

Abstract There has been an increasing interest in the automation of fringe analysis over the last decade. In recent years a number of researchers have addressed the problem of automatic phase unwrapping in fringe analysis. The advent of phase stepping and the development of FFT techniques has moved the emphasis in automated techniques away from fringe tracking and towards fringe counting or scanning approaches, but a major difficulty with such scanning techniques has been their susceptibility to noise. However, several alternative noise-immune phase unwrapping strategies have been proposed, but these approaches have not addressed the problems caused by large scale discontinuities such as those produced by aliasing. Other methods have been developed which confront this last difficulty. This paper gives an overview of the phase unwrapping problem and describes fringe tracking, fringe scanning, noise-immune and region processing techniques that have been developed in the search for the fully automated solution.

A Review of data fusion models and architectures: towards engineering guidelines

This paper reviews the potential benefits that can be obtained by the implementation of data fusion in a multi-sensor environment. A thorough review of the commonly used data fusion frameworks is presented together with important factors that need to be considered during the development of an effective data fusion problem-solving strategy. A system-based approach is defined for the application of data fusion systems within engineering. Structured guidelines for users are proposed.

Holographic deformation measurements by Fourier transform technique with automatic phase unwrapping

A carrier fringe technique for measuring surface deformation is described and verified by experiments. In contrast to conventional holography and fringe analysis, this holographic system is based on fiber optics and automatic spatial carrier fringe pattern analysis techniques. Single-mode optical fibers are used to transfer both the object and reference beams. Carrier fringes are generated by simply translating the object beam between two exposures. The fast Fourier transform method is used to process the interferograms. The experiment gives an example of the tile-level minimum spanning tree phase unwrapping technique and introduces a new pixel-level noise immune unwrapping strategy also based on minimum spanning trees. The test object is a centrally loaded disk. An excellent correlation between the theoretical deformation profile and that suggested by the technique is given.

Automatic interferogram analysis techniques applied to quasi-heterodyne holography and ESPI

Abstract This paper describes significant developments in methods for the automatic, quantitative analysis of interferograms. All areas of analysis have been considered: fringe field generation, pre-processing, and phase unwrapping. A new quasi-heterodyne holographic technique is described in which the image is reconstructed using a single beam. The errors in the reconstructed fringe field are mainly linear in form, and an error compensation scheme is proposed. The final error in the phase measurement using automatic analysis is λ/40. The process of image smoothing by an averaging filter is considered to reduce the effects of random noise. It is shown that by measuring the signal-to-noise ratio of the fringe field an optimum degree of smoothing may be applied. This is demonstrated on holographic and electronic speckle pattern interferometry (ESPI) data. Two methods for cosinusoidal fringe image combination are compared, using three or four fields. It is shown that an automatic analysis can be achieved using four phase stepped images. A new algorithm to automatically unwrap the phase of complex fringe patterns is described. The fringe field is segmented into small rectangular areas, called tiles. This allows local data to be obtained on fringe consistency and density. A confidence tree can then be formed to produce an optimal solution for the whole field. Results are presented and discussed for both holographic and ESPI data.

Contour measurement by fibre optic fringe projection and Fourier transform analysis

This paper describes an optical technique for contour measurement based on the fibre optic fringe projection and Fourier transform analysis. Sinusoidal fringes are produced by the interference of two spherical wavefronts emitted from the output fibre of a directional coupler. The fringe pattern is projected on an object surface and the deformed grating image is captured by a CCD camera for subsequent analysis by a microcomputer. In contrast to the conventional moire method, this method is relatively simple and accurate, and is capable of conducting fully automatic measurement.

Spatial phase stepping method of fringe-pattern analysis

A new method of fringe-pattern analysis by spatial phase stepping (SPS) is presented. SPS combines the computational simplicity of phase stepping with the single image analysis capability of the Fourier transform method. The technique works by subdividing a linearised fringe pattern into three component images, from which the phase is calculated by a three phase step algorithm. A computer simulation has been undertaken to demonstrate the theory and enable an analysis of systematic errors. In addition, two experimental transonic flow fields are solved by SPS and a comparison is made with results derived using the Fourier transform method.

Application of particle image velocimetry to large-scale transonic wind tunnels

Abstract Velocity measurements have been made in a large-scale transonic wind tunnel using particle image velocimetry. Diffraction-limited optics have been used to image seeding particles at up to 2.5 m from the flow. A practical system to record the data has been constructed such that the camera can be remotely focused on the plane of interest. Particle images were recorded onto 35 mm film. Results were obtained from Mach numbers 0.2 to 0.8, and automatically analysed to produce vector maps of the planar velocity field. It has also been shown that particle image velocity data can be successfully recorded onto a charged coupled device image sensor at transonic speeds. This approach eliminates the need for photographic processing and allows real time data acquisition in a digital form. The potential is then apparent for automatic, and near real-time processing, of particle image velocimetry data.

Two sub-pixel processing algorithms for high accuracy particle centre estimation in low seeding density particle image velocimetry

This article presents two algorithms for spatial processing of low seeding density PIV (particle image velocimetry) images which lead to sub-pixel precision in particle positioning. The particle centres are estimated to accuracies of the order of 0.1 pixel, yielding 1% error in velocity calculation. The first algorithm discriminates valid particles from the rest of the image and determines their centres in Cartesian coordinates by using a two-dimensional Gaussian fit. The second algorithm performs local correlation between particle pairs and determines instantaneous two-dimensional velocities. The methods have been applied initially to simulated data. Gaussian noise and distortion has then been added to simulate experimental conditions. It is shown that, in comparison with conventional methods, the new algorithms offer up to an order of magnitude higher accuracy for particle centre estimation. Finally, the Gaussian fit approach has been used to map an experimental transonic flow field from the stator trailing edge wake region of a cascade with an estimated error of 1%. The experimental results are found to be in good agreement with previous theoretical steady-state viscous calculations.

Phase difference determination by fringe pattern matching

A method of phase difference determination in interferometry is presented. In this method, the phase difference between two interferograms is determined by fringe pattern matching with subpixel accuracy. The signal-to-noise ratio is significantly improved due to the region-based fringe pattern matching and its effect of averaging noise. The experiment shows that this method is useful for the determination of phase difference between two equi-spaced fringe patterns, and it has the advantages of high precision of measurement and high resistance to noise.

Double-source holographic contouring using fibre optics

Abstract In this paper holographic contouring by the movement of the object beam through fibre optics is described. The contouring fringes can be visualized on the object surfaces. This contouring technique is based on a double-exposure hologram through fibre optics. Single-mode fibres are used to transfer both object and reference beams. Between the two exposures the object illumination fibre is displaced. The fringe type is analysed and several experimental results are given.