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.