NDTnet 1998 Aug, Vol.3 No.8

Abstract

A versatile phase stepping speckle shearing interferometer, that integrates a liquid crystal as a phase modulator is described. By implementation of the relevant algorithm upon phase stepped shearographic images it is possible to create a phase map, which provides data regarding the derivative of any out-of-plane surface displacement. The use of a liquid crystal cell as a phasemodulator in an optical arrangement has advantages over alternative methods in that it is a low voltage device that may be calibrated to operate over a range of wavelengths. However its use is normally limited by the spatial uniformity of the cells birefringence. The phase delay induced by a nematic liquid crystal cell under application of an electric field, was calculated for the whole area of the cell and two discreet points using the method described by Wu et al (1), where the intensities of the ordinary and extraordinary beams of a polarised laser source passing through the liquid crystal cell are monitored for a range of signal voltages. The results of the calibration are shown in Figure 1., which shows a close match between the three sets of data points. The small 'jumps' in the phase delay at 5 , 3 and are the result of the mathematical analysis used.

Figure 1. Calculated phase delay for 514nm, for the average and for discreet points on the liquid crystal cell.

An Argon ion laser was expanded onto an inspection surface, creating a speckle pattern that was imaged onto CCD camera array through a Michelson interferometric arrangement. The liquid crystal cell was integrated into an arm of the Michelson speckle shearing interferometer such that phase modulation of one pattern with respect to the other could be achieved. The system has been used for the non-destructive examination of sheets and pipes. Using images of stressed and unstressed samples, examples will be shown of phase maps where fringe anomalies reveal the location of sub-surface defects- This visual inspection technique offers flexibility not only in the extended range of monitoring wavelenghts, but also in the direction and amount of shear introduced by the interferometer.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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